Membrane process and apparatus for removing a component from a fluid stream

A pervaporation process and system for removing a component from a liquid stream. The process includes a pervaporation separation step and a recovery step. An auxiliary membrane module or set of modules is installed across a condenser and/or recovery unit on the downstream side of the main pervaporation unit. This module takes as its feed a stream from the recovery unit and returns a component-enriched stream to the inlet of the condenser or recovery unit. The module can be sized to produce a discharge stream containing the component in about the same concentration as the feed to be treated. This discharge stream can then be mixed with the feed without adverse effect on the efficiency of the system.

BACKGROUND OF THE INVENTION 
Vapor Separation 
Gas streams containing condensable vapors, such as water vapor, sulfur 
dioxide, ammonia or organic vapors, arise from numerous industrial and 
commercial processes. One method of removing the vapor from the gas stream 
is by means of a membrane separation step, followed by condensation of the 
vapor-enriched stream from the membrane separation step. 
A typical membrane vapor separation system includes a membrane unit, a pump 
for lowering the pressure on the permeate side of the membrane, and a 
condenser for liquefying the vapor. Membrane processes for removing vapors 
from gas streams are described, for instance, in U.S. Pat. Nos. 3,903,694, 
4,553,983 and 4,906,256, which all deal with removal of organic vapor from 
air or other gases, U.S. Pat. No. 4,444,571, which deals with removal of 
water vapor from gas streams, and U.S. Pat. Nos. 4,606,740 and 4,608,060, 
which describe membranes for removing polar gases such as hydrogen 
sulfide, sulfur dioxide and ammonia from other gases. 
In a vapor removal process characterized by membrane separation followed by 
condensation, the vapor concentration in the condenser vent gas after the 
condensation step depends on the vapor/liquid equilibrium at the operating 
conditions under which the condensation is performed. It is frequently the 
case that the condenser vent gas contains a much higher concentration of 
vapor than the original feed gas. The vent gas is often recirculated to 
the feed side of the membrane unit for further treatment. This type of 
scheme, performed via an oven, is shown for example, in U.S. Pat. Nos. 
4,553,983. 
There are several problems associated with returning the condenser vent gas 
to the membrane feed. First, the more concentrated is the vent gas 
compared with the feed gas, the less efficient the system becomes. 
Suppose, for example, the feed gas contains 2% vapor, the vapor-enriched 
stream from the membrane separation step contains 20% vapor, and the vent 
gas from the condenser contains 10% vapor. Then about half of the amount 
of vapor removed and concentrated by the membrane is recirculated to the 
front of the membrane. Much of the separation achieved by membrane is then 
negated, resulting in increased membrane area and pump capacity 
requirements for the system. 
What can be done to handle the condenser vent gas stream is to pass it to a 
second membrane stage. This stage can be designed to produce a discharge 
stream with a concentration about the same as the original feed, so as to 
minimize the impact of the recycled stream on the total process. The 
vapor-enriched stream from the second membrane stage is in turn condensed, 
and the vent gas from the second condenser is returned to the feed of the 
second membrane unit. Such an arrangement is shown, for example, in U.S. 
Pat. No. 4,906,256, FIG. 3. A two-stage system is complex compared with a 
one-stage, uses more controls and is more costly, since two sets of most 
components are needed. 
Pervaporation 
The discussion above concerns separations in which the feed to the membrane 
system is a gas or vapor. However, a similar situation obtains when 
components are removed from liquid streams by pervaporation. In 
pervaporation, the liquefied permeate may be subjected to recovery or 
further purification, by decantation, distillation, solvent extraction or 
adsorption, for example, and the impure, non-product stream from the 
decanter or other recovery unit may be passed to the feed side of the 
pervaporation unit for further treatment. This type of scheme is shown, 
for example, in U.S. Pat. No. 5,030,356. Mixing the non-product stream 
from the recovery unit with the raw, incoming feed solution can give rise 
to problems similar to those discussed above for gas or vapor separation. 
SUMMARY OF THE INVENTION 
The invention relates to an improved and advantageous arrangement of 
membrane modules that allows better performance of both vapor separation 
and pervaporation systems. 
Vapor Separation 
The invention is a vapor recovery system and process that permits condenser 
vent gas to be recirculated, using less energy and less membrane area than 
a one-stage membrane system, and without the complexity and cost of a 
two-stage system. In the system of the invention, a small auxiliary 
membrane module or set of modules is installed across the pump and 
condenser on the downstream side of the main membrane unit. This module 
takes as its feed the vent gas from the condenser, and returns a 
vapor-enriched stream upstream of the pump and condenser. If desired, the 
module can be sized to produce a discharge stream containing the vapor in 
about the same concentration as the feed to be treated. This discharge 
stream may then be mixed with the feed without adverse effect on the 
efficiency of the system. 
Using the arrangement of the invention can achieve substantial improvements 
in the performance and efficiency of a conventional one-stage vapor 
removal system, without the necessity of resorting to a two-stage system, 
with attendant complications and cost. 
In another aspect, the invention is a system and process that combines 
membrane vapor separation with any recovery system for the vapor. Besides 
condensation, the recovery process could be an extraction process, a 
physical or chemical absorption process or an adsorption process, for 
example. In each case, waste gas from the recovery process could be passed 
through the auxiliary module(s) before return to the main membrane unit or 
elsewhere. 
Pervaporation 
The invention is a system and process that combines pervaporation with any 
further recovery or purification method, such as decantation, 
distillation, extraction or adsorption. In each case, the non-product 
stream from the recovery or purification unit could be passed through an 
auxiliary membrane module, or set of modules, before returning to the main 
pervaporation unit or elsewhere. 
In the pervaporation embodiments of the invention, the efficiency 
considerations may be different from those in the gas separation 
embodiments. The auxiliary module(s) may be installed across the condenser 
and the recovery unit on the downstream side of the main pervaporation 
unit. In this case, the driving force for membrane permeation in the 
auxiliary module(s) may be provided by the main condenser. Alternatively, 
the auxiliary module(s) may be installed across the recovery unit only and 
provided with their own driving force, most simply a second condenser. 
This arrangement can offer advantages over a one-stage or a two-stage 
system, particularly if a three-component feed liquid is to be treated. 
It is an object of the invention to provide a process for removing vapors 
from gas streams. 
It is an object of the invention to improve the performance of membrane 
vapor removal systems and processes. 
It is an object of the invention to provide efficient membrane/condensation 
vapor removal processes in which the condenser vent gas is recirculated to 
the membrane unit. 
It is an object of the invention to provide membrane/condensation vapor 
removal processes in which the condenser vent gas composition is adjusted. 
It is an object of the invention to provide a pervaporation process. 
It is an object of the invention to improve the performance of 
pervaporation systems and processes. 
It is an object of the invention to provide efficient 
pervaporation/purification processes in which the non-product stream from 
the purification process is recirculated to the pervaporation unit. 
It is an object of the invention to provide pervaporation/purification 
processes in which the composition of the non-product stream from the 
purification process is adjusted. 
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 
Vapor Separation 
The term vapor as used herein refers to a liquefiable component of a gas 
stream. 
In the process of the invention, a feed gas stream containing a vapor is 
passed through a membrane separation step and a recovery step. For 
convenience, the figures and their descriptions show a recovery step 
achieved by condensation of the vapor. However, other recovery steps, 
including various kinds of extraction, absorption and adsorption steps are 
also within the scope of the invention. The vapor may be of an organic 
compound or mixture of compounds, such as a hydrocarbon, a halogenated 
hydrocarbon or the like, or an inorganic compound, such as water, sulfur 
dioxide, ammonia, etc. 
The other component or components of the feed gas stream may be other 
vapors, nitrogen, air or any other gas. 
A basic embodiment of the invention is shown in FIG. 1. Referring to this 
figure, a vapor-containing feed gas stream, 1, passes to a membrane 
separation unit, 2, containing one or more membranes. The membrane 
separation step normally involves running the feed gas stream across a 
membrane that is selectively permeable to the vapor that is to be removed. 
The vapor is concentrated in the stream, 4, permeating the membrane; the 
residue, non-permeating, stream, 3, is correspondingly depleted in vapor. 
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. 
Two types of membrane are preferred for use in the invention. The first is 
a composite membrane comprising a microporous support, onto which the 
permselective layer is deposited as an ultrathin coating. Composite 
membranes are preferred when a rubbery polymer is used as the 
permselective material. The second is an asymmetric membrane in which the 
thin, dense skin of the asymmetric membrane is the permselective layer. 
Both composite and asymmetric membranes are known in the art. References 
that teach the production of such membranes include U.S. Pat. Nos. 
4,243,701; 4,553,983; 4,230,463; and 4,840,646. 
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. Kremen, "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 driving force for membrane permeation is the pressure difference 
between the feed and permeate sides. The pressure drop across the membrane 
can be achieved by pressurizing the feed, by evacuating the permeate or by 
both. In FIG. 1, the feed gas is supplied to the membrane separation unit 
at atmospheric pressure or slightly above, and a vacuum pump, 5, is used 
to lower the pressure on the permeate side. 
Stream, 6, from the vacuum pump is subjected to a condensation step. In 
general, the condensation step may involve chilling, compression or a 
combination of these. In FIG. 1, the vapor-enriched stream passes without 
compression to condenser, 7. The condenser may be water cooled, or may 
employ refrigerants that can take the gas down to lower temperatures, and 
produces a stream, 8, of liquified vapor. 
The amount of vapor that can be removed from the vapor-enriched stream by 
condensation depends on the vapor concentration, the vapor/liquid 
equilibrium and the operating conditions under which the condensation is 
performed. In practice, the economics of achieving extremely high 
pressures and extremely low temperatures usually limit the performance of 
the condensation step in recovering liquified vapor. There are also 
constraints on the removal or recovery achieved by the other types of 
removal processes. 
The non-condensed gas fraction, 9, emerging from the condenser contains a 
higher vapor concentration than the feed gas, and may contain a 
concentration as high as five times, 10 times or more than the feed 
concentration. This condenser vent gas is passed through an auxiliary 
membrane module or modules, 10, which, like the main membrane separation 
unit, normally contains a vapor-selective membrane. The volume of the 
condenser vent gas stream is much smaller than that of the feed gas 
stream, so the membrane area required for the auxiliary module is small 
compared with the main unit. The auxiliary module, 10, is connected on its 
permeate side upstream of the vacuum pump, 6. Thus the driving force for 
auxiliary membrane permeation is provided by the pressure difference 
between the vacuum and exhaust sides of the vacuum pump. The concentrated 
vapor stream, 12, from the auxiliary module joins with vapor-enriched 
stream, 4, from the main membrane unit and passes again through the vacuum 
pump and condenser. The residue stream, 11, from the auxiliary unit is 
recirculated to the feed side of the main membrane unit. The concentration 
of vapor in the stream 11 depends on the membrane area contained in the 
auxiliary module. Preferably, the membrane area is such that there is not 
a big difference between the vapor concentrations in streams 11 and 1. 
Most preferably, stream 11 should have a concentration about the same as 
stream 1. 
An alternative embodiment of the invention is shown in FIG. 2. Referring 
now to this figure, a vapor-containing feed gas stream, 21, passes to a 
membrane separation unit, 22, containing one or more membranes. The vapor 
is concentrated in stream, 24, permeating the membrane; the residue, 
non-permeating, stream, 23, is correspondingly depleted in vapor. 
As in FIG. 1, the driving force for membrane permeation is provided by a 
vacuum pump, 25, which is used to lower the pressure on the permeate side. 
Stream, 26, from the vacuum pump is subjected to a condensation step. In 
this case, the condensation step involves both compression and chilling. 
The vapor-enriched stream, 26, passes to compressor, 27, emerging as 
pressurized stream, 28. It is then condensed in condenser, 29, and 
produces a stream, 30, of liquified vapor. 
The non-condensed gas fraction, 31, is passed through an auxiliary membrane 
module or modules, 32, connected on its permeate side as in FIG. 1, that 
is, upstream of the vacuum pump, 25. The driving force for auxiliary 
membrane permeation is provided by the pressure difference between the 
pressurized exhaust from the condenser and the low-pressure side of the 
vacuum pump. The concentrated vapor stream, 34, from the auxiliary module 
joins with vapor-enriched stream, 24, from the main membrane unit and 
passes again through the vacuum pump, compressor and condenser. The 
residue stream, 33, from the auxiliary unit is recirculated to the feed 
side of the main membrane unit. The concentration of vapor in stream 33 
may be tailored as discussed above. In this configuration, it would also 
be possible, although less desirable, to return stream 34 between the 
vacuum pump and compressor, so that it joined with stream 26. 
A third possible embodiment of the invention is shown in FIG. 3. This 
arrangement differs from those of FIG. 1 and FIG. 2 in that the feed gas 
stream is supplied to the main membrane unit at high pressure, so it is 
possible to operate at atmospheric pressure on the permeate side. 
Referring now to FIG. 3, a vapor-containing feed gas stream, 41, passes to 
a compressor, 42. Preferably, this raises the feed gas pressure to a value 
in the range of 1-20 atmospheres. Pressurized feed gas stream, 43, then 
passes to membrane separation unit, 44, containing one or more membranes. 
The vapor is concentrated in stream, 46, permeating the membrane; the 
residue, non-permeating, stream, 45, is correspondingly depleted in vapor. 
In this case, the driving force for membrane permeation is provided by the 
pressurized feed; the permeate side of the membrane is at, or close to, 
atmospheric pressure. Stream, 46, from the membrane passes to compressor, 
47, emerging as pressurized stream, 48. It is then condensed in condenser, 
49, and produces a stream, 50, of liquified vapor. 
The non-condensed gas fraction, 51, is passed through an auxiliary membrane 
module or modules, 52, connected on its permeate side upstream of 
compressor, 47. The driving force for auxiliary membrane permeation is 
provided by the pressure difference between the pressurized exhaust from 
the condenser and the low-pressure side of the compressor. The 
concentrated vapor stream, 54, from the auxiliary module joins with 
vapor-enriched stream, 46, from the main membrane unit and passes again 
through the compressor and condenser. The residue stream, 53, from the 
auxiliary unit is recirculated to the feed side of the main membrane unit. 
The concentration of vapor in stream 53 may be tailored as discussed 
above. 
From the above discussion it may be seen that various ways of providing the 
driving force for membrane permeation in the main and auxiliary membrane 
units are possible within the scope of the invention. What is required is 
that there be some component or set of components on the permeate side of 
the main membrane unit that changes the pressure of the gas stream passing 
through it, such as any type of pump, compressor, eductor or the like. 
What is further required is that at least part of the pressure change 
caused by that component or components is tapped to provide the driving 
means for the auxiliary module(s). 
FIGS. 1, 2 and 3 all show a one-stage main membrane unit. However, it will 
be apparent that the same principle can be applied where the main membrane 
unit includes multiple membrane stages or steps, such as a two-step 
system, a two-stage system or other combinations. 
FIGS. 1, 2 and 3 all show processes in which the residue stream from the 
auxiliary module(s) is recirculated to the feed side of the main membrane 
unit. Embodiments in which the residue stream passes to some other 
destination are also contemplated. In this case, the auxiliary module(s) 
can be tailored to achieve a residue stream concentration appropriate to 
that destination. 
Pervaporation 
FIGS. 5 and 6 show embodiments of the invention as it relates to 
pervaporation. A convenient mathematical method of describing 
pervaporation is to divide the separation into two steps. The first is 
evaporation of the feed liquid to form a hypothetical saturated vapor 
phase on the feed side of the membrane. The second is permeation of this 
vapor through the membrane to the low pressure permeate side of the 
membrane. Although no evaporation actually takes place on the feed side of 
the membrane during pervaporation, this approach is mathematically simple 
and is thermodynamically completely equivalent to the physical process. 
In pervaporation, transmembrane permeation is induced by maintaining the 
vapor pressure on the permeate side lower than the vapor pressure of the 
feed liquid. The permeate side vapor pressure can be reduced, for example, 
by drawing a vacuum on the permeate side of the membrane, by sweeping the 
permeate side to continuously remove permeating vapor, or by cooling the 
permeate vapor stream to induce condensation. The feed solution may also 
be heated to raise the vapor pressure on the feed side. 
FIG. 5 shows a pervaporation embodiment that corresponds to the gas 
separation embodiment of FIG. 1. In FIG. 1, the driving force is provided 
by the pump. The condenser provides a recovery step from which the 
purified condensate is removed and the impure, noncondensed fraction 
passes to the auxiliary module(s). The pervaporation system of FIG. 5 
differs from the gas separation design of FIG. 1 in that the condenser 
provides a driving force for transmembrane permeation and the recovery or 
further purification is provided by a decanter, distillation column, 
adsorbent bed, extraction process or the like. Referring now to FIG. 5, a 
solution, 101, containing a component to be separated, passes to a 
pervaporation unit, 102, containing one or more membranes. Preferably, 
although not essentially, solution 101 is warmed before entering the 
pervaporation unit to raise the vapor pressure on the feed side and 
augment the driving force provided by the condenser. The pervaporation 
step normally involves running the feed solution across a membrane that is 
selectively permeable to the component that is to be removed. That 
component is concentrated in the vapor stream, 104, permeating the 
membrane; the residue, non-permeating, stream, 103, is correspondingly 
depleted in the component. 
As with the gas separation designs, the membrane may take any of the 
membrane forms known in the art. For pervaporation, composite membranes, 
asymmetric membranes or ion-exchange membranes are preferred. For 
separating an organic compound from water, the following membrane 
materials, among others, might be used: nitrile rubber, neoprene, 
polydimethylsiloxane (silicone rubber), chlorosulfonated polyethylene, 
polysilicone-carbonate copolymers, fluororelastomers, plasticized 
polyvinylchloride, polyurethane, cis-polybutadiene, cis-polyisoprene, 
poly(butene-1), polystyrene-butadiene copolymers, 
styrene/butadiene/styrene block copolymers, styrene/ethylene/butylene 
block copolymers, polyesteramides, and block copolymers of polyethers and 
polyesters. For separating water from an organic compound, the following 
membrane materials, among others, might be used: polyvinylalcohol, 
cellulose and derivatives, such as cellulose diacetate, cellulose 
triacetate, cellulose nitrate and ethylcellulose, chitosan, crosslinked 
alginic acid, and ion-exchange membranes, such as the Nafion range from Du 
Pont. For separating two organic compounds, the following membrane 
materials, among others, might be used: polyamides, cellulose and 
derivatives, such as cellulose diacetate, cellulose triacetate, cellulose 
nitrate and ethylcellulose. 
For pervaporation, the form of the module containing the membrane may be 
any of the forms known in the membrane separation arts, including, for 
example, plate-and-frame modules, hollow-fiber modules and spiral-wound 
modules. Plate-and-frame module design and construction is discussed, for 
example, in U.S. Pat. No. 4,695,380. The preparation of spiral-wound 
modules is described, for example, in U.S. Pat. No. 3,966,616. The 
preparation of hollow-fiber membranes and modules is described, for 
example, in U.S. Pat. Nos. 3,798,185 and 4,230,463. 
In FIG. 5, the driving force for transmembrane permeation is provided by 
condenser, 105, which liquifies the permeating vapor and thereby maintains 
a low partial pressure on the permeate side. Liquid stream, 106, from the 
condenser passes to the recovery, second separation or further 
purification unit, 107. A variety of techniques can be used to further 
purify the condensed permeate. If the content of the permeate and the 
mutual solubilities of the components are appropriate, the permeate may 
form two phases, for example an aqueous phase and an organic phase. 
Further purification could then be achieved by separating the two phases 
in a decanter. If the permeate forms a single phase, it can be further 
separated by distillation, adsorption or solvent extraction, for example. 
In FIG. 5, stream 108 represents the stream rich in the desired component; 
stream 109 represents the residual, non-product stream. For example, if 
the purpose of the pervaporation/further purification process were to 
separate an organic compound from water, stream 108 would be the 
organic-rich stream and stream 109 would be the residual aqueous stream. 
As in the gas separation applications described above, the composition of 
stream 109 may be very different from that of incoming feed stream 101. 
Stream 109 may be saturated with organic, for example. 
The non-product stream, 109, is passed through an auxiliary pervaporation 
module or modules, 110. Preferably, before entering the module, stream 109 
is heated to increase the feed side vapor pressure. This may be done by 
running stream 109 through a heat exchanger in heat-exchanging contact 
with, for example, stream 103. The auxiliary module, 110, is connected on 
its permeate side upstream of the condenser, 105. Thus a driving force for 
auxiliary membrane permeation is provided by the condenser. The 
concentrated vapor stream, 112, from the auxiliary module passes, together 
with stream 104 or separately, through the condenser and the further 
purification process. The residue stream, 111, from the auxiliary unit is 
recirculated to the feed side of the main membrane unit. The composition 
of stream 111 depends on the membrane area contained in the auxiliary 
module. Preferably, the membrane area is such that there is not a big 
difference between the compositions of streams 111 and 101. Most 
preferably, stream 111 should have a concentration about the same as 
stream 101. 
In addition to the basic elements shown in FIG. 5, a prevaporation system 
commonly includes a small vacuum pump, on the permeate side, to remove any 
noncondensable gas that may be present in the system. The system may also 
include a pump to pump the condensed permeate to the recovery or further 
purification unit, 107, and a pump to pump the non-product stream from the 
recovery unit to the auxiliary module or modules. 
An alternative embodiment of the invention as it relates to pervaporation 
is shown in FIG. 6. The pervaporation system of FIG. 6 differs from that 
of FIG. 5 in that a separate condenser is used to drive the auxiliary 
modules, but the recovery or further purification unit handles the 
condensates from both condensers. Other optional equipment, such as pumps 
to remove noncondensable gas and to supply liquid to the recovery unit may 
serve both condensers. Less desirably, separate pumps may be provided to 
handle each condenser. 
Referring now to FIG. 6, a solution, 201, containing a component to be 
separated, passes to a pervaporation unit, 202, containing one or more 
membranes. The pervaporation step normally involves running the feed 
solution across a membrane that is selectively permeable to the component 
that is to be removed. That component is concentrated in the vapor stream, 
204, permeating the membrane; the residue, non-permeating, stream, 203, is 
correspondingly depleted in the component. As with the embodiment of FIG. 
5, the solution may be heated before entering the pervaporation unit. 
The membranes and modules may be chosen and configured according to the 
same teachings as given for the FIG. 5 embodiment. A driving force for 
transmembrane permeation is provided by condenser, 205, which liquefies 
the permeating vapor and thereby maintains a low partial pressure on the 
permeate side. Liquid stream, 206, from the condenser passes to the 
recovery or further purification unit, 207, which, as in FIG. 5, may be a 
decanter, distillation column, etc. Stream 208 represents the stream rich 
in the desired component; stream 209 represents the non-product stream. 
Stream 209 is passed through an auxiliary pervaporation module or modules, 
210. Preferably, before entering the module, stream 209 is heated to 
increase the feed side vapor pressure. This may be done by running stream 
209 through a heat exchanger in heat-exchanging contact with, for example, 
stream 203. The auxiliary module, 210, is connected on its permeate side 
to auxiliary condenser, 213. The concentrated vapor stream, 212, from the 
auxiliary module passes through condenser, 213, emerging as liquid stream, 
214, which in turn passes, together with stream 206 or separately, to the 
recovery or further purification unit, 207. The residue stream, 211, from 
the auxiliary unit is recirculated to the feed side of the main membrane 
unit. The design of FIG. 6 is particularly useful when the feed liquid 
contains three components of differing physical properties, such as a 
hydrophobic organic compound, an organic compound moderately soluble in 
water, and water. 
FIGS. 5 and 6 show a one-stage pervaporation unit. However, it will be 
apparent that the same principle can be applied where the main 
pervaporation unit includes multiple membrane stages or steps, such as a 
two-step system, a two-stage system or other combinations. 
FIGS. 5 and 6 show processes in which the residue stream from the auxiliary 
module(s) is recirculated to the feed side of the main membrane unit. 
Embodiments in which the residue stream passes to some other destination 
are also contemplated. In this case, the auxiliary module(s) can be 
tailored to achieve a residue stream concentration appropriate to that 
destination. 
FIGS. 5 and 6 show systems and processes in which a driving means for 
transmembrane permeation, equivalent to the pressure-changing means of the 
vapor separation embodiments, is provided by the condenser on the permeate 
side of the pervaporation unit, optionally augmented by heating the feed 
stream. Embodiments in which the driving force is provided by a vacuum 
pump instead of a condenser on the permeate side, a vacuum pump combined 
with a condenser, an eductor or any other means that would cause 
transmembrane permeation to occur are also within the scope of the 
invention. 
In preferred embodiments, the membranes used in the main pervaporation unit 
and the auxiliary modules are of the same type, selective to the component 
that is to be separated. However, useful embodiments are also possible 
using membranes of unlike selectivities in the main unit and the auxiliary 
modules. 
Representative Applications 
The systems and processes of the invention could be used for diverse 
applications, including: 
1. Removal of hydrocarbons, particularly C.sub.3 to C.sub.6 hydrocarbons, 
from gas streams emitted during storage or transfer of crude oil or 
gasoline. 
2. Removal of CFCs (chlorofluorocarbons) or HCFCs from streams emitted from 
refrigeration or air conditioning plants, foam manufacture, processes that 
use CFCs as solvents, CFC manufacture, storage or transfer. 
3. Removal of chlorinated solvents from streams generated during chemical 
manufacture and processing operations, film and laminate preparation, 
coating and spraying, solvent degreasing, industrial and commercial dry 
cleaning and many other sources. 
4. Removal of organic compounds, particularly volatile organic compounds 
(VOCs) from industrial wastewaters or process waters. 
5. Separation of closely boiling mixtures or azeotropes. 
6. Removal of small quantities of water from alcohols and other organic 
liquids. 
7. Clean up of groundwater contamination. 
8. Treatment of industrial waste streams containing mixtures of hydrophobic 
and/or volatile organic compounds (VOCs), particularly halogenated 
hydrocarbons or aromatic hydrocarbons, with water miscible, less volatile 
(hydrophilic) solvents. 
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 in any way. 
EXAMPLES 
Vapor Separation Examples 
Examples 1-3 compare the removal of a condensable vapor from a feed stream 
using (A) a one-stage membrane system and (B) a one-stage system fitted 
with an auxiliary module or modules. The examples are computer 
calculations, 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). In each case, the feed stream has a 
flow rate of 100 scfm and the feed is provided to the main membrane 
separation unit at a pressure of 80 cmHg. The driving force for membrane 
permeation is assumed to be a vacuum pump on the permeate side of the main 
membrane separation unit. The selectivity of the membranes for the vapor 
over the other components of the feed is assumed to be 40. This is a 
number typical of many vapor separations, such as hydrocarbons from air or 
nitrogen, chlorinated or fluorinated organic solvents from air or 
nitrogen, sulfur dioxide from nitrogen, oxygen or other gases, and so on. 
The examples are in three groups. The Group I examples assume a feed 
concentration of 2% vapor and a concentration in the residue leaving the 
main membrane unit of 0.5%, in other words 75% removal. The Group 2 
examples also achieve 75% removal, from 4% vapor in the feed to 1% in the 
residue. The Group 3 examples achieve 90% recovery, from 5% vapor in the 
feed to 0.5% in the residue. In each case, the condenser vent gas is 
assumed to contain 20% vapor, and the auxiliary module membrane area is 
tailored to achieve a residue stream having a vapor concentration about 
the same as that of the feed. 
GROUP 1 EXAMPLES 
EXAMPLE A 
Single-stage membrane separation unit. Not in accordance with the 
invention. 
The calculations described above were performed using the following 
assumptions: 
______________________________________ 
Feed concentration: 2% vapor 
Feed pressure: 80 cmHg 
Feed flow rate: 100 scfm 
Membrane selectivity: 40 
Residue concentration: 0.5% 
Condenser vent gas concentration: 
20% 
______________________________________ 
The pressure on the permeate side of the main membrane unit was set to 2 
cmHg, 5 cmHg and 10 cmHg. The calculated membrane areas and pump 
capacities required to achieve the desired performance with a one-stage 
membrane system are listed in Table 1. 
TABLE 1 
______________________________________ 
Permeate 
Membrane Area 
Pressure 
Main Unit Aux. Module 
Total Pump Capacity 
(cmHg) (m.sup.2) (m.sup.2) (m.sup.2) 
(acfm) 
______________________________________ 
10 430 -- 430 494 
5 163 -- 163 412 
2 72 -- 72 505 
______________________________________ 
EXAMPLE B 
Single-stage Membrane Separation Unit with Auxiliary Modules 
The calculations described in Example 1A were repeated using a system 
design as in FIG. 1. The assumptions were as before: 
______________________________________ 
Feed concentration: 2% vapor 
Feed pressure: 80 cmHg 
Feed flow rate: 100 scfm 
Membrane selectivity: 
40 
Residue concentration: 
0.5% 
Condenser vent gas concentration: 
20% 
Permeate pressure: 2 cmHg, 5 cmHg and 10 
cmHg 
______________________________________ 
The calculated membrane areas and pump capacities required to achieve the 
desired performance with a system design as in FIG. 1 are listed in Table 
2. 
TABLE 2 
______________________________________ 
Permeate 
Membrane Area 
Pressure 
Main Unit Aux. Module 
Total Pump Capacity 
(cmHg) (m.sup.2) (m.sup.2) (m.sup.2) 
(acfm) 
______________________________________ 
10 207 64 271 315 
5 98 15 113 294 
2 50 5 55 401 
______________________________________ 
Comparison of Tables 1 and 2 shows that, when the permeate pressure is 10 
cmHg for example, the membrane area needed for the process and system of 
the invention is only 63% of that needed for a conventional one-stage 
system, and the pump capacity is only 64%. Likewise, when the permeate 
pressure is 5 cmHg, the membrane area needed for the process and system of 
the invention is 69% of that needed for a conventional one-stage system, 
and the pump capacity is 71%. When the permeate pressure is 2 cmHg, the 
membrane area needed for the process and system of the invention is 76% of 
that needed for a conventional one-stage system, and the pump capacity is 
79%. 
The comparison of membrane areas and pump capacities for a one-stage system 
and process (curve A) and for the system and process of the invention 
(curve B) is shown in graph form in FIG. 4. 
GROUP 2 EXAMPLES 
EXAMPLE A 
Single-stage Membrane Separation Unit. Not in accordance with the 
invention. 
Calculations were performed as in Example 1A, but using the following 
assumptions: 
______________________________________ 
Feed concentration: 4% vapor 
Feed pressure: 80 cmHg 
Feed flow rate: 100 scfm 
Membrane selectivity: 
40 
Residue concentration: 
1% 
Condenser vent gas concentration: 
20% 
Permeate pressure: 2 cmHg, 5 cmHg, 10 
cmHg 
______________________________________ 
The calculated membrane areas and pump capacities required to achieve the 
desired performance with a one-stage membrane system are listed in Table 
3. 
TABLE 3 
______________________________________ 
Permeate 
Membrane Area 
Pressure 
Main Unit Aux. Module 
Total Pump Capacity 
(cmHg) (m.sup.2) (m.sup.2) (m.sup.2) 
(acfm) 
______________________________________ 
10 291 -- 291 350 
5 114 -- 114 318 
2 54 -- 54 452 
______________________________________ 
EXAMPLE B 
Single-Stage Membrane Separation Unit with Auxiliary Modules 
The calculations described in Example 2A were repeated using a system 
design as in FIG. 1. The assumptions were as before: 
______________________________________ 
Feed concentration: 4% vapor 
Feed pressure: 80 cmHg 
Feed flow rate: 100 scfm 
Membrane selectivity: 
40 
Residue concentration: 
1% 
Condenser vent gas concentration: 
20% 
Permeate pressure: 2 cmHg, 5 cmHg and 10 
cmHg 
______________________________________ 
The calculated membrane areas and pump capacities required to achieve the 
desired performance with a system design as in FIG. 1 are listed in Table 
4. 
TABLE 4 
______________________________________ 
Permeate 
Membrane Area 
Pressure 
Main Unit Aux. Module 
Total Pump Capacity 
(cmHg) (m.sup.2) (m.sup.2) (m.sup.2) 
(acfm) 
______________________________________ 
10 192 36 228 278 
5 88 9 97 278 
2 46 3 49 414 
______________________________________ 
Comparison of Tables 3 and 4 shows that, when the permeate pressure is 10 
cmHg for example, the membrane area needed for the process and system of 
the invention is only 78% of that needed for a conventional one-stage 
system, and the pump capacity is only 79%. Likewise, when the permeate 
pressure is 5 cmHg, the membrane area needed for the process and system of 
the invention is 85% of that needed for a conventional one-stage system, 
and the pump capacity is 87%. When the permeate pressure is 2 cmHg, the 
membrane area needed for the process and system of the invention is 90% of 
that needed for a conventional one-stage system, and the pump capacity is 
91%. 
GROUP 3 EXAMPLES 
EXAMPLE A 
Single-stage Membrane Separation Unit. Not in accordance with the 
invention. 
Calculations were performed as in Example 1A, but using the following 
assumptions: 
______________________________________ 
Feed concentration: 5% vapor 
Feed pressure: 80 cmHg 
Feed flow rate: 100 scfm 
Membrane selectivity: 
40 
Residue concentration: 
0.5% 
Condenser vent gas concentration: 
20% 
Permeate pressure: 1 cmHg, 2 cmHg, 5 cmHg, 
10 cmHg 
______________________________________ 
The calculated membrane areas and pump capacities required to achieve the 
desired performance with a one-stage membrane system are listed in Table 
5. 
TABLE 5 
______________________________________ 
Permeate 
Membrane Area 
Pressure 
Main Unit Aux. Module 
Total Pump Capacity 
(cmHg) (m.sup.2) (m.sup.2) (m.sup.2) 
(acfm) 
______________________________________ 
10 441 -- 441 527 
5 179 -- 179 496 
2 87 -- 87 711 
1 65 -- 65 1155 
______________________________________ 
EXAMPLE B 
Single-Stage Membrane Separation Unit with Auxiliary Modules 
The calculations described in Example 3A were repeated using a system 
design as in FIG. 1. The assumptions were as before: 
______________________________________ 
Feed concentration: 5% vapor 
Feed pressure: 80 cmHg 
Feed flow rate: 100 scfm 
Membrane selectivity: 
40 
Residue concentration: 
0.5% 
Condenser vent gas concentration: 
20% 
Permeate pressure: 1 cmHg, 2 cmHg, 5 cmHg 
and 10 cmHg 
______________________________________ 
The calculated membrane areas and pump capacities required to achieve the 
desired performance with a system design as in FIG. 1 are listed in Table 
6. 
TABLE 6 
______________________________________ 
Permeate 
Membrane Area 
Pressure 
Main Unit Aux. Module 
Total Pump Capacity 
(cmHg) (m.sup.2) (m.sup.2) (m.sup.2) 
(acfm) 
______________________________________ 
10 340 50 390 470 
5 152 11 163 454 
2 78 4 82 673 
1 59 3 62 1111 
______________________________________ 
Comparison of Tables 5 and 6 shows that, when the permeate pressure is 10 
cmHg for example, the membrane area needed for the process and system of 
the invention is 88% of that needed for a conventional one-stage system, 
and the pump capacity is 89%. When the permeate pressure is 5 cmHg, the 
membrane area needed for the process and system of the invention is 91% of 
that needed for a conventional one-stage system, and the pump capacity is 
91%. When the permeate pressure is 2 cmHg or 1 cmHg, the membrane area 
needed for the process and system of the invention is about 95% of that 
needed for a conventional one-stage system, and the pump capacity is 95%. 
Comparing Examples 1B, 2B and 3B, it may be seen that the greatest savings 
in membrane area and pump capacity is achieved when there is the greatest 
disparity between the vapor concentration in the condensor vent gas and 
the feed gas. 
Prevaporation Examples 
Examples 4, 5 and 6 concern pervaporation. Example 4 compares the 
performance that can be achieved using (A) a one-stage pervaporation unit 
followed by a decanter and (B) a one-stage pervaporation unit fitted with 
an auxiliary module or modules across the condenser and the recovery unit 
as shown in FIG. 5. Example 5 compares the performance that can be 
achieved using (A) a one-stage pervaporation unit and (B) a one-stage 
pervaporation unit fitted with an auxiliary module or modules across the 
recovery unit as shown in FIG. 6. Example 6 demonstrates the use of unlike 
membranes in the main pervaporation unit and the auxiliary modules. 
EXAMPLE 4 
EXAMPLE A 
Single-stage pervaporation unit not in accordance with the invention. 
Performance calculations were carried out to determine the performance of a 
conventional one-stage pervaporation system, followed by a decanter, in 
separating benzene from water. The separation factor chosen is typical for 
the separation of hydrophobic organic compounds from water. The 
calculations were done using the following assumptions: 
______________________________________ 
Raw solution concentration: 
20 ppm, 5 ppm and 1 ppm 
Raw solution flow rate: 
1,000 kg/h 
Membrane area: 10 m.sup.2 
Membrane separation factor: 
465 
Transmembrane flux: 
1 kg/m.sup.2 .multidot. h 
Condenser temperature: 
5.degree. C. 
Feed liquid temperature: 
60.degree. C. 
______________________________________ 
In each case, the condensed permeate stream is passed to the decanter for 
phase separation. The aqueous phase from the decanter is saturated with 
benzene at a concentration of 2,000 ppm and is returned at a rate of 10 
kg/h and mixed with the incoming raw solution to form a mixed feed to the 
pervaporation unit. The separation performance achieved at different raw 
feed concentrations is listed in Table 7. 
TABLE 7 
______________________________________ 
Raw solution 
Residue stream 
Mixed membrane 
concentration 
concentration 
feed concentration 
Removal 
(ppm) (ppm) (ppm) (%) 
______________________________________ 
20 0.4 40 98 
5 0.25 25 95 
1 0.21 21 79 
______________________________________ 
EXAMPLE B 
Single-stage pervaporation unit with auxiliary modules 
The calculations described in Example 4A were repeated using a system 
design as in FIG. 5. The assumptions were as before: 
______________________________________ 
Raw solution concentration: 
20 ppm, 5 ppm and 1 ppm 
Raw solution flow rate: 
1,000 kg/h 
Main unit membrane area: 
10 m.sup.2 
Membrane separation factor: 
465 
Transmembrane flux: 
1 kg/m.sup.2 .multidot. h 
Condenser temperature: 
5.degree. C. 
Feed liquid temperature: 
60.degree. C. 
______________________________________ 
In addition, it was assumed that the auxiliary module as shown in FIG. 5 
contains a membrane area of 0.1 m.sup.2, so that the total membrane area 
used for the separation is 10.1 m.sup.2. In each case, the condensed 
permeate stream from the main pervaporation unit is passed to the decanter 
for phase separation. The aqueous phase from the decanter is saturated 
with benzene at a concentration of 2,000 ppm and is passed to the 
auxiliary module. The permeate from the auxiliary module is passed to the 
condenser and thence to the decanter. The residue from the auxiliary 
module is reduced to a concentration of 20 ppm and mixed with the incoming 
raw solution to form the mixed feed to the pervaporation unit. The 
separation performance achieved at different raw feed concentrations is 
listed in Table 8. 
TABLE 8 
______________________________________ 
Raw solution 
Residue stream 
Mixed membrane 
concentration 
concentration 
feed concentration 
Removal 
(ppm) (ppm) (ppm) (%) 
______________________________________ 
20 0.2 20 99 
5 0.052 5.15 98.9 
1 0.012 1.2 98.8 
______________________________________ 
Comparison of Tables 7 and 8 shows that the separation performance of the 
system of the invention is improved compared with the performance of a 
conventional one-stage pervaporation unit followed by a decanter. The 
improvement is most marked at low feed concentrations. When the raw 
solution has a benzene concentration of only 1 ppm, the effect of mixing 
the saturated aqueous stream from the decanter is to increase the 
concentration of the feed to the pervaporation unit to 21 ppm. The residue 
stream increases in concentration, therefore, to 0.21 ppm. In other words, 
the removal of benzene from the raw solution is only 79%. Using the design 
of the invention, the feed concentration is only raised from 1 ppm to 1.2 
ppm and a removal of 98.8% can be sustained. Similar, but less marked 
effects are observed at higher raw solution concentrations. 
EXAMPLE 5 
EXAMPLE A 
Single-stage pervaporation unit not in accordance with the invention. 
Performance calculations were carried out to determine the performance of a 
conventional one-stage pervaporation system followed by a decanter in 
separating benzene from water. The separation factor chosen is typical for 
the separation of hydrophobic organic compounds from water. The 
calculations were done using the following assumptions: 
______________________________________ 
Raw solution concentration: 
20 ppm, 5 ppm and 1 ppm 
Raw solution flow rate: 
1,000 kg/h 
Membrane area: 10 m.sup.2 
Membrane separation factor: 
465 
Transmembrane flux: 
1 kg/m.sup.2 .multidot. h 
Condenser temperature: 
5.degree. C. 
Feed liquid temperature: 
60.degree. C. 
______________________________________ 
In each case, the condensed permeate stream is passed to the decanter for 
phase separation. The aqueous phase from the decanter is saturated with 
benzene at a concentration of 2,000 ppm and is returned at a rate of 10 
kg/h and mixed with the incoming raw solution to form a mixed feed to the 
pervaporation unit. The separation performance achieved by the system at 
different raw feed concentrations is listed in Table 9. 
TABLE 9 
______________________________________ 
Raw solution 
Residue stream 
Mixed membrane 
concentration 
concentration 
feed concentration 
Removal 
(ppm) (ppm) (ppm) (%) 
______________________________________ 
20 0.4 40 98 
5 0.25 25 95 
1 0.21 21 79 
______________________________________ 
Example B 
Single-stage pervaporation unit with auxiliary modules 
The calculations described in Example 5A were repeated using a system 
design as in FIG. 6. The assumptions were as before: 
______________________________________ 
Raw solution concentration: 
20 ppm, 5 ppm and 1 ppm 
Raw solution flow rate: 
1,000 kg/h 
Main unit membrane area: 
10 m.sup.2 
Membrane separation factor: 
465 
Transmembrane flux: 
1 kg/m.sup.2 .multidot. h 
Condenser temperature: 
5.degree. C. 
Feed liquid temperature: 
60.degree. C. 
______________________________________ 
In addition, it was assumed that the auxiliary module as shown in FIG. 6 
contains a membrane area of 0.1 m.sup.2, so that the total membrane area 
used for the separation is 10.1 m.sup.2. In each case, the condensed 
permeate stream from the main pervaporation unit is passed to the decanter 
for phase separation. The aqueous phase from the decanter is saturated 
with benzene at a concentration of 2,000 ppm and is passed to the 
auxiliary module. The permeate from the auxiliary module is passed to an 
auxiliary condenser and thence to the decanter. The residue from the 
auxiliary module is reduced to a concentration of 20 ppm and mixed with 
the incoming raw solution to form the mixed feed to the pervaporation 
unit. The separation performance achieved by the system at different raw 
feed concentrations is listed in Table 10. 
TABLE 10 
______________________________________ 
Raw solution 
Residue stream 
Mixed membrane 
concentration 
concentration 
feed concentration 
Removal 
(ppm) (ppm) (ppm) (%) 
______________________________________ 
20 0.2 20 99 
5 0.052 5.15 98.9 
1 0.012 1.2 98.8 
______________________________________ 
Comparison of Tables 9 and 10 and 7 and 8 shows that exactly the same 
improvement in performance is obtained with the design of FIG. 6 as with 
the design of FIG. 5. 
EXAMPLE 6 
Calculations were carried out to determine the performance of a 
pervaporation system as shown in FIG. 5, modified to contain membranes of 
unlike selectivities in the main unit and the auxiliary modules and 
further modified so that residue stream 111 from the auxiliary modules is 
not returned to the main pervaporation unit, in separating a raw solution 
containing 5 wt% water in butanol. The main pervaporation unit, 102, was 
assumed to contain a water-selective membrane with a water/butanol 
separation factor of approximately 20. The water-enriched permeate, 104, 
from the pervaporation unit is passed to the condenser, 105, and thence to 
the decanter, 107. The aqueous phase, 109, from the decanter is passed to 
the auxiliary module, 110, which contains a butanol-selective membrane 
with a butanol/water separation factor of approximately 50. The residue, 
111, from this membrane can be discharged. The permeate, 112, is returned 
upstream of the condenser. The butanol phase, 108, from the decanter can 
be mixed with the incoming raw solution, 101, or could be passed to a 
second auxiliary module containing a water-selective membrane. The 
compositions of the various streams are listed in Table 11. 
TABLE 11 
______________________________________ 
Stream (see Fig. 5) 
Composition (wt % butanol/wt % water) 
______________________________________ 
101 95/5 
103 99.5/0.5 
104 60/40 
108 85/15 
109 8/92 
111 1/99 
112 40/60 
______________________________________ 
It may be seen that both streams from the decanter, 108 and 109, 
"non-product" streams in this case; the stream from the auxiliary module, 
111, is the purified water product stream. This stream is discharged, not 
returned to the main pervaporation unit.