WO1998051620A1

WO1998051620A1 - Purification of a liquid stream

Info

Publication number: WO1998051620A1
Application number: PCT/AU1998/000334
Authority: WO
Inventors: Alastair Mcindoe Hodges; Peter Osvath
Original assignee: Usf Filtration And Separations Group Inc.
Priority date: 1997-05-09
Filing date: 1998-05-08
Publication date: 1998-11-19

Abstract

Method and apparatus for purifying a liquid stream comprising at least two ion exchange material beds (17, 18), one of anion exchange material and the second of cation exchange material. The beds (17, 18) are connected at at least one point such that a junction (4) which is permeable to ions is formed therebetween. Liquid to be purified is passed serially through one bed than the other. A pair of electrodes (2, 3) is provided one each being associated with a respective material bed such that at least some ion exchange material is sandwiched between each electrode (2, 3) and the junction (4), and an electrical potential is applied across the pair of electrodes (2, 3) so as to produce an electric field therebetween and regenerate the ion exchange material (10, 11). In another form the beds may contain a mixture of anion and cation exchange material. A method of forming a junction between the beds is also disclosed.

Description

Title: Purification of a Liquid Stream

Technical Field

The present invention relates to a method and apparatus for the purification of water

containing streams using ion exchange materials and the regeneration of said material

electrochemically.

Background of the Invention and Prior Art

There is an enormous market for high purity water. It is used extensively in industrial

applications, with notable major users including the pharmaceutical, electronics and power

generation industries.

In the final polishing step for most applications, dissolved salts are commonly

removed from the water by passage through ion exchange resins. A combination of a

cation exchange resin (in the H form) and an anion exchange resin (in the OH^" form) are

used to remove the cations and anions respectively. Two such beds may be used in tandem

for the removal of salts. If the aqueous solution is first passed through a bed of cation

exchange resin (in the H⁺ form), the cations in the solution are taken up by the cation

exchanger, and an equivalent amount of H ions are released into solutions (thus preserving

electroneutrality). This (now acidic) solution is then passed through a bed of anion

exchange resin (in the OH^" form), and the anions in the solution are taken up by the anion

exchanger. The OH^" that is released, neutralises the H in the solution, forming water. The

dissolved salts have effectively been removed from the solution. (If the sequence of the

desalting beds is reversed, a corresponding scenario, with an intermediate alkaline solution,

still applies).

However, for a number of applications, a mixed bed of ion exchange resin is

preferable, either in the place of the tandem bed configuration, or in addition to it, as a final polishing step. In this case, the cation and anion resins are intimately mixed, and the

cations and anions are removed from the solution at the same time, with immediate

neutralisation of the H and OH^" that are released by the resin. It is possible to achieve a

greater level of salt removal from aqueous solution with this configuration, since the

chemical equilibrium process is "driven" by the loss of H⁺ and OH^" by mutual

neutralisation.

In either configuration, once the resins are exhausted (i.e. once there is no more resin

in the H⁺ and OH^" form), the cation and anion beds must be regenerated by treatment with

acid (typically H₂ SO₄) and alkali (typically NaOH) respectively. This regeneration

involves the storage and handling of concentrated acid and caustic solutions, as well as the

disposal of the effluent regeneration and wash solutions after they have been used to treat

the resins. This is undesirable from the point of view of cost, safety and environmental

considerations. In the present application, we describe a method of regenerating the resin

using mild, inexpensive and ecologically acceptable electrochemical techniques. This

novel process does away with the regenerating chemicals, and has the added advantage that

the water purification process and ion exchanger regeneration can be carried out in the

same vessel, with no need to disturb the resin bed. In the case of the mixed bed systems,

this process also has the advantage that the regeneration can be carried out on the resin

beds without needing to separate the cation exchange resin from the anion exchange resin.

For the conventional chemical regeneration processes, the cation and anion resins in the

mixed resin bed must be physically separated before chemical regeneration (using acid and

alkali respectively) can be carried out.

Prior Art The use of conventional ion exchange technology, with chemical regeneration of the

exhausted resin, remains the principal means of removing dissolved salts from water to

very low concentration levels. The following patents and patent applications illustrate the

prior art relating to the present invention.

Electrodeionization apparatus (TJS Patent 4.632.745. Intl. Patent WO 92/1 1089

The Elix™ Electrodeionization Process, (marked by Millipore TM), is used to

deionise solutions almost to the level of purity attained by conventional mixed bed ion

exchange systems. In its simplest form, the system comprises four alternating cation (c)

and anion (A) permeable membranes sandwiched between a pair of electrodes ( + and -), in

the configuration + | C | A | C | A | -. The space between the central pair of membranes is

filled with mixed bed ion exchange resin (principally in order to improve electrical

conductivity). The feed water flows down between the sheets of ion exchange membrane

(which allow the flow of relevant ions but not water), perpendicular to the applied electric

field. The dissolved cations in the feed solution (in the central cavity) pass through the

cation permeable membrane towards the cathode, while OH^" ions generated at the cathode

pass through the anion permeable membrane. A concentrated stream of hydroxide salt of

the cations passes out through this (concentrating) chamber. Likewise, the dissolved

anions in the feed solution (in the central cavity) pass through the anion permeable

membrane towards the anode, while H ions generated at the anode pass through the cation

permeable membrane. A concentrated stream of the acid form of the anions passes out

through this (concentrating) chamber. The water flowing out of the central chamber is thus

depleted of dissolved salts, while the (waste) water flowing out of the two outer chambers

contains the cations and anions from the feed solution. A bank of such cells can be used to

generate water that has a low concentration of dissolved ions. In this case, the intermediate concentrating chambers have anions and cations entering through the membranes on

opposite sides of the chamber, and concentrated salt solutions (rather than concentrated

acid and alkali solutions) flow out through these chambers. It is only from the outermost

chambers (that house the electrodes) that acid and alkali solutions are released in this

configuration. Although this system is also used for polishing water, the technology is

totally different from that of the present invention. Furthermore, the modules involve a

stack of individual compartments, each of which has a complex design. The large number

of membranes increases the potential for leakage (even the smallest rupture in one

membrane renders the system ineffective) from the concentrating compartment into the

pure water compartment, as well as providing a large membrane surface area with a

potential for fouling and being relatively expensive. The current invention has significant

advantages both in simplicity of operation, reliability and lower cost.

Electrochemical deionisation (US Patent 5.584.981)

This is very similar to the above invention, except that the ion exchange material is

structured as a porous layer, so that water from the feed solution passes through the sheets

of ion exchanger along with the ions.

Purified ion exchange resi and process (US Patent 5.259.936)

The resin to be purified is placed in the ion-depletion chambers of the "Elix" type

configuration above (US Patent 4,632,745). Once again, this process operates using

extensive banks of ion exchange membranes.

Process for purifying resins using a bipolar interface (US Patent 5.211.823)

This patent discloses a method for the regeneration of ion exchange resins within the

compartments of an electrodeionization apparatus of the kind described in US Patent

4,632,745. However, each of the dilution compartments (comprising a compartment defined by an anion and a cation exchange membrane), is further divided by a bipolar

membrane, with an anion permeable face directed towards the anion permeable membrane,

and the cation permeable face directed towards the cation permeable membrane. The two

subcompartments thus formed are filled with anion exchange resin and cation exchange

resin respectively. Under the influence of an electric field (+ve potential applied on an

anion exchange side, -ve potential applied on the cation exchange side), water is split into

OH^" and H⁺ at the bipolar membrane, and these ions migrate through the anion and cation

resin beds respectively, replacing the associated anions and cations. These flow out of the

compartment through the anion and cation permeable membranes, and into concentrating

chambers that are formed by sandwiching a series of such cells in parallel. Concentrated

solutions of salt are thus removed, leaving the anion and cation resins in the OH^" and H⁺

forms respectively. In the terminal concentrating chambers (in which the electrodes are

housed, and where OH^" and H are generated), the cations and anions are removed as the

OH^" and H⁺ forms respectively. Once again, this system is exceedingly complex,

comprising multiple layers of anion, bipolar and cation membranes, with their attendant

potential for leakage and fouling. Furthermore, this system, in reality, is an electrodialysis

system rather than an electrochemical regeneration system, as it relies principally on the

electric field and the bipolar membrane to generate the H⁺ and OH^" that are used for

regeneration, rather than producing them as a consequence of the electrolysis of water.

Process and device for demineralising aqueous solutions using ion exchangers (US Patent

5.423.965.

This patent discloses a process for the electrochemical regeneration of a cation

exchange resin bed placed between a pair of electrodes, with the H ions displacing the

cations. These move toward the cathode, and combine with the OH^" produced there. This alkaline solution is then used to chemically regenerate the anion exchanger in a traditional

manner. However, here the regeneration of the two beds of resin does not take place

together as a result of the simultaneous generation of the H and OH^" coupled to their

electrically assisted migration through the resin bed. It more closely resembles the

electrochemical regeneration of a single resin bed, with the effluent then being used as a

conventional regenerant of the other bed.

Electrochemical ion exchange (UK Patent Appl. GB 2.178.759A: Appl. No. 8614401 ;.

This patent application discloses the electrochemical regeneration of a single bed of

either cation or anion exchange resin. It involves establishing an electrochemical cell with

the working electrode surrounded by ion exchange resin, which is held within a

compartment permeable to the ions generated at the electrode (and therefore, also the ions

exchanged from the resin). The whole is immersed in an electrolyte in which is placed the

counter electrode to complete the electrochemical cell. In the case of the working electrode

being the anode, H⁺ is generated, and displaces ions associated with the cation exchange

resin surrounding the electrode, and these migrate through the ion permeable compartment

and combine with the ions generated at the cathode (presumably OH^"). The regenerated

resin is then removed and is ready for use. A similar system of opposite polarity can be

used to regenerate anion exchange resin. This patent application also involves

electrochemical regeneration, but the novelty of the system of the present invention resides

in the simultaneous regeneration of a pair of anion and cation exchange resin beds due to

the generation of OH^" and H at the cathode and anode respectively, and the consequent

efficient use of all the generated ions, and simultaneous regeneration of a complete

deionising system. Even in the mixed bed form of the present invention, in any one cycle, the anion resin is one mixed bed and the cation resin is the other mixed bed, are being

simultaneously regenerated.

Process and Device for Regenerating Ion-Exchange Material (Intl. Patent Application No.

WO 89/00453)

This patent application discloses a process for the regeneration of an ion exchange

resin by the application of an alternating voltage or pulsating direct voltage to an

electrochemical cell containing either anion exchange or cation exchange resin. A two bed

system is also described, wherein one bed contains anion exchange resin, the other contains

cation exchange resin, and the beds are separated by an electrode and a liquid layer. A

high frequency (I KHz to 10 MHz) alternating potential (of tens of hundreds of volts) is

applied. This system is quite different in conception from that of the present invention, and

other than claiming to use electricity to regenerate an ion exchange resin, the two have

little if anything in common.

A process for regeneration of ion exchange resins .Indian Patent 15 08 70)

This patent discloses a method for partial regeneration of ion exchange resin in an

electrochemical cell. In this patent application, the electrodes are buried in the centre of

the compartments of ion exchange resin. By contrast, the sandwich design of the system of

the present invention (where the resin is contained between the two electrodes) is

fundamental to its successful operation. Furthermore, there is no release of liquid during

the regeneration process in the process described in this patent, which means that there is a

build up of salt, leading to equilibration between the salt in the water and on the resin,

preventing further regeneration. In the examples given, the amount of salt solution passed

through the resin bed amounts to -10% of the total capacity of the resin bed each time.

Only four aliquots of this solution were passed, so it is not clear whether the regeneration occurred to any significant extent, or whether the desalting was simply due to the presence

of more unused resin than required by the solution to be desalted. The difference in the

conductivity of the effluent solution after passage of the electric current may simply be due

to the difference in mobility of the H /Na and OH7CT ions. The highest purity level

achieved corresponded to water of 277 micromho conductivity (3600 ohms water).

Disclosure of the Invention

According to the present invention a method and apparatus is disclosed which allows

for the continuous production of purified water and regeneration of the ion exchange

material beds.

According to one aspect of the present invention there is provided apparatus for

purifying a liquid stream comprising at least two ion exchange material beds, one of anion

exchange material and the second of cation exchange material, the beds being connected at

at least one point such that a junction which is permeable to ions is formed therebetween,

means for passing liquid to be purified serially through one bed then the other, at least one

pair of electrodes one each being associated with a respective material bed such that at least

some ion exchange material is sandwiched between each said electrode and said junction,

means for applying an electrical potential across said pair of electrodes so as to produce an

electric field therebetween and regenerate said ion exchange material.

According a further aspect, the present invention provides apparatus for purifying a

liquid stream comprising at least two ion exchange material beds, each bed containing a

mixture of anion and cation exchange material, the beds being connected at at least one

point such that a junction which is permeable to ions is formed therebetween, means for

passing liquid to be purified through at least one of said beds, at least one pair of electrodes

one each being associated with a respective material bed such that at least some ion exchange material is sandwiched between each said electrode and said junction, means for

applying an electrical potential across said pair of electrodes so as to produce an electric

field therebetween and regenerate said ion exchange material.

The invention also provides a method of purifying a liquid stream comprising the

steps of passing liquid to be purified serially through at least a pair of ion exchange

material beds, one of anion exchange material and the second of cation exchange material,

the beds being connected at at least one point such that a junction which is permeable to

ions is formed therebetween, concurrently regenerating said ion exchange material beds by

applying an electrical potential across a pair of electrodes, one each being associated with a

respective material bed, so as to produce an electric field between the electrodes and said

electrodes being arranged such that at least some ion exchange material is sandwiched

between each said electrode and said junction.

When using separate anion and cation exchange material beds, the cathode is placed

in the anion exchange material bed and the anode is placed in the cation exchange material

bed. A liquid stream to be purified is fed into one of the beds, preferably to the base of one

of the beds. It is then allowed to flow through and exit the first bed and then be fed into a

second bed, preferably to the base of the second bed. The liquid stream then flows through

the second bed whereupon it is purified of ions. The liquid then exits the second bed as a

purified stream.

While these processes are occurring an electrical potential is applied between the

electrodes in contact with the ion exchange material beds such that a current flows between

the electrodes. This causes an electric field to exist across the ion exchange material

packed between the electrodes, and the reduction and oxidation of water to produce

hydroxide and hydrogen ions at the cathode are anode respectively. The electric field causes the contaminant ions that had been absorbed by the ion exchange material to collect

at the bed junction where they form salts. The contaminant ions are replaced in the ion

exchange material by the hydroxide and hydrogen ions that have been generated at the

electrodes. A small flow of liquid is caused in the vicinity of the bed junction, from one or

both sides of the junction, which carries away the salts formed, for external collection. In

this way, according to the present invention, the ion exchange material in the beds is

continuously regenerated while purified water is being produced. This allows the

continuous production of purified water for extended periods without the need to interrupt

the process to regenerate the ion exchange beds.

When using beds filled with mixed anion and cation exchange materials, the liquid to

be purified can be passed through a single bed or through multiple beds of ion exchange

material. In this case the hydrogen and hydroxide ions generated by the electrochemical

reactions do not necessarily come into contact with the bed of ion exchange material.

These products may be flushed away by a liquid stream passing over the electrodes.

Optionally, the cathode may be separated from the ion exchange beds by a cation exchange

membrane and the anode by a anion exchange membrane. The separation of salt in this

case is achieved by the salt being absorbed from the liquid stream by the mixed ion

exchange material. The presence of the applied electric field, by causing an electric current

to flow between the electrodes, causes the ions in the ion exchange material beds to

migrate towards and accumulate as salts at the bed junctions. As before, a small flow of

fluid in the vicinity of the bed junction serves to remove the accumulated salt. As this salt

is continuously removed from the bed it is continually replaced with salt from the stream to

be purified, resulting in a continuous purification of the liquid stream entering the

apparatus. Preferably, the direction of flow of liquid to be purified through the ion exchange

material beds should be such that, at least in part, it is different to the direction of flow of

ions under the influence of the electric field, more preferably in a counter-current or

orthogonal fashion.

In a second aspect of the present invention, the junction of the ion exchange material

beds is formed such that it is permeable to anions or cations or both anions and cations but

substantially restricts or prevents the flow of water through it. In this way an ion

connection path can exist between the ion exchange material beds without allowing the

substantial transport of water. This prevents hydraulic short-circuiting of the ion exchange

material beds by the liquid to be purified.

In a preferred embodiment of this aspect of the invention such a junction may be

formed by using a sheet of anion or cation exchange membrane to divide the two ion

exchange material beds. More preferably, a cation exchange membrane is used. Examples

of such membranes are Nafion™ (DuPont), Tosflex™ (TOSH) or Ionac™ (Sybron

Chemicals Inc.).

Another method for forming the junction is to pack ion exchange resin beads into a

grid such that there is a large resistance to water flow through the packed grid. The resin

particles could be compressed and held within the grids of a single mesh or sandwiched

between two or more grids. For example, a grid with a larger mesh size could be used as

the primary support for the resin beads, which are then further sandwiched between two

other grids with finer mesh size. Preferably, all these grids would be made of relatively

inert, electrically non-conductive material.

If desired, the flow in the vicinity of the bed junction used to flush the salt away can

be distributed via a flow distributor. It is desirable in operating the present invention to reduce the electrical resistance to

ion flow through the ion exchange material beds.

In a third aspect of the present invention it has been found that by applying pressure

to compress the bed of ion exchange material it is possible to lower the resistance of the

bed to ion flow by a factor of two or more.

A preferred embodiment of how to achieve this compression is to use the electrode

structure placed in or on the bed to apply pressure to the ion exchange material sandwiched

between the electrode and the bed junction. The electrode structure could be used as it is

or reinforced with suitable structural members to prevent undue flexing when compressive

force is applied.

In another embodiment of this aspect of the invention, a separate compressive

structure could be placed above the electrode structure in contact with the resin bed. This

could, for example be the lid of the vessel which contains the ion exchange material, where

by over-filling the vessel with ion exchange material during assembly the material would

be compressed as the lid was assembled onto the vessel.

Apart from compressing the ion exchange material there are other methods for

lowering its resistance to ion flow. The traditional form of ion exchange material for water

purification is in the form of beads. In a fourth aspect to the current invention it has been

discovered that ion exchange materials made into other structures can have lower

resistance to ion flow than resin particles when used in beds. Examples of other structures

are filaments of ion exchange material or a macroporous monolith of the material. These

structures can be made by any suitable method.

For example, a macroporous monolith could be fabricated by adding foaming agents

to a suitable polymer dope. Filaments of ion exchange material can be formed, for example, by melt blowing filaments of polymer which could subsequently be spun bonded

or similarly processed to form a sheet of conjoined non- woven filaments. These sheets

could then optionally be compressed and either stacked or wound to form a porous mass of

ion exchange material. The polymer which forms the filaments could contain ion exchange

groups prior to being formed into filaments or the ion exchange groups could be formed on

the polymer after formation of the sheets by chemical treatments. Examples of polymers

that would be suitable for use in this aspect of the current invention are polystyrene or

polyacrylates. Examples of suitable ion exchange groups are sulfonate, carboxylic acid,

amine and quaternary amines. A method of lowering resistance to ion flow of mixed ion

exchange materials is to use a filamental material, such as disclosed above, where some of

the filaments contain cation exchange groups and some contain anion exchange groups.

Where these two types of fibres are uniformly interspersed within the assembled mass of

ion exchange material.

Another method is to use filaments, beads or a monolith of suitable polymer material

and either before or after fabrication of the structure chemically react the materials such

that the final ion exchange materials contain mixed cation and anion exchange groups. For

example, polystyrene beads could be chemically treated such that each bead contained both

cation exchange groups and anion exchange groups.

According to a fifth aspect of the present invention there is provided an apparatus for

purifying a liquid stream comprising at least two ion exchange material containing beds,

one of anion exchange material and the second of cation exchange material, the beds being

connected at one point at least such that a junction is formed therebetween, means for

passing liquid to be purified through each bed, either through a single bed or through at

least one anion exchange material containing bed and at least one cation exchange material containing bed, said ion exchange material containing beds being arranged in a bank and

having at least one pair of electrodes associated with said bank and means for applying an

electrical potential across said electrodes and said bed junction or junctions.

In one embodiment of this aspect of the invention using a single pair of ion exchange

beds, two types of junction between the ion exchange material beds can be formed. In the

first type of junction the electrical potential is applied across the beds junction such that the

mobile cations and anions residing on the ion exchange material are attracted away from

the junction and into the cation and anion exchange material beds respectively. This will

tend to cause water to dissociate into protons and hydroxide ions in the junction. The

protons and hydroxide ions thus formed will then travel through the beds of ion exchange

material under the influence of the applied electric field to continuously regenerate the

material. In the second type of junction the electrical potential is applied across the

junction such that cations and anions present in the bed are attracted towards the junction

where they form salts. These salts can then be carried out of the beds by a small flow of

water caused to flow in the vicinity of the junction. A gutter may be provided at the base

of the junction to assist in carrying away the salt solution.

In an embodiment of this aspect of the invention using more than one pair of ion

exchange material beds, arranged in a bank, each bed of ion exchange material has at least

two junctions either with an adjacent bed or an electrode containing compartment. When

an electrical potential is applied in this case, one junction functions as a source of protons

or hydroxide ions and the second junction as a place for removing contaminant ions from

the beds.

According to one preferred form of this aspect of the invention, the junctions can be

formed, for example, by simply abutting the two exchange material beds, using a woven mesh or other macroporous separator between the two exchange material beds, forming a

mesh or other porous material into which ion exchange material is compressed, or using a

semipermeable membrane to separate the beds. In the case of the junction where water is

dissociated, the semipermeable membrane may be either a bipolar membrane or a cation or

anion or cation and anion permselective membrane. In the case of the salt collecting

junction the membrane may be either a cation or anion or a cation and anion permselective

membrane. Examples of such membranes are Nafion™ (DuPont), Tosflex™ (TOSH) or

Ionac™ (Sybron Chemicals Inc.).

The electrodes used in the embodiments of the present invention can be fabricated

from any material that is inert under the conditions used. Preferably, the anode is

constructed from gold, platinum, palladium, iridium, carbon or more preferably, titanium

coated with gold, platinum, palladium or iridium. Preferably the cathode is fabricated from

gold, platinum, palladium, iridium, carbon, stainless steel, silver, copper, titanium or

titanium coated with gold, platinum, palladium or iridium.

In a preferred form of the present invention the pressure of the liquid flowing

through each of the beds of ion exchange material can be substantially equal. When this is

the case, there is no need to have the junctions between the beds which substantially

restrict the flow of liquid through the junction and so a simple abutting of the exchange

materials is all that is necessary. The equalisation of hydraulic pressure between the beds

can be assisted by having connection pathways for the feed liquid between the feed inlets

to the separate beds. For example, a shared manifold for the feed liquid to all the beds may

be used.

When it is desired that the liquid to be purified is passed serially through more than

one bed in the bank of ion exchange material beds it is necessary that at least some of the bed junction means are such that they substantially restrict the flow of liquid through the

junction at the hydraulic pressure differences that exist across the junction during

operation. For example, if the flow of liquid to be purified was made such that for each

pair of cation/anion exchange material beds the liquid first flowed through an ion exchange

bed of one type then through an adjacent ion exchange bed of the opposite type, the salt

collecting junctions between these beds may restrict the flow of liquid but the water

dissociation junctions not. In this case it is desirable to manifold together the feeds into the

first ion exchange material beds to assist in pressure equalisation between the pairs of beds.

In a further aspect, the present invention provides a method for forming a junction

between at least two regions of ion exchange material comprising the step of providing a

densely packed structure of ion exchange material between said regions to form said

junction, said structure having a packing density sufficient to substantially restrict the flow

of liquid across said junction.

Brief Description of the Drawings

Preferred embodiments of the present invention will now be described, by way of

example only, with reference to the accompanying drawings in which;

Figure 1 shows a schematic of a one preferred embodiment of a first aspect of the

present invention;

Figure 2 shows a schematic of a second prefeπed embodiment of the first aspect of

the present invention;

Figure 3 shows a schematic of a third preferred embodiment of the first aspect of the

present invention. Figure 4 shows a schematic of a one preferred embodiment of the fifth aspect of the

present invention;

Figure 5 shows a schematic of a second preferred embodiment of the fifth aspect of

the present invention; and

Figure 6 shows a schematic of a third preferred embodiment of the fifth aspect of the

present invention.

Modes of Performance of the Invention

Referring to the Figure 1, two vessels 17 and 18 contain anion exchange material 10

and cation exchange material 11. They are connected by a piece of cation exchange

membrane 4 which forms an ion conducting junction between the two beds but

substantially prevents the passage of water therebetween..

The liquid to be purified is fed to the base of the anion bed 10 at 5 via a suitable

distributor which ensures the desired liquid flow patterns in the bed, it rises through the

bed and exits at port 6. Optionally, a collector could be positioned near the top of the bed

10 prior to port 6, again to ensure the desired flow patterns within the bed. The liquid is

then transferred via a pipe 7 to be fed to the base of the cation bed 11 at 8 through a second

distributor. Purified water exits the bed 11 at 9, again optionally through a suitable

collector.

As an example, let us assume that the liquid stream that we wish to purify is a

solution of water containing sodium chloride. As the solution rises through the bed 10 the

chloride ions are absorbed by the ion exchange material and replaced with hydroxide ions.

The absorption occurring mostly near the base of the bed 10. The liquid therefore exits at 6

as a solution of sodium hydroxide. This solution is then fed to the cation exchange

material bed 11. As the solution rises through this bed 11 the sodium ions are absorbed by the cation exchange material, again mostly near the bed base, and replaced with hydrogen

ions, which react with the hydroxide ions present to form water. The purified liquid then

exits the apparatus at 9 for use.

While liquid is being fed to the beds to be purified an electrical potential is applied

between electrodes 2 and 3 by a power supply 19. The potential is of sufficient size to

cause water to be reduced at the cathode 2 and oxidised at the anode 3, these electrodes

being fabricated from titanium mesh coated with platinum. The reduction of water

produces molecular hydrogen and hydroxide ions 14 and the oxidation of water produces

molecular oxygen and hydrogen ions 13. The small amount of gases produced could

dissolve in the liquid stream and exit with the purified water at 9. Optionally, suitable

vents could be incorporated into the tops of vessels 17 and 18 through which any build up

of gas could be removed.

Due to the electrical potential that is applied between the electrodes, an electric field

exists which attracts anions 14 and 16 towards the anode 3 and cations 13 and 15 towards

the cathode 2. This draws the anions 14 down through the anion exchange material bed

and the cations 13 down through the cation exchange material bed. Since sodium ions 15

and chloride ions 16 are being continuously deposited, mostly near the base of the ion

exchange material beds, it is these ions that will tend to arrive at the bed junction rather

than hydroxide or hydrogen ions. This is desirable as pairs of hydroxide 14 and hydrogen

ions 13 that arrive at the junction will react to form water, leading to coulombic

inefficiency in the system. The hydroxide and hydrogen ions 13, 14 formed at the

electrodes will tend to migrate down through the beds and replace the chloride or sodium

ions 15, 16 as the latter migrate towards the bed junction 4. This continual replenishment of the hydroxide and hydrogen ions in the ion exchange material ensures that the liquid

purification process can continue without interruption.

Anions will tend to migrate to the interface between the cation exchange membrane,

that forms the junction between the beds, and the anion exchange material, as the former is

substantially impermeable to anions. Cations from the bed 11 pass through the cation

exchange membrane and combine with the anions at the cation exchange membrane/anion

exchange material interface 4 to form salts. A small portion of the liquid feed stream is

allowed to exit the anion exchange bed at 12. This liquid serves to carry away the salts

formed at the cation exchange membrane/anion exchange material interface 4. The

removal of salts from this interface is aided by the fact that the concentrated salt solution

formed at the interface will be denser than the liquid in the rest of the bed and so will tend

to fall towards the exit point 12. This process can be further aided by sloping the base of

the vessel 18 in the vicinity of the bed junction such that the exit 12 is located at the lowest

point.

In a second preferred embodiment of the present invention as illustrated in Figure 2,

multiple beds 21 of ion exchange material 30 are placed between the electrodes 22 and 23.

According to this embodiment the ion exchange beds may contain mixed anion and cation

exchange material 30. The beds may be such that the liquid flow through each beds

follows a relatively straight path. For example, the liquid may enter the base of the bed at

25 and exit at the top of the bed at 26 such as is illustrated in Figure 2. In another preferred

embodiment the liquid to be purified may travel through several beds in the sandwich

sequentially either in the same direction or alternating directions in adjacent beds. In this

case the liquid would be successively purified as it travelled through the beds until it exits

as a purified stream. As the liquid to be purified travels through the stack of such beds, salt is removed at

each junction as in the previous embodiment such that by the time that the liquid has exited

the stack of bed it is purified to the required extent. By a suitable system of liquid

distributors and collectors placed at the ends of the beds the flow of the liquid to be

purified is directed such that it does not disturb the liquid near the membrane/exchange

material interface where the salt accumulates. This ensures that the accumulated salt is

washed out the exit points 24 and is substantially prevented from mixing in the stream to

be purified.

According to this embodiment the anode 23 is placed at one end of the stack of ion

exchange material beds 21 and optionally may be separated from the adjacent bed of ion

exchange material by a anion exchange membrane 28. Similarly, the cathode 22, placed at

the other end of the stack of ion exchange material beds may be separated from the

adjacent bed of ion exchange material by an cation exchange membrane 27.

A flow of liquid would be caused to flow past the electrodes to assist in carrying

away the products of the electrochemical reactions proceeding at the electrodes when a

potential is applied between them. This liquid may go directly to waste or be recirculated

around past the electrodes with a small bleed in of fresh liquid and a small bleed out of

liquid containing the products of the electrochemical reactions.

A third embodiment of the present invention is depicted in Figure 3. This figure

depicts three units sandwiched together for illustrative purposes however one unit or many

more units can be assembled to form the embodiment, if desired. In this embodiment the

vessels 21 are filled with mixed anion and cation exchange material 30. Each vessel is

separated by a central baffle 29 which extends part way up the height of the bed and the length of the bed. The liquid to be purified enters each unit at 25 travels up through the

bed, over the baffle 29 and down the other side to exit as a purified stream at 26.

While this liquid is being purified a potential is applied between the cathode 22 and

the anode 23. This causes the reduction and oxidation of water at the electrodes and causes

cations to be attracted towards the cathode 22 and anions to be attracted towards the anode

23. The beds of ion exchange material 30 in the vessels 21 are connected to one another

for ion transport by pieces of cation exchange membrane 27, for example. Salt tends to

accumulate at the interface between the cation exchange membrane 27 and the ion

exchange material on the side closest to the cathode 22 as in the previous embodiment.

Again these salts are removed by a small flow of liquid exiting the vessel at 24.

Optionally, rather than having mixed ion exchange material adjacent to the cation exchange

membrane 27, some anion exchange material can be placed adjacent to the cation exchange

membrane 27 on its side closest to the cathode 22 and/or cation exchange material adjacent

to the side of the membrane closest to the anode 23. This assists in ensuring the ions to be

removed at the interface do not re-enter the stream of liquid to be purified. Also,

optionally a piece of anion exchange membrane 28 can be used to prevent the hydroxide

ions formed at the cathode 22 from entering the exchange material beds.

While the potential is being applied between the electrodes 22 and 23 a small flow of

liquid is directed over them to provide water for the electrode reactions and to assist in

carrying away the electrode reaction products. This liquid could go directly to waste or be

recirculated around past the electrodes with a small bleed in of fresh liquid and a small

bleed out of liquid containing the products of the electrochemical reactions.

Referring to a first embodiment of a fifth aspect of the invention shown in Figure 4,

the liquid to be purified 49 enters the bank of ion exchange material beds 41 via a common manifold 54. The liquid travels up through either a cation, 44 or an anion, 45 exchange

material bed. Upon which either cations or anions in the feed stream are replaced with

protons or hydroxide ions. The product liquid 50 then exits the beds via the common

manifold 53. While this is occurring an electrical potential is applied between the cathode

42 and the anode 43 such that an electrical current flows between the electrodes. The

electrodes being separated from the beds of ion exchange material by a cation exchange

membrane 51 or an anion exchange membrane 52. This causes contaminant ions, shown

here as Na⁺ and Cl^", to migrate towards the bed junctions 47. At the bed junctions 47 the

contaminant ions combine to form salts. The gutters 48 have a drain point open to the

outside of the apparatus which causes a small flow of liquid towards the junction 47 and

into the gutter 48. This flow of liquid causes the salts formed at the junction 47 to be

flushed into the gutters 48 and so removed from the beds 44, 45. The gutters 48 shown at

the base of the junctions 46 are optional and may be required when treating feed streams

with relatively high concentrations of dissolved salts. At the junctions 46 there is only a

relatively low concentration of ions in the liquid compared to the concentration of mobile

ions on the ion exchange material phase. These mobile ions are attracted away from the

bed junctions 46 by the electric field and so in order to carry current across the junction

water will dissociate into protons and hydroxide ions. These ions will then migrate

through the cation and anion exchange material beds respectively, due to the presence of

the electric field, continuously regenerating the beds.

According to this embodiment, approximately half the contaminant salts will be

removed from the feed stream 49 and the junctions 46 and 47 need only be an interface

between the two material beds or a piece of macroporous material such as a mesh or

sintered sheet. Alternatively, the manifolding of the liquid flow through the beds may be changed such that the liquid to be purified first travels through one ion exchange material

bed and then a second bed of the opposite type of ion exchange material. This may

substantially remove all the dissolved salts from the liquid to be purified. This requires

that either the junctions 46 or 47 or both substantially restrict the flow of liquid through

them to restrict undesirable mixing of the liquids between adjacent beds of ion exchange

material.

In a further form of this embodiment, two banks of ion exchange material beds, of

the type depicted in Figure 4, may be used such that after the liquid to be purified passes

through the first bank of beds it enters a second bank of beds where it is further purified.

Further, according to this form of the embodiment, liquid that has passed through a cation

exchange material bed in the first bank of beds is transferred to the second bank of beds so

as to pass through an anion exchange material bed. Similarly, liquid that has passed

through an anion exchange material bed in the first bank of beds is transferred to the

second bank of beds so as to pass through a cation exchange material bed. In this way both

anions and cations are removed from the liquid to be purified.

According to this form of the embodiment, a separate pair of electrodes may be

associated with each bank of beds or they may have one common pair of electrodes.

In a second prefeπed embodiment of the fifth aspect of the invention, shown in

Figure 5, the two banks of beds are placed one on top of the other such that each bed of ion

exchange material in the first bank has a bed of the opposite type of ion exchange material

directly above it. A cathode 42 is placed at the one end of this double bank of ion

exchange material beds and an anode 43 at the other end. In this embodiment, a single

anode and cathode supply current to both banks of beds simultaneously. If desired, the

electrodes can be separated from the end beds of ion exchange material either by a cation exchange membrane 51 or an anion exchange membrane 52. The flow of liquid to be

purified 49 is straight through the two banks of beds. The other numbers in Figure 5

designate the similar structure as defined in Figure 4.

In a second form of this embodiment, the two banks of beds can be arranged in any

suitable fashion such that the liquid exiting from the first bank of beds can be fed into the

appropriate bed in the second bank via suitable connectors. In this form of the

embodiment, a separate pair of electrodes is used for each bank of beds. Preferably these

pairs of electrodes are connected together in series. In this form of the embodiment it is

desirable that the means for transferring the liquid from the first bank of beds to the second

bank of beds is long enough and of small enough cross-section such that the resistance to

ion flow through said means, due to the applied electric field,, is large compared to the

resistance to ion flow through the bank of ion exchange material beds. This prevents

significant electrical short-circuiting of the ion exchange material beds.

In a third preferred embodiment of the fifth aspect of the invention, an end bed of one

of the banks of ion exchange material beds has a junction with an end bed of the second

bank of ion exchange material, where this junction is such that it allows electrical

connection between the beds but restricts or substantially prevents the flow of liquid

through it. Such a junction means could be an ion exchange membrane. Means for

transferring the liquid exiting the first bank of beds to the second bank of beds is provided

as in the second preferred embodiment. According to this third embodiment, one pair of

electrodes is used, where the cathode is placed at one end of the joined banks of beds and

the anode is placed at the other end. In this embodiment the banks of beds could be joined

so as to form a rod shape or joined so as to form a U-shape, or other suitable shape. An

example of this embodiment in a rod shape is given is shown in Figure 6. In a further aspect of the present invention, a method for forming a bed junction is

disclosed. This method is applicable to junctions of the type referred to as 46 or 47 in the

Figures 4, 5 and 6. According to this further aspect, a structure of densely packed ion

exchange material is formed to make the junction. This ion exchange material may be in

the form of a fine powder, a compressed filament material, spun bonded filament material

or other material form that allows dense packing. The density of packing of the material is

such that in operation there is a sufficient resistance to liquid flow through the densely

packed material that only a small fraction of the flow of feed water can pass through. This

resistance may be such that, by itself it restricts the liquid flow to the desired level or, such

that the junction resistance is used in combination with a valve or capillary to give the

desired flow through the junction or out of the junction and into a gutter when one is used.

Preferably, the densely packed material has two regions abutting one another, one

containing anion exchange material and the second containing cation material. The region

containing the anion exchange material faces the anion exchange bed and that containing

cation exchange material faces the cation exchange bed. Optionally, one or more pieces of

macroporous material or mesh can be placed between the regions to give a relatively low

liquid flow resistance path compared to flow through the densely packed material.

When using spun bonded material for this aspect, the junction structure may be self

supporting. When using finely powdered material a bag formed out of mesh may be used

to contain the powder. The mesh is preferably fine enough to substantially retain the

powdered material but optionally also allows some of the material to extrude through the

mesh. This material which extrudes through the mesh allows more intimate contact

between the ion exchange material in the junction and that of the bed, lowering electrical

resistance. Optionally, this extruded material can be sprayed with a binder to increase the robustness of the structure. Fine powdered material may also be mixed with a binder and

formed into a self-supporting junction structure.

In a preferred embodiment of this further aspect of the present invention, the ion

exchange material is formed into a wedge shape with the thickest end of the wedge at the

base of the junction. According to this embodiment, the resistance to liquid flow through

the thinner portion of the wedge is less than through the thicker portion of the wedge so

that liquid entering the junction through the densely packed material tends to preferentially

enter towards the top of the junction. The liquid then tends to flow down the junction

assisting in the removal of salts if required.

It will be appreciated that further embodiments and exemplifications of the invention

are possible without departing from the spirit or scope of the invention described.

Claims

CLAIMS:

1. A method of purifying a liquid stream comprising the steps of passing liquid to be

purified serially through at least a pair of ion exchange material beds, one of anion

concurrently regenerating said ion exchange material beds by applying an electrical

potential across a pair of electrodes, one each being associated with a respective material

bed, so as to produce an electric field between the electrodes and said electrodes being

arranged such that at least some ion exchange material is sandwiched between each said

electrode and said junction.

2. A method of purifying a liquid stream according to claim 1 wherein the cathode is

placed in the anion exchange material bed and the anode is placed in the cation exchange

material bed.

3. A method of purifying a liquid stream according to claim 1 or claim 2 including the

step of feeding the liquid stream to be purified into one of the beds, allowing the stream to

flow through and exit said one of the beds and then be fed into a second bed, and allowing

the liquid stream to flow through the second bed whereupon it is purified of ions and exit

the second bed as a purified liquid stream.

4. A method of purifying a liquid stream according to any one of the preceding claims

including a step of providing a small flow of fluid in the vicinity of the bed junction which

serves to remove accumulated impurities.

5. A method of purifying a liquid stream according to any one of the preceding claims

wherein the direction of flow of liquid to be purified passing through the ion exchange material beds is such that, at least in part, it is different to the direction of flow of ions

under the influence of the electric field.

6. A method of purifying a liquid stream according to claim 5 wherein the direction of

flow of liquid is in a counter-current or orthogonal fashion to the direction of flow of ions

under the influence of the electric field.

7. A method of purifying a liquid stream according to any one of the preceding claims

including forming the junction of the ion exchange material beds such that it is permeable

to anions or cations or both anions and cations but substantially restricts or prevents the

flow of liquid therethrough.

8. A method of purifying a liquid stream according to claim 7 wherein the junction is

exchange material beds.

9. A method of purifying a liquid stream according to claim 7 wherein the junction is

formed by packing ion exchange resin beads into a grid.

10. A method of purifying a liquid stream according to claim 9 wherein the resin

particles forming said junction are compressed and held within the grids of a single mesh

or sandwiched between two or more grids.

11. A method of purifying a liquid stream according to claim 9 or claim 10 wherein the

grid is formed by of relatively inert, electrically non-conductive material.

12. A method of purifying a liquid stream according to any one of the preceding claims

including the step of applying pressure to compress the bed or beds of ion exchange

material so as to lower the resistance of the bed to ion flow.

13. A method of purifying a liquid stream according to claim 12 wherein the pressure is

applied by means an electrode structure placed in or on the bed to apply pressure to the ion

exchange material sandwiched between the electrode and the bed junction.

14. A method of purifying a liquid stream according to claim 13 wherein the electrode

structure is reinforced with suitable structural members to prevent undue flexing when

compressive force is applied.

15 A method of purifying a liquid stream according to claim 12 wherein the pressure is

applied by means of providing a lid on the bed which contains the ion exchange material,

and includes the step of over-filling the bed with ion exchange material during assembly

such that the material is compressed as the lid was assembled onto the bed.

16. A method of purifying a liquid stream according to any one of the preceding claims

wherein the ion exchange material is filamental.

17. A method of purifying a liquid stream according to any one of claims 1 to 15 wherein

the ion exchange material is a macroporous monolith of the material.

18. A method of purifying a liquid stream as claimed in any one of the preceding claims

wherein the ion exchange materials are formed from a chemically treated polymeric

material.

19. Apparatus for purifying a liquid stream comprising at least two ion exchange

ions is formed therebetween, means for passing liquid to be purified serially through one

bed then the other, at least one pair of electrodes one each being associated with a

respective material bed such that at least some ion exchange material is sandwiched

between each said electrode and said junction, means for applying an electrical potential across said pair of electrodes so as to produce an electric field therebetween and regenerate

said ion exchange material.

20. Apparatus for purifying a liquid stream comprising at least two ion exchange

material beds, each bed containing a mixture of anion and cation exchange material, the

beds being connected at at least one point such that a junction which is permeable to ions is

formed therebetween, means for passing liquid to be purified through at least one of said

beds, at least one pair of electrodes one each being associated with a respective material

bed such that at least some ion exchange material is sandwiched between each said

electrode and said junction, means for applying an electrical potential across said pair of

electrodes so as to produce an electric field therebetween and regenerate said ion exchange

material.

21. Apparatus for purifying a liquid stream comprising at least two ion exchange

material containing beds, one of anion exchange material and the second of cation

exchange material, the beds being connected at one point at least such that a junction is

formed therebetween, means for passing liquid to be purified through each bed, either

through a single bed or through at least one anion exchange material containing bed and at

least one cation exchange material containing bed, said ion exchange material containing

beds being arranged in a bank and having at least one pair of electrodes associated with

said bank and means for applying an electrical potential across said electrodes and said

bed j unction or j unctions .

22. Apparatus for purifying a liquid stream according to any one of claims 19 to 21

including means for applying pressure to compress the bed or beds of ion exchange

material so as to lower the resistance of the bed to ion flow.

23. Apparatus for purifying a liquid stream according to claim 22 wherein the means for

applying pressure includes an electrode structure placed in or on the bed to apply pressure

to the ion exchange material between the electrode and the bed junction.

24. Apparatus for purifying a liquid stream according to claim 23 wherein the electrode

compressive force is applied.

25. Apparatus for purifying a liquid stream according to claim 22 wherein the means for

applying pressure includes a lid on the bed containing the ion exchange material, and

wherein the bed is overfilled with ion exchange material during assembly such that the ion

exchange material is compressed as the lid is assembled onto the bed.

26. Apparatus for purifying a liquid stream according to any one of claims 19 to 25

wherein the ion exchange material is filamental.

27. Apparatus for purifying a liquid stream according to any one of claims 19 to 25

wherein the ion exchange material is a macroporous monolith of the material.

28. Apparatus for purifying a liquid stream according to claim 21 having a single pair of

ion exchange beds and two types of junction formed between the ion exchange material

beds, the first type of junction having an electrical potential applied across the junction

such that mobile cations and anions residing on the ion exchange material are attracted

away from the junction and into the cation and anion exchange material beds respectively,

and a second type of junction having an electrical potential applied across the junction such

that cations and anions present in respective beds are attracted towards the junction where

they form salts.

29. Apparatus for purifying a liquid stream according to claim 21 wherein said salts are

carried out of the beds in solution by a small flow of liquid caused to flow in the vicinity of the junction and a gutter is provided at the base of the junction to assist in carrying away

the salt solution.

30. Apparatus for purifying a liquid stream according to claim 21 having more than one

pair of ion exchange material beds, arranged in a bank, each bed of ion exchange material

has at least two junctions either with an adjacent bed or an electrode containing

compartment, one of said junctions having an electrical potential applied across the

junction such that mobile cations and anions residing on the ion exchange material are

attracted away from the junction and into the cation and anion exchange material beds

respectively and the other of said junctions having an electrical potential applied across the

junction such that cations and anions present in respective beds are attracted towards the

junction where they form salts.

31. Apparatus for purifying a liquid stream according to any one of claims 21 to 30

wherein the junction is formed by abutting the ion exchange material beds, using a woven

mesh or other macroporous separator between the ion exchange material beds, forming a

semipermeable membrane to separate the beds.

32. Apparatus for purifying a liquid stream according to claim 31 wherein at a junction

where water is dissociated, the semipermeable membrane may be either a bipolar

membrane or a cation or anion or cation and anion permselective membrane.

33. Apparatus for purifying a liquid stream according to claim 31 wherein at a junction

where salt is collected the semipermeable membrane may be either a cation or anion or a

cation and anion permselective membrane.

34. Apparatus for purifying a liquid stream according to any one of claims 21 to 33

wherein the pressure of the liquid flowing through each of the beds of ion exchange

material is substantially equal.

35. Apparatus for purifying a liquid stream according to claim 34 wherein the

equalisation of pressure between the beds is assisted by having connection pathways for

the feed liquid between feed inlets to the separate beds.

36. A method for forming a junction between at least two regions of ion exchange

material comprising the step of providing a densely packed structure of ion exchange

material between said regions to form said junction, said structure having a packing density

sufficient to restrict the flow of liquid across said junction.

37. A method for forming a junction between at least two regions of ion exchange

material according to claim 36 wherein the ion exchange material forming the densely

packed structure is in the form of a fine powder or a compressed, filament material.

38. A method for forming a junction between at least two regions of ion exchange

material according to claim 36 or 37 wherein the junction restriction is used in combination

with a valve or capillary to give the desired flow through the junction.

39. A method for forming a junction between at least two regions of ion exchange

material according to claim 36, 37 or 38 wherein the densely packed material is formed

from two regions abutting one another, one containing anion exchange material and the

second containing cation exchange material.

40. A method for forming a junction between at least two regions of ion exchange

material according to any one of claims 36 to 39 wherein each region containing one type

of ion exchange material is arranged to face an exchange bed of the same material type.

41. A method for forming a junction between at least two regions of ion exchange

material according to claim 40 wherein at least one piece of macroporous material or mesh

is placed between the regions to give a relatively low liquid flow resistance path compared

to flow through the densely packed material.

42. A method for forming a junction between at least two regions of ion exchange

material according to any one of claim 36 to 41 including the step of forming the ion

exchange material forming the junction into a wedge shape with the thickest end of the

wedge at the base of the junction, such that the resistance to liquid flow through the thinner

portion of the wedge is less than through the thicker portion of the wedge so that liquid

entering the junction through the densely packed material tends to preferentially enter

towards the top of the junction.