GB2573834A - Membrane for use in the purification of liquids, apparatus provided with such membrane; and method of purifying liquids - Google Patents

Abstract

A membrane for use in the purification of liquids which membrane (102) comprises a layer of hydrophobic material provided with at least one band (116,118) of hydrophilic material, such as cellulosic material. In use the membrane (102) is suspended vertically and at least some liquid such as water flowing down the membrane will wet the band (116, 118) and be distributed along the band and across said membrane (102) before leaving the band (116, 118) to facilitate the distribution of liquid across the membrane. The hydrophobic material may be polyamide, aramid, acrylic, modacrylic and polypropylene fibers, blends of these, and blends with polyester fibres. The membrane may be incorporated in the form of a loop (902, Fig 8) in an apparatus including an evaporation chamber (903, Fig 8), a fan, means for moving the membrane through the evaporation chamber, means for cooling air, means for collecting condensate, and means for heating air to produce relatively dry air in preparation for introduction through a dry air inlet (904, Fig 8). The membrane may be used in the production of potable water from seawater and brackish water, recovering water from commercial laundries and concentrating food and perfumery oils.

Description

Membrane for use in the purification of liquids,

Apparatus provided with such a membrane; and

Method of purifying liquids

This invention relates to a membrane for use in the purification of liquids and, more particularly but not exclusively, relates to a membrane for use in the production of potable water from seawater and brackish water. The present invention also relates to an apparatus provided with such a membrane and a method of purifying liquids .

In our PCT Patent Application PCT/GB2017/052404 we described, inter alia, a proof of concept desalination apparatus which constitutes the closest prior art.

In our proof of concept apparatus salt water was introduced to the upper edge of a truncated triangular hydrophobic membrane through which air was blown.

As the salt water moved downwardly across the triangular hydrophobic membrane under gravity it spread outwardly towards the side edges of the truncated triangular membrane assisted by the weave of the membrane.

As the air passed through the membrane it acquired water leaving salt on the membrane.

-2The moisture laden air, which had already been cooled as a result of losing heat to provide energy to evaporate the water, was then further cooled to allow the water to condense as very high purity potable water suitable for human use and for use in agricultural application, for example hydroponics.

Whilst the performance of the proof of concept apparatus was very promising the performance of the desalination apparatus fell off rapidly during scale up to a prototype apparatus and we abandoned our PCT Patent Application .

We subsequently realized that whilst the salt water spread substantially uniformly over substantially the entire area of the truncated triangular hydrophobic membrane in the proof of concept apparatus this was simply not happening with the bigger hydrophobic membranes and greater salt water flows in our prototype apparatus.

The technical problem we were faced with was encouraging the salt water to spread more uniformly over the entire hydrophobic membrane and thus increase the surface area where evaporation occurs.

The technical solution to this problem lay in the provision of one or more bands of hydrophilic material extending across the hydrophobic membrane.

Accordingly, the present invention provides a membrane for use in the purification of liguids which

-3membrane comprises a layer of hydrophobic material provided with at least one band of hydrophilic material positioned so that, when said membrane is suspended vertically and is in use, at least some of the liquid flowing down said membrane will wet said band and be distributed along said band and across said membrane before leaving said band to facilitate the distribution of liquid across said membrane.

The band(s) preferable extend across the entire width of the hydrophobic material and may be, for example straight, chevron shaped, undulant or a mixture thereof.

Our first attempt to address this problem was to hand stich a band of hydrophilic material across the entire width of the hydrophobic material substantially perpendicular to the downward flow of the seawater in our prototype apparatus .

Whilst this offered some improvement over a hydrophobic membrane the improvement was disappointing.

We then decided to distress the lower edge of the band.

Our first attempt at distressing the band involved cutting the lower edge of another strip of hydrophilic material with pinking shears so that it had a zigzag bottom edge with the appearance of a multiplicity of triangles with their apices facing downwardly. This was then hand stitched to the hydrophobic material.

-4This instantly provided a much better distribution of the salt water and the performance (kilowatt of energy input per litre of clean water produced) of the prototype desalination unit improved significantly.

We discovered that when we used scallop fabric shears rather than pinking shears we also obtained promising results although we cannot currently say which are best.

In the case of the un-distressed band it was somewhat unpredictable where the salt water was going to be dispensed.

However, with the lower edge carrying triangular or scalloped features the salt water spread itself along the hydrophilic band and flowed down each triangle/scallop exiting as a source of a separate and distinct stream of salt water .

Accordingly, at least one edge of at least one of the band(s) is preferably distressed.

The distressing may take the form of a multiplicity of generally triangular or scalloped projections.

The extremities of the triangles/scallops are preferably separated by something between 5 and 15 mm with 10 mm working well.

However, it is envisaged that a better distribution of salt water might be achieved by an arrangement in which

-5the triangles/scallops are not evenly spaced but are spaced at differing intervals.

Whilst triangular and scalloped features are clearly beneficial it is envisaged that a band with any suitably distressed lower edge may provide acceptable distribution.

The width of the band is preferably between 10 and 2 0mm.

If desired both sides of the hydrophobic material may be provided with bands which are either plain or have a distressed (lower) edge.

The bands will typically be spaced between 10cm and 2 5cm apart.

Having got back on track we addressed the problem of devising a continuous desalination apparatus.

As our starting point we replaced the triangular membrane of our protype desalination apparatus with a continuous loop of a hydrophobic membrane which we passed through a bath of seawater (collected from The Solent) and then moved it upwardly perpendicular to a stream of air.

The initial performance was not impressive. However, as soon as hydrophilic bands were hand stitched across the hydrophobic material the performance improved. As with the triangular filter above the performance improved when the lower edge of one or more of the bands was distressed. It

-6also improved further when a band was secured to either side of the hydrophobic membrane.

The continuous membrane could be arranged to pass through the air several times, in some cases travelling upwardly and in other downwardly.

For this use it is preferred that both the lower and the upper edges of the band are distressed. This ensures that, in the case of desalination, the seawater flowing downwardly on the membrane can be distributed by an appropriately orientated set of triangles or scallops or other equivalent distress features regardless of the direction of travel of the membrane.

If desired the hydrophilic band(s) (with or without their downwardly and upwardly extending projections) may be woven into the hydrophobic material either after, or preferably during, the production thereof. This is massively desirable as the textile business is well versed in techniques which can be applied to this requirement.

The preferred hydrophobic material remains the material described in our PCT Patent Application No. PCT/GB2017/052404.

For continuous purification/desalination the membrane preferably comprises a long web of substantially uniform width which cab be joined at its ends to form a continuous web. As used herein the term 'continuous web' includes such a membrane whether the end are connected or not.

-7Where the membrane is a continuous web the edges of the web are preferably hemmed to minimize fraying.

Whilst the hem may be formed by folding part of the hydrophobic web back on itself and securing it to the body of the web it is preferable to use hemming tape with a thickness of typically 0.5 to 2mm. In any event the thickness of the hem should be sufficient to help ensure that the portion of the membrane between the hems does not press hard on any rollers. The hemming tape is typically from 10 to 30mm in width.

The edges of the membrane may be provided with holes so that the membrane can be driven by sprocket wheels.

If required additional rows of hemming tape could be provided between the edges. Alternatively this feature could be formed in the membrane during manufacture.

By hydrophobic as used herein we mean any molecule or material that repels water. The surface contact angle Θ, which is the angle formed at the intersection of the liquid-solid phases, of a hydrophobic interaction has a value which ranges between ninety and one hundred and fifty degrees (90°< Θ < 150°).

By hydrophilic as used herein we mean any molecule or material that attracts water. The surface contact angle θ

-8of a hydrophilic interaction has a value which is less than ninety degrees (Θ < 90°).

Whilst any hydrophobic membrane can be used the hydrophobic membrane described in our PCT/GB2017/052404 remains the best option we have found.

So far as following have satisfactory to	concerns the been found date has been....	hydrophilic suitable...	membrane	the most
and	the
If desired	, the membrane	may be in	the	form	of a

continuous loop.

The present invention also provides an apparatus provided with a membrane in accordance with the present invention .

In one embodiment the membrane is generally truncated triangular and, in use, is maintained in approximately the same position whilst the liquid to be purified, typically

salt water,	brackish	water, and	even laundry	water	is
allowed	to	flow down	the	membrane whilst	air	is blown
through	the	membrane .	The	water	evaporates	into	the	air
whilst	salt/solids collect	on the	membrane .

The moist air leaving the membrane is brought into contact with a cold surface and water condenses to form a high purity condensate.

-9The process is stopped periodically and the salt scrapped off the membrane and further processed or discarded according to its content.

In another embodiment the apparatus comprises an evaporation chamber having a dry air inlet and a moist air outlet, a fan for blowing relatively dry air through said dry air inlet, across said evaporation chamber and relatively moist air out said moist air outlet, a membrane in accordance with the invention configured in a continuous loop, means for moving said membrane through said evaporation chamber across the intended flow of said air, means for cooling said relatively moist air to allow moisture to condense, means for collecting condensate, and means for heating the air to produce relatively dry air in preparation for introduction through said dry air inlet.

The apparatus of the present invention is relatively simple and relatively inexpensive compared with the more sophisticated alternatives such as reverse osmosis desalination plants. A unit suitable for providing potable water to a small village can easily be accommodated in an ISO shipping container. Furthermore, because the water is extremely pure it is ideal for use in hydroponic facilities where controlling the content of the water is absolutely critical. The apparatus also shows considerable promise for commercial laundries. At the present time the operators normally buy large volumes of water from the local water company. After the water has been used it is normally discarded. Proposals have been

- 10made for treating the used water and recycling it but these require quite knowledgeable and technically competent staff to operate. The apparatus of the present invention is very simple to operate and produces a most 5 excellent reusable water source.

The present invention also provides a method of purifying liquids which method comprises the step of introducing said liquid into an apparatus in accordance with the present invention.

It is anticipated that the liquid will normally be seawater or brackish water. However, it may also be effluent from a commercial laundry.

In a particular interesting embodiment the liguid comprises an aqueous solution containing oil, more 15 particularly but not exclusively, aqueous solutions of essential oils such as those used in perfumes and foodstuffs .

- 11 For a better understanding of the present invention and to show how the same may be carried into effect6 reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 is a side view of our prototype desalination apparatus;

Figure 2 is a front view of a membrane in accordance with the present invention which forms part of the apparatus of Figure 1 and with arrows representing the structure of the woven material;

Figure 3 is a photograph on an enlarged scale of the membrane of Fig. 2 showing a single opening and the adjoining fibre structure;

Figure 4 is a further photograph of a single opening at a lower edge of the membrane and towards a side edge thereof, showing salt accumulation following membrane use;

Figure 5 is a diagram, to an enlarged scale, of the top portion only of the membrane of Figure 2 showing the distribution of salinity levels across the membrane;

Figure 6 is a diagram showing a further embodiment of the membrane incorporating at its upper end hydrophilic material for providing a syphonic feed of saline water and incorporating at its lower end hydrophilic concentrated brine collection material;

Figure 7 is a simplified diagram of a second embodiment of desalination apparatus in accordance with

	- 12-
the present invention	incorporating a membrane in

accordance with the present invention;

Figure 8 is a schematic view of another embodiment of

a third embodiment of an

apparatus in accordance with the

invention being used for the desalination of seawater; and

Figure 9 is a view

of part of a membrane which is

used in the apparatus shown in Figure 8; and

Figure 10 is a photograph of a polyester mesh, said to be woven, downloaded from the web.

Referring to Figure 1 there is shown an apparatus for

recovering potable water

from seawater. The apparatus,

which is generally identified by reference numeral 100,

comprises a vertically

orientated membrane 102, a

reservoir 104, a stand 106 and a collecting dish 108. It will be appreciated that the membrane 102 may be at an inclination to the vertical provided that its inclination

does not interfere with

flow rate or other operation of

the evaporator, although verticality is preferred. The reservoir 104 is supported in an elevated position by the stand 106, while a collecting dish 108 forms the base of the apparatus 100.

An upper region 110	of the membrane 102 is attached
to the reservoir 104 and	extends into it, so that in use
an end region 112 of the	membrane 102 contacts with water

contained in the reservoir 104. The end region 112 is

secured to the reservoir

104 by attachment means 114 which

may comprise a clip, bracket, adhesive or any other suitable means .

- 13The reservoir 104 may comprise any suitable material for containing salt water. In the embodiment illustrated in Figure 1, the reservoir 104 is a glass vessel. The end region 112 of the membrane 102 within the reservoir 104 and the upper region 110 of the membrane 102 in contact with the glass surface of the reservoir 104 when wetted with water act as cooperating laminae through which water may be siphoned from within the reservoir 104, for which purpose the reservoir is periodically refilled so as to maintain the end region 112 at least partially in contact with the water.

Figure 2 shows a more detailed view of the membrane 102. In the present embodiment, it comprises a layer of hydrophobic material provided with two bands (116,118) of hydrophilic material which are spaced apart from one another and extend across the entire width of the hydrophobic material.

The hydrophobic material comprises a 100% polyester fibre network raschel warp knitted into an openwork or cellular structure of thickness in this instance about 38 mil (1mm) that is highly permeable to air and defines flow paths for directing water flowing down the membrane 102. It will be understood that the thickness of the membrane can be changed according to the mechanical strength required, e.g. if a larger membrane is used. The woven fibrous membrane is, as previously explained, made from 100% polyester which is known to be a hydrophobic material and therefore capillary action does not disturb or play a part in influencing the flow of liquid like water, whereas

- 14gravity does. The feed water encloses the fibrous structures of the membrane 102 by encircling them, so that the affinity of water for water uses the structures to direct the feed water down, and across the membrane. This is important because the support membrane is hydrophobic; it allows the water molecules to easily escape from the membrane when subjected to opposing osmotic forces. If the support membrane was made from a hydrophilic material there would be a tendency for the membrane to hold on to the feed water. Although 100% polyester fibres are presently preferred, it is believed that similar results may be obtained using membranes of other synthetic hydrophobic polymer fibres, e.g. polyamide, aramid, acrylic, modacrylic and polypropylene fibres, blends of these, and blends with polyester fibres.

In an embodiment, the membrane structure comprises somewhat polygonal or oval openings that allow a rod of 1.75 mm diameter to pass through and have largest dimension approximately 3mm and pattern repeat 7 mm formed in rows with the openings of each row offset from those in the immediately preceding and succeeding row so that they lie in a diagonal alignment as shown. In use, the largest dimension of the holes is aligned with the intended flow direction of the water as shown in Fig. 2. The generally oval shape of the holes is due to the manner of warp knitting in forming the sheet structures, which are suitably aligned for creating fluid pathways in the membrane as shown in Fig. 2 and make it highly permeable to air and water. These holes are created and enclosed by fibre structures that travel top to bottom and from side

- 15to side in such a way that they also create structures that travel diagonally. These diagonal structures that have been created by the design of the weave are very well defined, (see Fig 3) and play an important part in providing fluid pathways for water to travel, despite the fact the fibrous membrane is made from a hydrophobic material. The diagonal structures can however support the downward flow of water because the fibrous structure has a vertical component in its direction, and because the water is fed to both sides of the membrane. The openwork structure of the hydrophobic membrane provides paths that permit the water on one side of the membrane to contact the water on the opposite side of the membrane. Because the membrane is in use fed with water from both sides, both sides provide areas for evaporation.

In an alternative embodiment, the fabric may be a woven openwork netting e.g. in fine woven polyester mesh with holes of size e.g. 3-4mm and of hexagonal or oval shape, the holes being formed in rows with each row staggered so that the holes also form a diagonal pattern tending, as before, to direct flow to the sides of the sheet. As before the strands of the mesh advantageously are formed of multi-filaments woven with small holes between them to promote water flow and evaporation. Woven netting of this type is shown in Fig. 10 which is a macrophoto said to be of fine woven polyester mesh/netting, the photo being downloaded from Alamy.

The membrane 102, is of generally truncated triangular shape, and is flared outwardly from the top to

- 16the bottom, being narrower at the top and wider at the bottom. In the prototype embodiment, a membrane as shown in Fig 2 had a height of 47.5cm, a width at its top end of 5cm, a width at its bottom end of 22cm and sides diverging at an angle of 15°. The arrows A-D indicate the directing influence caused by the structure of the woven material. The middle vertical arrows A represent the initial downwards direction of seawater driven by the force of gravity. The middle line indicated by A also denotes an axis of symmetry of the structural design and shape of the membrane 102. The outwardly-directed diagonal arrows B represent the diverging structural fibre elements radiating out from the middle of the membrane 102. The pair of inwardly-directed diagonal arrows C, near the top of the membrane 102, represent fibre structures that converge towards the middle. The horizontal arrows D represent fibre structures that oppose each other in a horizontal direction, perpendicular to the seawater supply flowing down the middle of the membrane 102.

The structural lines created by the design of the weave each carry out different functions which are important to the separation process. The converging structures help to maintain the central column of the seawater on the membrane 102, and thus maintain a constant salinity level as a reference. It may be appreciated that, in the absence of capillary action or wicking, the route that the bulk of the seawater may take will be directed by the convergent structures, driven by gravity and the hydrophilic attraction of water molecules to each other. The diverging structures create increasingly longer

- 17pathways from the middle of the membrane 102 to its outside edges, progressively reducing the thickness of the layer of seawater travelling along the flow paths and thereby promoting evaporation of the water. This process causes an increase in the concentration of salinity on the two opposing sides of the membrane 102. Without being bound by theory, it is believed that this creates an osmotic pressure gradient across the membrane 102. The fibre structures at 90 degrees to the axis of symmetry of the membrane 102 provide lateral pathways that convert the osmotic pressure gradient into opposing force vectors that overcome the surface tension of the seawater to pull apart neighboring water molecules and thus promote super evaporation of the water.

A photographic enlargement of the knitted membrane material showing one of the openings and its surrounding knitted structure is shown in Fig. 3. It will be seen that the weave is in fact a structure formed by knitting together multiple individual polyester fibres, with the warp knitted structure providing a multitude of smaller openings which may also facilitate evaporation.

Figure 4 is similar photographic view showing an opening at the lower edge of the membrane towards one side

thereof after the apparatus	has been	in	use	for	24 hours
and	salt has	accumulated.
	Figure	5 shows a	representation	of	the	distribution
of	salinity	across the	upper	portion	of	the	membrane 102.
To	create	opposing	vector	forces	302,	the	f ibrous

structure is designed to have an axis of symmetry aligned

- 18to the vertical, in order to be able to use gravity to drive the system. Seawater of a nominal level of salinity is supplied to the middle of the membrane 304, where the thickness of the layer of seawater is greater and the normal evaporation rate is low. As the seawater travels from the middle 304, along the angled fibre structures of the membrane 102, towards the angled sides of the membrane 306, the layer becomes thinner due to normal evaporation over a distance from the source.

As diagrammatically shown in Figure 5, the resulting difference between the salinity in the middle 304 of the membrane 102 and its opposite angled sides 306 creates balanced, opposing osmotic pressure differentials across the horizontally opposed structures of the membrane 102, which can exert opposing force vectors 302 to pull apart neighbouring molecules of water from each other. The process starts at the top of the membrane 102, where the diverging pathways are shorter, and progresses down towards the bottom where the divergent pathways are longer. The super evaporation process begins when the osmotic pressure becomes great enough to overcome the hydrophilic attraction of water molecules to each other. The process becomes self-sustaining due to precipitation of the salt ions travelling to the angled sides of the membrane 306, which maintains the mineral concentration gradient.

It was appreciated that the rate of delivery of saline water to the top of the membrane could be limiting the rate of evaporation achieved. The rate of throughput

- 19of saline water to the membrane was increased using glass fibre wicks to provide the syphonic action. In further embodiments, to improve water delivery to the membrane a larger reservoir of plastics material was employed and as shown in Fig. 6 a strip 404 of cellulosic or other hydrophilic material is attached to membrane 402 as previously described, the attachment being a line of stitching 406. In use, the strip 406 is immersed in the reservoir 104, taking the positions previously occupied by immersed end region 112 and end region 110, and providing syphonic delivery to the membrane 402 at an enhanced flow rate. At the base corners of the membrane 402, attached long wicks of hydrophilic material 408 leads concentrated saline water from the membrane, whilst still collecting less concentratedly saline water from the middle of the membrane. The benefits of this change are expected to be increase the capacity / performance for a membrane of a given area .

Having said this when we tried to scale up from our proof of concept apparatus we ran into problems which we eventually determined were being caused by failure to distribute the salt water sufficiently uniformly over the surface of the hydrophilic membrane, ie whilst the distribution over the top portion 102 shown in Figure 5 was quite acceptable the distribution below the top portion was disappointing.

As can be seen in Figure 2 the membrane 102 of hydrophobic material is provided with two bands 116 and 118. Each band 116, 118 extends across the entire width of

-20the hydrophobic membrane 102 and has a respective lower edge 120, 122 which has been distressed by pinking shears to form a multiplicity of downwardly extending triangular projections 124, 126 respectively. Each band 116,118 is hand stitched to the hydrophobic membrane 102.

The bands 116 and 118 are respectively 9cm and 15cm wide and 1cm deep. The triangular projection are approximately 0.5cm deep and their apices are spaced at 0.5cm centers .

The band 116 and 118 are spaced 18cm apart vertically .

As the salt water runs down from the top of the membrane 102 it encounters the top of the hydrophilic band 116 and spreads along the band 116 to help distribute the salt water across the entire width of the membrane 102.

The water then gravitates downwardly and flows down the triangular projections leaving their apices in separate and discreet streams which pass down the hydrophilic membrane until they encounter the band 118 where the process is repeated.

The provision of the bands 116 and 118 helps distribute the salt water over the entire width of the membrane .

A stream of (relatively) dry air is blown onto the membrane substantially perpendicular thereto.

As the air passes through the membrane water evaporates and enters the air which becomes(relatively)

-21 moist and is also cooled due to the air providing energy to evaporate the water.

Salts simultaneously start building up on the membrane 102.

The relatively moist air is further cooled and high purity water condenses out.

The salt can be periodically recovered and used/discarded according to its composition.

Referring now to Figure 7 there is shown an apparatus in accordance with the present invention. The apparatus, which is generally identified by reference numeral 500

Figure 7 shows a desalination apparatus in accordance with the present invention which is generally identified by reference numeral 500. The apparatus 500 includes a membrane 502 of polyester mesh generally like that described in Figure 2 but of increased area. The membrane 502 is supported vertically and fed at its top with seawater or other brine from supply 504 through tap 506. The membrane 502 is provided with two horizontal bands 216, 218 which extend horizontally across the entire width of the membrane 502. The lower edges of each band is also distressed as described with reference to band 116 and 118 in Figure 2. The apparatus also includes a refrigeration circuit 508 connected to electrical power supply 510. The refrigeration apparatus comprises a compressor 512, a fan 514, a refrigerant evaporator 516, a refrigerant condenser 518 and a J-T valve 519. Potable water condensed by

-22contact with the relatively cold evaporator 516 is collected at drain 520 and discharged through line 522. Concentrated saline water reaching the bottom of the membrane 502 is collected in vessel 526 and is discharged through line 528. Fan 514 circulates air through the apparatus 500, air warmed by condenser 518 circulating as indicated by arrows 524a, 524b to one side of membrane 502, and water-laden air which has passed through the membrane 502 recirculating to evaporator 516 and fan 514 as indicated by arrows 524c and 524d. Evaporation of water from membrane 502 is in the air current indicated by arrows 502, 504 so that evaporation rates can be varied. Heat from the circulating air 524b replaces the heat loss at membrane 502 from water evaporation and maintains the membrane 502 at preferably at least ambient temperatures and more preferably at an elevated temperature to further promote evaporation of water vapour from the membrane 502, the temperature within the apparatus conveniently being between ambient temperature and water boiling temperature. The saturated vapour pressure of water at 15° is 12.8 mm Hg, at a normal room temperature of 20°is 17.5 mm Hg, warmed to 30°C is 32 mm Hg, warmed to 40°C, further warmed to 50°C is 92 mm Hg, and yet further warmed to 70°C is 238 mm Hg. It will be appreciated that operation of the apparatus with even mildly elevated temperatures at membrane 502 and downstream thereof significantly increases the rate of water evaporation and the ability of the airstream to carry the water vapour evaporated from the membrane assuming that relative humidities >,30% and preferably > 60-80% RH resulting in significant

-23improvements in the possible throughput of the apparatus and the rate of production of desalinated water.

Referring now to Figure 8 there is shown a continuous desalination apparatus in accordance with the present invention .

The apparatus, which is generally identified by reference numeral 900 comprises an evaporation chamber 903 having a dry air inlet 904 and moist air outlet 906.

A continuous membrane 902 which comprises an elongated membrane of substantially uniform width which is connected at its ends to form a continuous loop is arranged so that part of the continuous membrane passes vertically upwardly in the evaporation chamber 903 between the dry air inlet 904 and the moist air outlet 906, over a roller 908, and then downwardly through the evaporation chamber 903 parallel to but spaced apart from the previous part by approximately 15cm.

The continuous membrane 902 then passes around roller 910. It then passes around roller 912 which is positioned adjacent a roller 914 which is provided with bristles for removing deposits from the membrane 902 which are collected in recovery vessel 916.

The continuous membrane 902 then passes over roller 918 and around roller 920 in the bottom on a seawater tank 921 which is constantly topped up with seawater.

The continuous membrane 902 is kept in motion by a motor (not shown) which is coupled to roller 908. An insulated exit duct 922 is arranged to conduct moist air

from the evaporation chamber 903

through the coiling coils

924 of a refrigeration unit.

Water condenses out of

the moist air and the

condensate leaves through pipe 926 and is collected in a liquid recovery vessel 928.

The air is then passed

through duct 930 to a

circulating fan 934 and a heater 932 which it leaves warmed and relatively dry. The heater 932 is associated with the cooling coils 924 and contains a compressor and heat exchanger to cool hot compressed air against the air being circulated through the evaporation chamber 903.

As shown in Figure 9 the membrane 902 is of

substantially constant width.

It comprises a layer of

hydrophilic material provided with a plurality of bands which extend the entire width of the membrane perpendicular to its intended direction of travel.

Both the upper and lower

edges of each band are

distressed .

The side edges of the membrane are each provided with hemming tape which is approximately 1.3cm in width and ?? in thickness .

In use, and with reference to both Figures 8 and 9, the continuous membrane 902 is moved at a rate of Icm/s anticlockwise by the motor associated with roller 908.

As the membrane moves upwardly from the seawater tank

922 a certain volume of seawater attaches to the

-25hydrophobic membrane whilst significantly more seawater is absorbed into the hydrophilic bands.

As the membrane moves upwardly the seawater flows downwardly under the influence of gravity and is distributed substantially uniformly across the hydrophobic membrane by the bands of hydrophilic material assisted by the properties afforded by the fabrication of the hydrophobic membrane.

Relatively dry air (R/H=17%) is introduced through the dry air inlet 904 at a rate of 4.8m/s and at a temperature of 37°C.

As the dry air passes through the membrane it absorbs moisture and its temperature drops as it provides the energy necessary to vaporize the water.

The moist air (R/H=44%) leaves the evaporation chamber 903 through the moist air outlet 906 before passing through insulated exit duct 922 after which 27% of the water vapour present is condensed out by the cooling coils 924 which operate at about 7 degrees Centigrade.

The cold air leaves the cooling coils and passes through the circulating fan 934 and the heater 932 where it is warmed to 39 degrees C (R/H=17%) in readiness for being introduced through the dry air inlet 904.

The heater 932 and the cooling coils 924 conveniently form part of a conventional stand-alone refrigeration system.

-26Various modifications may be made to the embodiment described, for example the bands may be positioned on one or both sides of the hydrophobic membrane. The bands need not be distressed or may be distressed on one or both edges. Where bands are situated on opposite sides of a hydrophobic membrane they may be positioned back to back or may be staggered.

The bands may be straight or may be arranged in a pattern, for example a 'V' shape (chevron) pattern. The bands may also undulate.

Multiple passes of membrane may be arranged to be contacted by the air in series. In such an arrangement the streams are preferably arranged to meet in counter-current ie with the driest air coming first into contact with the most saline water.

If desired the cooling coil 924 could be arranged to further cool the moist air and the heater 932 could be replaced with a dehumidifier arranged to condense water from the pre-cooled moist air (the liquid leaving from the bottom of the dehumidifier) and pass warm air to the warm air inlet 904.

Whilst the membrane and apparatus are primarily being developed with the production of potable water from seawater and brackish water in mind early experiments indicate that the membrane and apparatus are capable of recovering potable and recyclable water from commercial laundries which use very considerable volumes of water which is normally discarded.

-27Another use is in the concentration of oils in the food and perfumery business. Depending on the physical properties of the oil the final product may either be condensate or what is recovered from the membrane - which 5 may be in liquid form.

Whilst the bands are preferably attached to or woven into the hydrophobic membrane it would also be possible for a length of hydrophobic membrane to be attached to one side (upper edge) of a band of hydrophilic material and 10 another length of hydrophobic membrane to be attached to the other side (lower edge) of the band. The bands can be attached to the hydrophobic membrane by any suitable means, for example hand stitching, machine stitching or using a suitable adhesive.

Claims (17)

Claims:

1 A membrane for use in the purification of liquids which membrane comprises a layer of hydrophobic material provided with at least one band of hydrophilic material positioned so that, when said membrane is suspended vertically and is in use, at least some of the liquid flowing down said membrane will wet said band and be distributed along said band and across said membrane before leaving said band to facilitate the distribution of liquid across said membrane.

2 A membrane as claimed in extend across the entire material.

3 . A membrane according to least one edge of the or at distressed .

Claim 1, wherein the band(s) width of the hyrdophobic

Claim 1 or 2, wherein at least one of the band(s) is

4. A membrane as claimed in Claim 3, wherein the distressing takes the form of a multiplicity of generally triangular or scalloped projections.

5. A membrane as claimed in Claim 4, wherein the extremities of the triangles/scallops are separated by between 5 and 15mm.

6. A membrane as claimed in Claim 5, wherein the extremities of the triangles/scallops are separated by

10mm.

7. A membrane as claimed in Claim 4, wherein the triangles/scallops are spaced at differing intervals.

8. A membrane as claimed in any preceding Claim, wherein the width of the band is between 10 and 20mm.

9. A membrane as claimed in any preceding Claim, wherein both sides of the hydrophobic material are provided with bands .

10. A membrane as claimed in any preceding Claim, wherein the bands are spaced between 10cm and 30cm apart.

11. A membrane as claimed in a any preceding Claim, wherein both the lower and the upper edges of the band are distressed .

12. A membrane as claimed in any preceding Claim, wherein the hydrophilic band(s) is woven into the hydrophobic material.

13. A membrane as claimed in any preceding Claim when in the form of a continues loop.

14. An apparatus provided with a membrane as claimed in any preceding claim.

15. An apparatus as claimed in Claim 13, which comprises an evaporation chamber having a dry air inlet and a moist air outlet, a fan for blowing relatively dry air through said dry air inlet, across said evaporation chamber and

-30relatively moist air out said moist air outlet, a membrane in accordance with any of Claims 1 to 13, configured in a continuous loop, means for moving said membrane through said evaporation chamber across the intended flow of said 5 air, means for cooling said relatively moist air to allow moisture to condense, means for collecting condensate, and means for heating the air to produce relatively dry air in preparation for introduction through said dry air inlet.

16. A method of purifying liquids which method comprises 10 the step of introducing said liquid into an apparatus as claimed in Claim 13 or 14.

17. A method according to Claim 15, wherein said liquid is seawater, brackish water, effluent from a commercial laundry or an aqueous solution containing oil, for example

15 essential oils such as those used in perfumes and foodstuffs .