Hybrid Systems and Methods with Forward Osmosis and Electrodeionization Using High-Conductivity Membranes

Abstract

Fluid desalination systems that include an FO reactor and an electrodeionization reactor with improved membranes and solvents, and a method of using such systems, are provided. A fluid having a first salt concentration is directed to the FO reactor, which uses a solute to draw salt away from the fluid across a membrane into the solute, where the electrodeionization reactor is salinized solute fluid and (i) generate substantially desalinated fluid and (ii) regenerate the solute for return to the forward osmosis reactor. The electrodeionization reactor is configured to draw positive and negative ions of the solute across cationic and anionic membranes, respectively, by applying a voltage across electrodes sandwiching the cationic and anionic membranes. In some cases, the cationic and anionic membranes are porous gelled polymer electrolyte membranes, wherein a saturated solution of the salinized solute fluid is absorbed.

Claims

1. A system for the desalination of fluid having a first salt concentration therein, the system comprising a forward osmosis reactor and an electrodeionization reactor in fluid communication therewith, where the forward osmosis reactor is configured to take the fluid having the first salt concentration into a first intake port in order to generate a fluid having a higher second salt concentration by directing into a second intake port a fluid having a first solute concentration with a higher osmotic pressure than the fluid having the first salt concentration in order to draw fluid having substantially no salt concentration across a forward osmosis membrane from the fluid having the first salt concentration so as to generate a fluid having a lower second solute concentration, and where the electrodeionization reactor is configured to take the fluid having the lower second solute concentration and (i) generate substantially desalinated fluid and (ii) regenerate the fluid having substantially the first solute concentration for return to the second intake port of the forward osmosis reactor, the electrodeionization reactor further configured to draw positive and negative ions in the fluid having the lower second solute concentration fluid across cationic and anionic membranes, respectively, by applying a voltage across electrodes sandwiching the cationic and anionic membranes.

2. The system of claim 1 wherein the positive and negative ions in the fluid are those associated with the solute, such that the positive and negative ions can be recombined to substantially regenerate the fluid having the first solute concentration.

3. The system of claim 1 wherein the electrodeionization reactor comprises a continuous electrodeionization reactor configured to introduce cation and anion resin into the fluid having the lower second solute concentration to permit substantially continuous operation of the continuation electrodeionization reactor.

4. The system of claim 1 wherein the solute comprises an ionic salt.

5. The system of claim 4, wherein the cationic and anionic membranes are each a porous polymer gelled electrolyte membrane comprising a substantially saturated solution of the ionic salt.

6. The system of claim 1 wherein the solute comprises a cloud point solute.

7. The system of claim 1 wherein the solute comprises a water-soluble polymer with high osmotic potential.

8. The system of claim 7, wherein the water-soluble polymer comprises non-cloud-point ethoxylates and/or propoxylates.

9. A method for desalinating fluid having a first salt concentration therein, the method comprising directing into a first intake port of a forward osmosis reactor the fluid having the first salt concentration, and further directing the fluid having a first salt concentration passed a first side of a forward osmosis membrane within the forward osmosis reactor; directing into a second intake port of the forward osmosis reactor a fluid having a first solute concentration, and further directing the fluid having the first solute concentration passed a second side of the forward osmosis membrane, where the fluid having the first solute concentration has a higher osmotic pressure than the fluid having the first salt concentration, so as to draw across the membrane fluid having substantially no salt concentration to thus generate a fluid having a lower second solute concentration; directing the fluid having the lower second solute concentration between a cationic membrane and an anionic membrane positioned between positive and negative electrodes; and applying a voltage across the electrodes so as to draw positive and negative ions across the cationic membrane and anionic membrane, respectively, thereby generating substantially desalinated fluid.

10. The method of claim 9 wherein the positive and negative ions are those associated with the solute.

11. The method of claim 10 further comprising recombining the positive and negative ions of the solute to regenerate fluid having substantially the first solute concentration for return to the second intake port of the forward osmosis reactor.

12. The method of claim 9 further comprising introducing cation and anion resin into the fluid having the lower second solute concentration to permit substantially continuous operation of the continuation electrodeionization reactor.

13. The method of claim 9 wherein the solute comprises an ionic salt.

14. The method of claim 13, wherein the cationic and anionic membranes are each a porous polymer gelled electrolyte membrane comprising a substantially saturated solution of the ionic salt.

15. The system of claim 9 wherein the solute comprises a cloud point solute.

16. The system of claim 9 wherein the solute comprises a water-soluble polymer with high osmotic potential.

17. The system of claim 16, wherein the water-soluble polymer comprises non-cloud-point ethoxylates and/or propoxylates.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0038] The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:

[0039] FIG. 1 identifies the types and characteristics of various prior art desalination processes;

[0040] FIG. 2 shows schematically one example of electrochemical desalination (ED) technology;

[0041] FIG. 3 shows schematically the water transport inefficiencies in current ED processes;

[0042] FIG. 4 shows schematically one embodiment of the present invention for use, for example, with an ionic solute;

[0043] FIG. 5 shows schematically one cell of the ED system of FIG. 4 and the separation of ions of the solute into adjacent cells;

[0044] FIG. 6 shows schematically an alternative embodiment of the present invention for use, for example, with a water-soluble polymer solute; and

[0045] FIG. 7 shows schematically one cell of the ED system of FIG. 6 and the separation of water ions while using a non-ionic polymer draw solute.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

[0046] Referring to FIG. 4, in one embodiment of the present invention, a system 10 is provided comprising a forward osmosis (FO) reactor 14 coupled to a downstream electrodeionization (EDI) reactor 16 (preferably a continuous electrodeionization (CEDI) reactor). The two are combined for the desalination of fluid 18 having a heavier than desired salt content, such as seawater, brackish water, industrial water or wastewater, to produce potable water 20. The FO reactor 14 preferably utilizes a solvent 24 having a generally high osmotic potential in solution that can be used to draw salt from the fluid 18 across membrane 26 within the FO reactor 14.

[0047] Any ionized salt which gives a high osmotic potential in its solution in water, can be used as the draw solution for the forward osmosis process. Examples of preferred salts include ammonium chloride, magnesium chloride, ammonium bicarbonate or ammonium carbonate. The downstream continuous electrodeionization enables recovery of the draw solution as a concentrated ionic solution in water, along with potable water as the main product. The use of a reverse CEDI process also enables use of non-ionic polymeric draw solutions, with high osmotic potentials, to also be used for the FO process, with regeneration of the concentrated polymer solution for recycling to the FO process and a stream of pure water as the permeated product. The use of a FO-CEDI process consumes much lower energy than reverse osmosis or thermal desalination, without issues with membrane fouling or bio-fouling, which plagues the reverse osmosis process, unless efficient pretreatment with ultra-filtration membranes is carried out prior to reverse osmosis processes.

[0048] In the example embodiment of FIG. 4, raw fluid having high saline content 18 enters a pre-filter 30, where particulates and suspended solids are removed. The result is filtered salt water 32 that is directed into the forward osmosis module 14, where water in the salt water 32 is removed due to osmotic gradients across the FO membrane 26. A solution 24 of solvent, for example, ionic hydrolytic agents, creates the necessary osmotic gradient across the FO membrane 26, although many type of solvents are contemplated, as discussed herein. A concentrated saline fluid 33 is carried away from the FO module 14 for disposal or use. The removal of water from the input raw saline water 32 converts the concentrated draw solution to a diluted draw solution 24, which is then sent to the EDI system 16, which comprises a plurality of cells 34a and 34b sandwiched between an cathode 36 and anode 38, explained in more detail below. In some embodiments, the EDI system 16 can is a continuous electrodeionization system (CEDI). Under a small applied voltage, typically 0.4-0.8 Volts/cell, the ionic draw solution is re-concentrated for supply to the FO module, and a pure stream of potable water is made available for human consumption. The diluted draw solution 42 is directed into the CEDI reactor 16 to separate the solvent 24 from clean water 20.

[0049] Forward osmosis, using for example ionic salts as the draw solute, enable substantially pure water-salt solutions to be sent down-stream to the EDI process, which alleviates membrane fouling and associated maintenance issues in the EDI system. Thus, the EDI process works at close to ideal efficiency. A newer process, called CEDI (continuous electrodeionization), also includes anionic and cationic exchange resins in the main electrode compartments, in addition to the anionic and cationic membranes lining the periphery of the cells. As the salts ions are transported across the respective membranes, typically at a voltage of around 0.4-0.6 V/cell, the conductivity of the solution decreases, leading to higher amperage needs and corresponding resistance effects. Operating the cell as a higher voltage, around 0.8 V/cell, allows water to break down into H+ and OH ions, which interact with the ion exchange resins in the cell, and restore ionic conductivity in the solution. Thus, the resins acts as a ionic pathway across each individual cell, keeping cell amperage and resistance low. The process is termed continuous, since the resins continuously get regenerated, there is no need for electrode polarity reversal, and product output is constant. Typical operating numbers for a CEDI system operating downstream of a dual RO system, to produce ultra-pure water for power generation, are a product rate of 9 m.sup.3/hr, for a CEDI power supply of 300 V, 16 A. This leads to a energy requirement of 0.53 kWh/m.sup.3 for highly purified water after initial water purification by reverse osmosis.

[0050] The preferred draw solutes would be suitable ionic salts, such as for example, ammonium bicarbonate, ammonium carbonate, magnesium chloride, calcium chloride, potassium chloride and sodium chloride salts. These salts have very high solubility in water and high osmotic potentials, as compared to NaCl and other salts commonly present in seawater and brackish water. Magnesium chloride is a strongly ionizable salt, while ammonium bicarbonate and ammonium carbonate are weakly ionized salts. Conversely, the low bonding strengths of ammonium bicarbonate and carbonate are low, since it can thermally be converted into ammonia and carbon dioxide at low temperatures, of around 60-65 C.

[0051] Magnesium chloride salt is one example of ionic hydrolytic agent, as shown in FIG. 4. It has excellent osmotic properties, as shown from laboratory experiments, well in excess of an equal concentration of NaCl. Thus, a 20% solution of MgCl.sub.2 yields an osmotic pressure of almost 300 atms, as compared to 168.54 atms for an equivalent concentration of NaCl. Similarly, ammonium chloride solutions have even higher osmotic potentials, and would be very suitable draw agents for the FO process, while also retaining high electrochemical conductivity for the downstream CEDI process. An equivalent concentration of NH.sub.4HCO.sub.3 generates only 124.76 atms. Thus, NH.sub.4Cl or MgCl.sub.2 would be an excellent ionic hydrolytic agent for the FO process, if it can be cost-effectively regenerated for repeat cycles in the FO process. Fortunately, the ionicity of NH.sub.4Cl or MgCl.sub.2 and the use of an EDI process downstream of an FO process enables the efficient recycling of a concentrated NH.sub.4Cl or MgCl.sub.2 solution for continuous use in the FO process for water recovery. In addition, commonly used FO membranes would allow no cross-over of the osmotic agent to the feed side, since MgCl.sub.2 is a bivalent salt, thus preventing any loss of the osmotic agent, unlike NH.sub.4HCO.sub.3 solutions. The MgCl.sub.2 hexahydrate has a solubility of 167 g/100 ml of water, and is a cheaply and commercially available salt.

[0052] In addition, MgCl.sub.2 solutions also serve as biocides and germicides, but still fit for human consumption in small amounts. This is an added advantage for emergency water supplies in remote areas with no need for chlorination or other biological disinfection systems.

[0053] NRGTEK Inc. has been working on FO-based desalination systems, though with different draw solutions, based on cloud-point polymers, using ethoxylate-butoxylate block-coplymers. NRGTEK synthesized several cloud-point polymers, based on glycerol ethoxylates and suitably butylated. All these polymers exhibited higher osmotic potentials, as tested against 20% MgCl.sub.2 solutions, and were able to pull water from the MgCl.sub.2 solution at good flux rates, across an HTI-CTA FO flat-sheet membrane. However, when run across a spiral-wound HTI-CTA FO membrane module, the very high viscosity of the polymers resulted in very low flux rates and water removal from the salt solutions.

[0054] A 20% MgCl.sub.2 solution has an osmotic potential of 300 atms, compared to 28 atms for a 3.5% NaCl solution. Hence, water recovery rates across the membrane are excellent. It is estimated that the 20% MgCl.sub.2 solution will need to go down to a 10% solution (OP=100 atms), while the 3.5% NaCl solution will go up to a 10.5% (OP=90 atms) solution before the osmotic potentials become close enough to prevent any significant water transfer. Thus, the water recovery from the feed solution is expected to be in the range of 75%, well in excess of commercial RO systems.

[0055] Referring to FIG. 5, an EDI system, preferably a continuous EDI system, is provided capable of concentrating a solute employed within the FO reactor 14. As discussed herein, the solute may be an ionic salt, for example, magnesium chloride, or a cloud-point solute, or a water-soluble polymer such as non-cloud-point ethoxylates and/or propoxylates. FIG. 5 illustrates use of an ionic salt as a solvent, whereas FIGS. 6 and 7 reflect use of a water-soluble polymer solute. In one example of a specific ionic salt, a 20% solution of MgCl.sub.2 may be used to yield potable water for human consumption. Referring specifically to FIG. 5, a single cell 34a is provided (between adjacent cells 34b) for the introduction of diluted FO solvent (e.g., 10% solution of MgCl.sub.2. The cell 34a is provided with an electrode mesh 44 and an anionic membrane 46, along one side of the cell, and an electrode mesh 48 and a cationic membrane 50, along the other side. The chloride ions are transported through the anionic 46 membrane to an adjacent cell 34b, and the magnesium ions are transported through the cationic membrane 50 to an opposing adjacent cell 34b, both under the influence of an electrical field (for example, approximately 0.4-0.6 VDC). Anionic 52 and cationic resin 54 may be provided to facilitate the transport process. The individual cationic and anionic exchange resins can be of several formulations; the cationic resin can be either in the protonic form (H+) or as an cationic ion format (Mg++ form or Na+ form). Similarly, the anionic exchange resin can be in the hydroxyl form (OH) or an anionic ion format (Cl). The individual anionic and cationic ions in the respective resins are the ions which are exchanged with the ions in the solute.

[0056] With multiple cells arranged in series to each other, as shown in FIG. 4, each of these adjacent cells containing the transported ions (e.g, the magnesium and chloride ions) recombine to form a concentrated magnesium chloride solutions for recycling to the FO module as a regenerated draw solution. The other cells from which these ions have been extracted now have substantially purified water, suitable for potable purposes.

[0057] Possible anionic and cationic membranes, which by example can be used for the multi-cell CEDI system, are listed in Table A below. These commercially available membranes are used for present-day CEDI systems, where ultra-pure water is produced after the process of reverse osmosis has initially purified the water and removed most of the salt, as well as other fouling agents from the raw water. However, for such membranes to be applied to the regeneration of a 20% MgCl.sub.2 or NH.sub.4Cl from the diluted permeate of a forward osmosis system, membranes with much higher conductivity and ion-exchange capacity are desirable.

TABLE-US-00001 TABLE A Conductivity Resistivity Thickness Areal Resist. Membrane (S/cm) (-cm) (cm) (-cm.sup.2) = RA Nafion NE-1135 0.10 min 10 max 0.0089 0.089 Protonic Nafion 115 0.10 min 10 max 0.127 0.127 Protonic Fumasep FAD 0.013 76.92 0.010 0.7692 Fumasep FAP 0.006 166.67 0.007 1.167 Excellion I-200 0.0034 294 0.034 9.996 Membranes Intl 0.00055 1818 0.0403 73.2654 AM-7001 Sybron Ultrex 0.0016 625 0.0406 25.375 MA-3475 Tokuyama AM1 0.0056 178.57 0.016 1.2-2.0 Tokuyama AFX 0.0127 78.74 0.014 0.7-1.5 Tokuyama AFN 0.04 23.08 0.013 0.3-1.0 Tokuyama A-010 0.018 55 0.004 0.22

[0058] The solid polymer membranes shown in Table A do not exhibit very high electrochemical conductivity for efficiently deionizing large amounts of salt without an energy penalty, nor do they have sufficient ion-exchange capacity for exchanging large amounts of salt ions. New membranes with much higher electrochemical conductivity and much higher ion-exchange capacity are desired. Such membranes, suitable for deionization of the FO permeate to pure water and regeneration of a concentrated FO draw solution, are described herewith.

[0059] In one embodiment, for example, porous polymer gelled liquid electrolyte membranes are provided that exhibit properties intermediate between liquid electrolytes and solids-state electrolyte membranes. These polymeric membranes have interconnected pores, filled with the desired electrolyte, which is held inside the pores by capillary forces. The pores are typically between 0.1-10 microns, or even smaller, and the porous polymer membrane may have a porosity between 85-90%, which is then filled with the desired liquid electrolyte by absorption. The polymers typically used for forming the porous membrane structures are well-known in literature, and range from polyethylene oxide (PEO), polyacrylonitrile (PAN), polydimethylsiloxane (PDMS), polyvinylidene difluoride (PVDF), poly(methyl-methlyacrylate) (PMMA) and other polymers. Some membranes cited in literature are also made from mixtures of these polymers with each other and other polymers. Thus, a few examples of porous membrane structures suitable for polymeric gelled electrolyte membranes are PVDF-HFP (PVDF-co-hexafluoropropylene) membranes, PDMS-PAN-PEO membranes, PVDF-NMP-EC-PC (PVDF with n-methylpyrolidine and ethylene and propylene carbonates), and even PVDF on glass mats. Such porous membranes, in which saturated solutions of MgCl.sub.2 or NH.sub.4Cl have been absorbed, would have much higher electrochemical conductivity and much higher ion-exchange capacity than conventional solid-state polymeric anionic and cationic membranes shown in Table A, especially for bivalent ions like magnesium ions. The ion exchange capacity for commercially available cationic and anionic ion-exchange membranes is only around 1.6-1.8 meq/g (milli-equivalents per gram), and a ten-fold to hundred-fold increase in ion-exchange capacity is desirable for energy-efficient EDI/CEDI. For example, magnesium chloride has a solubility in water in excess of 150 g/100 ml of water. This translates to a TDS (total dissolved solids) concentration of 1,500,000 ppm. Assuming the normal correlation of TDS with electrical conductivity (EC), at 500 ppm to 1,000 S/cm, a saturated solution of MgCl.sub.2 at a TDS level of 1,500,000 ppm computes to an electrical conductivity of 3 S/cm, a hundred-fold or higher than commercially available ionic membranes shown in Table A. If such a saturated solution can be used to make the porous polymeric gelled electrolyte ionic membranes, the presence of similar cations and anions, in a saturated solution inside the porous membrane architecture, also delivers very high ion-exchange capacity from one side of the membrane to the other, under the influence of an electrical voltage. The energy losses due to resistivity effects in the membranes in the CEDI system would also be substantially decreased. The fabrication of such a membrane is described herewith.

[0060] If the draw solution to be used for the FO process is MgCl.sub.2, a porous polymeric gelled electrolyte membrane filled with saturated MgCl.sub.2 solution can be used for both the anionic and the cationic sides in the subsequent CEDI process, instead of conventional anionic or cationic solid-state polymer membranes. Since the porous gelled MgCl.sub.2 cationic membrane is now used for transport of Mg ions across the membrane in the CEDI system, and the porous gelled MgCl.sub.2 anionic membrane is used for transport of chloride ions across the membrane in the CEDI system, the transport efficiency of these ions across their respective membranes are optimized, resulting in much higher ionic conductivity and ion-exchange capacity due to the saturated ionic solution filling the pores of the porous polymeric membrane scaffolding. A similar system is contemplated for CaCl.sub.2, KCl, NaCl and NH.sub.4Cl solutions, for example, if these solutions are alternatively used as the draw solution for the FO process. Such porous polymer gelled electrolyte membranes function as salt bridges with electrodes of suitable polarity attached to them to either enable anion or cation transport. No new ionic species are introduced into the system, and no other electrochemical or ionic interference effects takes place, since all the cells in the CEDI system contain the same ionic species, though in different concentrations in different cells in series. The equal impedance matching of these porous polymer gelled electrolyte membranes, if made by a procedure as described above, enables the minimization of polarization losses in the experimental cell, and the saturated nature of the solution filling the pores of the membrane enable high ion exchange capacity. The anode and cathode materials are platinized titanium meshes, in order to resist salt and chloride corrosion.

[0061] Referring back to FIG. 4, for example, the FO-CEDI system, when integrated together into a serial system, is capable of desalinating seawater, with at least a 75% water recovery, and with continuous regeneration of the draw solute for recycling to the FO module. In one example, if 100 liters of a saline salt solution, comprised of 3.5% NaCl (osmotic pressure of 28 atms) for example, is introduced into the feed side of a Forward Osmosis module, and 100 liters of a concentrated draw solution, comprised of 20% MgCl.sub.2 (osmotic pressure of 300 atms) is introduced into the draw side of the Forward Osmosis module, the osmotic pressure differential is sufficient between the two solutions to enable withdrawal of at least 75 liters of water across the FO membrane, from the feed side to the draw side, at practical flux rates. This results in fresh water recovery of 75% from the initial 100 liters of the saline salt solution. The 175 liters of the draw solution from the FO system, now diluted down to 12.5% MgCl.sub.2, is fed into a CEDI system, wherein the draw solution is re-concentrated back to 100 liters of a concentrated 20% MgCl.sub.2 draw solution, for recycling back to the Forward Osmosis module, while also resulting in production of fresh potable water of 75 liters.

[0062] Referring to FIGS. 6 and 7, in another variation of the CEDI process, polymeric draw solutions, similar to the cloud-point polymers already developed by NRGTEK Inc. can also be used for the FO-CEDI process, with a small variation in the CEDI cell. In such an application, the diluted draw solution permeate 142 from the FO process, containing high osmotic potential polymeric draw solutions are fed to one of several CEDI cells 134a. The CEDI feed cell is filled with strongly cationic and strongly anionic ion exchange resins 152, 154, and a voltage of around 0.8 VDC impressed across each cell. At this voltage, water breaks down into OH.sup. and H.sup.+ ions, which are now transferred across the anionic membrane 146 and cationic membrane 150, respectively, to the adjacent permeate cells 134b due to the voltage gradient present, wherein they recombine into pure water, as shown in FIG. 7. In spite of the polymeric draw solution not having any ionic conductivity, the use of strong ion-exchange resins enables the CEDI cell to still function electrochemically to transfer protons and hydroxyl ions across the membranes for recombination into pure water, leaving only a concentrated polymeric solution in the cell for recycling to the FO module. Typically, water electrolysis occurs at potentials greater than 1.23 VDC, typically 1.5 VDC, wherein oxygen and hydrogen gases are produced. However, in the proposed invention, water is not electrolyzed, but only ionic splitting and transport of hydrogen cations and hydroxyl anions take place across the relevant membranes under an impressed voltage of 0.8 VDC, after which they recombine into pure water.

[0063] Persons of ordinary skill in the art may appreciate that numerous design configurations may be possible to enjoy the functional benefits of the inventive systems. Thus, given the wide variety of configurations and arrangements of embodiments of the present invention the scope of the invention is reflected by the breadth of the claims below rather than narrowed by the embodiments described above.