ION EXCHANGE DEIONIZATION APPARATUS WITH ELECTRICAL REGENERATION
FIELD OF INVENTION
[0001] The invention pertains to membraneless methods and apparatus adapted
to perform electrodeionization functions including deionization of influent and
regeneration of active ion exchange sites on associated ion exchange materials.
BACKGROUND OF THE INVENTION
[0002] Electrodeionization (EDI) is a process that removes ionized and
ionizable species from liquids using electrically active media and an electrical potential
to influence ion transport. In many EDI processes, ion conducting membranes and an
imposed electrical current are used. Cations and anions in the feedwater are exchanged
for hydrogen and hydroxyl ions in the ion exchange resin or membrane, thus producing
demineralized feedwater.
[0003] Hydrogen and hydroxyl ions are needed to regenerate the exchange sites,
and these are continuously regenerated in EDI processes by the electrically driven water
splitting process by which H+ and OH ions are generated. The ion conducting
membranes utilized in the process are semipermeable anion and cation ion exchange
membranes and are provided in stacks between electrodes with spaces between
membranes configured to create fluid flow compartments. The anion membranes allow
only negatively charged ions (anions) to permeate while the cation membranes allow
only the positively charged ions (cations) to permeate. Ions migrating through the stack
toward their opposite polarity electrodes are trapped in certain "concentrating"
compartments while the influent feed is purified in so-called dilution compartments
from which the salt imparting ions have vacated. The reduced ion purified product is
taken from these dilution compartments to a header or the like for collection.
[0004] These EDI devices depend on the use of expensive ion exchange
membranes or carbon electrodes. There is accordingly, a need in the art for a device
that can perform intended deionization function that uses less expensive materials such
as ion exchange beads.SUMMARY OF THE INVENTION
[0005] In one embodiment, the invention pertains to an electrodechemical
deionization device and method which includes two operational steps. The first step or
phase involves regeneration of the active sites on the ion exchange resin. In this phase,
a flushing liquid is fed as influent with a current passing through the device. In the
second or deionization phase, the current is turned off or reversed, and the regenerated
ion exchange resin deionizes the water as it passes through the device. These two steps
alternate in time, cycling between the regeneration then deionization steps.
[0006] In accordance with one aspect of the invention, a method is provided for
reducing ion concentration in an influent solution. As part of the method, and in a
deionization phase thereof, the influent is fed to a membraneless electrodeionization
(EDI) stack. The stack comprises an array of alternating layers of porous anion
exchange materials (AIX) and cation exchange materials (CIX). In one embodiment,
the AIX and CIX materials are disposed in a sandwich like structure with at least one
interfacial area existing between contiguous AIX and CIX layers. The influent is
passed over the AIX and CIX array, whereby the ionic concentration of the influent is
reduced via ion exchange with the active sites of the AIX and CIX materials. The
deionized product is collected from the EDI stack.
[0007] In a regeneration phase of the process, a voltage is imparted across the
electrodes and a flushing solution is provided as influent across the membraneless
sandwich array of the EDI stack traversing the AIX and CIX materials. Water splitting
occurs along at least one of the interfacial areas resulting in the formation of hydrogen
T and OH ions which migrate to and replenish the AIX and CIX materials. A waste
stream is recovered as effluent from the stack as a result of this regeneration phase.
[0008] In accordance with another aspect of the invention, the influent to be
purified is water having dissolved sodium and chloride ions therein. In certain
exemplary embodiments, the CIX materials comprise beads having fixed S0 3 ions
therein, and in other embodiments, the AIX materials comprise beads having fixed
quaternary ammonium ions therein.[0009] In other embodiments, the EDI stack further comprises at least one
mixed ion exchange material interposed between AIX and CIX material layers.
[0010] From an apparatus perspective, the EDI apparatus comprises a
membraneless stack of alternating layers of AIX and CIX materials interposed between
an opposing cathode and anode. The stack of alternating layers defines at least one
interfacial area between an AIX and CIX. The EDI stack, in accordance with the
invention, is devoid of any ion exchange membranes therein save for membranes that
may border the electrodes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Fig. 1 is a schematic diagram of one membraneless EDI stack in
accordance with the invention; and
[0012] Fig. 2 is a schematic diagram of another membraneless EDI stack in
accordance with the invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0013] Turning first to Fig. 1, there is shown a first exemplary embodiment of
the invention comprising a membraneless electrodeionization (EDI) stack configuration.
As shown, the EDI stack 100 comprises opposing cathode 2 and electrode 4 with the
space between the electrodes composed entirely of alternating contiguous layers of
porous anion ion exchange materials 6, and porous cation ion exchange materials 8. As
shown, a plurality of interfacial AIX and CIX areas 14 are formed by this alternating,
sandwich structure.
[0014] The electrodes 2, 4 may be composed of conventional materials such as
corrosion resistant metals such as titanium, niobium, stainless, etc., and in some
instances, these materials may be provided with an outer coating of a noble metal such
as platinum. In the embodiment shown, cathode 2 is composed of stainless with anode
4 composed of Ti coated with Ir0 2.
[0015] The alternating contiguous layers of the stack construction span the
entirety of the distance between the electrodes and define a sandwich like array. As to
the ion exchange materials 6, 8, these can be in any form other than membrane form.Ion exchange beads are preferred, although the artisan will appreciate that these
materials may also be present in fiber, granule, or other ion absorptive form, save for
membrane form.
[0016] Exemplary anion ion exchange resins (AIX) include the strongly basic
type based on inclusion of quaternary amino groups such as trimethylammonium groups
including trimethylbenzyl ammonium and the weakly basic groups based on primary,
secondary, and/or tertiary amines such as polyethylene amine. These anion exchange
resins are sold commercially. As is known in the art, these materials provide for ion
exchange of counter ions such as Cl on the fixed ion portion of the structure.
[0017] Exemplary cation ion exchange (CIX) materials may include those based
on sulfonic acid groups such as sodium polystyrene sulfonate or polyAMPS
(acrylamidopropanesulfonate) or the weakly acidic groups such as the carboxylic acid
groups. These cation exchange resins are all well known and commercially available.
As is known in the art, these materials provide for ion exchange of counterions such as
Na+ on the fixed ion portion of the structure.
[0018] Fig. 2 shows another embodiment wherein a mixed bed ion exchange
(MBIX) layer 10 is included interposed between layers of anion ion exchange material
6 and cation exchange material 8. The inclusion of the mixed bed serves to increase the
contact surface area between the two types of ion exchange material to facilitate
appropriate decrease in the voltage that need be applied across the electrodes during the
regeneration phase as shall be referred to later. The MBIX layer preferably comprises a
mix of AIX and CIX materials in an AIX:CIX ratio of about 0.177-5.67: 1.
[0019] In operation, and with reference to Fig. 1, there is described a charging
phase of operation in which, for example, an influent stream is fed to the bottom of the
stack in the direction as shown by the arrows. It is important to note here that the
influent stream is fed across the entire surface of the sandwich array of alternating anion
ion exchange and cation exchange materials from the bottom through to the top, exiting
as an effluent waste stream. During this charging or regeneration phase, a direct
current, for example, 5 amperes, flows through the assembly from cathode to anode,
and the influent stream may be fed for instance at about 100 liters/hour. The influent
feed, or flush feed as referred to during this charging or regeneration phase, may havefor example, a conductivity of 2 µ / . The electric field causes water to split into
component ions H+ and OH that migrate through the ion exchange materials toward
the electrode having opposite polarity. That is, the H+ ion migrates toward the cathode
with the negatively charged OH migrating toward the anode. This water splitting
occurs at the AIX/CIX interfacial areas noted as "A" in Fig 1. The FT and OH ions
migrate into their respective IX resins, displacing salt ions and, in effect, regenerating
the IX materials. Salt ions held in the IX resins will also migrate in this electrical field
1, eventually reaching a second AIX/CIX interface at which point they release into
solution. This concentrated solution is removed as effluent or waste from the top part of
the stack in relation to Fig. 1 so as not to contaminate the newly generated IX resin, and
this effluent may, as expected, be highly conductive on the order of for example 800
µ / . Influent or flush feed may be maintained for a time to result in a waste stream
effluent ultimately exhibiting a diminished or steady state conductivity. Once the IX
resin has been partially or completely regenerated, the power to the stack can be
discontinued so that the stack can be operated then in its deionization phase.
[0020] During the deionization phase, influent such as salt water to be deionized
is fed across the array of IX materials as shown by the arrows in Fig. 1. The cations and
anions in the influent feed impinge upon the IX beads or the like, are adsorbed into the
IX materials, and the IX materials release H+ and OH which recombine to form water
into the influent steam as it travels (upwardly with respect to Fig. 1) through the stack.
Purified product is removed from the top of the stack.
[0021] As an example, during the deionization phase of operation, the feed may
be a slightly salt water such as measured as 25 µ / . (Similar to reverse osmosis
product water). The influent may be fed at for example, 1100 liters/hour. As above
stated, in one embodiment during the normal deionization phase, no voltage is applied
across the electrodes. The regenerated bed of IX beads or the like removes salt
imparting ions through conventional IX processes. The deionization would occur until
the IX beds are exhausted. It is noted that waters having up to about 1000 µ / can be
deionized in accordance with the invention.
[0022] The artisan will readily appreciate that a conductivity sensor may be
used to, for example, measure the conductivity of effluent during one or both of theregeneration or deionization cycles. For example, the sensor could be operatively
associated with a controller to initiate or terminate the application of the electrical feed
across the electrodes, or regulate the influent feed type, i.e., regeneration flush or water
to be purified during the deionization process. For example, during the regeneration
phase, effluent conductivity can be measured, and when this measurement would be less
than a preselected value, regeneration would terminate followed by feed of influent
water for purification during the deionization phase. Additionally, during the
deionization phase, product conductivity could be measured, and when this
measurement exceeded a preselected value, influent feed could be changed to
regeneration flush feed with commensurate application of an electrical potential across
the electrodes.
[0023] Also, it is to be noted that, in contrast to the flow direction shown in Fig.
1, if porous electrodes are used, the fluid flow could be made through the electrodes
perpendicular to the fluid flow direction shown in Fig. 1. Additionally, it may be
beneficial to provide opposite flow directions for the renegeration phase and
deionization phase.
[0024] Fig. 2 shows another embodiment in which a mixed bed of ion exchange
materials 10 such as beads, fibers, etc., are interposed between layers of the cation ion
exchange materials. The principles of operation of this embodiment are substantially
the same as set forth above in connection with Fig. 1. During the regeneration cycle,
water splitting occurs along some of the interfacial areas 14 with the salt imparting ions
trapped and concentrated at other ones of the interfacial areas 14.
[0025] As indicated briefly above, during the deionization phase, it may be
possible to enhance the demineralization process by applying an electrical field in a
polarity that is opposite from the regeneration polarity. It is thought that the current or
voltage for this would be some fraction of that required for the regeneration phase.
[0026] The artisan may appreciate that a membrane such as an ion exchange,
water permeable membrane such as a thin UF membrane or the like, or a very tight
mesh membrane, may be placed adjacent to but spaced apart from one or both of the
electrodes in order to prevent gasses from entering the bulk of the stack during
regeneration. For example, and with reference to Fig. 1, a membrane could be providedbetween the AIX material 6 next to cathode 2 and the cathode. Also, a membrane could
be located between the CIX layer adjacent anode 4 and the anode. The phrase
"membraneless deionization" or reference to the fact that the stack is devoid of ion
exchange membranes therein or equivalent verbiage shall not preclude such
constructions as envisioned above wherein a membrane may border one or both of the
electrodes. These phrases do however signify that the array of AIX, CIX, and MBIX
materials does not include a membrane located at any of the interfacial surfaces
between the adjacent AIX, CIX, or MBIX members of the array.
[0027] It will be apparent to those skilled in the art that other changes and
modifications may be made in the above methods and apparatus without departing from
the scope of the invention herein, and it is intended that all matter contained in the
above description shall be interpreted in an illustrative and not a limiting sense.
[0028] While the invention has been described in terms of preferred
embodiments, claims appended hereto are intended to encompass all other embodiments
which fall within the spirit of the invention.CLAIMS
1. Method of reducing ion concentration in an influent solution comprising:
in a deionization phase of operation, feeding said influent to a
membraneless electrodeionization (EDI) stack comprising alternating layers of porous
anion exchange materials (AIX) and cation exchange materials (CIX) interposed
between a cathode and an anode, of said AIX and CIX and defining at least one
interfacial area between an AIX and CIX, passing said influent over said AIX and CIX
whereby said ion concentration is reduced via ion exchange with said AIX and CIX;
collecting deionized product from said EDI stack; and
in a regeneration phase of operation, imparting a voltage across said
electrodes and feeding flush water as influent to said EDI stack and across said AIX and
CIX materials, whereby water splitting occurs along at least one of said interfacial areas
resulting in formation of H+ and OH ions which migrate to and replenish said AIX and
CIX materials, and recovering a waste stream as effluent.
2. Method as recited in claim 1 wherein said influent is water with
dissolved Na+ and CI ions therein.
3. Method as recited in claim 2 wherein said CIX materials comprise beads
having fixed SO3
2 ions therein.
4. Method as recited in claim 2 wherein said AIX materials comprise beads
having fixed quaternary ammonium ions therein.
5. Method as recited in claim 1wherein said EDI stack further comprises at
least one mixed ion exchange material therein interposed between an AIX and CIX.
6. Electrodeionization apparatus comprising a stack of alternating layers of
anion exchange materials AIX and cationic exchange materials CIX interposed between
a cathode and an anode, said stack of alternating layers defining at least one interfacial
area between an AIX and a CIX, said stack being devoid of ion exchange membranes.7. Apparatus as recited in claim 6 wherein said AIX comprises beads
having fixed SO3 ions therein.
8. Apparatus as recited in claim 6 wherein said AIX comprises beads
having fixed quaternary ammonium ions therein.
9. Apparatus as recited in claim 6 further comprising at least one mixed ion
exchange material in said stack and interposed between an AIX and CIX.