DESALINATION DEVICE AND PROCESS FOR RECOVERY AND VALORISATION OF CHLORIDES IN DILUTE SOLUTIONS

Abstract

The invention relates to a device and a process for the desalination of NaCl solutions employing a three-chamber electrochemical cell separated by relative ion exchange membranes, namely a succession of a cathode chamber, a cation exchange membrane, a central chamber for the saline solution, an anion exchange membrane and an anode chamber. The oxidation of OH.sup.? and the reduction of H.sub.3O.sup.+ under the formation of OH.sup.? and H.sub.2 causes the passage of Na and Cl.sup.? ions from the central chamber to the other chambers, thereby reducing the salt concentration. The feeding of the cathode chamber can be managed in a circuit with the insertion of a carbonation reactor to reduce the concentration of NaOH and eliminate CO.sub.2 from the air. Under certain conditions, the chlorides entering the anode chamber undergo oxidation and the chlorine formed therein reacts with water to produce HCl and HClO.

Claims

1. Desalination A desalination device comprising at least one electrochemical cell comprising: (a) an anode suitable for allowing the electrochemical reaction of oxidation of the OH.sup.? ion and consequent production of oxygen gas and release in solution of protons H.sup.+ wherein said anode is contained in an anode chamber suitable for containing or containing as an anolyte an acidic solution; (b) a cathode connected through an electrical connection to said anode and suitable to allow the electrochemical reaction of reduction of proton H.sup.+ and consequent production of hydrogen gas and release in solution of OH.sup.? ions wherein said cathode is contained in a cathode chamber suitable to contain or containing as a catholyte a basic solution, in particular an aqueous solution of NaOH; (c) a system for feeding said anode chamber with said acidic solution; (d) a system for feeding said cathode chamber with said basic solution; (e) a cation exchange membrane impermeable for OH.sup.? ions and permeable for cations, in particular Na.sup.+; (f) an anion exchange membrane impermeable for H.sup.+ ions and permeable for anions, particularly Cl.sup.?; wherein said anodic chamber and said cathodic chamber are separated by said cation exchange membrane and said anion exchange membrane which in turn are separated by a third chamber suitable for containing or containing an aqueous solution of chloride salt, in particular NaCl, where (?) said cation exchange membrane is simultaneously a wall or a portion thereof of said cathode chamber and of said third chamber such that from the third chamber to the cathode chamber a passage of salt cations, in particular Na+, is possible; (?) said anion exchange membrane is simultaneously a wall or a portion thereof of said anode chamber and of said third chamber such that from the third chamber to the anode chamber a passage of salt anions, in particular Cl.sup.?, is possible.

2. The desalination device according to claim 1, wherein said electrodes are made of stainless steel or graphite.

3. The desalination device according to claim 1, wherein said desalination device is configured to perform the following steps: (i) feeding the anode chamber with an anolyte, preferably water; (ii) feeding the cathode chamber with a catholyte, preferably water or the base produced therein at reduced concentration; (iii) feeding the third chamber with a concentrated chloride salt solution; (iv) oxidation of OH.sup.? on the anode with the formation of oxygen O.sub.2 and H.sup.+ protons; (v) reduction of H.sup.+ on the cathode with the formation of H.sub.2 hydrogen and OH.sup.? hydroxide ions.

4. The desalination device according to claim 1, wherein each of said chambers is provided with an inlet and an outlet, namely: (a) that the anode chamber is provided with an inlet for fresh anolyte and an outlet for anolyte enriched with salt anions, particularly Cl.sup.?, or their derivatives, and oxygen; (b) that the cathode chamber is provided with an inlet for the fresh catholyte and an outlet for the catholyte enriched in salt cations, particularly Na.sup.+, and hydrogen; and (c) that the third chamber has an inlet for the aqueous starting salt solution and an outlet for the reduced concentration salt solution.

5. The desalination device according to claim 1, further comprising (g) a gas-liquid separation device, in particular a gas-liquid scrubber, for recovering the hydrogen produced which is connected to the outlet of the cathode chamber and which is connected to a fuel cell.

6. The desalination device according to claim 1, further comprising: (h) a carbonation reactor wherein the cathode chamber comprises an inlet and an outlet which are connected in a circuit in which said carbonation reactor is inserted.

7. The desalination device according to claim 1, wherein said at least one electrochemical cell comprises: (i) a first plate supporting in a first window applied in said first plate said anode chamber of meandering path having a thickness with respect to the plane of the meandering path extension preferably not exceeding 6 mm and having an inlet and an outlet for the anolyte at its ends; (ii) a second plate supporting in a second window applied to said second plate said central chamber of meandering path having a thickness relative to the plane of the meandering path extension that preferably does not exceed 6 mm and having an inlet and outlet for saline solution at its ends; and (iii) a third plate supporting in a third window applied in said third plate said cathode chamber of meandering path having a thickness with respect to the plane of the meandering path extension that preferably does not exceed 6 mm and having an inlet and outlet for the catholyte at its ends, (iv) said anion exchange membrane interposed between said first and said second plate; (v) said cation exchange membrane interposed between said second and said third plate; (vi) a plate anode disposed alongside said first plate on the opposite side from the position of said anion exchange membrane; and (vii) a plate cathode disposed alongside said third plate on the opposite side with respect to the position of the cation exchange membrane; wherein plates, membranes and electrodes are superimposed in the following order: anode, anode chamber, anion exchange membrane, central chamber, cation exchange membrane, cathode, wherein each plate is optionally provided with a plurality of first holes arranged so that as the plates are overlapped, they are aligned so that they can be connected with relative fastening means; wherein each plate is provided with a plurality of second holes divided into three pairs of holes for conveying in separate channels the flows of anolyte, catholyte and saline solution, one hole of each pair provided for inlet of the respective flow into the system and the other hole for exit from the system wherein the corresponding holes are arranged so that with the overlapping of the plates they result aligned to be able to connect them to form separate channels for the respective flows, wherein each plate is preferably provided with a plurality of third holes for collecting gases formed in the anode and cathode chambers respectively and arranged in such a way that with the overlapping of the plates they result aligned to be able to connect them to form separate channels for the relative gas flows of the respective chambers, in which each flow traverses along the channel formed by the relative overlapping holes the whole system, but only in the plate carrying the chamber to which the relative channel flow is dedicated a communication with the chamber inlet is realized in such a way as to allow the flow to pass through the chamber and enter through the chamber outlet into the corresponding outlet channel formed by the respective set of holes.

8. The desalination device according to claim 7, further comprising a plurality of three-chamber electrochemical cells which is grouped into a cell stack according to variant (A) wherein the individual elements follow each other according to the following scheme: [+AZC?][+AZC?].sub.n with n=1, 2, . . . ; OR according to variant (B) in which the individual elements follow each other according to the following scheme: +AZC?CZA+AZC?CZA+ . . . +AZC? with varying number of AZC? and CZA? groups in which units with three adjacent chambers have the corresponding electrode in common, wherein for both variants (A) and (B) A is a plate with an anode chamber, Z is a plate with a central chamber, C is a plate with a cathode chamber, the + sign an anode and the ? sign a cathode, an anion exchange membrane is placed between adjacent chambers A and Z, and a cation exchange membrane is placed between adjacent chambers Z and C; and wherein for both variants (A) and (B) the chambers of the same type are connected through the relative inlets and outlets of the chambers and corresponding holes in the plates.

9. The desalination device according to claim 1, further comprising a plurality of electrochemical cells in a multipolar configuration in a stack containing the cells with alternating polarity.

10. The desalination device according to claim 1, further comprising pumping systems with adjustable speed for feeding said chambers.

11. The desalination device according to claim 1, wherein said electrochemical cell is configured to operate at a voltage between 2.5 and 3.5 V and a current between 0.5 and 6.5 mA/cm.sup.2.

12. A desalination process comprising the following steps: (a) providing a desalination device according to claim 1; (b1) feeding the anode chamber with an anolyte, preferably water; (b2) feeding the cathode chamber with a catholyte, preferably water or the base produced therein at reduced concentration; (b3) feeding of the third chamber with a concentrated chloride salt solution; (c1) oxidation of OH.sup.? on the anode with the formation of oxygen O.sub.2 and protons H.sup.+; (c2) reduction of H.sup.+ on the cathode with the formation of hydrogen H.sub.2 and hydroxide ions OH.sup.?; (d1) in reaction to increased concentration of H.sup.+ ions in the anode chamber passage of salt anions, particularly chlorides, from the third chamber into the anode chamber; (d2) in reaction to the increased concentration of OH.sup.? ions in the cathode chamber passage of salt cations, particularly Na.sup.+, from the third chamber into the cathode chamber.

13. The process according to claim 12, wherein said at least one electrochemical cell is supplied with a voltage between 2.5 and 3.5 V and a current between 0.5 and 6.5 mA/cm.sup.2, preferably between 4.5 and 6.5 mA/cm.sup.2 causing the oxidation of chloride within the anode chamber (14; 114) forming chlorine gas which in turn spontaneously undergoes a dismutation reaction resulting in the production of hydrochloric acid (HCl) and hypochlorous acid (HClO) in equal proportions.

14. The process according to claim 12, wherein (i) the anode chamber is fed with controlled flow, in particular with an acid solution or water, and that acid solutions containing oxygen and preferably HCl and HClO are extracted therefrom; (ii) the cathode chamber is fed with controlled flow with a basic solution, particularly NaOH, or with water and that concentrated basic solutions and hydrogen are extracted therefrom; (iii) the central chamber is fed at a controlled rate by concentrated salt solutions and that dilute salt solutions are extracted therefrom allowing control of the concentrations of the components of the aqueous solutions contained in the relevant chambers.

15. The process according to claim 12, wherein the feeding of the cathode chamber and the extraction of its contents take place in a circuit from which hydrogen is diverted, preferably feeding a fuel cell, and comprising a carbonation reactor from which carbonates and bicarbonates are diverted with the effect of buffering the pH of the basic solution which returns to the cathode chamber, preferably at values between 8.5 and 9.5, in which the energy produced in the fuel cell can be used to power the desalination device.

16. A method of using the desalination device according to claim 1 for reducing the concentration of chlorides in brackish water, industrial waste, mining, water treatment, or marine waters for one or more purposes selected from the group consisting of: production of carbonates and/or bicarbonates; removal of CO.sub.2 from the atmosphere; production of hydrogen for the purpose of producing energy; production of HCl and HClO for the production of disinfectants.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0102] FIG. 1 illustrates a schematic diagram of a desalination device according to the invention; and

[0103] FIG. 2 illustrates individual elements of an electrochemical cell usable in the device according to FIG. 1.

DETAILED DESCRIPTION

[0104] FIG. 1 illustrates a schematic diagram a desalination device according to the invention. The plant contains as a central element an electrochemical cell 10 comprising a central chamber 12, an anode chamber 14 and a cathode chamber 16. The anode chamber 14 contains an electrode, i.e. the anode 18. The corresponding electrode, the cathode 20, is located in the cathode chamber 16. The electrodes 18, 20 are connected by a circuit 22 powered by a photovoltaic cell 24. The electrons move from the negative electrode 18 to the positive electrode 20. The anode chamber 14 is separated by an anion exchange membrane 26 from the central chamber 12, while the cathode chamber 16 is separated from the central chamber 12 by a cation exchange membrane 28. Each of the three chambers 12, 14 and 16 is provided with an inlet and an outlet, the central chamber 12 of the inlet 30 and the outlet 32, the anode chamber 14 of the inlet 34 and the outlet 36 and the cathode chamber 16 of the inlet 38 and the outlet 40. Water (arrow a) is pumped by a pump 42 through the inlet 34 into the anode chamber 14. The central chamber 12 is fed through the inlet 30 with a concentrated NaCl solution (arrow b), for example 70 g NaCl/1. With a pump 44 a diluted basic aqueous solution, here NaOH with a concentration of 0.1 M, is pumped (arrow c) through the inlet 38 into the cathode chamber 16. At cathode 20, H.sub.3O.sup.+ cations are reduced to form hydrogen H.sub.2 according to reaction 2 H.sub.3O.sup.++4 e.sup.?.fwdarw.2 OH.sup.?+2 H.sub.2 (g). The basicity of the solution in the cathode chamber 16 is then increased. The NaOH solution, which leaves (arrow d) the cathode chamber 16 via the outlet 40, is thus more concentrated (here 1M). At the same time, the hydrogen formed exits (arrow e) via the outlet 46 separating it from the aqueous NaOH flow which is managed in a circuit 48 connecting the inlet 38 and the outlet 40 of the cathode chamber 16. A carbonation reactor 50 is installed in circuit 48, the concentrated NaOH solution leaving the cathode chamber 16 enters the reactor 50 which is fed (arrow f) through a line 52 with the help of a pump 54 with a gas (i.e. air) containing carbon dioxide CO.sub.2 that bubbles in the reactor 50. From the reaction between NaOH and CO.sub.2 in water respectively, sodium carbonate and/or bicarbonate are formed and can be discharged via an outlet 56 from the reactor 50. The reactor 50 has a separator wall 58 for separating the formation of the solid carbonate/bicarbonate salts from the circuit 48.

[0105] In the anode chamber 14 OH.sup.? ions are oxidised to form oxygen O.sub.2 according to reaction 4 OH.sup.?4e.sup.?2 H.sub.2O+O.sub.2 (g). The aqueous solution results in a lower pH as the H.sub.3O.sup.+ ion concentration increases.

[0106] The concentrated NaCl solution introduced into the central chamber 12 is diluted as reactions occur at the electrodes 18, 20 because the Na cations cross (arrows g) the cation exchange membrane 28 reacting to the increase in the concentration of OH.sup.?, while Cl.sup.? anions cross (arrows h) the anion exchange membrane 26 reacting to the increase in the concentration of H.sub.3O.sup.+. At the outlet 32 of the central chamber 12, a dilute NaCl solution of about 35 g/l exits. The Cl.sup.? ions entering the anode chamber 14 are oxidised at anode 18 forming Cl.sub.2 chlorine. Chlorine reacts with water to form HCl and HClO, here about 1 molar, and exits (arrow i) from outlet 36.

[0107] The diluted saline solution exiting the central chamber 12 is admixed with brine of similar concentration (arrow j) and pumped with a pump 60 into a reverse osmosis plant 62 from which a fresh water fraction (arrow k) and a concentrated NaCl solution fraction (70 g/L) are obtained and pumped with a pump 64 into the central chamber 12 forming the NaCl flow (arrow b).

[0108] FIG. 2 illustrates individual elements of an electrochemical cell usable in the plant according to FIG. 1. From left to right can be observed a first plate 166 with an anode chamber 114, a second plate 168 with a central chamber 112, and a third plate 170 with a cathode chamber 116. Each chamber follows a meandering or serpentine path.

[0109] In a three-chamber base cell which stands alone or in an isolated form according to the above variants (A) or (B) in a succession of a plurality of three-chamber units, an anion exchange membrane 127 is at all times interposed between the anode chamber 114 and central chamber 112 and between the central chamber 112 and cathode chamber 116 a cation exchange membrane 128. In the left-hand plate 166 the reference number 127 indicates the anion exchange membrane placed above the anode chamber 114; in the plate 170 on the right the reference number 128 indicates the cation exchange membrane placed above the cathode chamber 116, while in the plate 168 in the centre the reference number 126 indicates the set of the anion and cation exchange membranes (also represented individually in the drawings on the side) that include the central chamber 112 as in a sandwich.

[0110] By placing the second plate 168 above the first plate 166 and the third plate 170 above the second plate 168 with the anion exchange membrane 127 between the first 166 and second 168 plate, and the cation exchange membrane 128 between the second 168 and third 170 plate, a base electrochemical cell is obtained. To connect one plate to the other, a plurality of holes 172 is provided along the edge of each plate that serve to pass relative fixing means. By repeating the construction of electrochemical cells several times and placing one cell on top of the other, according to the sequences illustrated above, a stack of electrochemical cells is obtained that can be connected to work together. In this regard, each plate is provided with two three-hole assemblies, one for the outlets of the relative chambers 114, 112 and 116, and one for the inlets of the relative chambers. The anode chamber 114 of the first plate 166 connects at its ends to the outlet hole 136 and to the inlet hole 134; the central chamber 112 connects at its ends to the outlet hole 132 and to the inlet hole 130; and the cathode chamber 116 connects at its ends to the outlet hole 140 and to the inlet hole 138. Thus, in a stack of electrochemical cells, the outlets and inlets of the individual chambers are connected together, creating a unique flow of anodic solution, saline solution and cathodic solution between the chambers of the same category (anodic, central or cathodic).

[0111] Furthermore, there are holes 146, 147 on each plate that can be aligned in the stack that serve for the degassing of the anode and cathode chambers where gases are produced, in particular in the case of the connected cathode chambers 116, the relative holes 146 serve to create channels to convey the hydrogen formed.

[0112] The plates are made in the form of chips and have, by way of example, a thickness of approximately 6 mm.