WO2011150898A2

WO2011150898A2 - A process for the separation of a gaseous mixture and an apparatus for carrying out the same

Info

Publication number: WO2011150898A2
Application number: PCT/CZ2011/000051
Authority: WO
Inventors: Pavel IZÁK; Magda POLONCARZOVÁ; Jiří VEJRAŽKA
Original assignee: Česká Hlava S.R.O.
Priority date: 2010-06-02
Filing date: 2011-05-13
Publication date: 2011-12-08
Also published as: CZ303107B6; WO2011150898A3; CZ2010438A3; EP2576011A2

Abstract

A process for the separation of a gaseous mixture, which is different from biogas, to form two streams of which one is depleted and the other is enriched in at least one component of said gasous mixture, which comprises introducing said gaseous mixture to one side of a liquid membrane having higher affinity to at least one component of said gaseous mixture, said liquid membrane being immobilized in a porous support having good affinity to the membrane forming liquid, wherein the first stream is obtained as a retentate and the second stream is obtained as a permeate, wherein any losses of said liquid from said membrane, which are caused by driving away said liquid in said permeate stream, are compensated by saturating said introduced gaseous mixture with the vapour of said liquid at a temperature which is higher than the temperature at which said membrane separation takes place whereupon said vapour is condensed when said gaseous mixture is cooled in said membrane. An apparatus for carrying said process which comprises a membrane separator (6), which is divided by a liquid membrane immobilized in a porous support (10), into a retentate space (6a) and a permeate space (6b), said retentate space (6a) being connected with supply fittings (1, 3, 5, 11) for said gaseous mixture and discharge fittings (13, 7, 9) for said retentate stream whereas said permeate space is connected with discharge fittings (8, 14) for said permeate stream. A saturator (5) for saturating said gaseous mixture with said vapour of the used liquid is inserted into the supply tubing (11) for said introduced gaseous mixture.

Description

A process for the separation of a gaseous mixture and an apparatus for carrying out the same.

Field of Invention

The present invention relates to a process for the separation of a gaseous mixture, which is different from biogas, to form two streams of which one is depleted and the other is enriched in at least one component of the gaseous mixture. The process comprises introducing the gaseous mixture to one side of a liquid membrane having higher affinity to at least one component of the gaseous mixture. The liquid membrane is immobilized in a porous support having good affinity to the membrane forming liquid. The first stream is obtained as a retentate and the second stream is obtained as a permeate. The invention also relates to an apparatus for carrying out said process. The apparatus comprises a membrane separator, which is divided by a liquid membrane immobilized in/on a porous support, into a retentate space and a permeate space. The retentate space is connected with supply fittings for the gaseous mixture and discharge fittings for the retentate stream whereas the permeate space is connected with discharge fittings for the permeate stream.

Background Prior Art

Individual gas components have been separated by different separation techniques. These techniques e.g. include membrane processes, adsorption, absorption and cryogenic distillation. In view of the greenhouse effect, the separation of waste gases from industrial processes, particularly aimed at the regeneration of carbon dioxide, has acquired gaining importance recently. Flue gases depleted from the main portion of carbon dioxide can be vented into the atmosphere without substantially adding to global warming.

Membrane separations seem to be promising for the separation of a plurality of different gaseous mixtures. The term membrane separation is used to designate a process in which a mixture is separated on the basis of different permeability of individual components across a selective membrane by means of which two spaces are separated from one another. The transport of the components across the membrane l is influenced by their physical and chemical properties, and the interactions of the components between each other and the membrane, itself. With the use of porous membranes the components are most frequently separated on the basis of their particle size. With the use of non-porous membranes the transport properties are particularly dependent on the solubility of the components within the membrane or optional possibility of chemical interactions between the individual components and the membrane, and the diffusion coefficients of the components in the membrane.

In now-a-days practice the separation of gaseous mixtures is mostly carried out with the help of non-porous membranes made of polymeric materials (R. Barker: Recent Developements in Membrane Vapour Separation System. Membrane Technology 114 (1999) 9-12). Their main disadvantage is that they become contaminated by toxic materials. Due to such contamination the separation membranes lose their original selectivity and the permeation flow is also reduced. Contaminated membranes have to be replaced and moreover, they must be handled as toxic waste, which is very expensive.

Another possible approach is gas separation with the help of so called liquid membranes (J.E. Bara, S. Lessmann, C. J. Gabriel, E. S. Hatakeyama, R. D. Noble, D. L. Gin, Synthesis and Performance of Polymerizable Room-Temperature Ionic Liquids as Gas Separation Membranes, Ind. Eng. Chem. Res. 46 (2007) 5397). The most recent trend in this respect are so-called anchored ionic membranes (J.E. Bara, SE. Hatakeyama, DL. Gin, RD. Noble; Improving C<¾ Permeability in Polymerized Room-temperature Ionic Liquid Gas Separation Membranes through the Formation of a Solid Composite with a Room-temperature Ionic Liquid, Polym. Adv. Technol.; 19 (2008) 1415.

Immobilized ionic membranes comprise ionic liquids which, owing to their high selectivity and molecular diffusion, as well as negligible vapour tension, represent ideal materials for the preparation of liquid membranes. (J. E. Bara , Ch. J. Gabriel , E. S. Hatakeyamaa, T. K. Carlisle: Improving C0₂ Selectivity in Polymerized Room-temperature Ionic Liquid Gas Separation Membranes through Incorporation of Polar Substituents, Journal of Membrane Science 321 (2008) 3-7). In general, the main diadvantage of the liquid membranes consists in that their stability depends on miscibility with the separated materials which are not always capable of resisting to the long-termed (or even operational) duty. Moreover, ionic liquids more or less absorb moisture, which leads to relatively quick breakdown of the membrane structure and stability.

The hitherto conducted investigations have shown that it is the solubility of a separated component in a liquid membrane that is critical for the selectivity of the liquid membrane (P. Izak, L.Bartovska, K. Friess, M. Sipek, P. Uchytil; Comparison of Various Models Flat for Transport of Binary Mixtures through Dense Polymer Membrane, Polymer, 44 (2003) 2679-2687).

A suitable liquid for the separation has to comply with the requirement that the separated gases be at least by one order of magnitude more soluble in the selected liquid than the gas which is the predominant component of a gaseous mixture. Hence, the selection of a liquid for the impregnation of a porous membrane is a key factor.

For proper membrane function appropriate selection of the so-called support, in which the selected liquid is anchored, is very important, in particular in view of the rigidity and the stability of the system. Moreover, high affinity between the liquid and the porous support is a must.

The hitherto known liquid membranes for the separation of gases aimed at obtaining a stream enriched in valuable components and depleted in balast or strictly detrimental components have suffered from certain complexity and expensiveness of the used liquid systems, and, in particular, certain instability of a liquid membrane which is caused by varying amounts of the liquid embedded in the membrane support. If relatively volatile liquids, such as water, are used, such undesirable variation can be caused by the fact that under the operational conditions the liquid from the membrane is dissolved in the separated gas whereby the membrane is in fact getting dried. The membrane stability is however very important from the point of view of the continuous operation of a separation unit. Subject Matter of Invention

The above mentioned disadvantages are solved by a process and an apparatus according to the invention, which are described below.

The subject matter of the present invention is a process for the separation of a gaseous mixture, which is different from biogas, to form two streams of which one is depleted and the other is enriched in at least one component of said gaseous mixture, which comprises introducing said gaseous mixture to one side of a liquid membrane having higher affinity to at least one component of said gaseous mixture, said liquid membrane being immobilized in a porous support having good affinity to the membrane forming liquid, wherein the first stream is obtained as a retentate and the second stream is obtained as a permeate. The improvement of said process lies in that the liquid losses from said membrane, which are caused by driving away said liquid in said permeate stream, are compensated by saturating said introduced gaseous mixture with the vapour of said liquid at a temperature which is higher than the temperature at which said membrane separation takes place whereupon said vapour is condensed when said gaseous mixture is cooled in said membrane.

In general, as the porous supports any materials having high affinity to the selected liquid in the pores of which the vapour of the liquid with which the introduced gaseous mixture has been saturated can spontaneously condense as soon as it is cooled to the temperature below its dew point. Such materials can include polymer porous membranes e.g. made of polytetrafluoroethylene, polyatnide or other polymers or ceramic membranes e.g. on the basis of alumina.

In the preferred embodiment of said process the permeation takes place into sweeping gas. In another embodiment reduced pressure can be maintained on the permeate side of a permeation cell.

The subject matter of the present invention also includes an apparatus for carrying out said process. Said apparatus comprises a membrane separator, which is divided by a liquid membrane immobilized in a porous support, into a retentate space and a permeate space. Said retentate space is connected with supply fittings for said gaseous mixture and discharge fittings for said retentate stream whereas said permeate space is connected with discharge fittings for said permeate stream. The improvement of said apparatus lies in that a saturator for saturating said gaseous mixture with said vapour of the used liquid is inserted into the supply tubing for said introduced gaseous mixture.

For the correct function of both the process and the apparatus according to the invention it is critical that the solubility of the separated gases in the separation liquid may be substantially different. This can be achieved by appropriate selection of the separation liquid. For the correct function of the system it should be also ensured that no liquid may be getting lost from the pores of the support.. To this end, in a preliminary step the gaseous mixture introduced to the membrane is saturated with the separation liquid. The saturation takes place at a temperature which is higher than the temperature in the membrane separator in which the liquid membrane immobilized in the pores of the support is located. The temperature difference between the introduced gaseous mixture and the liquid membrane is such that spontaneous condensation of the liquid in the pores may take place. In this way the membrane can be prevented from drying which would impair the stability and the separation properties of the system.

The proposed separation method is advantageous in that it requires simple arrangement which is associated with reduced capital and operational expenditures, and it provides safe operation of the separation unit resulting from continuous regeneration of the membrane liquid in the course of the separation process.

Brief Description of Drawings

The attached figure contains a schematic diagram of an example of the apparatus according to the invention.

The process according to the invention is illustrated in the following working examples which are carried out in the apparatus depicted in the attached figure. The examples are only illustrative, and they do not limit the claimed scope in any respect. Working Examples

E x a m pi e 1

Enrichment of gaseous mixture CH₄/C0₂/H₂S in CH₄

A gaseous mixture consisting of a ternary mixture of 70% vol. methane, 30% vol.. carbon dioxide a 900 ppm sulfane is introduced from a pressure container 1 of the gaseous mixture over a flow controller 3 of the gaseous mixture into a saturator 5, in which the gaseous mixture is saturated with vapour, and then into a membrane separator 6. The temperature in the saturator 5 of the gaseous mixture with steam is higher than the temperature in the membrane separator 6, in which a water impregnated porous support of hydrophilized polytetrafluoroethylene is mounted. Partial gas flows through this condensing water membrane at the pressure over the

2 2

membrane 350 kPa are as follows: Jco₂ = 22.3 l/(ni h), J_CH4 = 1.26 l/(m^zh) a JH2S = 0.08 l/(m²h).

Table 2.

Separation of a gaseous mixture with the help of a condensing water membrane

The temperature difference is such that spontaneous water condensation in the pores of the porous membrane may take place. The driving force of the separation process is the different concentration of individual gas components and the pressure difference across the membrane. The permeation takes place into the carrier gas, which is nitrogen, introduced below the membrane from a pressure container 2 of the carrier gas over the carrier gas flow controller 4. Optimal total pressure over the membrane is controlled by a reverse pressure controller 7. E x a m l e

Removing C0₂ from a waste gaseous stream from an industrial process

The typical composition of waste gas from an industrial process is as follows: 25 % vol. 0₂ and 75 % vol. C0₂. For the evaluation of the separation as a feed a gaseous mixture having two different compositions, mentioned in Table 3 are used. The gaseous mixture is introduced from a pressure container l_of the gaseous mixture over a gaseous mixture flow controller 3 at a velocity of 10 ml/min into a saturator 5 of the gaseous mixture with steam, and then into a membrane separator 6. The saturator 5 is supplied with tap water. The temperature in the saturator 5 of the gaseous mixture with steam is higher (27 °C) than the temperature in the membrane separator 6_(14 °C). A porous mesh (with pores 3 μπι) with an attached membrane 10 of hydrophilized teflon (having pores 0.1 μπι, thickness 30 μιη and porosity 80 %), impregnated with water, is mounted, in the membrane separator 6. The effective area of the membrane having the radius of 13 cm is 132,7 cm². The permeation is carried out into the carrier gas, which is nitrogen. The conducted experiments show that C0₂ can be efficiently removed with the use of a porous membrane of hydrophilic teflon (Table 3) in which water condensation takes place. Partial gas flows through this „water membrane" at the pressure over the membrane 350 kPa are shown in Table 3.

Table 3

Removing C0₂ from a waste gaseous stream from a combustion process

The temperature difference (13 °C) is such that spontaneous water condensation in the pores of the porous membrane may take place. The driving force of the separation process is the different concentration of individual gas components and the overall pressure over and under the membrane. The permeation takes place into the carrier gas, which is nitrogen, introduced below the membrane from a pressure container 2 of the carrier gas over the carrier gas flow controller 4. Optimal total pressure over the membrane is controlled by a back pressure regulator 7. Industrial Applicability

In the above mentioned experimental part the invention is exemplified on an illustrative separation of the mixture CH₄/CO₂/H₂S a 0₂/C0₂ with the use of a water membrane immobilized in hydrophilized porous polytetrafluoroethylene as a support. The present invention is however not limited to the use of the exemplified separation. As can be seen, the improvement of a separation process which can be achieved with the use of the principle of "a condensing membrane" (i.e. while compensating for any losses of the liquid removed from the membrane by its continuous reintroduction through its condensation from the gaseous mixture stream) may be applied in general in numerous further applications, the examples of which include the separation of methane from hydrogen sulfide, such as in the case of processing pit gas, the separation of sulfane from the air, such as in the case of processing toxic volcanic gases, the separation of carbon dioxide or siloxanes or aromatic hydrocarbons from the air in the case of various environmental applications.

Claims

C l a i m s

1. A process for the separation of a gaseous mixture, which is different from biogas, to form two streams of which one is depleted and the other is enriched in at least one component of said gasous mixture, which comprises introducing said gaseous mixture to one side of a liquid membrane having higher affinity to at least one component of said gaseous mixture, said liquid membrane being immobilized in/on a porous support having good affinity to the membrane forming liquid, wherein the first stream is obtained as a retentate and the second stream is obtained as a permeate, characterized in that any losses of said liquid from said membrane, which are caused by driving away said liquid in said permeate stream, are compensated by saturating said introduced gaseous mixture with the vapour of said liquid at a temperature which is higher than the temperature at which said membrane separation takes place whereupon said vapour is condensed when said gaseous mixture is cooled in said membrane.

2. A process according to claim 1 characterized in that said permeation is carried out into the sweeping gas.

3. An apparatus for carrying out the process according to claim 1 or 2 characterized in that it comprises a membrane separator (6), which is divided by a liquid membrane immobilized in a porous support (10), into a retentate space (6a) and a permeate space (6b), said retentate space (6a) being connected with supply fittings (1, 3, 5, 11) for said gaseous mixture and discharge fittings (13, 7, 9) for said retentate stream whereas said permeate space is connected with discharge fittings (8, 14) for said permeate stream, characterized in that a saturator (5) for saturating said gaseous mixture with said vapour of the used liquid is inserted into the supply tubing (11) for said introduced gaseous mixture. List of Reference Numerals

1— pressure container for gaseous mixture

2 - pressure container for carrier gas

3 - gaseous mixture flow controller

4— carrier gas flow controller

5— saturator of gaseous mixture with vapour of selected

6 - membrane separator

6a - retentate space

6b - permeate space

7— reverse pressure controller

8 - permeate discharge valve

9 - valve for retentate effluent

10 - hydrophilic porous support

11 - 14 - piping

11 - feeding pipe for gaseous mixture

12 - feeding pipe for carrier gas

13 - discharging pipe for retentate

14 - discharging pipe for permeate