Multi-stage bipolar electrodialysis system for the production of highly concentrated acids or bases

Description

FIELD

[0001] This disclosure relates to bipolar electrodialysis systems and methods used to produce acids or bases from salt solution.BACKGROUND

[0002] The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.

[0003] A bipolar electrodialysis cell refers to an electrodialysis cell that includes a bipolar membrane. The bipolar membrane disassociates water into hydronium ions and hydroxyl ions on application of an electrical field. These generated ions combine with cations and anions from a process stream that includes salts, where the cations and anions are separated by one or more ion exchange membranes in the electrodialysis cell. The combination of the hydronium ions with the anions, and the hydroxyl ions with the cations, results in produced streams having acid and base.

[0004] A bipolar electrodialysis cell may be a two-compartment cell or a three- compartment cell. A two-compartment cell includes either a cation-exchange membrane or an anion-exchange membrane between two bipolar membranes. The choice of using a cation-exchange membrane or an anion-exchange membrane depends on which salts are being processed. Cation-exchange membranes are used to process solutions having salts of weak acids and strong bases, such as sodium salts of organic and amino acids. Examples of such organic and amino acids include: ascorbic acid, acetic acid, lactic acid, formic acid, gluconic acid, and glutamic acid. Anion-exchange membranes are used to process solutions having salts of weak bases and strong or weak acids, such as ammonium salts of chloride, sulfate or lactate.

[0005] A three-compartment cell includes an anion-exchange membrane and a cation-exchange membrane between two bipolar membranes, thereby forming three compartments. The three compartments are: an acidic solution-producing compartment between the first bipolar membrane and the anion-exchange membrane; a basic solution- producing compartment between the second bipolar membrane and the cation-exchange membrane; and a compartment between the cation-exchange membrane and the anion- exchange membrane that produces a salt-reduced solution. A three-compartment BPED cell is used for recovering an inorganic acid and base from its corresponding salt.

[0006] US 5,853,555 discloses a process for preparing onium hydroxide from a corresponding onium salt and for purifying onium hydroxide including providing an electrochemical cell containing a cathode, an anode, a divider and a bipolar membrane, the bipolar membrane having an anion selective side facing the anode and a cation selective side facing the cathode, wherein the divider is positioned between the cathode and the bipolar membrane, and the bipolar membrane is positioned between the divider and the anode, thereby defining a feed compartment between the divider and the bipolar membrane, a recovery compartment between the divider and the cathode, and a water compartment between the bipolar membrane and the anode; charging a solution containing at least one of the onium salt and the onium hydroxide to be purified to the feed compartment; charging a liquid electrolyte to the other compartments; passing a current through the electrochemical cell to produce the onium hydroxide in the recovery compartment; and recovering the onium hydroxide from the recovery compartment.

[0007] US 5,645,703 is directed to a method for recovering salt from a process stream containing organic contaminants is provided, comprising directing the waste stream to a desalting electrodialysis unit so as to create a concentrated and purified salt permeate and an organic contaminants containing stream, and contacting said concentrated salt permeate to a water-splitting electrodialysis unit so as to convert the salt to its corresponding base and acid, however US 5,645,703 teaches only the use of a single bipolar electrodialysis stage.

[0008] EP 3060699 There are provided processes for preparing lithium hydroxide comprising submitting an aqueous composition comprising lithium sulfate and / or lithium bisulfate to a first electromembrane process that comprises a two-compartment membrane process under suitable conditions for conversion of the lithium sulfate and / or lithium bisulfate to lithium hydroxide, and obtaining a first lithium-reduced aqueous stream and a first lithium hydroxide-enriched aqueous stream; and submitting the first lithium-reduced aqueous stream to a second electromembrane process that comprises a three-compartment membrane process under suitable conditions to prepare at least a further portion of lithium hydroxide and obtaining a second lithium-reduced aqueous stream and a second lithium-hydroxide enriched aqueous stream, however EP 3060699 does not teach or suggest the need to have proton-blocking membranes in some stages but not others.INTRODUCTION

[0009] The following introduction is intended to introduce the reader to this specification but not to define any invention. One or more inventions may reside in a combination or sub-combination of the apparatus elements or method steps described below or in other parts of this document. The inventors do not waive or disclaim their rights to any invention or inventions disclosed in this specification merely by not describing such other invention or inventions in the claims.

[0010] A bipolar electrodialysis (BPED) cell is able to convert salt solutions into acid and base solutions. However, protons migrate through the anion exchange membranes and tend to neutralize the base solution. With increasing acid concentration, the flow of protons increases. This reduces the energy efficiency of the cell and, in practice, limits the concentration of the acid produced. Typically, acids and bases are produced in conventional BPED systems at a concentration of about 1 mol / L and 70% current efficiency. In some cases, this concentration is enough to provide a usable product, but form most applications the product concentration is too low to reuse or sell the acid and base products.

[0011] In a bipolar electrodialysis system described herein, multiple BPED cells are arranged to provide a multi-stage treatment system. In a system with three compartment BPED cells, the feed solution flows in the opposite direction as the base solution and acid solution. In a two compartment BPED system, the feed / acid solution flows in the same direction as the base solution. Up to half, or up to one third, of the stages have cells with acid block anion membranes. For example one stage in a system having two or three stages may have acid block anion membranes, or one or two stages in a system having have four to eight stages may have acid block anion membranes. The one or more stages with acid block anion membranes are located at the acid product output end of the system, where the acid concentration in the system is the highest. The remainder of the stages have conventional, i.e. non-acid block, anion membranes.

[0012] Replacing the traditional anion membranes in some of the stages with acid block anion membranes reduces the migration of protons into the base solution. This allows higher concentration products to be produced. However, acid block anion membranes have less conductivity (higher resistance) compared to traditional anion membranes, which would result in a significant increase in energy consumption if all of the anion membranes were replaced. By using the acid block anion membranes only where the acid concentration in the system is high, product concentration can be increased with less increase in energy consumption compared to a system in which anion membranes are replaced in all of the stages.

[0013] Thus, according to the present invention, there is provided a bipolar electrodialysis (BPED) system (500) comprising, (a) a plurality of bipolar electrodialysis stages, wherein the plurality of the bipolar electrodialysis stages are all two-compartment bipolar electrodialysis cells or the plurality of the bipolar electrodialysis stages are all three-compartment bipolar electrodialysis cells, (b) wherein one or more stages and up to and including one half of the plurality of bipolar electrodialysis stages comprise acid block anion-exchange membranes (220) and wherein the remainder of the bipolar electrodialysis stage or stages comprise non-acid block anion-exchange membranes (206), (c) wherein the stage or stages comprising acid block anion-exchange membranes is or are located at an acid product output end of the BPED system, (d) wherein each two-compartment cell includes an anion-exchange membrane between two bipolar membranes, and each three-compartment cell includes an anion-exchange membrane and a cation-exchange membrane between two bipolar membranes; and wherein when the plurality of bipolar electrodialysis stages are three-compartment BPED cells (300); the BPED system comprises a feed-solution inlet (20) to the stage or stages comprising acid block anion-exchange membranes, and an effluent outlet (216) from the stage or stages lacking the acid block anion-exchange membranes, the feed-solution inlet and the effluent outlet being in fluid communication; a basifying-solution inlet (214) to the stage or stages lacking the acid block anion-exchange membranes, and a basic-solution outlet 26) from the stage or stages comprising acid block anion-exchange membranes, the basifying-solution inlet and the basic-solution outlet being in fluid communication; and an acidifying-solution inlet (210) to the stage or stages lacking the acid block anion-exchange membranes, and an acid-solution outlet (28) from the stage or stages comprising acid block anion-exchange membranes, the acidifying-solution inlet and the acid-solution outlet being in fluid communication or when the plurality of bipolar electrodialysis stages are two-compartment BPED cells (200); the BPED system comprises a feed-solution inlet (20) to the stage or stages lacking acid block anion-exchange membranes, and an effluent outlet (28) from the stage or stages comprising acid block anion-exchange membranes, the feed-solution inlet and the effluent outlet being in fluid communication; and an acidifying-solution inlet (210) to the stage or stages lacking acid block anion-exchange membranes, and an acid-solution outlet (28) from the stage or stages comprising acid block anion-exchange membranes, the acidifying-solution inlet and the acid-solution outlet being in fluid communication. BRIEF DESCRIPTION OF THE FIGURES

[0014] FIG. 1 is a schematic illustration of a three compartment bipolar electrodialysis cell. FIG. 2 is a schematic illustration of a two compartment bipolar electrodialysis cell with anion exchange membranes. FIG. 3 is a schematic illustration of a three compartment bipolar electrodialysis cell with acid block anion membranes. FIG. 4 is a schematic illustration of a two compartment bipolar electrodialysis cell with acid block anion membranes. FIG. 5 is a schematic illustration of a multi-stage three-compartment bipolar electrodialysis cell with counter current flow and some of the stages having acid block anion membranes. FIG. 6 is a schematic illustration of a multi-stage two-compartment bipolar electrodialysis cell with co-current flow and acid block anion membranes and some of the stages having acid block anion membranes. DETAILED DESCRIPTION

[0015] Bipolar membrane electrodialysis (or, bipolar electrodialysis, BPED) is a process that couples electrolysis and electrodialysis. The BPED device receives a salt solution and provides an acidic solution and a basic solution. A bipolar membrane electrodialysis cell may be a two or three compartment cell, depending on the acid and base to be produced.

[0016] A two compartment cell may include bipolar membranes and either cation exchange membranes or anion exchange membranes. In examples described herein, the two compartment cell includes anion exchange membranes. Two compartment cells that include bipolar membranes and anion exchange membranes are useful to convert the salts of strong acids and weak bases, such as, for example, ammonium chloride, ammonium sulfate, and ammonium lactate. In three compartment cells it is possible to convert an aqueous salt solution into strong bases and strong acids, such as, for example, the conversion of NaCl solution into NaOH solution and HCl solution. Other salts, for example KF, Na 2 SO 4 , NH 4 Cl, KCI, as well as the salts of organic acids and bases, can also be converted using three compartment cells.

[0017] An illustration of a three compartment bipolar electrodialysis cell (100) is shown in FIG. 1. The three compartment bipolar electrodialysis cell (100) illustrates two cells between cathode (202) and anode (204) to simplify the figure, though many cells are typically provided in a bipolar electrodialysis stack. Using electrolysis, bipolar electrodialysis disassociates water, which is found between a cation exchange membrane portion and an anion exchange membrane portion of the bipolar membrane (206), into H +< and -< OH. Application of an applied electric potential difference induces the produced H +< ions to move towards the cathode (202), through cation exchange membranes (208), into an acidifying solution (210). Similarly, the produced -< OH ions to move towards the anode (204), through anion exchange membranes (212), into a basifying solution (214). In a similar manner, cations (416) and anions (418) in the salt solution (20) are induced to move through the cation and anion exchange membranes, respectively, as charge balance for the H +< and -< OH ions, resulting in desalinated effluent (216) being discharged from the cell (200). The three compartment bipolar electrodialysis cell (100) shown is operating in a counter-current mode because the salt concentrated solution (20) (i.e. feed water) moves in the opposite direction as the acidifying solution (210) and the basifying solution (214).

[0018] With acceptance of the H +< ions, the acidifying solution (210) becomes acidic and is discharged from the bipolar electrodialysis cell (200) as the acid solution (28). Conversely, with acceptance of the -< OH ions, the basifying solution (214) becomes basic and is discharged from the bipolar electrodialysis cell (200) as the basic solution (26).

[0019] The acidifying solution (210) and the basifying solution (214) include ions to carry the applied current. These ions become the counter-ions of in the produced acids and bases. The acidifying solution (210), the basifying solution (214) and the salt-concentrated solution (20) may all be the same or different.

[0020] In one example, the acidifying solution, the basifying solution and the salt concentrated solution are all NaCl / water solutions, where the resulting acid solution is an HCl / water solution and the resulting basic solution is an NaOH / water solution. In another example, the acidifying solution, the basifying solution and the salt-concentrated solution are all sodium sulfate / water solutions, where the resulting acid solution is an H 2 SO 4 / water solution and the resulting basic solution is an NaOH / water solution. In yet another example, the acidifying solution, the basifying solution and the salt-concentrated solution are all mixtures of different salts, such as sodium sulfate and NaCl, and the resulting acid solution is an H 2 SO 4 / HCl / water solution and the resulting basic solution is an NaOH / water solution.

[0021] In still another example, the acidifying solution and the basifying solution are water, while the salt-concentrated solution is a NaCl / water solution, where the resulting acid solution is an HCl / water solution and the resulting basic solution is an NaOH / water solution.

[0022] An illustration of a two compartment bipolar electrodialysis cell (200) with anion exchange membranes is shown in FIG. 2. The bipolar electrodialysis cell (200) illustrates two full cells between cathode (202) and anode (204) to simplify the figure, though many cells are typically provided in a bipolar electrodialysis stack. Using electrolysis, bipolar electrodialysis disassociates water, which is found between a cation exchange membrane portion and an anion exchange membrane portion of the bipolar membrane (206), into H +< and -< OH. Application of an applied electric potential difference induces the produced H +< ions to move towards the cathode (202) into an acidifying solution (210), and the produced -< OH ions to move towards the anode (204) into the salt-concentrated solution (20). The bipolar electrodialysis cell (200) includes anion exchange membranes (212). The two compartment bipolar electrodialysis cell (200) is operating in a co-current mode because the acidifying solution (210) and salt-concentrated solution (20) flow in the same direction.

[0023] With acceptance of the H +< ions, the acidifying solution (210) becomes acidic and is discharged from the bipolar electrodialysis cell (200) as the acid solution (28). Conversely, with acceptance of the -< OH ions, the salt-concentrated solution (20) becomes basic and is discharged from the bipolar electrodialysis cell (200) as the basic solution (26).

[0024] The acidifying solution (210) and the salt-concentrated solution (20) include ions to carry the applied current. These ions become the counter-ions of in the produced acids and bases. The acidifying solution (210) and the salt-concentrated solution (20) may be the same or different.

[0025] In multi-stage bipolar electrodialysis systems to be described below, the anion exchange membranes in some, but not all, of the stages are replaced with acid block anion membranes. In one example, polymeric acid block anion selective membranes are prepared by impregnating a woven or non-woven cloth with the reaction products of three components. Component I is an ethelynically unsaturated aliphatic or aromatic tertiary or quaternary amine monomer. Component II is a crosslinking monomer. Component III is vinylbenzyl chloride. Membranes of this type are described in greater detail in US Patent Number 8,740,896, Acid Block Anion Membrane.

[0026] Figure 3 shows a three compartment electrodialysis cell (300) with acid block anion exchange membranes (220). The cell (300) of Figure 3 is similar to the three compartment electrodialysis cell (100) of Figure 1, and the description of Figure 1 applies to Figure 3, except that anion exchange membranes (212) of Figure 1 have been replaced with acid block anion membranes (220) in Figure 3.

[0027] Figure 4 shows a two compartment electrodialysis cell (400) with acid block anion exchange membranes (220). The cell (400) of Figure 4 is similar to the two compartment electrodialysis cell (200) of Figure 2, and the description of Figure 2 applies to Figure 4, except that anion exchange membranes (212) of Figure 2 have been replaced with acid block anion membranes (220) in Figure 4.

[0028] Figure 5 shows a multi-stage bipolar electrodialysis system (500) with three compartment electrodialysis cells (100, 300) operating in counter-current mode. In the example shown, there are two three compartment electrodialysis cells with acid block anion membranes (300) and five three compartment electrodialysis cells (100). Salt-concentrated solution (20) enters the system (500) through one of the three compartment electrodialysis cells with acid block anion membranes (300). The acidifying solution (210) and the basifying solution (214), which may start as make up water, enter the system (500) through one of the three compartment electrodialysis cells (100). Concentrated acidifying solution (210) flows through the three-compartment electrodialysis cell with acid block anion membranes (300) but the migration of protons is inhibited by the acid block anion membranes.

[0029] Figure 6 shows a multi-stage bipolar electrodialysis system (600) with two compartment electrodialysis cells (200, 400) operating in co-current mode. In the example shown, there is one two-compartment electrodialysis cell with acid block anion membranes (400) and three two-compartment electrodialysis cells (200). Salt-concentrated solution (20) enters the system (600) through one of the two-compartment electrodialysis cells (200). The acidifying solution (210), which may start as make up water, also enter the system (600) through one of the two compartment electrodialysis cells (200). Concentrated acidifying solution (210) flows through the two-compartment electrodialysis cell with acid block anion membranes (400) but the migration of protons is inhibited by the acid block anion membranes.

[0030] This written description uses examples to help disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. Alterations, modifications and variations can be effected to the particular examples by those of skill in the art without departing from the scope of the invention. The patentable scope of the invention is defined by the claims.

Claims

1. A bipolar electrodialysis (BPED) system (500) comprising, (a) a plurality of bipolar electrodialysis stages, wherein the plurality of the bipolar electrodialysis stages are all two-compartment bipolar electrodialysis cells or the plurality of the bipolar electrodialysis stages are all three-compartment bipolar electrodialysis cells, (b) wherein one or more stages and up to and including one half of the plurality of bipolar electrodialysis stages comprise acid block anion-exchange membranes (220) and wherein the remainder of the bipolar electrodialysis stage or stages comprise non-acid block anion-exchange membranes (206), (c) wherein the stage or stages comprising acid block anion-exchange membranes is or are located at an acid product output end of the BPED system, (d) wherein each two-compartment cell includes an anion-exchange membrane between two bipolar membranes, and each three-compartment cell includes an anion-exchange membrane and a cation-exchange membrane between two bipolar membranes; and wherein when the plurality of bipolar electrodialysis stages are three-compartment BPED cells (300); the BPED system comprises a feed-solution inlet (20) to the stage or stages comprising acid block anion-exchange membranes, and an effluent outlet (216) from the stage or stages lacking the acid block anion-exchange membranes, the feed-solution inlet and the effluent outlet being in fluid communication; a basifying-solution inlet (214) to the stage or stages lacking the acid block anion-exchange membranes, and a basic-solution outlet 26) from the stage or stages comprising acid block anion-exchange membranes, the basifying-solution inlet and the basic-solution outlet being in fluid communication; and an acidifying-solution inlet (210) to the stage or stages lacking the acid block anion-exchange membranes, and an acid-solution outlet (28) from the stage or stages comprising acid block anion-exchange membranes, the acidifying-solution inlet and the acid-solution outlet being in fluid communication or when the plurality of bipolar electrodialysis stages are two-compartment BPED cells (200); the BPED system comprises a feed-solution inlet (20) to the stage or stages lacking acid block anion-exchange membranes, and an effluent outlet (28) from the stage or stages comprising acid block anion-exchange membranes, the feed-solution inlet and the effluent outlet being in fluid communication; and an acidifying-solution inlet (210) to the stage or stages lacking acid block anion-exchange membranes, and an acid-solution outlet (28) from the stage or stages comprising acid block anion-exchange membranes, the acidifying-solution inlet and the acid-solution outlet being in fluid communication.

2. The bipolar electrodialysis (BPED) system of claim 1 wherein up to one third of the plurality of bipolar electrodialysis stages comprise acid block anion-exchange membranes.

3. The bipolar electrodialysis (BPED) system of claim 1 or 2 wherein one or two of the plurality of bipolar electrodialysis stages comprise acid block anion-exchange membranes.

4. A method comprising: treating a salt solution (20) to bipolar membrane electrodialysis (BPED) in a multi-stage BPED system (500) to produce an acid product from an acid-product output end, wherein the plurality of bipolar electrodialysis stages are all two-compartment bipolar electrodialysis cells or the plurality of bipolar electrodialysis stages are all three-compartment bipolar electrodialysis cells, and wherein one or more stages and up to and including one half of the plurality of stages of the BPED system comprise acid block anion-exchange membranes (220) and wherein the remainder of the bipolar electrodialysis stage or stages comprise non-acid block anion-exchange membranes (206), and wherein the stage or stages comprising acid block anion-exchange membranes is or are located at the acid-product output end of the BPED system, wherein each two-compartment cell includes either an anion-exchange membrane between two bipolar membranes, and each three-compartment cell includes an anion-exchange membrane and a cation-exchange membrane between two bipolar membranes; wherein when the stages comprise three-compartment BPED cells, the method comprises flowing a feed solution in the opposite direction as a basifying solution and an acidifying solution; or wherein when the stages comprise two-compartment BPED cells, and the method comprises flowing a feed solution in the same direction as an acidifying solution.

5. The method according to claim 4, wherein up to one third of the stages comprise acid block anion-exchange membranes.

6. The method according to claim 4 or 5, wherein one or two of the stages comprise acid block anion-exchange membranes.