U.S. patent application number 13/808281 was filed with the patent office on 2013-08-08 for spiral wound module including membrane sheet with regions having different permeabilities.
This patent application is currently assigned to DOW GLOBAL TECHNOLOGIES LLC. The applicant listed for this patent is Steven D. Jons, Allyn R. Marsh, III. Invention is credited to Steven D. Jons, Allyn R. Marsh, III.
Application Number | 20130199988 13/808281 |
Document ID | / |
Family ID | 44913398 |
Filed Date | 2013-08-08 |
United States Patent
Application |
20130199988 |
Kind Code |
A1 |
Jons; Steven D. ; et
al. |
August 8, 2013 |
SPIRAL WOUND MODULE INCLUDING MEMBRANE SHEET WITH REGIONS HAVING
DIFFERENT PERMEABILITIES
Abstract
The present invention is directed toward spiral wound modules
including membrane sheets with regions (70,72) having different
permeabilities. Said regions are arranged either axially or
radially or both.
Inventors: |
Jons; Steven D.; (Eden
Prairie, MN) ; Marsh, III; Allyn R.; (Lakeville,
MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Jons; Steven D.
Marsh, III; Allyn R. |
Eden Prairie
Lakeville |
MN
MN |
US
US |
|
|
Assignee: |
DOW GLOBAL TECHNOLOGIES LLC
Midland
MI
|
Family ID: |
44913398 |
Appl. No.: |
13/808281 |
Filed: |
October 17, 2011 |
PCT Filed: |
October 17, 2011 |
PCT NO: |
PCT/US11/56499 |
371 Date: |
January 4, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61406597 |
Oct 26, 2010 |
|
|
|
Current U.S.
Class: |
210/457 ;
156/188 |
Current CPC
Class: |
B01D 63/10 20130101;
B01D 69/02 20130101; B01D 2325/20 20130101; B01D 63/103 20130101;
B01D 2325/08 20130101; B29D 99/005 20130101 |
Class at
Publication: |
210/457 ;
156/188 |
International
Class: |
B01D 63/10 20060101
B01D063/10; B29D 99/00 20060101 B29D099/00 |
Claims
1. A spiral wound module comprising: a permeate collection tube, at
least one membrane envelope wound about collection tube and
defining a first and second scroll face, wherein the membrane
envelope comprises a section of membrane sheet having an active
membrane area, a length corresponding to the distance between the
first and second scroll faces, a width extending in a direction
perpendicular to the length, at least one longitudinal axis
extending along the length of the sheet and dividing the sheet into
an inner and outer region with the inner region located adjacent to
the permeate collection tube, and at least one latitudinal axis
extending along the width of the sheet and dividing the sheet into
an inlet and outlet region with the inlet region located adjacent
to the first scroll face, wherein the membrane sheet comprises a
semi-permeable membrane layer and a support layer, and wherein in
the spiral wound module is characterized by the membrane sheets
being prepared such that the active membrane area of the membrane
sheet has an average water permeability or average solute
permeability that varies by at least 10% between at least one of:
i) the inner and outer regions and ii) the inlet and outlet
regions.
2. The spiral wound module of claim 1 further characterized by the
membrane sheet having an average water permeability that varies by
at least 25% between at least one of: i) the inner and outer
regions and ii) the inlet and outlet regions.
3. The spiral wound module of claim 1 further characterized by the
membrane sheet having an average water permeability that varies by
at least 40% between at least one of: i) the inner and outer
regions and ii) the inlet and outlet regions.
4. The spiral wound module of claim 1 wherein the average water
permeability of the outer region of the membrane sheet is at least
25% greater than the average water permeability of the inner
region.
5. The spiral wound module of claim 1 wherein the average water
permeability of the outlet region of the membrane sheet is at least
25% greater than the average water permeability of the inlet
region.
6. The spiral wound module of claim 1 wherein the section of
membrane sheet has an area, wherein the inner and outer regions
each comprises 25% of said area and wherein the outer region has an
average water permeability at least 10% greater than the average
water permeability of the inner region.
7. The spiral wound module of claim 1 wherein the section of
membrane sheet has an area, wherein the inlet and outlet regions
each comprises 25% of said area and wherein the outlet region has
an average water permeability at least 25% greater than the average
water permeability of the inlet region.
8. A method for making a spiral wound module comprising: providing
a permeate collection tube; providing at least one roll of membrane
sheet, wherein the membrane sheet comprises: a semi-permeable
membrane layer and support layer which are rolled up in a roll
direction; removing a first and second rectangular section of
membrane sheet from at least one roll, wherein each section
includes: four edges, two opposing sides including a membrane side
and a support side, a length in the roll direction, a width
extending in a direction perpendicular to the length, at least one
longitudinal axis extending along the length of the section and
dividing the membrane sheet into an inner and outer region, and at
least one latitudinal axis extending along the width and dividing
the membrane sheet into an inlet and outlet region, wherein each
section has an average water permeability that varies by at least
10% between at least one of: i) the inner and outer regions and ii)
the inlet and outlet regions; forming a membrane envelope by:
overlaying the first section of membrane sheet upon the second
section so that: the roll direction of both sections are parallel
to each other, and the inner, outer, inlet and outlet regions of
each section of membrane sheet are directly opposed to each other;
aligning the edges of both sections of membrane sheet with each
other; sealing both sections of membrane sheet together along three
of the aligned edges such that an unsealed fourth edge is parallel
to the roll direction of both sections and defines a proximal edge;
and winding the membrane envelope concentrically about the permeate
collection tube such that the proximal edge of the membrane
envelope is in a proximal position along the permeate collection
tube.
9. The method of claim 8 further characterized by each section of
membrane sheet having an average water permeability that varies by
at least 25% between at least one of: i) the inner and outer
regions and ii) the inlet and outlet regions.
10. The method of claim 8 wherein the average water permeability of
the outer region of each section of membrane sheet is at least 25%
greater than the average water permeability of the inner
region.
11. The method of claim 8 wherein the average water permeability of
the outlet region of each section of membrane sheet is at least 25%
greater than the average water permeability of the inlet
region.
12. The method of claim 8 wherein the semi-permeable membrane layer
comprises a hyperfiltration membrane.
13. The method of claim 8 wherein each section of membrane sheet
has an elastic modulus, and wherein the elastic modulus in the roll
direction is at least 1.5 times greater than the elastic modulus in
a direction perpendicular to the roll direction.
14. The method of claim 8 wherein the permeate collection tube has
a length, the roll of membrane sheet has a width, and wherein the
step of removing rectangular sections of membrane sheet from the
roll comprises: unrolling and detaching rectangular sections of
membrane sheet from at least one roll, wherein each section has
length extending in the roll direction which corresponds to the
length of the permeate collection tube and a width which
corresponds to the width of the roll; and wherein the length of the
section of membrane sheet is at least twice as large as the
width.
15. The method of claim 8 wherein the wherein the spiral wound
module is at least 1.75 meters long.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed toward spiral wound
modules and methods for making and using the same.
DESCRIPTION OF THE RELATED ART
[0002] Spiral wound modules (also referred to as spiral wound
"elements") are well known for use in a variety of fluid
separations including hyperfiltration. "Hyperfiltration" is a
membrane-based separation process where pressure is applied to a
feed solution on one side of a semi-permeable membrane. The applied
pressure causes "solvent" (e.g. water) to pass through the membrane
(i.e. forming a permeate solution) while "solutes" (e.g. salts) are
rejected and remain in the feed solution. To overcome the natural
driving force of solvent to move from low to high concentration,
the applied feed pressure must exceed the osmotic pressure. For
this reason, the term "hyperfiltration" is often used
interchangeable with "reverse osmosis." For purposes of this
description, the term "hyperfiltration" encompasses both reverse
osmosis (RO) and nanofiltration (NF). And it is further recognized
that modules containing hyperfiltration membranes may also be used
in forward or direct osmosis processes.
[0003] The solvent flux (J.sub.s) of a hyperfiltration membrane is
proportional to the pressure differential across the membrane minus
the difference in osmotic pressure between the feed and permeate
solutions. See Mulder, Basic Principles of Membrane Technology,
2.sup.nd Ed. (Kluwer Academic Publishers (1996). For aqueous feeds,
water flux (J.sub.w) can be defined as:
J.sub.w=A(.DELTA.p-.DELTA..pi.) (Formula I)
wherein:
[0004] "A" is the water permeability coefficient or "water
permeability" of the membrane;
[0005] ".DELTA.p" is the difference in applied pressure across the
membrane (i.e. difference in pressure of feed solution and
permeate); and
[0006] ".DELTA..pi." the difference in osmotic pressure between the
feed solution and permeate at the membrane surfaces.
[0007] During operation feed solution flows through a spiral wound
module with a portion of solvent (e.g. water) passing through a
semi-permeable membrane. As a result, the feed solution becomes
increasingly concentrated in solute (e.g. salts) as feed flows from
the inlet to outlet end of the module. Also, the applied feed
pressure drops as the feed flows through the module. These combined
effects result in flux imbalances across the module between the
inlet and outlet ends. Similarly, a flux imbalance can also result
between the permeate collection tube and the distal end of a
membrane leaf (i.e. in a direction perpendicular to the permeate
collection tube). In this case, pressure drop across the permeate
spacer results in a higher net-driving-pressure near the center of
the module as compared with its outer periphery. These flux
imbalances contribute to polarization and fouling.
[0008] Various techniques for reducing flux imbalances have been
proposed. For example, US 2007/0272628 describes the combination of
modules having different flux characteristics within a common
vessel. Other techniques include the use of shorter membrane leaf
lengths to reduce pressure drop along a permeate spacer. Permeate
spacers may also be chosen that vary in permeability in the
direction from the permeate collection tube to a distal end (e.g.
U.S. Pat. No. 4,792,401 and JP 2009/220070). While each of these
approaches reduce flux imbalances, new approaches are desired.
BRIEF SUMMARY OF THE INVENTION
[0009] The present invention is directed toward a spiral wound
module including a membrane sheet with regions having distinct
permeabilities. In one embodiment the module includes a permeate
collection tube and at least one membrane envelope wound about
collection tube and defining a first and second scroll face. The
membrane envelope comprises a section of membrane sheet having a
length corresponding to the distance between the first and second
scroll faces, a width extending in a direction perpendicular to the
length, at least one longitudinal axis extending along the length
of the sheet and dividing the sheet into an inner and outer region
with the inner region located adjacent to the permeate collection
tube, and at least one latitudinal axis extending along the width
of the sheet and dividing the sheet into an inlet and outlet region
with the inlet region located adjacent to the first scroll face.
The membrane sheet is characterized by having an average water or
average solute permeability that varies by at least 10% between at
least one of: i) the inner and outer regions and ii) the inlet and
outlet regions. Many additional embodiments are described including
methods for making and using such modules.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The included Figures illustrate several embodiments of the
invention. The Figures are not to scale and include idealized views
to facilitate description. Where possible, like numerals have been
used throughout the Figures and written description to designate
the same or similar features.
[0011] FIG. 1 is a perspective, partially cut-away view of a spiral
wound filtration module.
[0012] FIG. 2A is a perspective view (partially cut-away) of a
partially assembled spiral wound module including two aligned
sections of membrane sheet.
[0013] FIG. 2B is a perspective view of a section of membrane
sheet.
[0014] FIG. 2C is a perspective view of a section of membrane
sheet.
[0015] FIG. 3A is a perspective view of an idealized set-up for
practicing one embodiment of the invention showing a roll of
membrane sheet being unrolled along a roll direction parallel to an
axis (X) of an adjacently positioned permeate collection tube.
[0016] FIG. 3B is an elevational view of an idealized set-up for
practicing another embodiment of the invention showing two rolls of
membrane sheet being unrolled along roll directions parallel to an
axis (X) of an adjacently positioned permeate collection tube.
[0017] FIG. 4A is a perspective view showing one embodiment of a
partially assembled membrane envelope.
[0018] FIG. 4B is a perspective view of an assembled membrane
envelope.
[0019] FIG. 4C is a perspective view of a partially assembled
spiral wound module including the membrane envelope of FIG. 4B.
[0020] FIG. 4D is a perspective view of the partially assembled
spiral wound module of FIG. 3C taken at a subsequent point of
assembly.
[0021] FIG. 5A is a perspective view (partially cut-away) of a
partially assembled spiral wound module including one embodiment of
a membrane leaf packet.
[0022] FIG. 5B is a perspective view (partially cut-away) of a
partially assembled spiral wound module including a membrane
envelope being assembled with two membrane leaf packets.
[0023] FIG. 6A is a perspective view (partially cut-away) of a
partially assembled spiral wound module including an alternative
embodiment of a membrane leaf packet.
[0024] FIG. 6B is an elevation view of an alternative embodiment of
a membrane leaf packet.
[0025] FIG. 6C is an elevation view of yet another embodiment of a
membrane leaf packet.
[0026] FIG. 7 is an end view of a partially assembled spiral wound
module showing six membrane envelopes wound about a permeate
collection tube.
[0027] FIG. 8 is a perspective view of a spiral wound module.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The invention encompasses spiral wound filtration modules
along with methods for making and using the same. The configuration
of the spiral wound module is not particularly limited.
Representative examples of spiral wound filtration modules,
corresponding fabrication techniques and modes of operation are
described in: U.S. Pat. No. 5,096,584, U.S. Pat. No. 5,114,582,
U.S. Pat. No. 5,147,541, U.S. Pat. No. 5,538,642, U.S. Pat. No.
5,681,467, U.S. Pat. No. 6,277,282, U.S. Pat. No. 6,881,336, US
2007/0272628, US 2008/0295951 and U.S. 61/224,092. The module
includes at least one membrane envelope concentrically wound about
a permeate collection tube. The membrane envelope is preferably
formed from one or more membrane sheets which are sealed about a
portion of their periphery. An edge of the membrane envelope is
axially aligned along a permeate collection tube such that the
membrane envelope is in fluid communication with the permeate
collection tube but is otherwise sealed from feed fluid passing
across the outer surface of the membrane envelope.
[0029] A preferred embodiment of a spiral wound filtration module
is generally shown at 2 in FIG. 1. The module (2) is formed by
concentrically winding one or more membrane envelopes (4) and
optional feed channel spacer sheet(s) ("feed spacers") (6) about a
permeate collection tube (8). Each membrane envelope (4) preferably
comprises two substantially rectangular sections of membrane sheet
(10, 10'). Each section of membrane sheet (10, 10') has a
semi-permeable membrane layer or front side (34) and support layer
or back side (36). The membrane envelope (4) is formed by
overlaying membrane sheets (10, 10') and aligning their edges. In a
preferred embodiment, the sections (10, 10') of membrane sheet
surround a permeate channel spacer sheet ("permeate spacer") (12).
This sandwich-type structure is secured together, e.g. by sealant
(14), along three edges (16, 18, 20) to form an envelope (4) while
a fourth edge, i.e. "proximal edge" (22) abuts the permeate
collection tube (8) so that the inside portion of the envelope (4)
(and optional permeate spacer (12)) is in fluid communication with
a plurality of openings (24) extending along the length of the
permeate collection tube (8).
[0030] The module (2) preferably comprises a plurality of membrane
envelopes (4) separated by a plurality of feed spacers sheets (6).
In the illustrated embodiment, membrane envelopes (4) are formed by
joining the back side (36) surfaces of adjacently positioned
"membrane leaf packets." Examples of representative membrane leaf
packets and methods for their fabrication are further described in:
U.S. Pat. No. 4,842,736; U.S. Pat. No. 5,147,541 and US
2010/0140161. In one preferred embodiment (as will be described in
more detail with reference to FIGS. 5A-5B), a membrane leaf packet
comprises a substantially rectangular membrane sheet (10) folded
upon itself to define two membrane "leaves" wherein the front sides
(34) of each leaf are facing each other and the fold is axially
aligned with the proximal edge (22) of the membrane envelope (4),
i.e. parallel with the permeate collection tube (8). A feed spacer
sheet (6) is shown located between facing front sides (34) of the
folded membrane sheet (10). The feed spacer sheet (6) facilitates
flow of feed fluid in an axial direction (i.e. parallel with the
permeate collection tube (8)) through the module (2). While not
shown, additional intermediate layers may also be included in the
assembly. During module fabrication, permeate spacer sheets (12)
may be attached about the circumference of the permeate collection
tube (8) with membrane leaf packets interleaved therebetween. The
back sides (36) of adjacently positioned membrane leaves (10, 10')
are sealed about portions of their periphery (16, 18, 20) to
enclose the permeate spacer sheet (12) to form a membrane envelope
(4). The membrane envelope(s) (4) and feed spacer(s) (6) are wound
or "rolled" concentrically about the permeate collection tube (8)
to form a first and second scroll face (30, 32) at opposing ends
and the resulting spiral bundle is held in place, such as by tape
or other means. The sealant (14) used for sealing the edges (16,
18, 20) of the membrane envelope (4) preferably permits relative
movement of the various sheet materials during the winding process.
That is, the cure rate or period of time before which the sealant
(14) becomes tacky is preferably longer than that required to
assemble and wind the membrane envelopes (4) about the permeate
collection tube (8).
[0031] Arrows shown in FIG. 1 represent the approximate flow
directions (26, 28) of feed and permeate fluid during operation.
Feed fluid enters the module (2) from an inlet scroll face (30) and
flows across the front side(s) (34) of the membrane sheet(s) and
exits the module (2) at the opposing outlet scroll face (32).
Permeate fluid flows along the permeate spacer sheet (12) in a
direction approximately perpendicular to the feed flow as indicated
by arrow (28). Actual fluid flow paths vary with details of
construction and operating conditions.
[0032] Materials for constructing various components of spiral
wound modules are well known in the art. Suitable sealants for
sealing membrane envelopes include urethanes, epoxies, silicones,
acrylates, hot melt adhesives and UV curable adhesives. While less
common, other sealing means may also be used such as application of
heat, pressure, ultrasonic welding and tape. Permeate collection
tubes are typically made from plastic materials such as
acrylonitrile-butadiene-styrene, polyvinyl chloride, polysulfone,
poly (phenylene oxide), polystyrene, polypropylene, polyethylene or
the like. Tricot polyester materials are commonly used as permeate
spacers. Additional permeate spacers are described in US
2010/0006504. Representative feed spacers include polyethylene,
polyester, and polypropylene mesh materials such as those
commercially available under the trade name VEXAR.TM. from Conwed
Plastics. Additional feed spacers are described in U.S. Pat. No.
6,881,336.
[0033] During module fabrication, long glass fibers may be wound
about the partially constructed module and resin (e.g. liquid
epoxy) applied and hardened. In an alternative embodiment, tape may
be applied upon the circumference of the wound module as described
in U.S. 61/255,121 to McCollam. The ends of modules are often
fitted with an anti-telescoping device or end cap (not shown)
designed to prevent membrane envelopes from shifting under the
pressure differential between the inlet and outlet scroll ends of
the module. The end cap is commonly fitted with an elastomeric seal
(not shown) to form a tight fluid connection between the module and
a pressure vessel (not shown). Examples of end cap designs include
those available from The Dow Chemical Company, i.e. iLEC.TM.
interlocking end caps and those described in U.S. Pat. No.
6,632,356 and U.S. Ser. No. 12/545,098 to Hallan, et al. The outer
housing of a module may include fluid seals to provide a seal
within the pressure vessel as described in U.S. Pat. No. 6,299,772
and U.S. Pat. No. 6,066,254 to Huschke et al. and US 2010/0147761
to McCollam. Additional details regarding various components and
construction of spiral wound modules are provided in the literature
see for example U.S. Pat. No. 5,538,642 to Solie which describes a
technique for attaching the permeate spacer to the permeate
collection tube and WO 2007/067751 to Jons et al. which describes
trimming operations and the use of a UV adhesive for forming an
insertion point seal.
[0034] The membrane sheet comprises at least one semi-permeable
membrane layer (front side) and a support layer (back side) in
planer arrangement with each other to form a composite structure.
The support layer is not particularly limited but preferably
comprises a non-woven fabric or fibrous web mat including fibers
which may be orientated. Alternatively, a woven fabric such as sail
cloth may be used. Representative examples of support layers are
described in U.S. Pat. No. 4,214,994, U.S. Pat. No. 4,795,559, U.S.
Pat. No. 5,435,957, U.S. Pat. No. 5,919,026, U.S. Pat. No.
6,156,680, U.S. Pat. No. 7,048,855, US 2008/0295951 and US
2010/0193428. In preferred embodiments, the support layer is
provided as a roll of sheet material upon which a membrane layer is
applied. The support layer preferably comprises non-woven fibers
orientated in the roll direction such that the support layer has an
elastic modulus in the roll direction (i.e. length direction) which
is at least 1.5 times greater and more preferably at least 3 times
greater than the elastic modulus in a direction perpendicular to
the roll direction (i.e. width direction). Similarly, the membrane
sheet formed with the support layer also preferably has a elastic
modulus in the roll direction (i.e. length direction) which is at
least 1.5 times greater and more preferably at least 3 times
greater than the elastic modulus in a direction perpendicular to
the roll direction (i.e. width direction). As used herein, the term
"elastic modulus" refers to Young's modulus or tensile elasticity,
i.e. the ratio of tensile stress to tensile strain, as measured by
ASTM (D882-09). A support layer including fibers orientated in the
roll direction provides a module with improved dimensional strength
along the modules' length. This added strength can be particularly
useful when making long modules, i.e. modules over 1 meter long. It
will be understood by those skilled in the art that fibers of
support layers extend along a variety of directions and that term
"orientated" is intended to refer to a relative value, i.e. a
dominate alignment direction of fibers, rather than an absolute
value.
[0035] The support layer preferably includes a microporous polymer
support that may be cast upon the aforementioned non-woven fabric
or fibrous web mat. The microporous support is preferably about
25-125 microns in thickness. The microporous support preferable
comprises a polymeric material having pore sizes which are of
sufficient size to permit essentially unrestricted passage of
permeate but not large enough so as to interfere with the bridging
over of a semi-permeable membrane layer formed thereon. For
example, the pore size of the support preferably ranges from about
0.001 to 0.5 micron. In some instances pore diameters larger than
about 0.5 micron permit the semi-permeable membrane layer to sag
into the pores and disrupt a flat sheet configuration. Examples of
microporous supports include those made of: polysulfone, polyether
sulfone, polyimide, polyamide, polyetherimide, polyacrylonitrile,
poly(methyl methacrylate), polyethylene, polypropylene, and various
halogenated polymers such as polyvinylidene fluoride. The
microporous support may also be made of other materials. The
microporous support provides strength but offers little resistance
to fluid flow due to its relatively high porosity.
[0036] In a preferred embodiment, the semi-permeable membrane layer
of the membrane sheet comprises a hyperfiltration membrane, i.e. a
semi-permeable membrane suitable for hyperfiltration processes.
Such membranes and processes are often referred to as reverse
osmosis (RO) and nanofiltration (NF). For purposes of this
description, the term "hyperfiltration" encompasses both RO and NF
processes. RO membranes are relatively impermeable to virtually all
dissolved salts and typically reject more than about 95% of salts
having monovalent ions such as sodium chloride. RO composite
membranes also typically reject more than about 95% of inorganic
molecules as well as organic molecules with molecular weights
greater than approximately 100 Daltons. NF membranes are more
permeable than RO composite membranes and typically reject less
than about 95% of salts having monovalent ions while rejecting more
than about 50% (and often more than 90%) of salts having divalent
ions--depending upon the species of divalent ions. NF membrane also
typically reject particles in the nanometer range as well as
organic molecules having molecular weights greater than
approximately 200 to 500 Daltons.
[0037] In a preferred embodiment, the hyperfiltration membrane
layer comprises a thin film polyamide layer having a thickness of
less than about 1 micron and more preferably from about 0.010 to
0.1 micron. Due to its relative thinness, the polyamide layer is
commonly described in terms of its coating coverage or loading upon
the microporous support, e.g. from about 2 to 5000 mg of polyamide
per square meter surface area of microporous support and more
preferably from about 50 to 500 mg/m.sup.2. The polyamide layer is
preferably formed by an interfacial polycondensation reaction
between a polyfunctional amine monomer and a polyfunctional acyl
halide monomer upon the surface of the microporous polymer support,
as generally described in U.S. Pat. No. 4,277,344 and U.S. Pat. No.
5,658,460 to Cadotte et al. and U.S. Pat. No. 6,878,278 to Mickols.
More specifically, the polyamide membrane layer may be prepared by
interfacially polymerizing a polyfunctional amine monomer with a
polyfunctional acyl halide, (wherein each term is intended to refer
both to the use of a single species or multiple species), on at
least one surface of a microporous support. As used herein, the
term "polyamide" refers to a polymer in which amide linkages
(--C(O)NH--) occur along the molecular chain. The polyfunctional
amine monomer and polyfunctional acyl halide are most commonly
delivered to the microporous support by way of a coating step from
solution, where the polyfunctional amine monomer is typically
coated from an aqueous-based solution and the polyfunctional acyl
halide from an organic-based solution. Although the coating steps
need not follow a specific order, the polyfunctional amine monomer
is preferably coated on the microporous support first followed by
the polyfunctional acyl halide. Coating can be accomplished by
spraying, film coating, rolling, or through the use of a dip tank
among other coating techniques. Excess solution may be removed from
the support by air knife, dryers, ovens and the like.
[0038] The polyfunctional amine monomer may have primary or
secondary amino groups and may be aromatic (e.g.,
m-phenylenediamine, p-phenylenediamine, 1,3,5-triaminobenzene,
1,3,4-triaminobenzene, 3,5-diaminobenzoic acid, 2,4-diaminotoluene,
2,4-diaminoanisole, and xylylenediamine) or aliphatic (e.g.,
ethylenediamine, propylenediamine, and tris (2-diaminoethyl)
amine). Examples of preferred polyfunctional amine monomers include
primary amines having two or three amino groups, for example,
m-phenylene diamine, and secondary aliphatic amines having two
amino groups such as piperazine. The polyfunctional amine monomer
may be applied to the microporous support as an aqueous-based
solution. The aqueous solution may contain from about 0.1 to about
20 weight percent and more preferably from about 0.5 to about 6
weight percent polyfunctional amine monomer. Once coated on the
microporous support, excess aqueous solution may be optionally
removed.
[0039] The polyfunctional acyl halide is preferably coated from an
organic-based solution including a non-polar solvent.
Alternatively, the polyfunctional acyl halide may be delivered from
a vapor phase (e.g., for polyfunctional acyl halide species having
sufficient vapor pressure). The polyfunctional acyl halide is
preferably aromatic and contains at least two and preferably three
acyl halide groups per molecule. Because of their lower cost and
greater availability, chlorides are generally preferred over other
halides such as bromides or iodides. One preferred polyfunctional
acyl halide is trimesoyl chloride (TMC). The polyfunctional acyl
halide may be dissolved in a non-polar solvent in a range from
about 0.01 to 10 weight percent, preferably 0.05 to 3 weight
percent, and may be delivered as part of a continuous coating
operation. Suitable solvents are those which are capable of
dissolving the polyfunctional acyl halide and which are immiscible
with water, e.g. hexane, cyclohexane, heptane and halogenated
hydrocarbons such as the FREON series. Preferred solvents include
those which pose little threat to the ozone layer and which are
sufficiently safe in terms of flashpoints and flammability to
undergo routine processing without taking special precautions. A
preferred non-polar solvent is ISOPAR.TM. available from Exxon
Chemical Company. The organic-based solution may also include small
quantities of other materials.
[0040] Once brought into contact with one another, the
polyfunctional acyl halide and the polyfunctional amine monomer
react at their surface interface to form a polyamide layer or film.
This layer, often referred to as a polyamide "discriminating layer"
or "thin film layer," provides the composite membrane with its
principal means for separating solute (e.g. salts) from solvent
(e.g. water). The reaction time of the polyfunctional acyl halide
and the polyfunctional amine monomer may be less than one second
but contact times typically range from about 1 to 60 seconds, after
which excess liquid may be optionally removed by way of an air
knife, water bath(s), dryer or the like. The removal of the excess
water or organic solvent can be achieved by drying at elevated
temperatures, e.g. from about 40.degree. C. to about 120.degree.
C., although air drying at ambient temperatures may be used.
[0041] The polyamide layer may include one or more coatings
including those described in U.S. Pat. No. 6,280,853, U.S. Pat. No.
6,878,278, US 2009/0159527 and US 2010/0143733 to Mickols and US
2007/0251883 and US 2008/0185332 to Niu et al.
[0042] In a preferred embodiment, the membrane sheet has regions
with distinct water permeabilities, e.g. the water permeability
varies across portions of the length or width (or both) of the
membrane sheet. For purposes of this description, the term "water
permeability" ("A") is defined as:
A=J.sub.w/(.DELTA.p-.DELTA..pi.) (Formula II)
wherein:
[0043] "J.sub.w" is the water flux of the membrane,
[0044] ".DELTA.p" is the difference in applied pressure across the
membrane (i.e. difference in pressure of feed solution and
permeate), and
[0045] ".DELTA..pi." the difference in osmotic pressure between the
feed solution and permeate at the membrane surfaces.
[0046] The value of "A" (A-value) is a constant of the membrane
that depends principally on temperature and to a much lesser
extent, other operating conditions. For purposes of the present
description, the A-value for a membrane region may be measured by
dividing the water flux by the differential pressure, where flux is
measured at 25.degree. C., using pure water at a pH of 7 with a
differential pressure sufficient to result in a flux of
approximately 1 L/m.sup.2/day. Accurate measurements of the A-value
may require wetting out the membrane by placing it first into an
alcohol solution, such as 25% isopropyl alcohol, and then
equilibrating it in water before measurement.
[0047] The membrane sheet may also, or alternatively, have regions
of distinct solute permeabilities, e.g. the solute permeability
varies across portions of the length or width (or both) of the
membrane sheet. For purposes of this description, the term "solute
permeability" ("B") is defined as:
B=(J.sub.w.times.SP)/(1-SP) (Formula III)
wherein:
[0048] "J.sub.w" is the water flux of the membrane and "SP" is the
fractional solute passage through the membrane.
[0049] Similar to the A-value, the value of B (B-value) of a
membrane changes with temperature. B-values may also change with
ionic strength and pH. One skilled in the art will recognize that
polarization can cause measurements to over estimate solute passage
through the membrane. An accurate B-value is best obtained in the
limit of high surface mixing. For purposes of the present
description, the B-value for a solute over a region of membrane may
be calculated by measuring the flux and solute passage using an
aqueous solution comprising of 500 ppm of the solute (e.g. NaCl) at
25.degree. C., pH 7, with a differential pressure sufficient to
result in a flux of approximately 1 L/m.sup.2/day.
[0050] The variation in average water permeability (or average
solute permeability) of the membrane sheet preferably corresponds
to specific regions of membrane sheet used to make membrane
envelopes or leaf packets. For example, the average water
permeability of a section of membrane sheet may be varied as
between an inner region (adjacent to the permeate collection tube
and extending the length of the section) and an outer region
(distal to the permeate collection tube and extending the length of
the section), between inlet and outlet regions located adjacent to
the opposing scroll faces of the module, or both. In this way, flux
imbalances across the membrane sheet can be reduced. The difference
in average water permabilities (or average solute permeability)
between the above-mentioned regions is preferably at least 10%, 25%
or even 40%. The term "average water permeability" refers to a
numerical average of at least 10, (but more preferably at least 25)
A-values measured at locations evenly distributed across the entire
region of membrane sheet using pure water at 25.degree. C., pH 7,
and a differential pressure sufficient to result in a flux of
approximately 1 L/m.sup.2/day. The term "average solute
permeability" refers to a numerical average of at least 10, (but
more preferably at least 25) B-values measured at locations evenly
distributed across the entire region of membrane sheet using an
aqueous solution comprising 500 ppm of solute (e.g. NaCl) at
25.degree. C., pH 7 with a differential pressure sufficient to
result in a flux of approximately 1 L/m.sup.2/day. In calculating
average water and solute permeabilities, locations on the membrane
sheet exhibiting essentially no flux are excluded from the
calculation. Such locations typically correspond to glue lines.
Similarly, locations without an intact barrier layer are also
excluded, such as may result from scratches or other damage. In
order to clarify that such locations are excluded, the phrase
"active membrane area" may be used.
[0051] Methods for preparing membrane sheets having such variances
in average water permeability and/or average solute permeability
are not particularly limited and may involve controlling the
conditions under which the semi-permeable layer is formed. For
example, during formation of a thin film polyamide layer, the
concentration, stoichiometric ratio or temperature of the polyamide
forming reactants (e.g. polyfunctional amine monomer and
polyfunctional acyl halide monomer) may be controllably varied
during coating. Alternatively, the thickness or composition of an
optional coating (as described in U.S. Pat. No. 6,280,853; US
2009/0159527 and US 2010/0143733 to Mickols and US 2007/0251883 and
US 2008/0185332 to Niu et al.) may be varied across the width
and/or length of the membrane sheet to impart the desired
differences in average water permeability.
[0052] FIG. 2A illustrates a partially assembled spiral would
module including two aligned sections (10, 10') of membrane sheet
having lengths approximately corresponding to the length of a
permeate collection tube (8) and a width extending in a direction
perpendicular to the length. Each section (10, 10') includes at
least one longitudinal axis (Y) extending along the length of the
sheet and dividing the sheet into an inner (37) and outer (39)
region with the inner region (37) located adjacent to the permeate
collection tube (8) and the outer region (39) located distal to the
tube (8). Each section (10, 10') further includes at least one
latitudinal axis (Z) extending along the width of the membrane
sheet and dividing the sheet into an inlet (41) and outlet (43)
region with the inlet region (41) located adjacent to the first
scroll face (30) and the outlet region (43) located adjacent to the
second scroll face (32). While not shown in FIG. 2A, scroll faces
(30, 32) are formed upon winding the membrane sections (10, 10')
about the permeate collection tube (as shown in FIG. 1).
[0053] In a preferred embodiment, at least one but preferably both
sections (10, 10') of membrane sheet are characterized by having an
average water permeability (or average solute (e.g. NaCl)
permeability) that varies by at least 10% between at least one of:
i) the inner (37) and outer (39) regions and ii) the inlet (41) and
outlet (43) regions. In other embodiments, the average water
permeability varies by at least 25% or even 40% between such
regions. In one preferred embodiment, the average water
permeability of the outer region (39) of the membrane sheet is at
least 10%, 25% or even 40% greater than the average water
permeability of the inner region (37). In another embodiment, the
average water permeability of the outlet region (43) of the
membrane sheet is at least 10%, 25% or even 40% greater than the
average water permeability of the inlet region (41). In yet another
embodiment, both of the preceding conditions exist, i.e. both the
outer and outlet regions have average water permeabilities at least
10%, 25% or event 40% greater than the average water permeability
of the inner and inlet regions, respectively. In still another
embodiment, the average solute (e.g. NaCl) permeability of the
outlet region (43) of the membrane sheet is at least 10%, 25% or
even 40% greater than the average solute permeability of the inlet
region (41). Embodiments may include these features in combination.
For instance, the outer region (39) may have an average water
permeability that exceeds the inner region (37) by 10%, while the
outlet region (43) of the membrane sheet is at least 25% or even
40% greater than the inlet region (41).
[0054] While shown as being equal in size, the inner (37) and outer
(39) regions; and inlet (41) and outlet (43) regions need not be of
equivalent size. While shown as being divided into two regions (37,
39) along the length and two regions (41, 43) along the width,
additional regions may be included. For example, as illustrated in
FIGS. 2B and 2C, the section of membrane sheet may include multiple
longitudinal (Y, Y') and latitudinal (Z, Z') axes that define a
plurality of regions along the length (37, 37', 39) and width (41,
41', 43) of the sheet (10). Each longitudinal and latitudinal
region is preferably at least 25 mm wide, and more preferably at
least 50 mm wide. In one preferred embodiment, the membrane sheet
comprises an inner and outer region that each comprise 25% of the
active membrane area of the sheet closest to the permeate
collection tube (8) and distal edge, respectively, (with the
remaining 50% constituting a middle region). In another embodiment,
the membrane sheet comprises an inlet and outlet region that each
comprise 25% of the active membrane area of the sheet closest to
the first and second scroll faces (with the remaining 50%
constituting a center region). In yet another embodiment, both the
preceding conditions are present.
[0055] FIG. 3A is a perspective view of an idealized set-up for
practicing several embodiments of the invention. During fabrication
of a spiral wound module, first and second sections (10, 10') of
membrane sheet are removed from a common roll (38) having a width
(W) and assembled into a membrane envelope or membrane leaf packet
(not shown). The manner in which the sections (10, 10') are removed
from the roll (38) is not particularly limited but preferably
comprises unrolling (depicted by curved bi-direction arrow (40))
membrane sheet from the roll (38) along a roll direction (42, 42')
and detaching, e.g. cutting (as depicted by dotted lines (44))
rectangular sections of membrane sheet from the roll (38). Once
removed from the roll (38) the rectangular sections (10, 10') have
a width corresponding to the width of the roll (38) and a length
that preferably corresponds to the length of permeate collection
tube (8), (e.g. the length of the sections need not be exactly the
same as the tube (8) as excess sheet can be subsequently trimmed
away). The length of the sections (10, 10') is preferably at least
twice as large as the width, but more preferably at least 2.5, 3,
5, 7, 10 or in some embodiments at least 15 times as large. As will
be subsequently described, modules made pursuant to embodiments of
the present invention may have lengths over 1 meter long and in
some embodiments, lengths at least 1.75 meters, 2.75 meters, 3.75
meters, 4.75 meters and even 5.75 meters long.
[0056] As will be described in connection with other Figures, a
membrane envelope or membrane leaf packet can be formed by
overlapping and aligning rectangular sections of membrane sheet. In
the idealized set-up of FIG. 3A, sections (10, 10') of membrane
sheet are provided in an overlapping orientation by unrolling the
membrane sheet from a common roll (38) and reversing the roll
direction by way of a roller (46). In the idealized set-up of FIG.
2A, the membrane sheet is unrolled along a roll direction (42, 42')
which is parallel and adjacently aligned with an axis (X) defined
by the permeate collection tube (8). While this alignment is
preferred, it is not required. That is, membrane leaf packets or
membrane envelopes may be prepared at a remote location and
subsequently be aligned with a permeate collection tube (8) during
module assembly. However, in either embodiment the roll direction
(42, 42') of each section (10, 10') of membrane sheet is preferably
parallel with the axis (X) defined by the permeate collection tube
(8).
[0057] FIG. 3B illustrates another idealized set-up for practicing
an embodiment of the invention using two separate membrane rolls
(38, 38'), both shown with partially unrolled sections of membrane
sheet with opposing roll directions extending along a path parallel
to the axis (X) of an adjacently positioned permeate collection
tube (8). As with the embodiment of FIG. 3A, the illustrated set-up
provides membrane sheets (10, 10') in an overlapping orientation
which is adjacently aligned with the permeate collection tube (8).
While shown at opposite ends of the permeate collection tube (8),
the membrane rolls (38, 38') may also be positioned and unrolled
from a common end.
[0058] The roll (38) upon which the membrane sheet is wound may be
provided with either the membrane side (34) or support side (36)
facing outward. In the embodiment of FIG. 3A, the membrane sheet is
unrolled in a roll direction (42, 42') such that the support side
(36) faces outward and the overlapping sections (10, 10') are
orientated such that their membrane sides (34) are facing each
other. As will be described in connection with FIGS. 5-6, this
set-up is useful for making membrane leaf packets. Whereas, if the
membrane sheet were reversed, (i.e. such that the support sides
(36) of the overlapping sections (10, 10') are facing), the set-up
is useful for making membrane envelopes as described in connection
with FIGS. 4A-4D. Both approaches are applicable to the present
invention.
[0059] As previously described, the methods for preparing membrane
sheets having variances in average water permeability and/or
average solute permeability are not particularly limited. In
further reference to FIGS. 3A and 3B, a continuous membrane
manufacturing process may be used wherein membrane chemistry and or
coatings are varied in the width (W) direction during production of
the membrane roll (38). For example, during formation of a
semi-permeable layer, different reactants or different
concentrations of reactants may be applied at to different
positions on a support surface. In a reaction between
m-phenylenediamine and trimesoyl chloride, applying a lower
concentration of amine to the left side of the membrane than to the
right will result in higher water permeability for the left side of
the sheet. Similarly, additional of a small amount of a different
monomer, such as m-phenylenediamine, to a piperazine and trimesoyl
chloride reaction can reduce water and salt permeability across the
width (W) of the membrane sheet. Likewise, directed heat or cooling
may also be used to vary permeability across the membrane sheet.
Reactants may also be applied or removed at different locations
down the line for different positions across the membrane sheet, as
reaction time can also be a relevant parameter in determining
performance properties. Alternatively, the thickness or composition
of an optional coating (as described in U.S. Pat. No. 6,280,853; US
2009/0159527 and US 2010/0143733 to Mickols and US 2007/0251883 and
US 2008/0185332 to Niu et al.) may be varied across the membrane
width to impart the desired differences in average water or solute
permeability.
[0060] Another approach to obtaining a membrane sheet with regions
of distinct water permeability or solute permeability is to vary
properties along the roll direction (42). In a continuous membrane
manufacturing process this can be done by periodically cycling
conditions influential on membrane formation as the membrane sheet
(10) moves in the roll direction (42). As described for variations
across the membrane width, there are many parameters that may be
varied in the process to induce desired variations in
permeabilities. Conditions under which the semi-permeable layer is
formed may be varied (time, temperature, concentrations, monomers)
or the membrane properties may be modified after formation (as by
coating or post-treatment). For example, in a reaction between
m-phenylenediamine and trimesoyl chloride, the position at which
the trimesoyl chloride is applied is one parameter that may be
cycled on the time scale required. The concentration of amine
applied can also be cycled at the desired rate. As will be
appreciated by those skilled in the art, the rate at which
conditions need to cycle depends upon line speed. The cycle rate
becomes less challenging for embodiments wherein the membrane sheet
is oriented with the roll direction (42) parallel to the permeate
collection tube, particularly if the spiral wound module is longer
than 1 meter. While spiral wound modules are typically at most one
meter in length, there is an advantage in using lengths longer than
1.75 meters, 2.75 meters, 3.75 meters, 4.75 meters and even 5.75
meters. In one embodiment, membrane sheet used to make modules is
provided on a continuous roll (38) and has periodic variations in
average water or solute (e.g. NaCl) permeability in the roll
direction (42), with a period between 2 and 20 meters.
[0061] FIGS. 4A-4D illustrate one embodiment of a membrane envelope
and spiral wound module. Turning to FIG. 4A, a partially assembled
membrane envelope is generally shown at 4 including a first and
second rectangular section (10, 10') of membrane sheet. The
membrane envelope (4) is formed by overlaying sections (10, 10')
such that the roll directions (42) of both sheets (10, 10') are
parallel. The overlaying sections (10, 10') are preferably arranged
so that the roll direction of both sections are parallel to each
other, and the inner, outer, inlet and outlet regions of each
section of membrane sheet are directly opposed to each other (as
best shown in FIG. 2A). The edges of the sections (10, 10') are
aligned and sealed together along three edges. The method for
sealing the sections together is not particularly limited, (e.g.
application of adhesive or sealant (48), application of tape,
localized application of heat and pressure, etc.). Once sealed
together as shown in FIG. 4B, the membrane envelope (4) includes an
unsealed edge or "proximate edge" (22) which is parallel to the
roll directions (42) of the sections (10, 10'). FIG. 4C shows the
membrane envelope (4) in alignment along the permeate collection
tube (8) such that the proximal edge (22) is parallel with the axis
(X) and in a proximal position along the permeate collection tube
(8). Once aligned, the proximal edge (22) is in fluid communication
with the opening(s) (24) along the permeate collection tube (8) but
is preferably sealed such that feed fluid flowing through the
module (shown as arrow 26 in FIG. 1) is prevented from passing
directly into the permeate collection tube (8). FIG. 4D shows the
membrane envelope (4) being concentrically wound about the permeate
collection tube (8). As previously described, the membrane envelope
(4) may be formed at a remote location and subsequently be aligned
along a permeate collection tube (as shown in FIG. 4C) during
module assembly. Alternatively, the membrane envelope may be formed
from membrane sheets which are already aligned with the permeate
collection tube as illustrated in FIGS. 3A-3B.
[0062] FIG. 5A illustrates an embodiment of a partially assembled
membrane leaf packet, generally shown at 50. The membrane leaf
packet (50) has four edges and may be prepared by removing a
rectangular portion of membrane sheet from a roll (not shown). The
portion is then folded along an axis parallel with the roll
direction (42) of the membrane sheet to form a first (52) and
second (54) leaf extending from a fold (56). The portion is folded
such that the membrane sides (34) of the leaves (52, 54) face each
other, preferably with their edges aligned (i.e. both leaf 52, 54
have approximately the same dimension).
[0063] As shown in FIG. 5B, a membrane envelope (4) may be formed
by overlaying a first membrane leaf packet (50') upon a second
membrane leaf packet (50) such that the support side (not shown) of
a membrane leaf (54') of the first membrane leaf packet (50') faces
the support side (36) of a membrane leaf (52) of the second
membrane leaf packet (50). The edges of the first and second
membrane leaf packets (50, 50') are aligned such that the folds
(56, 56') of each are aligned and parallel with each other. The
facing membrane leaves (54', 52) are sealed together along three
peripherally edges (48) such that an unsealed fourth edge defines a
proximal edge (22) which is aligned and parallel to the folds (56,
56') of the first and second membrane leaf packets (50, 50'). As
with the embodiments of FIG. 1 and FIGS. 4C-4D, the proximal edge
(22) of the membrane envelope (22) is in fluid communication with
the permeate collection tube via openings (24).
[0064] FIG. 6A illustrates an alternative embodiment of a membrane
leaf packet (50'') comprising a first and second rectangular
section (10, 10') of membrane sheet. The sections (10, 10') are
removed from at least one roll (not shown). Each section (10, 10')
has four edges and two opposing sides including a membrane side
(34) and a support side (36). The membrane leaf packet (50'') is
formed by overlaying the first section (10) upon the second (10')
such that the roll direction (42) of both sections (10, 10') are
parallel to each other and the membrane side (34) of both sections
(10, 10') are facing each other. The edges of the sections (10,
10') are aligned and both sections are sealed together along an
aligned edge (58) which is parallel to the roll direction (42) of
both sections (10, 10'), hereinafter referred to as a "sealed edge"
(58). The means for sealing the sealed edge (58) are not limited.
For example, in the embodiments of FIGS. 6A and 6B, tape (60) is
disposed along the length of the sealed edge (58); whereas FIG. 6C
illustrates an embodiment wherein heat and pressure (depicted by
inward facing arrows) are applied to seal the sections (10, 10')
together to form the sealed edge (58). While not shown, sealants
such as adhesives may also be used to form the sealed edge (58).
Membrane envelopes can be formed using the membrane leaf packets
(50'') of FIG. 6A in the same manner as described in connection
with FIG. 5B.
[0065] As shown in FIG. 7, the spiral wound module (2) may include
a plurality of membrane envelops (4). While six membrane envelopes
are shown, preferred embodiments include at least 3, and in some
embodiments at least 20 or even 50. The spiral would module (2)
comprises an inner domain (62) (represented by the area within an
inner concentric circle shown with dashed lines) that comprises 25%
of the total membrane area of the module and is located closest
(i.e. concentrically about) the permeate collection tube (8). The
module (2) further comprises an outer domain (64) (represented by
the area outside the outer concentric circle shown with dashed
lines) that comprises 25% of the total membrane area of the module
and is located most distal about the permeate collection tube (8).
The remaining 50% of total membrane area is located within a middle
domain (68). The average water permeability (or average solute
permeability) of membrane located within the outer domain (64) of
the module is preferable at least 10%, 25% or even 40% greater than
the average water permeability (or solute permeability) of the
membrane sheet in the module's inner domain (62).
[0066] FIG. 8 illustrates another embodiment of the invention
wherein the spiral would module (2) comprises an inlet and outlet
domain (70, 72) that each comprise 25% of the total membrane area
of the module and are located adjacent to the first and second
scroll faces (30, 32), with the remaining 50% of membrane area
constituting a center domain (74). The average water permeability
(or average solute permeability) of membrane located within the
inlet domain (70) is preferably at least 10%, 25% or even 40%
greater than the average water permeability (or average solute
permeability) of membrane in the module's outlet domain (72).
[0067] Embodiments may include a combination of features described
in connection with the embodiments of FIGS. 7 and 8. For instance,
the outer domain may have an average water permeability that
exceeds the inner domain by 10%, 25%, or even 40%, while membrane
in the outlet domain of the module is at least 25% or even 40%
greater than membrane in the inlet domain of the module.
[0068] As shown in FIG. 1, spiral wound modules of the present
invention may optionally include one or more feed channel spacer
sheets (6). During the fabrication of a spiral wound module, a feed
channel spacer sheet may be positioned in planer alignment (i.e.
overlaid) with a membrane envelope prior to winding the membrane
envelope about the permeate collection tube. A spiral wound module
with inlet and outlet domains that differ substantially in average
water and/or solute permeabilities is particularly advantageous
when using a feed spacer that is characterized as having a pressure
drop greater than 1.5 bar when 0.12 m/sec water at 25.degree. C. is
flowed therethrough.
[0069] As also shown in FIG. 1, spiral wound modules of the present
invention may optionally include one or more permeate channel
spacer sheets (12). A permeate channel spacer sheet may be
positioned within the membrane envelope such that the permeate
sheet extends from the proximal edge of the membrane envelope prior
to winding the envelope about the permeate collection tube. A
spiral wound module with inner and outer domains that differ
substantially in water and/or solute permeabilities is particularly
advantageous when the permeate channel spacer has relatively high
resistance to flow in the direction perpendicular to permeate
collection tube. In one preferred geometry, the coefficient of
pressure drop "C.sub.p" in the permeate spacer sheet (in the
direction perpendicular to the permeate collection tube) is chosen
for the module based on the equation below:
C p .ident. 1 Q p ( x ) P ( x ) x > K A avg W 2 ( Formula IV )
##EQU00001##
wherein:
[0070] "W" is the width of the membrane sheet (i.e. from the
permeate collection tube to the distal end of the membrane
envelope,
[0071] "P" is the pressure at location "x" located along the width
(W) of the membrane sheet,
[0072] "A.sub.avg" is the average water permeability of the
membrane sheet within the module,
[0073] "Q.sub.p" is the permeate flow rate per unit width, and
[0074] "K" is a numerical value greater than 0.6, more preferably
greater than 0.75 and in some embodiments greater than 0.9.
[0075] While C.sub.p is approximately a constant of the permeate
spacer sheet, for purposes of the present description, C.sub.p may
be determined using a flow rate Q.sub.p within the permeate spacer
sheet that is an average for a spiral wound module when operating
at an average flux of 1 L/m.sup.2/day with pure water at a pH of 7
and 25.degree. C.
[0076] While not shown, feed channel spacer sheets and permeate
channel spacer sheets may be provided from rolls in a manner
similar to that of the membrane sheet, as shown in FIGS. 3A and 3B.
The use of aligned rolls of sheet materials facilitates the
production of long modules, (e.g. longer than 1 meter and
preferably at least 1.75 meters, 2.75 meters, 3.75 meters, 4.75
meters and even 5.75 meters long). The use of sheet materials
having higher elastic modulus values in the length or roll
direction as compared with the width direction (e.g. preferably
3.times. greater) further facilitates the production of such long
modules due to increased dimensional strength along the modules'
length. The spiral module may also include glass fiber or a tape on
the circumference of the module that aligns with the permeate tube
(as described in U.S. 61/255,121), and this can provide strength to
modules, especially modules of at least at least 1.75 meters, 2.75
meters, 3.75 meters, 4.75 meters and even 5.75 meters long.
[0077] Many embodiments of the invention have been described and in
some instances certain embodiments, selections, ranges,
constituents, or other features have been characterized as being
"preferred". Such designations of "preferred" features should in no
way be interpreted as an essential or critical aspect of the
invention. While membrane sheets including thin film polyamide
layers have been described in detail, other types of
hyperfiltration membrane layers may be used.
[0078] The entire content of each of the aforementioned patents and
patent applications are incorporated herein by reference.
* * * * *