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Description  |
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BACKGROUND OF INVENTION
A. Field
This invention relates to an improved membrane separation device of the
spiral wound type useful for ultrafiltration, microfiltration and reverse
osmosis applications and capable of obtaining high conversions while
maintaining turbulent or chopped laminar hydrodynamic flow conditions,
including methods of use. More specifically, the invention relates to a
spiral wound membrane element device having a radial feed path ("RFP"
herein) and thereby providing a potential for much higher conversion rates
in a single element than heretofore possible.
B. Description of the Prior Art
Spiral membrane elements for ultrafiltration, microfiltration and reverse
osmosis have long been regarded as efficient devices for separating
components of fluid mixtures. In a typical process, a pressurized fluid
mixture is brought into contact with a membrane surface whereby one or
more components of that fluid mixture pass through the membrane because of
a difference in chemical potential and, due to varying mass transport
rates through the membrane, a separation is achieved.
The most common spiral membrane element known heretofore is designed to
have the fluid feed mixture enter at one end of the cylindrical membrane
element and travel across the spiral windings between parallel membrane
surfaces along the longitudinal axis of the element (axial feed path-"AFP"
herein). Separation occurs at the membrane-fluid interface resulting in
(1) a more concentrated feed stream and (2) a permeate, which is the fluid
passing through the membrane barrier layer. The permeate stream travels in
a spiraling radial direction within the separate sealed channel defined by
the permeate sides of two membranes until it reaches the porous central
core tube where it is collected and expelled out one or both ends of the
core tube (see, U.S. Pat. Nos. 4,235,723, 3,367,504, 3,504,796, 3,493,496,
3,417,870).
Spiral wound membranes invariably contain a flow path or channel for the
feed enclosed by membrane sheets with active membrane barrier layers
facing said flow path. In the case of anisotropic membranes containing a
single barrier layer on only one side of the sheet, it is conventional for
the membrane sheets to have the barrier layers facing each other and
separated by a spacer which promotes turbulence in the feed flow path. The
membranes are edge-sealed with adhesives or heat sealed, etc. in such a
manner as to furnish an inlet for feed and an outlet for concentrate
(since "feed" becomes "concentrate" as it passes along the membrane, the
stream within the membrane element may be optionally termed
"feed-concentrate" herein).
The conversion (i.e. the ratio of permeate volume to feed volume) for the
common prior art spiral elements is governed by the element's length (see,
Desalination by Reverse Osmosis, Ulrich Merten, 1966, Chapter 5).
Typically, unit conversions are far below commercial process requirements,
requiring numerous elements in series to achieve acceptable converions.
For example, a typical remote osmosis system operating at 75% conversion
might require eighteen one meter long elements in a 2-1 array of pressure
vessels producing a feed-concentrate flow path length of 12 meters (i.e.,
first stage has six elements in series in each of two parallel trains and
the second stage has six elements in series in a single train). The
requirement for arraying spiral elements in series depending on the
fouling potential of the feed water with the above example being most
commonly employed on municipal, well, and surface-water feeds without
extraordinary pretreatment.
For desalination systems requiring high conversions and permeate flows
below 75,000 to 100,000 GPD, (gallons per day), small diameter elements
(less than 8 inches) must be used to maintain arrays with 12 meter
feed-concentrate path lengths. The drawbacks to this method of obtaining
high conversion include (a) increased floor space requirements, (b)
increased membrane module cost on a cents per gallon basis, (c) increased
process and pressure vessel costs, and (d) added complexity of expanding
systems due to array requirements.
If it were possible to change the element flow path from the standard axial
(AFP) to a radial direction (RFP), the flow path may be tailored to the
desired conversion rate or even increased; thus such module's conversion
would be governed by its diameter rather than length.
Unfortunately, it is not a simple matter to design a practical radial flow
path element since the permeate collected within the permeate channel must
not travel more than one to two meters before exiting the module, or
excess back pressure is generated in the permeate carrier fabric reducing
the element's efficiency. This constraint eliminates the possibility of
successfully utilizing the principle of the flowpath design of U.S. Pat.
No. 3,933,646 containing one or more very long membrane envelopes in which
the permeate travels the length of the membrane envelope before entering
the core tube and exiting the module.
SUMMARY OF INVENTION
A. We have now discovered a spiral wound membrane element device that can
operate at high conversions (i.e. greater than 30% and up to but not
limited to 90%) while maintaining turbulent or chopped laminar flow
conditions. This is accomplished by designing the feedconcentrate path to
spiral radially (RFP), preferably outwardly from the central core tube
while collecting the permeate through one or both open lateral edges of
the membrane element. This latter feature provides a maximum permeate flow
path not greater than the element's axial length and independent of the
feed-concentrate flow path length. The high pressure seal between
individual membrane sheets and the permeate fluid is accomplished by
sealing the product water carrier fabric with an adhesive (or thermally)
while recessing the membrane and spacer materials from the edge of the
permeate carrier fabric.
B. This new spiral wound element does not require serial staging in order
to operate at commercially viable system conversions for either
ultrafiltration ("UF"), microfiltration ("MF"), or reverse osmosis ("RO")
applications. Accordingly, small reverse osmosis systems ranging in flow
from 3000 to 75,000 GPD may be produced with large diameter elements (8-12
inches or larger). System design can be modular, i.e. elements may be
added on a unit basis without the need for maintaining proper arrays.
Smaller systems (less than 3000 GPD) may be produced by decreasing the
element length and/or diameter.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention may be better understood and its numerous objectives and
advantages will become apparent to those skilled in the art by reference
to the accompanying drawings. A reverse osmosis element is illustrated
since RO design requirements are perhaps the most critical because of the
high hydraulic pressures needed for filtration.
FIG. 1 is a cut-away view illustrating the spiral wound RFP membrane
element of the invention within an external cylindrical pressure housing.
FIG. 2 is a perspective view of a typical layer arrangement to be wound
about a porous tube to produce the spiral wound RFP element of the
invention.
FIG. 3 is an enlarged section of a feed-side view of the RFP element of
FIG. 1 taken along line 3--3.
FIG. 4 is an enlarged cross-section view of a recessed portion of the
permeate side of the RFP element of FIG. 1 taken along line 4--4. A
portion of this section has been further enlarged in section 4a to show
the positional relationship between the constituents.
FIG. 5 is an enlarged cross-section view of a nonrecessed portion of the
permeate side of the RFP element of FIG. 1 taken along line 5--5. A
portion of this section has been further enlarged in 5a to show the
positional relationship between the constituents.
FIGS. 6 and 7 which diagrammatically illustrate the prior art elements and
the RFP membrane elements of the present invention, respectively, together
with their sections 6a and 7a, are included for the purpose of explaining
the different membrane flow paths.
The manufacture of spiral wound membrane elements for reverse osmosis,
ultrafiltration and microfiltration applications is well known in the art.
Flat sheet membranes, alternately containing, between the membrane sheets,
open porous fabric or plastic sheets or webs are attached to (normally
with adhesive) and wound about a central porous "core" tube. The open
porous sheets between membranes serve primarily to transport the fluid,
create turbulence (in the case of a brine spacer, etc.) and prevent the
collapse of the flow channel.
As used herein, and in the appended claims, for convenience the term
"spacer" or "spacer sheets" without qualification is intended to define
the common porous sheet materials known to the membrane filtration art,
particularly the reverse osmosis field, as useful for providing a solid
but porous conduit for permeate fluid or turbulence in a stream flowing
within a confined channel or merely to space membrane sheets or prevent
collapse of the channel between membranes under elevated pressures. Such
"spacer sheets" as used heretofore are normally of fabric or plastic
composition but any durable flat sheet material capable of performing the
above-stated functions upon assembly into spiral wound membrane elements
should be deemed within the purview of the term "spacer."
In most instances, there is a feed (brine in the case of RO) spacer on the
feed-condensate side of the membrane leaf (i.e., the side with the active
barrier membrane surface of "skin") and a knitted fabric sheet spacer for
permeate transport on the opposite, i.e., permeate side. Using industrial
adhesives and cements well known to this art or other sealing means such
as heat sealing the various sheets or leaves of membranes and spacers are
"glued" to form the flow paths, normally immediately before winding the
membrane into a rigid cylinder. After the optional end-cups, and other
housing parts are glued onto this cylinder and all of the seals are cured,
the spiral membrane element may be inserted or connected to apparatus as
the case may be for filtration purposes. In reverse osmosis applications
the element with end cups attached is usually inserted in a pressure
vessel tube for high pressure filtration (e.g., 4-100 atmospheres).
Obviously, depending upon the desired flow configuration, it is
conventional in the art to have a number of repeating membrane envelopes
(5-15) and spacers wonnd about a single porous core tube. Unlike the
spirals of the prior art, in the RFP element which can provide a very long
flow path for the feed, it is desirable that fewer, even a single or
double membrane envelope will prove most useful for the application
intended. Unlike the conventional spiral wound membrane elements which
must be staged in a series to achieve high conversion, the RFP element can
be designed to achieve the desired conversion within a single element with
overall capacity increased by parallel flow through additional elements.
DETAILED DESCRIPTION OF DRAWINGS
FIG. 1 shows a cut-away view of a typical reverse osmosis spiral wound
membrane element 1 of this invention within a cylindrical RO pressure
vessel 2. The membrane element 1 is sealed within the pressure vessel 2
with end plates 2a and 2b containing ports for the various feed,
concentrate and permeate nozzles and retained in position by ring clamps
10a and 10b. "O" ring seals 6, 6a, 6b, 6c, 6d, 6e and 6f are shown as
solid dark rectangles at the principle points where the membrane element
or its various nozzles are connected to ports of the membrane element 1,
the pressure vessel 2, its end plates 2a and 2b. At the center of the RFP
membrane element 1 is a porous core tube 3 around which the membrane
sheets 15 and spacers are spirally wound. In FIG. 1, the porous core tube
3 contains a tube plug 4 (an adhesive plug) at the product end of membrane
element 1 to prevent mixing of feed and permeate at the product end in the
porous plate 5 which serves as a conduit for the product leaving the
pressure vessel 2 through a permeate nozzle 11. Both lateral edges of the
membrane element 1 are potted in a low viscosity adhesive 7a and 7b which
seals the membrane and spacer ends and bonds the membranes and spacers to
the optional end cups 9a and 9b. In place of end cups 9a and 9b the
membrane element 1 can be optionally sealed in a glue "cup" of the same
dimensions.
The product-side end cup 9b shown in FIG. 1 is cylindrical in shape and
contains an "O" ring seal 6b to prevent leakage of concentrate into the
porous plate 5. To insure sufficient encapsulation of the membrane element
1 it is preferable that the product end cup 9b length be about 6 inches,
and about 3 inches in the case of the feed-side end cup 9a. The two ends
of the membrane element 1 are potted in the end cups 9a and 9b
individually, usually starting with the product end followed by the feed
end, often on the following day. During the encapsulation process the
element may be placed in a pressure chamber and blanketed with nitrogen,
at e.g., 50 psig, to insure a bubble and void free seal.
At the product end of the membrane element 1 (see FIG. 1) the element abuts
a rigid porous plate 5 which serves to transport the permeate fluid
(product) from the product carrier fabric 14 through an outlet nozzle 11.
The spacers used in the feed-concentrate channel are not shown in FIG. 1.
The concentrated feed stream flows out of the membrane element 1 from an
unsealed end (not illustrated in FIG. 1) of the spiralling radial membrane
flow path into an open circumferential chamber 8, defined by the space
between the cylindrical element 1 and the cylindrical pressure vessel 2.
Openings contained in feed end cup 9a allow the concentrate to pass out of
the circumferential chamber 8 into an open space about a feed nozzle 12
and thence exit from the pressure vessel 2 through a concentrate nozzle
13.
Although for purposes of illustration only the figures show a membrane
design having a permeate exit on only one side of the RFP element 1, a
permeate exit may be contained on both ends with only minor changes in
design. To achieve dual permeate ports the sealing technique used for the
permeate side in FIG. 1 may be repeated on the opposite side together with
a second permeate nozzle 11. Relocation of the concentrate nozzle 13 to
another location along the pressure vessel 2 would be simple matter of
design convenience.
FIG. 2 is a diagrammatic illustration of a typical layer arrangement of an
RFP spiral wound membrane of the invention containing product carrier
fabric 14, a feed spacer, and two membrane sheets 15. The product carrier
fabric 14 is typically a knit fabric capable of transporting the product
fluid (usually water) along the defined permeate flow path. As
illustrated, the membrane sheets 15 and the feed spacer material are
recessed in width with respect to the width of the product carrier fabric
14. In order to produce effective seals, on the product side the preferred
recess is about four inches, but at least about one inch, and on the feed
side the preferred recess is about one inch, but at least about one-half
inch.
Referring to FIG. 3, the feed side end cup 9 is of a molded hexagonal
configuration with a central opening to accommodate the core tube 3.
FIGS. 4 and 4a are enlarged cross sections of the element 1 of FIG. 1 and a
further enlarged view of the potted permeate fabric of the element,
respectively. In FIG. 4 the outer ring is the pressure vessel 2 into which
is fitted the product-side end cap 9b. The adhesive 7b hydraulically seals
the end cup 9b to the spiralling potted carrier fabric 14 (represented by
the solid spiralling line) the only sheet of the membrane layer
arrangement illustrated in FIG. 2 which extends to the product end of the
element 1. In the further expanded view of FIG. 4a, the permeate carrier
fabric with a thin impervious film on either side thereof is shown potted
in adhesive 7b. The product carrier fabric 14 spirals outwardly from the
porous core tube 3, with a central adhesive tube plug 4, to the adhesive
layer connecting it to the end cup 9b.
FIGS. 5 and 5a are views of the non-recessed portion of the membrane
element 1 of FIG. 1 along line 5--5. In FIG. 5 the solid spiralling lines
represent the membrane sheets 15 and the spaces between represent the
feed-concentrate and permeate flow channels with spacers and adhesive 7a
omitted. In FIG. 5a the hatched lines represent spacers between membrane
leaves. (again the adhesive 7a is omitted).
FIG. 6 is a diagrammatic representation of a conventional spiral element of
the prior art in an exaggerated "unwound" state intended to illustrate the
flow direction of the feed-concentrate and permeate in such elements. FIG.
6a is a view of the spiral element of FIG. 6 along the line 6a-6a.
Channels a and c are feed-concentrate channels with b the permeate
channel. The barrier layer ("skin") sides of the membrane pairs face each
other in channels a and c, with spacers not shown, and channel b for
permeate is defined by the opposite (permeate) sides of the membrane
pairs. The feed stream flows axially into one end of the open membrane
channels a and c wherein a portion of the feed permeates the membrane skin
into the adjacent permeate channel b and the remaining feed (now
concentrate) exits through the opposite axial end of the membrane
channels. The permeate flows inward to the core tube at right angles to
the feed, and spirals down to ultimately leave the spiral winding through
the porous core tube and out of the element. To direct the flow path as
described, the membrane and spacer leaves are sealed at the indicated
places represented by shaded areas in FIG. 6 and 6a. Thus it may be seen
that the permeate channel b is sealed on all sides except at the openings
in the porous core tube. Seals at the core tube between permeate and
feed-concentrate channels illustrated in FIG. 6a are essential to prevent
mixing at that location.
FIG. 7 represents a preferred embodiment of the novel RFP element of the
invention. The feed enters a porous core tube where it is distributed at
right angles (see arrows) into the outwardly spiralling membrane channel
b. FIG. 7a is a view of the spiral element of FIG. 7 along the line
7a--7a. These figures do not accurately show the geometry of the RFP
element but are intentionally distorted from scale to more clearly
illustrate the flow patterns of the various fluid fractions within the
membrane channels. The concentrate leaves the membrane channel b at the
outer edge of the spiral winding after passing the full length of the
membrane channel. As illustrated, the feed-concentrate channel b is sealed
at both lateral edges (see FIGS. 7a) whereas the permeate channel on the
permeate sides of the membranes (illustrated as channels a and b) are
sealed at one lateral edge, (see FIG. 7a) longitudinally at the porous
core tube, and at the terminal edge of the spiralling membrane sheets (see
FIGS. 7 and 7a). Illustrated in FIG. 7a are seals for each membrane at the
core tube to prevent lateral mixing of permeate and feed-concentrate.
Because the length of the membrane in the RFP spiral is not constrained by
any operating limitations, such as backpressure from the permeate side,
the flow path can be shortened or lengthened to "tailor" the flow path to
the desired degree of conversion or concentration of the feed. In this
regard the area of the flow path and to a certain extent the type of fluid
flow, i.e., whether laminar or turbulent, determines the transmembrane
passage of the permeate. Prohibitive back pressure is avoided by allowing
the permeate to leave the spiral at right angles to the feed-concentrate
flow at one or both axial ends of the cylindrical element.
Permeation of a portion of the feed through the membrane along the
feed-concentrate flow path causes a gradual reduction of the feed volume,
thereby diminishing feed velocity in a fixed-dimension channel and
reducing the downstream permeation efficiency. This phenomenon is
exacerbated by the present invention which provides the possibility of a
much longer feed flow path (RFP). Design modifications of the RFP element
can reduce or virtually eliminate such feed velocity changes. Some of the
more obvious design changes include (1) using a tapered spacer to
progressively reduce the distance between membranes thereby constricting
the downstream flow path and increasing fluid velocity or (2) taper the
width of the flow path by sealing the edges closer to the middle along the
spiral path. A preferred embodiment of the invention utilizes a novel
option inherently provided by the RFP to internally "stage" a single
element. Accordingly, two, three or more membrane envelopes of different
lengths (measured radially from the core) can be wound about a single core
tube yielding multiple stages as the feed volume decreases along its
spiral path (see, e.g., Examples III and IV, infra). Various other options
should be obvious to those skilled in the art.
The radially spiralling feed flow path of the invention offers a much
longer potential net flow path length than the traditional axial flow
direction for the industry's standard spiral modules. This affords
correspondingly greater flow conversions without reduction in permeate
volume or quality. However, this novel flow path design requires a high
pressure seal between the feed and permeate streams located outside of the
membrane envelope; a requirement which is not necessary in the standard
spiral module flow geometry. Such a pressure seal is producable using an
adhesive and a compatible bonding surface. Not only must the bonding
surface be compatible with the sealing adhesive, it must also act as a
shield for the product water carrier to insure an unobstructed pathway for
the exiting permeate.
In a preferred embodiment, it is most desirable to coat or laminate a
hydraulically impervious film onto the product (permeate) carrier fabric
at the product end thereof to achieve suitable bonding to seal the product
end of the element. This coating or film, preferably a polymer film or
metal foil must be carefully applied to avoid substantial penetration into
the knit permeate fabric which could reduce transport of product through
the fabric particularly in a reverse osmosis operation. We have found that
this may be accomplished by applying a uniform non-porous polyurethane
coating to the surface of the fabric which is to be located at the product
end. The polymer coating is of such composition and thickness that it will
adhere uniformly to the surface of the fabric even when the fabric is
rolled into a tight cylinder in a spiral membrane element. The length of
the coating or film should be sufficient to form parallel planar fluid
seals about the knitted fabric, usually about 3-12 inches and preferably
6-10 inches long. To obtain an effective seal at a product (permeate) end
of the RFP spiral element of the invention it is normally necessary to
recess the membrane and feed spacer materials, allowing only the permeate
fabric to extend to the end of the element. Thus only the knit fabric,
which serves as the permeate conduit is visible from the product end of
the RFP element.
For convenience, the term "product" is used herein to identify the permeate
of a reverse osmosis desalination element. In some types of membrane
separations the feed-concentrate stream is the true product and the
permeate is a waste or recycled stream. Further, it is even possible that
both the permeate and feed-concentrate streams are considered as product
streams in the sense that both have uses after separation, e.g.,
ultrafiltration of electrocoat paint where both paint solids (concentrate)
and permeate water are reused in the painting-rinsing operation.
The principle of this invention is useful in any spiral wound membrane
device employing flat sheet membrane for reverse osmosis, ultrafiltration,
membrane softening, microfiltration, and gas separation, requiring the use
of recoveries greater than 20/30%, the limit of currently available RO
spiral wound elements based on present engineering practice. This
invention allows a single element ranging in lengths of about 12-60 inches
to operate under turbulent or chopped laminar flow conditions at
recoveries up to 90% while maintaining boundary layer conditions similar
to current brine staged spiral system designs using 12 to 18 elements in
series. Said another way, the degree of conversion/recovery of the feed
stream is independent of the length of a module, but rather depends upon
the length of the radial flow path which affects only the diameter of the
module.
Membranes for UF, RO MF and gas filtration are wellknown in the prior art.
Both anisotropic (asymmetric) membranes having a single or double barrier
layer (skin) and isotropic membranes are presently made in flat sheet form
for UF, RO, MF and gas filtration (see e.g., U.S. Pat. Nos. 3,615,024;
3,597,393; and 3,567,632). The membranes may be of a single polymer or of
a copolymer, laminated or of a composite structure wherein a thin barrier
coating or film, charged or uncharged is formed over a thicker substrate
film, the latter being either porou | | |
