Immobilized liquid membrane

The present invention is a single-ply immobilized liquid membrane comprising an aqueous liquid membrane immobilized within a hydrophobic microporous support, and a method of preparing such an immobilized liquid membrane. The present invention also includes a method of preparing an ultrathin single-ply immobilized liquid membrane.

BACKGROUND OF THE INVENTION 
The present invention provides a stable, efficient, single-ply, immobilized 
liquid membrane comprising an aqueous liquid membrane immobilized within a 
hydrophobic microporous support, and a method of preparing such an 
immobilized liquid membrane. The present invention also includes a method 
of preparing an ultrathin immobilized liquid membrane having a thickness 
of about 0.0084 mm or less. 
The removal of a gaseous component from a gaseous mixture by an immobilized 
liquid membrane is well-known. Typically, such conventional immobilized 
liquid membranes are prepared by immersing a hydrophilic microporous 
membrane in a suitable aqueous solution. The hydrophilic membrane draws up 
the aqueous solution into its pores such that the solution in the pores 
acts as membrane so as to preferentially separate a gaseous component from 
a gaseous mixture. That is, one of the species in a gaseous mixture 
preferentially permeates through the liquid in the pores. 
The conventional immobilized liquid membrane consists of an aqueous liquid 
membrane immobilized within a hydrophilic microporous membrane. The 
immobilized liquid membrane is then supported on a hydrophobic microporous 
support membrane on the lower pressure side to prevent expulsion of the 
immobilized aqueous liquid serving as the membrane. Examples of such a 
sandwich structure may be found in U.S. Pat. Nos. 3,819,806; 4,089,653; 
4,115,513; 4,119,408; 4,147,754 and 4,174,374. When flat microporous 
membranes are used and a positive pressure difference exists between the 
two sides of the immobilized liquid membrane, the sandwich structure 
described above is further supported by a flat, fine mesh stainless steel 
screen. See, Kimura and Walmet, "Fuel Gas Purification With Permselective 
Membranes", Separation Science and Technology, 15 (4), p.p. 1115-1133 
(1980). When hydrophilic microporous hollow fiber support membranes are 
used to immobilize aqueous liquids, the application of a positive pressure 
difference is avoided to prevent the expulsion of the aqueous liquid from 
the membrane. Id. 
Conventional immobilized liquid membranes also inherently suffer from their 
hydrophilic composition, for when a gaseous component is removed from a 
feed gas mixture that contains water, the residual feed gas mixture 
becomes supersaturated. Water condenses on the hydrophilic membrane, and 
floods it. Thus such immobilized liquid membranes require very careful 
humidity control. Id. at p. 1128. 
Furthermore, as disclosed in U.S. Pat. No. 4,119,408, it requires 
specialized process steps to replenish liquid loss in a conventional 
sandwich structure immobilized liquid membranes in an online gas 
separation system. However, the ultrathin immobilized liquid membrane of 
the present invention may be easily replenished from the downstream side 
to compensate for any liquid loss. 
U.S. Pat. No. 3,625,734 discloses an immobilized liquid membrane comprising 
aqueous polyethylene glycol supported on a porous, inert backing membrane 
having deposited thereon a non-wetting microporous film of particles of 
polytetrafluoroethylene. To deposit the immobilized liquid film of 
polyethylene glycol directly on the polytetrafluoroethylene coated backing 
membrane, the membrane is made wettable by spraying it with a dilute 
aqueous solution that contains hydroxymethyl cellulose. Such an 
immobilized liquid membrane is thus difficult to prepare and will suffer 
from problems such as liquid expulsion at even low applied pressure 
differences as well as from liquid membrane flooding on the feed side. 
Japanese Patent Publications No. 52123/1981 discloses that porous, hollow 
polypropylene filaments can be made hydrophilic by immersing the filaments 
in ethanol and then passing water through the filaments. The fibers are 
used to filter fine particles from aqueous solution. This reference does 
not mention immobilized liquid membrane technology, and does not suggest 
immobilizing aqueous solutions within the porous, hollow polypropylene 
fibers. 
The disadvantages of the conventional immobilized liquid membranes are 
overcome by the present invention. First, once the aqueous membrane is 
immobilized within the hydrophobic microporous support, it is not expelled 
under a substantial positive pressure difference, e.g. 175 psig, applied 
across the membrane. Such positive pressure difference stability is wholly 
unexpected in a single-ply immobilized liquid membrane. 
This high positive pressure difference stability means that if the 
microporous hydrophobic support can support mechanically the pressure 
difference in any given application, the conventional sandwich structure 
is not needed, and the immobilized liquid membrane of the present 
invention can stand alone. This ability to function without support 
provides for ease of service and cost-effectiveness as compared to 
conventional immobilized liquid membranes. 
Second, if condensation of water due to supersaturation of the feed gas 
mixture occurs, the liquid membrane immobilized in the hydrophobic 
microporous support of the present invention will not flood. Also, if the 
aqueous liquid membrane is immobilized within hydrophobic microporous 
hollow fiber, it is stable at significant levels of applied pressure 
difference, especially if the higher pressure exists on the outside of the 
hollow fiber. 
To maximize flux through a membrane and reduce the area required for a 
given separation, it is generally preferable to utilize as thin a membrane 
as possible. The present invention thus also includes a method of 
preparing an ultrathin, single-ply immobilized liquid membrane. 
Specifically, the thickness of the aqueous liquid membrane immobilized in 
the microporous hydrophobic support is reduced by partially removing, 
e.g., by evaporation, the aqueous liquid in the membrane. As the support 
is hydrophobic, the remaining aqueous liquid does not migrate to dried 
sections of the support. The ultrathin immobilized liquid membranes of the 
present invention have a thickness of less than about 0.0084 mm. 
Therefore, it is an object of the present invention to provide a single-ply 
immobilized liquid membrane comprising an aqueous liquid membrane 
immobilized within a hydrophobic microporous support, and a method of 
preparing such an immobilized liquid membrane. 
It is also an object of the present invention to provide a single-ply 
immobilized liquid membrane stable at substantial positive pressure 
differences applied across the membrane. 
It is also an object of the present invention to provide a single-ply 
immobilized liquid membrane that is easy to service and relatively 
inexpensive to fabricate. 
It is also an object of the present invention to provide an immobilized 
liquid membrane that resists flooding. 
It is also an object of the present invention to provide an aqueous liquid 
membrane immobilized within hydrophobic microporous hollow fiber with the 
immobilized liquid membrane being stable to high levels of applied 
pressure difference. 
It is also an object of the present invention to provide an ultrathin, 
single-ply immobilized liquid membrane, and a method of preparing such a 
membrane. 
It is also an object of the present invention to provide an ultrathin, 
single-ply immobilized liquid membrane that exhibits superior separation 
ability. 
SUMMARY OF THE INVENTION 
The present invention provides a single-ply immobilized liquid membrane 
comprising an aqueous liquid membrane immobilized within a hydrophobic 
microporous support, and the process for making such an immobilized liquid 
membrane. That process comprises the following steps: (a) contacting a 
hydrophobic microporous support with an aqueous solution containing from 
about 40 to about 95 percent by volume of an exchange component until 
steady state is achieved; (b) removing the support from the aqueous 
solution; (c) contacting the support with water without appreciable 
surface drying until steady state is achieved; (d) removing the support 
from the water; and (e) repeating steps (a)-(d) until a water membrane is 
immobilized within substantially the entire thickness of the support. 
The present invention also provides a method for immobilizing a aqueous 
salt solution within the hydrophobic microporous support, by including a 
further step (f) whereby the support contacts an aqueous salt solution 
bath until an aqueous salt solution membrane replaces the water membrane 
in the support. 
Finally, the present invention provides a method for preparing a 
single-ply, ultrathin immobilized liquid membrane comprising the following 
steps: (a) contacting a hydrophobic microporous support with an aqueous 
solution containing from about 40 to about 95 percent by volume of an 
exchange component until steady state is achieved; (b) removing the 
support from the aqueous solution; (c) contacting the support with water 
without appreciable surface drying until steady state is achieved; (d) 
removing the support from the water; and (e) repeating steps (a)-(d) until 
a water membrane is immobilized within substantially the entire thickness 
of the support; and (f) partially reducing the thickness of the membrane 
immobilized within the support. The ultrathin immobilized liquid membrane 
of the present invention may comprise an aqueous salt membrane immobilized 
within the support.

DETAILED DESCRIPTION OF THE INVENTION 
The product and process provided by the present invention relates to a 
single-ply, immobilized liquid membrane comprising an aqueous liquid 
membrane immobilized with a hydrophobic microporous support. 
THE MICROPOROUS SUPPORT 
In one embodiment of the present invention, the hydrophobic microporous 
support are the microporous films of the type described below and 
disclosed in U.S. Pat. Nos. 3,801,404; 3,801,444; 3,839,516; 3,843,761; 
4,255,376; 4,257,997; and 4,276,179 which are all herein incorporated by 
reference. It should be noted that any hydrophobic microporous material 
may be used in the present invention. These include any microporous 
material not spontaneously wet by water. 
Porous or cellular films can be classified into two general types: one type 
in which the pores are not interconnected, i.e., a closed-cell film, and 
the other type in which the pores are essentially interconnected through 
tortuous paths which may extend from one exterior surface or surface 
region to another, i.e., an open-celled film. The porous films of the 
present invention are of the latter type. 
Further, the pores of the microporous films of the present invention are 
microscopic, i.e., the details of their pore configuration or arrangement 
are discernible only by microscopic examination. In fact, the open cells 
or pores in the films prepared by the "dry stretch" or "solvent stretch" 
techniques described herein generally are smaller than those which can be 
measured using an ordinary light microscope, because the wavelength of 
visible light, which is about 5,000 Angstroms (an Angstrom is one 
tenbillionth of a meter), is longer than the longest planar or surface 
dimension of the open cell or pore. The microporous films prepared by the 
"solvent stretch" or "dry stretch" method may be identified, however, by 
using electron microscopy techniques which are capable of resolving 
details of pore structure below 5,000 Angstroms. 
The microporous films of the present invention are also characterized by a 
reduced bulk density, sometimes hereinafter referred to simply as a "low" 
density. That is, these microporous films have a bulk or overall density 
lower than the bulk density of corresponding films composed of identical 
polymeric materials but having no open celled or other voidy structure. 
The term "bulk density" as used herein means the weight per unit of gross 
or geometric volume of the film, where gross volume is determined by 
immersing a known weight of the film in a vessel partly filled with 
mercury at 25.degree. C. and atmospheric pressure. The volumetric rise in 
the level of mercury is a direct measure of the gross volume. This method 
is known as the mercury volumenometer method, and is described in the 
Encyclopedia of Chemical Technology, Vol. 4, page 892 (Interscience 1949). 
Thus, the adsorbent (e.g., porous film) of the present invention possess a 
microporous open-celled structure, and is also characterized by a reduced 
bulk density. 
For example, suitable microporous films may be prepared in accordance with 
the processes described in U.S. Pat. No. 3,801,404, which defines a method 
for preparing microporous films herein referred to as the "dry stretch" 
method and U.S. Pat. No. 3,839,516 which defines a method for preparing 
microporous films herein referred to as the "solvent stretch" method, both 
of which are herein incorporated by reference. Each of these patents 
discloses preferred alternative routes for obtaining a microporous film by 
manipulating a precursor film in accordance with specifically defined 
process steps. 
The most preferred hydrophobic microporous films for use as supports in the 
present invention are the CELGARD.RTM. 2000 series polypropylene 
microporous films available from Celanese Separations Products, Celanese 
Corporation, Charlotte, N.C. 
In another embodiment of the present invention, the hydrophobic microporous 
support is a microporous hollow fiber. 
Again, it should be understood in characterizing the microporous hollow 
fibers of the present invention that porous or cellular fiber structures 
can be classified into two general types: one type in which the pores are 
not interconnected, i.e., a closed-cell structure, and the other type in 
which the pores are essentially interconnected through more or less 
tortuous paths which may extend from one exterior surface or surface 
region to another, i.e., an open-celled structure. The porous hollow 
fibers of the present invention are of the latter type. 
U.S. Pat. No. 4,055,696, hereby incorporated by reference, describes a 
process for the preparation of microporous polypropylene hollow fibers 
wherein a cold stretching technique is employed to prepare the hollow 
polypropylene microporous fibers. This process requires that the size of 
the pores be kept within a specified range by limiting the degree and 
temperature of cold stretch to 30 to 200% of the original fiber length and 
less than 110.degree. C., respectively. The resulting cold stretched 
fibers which have been previously anhealed are heat set at a temperature 
at or above the initial annealing temperature, employed prior to 
stretching as described above. Annealed, cold stretched, heat set, hollow 
fibers prepared in accordance with this patent tend to exhibit varying 
degrees of shrinkage depending on the relationship of the prior annealing 
temperature and duration to the heat setting temperature and duration. 
A preferred method of producing the microporous hollow fiber utilized as 
the support in present invention is disclosed in U.S. Pat. No. 4,405,688, 
hereby incorporated by reference, wherein a microporous polyolefinic 
hollow fiber is made by melt spinning a polyolefinic resin in a 
substantially vertically upward direction at a temperature of from about 
10.degree. to about 90.degree. C. above the crystalline melting point of 
the polymer into a nonporous hollow precursor fiber while contacting the 
precursor with a substantially symmetric flow of a quenching medium such 
as air or other gas, and then converting the resulting non-porous hollow 
precursor fiber into a microporous hollow fiber by stretching the 
precursor fiber and then heat setting the stretched fiber. Preferably the 
precursor fiber is also annealed prior to stretching. 
The most preferred hydrophobic microporous hollow fibers utilized as 
supports in the present invention are CELGARD.RTM. microporous hollow 
fiber available from Celanese Separations Products, Celanese Corporation, 
Charlotte, N.C. 
THE SINGLE-PLY IMMOBILIZED LIQUID MEMBRANE OF THE PRESENT INVENTION 
The single-ply, immobilized liquid membrane of the present invention 
comprises a hydrophobic microporous support which has immobilized within 
its pores an aqueous liquid membrane. The aqueous liquid membrane is 
incorporated within the support by the following exchange process. 
The hydrophobic microporous support is placed into contact with an aqueous 
solution containing from about 40 to about 95 percent by volume of an 
exchange component. The exchange component is most preferably ethyl 
alcohol but may be any water miscible liquid or mixture of liquids that, 
when mixed with water in an appropriate amount, renders the support 
spontaneously wettable by an aqueous solution. To render CELGARD.RTM. 
hydrophobic microporous films wettable, the exchange component will have a 
surface tension value of less than or equal to 35 dyne/cm at 25.degree. C. 
Preferred exchange components include methyl alcohol, acetone, and ethyl 
alcohol; with ethyl alcohol being most preferred. 
A preferred method of contacting the support with the aqueous solution of 
the exchange component is to place the support in a stream of the aqueous 
solution within a bath with gentle agitation. The support remains in 
contact with the aqueous solution until steady state is achieved. 
The support is then removed from contact with the aqueous solution of the 
exchange component and placed in contact with water, preferably, without 
appreciable surface drying, until steady state is again achieved. 
The support is removed from the water and the successive steps of 
contacting the support with an aqueous solution of an exchange component, 
removal from such contact, and contacting the support with water are 
repeated until a water membrane is immobilized within substantially the 
entire thickness of the support. Such a point may be determined 
qualitatively by observing when the support is completely transparent to 
light. A test with a UV spectrophotometer for light transmission will 
quickly verify complete transparency. 
The so-called "fully exchanged " support is a single-ply, immobilized 
liquid membrane comprising a water membrane immobilized within a 
hydrophobic microporous support. If it is desired to immobilize an aqueous 
salt solution within the support, the "fully exchanged " support may be 
placed into contact with the aqueous salt solution until an aqueous salt 
solution membrane replaces the water membrane. A preferred method of 
achieving such contact is by immersing the "fully exchanged" support in 
the aqueous salt solution for several hours. The aqueous salt solution may 
contain any ion or mixture of ions compatible with the support, and which 
promote the selective passage of a gaseous molecule through the 
immobilized liquid membrane of the present invention. These include, 
CO.sub.3.sup.-2, HCO.sub.3.sup.-1, Cl.sup.-1, I.sup.-1, SO.sub.4.sup.-2, 
ClO.sub.4.sup.-l, NO.sub.3.sup.-l, PO.sub.4.sup.-3, HPO.sub.4.sup.-1, 
S.sup.-2. 
The present invention also includes a single-ply, ultrathin immobilized 
liquid membrane, and the method for producing such a membrane. It is well 
known that to maximize the flux through any given membrane and to reduce 
the area needed for any given separation, it is advantageous to use as 
thin a membrane as possible. In the present invention, the thickness of 
the aqueous liquid membrane immobilized in the microporous hydrophobic 
support may be reduced by partially removing, e.g., by evaporation, the 
aqueous liquid in the membrane. 
This reduction in the thickness of the membrane immobilized in the support 
may be accomplished in numerous ways. Preferably, a stream of dry gas or 
partially humidified gas is blow over the immobilized liquid membrane. The 
blowing may be done on either side of the support by an gas that does not 
interact with the membrane or the support. Preferably, the side of the 
support not in contact with the gas stream is not in contact with any gas 
flow. So long as there is a gradient in moisture partial pressure from the 
surface of the support to the gas stream, evaporation will occur and the 
thickness of the aqueous liquid membrane immobilized within the support 
will be reduced. 
Alternatively, a vacuum may be applied so as to remove a portion of the 
aqueous liquid membrane. Also, the immobilized liquid membrane may be 
passed through a chamber at a controlled rate. The atmosphere of the 
chamber is maintained at conditions, e.g., elevated temperature, 
facilitating transfer of liquid from the pores of the support to the 
atmosphere of the chamber. It should be noted that if evaporation occurs 
from both sides of the support, no subsequent exchange with the liquid 
membrane is possible, i.e., the exchange of a water membrane with an 
aqueous salt solution, as the evaporated portions of the hydrophobic 
support are nonwettable by aqueous solutions that do not contain an 
exchange component. 
It is also within the ambit of the present invention to produce single-ply, 
ultrathin immobilized liquid membrane wherein an aqueous salt solution 
membrane is immobilized within a hydrophobic microporous support. Such an 
immobilized liquid membrane may be produced by contacting a support 
wherein a water membrane is immobilized within substantially the entire 
thickness of the support with an aqueous salt solution until the salt 
solution replaces the water membrane, and subsequently partially reducing, 
as described above, the thickness of the aqueous salt solution membrane 
immobilized within the support. 
Alternatively, an immobilized liquid membrane of the present invention 
wherein the thickness of the water membrane immobilized within the support 
has been partially reduced, may be placed in contact with an aqueous salt 
solution until salt solution of reduced thickness replaces the water 
membrane of reduced thickness. 
The following Examples are given as specific illustrations of the 
invention. It should be understood, however, that the invention is not 
limited to the specific details set forth in the Examples. 
EXAMPLE 1 
A CELGARD.RTM. 2400 hydrophobic microporous film 0.00254 cm thick and 5.08 
cm in diameter was placed in an aqueous solution comprising 40 percent by 
volume of ethyl alcohol. The solution was gently stirred for 1 minute and 
the film was removed. The film was placed in water without appreciable 
surface drying, gently stirred for 5 minutes, and removed. The film was 
placed in an aqueous solution comprising 40 percent by volume of ethyl 
alcohol, gently stirred for 1 minute, and transferred to a water bath. The 
water was stirred for 5 minutes. These steps of contacting the film with 
an aqueous solution comprising 40 percent by volume of ethyl alcohol for 
one minute and transferring the film to a water bath for 5 minutes was 
repeated until the film became completely transparent to light. This 
transparency indicated that the microporous film had become fully 
exchanged with water. 
EXAMPLE 2 
A microporous polyethylene film 0.00254 cm thick and 5.08 cm in diameter 
was placed in an aqueous solution comprising 40 percent by volume ethyl 
alcohol. The film was processed in the same manner as elaborated in 
Example 1, and was observed to become completely transparent to light. 
Again, this transparency indicated that the microporous film had become 
fully exchanged with water. 
EXAMPLE 3 
A microporous polytetrafluorethylene film 0.00381 cm thick was placed in an 
aqueous solution comprising 95 percent by volume ethyl alcohol. The film 
was processed in the same manner as elaborated in Example 1, and was 
observed to become completely transparent to light. This transparency 
indicated that the microporous film had become fully exchanged with water. 
EXAMPLE 4 
A fully water-exchanged CELGARD.RTM. 2400 film was immersed in an aqueous 
K.sub.2 CO.sub.3 solution for several hours with occasional gentle 
stirring. Gas permeation tests with pure N.sub.2 were subsequently run on 
the film. The resultant gas permeation rate was found to be much lower 
than water and, when compared to theoretical rates and consideration, such 
rate was indicted that the microporous film had become fully exchanged 
with the aqueous K.sub.2 CO.sub.3 solution. 
EXAMPLE 5 
A fully water-exchanged CELGARD.RTM. 2400 film 0.00254 cm thick was 
scrubbed with a dry CELGARD.RTM. 2400 film to remove any surface moisture 
and was placed in permeation cell shown in the figure. The permeation cell 
1 has a top half 2 and a bottom half 3 such that the film was disposed 
between top half 2 and bottom half 3. A pure, completely humidified (i.e., 
100% relative humidity) nitrogen feed gas at 593 cm Hg and a flow rate of 
15 cm.sup.3 /min was fed through line 4 into entrance 5 and out exit 6 of 
top half 2. A pure, completely humidified helium sweep gas at 
approximately atmospheric pressure and a flow rate of 10 cm.sup.3 /min was 
fed through line 7 into entrance 8 and at exit 9 of bottom half 3. At 
24.degree. C., a permeation rate of N.sub.2 of 1.42.times.10.sup.-3 std 
cm.sup.3 /sec for the fully water exchanged film 0.00254 cm thick was 
conventionally determined by gas chromatograph analysis of the helium 
sweep gas after the sweep gas exited permeation cell 1 at exit 9. 
The following evaporation procedure was then used to partially reduce the 
thickness of the membrane immobilized within the CELGARD.RTM. film. The 
nitrogen feed gas flow was bypassed around permeation cell 1 by opening 
bypass valve 10 and closing valves 11 and 12. A pure dry helium sweep gas 
at approximately atmospheric pressure and flowing at 15 cm.sup.3 /min was 
then fed through line 7 into entrance 8 and out exit 9 of bottom half 3. 
After 6 minutes elapsed, the sweep gas flow was stopped and restarted for 
2 minutes. The pure dry helium sweep gas flow was stopped. 
Bypass valve 10 was closed and valves 11 and 12 were opened. A pure, 
completely humidified nitrogen feed gas at 593 cm Hg and a flow rate of 15 
cm.sup.3 /min was fed through line 4 into entrance 5 and at exit 6 of top 
half 2. A pure, completely humidified helium sweep gas approximately 
atmospheric pressure and a flow rate of 10 cm.sup.3 /min was fed through 
line 7 into entrance 8 and at exit 9 of bottom half 3. At 24.degree. C. a 
permeation rate of N.sub.2 of 2.12.times.10.sup.-3 std cm.sup.3 /sec was 
conventionally determined by gas chromatograph analysis as before. From 
such a permeation rate, the thickness of the liquid membrane immobilized 
within the CELGARD.RTM. film was conventionally calculated to have been 
reduced from 0.00254 cm to 0.001675 cm. The permeation rate of N.sub.2 was 
found to be constant through this ultrathin immobilized liquid membrane 
for hours, indicating a highly stable immobilized liquid membrane. 
The same evaporation technique described above was repeated for a shorter 
period (1 or 2 minutes) and a permeation rate of N.sub.2 of 
4.26.times.10.sup.- std cm.sup.3 /sec was calculated. Such a permeation 
rate indicated that the thickness of the liquid membrane immobilized 
within the CELGARD.RTM. film had been reduced to 0.000838 cm. Again, this 
ultrathin immobilized liquid membrane was found to be highly stable. 
The same evaporation technique was repeated for a short period (1 or 2 
minutes) and a permeation rate of N.sub.2 of 9.46.times.10.sup.-3 std 
cm.sup.3 /sec was calculated. This permeation rate indicated that the 
thickness of the liquid membrane immobilized within the CELGARD.RTM. film 
had been reduced to 0.000406 cm. This ultrathin immobilized liquid 
membrane was also found to be highly stable. Thus, the final membrane 
thickness of 0.000406 cm in the film was obtained through a stepwise 
reduction of liquid film thickness starting with a fully water exchanged 
film, 0.00254 cm thick. However, the stepwise thickness reduction may be 
replaced by a one step process. 
EXAMPLE 6 
A fully water-exchanged CELGARD.RTM. film 0.00254 cm thick was placed in 
permeation cell 1 as in Example 5. A completely humidified feed gas 
mixture containing 10.1% CO.sub.2 and 89.9% N.sub.2 at approximately 175 
psig and a flow rate of 15 cm.sup.3 /min was fed through permeation cell 1 
in like manner as the feed gas in Example 5. A completely humidified 
helium gas at approximately atmospheric pressure and a flow rate of 10 
cm.sup.3 /min was passed through permeation cell 1 as the sweep gas as in 
Example 5. 
The partial pressure difference of N.sub.2, .DELTA.P(N.sub.2), across the 
film was 880.8 cm Hg while that for CO.sub.2, .DELTA.P(CO.sub.2), was 88.2 
cm Hg. A permeation rate of N.sub.2, R(N.sub.2), of 1.88.times.10.sup.-3 
std cm.sup.3 /sec was calculated while that of CO.sub.2, R(CO.sub.2), was 
5.38.times.10.sup.-3 std cm.sup.3 /sec. The separation factor between 
CO.sub.2 and N.sub.2, .alpha.CO.sub.2 --N.sub.2, defined as 
(R(CO.sub.2)/.DELTA.P(CO.sub.2)/(R(N.sub.2)/.DELTA.P(N.sub.2)) was found 
to be 28.6. 
The evaporation procedure as in Example 5 was performed except that the 
high pressure CO.sub.2 --N.sub.2 humidified feed gas mixture was fed with 
the humidified helium sweep feed gas into bottom half 3 before permeation 
rate measurements were taken. The nitrogen permeation rate R(N.sub.2), was 
calculated to be 3.96.times.10.sup.-3 std cm.sup.3 /sec, while that for 
CO.sub.2, R(CO.sub.2), was 9.88.times.10.sup.-3 std cm.sup.3 /sec. 
.DELTA.P(N.sub.2) was now found to be 888.0 cm Hg and .DELTA.P(CO.sub.2) 
was now 78.3 cm Hg. A separation factor .alpha.CO.sub.2 --N.sub.2 was 
calculated to be 28.0. Thus, the species flux had increased by two times 
indicating a reduction in the thickness of the liquid membrane from 
0.00254 cm to about 0.00127 cm. Moreover, the separation factor had 
remained almost unchanged even with a partial reduction in the thickness 
of the liquid membrane immobilized within the CELGARD.RTM. film. 
The evaporation procedure described above was repeated and R(N.sub.2) and 
R(CO.sub.2) were found to be 8.05.times.10.sup.-3 std cm.sup.3 /sec and 
14.71.times.10.sup.-3 std cm.sup.3 /sec, respectively, with 
.DELTA.P(N.sub.2)=878.4 cm Hg and .DELTA.P(CO.sub.2)=66.5 cm Hg. The value 
of .alpha.CO.sub.2 --N.sub.2 was calculated to be 24.2. The thickness of 
the liquid membrane was calculated to be about 0.00059 cm yet the 
separation factor .alpha.CO.sub.2 --N.sub.2 was basically unchanged. 
EXAMPLE 7 
A fully water-exchanged CELGARD.RTM. film 0.00254 cm thick was placed in a 
permeation cell 1 as in Example 5. A completely humidified 9.91% CO.sub.2 
--90.09% N.sub.2 feed gas mixture at approximately 102 psig and a flow 
rate of 15 std cm.sup.3 /min was fed through permeation cell 1 in like 
manner as the feed gas in Example 5. A completely humidified helium sweep 
gas at approximately atmospheric pressure and a flow rate of (15 cm.sup.3 
/min) was passed through permeation cell 1 as the sweep gas as in Example 
5. 
The .DELTA.P(N.sub.2) and .DELTA.P(CO.sub.2) values were found to be 543.2 
cm Hg and 55.70 cm Hg, respectively. A permeation rate of N.sub.2, 
R(N.sub.2), of 1.12.times.10.sup.-3 std cm.sup.3 /sec was calculated, 
while that of CO.sub.2, R(CO.sub.2), was 3.3.times.10.sup.-3 std cm.sup.3 
/sec. The separation factor, .alpha.CO.sub.2 --N.sub.2, was found to be 
28.7. 
The evaporation procedure of Example 5 was performed. The film was found to 
have R(N.sub.2)=2.5.times.10.sup.-3 std cm.sup.3 /sec and 
R(CO.sub.2)=6.1.times.10.sup.-3 std cm.sup.3 /sec. The thickness of the 
liquid membrane immobilized within the film was calculated to be about 
0.00127 cm. Separation factor .alpha.CO.sub.2 --N.sub.2 was found to be 
25.4. 
All processes were halted and the film was transferred from permeation cell 
1 to a bath of an aqueous salt solution of 30 weight percent K.sub.2 
CO.sub.3. After 3 hours the exchange was considered complete such that an 
aqueous salt solution of 30 weight percent K.sub.2 CO.sub.3 comprised a 
0.00127 cm thick liquid membrane immobilized within the film. 
The immobilized liquid membrane comprising an aqueous liquid salt solution 
membrane immobilized within the hydrophobic microporous CELGARD.RTM. film 
support was placed in permeation cell 1 and evaluated for permeation and 
separation characteristics as described above. .DELTA.T(N.sub.2) was 543.2 
cm Hg and .DELTA.P(CO.sub.2) was 54.40 cm Hg. R(N.sub.2) was found to be 
drastically reduced by almost a factor of ten to 0.28.times.10.sup.-3 std 
cm.sup.3 /sec, while R(CO.sub.2) was marginally reduced to 
4.24.times.10.sup.-3 std cm.sup.3 /sec. The separation factor 
.alpha.CO.sub.2 --N.sub.2 was increased dramatically to 151.4 
The principles, preferred embodiments and modes of operation of the present 
invention have been described in the foregoing specification. The 
invention which is intended to be protected herein, however, is not to be 
construed was limited to the particular forms disclosed, since there are 
to be regarded as illustrative rather than restrictive. Variations and 
changes may be made by those skilled in the art without departing from the 
spirit of the invention.