Refrigeration process with purge and recovery of refrigerant

A refrigeration process including a refrigeration cycle, and refrigerant purge and recovery operations is disclosed. The refrigeration cycle may be a vapor compression cycle or an absorption cycle, for example. A purge stream is withdrawn from the refrigeration cycle and subjected to treatment by means of a membrane separation unit. The purge-stream treatment operation produces an essentially pure refrigerant stream, suitable for return to the refrigeration cycle, and an air stream, clean enough for direct discharge to the atmosphere. The process is applicable to most refrigerants, but is particularly useful in minimizing atmospheric emissions of chlorofluorocarbons, such as CFC-11 and CFC-12.

FIELD OF THE INVENTION 
This invention relates to a refrigeration process. More particularly, the 
invention relates to a refrigeration process in which refrigerant/air 
mixtures are purged from the refrigeration cycle and treated by means of a 
membrane separation process to recover refrigerant and to reduce 
atmospheric pollution. 
BACKGROUND OF THE INVENTION 
Refrigeration is the use of mechanical or heat-activated machinery for 
cooling purposes. Refrigeration is commonly accomplished in a reverse 
Carnot cycle, by using as refrigerant a fluid that evaporates and 
condenses at suitable pressures and temperatures to enable practical 
equipment to be manufactured. In a vapor compression refrigeration cycle, 
the vapor is typically compressed, then condensed by chilling with air or 
water, then expanded to a low pressure and correspondingly low temperature 
through an expansion valve. Subsequent evaporation of the refrigerant 
provides the cooling action. In an absorption refrigeration cycle, cooling 
is also achieved by expansion of a high-pressure vapor into a low-pressure 
region. The resulting low-pressure vapor is absorbed into water, then 
separated from the water at high pressure in a stripper. 
Many fluids that can serve as refrigerants under appropriate conditions are 
known. Refrigerants are generally grouped into three classes, depending on 
their toxicity and flammability. Group 3 refrigerants are highly toxic or 
flammable, and are therefore used only in special circumstances, such as 
where the refrigerant is available on-site as a process or product 
chemical, and the existing hazard is not exacerbated by the use. Such 
refrigerants include hydrocarbons such as methane, propane and butane. 
Group 2 refrigerants are slightly toxic or flammable, and include ammonia, 
which is still used widely, as well as sulfur dioxide. Group 1 
refrigerants are non-toxic and non-flammable, and are, therefore, the most 
widely used over a broad spectrum of refrigeration needs. Mosts of the 
Group 1 refrigerants are halogenated hydrocarbons, containing one or more 
chlorine, fluorine or bromine atoms in their structures. For example, 
industrial refrigerators use vast quantities of CFC-12 and other 
chlorofluorocarbons (CFCs), which, although they are non-toxic and 
non-flammable, are now recognized to have a disastrous environmental 
impact. 
Refrigeration can be carried out either as a closed-cycle or open-cycle 
process. Open-cycle operation is mostly used in the chemical process 
industry, where advantage is taken of the presence in the chemical process 
of a product that can also serve as refrigerant. For example, natural gas 
liquids removed by cooling and compressing raw natural gas may be expanded 
in a refrigeration cycle to further lower the temperature of the raw gas, 
thereby recovering more of the heavier hydrocarbons. Ammonia synthesis 
plants use the product stream to refrigerate ammonia storage tanks. 
For most other industrial purposes, closed-cycle refrigerators are used. 
The refrigerant is contained in an essentially closed loop, where it 
cycles round from high-pressure vapor to high-pressure liquid to 
low-pressure liquid to low-pressure vapor. The low-pressure, evaporating 
portion of the system may be at atmospheric pressure or may be below 
atmospheric pressure, depending on the thermodynamic properties of the 
refrigerant and the cooling temperature. For practical reasons, 
refrigeration systems using CFC refrigerants are frequently operated with 
the evaporating pressure as low as 2-5 psia. 
Because a large portion of the refrigeration system is at sub-atmospheric 
pressure, air leaks into the system on the low pressure side. Air leaks 
are almost unavoidable in large industrial refrigerators; thus air 
contaminated with refrigerant vapor must be periodically purged from the 
system. In conventional purge systems, a gas stream, containing 
refrigerant and air, is withdrawn from the high-pressure side of the 
cycle. To reduce the refrigerant loss, the stream is maintained at the 
high purge pressure and then cooled, typically down to as low as 
-50.degree. F. or below. The low-temperature refrigerant can conveniently 
be used to effect the cooling. Under these conditions, the bulk of the 
refrigerant contained in the stream is condensed and passed back to the 
refrigerator. The remainder is vented to the atmosphere. The frequency and 
thoroughness with which the purging operation is carried out is dictated 
by energy and economic considerations. If the air content within the loop 
is allowed to build up over a prolonged period, the partial pressure of 
the air in the system may become substantial. As a result, the total 
compressor pressure required to maintain the refrigerant partial pressure 
at an adequate level becomes higher and higher, with a corresponding 
increase in energy consumption and costs. 
The air content of the refrigerator can be kept at a constant low level by 
continuous purging. Cooling the purge gas typically enables as much as 90% 
or more of the refrigerant to be recovered from the purge stream by 
condensation. Nevertheless the air that is vented to the environment may 
contain as much as 15% refrigerant. Running the purge-gas treatment 
condenser at pressure and temperature conditions where essentially no 
refrigerant is lost imposes an excessively heavy load on the condenser, 
consumes excessive energy, and becomes impractical economically. The need 
to drastically control or eliminate CFC emissions to the atmosphere has 
been recognized throughout the world and is the subject of increasingly 
stringent regulatory laws. CFC refrigerants, besides their environmental 
unacceptability, are becoming increasingly expensive. Refrigerator 
discharges represent a serious environmental problem and waste of 
resources. A 10 scfm condenser vent stream containing 5% or more CFC is 
typical of many that are found throughout the food and pharmaceutical 
industries, for example. Such a discharge corresponds to a CFC loss of 
0.16 lb/min, or approximately 80,000 lb/yr. When multiplied by the many 
hundred industrial refrigeration plants in use nationwide, this rate of 
loss represents a large source of CFC pollution and waste resources. Thus 
there is an urgent need to improve refrigeration technology to drastically 
reduce or eliminate CFC discharges. Similar, if less critical, concerns 
apply to other refrigerants. Because of the adverse effect on the 
operation of the refrigeration cycle, there is also a need for improved 
methods of keeping the air content of the cycle as low as possible. 
Attempts have been made to monitor and/or treat purge streams from 
refrigeration operations by various means besides condensation. For 
example, U.S. Pat. No. 4,485,289 to Lofland describes a distillation 
process for recovering CFCs from refrigerator purge streams. U.S. Pat. No. 
4,531,375 to Zinsmeyer describes a refrigeration system including means 
for monitoring a refrigerator purge system and correcting excess discharge 
of purge gases. U.S. Pat. No. 4,484,453 to Niess describes a method for 
controlling non-condensable gases at a predetermined concentration in an 
ammonia refrigerator by sensing the temperature at which the ammonia 
condenses. 
Separation of gas or vapor mixtures by means of permselective membranes has 
been known to be possible for many years, and membrane-based gas 
separation systems are emerging to challenge conventional separations 
technology in a number of areas. That membranes have the potential to 
separate organic or inorganic vapors from air is known. For example, U.S. 
Pat. No. 4,553,983, commonly owned with the present invention, describes a 
process for separating airstreams containing low concentrations of organic 
vapor (2% or less) from air, using highly organic-selective membranes. 
U.S. Pat. No. 3,903,694 to Aine describes a concentration driven membrane 
process for recycling unburnt hydrocarbons in an engine exhaust. U.S. Pat. 
No. 2,617,493 to Jones describes separation of nitrogen from concentrated 
hydrocarbon feedstreams. Pending patent application Ser. No. 327,860, now 
U.S. Pat. No. 4,906,256, commonly owned with the present invention, 
describes a membrane separation process for treating air or other gas 
streams containing fluorinated hydrocarbons, such as CFCs. 
SUMMARY OF THE INVENTION 
The invention is an improved refrigeration process, involving the 
combination of a refrigeration cycle, a purge operation to remove air or 
other non-condensable gases from the refrigerator, and treatment of the 
purged gas by a membrane separation system to recover the refrigerant. 
The refrigeration cycle is preferably a closed-cycle operation in which a 
refrigerant is brought to a low temperature, for example, either by vapor 
compression or absorption. The refrigeration cycle may take the form of 
(a) a simple cycle, in which a single refrigerant circulating in a single 
cycle is used, (b) a compound cycle in which more than one 
compression/expansion cycle is used, but a common refrigerant circulates 
throughout, or (c) a cascade, in which a series of separate refrigeration 
cycles are used to achieve successively lower temperatures. 
Any of the refrigerants known in the art may be used, including inorganic 
compounds such as ammonia, sulfur dioxide or carbon dioxide, saturated and 
unsaturated hydrocarbons such as propane, butane, ethylene or propylene, 
and halogenated hydrocarbons, such as many of the chlorofluorocarbons 
(CFCs) and hydrochlorofluorocarbons (HCFCs). The purpose of the purge 
operation is to remove air or any other non-condensable gases that have 
entered the refrigeration system. It is desirable to maintain the amount 
of air circulating with the refrigerant at a very low level, because the 
presence of air in the refrigerant vapor means that the compressor has to 
operate at higher pressures than would otherwise be necessary. As the 
level of air in the system builds up, the system becomes more and more 
inefficient. The purge operation involves withdrawing a portion of the 
refrigerant vapor, either continuously or periodically. For example, the 
vapor may be withdrawn through a pressure-actuated valve, connected in the 
high-pressure vapor portion of the cycle, that opens automatically when 
the pressure within the system exceeds a certain value. The purged vapor 
may contain from only very small amounts of non-condensable gas, such as 
less than 1%, up to a substantial percentage, say 5%, 10%, 15%, or above. 
The treatment of the purge stream is designed to recover as much of the 
refrigerant as possible, and to leave a residual air stream that is clean 
enough for discharge to the atmosphere without adverse environmental 
effects. The purge-stream treatment operation involves separating the 
refrigerant and air by running the purge gas stream across a membrane that 
is selectively permeable to the refrigerant. The refrigerant is therefore 
concentrated in the stream permeating the membrane; the residue, 
non-permeating, stream is correspondingly depleted in refrigerant. The 
driving force for permeation across the membrane is the pressure 
difference between the feed and permeate sides. If the purge stream is 
withdrawn from the high-pressure portion of the cycle, as is normally 
done, no additional driving force for membrane permeation need be 
provided. The membrane separation process produces a permeate stream 
enriched in the refrigerant compared with the feed and a residue stream 
depleted in the refrigerant. 
The efficiency of the process, in terms of the relative proportions of 
refrigerant and air in the feed, permeate and residue streams, will be 
determined by a number of factors, including the pressure difference, the 
selectivity of the membrane, the proportion of the feed that permeates the 
membrane, and the membrane thickness. The present invention is applicable 
to feedstreams with a broad range of refrigerant concentrations. Effective 
membrane separation is possible, even when the membrane selectivity is 
modest. In one possible embodiment, the process produces a permeate stream 
from which the pure liquid refrigerant can be recovered by cooling and/or 
compressing the permeate stream. The liquid refrigerant might then be 
returned to the refrigeration cycle in the liquid portion of the cycle. As 
another option, it may be possible to return the permeate vapor directly 
to the refrigeration cycle on the low-pressure vapor side. 
The membrane separation process may be configured in many possible ways, 
and may include a single membrane stage or an array of two or more units 
in series or cascade arrangements. Eighty to 99% or above removal of the 
refrigerant content of the feed to the membrane system can be achieved 
with an appropriately designed membrane separation process, leaving a 
residue stream containing only traces of the refrigerant. The permeate 
stream is typically concentrated 5- to 100-fold compared with the 
feedstream. 
The discussion above describes embodiments of the invention in which the 
membrane used to separate the refrigerant from air is preferentially 
permeable to the refrigerant. Embodiments of the invention in which the 
membrane is selectively permeable to air are also possible. In this case 
the non-permeating, or residue, stream is enriched in the refrigerant. The 
particular advantage of this method of operation is that, depending on the 
selectivity of the membrane, the nature of the refrigerant, and operating 
parameters, it may be possible to maintain a substantially lower air level 
within the refrigerator than can be economically achieved using the 
refrigerant-selective membrane options. In this case also, the membrane 
separation step may be configured as a single or multiple stage operation. 
If refrigerant-selective membranes are used, a preferred embodiment of the 
invention incorporates a purge-gas treatment step in which the purge gas 
is passed through a condenser prior to entering the membrane separation 
unit. Purge-gas streams have previously been treated by condensation and 
many industrial refrigerators are already fitted with condensers for this 
purpose. The purge gas withdrawn from the refrigerator is normally at high 
pressure, for example, around 100 psi, so simply cooling the purge gas 
will cause a fraction of the refrigerant to condense out. Conventional 
condensation units attached to refrigerators may remove up to about 90% of 
the refrigerant that passes through them. If the condenser is followed by 
a membrane separation unit, the requirement for a high degree of removal 
by the condenser may be eased. The condenser may be operated at a less 
cold temperature, thereby saving energy and costs, and yet achieving 
essentially complete recovery of the refrigerant. The membrane system can 
typically remove 90 or 95% of the refrigerant that reaches it from the 
condenser. Thus the combination of the condenser and the membrane 
separator will achieve a much higher degree of refrigerant recovery than 
could be achieved by the condenser alone. For example, suppose the 
condensation step removes only 50% of the refrigerant. If the condensation 
step is followed by a membrane separation step that can remove 80% of the 
refrigerant reaching it, then the total removal obtained by the process is 
90%. If the condensation step removes 80%, and is followed by a membrane 
separation step that also removes 80%, then the total removal obtained by 
the process is 96%. If the condensation step removes 80% and the membrane 
separation step 90%, the total removal is 98%. 
The process of the invention exhibits a number of advantages over 
conventional refrigeration processes. Membrane separation systems are 
characterized by low energy consumption compared with other separation 
techniques. The driving force for permeation through the membrane is 
provided by a pressure difference between the feed and permeate sides of 
the membrane. In the present process, the purge gas from the refrigerator 
is already at a high pressure compared with atmospheric pressure, so the 
membrane separation can be achieved in some embodiments without supplying 
any additional energy at all. The value of the additional refrigerant that 
is recovered may be substantial, particularly in the case of CFCs and 
HCFCs. Thus the process of the invention can provide a much improved 
recovery rate for the refrigerant, for example from 80% to 95%, or from 
90% to &gt;99%, coupled with a net reduction in the operating cost. Another 
significant advantage, again particularly important for CFCs and HCFCs, is 
that the amount of refrigerant discharged to the environment as vent gas 
is reduced by 90% or more. By providing a more efficient purge cycle, the 
process of the present invention also reduces the energy demand on the 
compressor in the refrigeration cycle, because it becomes easier and cost 
effective to maintain a lower circulating air content. 
It is an object of the invention to provide a refrigeration process in 
which emissions of refrigerant to the atmosphere are eliminated or 
minimized. 
It is an object of the invention to provide an improved method of treating 
refrigerator purge gases. 
It is an object of the invention to provide an energy-saving refrigeration 
process. 
It is an object of the invention to separate refrigerants from air. 
It is an object of the invention to reduce the load on a compressor used in 
a refrigeration cycle. 
It is an object of the invention to reduce the load on a condenser used to 
recover purged refrigerant. 
Other objects and advantages of the invention will be apparent from the 
description of the invention to those of ordinary skill in the art. 
It is to be understood that the above summary and the following detailed 
description are intended to explain and illustrate the invention without 
restricting its scope.

DETAILED DESCRIPTION OF THE INVENTION 
The term vapor as used herein refers to organic or inorganic chemical 
compounds in the gaseous phase below their critical temperatures. 
The term CFC as used herein refers to hydrocarbons containing at least one 
fluorine atom and one chlorine atom. 
The term HCFC as used herein refers hydrocarbons containing at least one 
fluorine atom, one chlorine atom and one hydrogen atom. 
The term hydrocarbon as used herein means saturated or unsaturated 
hydrocarbons. 
The term permselective as use herein refers to polymers, or membranes made 
from those polymers, that exhibit selective permeation for at least one 
gas or vapor in a mixture over the other components of the mixture, 
enabling a measure of separation between the components to be achieved. 
The term permeability of a polymer film means the rate at which a gas or 
vapor passes through a unit cross section of that film of unit thickness 
under a unit driving force. 
The term selectivity means the ratio of the permeabilities of two gases or 
vapors, the permeabilities being determined with a mixture of gases or 
vapors at concentrations and under operating conditions representative of 
an actual membrane separation system. 
The term air-selective means selective for nitrogen and selective for 
oxygen over the refrigerant. 
The term multilayer as used herein means comprising a support membrane and 
one or more coating layers. 
The term residue stream means that portion of the feedstream that does not 
pass through the membrane. 
The term permeate stream means that portion of the feedstream that passes 
through the membrane. 
The term stage-cut as used herein means the ratio of the membrane permeate 
volume flow to the membrane feed volume flow. 
The term membrane unit as used herein means one or more membrane modules 
arranged in parallel, so that a portion of the incoming stream passes 
through each one. 
The term series arrangement means an arrangement of membrane modules or 
units connected together such that the residue stream from one module or 
unit becomes the feedstream for the next. 
The term cascade arrangement means an arrangement of membrane modules or 
units connected together such that the permeate stream from one module or 
unit becomes the feedstream for the next. 
The term membrane array means a set of one or more individual membrane 
modules or membrane units connected in a series arrangement, a cascade 
arrangement, or mixtures or combinations of these. 
The term product residue stream means the residue stream exiting a membrane 
array when the membrane separation process is complete. This stream may be 
derived from one membrane unit, or may be the pooled residue streams from 
several membrane units. 
The term product permeate stream means the permeate stream exiting a 
membrane array when the membrane separation process is complete. This 
stream may be derived from one membrane unit, or may be the pooled 
permeate streams from several membrane units. 
All percentages cited herein are by volume unless specifically stated 
otherwise. 
The refrigeration process of the invention is a combination of a 
refrigeration cycle, withdrawing a purge stream, and treating the purge 
stream. The refrigeration cycle of the invention is a conventional 
mechanical cycle of either the vapor or the absorption type. The vapor 
cycle includes as basic elements a compressor, a condenser, an expansion 
valve and an evaporator, in that order. The compressor raises the pressure 
of the refrigerant vapor so that its saturation temperature is above the 
temperature of the coolant in the condenser. A transfer of heat from the 
refrigerant vapor to the coolant takes place, and the refrigerant 
condenses. The condensed liquid passes through the expansion valve to a 
low-pressure region of the cycle, where its saturation temperature is 
below the temperature of the material to be refrigerated. A transfer of 
heat takes place to the refrigerant, causing the refrigerant to evaporate. 
The low pressure vapor is removed by the compressor and the cycle 
continues. The compressors and condensers used in the refrigeration cycle 
may take any of the common forms known in the art. For example, the 
compressor may be of the reciprocating or centrifugal types. Multiple 
compressors arranged in series or parallel may be used. The condenser may 
conveniently be water cooled, such as a basic shell-and-tube condenser, or 
air cooled, in which case air may be blown over the condenser by propeller 
fans. The evaporator may also be of the shell-and-tube design, where the 
exchange surfaces are either flooded or sprayed with refrigerant liquid. 
In an absorption cycle, the refrigerant, usually ammonia, is alternately 
absorbed into water, then stripped from the water at high pressure. The 
cycle includes the stripper, a condenser, an expansion valve, an 
evaporator and an absorber. Ammonia at high pressure is withdrawn from the 
top of the stripper, condensed and passed through an expansion valve to 
form a low-pressure liquid. As in the vapor cycle, the liquid then passes 
through the evaporator. The resulting low-pressure vapor is absorbed into 
water in a conventional absorber, and the resulting liquid is pumped back 
to the stripper. 
The construction and operation of refrigeration cycles is well known in the 
art. For more details, the four-volume handbook published by the American 
Society of Heating, Refrigerating and Ventilating Engineers, Publications 
Dept., Atlanta, Ga., provides comprehensive information on refrigerator 
design considerations and equipment. 
The refrigeration cycle used in the process of the invention may be simple, 
complex or cascade. 
Any refrigerant may be used in the process of the invention. Representative 
refrigerants include, from Group 1, carbon dioxide, CFC-11, CFC-12, 
CFC-13, CFC-22, CFC-23, CFC-113 and CFC-114, HCHF-123, HCFC-142b and 
HCFC-134a from Group 2, carbon tetrachloride, ammonia and sulfur dioxide, 
and from Group 3, methane, ethane, propane, butane, isobutane, ethylene 
and propylene. 
The purge stream, containing the refrigerant and air that has entered the 
refrigeration cycle, is withdrawn from the refrigeration cycle either 
continuously or periodically. The rate of withdrawal can be chosen, 
depending on the amount of air leakage into the system and the level to 
which the air content within the cycle is to be reduced or at which it is 
to be maintained. It is preferable to withdraw the purge continuously, or 
for lengthy periods, if possible, so that the membrane unit can also 
operate without the need for frequent shut-down and start-up. A valve is 
used to control the refrigerant withdrawal. The valve may be manually 
operated, or may be automatically opened at certain time intervals or at a 
chosen pressure, for example. The purge stream contains refrigerant and 
air, preferably, although not necessarily, with air as the minor component 
of the stream. Typically the air content of the purge stream may range 
from less than 1% to 50%. Preferably, the purge stream is withdrawn from 
the high-pressure vapor segment of the refrigeration cycle. In a cascade 
cycle, separate purge streams may be withdrawn from each stage. 
A membrane system is used to treat the purge stream. The permeability of a 
gas or vapor through a membrane is a product of the diffusion coefficient, 
D, and the Henry's law sorption coefficient, k. D is a measure of the 
permeant's mobility in the polymer; k is a measure of the permeant's 
sorption into the polymer, and depends in part on the condensability of 
the vapor. The diffusion coefficient tends to decrease as the molecular 
size of the permeate increases, because large molecules interact with more 
segments of the polymer chains and are thus less mobile. Depending on the 
nature of the polymer, either the diffusion or the sorption component of 
the permeability may dominate. In rigid, glassy polymer materials, 
diffusion is usually the predominant factor controlling permeation, so 
glassy membranes tend to permeate nitrogen or oxygen, for example, faster 
than the larger organic molecules. For embodiments of the invention 
requiring an air-selective membrane, therefore, membranes made from glassy 
polymers are preferred. 
In elastomeric membrane materials, the effect of the sorption coefficient 
can be dominant, so that the condensable refrigerant will permeate the 
membrane much faster than nitrogen or oxygen. For refrigerant-selective 
embodiments of the invention, therefore, elastomeric membrane materials 
are preferred. Hydrophobic elastomers are preferred for hydrophobic 
refrigerants; for hydrophilic refrigerants, such as ammonia, carbon 
dioxide and sulfur dioxide, more hydrophilic materials may be more 
suitable. 
In the process of the present invention, the purge stream from the 
refrigerator cycle, which may optionally have been subjected to 
condensation to recover a portion of the refrigerant, and which contains 
refrigerant vapor and air is passed across a thin, permselective membrane. 
The permselective membrane forms a barrier that is relatively permeable to 
one component of the stream, but relatively impermeable to the other. The 
membrane may take the form of a homogeneous membrane, a membrane 
incorporating a gel or liquid layer, or any other form known in the art. 
To achieve a high flux of the permeating components, the permselective 
membrane should be made as thin as possible. Preferred embodiments of the 
invention involve the use of a composite membrane comprising a microporous 
support, onto which the permselective layer is deposited as an ultrathin 
coating. The microporous support membrane should have a flow resistance 
that is very small compared to the permselective layer. A preferred 
support membrane is an asymmetric membrane, consisting of a relatively 
open, porous substrate with a thin, dense, finely porous skin layer. 
Suitable polymers for making asymmetric support include polysulfones, 
polyimides, polyamides and polyvinylidene fluoride. Asymmetric polysulfone 
and polyimide membranes available commercially for ultrafiltration 
applications are also suitable as supports. Isotropic support membranes, 
such as microporous polypropylene or polytetrafluoroethylene may also be 
used in some cases. The thickness of the support membrane is not critical, 
since its permeability is high compared to that of the permselective 
layer. However the thickness would normally be in the range 100 to 300 
microns, with about 150 microns being the preferred value. 
Optionally, the support membrane may be reinforced by casting it on a 
fabric or paper web, made from polyester or the like. The multilayer 
membrane then comprises the web, the microporous membrane, and the 
ultrathin permselective membrane. 
The permselective membrane is deposited on the dense skin of the support 
membrane, for example by dip coating. The dip coating method is described, 
for example, in U.S. Pat. No. 4,243,701 to Riley et al. In a typical 
dip-coating process, the support membrane from a feed roll is passed 
through a coating station, then to a drying oven, and is then wound onto a 
product roll. The coating station may be a tank containing a dilute 
polymer or prepolymer solution, in which a coating typically 50 to 100 
microns thick is deposited on the support. Assuming a 1% concentration of 
polymer in the solution, after evaporation a film 0.5 to 1 micron thick is 
left on the support. 
Alternatively, the permselective membrane may be cast by spreading a thin 
film of the polymer solution on the surface of a water bath. After 
evaporation of the solvent, the permselective layer may be picked up onto 
the microporous support. This method is more difficult in practice, but 
may be useful if the desired support is attacked by the solvent used to 
dissolve the permselective material. 
The thickness of the permselective layer should normally be in the range 
0.1 to 20 microns, preferably 5 microns or less, and more preferably 0.1 
to 2 micron. 
Preferred polymers for use as refrigerant-selective membranes include 
rubbery non-crystalline polymers, i.e. those that have a glass transition 
temperature below the normal operating temperature of the system. 
Thermoplastic elastomers are also useful. These polymers combine hard and 
soft segments or domains in the polymer structure. Provided the soft 
segments are rubbery at the temperature and operating conditions of the 
invention, polymers of this type could make suitable membranes for use in 
the invention. Polymers that may be used include, but are not limited to, 
nitrile rubber, neoprene, polydimethylsiloxane (silicone rubber), 
chlorosulfonated polyethylene, polysilicone-carbonate copolymers, 
fluoroelastomers, plasticized polyvinylchloride, polyurethane, 
cis-polybutadiene, cis-polyisoprene, poly(butene-1), polystyrene-butadiene 
copolymers, styrene/butadiene/styrene block copolymers, 
styrene/ethylene/butylene block copolymers, thermoplastic polyolefin 
elastomers, and block copolymers of polyethers, polyamides and polyesters. 
To maximize the flux of permeating components, the permselective layer 
should be made as thin as possible. However, the permselective layer must 
also be free of pinholes or other defects that could destroy the 
selectivity of the membrane by permitting bulk flow-through of gases. In 
the context of the invention, a particularly preferred rubber is silicone 
rubber. Silicone rubber solutions can wet a finely microporous support and 
leave a uniform, defect-free coating after solvent evaporation, so the 
preferred membrane is one in which the permselective coating is deposited 
directly on the microporous support. However optional embodiments that 
include additional sealing or protective layers above or below the 
permselective layer are also intended to be encompassed by the invention. 
Preferred polymers for use as air-selective membranes include glassy 
materials such as polysulfones, polyimides, polyamides, polyphenylene 
oxide, polycarbonates, ethylcellulose or cellulose acetate. Glassy 
materials are more difficult than elastomers to form into thin-film 
composite membranes. Preferred embodiments of the process employing 
air-selective membranes use asymmetric glassy membranes in which the thin, 
dense skin serves as the permselective layer. Such membranes are known in 
the gas-separation art, and may be prepared, for example, by various 
modifications of the Loeb-Sourirajan process. This process involves 
preparing a solution of the polymer in a suitable solvent, casting a thin 
film and then immersing the film in a precipitation bath. The resulting 
membrane has an asymmetric structure graded from openly microporous on the 
support surface to non-porous or very finely microporous on the skin side. 
Such gas-separation membranes are frequently overcoated with a sealing 
layer on the skin side, to prevent bulk flow of gases through pores or 
other defects. The preparation and properties of asymmetric gas-separation 
membranes are described, for example, in U.S. Pat. No. 4,230,463 to Henis 
and Tripodi, or U.S. Pat. No. 4,840,646 to Dow Chemical. 
The permselective membranes used in the present invention should preferably 
have a selectivity for refrigerant/nitrogen or nitrogen/refrigerant of at 
least 5, and more preferably at least 10, and most preferably at least 20. 
Table 1 lists experimentally measured selectivities for a number of common 
refrigerants. In each case, the membrane used was a thin-film composite 
membrane with a silicone rubber permselective layer. The measurements were 
made at 20.degree. C. As a general rule, lowering the temperature will 
increase the selectivity and vice versa. For refrigerants such as 
CHF.sub.3, where the membrane selectivity is only 4 at room temperature, a 
much better membrane performance would be obtained by performing the 
membrane separation operation at low temperature. As shown in the table, 
the membrane selectivity increases to 14 at -39.degree. C. 
TABLE 1 
______________________________________ 
Membrane Selectivity for Refrigerant Over Nitrogen for 
Common Refrigerants, measured with a silicone rubber membrane 
at 20.degree. C. 
Boiling point 
Selectivity 
Refrigerant Group (.degree.C.) 
(at 20.degree. C.) 
______________________________________ 
Inorganic Compounds 
Ammonia 2 -33 20 
Sulfur dioxide 
2 -10 50 
Carbon dioxide 
1 -18 11 
Hydrocarbons 
Methane 3 -161 3 
Ethane 3 -89 10 
Propane 3 -42 20-40 
Butane 3 0 70-100 
Isobutane -12 70-100 
Ethylene 3 -104 2-4 (est) 
Propylene 3 -48 15-20 
Chlorinated 
Hydrocarbons 
CCl.sub.4 2 77 100-200 
CCl.sub.3 F 1 24 30-50 
CCl.sub.2 F.sub.2 
1 -30 6 
CClF.sub.3 1 -81 0.6 
CHClF.sub.2 1 -41 15 
CHF.sub.3 1 -82 4 (14 at -39.degree. C.) 
CCl.sub.2 FCClF.sub.2 
1 48 20 
CClF.sub.2 CClF.sub.2 
1 4 9-11 
C.sub.2 HCl.sub.2 F.sub.3 
1 25 
C.sub.2 H.sub. 3 ClF.sub.2 
1 13-15 
______________________________________ 
The form in which the membranes are used in the invention is not critical. 
They may be used, for example, as flat sheets or discs, coated hollow 
fibers, or spiral-wound modules, all forms that are known in the art. 
Spiral-wound modules are a preferred choice. References that teach the 
preparation of spiral-wound modules are S. S. Kreman, "Technology and 
Engineering of ROGA Spiral Wound Reverse Osmosis Membrane Modules", in 
Reverse Osmosis and Synthetic Membranes, S. Sourirajan (Ed.), National 
Research Council of Canada, Ottawa, 1977; and U.S. Pat. No. 4,553,983, 
column 10, lines 40-60. Alternatively the membranes may be configured as 
microporous hollow fibers coated with the permselective polymer material 
and then potted into a module. 
The flux of a gas or vapor through a polymer membrane is proportional to 
the pressure difference of that gas or vapor across the membrane. To 
achieve high fluxes of the permeating components, it is desirable not only 
to make the permselective membrane very thin, but also to operate the 
system with a substantial pressure drop across the membrane. The purge gas 
stream is withdrawn from the high-pressure vapor segment of the 
refrigeration cycle. The purge gas stream is therefore normally at a 
pressure substantially above atmospheric, and may be at a pressure as high 
as 100 psia or 200 psia. Consequently an adequate driving force for many 
embodiments of the invention may be provided by keeping the permeate side 
of the membrane at atmospheric pressure and using the high pressure 
inherently available in the refrigeration cycle. The performance of the 
membrane system depends not only on the membrane selectivity and the 
pressure drop across the membrane, but also on the ratio feed pressure: 
permeate pressure. It can be shown theoretically that, even for an 
infinitely selective membrane, the concentration of the preferentially 
permeating component on the permeate side of the membrane can never be 
greater than .phi. times the concentration in the feed, where .phi. is 
feed pressure/permeate pressure. To achieve an adequate pressure ratio 
with the permeate pressure at atmospheric requires that the feed pressure 
preferably be above about 60 psi. In many cases, the purge gas withdrawn 
from the refrigeration cycle will be at a pressure substantially above 60 
psi. If the feed pressure to the membrane is not sufficiently high to 
provide a useful pressure ratio, then a pressure drop across the membrane 
can be provided by drawing a partial vacuum on the permeate side of the 
membrane. Subatmospheric pressure on the permeate side can also be 
sustained in some cases simply by continuously condensing and withdrawing 
the permeate stream. 
In embodiments using refrigerant-selective membranes, the residue stream 
will be the air stream. The refrigerant content of the air should be 
reduced to a level at which the air can be vented to the atmosphere with 
minimal loss of refrigerant or environmental pollution. Preferably the 
residue stream should contain less than 10%, more preferably less than 5% 
of the refrigerant that was in the feed to the membrane unit. If 
air-selective membranes are used, the permeate will be the air stream and 
similarly should be clean enough for discharge. 
The process of the invention can be carried out using membrane system 
designs tailored to particular requirements in terms of the composition of 
the feed to the membrane unit, and the desired compositions of the residue 
and permeate streams. The purge gas stream may optionally be subjected to 
a condensation step to recover a substantial portion of the refrigerant, 
followed by the membrane treatment step. Some representative embodiments 
of the invention are described below. These embodiments are illustrative 
of workable configurations, but are not intended to limit the scope of the 
invention in any way. Those of skill in the refrigeration or membrane arts 
will appreciate that many other embodiments in accordance with the 
invention are possible. 
REPRESENTATIVE EMBODIMENTS USING REFRIGERANT-SELECTIVE MEMBRANE 
1. Purge-gas treatment step comprises condensation followed by membrane 
separation: 
A preferred mode for carrying out the invention is to subject the withdrawn 
purge gas to a condensation step that precedes the membrane treatment 
step. If the purge gas contains a high percentage of refrigerant, and is 
at a high pressure, both of which will usually be the case, then cooling 
the gas stream will cause a fraction of the refrigerant process using this 
treatment scheme is shown in FIG. 1. Referring now to this figure, the 
refrigeration cycle, 1, is a single vapor cycle. Compressor, 2, creates a 
region of high-pressure refrigerant vapor, 3. The vapor passes into a 
heat-exchange zone, 4, where heat is given off to a coolant and the vapor 
condenses to create a high-pressure liquid zone, 5. The refrigerant then 
passes through expansion valve, 6, to a low-pressure liquid zone, 7. Heat 
exchange between the product to be refrigerated and the refrigerant takes 
place in evaporator zone, 8. The resulting low-pressure vapor in zone, 9, 
is recompressed and the cycle starts again. A high-pressure purge stream, 
12, is withdrawn from the refrigeration cycle through outlet, 10, and 
valve, 11. The purge stream is passed through condenser, 13, which may be 
simply a water or air-cooled condenser operating at above 0.degree. C., or 
may be refrigerated, either by making use of the existing refrigeration 
cycle or by means of a separate, smaller refrigerator. Condenser 
temperatures down to about -45.degree. C. can be reached in a single-cycle 
chilling operation. If a lower condenser temperature is used, a compound 
or cascade system could be used. This would be a very undesirable mode of 
operation, because of the complexity and high energy consumption, unless 
the refrigeration cycle itself were a compound or cascade cycle, through 
which the purge stream could easily be fed. The presence of the membrane 
treatment unit means that the amount of refrigerant removed by the 
condenser is not a critical factor in the design. The combined 
condensation/membrane separation treatment step may be tailored so that 
the condensation step can be performed above 0.degree. C. This can be 
advantageous in situations where the purge gas stream contains water 
vapor, for example, in embodiments using an absorption refrigeration 
cycle, because the need to defrost the condenser regularly will then be 
avoided. On the other hand, the selectivity of some polymers for organic 
vapors over air increases with decreasing temperature. In cases where the 
refrigerant selectivity at room temperature is poor, therefore, a better 
separation may be obtained in the membrane step by chilling the purge gas 
to a relatively low temperature before it passes through the membrane 
unit. 
The fraction of refrigerant remaining in the purge gas stream after the 
condensation step depends on the vapor/liquid equilibrium at the operating 
conditions under which the condensation step is performed. If a 
condensation step is used, it is generally preferable that the 
condensation step be designed to remove at least 50% of the refrigerant 
present in the withdrawn purge stream. Operation under extreme conditions, 
to achieve 95% or more refrigerant removal is usually unnecessary, because 
of the presence of the membrane step. The overall degree of condensable 
removal and recovery that can be achieved by the combined 
condensation/membrane separation step is a multiple of the removal 
achieved in the individual steps. For example, suppose the condensation 
step removes 50% of the refrigerant. If the condensation step is followed 
by a membrane separation step that can remove 80% of the refrigerant 
reaching it, then the total removal obtained by the process is 90%. If the 
condensation step removes 80%, and is followed by a membrane separation 
step that also removes 80%, then the total removal obtained by the process 
is 96%. If the condensation step removes 80% and the membrane separation 
step 90%, the total removal is 98%. 
The above discussion is intended to show that the process can be tailored 
to achieve a desired degree of refrigerant in a highly efficient manner. 
The tailoring can be done by comparing estimates of the energy and dollar 
costs with several sets of system configurations and operating conditions. 
For example, the costs and energy requirements to achieve 95% total 
removal, using an initial condensation step removing 50, 75 or 90% of 
refrigerant component, followed by a membrane separation step removing 90, 
80 or 50% of the remaining refrigerant, could be compared. 
The liquid refrigerant stream, 14, from the condenser may be drawn off for 
reuse in the refrigeration cycle. Stream, 15, which is at high pressure 
compared with atmospheric and which contains non-condensed refrigerant and 
air, passes to membrane unit, 16, containing one or more membranes that 
are selectively permeable to the refrigerant. The permeate stream, 18, is 
therefore enriched in refrigerant compared with stream 15. If the 
selectivity is high, and the stream from the condenser is not too dilute, 
permeate stream, 18, may be sufficiently concentrated to be fed back to 
the refrigeration cycle. FIGS. 2 and 3 show such options. The residue 
stream, 17, from the membrane operation, which may still be at above 
atmospheric pressure, is sufficiently reduced in refrigerant concentration 
that it can be discharged to the atmosphere. Preferably this residue 
stream contains less than 1% refrigerant, more preferably less than 0.5% 
refrigerant. 
2. Permeate vapor is returned to refrigeration cycle: 
Referring now to FIG. 2, this is a variation of the embodiment of FIG. 1. 
In this case, the permeate stream, 19, is withdrawn from the membrane 
separation operation and returned directly to the low-pressure vapor zone 
of the refrigeration cycle. This option requires no additional compressor 
or condenser to handle the permeate stream. On the other hand, because 
membranes are not infinitely selective, the permeate stream will always 
contain some air that will, therefore, reenter the refrigeration cycle, 
and have to pass through the main compressor again. 
3. Permeate vapor is recompressed, then returned to condenser: 
Referring now to FIG. 3, permeate stream, 20, is compressed in compressor, 
20, and returned to the condenser, 15. This may be preferable to feeding 
the permeate vapor directly back to the refrigeration cycle, because 
compressor, 20, is relatively small, and no air is fed back to the 
refrigeration cycle. The overall energy consumption of this system may 
therefore be less than that of FIG. 2. 
REPRESENTATIVE EMBODIMENTS USING AIR-SELECTIVE MEMBRANE 
1. Air-selective membrane treatment only: 
Other embodiments of the invention involve a membrane treatment step in 
which air-selective membranes are used to directly treat the purge gas 
stream, without a prior condensation. A basic form of such an embodiment 
is shown in FIG. 4. Referring now to this figure, the refrigeration cycle 
and the purge withdrawal operate in the same manner as in the embodiments 
of FIGS. 1, 2 and 3. The purge stream, 12, at high pressure compared with 
atmospheric pressure, is passed directly to membrane unit, 22, containing 
an air-selective membrane. The permeate stream, 24, is thus enriched in 
nitrogen and oxygen compared with stream 12. The residue stream contains 
concentrated refrigerant. If the purge gas concentration, membrane 
selectivity and operating parameters are such that a very good separation 
is achieved in the simple, single-stage membrane operation, it may be 
possible to vent the permeate stream and to return the residue stream, 
with or without liquifying it, to the refrigeration cycle, as in the 
refrigerant-selective embodiments of FIGS. 2 and 3. Unless extremely 
selective membranes, having a selectivity for nitrogen over refrigerant of 
200, 500 or even more are available, it is likely that the permeate 
stream, at least, will require further treatment. However, in a membrane 
separation process, it is usually desirable to keep the stage-cut low to 
achieve a good separation. Thus the volume of the permeate stream is 
normally much smaller than the volume of the feed and residue streams. An 
advantage of air-selective embodiments such as FIG. 4, therefore, is that 
the volume of the permeate air stream containing refrigerant vapor that 
must be condensed or otherwise treated is very small compared with the 
total purge gas volume. This means it may become economically viable to 
operate the refrigeration cycle with the air content maintained at a lower 
level than was previously possible, thereby reducing the head pressure 
that must be achieved by the refrigerant cycle compressor. 
2. Combination of air-selective and refrigerant-selective membranes: 
The purge gas stream as it is withdrawn from the refrigeration cycle may 
typically contain 90% refrigerant and 10% air, for example. If the purge 
stream is passed across a membrane having a selectivity for nitrogen over 
the refrigerant of about 20, for example, then even if the stage-cut is 
kept low, the permeate stream may still contain 20-30% refrigerant. This 
level is far too high to discharge the permeate stream directly to the 
atmosphere. However if the permeate stream, 24, is now passed through a 
refrigerant-selective membrane unit, the refrigerant concentration can be 
reduced to a point where discharge is possible. The residue stream from 
the refrigerant-selective membrane operation could now be discharged, and 
the permeate stream from the refrigerant-selective unit could be recycled 
to the feed side of the air-selective unit. 
In embodiments incorporating an air-selective membrane unit followed by a 
refrigerant-selective membrane unit, it will normally be necessary to 
lower the pressure on the permeate side of the refrigerant-selective unit 
below atmospheric pressure, because the feed to this unit will be at, or 
close to, atmospheric pressure. 
3. Combination of air-selective membrane and condensation: 
a) Purge-gas treatment step comprises membrane treatment followed by 
condensation step: 
This process is the same as that described in embodiment 1 as far as the 
refrigeration cycle, the purge gas withdrawal and the passage through the 
air-selective membrane are concerned. In this case, however, the permeate 
stream, 24, is optionally recompressed and then cooled, preferably using 
the cooling portion of the refrigerant cycle, to recover a stream of 
liquid refrigerant suitable for return to the refrigeration cycle. The 
non-condensed gases, containing only very small quantities of refrigerant 
vapor, may now be discharged. 
b) Purge-gas treatment step comprises membrane treatment preceded by 
condensation step: 
To maximize the advantages of the embodiments of the invention described 
above, using air-selective membranes in the membrane treatment step, it is 
very desirable to use membranes that are highly selective for nitrogen 
over the refrigerant vapor. 
However, if membranes with selectivities for nitrogen over refrigerant of 
10 or 20, for example, are available, it is still possible to design 
useful embodiments, such as that shown in FIG. 5. Referring now to this 
figure, the refrigeration cycle and the purge gas withdrawal operation are 
as described for FIG. 4. In this case, however, purge stream, 12, at high 
pressure compared with atmospheric pressure, passes to condenser, 13. As 
in the refrigerant-selective embodiments, this condenser may be a water or 
air-cooled condenser operating at above 0.degree. C., or may be 
refrigerated, either by making use of the existing refrigeration cycle or 
by means of a separate, smaller refrigerator. Condenser temperatures down 
to about -45.degree. C. can be reached in a single-cycle chilling 
operation; lower temperatures require a compound or cascade system. As 
with the refrigerant-selective options, it is convenient and cheap to use 
the existing refrigeration cycle to chill the condenser. Liquid 
refrigerant stream, 14, from the condenser is suitable for reuse. The 
non-condensed fraction, 15, from the condenser, which is at high pressure 
compared with atmospheric, is fed to membrane unit, 16, containing an 
air-selective membrane. The permeate stream, 28, from the membrane unit is 
mostly air containing only a very low concentration of refrigerant vapor. 
The residue stream, 29, contains refrigerant vapor at above atmospheric 
pressure. FIG. 5 shows an option in which this stream is returned to the 
high-pressure vapor segment of the refrigeration cycle. A recirculation 
blower or pump may optionally be used to maintain adequate flow of the 
recirculating vapor stream. Alternatively stream 29 could be liquified and 
returned to the liquid segment of the refrigeration cycle. 
It may be seen from the above discussions that the purge-gas treatment 
operation may be configured in many different ways, tailored to achieve a 
highly efficient and economic recovery of refrigerant, and to minimize the 
atmospheric discharge of waste refrigerant vapors. Depending on the 
refrigerant that is used, the operating conditions of the refrigeration 
cycle, and the ability to use existing compression and/or condensation 
equipment, many different workable and practical embodiment could be 
designed. The goal of all modes of operation is that the purge-gas 
treatment operation produce only two streams: one a vent stream 
sufficiently free of refrigerant vapor that its discharge to the 
atmosphere has no adverse environmental effects; the other a product 
stream containing sufficiently pure refrigerant for return to the 
refrigeration cycle. 
For simplicity, all the refrigeration processes discussed above have been 
described in terms of a simple, single-stage membrane operation. As will 
be appreciated by those of skill in the art, the membrane separation 
operation may be configured in many possible ways, and may include a 
single membrane stage or an array of two or more units in series or 
cascade arrangements. For example, a membrane array consisting of a 
two-stage cascade is shown schematically in FIG. 6. This type of membrane 
configuration could be used, for example, when the purge stream has been 
first subjected to condensation, and where the non-condensed gas from the 
condenser contains the refrigerant in a low concentration, such that a 
single pass through a membrane unit would not concentrate the refrigerant 
vapor to make it return to the refrigeration cycle desirable. 
Referring now to FIG. 6, incoming purge stream, 30, containing refrigerant 
and air, passes to first membrane separation unit, 31, which contains 
membranes selectively permeable to the refrigerant. The non-permeating, 
residue stream, 32, is thus depleted in the refrigerant. The permeate 
stream, 33, is enriched in refrigerant, but still contains significant 
amounts of nitrogen and oxygen. The permeate from the first membrane unit, 
now at atmospheric pressure, is fed to second membrane unit, 34. A 
pressure difference across the second membrane unit is provided by vacuum 
pump, 35. The permeate stream, 36, from the second membrane unit is now 
highly concentrated in refrigerant and can be condensed in condenser, 37, 
to produce a liquid refrigerant stream, 38. Any non-condensed fraction, 
39, can be recycled to the second membrane unit. The residue stream, 40, 
from the second membrane unit, depleted in refrigerant compared with 
stream 33, may optionally be recycled to the feed side of the first 
membrane unit. In this way the membrane operation produces only two 
streams, the liquid refrigerant stream, 38, suitable for return to the 
refrigeration cycle, and the essentially clean residue stream, 32, for 
discharge. 
A second membrane array, consisting of a two-step series arrangement, is 
shown schematically in FIG. 7. This type of membrane unit could be used, 
for example, when essentially complete removal of refrigerant before 
venting is required. A two-step process will typically remove 99% or more 
of the refrigerant reaching it. Referring now to FIG. 7, incoming purge 
gas stream, 42, containing refrigerant and air, is passed to first 
membrane separation unit, 43, which contains membranes selectively 
permeable to the refrigerant. The permeate stream, 49, is enriched in the 
refrigerant and can be optionally liquified by means of compressor, 50, 
and condenser, 51, to yield a liquid refrigerant stream, 52, suitable for 
return to the refrigeration cycle. The residue stream, 44, is depleted in 
refrigerant compared with stream 42, but still contains too much 
refrigerant for discharge. Stream 44 is therefore fed to second membrane 
unit, 45. The residue stream, 46, from the second membrane unit is now 
sufficiently clean for discharge. The permeate stream, 48, from the second 
membrane unit, enriched in refrigerant compared with stream 42, may be 
recompressed by compressor, 47, and recycled to the feed side of the first 
membrane unit. In this way the membrane operation produces only two 
streams, the liquid refrigerant stream, 52, and the relatively clean air 
stream, 46. 
Multiple-stage and multiple-step membrane operations, and combinations of 
these, could be used with embodiment using refrigerant-selective or 
air-selective membranes. 
The invention is now further illustrated by the following examples, which 
are intended to be illustrative of the invention, but are not intended to 
limit the scope or underlying principles of the invention in any way. The 
example are divided into two groups. The first group, Examples 1-13 shows 
the separation performance that can be achieved for a number of common 
refrigerants using a typical thin-film composite membrane. The second 
group, Examples 14-27, shows typical performances achieved in the purge 
stream treatment operation. 
EXAMPLES 
Group 1 Examples 
Examples 1-11. Experimental results with refrigerant-selective membranes. 
Experimental Procedure 
All sample feedstreams were evaluated in a laboratory test system 
containing one membrane module with a permselective silicone rubber 
membrane and membrane area of approximately 2,000 cm.sup.2. The air in the 
feed cycle was replaced with nitrogen from a pressure cylinder prior to 
the experiment. Nitrogen was continuously fed into the system during the 
experiment to replace the lost nitrogen into the permeate. Solvent vapor 
was continuously fed into the system by either pumping liquid solvent into 
the residue line using a syringe pump and evaporating the solvent using 
additional heating, or sending a bypass stream of the residue through a 
wash bottle containing the liquid solvent. The feed and residue organic 
concentrations were determined by withdrawing samples from the appropriate 
lines by syringe and then subjecting these to gas chromatograph (GC) 
analysis. A small bypass stream was used to take the samples at 
atmospheric pressure instead of the elevated pressure in the lines. Two 
liquid nitrogen traps were used to condense the solvent contained in the 
permeate stream. For long-term experiments, a non-lubricated rotary-vane 
vacuum pump was used on the permeate side of the module. The samples from 
the permeate stream were taken using a detachable glass vessel constantly 
purged with a bypass stream of the permeate. Upon sampling, the vessel was 
detached and air was allowed to enter the vessel. The concentration in the 
vessel was determined by gas chromatography. The permeate concentration 
was then calculated from the relationship: 
##EQU1## 
The procedure for a test with the system was as follows: 
1. The system was run without solvent under maximum permeate vacuum to 
replace the air in the loop with nitrogen. 
2. The nitrogen permeate flow rate was determined by measuring the vacuum 
pump exhaust flow rate. This provided a quality check on the module. 
3. The feed flow, feed pressure and permeate pressure were adjusted to the 
desired values. The cold trap was filled with liquid nitrogen. 
4. The solvent input was started and the feed concentration was monitored 
with frequent injections into the GC. The permeate pressure was adjusted 
if necessary. 
5. The system was run until the feed analysis showed that steady state had 
been reached. 
6. All parameters were recorded and a permeate sample was taken and 
analyzed. 
7. Step 6 was repeated after 10-20 minutes. The feed concentration was 
monitored after each parameter change to ensure steady state had been 
reached. 
EXAMPLE 1 
CFC-11. Low Concentrations 
The experimental procedures described were carried out using a feedstream 
containing CFC-11 (CCl.sub.3 F) in concentrations from 100-2,000 ppm. The 
results are summarized in FIGS. 8 and 9. The calculated CFC/N.sub.2 
selectivity of the module increased slightly from 22 at 100 ppm to 28 at 
2,000 ppm. As can be seen from FIG. 6, up to about 4 lb/m.sup.2.day of 
CFC-11 could be recovered, even from a very dilute stream in a very simple 
one-step process. 
EXAMPLE 2 
CFC-11. Higher Concentrations 
The experimental procedures described were carried out using a feedstream 
containing CFC-11 (CCl.sub.3 F) in concentrations from 1-35 vol %. The 
results are summarized in FIGS. 10 and 11. The calculated CFC/N.sub.2 
selectivity of the module increased from 30 at 1 vol % to 50 at 35 vol %. 
This effect may be attributable to plasticization of the membrane material 
by sorbed hydrocarbon. Both hydrocarbon and nitrogen fluxes increased with 
increasing hydrocarbon feed concentration. 
EXAMPLE 3 
CFC-113. Low Concentrations 
The experimental procedures described were carried out using a feedstream 
containing CFC-113 (C.sub.2 Cl.sub.3 F.sub.3) in concentrations from 
500-2,000 ppm. The results are summarized in FIG. 12. The calculated 
CFC/N.sub.2 selectivity of the module remained constant at about 20 over 
the feed concentration range. 
EXAMPLE 4 
CFC-113. Higher Concentrations 
The experimental procedures described were carried out using a feedstream 
containing CFC-113 (C.sub.2 Cl.sub.3 F.sub.3) in concentrations from 0.5-6 
vol %. The results are summarized in FIG. 13. The calculated CFC/N.sub.2 
selectivity of the module remained constant at about 25 over the feed 
concentration range. 
EXAMPLE 5 
HCFC-123. Low Concentrations 
The experimental procedures described were carried out using a feedstream 
containing HCFC-123 (C.sub.2 HCl.sub.2 F.sub.3) in concentrations from 
500-2,000 ppm. The results are summarized in FIG. 14. The calculated 
CFC/N.sub.2 selectivity of the module remained constant at about 25 over 
the feed concentration range. 
EXAMPLE 6 
HCFC-123. Higher Concentrations 
The experimental procedures described were carried out using a feedstream 
containing HCFC-123 (C.sub.2 HCl.sub.2 F.sub.3) in concentrations from 
0.5-8 vol %. The results are summarized in FIG. 15. The calculated 
CFC/N.sub.2 selectivity of the module remained constant at about 25 over 
the feed concentration range. 
EXAMPLE 7 
HCFC-142b. 
The experimental procedures described were carried out using a feedstream 
containing HCFC-142b (C.sub.2 H.sub.3 ClF.sub.2) in concentrations from 
300-3,500 ppm. The results are summarized in FIG. 16. The calculated 
CFC/N.sub.2 selectivity of the module increased very slightly from 13 to 
15 over the feed concentration range. 
EXAMPLE 8 
CFC-114 
The experimental procedures described were carried out using a feedstream 
containing CFC-114 (C.sub.2 Cl.sub.2 F.sub.4) in concentrations from 2-25 
vol %. The results are summarized in FIG. 17. The calculated CFC/N.sub.2 
selectivity of the module increased very slightly from about 9 to 12 over 
the feed concentration range. 
EXAMPLE 9 
Halon-1301 
The experimental procedures described were carried out using a feedstream 
containing Halon-1301 (CF.sub.3 Br) in concentrations from 0.1-5 vol %. A 
Halon/nitrogen selectivity of about 4 was obtained. 
EXAMPLE 10 
Carbon Dioxide 
Membranes particularly suited to the separation of carbon dioxide, sulfur 
dioxide and ammonia from air can be prepared from commercially available 
poly(ether amide ester) block copolymers. A thin-film composite membrane 
was prepared by coating a 1-3% solution of Pebax.RTM. 4011 (Atochem, Inc.) 
in butanol onto a polysulfone microporous support membrane. Permeability 
experiments were carried out using a feed stream containing 8% carbon 
dioxide in an air mixture. The carbon dioxide flux at 61.degree. C. was 
6.05.times.10.sup.-4 cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg. 
The calculated carbon dioxide/nitrogen selectivity was 26. 
EXAMPLE 11 
Sulfur Dioxide 
A thin-film composite membrane was prepared as in Example 10. Permeability 
experiments were carried out using a feed stream containing 0.33% sulfur 
dioxide in an air mixture. The sulfur dioxide flux at 61.degree. C. was 
6.12.times.10.sup.-3 cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg. 
The calculated sulfur dioxide/nitrogen selectivity was 251. 
EXAMPLE 12 
Air-selective Membranes 
Asymmetric Loeb-Sourirajan membranes were prepared using a casting solution 
of 14.4 wt % polyethersulfone (Victrex 52009ICI Americas) dissolved in 
47.9% methylene chloride, 24% 1,1,2-trichloroethane, 6% formic acid and 
7.7% butanol. The casting solution was spread on a glass plate using a 
hand-held spreader roll. The glass plate was then immersed in a methanol 
bath, causing the polymer to precipitate. After the precipitation was 
complete, the membranes were removed and dried. The membranes were 
overcoated with a 0.5- to 2-.mu.m-thick layer of silicone rubber dissolved 
in octane. This silicone rubber layer sealed the membrane defects and the 
permselectivity of the membrane was then close to the intrinsic values 
obtained with thick isotropic films of the polymer. 
Permeation experiments were carried out as above, using various dilute 
mixtures of CFC-11 in air. The membranes exhibited an oxygen flux of 
2.85.times.10.sup.-6 cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg, a 
nitrogen flux of 7.45.times.10.sup.-7 cm.sup.3 (STP)/cm.sup.2 
.multidot.s.multidot.cmHg and a CFC-11 flux of 4.75.times.10.sup.-8 
cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg. The nitrogen/CFC-11 
selectivity was 16. 
EXAMPLE 13 
Air-selective Membranes 
Asymmetric Loeb-Sourirajan membranes were prepared by a similar technique 
to that described in Example 12, but using a casting solution of 10 wt % 
polyphenylene oxide dissolved in 85% 1,1,2-trichloroethylene and 5% 
octanol. The casting solution was spread on a glass plate using a 
hand-held spreader roll. The glass plate was then immersed in a methanol 
bath, causing the polymer to precipitate. After the precipitation was 
complete, the membranes were removed and dried. The membranes were 
overcoated with a 0.5- to 2-.mu.m-thick layer of silicone rubber dissolved 
in octane. This silicone rubber layer sealed the membrane defects and the 
permselectivity of the membrane was then close to the intrinsic values 
obtained with thick isotropic films of the polymer. 
Permeation experiments were carried out as above, using various dilute 
mixtures of CFC-11 in air. The membranes exhibited an oxygen flux of 
1.20.times.10.sup.-6 cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg, a 
nitrogen flux of 2.90.times.10.sup.-7 cm.sup.3 (STP)/cm.sup.2 
.multidot.s.multidot.cmHg and a CFC-11 flux of 3.68.times.10.sup.-9 
cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg. The nitrogen/CFC-11 
selectivity was 79. 
GROUP 2 EXAMPLES 
EXAMPLES 14-18 
Design and Analysis of Different Purge-Gas Treatment Operations 
This set of examples compares treatment of a CFC-11 laden stream by 
condensation alone and by the purge-gas treatment operation of the 
invention. Examples 14-16 are not in accordance with the invention. The 
stream has a flow rate of 10 scfm and contains 50% CFC-11 in all cases. 
The membrane calculations are all based on CFC-11 selectivities determined 
in single module experiments of the type described in the first group of 
examples. The calculations were performed using a computer program based 
on the gas permeation equations for cross flow conditions described by 
Shindo et al., "Calculation Methods for Multicomponent Gas Separation by 
Permeation," Sep. Sci. Technol. 20, 445-459 (1985). The membrane area 
required was generated by the computer program. The chiller capacity was 
extrapolated from product literature provided by Filtrine Manufacturing 
Company, of Harrisville, N.H. The capacities of the vacuum pumps and 
compressors were obtained or extrapolated from performance specification 
charts and other data from the manufacturers. Energy calculations were 
done by calculating the adiabatic ideal work of compression and dividing 
by the efficiency of the unit. Compressor efficiency was taken to be 60%: 
vacuum pump efficiency was taken to be 35%. 
EXAMPLE 14 
Compression to 5 Atmospheres Plus Chilling to 7.degree. C. 
The CFC-11 laden purge stream is considered to be at a pressure of 5 
atmospheres, and is chilled to 7.degree. C. and condensed. The performance 
is characterized as shown in Table 2. 
TABLE 2 
______________________________________ 
Stream Composition Flow rate 
______________________________________ 
Feed 50% CFC-11 in air 
10 scfm 
Liquid condensate 
Pure CFC-11 0.77 kg/min 
Non-condensed off- 
10.9% CFC-11 5.6 scfm 
gas from condenser: 
Fractional recovery of CFC from feed: 88% 
Energy requirement (hp) 
Total: 2.96 Compressor: 1.96 
Chiller/condenser: 1 
______________________________________ 
EXAMPLE 15 
Compression to 25 Atmospheres Plus Chilling to 7.degree. C. 
The CFC-11 laden purge stream is compressed to 25 atmospheres, then chilled 
to 7.degree. C. and condensed. The performance is characterized as shown 
in Table 3. 
TABLE 3 
______________________________________ 
Stream Composition Flow rate 
______________________________________ 
Feed 50% CFC-11 in air 
10 scfm 
Liquid condensate 
Pure CFC-11 0.86 kg/min 
Non-condensed off- 
2.18% CFC-11 5.11 scfm 
gas from condenser: 
Fractional recovery of CFC from feed: 98% 
Energy requirement (hp) 
Total: 6.14 Compressor: 5.04 
Chiller/condenser: 1.1 
______________________________________ 
EXAMPLE 16 
Compression to 5 Atmospheres Plus Chilling to -27.degree. C. 
This example achieves the same performance as Example 15 above, by taking 
the purge gas at 5 atm pressure, but using a lower chiller temperature of 
-27.degree. C. The performance is characterized as shown in Table 4. 
TABLE 4 
______________________________________ 
Stream Composition Flow rate 
______________________________________ 
Feed 50% CFC-11 in air 
10 scfm 
Liquid condensate 
Pure CFC-11 0.86 kg/min 
Non-condensed off- 
2.18% CFC-11 5.11 scfm 
gas from condenser: 
Fractional recovery of CFC from feed: 98% 
Energy requirement (hp) 
Total: 7.46 Compressor: 1.96 
Chiller/condenser: 5.5 
______________________________________ 
EXAMPLE 17 
Purge-Gas Treatment Operation In Accordance With An Embodiment of The 
Invention 
A process was designed to achieve the same level of performance as in 
Examples 15 and 16. The process involved a condensation step followed by a 
membrane separation step. In the condensation step, the CFC-11 laden 
stream, at 5 atmospheres pressure, is chilled to 7.degree. C. and 
condensed. The non-condensed off-gas from the condensation step is then 
subjected to a membrane separation step, using a membrane with a 
selectivity for CFC-11 over air of 30. A pressure drop across the membrane 
is provided only by the elevated pressure of the compressed feed. The 
permeate stream from the membrane separation step is returned for 
treatment in the condensation step. The performance is characterized as 
shown in Table 5. 
TABLE 5 
______________________________________ 
CONDENSATION STEP: 
Stream Composition Flow rate 
______________________________________ 
Feed 50% CFC-11 in air 
10 scfm input + 3.33 
input + 24.3% from 
scfm returned from 
membrane = 43.6% 
membrane step = 
13.33 scfm 
Liquid condensate 
Pure CFC-11 0.86 kg/min 
Condenser off-gas 
10.9% CFC-11 8.44 scfm 
______________________________________ 
MEMBRANE SEATION STEP: 
Stream Composition Flow rate 
______________________________________ 
Feed 10.9% CFC-11 8.44 scfm 
Residue 2.18% CFC-11 5.11 scfm 
Permeate 24.3% CFC-11 3.33 scfm 
Membrane area: 4.17 m.sup.2 
Stage cut: 40% 
Fractional recovery of CFC from feed: 98% 
Energy requirement (hp) 
Total: 3.91 
Compressor: 2.61 
Chiller/condenser: 1.3 
______________________________________ 
Comparing this example with Examples 15 and 16, it may be seen that the 
process of the invention can reduce the energy demands for a treatment 
system to remove and recover 98% of the CFC from either 7.46 hp or 6.14 hp 
to 3.91 hp. In other words, the energy usage of the process is only 52% or 
64% that of the comparable condensation process alone. 
EXAMPLE 18 
Purge-Gas Treatment Operation Employing the Process of the Invention 
The process as in Example 17 was again considered. The only difference was 
the inclusion of a small vacuum pump on the permeate side of the membrane 
to lower the permeate pressure to 15 cmHg. The performance is 
characterized as shown in Table 6. 
TABLE 6 
______________________________________ 
CONDENSATION STEP: 
Stream Composition Flow rate 
______________________________________ 
Feed 50% CFC-11 in air 
10 scfm input + 1.16 
input + 49.3% from 
scfm returned from 
membrane = 49.9% 
membrane step = 
11.16 scfm 
Liquid condensate 
Pure CFC-11 0.86 kg/min 
Condenser off-gas 
10.9% CFC-11 6.27 scfm 
______________________________________ 
MEMBRANE SEATION STEP: 
Stream Composition Flow rate 
______________________________________ 
Feed 10.9% CFC-11 6.27 scfm 
Residue 2.18% CFC-11 5.11 scfm 
Permeate 49.3% CFC-11 1.16 scfm 
Membrane area: 0.81 m.sup.2 
Stage cut: 18% 
Fractional recovery of CFC from feed: 98% 
Energy requirement (hp) 
Total: 3.93 
Compressor: 2.19 
Chiller/condenser: 1.3 
Vacuum pump 0.44 
______________________________________ 
Comparing this example with Example 17, several differences are apparent. 
To reduce the residue concentration to 2.18% in Example 17 requires a 
relatively high stage cut of 40%. The permeate volume flow is high, 3.33 
scfm, so a more powerful compressor is needed to handle the additional 
load returned from the membrane unit. The membrane area, 4.17 m.sup.2, is 
also large. The use of a vacuum pump to lower the pressure on the permeate 
side means that the same degree of CFC removal can be achieved with a much 
smaller membrane area, 0.81 m.sup.2, and a much lower stage cut, 18%. 
There is a corresponding saving in the energy requirements of the 
compressor. However, the energy used by the vacuum pump makes the overall 
energy demand of the system about the same in both cases. Both schemes 
achieve major improvements in performance compared with condensation 
alone. 
EXAMPLES 19-21 
This set of examples compares treatment of a gas stream containing sulfur 
dioxide in air by condensation alone and by a representative purge-gas 
treatment operation in accordance with the invention. The stream has a 
flow rate of 10 scfm and contains 50% sulfur dioxide in all cases. The 
calculations are performed in similar manner to those for the CFC-11 
examples. The membrane calculations were based on the performance of 
composite membranes having a permselective layer of polyamide-polyether 
block copolymer. The membrane selectivity for sulfur dioxide over air was 
taken to be 100, and the normalized sulfur dioxide flux was 
6.times.10.sup.-3 cm.sup.3 (STP)/cm.sup.2 .multidot.s.multidot.cmHg. 
EXAMPLE 19 
Compression to 8 Atmospheres Plus Chilling to 6.degree. C. 
The sulfur dioxide laden purge stream is considered to be available at 8 
atmospheres pressure, and is chilled to 6.degree. C. and condensed. The 
boiling point of sulfur dioxide is -10.degree. C., so under these 
conditions 25% sulfur dioxide remains in the vent gas from the condenser. 
The performance is characterized as shown in Table 7. 
TABLE 7 
______________________________________ 
Stream Composition Flow rate 
______________________________________ 
Feed 50% SO.sub.2 in air 
10 scfm 
Liquid condensate 
Pure SO.sub.2 
0.3 kg/min 
Non-condensed 25% SO.sub.2 6.25 scfm 
off-gas from condenser: 
______________________________________ 
EXAMPLE 20 
Compression to 40 Atmospheres Plus Chilling to 6.degree. C. 
The sulfur dioxide laden stream is compressed to 40 atmospheres, then 
chilled to 6.degree. C. and condensed. The sulfur dioxide content of the 
vent gas is reduced to 5% under these conditions, but the energy and cost 
requirements of the system are more than double those of Example 19. The 
performance is characterized as shown in Table 8. 
TABLE 8 
______________________________________ 
Stream Composition Flow rate 
______________________________________ 
Feed 50% SO.sub.2 in air 
10 scfm 
Liquid condensate 
Pure SO.sub.2 
0.38 kg/min 
Non-condensed 5% SO.sub.2 5.26 scfm 
off-gas from condenser: 
______________________________________ 
EXAMPLE 21 
Purge-Gas Treatment Operation in Accordance With the Invention 
A process was designed employing the condensation step exactly as in 
Example 19, followed by a membrane separation step to treat the 
condensation step vent gas stream, using a membrane with a selectivity for 
sulfur dioxide over air of 100. A pressure drop across the membrane is 
provided only by the elevated pressure of the compressed feed. The 
performance is characterized as shown in Table 9. 
TABLE 9 
______________________________________ 
Stream Composition Flow rate 
______________________________________ 
CONDENSATION STEP 
Feed 50% SO.sub.2 in air 
10 scfm 
Liquid condensate 
Pure SO.sub.2 
0.40 kg/min 
Non-condensed 25% SO.sub.2 6.25 scfm 
off-gas from condenser: 
MEMBRANE SEATION STEP: 
Feed 25% SO.sub.2 6.25 scfm 
Residue 1.0% SO.sub.2 
5.05 scfm 
Permeate 55.6 SO.sub.2 
1.20 scfm 
______________________________________ 
The permeate from the membrane separation step is richer in sulfur dioxide 
content than the original gas stream to be treated, and can be returned 
for treatment by the condensation step. The process is able to reduce the 
concentration of sulfur dioxide in the vented gas stream from 25% to 1%, 
with no extra energy consumption whatsoever, because the driving force for 
membrane permeation is provided by the relatively high pressure of the 
already compressed feed. 
EXAMPLE 22 
Propylene/Ethylene Cascade Refrigeration Cycle 
A two-stage cascade refrigeration cycle employing ethylene and propylene is 
used to produce refrigeration at -145.degree. F. A system of this type is 
described in FIG. 8.26, page 226 in Chemical Process Equipment Handbook, 
Butterworth's Series in Chemical Engineering. In this system, propylene 
vapor in the first stage is compressed to 245-250 psia and cooled with 
water to 116.degree. F. forming the liquid propylene. This liquid is then 
expanded to 16 psia to produce a cold vapor. This cold vapor is passed 
through a heat exchanger and provides cooling to liquify ethylene vapor at 
a pressure of 230-240 psia in the second stage of the cascade. Expansion 
of this ethylene liquid to 12 psia produces an ethylene vapor at a 
temperature of -145.degree. F. The low-pressure portion of the ethylene 
cycle is subject to air leaks. Suppose that the purge stream from the 
ethylene cycle contains 2-10% air. Typically, this purge stream will first 
be cooled to -140.degree. F. in a purge-gas condenser. At a purge gas 
pressure of 240 psia, the condensation operation will produce a liquid 
ethylene stream and a non-condensed stream, consisting of 90% air and 10% 
ethylene, at a rate of approximately 10 scfm. This pressurized vent gas is 
most economically passed across the surface of a silicone rubber membrane. 
This membrane is 8 times more permeable to ethylene than nitrogen and 4 
time more permeable to ethylene than oxygen. The membrane thus 
fractionates the gas into a 5.2 scfm residue stream containing 99% air and 
1% ethylene, which can be discharged to the atmosphere, and a low-pressure 
permeate stream containing 19.7% ethylene and 80.3% air. The permeate 
stream may be returned directly to the low-pressure side of the 
refrigeration cycle, or recompressed to 240 psia and introduced in front 
of the -140.degree. F. purge-gas condenser. 
EXAMPLE 23 
Ammonia Refrigeration Cycle 
Ammonia is often used as a refrigerant in compression refrigeration systems 
to provide cooling in the 20.degree. to -50.degree. F. range. In these 
systems, non-condensable gases collect in high pressure side of the cycle 
and must be removed as a purge stream. Consider, for example, a 
refrigerator using ammonia with a condenser liquid ammonia temperature on 
the high-pressure side of 95.degree. F. At this temperature the vapor 
pressure of ammonia is 197 psia. However, not uncommonly, the actual 
operating pressure will be on the order of 210 psia. The extra 13 psia 
represents 6% non-condensable gases (air, hydrogen, nitrogen, etc.). This 
gas must be purged at a rate determined by the rate of appearance of 
non-condensable gas in the refrigeration cycle. Suppose that the purge-gas 
stream, containing 6% air or other non-condensable gases, is first 
subjected to a condensation step using cooling to -40.degree. C. provided 
by the refrigeration cycle. The ammonia concentration in the vent gas 
leaving the condenser will be 4.9%. The condenser vent gas is then passed 
to a membrane separation unit, containing a thin-film composite membrane 
with a silicone rubber permselective layer. Such a membrane has a 
selectivity of 20 for ammonia over nitrogen and 10 for ammonia over 
oxygen. Depending on the stage-cut, the membrane operation could produce a 
residue stream containing 0.5% ammonia, and a permeate stream containing 
16% ammonia, down to a residue stream containing about 0.05% ammonia, and 
a permeate stream containing 10% ammonia. By using a two-step process, the 
concentration of ammonia in the residue stream could be reduced even 
further if necessary. 
EXAMPLE 24 
CFC-12 Recovery 
Consider an embodiment of the invention as shown in FIG. 1, with CFC-12 as 
refrigerant. Based on our experimental data, we assume a membrane 
selectivity for CFC-12 over air of 6-10. Consider a purge gas stream 
containing 10 scfm air, contaminated with 67 scfm of CFC-12, to produce an 
87% CFC-12 stream. The stream emerges from the purge withdrawal operation 
at 90 psia, and is first passed through a condenser operating at 
-60.degree. F. The condenser reduces the CFC content of the gas to 5% 
CFC-12. This resulting stream is passed to a membrane unit, which 
selectively permeates the CFC. As a result, a 10 scfm stream containing 
0.5% CFC-12 is formed and can be vented. The permeate stream from the 
membrane separation operation contains 11% CFC-12, and could be 
recompressed with a small compressor and passed back to the cold 
condenser. 
As an alternative, the CFC-12 content of the vent stream could be reduced 
to 0.05% by using a two-step membrane separation operation as shown in 
FIG. 7. In this case the permeate from the second step could be returned 
to the inlet of the first step. The purge stream treatment operation still 
only produces two streams therefore: the vent gas stream containing 0.05% 
CFC-12, and the liquid CFC-12 stream from the condensation step. 
EXAMPLE 25 
Purge-Gas Treatment Using Air-Selective Membrane Step 
Consider an embodiment of the invention as shown in FIG. 4, used to treat a 
purge-gas stream at a pressure of 100 psig, containing 90% of an 
unspecified refrigerant, mixed with 9.4% nitrogen and 0.6% oxygen. Assume 
that the membrane selectivity for oxygen over nitrogen is 4, and that the 
membrane unit is run with a stage-cut of 1%. Assume further that, to 
increase the pressure difference across the membrane, a vacuum pump is 
used on the permeate side of the membrane to lower the pressure on the 
permeate side to 1 cmHg. Using a computer model based on the computational 
methods of Shindo et al. (Shindo et al., "Calculation Methods for 
Multicomponent Gas Separation by Permeation," Sep. Sci. Technol. 20, 
445-459 (1985)), the amount of refrigerant remaining in the permeate 
stream from the membrane unit was calculated as a function of the membrane 
selectivity. The results are summarized in Table 10. 
TABLE 10 
______________________________________ 
Refrigerant in vent stream as a function of membrane selectivity 
for air-selective membranes used to treat refrigerator purge gas. 
Selectivity Refrigerant in exhaust 
N.sub.2 /refrigerant 
(%) 
______________________________________ 
10 44.4 
20 28.9 
50 14.2 
100 7.7 
200 4.0 
500 1.64 
1,000 0.83 
______________________________________ 
It may be seen that extremely air-selective membranes are required to 
reduce the refrigerant content of the purge gas to an acceptable level in 
a single pass. 
EXAMPLE 26 
Purge-Gas Treatment Using Air-Selective Membrane Step and Condensation Step 
Consider an embodiment of the invention as shown in FIG. 5, in which a 
condensation step is used to treat the purge-gas stream prior to the 
membrane separation step. Suppose that as a result, the refrigerant 
content of the stream being passed to the membrane separation operation is 
reduced to 5%. Suppose that the feed to the membrane separation step 
remains at 100 psig, and that a vacuum pump is used as before to lower the 
permeate pressure to 1 cmHg. The calculation as in Example 22 was 
repeated, and the results are summarized in Table 11. 
TABLE 11 
______________________________________ 
Refrigerant in vent stream as a function of membrane selectivity 
for air-selective membranes used to treat refrigerator purge gas. 
Selectivity Refrigerant in exhaust 
N.sub.2 /refrigerant 
(%) 
______________________________________ 
10 0.45 
20 0.23 
50 0.090 
100 0.045 
200 0.0225 
500 0.0090 
1,000 0.0045 
______________________________________ 
As can be seen from the Table, the residue of refrigerant remaining in the 
vent gas is now reduced to an extremely low level, even when membranes 
with modest selectivities are used. 
EXAMPLE 27 
Purge-Gas Treatment Using Air-Selective Membrane Step and Condensation Step 
The calculation of Example 26 was repeated, the only difference being that 
no vacuum pump was used, so that the pressure on the permeate side of the 
membrane remained at 76 cmHg. The results are summarized in Table 12. 
TABLE 12 
______________________________________ 
Refrigerant in vent stream as a function of membrane selectivity 
for air-selective membranes used to treat refrigerator purge gas. 
Selectivity Refrigerant in exhaust 
N.sub.2 /refrigerant 
(%) 
______________________________________ 
10 0.53 
20 0.27 
50 0.11 
100 0.054 
200 0.027 
500 0.011 
1,000 0.0054 
______________________________________ 
As can be seen from the Table, the residue of refrigerant remaining in the 
vent gas is reduced to an extremely low level, even when membranes with 
modest selectivities are used, and a smaller pressure drop is available.