An improved cross-flow membrane module having a cylindrical housing containing semi-permeable membranes for separating a feed fluid is provided wherein a seal device is positioned in the permeate compartment of the module to separate the permeate stream passing through the semi-permeable membranes into two or more streams having differing component concentrations.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to cross-flow permeator designs for the 
separation of a feed stream into two or more streams having different 
concentrations. 
BACKGROUND OF THE INVENTION 
Conventional cross-flow membrane modules used to separate gas or liquid 
mixtures in separations where the product is the retentate do not provide 
a means of separately capturing the fluid permeating through the membrane 
in the downstream, or product section of the module. Past a certain point 
in the module, which varies from application to application, this fluid 
has a higher concentration of the desired product than the incoming feed. 
Various permeator module designs for fluid separation are described in the 
patent literature. U.S. Pat. No. 4,080,296 discloses a hollow fiber 
permeator comprising an elongated, ordered bundle of selectively permeable 
hollow fibers passing through a central tubesheet formed around a 
perforated feed tube containing a coaxial permeate conduit. U.S. Pat. No. 
4,652,373 discloses a hollow fiber reverse osmosis permeator separtion 
unit comprising hollow fibers spirally wound on a hollow core creating a 
bundle of fiber lengths. Each of the fiber lengths pass through a 
tubesheet which is drilled with a hole pattern that cuts through all of 
the hollow fibers. 
U.S. Pat. No. 4,666,469 discloses a hollow fiber membrane device comprising 
a plurality of hollow fiber membranes assemblies in a bundle about a feed 
flow distribution means with said membranes embedded in at least one 
tubesheet and the lumens of the fibers communicating through the 
tubesheet. A wrap of material envelopes a major portion of the 
longitudinal dimensions of the plurality of inner fibers assembled about 
the feed flow distribution means. The wrap serves to constrain the 
movement of the fibers within the wrap, but is formainous or contains 
other openings which permit passage of feed fluid through the inner fibers 
to the hollow fiber membranes. The permeate is collected in a header space 
of the device and removed at essentially the same concentration via two 
exit ports. 
U.S. Pat. No. 4,119,417 discloses a gas separator comprising two membrane 
separation cells arranged in series. The first membrane separation cell 
comprises a feed pipe for conducting a gas mixture to a high pressure 
chamber of the first cell and a leadout pipe for drawing off either 
permeate or reject gas from the membrane located within the cell to a 
second separation cell. Either the permeate gas or reject gas from the 
second membrane cell is carried via a feedback pipe to the feed to the 
first separation cell. 
U.S. Pat. No. 4,083,780 discloses a fluid purification system comprising a 
plurality of spiral wrap membrane elements placed in parallel within a 
pressure vessel. The spiral wrap membrane elements are designed to have 
product fluid passageways of short length to insure a low pressure drop in 
the passageways. The impurified fluid, i.e., that not collected as the 
main products, exits the vessel through a single fluid outlet. 
BRIEF SUMMARY OF THE INVENTION 
The present invention is an improved cross-flow membrane separation module 
for the separation of a feed fluid. Typical separation modules comprise a 
housing means having a fluid inlet positioned such that incoming feed 
fluid contacts semi-permeable membranes contained in the housing means 
such that the feed flow is separated into a non-permeate product stream 
and a permeate stream. The housing means may be simply a mechanical 
structure to hold the membranes in place, or optionally may be a 
pressurized shell. Such housing means is typically an elongated, 
cylindrical structure. The module may be designed in accordance with a 
wide variety of configurations, examples of which include a design wherein 
the incoming feed directly contacts a series of membrane elements, such as 
a series of hollow fiber disks, a design wherein the incoming feed passes 
through the feed passages in a flat sheet spiral wound element to directly 
contact the membranes, or a design in which the feed flows through a feed 
pipe running axially through the housing means wherein the feed pipe has 
openings at various points along its length to allow feed to contact 
membranes positioned within the housing means. A product outlet, in fluid 
communication with the housing means, is positioned to remove the 
non-permeate product stream. A permeate compartment runs co-axially with 
the long axis of the housing means and serves to collect permeate fluid 
from the membranes. A permeate outlet or outlets in fluid communication 
with the permeate compartment serve to remove the permeate fluid from the 
housing means. 
The present invention provides an improvement in the type of separation 
module described above wherein a seal device is positioned in the permeate 
compartment at a point along the long axis of the module to segment said 
permeate compartment into an area where the permeate has a lower product 
concentration than the feed fluid, and an area where the permeate has a 
higher concentration than the feed. Additionally, at least two permeate 
outlets are provided such that each is positioned to separately recover 
permeate fluid from each area of the permeate compartment. 
The membrane separation separation modules of the present invention provide 
for the recovery of fluid permeating the membrane elements which has a 
higher product concentration than the feed. This recovered permeate can be 
used for applications requiring fluids of lesser purity than the required 
product, or alternatively can be recycled to the feed fluid entering the 
module to increase the efficiency of the module. Additionally, in 
applications in which the permeate component is recovered as a useful 
product, the present module design may be used to recover two or more 
permeate streams having different concentrations of permeate gasses.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is an improved cross-flow membrane separation module 
design for the separation of a feed fluid. The improvement resides in a 
seal device positioned in the permeate compartment of a typical cross-flow 
separation module to segment the permeate compartment into an area where 
the permeate has a lower product concentration than the feed fluid and an 
area where the permeate has a higher product concentration than the feed 
fluid. In instances in which the separation module has a permeate 
compartment which comprises the annulus between the outer disk wall and 
the inside of the housing, such as in a module employing one or more 
hollow fiber disk elements, the seal device is typically a seal ring 
segregating the permeate compartment. In embodiments wherein the permeate 
is collected in a permeate pipe located on the center axis of the module, 
such as in a module containing a spiral wound membrane element, the seal 
device may be a plug positioned within the permeate pipe. This module 
design allows for the recovery of permeate fluid having a higher 
concentration than the feed but a lower concentration than the 
non-permeate product. This recovered permeate stream can be used for 
applications requiring product of lesser purity, or alternatively may be 
recycled and mixed with the feed stream to the module to increase the 
efficiency of the system. Additionally, in applications in which the 
permeate component is recovered as a useful product, the present module 
design may be used to recover two or more permeate streams having 
different concentrations of permeate gasses. 
In instances wherein the permeate fluid having a higher product 
concentration than the feed is recycled back to the feed, the outlet for 
removing this permeate fluid from the module may be positioned in an 
enclosure which also includes the feed compression suction to allow 
simultaneous compression of the incoming feed and recycle permeate stream. 
An embodiment of the present invention wherein hollow fiber membrane disk 
elements are positioned in a cylindrical pressure shell housing is 
depicted in the drawing of FIG. 1. A housing means 2, has a fluid inlet 4 
positioned such that incoming feed fluid, represented by heavy arrows 3, 
travels axially through said housing means 2 and contacts a series of 
semi-permeable hollow fiber membrane disks 10 located within said housing 
means 2. The outsides of the ends of the hollow fiber membranes which make 
up the disks are encapsulated by a circumferential tubesheet 11 to form a 
hollow fiber disk element. The hollow fiber membrane disks 10 may be any 
material which is capable of separating one or more components of the feed 
stream, such as cellulose acetate, polysulfone, polyolefins, silicone 
rubber, poly(trimethylsilylpropyne) and the like. In other embodiments, 
the semi-permeable membrane elements may be in the form of a bundle of 
hollow fibers, spiral wrapped sheets or any other suitable configuration. 
The hollow fiber disk element is held in place within the housing means 2 
by securing means 13 located along the circumference at each end of the 
disk element which also serve to form the end walls of the permeate 
compartment. 
A product outlet 22 in fluid communication with the housing means 2 is 
provided for removing non-permeate product, represented by hollow arrows 
9, from the housing means. The portion of feed fluid which permeates 
through the hollow fibers which make up the hollow fiber membrane disks 10 
is collected as permeate fluid in a permeate compartment in fluid 
communication with the openings of the hollow fibers and which is 
segmented into an upstream section 12 and a downstream section 14 by a 
seal device 16. The seal device 16 is positioned in the permeate 
compartment at a point along the long axis of the module 2 such that 
permeate fluid, represented by thin arrows 5, entering the upstream 
segment 12 of the permeate compartment has a lower product concentration 
than the feed entering the module, and the permeate fluid, represented by 
dashed arrows 7, entering the downstream segment 14 has a higher product 
concentration than the feed. In a preferred embodiment, the seal device 16 
is adjustable in that it can be moved within the permeate compartment to 
different locations depending upon the specific application and feed 
concentration. Since the pressure of the permeate fluid on both sides of 
the seal device 16 is virtually the same, and because some leakage across 
the seal device is permissible for most applications, the seal device 16 
typically does not have to withstand high pressure or be fluid tight. 
The permeate fluid from permeate sections 12 and 14 are collected 
separately via permeate outlets 18 and 20 respectively which are in fluid 
communication with the respective sections of the permeate compartment. 
A second embodiment of the present invention is illustrated by the drawing 
of FIG. 2. All elements of the module in FIG. 2 which coincide with those 
of FIG. 1 are numbered the same. In the embodiment of FIG. 2, an enclosure 
24 is positioned to include the permeate outlet 20 such that permeate 
fluid exiting the module through this fluid outlet 20 is mixed with 
incoming feed entering the enclosure 24 via inlet 25 and subsequently 
passed as a single recycle stream 29 to a feed compressor 27 prior to 
recycle back to the fluid inlet 4. A subsequent treatment unit 26 such as 
a cooling unit and/or oil or water removal unit may optionally be 
positioned downstream of the feed compressor 27 to treat the stream 29 
being recycled to the fluid inlet 4. 
The membrane separation modules of the present invention can be employed in 
a wide variety of separations, including liquid separations, gas 
separations and pervaporation applications. 
EXAMPLE 1 
Calculations were carried out to compare the performance of the module 
depicted in the drawing of FIG. 2 with the performance of a similar module 
without the seal device, and hence without any recycle under the same 
conditions of pressure, temperature, flow rate and feed composition. The 
module used in the calculations was a pressure shell configuration 
containing a plurality of polyolefinic hollow fiber disk elements. The 
calculations are for the production of nitrogen as the non-permeate 
product at purity levels of 98% and 99% from an air feed stream. Table 1 
below sets out the membrane surface area reduction per unit of product 
produced along with the recovery increase realized by using the separation 
module of this invention. 
TABLE 1 
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N.sub.2 Purity 
98% 99% 
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Membrane Surface 9.5% 14.5% 
Area Reduction 
Recovery Increase 9.7% 17.2% 
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From the results reported above, it can be seen that the present module 
design is more efficient in terms of product recovery and also requires 
significantly less membrane area than conventional membrane modules, 
especially for operations requiring high product purity. 
Having thus described the present invention, what is now deemed appropriate 
for Letters Patent is set out in the following appended claims.