CA1286865C - Porous liquid polytetrafluoroethylene membrane with hydrophobic and hydrophilic surfaces - Google Patents
Porous liquid polytetrafluoroethylene membrane with hydrophobic and hydrophilic surfacesInfo
- Publication number
- CA1286865C CA1286865C CA000491386A CA491386A CA1286865C CA 1286865 C CA1286865 C CA 1286865C CA 000491386 A CA000491386 A CA 000491386A CA 491386 A CA491386 A CA 491386A CA 1286865 C CA1286865 C CA 1286865C
- Authority
- CA
- Canada
- Prior art keywords
- liquid membrane
- solvent
- active species
- membrane
- liquid
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
- B01D67/0081—After-treatment of organic or inorganic membranes
- B01D67/0086—Mechanical after-treatment
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
- B01D61/38—Liquid-membrane separation
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
- B01D67/0081—After-treatment of organic or inorganic membranes
- B01D67/0088—Physical treatment with compounds, e.g. swelling, coating or impregnation
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
- B01D69/12—Composite membranes; Ultra-thin membranes
- B01D69/125—In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction
- B01D69/127—In situ manufacturing by polymerisation, polycondensation, cross-linking or chemical reaction using electrical discharge or plasma-polymerisation
Abstract
ABSTRACT OF THE DISCLOSURE
A liquid membrane is disclosed, which comprises a solvent, an active species capable of facilitated transport of a specific gas, and a support for maintaining a liquid body which comprises a mixture of said solvent and active species dissolved therein, said support being a porous polytetrafluoroethylene film one surface of which is hydrophobic and the other of which is hydrophilic.
A liquid membrane is disclosed, which comprises a solvent, an active species capable of facilitated transport of a specific gas, and a support for maintaining a liquid body which comprises a mixture of said solvent and active species dissolved therein, said support being a porous polytetrafluoroethylene film one surface of which is hydrophobic and the other of which is hydrophilic.
Description
l'~ f`~6~6~
LIQUID MEMBRANE
BACKGROUND OF THE INVENTION
The present invention relates to a liquid membrane used ;n facilitated transport of a specific gasO
More specifically! the invention relates to a liquid membrane using a porous polytetrafluoroethylene support one surface of which is first roughened by a physical means, wherein the support has a polymerized layer of a nitrogen-con~aining compound chemically deposited thereon so as to provide an increased affinity for liquids.
Membrane technology using ethyl cellulose r acetyl cellulose or silicon-carbonate copolymer membranes to separate a specific component gas from a gaseous mixture containing the same has been the subject of extensive research. These membranes are made of solid materials and are hence referred to as solid membranes~ In order to separate the specific gas component in an economical and efficient manner, ~wo requirements must be met: (1) a membrane material that permits the selective passage of the gas component to be separated must be selected; and ~2) the membrane must be made as thin as possible. Most of the materials known today for use in solid membranes have low gas selectivities and those materials which have relatively high selctivities are only capable of very low rates of gas permeation.
Facilitated tranport of a particular gas can be 1 realized by using a liquid membrane of a material having a high degree o~ affinity for the ~as to be separated. As shown in U.S. Patent Nos. 3,865,890, 3,951,621, 4,015,955 and 4,060,566, ethylene can be selectively concentrated from a mixture of methane, ethane and ethylene using a nylon-6.6 membrane impregnated with an aqueous solution of AgNO3. The membrane disclosed in these patents is rendered hydrophilic by incorporation of a hydrophilic polymer such as polyvinyl alcohol, but the life of the membrane is not very long since water used as the solvent for the aqueous solution of Ag ions will unavoidably evaporate during the use of the membrane.
U.S. Patent Nos. 3,396,510, 3,819,806 and 4,119,408 show that acidic gas components such as CO2, H2S
and SO2 can be selectively permeated through a polyethersulfone membrane impregnated with an aqueous solution of K2CO3. But this membrane has the same problem as in the case of the membrane used for ethylene separation discussed above.
~O European Patent Application No. 98731/1984 ~a laid-open publication Of January 18, 1984, naming I.C. Roman and R.W. Baker as inventors and assigned to Bend Research Inc.) shows that oxygen can be selectively separated from the air using a nylon-6.6 membrane impregnated with a transition metal complex of a Schiff base dissolved in a solvent such as lactone or amide.
In the Example therein, a nylon-6.6 membrane with a thickness of 130 ,um was used and this suggests the presence of a liquid membrane that was at least 130Jum thick.
8~i.r;
1 l'his membrane system is substantially free ~rom the problem of solvent evaporation because the li~uid barrier is made not of water, but of organic solvents such as lactone and amide. ~owever, the life of the system is short because the transition metal complex of a Schiff base is irreversibly oxidized during operation. In addition, the liquid barrier that is impregnated into the nylon-6.6 membrane cannot be made thinner than 130 ~m-Sl~MMARY OF THE_INVENTION
One object, therefore, of the present inven~ionis to provide a liquid barrier in a liquid membrane that is not thicker than a few micrometers and which yet is capable of enhanced transport of a specific gas component.
Another object of the inventlon is to provide a membrane support one surface of which does not dissolve in solvents but has a suitable degree of affinity therefore.
rn or~er to make a liquid barrier no~ in the form of an aqueous solu~ion but in the form of an organic solution with low volatility solvents such as lactone, dimethylformamide and N-methylpyrrolidone are used. These solvents will dissolve known porous support materials such as polysulfones and polyamides, so that it is necessary touse a support material that will not dissolve in these organic solvents.
1.'~'`sfifi~
DETAILE~D DES(~RIPTION OF TIIE IIJVENTION
The liquid ~embrane in accordance with the invention is composed of a solvent, an active species for performing facilitated transport of a specific gas, and a S support. The support is made of porous poly-tetrafluoroethylene (hereinafter abbreviated as PTF~) ana is characterized by having one hydrophilic surface in order to make contact with a mixed solution of the solvent and active species and to retain this mixed solution in the form of a thin film. In order to provide such hydrophilic surface, the support must be given both a physical treatment for roughening one surface and a chemical ~rea~ment for depositiny a iayer of a non-PTFE
compound on that surface. If only one of th~ese two treatments is applied, a liquid barrier that will maintain a uniform thickness cannot be obtained, Ol even if this is possible, the resulting membrane has a very short life.
The PTFE used as the support material is insoluble in almost all solvents known today, so it can be used in supports for liquid barriers employing solvents of high polarity such as dimethylformamide (hereinafter DMF) and N-methylpyrrolidone (hereinafter NMP). However, because of high polarity, DMF and NMP exhibit such a great surface tension that they are incapable of wetting the s~rface of PTFE satisfactorily. As a result, the porous PTFE membrane cannot be put to service in any manner other than where a pool of a mixed solution of solvent and 6~
1 active species is placed on the membrane, thus producing an undesirably thick liquid barrier.
It therefore becomes important for the purposes of the present invention to render one surface of the porous PTFE hydrophilic so that it is wettable by polar solvents. The intensity of this hydrophilic treatment determines the thickness in which the liquid barrier made of the solvent and active species can remain stable.
The physical treatments for providing a roughened surface include graining with a metal brush, etc., heat treatment in which only the surface layer of PT~E is thermally decomposed, laser treatment using the laser energy as a heat source, low-temperature plasma etching using a non-polymerizable gas, and sputter etchinq using Rf energy, which are described in U.S. Patent Nos. 4,297,187, and 4,311,828. By selectinq a proper method from among these methods, a suIface roughness in the range of 1 to 100 ~m preferably 2 to 20 ~m, can be obtalned. If a very fine texture with a roughness of about 2G several micrometers is desired, lasers or Rf energy is preferably used.
U.S. Patent Nos. 3,664,915, 3,953,566, 4,082,893, and 4,248,942 provide porous PTFE by stretching and expanding operations. The structures produced by such techniques consist of fibers and nodes interconected by the fibers. Such structures are preferred starting 1 materialssince they provide for a great latitude in selection of porosity and pore size. If the porous PTFE
is subjected to sputtering or plasma etching, the fibers in the treated surface are cut and subsequently decomposed to provide a texture structure consisting essentially of nodes. If, on the other hand, only the surface layer of the porous PTFE is thermally decomposed by scanning with a C2 laser beam, not only the fibers but also a part of the nodes is volatilized.
The choice of a suitable method of physical treatment depends on the required thickness of the liquid barrier.
The physical treatment for roughening one surface of the porous PTFE is followed by a chemical treatment. The surface subjected to roughening by a physical treatment has a reduced angle of contact with the highly polar DMF or NMP and its affinity for these solvents appears to have increased in comparison with the untreated surface. In fact, however, the roughened surface of PTFE causes little effects on its inherent nature and the increase in its affinity for polar solvents is negligible. In order to achieve an increased affinity for such solvent, the surface of the PTFE must be covered with a thin layer of a compound having a chemical structure similar to that of the polar solvents. This object is insufficiently achieved by merely performing "tetra-1~6~
1 etching", i.e., withdrawal of fluorine atoms using sodium-naphthalene complex or other suitable etchants. The preferred chemical treatment is one depending on plasma polymerization for depositing a compound similar to the solvent component of the final liquid barrier. As described in U.S. Patent Nos. 3,657,113, and 4,199,448, the plasma polymerization is realized by introducing a polymerizable gas into a bell jar or a tubular reactor while a glow discharge is maintained by application of radio frequencies, microwaves or d,c. current.
Polymerizable gases that can be used with advantage are nitrogen-containing cyclic compounds that include pyridines such as 4-vinylpyridine, 2-vinylpyridine, 4-ethylpyridine and 5-vinyl-2-methylpyridine; pyrrolidone derivatives such as N-methylpyrrolidone and N-vinylpyrrolidone amines such as 4-methylbenzylamine and ~-butylamine; and pyridine derivatiYes such as picoline and lutidine. Oxygen-containing compounds could be used as polymerizable gases but they are by no means advantageous 2~ over the nitroqen-containing compounds from the viewpoint of deposition rates of plasma polymers.
If nitrogen-containing compounds, preferably cyclic compounds, are used as polymerizable gases, films of plasma polymer are produced that are about 1 ~m thick and which are composed of a highly cross-linked structure.
Because of the cross-linked structure, such polymer films 36~
1 will not dissolve in the highly polar solvents which are used to make up a liquid barrier, but they can be swelled by such solvents. This will provide a great advantage for the purpose of maintaining a liquid barrier in the form of a very thin film of a thickness of about several micrometers. The nitrogen-containing compounds listed above will also act as "axial bases" in the sense of the term used in European Patent Application No. 98731/1984.
It would of course be possible to form films of plasma polymer even thinner than 0.1 ~m or thicker than 10 ~m by changing the polymerization conditions. However, polymer films thinner than 0.1 ~m have a smaller ability to maintain a liquid barrier stably and it becomes difficult to have a uniform and sound liquid barrier spread over a large surface area. If polymer film is thicker than 10 ~m, cracks will develop because of the internal stress that has occurred in the film during plasma polymerizationO
Even in the absence of any cracking, the film is unstable and may sometimes separate from the substrate.
Under these cirumstances, the particularly preferred thickness of the plasma polymer film may range from 0.3 ~m to 3 ~m. Polymer films having this thickness range are capable of maintaining a liquid barrier~with the thickness ranging from 0.1 ~m to 6 ~m.
The other features of the liquid membrane in accordance with the present invention may substantially be . ~;
- .
.
. , . , :
1 the same as described in European Patent Application No. 98731/1984.
The solvents of high polarity include lactams, sulfoxides and amides, and preferably, dimethyl sulfoxide, NMP, propylene carbonate, DMF and gamma-butyrolactam are used. These solvents may contain nitrogen-containing com-pounds such as polyethyleneimine and tetra-ethylene-pentamine, or pyridine and pyrrolidone derivatives used as polymerizable gases in plasma polymerization.
Examples of the active species capable of enhanced transport of oxygen or carbon monoxide gas are complex compounds of transition metals with Schiff bases prepared by dehydration condensation from ethylenediamine and aldehyde compounds. Illustrative Schiff bases are N,N-bis(salicylidene)ethylenediamine and bis(2-amino-1-benz-aldehyde)ethylenediamine. Exemplary transition metals are divalent elements ~uch as Co, Cu and Fe. These active species a~e d s~lved in polar so~vents in amounts ranginq from 10 5 to 10 3 mole, ~referably 10 4 to 10 3 mole, per unit weight of the liquid barrier. Higher concentrations of the active species will provide increased initial gas selectivities, but their characteristics will vary with time as a result of dimerization or other side reactions.
If the concentration of the active species is less than 10 5 mole the intended effect of the active species is difficult to obtain and only low gas selectivities will 1 result. Therefore, the preferred concentration of the active species is in the range of 10 5 to 10 3 mole per g of the liquid barrier.
The process of producing a large-scale module from the liquid membrane of the present invention will start with the shaping of a support that has been provided with a hydrophilic surface but on wihch no liquid barrier is maintained. In order to provide a large surface area, a support in the form of a bundle of tubes or hollow fibers is packed in a molding container and both ends of the bundle are sealed. The sealant may be an epoxy resin but silicone rubbers having a greater adhesive strength are preferred. After the sealant pas solidified, part of the sealed portion is cut open so as to provide a separation module having respective channels for the feed gas, permeate gas and reject gas.
After forming such module, a mixed solution of solvent and active species is introduced in excess amount into the module at the feed gas inlet and the module is pressurized at 1 to 2 kg/cm2 with the reject gas outlet closed. But this pressurization step, the mixed solution is impregnated into the entire part of the hydrophilic surface of the support. Desirably, complete impregnation of the mixed solution is ensured by shaking the whole part ?5 Of the module. Uniform impregnation of the mixed solution can be realized by monitoring the flow rate of gas coming 1 from the permeate gas outlet because as the impregnation proceeds, a decreasing amount of gas will come out of the module until the point i5 reached where a minimum flow rate occurs, which indicates that the mixed solution has been impregnated uniformly in every part of the hydrophilic surface of the support.
The following Examples are provided for further illustration of the claimed liquid membrane but are not to be construed as limiting the invention.
Example 1 A reactor of the bell iar type having a sheet of *
Fluoropore FP-010 (porous PTFE membrane manufactured by Sumitomo Electric Industries, Ltd. having an average pore size of 0.1 ~m) placed on an electrode was evacuated to a pressure of 0.01 Torr. Glow discharge was conducted with a radio wave ~13.~6 MHz) applied at a power of 60 watts.
The reactor was supplied with 4-vinylpyridine until the pressure in the reactor was increased to 0.2 Torr. Plasma polymerization was performed for 30 minutes at a po~er of 30 watts. The thickness of the polymer deposit as estimated from the increase in weight was 0.5 ~m. The plasma polymer coat on one side was dipped in a dimethylformamide bath. When it was recovered from the solvent bath, the treated surface was found to have a uniform layer of the solvent but only drops of sovent adhered to the untreated surface, The drops of solvent *Trade Mark 1 ~3~>~
1 were wiped off and the thickness of the weight of the support resulting from the deposition of that layer was approximately 3 ~m.
Example 2 Hollow PTFE fibers (outside diameter: 1.1 mm, inside diameter: 0.6 mm, porosity, 35%, average pore size, 0.1 ~m) were used as a starting material.
The hollow fibers were prepared by first extruding PTFE tubes by the paste method ~U.S. Paten~
No. 4,22~,547), then stretching the tubes longitudinal-y at a stretch ratio of 2, and finally sintering the stretched tubes at temperatures not lower than 327 C
(U.S. Patent No. 4,082,893). ~nly the surface of the outermost layer of each tube was roughened by passing it at a linear speed of 8 m/min. through a furnace wherein flames issued uniformly in the radial direction.
observation with a scanning electron microscope showed that the scorched surface had a roughness of about 10 to 30 ~m.
A plasma polymerization system was supplied with N-vinylpyrrolidone and a plasma polymer was deposited on the outer sur~ace of each of the hollow PTFE fibers under the same conditions as used in Example 1. The deposit of ~he polymer coat was estimated to have a thickness of 0.35 ~m.
The fibers were dipped in a dimethyl sulfoxide 36~
1 bath. ~ unificrm layer of the solvent was formed on the entire periphery of each fiber, and the thickness of the layer was estimated to be about 8 ~m by measuring the increase in the weight of the fiber.
The interior of each of the fibers was pressurized by introducing air at a pressure of 2 kg/cm~
with the other end of the fiber closed. The dimethyl sulfoxide layer remained strongly adherent on the outer surface of each fiber, causing no foam in the liquid barrier.
Example 3 Hollow PTFE fikres having an outside diameter of 1.0 mm, an inside diameter of 0.4 mm, a porosity of 30~
and an average pore size of 0.05 ~m were prepared as a starting material by modifying the paste extrusion and reducing longitudinal stretching ratio.
~he surface of the outermost layer of each of the hollow fibers was thermally decomposed by scanning with a CO2 laser (100 watts) beam obtained by focusinq with a lens system consisting of a condenser, reflector and a conical mirror. By this physical treatment~ not only were the fiber portions cut but also the nodes forming the porous structure were volatilized.
The so treated hollow PTFE fibers were set in a plasma treatment unit with a tubular react~r so that they could be transported through the unit. The surface of each 6~
1 fibe~ was treated by an oxygen gas plasma at 40 watts, producing a fine texture (0.5 ~m roughness) on the surface of the outermost layer of each fiber.
The reactor was supplied with 4-vinylpyridine as a polymerizable gas and plasma polymerization was performed at 30 watts on the fibers running at 1 m/min.
Assuming a uniform deposition was effected, the deposited thickness of plasma polymer was calculated to be 0.3 ~m.
A bundle of 3,000 PTFE fibers thus treated was packed in a cylindrical container so that it would have an effective length of 30 cm, leaving 2 5-cm portion at each end for subsequent sealing. A silicone rubber of the addition reacti~e type was applied ,t~ both ends of the bundle and cured to cross-link. One sealed end of the bundle was cut open to provide an outlet for permeate gas.
A solution having 10 4 mole/g of N,N-bis(2-aminobenzyl)e~hylenediamine dissolved in dimethyl sulfoxide was injected into the cylindrical container at the feed gas inlet and the container was pressurized for 10 minutes with a nitrogen gas supplied at 1.5 kg/cm2. In the meantime, the cylinder was rotated to ensure that the solution could be impregnated into the entire surface of the hollow fibers. Excess solution was withdrawn from the container, and immediately thereafter, the container was placed in a refrigerator for storage at 5 C or below.
~he module was taken out of the refriqerator and 68~
1 air at one atmosphere was pumped into the module at the feed inlet while the outlet for permeate gas was held at 30 mmHg. On the product side, an oxygen-rich air (66% 2) was ob~ained with a calculated 2 permeation rate of 4.2 x 1~ om /cm /sec cmHg.
While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various chan~es and modifications can be made therein without departing from the spirit and scope thereof.
LIQUID MEMBRANE
BACKGROUND OF THE INVENTION
The present invention relates to a liquid membrane used ;n facilitated transport of a specific gasO
More specifically! the invention relates to a liquid membrane using a porous polytetrafluoroethylene support one surface of which is first roughened by a physical means, wherein the support has a polymerized layer of a nitrogen-con~aining compound chemically deposited thereon so as to provide an increased affinity for liquids.
Membrane technology using ethyl cellulose r acetyl cellulose or silicon-carbonate copolymer membranes to separate a specific component gas from a gaseous mixture containing the same has been the subject of extensive research. These membranes are made of solid materials and are hence referred to as solid membranes~ In order to separate the specific gas component in an economical and efficient manner, ~wo requirements must be met: (1) a membrane material that permits the selective passage of the gas component to be separated must be selected; and ~2) the membrane must be made as thin as possible. Most of the materials known today for use in solid membranes have low gas selectivities and those materials which have relatively high selctivities are only capable of very low rates of gas permeation.
Facilitated tranport of a particular gas can be 1 realized by using a liquid membrane of a material having a high degree o~ affinity for the ~as to be separated. As shown in U.S. Patent Nos. 3,865,890, 3,951,621, 4,015,955 and 4,060,566, ethylene can be selectively concentrated from a mixture of methane, ethane and ethylene using a nylon-6.6 membrane impregnated with an aqueous solution of AgNO3. The membrane disclosed in these patents is rendered hydrophilic by incorporation of a hydrophilic polymer such as polyvinyl alcohol, but the life of the membrane is not very long since water used as the solvent for the aqueous solution of Ag ions will unavoidably evaporate during the use of the membrane.
U.S. Patent Nos. 3,396,510, 3,819,806 and 4,119,408 show that acidic gas components such as CO2, H2S
and SO2 can be selectively permeated through a polyethersulfone membrane impregnated with an aqueous solution of K2CO3. But this membrane has the same problem as in the case of the membrane used for ethylene separation discussed above.
~O European Patent Application No. 98731/1984 ~a laid-open publication Of January 18, 1984, naming I.C. Roman and R.W. Baker as inventors and assigned to Bend Research Inc.) shows that oxygen can be selectively separated from the air using a nylon-6.6 membrane impregnated with a transition metal complex of a Schiff base dissolved in a solvent such as lactone or amide.
In the Example therein, a nylon-6.6 membrane with a thickness of 130 ,um was used and this suggests the presence of a liquid membrane that was at least 130Jum thick.
8~i.r;
1 l'his membrane system is substantially free ~rom the problem of solvent evaporation because the li~uid barrier is made not of water, but of organic solvents such as lactone and amide. ~owever, the life of the system is short because the transition metal complex of a Schiff base is irreversibly oxidized during operation. In addition, the liquid barrier that is impregnated into the nylon-6.6 membrane cannot be made thinner than 130 ~m-Sl~MMARY OF THE_INVENTION
One object, therefore, of the present inven~ionis to provide a liquid barrier in a liquid membrane that is not thicker than a few micrometers and which yet is capable of enhanced transport of a specific gas component.
Another object of the inventlon is to provide a membrane support one surface of which does not dissolve in solvents but has a suitable degree of affinity therefore.
rn or~er to make a liquid barrier no~ in the form of an aqueous solu~ion but in the form of an organic solution with low volatility solvents such as lactone, dimethylformamide and N-methylpyrrolidone are used. These solvents will dissolve known porous support materials such as polysulfones and polyamides, so that it is necessary touse a support material that will not dissolve in these organic solvents.
1.'~'`sfifi~
DETAILE~D DES(~RIPTION OF TIIE IIJVENTION
The liquid ~embrane in accordance with the invention is composed of a solvent, an active species for performing facilitated transport of a specific gas, and a S support. The support is made of porous poly-tetrafluoroethylene (hereinafter abbreviated as PTF~) ana is characterized by having one hydrophilic surface in order to make contact with a mixed solution of the solvent and active species and to retain this mixed solution in the form of a thin film. In order to provide such hydrophilic surface, the support must be given both a physical treatment for roughening one surface and a chemical ~rea~ment for depositiny a iayer of a non-PTFE
compound on that surface. If only one of th~ese two treatments is applied, a liquid barrier that will maintain a uniform thickness cannot be obtained, Ol even if this is possible, the resulting membrane has a very short life.
The PTFE used as the support material is insoluble in almost all solvents known today, so it can be used in supports for liquid barriers employing solvents of high polarity such as dimethylformamide (hereinafter DMF) and N-methylpyrrolidone (hereinafter NMP). However, because of high polarity, DMF and NMP exhibit such a great surface tension that they are incapable of wetting the s~rface of PTFE satisfactorily. As a result, the porous PTFE membrane cannot be put to service in any manner other than where a pool of a mixed solution of solvent and 6~
1 active species is placed on the membrane, thus producing an undesirably thick liquid barrier.
It therefore becomes important for the purposes of the present invention to render one surface of the porous PTFE hydrophilic so that it is wettable by polar solvents. The intensity of this hydrophilic treatment determines the thickness in which the liquid barrier made of the solvent and active species can remain stable.
The physical treatments for providing a roughened surface include graining with a metal brush, etc., heat treatment in which only the surface layer of PT~E is thermally decomposed, laser treatment using the laser energy as a heat source, low-temperature plasma etching using a non-polymerizable gas, and sputter etchinq using Rf energy, which are described in U.S. Patent Nos. 4,297,187, and 4,311,828. By selectinq a proper method from among these methods, a suIface roughness in the range of 1 to 100 ~m preferably 2 to 20 ~m, can be obtalned. If a very fine texture with a roughness of about 2G several micrometers is desired, lasers or Rf energy is preferably used.
U.S. Patent Nos. 3,664,915, 3,953,566, 4,082,893, and 4,248,942 provide porous PTFE by stretching and expanding operations. The structures produced by such techniques consist of fibers and nodes interconected by the fibers. Such structures are preferred starting 1 materialssince they provide for a great latitude in selection of porosity and pore size. If the porous PTFE
is subjected to sputtering or plasma etching, the fibers in the treated surface are cut and subsequently decomposed to provide a texture structure consisting essentially of nodes. If, on the other hand, only the surface layer of the porous PTFE is thermally decomposed by scanning with a C2 laser beam, not only the fibers but also a part of the nodes is volatilized.
The choice of a suitable method of physical treatment depends on the required thickness of the liquid barrier.
The physical treatment for roughening one surface of the porous PTFE is followed by a chemical treatment. The surface subjected to roughening by a physical treatment has a reduced angle of contact with the highly polar DMF or NMP and its affinity for these solvents appears to have increased in comparison with the untreated surface. In fact, however, the roughened surface of PTFE causes little effects on its inherent nature and the increase in its affinity for polar solvents is negligible. In order to achieve an increased affinity for such solvent, the surface of the PTFE must be covered with a thin layer of a compound having a chemical structure similar to that of the polar solvents. This object is insufficiently achieved by merely performing "tetra-1~6~
1 etching", i.e., withdrawal of fluorine atoms using sodium-naphthalene complex or other suitable etchants. The preferred chemical treatment is one depending on plasma polymerization for depositing a compound similar to the solvent component of the final liquid barrier. As described in U.S. Patent Nos. 3,657,113, and 4,199,448, the plasma polymerization is realized by introducing a polymerizable gas into a bell jar or a tubular reactor while a glow discharge is maintained by application of radio frequencies, microwaves or d,c. current.
Polymerizable gases that can be used with advantage are nitrogen-containing cyclic compounds that include pyridines such as 4-vinylpyridine, 2-vinylpyridine, 4-ethylpyridine and 5-vinyl-2-methylpyridine; pyrrolidone derivatives such as N-methylpyrrolidone and N-vinylpyrrolidone amines such as 4-methylbenzylamine and ~-butylamine; and pyridine derivatiYes such as picoline and lutidine. Oxygen-containing compounds could be used as polymerizable gases but they are by no means advantageous 2~ over the nitroqen-containing compounds from the viewpoint of deposition rates of plasma polymers.
If nitrogen-containing compounds, preferably cyclic compounds, are used as polymerizable gases, films of plasma polymer are produced that are about 1 ~m thick and which are composed of a highly cross-linked structure.
Because of the cross-linked structure, such polymer films 36~
1 will not dissolve in the highly polar solvents which are used to make up a liquid barrier, but they can be swelled by such solvents. This will provide a great advantage for the purpose of maintaining a liquid barrier in the form of a very thin film of a thickness of about several micrometers. The nitrogen-containing compounds listed above will also act as "axial bases" in the sense of the term used in European Patent Application No. 98731/1984.
It would of course be possible to form films of plasma polymer even thinner than 0.1 ~m or thicker than 10 ~m by changing the polymerization conditions. However, polymer films thinner than 0.1 ~m have a smaller ability to maintain a liquid barrier stably and it becomes difficult to have a uniform and sound liquid barrier spread over a large surface area. If polymer film is thicker than 10 ~m, cracks will develop because of the internal stress that has occurred in the film during plasma polymerizationO
Even in the absence of any cracking, the film is unstable and may sometimes separate from the substrate.
Under these cirumstances, the particularly preferred thickness of the plasma polymer film may range from 0.3 ~m to 3 ~m. Polymer films having this thickness range are capable of maintaining a liquid barrier~with the thickness ranging from 0.1 ~m to 6 ~m.
The other features of the liquid membrane in accordance with the present invention may substantially be . ~;
- .
.
. , . , :
1 the same as described in European Patent Application No. 98731/1984.
The solvents of high polarity include lactams, sulfoxides and amides, and preferably, dimethyl sulfoxide, NMP, propylene carbonate, DMF and gamma-butyrolactam are used. These solvents may contain nitrogen-containing com-pounds such as polyethyleneimine and tetra-ethylene-pentamine, or pyridine and pyrrolidone derivatives used as polymerizable gases in plasma polymerization.
Examples of the active species capable of enhanced transport of oxygen or carbon monoxide gas are complex compounds of transition metals with Schiff bases prepared by dehydration condensation from ethylenediamine and aldehyde compounds. Illustrative Schiff bases are N,N-bis(salicylidene)ethylenediamine and bis(2-amino-1-benz-aldehyde)ethylenediamine. Exemplary transition metals are divalent elements ~uch as Co, Cu and Fe. These active species a~e d s~lved in polar so~vents in amounts ranginq from 10 5 to 10 3 mole, ~referably 10 4 to 10 3 mole, per unit weight of the liquid barrier. Higher concentrations of the active species will provide increased initial gas selectivities, but their characteristics will vary with time as a result of dimerization or other side reactions.
If the concentration of the active species is less than 10 5 mole the intended effect of the active species is difficult to obtain and only low gas selectivities will 1 result. Therefore, the preferred concentration of the active species is in the range of 10 5 to 10 3 mole per g of the liquid barrier.
The process of producing a large-scale module from the liquid membrane of the present invention will start with the shaping of a support that has been provided with a hydrophilic surface but on wihch no liquid barrier is maintained. In order to provide a large surface area, a support in the form of a bundle of tubes or hollow fibers is packed in a molding container and both ends of the bundle are sealed. The sealant may be an epoxy resin but silicone rubbers having a greater adhesive strength are preferred. After the sealant pas solidified, part of the sealed portion is cut open so as to provide a separation module having respective channels for the feed gas, permeate gas and reject gas.
After forming such module, a mixed solution of solvent and active species is introduced in excess amount into the module at the feed gas inlet and the module is pressurized at 1 to 2 kg/cm2 with the reject gas outlet closed. But this pressurization step, the mixed solution is impregnated into the entire part of the hydrophilic surface of the support. Desirably, complete impregnation of the mixed solution is ensured by shaking the whole part ?5 Of the module. Uniform impregnation of the mixed solution can be realized by monitoring the flow rate of gas coming 1 from the permeate gas outlet because as the impregnation proceeds, a decreasing amount of gas will come out of the module until the point i5 reached where a minimum flow rate occurs, which indicates that the mixed solution has been impregnated uniformly in every part of the hydrophilic surface of the support.
The following Examples are provided for further illustration of the claimed liquid membrane but are not to be construed as limiting the invention.
Example 1 A reactor of the bell iar type having a sheet of *
Fluoropore FP-010 (porous PTFE membrane manufactured by Sumitomo Electric Industries, Ltd. having an average pore size of 0.1 ~m) placed on an electrode was evacuated to a pressure of 0.01 Torr. Glow discharge was conducted with a radio wave ~13.~6 MHz) applied at a power of 60 watts.
The reactor was supplied with 4-vinylpyridine until the pressure in the reactor was increased to 0.2 Torr. Plasma polymerization was performed for 30 minutes at a po~er of 30 watts. The thickness of the polymer deposit as estimated from the increase in weight was 0.5 ~m. The plasma polymer coat on one side was dipped in a dimethylformamide bath. When it was recovered from the solvent bath, the treated surface was found to have a uniform layer of the solvent but only drops of sovent adhered to the untreated surface, The drops of solvent *Trade Mark 1 ~3~>~
1 were wiped off and the thickness of the weight of the support resulting from the deposition of that layer was approximately 3 ~m.
Example 2 Hollow PTFE fibers (outside diameter: 1.1 mm, inside diameter: 0.6 mm, porosity, 35%, average pore size, 0.1 ~m) were used as a starting material.
The hollow fibers were prepared by first extruding PTFE tubes by the paste method ~U.S. Paten~
No. 4,22~,547), then stretching the tubes longitudinal-y at a stretch ratio of 2, and finally sintering the stretched tubes at temperatures not lower than 327 C
(U.S. Patent No. 4,082,893). ~nly the surface of the outermost layer of each tube was roughened by passing it at a linear speed of 8 m/min. through a furnace wherein flames issued uniformly in the radial direction.
observation with a scanning electron microscope showed that the scorched surface had a roughness of about 10 to 30 ~m.
A plasma polymerization system was supplied with N-vinylpyrrolidone and a plasma polymer was deposited on the outer sur~ace of each of the hollow PTFE fibers under the same conditions as used in Example 1. The deposit of ~he polymer coat was estimated to have a thickness of 0.35 ~m.
The fibers were dipped in a dimethyl sulfoxide 36~
1 bath. ~ unificrm layer of the solvent was formed on the entire periphery of each fiber, and the thickness of the layer was estimated to be about 8 ~m by measuring the increase in the weight of the fiber.
The interior of each of the fibers was pressurized by introducing air at a pressure of 2 kg/cm~
with the other end of the fiber closed. The dimethyl sulfoxide layer remained strongly adherent on the outer surface of each fiber, causing no foam in the liquid barrier.
Example 3 Hollow PTFE fikres having an outside diameter of 1.0 mm, an inside diameter of 0.4 mm, a porosity of 30~
and an average pore size of 0.05 ~m were prepared as a starting material by modifying the paste extrusion and reducing longitudinal stretching ratio.
~he surface of the outermost layer of each of the hollow fibers was thermally decomposed by scanning with a CO2 laser (100 watts) beam obtained by focusinq with a lens system consisting of a condenser, reflector and a conical mirror. By this physical treatment~ not only were the fiber portions cut but also the nodes forming the porous structure were volatilized.
The so treated hollow PTFE fibers were set in a plasma treatment unit with a tubular react~r so that they could be transported through the unit. The surface of each 6~
1 fibe~ was treated by an oxygen gas plasma at 40 watts, producing a fine texture (0.5 ~m roughness) on the surface of the outermost layer of each fiber.
The reactor was supplied with 4-vinylpyridine as a polymerizable gas and plasma polymerization was performed at 30 watts on the fibers running at 1 m/min.
Assuming a uniform deposition was effected, the deposited thickness of plasma polymer was calculated to be 0.3 ~m.
A bundle of 3,000 PTFE fibers thus treated was packed in a cylindrical container so that it would have an effective length of 30 cm, leaving 2 5-cm portion at each end for subsequent sealing. A silicone rubber of the addition reacti~e type was applied ,t~ both ends of the bundle and cured to cross-link. One sealed end of the bundle was cut open to provide an outlet for permeate gas.
A solution having 10 4 mole/g of N,N-bis(2-aminobenzyl)e~hylenediamine dissolved in dimethyl sulfoxide was injected into the cylindrical container at the feed gas inlet and the container was pressurized for 10 minutes with a nitrogen gas supplied at 1.5 kg/cm2. In the meantime, the cylinder was rotated to ensure that the solution could be impregnated into the entire surface of the hollow fibers. Excess solution was withdrawn from the container, and immediately thereafter, the container was placed in a refrigerator for storage at 5 C or below.
~he module was taken out of the refriqerator and 68~
1 air at one atmosphere was pumped into the module at the feed inlet while the outlet for permeate gas was held at 30 mmHg. On the product side, an oxygen-rich air (66% 2) was ob~ained with a calculated 2 permeation rate of 4.2 x 1~ om /cm /sec cmHg.
While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various chan~es and modifications can be made therein without departing from the spirit and scope thereof.
Claims (12)
1. A liquid membrane comprising a solvent, an active species capable of enhanced transport of a specific gas, and a support for maintaining a liquid body which comprises a mixture of said solvent and active species dissolved therein, wherein said support is a porous polytetrafluoro-ethylene film, one surface of which is hydrophobic and the other surface of which is hydrophilic.
2. The liquid membrane as claimed in Claim 1, wherein said porous polytetrafluoroethylene film is composed of fibers and nodes interconnected with said fibers, wherein the hydrophilic surface of said polytetrafluoroethylene film has been formed by both physical and chemical treatments.
3. The liquid membrane according to Claim 2, wherein the physical treatment used to produce the hydrophilic surface was surface roughening.
4. The liquid membrane as claimed in Claim 2, wherein the chemical treatment used to produce the hydrophilic surface consists of deposition of a plasma polymerized coat of a nitrogen-containing compound, said coat having a thickness not greater than about 10 µm.
5. The liquid membrane as claimed in Claim 4, wherein the nitrogen-containing compound is at least one member selected from the group consisting of vinylpyridine, 4-ethylpyridine, N-vinylpyrrolidone, N-methylpyrrolidone, 4-picoline, and 3,5-lutidine.
6. The liquid membrane as claimed in Claim 1,2,3,4 or 5, wherein the solvent is one or more members selected from the group consisting of gamma-butyrolactone, N-methylpyrrolidone, 4-dimethylaminopyridine and 4-aminopyridine, and the active species is a complex compound of a Schiff base and a transition metal.
7. The liquid membrane as claimed in Claim 2, wherein said physical treatment was selected from the group consisting of graining with a metal brush, heat treatment in which only the surface layer of PTFE is thermally decomposed, laser treatment using laser energy as a heat source, low-temperature plasma etching using a non-polymerizable gas and sputter etching using Rf energy.
8. The liquid membrane as claimed in Claim 3, 4, 5 or 7, wherein the resulting surface roughness is in the range of 1 to 100 µm.
9. The liquid membrane as claimed in Claim 4 or 5, wherein the plasma polymerized coat has a thickness of 0.1 µm to
10 µm.
10. The liquid membrane as claimed in Claim 3, wherein the plasma polymerized coat has a thickness of 0.3 µm to 3 µm.
10. The liquid membrane as claimed in Claim 3, wherein the plasma polymerized coat has a thickness of 0.3 µm to 3 µm.
11. The liquid membrane as claimed in Claim 6, wherein said Schiff base is selected from the group consisting of N,N-bis(salicylidene)ethylenediamine and bis(2-amino-1-benzaldehyde)ethylenediamine and said transition metal is selected from the group consisting of cobalt, copper and iron.
12. A liquid membrane comprising:
i) a solvent ii) an active species capable of enhanced transport of a specific gas, and iii) a support for maintaining a liquid barrier layer thereon comprising a solution of said active species dissolved in the solvent, wherein said support comprises a porous polytetrafluoro-ethylene film, one surface of which is hydrophobic and the other surface of which has been treated to be hydrophilic by physical roughening to a resulting surface roughness in the range of 0.5 µm to 100 µm and subsequent deposition of a coating having a thickness not greater than about 10 µm and being composed of a polymer having a structure similar to the solvent; which coating polymer is highly cross-linked, does not dissolve in the solvent and is swelled by the solvent.
i) a solvent ii) an active species capable of enhanced transport of a specific gas, and iii) a support for maintaining a liquid barrier layer thereon comprising a solution of said active species dissolved in the solvent, wherein said support comprises a porous polytetrafluoro-ethylene film, one surface of which is hydrophobic and the other surface of which has been treated to be hydrophilic by physical roughening to a resulting surface roughness in the range of 0.5 µm to 100 µm and subsequent deposition of a coating having a thickness not greater than about 10 µm and being composed of a polymer having a structure similar to the solvent; which coating polymer is highly cross-linked, does not dissolve in the solvent and is swelled by the solvent.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP205329/84 | 1984-09-28 | ||
| JP59205329A JPS6182806A (en) | 1984-09-28 | 1984-09-28 | Liquid membrane |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1286865C true CA1286865C (en) | 1991-07-30 |
Family
ID=16505120
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000491386A Expired - Fee Related CA1286865C (en) | 1984-09-28 | 1985-09-24 | Porous liquid polytetrafluoroethylene membrane with hydrophobic and hydrophilic surfaces |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US4734112A (en) |
| EP (1) | EP0176986B1 (en) |
| JP (1) | JPS6182806A (en) |
| CA (1) | CA1286865C (en) |
| DE (1) | DE3583720D1 (en) |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4824443A (en) * | 1986-12-30 | 1989-04-25 | Bend Research, Inc. | Gas separation by composite solvent-swollen membranes |
| DE3871462D1 (en) * | 1987-08-22 | 1992-07-02 | Agency Ind Science Techn | GAS SEPARATION MEMBRANE. |
| JPH04122412A (en) * | 1990-09-12 | 1992-04-22 | Hitachi Ltd | Method and equipment for dissolving oxygen |
| US5507949A (en) * | 1992-03-20 | 1996-04-16 | Monsanto Company | Supported liquid membrane and separation process employing same |
| DK0561760T3 (en) * | 1992-03-20 | 1999-05-25 | Monsanto Co | Extraction of organic compounds from aqueous solutions |
| DE4243995C2 (en) * | 1992-12-23 | 1996-07-25 | Gore W L & Ass Gmbh | Permanent hydrophilic modification for fluoropolymers and methods for generating the modification |
| US6126723A (en) * | 1994-07-29 | 2000-10-03 | Battelle Memorial Institute | Microcomponent assembly for efficient contacting of fluid |
| US6780497B1 (en) * | 1999-08-05 | 2004-08-24 | Gore Enterprise Holdings, Inc. | Surface modified expanded polytetrafluoroethylene devices and methods of producing the same |
| SE515567C2 (en) * | 1999-10-27 | 2001-08-27 | Esy Tech Ab | Liquid-liquid extraction and direct transfer of the analyte to gas chromatography apparatus |
| US6579343B2 (en) * | 2001-03-30 | 2003-06-17 | University Of Notre Dame Du Lac | Purification of gas with liquid ionic compounds |
| EP2060315A3 (en) | 2007-11-15 | 2009-08-12 | DSMIP Assets B.V. | High performance membrane |
| US9018584B2 (en) | 2011-04-14 | 2015-04-28 | Emd Millipore Corporation | Devices and methods for infrared (IR) based quantitation of biomolecules |
| WO2013080994A1 (en) * | 2011-12-01 | 2013-06-06 | 株式会社ルネッサンス・エナジー・リサーチ | Facilitated transport membrane manufacturing method |
| JP6186286B2 (en) * | 2014-01-16 | 2017-08-23 | 富士フイルム株式会社 | Method for producing laminate for acid gas separation, laminate for acid gas separation, and acid gas separation module |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3657113A (en) * | 1970-02-03 | 1972-04-18 | Mobil Oil Corp | Separating fluids with selective membranes |
| US3957504A (en) * | 1974-11-11 | 1976-05-18 | Allied Chemical Corporation | Membrane hydro-metallurgical extraction process |
| JPS5254688A (en) * | 1975-10-30 | 1977-05-04 | Sumitomo Chem Co Ltd | Method of producing semipermeable memebrane |
| US4199448A (en) * | 1976-06-09 | 1980-04-22 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Reverse osmosis membrane of high urea rejection properties |
| US4230463A (en) * | 1977-09-13 | 1980-10-28 | Monsanto Company | Multicomponent membranes for gas separations |
| JPS5814453B2 (en) * | 1979-07-16 | 1983-03-19 | 信越化学工業株式会社 | Surface treatment method for plastic molded products |
| US4542010A (en) * | 1982-06-30 | 1985-09-17 | Bend Research, Inc. | Method and apparatus for producing oxygen and nitrogen and membrane therefor |
-
1984
- 1984-09-28 JP JP59205329A patent/JPS6182806A/en active Granted
-
1985
- 1985-09-24 CA CA000491386A patent/CA1286865C/en not_active Expired - Fee Related
- 1985-09-27 DE DE8585112299T patent/DE3583720D1/en not_active Expired - Fee Related
- 1985-09-27 EP EP85112299A patent/EP0176986B1/en not_active Expired - Lifetime
- 1985-09-30 US US06/781,602 patent/US4734112A/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| EP0176986A2 (en) | 1986-04-09 |
| JPH0479683B2 (en) | 1992-12-16 |
| JPS6182806A (en) | 1986-04-26 |
| US4734112A (en) | 1988-03-29 |
| EP0176986A3 (en) | 1988-03-30 |
| DE3583720D1 (en) | 1991-09-12 |
| EP0176986B1 (en) | 1991-08-07 |
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