WO2020190212A1

WO2020190212A1 - Electrically conductive spacers to enhance membrane distillation

Info

Publication number: WO2020190212A1
Application number: PCT/SG2020/050141
Authority: WO
Inventors: Yong Zen TAN; Huixiang Edison ANG; Jia Wei CHEW; Anthony Gordon Fane
Original assignee: Nanyang Technological University
Priority date: 2019-03-18
Filing date: 2020-03-18
Publication date: 2020-09-24
Also published as: MX2021010999A

Abstract

A membrane distillation system, provided herein, comprises a membrane module comprising a membrane arranged in the membrane module to define a feed channel and a distillate channel, and one or more electrically and thermally conductive spacers arranged proximal to the membrane, an induction heating module coupled to the one or more electrically and thermally conductive spacers, wherein the induction heating module is operable to heat the one or more electrically and thermally conductive spacers by electromagnetic induction. A membrane module, operable for membrane distillation, comprises a membrane arranged in the membrane module to define a feed channel and a distillate channel, and one or more electrically and thermally conductive spacers arranged proximal to the membrane, wherein the one or more electrically and thermally conductive spacers are configurable to be coupled to an induction heating module operable to heat the one or more electrically and thermally conductive spacers by electromagnetic induction.

Description

ELECTRICALLY CONDUCTIVE SPACERS TO ENHANCE MEMBRANE

DISTILLATION

Cross-Reference to Related Application

[0001] This application claims the benefit of priority of Singapore Patent Application No. 10201902404S, filed 18 March 2019, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] The present disclosure relates to a membrane module having one or more electrically and thermally conductive spacers operably heat-able by electromagnetic induction, and a membrane distillation system including such a membrane module.

Background

[0003] Membrane distillation (MD) may be deemed a promising thermal-driven separation technology, as it may be low-cost, easily integrated to existing systems, a green alternative (i.e. utilization of low-grade waste heat), compared to conventional water treatment processes like thermal distillation and reverse osmosis (RO). As membrane distillation happens to be a thermal-driven process, heat-transfer in addition to mass transfer have to be considered.

[0004] Membrane distillation, however, seems to remain commercially unfeasible at least due to lack of suitable membranes. Studies then focused on membrane fabrication and modification that optimize existing membranes specifically for use in membrane distillation, e.g. to increase permeability, mitigate fouling, increase resistance to membrane pore-wetting, and improve energy-efficiency by altering material surface energy, pore size, thickness, porosity, and tortuosity. Such efforts tend to focus on imbuing commercial membranes with additional functionalities, e.g. coating with superhydrophobic T1O2 and/or S1O2, which may not provide for long-lasting results in membrane distillation operations, and the implementation of membrane distillation gets restricted. Moreover, studies showed that incorporation of photothermal materials, such as T1O2, may provide localized heating via light irradiation, wherein such localized heating purports to uniformly increase surface temperature of the membrane and enhance vapor flux across the membrane.

[0005] While the photothermal materials enable solar energy to be utilized for the localized heating, may boost flux recovery after mild washing of membrane or produce better solute rejection rate, addition of photothermal materials on membrane may have limitations. For example, such membranes may suffer a decline in flux due to additional mass transfer resistance and/or vapor pressure depression arising from smaller pores of a membrane coated with the photothermal materials. Said differently, the photothermal materials may block the pores of membrane, thereby decreasing flux.

[0006] Other than photothermal materials, incorporation of electrically conductive materials to the membrane have been studied. The electrically conductive materials provide localized heating by directly passing an electrical current therein to generate heat (i.e. Joule heating). Similarly, the localized heating purports to uniformly increase surface temperature of the membrane and enhance vapor flux across the membrane. This approach, however, may suffer the same limitations from use of photothermals materials as mentioned above, e.g. undesirable increase in mass transfer resistance. In addition, Joule heating may not be effectively provided across electrical insulators, such as the plastic casing of a membrane module, potentially rendering it necessary for electrodes to contact wastewater to be treated for localized heating (e.g. electrodes may have to be fitted in a membrane module to be in electrical contact with the membrane - invasive heating), which may lead to corrosion of electrodes.

[0007] In view of the reported studies described above, it appears that few efforts were targeted at understanding the effect of flow patterns on performance of membrane distillation, most of which may either be related to module orientation and design or use and design of spacers. In particular, the tailoring of spacers for use in membrane distillation may not have been deeply investigated.

[0008] A spacer may function as a stationary turbulence promoter to mitigate membrane fouling and enhance permeation by lowering the boundary-layer mass transfer coefficient through the membrane. As for design of spacers tested in membrane distillation, research showed that the shape, configuration, diameter, and number of spacer filaments may significantly impact water vapor flux. Spacers tested may be made of low-cost polymers (e.g. polypropylene). As membrane distillation happens to be a thermal-driven process, it may be desirable to improve heat transfer apart from mass transfer rendered by spacers. For example, in conductive gap membrane distillation (CGMD), metallic spacers were proposed for improving performance based on their higher thermal conductivity. This may suggest that selection of spacer material potentially benefits membrane distillation performance.

[0009] There is thus a need to provide for a solution that ameliorates one or more of the limitations mentioned above. The solution should at least provide for beneficial alterations that avoid direct modification of the membrane. The solution should also provide for non-invasive heating to improve heat transfer in membrane distillation.

Summary

[0010] In one aspect, there is provided for a membrane distillation system comprising:

a membrane module comprising

a membrane arranged in the membrane module to define a feed channel and a distillate channel; and

one or more electrically and thermally conductive spacers arranged proximal to the membrane;

an induction heating module coupled to the one or more electrically and thermally conductive spacers, wherein the induction heating module is operable to heat the one or more electrically and thermally conductive spacers by electromagnetic induction.

[0011] In another aspect, there is provided for a membrane module operable for membrane distillation, the membrane module comprising:

one or more electrically and thermally conductive spacers arranged proximal to the membrane, wherein the one or more electrically and thermally conductive spacers are configurable to be coupled to an induction heating module operable to heat the one or more electrically and thermally conductive spacers by electromagnetic induction.

Brief Description of the Drawings

[0012] The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0013] FIG. 1 is a schematic of a direct contact membrane distillation (DCMD) module, wherein the feed and distillate (i.e. permeate) channels are of equal dimensions, the membrane is arranged to define the feed and distillate channels, and the spacer (a mesh with dimension of 3 mm on each side) is arranged at the feed side. The arrows in the top and bottom halves denote the countercurrent flows, wherein the arrows in the top and bottom halves denote feed and distillate flows, respectively. [0014] FIG. 2A shows a spacer of the present disclosure having a mesh density of 3 mm, drawn with the aid of COMSOL multiphysics software.

[0015] FIG. 2B shows a spacer of the present disclosure having a mesh density of 1.5 mm, drawn with the aid of COMSOL multiphysics software.

[0016] FIG. 2C shows a foam spacer of the present disclosure, drawn with the aid of COMSOL multiphysics software.

[0017] FIG. 3 A is a schematic of the experimental DCMD setup, consisting of (1) a feed tank (i.e. 2-L round-bottom flask) heated by a hot-plate and agitated with a magnetic stirrer, (2) three peristaltic pumps, (3) a cross flow flat-sheet acrylic membrane module, (4) 50 W LED lamp placed 4 cm above the membrane surface, (5) a distillate tank (i.e. 1-L acrylic cylinder with a spout) cooled by a recirculating chiller and with a conductivity meter inserted (denoted by the circle marked with C), and (6) an overflow distillate tank (300-mL beaker) placed on a mass balance. The circles marked with T each denotes a temperature sensor. NI DAQ represents a data acquisition module that records the readings from the temperature sensors.

[0018] FIG. 3B is a table indicating properties of Durapore GVHP hydrophobic polyvinylidene fluoride (PVDF) membrane.

[0019] FIG. 3C is a table indicating specifications of the channel in the cross flow flat-sheet acrylic membrane module of FIG. 3 A.

[0020] FIG. 3D is a table indicating specifications of a 3 mm polypropylene spacer.

[0021] FIG. 3E is a table indicating specifications of a nickel foam of the present disclosure.

[0022] FIG. 3F is a table indicating specifications of a copper foam of the present disclosure.

[0023] FIG. 4A is a scanning electron microscopy (SEM) image of the nickel foam. Scale bar denotes 100 mm.

[0024] FIG. 4B is a SEM image of the copper foam. Scale bar denotes 100 mm.

[0025] FIG. 5 shows the temperature contour plots of the feed-membrane interface obtained from simulations. The first, second and third columns respectively denote the polypropylene, nickel and copper spacers. The top, middle and bottom rows respectively denote the spacer with 3 mm mesh, spacer with 1.5 mm mesh and the foam spacer. The feed flow is from the right to the left.

[0026] FIG. 6 shows the temperature contour plots of the distillate-membrane interface obtained from simulations. The distillate side has a polypropylene spacer with 3 mm mesh. The first, second and third columns respectively denote the polypropylene, nickel and copper spacers at the feed side. The top, middle and bottom rows respectively denote the spacer with 3 mm mesh, spacer with 1.5 mm mesh and the foam spacer at the feed side. The distillate flow is from the left to the right.

[0027] FIG. 7A shows the surface-averaged temperature values obtained from simulations at the feed-membrane interface for the three spacer materials and three spacer densities.

[0028] FIG. 7B shows the surface-averaged temperature values obtained from simulations at the distillate-membrane interface for the three spacer materials and three spacer densities.

[0029] FIG. 7C shows the vapor pressure difference (p°_h- p° _c) between the two faces of the membrane from simulations for the three spacer materials and three spacer densities.

[0030] FIG. 8 shows the experimental flux magnitudes of the 3 mm mesh polypropylene spacer, nickel foam and copper foam. Each error bar denotes the span of two repeated experiments.

[0031] FIG. 9A shows the spatial flow velocity profile at the cross-section of the feed outlet and distillate inlet as simulated for the polypropylene 3 mm mesh spacer.

[0032] FIG. 9B shows the spatial flow velocity profile at the cross-section of the feed outlet and distillate inlet as simulated for the nickel foam.

[0033] FIG. 9C shows the spatial flow velocity profile at the cross-section of the feed outlet and distillate inlet as simulated for the copper foam.

[0034] FIG. 9D shows the surface-averaged spatial flow velocity at the feed side of the membrane surface for the polypropylene (Pp) 3 mm mesh spacer, nickel and copper foams.

[0035] FIG. 10 shows the experimental energy per unit volume distillate for the polypropylene 3 mm mesh spacer, nickel and copper foams. Each error bar denotes span of two repeated experiments.

[0036] FIG. 11 shows the experimental rate of heat loss across the membrane cell for the polypropylene 3 mm mesh spacer, nickel and copper foams. Each error bar denotes the span of two repeated experiments.

[0037] FIG. 12A shows a temperature contour plot of the feed-membrane interface obtained from simulations for the nickel foam at the feed side.

[0038] FIG. 12B shows a temperature contour plot of the distillate-membrane interface obtained from simulations for the nickel foam at the feed side.

[0039] FIG. 12C shows a temperature contour plot of the feed-membrane interface obtained from simulations for the nickel foam at the distillate side.

[0040] FIG. 12D shows a temperature contour plot of the distillate-membrane interface obtained from simulations for the nickel foam at the distillate side. [0041] FIG. 13 A shows the surface-averaged temperature of the feed-membrane interface from simulation when the nickel foam was at the feed and distillate sides of the membrane.

[0042] FIG. 13B shows the surface-averaged temperature of the distillate-membrane interface when the nickel foam was at the feed and distillate sides of the membrane.

[0043] FIG. 13C shows the (p°_h- p°_c) when the nickel foam was at the feed and distillate sides of the membrane.

[0044] FIG. 14A shows experimental results comparing the difference between the placement of the nickel foam at the feed versus distillate sides of the membrane in terms of distillate flux.

[0045] FIG. 14B shows experimental results comparing the difference between the placement of the nickel foam at the feed versus distillate sides of the membrane in terms of heater input energy per unit volume of distillate.

[0046] FIG. 14C shows experimental results comparing the difference between the placement of the nickel foam at the feed versus distillate sides of the membrane in terms of rate of heat loss across the membrane cell.

[0047] FIG. 15A is a SEM image of the nickel foam of present disclosure. The scale bar denotes 10 mm.

[0048] FIG. 15B is a transmission electron microscopy (TEM) image of platinum nanosheets (Pt NSs). The scale bar denotes 2 mm.

[0049] FIG. 15C is a SEM image of Pt NSs grown on the nickel foam of FIG. 15A. The inset shows energy dispersive x-ray (EDX) mapping images. The scale bar denotes 10 mm.

[0050] FIG. 15D shows a SEM image of the Pt NSs grown on nickel foam of FIG. 15A, that is of higher magnification compared to FIG. 15C. The scale bar denotes 1 mm.

[0051] FIG. 16 shows the EDX spectrum of the Pt-coated nickel foam.

[0052] FIG. 17 shows the magnitudes of flux for various spacers at the feed side (i.e. polypropylene (pp) 3 mm mesh spacer, nickel foam, Pt-coated nickel (Pt-Ni) foam and copper foam) in the absence and presence of visible light irradiation. Each error bar denotes the span of two repeated experiments. The expressions “Pt-coated nickel” and “Pt-nickel” are interchangeably used herein.

[0053] FIG. 18 shows the heater input energy per unit volume distillate of the DCMD system based on 3 mm mesh polypropylene spacer, nickel foam, Pt-Ni foam and copper foam, with and without visible light irradiation. Each error bar denotes the span of two repeated experiments. [0054] FIG. 19A is an infrared (IR) thermal image of the membrane with nickel foam before irradiation.

[0055] FIG. 19B is an infrared (IR) thermal image of the membrane with nickel foam after 1 min of light irradiation.

[0056] FIG. 19C is an infrared (IR) thermal image of the membrane with Pt-nickel foam before irradiation.

[0057] FIG. 19D is an infrared (IR) thermal image of the membrane with Pt-nickel foam after 1 min of light irradiation.

[0058] FIG. 20A is graphic representation of the separation process in a conventional DCMD with feed being externally heated.

[0059] FIG. 20B is graphic representation of the separation process in a DCMD using nickel foam at the feed side with localized induction heating.

[0060] FIG. 21 is a schematic of the experimental DCMD setup consisting of (1) a feed tank (i.e. 2 L round-bottom flask), (2) three peristaltic pumps, (3) a cross flow flat-sheet acrylic membrane distillation module, (4) a induction coil with a controller board, (5) a distillate tank (i.e. 1 L acrylic tank with a spout) cooled by a recirculating chiller and with a conductivity meter inserted (denoted by the circle marked with C), and (6) an overflow distillate tank (300 mL beaker) on a mass balance. The circles marked with T each denotes a temperature sensor.

[0061] FIG. 22 is a table comparing the experimental conditions for different modes of heating.

[0062] FIG. 23A compares the flux results from the three heating modes. Each error bar denotes the span of two repeated experiments.

[0063] FIG. 23B compares the total input heating energy per unit volume distillate from the three heating modes. Each error bar denotes the span of two repeated experiments.

[0064] FIG. 23C compares the average feed temperature from the three heating modes. Each error bar denotes the span of two repeated experiments.

[0065] FIG. 24A shows the temperature contour plots of the feed-membrane interface obtained from simulations of external hot-plate heating to the feed side. The feed flow is from the right to the left.

[0066] FIG. 24B shows the temperature contour plots of the feed-membrane interface obtained from simulations of localized induction heating to the feed side. The feed flow is from the right to the left.

[0067] FIG. 24C shows the temperature contour plots of the feed-membrane interface obtained from simulations of a heating mode comprising combination of external hot-plate and localized induction heating (i.e. combined heating) to the feed side. The feed flow is from the right to the left.

[0068] FIG. 25A shows the temperature contour plots of the distillate-membrane interface obtained from simulations of external hot-plate heating to the feed side. The distillate flow is from the left to the right.

[0069] FIG. 25B shows the temperature contour plots of the distillate-membrane interface obtained from simulations of localized induction heating to the feed side. The distillate flow is from the left to the right.

[0070] FIG. 25C shows the temperature contour plots of the distillate-membrane interface obtained from simulations of the combined heating mode to the feed side. The distillate flow is from the left to the right.

[0071] FIG. 26A compares the simulated results for surface-averaged temperature of the feed- membrane interface for the three heating modes.

[0072] FIG. 26B compares the simulated results for surface-averaged temperature of the distillate-membrane interface for the three heating modes.

[0073] FIG. 26C compares the simulated results for (p°_h- p°_c) for the three heating modes.

[0074] FIG. 27A shows a stacked membrane module with an electromagnetic induction coil designed to compensate for the loss in driving force along the membrane.

[0075] FIG. 27B shows the stacking configuration in a stacked membrane distillation module used in the present membrane distillation system n may be an integer ranging from, for example, 1 to 3 per induction coil.

[0076] FIG. 28A shows the use of the present electromagnetic induction heating to heat the feed flow in the core of the fiber using an electrically and thermally conductive spacer material placed in the core.

[0077] FIG. 28B shows the use of the present electromagnetic induction heating to heat the feed flow around the fiber using using an electrically and thermally conductive spacer material wrapped around the membrane fibers.

[0078] FIG. 29A is a diagram showing the placement of the induction heating coil at bottom of the membrane distillation module (i.e. proximal to distillate side). The arrow distal to the induction heating coil and the arrow proximal to the induction heating coil represent the direction and channel of the feed and distillate flows, respectively. The grey block between the two arrows represents a spacer of the present disclosure, e.g. nickel foam. [0079] FIG. 29B is a diagram showing the placement of the induction heating coil at the top of the membrane distillation module (i.e. proximal to feed side). The arrow distal to the induction heating coil and the arrow proximal to the induction heating coil represent the direction and channel of the distillate and feed flows, respectively. The grey block between the two arrows represents a spacer of the present disclosure, e.g. nickel foam.

[0080] FIG. 30 shows digital images of metal foams after an experimental run of 24 hours.

[0081] FIG. 31 shows the X-ray diffraction (XRD) patterns of pristine and used nickel (Ni) foam. The pristine nickel foam XRD pattern was offset by a 2Q of -2° for ease of comparison.

[0082] FIG. 32A is a SEM image of pristine nickel foam of the present disclosure. Scale bar denotes 100 mm.

[0083] FIG. 32B is a SEM image of used nickel foam of FIG. 32A. Scale bar denotes 100 mm.

[0084] FIG. 33A shows the flux for graphene oxide (GO) coated nickel foam with and without induction heating at a feed flow rate of 250 mL/min and 50mL/min.

[0085] FIG. 33B shows the heater energy per unit volume distillate for graphene oxide (GO) coated nickel foam with and without induction heating at a feed flow rate of 250 mL/min and 50mL/min.

[0086] FIG. 34A shows a plot of eddy current against voltage for various coated nickel foam.

[0087] FIG. 34B shows a plot of surface temperature against voltage for various coated nickel foam.

[0088] FIG. 35A shows the flux from the scaled-up version of the membrane distillation module with an electromagnetic induction coil, with respect to different heat supplied. IH supplemented refers to the induction heating approach of the present disclosure.

[0089] FIG. 35B shows the heat input to the scaled-up version of the membrane distillation module with the electromagnetic induction coil, with respect to different heat supplied. IH supplemented refers to the induction heating approach of the present disclosure.

[0090] FIG. 35C shows the total input heating energy per unit volume distillate for the scaled- up version of the membrane distillation module with an electromagnetic induction coil, with respect to different heat supplied. IH supplemented refers to the induction heating approach of the present disclosure.

[0091] FIG. 36A shows a scaled-up of the membrane distillation module with the electromagnetic induction coil. [0092] FIG. 36B compares the lab-scale membrane distillation module and the scaled-up version, both of the present disclosure. In FIG. 36B, the lab-scale membrane distillation module is placed bottom to the scaled-up module.

[0093] FIG. 37A shows the vapor pressure difference between the two faces of the membrane (p°_h- p° _c) for the polypropylene mesh and nickel foam setups. Each error bar denotes the span of two repeated simulations.

[0094] FIG. 37B shows the membrane surface temperature difference between the feed and distillate sides (DT_m) for the polypropylene mesh and nickel foam setups. Each error bar denotes the span of two repeated simulations.

[0095] FIG. 37C shows the flux from the polypropylene mesh and nickel foam setups. Each error bar denotes the span of two repeated experiments.

[0096] FIG. 37D shows heater input energy per unit volume distillate for the polypropylene mesh and nickel foam setups. Each error bar denotes the span of two repeated experiments.

[0097] FIG. 38A highlights the effect of imperfect contact between the membrane surface and the nickel foam on vapor pressure difference between the two faces of the membrane (p°_h - p° _c).

[0098] FIG. 38B highlights the effect of imperfect contact between the membrane surface and the nickel foam on membrane surface temperature difference between the feed and distillate side ( DT_m ).

[0099] FIG. 39A shows the flux at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating. Each error bar denotes the span of two repeated experiments.

[00100] FIG. 39B shows the heater input energy per unit volume distillate at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating. Each error bar denotes the span of two repeated experiments.

[00101] FIG. 39C shows the simulated vapor pressure difference between the two faces of the membrane at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating.

[00102] FIG. 39D shows the simulated membrane surface temperature difference between the feed and distillate sides (DT_m) at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating.

[00103] FIG. 40A compares rate of heat loss across the membrane at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating, to illustrate the effect of different feed flow rates coupled with different modes of heating. Each error bar denotes the span of two repeated experiments.

[00104] FIG. 40B compares the thermal efficiency (TE) of the membrane distillation process at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating, to illustrate the effect of different feed flow rates coupled with different modes of heating. Each error bar denotes the span of two repeated experiments.

[00105] FIG. 40C compares the temperature polarization coefficient (TPC) of the membrane distillation process at different flow rates calculated using simulation results for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating, to illustrate the effect of different feed flow rates coupled with different modes of heating.

[00106] FIG. 41 A compares the flux at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating. Each error bar denotes the span of two repeated experiments.

[00107] FIG. 4 IB compares the heater input energy per unit volume distillate at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating. Each error bar denotes the span of two repeated experiments.

[00108] FIG. 41C compares the simulated vapor pressure difference between the two faces of the membrane (p°_h- p°_c) at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating.

[00109] FIG. 4 ID compares the membrane surface temperature difference between the feed and distillate side (DT_m) at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating.

[00110] FIG. 42A compares the flux from membrane distillation processes using polypropylene mesh and nickel foam with external feed heating, and nickel foam with localized induction heating. Each error bar denotes the span of two repeated experiments.

[00111] FIG. 42B compares the total heater input energy per unit volume distillate for membrane distillation processes using polypropylene mesh and nickel foam with external feed heating, and nickel foam with localized induction heating. Each error bar denotes the span of two repeated experiments. [00112] FIG. 42C compares the TE of membrane distillation processes using polypropylene mesh and nickel foam with external feed heating, and nickel foam with localized induction heating. Each error bar denotes the span of two repeated experiments.

[00113] FIG. 42D compares the simulated vapor pressure difference between the two faces of the membrane (p°_h- p°_c) for membrane distillation processes using polypropylene mesh and nickel foam with external feed heating, and nickel foam with localized induction heating.

[00114] FIG. 43A compares the change in feed temperature, DT_f, for experiments conducted to investigate effect of feed flow rate coupled with different modes of heating (see example 7E). Each error bar denotes the span of two repeated experiments.

[00115] FIG. 43B compares the change in feed temperature, DT_f, for experiments conducted to investigate effect of varying induction heater power input (see example 7D). Each error bar denotes the span of two repeated experiments.

[00116] FIG. 43C compares the change in feed temperature, DT_f, for experiments conducted to investigate use of induction heating to mitigate fluctuations in supply of waste heat. Each error bar denotes the span of two repeated experiments.

[00117] FIG. 44 compares the flux for the induction assisted membrane distillation experiments. Each error bar represents the span of two repeated experiments. FIG. 44 further substantiates the results from FIG. 35, in that a higher flux may be achieved for low external heat input supplemented with induction heating.

[00118] FIG. 45 compares the flux of the pristine nickel foam (before use) and the used nickel foam (after 168 hrs of use) under different heating modes at a feed flow rate of 250 mL/min.

[00119] FIG. 46A compares rate of heat loss across the membrane at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating, to illustrate the effect of varying induction heater power input. Each error bar denotes the span of two repeated experiments.

[00120] FIG. 46B compares the thermal efficiency (TE) of the membrane distillation process at different flow rates for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating, to illustrate the effect of varying induction heater power input. Each error bar denotes the span of two repeated experiments.

[00121] FIG. 46C compares the TPC of the membrane distillation process at different flow rates calculated using simulation results for polypropylene mesh and nickel foam setups with external feed heating, and nickel foam setup with the localized induction heating, to illustrate the effect of varying induction heater power input. Each error bar denotes the span of two repeated experiments.

Detailed Description

[00122] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[00123] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[00124] The present disclosure relates to a membrane distillation system having a membrane module and an induction heating module. The membrane module may have a membrane that separates undesirable substances, for example, from water in water and/or wastewater treatment. The membrane may also be suitable for separating any substance from another. Apart from the membrane, the membrane module may have one or more electrically and thermally conductive spacers. For brevity, the electrically and thermally conductive spacer may be termed herein as a“metallic spacer” where the spacer is formed of a metal, or simply a “spacer”. The electrically and thermally conductive spacer may be formed of an electrically and thermally conductive material, apart from metal, which can be heated by electromagnetic induction. Membrane distillation is a thermal process, and the one or more electrically and thermally conductive spacers are advantageous therefor. The one or more electrically and thermally conductive spacers render membrane distillation more energy efficient and economically viable. The one or more spacers of the present disclosure, being both electrically and thermally conductive, can be induced to have a current generated therein, which converts to thermal energy effectively, conferring a higher temperature to the feed at the surface of the membrane. This minimizes or eliminates heat loss, wherein the heat is needed to maintain a thermal-driving force for separating materials through the membrane in membrane distillation. The one or more electrically and thermally conductive spacers, which may be of the same length as or shorter than the membrane, also provides for more uniform heat distribution to the feed at the surface of the membrane, when placed at the feed side (i.e. in the feed channel). The feed channel refers to the part of the membrane module which the feed flows to after entering the membrane module. The one or more electrically and thermally conductive spacers distribute heat more evenly to the feed when placed in the feed channel. This means that less or no parts of the membrane become susceptible to poor heat transfer, which may render poor separation results (e.g. flux, rejection rate).

[00125] Advantageously, with induction heating (interchangeably termed herein as “electromagnetic induction heating”,“localized induction heating”, or“non-invasive heating”) of such spacers, direct contact heating may be avoided. Direct contact heating requires electrodes to be connected to electrical parts within the membrane module to pass a current therethrough (i.e. invasive heating). In some instances, the electrodes may have to be disposed in the membrane module as electrical contact may be hindered by insulating parts. Such electrical parts and/or the electrodes undesirably require additional chemical protection, which may otherwise corrode if they contact a feed containing substances that react them away. The induction heating of the one or more electrically and thermally conductive spacers relies on electromagnetism, which does not require any direct contact to generate a current, avoids the limitations of invasive heating. Since induction heating requires no heating element to be in physical contact with the present spacers nor the present membrane module, the present membrane distillation system is thus not limited to one membrane module. In other words, one induction heating module may be used to heat one or more of the present membrane modules in the present membrane distillation system. For example, where three membrane modules each having the present spacers fitted therein are used, the induction heating module may be arranged between any two of the three membrane modules. The induction heating module need not be sandwiched between any two of the three membrane modules, e.g. arranged proximal to a side of one out of the three membrane modules.

[00126] With use of the one or more spacers, modifications to membrane can be avoided or considered an alternative. With the one or more spacers, photothermal materials can be coated thereon instead of the membrane. If coated on membrane, the photothermal materials may be susceptible to coating stability issues that result in the photothermal materials detaching from the membrane over time, or the photothermal materials may block membrane pores and compromise flux.

[00127] Other advantages of the present membrane distillation system and membrane module having the one or more electrically and thermally conductive spacers, are already described below and demonstrated in the examples herein. Details of the present membrane distillation system and membrane module, and their various embodiments are described as follows.

[00128] The first aspect of the present disclosure provides for a membrane distillation system. The membrane distillation system may comprise a membrane module comprising a membrane arranged in the membrane module to define a feed channel and a distillate channel, and one or more electrically and thermally conductive spacers arranged proximal to the membrane. The membrane distillation module may also comprise an induction heating module coupled to the one or more electrically and thermally conductive spacers, wherein the induction heating module may be operable to heat the one or more electrically and thermally conductive spacers by electromagnetic induction.

[00129] As mentioned above, the one or more electrically and thermally conductive spacers may be arranged proximal to the membrane. That is to say, the one or more electrically and thermally conductive spacers may be incorporated in the membrane module at the side of the membrane that faces the incoming feed. In various embodiments, the one or more electrically and thermally conductive spacers may be arranged proximal to a surface of the membrane facing the feed channel. This means that the one or more electrically and thermally conductive spacers may or may not be in contact with the surface of the membrane facing the feed channel. The one or more electrically and thermally conductive spacers may be arranged as close as possible to the membrane surface. For example, the one or more electrically and thermally conductive spacers may be arranged in the feed channel at most 0.05 mm, at most 0.1 mm, at most 0.2 mm, at most 0.4 mm, or at most 1 mm, etc. away from the surface of the membrane. For example, the one or more electrically and thermally conductive spacers may be arranged in the feed channel at about, for example, 0.1 mm to 0.2 mm, or 0.2 mm to about 0.4 mm, etc., away from the surface of the membrane. As there may be liquid flowing between the membrane and the one or more electrically and thermally conductive spacers, and given that the membrane may be flexible, the distance between the membrane and the one or more electrically and thermally conductive spacers may not be uniform with respect to the entire membrane. That is to say, not all parts of the membrane may be at the same distance to the one or more electrically and thermally conductive spacers during operation in the presence of a liquid. Nevertheless, the arrangement is advantageous as it helps to maintain or heat up the bulk of the feed, so the feed that contacts the membrane’s surface may be of a desirable uniform temperature. The arrangement also helps to distribute heat uniformly in the feed, and hence render temperatures at the surface of the membrane more uniform. The one or more electrically and thermally conductive spacers may be arranged in the feed channel such that the one or more electrically and thermally conductive spacers prevent the feed bypassing therefrom. For example, the one or more electrically and thermally conductive spacers may be arranged to adopt a shape of housing of the membrane module such that the feed has to pass through the membrane before the feed can come into contact with the membrane. For example, the one or more electrically and thermally conductive spacers may be wrapped around the feed side of the membrane when the membrane is a hollow fiber, or each of the one or more electrically and thermally conductive spacers may be placed in a core (i.e. lumen) of a hollow fiber, wherein the lumen happens to be configured as the feed channel.

[00130] In some embodiments, the one or more electrically and thermally conductive spacers may be arranged in contact with the surface of the membrane facing the feed channel. The arrangement is advantageous as it may be easier to render a more uniform distribution of heat at membrane’s surface. The advantages of arranging the one or more electrically and thermally conductive spacers proximal to the membrane’s surface may be applicable for this arrangement.

[00131] Where the one or more electrically and thermally conductive spacers are arranged at the feed side (i.e. in the feed channel), the distillate side may be supported with a polymeric spacer to provide mechanical support to the membrane. The distillate side or distillate channel may be defined herein as the side which substances separated through the membrane flows therefrom and out of the membrane module.

[00132] In various embodiments, the one or more electrically and thermally conductive spacers may comprise or consist of carbon, cobalt, copper, graphene, iron, nickel, or stainless steel. Such materials are advantageous as they may be electromagnetically induced to generate a desirable amount of heat sufficient for membrane distillation more efficiently than conventional heaters and power supply units that rely on Joule heating using electrodes, as heat generated by induction heating of such materials may be higher in terms of the input energy per unit volume distillate, rendering the present membrane distillation system and membrane module more energy efficient. The present system involving electromagnetic induction heating induces eddy currents, which led to heating near and/or at a surface of the membrane facing the feed channel. In contrast, Joule heating utilizes the passing of a current through electrodes, which the present system circumvents to advantageously avoid corrosion issues of the electrodes.

[00133] The membrane distillation system may further comprise a material, which absorbs electromagnetic waves, incorporated to the one or more electrically and thermally conductive spacers. The material may comprise or consist of carbon nanotubes or graphene oxide. These materials may enhance electromagnetic shielding and increase in surface eddy current, both of which improves the heat produced and heat transfer. The electromagnetic shielding from such materials means that these materials may absorb electromagnetic waves from the induction heating unit and get induced to produce heat. The increase in surface eddy current promotes turbulence in the feed channel and at the membrane’s surface, thereby improving heat transfer. Moreover, the turbulence may mitigate fouling of the membrane surface.

[00134] In various embodiments, the one or more electrically and thermally conductive spacers may comprise or consist of a porous structure, or the one or more electrically and thermally conductive spacers may be in the form of particulates. The porous structure may comprise or consist of a mesh or a foam. The porous structure may comprise or consist of pores of a size ranging from 1.5 mm to 3.5 mm, 1.5 mm to 1.5 mm, 10 mm to 1.5 mm, 100 mm to 1.5 mm, 1000 mm to 1.5 mm, 1.5 mm to 3.5 mm, 2 mm to 3.5 mm, 2.5 mm to 1.5 mm, 3 mm to 3.5 mm, etc. For example, the mesh may have a pore size in the range of 1.5 mm to 3.5 mm, e.g. 1.5 mm or 3 mm. The foam may have a pore size of 3.5 mm or less, 3 mm or less, 2.5 mm or less, 2 mm or less, 1.5 mm or less, 1 mm or less, 100 mm or less, 10 mm or less, etc. The mesh may help to promote turbulence, which in turn improves heat transfer. The foam may render more uniform heat transfer across the membrane surface due to less pores therein. Where the one or more electrically and thermally conductive spacers are in the form of particulates, the particulates may be disposed at the feed side of the membrane, e.g. disposed in feed channel of membrane module.

[00135] The present membrane distillation system may further comprise a photothermal material incorporated to the one or more electrically and thermally conductive spacers, wherein the photothermal material may comprise or consist of platinum nanosheets. The photothermal material is advantageous as it allows the membrane distillation to be operated using solar energy in addition to the induction heating. For example, when solar energy is insufficient or fluctuates, the induction heating may be used to mitigate such instances. [00136] The membrane distillation system includes the induction heating module, which may comprise an induction heating element arranged to electromagnetically induce heating of the one or more electrically and thermally conductive spacers. The present system is versatile in that the induction heating element need not be restricted to a position relative to the one or more electrically and thermally conductive spacers in the membrane module. This is because the induction heating allows for contactless heating, i.e. the induction heating element need not be in physical contact with the one or more electrically and thermally conductive spacers. Therefore, the induction heating element, including the entire induction heating module may be arranged external to the membrane module, or even at any suitable distance, for working around any space contraints without compromising the induction heating. The induction heating element may be in the form of a coil. The coil may be coiled around the housing of the membrane module, or placed at one side of the housing. Other suitable induction heating element may be used as long as they are able to induce a sufficient amount of heat generation by electromagnetism.

[00137] The membrane module may be in fluid communication with a feed pump operable to deliver a feed to the membrane module and produce a cross flow velocity along the surface of the membrane ranging from 0.8 mm/s to 21 mm/s, 1 mm/s to 21 mm/s, 5 mm/s to 21 mm/s, 10 mm/s to 21 mm/s, 15 mm/s to 21 mm/s, etc. The feed pump, together with induction heating of the one or more electrically and thermally conductive spacers, help to improve cross flow velocity at the membrane surface, which in turn aids in better heat transfer. For brevity, other advantages of such cross flow velocity shall not be iterated as they have been demonstrated via the examples herein.

[00138] The membrane distillation system may further comprise a heating module coupled to the feed pump, wherein the heating module is operable to render the feed pump to deliver the feed at a temperature ranging from 35°C to 70°C, 40°C to 70°C, 45°C to 70°C, 50°C to 70°C, 55°C to 70°C, 60°C to 70°C, 65°C to 70°C, etc., to the membrane module. The temperature range allows for various design of the present spacers to be used. For instance, higher range temperatures, e.g. more than 45°C, may be more compatible with foam spacers of the present disclosure. The higher temperature may compensate for the decrease in cross flow velocity when foam spacers are used, as the foam spacers may be for better heat transfer compared to mesh spacers that may be for better mass transfer. The examples herein already demonstrate the respective advantages for each spacer types and their compatible temperature range. [00139] The membrane in the membrane module may comprise or consist of a hydrophobic membrane comprising polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE). Other membranes suitable for membrane distillation may also be used. The membrane may comprise or may be a hollow fiber membrane or a flat sheet membrane. Where the membrane comprise a hollow fiber membrane, there may be a plurality of hollow fiber membranes.

[00140] The present disclosure also provides for a membrane module operable for membrane distillation. The membrane module may comprise a membrane arranged in the membrane module to define a feed channel and a distillate channel, and one or more electrically and thermally conductive spacers arranged proximal to the membrane, wherein the one or more electrically and thermally conductive spacers may be configurable to be coupled to an induction heating module operable to heat the one or more electrically and thermally conductive spacers by electromagnetic induction. Embodiments and advantages described for the present membrane distillation system of the first aspect can be analogously valid for the present membrane module subsequently described herein, and vice versa. As the various embodiments and advantages have already been described above and examples demonstrated herein, they shall not be iterated for brevity. In various embodiments, the one or more electrically and thermally conductive spacers may be coupled to the induction heating module.

[00141] In the membrane module, the one or more electrically and thermally conductive spacers may be arranged proximal to a surface of the membrane facing the feed channel. The distance at which the one or more electrically and thermally conductive spacers may be arranged from the membrane has already been described above. The one or more electrically and thermally conductive spacers may be arranged in contact with the surface of the membrane facing the feed channel.

[00142] In various embodiments, the one or more electrically and thermally conductive spacers may comprise or consist of carbon, cobalt, copper, graphene, iron, nickel, or stainless steel.

[00143] The membrane module may further comprise a material which absorbs electromagnetic waves, wherein the material may be incorporated to the one or more electrically and thermally conductive spacers. The material may comprise or consist of carbon nanotubes or graphene oxide, as already mentioned in various embodiments of the first aspect.

[00144] The one or more electrically and thermally conductive spacers may comprise or consist of a porous structure, or the one or more electrically and thermally conductive spacers may be in the form of particulates. As already described above, the porous structure may comprise or consist of a mesh or a foam. The porous structure may comprise or consist of pores of a uniform size ranging from 1.5 mm to 3.5 mm, 1.5 mm to 1.5 mm, etc. The other ranges for size of the pores and other embodiments related to the porous structure are already described above and shall not be iterated for brevity. Similarly, as already described above, the particulates may be disposed at the feed side of the membrane, e.g. disposed in feed channel of membrane module.

[00145] The membrane module may further comprise a photothermal material incorporated to the one or more electrically and thermally conductive spacers, wherein the photothermal material may comprise or consist of platinum nanosheets.

[00146] The membrane in the membrane module may comprise or consist of a hydrophobic membrane comprising polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE). The membrane may comprise or may be a hollow fiber membrane or a flat sheet membrane. Where the membrane comprises a hollow fiber membrane, there may be a plurality of hollow fiber membranes.

[00147] The membrane module may be coupled to the induction heating module. The term “coupled” used herein means that the membrane module and the induction heating module need not be in physical contact, but are arranged such that the one or more electrically and thermally conductive spacers can be heated by electromagnetic induction from the induction heating module. The induction heating module may comprise or consist of an induction heating element arranged to electromagnetically induce heating of the one or more electrically and thermally conductive spacers. Embodiments regarding the induction heating element have been described above and shall not be iterated for brevity.

[00148] In the present disclosure, the word“substantially” does not exclude“completely” e.g. a composition which is“substantially free” from Y may be completely free from Y. Where necessary, the word“substantially” may be omitted from the definition of the invention.

[00149] In the context of various embodiments, the articles“a”,“an” and“the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[00150] In the context of various embodiments, the term“about” or“approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

[00151] As used herein, the term“and/or” includes any and all combinations of one or more of the associated listed items. [00152] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[00153] While the methods described above are illustrated and described as a series of steps or events, it will be appreciated that any ordering of such steps or events are not to be interpreted in a limiting sense. For example, some steps may occur in different orders and/or concurrently with other steps or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement one or more aspects or embodiments described herein. Also, one or more of the steps depicted herein may be carried out in one or more separate acts and/or phases.

Examples

[00154] The present disclosure relates to a membrane module for membrane distillation, and a membrane distillation system including such membrane module. Particularly, the present disclosure relates to an electrically and thermally conductive spacer for use in a membrane distillation module, and induction heating of the electrically and thermally conductive spacer by electromagnetic induction. For brevity, the electrically and thermally conductive spacer may be termed herein as“metallic spacer”.

[00155] In relation to the spacer and based on findings herein that an electrically conductive material that conducts more electricity may facilitate better heat conduction across the membrane, metallic spacers were hence investigated to assess the benefits from higher thermal conductivity from use of such spacers. In addition, the effect of incorporating additional functionalities (e.g. photothermal materials) to the spacer to improve membrane disillation performance was investigated. In the examples below, the spacer was investigated in a direct contact membrane distillation (DCMD) system via both computer simulations and actual experiments. A polypropylene spacer having a mesh dimension of 3 mm was used as a benchmark for comparison. The results from two metals investigated, nickel and copper, are discussed in the examples below for brevity. Effect of spacer densities, e.g. three spacer densities of 3 mm mesh, 1.5 mm mesh, and foam, have been studied as well. Further, the examples below demonstrated that a nickel spacer coated with photocatalytic platinum enhances energy efficiency via photothermal conversion. From the simulations, surface temperatures at both feed and distillate sides of the membrane, and velocity profiles were obtained. From the experiments, the distillate flux, heater input energy per unit volume distillate, and rate of heat loss were obtained. The results showed improvement to energy efficiency is possible using the present electrically and thermally conductive spacers and such spacers that have been photocatalytic-enhanced.

[00156] Use of the electrically and thermally conductive spacer renders generation of electrical current via electromagnetic induction, so as to provide localized heating, possible. This is advantageous due to the no-contact characteristic that involves fast-switching poles of electromagnetic fields that heats up an electric conductor affected by the electromagnetic field, allowing for heating and enhancing heat transfer even in the presence of electrical insulators, such as the membrane module casing that houses the spacer and the membrane. Hence, with electromagnetic induction heating of the electrically and thermally conductive spacer, it becomes possible to provide non-invasive heating across electrical insulators, which in turn heats up wastewater that are fed to the membrane for treatment. Moreover, corrosion of electrodes can be circumvented. In fact, such non-invasive heating is favored in food industries for its faster, better-controlled, uniform and more energy efficient heating. Prior to this, induction heating of the wastewater feed suffered heat loss or detrimentally requires a high heating rate, due to inability to heat the feed-membrane interface effectively and directly. For instance, coating of the membrane with metallic materials for heating had to be as thin as possible for not compromising flux but this produces a lower heating rate. The present disclosure circumvents such a limitation by the localized induction heating of the electrically and thermally conductive spacer for membrane distillation. For brevity, the examples for elucidating the above refer to tests conducted using a nickel foam spacer, which is not meant to be limiting, as other spacer materials have been used.

[00157] Details of the present membrane distillation system and membrane module including the electrically and thermally conductive spacer, are further discussed, by way of non-limiting examples set forth below.

[00158] Example 1A: Computational Fluid Dynamics Simulations - Simulation Setup

[00159] Three-dimensional (3D) computational fluid dynamics (CFD) simulations of a DCMD module as shown in FIG. 1 were carried out using Comsol Multi-Physics 5.1. The simulations investigated the effect different spacers had on the heat and mass transfer near the membrane surface. The governing equations used were the Navier-Stokes and continuity equations (listed in example 8), and the physics packages in Comsol used were Laminar Flow (because Reynolds number ranged from 9 to 229 (e.g. 149) in the 4 mm channel) and Heat Transfer. Laminar flow was used in this case to eliminate the effect of turbulence on heat transfer to better study the effect of thermal conductivity of the different spacers on membrane distillation performance. To simplify the simulation, the module walls were adiabatic and heat transfer was from the hot feed to the cold distillate. This simplification does not affect the comparison of the spacers as the spacers did not have contact with the top of the acrylic cell, rendering heat loss to the environment similar regardless of spacers used. Thermal conductivity of the membrane (k_membrane = 0.06715 W/nrK) was calculated using equation (1):

[00160] where e is the membrane porosity, fe_vdf is the thermal conductivity of polyvinylidene fluoride (PVDF) (0.19 W/mK) and k_ai is the thermal conductivity of air at 100 % relative humidity (0.0262 W/m K). The feed and permeate flowed counter-currently, with each inlet flow rate set at 250 mL/min.

[00161] As different pre-defined mesh sizes with tetrahedral elements gave similar results, the ‘normal’ mesh (with the finest dimension being 0.412 mm and the largest dimension being 1.38 mm) was selected to shorten the computation time.

[00162] Example IB: Simulation - Effect of Spacer Material and Mesh Density

[00163] To study the effect of spacer material and spacer mesh density, a total of 9 simulations were carried out with three different spacer materials (polypropylene (pp), nickel (Ni) and copper (Cu)), each with three different mesh densities (FIG. 2A to 2C). Specifically, the spacer with lowest mesh density was such that each grid had four equal sides of 3 mm each (with a filament diameter of 0.26 mm and a thickness of 0.52 mm, FIG. 2A), followed by one of intermediate density with each grid that had four equal sides of 1.5 mm each (with the same filament diameter and thickness as the less dense spacer, FIG. 2B), and lastly the densest one being the 1 mm thick foam (i.e. sponge-like with random pores, FIG. 2C). These were simulated using the porous media sub-model in Comsol. It should be noted that the spacer with lowest mesh density (FIG. 2A) was designed to match that of the polypropylene spacer used in the experiments. The material properties such as density, thermal conductivity and heat capacity were obtained from the Comsol material library and used as is.

[00164] Example 1C: Simulation - Effect of Spacer Position

[00165] To investigate whether positioning of the spacer in the feed or permeate channel impacts membrane distillation, the nickel foam (FIG. 2C) was simulated in the feed channel and the permeate channel separately.

[00166] Example 2A: Experimental Study - Experimental Setup [00167] An experimental study was carried out in parallel with the simulations above to evaluate the effect of spacer material and spacer mesh density on DCMD performance (e.g. flux and energy efficiency).

[00168] A schematic of the experimental DCMD setup is shown in FIG. 3A. The membrane module was made of clear acrylic (detailed specifications listed in FIG. 3B) without additional thermal insulation, allowing visible light to pass through to the feed channel with minimal absorption and diffraction losses, thereby allowing the spacers conferred with photothermal capability to convert light to heat in the feed channel. The membrane module was operated in counter-current cross flow mode, which was similar to that of the simulation (FIG. 1). The circulation of the feed and distillate streams from their respective tanks was carried out using a peristaltic pump (Masterflex L/S Digital Drive) for each. The feed tank (2-L round-bottom flask) was heated and agitated using a hot-plate stirrer (Heidolph MR Hei-Tec), and the feed was circulated at 250 mL/min through heat-resistant Masterflex Norprene tubings between the membrane module and feed tank. The distillate tank (1-L acrylic cylinder with a spout) was cooled by a recirculating chiller (Julabo ME), and the distillate was recirculated at 250 mL/min through Masterflex Tygon E-LFL tubings between the membrane module and distillate tank.

[00169] The conductivity of the distillate in the distillate tank was measured at the end of every experiment using a conductivity meter (Eutech Instruments Alpha Cond 500) to ensure that the conductivity remained within 2 mS/cm, which was the value corresponding to the deionized (DI) water used. The overflow tank (300-mL beaker) was placed on a mass balance (Mettler- Toledo ME4002) for the measurement of distillate flux, which was derived from the accumulated distillate which overflowed into the overflow tank per unit time. The energy consumption (measured using a Uni-T UT230B-UK energy meter) of the hot-plate stirrer was measured at the end of every 3-hour while mass and conductivity of the distillate was measured every 5 mins over every 3 hours.

[00170] A virgin PVDF hydrophobic flat-sheet microfiltration membrane (Durapore GVHP; detailed specifications listed in FIG. 3B) with an active membrane area of 0.00371 m² (i.e. 53 mm by 70 mm) and a new spacer were used for every experiment. Membranes and spacers were not re-used to avoid fouling effects from affecting subsequent experiments.

[00171] The spacers investigated included a polypropylene spacer (Sterlite; each grid with equal sides of 3 mm), and nickel and copper foams (Latech), with detailed specifications listed in FIG. 3D to 3F, respectively. The nickel and copper foams used in this experiment had 100 to 110 pores per inch (PPI) and were structurally similar, as evidenced by the SEM images in FIG. 4A and 4B. Furthermore, to harness the photothermal effects to further improve performance of membrane distillation through localized heating at the feed-membrane interface, the nickel foam was coated with platinum (Pt), wherein the modification and characterization procedures are described in the following example. For studying effect of photothermal materials on the spacers, all the spacers with and without modification were tested with and without light irradiation. The light source (50 W LED) used had an illuminance of 700000 lux (i.e. 7 times that of the sun) and placed 4 cm from the spacers. This strong light source was used to compensate for the small area for light irradiation in the laboratory membrane module, thus lowering the relative contribution of systematic error towards the result. The spacer in the feed channel was varied to evaluate the effect of spacer type, while that in the distillate channel was consistently a polypropylene spacer (with grids of equal sides of 3 mm) to reduce the heat-transfer resistance and provide mechanical support for the membrane.

[00172] Example 2B: Experimental Study - Foam Modification and Characterization

[00173] To coat platinum (Pt) nanosheets (NSs) onto the nickel foam, the thermal decomposition method was employed. Specifically, the nickel foam (dimensions of 5.3 cm by 7 cm, and 0.001 cm thick) was first cleaned with ethanol and DI water, and then dried at ambient temperature (e.g. 20 °C to 30 °C), before being immersed into a quartz boat containing 200 mg of H₂PtCl₆.6H₂0 in 80 mL of a solvent mixture (1: 1 volume ratio of ethanol:2- propanol). After that, the quartz boat was placed in the furnace and the temperature was increased at a rate of 20°C/min to 450 °C, and held at 450°C for 15 mins while a gaseous mixture of 95% Ar:5% H₂ continuously flowed through. Finally, the sample was cooled to room temperature naturally followed by cleaning using DI water then drying in the vacuum oven at 60°C overnight.

[00174] X-ray diffraction (XRD) patterns were obtained with a powder diffractometer (Bruker D2 phaser XRD) using Cu Ka radiation. A field emission scanning electron microscope (FESEM JEOL JSM-6701F), operating at 5 kV accelerating voltage and high vacuum (9.63 x 10⁵ Pa) mode, was used to obtain high magnification images of the spacer samples. All FESEM samples were coated with platinum to prevent electron build-up during imaging.

[00175] Example 2C: Experimental Study - Infrared (IR) Thermal Imaging

[00176] A digital IR thermal imaging camera (Cat S60) equipped with a FLIR Lepton IR sensor, was used to take thermal maps of the metallic foam spacers investigated in this study before irradiation and after 1 min of irradiation with a 50 W LED lamp placed 4 cm away from the metallic foam spacers. The metallic foam spacers rested on a 6 mm thick clear acrylic, which mimicked the acrylic window of the DCMD module used in the experiments.

[00177] Example 2D: Experimental Study - Experimental Protocol

[00178] The following protocol was used for each experiment. Firstly, the PVDF membrane and targeted spacers were positioned in the acrylic membrane module, and the necessary fittings secured. Secondly, 5 g/L of sodium chloride (NaCl; Merck-Millipore CAS No. 7647- 14-5) and DI water were, respectively, circulated through the feed and distillate loops at 250 mL/min. The temperatures of the feed and distillate streams of the membrane module were set at 65°C and 15°C, respectively. Thirdly, the system was given an hour to stabilize before the mass and heater energy consumption measurements were recorded every five minutes over three hours, with the distillate recycled back to the feed tank every hour to maintain a constant feed composition.

[00179] Furthermore, for investigation of photothermal effects, an LED lamp was used. The additional step was such that the 50 W LED lamp was switched on, and step three above was repeated. Since the temperature and mixing rate of the hot-plate stirrer were fixed, the energy consumed by the hot-plate was representative of the amount of energy required to sustain the feed temperature at 65°C. Therefore, comparison of the heater energy consumption per unit volume of distillate would reflect the energy efficiency of each test (as denoted by equation (2) below):

Heater energy consumption per unit volume of distillate

[00180] Every experiment was carried out twice to check for reproducibility.

[00181] Example 3A: Results and Discussion - Effect of Spacer Material and Mesh Density

[00182] Simulations were first carried out to assess the benefits of using metallic spacers in membrane distillation. Since commercially available metallic supports come in the form of different materials and mesh densities, the first set of simulations was targeted at investigating the effect of spacer material and mesh density on membrane surface temperatures, which provide the driving force for the flux across the membrane. FIG. 5 and FIG. 6 present the temperature contour maps of the feed-membrane and distillate-membrane interfaces, respectively, while FIG. 7A to 7C reflect the surface-averaged temperatures and vapor pressure differences (p°_h- p° _c) across the membrane. [00183] With respect to the feed-membrane interface, three observations can be made.

[00184] First, FIG. 5 and FIG. 7 A indicate that the metallic spacers (i.e. nickel and copper) conferred higher temperatures than the polypropylene one, with the copper spacer performing slightly better than nickel due to copper’s relatively higher thermal conductivity.

[00185] Second, FIG. 5 apparently shows that temperatures are most uniform across the membrane surface for the densest foam, which indicates the beneficial effect in terms of the even distribution of heat and thus the driving force along the membrane surface.

[00186] Third, FIG. 7A indicates that the surface-averaged feed-membrane interface temperature decreases monotonically with spacer density for the polypropylene spacer, but non-monotonically (i.e. decreases then increases) for the other two metallic spacers. The monotonic relationship for polypropylene is likely due to its low thermal conductivity, which resulted in poorer heat transfer to the membrane surface as the spacer density increased. On the other hand, the non-monotonic relationship for the metallic spacer suggests an interplay between the heat transfer rate from the bulk fluid to the feed-membrane interface and the heat transfer rate across the membrane. While the lowest temperature was from the spacer with intermediate density (i.e. 1.5 mm mesh) because of the enhanced heat transfer across the membrane, the highest temperature was from the dense foam because of the greater heat accumulation associated with the highest thermal conductivity. This likely implies that the mesh density of the metallic spacer has to be increased to a value to enhance the temperature at the feed-membrane interface.

[00187] Analogously, FIG. 6 and FIG. 7B display, respectively, the temperature contour maps and surface-averaged temperatures of the distillate-membrane interface for the different spacers (i.e. three spacer materials and three spacer densities) at the feed side. Clearly, variation of temperatures caused by the different spacers at the feed side is lesser but not negligible. It can be observed from the results that (i) an increase in the thermal conductivity of the feed- side spacer increased the surface-averaged temperatures of the distillate-membrane interface (FIG. 7B) and (ii) an increase in spacer density decreased then increased the temperatures of the distillate-membrane interface for all three spacer materials (FIG. 7B).

[00188] FIG. 7C displays the (p°_h- p°_c ) trends, where p°_h and p°_c are the vapor pressure of water at the hot feed side and cold distillate of the membrane, respectively. The flux across the membrane in membrane distillation gets driven by the vapor pressure differential between the two faces of the membrane (equation (3)):

[00189] where N is the mass transfer flux and K_p is the overall mass transfer coefficient of water vapor through the membrane. Specifically, the vapor pressure is calculated by (equation

(4)):

[00190] where T is the surface-averaged temperature of the membrane. Accordingly, the vapor pressures across the membrane were derived using equation (4), and the (p°_h - p° _c) values were calculated and compared in FIG. 7C. The (p°_h- p°_c) trends in FIG. 7C are similar to that for the feed-membrane temperature in FIG. 7A, which likely implies that the relative fluxes in this study can be better predicted using temperatures at the feed-membrane interface/side rather than those at distillate-membrane interface/side.

[00191] In parallel with the simulations, experiments were carried out to compare the effect of the different spacers. FIG. 8 shows the distillate flux values for the three different spacer types on the feed side, namely, 3 mm polypropylene spacer, nickel foam and copper foam. Although the simulations predict larger (p°_h - p° _c) values for nickel and copper foams (FIG. 7C), and thereby higher flux from equations (3) and (4), the experiments indicate, unexpectedly, that the flux values were similar for all three feed-side spacers (FIG. 8). The negligible variation in the experimental flux can be attributed to the decrease in flow velocity near the surface of the membrane when the highly dense foam was used in the feed channel, as shown by the simulated velocity profiles in FIG. 9A to 9C. More specifically, FIG. 9D indicates that the surface- averaged spatial flow velocity near the feed side of the membrane surface was 36% to 38% lower for the foam (FIG. 9D). This, along with the greater coverage of membrane area by the dense metal spacer, result in a decrease of the mass transfer coefficient (K_p, equation (3)), which in turn negated the improvements provided by the increase in AT across the membrane.

[00192] Further analysis of the heater input energy per unit volume of distillate shows that the highest magnitude was for the polypropylene 3 mm mesh, followed by nickel foam and then the copper foam (FIG. 10), which indicates that the copper foam conferred the best energy efficiency. This is tied to the reduced spatial flow velocity near the feed side of the membrane surface (FIG. 9C to 9D). As the flow velocity reduced near the feed side of the membrane surface, the boundary-layer heat transfer coefficient reduced, thereby reducing the heat loss across the membrane cell. Together with the even distribution of heat on the feed side as well as a low thermal conductivity of the membrane, high thermal conductivity metal spacers produced a higher feed outlet temperature. This translates to a lower rate of heat loss across the membrane cell (Q) quantified by following equation (5a):

[00193] Q = mC_p( T_in - T_out) (5a)

[00194] where m is the mass flow rate, C_p is the heat capacity of water, and T_in and T_out are the average (over 1 hour) temperatures at the feed inlet and outlet, respectively. This equation considers heat loss to the environment through the acrylic module without additional thermal insulation. The measured rate of heat loss is displayed in FIG. 11, which shows a similar trend as that in FIG. 10. This affirms that the superior thermal conductivity of the copper foam and its dense porous structure were advantageous in reducing the heat loss (FIG. 11), thereby improving the energy efficiency (FIG. 10).

[00195] Example 3B: Results and Discussion - Effect of Spacer Position

[00196] When the metallic spacer was used on the distillate side, the heat absorbed was used to preheat the feed, which improved the overall energy efficiency of a conductive air gap membrane distillation. Based on this, whether the placement of the nickel foam at the feed or distillate side of the membrane makes a difference, for example, in DCMD, was also assessed. The nickel foam at the feed side enhanced the temperature both in terms of magnitude and uniformity at the feed side only while the nickel foam when placed at the distillate side enhanced temperature in terms of only the magnitude at the distillate side only (FIG. 12A to 12D). This indicates that, due to superior thermal conductivity, the metallic foam provided for a more evenly distributed temperature profile at the membrane surface where it was placed at. FIG. 13 quantifies the surface-averaged temperatures at both surfaces of the membrane and also (p°_h- p° _c) values when the nickel foam was at the feed versus distillate side. Clearly, presence of the nickel foam at either side of the membrane improved the temperature at that side due to the high thermal conductivity. In terms of the driving force (i.e. (p°_h- p° _c), FIG. 13C), it was greater when the nickel foam was at the feed side, which translates into a relative flux improvement of 30.7% as per equations (3) and (4) (assuming a constant K_p).

[00197] FIG. 14A shows that the flux was slightly higher when the nickel foam was at the feed side rather than the distillate side, which agrees with the simulated (p°_h - p° _c) results (FIG. 13C). Moreover, FIG. 14B indicates that the heater input energy per unit volume distillate was slightly greater when the nickel foam was on the distillate side, which indicates that the nickel foam was more beneficial at the feed side. The heat loss values were similar with overlapping error bars regardless of which side the nickel foam was at (FIG. 14C), indicating similar effectiveness. Overall, the experimental results agree with the simulation results in that the nickel foam was slightly more beneficial at the feed side rather than the distillate side.

[00198] Example 3C: Results and Discussion - Photothermal Effect of Present Modified Spacers

[00199] In view of the superiority of the present electrically and thermally conductive spacer having higher thermal conductivity, this study targets further improving energy efficiency of membrane distillation. Specifically, the nickel foam was conferred photothermal properties to harness the readily available solar energy to reduce the energy requirements from use of an external heater. Since the metallic spacers were placed in the feed channel, the photothermal conversion led to localized heating near the membrane surface itself, which improves energy efficiency of membrane distillation. To this end, noble metals, e.g. platinum (Pt), exhibit significant photothermal effects due to its marked plasmonic response in the visible region of the electromagnetic spectrum. Moreover, Pt has excellent chemical stability and catalytic activity. In this study, Pt NSs was adhered to the nickel foam and the improvement in membrane distillation performance or lack thereof was quantified experimentally.

[00200] FIG. 15A shows the SEM image of the highly porous network of a pristine nickel foam while FIG. 15B shows a TEM image of the Pt NSs, which was coated onto the nickel foam via thermal decomposition. FIG. 15C displays the Pt-coated nickel foam, which is clearly rougher than the pristine nickel foam (FIG. 15A) due to the Pt deposits, with the EDX mapping further confirming that the nickel foam was uniformly coated with Pt. EDX analysis (FIG. 16) further revealed that the Pt loading on the nickel foam was about 37 wt%. The higher- magnification SEM image in FIG. 15D shows the sheet-like structure of the Pt deposited the nickel foam, which provides significant surface area for photothermal conversion.

[00201] The flux results in the absence and presence of visible light irradiation (i.e. 50 W LED) for the various spacers (i.e. polypropylene 3 mm mesh spacer, nickel foam, Pt-coated nickel foam and copper foam) at the feed side are depicted in FIG. 17. Regardless of spacer type, the average flux values were similar in the absence and presence of the light source. The greater error bars for the Pt-coated nickel foam were likely because of the variations in the extents of Pt coating.

[00202] As for the heater input energy per unit volume distillate (FIG. 18), the benefits of the Pt-nickel spacer were apparent in terms of the lowest magnitude in the presence of irradiation, and also the metallic spacers required less energy in the presence of irradiation (relative to the absence) due to the enhanced absorption of the heat from the light source. Specifically, under light irradiation, the nickel, Pt-nickel and copper foams gave remarkable reductions in heater input energy per unit volume of distillate of 14.8%, 27.6% and 21.3%, respectively, compared to the polypropylene spacer. It should be noted that the Pt-nickel gave a photothermal conversion of 5.0 kW/m², which represents 18.5 W from the 50 W light source and a photothermal conversion efficiency of 37%. The photothermal conversion efficiency can be further improved to be even more advantageous for such solar-assisted membrane distillation.

[00203] The photothermal effects were validated using thermal images. FIG. 19A to 19D show the thermal images of the nickel and Pt-nickel spacers before and after 1 min of irradiation to elucidate the effect of irradiation. The change in temperatures was not significant before and after irradiation for the used nickel foam (FIG. 19A and 19B) while the temperature change after irradiation was marked for the used Pt-nickel foam (FIG. 19C and 19D), with increases of up to 22.8°C. This clearly indicates the significant photothermal effects conferred by the Pt coated.

[00204] Example 3D: Results and Discussion - Implications and Potential Applications

[00205] The non-limiting examples set out above demonstrates for the feasibility of using electrically and thermally conductive spacers, such as those in the form of a metal mesh and foam, in membrane distillation, e.g. DCMD process. The above examples include demonstrating for imbuing metal spacers with photothermal properties to carry out solar- assisted membrane distillation.

[00206] The present electrically and thermally conductive spacers may be modified to reduce fouling propensity of the membrane. Further, modifications made to the spacers may alleviate trapping of foulants at the membrane or adsorb foulants on the metal foam instead of the membrane. Hence, the present spacers, which allow for photocatalysts to be grown thereon, i.e. having the photocatalysts incorporated to a metallic foam’s large surface, may help to degrade the trapped and/or adsorbed foulants at the spacers in membrane distillation. This reduces fouling at the membrane as the foulants get trapped at the present spacers, and the photocatalysts incorporated to the present spacers break down the foulants. This avoids the compromise of a membrane’s flux which may occur when photocatalysts are incorporated to the membrane. This also avoids the downtime needed for a membrane’s flux to be recovered as photocatalysts incorporated to the membrane still require time to degrade foulants, which causes a delay in a membrane’s flux recovery.

[00207] Secondly, the present spacers, being metallic in nature, does not require coarse filaments and lower mesh density of polymeric spacers. This opens up a possibility of using metallic spacers of different mesh sizes, to identify a metallic spacer mesh density and analyze how metallic spacer mesh density changes with thermal conductivity of the different spacer materials, allowing for spacers to be tuned for different applications.

[00208] Third, due to the intermittent nature of solar energy source, photothermal conversion localized heating through the use of spacer materials such as Pt NSs coated nickel foam discussed above reduces the heating load of the external heater during the day, instead of replacing the external heater. That said, the present spacer being metallic, may provide localized heating via electromagnetic induction heating as an alternative and stable source of heating, which is described by way of non-limiting examples further set out below. Development of such localized heating technology can be extended to other flat-sheet membrane distillation configurations to incorporate solar heating to existing process. Furthermore, since the improvement done on spacers are independent of the technology used to heat the feed in the feed tank and membrane modifications, by coupling solar heating of the bulk feed or other heating methods such as electromagnetic induction heating and/or microwave heating, membrane distillation may be rendered much more energy efficient.

[00209] Example 4: Summary of Present Electrically and Thermally Conductive Spacers

[00210] The efficacy of metallic foams as the present spacers to improve performance of membrane distillation was investigated via CFD simulations and actual experiments. The frequently used polypropylene spacer with a mesh dimension of 3 mm was used as a benchmark for comparison. Two metals were investigated, e.g. nickel and copper, along with three spacer densities (a 3 mm mesh, a 1.5 mm mesh, and a foam). In addition, the feasibility of coating the spacers with photocatalysts (e.g. platinum nanosheets (Pt NSs) that exhibit excellent chemical stability and catalytic activity to further enhance the energy efficiency by harnessing solar energy was assessed. The following results are noteworthy.

[00211] First, because of higher thermal conductivity, the present metallic spacers placed at the feed side of the membrane gave higher temperatures that were more uniform along the membrane surface.

[00212] Second, while the surface-averaged membrane temperature for the polypropylene material decreased with spacer density, the observation for the present metallic spacers was different in that the surface-averaged temperature decreased then increased with spacer density. This is likely due to the interplay between the heat transfer rate from the bulk fluid to the feed- membrane interface and the heat transfer rate across the membrane, which implies that the mesh density of the metallic spacer may have to be increased to a value to enhance the temperature at the feed-membrane interface.

[00213] Third, although the experimental distillate fluxes were similar, the heater input energy per unit volume distillate was lower for the present metallic foam spacers by up to 16% relative to the polypropylene spacer because of the lower rate of heat loss associated with the lower velocity at the membrane surface.

[00214] Fourth, the placement of nickel foam was slightly more beneficial at the feed side rather than distillate side.

[00215] Fifth, relative to the polypropylene spacer, the present metallic foams significantly reduced up to 21% in heater input energy per unit volume of distillate under irradiation due to the absorption of heat from the light source, while the Pt-coated nickel foam provide an even better reduction of 28% due to photothermal conversion.

[00216] Holistically, the above non-limiting examples demonstrate for the practical use of the present metallic spacers for improving energy efficiency of membrane distillation, e.g. DCMD. Other suitable metallic spacers may be configured as described above. Furthermore, as the spacer is independent of the membrane, existing membrane modification methods may instead be applied to the spacer to avoid issues with pore plugging.

[00217] Example 5A: Brief Discussion on Localized Induction Heating in Membrane Distillation

[00218] The concept of localized heating as highlighted above may be about providing heat at the feed-membrane interface (i.e. p°_h) where it has the highest beneficial impact on membrane distillation flux. Most of such localized heating experiments carried out in membrane distillations utilized surface-modified membranes, which may be constrained either by the limited surface area or the limited exposure to solar irradiance (flux density of about 1.362 kW/m²). This results in limiting such modifications for use as a supplement to external heating to reduce the heating load. For example, it has been reported that Joule heating of porous carbon nanotubes (CNT) is only sufficient to heat up a feed of a single pass DCMD setup.

[00219] Compared to Joule heating of CNT (i.e. directly passing current through the CNT via platinum electrodes), the present induction heating method of a nickel foam of the present disclosure at the feed channel of a membrane distillation cell creates the same localized membrane heating as well as feed heating effects (FIG. 20B), which have been proven to be advantageous for membrane distillation. Beside, there are significant advantages of using nickel foam compared to CNT. [00220] First, nickel foams are readily available and low-cost (30 USD/kg in bulk quantities), rendering the heating approach much more economically feasible compared to use of CNT.

[00221] Second, induction heating does not cause degradation of the nickel foam, whereas degradation of CNT may occur in ionizable media depending on the alternating current (AC) frequency and salinity.

[00222] Third, expensive electrodes such as the platinum electrode conventionally used to avoid and/or prevent corrosion at the contact interface, can be circumvented as no electrical contacts in the membrane module is required for induction heating.

[00223] Fourth, the nickel foam can be easily shaped, e.g. into cylinders or cut into small packings, allowing them to be used with hollow fiber membrane modules.

[00224] Nevertheless, the main feature is that the heat generated from the induction heating of the nickel foam near the surface of the membrane can improve the energy efficiency of membrane distillation by increasing the clean water produced per unit energy consumed.

[00225] Example 5B: Proof-of-Concept of Present Induction Heating

[00226] Experiments have been performed to demonstrate the efficacy of the present induction heating approach using, as a non-limiting example, the nickel foam spacer of the present disclosure in membrane distillation.

[00227] The piece of nickel foam (specifications already listed in FIG. 3E) was used as a spacer in the feed channel, while a polypropylene mesh (specifications in FIG. 3D) was used in distillate channel of the acrylic membrane distillation module (flow channel specifications are in FIG. 3C), as depicted in FIG. 20B. Sandwiched between the feed and distillate channels was a hydrophobic polyvinylidene difluoride (PVDF) membrane (specifications in FIG. 3B). The membrane distillation module was then operated in counter-current cross flow mode, as shown in the schematic of FIG. 21.

[00228] The feed and distillate were recirculated through the membrane distillation module from their respective tanks using two peristaltic pumps (Masterflex L/S Digital Drive). The feed contained in the feed tank (a 2-L round bottom flask) was recirculated at 200 mL/min through Masterflex Norprene tubings between the membrane module and feed tank. In this proof-of-concept, three different modes of heating (i.e. external heating using a hot-plate, induction heating of a nickel spacer, and a combination of both) were investigated. The operating parameters are summarized in FIG. 22. Whenever the electromagnetic induction coil was used, the power input was set to be approximately equal to the power consumed by the external hot-plate heater (Heidolph MR Hei-Tec) to maintain the feed temperature at 45 °C. The cold distillate contained in the distillate tank (a 1L acrylic tank with a spout) was cooled by a recirculating chiller (Julabo ME), while being recirculated at 200 mL/min through Masterflex Tygon E-LFL tubings between the membrane module and distillate tank. The overflow tank (a 300 mL beaker) was placed on a mass balance (Mettler-Toledo ME4002) for the measurement of distillate flux, which was derived from the accumulated distillate that overflowed into the overflow tank per unit time.

[00229] The mass of overflow distillate, temperatures of the inlets and outlets of both the feed and distillate (measured with HYXC TM6-1003 PT100 temperature sensors), and electrical conductivity of the distillate (measured with Eutech Instruments Alpha Cond 500 conductivity meter) were recorded every 5 mins. The energy consumed by the hot-plate stirrer and/or induction heater was recorded using a power meter (UNTTrend UT230B) at the end of each 3- hour long experiment.

[00230] Simulations of the MD module channels were also carried out using Comsol Multiphysics 5.1 to provide a better understanding of the experimental results obtained.

[00231] Example 5C: Discussion on Results from Proof-of-Concept of Present Induction Heating

[00232] FIG. 23A to 23C display the results for the three modes of heating. FIG. 23A shows that, relative to external hot-plate feed heating, localized induction heating via the use of the nickel foam of the present disclosure improved flux by 36.3% despite having a lower average feed temperature (FIG. 23C). This can be attributed to the localized heating, with simulations indicating a feed-membrane interface temperature that is 1.3 °C higher (FIG. 24A to 24C and FIG. 26A) and a distillate-membrane interface that is 2.9 °C lower (FIG. 25A to 25C and FIG. 26B) than that of external hot-plate feed heating, thereby enhancing the driving force for vapor flux across the membrane. It should be noted that the (p°_h - p° _c) trends obtained by simulations (FIG. 26C) agree with the experimental flux trends (FIG. 23 A). Regarding use of both heating modes simultaneously, this combination gave the highest flux, which is due to the highest feed- side temperatures (FIG. 23C). With respect to energy efficiency, FIG. 23B shows that the total input heating energy per unit volume of distillate for external hot-plate feed heating and combined heating were respectively 39.8% and 20.8% higher than that of localized induction heating alone. This demonstrates the significant improvement of the energy efficiency of the DCMD system through the induction heating of the present nickel spacer.

[00233] Example 5D: Brief Discussion on Advantages from Proof-of-Concept of Present

Induction Heating [00234] From the proof-of-concept study, it has been demonstrated that localized induction heating in membrane distillation has a huge potential in improving energy efficiency of membrane distillation (by up to 39.7% compared to external hot-plate feed heating) through increasing the temperature at the feed-membrane interface and reducing heat loss to the environment by such directed heating. Since induction heating can be extended to other electrically conductive materials besides metal foams (e.g. carbon-fibre composites), this suggests the possibilities of designing electrically conductive spacers such as those using carbon nanotubes and graphene that can be used with induction heating. Furthermore, the induction heating coil can be designed to compensate the loss in driving force along larger stacked membrane module (as the feed gets cooled down along the membrane module) by increasing the density of the induction coil along the membrane module in the direction of the feed flow (FIG. 27 A). Such stacked membrane modules can be stacked according to FIG. 27B to increase membrane active area with improvement to heating power output of the induction coil.

[00235] Alternatively, the use of membrane distillation with hollow fiber membranes can be done either by (1) heating the feed flow in the core of the fiber using an electrically and thermally conductive spacer material placed in the core (FIG. 28A) or (2) heating the feed flow around the fiber using an electrically and thermally conductive spacer wrapped around the membrane fibers (FIG. 28B).

[00236] Another approach to the use of local induction heating for membrane distillation may be to disperse, into the feed, suitable agents that can be inductively heated. These may be particulates (subdivided metal foam, graphene, etc.) of sub- or supra-micron size or immiscible magnetic fluids (e.g. an oil containing magnetic fragments). These agents may facilitate local heating in the boundary layer adjacent to the membrane. With further modification of the membrane distillation by utilizing energy efficient industrial induction heater with respect to a larger membrane distillation module, heat recovery of heat generated from electrical current flow in the induction coil itself, the flow across the nickel foam, and the surface modification of nickel foam with photothermal materials, further improve the energy efficiency for operation of a solar-enhanced membrane distillation system to become economically viable for desalination.

[00237] Example 6A: Further Examples - Utilization of Iron-Nickel and Cobalt-Nickel Spacer Materials [00238] The present example further demonstrates for different electrically and thermally conductive spacer materials. Apart from nickel and copper spacers demonstrated above, iron- nickel (Fe-Ni) and cobalt-nickel (Co-Ni) spacers at the feed side have been experimented to evaluate the effect of spacer materials on the performance of membrane distillation with external heating and localized induction heating.

[00239] The experimental setup used to test the different spacer materials have already been described above in example 5B and FIG. 21.

[00240] From the results obtained for Fe-Ni and Co-Ni foams, corrosion may be visually observed when these two spacer materials were used (FIG. 30). High content of iron and cobalt (more than 70 weight% (wt%) of the foam) may lead to corrosion in saline solutions or feed. Results also showed a decrease in flux due to fouling of the membrane caused by the metal oxide particulates from the corroded spacers. Nevertheless, such spacer materials can still be used where the substances to be processed by membrane distillation do not react therewith, or if such spacer materials contain less iron and/or carbon (less than 70 wt%). Nickel foam was the only spacer which had the relatively best cycle performance (168 hours of usage) with no deterioration in flux in the present setup, with and without localized induction heating. The X- ray diffraction results (FIG. 31) of the nickel foam before and after 168 hours of usage, show no change in crystal phase composition while the SEM image only show a layer of salt deposition (FIG. 32).

[00241] Example 6B: Further Examples - Coating of Spacer Foams to Enhance Induction Heating Effect

[00242] The present example further demonstrates for coatings of different electrically and thermally conductive spacer materials, including coating of nickel spacer as a non-limiting example, to enhance induction heating effects.

[00243] Platinum nanosheets grown on nickel foam have been demonstrated and tested hereinabove for its photothermal properties. Other materials, which may be able to shield electromagnetic waves by absorbing them, i.e. improving heat generation using electromagnetic induction heating by absorbing the electromagnetic waves, are investigated. That is to say, a series of experiment were included to test coating of different materials, e.g. carbon nanotubes (CNT) and graphene oxide (GO). The inductive heating was carried out with the coated spacer placed within the feed channel of the membrane distillation module of the setup in FIG. 21. [00244] Coated nickel foam showed improvement in flux and decrease heater energy per unit volume of distillate when inductively heated (FIG. 33A and 33B). This improvement can be attributed to the electromagnetic shielding effect, in which the coating absorbs the electromagnetic waves to be inductively heated, as well as the increase in eddy current (FIG. 34A) which resulted in an increase in the foam’s surface temperature (FIG. 34B). However, GO coating resulted in a reduction in flux when externally heated (i.e. heating the feed with the hot plate). This was found to be due to the lower thermal conductivity of the GO coating which results in less heat being transferred from the bulk feed to the surface of the membrane.

[00245] The coating parameters can be further improved to increase the absorption of electromagnetic waves and improve heat transfer from coated foam to the feed.

[00246] Example 6C: Further Examples - Scaling Up of Experimental Setup

[00247] The present example further demonstrates for scaling up of the localized induction heating membrane distillation setup.

[00248] A stacked membrane module was designed and put together as per FIG. 27 A and 27B. The module and coil configuration shown in FIG. 27A was used, and the feed and distillate flow rates were adjusted to achieve a cross flow velocity (calculated using equation (5b) below) similar to that for the laboratory- scale setup.

[00249] This scaled-up module was able to produce about 15 times the amount of distillate at the same feed and distillate temperatures and cross flow velocities with a 100% increase in physical footprint. Results for the scaled-up model are illustrated in FIG. 35A to 35C.

[00250] Heat energy input indeed increased at a rate higher than the increase in volume of distillate produced for the scaled-up version. This can be attributed to the higher heat loss across the larger active membrane area. Preliminary results (FIG. 35A to 35C) show that with the current induction coil set up (FIG. 36A), the localized induction heating in tandem with external heating to simulate mitigation of waste heat fluctuations proved to be feasible. The localized induction heating was able to be used as a supplement to the waste heat when it was low, sustaining the flux at a peak waste heat input of 640 W, with similar total energy input per unit volume of distillate.

[00251] The coil design and placement may be altered to improve the results further for different applications. Furthermore, the experimental setup can be improved to better simulate actual waste heat recovery for membrane distillation, to better compare the use of localized induction heating to mitigate waste heat fluctuations.

[00252] Example 6D: Summary of Further Examples 6A to 6C

[00253] Examples 6A to 6C show the various potential uses of different electrically and thermally conductive spacers for enhancing membrane distillation. The results show that coating of graphene oxide on metal foam spacers improves the heat produced via induction heating due to its improved electromagnetic wave shielding (absorption) and increase in surface eddy currents. The coating may be developed into an anti-corrosion coating that allows other materials, which may corrode too easily, to be used as spacers in membrane distillation. Scaling-up has proven to be at least comparable to external heating in mitigating waste heat fluctuations.

[00254] Example 7A: Further Examples - Simulations for Feasibility and Efficacy of Induction Heating of the Present Spacer at Different Feed Temperatures and Flow Rates

[00255] The simulation setup described in example 1A was adopted, where applicable and suitable.

[00256] The hot feed flowed at rates between 10 mL/min to 250 mL/min and counter-current to the distillate fixed at a flow rate of 250 mL/min. The governing equations, namely, Navier- Stokes and continuity equations are listed in the example 8A, and the physics packages in Comsol used were‘Laminar Llow’ (since experimental Reynolds number ranged from 9 to 229 in the 4 mm channel) and‘Heat Transfer’ .

[00257] Adiabatic conditions were applied as set out in example 1A for simulations.

[00258] Two different spacers used in the simulations were designed to mimic those used experimentally. The surface-averaged temperatures of the membrane on the feed and distillate side were derived, and used to calculate the temperature polarization coefficient (TPC), , using equation (6):

[ 00259 ] where T_b,f and T_{b, d} are the bulk temperatures of feed and distillate side, respectively, and T_m,f and T_{m, d} are the membrane surface temperatures on feed and distillate side, respectively. The TPC provides an indication of performance enhancement with the use of metal spacers and localized induction heating. The bulk temperatures in the feed and distillate channels were volume-averaged, while the membrane surface temperatures on the feed and distillate sides were area- averaged. [00260] Similarly, the 0.52 mm thick polypropylene spacer with 3 mm by 3 mm parallelepiped (with filament diameters of 0.26 mm, FIG. 2A) and a 1 mm thick, sponge-like nickel foam (FIG. 2C) were simulated using the porous media sub-model in Comsol. The material properties, such as density, thermal conductivity and heat capacity, were used as is from the Comsol material library. A sensitivity analysis on the spatial resolution (i.e. mesh size) was carried out, and it was determined that the‘normal’ mesh (with the finest dimension being 0.412 mm and the largest dimension being 1.38 mm) was adequate to shorten the computation time without sacrificing accuracy.

[00261] Example 7B: Further Examples - Experiment Setup for Feasibility and Efficacy of Induction Heating of the Present Spacer at Different Feed Temperatures and Flow Rates

[00262] An experimental study was carried out in parallel with simulations to evaluate the effect of spacer (e.g. polypropylene mesh and nickel foam) at various feed temperatures and flow rates, as well as the different modes of heating on performance (e.g. flux and energy efficiency) of membrane distillation, e.g. DCMD.

[00263] The experimental setup described in example 5B was adopted, wherein the induction coil and controller board described in example 5B and FIG. 21 are used. The acrylic membrane module of FIG. 3C was used. The induction coil was arranged 1 cm away from top surface of the metallic spacer. The membrane module was operated in counter-current cross flow mode. A peristaltic pump (Masterflex L/S Digital Drive) was each used to circulate the feed and distillate streams from their respective tanks. The feed tank, a 2 L round-bottom flask, was stirred using a hot-plate stirrer (Heidolph MR Hei-Tec), and the feed was circulated through a heat-resistant Masterflex Norprene tubings between the membrane module and feed tank at a flowrate ranging from 250 mL/min to 10 mL/min. The feed could either be heated externally by heating the feed tank using the hot-plate stirrer or adding heat via the induction heater placed at the top of the membrane module using a 12 to 24 V direct current zero voltage switching (ZVS) induction heater (ZQC module). The distillate tank, a 1 L acrylic cylinder with a long spout which leads to the overflow tank, was cooled by a recirculating chiller (Julabo ME), and the distillate was recirculated at 250 mL/min through a Masterflex Tygon E-LFL tubings between the membrane module and distillate tank. A 300 mL beaker was used as an overflow tank and placed on a mass balance (Mettler-Toledo ME4002) for the measurement of distillate flux, which may be calculated from the accumulated distillate that overflowed into the overflow tank per unit time. [00264] In order to maintain a constant feed sodium chloride (NaCl) concentration of 5 g/F, the accumulated distillate from the overflow tank was recycled every 1 hour throughout the experiment back to the feed tank by the peristaltic pump (Masterflex L/S Digital Drive) in a time dispensing mode, where it turns on for 1 min every hour. In order to ensure that the conductivity remained within 2 mS/cm (which was the value corresponding to the DI water used) throughout the experiments, the conductivity of the distillate in the distillate tank was measured using a conductivity meter (Eutech Instruments Alpha Cond 500).

[00265] The mass and conductivity of the distillate was logged every 5 mins over 3 hours, while the energy consumption (measured using a Uni-T UT230B-UK energy meter) of the hot plate stirrer and/or ZVS induction heater was recorded at the end of each 3 -hour experiment.

[00266] In these examples, only the spacer in the feed channel was interchanged between the nickel foam and polypropylene mesh, to provide a comparison between metallic spacers and conventional polypropylene mesh (in this case, with grids of equal sides of 3 mm) with and without localized induction heating, while that in the distillate channel was consistently the conventional polypropylene mesh to provide mechanical support for the membrane and local turbulence near the membrane surface. The polypropylene mesh and nickel foam were reused throughout the whole study (168 hours of operation), further proving the robustness of the metallic spacer and polypropylene mesh alike. The specification of the nickel foams (Latech) and polypropylene spacer (Sterlite) are already listed in FIG. 3E and 3D, respectively. For the induction heating of nickel foam, the ZVS induction heater was supplied with DC power from a RS PRO 1.1 kW IPS series DC power supply. The membrane used was a PVDF hydrophobic flat sheet microfiltration membrane (Durapore GVHP with detailed specifications listed in FIG. 3B) having an active membrane area of 0.00371 m² (i.e. 53 mm by 70 mm).

[00267] Experimental protocol was similar to that described in example 2D.

[00268] Briefly, the membrane module was first prepared with specific spacers positioned within the channel and the PVDF membrane was placed between the two part module and secured with the necessary fittings.

[00269] Second, 5 g/L of NaCl (Merck-Millipore CAS No. 7647-14-5) and DI water were circulated through the feed and distillate loops at 250 mF/min. The temperature of the distillate stream was set at 15 °C for all experiments in this study. While the feed temperature was either set at 45°C, 50°C or 65°C when external heating was used. Focalized induction heating experiments were carried out by controlling the DC power input without any temperature feedback control. [00270] Third, every time any system parameter was changed, it was given an hour to stabilize before the distillate overflow mass and distillate conductivity measurements were recorded every five minutes over three hours, with the distillate recycled back to the feed tank every hour to maintain a constant feed composition.

[00271] Fourth, the heater energy consumption was recorded at the end of the three hour, before the system parameter was varied and energy meter reset for the next set of data.

[00272] The energy consumed by the hot-plate was representative of the amount of heat energy required to distill the amount of water produced over the time period. Therefore, the comparison of the heater energy consumption per unit volume of distillate would reflect the energy efficiency of each test (as denoted by equation (7) below):

Heater energy consumption per unit volume of distillate

Energy consumed over 3 hrs (obtained from the energy meter )

Distillate produced over 3 hrs

-1-Power input to ZVS induction heater (7)

[00273] Every experiment was carried out twice to check for reproducibility.

[00274] Example 7C: Further Examples - Effect of Spacer on Flux at Different Feed Temperatures

[00275] Since the temperature difference between the feed and distillate directly affects the flux and efficiency of the membrane distillation operation, and the present spacers have been shown to improve the temperature driving force due to better thermal conductivity, an understanding of the effect of different feed temperatures on the augmentation by present spacers may help better utilization of localized induction heating. Also, this shed insights on whether the induction heating should be a standalone heating method for membrane distillation or to mitigate waste heat fluctuations in membrane distillation.

[00276] Two feed temperatures were chosen to study this effect. Since higher feed temperature tends to produce higher flux, a feed temperature of 65°C was chosen as the higher feed temperature to provide comparison against the lower feed temperature of 45°C, which was chosen because of the power input limitation of the induction heater used in the actual experiment. Hence, the simulation with the nickel foam and polypropylene mesh at 45 °C and 65 °C were carried out to evaluate the improvement that the present spacers provide at different feed temperature.

[00277] Simulations were carried out to evaluate the use of metallic spacers at different feed temperatures. The flux across the membrane, N, was calculated using equation (3) as set out above in example 3A. The vapor pressure was calculated using equation (4) as set out in example 3 A. Simulation results are shown in FIG. 37 A to 37B.

[00278] An observation may be made between the simulation and experimental results. Regarding the simulation results, FIG. 37A shows that, at a higher feed temperature, the difference in the (p°_h- p°_c) values between the nickel foam and polypropylene mesh was greater at a higher temperature, because of the larger temperature difference across the membrane for the former ( DT_m , FIG. 37B). This can be attributed to the better heat conductivity of the metallic spacer, leading to better heat transfer via conduction from the bulk feed to the membrane surface. The experimental results showed that, at the lower feed temperature of 45°C, the flux values were slightly higher (FIG. 37C) and the heater input energy per unit volume were lower (FIG. 37D) for the polypropylene mesh, indicating the polypropylene mesh may perform comparably to the nickel foam (having 1 mm thickness and porosity of 110 pores per inch) only for such instance. This is because of the decrease in flow velocity near the surface of the membrane and the greater coverage of membrane area by the highly dense nickel foam, which resulted in a decrease of the mass transfer coefficient (K_p, equation (3)) that balanced out the improvements provided by the increase in DT across the membrane. Furthermore, perfect contact between the spacer and membrane surface was not possible in the experiments, and the lack of contact may reduce the benefits of better thermal conductivity from the nickel foam. To verify the effect of imperfect spacer-membrane contact, simulations were carried out such that the nickel foam was, for example, 0.2 mm and 0.4 mm from the membrane. FIG. 38A and 38B show that a mere 0.2 mm between the nickel foam and membrane was enough to cause the (p°_h- p° _c) and DT_m values to be similar to that of the polypropylene mesh. This indicates that the difference between the simulation and experimental results (FIG. 37A to 37D) at the lower temperature is because of differences in spacer density and also spacer-membrane contact, and more importantly that the nickel foam consistently outperformed the polypropylene mesh at the higher temperature. Higher temperatures may help render better utilization of the higher thermal conductivity of nickel foam to bring out its benefits therefrom.

[00279] Example 7D: Further Examples - Effect of Feed Flow Rate Coupled with Different Modes of Heating

[00280] Since induction heating heats up the feed as it flows through the porous nickel foam, the temperature at the feed-membrane interface gets affected by the feed flow rate. The effect of feed flow rate on the efficiency of localized induction heating of the spacers in membrane distillation and the heat loss across the membrane was studied to evaluate feasibility of the present method of induction heating in membrane distillation.

[00281] Membrane distillation experiments with either the polypropylene mesh or nickel foam at the feed side were carried out at a feed temperature of 45°C (maintained by a hot plate stirrer; termed“external heating”) and feed flow rates of between 10 ml/min to 250 mL/min. Since induction heating was only possible with the thermally conductive nickel foam, the same set of experiments with power input to the induction heater fixed at 78 W was not carried out for the polypropylene mesh. For external heating, FIG. 39A and 39B show that experimentally, (i) a lower flow rate translates to lower flux and higher heater input energy per unit volume distillate, which was attributed via simulation results to the higher (p°_h- p°_c) and DT_m values (FIG. 39C and 39D), and (ii) the nickel foam consistently gave lower flux and required higher heater energy per distillate volume than the polypropylene mesh. As for induction heating, at the lowest flow rate of 10 mL/min, the flux, (p°_h- p°_c) and DT_m values were the highest, while the heater energy per distillate volume was the lowest. In all cases, flow rate exerts a significant influence for flow rates lower than 150 mL/min. Notably, between the best-performing cases for the nickel foam (at the lowest flow rate of 10 mL/min) and polypropylene mesh (at the highest flow rate of 250 mL/min), the former gave a 10.8 % higher flux at 16.6% lower energy per distillate volume.

[00282] The rate of heat loss, between the inlet and outlet of the membrane module, was calculated using the following (equation (8)):

[00283] where DT_d denotes temperature difference at inlet (T_{d, in}) and outlet (T_{d, out}) of distillate side. The distillate temperatures were used instead of the feed temperatures, as the induction heating would result in an apparent decrease in heat loss if the feed temperatures were used. The thermal efficiency, TE, for the different modes of heating at different feed flow rates was calculated using equation (9) below:

[00284] where N is the distillate flux, A is the active area of the membrane and H_v is the latent heat of vaporization of water.

[00285] Based on the above equations, the results obtained are depicted in FIG. 40A to 40C. FIG. 40A shows that the experimental heat loss was consistently the greatest for induction heating of nickel foam, followed by external heating of nickel foam and then polypropylene mesh. The greatest heat loss for induction heating is because the higher feed-side membrane temperature created a greater driving force for heat to be lost to the distillate side. However, FIG. 40B indicates that the thermal efficiency was the best for induction heating of nickel foam at the lowest flow rate of 10 mL/min, which agrees with FIG. 39B. Therefore, FIG. 40A and 40B show that, despite the greater heat loss associated with induction heating, the flux increases more than compensated for the increased heat loss at the lowest flow rate. Furthermore, FIG. 40C shows that the TPC values (i) were greatest for induction heating, followed by external heating of nickel foam, and then finally external heating of polypropylene mesh, and (ii) increased with flow rate for external heating, but decreased with flow rate for induction heating. TPC refers to a ratio of the membrane surface temperature and the bulk fluid temperature, wherein it is better for this ratio to be 1 or close to 1 as this means more heat gets supplemented to the surface of the membrane. The results for FIG. 40A to 40C are obtained under the conditions of having the power of the induction heater fixed at the maximum, i.e. 78 W, while the flow rate of the feed is varied. As the induction heater power is fixed at the maximum, lower energy-saving feed flowrates may be used, allowing the feed to be heated for longer and thus a higher temperature reached, thus resulting in the higher flux and energy efficiency. Based on the results, especially the TPC results, it can be observed that the heat energy used indeed provides a higher feed temperature near the surface of the membrane.

[00286] Example 7E: Further Examples - Detailed Comparison on Varying Induction Heater Power Input with Different Modes of Heating

[00287] A comparison among the different modes of heating in the membrane distillation was carried out to evaluate feasibility of the standalone use of induction heating of a spacer of the present disclosure as well as use of induction heating of the spacers in membrane distillation.

[00288] In order to study the effect of induction heating, the spacer structure was specified as a heat source in the heat transfer physics package simulation. Accounting for the efficiency of the small induction heater, the induction heating heat flux, Q_IH, was specified on the volume of the spacer in the simulation according to the following equation (10):

[00289] where h_IH is the efficiency of the induction heater, P_in is the power input to the induction heater and s_spacer is the void fraction of the nickel foam spacer.

[00290] The results obtained are accordingly discussed below. [00291] As the induction heater power was fixed but the external heating power consumption decreased with decreasing feed flow rate, a fair comparison may be to carry out the induction heating experiments with a power input similar to that of the external heating power consumption at that particular feed flow rate. Hence, both experiments and simulations were carried out with induction heater power input similar to that of the external heating power consumption at 100, 50 and 10 mL/min feed flow rate.

[00292] Overall, the flux for all cases studied in this example decreased with decreasing flow rate. Results show that the flux produced by the induction heating of nickel foam remained higher, while the heater energy input per unit volume of distillate remained lower, compared to that produced by the external heating with polypropylene mesh and nickel foam (FIG. 41 A and 4 IB). These indicate that induction heating of nickel foam at the selected flow rates with power input adjusted to that of the external heating, continued to perform better than that of external heating. With the difference in flux being higher at lower feed flow rate, at 10 mL/min feed flow rate, induction heating of the nickel foam showed a flux 1.9 and 2.8 times that of the external heating of polypropylene mesh and nickel foam, respectively. Interestingly, the heater energy input per unit volume of distillate for the induction heating experiments in this example, remained almost similar at the different feed flow rates (FIG. 41B). Simulation results shows the (p°_h- p° _c) (FIG. 41C) and DT difference across the membrane (FIG. 4 ID) for induction heating of nickel foam was only able to surpass that of the polypropylene mesh and nickel foam with external feed heating at a feed flow rate of 150 mL/min and below.

[00293] Further calculating the heat loss across the membrane and TE from the results obtained experimentally, led to the following observations.

[00294] The heat loss across the membrane for the induction heating of nickel foam increased with decreasing feed flow rate similar to that observed in example 7E below, with heat loss being less than the external feed heating experiments only when the feed flow rate was set to 100 mL/min (also see FIG. 46A). Similar to example 7E below, the TE for the induction heating of nickel foam, increased with decreasing feed flow rate, a trend opposite to that of the external heating experiments (also see FIG. 46B). TPC in this case (see FIG. 46C), decreased with decreasing feed flowrate for all modes of heating, with the rate of decrease being the highest for polypropylene mesh with external feed heating, followed by nickel foam with external feed heating and nickel foam with localized induction heating. The results for FIG. 46A to 46C are obtained under the conditions of having the power of the induction heater matched with the power of external feed heating at flowrates of 100 mL/min, 50 mL/min, and 10 mL/min, wherein the power used at these three flowrates are 68 W, 55 W, and 34 W, respectively. Based on the results, especially the TPC results, the localized induction heating does provide heat at the surface of the membrane where the most significant impact on flux and/or efficiency can be observed.

[00295] Example 7F: Further Examples - Feasibility of Induction Heating in Complementing Waste Heat Utilization in Membrane Distillation

[00296] From the above experiments and simulations, standalone localized induction heating of metallic spacers seems to only be more efficient than external feed heating at low feed flow rates. However, to use localized induction heating to mitigate the fluctuations in waste heat provided for membrane distillation, the experiments using the present spacers may be re designed to simulate a reduction in waste heat of a certain fixed temperature. In this regard, the temperature of the feed was fixed at 65 °C while the induction heater was operated in 3 modes, that is, in switch-off mode, at 50 W and at 78 W, and the feed flow rate was set at 50 and 250 mL/min.

[00297] Flux increased with higher induction heating power input at both feed flow rates tested (FIG. 42A), which is also reflected in the simulation results of (p°_h- p°_c) (FIG. 42D). However, the total heater input energy per unit volume of distillate remained almost constant regardless of the addition of induction heating at a feed flowrate of 250 mL/min and decreased with the increase in induction heating power when a feed flowrate of 50 mL/min was used (FIG. 42B). This is due to the resultant efficiency of the combined system being improved only at the lower feed flowrate of 50 mL/min where the induction heater is more efficient at producing water. However, with reference to the result in example 7E (FIG. 39B), the 78.2% increase in external feed heating power input per unit volume of distillate far outweighs the 24% decrease in induction heating power input per unit volume of distillate as the feed flowrate decreased from 250 mL/min to 50 mL/min. Furthermore, the TE decreased as feed flowrate varied from 250 mL/min to 50 mL/min (FIG. 42C). Hence, the MD system operated at a lower flowrate in this condition did not help to improve flux nor total heater input energy per unit volume of distillate.

[00298] From further analysis of the experimental temperature data, it can be observed from the runs in examples 7D and 7E that induction heating was able to heat up the feed (FIG. 43A and 43B), wherein the different DT_f with respect to induction heating of a nickel foam for the same feed flowrate in FIG. 43A and 43B was due to different conditions for example 7D wherein different flow rates were studied at maximum power, while in example 7E, the power of the induction heater was varied. However, for the present example of induction assisted membrane distillation (FIG. 43C), induction heating was merely able to reduce the temperature drop across the feed channel as the present setup for the example of FIG. 43C utilizes an induction heating module that has an induction heating circuit board having small heating area and low heating rate. Hence, the less significant reduction in the total heater input energy per unit volume of distillate for induction heating assisted MD. However, since it does not increase the total heater input energy per unit volume of distillate, it remains a feasible option to use the present induction heating approach to mitigate waste heat fluctuations in membrane distillation systems

[00299] Further simulating the decrease in heat energy input due to the lack of localized induction heating, the feed solution recirculated at 250 mL/min was heated using the external heater to a lower temperature which reduced its power consumption to that of the power consumed by the external heater alone when it is used in parallel with the induction heater. This resulted in a steady-state feed temperature of 50°C and a flux reduction of 29.5% and 44.4% compared to external heating and a combination of external heating and 78 W of localized induction heating.

[00300] Example 7G: Further Examples - Summary of Examples 7A to 7F

[00301] Examples 7A to 7F relate to studies on the feasibility of using induction heating for spcaers for the present disclosure. Based on the results, various frequency, coil design and other induction heating parameters may be used to improve the membrane distillation efficiency.

[00302] Conventionally, heating in membrane distillation has not been very ideal due to coating on membranes, which affects flux, and the lack of contact area for thermal conduction, resulting in low thermal efficiency. Conversely, the present localized induction heating of spacers of the present disclosure provides a larger contact area for heating without affecting the flux and avoids coating of membrane. The nickel foam used in the examples showed great cycle performance even after 168 hours of repeated usage with no significant material degradation and change in performance (FIG. 32A and 32B). Hence, rendering heating of the feed closer to the surface of the membrane as well as using it as a method to mitigate fluctuations of supplied waste heat that may be used to heat up feed to a membrane distillation unit.

[00303] Efficacy of the present localized induction heating of metallic spacer to improve DCMD performance was investigated via CFD simulations and experiments, as already demonstrated above. Use of the present spacers in a lower feed temperature condition was first investigated and compared to higher feed temperature. This sets a basis of comparison and allows for better comparison with standalone induction heating of metallic spacer due to the low power input for the induction heater circuit. The standalone induction heating of metallic spacer was investigated at different feed flow rates and induction heater power input. Lastly, induction heating was used as a supplement to the heat from external feed heating to investigate feasibility of it mitigating the fluctuations from heating MD feed with waste heat recovery. The results are summarized as follows.

[00304] First, lower feed temperature resulted in lesser heat transfer via conduction through the present metallic spacers from the bulk solution to the surface of the membrane. This along with the metallic spacer being placed in a deeper channel and possibly not in contact with the membrane may render the membrane distillation unit suitable for process with lower requirements.

[00305] Second, the standalone induction heating at a fixed input power corresponding to an external feed heating power input of 250 mL/min, showed that operating the present localized induction heating setup at lower feed flow rates potentially improves flux and energy efficiency beyond that of an external feed heating setup at higher feed flow rates.

[00306] Third, by varying the standalone induction heating power supply to match the external feed heating power input at a flow rate ranging from 10 mL/min to 100 mL/min, it was proven that even at lower power input, the flux and energy efficiency improved when standalone localized induction heating was used as compared to external feed heating. Lastly, by adding localized induction heating of metallic spacer as a supplement to the external heating of feed in an induction heating assisted MD, the feasibility of mitigating fluctuating waste heat with localized induction heating of metallic spacers in MD was proven at a higher feed flow rate

[00307] Holisitically, examples 7A to 7F proved for the use of both standalone localized induction heating of metallic spacers and localized induction heating of metallic spacers as a supplement to external feed heating, via DCMD as an example. This in turn provides insights as to when the use of localized heating of metallic spacers becomes more efficient and how to further improve efficiency of membrane distillation. The present method and spacers can also be used for processing more saline solutions such as reverse osmosis brine, as the present induction heating does not require electrodes to be in contact with such saline solution that may cause corrosion to the electrodes. Furthermore, the present spacers modified with photothermal materials advantageously reduce pore plugging arising from modifications of membrane, as the photothermal materials are incorporated to the present spacers and not the membrane, which allows for other modifications to be made to the membrane that may be applied to further improve membrane distillation.

[00308] Example 8A: Navier-Stokes and Continuity Equations

[00309] The continuity and Navier-Stokes equations for a 3D steady-state incompressible flow used in the laminar flow stationary study are as follows:

[00310] Continuity equation

[00311] (Al)

[00312] Momentum equation

[00313] (A2)

[00314] The flow through the metallic foams were simulated as flow through a porous media which can be simulated using the following equations.

[00315] Momentum equation (Brinkman equations)

[00316]

[00317] Of which, the metallic foam permeability, K, was found to be 9.785 X 10^-9 m². I denotes an identity matrix.

[00318] Continuity equation

[00319] (A4)

[00320] The two boundary conditions of the simulated flow system are listed as follows:

[00321] a) u_x, u_y, u_z = 0 at the walls (i.e., no-slip condition)

[00322] b) Outlet pressure is set as atmospheric

[00323] The heat transfer energy balance equations for a 3D steady-state incompressible flow used in the heat transfer study are presented as follows:

[00324] Heat transfer in fluid

[00325] (A5)

[00326] (A6)

[00327] Heat transfer in porous media

[00328] (A7)

[00329] (A8)

[00330]

(A9)

[00331] Heat source (Induction heating of Ni foam)

[00333] The two heat transfer boundary conditions of the simulated flow system are listed as follows:

[00334] a) q = 0 at the walls (i.e., thermally insulated)

[00335] b) Heat transfer occurs through a simulated thin layer with a thermal resistance

(Rmembrane)^'·

[00336] (Al l)

[00337] Example 8B: List of Symbols

[00338] C_p Fluid specific heat capacity at constant pressure (J/kg.K)

[00339] (pC_p) Effective volumetric heat capacity at constant pressure (J/m³.K) ff

[00340] T Temperature (K)

[00341] T_m,f Temperature at feed side membrane surface (K)

[00342] T_m,d Temperature at distillate side membrane surface (K)

[00343] T_b,f Bulk feed temperature (K)

[00344] T_b,d Bulk distillate temperature (K)

[00345] P Pressure (Pa)

[00346] Pin Induction heater power input (W)

[00347] u Velocity of fluid (m/s)

[00348] d_membrane Membrane thickness (m)

[00349] k_memPrane Thermal conductivity of membrane (W/m.K)

[00350] k_air Thermal conductivity of air (W/m.K)

[00351] k_pvdf Thermal conductivity of PVDF (W/m.K)

[00352] k_eff Effective thermal conductivity of metallic foam (W/m.K)

[00353] k_metai Thermal conductivity of metal (W/m.K)

[00354] N Mass-transfer flux (kg/m².s)

[00355] A Membrane active area (m²)

[00356] H_v Latent heat of vaporization (J/kg)

[00357] p^° Vapor pressure (Pa)

[00358] p_h ^° Vapor pressure of water on the hot feed side of the membrane

(Pa)

[00359] p_c ^° Vapor pressure of water on the cold distillate side of the membrane (Pa)

[00360] K_p Overall mass-transfer coefficient of water vapor through the membrane (kg/m².s.kPa)

[00361] Q_br Mass source/sink (kg/m³.s)

[00362] Rmembrane Thermal resistance of membrane (K.m²/W)

[00363] p Mass density (kg/m³)

[00364] m Dynamic viscosity (kg/m· s)

[00365] Q Temperature polarization coefficient, TPC(dimensionless)

[00366] DT Temperature difference between membrane surface at the feed side and membrane surface at the distillate side (°C)

[00367] DT_f Temperature difference between feed outlet and feed inlet (K)

[00368] DT_d Temperature difference between distillate outlet and distillate inlet (K)

[00369] e or e_mernbrane Membrane porosity (dimensionless)

[00370] e_spacer Membrane porosity (dimensionless)

[00371] h_IH Induction heater efficiency (dimensionless)

[00372]€_p Metal foam volumetric porosity (dimensionless)

[00373] K Metal foam permeability (m²)

[00374] Example 9: Summary

[00375] In summary, the present disclosure includes a membrane distillation system comprising an electrically conductive porous module, a heating element which may be coupled to the electrically conductive porous module, wherein the electrically conductive porous module may be a spacer module that may be positioned in close proximity to a membrane module and/or the electrically conductive porous module may be particulates that may be dispersed in the feed, the electrically conductive porous module may be preferably placed in the feed compartment of the membrane distillation system, and preferably the heating element heats up the electrically conductive porous module via electromagnetic induction heating. The present membrane distillation system is able to harness high thermal conductivity of the present electrically and thermally conductive spacers (e.g. metal foam spacers) for energy efficient membrane distillation. The present disclosure compares the superior performance of nickel and copper foams against conventional polymeric spacers. The present electrically and thermally conductive spacers provided higher and more uniform feed-side temperatures, and energy per unit volume distillate for the metal foam spacers was reduced by up to 16%.

[00376] Photocatalytic coatings on the present nickel foam reduced energy requirements by 28% under light irradiation. In the present disclosure, the examples demonstrate the capability of harnessing the high electrical and thermal conductivity of the present spacers to improve the energy efficiency in DCMD. Results were studied via both simulations and actual experiments.

[00377] The present invention also relates to a membrane-filtration device designed to efficiently convert electrical energy to heat energy near the surface of the membrane via the electromagnetic induction heating of the electrically and thermally conductive spacers in membrane distillation. Spacers, which are typically used for membrane fouling mitigation and mechanical support, are conventionally polymeric. With use of the present electrically and thermally conductive spacers, the electromagnetic induction heating of the spacers not only provides localized heating near the membrane surface, but also heats up the feed, thereby improving the energy efficiency for water and wastewater treatment. Since electromagnetic induction heating uses rapidly changing electromagnetic field from an induction coil and the induction coil can be shaped in many ways, the present method of heating can readily be used for many different membrane types (e.g. flat-sheet, hollow fiber, tubular) as well as different membrane distillation configurations (e.g. DCMD, sweeping-gas MD (SGMD), air gap MD (AGMD), vacuum MD (VMD)). The present membrane module having the present spacers, along with the induction coil, can be used for, but is not limited to, lowering the footprint of a membrane distillation system and improving efficiency thereof, making membrane distillation more commercially viable.

[00378] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A membrane distillation system comprising:

a membrane module comprising

2. The membrane distillation system of claim 1, wherein the one or more electrically and thermally conductive spacers are arranged proximal to a surface of the membrane facing the feed channel.

3. The membrane distillation system of claim 2, wherein the one or more electrically and thermally conductive spacers are arranged in contact with the surface of the membrane facing the feed channel.

4. The membrane distillation system of any one of claims 1 to 3, wherein the one or more electrically and thermally conductive spacers comprise carbon, cobalt, copper, graphene, iron, nickel, or stainless steel.

5. The membrane distillation system of any one of claims 1 to 4, further comprising a material which absorbs electromagnetic waves, wherein the material is incorporated to the one or more electrically and thermally conductive spacers, wherein the material comprises carbon nanotubes or graphene oxide.

6. The membrane distillation system of any one of claims 1 to 5, wherein the one or more electrically and thermally conductive spacers comprise a porous structure.

7. The membrane distillation system of claim 6, wherein the porous structure comprises a mesh or a foam.

8. The membrane distillation system of claim 6 or 7, wherein the porous structure comprises pores of a size ranging from 1.5 mm to 1.5 mm.

9. The membrane distillation system of any one of claims 1 to 5, wherein the one or more electrically and thermally conductive spacers are in the form of particulates.

10. The membrane distillation system of any one of claims 1 to 9, further comprising a photothermal material incorporated to the one or more electrically and thermally conductive spacers, wherein the photothermal material comprises platinum nanosheets.

11. The membrane distillation system of any one of claims 1 to 10, wherein the induction heating module comprises an induction heating element arranged to electromagnetically induce heating of the one or more electrically and thermally conductive spacers.

12. The membrane distillation system of any one of claims 1 to 11, wherein the membrane module is in fluid communication with a feed pump operable to deliver a feed to the membrane module and produce a cross flow velocity along the surface of the membrane ranging from 0.8 mm/s to 21 mm/s.

13. The membrane distillation system of claim 12, further comprising a heating module coupled to the feed pump, wherein the heating module is operable to render the feed pump to deliver the feed at a temperature ranging from 35°C to 70°C to the membrane module.

14. The membrane distillation system of any one of claims 1 to 13, wherein the membrane comprises a hydrophobic membrane comprising polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE).

15. The membrane distillation system of any one of claims 1 to 14, wherein the membrane comprises a hollow fiber membrane or a flat sheet membrane.

16. A membrane module operable for membrane distillation, the membrane module comprising:

17. The membrane module of claim 16, wherein the one or more electrically and thermally conductive spacers are coupled to the induction heating module.

18. The membrane module of claim 16 or 17, wherein the one or more electrically and thermally conductive spacers are arranged proximal to a surface of the membrane facing the feed channel.

19. The membrane module of claim 18, wherein the one or more electrically and thermally conductive spacers are arranged in contact with the surface of the membrane facing the feed channel.

20. The membrane module of any one of claims 16 to 19, wherein the one or more electrically and thermally conductive spacers comprise carbon, cobalt, copper, graphene, iron, nickel, or stainless steel.

21. The membrane module of any one of claims 16 to 20, further comprising a material which absorbs electromagnetic waves, wherein the material is incorporated to the one or more electrically and thermally conductive spacers, wherein the material comprises carbon nanotubes or graphene oxide.

22. The membrane module of any one of claims 16 to 21, wherein the one or more electrically and thermally conductive spacers comprise a porous structure.

23. The membrane module of claim 22, wherein the porous structure comprises a mesh or a foam.

24. The membrane module of claim 22 or 23, wherein the porous structure comprises pores of a size ranging from 1.5 mm to 1.5 mm.

25. The membrane module of any one of claims 16 to 21, wherein the one or more electrically and thermally conductive spacers are in the form of particulates.

26. The membrane module of any one of claims 16 to 25, further comprising a photothermal material incorporated to the one or more electrically and thermally conductive spacers, wherein the photothermal material comprises platinum nanosheets.

27. The membrane module of any one of claims 16 to 26, wherein the membrane comprises a hydrophobic membrane comprising polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE).

28. The membrane module of any one of claims 16 to 27, wherein the membrane comprises a hollow fiber membrane or a flat sheet membrane.

29. The membrane module of any one of claims 16 to 28, wherein the induction heating module comprises an induction heating element arranged to electromagnetically induce heating of the one or more electrically and thermally conductive spacers.