WO2004037727A1

WO2004037727A1 - Applications of an ion bridge and a countercurrent flow design for use in water purification

Info

Publication number: WO2004037727A1
Application number: PCT/US2003/033843
Authority: WO
Inventors: Edward T. Knobbe; Robert M. Taylor
Original assignee: Sciperio, Inc.
Priority date: 2002-10-24
Filing date: 2003-10-24
Publication date: 2004-05-06

Abstract

A method and apparatus are provided for fluid purification. A charged species is separated from the fluid using techniques such as Lorentz ionic separation and electrodialysis. An ionic bridge is used to compensate for effects resulting from the separation. A countercurrent flow design improves the fluid purification process.

Description

APPLICATIONS OF AN ION BRIDGE AND A COUNTERCURRENT FLOW DESIGN FOR USE IN WATER PURIFICATION

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to U.S. Provisional Application No. 60/7421,340 filed on October 24, 2002 and PCT Application No. PCT/US02/38858, filed 12/5/02. These applications are hereby incorporated by reference.

BACKGROUND The present invention is directed to a method and system for fluid purification.

More particularly, the present invention is directed to methods and systems for improving fluid purification.

Recently, numerous reports have described the pending shortage of potable water as a result of rapidly changing agricultural and industrial uses as well as the rapid population increases being seen in arid regions of the world. Thus, much interest has been focused on the development of new technologies aimed at the economical purification of water from non traditional sources. Sources of interest include saline waters, brackish waters, and sea water.

At the present time, reverse osmosis (RO) exists as the dominant water desalination process. However, reverse osmosis has proven be a comparatively expensive way to desalinate water due to the large capital costs and energy-intensive nature of the RO process, which drives the solvent (generally > 95% of the total solution mass) through a semi -permeable membrane at pressures of typically 1000 psi or greater. Newer, potentially less expensive technologies such as electrodialysis and the flow-through capacitor have met with problems that have, to date, prevented their widespread acceptance. Both electrodialysis and the flow-through capacitor systems experience problems due to electrochemical and other processes that take place at the electrodes.

The Lorentz Ionic Separation Apparatus (LISA) provides a methodology that obviates problems with electrodes by employing an electrode-free method through which to introduce an electrostatic potential for water desalination. This methodology is described in detail in the above-referenced patent applications. This methodology is also described in U.S. Provisional Patent Application No. 60/335,592 filed December 5, 2001 and U.S. Patent Application No. 10/301,550 filed November 21, 2002, both of which are hereby incorporated by reference.

While each of the processes described above is useful in purifying fluid, there is room for improvement.

SUMMARY It is therefore an object of the present invention to improve fluid purification.

According to exemplary embodiments, fluid purification is improved by using an ionic bridge and a countercurrent flow design.

The objects, advantages and features of the present invention will become more apparent when reference is made to the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and IB illustrate movement of magnetic lines of flux through a fluid reservoir; FIGS. 2A and 2B illustrate Lorentz forces imposed on charged particles arising from a magnetic field;

FIG. 3A is an exemplary schematic representation of ion migration and water desalination using the LISA approach;

FIG. 3B illustrates exemplary chemical functionality association with a cation exchange membrane;

FIG. 4 illustrates an exemplary multilayer cartridge with ion-selective membranes;

FIG. 5 is an exemplary schematic two-dimensional representation of LISA ion transport processes within a separation cartridge; FIG. 6 illustrates establishment of a long range space-field in an uncompensated cartridge configuration;

FIG. 7 illustrates charge compensation and obviation of long range space- charge effects using an ion transport bridge according to exemplary embodiments; FIG. 8 is an exemplary schematic representation of water desalination via electrodialysis; and

FIG. 9 illustrates an exemplary counter current flow design.

DETAILED DESCRIPTION According to exemplary embodiments, fluid purification processes are made more efficient through the use of an ion bridge which serves to eliminate electrochemical and/or space-charge effects that would otherwise develop within the water deionization apparatus. Application of a counter-current flow provides further improvements. Summary of the Lorentz Ionic Separation Apparatus (LISA)

In electromagnetism, the Lorentz equation is expressed as:

F_L = ₉(v x B) This equation describes the vector force experienced by a charged particle as its position changes with respect to a local magnetic field (where q is a scalar that denotes the charge of the perturbed particle, B is a vector quantity indicating the local magnetic field, and v denotes the particle velocity relative to B).

Figures 1 A-2B provide a schematic representation of the utilization of Lorentz forces as they may be applied for the purpose of driving ions in a prescribed direction. Figure 1(a) represents a long rectangular reservoir filled with a liquid that contains charged species (e.g., cations and anions). A locally applied magnetic field, B, is experienced at one end of the reservoir. The magnetic field is shown to be swept, with velocity v, from the near to far ends of the reservoir in transitioning from figure 1(a) to 1(b). The vector quantity V_B indicates the rate of movement of magnetic flux lines through the stationary liquid reservoir. Thus, for the particular case posed, B points to the left and is oriented in the plane of the page, while V_B is directed into the page. The significance of the vector orientations selected in Figures l(a)-3(b) is that they are consistent with, and provide a more easily identifiable analogue for, the physical processes of the LISA embodiment described in the following section.

For the field and velocity vector orientations given in Figures 1(a) and 1(b), Lorentz' Law indicates an orthogonally-oriented force experienced by all charged particles within the reservoir. Dissolved ions, such as Na⁺ and Cl^", are accelerated in opposite directions by F_L ⁺ and F_L ^", respectively, as shown in Figures 2(a) and 2(b).

When used in conjunction with ion-selective membranes, it is possible to purify water by driving solute ions to an ion "reject" region, as depicted in Figures 3(a) and 3(b). Figure 3(a) schematically illustrates ion migration and water desalination envisioned in the LISA approach. Figure 3(b) illustrates chemical functionality associated with a cation exchange membrane.

Multiple unidirectional passes of a magnetic field through a fluidic reservoir system, of the type depicted in Figures l(a)-(b), would effectively enhance counterion separation until space-charge and Lorentz forces counterbalance. In the absence of an ionic discharge mechanism, establishment of the counteracting space-charge field at equilibrium precludes further deionization within the purification region, It is possible, however, to overcome short range space-charge effects by constructing and laminating a multilayer cartridge that incorporates a cation-anion recombination (or discharge) channel for continuous solute removal, as indicated in Figure 4. Figure 4 illustrates a multilayer laminate construct using ion-selective membranes which provides cation- anion discharge channels, obviating the space-charge effects and reducing the separation cartridge footprint. Such constructs facilitate the establishment of separate deionized water and reject brine output channels. Improvements Resulting from the Use of an Ionic Bridge

A key component to the success of water desalinization by the LISA process is the design of the membrane cartridge. Figure 5 represents a 2-dimensional depiction of the ion transport processes that are targeted with the LISA separation method. The ion separation cartridge, including alternating cation- and anion-exchange membranes, allows for the passage of positive and negative ions as depicted (F_L ⁺ and F ^' oriented to the left and to the right, respectively), ultimately resulting in alternating channels of deionized (diluate) water and concentrated (brine) waste product streams. At the outset of the separation process it is assumed that the ionic strength in all cells is the same (as indicated by the initial placement of one ionized ionic Na⁺ and Cl^" pair in each cell). A simple cartridge of the type shown in Figure 5, however, suffers from the accumulation of an excess of like-charge ions in the terminal end channels as the separation process occurs, as indicated in Figure 6 which illustrates establishment of a long range space-charge field in an uncompensated cartridge configuration. Excess charge buildup is indicated by 2 molar equivalents of Na⁺ to one of Cl^" in the left-most cell of Figure 6, with the simultaneous formation of two molar equivalents of Cl^" to one of Na⁺ in the right-most cell. The rapid development of a long-range space-charge voltage will effectively counteract the Lorentz forces, thereby establishing an equilibrium state with concomitant cessation of the separation process.

One possible solution is to provide a charge-compensation mechanism to the cartridge ends. This can be accomplished by implementing an ionic transport bridge that couples the outermost concentrate/reject cells. The bridge, which contains mobile cationic and anionic species, should be implemented in a manner such that the ions in the bridge are shielded from, or otherwise unaffected by, the external magnetic source. In this way the ion bridge facilitates charge compensation through ion migration, resulting in a "shorting out" or cancellation of the inherent Hall voltage.

One possible realization is shown in Figure 7, where compensating ions can be seen to flow through the bridge into the outermost cells, thereby obviating the long range space-charge effects that would otherwise cause separation to cease. Other Potential Uses for Ion Bridge Devices in Water Deionization Processes An ion bridge can be useful in other water deionization applications by helping to eliminate electrochemical processes that transpire at the electrode locations in electrostatically-driven desalination processes, such as electrodialysis.

A basic summary of electrodialysis processes can be found on the website published by PCA-Polymerchemie Altmeier GmbH (http://www.pca- gmbh.com/appli/ed.htm); an excerpted and slightly modified narrative from that source appears hereinafter:

Electrodialysis is used to transport salt from one solution (e.g., the diluate) to the other solution (concentrate) by applying an electrical current as shown, e.g., in Figure 8.

Within an electrodialysis unit, the solutions are separated by alternatively arranged anion exchange membranes (peπneable only for anions) and cation exchange membranes (permeable only for cations). By this, the two kinds of cells are formed, distinguishing in the membrane type facing the cathode's direction. The application of an electrostatic potential, with concomitant current flow, causes cations within the diluate to move toward the cathode by passing through a cation exchange membrane. Similarly, anions move from the diluate cell into the concentrate cell, towards the anode, by passing through an anion exchange membrane. Further transport of these ions, now being in a chamber of the concentrate, is stopped by the next membrane. The problematic issue for electrodialysis, from an electrochemical perspective, occurs at the electrodes locations where coupled oxidation and reduction (redox) chemistry occurs. In the case of water desalination, this can be found in the form of electrolysis (splitting of water into H₂ and 0₂) and/or the formation of metallic sodium and gaseous chlorine (via the electrochemical reduction of Na⁺ at the cathode in concert with the oxidation of Cl^" at the anode). Corrosion and/or the formation of insoluble sodium products creates a need for frequent backflushing and electrode maintenance, thereby lowering the efficiency and raising the operational costs of such systems. The ionic transport bridge can be used, in conjunction with a modified external electrode configuration, to couple the outermost cells of an electrodialysis apparatus and to facilitate ion removal and charge compensation without the need to initiate redox reactions. Improvements Resulting from Application of a Counter-Current Flow

In Figure 8, water desalinization via electrodialysis is schematically represented with parallel, concurrent flow directions in the deionized (DI) and concentrate water channels (e.g., adjacent solution streams flow in the same direction). Also, in the LISA approach described in the aforementioned patent applications, adjacent deionized and brine concentrate streams were envisioned to occur in, e.g., a parallel, concurrent manner. This is depicted in Figure 4.

The LISA approach may be improved through the use of parallel DI solution streams that flow in a countercurrent direction with respect to the interleaved concentrate streams. Performance improvements in such an embodiment would be derived through a reduction in osmotic pressure differentials across the membranes. In a parallel, concurrent flow scheme, the lowest ionic strength DI solution (e.g., the "cleanest" portion of the product water stream) is immediately adjacent to the highest ionic strength concentrate solution (e.g., the "dirtiest" portion of the reject water stream). Parallel, concurrent flow designs are shown in Figures 8 (electrodialysis schematic) and 4 (LISA schematic). The osmotic pressure difference across the ion- exchange membranes is maximized under such circumstances, thereby tending to cause water from the low ionic strength side to diffuse across the membrane to the high ionic strength side. In such cases there is a clear loss of "clean" water product to the reject stream, with concomitant losses in product rate and energetic efficiency. An example of osmotic pressure differences can be seen through the use of a simple example. Example:

Assume that the input solution for the DI (C_O,_DI) and reject streams (Co.bπne) are identical: idealized seawater (dissolved NaCl at a concentration of 35,000 ppm) at 30 °C. The concentration difference across the membrane at the input (beginning) of the flow channels in a parallel, concurrent flow scheme is 0 ppm (δC,_ oi_{/ bπn}e = 0 ppm). Total fluid flow in each stream is the same (e.g., QDI = Qbπ_ne)- Salinity in the DI stream is brought down to the potable water target of 500 ppm dissolved salt (C_f, _DI); given that all of the salt removed from the DI stream is deposited into the reject stteam, the end of the reject stream must contain 69,500 ppm of dissolved salt (C_f, ri_ne = 69,500 ppm). Therefore, the concentration difference across the membrane at the end of the flow channels in a parallel, concurrent flow scheme is 69,000 ppm (δCr, DI / b_rine = 69,000 ppm). Osmotic pressure differences across a membrane may be expressed as a function of the difference in solution concentration according to the following: δπ = δ(n/N)RT = δMRT where δπ is the osmotic pressure difference, δ(n/N) is the difference in solute molarity across the membrane (n/N is the quantity moles per liter, or molarity, M), R is the ideal gas constant (0.0821 L atm mol^"1 K^"1, and T is the absolute temperature (303 K in this example).

At 30 °C: δπ_f = δM_fRT = δ(C_f, _Di_/bri_ne)(MW_Νacι)(0.0821 L atm mol^"1 K^"')(303K) δπf ~ 29 atm (approx 420 psi) {δπj = 0, since δCo, _{DI /} b_ri_ne = 0 ppm}

This is a substantial pressure that would be expected, under most conditions, to cause "clean" water to permeate through the ion-exchange membranes into the reject stream. The slower the flow rates in each channel, the greater the potential for loss. In a countercurrent flow scheme, depicted in Figure 9 for one DI / reject channel pair, the osmotic pressure across the membrane at the product end is reduced by 50% as in the following discussion.

Using the same DI and reject stream sources (35,000 ppm seawater at 30 °C) in a countercurrent flow design produces the following values by analogous reasoning:

CO,DI = Co.bnne = 35,000 ppm; C_f, DI = 500 ppm; C_f, _bnn_e = 69,500 ppm.

However, the concentration difference at the product end of the countercurrent flow scheme (right-hand side in Figure CC) is essentially half of what it was before, as the lowest ionic strength DI product (C_f, D_I = 500 ppm) is adjacent to the lowest ionic strength reject stream (Co,b_ine = 35,000 ppm). δπprøduct end = δM_product e«rfRT = δ(C_pr0duct end)(MW_Nacι)(0.0821 L atm mol^"1 K^_1)(303K) δπ product end ~ 14.5 atm (approx 210 psi) {δπ_S0Urce end is also 14.5 atm} This 50% reduction in osmotic pressure across the membrane, for the simple example given, would be expected to significantly system losses due to osmotic pressure-induced water permeation through the ion-exchange membranes. Other Potential Uses for Counter-Current Flow Design

Electrostatically-driven water deionization processes (e.g., electrodialysis, the flow-through capacitor) that use concurrent parallel flow designs, such as that shown in Figure 8, can also implement countercurrent flow designs to reduce osmotic pressure differentials at the product end of the apparatus. As in the case described for the LISA device, countercurrent flow schemes can significantly reduce ionic strength differences at the product end, thereby creating the possibility for loss reduction via water permeation across the ion-exchange membranes.

It should be understood that the foregoing description and drawings are by way of example only and are not intended to limit the present invention in any way. A variety of modifications are envisioned that do not depart from the scope and spirit of the invention.

It should be understood that the foregoing description and accompanying drawings are by example only and are not intended to limit the present invention in any way. A variety of modifications are envisioned that do not depart from the scope and spirit of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method for purifying fluid, comprising: separating a charged species from the fluid by passing the fluid through an ion separation cartridge including alternating cation-exchange and anion-exchange membranes, wherein the step of separating creates charge buildup that counteracts separation of the charged species from the fluid; and compensating for the charge buildup using an ionic bridge.

2. The method of claim 1, wherein the step of separating includes applying a magnetic field to the cartridge.

3. The method of claim 1, wherein the ionic bridge contains mobile cationic and anionic species.

4. The method of claim 1, wherein the charge buildup is a like-charge buildup that occurs at ends of the cartridge.

5. The method of claim 4, wherein the ionic bridge is connected to the ends of the cartridge to compensate for the charge buildup.

6. The method of claim 1, further comprising shielding the ions in the ionic bridge from the magnetic field.

7. A method for purifying fluid, comprising: separating a charged species from the fluid by passing the fluid through a unit including alternately arranged anion exchange membranes and cation exchange membranes, wherein the step of separating results in undesirable electrochemical processes; and eliminating the undesirable electrochemical processes using an ionic bridge.

8. The method of claim 7, wherein the step of separating includes applying an electrical current to the unit via electrodes, and the undesirable electrochemical processes occur at the electrodes.

9. The method of claim 7, wherein the electrochemical processes include at least one of electrolysis and formation of metallic sodium and gaseous chlorine.

10. The method of claim 7, wherein the ionic bridge facilitates ion removal and charge compensation.

11. The method of claim 7, wherein the ionic bridge is coupled to the outermost portions of the unit.

12. The method of claim 7, wherein the alternating cation-exchange and anion- exchange membranes form cells in the unit, and the ionic bridge is connected to the outermost cells in the unit.

13. An apparatus for purifying fluid, comprising: an ion separation cartridge, including alternating cation-exchange and anion- exchange membranes, for separating a charged species from the fluid when the fluid is passed through the cartridge, wherein the separation creates charge buildup that counteracts separation of the charged species from the fluid; and an ionic bridge coupled to the cartridge for compensating for the charge buildup.

14. The apparatus of claim 13, wherein for facilitating separation, a magnetic field is applied tq the cartridge.

15. The apparatus of claim 13, wherein the ionic bridge contains mobile cationic and anionic species.

16. The apparatus of claim 13, wherein the charge buildup is a like-charge buildup that occurs at ends of the cartridge.

17. The apparatus of claim 16, wherein the ionic bridge is connected to the ends of the cartridge to compensate for the charge buildup.

18. The apparatus of claim 13, wherein the ions in the ionic bridge are shielded from the magnetic field.

19. An apparatus for purifying fluid, comprising: a unit including alternately arranged anion exchange membranes and cation exchange membranes for separating a charged species from the fluid by passing the fluid through the unit, wherein the step of separation results in undesirable electrochemical processes; and an ionic bridge coupled to the unit for eliminating the undesirable electrochemical processes.

20. The apparatus of claim 19, wherein an electronic current is applied to the unit via electrodes to facilitate separation, and the undesirable electrochemical processes occur at the electrodes.

21. The apparatus of claim 19, wherein the electrochemical processes include at least one of electrolysis and formation of metallic sodium and gaseous chlorine.

22. The apparatus of claim 19, wherein the ionic bridge facilitates ion removal and charge compensation.

23. The apparatus of claim 19, wherein the ionic bridge is coupled to the outermost portions of the unit.

24. The apparatus of claim 19, wherein the alternately arranged anion exchange membranes and cation exchange membranes form cells in the unit, and the ionic bridge is coupled to the outermost cells in the unit.

25. A method for purifying fluid, comprising: separating a charged species from the fluid by passing the fluid through a unit, thereby producing a stteam of high ionic strength and a stteam of low ionic sttength; and causing the streams to flow in opposite directions, an ion separation cartridge including alternating.

26. The method of claim 25, wherein the unit includes alternating cation-exchange and anion-exchange membranes.

27. The method of claim 25, wherein the step of separating includes applying at least one of a magnetic field and an electronic current to the unit.

28. The method of claim 25, wherein the unit is a flow-through capacitor.

29. The method of claim 25, wherein causing the streams to flow in opposite directions reduces pressure buildup in the unit.

30. An apparatus for purifying fluid, comprising: a unit for separating a charged species from the fluid by passing the fluid through the unit, thereby producing a stream of high ionic sttength and a stteam of low ionic sttength; and means for causing the streams to flow in opposite directions.

31. The apparatus of claim 30, wherein the unit includes alternating cation- exchange and anion-exchange membranes.

32. The apparatus of claim 30, wherein at least one of a magnetic field and an electronic current is applied to the unit to facilities separation.

33. The apparatus of claim 30, wherein causing the streams to flow in opposite directions reduces pressure buildup in the unit.

34. The apparatus of claim 30, wherein the unit is a flow-through capacitor.