METHOD AND ELECTROLYSIS DEVICE FOR THE PRODUCTION OF CHLORINE, CARBON MONOXIDE AND OPTIONALLY HYDROGEN

Abstract

The invention relates to a method and an electrolysis device for the production of chlorine, carbon monoxide and optionally hydrogen via the electrochemical conversion of carbon dioxide and alkali chloride solution, wherein the carbon dioxide from a carbon dioxide gas source (55) is electrochemically reduced at a gas diffusion electrode, designed as a cathode (11), in an aqueous alkali-chloride-containing solution as the catholyte (17), and chlorine is simultaneously anodically generated from an aqueous alkali-chloride-containing solution as the anolyte (15a).

Claims

1.-22. (canceled)

23. A process for preparing carbon monoxide, optionally hydrogen and chlorine by electrochemical reaction of carbon dioxide and alkali metal chloride solution, wherein the carbon dioxide is electrochemically reduced at a gas diffusion electrode as cathode in an aqueous alkali metal chloride-containing solution as catholyte and, at the same time, chlorine is produced anodically from an aqueous alkali metal chloride-containing solution as anolyte (15a), where the alkali metal salt of carbonic acid formed in the catholyte, selected from alkali metal carbonate, alkali metal hydrogencarbonate or mixtures thereof, is then reacted with hydrogen chloride to give carbon dioxide and alkali metal chloride, and the carbon dioxide (38a) released is returned to the cathode space for the gas diffusion electrode and the alkali metal chloride produced is returned either to the anode space and/or to the cathode space.

24. The process as claimed in claim 23, wherein the hydrogen chloride is taken from a connected process for preparing isocyanates via phosgene as intermediate, and the chlorine formed in the electrochemical reaction is recycled into the phosgene production as precursor for isocyanate production.

25. The process as claimed in claim 23, wherein the hydrogen formed together with CO as optional by-product from the electrochemical reaction is separated from the mixture of hydrogen, CO and CO.sub.2 and utilized.

26. The process as claimed in claim 25, wherein the hydrogen is utilized for preparation of diamines as precursor for the isocyanate production process.

27. The process as claimed in claim 23, wherein the alkali metal chloride used for anolyte and catholyte is potassium chloride.

28. The process as claimed in claim 23, wherein the concentration of the alkali metal chloride solution of the anolyte and/or of the catholyte is independently up to 25% by weight.

29. The process as claimed in claim 23, wherein the temperature of the catholyte in the feed to the electrochemical reaction is at least 60 C.

30. The process as claimed in claim 23, wherein the electrochemical conversion of CO.sub.2 is conducted on an industrial scale by the membrane electrolysis method at a gas diffusion electrode as cathode.

31. The process as claimed in claim 23, wherein the CO.sub.2 is fed to the gas diffusion electrode via a gas space divided from the electrolyte space by the gas diffusion electrode.

32. The process as claimed in claim 31, wherein the gas velocity in the gas space close to the reverse side of the gas diffusion electrode is from 0.001 to 15 m/s.

33. The process as claimed in claim 30, wherein the drift velocity of the catholyte in the interspace between ion exchange membrane and gas diffusion electrode is from 0.8 to 10 cm/s.

34. An electrolysis apparatus for electrochemical conversion of carbon dioxide and alkali metal chloride solution by process as claimed in claim 23, at least comprising (i) at least one carbon dioxide gas source and (ii) at least one electrolysis cell, at least comprising a cathode half-shell having a cathode, a catholyte feed, a catholyte drain, and a gas space in fluid connection to the carbon dioxide gas source via a first gas inlet, and connected to a first gas outlet for gaseous reaction product-containing gas, further comprising an anode half-shell, wherein the anode half-shell has been provided at least with a second gas outlet for the anode reaction product, especially chlorine and optionally oxygen, an anolyte feed for the introduction of an aqueous alkali metal chloride-containing solution as anolyte and an anolyte drain, and an anode, and a separator disposed between the anode half-shell and cathode half-shell, for separation of anode space and cathode space, further comprising electrical power leads for connection of anode and cathode to a DC voltage source, wherein the cathode is designed as a gas diffusion electrode for conversion of carbon dioxide gas, and cathode, anode and the separator are arranged with their main extent vertically, and a gap as electrolyte space for passage of the catholyte by the principle of a falling liquid film is disposed between separator and cathode.

35. The electrolysis apparatus as claimed in claim 34, wherein the separator is an ion exchange membrane or a diaphragm.

36. The electrolysis apparatus as claimed in claim 34, wherein the vertical main extent of the cathode is at least 30 cm.

37. The electrolysis apparatus as claimed in claim 34, wherein the cathode is in a compact design as a gas diffusion electrode based on silver and/or silver oxide, as electrocatalyst and with a pulverulent fluoropolymer, as nonconductive binder on a metallic or nonmetallic, conductive or nonconductive support.

38. The electrolysis apparatus as claimed in claim 34, wherein the first gas outlet is connected at the upper end of the gas space and the second gas outlet at the upper end of the anode space, and the first gas inlet is connected at the lower end of the gas space.

39. The electrolysis apparatus as claimed in claim 34, wherein the second gas outlet for the anode reaction product is connected to a second gas separation unit for separation of oxygen from chlorine from the anode gas.

40. The electrolysis apparatus as claimed in claim 34, wherein the first gas outlet is connected, especially via a collecting conduit, to a first gas separation unit for separation of carbon monoxide, hydrogen and unconsumed carbon dioxide gas.

41. The electrolysis apparatus as claimed in claim 40, wherein the first gas separation unit has a recycle conduit for carbon dioxide gas separated off, connected to the first gas inlet for carbon dioxide gas especially via a distributor pipe conduit.

42. The electrolysis apparatus as claimed in claim 40, wherein the gas separation unit has an outlet for carbon monoxide separated off, connected to a chemical production plant for chemical conversion of carbon monoxide and chlorine to phosgene.

43. The electrolysis apparatus as claimed in claim 34, wherein a flow retarder for the catholyte stream is provided in the gap, where the flow retarder takes the form of an electrically nonconductive, chemically inert textile fabric.

44. The electrolysis apparatus as claimed in claim 34, wherein the catholyte drain and the anolyte drain are connected directly or indirectly via pipe conduits to an electrolyte collector, the electrolyte collector is provided via pipe conduit with a carbonate breakdown unit, and the carbonate breakdown unit at least with a recycle line for dissociated carbon dioxide, a controllable feed for hydrogen chloride and a recycle conduit for electrolyte, and the recycle conduit is connected both to the catholyte feed and to the anolyte feed.

Description

[0097] The invention is illustrated in detail by way of example hereinafter with reference to the figures. The figures show:

[0098] FIG. 1 a schematic overview of the overall process and of the elements of the electrolysis apparatus usable for performance

[0099] FIG. 2 a schematic vertical cross section through an electrolysis cell Z of the electrolysis apparatus

[0100] In the figures, reference symbols have the following meaning: [0101] Z electrolysis cell [0102] 1 cathode half-shell [0103] 2 anode half-shell [0104] 3 separator (diaphragm, ion exchange membrane) [0105] 4 gas space (cathode) [0106] 5 first gas inlet for carbon dioxide (cathode space) [0107] 6 first gas outlet for gaseous reaction products (cathode space) [0108] 7 second gas outlet for anode reaction product [0109] 8 anolyte feed [0110] 9 chlorine-containing anolyte output to gas collection pipe conduit 20 [0111] 10 anode [0112] 11 cathode (gas diffusion electrode) [0113] 12 catholyte space (electrolyte space) [0114] 12a catholyte gap (electrolyte space) [0115] 13 catholyte feed [0116] 14 catholyte drain [0117] 14a CO/H.sub.2-containing catholyte output from collection pipe conduit for catholyte 65 to gas separation unit 66 [0118] 15 anode space [0119] 15a anolyte [0120] 16 cathode space [0121] 17 catholyte [0122] 18 feed of water or concentrated or diluted electrolyte for concentration adjustment [0123] 18a electrolyte discharge [0124] 20 collection pipe conduit for anolyte & Cl.sub.2 gas mixture [0125] 22 Cl.sub.2 gas drying [0126] 23 anolyte dechlorination [0127] 24 flow resistance [0128] 25 Cl.sub.2O.sub.2 gas separation unit [0129] 25a cleaned Cl.sub.2 gas [0130] 25b Cl.sub.2/O.sub.2 tail gas [0131] 31 cathode power lead [0132] 32 anode power lead [0133] 34 distributor channel [0134] 35 electrically conductive top structure [0135] 36 elastic electrically conductive connection between GDE 11 and top structure 35 [0136] 38 carbonate breakdown unit [0137] 38a conduit for CO.sub.2 stream from carbonate breakdown unit [0138] 38b recycle conduit for alkali metal chloride solution from the carbonate breakdown unit [0139] 40 anolyte distributor pipe conduit [0140] 42 anolyte heat exchanger [0141] 44 distributor pipe conduit for CO.sub.2 [0142] 46 metered addition of acid or base to adjust the anolyte pH [0143] 50 catholyte distributor pipe conduit [0144] 53 recycle conduit for carbon dioxide gas separated off [0145] 54 catholyte heat exchanger [0146] 55 carbon dioxide gas source [0147] 56 metered addition of acid or base to adjust the catholyte pH [0148] 60 isocyanate production [0149] 61 phosgene synthesis [0150] 62 HCl gas feed [0151] 65 catholyte collection pipe conduit [0152] 66 CO/H.sub.2 gas separation unit [0153] 66a separated CO/H.sub.2 gas from the catholyte from gas separation unit 66 [0154] 66b cleaned catholyte [0155] 67 electrolyte collection apparatus [0156] 68 collection pipe conduit for CO/CO.sub.2/H.sub.2 gas mixture [0157] 68a mixture of CO.sub.2, CO, H.sub.2 [0158] 69 CO.sub.2 gas separation for separation of CO.sub.2, H.sub.2 and unconverted CO.sub.2 [0159] 70 CO/H.sub.2 gas separation [0160] 70a carbon monoxide gas [0161] 70b hydrogen gas [0162] 90 electrically conductive connection of anode to anode half-shell [0163] 91 electrically conductive connection of GDE 11, elastic structure 36, electrically conductive top structure 35 to the cathode half-shell 1 [0164] 100 electrolyzer with electrolysis cells Z (n), n=number of electrolysis cells Z

EXAMPLES

More General Illustrative Description of the Process and the Electrolysis Apparatus

[0165] An electrolyzer 100 is equipped with a number of 30 to 100 electrolysis cells (Z)also called electrolysis cells by way of simplification hereinafterper electrolyzer frame. An electrolysis cell Z (FIG. 2) consists of an anode half-shell 2 and a cathode half-shell 1, and these are separated from one another in each case by an ion exchange membrane 3. The anode half-shell 2 is equipped with an anode 10 having a commercial coating based on a mixed oxide of ruthenium and indium for chlorine production (DSA coating, Denora Deutschland). The cathode 11 is set up as a silver/PTFE-based gas diffusion electrode from Covestro Deutschland AG. The gas diffusion electrode 11 separates the gas space 4 from an electrolyte space 12. The electrolyte space 12 is bounded by the gas diffusion electrode 11 and the ion exchange membrane 3, and forms a gap 12a. The gap 12a is filled with a PTFE-based fabric 24 that acts as flow resistance for the catholyte 17, which flows from the top downward through the gap 12a filled with the fabric 24. The catholyte 17 collects at the base of the cathode half-shell 1 and leaves it via a preferably flexible pipe connection and is guided to a collection pipe conduit (catholyte) 65. In order to prevent gases from leaving the cathode half-shell via the pipe connection, the pipe connection is executed such that it is guided into the collection pipe conduit (catholyte) 65 in an immersed manner.

[0166] Anode half-shell 2 and cathode half-shell 1 are separated from one another by a separator 3, an ion exchange membrane 3. It is possible here to use commercial perfluorinated ion exchange membranes of the Asahi Glass F8080 type (manufacturer: Asahi Glass) or Chemours N2050 (manufacturer: Chemours). This prevents the chlorine produced at the cathode 11 from being reduced again and any carbon monoxide produced at the cathode from being oxidized at the anode 10. Likewise prevented here is the mixing of chlorine with hydrogen or CO, which is necessary for safety reasons (risk of hydrogen chloride gas explosion).

[0167] Electrical contact in the case of monopolar connection of the electrolysis elements to a DC voltage source (not shown) is from the anode 10 via an electrically conductive connection 90 to the anode half-shell 2, and from the anode half-shell 2 via an anode power lead 32. From the cathode 11, electrical contact is via an elastic electrically conductive mat 36, further via an electrically conductive top structure 35, and further via an electrically conductive connection 91, to the cathode half-shell 1 and thence to the DC voltage source. The gas space 4 of the cathode half-shell 1 is supplied via a distributor pipe conduit 44 with CO.sub.2 (see FIG. 1), and then unconverted CO.sub.2 and the reaction products from the cathode half-shell 1 are fed via a preferably flexible connection, connected to outlet 6, to a collection pipe conduit 68 for CO/CO.sub.2 gas mixture.

[0168] The catholyte 14a from the collection pipe conduit (catholyte) 65 is sent to a CO.sub.2/CO/H.sub.2 gas separation unit 66 in order to separate the still dissolved or dispersed gases such as CO.sub.2, CO and any H.sub.2 from the catholyte 14a. The separation can be effected, for example, by means of a stripping column with the aid of an inert gas. The stripped gas mixture 66a (tail gas) is sent, for example, to an incineration. The cleaned catholyte 66b is sent to an electrolyte collection apparatus 67 or directly to a carbonate breakdown unit 38.

[0169] The gas mixture withdrawn from the gas space 4 of the cathode half-shell 1 is sent to a collection pipe conduit for CO.sub.2/CO/H.sub.2 gas mixture 68. Subsequently, excess or unconverted CO.sub.2 is separated from the gas mixture (CO.sub.2 separation 69). This can be effected, for example, by an amine scrub. The CO.sub.2 gas separated off is fed via recycle conduit 53 to the gas space 4 of the cathode half-shell 1, again via the CO.sub.2 distributor pipe conduit 44. The CO.sub.2 converted is replenished here by an appropriate amount of fresh CO.sub.2 from a carbon dioxide gas source 55.

[0170] The anode half-shell 2 is supplied with the anolyte 15a via a distributor pipe conduit (anolyte) 40. In the anode half-shell 2, chlorine is produced at the anode 10 from an aqueous alkali metal chloride solution. As a side reaction, a small amount of oxygen may be formed as well as the chlorine at the anode. The mixture of Cl.sub.2, any O.sub.2 and the anolyte is withdrawn from the anode half-shell 2 via outlets 7 and 9 and sent to a collection pipe conduit for anolyte & Cl.sub.2 gas mixture 20. The Cl.sub.2 still containing oxygen and water vapor is withdrawn from the collection pipe conduit 20, and sent to a Cl.sub.2 drying 22, for example by sulfuric acid drying. Depending on the required purity of the chlorine, a further optional purification is connected downstream. For instance, the Cl.sub.2 can be sent to the Cl.sub.2 gas separation unit 25 in order to separate residues 25b of O.sub.2 with traces of chlorine. This can be effected, for example, by means of recuperative liquefaction. Thereafter, the chlorine can be compressed and/or liquefied, and/or sent to a chemical synthesis (e.g. phosgene synthesis 61).

[0171] A portion of the Cl.sub.2 removed, 25a, is thus sent to the phosgene synthesis 61 as precursor for isocyanate production 60.

[0172] The cleaned anolyte from the collection pipe conduit 20 is sent to a dechlorination unit 23 for removal of compounds in which chlorine is present in an oxidation state greater than zero (active chlorine). This can be effected either by a vacuum dechlorination and/or a chemical dechlorination by addition of an alkali metal-containing bisulfite solution or by addition of hydrogen peroxide. After the dechlorination, the active chlorine content of the anolyte should preferably be less than 20 ppm.

[0173] The dechlorinated anolyte is sent to the electrolyte collection apparatus 67. The collected electrolytes in the electrolyte collection apparatus, as well as the alkali metal chloride, contain the alkali metal hydrogencarbonate or carbonate formed, and are sent to the carbonate breakdown unit 38. The carbonate breakdown unit 38 is supplied with hydrogen chloride 62 from isocyanate production 60, with reaction of the hydrogen chloride 62 with the alkali metal carbonate or alkali metal hydrogen carbonate present in the electrolyte to give alkali metal chloride, water and CO.sub.2. It is optionally possible to add a stoichiometric excess of hydrogen chloride 62. The separated CO.sub.2 is fed back to the gas space 4 of the cathode half-shell together with that from the CO.sub.2 separation, the amine scrub 69 and the CO.sub.2 to be replenished from a carbon dioxide gas source 55 via the distributor pipe conduit for CO.sub.2 44.

[0174] The electrolyte from the carbonate breakdown unit 38 is fed back to the anode half-shell 2 after adjustment of pH by feeding in mineral acid/alkali 46, heating in the heat exchanger 42, via the distributor pipe conduit 40. The temperature of the electrolyte supplied to the anode half-shell is more than 50 C. downstream of the heat exchanger. The pH of the alkali metal chloride solution supplied to the anode half-shell is between 2 and 8; the concentration of alkali metal chloride is 14% by weight to 23% by weight.

[0175] In addition, the electrolyte from the CO.sub.2 carbonate breakdown unit 38 is fed back to the cathode half-shell 1 via a pH adjustment by feeding in mineral acid/alkali 56 and a heat exchanger 54 via the distributor channel (catholyte) 50. The pH of the electrolyte supplied to the cathode half-shell 1 is between 6 and 14; the temperature is greater than 50 C. The concentration of alkali metal chloride corresponds to that of the electrolyte supplied to the anode half-shell.

[0176] The phosgene produced from the phosgene synthesis 61 is used for the production of isocyanates 60, by reacting it with an appropriate, for example aromatic, amine. If the amine is prepared from an aromatic nitro compound, it can be reduced using the hydrogen 70b that has been produced in the CO/H.sub.2 gas separation 70 and separated off.

[0177] A portion of the HCl gas obtained in the isocyanate production 60 is fed to the carbonate breakdown unit 38. The alkali metal carbonate, and possibly also alkali metal hydrogencarbonate, formed by the hydroxide ions formed and CO.sub.2 in the cathode half-shell are converted here to alkali metal chloride, water and CO.sub.2. The CO.sub.2 is sent here to the distributor pipe conduit for CO.sub.2 44.

[0178] The concentration of anolyte and catholyte can be adjusted by addition of water or alkali metal chloride salt or by diluted or concentrated salt solutions 18. For avoidance of the accumulation of impurities or of sulfate formed in the dechlorination by addition of bisulfite in the anolyte dechlorination 23, it is possible to dispose of a portion of the electrolyte. This is effected via a disposal conduit 18a.

Example (Inventive)

[0179] An electrolysis cell Z (see FIG. 2) having an area of 2.5 m.sup.2, formed from an anode half-shell 2 and a cathode half-shell 1, is separated by an ion exchange membrane 3 of the Nafion 982 WX type.

[0180] The anode 10 consists of expanded titanium metal with a customary coating set up for the preparation of chlorine (with mixed Ru/Ir oxide) for chlorine production from Denora (DSA coating).

[0181] The cathode, a gas diffusion electrode (GDE) 11, is installed vertically into the cell Z by the principle of falling-film cell technology, in which the GDE 11 lies on an elastically mounted flow-guiding element 36, which in turn rests on an electrically conductive support structure 35 with openings for access of gas. The GDE 11 is a silver- and PTFE-based GDE on a silver metal mesh from Covestro Deutschland AG (in accordance with published specification EP1728896A2) for chloralkali electrolysis. Between GDE 11 and membrane 3, a PTFE-based two-dimensional fabric 24 was used as flow resistance, through which the catholyte 17 flows from the top downward in free fall.

[0182] The current density in electrolysis operation is 3 kA/m.sup.2.

[0183] The anode space 15 is supplied via a distributor pipe 40 with 229.02 kg/h of an electrolyte (anolyte 15a) consisting of 2.68 kg/h of K.sub.2SO.sub.4, 45.8 kg/h of KCl and 180.5 kg/h of water with a pH of 7 and a temperature of 80 C. In addition, the chemical dechlorination described below, in steady-state operation of the process, establishes a proportion of 2.68 kg/h K.sub.2SO.sub.4 in the anolyte 15a.

[0184] Withdrawn from the anode space 15 via the outlet 9 are 9.92 kg/h of Cl.sub.2 and 0.58 kg/h of O.sub.2, and also 193.05 kg/h of electrolyte with 24.9 kg/h of KCl, 10.4 g/h of active chlorine and 165.4 kg/h of H.sub.2O and, in steady-state operation, 2.68 kg/h of K.sub.2SO.sub.4, and these are fed to the collection pipe conduit 20.

[0185] The electrolyte (spent anolyte) taken from the anode space 15 has a pH of 3.5 and is sent to an anolyte dechlorination 23 which consists, in a first stage, of a vacuum dechlorination, which removes 10.3 g of the Cl.sub.2 formed. The spent anolyte thus treated is sent to a second stage of anolyte dechlorination 23 and here to a pH of 9 by addition of 19 g of an 18% by weight potassium hydroxide solution and then sent to a chemical dechlorination in which 0.5 g of a 38% by weight KHSO.sub.3 solution is added to the anolyte.

[0186] For avoidance of accumulation of K.sub.2SO.sub.4 which is formed in the chemical dechlorination of the anolyte, 74.5 g/h of the treated anolyte is discharged (conduit 18a) and discarded.

[0187] After the chemical dechlorination, the pH of the treated anolyte is lowered to 3.5 by addition of 13.4 g of 18% by weight hydrochloric acid.

[0188] The anolyte thus treated can be sent to a storage vessel, the electrolyte collection apparatus 67, or fed directly to the carbonate breakdown unit 38.

[0189] The cathode space 16 of the electrolysis cell Z is supplied with 618 kg/h of a catholyte 17 having a temperature of 72 C., consisting of 7.25 kg/h of K.sub.2SO.sub.4, 123.6 kg/h of KCl and 487.15 kg/h of H.sub.2O. The gas space 4 in the cathode half-shell 1 is supplied with 30.34 kg/h of CO.sub.2.

[0190] Withdrawn from the cathode space 16 is 651.6 kg/h of catholyte consisting of 123.6 kg/h of KCl, 19.3 kg/h of K.sub.2CO.sub.3, 7.25 kg/h of K.sub.2SO.sub.4 and 501.4 kg/h of water. The catholyte output had a temperature of 87.3 C.

[0191] Additionally withdrawn from the gas space 4 in the cathode half-shell 1 is 22.75 kg/h of gas consisting of 2.66 kg/h of CO, 0.09 kg/h of H.sub.2 and 20 kg/h of CO.sub.2.

[0192] The cathodic conversion of CO.sub.2 to CO was 68%.

[0193] The anodically produced Cl.sub.2 (9.92 kg/h), after drying 22 and removal 25 of O.sub.2, together with the CO produced and dried and further CO, was converted to phosgene and sent to isocyanate production 60.

[0194] The HCl gas 62 separated from the isocyanate production 60 was sent to the carbonate breakdown unit 38 (10.2 kg/h).