Indian Patents. 279785:"A PROCESS FOR THE PREPARATION OF ASYMMETRIC MEMBRANE FOR GAS SEPARATION"

Title of Invention	"A PROCESS FOR THE PREPARATION OF ASYMMETRIC MEMBRANE FOR GAS SEPARATION"
Abstract	The present invention relates to a process for preparation of thin, mechanically strong asymmetric membranes for gas separation. More particularly, the present invention relates to a process for the preparation of asymmetric membranes for separation of oxygen from atmospheric air.

Full Text	PROCESS FOR PREPARATION OF ASYMMETRIC MEMBRANES FOR GAS SEPARATION FIELD OF INVENTION The present invention relates to a process for the preparation of asymmetric membranes for gas separation. More particularly, the present invention relates to a process for the preparation of thin, mechanically strong asymmetric membranes for gas separation. More particularly, the present invention relates to a process for the preparation of asymmetric membranes for separation of oxygen from atmospheric air. PRIOR ART Separation of one or more gases from a mixture of gases forms an important and often critical step in several industries. Several methods are known in the art for the separation of gases such as cryogenics, pressure swing adsorption and membrane separation. Membrane based separation of gases are found to be more viable economically in some industries than other methods of separation. It is known in the art that the permeation flux across a membrane is inversely proportional to the film thickness. Several efforts have therefore been made therefore, to reduce the thickness of a gas separation membrane. However, reduction of thickness has an inherent limitation in that very thin membranes (below 20µ) cannot be made with adequate mechanical integrity and defect-free structure using existing materials. As a result, efforts in the art to overcome this limitation in obtaining thin gas separation membranes with mechanical integrity and defect free structure have focused on two different membrane morphologies. One area of research has been to develop asymmetric membrane having graded porosity and another area of research has been to develop a composite membrane where a porous support is coated with an ultra thin dense top layer of a selective polymer. Asymmetric membranes consist of a dense surface and a porous support prepared by phase inversion process. The surface skin layers of these membranes have semi-permeability. Asymmetric membranes are used for reverse osmosis as well as gas separation applications. Loeb and Sourirajan made the first asymmetric membrane from cellulose acetate (US Patent No. 3,133,132 and US Patent No. 3, 133, 137). However, the above symmetric membranes suffer from the limitation that the separation property for smaller molecules such as gas molecules is not very high. In composite membranes the superimposed layer discriminates or separates gases while the porous support is needed for the mechanical integrity of the membrane. Klass et al, US Patent No. 3616607, Stancell et al US Patent No. 3657113 and Yasuda US Patent No. 3775303 and Browall US Patent No. 3980456 exemplify making of such composite gas separation membranes. The main disadvantage of fabricating these membranes having ultra-thin defect-free skin is that the processes of preparation are to be carried out under stringent conditions such as 'Clean room' atmosphere. Cabasso, et al US Patent No. 4,602922 teaches the use of in-situ crosslinking of amino-organic functional polysiloxane with diisocynate on polysulfone to form a gas separation membrane. The drawback of the above known process is that it uses tri-layer system having a gutter layer of poly silylated propyne a costly polyacetylene. Another disadvantage of this thin-film composite is that all the three distinct layers are to be laid down separately from three different sols. Yet another disadvantage of the above known processes is that in order to ensure integrity of the final membrane the choice of the sols is limited by the fact that solvent for any superimposed layer should essentially be a non-solvent for the previous layer. Another disadvantage of the above known process is that the procedure of membrane formation is tedious and time-consuming. Another process for preparing membrane having improved permeability is disclosed in US Patent Nos. 4,871,494 and 4,880,441 by Kesting et al. This comprises of preparing the membrane using an acid-base complex solvent and then coating the surface with polysiloxane. The disadvantage of the above known process is that the membranes essentially have defective skins having surface pores. Other known membranes (US Patent No. 4,685,940) comprise carbon fiber membranes or zeolite membranes, which provide gas separation properties. However, such membranes are expensive, difficult to produce, fragile and / or highly sensitive to impurities, such as oils in the gas mixture. A need therefore exists for improved membrane and a process of preparation of such membrane. This necessitates a dope which is cost-effective, easy to handle, having less toxic component on the one hand and good membrane forming properties on the other. OBJECT OF THE INVENTION The main object of the present invention is to provide a process for the preparation of asymmetric membranes for gas separation. Another object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the membranes can be a flat sheet asymmetric membranes or hollow fiber membranes. Yet another object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the chemicals used are less toxic with lower solvent bulk. Another object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the dope comprises of a polymer, two solvents, a co-solvent, an internal non-solvent and a coagulant like steam or vapour of alcohol or a mixture of water and alcohol vapour. Other object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the dope composition has better stability and maneuverability in terms of flexibility in handling and casting. A further object of the present invention is to provide a process for the preparation of asymmetric membranes which are thin and mechanically strong. Yet further object of the present invention is to provide a process for rhe preparation of asymmetric membranes which is cost effective. STATEMENT OF INVENTION Accordingly, the present invention provides a process for the preparation of asymmetric membrane for gas separation, the process comprising: a) preparing a dope composition by dissolving a polymer in a binary solvent with a co-solvent system, allowing the said polymer to swell and then stirring it to obtain a homogenous solution without allowing loss of solvents from the composition, followed by adding a non-solvent drop-wise till the cloud point is reached; b) casting a membrane by spreading the said dope composition prepared in step a) on a glass plate with a fixed casting gap, exposing it to saturated non solvent vapour till an opaque membrane is formed followed by stabilizing the opaque membrane by immersing it in a methanol bath; c) coating the stabilized opaque membrane prepared in step b) by drying it in a step wise manner and coating the said dried membrane with a coating of polymer in a solvent containing a curing agent. In one embodiment of the invention, in step (a) the polymer is taken in an amount of 10 to 20% w/v preferably 13 to 17 w/v to the binary solvent. In another embodiment of the invention, the polymer is allowed to swell in the solvent system for a period of about 1 hour and then the mixture is stirred for about 2 hrs at a temperature of 25 to 27°C to obtain the homogenous solution without loss of solvents from the composition. In another embodiment of the invention, the non-solvent added in step (a) is preferably tertiary amyl alcohol (TAA). In another embodiment of the invention, in step (b) the casting gap is about 500µ and the casting step is carried out at a temperature of about 25°C. In another embodiment of the invention, the saturated non solvent vapour is preferably water vapour and the cast membrane is exposed to the said non-solvent vapour in a chamber for a period of about 45 seconds to form the opaque membrane. In yet another embodiment of the invention, the opaque membrane is stabilized by immersing it in a methanol bath for 24-30 hours at about 20°C. In yet another embodiment of the invention, the step wise drying of the opaque membrane formed in step (b) comprises first drying it at 25°C - 30°C for about 24 hours followed by drying at an elevated temperature of 80°C - 100°C for 6-24 hours. In another embodiment of the invention, the polymer used to coat the dried membrane in step (c) comprises 2-5% v/v poly dimethylsiloxane in hexane and wherein the curing agent comprises 0.05 to 1% v/v of dicumyl peroxide. In another embodiment of the invention, the polymer is selected from the group consisting of Polysulfone, Polyethersulfone and Polycarbonate, preferably Polysulfone. In another embodiment of the invention, the binary solvent comprises a high boiling point solvent such as N-methyl pyrolidone (NMP) and a low boiling point solvent such as dichloromethane, in a ratio of 1:5 to 5:1 preferably 1:4 to 1:1. In another embodiment of the invention, the co-solvent is preferably ethyl acetate and is taken in a quantity of 2.5 to 10% v/v preferably 4 to 7.5% v/v. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process for the preparation of asymmetric membranes for the separation of mixture of gases, for example oxygen from atmospheric air. The process utilizes a dope comprising a multi-component system consisting of a polymer such as polysulfone, polyether sulphone or polycarbonate, solvents such as n-methyl pyrrolidone (NMP), dichloromethane; a co-solvent like methyl acetate, ethyl acetate or propyl acetate; an internal non-solvent namely tertiary amyl alcohol, a coagulant like steam or alcohol vapour or a mixture of water and alcohol vapour. The asymmetric membrane of the present invention is prepared by a dope which is prepared by phase inversion process. The dense skin layer forms due to solvent evaporation from the top layer of the cast membrane dope. The bulk of solution underneath the skin slowly transforms in a controlled manner into two phases - a polymer-rich phase and a polymer-lean-phase. At a certain stage, under optimum conditions the polymer-rich-phase solidifies into a solid matrix framework while the polymer-lean-phase forms the porous cellular part. The process of the present invention provides dope composition having better stability and maneuverability in terms of flexibility in casting and handling of the dope. The dope has superior solution spreading properties to form thin mechanically strong asymmetric membranes. The average selectively of (O2/N2) of these asymmetric membranes is from 4 to 7.6 with a permeation flow of 0.25 to 0.33 ml/min for nitrogen and 1.0 to 1.9 ml/min for oxygen. The process for the preparation of asymmetric membranes for separation of gases comprises of the following steps: a) Preparation of dope: 10 to 20% w/v preferably 13 to 17% w/v of a polymer namely Polysulfone, Poly ethersulfone or Polycarbonate, preferably Polysulfone is dissolved in a binary solvent with co-solvent system, consisting of a high boiling point solvent preferably N-methyl pyrolidone (NMP) and a low boiling point solvent preferably dichloromethane in a ratio of 1:5 to 5:1 preferably 1:4 to 1:1 and 2.5 to 10% v/v preferably 4 to 7.5% v/v of a co-solvent such as ethyl acetate. The polymer is allowed to swell for about 1 hour and is then stirred for about 2 hrs, to obtain homogenous solution without allowing the loss of solvents from the composition at temperature 25 to 27°C. To this above solution a non-solvent preferably tertiary amyl alcohol (TAA) is added drop-wise till the cloud point is reached. b) Casting of membrane: The dope prepared in step a) is spread with the help of a knife with a fixed casting gap of 500uonto a glass plate at about 25°C. The spread solution is exposed for about 45 seconds in a chamber having saturated non-solvent vapour preferably water vapour, till it changes into an opaque membrane. This opaque membrane is immersed in a methanol bath at about 20°C for 24 hrs to 30 hrs to stabilize the membrane. c) Coating of membranes: The membrane prepared in step b) is dried at 25 to 30°C for 24 hrs and then at elevated temperature of 80-100°C for 6 to 24 hrs. The dried membrane is then coated with 2 to 5% poly dimethyl siloxane in hexane having 0.05 to 1% v/v of di-cumyl peroxide as curing agent. This invention will now be illustrated with working examples, which are intended to be typical examples to explain the technique of the present invention and are not intended to be taken restrictively to imply any limitation to the scope of the present invention. EXAMPLE 1: A casting solution was prepared by dissolving 13.43g polysulfone in a solvent mixture containing of 39.54 ml N-methylpyrrolidone and 50.67 ml of dichloromethane and co-solvent. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.94 ml of internal non-solvent additive ternary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of 24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air-dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EXAMPLE II: A casting solution was prepared by dissolving 15.09g polysulfone in a solvent mixture containing of 38.85 ml n-methylpyrolidone and 49.79 ml of dichloromethane and co-solvent. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.67 ml of nonsolvent tertiary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of 24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EXAMPLE HI: A casting solution was prepared by dissolving 16.73g polysulfone in a solvent mixture containing of 38.85 ml n-methylpyrolidone and 49.07 ml of dichloromethane. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.23 ml of non-solvent tertiary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of 24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EXAMPLE IV: A casting solution was prepared by dissolving 18.22g polysulfone in a solvent mixture containing of 37.53 ml n-methylpyrolidone and 48.10ml of dichloromethane and co-solvent. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.94 ml of nonsolvent tertiary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of 24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EVALUATION OF THE MEMBRANE The productivity (permeation and selectively) of uncoated asymmetric membranes was measured with the help of a permeation cell and bubble flow meter. The effective membrane area was 12.75 cm2. The permeation of pure gases like oxygen and nitrogen was measured at a pressure of 7kg/cm2. The average selectively of (02/N2) these membranes registered was from 4 to 7.6 with a permeation flow of 0.25 to 0.33 ml/min for nitrogen and 1.0 to 1.9ml/min for oxygen. The usefulness of the prepared membrane has been demonstrated by testing the oxygen enrichment of the membrane using atmospheric air as an input of mixture of gases. The ideal selectively measured by soap bubble meter and the percent oxygen content in the permeate gas shows enrichment up to 43.0% O2. The corresponding decrease in the nitrogen content comes to 57.0% N2 from the standard initial value of 78.08%. WE CLAIM: 1. A process for the preparation of asymmetric membrane for gas separation, the process comprising: (a) preparing a dope composition by dissolving a polymer in a binary solvent with a co-solvent system, allowing the said polymer to swell and then stirring it to obtain a homogenous solution without allowing loss of solvents from the composition, followed by adding a non-solvent drop-wise till the cloud point is reached; (b) casting a membrane by spreading the said dope composition prepared in step a) on a glass plate with a fixed casting gap, exposing it to saturated non solvent vapour till an opaque membrane is formed followed by stabilizing the opaque membrane by immersing it in a methanol bath; (c) coating the stabilized opaque membrane prepared in step b) by drying it in a step wise manner and coating the said dried membrane with a coating of polymer in a solvent containing a curing agent. 2. A process as claimed in claim 1 wherein in step (a) the polymer is taken in an amount of 10 to 20% w/v, preferably 13 to 17 w/v to the binary solvent. 3. A process as claimed in claim 1 and 2 wherein the polymer is allowed to swell in the solvent system for a period of about 1 hour and then the mixture is stirred for about 2 hrs at a temperature of 25 to 27°C to obtain the homogenous solution without loss of solvents from the composition. 4. A process as claimed in any of claims 1 to 3 wherein the non-solvent added in step (a) is preferably tertiary amyl alcohol (TAA). 5. A process as claimed in claim 1 wherein in step (b) the casting gap is about 500µ and the casting step is carried out at a temperature of about 25°C. 6. A process as claimed in claim 1 and 5 wherein the saturated non solvent vapour is selected from the group consisting of water vapour, alcohol vapour or any mixture thereof, preferably water vapour. 7. A process as claimed in claim 6 wherein the cast membrane is exposed to the said non-solvent vapour in a chamber for a period of about 45 seconds to form the opaque membrane. 8. A process as claimed in any of claims 1 and 5 to 7 wherein the opaque membrane is stabilized by immersing it in a methanol bath for 24-30 hours at about 20°C. 9. A process as claimed in any preceding claim wherein the step wise drying of the opaque membrane formed in step (b) comprises first drying it at 25°C - 30°C for about 24 hours followed by drying at an elevated temperature of 80°C - 100°C for 6-24 hours. 10. A process as claimed in claim 1 or 9 wherein the polymer used to coat the dried membrane in step (c) comprises 2-5% v/v poly dimethylsiloxane in hexane and wherein the curing agent comprises 0.05 to 1% v/v of dicumyl peroxide. 11. A process as claimed in any preceding claim wherein the polymer is selected from the group consisting of Polysulfone, Polyethersulfone and Polycarbonate, preferably Polysulfone. 12. A process as claimed in any preceding claim wherein the binary solvent comprises a high boiling point solvent such as N-methyl pyrolidone (NMP) and a low boiling point solvent such as dichloromethane, in a ratio of 1:5 to 5:1, preferably 1:4 to 1:1. 13. A process as claimed in any preceding claim wherein the co-solvent is selected from the group consisting of methyl acetate, ethyl acetate and propyl acetate, preferably ethyl acetate. 14. A process as claimed in any preceding claim wherein the co-solvent is taken in a quantity of 2.5 to 10% v/v preferably 4 to 7.5% v/v. 15. A process for the preparation of an asymmetric membrane for gas separation substantially as described hereinbefore and with reference to the foregoing examples.

Full Text

PROCESS FOR PREPARATION OF ASYMMETRIC MEMBRANES FOR GAS SEPARATION
FIELD OF INVENTION
The present invention relates to a process for the preparation of asymmetric membranes for gas separation. More particularly, the present invention relates to a process for the preparation of thin, mechanically strong asymmetric membranes for gas separation. More particularly, the present invention relates to a process for the preparation of asymmetric membranes for separation of oxygen from atmospheric air. PRIOR ART
Separation of one or more gases from a mixture of gases forms an important and often critical step in several industries. Several methods are known in the art for the separation of gases such as cryogenics, pressure swing adsorption and membrane separation. Membrane based separation of gases are found to be more viable economically in some industries than other methods of separation.
It is known in the art that the permeation flux across a membrane is inversely proportional to the film thickness. Several efforts have therefore been made therefore, to reduce the thickness of a gas separation membrane. However, reduction of thickness has an inherent limitation in that very thin membranes (below 20µ) cannot be made with adequate mechanical integrity and defect-free structure using existing materials. As a result, efforts in the art to overcome this limitation in obtaining thin gas separation membranes with mechanical integrity and defect free structure have focused on two different membrane morphologies. One area of research has been to develop asymmetric membrane having graded porosity and another area of research has been to develop a composite membrane where a porous support is coated with an ultra thin dense top layer of a selective polymer.
Asymmetric membranes consist of a dense surface and a porous support prepared by phase inversion process. The surface skin layers of these membranes have semi-permeability. Asymmetric membranes are used for reverse osmosis as well as gas separation applications. Loeb and Sourirajan made the first asymmetric membrane from cellulose acetate (US Patent No. 3,133,132 and US Patent No. 3, 133, 137). However, the above symmetric membranes suffer from the limitation that the separation property for smaller molecules such as gas molecules is not very high.
In composite membranes the superimposed layer discriminates or separates gases while the porous support is needed for the mechanical integrity of the membrane. Klass et al, US Patent No. 3616607, Stancell et al US Patent No. 3657113 and Yasuda US Patent No.

3775303 and Browall US Patent No. 3980456 exemplify making of such composite gas separation membranes. The main disadvantage of fabricating these membranes having ultra-thin defect-free skin is that the processes of preparation are to be carried out under stringent conditions such as 'Clean room' atmosphere.
Cabasso, et al US Patent No. 4,602922 teaches the use of in-situ crosslinking of amino-organic functional polysiloxane with diisocynate on polysulfone to form a gas separation membrane. The drawback of the above known process is that it uses tri-layer system having a gutter layer of poly silylated propyne a costly polyacetylene. Another disadvantage of this thin-film composite is that all the three distinct layers are to be laid down separately from three different sols. Yet another disadvantage of the above known processes is that in order to ensure integrity of the final membrane the choice of the sols is limited by the fact that solvent for any superimposed layer should essentially be a non-solvent for the previous layer. Another disadvantage of the above known process is that the procedure of membrane formation is tedious and time-consuming.
Another process for preparing membrane having improved permeability is disclosed in US Patent Nos. 4,871,494 and 4,880,441 by Kesting et al. This comprises of preparing the membrane using an acid-base complex solvent and then coating the surface with polysiloxane. The disadvantage of the above known process is that the membranes essentially have defective skins having surface pores.
Other known membranes (US Patent No. 4,685,940) comprise carbon fiber membranes or zeolite membranes, which provide gas separation properties. However, such membranes are expensive, difficult to produce, fragile and / or highly sensitive to impurities, such as oils in the gas mixture.
A need therefore exists for improved membrane and a process of preparation of such membrane. This necessitates a dope which is cost-effective, easy to handle, having less toxic component on the one hand and good membrane forming properties on the other. OBJECT OF THE INVENTION
The main object of the present invention is to provide a process for the preparation of asymmetric membranes for gas separation.
Another object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the membranes can be a flat sheet asymmetric membranes or hollow fiber membranes.
Yet another object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the chemicals used are less toxic with lower solvent bulk.

Another object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the dope comprises of a polymer, two solvents, a co-solvent, an internal non-solvent and a coagulant like steam or vapour of alcohol or a mixture of water and alcohol vapour.
Other object of the present invention is to provide a process for the preparation of asymmetric membranes wherein the dope composition has better stability and maneuverability in terms of flexibility in handling and casting.
A further object of the present invention is to provide a process for the preparation of asymmetric membranes which are thin and mechanically strong.
Yet further object of the present invention is to provide a process for rhe preparation of asymmetric membranes which is cost effective. STATEMENT OF INVENTION
Accordingly, the present invention provides a process for the preparation of asymmetric membrane for gas separation, the process comprising:
a) preparing a dope composition by dissolving a polymer in a binary solvent with a co-solvent system, allowing the said polymer to swell and then stirring it to obtain a homogenous solution without allowing loss of solvents from the composition, followed by adding a non-solvent drop-wise till the cloud point is reached;
b) casting a membrane by spreading the said dope composition prepared in step a) on a glass plate with a fixed casting gap, exposing it to saturated non solvent vapour till an opaque membrane is formed followed by stabilizing the opaque membrane by immersing it in a methanol bath;
c) coating the stabilized opaque membrane prepared in step b) by drying it in a step wise manner and coating the said dried membrane with a coating of polymer in a solvent containing a curing agent.
In one embodiment of the invention, in step (a) the polymer is taken in an amount of 10 to 20% w/v preferably 13 to 17 w/v to the binary solvent.
In another embodiment of the invention, the polymer is allowed to swell in the solvent system for a period of about 1 hour and then the mixture is stirred for about 2 hrs at a temperature of 25 to 27°C to obtain the homogenous solution without loss of solvents from the composition.
In another embodiment of the invention, the non-solvent added in step (a) is preferably tertiary amyl alcohol (TAA).

In another embodiment of the invention, in step (b) the casting gap is about 500µ and the casting step is carried out at a temperature of about 25°C.
In another embodiment of the invention, the saturated non solvent vapour is preferably water vapour and the cast membrane is exposed to the said non-solvent vapour in a chamber for a period of about 45 seconds to form the opaque membrane.
In yet another embodiment of the invention, the opaque membrane is stabilized by immersing it in a methanol bath for 24-30 hours at about 20°C.
In yet another embodiment of the invention, the step wise drying of the opaque membrane formed in step (b) comprises first drying it at 25°C - 30°C for about 24 hours followed by drying at an elevated temperature of 80°C - 100°C for 6-24 hours.
In another embodiment of the invention, the polymer used to coat the dried membrane in step (c) comprises 2-5% v/v poly dimethylsiloxane in hexane and wherein the curing agent comprises 0.05 to 1% v/v of dicumyl peroxide.
In another embodiment of the invention, the polymer is selected from the group consisting of Polysulfone, Polyethersulfone and Polycarbonate, preferably Polysulfone.
In another embodiment of the invention, the binary solvent comprises a high boiling point solvent such as N-methyl pyrolidone (NMP) and a low boiling point solvent such as dichloromethane, in a ratio of 1:5 to 5:1 preferably 1:4 to 1:1.
In another embodiment of the invention, the co-solvent is preferably ethyl acetate and is taken in a quantity of 2.5 to 10% v/v preferably 4 to 7.5% v/v. DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a process for the preparation of asymmetric membranes for the separation of mixture of gases, for example oxygen from atmospheric air. The process utilizes a dope comprising a multi-component system consisting of a polymer such as polysulfone, polyether sulphone or polycarbonate, solvents such as n-methyl pyrrolidone (NMP), dichloromethane; a co-solvent like methyl acetate, ethyl acetate or propyl acetate; an internal non-solvent namely tertiary amyl alcohol, a coagulant like steam or alcohol vapour or a mixture of water and alcohol vapour.
The asymmetric membrane of the present invention is prepared by a dope which is prepared by phase inversion process. The dense skin layer forms due to solvent evaporation from the top layer of the cast membrane dope. The bulk of solution underneath the skin slowly transforms in a controlled manner into two phases - a polymer-rich phase and a polymer-lean-phase. At a certain stage, under optimum conditions the polymer-rich-phase

solidifies into a solid matrix framework while the polymer-lean-phase forms the porous cellular part.
The process of the present invention provides dope composition having better stability and maneuverability in terms of flexibility in casting and handling of the dope. The dope has superior solution spreading properties to form thin mechanically strong asymmetric membranes. The average selectively of (O2/N2) of these asymmetric membranes is from 4 to 7.6 with a permeation flow of 0.25 to 0.33 ml/min for nitrogen and 1.0 to 1.9 ml/min for oxygen.
The process for the preparation of asymmetric membranes for separation of gases comprises of the following steps:
a) Preparation of dope: 10 to 20% w/v preferably 13 to 17% w/v of a polymer namely Polysulfone, Poly ethersulfone or Polycarbonate, preferably Polysulfone is dissolved in a binary solvent with co-solvent system, consisting of a high boiling point solvent preferably N-methyl pyrolidone (NMP) and a low boiling point solvent preferably dichloromethane in a ratio of 1:5 to 5:1 preferably 1:4 to 1:1 and 2.5 to 10% v/v preferably 4 to 7.5% v/v of a co-solvent such as ethyl acetate. The polymer is allowed to swell for about 1 hour and is then stirred for about 2 hrs, to obtain homogenous solution without allowing the loss of solvents from the composition at temperature 25 to 27°C. To this above solution a non-solvent preferably tertiary amyl alcohol (TAA) is added drop-wise till the cloud point is reached.
b) Casting of membrane: The dope prepared in step a) is spread with the help of a knife with a fixed casting gap of 500uonto a glass plate at about 25°C. The spread solution is exposed for about 45 seconds in a chamber having saturated non-solvent vapour preferably water vapour, till it changes into an opaque membrane. This opaque membrane is immersed in a methanol bath at about 20°C for 24 hrs to 30 hrs to stabilize the membrane.
c) Coating of membranes: The membrane prepared in step b) is dried at 25 to 30°C for 24 hrs and then at elevated temperature of 80-100°C for 6 to 24 hrs. The dried membrane is then coated with 2 to 5% poly dimethyl siloxane in hexane having 0.05 to 1% v/v of di-cumyl peroxide as curing agent.
This invention will now be illustrated with working examples, which are intended to be typical examples to explain the technique of the present invention and are not intended to be taken restrictively to imply any limitation to the scope of the present invention.

EXAMPLE 1:
A casting solution was prepared by dissolving 13.43g polysulfone in a solvent mixture containing of 39.54 ml N-methylpyrrolidone and 50.67 ml of dichloromethane and co-solvent. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.94 ml of internal non-solvent additive ternary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of 24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air-dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EXAMPLE II:
A casting solution was prepared by dissolving 15.09g polysulfone in a solvent mixture containing of 38.85 ml n-methylpyrolidone and 49.79 ml of dichloromethane and co-solvent. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.67 ml of nonsolvent tertiary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of 24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EXAMPLE HI:
A casting solution was prepared by dissolving 16.73g polysulfone in a solvent mixture containing of 38.85 ml n-methylpyrolidone and 49.07 ml of dichloromethane. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.23 ml of non-solvent tertiary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of

24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EXAMPLE IV:
A casting solution was prepared by dissolving 18.22g polysulfone in a solvent mixture containing of 37.53 ml n-methylpyrolidone and 48.10ml of dichloromethane and co-solvent. After adequate swelling time for complete dissolution of the polymer the mixture was stirred for 2 hrs, and then 7.94 ml of nonsolvent tertiary amyl alcohol (TAA) was added slowly to the casting dope while stirring. Membranes were cast by pour-casting method, on a glass plate of 24X24 cm2 size having raised boundary at 25°C. Immediately after casting the membranes, the glass plate frame was introduced into a chamber having saturated water vapour. The initially clear, thermodynamically stable nascent membrane turned opaque immediately after stem exposure. After a free convection period of 45 seconds, the opaque membranes were immersed into a methanol bath at 20°C for 24 hrs. Thereafter the membranes were air dried at 25°C for another 24 hrs and then dried at elevated temperature of 100°C for 8 hrs. The membranes were then coated with 2% poly dimethylsiloxane in hexane solution containing 0.05% dicumyl peroxide as curing agent. EVALUATION OF THE MEMBRANE
The productivity (permeation and selectively) of uncoated asymmetric membranes was measured with the help of a permeation cell and bubble flow meter. The effective membrane area was 12.75 cm2. The permeation of pure gases like oxygen and nitrogen was measured at a pressure of 7kg/cm2. The average selectively of (02/N2) these membranes registered was from 4 to 7.6 with a permeation flow of 0.25 to 0.33 ml/min for nitrogen and 1.0 to 1.9ml/min for oxygen.
The usefulness of the prepared membrane has been demonstrated by testing the oxygen enrichment of the membrane using atmospheric air as an input of mixture of gases. The ideal selectively measured by soap bubble meter and the percent oxygen content in the permeate gas shows enrichment up to 43.0% O2. The corresponding decrease in the nitrogen content comes to 57.0% N2 from the standard initial value of 78.08%.

WE CLAIM:
1. A process for the preparation of asymmetric membrane for gas separation, the process
comprising:
(a) preparing a dope composition by dissolving a polymer in a binary solvent with a co-solvent system, allowing the said polymer to swell and then stirring it to obtain a homogenous solution without allowing loss of solvents from the composition, followed by adding a non-solvent drop-wise till the cloud point is reached;
(b) casting a membrane by spreading the said dope composition prepared in step a) on a glass plate with a fixed casting gap, exposing it to saturated non solvent vapour till an opaque membrane is formed followed by stabilizing the opaque membrane by immersing it in a methanol bath;
(c) coating the stabilized opaque membrane prepared in step b) by drying it in a step wise manner and coating the said dried membrane with a coating of polymer in a solvent containing a curing agent.

2. A process as claimed in claim 1 wherein in step (a) the polymer is taken in an amount of 10 to 20% w/v, preferably 13 to 17 w/v to the binary solvent.
3. A process as claimed in claim 1 and 2 wherein the polymer is allowed to swell in the solvent system for a period of about 1 hour and then the mixture is stirred for about 2 hrs at a temperature of 25 to 27°C to obtain the homogenous solution without loss of solvents from the composition.
4. A process as claimed in any of claims 1 to 3 wherein the non-solvent added in step (a) is preferably tertiary amyl alcohol (TAA).
5. A process as claimed in claim 1 wherein in step (b) the casting gap is about 500µ and the casting step is carried out at a temperature of about 25°C.
6. A process as claimed in claim 1 and 5 wherein the saturated non solvent vapour is selected from the group consisting of water vapour, alcohol vapour or any mixture thereof, preferably water vapour.
7. A process as claimed in claim 6 wherein the cast membrane is exposed to the said non-solvent vapour in a chamber for a period of about 45 seconds to form the opaque membrane.
8. A process as claimed in any of claims 1 and 5 to 7 wherein the opaque membrane is stabilized by immersing it in a methanol bath for 24-30 hours at about 20°C.
9. A process as claimed in any preceding claim wherein the step wise drying of the opaque membrane formed in step (b) comprises first drying it at 25°C - 30°C for about 24 hours followed by drying at an elevated temperature of 80°C - 100°C for 6-24 hours.

10. A process as claimed in claim 1 or 9 wherein the polymer used to coat the dried membrane in step (c) comprises 2-5% v/v poly dimethylsiloxane in hexane and wherein the curing agent comprises 0.05 to 1% v/v of dicumyl peroxide.
11. A process as claimed in any preceding claim wherein the polymer is selected from the group consisting of Polysulfone, Polyethersulfone and Polycarbonate, preferably Polysulfone.
12. A process as claimed in any preceding claim wherein the binary solvent comprises a high boiling point solvent such as N-methyl pyrolidone (NMP) and a low boiling point solvent such as dichloromethane, in a ratio of 1:5 to 5:1, preferably 1:4 to 1:1.
13. A process as claimed in any preceding claim wherein the co-solvent is selected from the group consisting of methyl acetate, ethyl acetate and propyl acetate, preferably ethyl acetate.
14. A process as claimed in any preceding claim wherein the co-solvent is taken in a quantity of 2.5 to 10% v/v preferably 4 to 7.5% v/v.
15. A process for the preparation of an asymmetric membrane for gas separation substantially as described hereinbefore and with reference to the foregoing examples.

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Patent Number

279785

Indian Patent Application Number

2071/DEL/2004

PG Journal Number

05/2017

Publication Date

03-Feb-2017

Grant Date

31-Jan-2017

Date of Filing

21-Oct-2004

Name of Patentee

THE DIRECTOR GENERAL, DEFENCE RESEARCH AND DEVELOPMENT ORGANISATION

Applicant Address

DEFENCE, GOVT. OF INDIA, OF WEST BLOCK-VIII, WING-1, SECTOR-1, RK PURAM, NEW DELHI-110 066

Inventors:

#	Inventor's Name	Inventor's Address
1	PRATIBHA PANDEY	DEFENCE RESEARCH AND DEVELOPMENT ESTABLISHMENT, JHANSI ROAD, GWALIOR-474 002
2	RAM SINGH CHAUHAN	DEFENCE RESEARCH AND DEVELOPMENT ESTABLISHMENT, JHANSI ROAD, GWALIOR-474 002

PCT International Classification Number

B01D 9/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA