U.S. patent application number 11/291528 was filed with the patent office on 2006-07-27 for spiral electrodeionization device with uniform operating characteristics.
Invention is credited to William W. Carson, Oleg Grebenyuk, Vladimir Grebenyuk, Russell J. MacDonald, Keith J. Sims, Li Zhang.
Application Number | 20060163056 11/291528 |
Document ID | / |
Family ID | 34549210 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060163056 |
Kind Code |
A1 |
Grebenyuk; Vladimir ; et
al. |
July 27, 2006 |
Spiral electrodeionization device with uniform operating
characteristics
Abstract
EDI apparatus for demineralizing a liquid flow is assembled in a
housing having a cylindrical shape, and includes two metal
electrodes, and one or more leafs, each leaf comprising a pair of
selectively ion-permeable membranes arranged parallel to each other
and spaced apart by spacing elements that allow liquid to flow in
the interstitial space between membranes, thus forming an
arrangement of dilute and concentrate cells in a desired flow
configuration. Spacing elements between membranes, as well as
between leaves, can be formed of inert polymer material, ion
exchange beads, ion exchange fibers, a combination of two or more
these elements, or a porous media incorporating one or more of such
elements as an intrinsic part. An inner or central electrode and an
outer or perimeter electrode establish a generally uniform and
radially-oriented electrical or ionic current between the inner and
the outer electrodes, across the helical flow spaces defined by the
membrane/spacer windings. One or both electrodes may include a
pocket, and the adjacent flow cells lie parallel to the electrode
and free of shadowing and field inhomogeneity around a full
circumference of the electrode. Flow paths within the helical cells
are defined by barrier seals, which may form a path-lengthening
maze, while unfilled cell regions may disperse or collect flow
within a cell and define pressure gradients promote directional
flows. Impermeable barriers between membranes further prevent the
feed and concentrate flows from mixing. In various embodiments,
seals along or between portions of the flow path may define a
multi-stage device, may define separate feed and/or concentrate
flows for different stages, and/or may direct the feed and
concentrate flows along preferred directions which may be
co-current, counter-current or cross-current with respect to each
other within the apparatus.
Inventors: |
Grebenyuk; Vladimir;
(Woburn, MA) ; Grebenyuk; Oleg; (Ashland, MA)
; Sims; Keith J.; (Wayland, MA) ; Carson; William
W.; (Hopkington, MA) ; MacDonald; Russell J.;
(Wilmington, MA) ; Zhang; Li; (Belmont,
MA) |
Correspondence
Address: |
Schwegman, Lundberg, Woessner & Kluth, P.A.
P.O. Box 2938
Minneapolis
MN
55402
US
|
Family ID: |
34549210 |
Appl. No.: |
11/291528 |
Filed: |
December 1, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/US04/34909 |
Oct 20, 2004 |
|
|
|
11291528 |
Dec 1, 2005 |
|
|
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60512661 |
Oct 20, 2003 |
|
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Current U.S.
Class: |
204/260 |
Current CPC
Class: |
C02F 2001/46152
20130101; B01D 61/48 20130101; C02F 2201/4611 20130101; B01D 61/52
20130101; C02F 2201/003 20130101; B01D 61/46 20130101; C02F 1/4695
20130101; B01J 47/08 20130101; B01D 63/10 20130101 |
Class at
Publication: |
204/260 |
International
Class: |
C25C 7/02 20060101
C25C007/02 |
Claims
1. An electrodeionization device comprising a generally cylindrical
housing including a) a cylindrical inner core b) an inner electrode
extending around the inner core c) a leaf arranged as a spiral
winding about the inner electrode d) an outer electrode extending
about the spiral winding, wherein active treatment cells are
defined by spaces within said spiral winding and by interleaf
spaces thereof arranged to limit increase of current density across
said active treatment cells.
2. The electrodeionization device of claim 1, wherein said
cylindrical core and inner electrode are arranged to define a
current density varies by a factor of less than two between a
position proximate the inner electrode and a position proximate the
inner electrode and a position proximate the outer electrode.
3. The electrodeionization device of claim 1, wherein an end region
of the leaf is disposed essentially parallel to one of said
electrodes whereby a substantially uniform electric field is
imposed across the leaf at said end region.
4. The electrodeionization device of claim 1, wherein at least one
of said inner and said outer electrode includes a volute.
5. The electrodeionization device of claim 4, wherein the volute
presents an electrode pocket, and said leaf terminates in the
pocket.
6. The electrodeionization device of claim 5, wherein the leaf
seals to the electrode pocket, and a flow cell defined by the leaf
communicates through the electrode.
7. The electrodeionization device of claim 1, wherein interior of
the leaf forms a concentrate cell.
8. The electrodeionization device of claim 1, wherein one or more
sealing bands within the leaf define concentrate flow.
9. The electrodeionization device of claim 8, wherein said bands
are arranged transverse to a flow of fluid that is to be
demineralized, and are positioned and oriented to receive and
maintain different scale-forming components in separate
channels.
10. The electrodeionization device of claim 8, wherein the sealing
bands effect one or more functions selected from the group
consisting of the function of a) defining flow paths of a length
longer than the leaf; b) defining a brine cell as an electrolyte
cell isolated from dilute cells; c) defining a brine path
orientation relative to a dilute path; d) defining brine flow such
that the brine is acidified to resist scaling; e) defining a
two-stage spiral EDI treatment device; f) defining pressure drops
to induce flow in a desired direction; g) defining brine cell inlet
and/or outlet port positions; and h) defining brine path impedance
that enables brine feed by a passive internal bleed from a feed or
product flow of dilute cells.
11. An electrodeionization device comprising a generally
cylindrical housing including a) an inner electrode extending
around the inner core b) at least one leaf arranged as a spiral
winding about the inner electrode c) an outer electrode extending
about the spiral winding, wherein spaces within said spiral winding
and interleaf spaces thereof define flow cells of the device, at
least one of said inner and said outer electrodes being configured
such that a leaf terminates at the electrode in a substantially
tangential direction.
12. An electrodeionization device comprising a generally
cylindrical housing including a) an inner electrode extending
around the inner core b) at least one leaf containing two
spaced-apart membranes and arranged as a spiral winding about the
inner electrode c) an outer electrode extending about the spiral
winding, and d) a plurality of seals applied in contact with said
membranes in a pattern within said leaf and/or an interleaf space
of said spiral winding so as to define a pattern, said pattern
controlling direction or location of flow for electrodeionization
of a feed flow within the housing.
13. An electrodeionization device comprising a generally
cylindrical housing and including an inner electrode and an outer
electrode defining therebetween an annular cylindrical space, and a
membrane/spacer leaf rolled in said an annular cylindrical space
such that the device constitutes an electrodeionization device
having dilute and concentrate flow paths defined between membranes
of said membrane/spacer leaf, and lying parallel one or more of
said electrodes around substantially the full circumference
thereof.
14. The electrodeionization device of claim 13, wherein diameters
of said outer and said inner electrode bear a ratio below 2.0 and
preferably below 1.5 effective to limit current density at the
inner electrode.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of earlier-filed PCT
International Application US2004/034909 filed Oct. 20, 2004, which
claimed priority of U.S. Provisional Application Ser. No.
60/512,661 filed Oct. 20, 2003.
BACKGROUND
[0002] The present invention relates to methods and devices for
demineralizing fluids, and relates to filtration or treatment
cartridges or modules, having a generally cylindrical aspect and
constructed with plies of spirally wound selectively-permeable
membranes. It particularly relates to electrodialysis and
electrodeionization devices, wherein the membranes possess ionic
selectivity, and the device includes electrodes for inducing
transport of species across the membranes by ionic conduction.
[0003] In general, electrodialysis (ED) and so-called
electrodeionization (EDI) devices operate by providing a structure
that arranges flow channels such that a flow of a feed fluid that
is to be treated is channeled between two ion exchange membranes of
opposite exchange type, while an electrical potential is applied
across the membranes transverse to the flow to maintain an ionic
current that demineralizes the feed fluid, moving ionizable species
from the feed fluid in one channel, through the membranes, and into
adjacent channels, thereby producing a demineralized product flow
from the feed. Spacers position successive membranes apart to
define the fluid treatment channels or "dilute" flow spaces.
[0004] A subclass of electrodialysis (ED) devices, often referred
to as electrodeionization (EDI) devices, further include a packing
of ion exchange material, typically beads or felt, as a
flow-pemeable packing within the flow treatment channels and, in
certain constructions, within the adjacent mineral-receiving
channels. The presence of exchange material in the treatment
channels or cells enhances the active fluid interaction area and
the capture of ions from the feed, and provides a stationary
transfer medium of good electrical and ionic conductivity for
transporting the captured ions to and across the surrounding
membranes. This construction offers a robust and efficient
mechanism for effectively separating many dissolved materials from
the flow along a relatively short flow path. The ion exchange
material is continuously maintained in an at least partially
regenerated (active) state by water splitting.
[0005] Over many years, developers of these units have explored the
suitability and operating characteristics of ED and EDI devices
with a range of flow channel geometries and flow velocities,
various membranes defining cells of different fixed or even
progressive thickness, and a variety of ion exchange fillings
distributed in various localized patterns (such as stripes, bands,
special monotype or mixed beds) and other variations. For certain
applications, the use of beads with special sorption, catalytic or
other properties has been described to stabilize operating
characteristics or effect other aspects of treatment.
[0006] In these devices, the feed fluid flows one or more times
through "dilute" spaces or cells, giving up its ions, to emerge as
a substantially demineralized or treated product flow, while a
separate fluid in adjacent "concentrate" or "brine" cells receives
the minerals stripped from the feed by ionic conduction through the
membranes, together with such non-ionic small molecules as may pass
through the membranes. Various physical implementations of ED and
EDI units are known. The majority of commercial devices,
particularly EDI devices, have historically employed an
architecture based on flat plate "stacks"--arrangements of many
cells formed by stacking substantially oblong membranes, spacers,
and screens--collectively forming many cells--between endplates,
with electrodes and usually ports or manifolds positioned at the
ends of the stack. Similar stacks of disk-shaped cells are
historically known. In addition to these "stack" constructions,
many publications also describe, and several companies have
commercially marketed, cylindrically-shaped ED or EDI devices
having cells formed between ion exchange membranes that are
spirally-wound about a pipe or core. These devices have electrodes
at radially inner and outer positions to apply a substantially
radial electrical field between the core and the outer shell of the
cylindrical device.
[0007] ED (unfilled) devices have found use in treating a number of
food industry fluids. A rolled spiral construction similar to the
spiral ED or EDI units has also long been used in fabricating
cross-flow reverse osmosis (RO), microfiltration (MF) and other
types of filtration/separation modules for use with feed streams of
alimentary fluids or fermentation product streams, so the spiral
architecture is well accepted in that industry for its flow dynamic
characteristics, plumbing requirements, ability to handle elevated
pressure and other desirable properties. These other spiral-wound
filtration devices typically rely upon elevated pressure to drive
the filtration process or product through a membrane, rather than
upon an electric potential to transport ionizable components across
a membrane. Such spiral filter constructions typically permit only
small deflections, and are able to sustain high pressures without
rupturing membranes. Applicant believes that a spiral EDI
construction may potentially enjoy a pressure resistant
construction that would desirably permit enhanced throughput,
longer, more effective treatment path length or other improved
property.
[0008] Among the published or commercially promoted spiral ED and
EDI products, early examples of Ionics, Incorporated, as shown in
U.S. Pat. No. 2,741,591, describe various directions for the
respective dilute and concentrate flows, both in relation to the
inner and outer electrode and with respect to each other. The
Christ, A. G. company of Switzerland has more recently marketed
spiral EDI devices, of which examples are shown in their U.S. Pat.
No. 5,376,253, entitled Apparatus for the Continuous
Electrochemical Desalination of Aqueous Solutions, naming inventors
Rychem et al. The construction shown in that patent is a spiral
wound EDI with inner and outer electrodes, having its fluid
treatment dilute cells sealed to the wall of, and opening into, the
inner electrode (which also serves as a central flow pipe), and
having its concentrate cells open to the surrounding cylindrical
wall that forms a counter-electrode.
[0009] Another commercial EDI unit of spiral architecture,
originally developed in China, employs mesh-filled wound
concentrate envelope and provides an axially oriented dilute flow
between the windings. This device is marketed in the United States
by Omexell, Inc. of Houston, Tex. The Omexell device is illustrated
in U.S. Pat. No. 6,190,528, naming inventors Xiang Li and Gou-Lin
Luo. In that construction, a central pipe is both an electrode and
a water distributor, while wound metal strip or wire forms the
outer electrode. Two membranes surrounding a mesh web form an
envelope without any exchange bead filling, and the envelope is
spirally-wound about the central pipe to form the concentrate flow
space(s) of the device. The alternate regions between successive
turns of the envelope are filled with ion exchange resin beads to
constitute the dilute channels. The input feed flow and the treated
product output proceed through the exchange bead-filled space along
an axial direction, from one end of the cylinder to the other,
while the concentrate flows from the product feed inlet (embodiment
#2, shown in FIG. 4 of the aforesaid '528 patent) or from a slot
along half the central electrode/pipe (embodiment #1, shown in
FIGS. 1-3 of that patent), along a helical path through the wound
concentrate envelope and into (or back into) the central
electrode/pipe. Thus, the Omexell construction winds a
membrane/spacer/membrane concentrate envelope, and fills the space
between windings with resin to form the dilute passages. The resin
filling is stated to be replaceable.
[0010] Some spiral EDI devices may employ a central pipe as an
electrode that doubles as a fluid manifold. Early flat plate EDI
stacks were arranged with their dilute and concentrate flows in
parallel planes but at a right angle to each other, or at a
meandering angle with respect to each other, while many modern flat
plate rectangular or oblong EDI stacks are now configured so that
dilute and concentrate flows are arranged in closely-spaced
parallel sheets in either a co-current or counter-current
arrangement. Spiral EDI devices tend to arrange a major portion of
the two flow paths cross-current, with one flow being axial and the
other locally across the axis along a globally helical path
following the spiral contour of the membrane envelopes that define
the dilute and/or brine cells. The spiral architecture permits one
to define different relative path lengths and flow rates of the two
fluids (for example, the axial path may be shorter than the spiral
path), and may allow some flexibility or advantages in other
respects, such as ease of re-filling or refurbishment, over
clamped-plate stack designs.
[0011] The Omexell spiral EDI construction is advertised as being
readily serviceable, and the '528 patent mentions replacing the
dilute cell exchange beads every day by opening the ends of the
cylinder, blowing out the exchange beads, and re-filling. That
Company has filed a number of This accessibility of the beads in
the construction of the '528 patent has been advertised to promote
the product by contrasting it to the situation applying to
conventional stacks of rectangular construction mentioned above, in
which the separate replacement of the exchange beads is generally
either quite cumbersome (for example, requiring disassembly and
re-assembly of the stack, or requiring a complex emptying and
filling regimen), or else is not feasible (because the dilute cells
are each formed as discrete permanently sealed envelope-cells that
cannot be opened). However, it is not entirely clear from the '528
patent or from the commercial product description why bead
replacement is deemed necessary. It is possible that the patent,
being a short technical description drafted by a third party at an
early stage of development, contains an erroneous description. It
is also possible that the common practice in China of relying upon
ion exchange beds for primary water treatment influenced the
inventors to emphasize, in the '528 patent, the replaceability of
exchange beads, so that the new EDI technology would be seen not as
an unproven and different technology, but as simply an augmented
form of the accepted and proven treatment involving periodic
renewal of an ion exchange bed. It is also possible, however, that
the device described in the '528 patent was prone to scaling as a
result of the minerals (such as calcium and silica) present in the
local waters and the nature of fluid flows and electrical fields
within the device, and that resin replacement was necessary in that
particular context.
[0012] EDI units were first developed forty or fifty years ago. At
a historically early period of this development, the bead filling
was often more or less readily accessible, and one could replace or
regenerate the beads separately at frequent intervals to achieve a
desired degree of treatment. This allowed the treatment regimen to
rely in part on the bead storage capacity (like that of a
conventional ion exchange bed) to accommodate part of the removal
burden or to effectively remove certain ones of the less mobile
ions. Generally, however, modern stacks and EDI devices are
designed to operate without disassembly or resin replacement for
extended times--a period up to several years. During operation, a
portion of the exchange bead filling is continuously electrically
regenerated, and the devices are operated in a steady state. While
certain feed water quality standards may be specified to assure
long term stability, occasional total regeneration and/or cleaning
or reversal cycles my be performed to address scale-like build-up
or performance deterioration, and to prevent any fouling or scaling
from irreversibly impairing operation.
[0013] Without dwelling further on generalities or specific
constructions, it may be said that EDI constructions of both the
stack and the spiral architectures rely on the capture of ions by
exchange beads and the transport of captured ions through a chain
of one or more beads either to, or closer to, the exchange
membranes that actually transfer the ions out of and separate the
ions from the feed flow/dilute path. The exchange beads are
continuously regenerated (for example, by hydronium or hydroxide
ions that are created by water splitting at places of high field
intensity, such as heterogeneous bead/bead or bead/membrane
junctions), and the devices are generally set up to operate in a
steady state on a given feed for extended periods of time. However,
the rate or flow distribution and other factors governing all these
effects are such that conditions of high concentration of specific
ions, extreme pH, or flow stagnation may all arise in use, and
certain combinations of these conditions may pose control problems,
impair the efficiency or degree of treatment, or risk introducing
irreversible membrane damage and/or localized occurrences of resin
or membrane scaling within the device. The dimensions and geometry
of the flow cells, the nature of the exchange filling formulations,
and details of the hydraulic plumbing may all be important in
addressing such problems, and a certain amount of pretreatment of
the feed fluid is also generally required to assure a suitable
initial feed quality that will not give rise to problems over the
long term. Extensive industrial operating experience further allows
one to specify operating parameters and protocols to follow for
each device with various feeds in order to safely avoid, address or
minimize long term performance deterioration.
[0014] One aspect of EDI device construction deserves special
mention, namely that the membranes as well as the exchange beads
employed in these devices are swellable, and generally undergo
changes in dimension between their dry and hydrated forms. Some
heterogeneous exchange membranes may swell by twenty percent, and
wetted beds of exchange beads also increase their volume and may
exert high pressure if unduly confined. Such swelling may impair
the flow impedance, or may affect the integrity of membranes or
structural elements. This has lead various manufacturers of EDI
stacks to propose assembly steps such as pre-soaking membranes for
lengthy periods before assembly; using more rigid intermediate
frame or spacer assemblies having multiple lands, bosses, beads
and/or registration pins to secure the membranes, confine the
exchange beads and maintain alignment and sealing; filling of beads
by precisely-measured quantities in a dry or salted form to achieve
precisely quantified swelling, or filling as pre-formed blocks or
gels of exchange media; or dynamic filling of cells by a fluidized
and possibly salted slurry, to assure a desired cell packing.
[0015] For spiral constructions, the dimensional instability of
membrane and bead media, together with the local slippage
introduced by winding at different radii, and the relatively large
length of individual membranes, raise additional potential problems
of membrane spacing or support, stress, shrinking or buckling, and
cracking. A number of investigators have proposed the use of fixed
and pre-formed spacing elements such as bumps, posts or ribs rather
than beads, either as separate elements, or as features formed on
the membrane surface, to avoid irregular spacing or undue
mechanical stresses and to maintain a desired membrane-to-membrane
spacing.
[0016] Within this general picture, various problems or perceived
problems or design constraints may arise. For example, in the
1960's it had been shown that certain properties of EDI operation
are optimized with uniform sized ion exchange beads, and with thin
filled cells; in the commercial field, some industry advocates have
long urged that a cell thickness defined by a low number of
exchange beads (e.g., 4-10 beads) is optimal. Thick cells have also
been advocated for specific purposes, such as high silica removal
achieved by inducing an upward pH shift due delayed hydroxyl
removal under polarized operation. It is apparent that a small cell
thickness introduces hydraulic flow limitations that will vary
greatly as a function of exchange bead size and feed fluid
viscosity; theoretical or empirical modeling done with water would
not necessarily apply to systems for treating common alimentary
fluids. Moreover, with any feed, local current density may vary
within the many cells of a conventional EDI stack or device, and is
substantially affected by local variations in distribution of
exchange beads, as well as by channeling or local variations in
flow that may occur. These current variations and resulting
potentials may profoundly alter the intended operating performance.
In addition, in spiral devices, current density increases inversely
with radial position, raising further control or operational
difficulties. Moreover, fluids such as alimentary or fermentation
fluids are notoriously prone to fouling--both functional fouling of
exchange bead surfaces and functionality, and physical blockage of
flow through the exchange beds. Fluidized exchange beds have been
employed to address the latter problem, but this approach cannot be
employed with the exchange bead filling of EDI devices, because it
is inconsistent with the requirement of direct contact between
exchange beads and the constricted space existing between the
exchange membranes.
[0017] For such reasons, the fabrication and operation of EDI
demineralization devices remain rather complex and costly, and each
particular construction may have its own limitations or
drawbacks.
[0018] There is thus a need for new constructions of such devices,
for devices that offer improved cost or ease of manufacture, and
for EDI devices that provide different or improved operating
abilities.
SUMMARY OF INVENTION
[0019] One or more of these and other desirable features are
achieved in accordance with the present invention by an apparatus
for demineralizing a fluid flow. The apparatus includes two
conductive electrodes, and one or more windings, each winding
comprising at least a pair of selectively permeable membranes,
generally a cation exchange membrane and an anion exchange
membrane, together with a spacing element. The cation and anion
exchange membranes are arranged parallel to each other, and the
spacing element maintains a separation or gap that allows liquid to
flow in the space between membranes. In accordance with one aspect
of the invention, the assembly of membranes and spacer (herein
called a "leaf") or several such leaves, are spirally-wound around
a central cylindrical core formed of electrically non-conducting
material, while maintaining a space, for example by means of a
further spacer, between successive leafs, or between the successive
windings of a single leaf. The assembled device comprises two types
of cell or chambers which are adjacent to each other in
alternation, and the cells are defined by the spacing elements and
the further spacers, forming flow chambers, i.e., dilute and
concentrate chambers within the device. In a preferred embodiment
both the dilute and the concentrate chambers each include ion
exchange material. In accordance with another aspect of the device,
the spacers are sheets that may be handled, rolled and manipulated
during assembly of the device.
[0020] The spacing element between membranes, as well as the
further spacer, can be formed of inert polymer material, ion
exchange beads, ion exchange fibers, a combination of two or more
these elements, or a porous medium (such as a sponge, felt or
sheet) incorporating one or more of such elements as an intrinsic
part.
[0021] In accordance with another aspect of the invention, the
apparatus is assembled in a housing having a cylindrical shape, and
includes two radially spaced apart metal or conductive components
which act as electrodes. At least one of these conductive
components is placed at a radially inner position surrounding the
central core and the other of the conductive components is placed
at a radially outer position near the perimeter, thus establishing
a generally radially-oriented electrical or ionic current between
the inner and the outer electrode, across the helically disposed
flow spaces defined within and between the membrane/spacer windings
of a rolled leaf assembly. One or preferably each of the electrodes
may be formed as a sheet metal spiral, with a radially-extending
opening that receives the end of a leaf, and positions the active
flow cells of the device in a uniform electric field free of
shadowing and hot spots. The opening forms a sealed and isoelectric
cage about the end of the cell-defining layers.
[0022] A preferred electrode is formed as a conductive sheet wound
in a volute circumscribing more than one full turn, with an
axially-extending strip-like gap or opening defined in an overlap
region between the radially inner and the radially outer edges of
the volute. The opening accommodates passage of the end of a leaf
along a tangential path parallel to the electrode surface into a
sealed pocket of the electrode (e.g., between the inner surface and
the edge of the outer surface of the volute), while fluid
communication with the inter-membrane flow space and connection to
a fluid port or manifold may be effected through the electrode. The
leaf entering or exiting the electrode pocket approaches at a
tangential angle, and may therefore wrap closely parallel to the
electrode surface, providing an exceptionally uniform current
distribution around the electrode, free of the shadowing and
inhomogeneities that occur with prior art constructions that employ
clamping, membrane doubling or other irregular fixing or
termination structures at the electrode. The construction also
avoids introducing brine manifold shorting or back diffusion that
may, to some extent, plague prior art constructions. A membrane may
also be sealed or attached to the electrode in the overlap region,
simplifying fabrication of the spiral-rolled cell structure.
[0023] While it is preferred that the electrodes be formed of
continuous metal sheet in this aspect of the invention, in other
embodiments the electrode may have openings and may take the form,
e.g., of a metal screen, or of multiple discrete but electrically
interconnected segments that are arranged to form a generally
equipotential surface contour. For example, discrete elements may
be shingled or arranged adjacent each other to form a cylinder (of
substantially constant radius), open volute (of somewhat increasing
radius) or similar shape that defines a complete circumferential
turn of equipotential surface.
[0024] In EDI devices of the present invention, the leaf (or
leaves) are wound such that, as viewed in cross-section, dilute and
concentrate spaces alternate adjacent each other along the radial
direction, and preferably the envelope forms the concentrate
channel. Preferably the feed flow of liquid to be treated enters
the apparatus (e.g., enters the dilute cells) at one end of the
device between the inner and outer core, and passes along a
treatment path parallel to the axis of the device through the
dilute cells between membranes, while an ion-receiving concentrate
flow is maintained in a corresponding concentrate cells defined on
the other side of each membrane, within the helical inter-membrane
spaces described above.
[0025] Flow paths taken by flows within the helical cells are
defined by one or more seals that extend between pairs of adjacent
membranes, and which may constitute edge seals, blocking or
channeling barriers, or a path-lengthening maze to direct the flow.
Other seals on one or more membranes may define a dry or inert
region of the spiral in a position effective to prevent the feed
and concentrate flows from mixing, for example, at the ends of a
leaf.
[0026] In accordance with embodiments of this aspect of the
invention, seals along or between portions of the flow path may
define a multi-stage device, may define separate feed and/or
concentrate flows for different stages, may define relative flow
rates of the concentrate and/or dilute fluids, and/or may operate
to define functional sub-regions of the treatment path. The seals
may also direct the feed and concentrate flows along preferred
directions or along preferred relative orientations, or may direct
the concentrate to maintain separate flows of different groups of
removed species in one or more portions of the paths so defined.
The relative orientation of feed and concentrate flow on opposite
sides of a membrane may, for example, be different at different
positions along the flow path, based on a considerations such as
the prevailing ionic species and their concentrations, the
electrical resistance, the polarization state and/or pH in that
region, the type of ions (such as scale-forming, monovalent,
divalent or specific ions) transported in that or upstream flow
regions, and the mineral or gas burden, of the dilute, the
concentrate, or both flows. In some embodiments, the seals may
operate to form internal distribution manifolds, for example
directing a portion of the feed or partially treated feed into the
concentrate path; or may operate as pressure regulators to adjust
the pressure in a cell relative to adjacent cells in order to
assure proper flow or to resist pinching of cell walls.
[0027] Flow may also be segregated on the concentrate side by
oriented strands of a mesh spacer, or by bands of impermeable
material placed on the spacer in an oriented pattern. In one
preferred embodiment of this aspect of the invention, the dilute
flow follows a path parallel to the axis of the cylindrical
winding, and the concentrate flow is directed such that it remains
in or flows parallel to a plane perpendicular to the axis. Thus, as
bivalent metals such as calcium, followed by ions such as sulfate
or carbonate, are successively removed from the axial dilute flow
and enter the concentrate cells, each of the removed impurities
remains in a band within the cell as it flows toward the cell
outlet, and does not mix with the other removed species. The
segregated flow effectively prevents scale from forming. In another
or further embodiment of this aspect of the invention, the dilute
cells may have a banded filling wherein the resin in a band at a
stage along the feed-product flow path is of a type selected to
enhance removal of the species (for example, of, to capture scale
forming metallic ions) in that stage, or to selectively block
capture or transmembrane passage of a species (for example, sulfate
or carbonate) until a later position where it may encounter a resin
selected to promote capture. Such banded resin filling thus
sharpens the separation of the different potentially scale-forming
species that may be present in the feed.
[0028] In accordance with another aspect of the invention,
structures normalize current density to promote uniform and
effective demineralization. A distribution of dilute inlet and/or
outlet passages may define a radially varying flow distribution
tailored to the prevailing electrical current density.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The invention will be understood by those skilled in the art
from the description herein of several embodiments and illustrative
details of construction, and some of its desirable variations and
features, together with figures thereof, wherein
[0030] FIG. 1 is a schematic plan view of a first embodiment of a
cylindrically-shaped spiral EDI apparatus in accordance with the
present invention, illustrating one layout of representative
components;
[0031] FIG. 1A illustrates a method according to one aspect of the
invention for forming cells of the EDI apparatus of FIG. 1;
[0032] FIG. 1B is a plan view of a first membrane envelope
illustrating aspects of sealing in accordance with the present
invention for defining cells of an EDI device such as schematically
illustrated in FIG. 1;
[0033] FIG. 1C is a cross sectional view of the cells constructed
as in FIGS. 1A and 1B;
[0034] FIG. 1D illustrates a manifold construction useful in the
cells of FIGS. 1A-1C;
[0035] FIG. 2A is a plan view of a second membrane envelope and
associated components for a spiral EDI device as illustrated in
FIG. 1;
[0036] FIG. 2B is a vertical section taken parallel to the roll
axis of a spiral EDI device made with the membrane envelope of FIG.
2A;
[0037] FIG. 3 illustrates an inner electrode construction of one
prototype embodiment in accordance with one aspect of the
invention;
[0038] FIG. 4 illustrates an outer electrode construction of the
prototype embodiment in accordance with this aspect of the
invention;
[0039] FIGS. 5, 6, 7 and 7A are plan views of additional
embodiments of exchange membrane envelopes and associated details
for other embodiments of a spiral EDI device as illustrated in FIG.
1;
[0040] FIG. 8 illustrates a detail of a sparse bilayer screen
spacer construction having cross-path deflectors for uniform cell
conductivity and enhanced treatment;
[0041] FIGS. 9A-B illustrates details of a flow port construction
for practice of embodiments of the invention;
[0042] FIG. 10 illustrates flow profiling of the invention with the
ports of FIGS. 9A-B; and
[0043] FIGS. 11A-B illustrate concentrate segregation with banded
spiral filling according to another aspect of the invention.
DETAILED DESCRIPTION
[0044] FIG. 1 is a schematic plan view of a first embodiment 10 of
a cylindrical EDI apparatus in accordance with the present
invention, showing general layout of components. The EDI apparatus
10 includes a housing illustratively comprised of a bottom flange
plate B, a top flange plate T and a cylindrical body C that
together define a generally cylindrical vessel or fluid-confining
enclosing chamber. A membrane roll 15, of which several examples
are described below, is wound around a central core 12 within the
housing. Illustratively, a membrane roll support 18, the structure
of which may take various forms, supports the membrane roll. An
inner electrode 14 surrounds the central core 12, and is coupled to
a first current leader 14a for connection to an external power
source, and an outer electrode 16, coupled to a second current
leader 16a, substantially surrounds the membrane roll 15. The
membrane roll is wound in a spiral in the annular space between the
two electrodes 14, 16. Within various constraints discussed below,
one or both of these electrodes may be a continuous sheet, may be
perforated, may be a mesh or screen-like sheet, or may be formed of
interconnected discrete electrically conductive elements which
generally span a contour surrounding the center or the periphery,
respectively. The electrode, whether screen-like, perforated, or
continuous has its conductive surface sufficiently extensive to
provide what is substantially an equipotential surface around the
inside axis (respectively, outside periphery) of the membrane roll.
Preferably, as described further below in connection with FIGS. 2A
and 2B, at least one and preferably both of the electrodes are
continuous sheet electrode that span more than one full turn to
define a pocket or opening to receive the end of the membrane roll
tangentially into the electrode.
[0045] This overall geometry positions the flow spaces defined
between a pair membranes generally traverse to the (radial)
direction of current paths which extend between the inner and outer
electrodes 14, 16. The electrode opening allows an inter-membrane
flow space to terminate in an inert or field-free region, while
avoiding the localized regions of shadowing and field inhomogeneity
previously caused by clamping, blocking or doubled membranes, and
sealing structures of the prior art. The electrode pocket
construction also allows a dilute chamber sealing that isolates the
treated stream from excessive electrolyte concentrations near the
electrodes. No separate electrolyte chambers or streams are
needed.
[0046] The central core 12, which may be substantially contiguous
to the inner electrode, is preferably a non-conductive structural
element such as a polymer pipe, or if conductive is not energized
to define a driving field. It may be sized so that the inner
electrode sits directly against its outer surface, thus serving as
a structural or supporting member, and it may include appropriate
apertures to serve as the fluid inlet or outlet for portions of the
device. In accordance with one important aspect of the present
invention, the inner electrode may have a diameter or
cross-dimension that is a substantial fraction of the diameter of
the outer electrode, so that the active windings of the membrane
roll are positioned in a relatively narrow cylindrical annulus
between the inner and the outer electrodes which experiences a
fairly uniform field. This annular region has a radial extent less
than, e.g., half the diameter of the outer electrode, preferably
under about thirty percent, and most preferably under about twenty
percent. The relatively similar magnitude of the radii of the two
electrodes enhances electrical operation by keeping the
distribution of current density fairly uniform: the current density
near the inner turns of the membrane roll remains relatively
limited. For example, other things being equal, the relative
current densities would vary inversely with the relative areas,
hence radii, of the electrodes. Setting the inner electrode
diameter close to the outer electrode diameter may be done such
that the ratio of inner and outer electrode areas, hence the
nominal relative current densities at the outer and inner electrode
surfaces, may be under 1:2, under 1:(1.5) or even below about
1:(1.2). While not explicitly shown, various fittings, passages,
ports and conduits may pass through the housing and/or the support
18 to introduce, distribute, collect or deliver the feed stream,
treated product and concentrate flows. Some example will be
discussed below with regard to particular constructions.
[0047] The membrane roll 15 includes one or more "leafs", each leaf
being rolled or spirally-wound about the inner electrode and
central core. A leaf includes two selectively ion permeable
membranes, and these are arranged so that the membrane roll defines
adjacent or parallel sets of dilute (feed) and concentrate (brine)
flow paths positioned in the annular space between electrodes. In
accordance with a principal feature of one aspect of the present
invention, the dilute and brine cells are constituted by spacers
having flow paths defined by various flow-blocking or
flow-directing seals between adjacent membrane surfaces of a leaf.
In some cases, flow-enhancing spacer regions are also positioned to
achieve a desired pattern of flow direction and magnitude within
the housing. The geometry and pattern of the flow paths may be
configured in several ways according to the invention so as to
enhance flow treatment characteristics, such as increasing ion
removal, decreasing back-diffusion, scaling or other undesired
effect, or enhancing or controlling other characteristics of
operation. These features will be understood from the following
specific examples.
[0048] FIG. 1A schematically illustrates a method of manufacture in
accordance with one aspect of the present invention. As shown, a
membrane roll 15 as described above is formed of a single
continuous leaf 20 using a continuous sheet of anion exchange
membrane 22, a brine cell spacer 24, a continuous sheet of cation
exchange membrane 26 and a dilute cell spacer 28. The brine cell
spacer 24 may consist of one or more plies of a flexible non-woven
screen mesh, such as a ten to seventy mil thick polyethylene or
other fluid-compatible material of suitable thickness and screen
size that, after assembly, provides a distributed support between
the two membranes adjacent to it while permitting fluid to flow
through the layer occupied by the mesh. The spacer in preferred
constructions includes other elements in its assembly, particularly
conductive and/or ion exchange beads distributed to constitute a
suitably fluid-permeable but ionically- or electrically-conductive
inter-membrane filling. Preferably, the spacer is a web comprised
of one or more mesh sheets having ion exchange beads permanently or
temporarily stabilized thereby, interspersed therein, or affixed or
adhered thereto. Such a sheet may be handled with ease during
assembly of the EDI devices. The body of the screen, e.g., the
filaments or crossed filaments, assures a minimum inter-membrane
spacing, while the beads provide distributed membrane-contacting
support and a certain level of electrical conduction (in the
concentrate space) or of ion-capture and ion-conductivity or
transport activity (in the dilute space). In a presently preferred
embodiment, the concentrate compartment preferably employs only
cation exchange beads, while in the dilute compartment a spacer
layer having beads of both cation and anion exchange types will
generally be desired for capture and removal of all ionizable
species. A preferred spacer assembly 24 includes both a screen
mesh, and ion exchange beads or conductive media, which together
determine the cell thickness, flow impedance and other flow
characteristics, and the conductivity and overall efficacy of
demineralization. The dilute screen preferably also serves a
bead-segregation function, allowing dilute cell fillings in which
beads are segregated by type- the anion exchange beads and cation
exchange beads are retained in separate physical positions.
[0049] A few representative dimensions will be given here simply as
an aid to visualization of commercially useful sizes of the spiral
EDI devices contemplated by the invention, without, however
limiting the invention to such sizes. The axial length of the
apparatus may be on the order of about one-quarter to about one
meter or longer, while the device may have a diameter of under ten
to about sixty centimeters. The spiral flow cells defined by the
rolled membrane and spacers may extend up to several meters or
longer, and the path of fluid flow within the spiral space or layer
of a roll of given diameter may be constrained to an axial or a
helical direction or may follow a path comprised of several
segments changing in direction or type. The inter-membrane space is
well-defined with spacer material comprising one or more screens
and a defined exchange bead filling. It is preferred that the
filling of ion exchange beads be a relatively spare filling, as
described in commonly owned PCT International Application
PCT/US03/28815 entitled SPARSE MEDIA EDI APPARATUS AND METHOD,
filed in the United States PCT Receiving Office on Sep. 12, 2003.
The disclosure of that international application is hereby
incorporated by reference herein in its entirety. Briefly, that
patent application describes methods of forming ion exchange
fillings consisting of a layer of scattered beads, or a relatively
complete monolayer, bi-layer or various striped, banded, or
otherwise segregated layers of ion exchange beads, in thin EDI
cells that operate with enhanced electrical efficiency or control
and exhibit low and well controlled flow impedance characteristics.
These layers, which preferably include a screen in addition to the
exchange beads, promote extremely uniform inter-membrane spacing
and support, and in operation they achieve enhanced electrical
efficiency and reduced residual contamination. They also operate
dependably with long flow paths, allow a greater number of parallel
cells in a given radial length of the winding, and present a flow
cross-section that remains relatively free of channeling.
[0050] Thus, in accordance with one important aspect of the present
invention, a spiral EDI device is constructed having cells formed
by the interior of one or more thin rollable envelope assemblies
that feature both a mesh web and a sparse filling of ion exchange
beads. Advantageously, by permanently or temporarily bonding or
sticking the beads to the mesh web, a sheet or a continuous web of
screen/bead spacer material may be formed, and the leaf structures,
e.g., a bead-loaded spacer layer, a spacer/membrane or a
membrane/spacer/membrane layer or membrane/spacer/membrane/spacer
layer, may then be rolled in a discrete or a continuous operation
as shown in FIG. 1A.
[0051] When the layer or cell structure employs a sparse filling, a
so-called "short diffusion path" or "shallow shell" resin may be
used to help control or match the electrical resistances of the
resins in constructions wherein two or more resins are placed next
to each other in a common electrical field, or to enhance
efficiency when resins of different diameters are to be placed
together. These beads will also regenerate faster after a
clean-in-place procedure or after being assembled in salt form, and
are expected to generally operate and regenerate noticeably more
efficiently in a sparsely filled EDI device, where they will
exhibit very sharply defined bleed-down times after regeneration or
reversal.
[0052] Various protocols may be employed to attach a scattered or
continuous monolayer of beads to the screen (for example, with
individual beads held in the openings of a screen having a mesh
size roughly equal to the bead diameter), or to attach a
substantially full monolayer of beads to each side of the screen
(for example by treating the screen with adhesive and then
cascading ion exchange beads against each side of the screen to
capture and bind the beads.) In each case, the bead/spacer assembly
may then be handled and manipulated freely, enabling a bulk or
semi-continuous process of rolling and assembly to produce a
finished EDI device. When cation and anion exchange beads are
placed on opposite sides of the dilute screen, the dilute screen
should be oriented in the final assembly to have the cation
exchange beads on its side contiguous to the cation exchange
membrane, and to have the anion exchange beads on its anion
exchange membrane side. Furthermore, when rolled between an inner
and an outer electrode, the dilute cell spacers are preferably
oriented such that a cation exchange membrane lies on the cathode
side of the cell and the anion exchange membrane lies on the anode
side of the cell.
[0053] In thin spacers cells of the above-described construction,
the screen mesh and bead size may be selected such that the screen
maintains separation of the bead layers on opposite sides of the
mesh, and such that the beads and screen assure an appropriate
total inter-membrane spacing, which may be selected in the range of
under about one millimeter to at most several millimeters. When
beads are attached to respective sides of the mesh in this manner,
the mesh size may also be selected such that beads of opposite type
contact each other through the mesh openings, without migrating
through the openings. In these constructions, it is preferred that
during fabrication, the screen be coated with a contact adhesive to
capture and hold the respective beads, but that the beads
themselves lack adhesive; this assures that the bead surface
remains active, and that the bead-to-bead junctions that occur in
the completed assembly are direct conductive contacts with no
adhesive interlayer or other impairment of electrical or ionic
contact. As described in the aforesaid international patent
application, these constructions assure a useful level of water
splitting, but do not introduce extraneous reverse junctions that
would throw salt or cause electrical inefficiency. When different
exchange types are segregated on opposite sides of the mesh,
certain one-sided barriers or diverters may also be provided along
the flow path to further enhance efficiency by causing the dilute
stream to meander back and forth across the screen, i.e., from the
cation side of the cell to the anion side and back to the cation
side so that the fluid passes through both exchange bead layers.
This construction which applicant refers to as "s-layering" and
which is schematically shown in FIG. 8 below, assuring that the
treated fluid contacts both types of exchange bead, offers the
performance advantages associated with zebra- or layered-filling,
and also avoids the creation of localized regions of irregular
conductivity and promotes a more uniform current distribution,
because the two types of beads contact each other in series across
the cell. Therefore, the same current must pass through both the
anion and cation exchange beads, despite their relative capture
affinities or ion transport efficiencies and the prevalent ionic
burden in the feed fluid. As a result, the removal of anions and
cations from the flow therefore each proceed at comparable rates,
and the flow is not subjected to irregular patches of species
depletion and polarization. Moreover, the lack of "granularity"
tends to prevent localized regions of high pH that might otherwise
be prone to membrane scaling.
[0054] As shown schematically in FIG. 1A, a process 100 for forming
the leafs of EDI devices of the present invention involves
providing a sheet 110 of anion exchange membrane, placing a sheet
115 comprised of one or more layers of a screen or mesh loaded with
exchange beads on or adjacent the sheet 110, and covering the
mesh/bead sheet 115 with a sheet of cation exchange membrane 120.
The two membranes may be sealed together along one or more edges,
as discussed further below, forming an envelope structure about the
mesh; this may be done, for example, to form a concentrate cell
envelope structure. The envelope is assembled with an additional
screen layer 125 having suitable exchange beads, which, for the
dilute cell spacer layer, will generally be of both anion and
cation exchange type, and the membranes and spacers are then rolled
to form a spiral EDI unit. Such process of manufacture is indicated
schematically by the arrangement of rollers 130 in FIG. 1A. In
practice, the assembly process will employ various guides and
brakes to maintain web tension, and particular ones of the layers
may terminate or extend beyond others, as discussed below, to
effect suitable end geometries and to suitably position electrode,
spacer cell or other functional components of the device. Various
subsidiary steps or components, such as addition of flow
deflectors, spacer shims, and edge gaskets or seals, some of which
steps or components are discussed further below, are not
specifically shown but these may be effected at appropriate points
along the line as the first membrane, spacer or other web passes
along the stages for assembly of the spacers and membranes in an
envelope/spacer roll. This basic structure is wound and assembled
with the electrode and core structures, and mounted within a
cylindrical vessel to form the complete EDI device. An electrode
may itself constitute a wall of the a vessel, although it is
necessary to have at least some portions of the vessel be
electrically non-conductive to avoid electrode shorting and/or
potential shock hazards. In one prototype embodiment, electrodes
are formed of foil, and may be assembled in a process wherein the
first and last turns of the membrane/spacer assembly are wound with
and conform to the electrodes.
[0055] On a global scale, flow paths in the assembled EDI device
are implemented with the dilute (feed-product) flow proceeding
within the spiral-rolled spacers that constitute the dilute cells
and brine cells. In the dilute cell, the flow may preferably be
parallel to the longitudinal axis of the device, while in the
concentrate or brine cells, flow proceeds along one or more
directions, examples of which are illustrated below, within the
spiral-rolled envelope(s) that define the cells. Within at least
some cells--illustratively the concentrate cells--the flow is
confined and its direction further determined by impermeable seals
extending between the opposed surfaces of the two membranes on both
sides of a spacer. These seals may confine, deflect, orient or
concentrate the flow in various ways, discussed further below, and
they are preferably implemented by laying down one or more strips
of a viscous sealant, adhesive tape or band in desired positions.
Seals may be formed, e.g., by applying a liquid formulation via an
applicator nozzle as the membrane/spacer/membrane assembly is being
laid out, or as it is being rolled if rolling of multiple lamina is
performed directly as shown in FIG. 1A. When sealing bands are
applied to form dilute cell flow paths, these are preferably
applied during the rolling process (FIG. 1A), while the concentrate
cell paths may be applied either during a preliminary rectilinear
layout operation, or during a roll-forming procedure.
[0056] Embodiments of the invention may be implemented with
different sealing band patterns to achieve different patterns of
desired flow.
[0057] FIGS. 1B and 1C illustrate a pattern of brine cell sealing
bands for effecting one basic spiral-brine cell construction. In
this embodiment, the spacer mesh 115 is impregnated with a suitable
sealant along first and second sealing bands 116a, 116b extending
along the top and bottom of the spacer (corresponding to upper and
lower ends of the device in the orientation shown in FIG. 1), above
and below the bead-bearing central region 117 of the spacer between
the two opposed membranes 120, 110. This results in an envelope
structure wherein the ends of the membrane/spacer/membrane envelope
(the left and right ends in FIG. 1B) open to the interior of the
envelope for providing and receiving the concentrate flow. The
sealant may, for example be a suitable (poly) ethylene vinyl
acetate ("EVA" or "PVA") material applied to both sides of (and
through) the edge of the screen, which otherwise constitutes a
flow-permeable spacer. The sealant may be formed of another
suitable material, e.g., a viscous, preferably curable, sealant or
a sticky two-sided tape effective to provide an impermeable seal
extending for the thickness of the spacer between and bonded to
both membranes along the edges of the spacer assembly. In these
constructions, the spacer in the region of the sealing bands is
preferably, but not necessarily, free of ion exchange beads. When
assembled with the membranes 110, 120 in a rolled winding, the
spacer then defines a brine flow cell having closed top and bottom
wherein the brine flows within, and may follow, the helical winding
space of the membrane roll between its inner and outer ends.
Advantageously, when the pattern of sealant is applied on the
component at the time of assembly, the various membrane and spacer
layers may slip to accommodate differential movements as the as the
leaf is rolled (with suitable tension on the webs) into a spiral.
Such slippage results in a substantially stress-free and
buckle-free assembly process; the sealant may further polymerize
and cross-link to form a stronger, flexible or inflexible,
impermeable barrier between components (e.g., a "form-in-place"
gasket). In other embodiments, however, a seal may be achieved by
placing flexible liquid-impermeable electrically non-conducting
strips between the membranes, for example, by placing gaskets along
bands at the edges of the membranes and outside the screen area.
EDI devices of the invention may also be constructed that achieve
sealing by forming the spacer assembly itself with
liquid-impermeable electrically non-conducting rubber or plastic
solid (non-mesh) edges that span the gap between the membranes, a
construction similar to the one-piece "screen spacers" commonly
used in brine cells of most commercial flat plate type EDI stacks
which have a strip gasket formed by co-moulding around their
periphery. However, in this case the modulus, finish and
dimensional tolerances of the edge region gasket material must be
appropriately set to assure that the spacer edges will seal
effectively against the surface of the adjacent membranes. It may
further be advisable to employ liquid sealant or gasket cement on
the relevant membrane-contacting surfaces of the solid periphery.
However, use of a viscous sealant applied through the spacer mesh,
possibly with additional lamina of mesh to provide a cell thickness
identical to the mesh-plus-bead spacer of the center, is preferred
for its ease of implementation. In each case, the sealant, cement
or gasket material is preferably of a composition selected to be
non-leaching so that it does not bleed solvent or polymer into the
stream, and is of a composition proven to tolerate the EDI
conditions and the treatment or conditioning chemicals that may be
present in the process flow. Sealants compounded with a filler
material (such as titanium dioxide or other inorganic powder) are
similarly to be avoided. When the leaf structure or roll assembly
is to be assembled wet (including soaked with a non-aqueous
solvent), it is preferred that the sealant or adhesive be
compatible with such assembly, and when the assembly is to be
sealed before rolling, the sealant should be flexible, or
non-hardening at least during the assembly process. It is also
desirable that the curing or drying of sealant not introduce such
stiffness or dimensional changes as might introduce mechanical
stresses in the adjacent membrane, or cause cracking.
[0058] Applicant has found a wide variety of adhesives to be
serviceable. These include a two-part epoxy sealant for wet
surfaces made by the Hardman Company; a one part marine adhesive
sealant marketed by the 3M Company; DAP two-part resorcinal glue;
DAP liquid neoprene rubber cement; a two-part polyurethane sealant
of H. B. Fuller Company; Wellbond.TM. sealant; and a one-part
water-cured polyurethane sealer 4R-0215MF of H. B. Fuller
Company.
[0059] FIG. 1C shows a cross sectional view of the spacer 115 of
FIG. 1B assembled between two membranes 110, 120 as in FIG. 1. The
cation exchange membrane 120 lies on one side of the spacer, and
the anion exchange membrane 110 lies on the opposite side, with the
seals 116a, 116b forming impermeable barriers at the top and bottom
edges of the membranes. When rolled in the housing, the brine cells
thus constitute the helically disposed space between the two
sealing bands. Fluid may be introduced and removed at the inner and
outer ends of the winding.
[0060] FIG. 1D illustrates a useful construction for applying or
removing fluid from the concentrate cells formed in the membrane
roll. In accordance with this embodiment, the spacer element 115
comprises a screen, or several plies of screen, together with some
exchange beads that maintain a desired minimum level or threshold
value of electrical conductivity (not specifically shown), and as
discussed above, and which generally maintains separation between
membranes 110, 120 and assures distributed support against pinching
off of the inter-membrane flow space. A band or region of the
spacer, which may be one of the bands 116a, 116b or may be a band
placed elsewhere, such as at an end of the spiral, is closed by a
further seal 119, and a portion of the screen adjacent the end
remains free of such beads, thus offering a more open flow path and
decreased flow resistance. This open or unfilled spacer, if
positioned near the inlet of the flow may advantageously serve as a
flow-distributing region to efficiently allocate flow across the
width of the spacer into the adjacent region of bead loaded mesh;
alternatively, when positioned distal to the inlet, it may operate
as a flow-collecting outlet region to efficiently receive the
outflow from across the bead loaded spacer mesh and conduct the
combined outflow. It thus presents a low pressure drop (at the
inlet end for distribution) or a high pressure drop relative to the
filled-path impedance (at the outlet end) and low impedance outlet
conduit that serves to define the general direction of flow within
the cell from a supply inlet to an outlet.
[0061] A separate inlet/outlet conduit 128, such as a perforated
tube, may optionally be placed in this area, to deliver to or
collect fluid present in the low impedance unfilled screen, and in
that case the inlet (respectively, outlet) tube may pass through
one of the seals 116a, 116b, 119 or other structure to connect with
a vessel inlet or outlet port of the assembled device. Such port
may be internal (when, for example the brine is fed by a bleed from
the feed in, from the dilute mid-path, or from the dilute product
out flows), or may be external as may be desired when the brine is
to be fed via outside piping, or is to be actively recirculated or
have its pressure or flow set via an external valve or regulator.
In general, it is expected that with the flow cells defined by an
open mesh and the presence or absence of a relatively spare filling
of exchange material, the leaf or leafs of the spiral EDI devices
may be arranged to passively provide effective dilute and
concentrate flows by use of suitable seals, path lengths and
intercommunicating path openings, without reliance upon circulation
pumps or complex flow control systems.
[0062] The concentrate cell screen may have conductivity-enhancing
material, such as conductive metal, polymer or carbon beads
temporarily or permanently fixed thereto or captured therein, to
augment its structural support and/or electrical conduction
properties, either instead of or in addition to cation exchange
material. The inlet or outlet pipe, if provided to apply or conduct
away fluid from the inter-membrane space, may be formed by a
perforated stainless steel tube or other suitable
conduit/distributor structure extending into the open (unfilled)
mesh or extending along the sealed edge.
[0063] These elements of construction may be carried out to
implement different flow paths. FIG. 2A illustrates one such
arrangement, showing an (unfurled) leaf 40 having a pattern of
sealing bands 1, unfilled mesh 2, and bead-filled mesh region 3
forming a half-sealed envelope that remains open along the bottom
edge. The half-envelope is configured to receive an inlet flow at
the bottom. The low impedance unfilled mesh region 2 extending
along the top edge promotes a generally upward flow through the
filled mesh 3, parallel to the axis of the device, as indicated by
arrows in the figure. The filled mesh may, for example employ a 28
mil (0.7 mm) thick screen with a coating of 650 C cation exchange
beads, to define brine cell regions 3 of suitable hydraulic
resistance and good electrical conductivity, and a thicker unfilled
screen (e.g. 70 mil) may be employed in region 2 to maintain
membrane spacing while presenting lowered resistance that promotes
the desired flow distribution. As indicated by arrows in FIG. 2A,
the flow follows the pressure drop toward the unfilled spacer
region 2, and then turns to proceed along an outflow path that
branches, running horizontally along the top of the leaf to conduct
the brine flow out both of the two ends (e.g., the inner and outer
ends of the spiral when the envelope is rolled in an EDI device.) A
short concentrate outlet conduit may be inserted into the outflow
region 2 in the innermost and outermost turns of the roll to
connect this flow to one or more vessel ports. FIG. 2B shows a
vertical cross-section through several turns of the spiral wound
assembly, omitting the electrodes, to illustrate relative
directions of the feed and concentrate flow paths in the central
treatment area of the device that occur with the leaf of FIG. 2A
when the brine cell is fed at the bottom (e.g., with the product
water as illustrated, or with a separate fluid connection of feed
or conditioned brine). The feed water proceeds downward along the
axis of the device, while on the other side of the membranes there
is an axial flow upward from the concentrate inlet end which
becomes a faster exit flow of the concentrate along the band 2 of
unfilled mesh, which defines a spiral outflow path lying in a
generally horizontal plane at the top of the device. In general,
when fed from the feed or product flow in this manner, the
concentrate flow need be only a small portion, e.g., 1%-10%, of the
total flow, and such minor fraction may be automatically and
passively diverted into the concentrate cells by using suitable
mesh and filling parameters. The thin concentrate cells having some
cation exchange beads retain, or quickly attain suitable
conductivity even when fed with product water, and recovery is
high. Apportionment or deflector elements or one or more valves
(such as a product back-pressure valve and/or a brine inlet valve)
may be positioned at the bottom of the unit if desired to set, or
to control or adjust, the amount of flow diverted to or passed into
the brine cells. Brine back pressure may alternatively be set at
the brine outlet to control brine flow.
[0064] These arrangements of the concentrate path present
advantageous operating characteristics. When using a single pass,
rather than recirculating, brine fed into a filled cell, the
product outlet end of the dilute may be highly polarized, resulting
in very stable removal characteristics and robust ability to deal
with upset or start-up conditions and changes in feed quality.
[0065] The leaf of FIG. 2A may alternatively be installed in an
inverted orientation, with its seal band 1 positioned along the
bottom or product out end of the unit. In that case, the
concentrate cells may be fed in a similar manner by a bleed from
the feed, and both the concentrate and dilute flows will proceed
along the same (downward axial) direction during the initial
portion of their path, with the concentrate flow turning to form a
faster cross path near the exit end of the device.
[0066] Other path geometries are implemented in accordance with the
invention by employing different leaf sealing patterns, several of
which will be described later below with reference to FIGS. 5-7.
These may include leaves with an at least partly open (inner or
outer) end, and at least partly open sides to define paths running
in segments along one or more spiral directions. The seals provide
great latitude in setting path length, path direction, and such
characteristics as flow impedance, pressure drop and dilute/brine
pressure difference.
[0067] In accordance with another aspect of the invention, a
membrane/spacer leaf is fitted in a device fabricated with one or
more electrode assemblies that are implemented as a conductive
electrode sheet winding parallel to the rolled leaf, and having
more than a full turn of electrode surface such that a pocket is
formed in an electrode overlap region. The electrode forms a
one-turn volute or helix, and the leaf passes into the
radially-open gap extending between the end of the electrode and
the next turn. This construction positions the leaf flat and
parallel against the electrode surface around its full
circumference, and places the envelope end or termination in a
field-free region, e.g., the interior of the electrode or pocket.
The envelope/electrode geometry wherein the envelope remains
substantially entirely parallel to a continuous cylindrical
electrode contour, without abrupt turns or doubling of plies at the
regions of attachment, and without projecting clamp structures,
results in a more uniform field through the dilute and concentrate
cells near the electrodes, free of the shadowing, inhomogeneities
and shielding that arise from prior art arrangements of slotted
openings, sheet clamping structures and the like. When applied to
the concentrate cells, it permits passage of concentrate through
the electrode and into a field-free region while maintaining
substantial isolation from the feed-product flow path, and thus
avoids the problems of brine short-circuiting or back-diffusion
that may occur near the concentrate manifolds and electrolyte cells
of prior art EDI devices.
[0068] FIG. 3 is a cross-sectional view of an inner electrode and
spiral-wound leaf, taken in a plane normal to the winding axis,
illustrating such an electrode pocket and termination of the
membrane/cell structure, such as the spacer/leaf structure of FIGS.
1A-1D, at the electrode in one prototype spiral EDI device. FIG. 4
shows a corresponding view of an outer electrode structure for the
prototype embodiment. Each electrode is preferably formed of sheet
stock of a suitable conductive sheet material, such as stainless
steel, titanium or platinum (e.g., platinum-painted or or with its
surface otherwise platinized) or other inert or conductive
metallic-surfaced sheet. Preferably a non-oxidizing material, such
as a platinum-surfaced sheet is employed for the anode. Ancillary
structures such as suitable current leads, electrical connector
tabs and the like (not shown) may connect to the electrode and
extend or electrically communicate through the vessel housing (FIG.
1), and the very end of a membrane, spacer or leaf may be affixed
to the electrode surface by a cement, by one or more fastening
clips, a screwed-down metal strip or the like (also not shown).
[0069] As shown in FIG. 3, according to this aspect of the
invention, the central electrode 114 (corresponding to electrode 14
of FIG. 1) which in the prototype device is the anode, is comprised
of a wound sheet of which the end regions 114a, 114b overlap in an
angular sector 114c that extends for a few centimeters and in which
the sealed ends of the leaf terminate. This overlap region forms an
electrode pocket which receives the leaf end--membranes, dilute and
brine spacers, and lies at a single potential, so that there is no
electrical field in the pocket region. The pocket termination of
the leaf, including spacers, may be sealed, for example with a
curable polyurethane assuring that the dilute and brine fluids
cannot leak out or intermingle. The individual spacer and membrane
layers may terminate successively, or with slight offsets, as
illustrated, to form a tapered-end insertion, rather than a
butt-ended insert, where the electrode's outer edge overlaps the
inner one. The electrode winding thus fits well against the angled
surface in the pocket region and is fully supported. Thin foil or
other metal sheet may be used to form the electrode surface. The
spacer/membrane roll lies exactly parallel and flat against the
outer surface of the central electrode as the spacer/leaf exits the
pocket.
[0070] In this prototype construction, the winding structure
comprises a repeating sequence of four layers, namely a brine
spacer assembly B, an anion exchange membrane A, a dilute spacer
assembly D and a cation exchange membrane C. In the construction of
the illustrated prototype with a central anode, the brine spacer
layer B extends for a length of one full circumference of the
central electrode, beyond the end of the adjacent cation exchange
membrane C.sub.x. Thus, when the leaf is mounted in the pocket, the
brine spacer B lies directly against the outer surface of the anode
114 for a full turn, and the adjacent dilute cell is bounded on the
anode-facing side by an anion exchange membrane. The first winding
or end length of the brine spacer path thus functions like the
anolyte cell of a conventional EDI device. Preferably the anode,
electrode 114, has one or more openings 114d therethrough placing
the brine space in fluid communication, for example, with a port in
the bottom flange as shown in FIG. 1, e.g., a brine port,
permitting the concentrate fluid to pass through the inner
electrode.
[0071] The brine concentrate spacer and anion exchange membrane are
placed adjacent to the electrode, and wound and sealed with the
remaining plies into the pocket at the region 114c, after which the
leaf/spacer assembly is wound multiple times and then terminated at
the outer electrode. The outer electrode structure, one example of
which is illustrated in FIG. 4, preferably employs a similar
construction as a winding with a pocket. After winding the outer
electrode, screws or other fasteners may be placed through the
overlapping electrode layers, or the circumference may be banded
and clamped to seal the unit together. Thus one of the two
electrodes conforms to the shape of the first turn of
membrane/spacer assembly and the other one follows the shape of the
last turn. In the case of the outer cathode assembly, termination
may be effected by extending the brine spacer layer for a length of
one full electrode circumference beyond the anion exchange
membrane, so that the final brine spacer layer lies directly
against the inside surface of the outer electrode (the cathode, as
illustrated), and a cation membrane lies on the cathode side of the
next adjacent dilute cell. All or a portion of an electrode may be
made from metal screen, wire or conductive mesh, rather than from a
sheet of conductive foil, or may include conductive mesh or wire on
a support sheet, but conductive foil is preferred. Furthermore, the
foil (for example, a two mil [0.05 mm] foil) may be wound several
additional turns around the outside (when used to form the outer
electrode as indicated schematically in FIG. 4) so as to constitute
a containment vessel for the assembly, or may be initially wound
for several turns at the inner electrode, before attachment of the
leaf, so as to constitute a central pipe, making the corresponding
structural portion of the housing element, vessel or support
unnecessary. In this case, one or more ports P as shown in FIGS. 3
and 4 may be drilled through the electrode and fitted with suitable
fittings to allow fluid communication through the electrode winding
with the brine cells.
[0072] FIG. 4 is a cross-sectional view of the outer electrode 116
formed with a similar wound sheet and pocket structure overlapping
in an angular sector 116c where the sealed ends of the leaf
terminate. The envelope/spacer roll of the prototype is constructed
so that here too, the brine spacer B lies adjacent the electrode
surface, and communicates through a suitable port P. When the brine
spacer is a spacer as shown in FIGS. 1A-1C, constrained between two
membranes sealed at top and bottom, then the concentrate flow
passes through the spiraling concentrate spacer layer B, and may
pass directly through one or more of the electrodes. When the brine
envelope of FIG. 2A is used, having one open edge for receiving the
brine inlet, the both the inner and outer electrode ports may be
brine outlets. However, in other embodiments, it may be preferable
to have the brine enter at one end of the spiral, e.g., at the
anode, which is preferably the inner (smaller) electrode, and
proceed toward the cathode so that the brine stream is initially
acidified by anolyte and better resists scaling. In still other
embodiments, the brine layer need not lie immediately adjacent to
the electrode, but instead separate electrode spacer cells
(electrolyte cells) may be provided at one or both electrodes to
allow a separate flow of fluid adjacent to one or both electrodes.
This allows one or both electrolyte flows to be supplied, treated
or conditioned separately from the bulk dilute and brine flows, in
a manner analogous to electrolyte treatment of prior art EDI device
constructions.
[0073] The foregoing examples illustrate several generally
advantageous properties. Spiral EDI units of the invention employ a
relatively small number of hardware and spacer components, and
these are of low cost. The use of a sparse bead filling allows
cells of small width to support high product flow velocities, and
the roll sealing and assembly process allows path lengths to be
readily defined and optimized to accommodate flows or apportion
flows without causing occlusion or cross-contamination of cells.
This also allows the EDI device to be easily assembled in a dry or
non-swelled state, without membrane pre-treatment. Subsequent
conversion (wetting and/or de-salting) then leads to expansion and
enhanced sealing as well as enhanced membrane-bead contact and
highly uniform electrical and ionic conductivity. Constructions
with an envelope as shown in FIG. 2A that seals the brine cell on
only a single top or bottom end of the envelope also allow the
concentrate cells to be easily fed by a portion of the feed or
product water internally of the vessel. Furthermore, the use of an
inter-membrane pattern of multiple seal line segments to form
bead-filled envelopes results in very efficient membrane
utilization--up to 95% of the membrane area actively participates
in electrodeionization, far more than in current flat plate EDI
architectures--and provides great control over flow direction in
both dilute and brine cells. The sparsely-filled mesh and unfilled
mesh regions have low hydraulic resistance which may be exploited
to define distribution or collection manifolds or determine flow
direction and paths within the device that are substantially free
of channeling.
[0074] The spacer structure, consisting of one or more layers of
mesh (e.g., polymer screen) with exchange beads fixed in/on the
screen efficiently determines the distance between membranes,
providing both membrane support and a medium for ion capture and
transport, and effectively prevents migration or loss of the resin.
Moreover, in localized regions (e.g., adjacent to a port or at the
ends of the flow) a screen may be used with a smaller mesh sized to
also serve as a bead trap, or with unfilled regions or a larger
overall thickness intended to serve as a flow distributor or flow
collector. Ion exchange felt or suitable (e.g., polypropylene)
cotton may also be applied near edges or at ends of the containment
vessel to assure that the beads are retained in the unit, or in
active treatment areas.
[0075] Thin constructions of a spacer with exchange beads, or a
defined pattern and distribution of bead types, are readily formed
by attaching the beads to the screen with adhesive, and specialized
spacer assemblies may be so formed before final rolling and
assembly of the device. The screen dimensions may be selected so
the gap between any strand and one of two neighboring membranes is
smaller than bead size, preventing the beads from moving around or
clumping, and thus assuring effective flow a effective exchange or
conductivity characteristics. Spacing of the adjacent membranes in
a roll may also be achieved by employing a "bumpy" membrane, i.e, a
membrane formed with bumps or other features protruding above the
nominal surface plane; raised portions of the surface then contact
opposite membrane. In this case, a screen or mesh is not
necessarily required to determine the inter-membrane spacing or the
exchange particle distribution, and may be omitted in some
embodiments. It should be noted, however, that in the past, it has
proven difficult to manufacture exchange membranes with surface
projections, and applicant does not believe that any bumpy exchange
membranes are now available commercially. An alternative, however,
is to attach scattered ion exchange beads to the surface of at
least one of the ion exchange membranes using, for example, a
non-insulating fixation compound, such as a soluble glue. The
beads, once assembled contact the opposite membrane (which may be
similarly prepared) to determine the membrane spacing, cell
conductivity and/or ionic conductivity. The assembly may be
hydrated and swelled after assembly, which flushes the glue from
the assembly, further enhances membrane-bead contact, and prevents
the beads from shifting position. In dilute cells, preferably ion
exchange material placed between membranes by any of these methods
is positioned so that the anion exchange mass contacts the anion
exchange membrane on the anode side, and the cation exchange mass
contacts the cation exchange membrane on the cathode side. Because
the quantity or distribution of exchange beads remains limited,
swelling will be small and manageable in the device as a whole and
should not impair either the structural properties or the flow
properties of the device. The units may be rolled and assembled
"dry" or using a non-water solvent different from the solvent
intended during normal operation. The solvent may then be
substituted/removed after assembly, and the expansion of ion
exchange material and membranes resulting from this conversion will
assure good contact between components.
[0076] Homogeneous anion and cation exchange membranes such as
those made by Ionics, Incorporated of Watertown, Mass. were
employed in construction of several prototypes. These are preferred
because of their strength, relatively low swelling and limited
transmembrane water leakage. Heterogeneous membranes may also be
used, but in the latter case it is preferable that at least some
degree of pre-swelling or membrane hydration be effected before
assembly, and steps such as web tensioning during a leaf rolling or
assembly may require closer control due to the lesser strength,
greater swelling and general looseness and flacidity of
heterogeneous membrane.
[0077] As noted above, the described constructions provide a
flexible approach toward defining different fluid flow paths within
the spiral EDI device by suitable patterning of the sealing bands
and manifold regions of an envelope. FIG. 5 illustrates one
embodiment of a two-membrane envelope for defining spiral EDI flow
cells, wherein the brine flow is fed at one end of the cylindrical
housing, between the center and the outside of the spiral, and
branches to flow inwardly and outwardly within the spiral brine
flow space. Each branch turns at the respective inner/outer end of
a blocking seal 1a to reverse its direction along the spiral, and
reverses once more as it passes through the central opening between
two further flow deflectors 1b, 1c. The distal ends of the
bifurcated flow paths then pass out through openings at the inner
and outer edges of the roll, having followed two generally
spiraling paths of length somewhat greater than the length of the
winding itself.
[0078] FIG. 6 illustrates another path configuration defined by
envelope seal lines 1. In this embodiment, the brine enters at the
lower right corner, is constrained along two "race track" turns to
travel approximately three times the spiral length, and exits at
the upper left corner. "Lower right" and "Upper left" refer to the
positions in the unrolled membrane, but will correspond to
positions at the inside (center) and at the outside (periphery) at
opposite ends when the envelope is rolled in the device. Entry or
exit may be effected by any means discussed above--e.g., a conduit
passing into the cell, an opening through a bounding electrode or
other construction.
[0079] FIG. 7 illustrates another configuration, similar to that of
FIG. 5 but employing separate inlets for the two brine flow
branches that are maintained separate while arranging the barrier
seals 1 so that the two flow paths are of different lengths.
[0080] The seals may further define flow paths that are restricted
to the region of the electrodes, for example by a seal line extend
parallel to the winding axis at the edge of the electrode
region--that is a distance one electrode circumference inward from
the leaf end. FIG. 7A illustrates such an embodiment, showing the
glue/sealant bands (dark lines and flow paths through a brine cell
wherein the right-hand end corresponds to the anode or anode cell,
and the left-hand end to the cathode or cathode cell. As shown, the
brine inlet flow bi enters at the bottom of the anode cell and is
constrained to flow axially along the length of the anode by seal
line Sa becoming acidic. The acidified brine then turns along
successive helical path segments a, b, c before entering the
cathode area of the brine or catholyte cell, where seal line sc
retains the flow in the cathode or cathode cell. The dilute spacer
covers only the area between the anode and cathode area seal lines
s.sub.a and s.sub.c away from the highly concentrated electrolyte
area (see FIGS. 3 and 4), thus limiting effects such as back
diffusion. Moreover, the initially acidified brine guards against
the occurrence of excessively high pH conditions in the brine cell
or at the brine side of the anion exchange membrane that might
otherwise contribute or induce susceptibility to scaling.
[0081] In addition to seals defining brine cell flow paths, devices
of the invention may employ seals to restrict or delimit the dilute
flow paths, so that the spiral wound device effectively operates
with the dilute flowing in a longer path or in series through two
or more cells, becomes a two-stage device, or becomes a two stage
device with different brine flows or path configurations for each
stage.
[0082] Another important aspect of construction that may be
employed in dilute cells of an EDI device of the present invention
is to employ a screen mesh within the sparsely-filled dilute cell
wherein the screen both segregates the different types of exchange
beads, and deflects flow to assure adequate contact between the
dilute flow and both types of beads. One construction, which the
inventors refer to as s-layering, is illustrated in FIG. 8, in a
schematic view, taken normal to the tangent plane of the dilute
cell and along a line extending in the nominal flow direction. As
shown in FIG. 8, a dilute cell is defined between an anion exchange
membrane A.sub.x and a cation exchange membrane C.sub.x such that
cation exchange material C lies adjacent to the membrane C.sub.x
and anion exchange material lies adjacent to the membrane A.sub.x.
This may be accomplished as described above by selectively coating
opposite sides of an adhesive-coated screen S with the different
types of exchange beads to form a dilute cell spacer assembly. The
s-layering construction according to this aspect of the invention
is further characterized by a plurality of one-sided obstructions
or flow deflectors D.sub.c (positioned on the cation side to
deflect flow toward the anion side) and D.sub.a (positioned on the
anion side to deflect flow toward the cation side) which are
alternately placed across the general direction of flow to divert
the flow to the opposite side of the dilute cell. The flow
deflectors may be part of the screen itself, such as filaments of
greater height or cross-section running across the flow direction
and projecting on alternate sides of the screen by an amount that
obstructs flow across a substantial portion of one side.
Alternately, the deflectors may be separately formed or placed, for
example as filaments or lines of sealant laid down at the indicated
positions to deflect flow from alternate halves of the channel. The
screen or spacer construction may also be effected using more than
one layer of screen, or may be effected by using screen or mesh
formed of the corresponding ion exchange materials, in which case
exchange beads may be omitted. This aspect of the invention is
advantageously employed in flat plate EDI devices of thin cell
construction and is not limited to use in EDI devices of
rolled-leaf or spiral construction.
[0083] Various spiral EDI devices may be configured with membranes
and spacers according to one or more of the above specific types.
One particularly advantageous construction is achieved in
accordance wit the present invention by providing a brine cell
spacer having segregation bands that extend across the general
direction of the dilute flow (which may be axial), and which
operate to isolate the species entering the concentrate cell near
the front of the dilute path from the species entering the
concentrate cell further along the dilute path. This aspect is
illustrated in FIG. 9A.
[0084] As shown in FIG. 9A, a brine spacer, denoted generically B
has one or more bands BB that extend at least the full thickness of
the spacer to contact the adjacent membranes and constrain the
concentrate flow within a horizontal (as shown) region which
corresponds to an initial or subsequent segment of the dilute flow
path. Three such bands BB are shown, corresponding to different
characteristic regions of demineralization along the dilute path as
schematically illustrated in FIG. 9B. While certain species may be
absent or of negligible effect in many feed fluids, these regions
illustratively include a first region a of the dilute path wherein
cations in the dilute flow such as certain bivalent metal ions like
calcium or magnesium enter the concentrate cell; a second region b
where monovalent ions and larger or less mobile higher valence ions
such as CO3, sulfate and the like pass from the dilute flow, and a
third band c located toward the product outlet where the device may
operate in a more polarized mode with substantial generation of
hydroxyl and hydronium ions and their passage into the concentrate
cell. The bands BB separate these regions of the concentrate cell
into distinct and separate flow strips, so that flow of each
species or group of species proceeds along a separate path toward
the concentrate outlet or outlets. In this manner, the various
complementary components that might otherwise give rise to scale
are prevented from meeting. Certain species, such as the neutral
gas CO2, which may pass through the membranes relatively freely,
back-diffuse into the dilute stream and re-enter the concentrate
cell, may enter all the concentrate regions a, b, c, but at each
location the absence of a component segregated elsewhere would
render this combination essentially non-scaling.
[0085] The segregation bands BB may be implemented by several
alternative means. One approach is to deposit a band of impermeable
sealant along a strip to fill the brine screen spacer and prevent
fluid movement across the band. Another approach is to employ, as a
spacer, an asymmetric screen wherein the larger-dimension strands
of a network extend continuously and parallel to each other for the
full thickness of the spacer, while smaller-dimension cross strands
permit flow to proceed parallel to the large strands. In this case,
the screen spacing may be relatively small, with a mesh of 0.5 to 5
centimeters, so that the major strands would create dozens or
hundreds of segregated concentrate flow paths along a one-meter
long dilute flow path, rather than the three general regions
illustrated in FIGS. 9A-B. For prevention of scaling, it is
important that the bands be sufficiently distinct in composition
that the presence of complementary scale forming species does not
come about under pH conditions that would cause them to deposit.
This function can be achieved by a few bands BB, or by the tens or
hundreds of bands provided by orienting the strands of an
asymmetric screen along the direction transverse to the dilute
flow.
[0086] The spatial separation of the relevant species may be
enhanced or more precisely defined according to another or further
aspect of the present invention, by arranging selected exchange
resins in regions along the dilute path so as to selectively strip
one type of ion in that region, or inhibit the passage of a
complementary ion into the concentrate, thus more quickly and
clearly separating the scaling or other components. FIG. 10
illustrates this aspect of the invention, showing schematically a
dilute cell of a rolled EDI device (or of three devices arranged
for serial flow. As shown, the initial region of the path,
corresponding to region a is filled with a cation exchange resin C
to more completely capture and transport the scale-forming metal
cations into the upper concentrate band, while more effectively
discriminating against certain potentially deleterious co- or
counter-ions. A subsequent portion of the dilute cell has a filling
of anion exchange material A to sharpen the removal of the bulky or
hindered sulfate ions and other components, while a third region or
stage contains a conventional mixed filling of exchange material
for better polishing. Three separately-energized electrodes E1, E2,
E3 may be employed to adjust or control operation more closely for
the specific distribution of material present in the feed.
[0087] As noted above, embodiments of the invention address certain
intrinsic inhomogeneities of prior art constructs by features such
as a shielded electrode pocket for envelope termination, arranging
the windings to avoid shadowing effects, and employing a relatively
large core to limit the increase in current density that occurs at
inner cells of the winding. In accordance with another aspect of
the invention, the spiral is equipped with an end port structure
that provides a compensatory flow profile.
[0088] FIGS. 11A and 11B illustrate this aspect of the invention. A
spacer S, which illustratively is a relatively large-strand mesh as
described in Applicant's international application WO03/043721, is
slotted to receive a plurality of tubes or rods T along one edge,
and the screen and tubes are embedded in a full-width sealing band.
The band may be formed of a polyurethane or epoxy material which
cures and provides sufficient flexibility to allow the screen to be
rolled together with the exchange membrane and another spacer as
described above, into a rolled EDI assembly. The end of the rolled
assembly is preferably then potted, in a manner similar to that
employed for forming hollow-fiber MF modules, so that all the
membranes and spacers are sealed at the bottom (as shown) edge and
the concentrate cells are well isolated from the dilute cells. The
rods/tubes T project through the potting material. If rods rather
than tubes are used, these may then be pulled from the assembly
leaving through-holes communicating with the mesh S, which, like
the tubes, operate as end-ports into the cells defined by the
spacer S. As further shown in FIG. 11A, the elements T are spaced
at progressively greater intervals toward one end of the spacer S.
this results in a greater number of ports, hence increased flow at
one end region of the spacer, which is preferably the inside
(smaller diameter) portion of the rolled assembly.
[0089] FIG. 11B schematically illustrates this effect. The greater
number of inlet or outlet ports allows more flow, or higher flow
velocity at the radially inner portion of the spiral, the region
that also experiences a higher current density. The fluid flowing
in that region thus has a shorter residence time, yet may be
treated to the same end point (for example, 15-16 MegOhm
conductivity, as the fluid passing through the outer windings,
without causing excessive depletion and polarized operation. The
result is a high throughput, uniform quality product, free of
extreme or inefficient operating regions.
[0090] The devices described above thus embody a number of novel,
inventive and advantageous constructions for EDI devices that
enhance the ease of manufacture, effectiveness of operation and
overall performance or capabilities of the devices so constructed.
In the foregoing description of illustrative embodiments, various
novel elements and salient features have been emphasized, but these
may be varied or supplemented with variations of overall
architecture and other details of construction known from the
technical literature of flat plate and spiral EDI devices, many of
which are now on the market. Ancillary details relating to aspects
such as bead catchers, ports, valves and electrode constructions as
well as operating control are well known to those skilled in the
art, and may be applied with suitable modification to the
constructions described herein. The invention being thus disclosed,
further variations and modifications will occur to those skilled in
the art, and all such variations and modifications are considered
to be within the scope of the invention as described herein and
defined by the claims appended hereto.
* * * * *