Title of Invention	"A PROCESS FOR THE PRODUCTION OF CHLORINE FROM HYDROGEN CHLORIDE AND OXYGEN"
Abstract	A process for the production of chlorine by means of multi-stage oxidation by thermal reaction of hydrogen chloride with oxygen using catalysts or by non-thermal activated reaction of hydrogen chloride with oxygen, is described, in which the gas mixture forming in the reaction, consisting of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents, in particular carbon dioxide and nitrogen, a) is cooled to condense hydrochloric acid and b) the aqueous solution of hydrogen chloride (hydrochloric acid) forming is separated off from the gas mixture, characterised in that c) the hydrochloric acid separated off is fed at least partially to an electrochemical oxidation in which at least part of the aqueous hydrochloric acid is oxidised to chlorine, d) the chlorine gas occurring in step d) is optionally added to the gas mixture occurring in step c), e) the residues of water present in the gas mixture from steps c) and e), in particular by washing with sulfuric acid, are removed, f) the chlorine-rich gas mixture forming is freed of oxygen and optionally of secondary constituents.

Title of Invention

"A PROCESS FOR THE PRODUCTION OF CHLORINE FROM HYDROGEN CHLORIDE AND OXYGEN"

Abstract

A process for the production of chlorine by means of multi-stage oxidation by thermal reaction of hydrogen chloride with oxygen using catalysts or by non-thermal activated reaction of hydrogen chloride with oxygen, is described, in which the gas mixture forming in the reaction, consisting of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents, in particular carbon dioxide and nitrogen, a) is cooled to condense hydrochloric acid and b) the aqueous solution of hydrogen chloride (hydrochloric acid) forming is separated off from the gas mixture, characterised in that c) the hydrochloric acid separated off is fed at least partially to an electrochemical oxidation in which at least part of the aqueous hydrochloric acid is oxidised to chlorine, d) the chlorine gas occurring in step d) is optionally added to the gas mixture occurring in step c), e) the residues of water present in the gas mixture from steps c) and e), in particular by washing with sulfuric acid, are removed, f) the chlorine-rich gas mixture forming is freed of oxygen and optionally of secondary constituents.

Full Text	Process for the production of chlorine from hydrogen chloride and oxygen The invention is based on known processes for the production of chlorine by thermal reaction of hydrogen chloride with oxygen using catalysts, or by non-thermal activated reaction of hydrogen chloride with oxygen, in which the gas mixture formed during the reaction, consisting at least of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents such as carbon dioxide and nitrogen, is cooled to condense hydrochloric acid, the liquid hydrochloric acid formed is separated off from the gas mixture, the residues of water remaining in the gas mixture in particular are removed by washing with concentrated sulfuric acid. The invention relates especially to the combination with other HC1 oxidation processes. Hydrogen chloride occurs as a by-product in the production of a plurality of chemical reactions with chlorine and or phosgene, for example the production of isocyanates or the chlorination of aromatics. This hydrogen chloride can be again converted by electrolysis or by oxidation with oxygen to chlorine which is again used in the chemical reactions. The oxidation of hydrogen chloride (HC1) to chlorine (C12) takes place by reaction of hydrogen chloride and oxygen (O2) according to 4HC1 + O2 →2 Cl2 + 2 H2O The reaction can be carried out in the presence of catalysts at a temperature of approx. 250 to 450°C. The normal reaction pressure is in the range of 1 to 10 bar. Various embodiments of the process which is generally known as the Deacon process, are described : Shell-Chlor process, MT-Chlor process, KEL-Chlor process, Carrier Catalyst process and Sumitomo-Chlor process. Suitable catalysts for the Deacon process contain transition metal compounds such as copper and ruthenium compounds or even compounds of other noble metals such as gold and palladium. Such catalysts are described for example in specifications DE 1 567 788 Al, EP 251 731 A2, EP 936 184 A2, EP 761 593 Al, EP 711 599 Al and DE 102 50 131 Al. The catalysts are as a rule applied to a support. These supports consist for example of silicon dioxide, aluminium oxide, titanium dioxide or zirconium oxide. The Deacon processes are carried out as a rule in fluid-bed reactors or fixed-bed reactors, e.g. in multi-tube reactors. Hydrogen chloride is freed of impurities before the reaction in order to avoid poisoning the catalysts used. Alternatively, processes in which the reaction of hydrogen chloride with oxygen is non-thermally activated, are known. Such processes are described in "W. Stiller, Nichtthermische aktivierte Chemie, Birkhauser Verlag, Basel, Boston, 1987, pp 33-34, pp 45-49, pp 122-124, pp 138-145". Special embodiments are disclosed for example in specification RU 2253607, JP-A-59073405, DD-A-88 309 and SU 1801943 Al. By non-thermally activated reactions is understood for example excitations of the reaction with the following means or processes : energy radiation, e.g. laser radiation, photochemical radiation sources, UV radiation, infrared radiation, etc - a low-temperature plasma, e.g. created by electrical discharge - magnetic field excitation - tribomechanical activation, e.g. excitation by shock waves ionising radiation, e.g. gamma-ray and X-ray radiation, a- and ß-rays from nuclear disintegration, high-energy electrons, protons, neutrons and heavy ions microwave irradiation Oxygen is used both in the thermal and in the non-thermal activated reaction of hydrogen chloride with oxygen as a rule as a pure gas with an O2 content of > 98 vol.%. It is common to all known processes that in the reaction of hydrogen chloride with oxygen, a gas mixture is obtained which in addition to the target product chlorine also contains water, unreacted hydrogen chloride and oxygen, and other secondary constituents such as carbon dioxide. To obtain pure chlorine, the product gas mixture is cooled after the reaction until reaction water and hydrogen chloride condense out in the form of concentrated hydrochloric acid. The hydrochloric acid forming is separated off and the remaining gaseous reaction mixture is freed from residual water by washing with sulfuric acid or other methods such as drying with zeolites. The hydrochloric acid separated off is preferably fed to a desorption stage in which gaseous hydrogen chloride is again released. This gaseous hydrogen chloride can be partially or preferably completely returned to the reaction of hydrogen chloride with oxygen. The dilute hydrochloric acid occurring in the desorption stage can be returned to the hydrochloric acid condensation stage. Here the dilute hydrochloric acid serves as an absorbing agent for the gaseous hydrogen chloride to be separated off. A procedure of this type is described for example in specification DE 102 35 476 Al. Alternatively, the hydrochloric acid separated off can also be fed to recycling. The chlorine-containing reaction gas mixture freed from residual water is then compressed, wherein oxygen and other gas constituents can remain in the gas phase and be separated off from the liquefied chlorine. Processes of this type for obtaining pure chlorine from gas mixtures are described for example in Offenlegungsschrift DE 195 35 716 Al and Offenlegungsschrift DE 102 35 476 Al. The now purified chlorine is then fed for use, for example in the production of isocyanates. A substantial disadvantage of these processes is the comparatively high expenditure of energy for the desorption of hydrogen chloride from the hydrochloric acid occurring in the purification of the chlorine gas stream after the reaction. Recycling of the occurring hydrochloric acid in other processes, however, is not as a rule economic. It has now been found that the disadvantages can be overcome if the hydrochloric acid occurring in the purification of the chlorine gas stream after the reaction is fed to an electrochemical oxidation in which at least part of the hydrochloric acid is oxidised to chlorine. The subject matter of the invention is a process for the production of chlorine by means of multi-stage oxidation by a) thermal reaction of hydrogen chloride with oxygen using catalysts or by non thermal activated reaction of hydrogen chloride with oxygen, in which the gas mixture forming in reaction a), consisting of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents, in particular carbon dioxide and nitrogen, b) is cooled to condense hydrochloric acid and c) the aqueous solution of hydrogen chloride (hydrochloric acid) forming is separated off from the gas mixture, characterised in that d) the hydrochloric acid separated off is fed at least partially to an electrochemical oxidation in which at least part of the aqueous hydrochloric acid is oxidised to chlorine, e) the chlorine gas occurring in step d) is optionally added to the gas mixture occurring in step c), f) the residues of water present in the gas mixture from steps c) and e), in particular by washing with sulfuric acid, are removed, g) the chlorine-rich gas mixture forming is freed of oxygen and optionally of secondary constituents. The process is preferably carried out continuously because an at the same time possible batch or semi-batch operation is industrially somewhat more costly than the continuous process. In a preferred embodiment, hydrogen chloride is separated off completely from the gas mixture as hydrochloric acid according to steps b) and c). Separation of the hydrogen chloride from the gas mixture according to step b) and c) takes place particularly preferably by additional washing of the gas mixture with water or dilute hydrochloric acid, in particular with water. The electrochemical oxidation of the hydrochloric acid according to step d) can be carried out in accordance with the membrane process or in accordance with the diaphragm process, in particular in a two-chamber electrolysis cell consisting of an anode chamber and a cathode chamber, or in particular in a three-chamber electrolysis cell consisting of an anode chamber, cathode chamber and an electrolyte chamber between the anode and cathode chamber. A two-chamber electrolysis cell is preferably selected. In the membrane process, the anode chamber is separated from the cathode chamber by an ion exchange membrane (hereafter referred to simply as membrane), in particular a cation exchange membrane, and in the diaphragm process the cathode and anode chambers by a diaphragm. The distance of the electrodes (anode and cathode) from the diaphragm or the membrane is preferably 0 to 3 mm, particularly preferably 0 to 2 mm. Suitable ion exchange membranes are commercially available, e.g. single-layer ion exchange membranes with sulfonic acid groups. A membrane of the Nafion® 324 or Nafion® 117 type from the company DuPont for example can be used. A woven diaphragm according to DE 3 321 159 Al for example can be used as diaphragm. Threads made of plastic can be used for the fabric. Thus for example fabrics made of polyvinyl chloride (PVC) or polyvinylidene fluoride (PVDF) or even made of mixed fabrics with PVC and PVDF threads can be used. Warp threads or weft threads can consist, in addition to monofilament threads, of multifilament threads, as described in DE 3 321 159 A. Depending on the weaving of the diaphragm, the fabric can be compressed to optimise the gas permeability, e.g. by calendering. Electrodes based on graphite can be used in the electrolysis of hydrochloric acid in accordance with the diaphragm process or the membrane process, the anode and/or the cathode preferably substantially consisting of graphite. Bipolar electrodes made of graphite are particularly preferably used. A particularly advantageous embodiment of the cathodes is that in particular the cathodes but also the anodes made of graphite are equipped with a coating containing noble metal, for example iridium, e.g. according to DE 4 417 744 Al. The graphite anodes have in particular a geometric shape as is known e.g. from DE 3 041 897 A. The cathodes preferably have a similar structure to the anodes. The shape of the anode and/or cathode preferably has vertically arranged grooves, flutes, notches, depressions or similar. These grooves serve substantially to lead off upwards the gas forming during electrolysis, i.e. chlorine and hydrogen, from the narrow gap between the respective electrode and the diaphragm or the membrane. Particularly preferably the grooves have a depth of 5 to 35 mm, very particularly preferably 15 to 25 mm and a width of preferably 1 to 5 mm. The distance between two adjacent grooves substantially arranged parallel to one another is preferably 4 to 6 mm. In a further possible embodiment of the grooves, the depth and/or width of the grooves vary along their length. Thus the depth of the grooves can be 12 to 15 mm at the lower end of the grooves and 20 to 30 mm at the upper end of the grooves. The hydrochloric acid is used in a preferred diaphragm process as an electrolyte both in the anode chamber and in the cathode chamber. During electrolysis, chlorine is produced on the anode and hydrogen on the cathode. A preferred mode of operation of electrochemical oxidation of hydrochloric acid in which hydrogen is developed cathodically consists in metal ions from the group of platinum metals, preferably platinum and/or palladium, being added to the hydrochloric acid which serves as electrolyte in the cathode chamber. Thus for example solutions of hexachloroplatinic (IV) acid (H2PtCl6) or solutions of disodium tetrachloropalladate (II) (Na2PdC14) or even mixtures thereof are added. Addition can take place continuously or discontinuously. The addition of metal ions to the hydrochloric acid in the cathode chamber serves to maintain a low electrolysis voltage in the range of 1.6 to 2.1 V, compared with 2.2 to 2.3 V without addition of metal ions, e.g. at 5 kA/m and 70 to 80°C and a preferably 15 to 25%, particularly preferably approx. 20%, hydrochloric acid. A quantity of metal ions which is sufficient to maintain the electrolysis voltage in the range of 1.8 to 2.1 should therefore be added. This means that as the electrolysis voltage rises during operation the addition of metal ions is increased. Electrolysis according to step d) is preferably carried out at a temperature of 50 to 90°C. The concentration of the aqueous solution of hydrogen chloride to be used in electrolysis is generally preferably 10 to 25 wt.%. In the diaphragm processes, hydrochloric acid of a concentration of 12 to 20 wt.% is particularly preferably used. Electrolysis with diaphragm can be carried out at an absolute pressure of approx. 1 bar taking into consideration a differential pressure between anode and cathode chamber optionally still to be set or even at higher pressure of up to 2 bar. Even higher pressures are generally possible but require a correspondingly higher expenditure in the construction of the electrolysis cell. The differential pressure between anode chamber and cathode chamber is preferably 0 to 10 mbar, particularly preferably 1 to 2 mbar, so that because of the higher pressure on the anode side traces of the chlorine gas formed passes via the diaphragm on the cathode side and thus can mix with the cathodically formed hydrogen. In an alternative embodiment, the electrochemical oxidation of the aqueous solution of hydrogen chloride takes place according to step d) in accordance with the membrane process with a gas diffusion electrode as the cathode. The electrolysis cell can therefore consist both of two chambers and of three chambers, but preferably of two chambers. An oxygen-containing gas, e.g. oxygen, air or air enriched with oxygen, is fed to the cathode half-cell. The oxygen is reduced on the gas diffusion electrode, water being formed. The aqueous hydrogen chloride solution is fed to the anode half-cell, the hydrogen chloride being oxidised to chlorine on the anode. The anode half-cell and the cathode half-cell are separated from one another by a cation exchange membrane. The electrolysis of hydrochloric acid using a gas diffusion electrode as the cathode is described e.g. in WO 00/73538 Al, e.g. a chlorine-resistant plastic. The gas diffusion electrode is particularly preferably based at least on an electrically conductive fabric, woven, knitted fabric, net or nonwoven of carbon which is arranged between a carbon-containing catalyst layer and a gas diffusion layer. The electrolysis cell can in particular consist of either a non-metallic material according to DE 103 47 703 Al or a metallic material. Titanium or a titanium alloy particularly such as a titanium-palladium alloy for example is suitable as a metallic material for the electrolysis cell. The half-shells for the anode and cathode half-cell, the current distributor and the current feeds are thereby manufactured from titanium or a titanium alloy. The anode can be preferably executed according to DE 102 34 806 Al with a particular gas lead-off construction. The anode thereby consists of metal, preferably titanium with a coating of noble metal oxide, e.g. of ruthenium oxide. Furthermore, the anode particularly preferably made of titanium according to specification DE 102 00 072 Al can have an intermediate layer of titanium carbide or titanium boride which is applied to the titanium anode by means of plasma or flame spraying before the coating of a noble metal oxide is applied. According to specification DE 102 34 806 Al, the metal anode in a further preferred embodiment has openings for the passage of the gas formed during electrolysis, the openings having preferably guide structures which lead off the gas formed to the side of the metal anode turned away from the ion exchange membrane. The total cross-sectional area of the openings should thereby be in the range of 20% to 70% of the area which is formed by the height and width of the anode. The metal anode can in addition have a wavy, zigzag or rectangular cross-section. The depth of the anode should thereby be at least 1 mm. The ratio of electrochemically active area of the metal anode to the area which is formed by the height and width of the metal electrode should be at least 1.2. In a special embodiment, the metal anode can consist of two adjacent expanded metals, the expanded metal showing to the ion exchange membrane being more finely structured than the expanded metal turning away from the ion exchange membrane. Furthermore, the finer-structured expanded metal is thereby rolled flat and the coarser-structured expanded metal arranged so that the mesh webs are inclined in the direction of the cathode and serve as guide structures. Alternatively, the anode can also consist of an expanded metal. In principle, the anode should have a free area of 15 to 70%. The thickness of the expanded metals should be selected so that no additional electrical resistance with a bipolar switching of the individual electrolysis cells (cell element) to an electrolyser occurs. The electrical resistance depends substantially on the electrical contacting of the anode, such as for example number of current feed connection elements between the anode and back wall of the anode half-cell. During electrolysis by means of gas diffusion electrode, the anode chamber and the cathode chamber can be separated by a commercial ion exchange membrane. DuPont ion exchange membranes of the Nafion® 324 or Nafion® 117 type for example can be used. A membrane which, as described in WO 05//12596 Al, has the side of which turned towards the gas diffusion electrode has a smooth surface structure, is preferably used. The smooth surface structure of the membrane enables the gas diffusion electrode and the membrane to be adjacent in such a way that at a pressure of 250 g/cm2 and a temperature of 60°C the contact area is at least 50% of the geometric area of the membrane. The cathodic current distributor to which the gas diffusion electrode is applied, is preferably executed according to DE 102 03 689 Al. This has a free area of less than 65%, but more than 5%. The thickness of the current distributor is at least 0.3 mm. It can consist of an expanded metal, net, fabric, foam, nonwoven, slotted sheet or perforated plate of metal. The cathodic current distributor preferably consists of an expanded metal with a mesh length of 4 to 8 mm, a mesh width of 3 to 5 mm, a web width of 0.4 to 1.8 mm and a thickness of 0.4 to 2 mm. In addition, the cathodic current distributor can have a second expanded metal as a support for the first expanded metal. The second expanded metal as support preferably has a mesh length of 10 to 40 mm, a mesh width of 5 to 15 mm, a web width of 2 to 5 mm and a thickness of 0.8 to 4 mm. A net which preferably has a wire thickness of 1 to 4 mm and a mesh width of 7 to 25 mm can also be used as a support. Furthermore, a perforated sheet or slotted sheet which preferably has an open area of less than 60 % and a thickness of 1 to 4 mm can furthermore be used as a support. For example, titanium or a noble metal-containing titanium alloy, such as e.g. titanium-palladium, can be used as a material for the cathodic current distributor. If the current distributor is an expanded metal, this is preferably rolled. A commercial gas diffusion electrode which is fitted with a suitable catalyst can be used as the gas diffusion electrode. Suitable catalysts preferably contain according to WO 00/73538 Al rhodium and/or at least one rhodium sulfide or a mixture of rhodium and at least one rhodium sulfide. According to EP 931 857 Al, rhodium and/or rhodium oxide or mixtures thereof can also be used. The gas diffusion electrode preferably consists of an electrically conductive fabric, paper or nonwoven made of carbon, the fabric, paper or nonwoven having on one side a carbon-containing catalyst layer and on the other side a gas diffusion layer. The catalyst is preferably applied to a support, preferably made of carbon, polytetrafluoroethylene particles being integrated which are coupled to the support structure. The gas diffusion layer consists preferably of carbon and polytetrafluoroethylene particles in which for example the ratio of carbon to PTFE is 50:50. The gas diffusion electrode can for example be arranged so that it is not firmly connected to the ion exchange membrane. Contacting of the gas diffusion electrode to the current distributor and the ion exchange membrane takes place preferably by pressure contact, i.e. the gas diffusion electrode, the current distributor and the membrane are pressed together. The gas diffusion electrode can be connected to the current collector according to DE10148 600A1. The electrolysis of hydrochloric acid in accordance with the membrane process with gas diffusion electrode is preferably carried out at a temperature of 40 to 70°C. The concentration of the aqueous solution of hydrogen chloride in the anode chamber is in particular 10 to 20 wt.%, particularly preferably 12 to 17 wt.%. The cell can be operated for example so that the pressure in the anode chamber is higher than the pressure in the cathode chamber. The cation exchange membrane is thereby pressed on to the gas diffusion electrode and this in turn on a current distributor. Alternatively, a construction of the electrolysis cell which is described in DE 101 38 214A1 can be selected. The anode and/or current distributor are stored elastically, for example in that they are connected by means of springs to the back wall of the respective "half-cell". When assembling the cell, a "zero gap" arrangement is produced where the anode is in direct contact with the ion exchange membrane, this in turn in direct contact with the gas diffusion electrode and this in turn indirect contact with the current distributor. The elastic storage causes the anode, membrane, gas diffusion electrode and current distributor to press together. In a particularly preferred embodiment of the electrolysis process, the anode half-element is filled with a 5 to 20 wt.% hydrochloric acid when the electrolysis cell is started according to DE 10 152 275 Al, the hydrochloric acid containing at least 10 ppm free chlorine and the concentration of the hydrochloric acid during start-up being more than 5 wt.%. The volumetric flow rate of the hydrochloric acid through the anode chamber is set so that at the beginning of electrolysis the hydrochloric acid flows at a rate in the anode chamber of 0.05 to 0.15 cm/s. Electrolysis is started at a current density of 0.5 to 2 kA/m2 and increased by 0.5 to 1.5 kA/m2 at a time at intervals of 5 to 25 minutes. After a given current density of preferably 4 to 7 kA/m2 is achieved, the volumetric flow rate of the hydrochloric acid is set so that the hydrochloric acid in the anode half-element flows at a rate of 0.2 to 0.4 cm/s. A particularly advantageous mode of operation of the electrolysis cell can take place according to DE 101 38 215 Al wherein the electrolysis cell is operated at an elevated pressure in the cathode chamber to lower the cell voltage. The differential pressure between anode chamber and cathode chamber should be 0.01 to 1000 bar and the oxygen pressure in the cathode chamber at least 1.05 bar absolute. In a further preferred variation of the new process, the hydrochloric acid is oxidised in step d) together with alkali chlorides, in particular with sodium chloride in an anodic alkali chloride electrolysis. The process of sodium chloride electrolysis is described in more detail in the following. Normally membrane electrolysis processes are used for the electrolysis of sodium chloride-containing solutions (see also Peter Schmittinger, CHLORINE, Wiley-VCH Verlag, 2000, page 77 ff). A two-part electrolysis cell which consists of an anode chamber with an anode and a cathode chamber with a cathode, is hereby used. Anode and cathode chambers are separated by an ion exchange membrane. Commercially available ion exchange membranes such as e.g. Nafion® 982 from DUPONT can be used here. A sodium chloride-containing solution with a sodium chloride concentration of normally approx. 300 g/1 is introduced into the anode chamber consisting of titanium or a noble metal-coated titanium with a noble metal oxide-coated titanium anode. The chloride ion is oxidised to chlorine which is taken from the cell with the depleted sodium chloride-containing solution (approx. 200 g/1), on the anode. The sodium ions migrate under the influence of the electrical field through the ion exchange membrane into the cathode chamber. During this migration, each mole of sodium takes with it per membrane between 3.5 and 4.5 moles water. This leads to the anolyte becoming low in water. In contrast to the anolyte, on the cathode side water is consumed by the electrolysis of water to hydroxide ions. The water passing with the sodium ions into the catholytes is sufficient to maintain the sodium hydroxide solution concentration in the discharge at 31-32 wt.%; this applies at a NaOH inlet concentration of 30% and a current density of 4 kA/m2. Water is electrochemically reduced in the cathode chamber, hydroxide ions and hydrogen being formed. Alternatively, a gas diffusion electrode on which oxygen are reacted with electrons to hydroxide ions, can also be used as cathode, no hydrogen being formed. The hydroxide ions form sodium hydroxide solution with the sodium ions passing via the ion exchange membrane into the cathode chamber. A sodium hydroxide solution with a concentration of 30 wt.% is normally fed into the cathode chamber and a sodium hydroxide solution with a concentration of 31-32 wt.% taken off. In order to feed maximum quantities of sodium chloride-containing solutions economically to sodium chloride electrolysis, it is necessary that the water transport via the membrane is-increased. This can be done by selecting suitable membranes, such as described e.g. in US 4 025 405. The effect of an increased water transport is that the otherwise normal water addition is not needed to maintain the caustic solution concentration. According to US 4 025 405, the operating conditions for electrolysis are very broadly worded, so a pH range of 1 to 5, a temperature of 35-100°C and a sodium chloride concentration of 100-325 g/1 is given. According to US 3 773 634, electrolysis can be operated with a higher current yield at higher water transport through the membrane if a caustic solution concentration of 31-34 wt.%, particularly preferably 36-38 wt.% and a sodium chloride concentration of 120-250 g/1 is used. A further alternative is the use of a modified cell design according to EP 1 609 887. Here the electrolysis cell is modified so that the distance between membrane and anode which is normally zero, i.e. the membrane lies on the anode, is increased. This has the disadvantage that the use can only take place using a new electrolysis cell with adapted design or by converting existing electrolysis cells to the new design which is linked to high investment. With existing plants, this process concept is economically not practicable. When the process according to the invention is carried out, a very pure chlorine gas which can be used without problems in chemical synthesis such as for example the production of isocyanates, is obtained. Furthermore, a process which is characterised in that the hydrogen chloride used as starting material for the first oxidation a) is the product of a production process for isocyanates and that the purified chlorine gas freed of oxygen and optionally secondary constituents of step g) is used in the production of isocyanates, in particular is reused as part of a material cycle is preferred. In a first step of a preferred process that has as its subject matter the integration of the new combined chlorine production process into an isocyanate production, the production of phosgene takes place by reacting chlorine with carbon monoxide. The synthesis of phosgene is sufficiently known and is described e.g. in Ullmanns Enzyklopadie der industriellen Chemie, 3rd edition, volume 13, pages 494-500. Phosgene is produced on an industrial scale predominately by reacting carbon monoxide with chlorine preferably on activated carbon as catalyst. The strongly exothermic gas phase reaction takes place typically at a temperature of minimum 250°C to maximum 600°C as a rule in multi-tube reactors. Taking off the heat of reaction can take place in a different way, for example by a liquid heat exchange agent, as described e.g. in specification WO 03/072237 Al, or by hot cooling via a secondary cooling cycle with simultaneous use of the heat of reaction to produce vapour, as disclosed e.g. in US-A 4764308. At least one isocyanate is formed from the phosgene formed in the first step by reaction with at least one organic amine or a mixture of two or more amines in a next process step. This second process step is designated phosgenation in the following. The reaction takes place with formation of hydrogen chloride as a byproduct which occurs as a mixture with the isocyanate. The synthesis of isocyanates is likewise known in principle from the prior art, phosgene as a rule being used in a stoichiometric excess based on the amine. Phosgenation normally takes place accordingly in the liquid phase, wherein the phosgene and the amine can be dissolved in a solvent. Preferred solvents for phosgenation are chlorinated aromatic hydrocarbons such as chlorobenzene, o-dichlorobenzene, p-dichlorobenzene, trichlorobenzene, the corresponding chlorotoluenes or chloroxylenes, chloroethylbenzene, monochlorodiphenyl, a- or ß-naphthyl chloride, benzoic acid ethyl esters, phthalic acid dialkyl esters, diisodiethyl phthalate, toluene and xylenes. Further examples of suitable solvents are in principle known from the prior art. As also known from the prior art, e.g. according to specification WO 96/16028, the isocyanate formed can likewise function as solvent for phosgene. In another, preferred embodiment, phosgenation, in particular of suitable aromatic and aliphatic diamines, takes place in the gas phase, i.e. above the boiling point of the amine. The gas phase phosgenation is described e.g. in EP 570 799 Al. Advantages of this process over the otherwise normal liquid phase phosgenation lie in energy saving caused by the minimisation of a costly solvent and phosgene cycle. All primary amines with one or more primary amino groups which can react with phosgene forming one or more isocyanates with one or more isocyanate groups, are in principle suitable as organic amines. The amines have at least one, preferably two, or optionally three and more primary amino groups. Aliphatic, cycloaliphatic, aliphatic-aromatic, aromatic amines, di- and/or polyamines, such as aniline, halogen- substituted phenyl amines, e.g. 4-chlorophenylamine, 1,6-diaminohexane, 1-amino- 3,3,5-trimethyl-5-amino cyclohexane, 2,4-, 2,6-diaminotoluene or mixtures thereof, 4,4'-, 2,4'- or 2,2'-diphenylmethane diamine or mixtures thereof, and also higher- molecular isomer, oligomer or polymer derivatives of the named amines and polyamines, are thus possible as organic primary amines. Other possible amines are known in principle from the prior art. Preferred amines for the present invention are the amines of the diphenyl methane diamine range (monomer, oligomer and polymer amines), 2,4'-, 2,6'-diaminotoluene, isophorone diamine and hexamethylenediamine. The corresponding isocyanates diisocyanatodiphenylmethane (MDI, monomer, oligomer and polymer derivatives), toluylene diisocyanate (TDI), hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) are obtained during phosgenation. The amines can be reacted with phosgene in a one-stage or two-stage or optionally multi-stage reaction. A continuous and also discontinuous mode of operation is thereby possible. If a single-stage phosgenation in the gas phase is selected, the reaction takes place above the boiling point of the amine preferably within an average contact time of 0.5 to 5 seconds and at a temperature of 200 to 600°C. Phosgenation in the liquid phase is normally carried out at a temperature of 20 to 240°C and a pressure of 1 to approx. 50 bar. Phosgenation in the liquid phase can be carried out in a single stage or in multiple stages, wherein phosgene can be used in stoichiometric excess. The amine solution and the phosgene solution are thereby combined via a static mixing element and then passed for example from bottom to top through one or more reaction towers where the mixture reacts out to the required isocyanate. In addition to reaction towers which are provided with suitable mixing elements, reaction vessels with stirring device can also be used. Apart from static mixing elements, special dynamic mixing elements can also be used. Suitable static and dynamic mixing elements are in principle known from the prior art. As a rule, the continuous liquid phase isocyanate production is carried out on an industrial scale in two stages. In the first stage carbamoyl chloride is formed from amine and phosgene and amine hydrochloride from amine and split off hydrogen chloride in general at a temperature of maximum 220°C, preferably maximum 160°C. This first stage is strongly exothermic. In the second stage, both the carbamoyl chloride is split to isocyanate and hydrogen chloride and the amine hydrochloride reacted to the carbamoyl chloride. The second stage is carried out as a rule at a temperature of at least 90°C, preferably 100 to 240°C. After phosgenation, separation of the isocyanates formed during phosgenation takes place in a third step. This occurs by first of all separating the phosgenation reaction mixture into a liquid and a gaseous product stream in a way known in principle to the skilled person. The liquid product stream contains substantially the isocyanate or isocyanate mixture, the solvent and a small part of unreacted phosgene. The gaseous product stream consists substantially of hydrogen chloride gas, stoichiometrically excess phosgene, and small quantities of solvent and inert gases, such as for example nitrogen and carbon monoxide. Furthermore, the liquid stream is then fed to a processing, preferably a distillative processing, with phosgene and the solvent for phosgenation being separated off in succession. Optionally a further processing of the isocyanates formed also takes place. This occurs for example by fractionating the isocyanate product obtained in a way known to the skilled person. The hydrogen chloride obtained in the reaction of phosgene with an organic amine in general contains organic secondary constituents which can interfere both in the thermal catalysed or non-thermal activated HC1 oxidation and in the electrochemical oxidation of an aqueous hydrogen chloride solution according to step (c). These organic constituents include for example the solvents used in isocyanate production such as chlorobenzene, o-dichlorobenzene or p-dichlorobenzene. If a gas diffusion electrode is used as cathode during electrolysis, the catalyst of the gas diffusion electrode can also be deactivated by the organic impurities. In addition, these impurities can be deposited on the current collector and thereby impair the contact between gas diffusion electrode and current collector, resulting in an undesired rise in voltage. If the diaphragm process is used for electrolysis of the hydrochloric acid, the named organic constituents can be deposited on the graphite electrodes and/or diaphragm and thereby likewise increase the electrolysis voltage. The separation of the hydrogen chloride produced during phosgenation from the gaseous product stream accordingly takes place preferably in a further process step. The gaseous product stream which is obtained during separation of the isocyanate is treated so that the phosgene can again be fed to phosgenation and the hydrogen chloride to an electrochemical oxidation. The separation of the hydrogen chloride takes place preferably first by separating off phosgene from the gaseous product stream. Separation of the phosgene occurs by liquefaction of phosgene, for example on one or more series-connected condensers. Liquefaction takes place preferably at a temperature in the range of-15 to -40°C as a function of the solvent used. In addition, parts of the solvent residues can be removed from the gaseous product stream by this deep cooling. In addition or alternatively, the phosgene can be washed out from the gas stream with a cold solvent or solvent-phosgene mixture in one or more stages. The solvents chlorobenzene and o-dichlorobenzene already used in the phosgenation are for example suitable as solvent for this. The temperature of the solvent or the solvent-phosgene mixture for this is in the range of-15 to -46°C. The phosgene separated off from the gaseous product stream can be again fed to phosgenation. The hydrogen chloride obtained after separation of the phosgene and part of the solvent residue can contain, in addition to the inert gases such as nitrogen and carbon monoxide, 0.1 to 1 wt.% solvent and 0.1 to 2 wt.% phosgene. Optionally a purification of the hydrogen chloride then takes place to reduce the content of traces of solvent. This can take place for example by means of freezing, by passing the hydrogen chloride for example through one or more cold traps as a function of the physical properties of the solvent. In a particularly preferred embodiment of the purification of the hydrogen chloride optionally provided, the hydrogen chloride stream flows through two series-connected heat exchangers, the solvent to be separated being frozen as a function of the fixed point for example at -40°C. The heat exchangers are preferably operated alternately, the gas current thawing the previously frozen solvent in the heat exchanger through which it has first flowed. The solvent can be used again for the production of a phosgene solution. In the downstream second heat exchanger which is treated with a normal heat exchange medium for refrigerators, e.g. a compound from the freon range, the gas is preferably cooled below the fixed point of the solvent so that this crystallises out. When the thawing and crystallisation procedures are complete, the gas stream and the coolant stream are switched so that the function of the heat exchangers is reversed. The hydrogen chloride-containing gas stream can be depleted in this way to preferably maximum 500 ppm, particularly preferably maximum 50 ppm, very particularly preferably to maximum 20 ppm solvent content. Alternatively, purification of the hydrogen chloride can take place preferably in two series-connected heat exchangers, e.g. according to US-A-6 719 957. The hydrogen chloride is thereby preferably compressed at a pressure of 5 to 20 bar, preferably 10 to 15 bar, and the compressed gaseous hydrogen chloride fed to a first heat exchanger at a temperature of 20 to 60°C, preferably 30 to 50°C. In this, the hydrogen chloride is cooled with a cold hydrogen chloride with a temperature of -10 to -30°C which comes from a second heat exchanger. Organic constituents which can be passed for disposal or recycling are thereby condensed. The hydrogen chloride passed into the first heat exchanger leaves this at a temperature of -20 to 0°C and is cooled in the second heat exchanger to a temperature of -10 to -30°C. The condensate occurring in the second heat exchanger consists of further organic constituents and small quantities of hydrogen chloride. To avoid a loss of hydrogen chloride, the condensate discharging from the second heat exchanger is fed to a separation and evaporation unit. This can for example be a distillation column in which the hydrogen chloride is expelled from the condensate and is returned to the second heat exchanger. It is also possible to return the expelled hydrogen chloride to the first heat exchanger. The hydrogen chloride cooled in the second heat exchanger and freed from organic constituents is passed into the first heat exchanger at a temperature of -10 to -30°C. After heating to 10 to 30°C, the hydrogen chloride freed of organic constituents leaves the first heat exchanger. In a likewise preferred alternative process, the optionally provided purification of the hydrogen chloride from organic impurities, such as solvent residues, takes place on activated carbon by means of adsorption. The hydrogen chloride is for example thereby passed over or through an activated carbon bed after removing excess phosgene at a pressure difference of 0 to 5 bar, preferably 0.2 and 2 bar. The flow rate and dwell time is thereby adapted in a way known to the skilled person to the content of impurities. The adsorption of organic impurities is likewise possible on other suitable adsorption agents, e.g. on zeolites. In a further, also preferred, alternative process, a distillation of the hydrogen chloride can be provided for the optionally provided purification of the hydrogen chloride from phosgenation. This takes place after condensation of the gaseous hydrogen chloride from phosgenation. In the distillation of the condensed hydrogen chloride, the purified hydrogen chloride is removed as the overhead product of the distillation, distillation taking place at standard conditions of pressure, temperature etc for such a distillation known to the skilled person. The hydrogen chloride separated off and optionally purified according to the process described above can then be fed to HC1 oxidation with oxygen. This HC1 oxidation takes place as described preferably according to the Deacon process. As already described above, the catalytic process known as the Deacon process is preferably used. Hydrogen chloride is herein oxidised with oxygen in an exothermic equilibrium reaction to chlorine, water vapour being produced. The reaction temperature is normally 150 to 500°C, the normal reaction pressure is 1 to 25 bar. Since this is an equilibrium reaction, it is expedient to work at the lowest temperatures at which the catalyst still has sufficient activity. Since there is no fear of selectivity losses, it can be economically advantageous to work at relatively high pressure and correspondingly at a longer dwell time compared to standard pressure. Suitable preferred catalysts for the Deacon process contain ruthenium oxide, ruthenium chloride or other ruthenium compounds on silicon dioxide, aluminium oxide, titanium dioxide or zirconium dioxide as support. Suitable catalysts can for example be obtained by applying ruthenium chloride to the support and subsequent drying or drying and calcining. Suitable catalysts can also contain in addition to or in place of a ruthenium compound, compounds of other noble metals, for example gold, palladium, platinum, osmium, iridium, silver, copper or rhenium. Suitable catalysts can furthermore contain chromium(III) oxide. The catalytic hydrogen chloride oxidation can be carried out adiabatically or preferably isothermally or approximately isothermally, discontinuously, but preferably however continuously as a fluid-bed or fixed-bed process, preferably as a fixed-bed process, particularly preferably in multi-tube reactors on heterogeneous catalysts at a reactor temperature of 180 to 500°C, preferably 200 to 400°C, particularly preferably 220 to 350°C and a pressure of 1 to 25 bar (1000 to 25000 hPa), preferably 1.2 to 20 bar, particularly preferably 1.5 to 17 bar and in particular 2.0 to 15 bar. Normal reaction apparatus in which the catalytic hydrogen chloride oxidation is carried out are fixed-bed or fluid-bed reactors. The catalytic hydrogen chloride oxidation can preferably also be carried out in multiple stages. Several, thus 2 to 10, preferably 2 to 6, particularly preferably 2 to 5, in particular 2 to 3, series-connected reactors with additional intermediate cooling can be used in the isothermal or approximately isothermal procedure. The oxygen can be added in full together with the hydrogen chloride before the first reactor or distributed over the various reactors. This series connection of individual reactors can also be combined in one apparatus. A further preferred embodiment of a device suitable for the process consists of using a structured catalyst bed in which the catalyst activity increases in the direction of flow. Such a structuring of the catalyst bed can take place by different impregnation of the catalyst supports with activated material or by different dilution of the catalyst with an inert material. Rings, cylinders or pellets of titanium dioxide, zirconium dioxide or mixtures thereof, aluminium oxide, steatite, ceramic, glass, graphite or noble steel for example can be used as inert material. With the preferred use of catalyst shaped bodies, the inert material should have preferably similar external dimensions. Shaped bodies with any shapes are suitable as catalyst shaped bodies; tablets, rings, cylinders, stars, cartwheels or pellets are preferred as the shape, particularly preferred are rings, cylinders or star strands. In particular, ruthenium compounds or copper compounds on support materials which can also be doped, are suitable as heterogeneous catalysts; optionally doped ruthenium catalysts are preferred. Silicon dioxide, graphite, and titanium dioxide with a rutile or anatase structure, zirconium dioxide, aluminium oxide or mixtures thereof, preferably titanium dioxide, zirconium dioxide, aluminium oxide or mixtures thereof, particularly preferably y- or 8-aluminium oxide or mixtures thereof, are suitable as support materials. The copper or ruthenium support catalysts can be obtained for example by impregnating the support material with aqueous solutions of CuC12 or RuCl3 and optionally a promoter for doping, preferably in the form of their chlorides. The shaping of the catalyst can take place after or preferably before impregnation of the support material. Alkali metals such as lithium, sodium, potassium, rubidium and caesium, preferably lithium, sodium and potassium, particularly preferably potassium, alkaline-earth metals such as magnesium, calcium, strontium and barium, preferably magnesium and calcium, particularly preferably magnesium, rare earth metals such as scandium, yttrium, lanthanum, cerium, praseodymium and neodymium, preferably scandium, yttrium, lanthanum and cerium, particularly preferably lanthanum and cerium, or mixtures thereof, are suitable as promoters for doping the catalysts. The shaped bodies can then be dried and optionally calcined at a temperature of 100 to 400°C, preferably 100 to 300°C for example under a nitrogen, argon or air atmosphere. The shaped bodies are preferably first of all dried at 100 to 150°C and then calcined at 200 to 400°C. The reaction to hydrogen chloride in the single pass can be limited preferably to 15 to 90%, preferably 40 to 85%, particularly preferably 50 to 70%. Unreacted hydrogen chloride can be returned after separation partially or completely to the catalytic hydrogen chloride oxidation. The heat of reaction of the catalytic hydrogen chloride oxidation can be used in an advantageous way to produce high-pressure water vapour. This can be used to operate a phosgenation reactor and or distillation columns, in particular isocyanate distillation columns. In a next step, the chlorine formed is separated off. The separation step normally comprises several stages, namely the separation and optionally return of unreacted hydrogen chloride from the product gas stream of the catalytic hydrogen chloride oxidation, the drying of the stream obtained containing substantially chlorine and oxygen and the separation of chlorine from the dried stream. The separation of unreacted hydrogen chloride and of water vapour formed can take place by condensing out aqueous hydrochloric acid from the product gas stream of the hydrogen chloride oxidation by cooling. Hydrogen chloride can be absorbed even in dilute hydrochloric acid or water. The process according to the invention is illustrated for example in greater detail in the following by means of Fig. 1 Fig. 1 shows a preferred embodiment of the new process here in combination with the production of isocyanates. Examples: Fig. 1 shows an example for the use of the process as a supplement and part of an isocyanate production. In a first stage 1 of the isocyanate production, chlorine 11 is reacted with carbon monoxide 10 to phosgene 13. In the following stage 2, phosgene 13 from stage 1 is reacted with an amine 14 (e.g. toluene diamine) to a mixture 15 of isocyanate (e.g. toluene diisocyanate, TDI) and hydrogen chloride, the isocyanate 16 is separated off (in stage 3) and utilised and the HC1 gas 17 subjected to a purification 4. The purified HC1 gas 18 is reacted in the HC1 oxidation process 5 with oxygen, here in a Deacon process by means of catalyst. The reaction mixture 19 from stage 5 is cooled (step 6). Aqueous hydrochloric acid 26 which thereby occurs mixed optionally with water or dilute hydrochloric acid, is channelled out. The gas mixture 20 thus obtained, consisting at least of chlorine, oxygen and secondary constituents such as nitrogen, carbon dioxide etc and [sic] is dried with cone, sulfuric acid 21 (96%) (step 7). In a purification stage 8, chlorine 11 is separated from the gas mixture 22 from stage 7. The residual stream 23 having oxygen and secondary constituents is fed to electrolysis stage 9. The electrolysis stage is an ODC electrolysis in which on the cathode side oxygen is used as reactant. The chlorine gas obtained from the purification stage 8 can be used again directly in the phosgene synthesis 1. The oxygen-containing stream 23 occurring in this step is used in step 5 (HC1 oxidation) and step 9 (electrolysis cell). The concentration of the hydrochloric acid 27 fed to the electrolysis cell 9 is 14 to 15 wt.% HC1, that of the hydrochloric acid 28 discharging from electrolysis 9 is 11 to 13 wt.% HC1. Concentrated hydrochloric acid 26 from the separation stage 6 is added to the hydrochloric acid stream 28 and again fed to cell 9. The oxygen consumed in step 5 and step 9 is replaced by oxygen from an external source 24. The oxygen 25 not consumed in the cathode chamber of the electrolysis cell is passed in the cycle and fresh oxygen from an external source 24 added. The approx. 2 wt.% hydrochloric acid stream 29 likewise occurring in the cathode chamber is fed to the hydrochloric acid separation 6 and serves there as absorption agent for excess gaseous hydrogen chloride. The chlorine 30 produced according to step 9 is combined with the chlorine-containing gas stream 20. The combined gas stream 30, 20 is cooled in a single- or multi-stage cooling by means of a cooling unit, e.g. a tubular heat exchanger, and dried. Drying 7 can take place for example by means of a suitable drying agent in an absorption column fitted with material exchange elements. A suitable drying agent can, as described e.g. in DE 10 235 476 A, be, in addition to molecular sieves or hygroscopic adsorbents, e.g. sulfuric acid. Drying can take place in a single or multiple stages. Drying preferably takes place in two stages, by the chlorine to be dried being brought into contact in a first stage with a sulfuric acid of lower concentration, preferably 70 to 80%), particularly preferably 75 to 80%. In a second stage, the residual moisture is removed from the chlorine by means of a higher-concentrated sulfuric acid of preferably 88 to 96%, particularly preferably 92-96%. The chlorine (22) dried in this way with a residual moisture of preferably maximum 100 ppm, particularly preferably maximum 20 ppm, can be passed through a droplet separator in order to optionally remove sulfuric acid droplets still contained therein. The dried chlorine gas stream 22 is then subjected to a chlorine purification 8. Example 1 A stream of 35.9 t/h purified hydrogen chloride from an isocyanate plant is divided into two partial streams. 29.5 t/h are fed to a HCl oxidation and 6.4 t/h to a HCl absorption. 29.5 t/h HCl are fed with 12.9 t/h oxygen (content greater than 99%) of a catalytic HCl oxidation. Oxidation takes place at 333°C and 3.4 bar. The HCl conversion in the reactor is 85%. The gas mixture leaving the reactor is cooled to 100°C, the HCl condensed with the reaction water in a HCl absorption. A partial stream of the hydrochloric acid-depleted anolytic acid stream 28 coming from electrolysis is introduced into the hydrochloric acid 26 from the HCl absorption. 32.1 t/h of the hydrochloric acid-depleted analytic acid 28 with a HCl concentration of 12.2 wt.% is fed to HCl absorption. The cooled process gas stream (4.4 t/h HCl, 7.4 t/h oxygen, 24.4 t/h chlorine, 6.18 t/h water) is passed with 6.4 t/h purified hydrogen chloride into the HCl absorption. In this HCl absorption unit, a 30 wt.% hydrochloric acid 26 which is combined with the residue of the depleted anolytic acid 28 and again fed to the electrolysis cell, is produced. 2.96 t/h of the depleted anolytic acid 28 are channelled from the anolytic acid cycle (not shown). Electrolysis is operated with a current density of 5 kA/m2 at 55°C and a voltage of 1.39 V. A palladium-stabilised titanium is used as the anode and cathode material. 10.1 t/h chlorine is developed on the ruthenium oxide-coated anode manufactured by DENORA, Germany. The anode and cathode half-shells are separated by an ion exchange membrane, type Nation 324, manufactured by DUPONT. An oxygen absorption cathode manufactured by ETEK which contains a rhodium sulfide-supported catalyst is used as the cathode. Oxygen is fed to the cathode half-element with 100%o excess, i.e. at 9.17 t/h. The oxygen is recycled in the electrolysis, a purge stream of 1% of the feed quantity is led off behind electrolysis 9 (not shown) or used in HCl oxidation 5. The pressure in the anode half-cell is higher than that in the cathode half-cell. The differential pressure is 200 mbar. A condensate stream of 8.8 t/h is removed from the cathode half-cell. The electrolysis unit 9 consists of 615 electrolysis cells, one element consisting of an anode half-shell with anode, an ion exchange membrane and a cathode half-shell with oxygen absorption cathode. Example 2 As in Example 1, a purified HCl gas stream is fed at 35.9 t/h to the HCl oxidation with 100% oxygen excess, i.e. 15.7 t/h oxygen. The conversion is 85%, so that 5.4 t/h HCl, 9.0 t/h oxygen, 7.5 t/h water and 29.7 t/h chlorine leave the reactor. This process gas 19 is fed to a HCl absorption which is operated with a first partial stream of 177.8 t/h of a NaCl-depleted NaCl-containing solution (18.3 wt.% NaCl) coming from NaCl electrolysis. The water and hydrogen chloride of the process gas 19 is absorbed in this NaCl-containing solution. The stream leaving the absorption is composed as follows : 152.8 t/h water, 32.5 t/h NaCl, 5.4 t/h hydrogen chloride. This stream is then combined with the second partial stream of the NaCl-containing solution of 118.2 t/h, 26.4 t/h solid NaCl are added and it is again fed to the NaCl electrolysis. NaCl electrolysis consists of 1475 biopolar electrolysis elements per 2.71 m2 membrane surface. NaCl electrolysis is operated with anode half-shells made of titanium which has a noble metal-coated titanium anode. The cathode half-shell consists of nickel and supports a noble metal oxide-coated nickel cathode. The anode and cathode half-shells are separated by an ion exchange membrane from Nafion 982 manufactured by DUPONT. 21.2 t/h chlorine are produced anodically, 74.8 t/h sodium hydroxide solution with a concentration of 32 wt.% and 0.6 t/h hydrogen cathodically. The NaCl-depleted anolyte is partially fed again to the HCl absorption. Claims 1. Process for the production of chlorine by means of multi-stage oxidation by a) thermal reaction of hydrogen chloride with oxygen using catalysts or by non-thermal activated reaction of hydrogen chloride with oxygen, in which the gas mixture forming during reaction a), consisting of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents, in particular carbon dioxide and nitrogen, b) is cooled to condense hydrochloric acid and c) the aqueous solution of hydrogen chloride (hydrochloric acid) forming is separated off from the gas mixture, characterised in that d) the hydrochloric acid separated off is fed at least partially to an electrochemical oxidation in which at least part of the aqueous hydrochloric acid is oxidised to chlorine, e) the chlorine gas produced in step d) is optionally added to the gas mixture occurring in step c), f) the residues of water present in the gas mixture from steps c) and e), in particular by washing with sulfuric acid, are removed, g) the chlorine-rich gas mixture forming is freed of oxygen and optionally of secondary constituents. 2. Process according to Claim 1, characterised in that the electrochemical oxidation of the aqueous hydrogen chloride solution takes place according to step d) by means of diaphragm electrolysis. 3. Process according to Claim 1, characterised in that the electrochemical oxidation of the aqueous hydrogen chloride solution according to step d) takes place by means of diaphragm electrolysis. 4. Process according to Claim 3, characterised in that electrodes based on graphite are used as electrodes for the anode and/or the cathode. 5. Process according to Claim 3 or 4, characterised in that cathodes based on graphite which have a coating containing iridium, are used 6. Process according to at least one of Claims 1 to 5, characterised in that hydrogen chloride is separated off completely from the gas mixture as hydrochloric acid according to steps b) and c). 7. Process according to at least one of Claims 1 to 6, characterised in that the separation of the hydrogen chloride from the gas mixture according to step b) and c) takes place by additional washing of the gas mixture with water or dilute hydrochloric acid, in particular with water. 8. Process according to at least one of Claims 1, 2 and 4 to 7, characterised in that a gas diffusion electrode which is operated as an oxygen absorption cathode is used as the cathode for the electrolysis according to step d). 9. Process according to Claim 8, characterised in that the gas diffusion electrode is based at least on an electrically conductive fabric, woven, knitted fabric, net or nonwoven of carbon which is arranged between a carbon-containing catalyst layer and a gas diffusion layer. 10. Process according to Claim 9, characterised in that the catalyst layer contains rhodium and/or a rhodium sulfide. 11. Process according to at least one of Claims 1, 2 and 4 to 10, characterised in that a titanium-containing electrode which has a coating of at least one noble metal oxide, preferably of ruthenium oxide, is used as the anode. 12. Process according to at least one of Claims 1, 2 and 4 to 11, characterised in that titanium and/or a titanium alloy is used as the material for the electrolysis cell. 13. Process according to one of Claims 1 to 12, characterised in that the hydrogen chloride used as the starting material for the first oxidation a) is the product of a production process of isocyanates and that the purified chlorine gas of step g) freed of oxygen and optionally of secondary constituents is reused in the production of isocyanates, in particular as part of a material cycle. 14. Process according to at least one of Claims 1, 2 and 6 to 13, characterised in that the hydrochloric acid in step d) is oxidised together with alkali chlorides, in particular with sodium chloride in an anodic alkali chloride electrolysis. 15. Process according to at least one of Claims 1, 2 and 6 to 14, characterised in that the first oxidation a) takes place by thermal catalysed oxidation (Deacon process). 16. Process according to Claim 14 or 15, characterised in that a depleted alkali chloride solution coming from the alkali chloride electrolysis is used as alkali chloride solution. 17. Process according to at least one of Claims 1, 2 and 6 to 16, characterised in that part of the hydrogen chloride provided for the first HCl oxidation is branched, subjected to an absorption with water or dilute hydrochloric acid, in particular dilute hydrochloric acid, and the concentrated hydrochloric acid forming combined with the hydrochloric acid from step c) and fed to electrolysis according to step d). 18. Process according to at least one of Claims 1 to 17, characterised in that the first HCl oxidation a) is operated with a conversion of 50 to 98%, in particular with a conversion of 60 to 95%.

Full Text

Process for the production of chlorine from hydrogen chloride and oxygen
The invention is based on known processes for the production of chlorine by thermal reaction of hydrogen chloride with oxygen using catalysts, or by non-thermal activated reaction of hydrogen chloride with oxygen, in which the gas mixture formed during the reaction, consisting at least of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents such as carbon dioxide and nitrogen, is cooled to condense hydrochloric acid, the liquid hydrochloric acid formed is separated off from the gas mixture, the residues of water remaining in the gas mixture in particular are removed by washing with concentrated sulfuric acid. The invention relates especially to the combination with other HC1 oxidation processes.
Hydrogen chloride occurs as a by-product in the production of a plurality of chemical reactions with chlorine and or phosgene, for example the production of isocyanates or the chlorination of aromatics. This hydrogen chloride can be again converted by electrolysis or by oxidation with oxygen to chlorine which is again used in the chemical reactions. The oxidation of hydrogen chloride (HC1) to chlorine (C12) takes place by reaction of hydrogen chloride and oxygen (O2) according to
4HC1 + O2 →2 Cl2 + 2 H2O The reaction can be carried out in the presence of catalysts at a temperature of approx. 250 to 450°C. The normal reaction pressure is in the range of 1 to 10 bar. Various embodiments of the process which is generally known as the Deacon process, are described : Shell-Chlor process, MT-Chlor process, KEL-Chlor process, Carrier Catalyst process and Sumitomo-Chlor process.
Suitable catalysts for the Deacon process contain transition metal compounds such as copper and ruthenium compounds or even compounds of other noble metals such as gold and palladium. Such catalysts are described for example in specifications DE 1 567 788 Al, EP 251 731 A2, EP 936 184 A2, EP 761 593 Al, EP 711 599 Al
and DE 102 50 131 Al. The catalysts are as a rule applied to a support. These supports consist for example of silicon dioxide, aluminium oxide, titanium dioxide or zirconium oxide.
The Deacon processes are carried out as a rule in fluid-bed reactors or fixed-bed reactors, e.g. in multi-tube reactors. Hydrogen chloride is freed of impurities before the reaction in order to avoid poisoning the catalysts used.
Alternatively, processes in which the reaction of hydrogen chloride with oxygen is non-thermally activated, are known. Such processes are described in "W. Stiller, Nichtthermische aktivierte Chemie, Birkhauser Verlag, Basel, Boston, 1987, pp 33-34, pp 45-49, pp 122-124, pp 138-145". Special embodiments are disclosed for example in specification RU 2253607, JP-A-59073405, DD-A-88 309 and SU 1801943 Al. By non-thermally activated reactions is understood for example excitations of the reaction with the following means or processes :
energy radiation, e.g. laser radiation, photochemical radiation sources, UV radiation, infrared radiation, etc
- a low-temperature plasma, e.g. created by electrical discharge
- magnetic field excitation
- tribomechanical activation, e.g. excitation by shock waves
ionising radiation, e.g. gamma-ray and X-ray radiation, a- and ß-rays from
nuclear disintegration, high-energy electrons, protons, neutrons and heavy
ions
microwave irradiation
Oxygen is used both in the thermal and in the non-thermal activated reaction of hydrogen chloride with oxygen as a rule as a pure gas with an O2 content of > 98 vol.%.
It is common to all known processes that in the reaction of hydrogen chloride with oxygen, a gas mixture is obtained which in addition to the target product chlorine
also contains water, unreacted hydrogen chloride and oxygen, and other secondary constituents such as carbon dioxide. To obtain pure chlorine, the product gas mixture is cooled after the reaction until reaction water and hydrogen chloride condense out in the form of concentrated hydrochloric acid. The hydrochloric acid forming is separated off and the remaining gaseous reaction mixture is freed from residual water by washing with sulfuric acid or other methods such as drying with zeolites.
The hydrochloric acid separated off is preferably fed to a desorption stage in which gaseous hydrogen chloride is again released. This gaseous hydrogen chloride can be partially or preferably completely returned to the reaction of hydrogen chloride with oxygen. The dilute hydrochloric acid occurring in the desorption stage can be returned to the hydrochloric acid condensation stage. Here the dilute hydrochloric acid serves as an absorbing agent for the gaseous hydrogen chloride to be separated off. A procedure of this type is described for example in specification DE 102 35 476 Al. Alternatively, the hydrochloric acid separated off can also be fed to recycling.
The chlorine-containing reaction gas mixture freed from residual water is then compressed, wherein oxygen and other gas constituents can remain in the gas phase and be separated off from the liquefied chlorine. Processes of this type for obtaining pure chlorine from gas mixtures are described for example in Offenlegungsschrift DE 195 35 716 Al and Offenlegungsschrift DE 102 35 476 Al. The now purified chlorine is then fed for use, for example in the production of isocyanates.
A substantial disadvantage of these processes is the comparatively high expenditure of energy for the desorption of hydrogen chloride from the hydrochloric acid occurring in the purification of the chlorine gas stream after the reaction. Recycling of the occurring hydrochloric acid in other processes, however, is not as a rule economic.
It has now been found that the disadvantages can be overcome if the hydrochloric acid occurring in the purification of the chlorine gas stream after the reaction is fed to an electrochemical oxidation in which at least part of the hydrochloric acid is oxidised to chlorine.
The subject matter of the invention is a process for the production of chlorine by means of multi-stage oxidation by
a) thermal reaction of hydrogen chloride with oxygen using catalysts or by non
thermal activated reaction of hydrogen chloride with oxygen,
in which the gas mixture forming in reaction a), consisting of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents, in particular carbon dioxide and nitrogen,
b) is cooled to condense hydrochloric acid and
c) the aqueous solution of hydrogen chloride (hydrochloric acid) forming is separated off from the gas mixture, characterised in that
d) the hydrochloric acid separated off is fed at least partially to an electrochemical oxidation in which at least part of the aqueous hydrochloric acid is oxidised to chlorine,
e) the chlorine gas occurring in step d) is optionally added to the gas mixture occurring in step c),
f) the residues of water present in the gas mixture from steps c) and e), in particular by washing with sulfuric acid, are removed,
g) the chlorine-rich gas mixture forming is freed of oxygen and optionally of
secondary constituents.
The process is preferably carried out continuously because an at the same time possible batch or semi-batch operation is industrially somewhat more costly than the continuous process.
In a preferred embodiment, hydrogen chloride is separated off completely from the gas mixture as hydrochloric acid according to steps b) and c). Separation of the hydrogen chloride from the gas mixture according to step b) and c) takes place particularly preferably by additional washing of the gas mixture with water or dilute hydrochloric acid, in particular with water.
The electrochemical oxidation of the hydrochloric acid according to step d) can be carried out in accordance with the membrane process or in accordance with the diaphragm process, in particular in a two-chamber electrolysis cell consisting of an anode chamber and a cathode chamber, or in particular in a three-chamber electrolysis cell consisting of an anode chamber, cathode chamber and an electrolyte chamber between the anode and cathode chamber. A two-chamber electrolysis cell is preferably selected.
In the membrane process, the anode chamber is separated from the cathode chamber by an ion exchange membrane (hereafter referred to simply as membrane), in particular a cation exchange membrane, and in the diaphragm process the cathode and anode chambers by a diaphragm. The distance of the electrodes (anode and cathode) from the diaphragm or the membrane is preferably 0 to 3 mm, particularly preferably 0 to 2 mm. Suitable ion exchange membranes are commercially available, e.g. single-layer ion exchange membranes with sulfonic acid groups. A membrane of the Nafion® 324 or Nafion® 117 type from the company DuPont for example can be used.
A woven diaphragm according to DE 3 321 159 Al for example can be used as diaphragm. Threads made of plastic can be used for the fabric. Thus for example fabrics made of polyvinyl chloride (PVC) or polyvinylidene fluoride (PVDF) or even made of mixed fabrics with PVC and PVDF threads can be used. Warp threads
or weft threads can consist, in addition to monofilament threads, of multifilament threads, as described in DE 3 321 159 A. Depending on the weaving of the diaphragm, the fabric can be compressed to optimise the gas permeability, e.g. by calendering.
Electrodes based on graphite can be used in the electrolysis of hydrochloric acid in accordance with the diaphragm process or the membrane process, the anode and/or the cathode preferably substantially consisting of graphite. Bipolar electrodes made of graphite are particularly preferably used. A particularly advantageous embodiment of the cathodes is that in particular the cathodes but also the anodes made of graphite are equipped with a coating containing noble metal, for example iridium, e.g. according to DE 4 417 744 Al.
The graphite anodes have in particular a geometric shape as is known e.g. from DE 3 041 897 A. The cathodes preferably have a similar structure to the anodes. The shape of the anode and/or cathode preferably has vertically arranged grooves, flutes, notches, depressions or similar. These grooves serve substantially to lead off upwards the gas forming during electrolysis, i.e. chlorine and hydrogen, from the narrow gap between the respective electrode and the diaphragm or the membrane. Particularly preferably the grooves have a depth of 5 to 35 mm, very particularly preferably 15 to 25 mm and a width of preferably 1 to 5 mm. The distance between two adjacent grooves substantially arranged parallel to one another is preferably 4 to 6 mm. In a further possible embodiment of the grooves, the depth and/or width of the grooves vary along their length. Thus the depth of the grooves can be 12 to 15 mm at the lower end of the grooves and 20 to 30 mm at the upper end of the grooves.
The hydrochloric acid is used in a preferred diaphragm process as an electrolyte both in the anode chamber and in the cathode chamber. During electrolysis, chlorine is produced on the anode and hydrogen on the cathode.
A preferred mode of operation of electrochemical oxidation of hydrochloric acid in which hydrogen is developed cathodically consists in metal ions from the group of platinum metals, preferably platinum and/or palladium, being added to the hydrochloric acid which serves as electrolyte in the cathode chamber. Thus for example solutions of hexachloroplatinic (IV) acid (H2PtCl6) or solutions of disodium tetrachloropalladate (II) (Na2PdC14) or even mixtures thereof are added. Addition can take place continuously or discontinuously. The addition of metal ions to the hydrochloric acid in the cathode chamber serves to maintain a low electrolysis voltage in the range of 1.6 to 2.1 V, compared with 2.2 to 2.3 V without addition of metal ions, e.g. at 5 kA/m and 70 to 80°C and a preferably 15 to 25%, particularly preferably approx. 20%, hydrochloric acid. A quantity of metal ions which is sufficient to maintain the electrolysis voltage in the range of 1.8 to 2.1 should therefore be added. This means that as the electrolysis voltage rises during operation the addition of metal ions is increased.
Electrolysis according to step d) is preferably carried out at a temperature of 50 to 90°C. The concentration of the aqueous solution of hydrogen chloride to be used in electrolysis is generally preferably 10 to 25 wt.%. In the diaphragm processes, hydrochloric acid of a concentration of 12 to 20 wt.% is particularly preferably used. Electrolysis with diaphragm can be carried out at an absolute pressure of approx. 1 bar taking into consideration a differential pressure between anode and cathode chamber optionally still to be set or even at higher pressure of up to 2 bar. Even higher pressures are generally possible but require a correspondingly higher expenditure in the construction of the electrolysis cell. The differential pressure between anode chamber and cathode chamber is preferably 0 to 10 mbar, particularly preferably 1 to 2 mbar, so that because of the higher pressure on the anode side traces of the chlorine gas formed passes via the diaphragm on the cathode side and thus can mix with the cathodically formed hydrogen.
In an alternative embodiment, the electrochemical oxidation of the aqueous solution of hydrogen chloride takes place according to step d) in accordance with the membrane process with a gas diffusion electrode as the cathode. The electrolysis
cell can therefore consist both of two chambers and of three chambers, but preferably of two chambers. An oxygen-containing gas, e.g. oxygen, air or air enriched with oxygen, is fed to the cathode half-cell. The oxygen is reduced on the gas diffusion electrode, water being formed. The aqueous hydrogen chloride solution is fed to the anode half-cell, the hydrogen chloride being oxidised to chlorine on the anode. The anode half-cell and the cathode half-cell are separated from one another by a cation exchange membrane. The electrolysis of hydrochloric acid using a gas diffusion electrode as the cathode is described e.g. in WO 00/73538 Al, e.g. a chlorine-resistant plastic. The gas diffusion electrode is particularly preferably based at least on an electrically conductive fabric, woven, knitted fabric, net or nonwoven of carbon which is arranged between a carbon-containing catalyst layer and a gas diffusion layer.
The electrolysis cell can in particular consist of either a non-metallic material according to DE 103 47 703 Al or a metallic material. Titanium or a titanium alloy particularly such as a titanium-palladium alloy for example is suitable as a metallic material for the electrolysis cell. The half-shells for the anode and cathode half-cell, the current distributor and the current feeds are thereby manufactured from titanium or a titanium alloy.
The anode can be preferably executed according to DE 102 34 806 Al with a particular gas lead-off construction. The anode thereby consists of metal, preferably titanium with a coating of noble metal oxide, e.g. of ruthenium oxide. Furthermore, the anode particularly preferably made of titanium according to specification DE 102 00 072 Al can have an intermediate layer of titanium carbide or titanium boride which is applied to the titanium anode by means of plasma or flame spraying before the coating of a noble metal oxide is applied. According to specification DE 102 34 806 Al, the metal anode in a further preferred embodiment has openings for the passage of the gas formed during electrolysis, the openings having preferably guide structures which lead off the gas formed to the side of the metal anode turned away from the ion exchange membrane. The total cross-sectional area of the openings should thereby be in the range of 20% to 70% of the area which is formed
by the height and width of the anode. The metal anode can in addition have a wavy, zigzag or rectangular cross-section. The depth of the anode should thereby be at least 1 mm. The ratio of electrochemically active area of the metal anode to the area which is formed by the height and width of the metal electrode should be at least 1.2. In a special embodiment, the metal anode can consist of two adjacent expanded metals, the expanded metal showing to the ion exchange membrane being more finely structured than the expanded metal turning away from the ion exchange membrane. Furthermore, the finer-structured expanded metal is thereby rolled flat and the coarser-structured expanded metal arranged so that the mesh webs are inclined in the direction of the cathode and serve as guide structures. Alternatively, the anode can also consist of an expanded metal. In principle, the anode should have a free area of 15 to 70%. The thickness of the expanded metals should be selected so that no additional electrical resistance with a bipolar switching of the individual electrolysis cells (cell element) to an electrolyser occurs. The electrical resistance depends substantially on the electrical contacting of the anode, such as for example number of current feed connection elements between the anode and back wall of the anode half-cell.
During electrolysis by means of gas diffusion electrode, the anode chamber and the cathode chamber can be separated by a commercial ion exchange membrane. DuPont ion exchange membranes of the Nafion® 324 or Nafion® 117 type for example can be used. A membrane which, as described in WO 05//12596 Al, has the side of which turned towards the gas diffusion electrode has a smooth surface structure, is preferably used. The smooth surface structure of the membrane enables the gas diffusion electrode and the membrane to be adjacent in such a way that at a pressure of 250 g/cm2 and a temperature of 60°C the contact area is at least 50% of the geometric area of the membrane.
The cathodic current distributor to which the gas diffusion electrode is applied, is preferably executed according to DE 102 03 689 Al. This has a free area of less than 65%, but more than 5%. The thickness of the current distributor is at least 0.3 mm. It can consist of an expanded metal, net, fabric, foam, nonwoven,
slotted sheet or perforated plate of metal. The cathodic current distributor preferably consists of an expanded metal with a mesh length of 4 to 8 mm, a mesh width of 3 to 5 mm, a web width of 0.4 to 1.8 mm and a thickness of 0.4 to 2 mm. In addition, the cathodic current distributor can have a second expanded metal as a support for the first expanded metal. The second expanded metal as support preferably has a mesh length of 10 to 40 mm, a mesh width of 5 to 15 mm, a web width of 2 to 5 mm and a thickness of 0.8 to 4 mm. A net which preferably has a wire thickness of 1 to 4 mm and a mesh width of 7 to 25 mm can also be used as a support. Furthermore, a perforated sheet or slotted sheet which preferably has an open area of less than 60 % and a thickness of 1 to 4 mm can furthermore be used as a support. For example, titanium or a noble metal-containing titanium alloy, such as e.g. titanium-palladium, can be used as a material for the cathodic current distributor. If the current distributor is an expanded metal, this is preferably rolled.
A commercial gas diffusion electrode which is fitted with a suitable catalyst can be used as the gas diffusion electrode. Suitable catalysts preferably contain according to WO 00/73538 Al rhodium and/or at least one rhodium sulfide or a mixture of rhodium and at least one rhodium sulfide. According to EP 931 857 Al, rhodium and/or rhodium oxide or mixtures thereof can also be used. The gas diffusion electrode preferably consists of an electrically conductive fabric, paper or nonwoven made of carbon, the fabric, paper or nonwoven having on one side a carbon-containing catalyst layer and on the other side a gas diffusion layer. The catalyst is preferably applied to a support, preferably made of carbon, polytetrafluoroethylene particles being integrated which are coupled to the support structure. The gas diffusion layer consists preferably of carbon and polytetrafluoroethylene particles in which for example the ratio of carbon to PTFE is 50:50. The gas diffusion electrode can for example be arranged so that it is not firmly connected to the ion exchange membrane. Contacting of the gas diffusion electrode to the current distributor and the ion exchange membrane takes place preferably by pressure contact, i.e. the gas diffusion electrode, the current distributor and the membrane are pressed together. The gas diffusion electrode can be connected to the current collector according to DE10148 600A1.
The electrolysis of hydrochloric acid in accordance with the membrane process with gas diffusion electrode is preferably carried out at a temperature of 40 to 70°C. The concentration of the aqueous solution of hydrogen chloride in the anode chamber is in particular 10 to 20 wt.%, particularly preferably 12 to 17 wt.%. The cell can be operated for example so that the pressure in the anode chamber is higher than the pressure in the cathode chamber. The cation exchange membrane is thereby pressed on to the gas diffusion electrode and this in turn on a current distributor. Alternatively, a construction of the electrolysis cell which is described in DE 101 38 214A1 can be selected. The anode and/or current distributor are stored elastically, for example in that they are connected by means of springs to the back wall of the respective "half-cell". When assembling the cell, a "zero gap" arrangement is produced where the anode is in direct contact with the ion exchange membrane, this in turn in direct contact with the gas diffusion electrode and this in turn indirect contact with the current distributor. The elastic storage causes the anode, membrane, gas diffusion electrode and current distributor to press together.
In a particularly preferred embodiment of the electrolysis process, the anode half-element is filled with a 5 to 20 wt.% hydrochloric acid when the electrolysis cell is started according to DE 10 152 275 Al, the hydrochloric acid containing at least 10 ppm free chlorine and the concentration of the hydrochloric acid during start-up being more than 5 wt.%. The volumetric flow rate of the hydrochloric acid through the anode chamber is set so that at the beginning of electrolysis the hydrochloric acid flows at a rate in the anode chamber of 0.05 to 0.15 cm/s. Electrolysis is started at a current density of 0.5 to 2 kA/m2 and increased by 0.5 to 1.5 kA/m2 at a time at intervals of 5 to 25 minutes. After a given current density of preferably 4 to 7 kA/m2 is achieved, the volumetric flow rate of the hydrochloric acid is set so that the hydrochloric acid in the anode half-element flows at a rate of 0.2 to 0.4 cm/s.
A particularly advantageous mode of operation of the electrolysis cell can take place according to DE 101 38 215 Al wherein the electrolysis cell is operated at an elevated pressure in the cathode chamber to lower the cell voltage. The differential
pressure between anode chamber and cathode chamber should be 0.01 to 1000 bar and the oxygen pressure in the cathode chamber at least 1.05 bar absolute.
In a further preferred variation of the new process, the hydrochloric acid is oxidised in step d) together with alkali chlorides, in particular with sodium chloride in an anodic alkali chloride electrolysis.
The process of sodium chloride electrolysis is described in more detail in the following. Normally membrane electrolysis processes are used for the electrolysis of sodium chloride-containing solutions (see also Peter Schmittinger, CHLORINE, Wiley-VCH Verlag, 2000, page 77 ff). A two-part electrolysis cell which consists of an anode chamber with an anode and a cathode chamber with a cathode, is hereby used. Anode and cathode chambers are separated by an ion exchange membrane. Commercially available ion exchange membranes such as e.g. Nafion® 982 from DUPONT can be used here. A sodium chloride-containing solution with a sodium chloride concentration of normally approx. 300 g/1 is introduced into the anode chamber consisting of titanium or a noble metal-coated titanium with a noble metal oxide-coated titanium anode. The chloride ion is oxidised to chlorine which is taken from the cell with the depleted sodium chloride-containing solution (approx. 200 g/1), on the anode. The sodium ions migrate under the influence of the electrical field through the ion exchange membrane into the cathode chamber. During this migration, each mole of sodium takes with it per membrane between 3.5 and 4.5 moles water. This leads to the anolyte becoming low in water. In contrast to the anolyte, on the cathode side water is consumed by the electrolysis of water to hydroxide ions. The water passing with the sodium ions into the catholytes is sufficient to maintain the sodium hydroxide solution concentration in the discharge at 31-32 wt.%; this applies at a NaOH inlet concentration of 30% and a current density of 4 kA/m2. Water is electrochemically reduced in the cathode chamber, hydroxide ions and hydrogen being formed.
Alternatively, a gas diffusion electrode on which oxygen are reacted with electrons to hydroxide ions, can also be used as cathode, no hydrogen being formed. The
hydroxide ions form sodium hydroxide solution with the sodium ions passing via the ion exchange membrane into the cathode chamber. A sodium hydroxide solution with a concentration of 30 wt.% is normally fed into the cathode chamber and a sodium hydroxide solution with a concentration of 31-32 wt.% taken off.
In order to feed maximum quantities of sodium chloride-containing solutions economically to sodium chloride electrolysis, it is necessary that the water transport via the membrane is-increased. This can be done by selecting suitable membranes, such as described e.g. in US 4 025 405. The effect of an increased water transport is that the otherwise normal water addition is not needed to maintain the caustic solution concentration. According to US 4 025 405, the operating conditions for electrolysis are very broadly worded, so a pH range of 1 to 5, a temperature of 35-100°C and a sodium chloride concentration of 100-325 g/1 is given.
According to US 3 773 634, electrolysis can be operated with a higher current yield at higher water transport through the membrane if a caustic solution concentration of 31-34 wt.%, particularly preferably 36-38 wt.% and a sodium chloride concentration of 120-250 g/1 is used.
A further alternative is the use of a modified cell design according to EP 1 609 887. Here the electrolysis cell is modified so that the distance between membrane and anode which is normally zero, i.e. the membrane lies on the anode, is increased. This has the disadvantage that the use can only take place using a new electrolysis cell with adapted design or by converting existing electrolysis cells to the new design which is linked to high investment. With existing plants, this process concept is economically not practicable.
When the process according to the invention is carried out, a very pure chlorine gas which can be used without problems in chemical synthesis such as for example the production of isocyanates, is obtained.
Furthermore, a process which is characterised in that the hydrogen chloride used as starting material for the first oxidation a) is the product of a production process for isocyanates and that the purified chlorine gas freed of oxygen and optionally secondary constituents of step g) is used in the production of isocyanates, in particular is reused as part of a material cycle is preferred.
In a first step of a preferred process that has as its subject matter the integration of the new combined chlorine production process into an isocyanate production, the production of phosgene takes place by reacting chlorine with carbon monoxide. The synthesis of phosgene is sufficiently known and is described e.g. in Ullmanns Enzyklopadie der industriellen Chemie, 3rd edition, volume 13, pages 494-500. Phosgene is produced on an industrial scale predominately by reacting carbon monoxide with chlorine preferably on activated carbon as catalyst. The strongly exothermic gas phase reaction takes place typically at a temperature of minimum 250°C to maximum 600°C as a rule in multi-tube reactors. Taking off the heat of reaction can take place in a different way, for example by a liquid heat exchange agent, as described e.g. in specification WO 03/072237 Al, or by hot cooling via a secondary cooling cycle with simultaneous use of the heat of reaction to produce vapour, as disclosed e.g. in US-A 4764308.
At least one isocyanate is formed from the phosgene formed in the first step by reaction with at least one organic amine or a mixture of two or more amines in a next process step. This second process step is designated phosgenation in the following. The reaction takes place with formation of hydrogen chloride as a byproduct which occurs as a mixture with the isocyanate.
The synthesis of isocyanates is likewise known in principle from the prior art, phosgene as a rule being used in a stoichiometric excess based on the amine. Phosgenation normally takes place accordingly in the liquid phase, wherein the phosgene and the amine can be dissolved in a solvent. Preferred solvents for phosgenation are chlorinated aromatic hydrocarbons such as chlorobenzene, o-dichlorobenzene, p-dichlorobenzene, trichlorobenzene, the corresponding
chlorotoluenes or chloroxylenes, chloroethylbenzene, monochlorodiphenyl, a- or ß-naphthyl chloride, benzoic acid ethyl esters, phthalic acid dialkyl esters, diisodiethyl phthalate, toluene and xylenes. Further examples of suitable solvents are in principle known from the prior art. As also known from the prior art, e.g. according to specification WO 96/16028, the isocyanate formed can likewise function as solvent for phosgene. In another, preferred embodiment, phosgenation, in particular of suitable aromatic and aliphatic diamines, takes place in the gas phase, i.e. above the boiling point of the amine. The gas phase phosgenation is described e.g. in EP 570 799 Al. Advantages of this process over the otherwise normal liquid phase phosgenation lie in energy saving caused by the minimisation of a costly solvent and phosgene cycle.
All primary amines with one or more primary amino groups which can react with
phosgene forming one or more isocyanates with one or more isocyanate groups, are
in principle suitable as organic amines. The amines have at least one, preferably
two, or optionally three and more primary amino groups. Aliphatic, cycloaliphatic,
aliphatic-aromatic, aromatic amines, di- and/or polyamines, such as aniline, halogen-
substituted phenyl amines, e.g. 4-chlorophenylamine, 1,6-diaminohexane, 1-amino-
3,3,5-trimethyl-5-amino cyclohexane, 2,4-, 2,6-diaminotoluene or mixtures thereof,
4,4'-, 2,4'- or 2,2'-diphenylmethane diamine or mixtures thereof, and also higher-
molecular isomer, oligomer or polymer derivatives of the named amines and
polyamines, are thus possible as organic primary amines. Other possible amines are
known in principle from the prior art. Preferred amines for the present invention are
the amines of the diphenyl methane diamine range (monomer, oligomer and polymer
amines), 2,4'-, 2,6'-diaminotoluene, isophorone diamine and
hexamethylenediamine. The corresponding isocyanates
diisocyanatodiphenylmethane (MDI, monomer, oligomer and polymer derivatives), toluylene diisocyanate (TDI), hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) are obtained during phosgenation.
The amines can be reacted with phosgene in a one-stage or two-stage or optionally multi-stage reaction. A continuous and also discontinuous mode of operation is thereby possible.
If a single-stage phosgenation in the gas phase is selected, the reaction takes place above the boiling point of the amine preferably within an average contact time of 0.5 to 5 seconds and at a temperature of 200 to 600°C.
Phosgenation in the liquid phase is normally carried out at a temperature of 20 to 240°C and a pressure of 1 to approx. 50 bar. Phosgenation in the liquid phase can be carried out in a single stage or in multiple stages, wherein phosgene can be used in stoichiometric excess. The amine solution and the phosgene solution are thereby combined via a static mixing element and then passed for example from bottom to top through one or more reaction towers where the mixture reacts out to the required isocyanate. In addition to reaction towers which are provided with suitable mixing elements, reaction vessels with stirring device can also be used. Apart from static mixing elements, special dynamic mixing elements can also be used. Suitable static and dynamic mixing elements are in principle known from the prior art.
As a rule, the continuous liquid phase isocyanate production is carried out on an industrial scale in two stages. In the first stage carbamoyl chloride is formed from amine and phosgene and amine hydrochloride from amine and split off hydrogen chloride in general at a temperature of maximum 220°C, preferably maximum 160°C. This first stage is strongly exothermic. In the second stage, both the carbamoyl chloride is split to isocyanate and hydrogen chloride and the amine hydrochloride reacted to the carbamoyl chloride. The second stage is carried out as a rule at a temperature of at least 90°C, preferably 100 to 240°C.
After phosgenation, separation of the isocyanates formed during phosgenation takes place in a third step. This occurs by first of all separating the phosgenation reaction mixture into a liquid and a gaseous product stream in a way known in principle to
the skilled person. The liquid product stream contains substantially the isocyanate or isocyanate mixture, the solvent and a small part of unreacted phosgene. The gaseous product stream consists substantially of hydrogen chloride gas, stoichiometrically excess phosgene, and small quantities of solvent and inert gases, such as for example nitrogen and carbon monoxide. Furthermore, the liquid stream is then fed to a processing, preferably a distillative processing, with phosgene and the solvent for phosgenation being separated off in succession. Optionally a further processing of the isocyanates formed also takes place. This occurs for example by fractionating the isocyanate product obtained in a way known to the skilled person.
The hydrogen chloride obtained in the reaction of phosgene with an organic amine in general contains organic secondary constituents which can interfere both in the thermal catalysed or non-thermal activated HC1 oxidation and in the electrochemical oxidation of an aqueous hydrogen chloride solution according to step (c). These organic constituents include for example the solvents used in isocyanate production such as chlorobenzene, o-dichlorobenzene or p-dichlorobenzene. If a gas diffusion electrode is used as cathode during electrolysis, the catalyst of the gas diffusion electrode can also be deactivated by the organic impurities. In addition, these impurities can be deposited on the current collector and thereby impair the contact between gas diffusion electrode and current collector, resulting in an undesired rise in voltage. If the diaphragm process is used for electrolysis of the hydrochloric acid, the named organic constituents can be deposited on the graphite electrodes and/or diaphragm and thereby likewise increase the electrolysis voltage.
The separation of the hydrogen chloride produced during phosgenation from the gaseous product stream accordingly takes place preferably in a further process step. The gaseous product stream which is obtained during separation of the isocyanate is treated so that the phosgene can again be fed to phosgenation and the hydrogen chloride to an electrochemical oxidation.
The separation of the hydrogen chloride takes place preferably first by separating off phosgene from the gaseous product stream. Separation of the phosgene occurs by
liquefaction of phosgene, for example on one or more series-connected condensers. Liquefaction takes place preferably at a temperature in the range of-15 to -40°C as a function of the solvent used. In addition, parts of the solvent residues can be removed from the gaseous product stream by this deep cooling.
In addition or alternatively, the phosgene can be washed out from the gas stream with a cold solvent or solvent-phosgene mixture in one or more stages. The solvents chlorobenzene and o-dichlorobenzene already used in the phosgenation are for example suitable as solvent for this. The temperature of the solvent or the solvent-phosgene mixture for this is in the range of-15 to -46°C.
The phosgene separated off from the gaseous product stream can be again fed to phosgenation. The hydrogen chloride obtained after separation of the phosgene and part of the solvent residue can contain, in addition to the inert gases such as nitrogen and carbon monoxide, 0.1 to 1 wt.% solvent and 0.1 to 2 wt.% phosgene.
Optionally a purification of the hydrogen chloride then takes place to reduce the content of traces of solvent. This can take place for example by means of freezing, by passing the hydrogen chloride for example through one or more cold traps as a function of the physical properties of the solvent.
In a particularly preferred embodiment of the purification of the hydrogen chloride optionally provided, the hydrogen chloride stream flows through two series-connected heat exchangers, the solvent to be separated being frozen as a function of the fixed point for example at -40°C. The heat exchangers are preferably operated alternately, the gas current thawing the previously frozen solvent in the heat exchanger through which it has first flowed. The solvent can be used again for the production of a phosgene solution. In the downstream second heat exchanger which is treated with a normal heat exchange medium for refrigerators, e.g. a compound from the freon range, the gas is preferably cooled below the fixed point of the solvent so that this crystallises out. When the thawing and crystallisation procedures are complete, the gas stream and the coolant stream are switched so that the function
of the heat exchangers is reversed. The hydrogen chloride-containing gas stream can be depleted in this way to preferably maximum 500 ppm, particularly preferably maximum 50 ppm, very particularly preferably to maximum 20 ppm solvent content.
Alternatively, purification of the hydrogen chloride can take place preferably in two series-connected heat exchangers, e.g. according to US-A-6 719 957. The hydrogen chloride is thereby preferably compressed at a pressure of 5 to 20 bar, preferably 10 to 15 bar, and the compressed gaseous hydrogen chloride fed to a first heat exchanger at a temperature of 20 to 60°C, preferably 30 to 50°C. In this, the hydrogen chloride is cooled with a cold hydrogen chloride with a temperature of -10 to -30°C which comes from a second heat exchanger. Organic constituents which can be passed for disposal or recycling are thereby condensed. The hydrogen chloride passed into the first heat exchanger leaves this at a temperature of -20 to 0°C and is cooled in the second heat exchanger to a temperature of -10 to -30°C. The condensate occurring in the second heat exchanger consists of further organic constituents and small quantities of hydrogen chloride. To avoid a loss of hydrogen chloride, the condensate discharging from the second heat exchanger is fed to a separation and evaporation unit. This can for example be a distillation column in which the hydrogen chloride is expelled from the condensate and is returned to the second heat exchanger. It is also possible to return the expelled hydrogen chloride to the first heat exchanger. The hydrogen chloride cooled in the second heat exchanger and freed from organic constituents is passed into the first heat exchanger at a temperature of -10 to -30°C. After heating to 10 to 30°C, the hydrogen chloride freed of organic constituents leaves the first heat exchanger.
In a likewise preferred alternative process, the optionally provided purification of the hydrogen chloride from organic impurities, such as solvent residues, takes place on activated carbon by means of adsorption. The hydrogen chloride is for example thereby passed over or through an activated carbon bed after removing excess phosgene at a pressure difference of 0 to 5 bar, preferably 0.2 and 2 bar. The flow
rate and dwell time is thereby adapted in a way known to the skilled person to the content of impurities. The adsorption of organic impurities is likewise possible on other suitable adsorption agents, e.g. on zeolites.
In a further, also preferred, alternative process, a distillation of the hydrogen chloride can be provided for the optionally provided purification of the hydrogen chloride from phosgenation. This takes place after condensation of the gaseous hydrogen chloride from phosgenation. In the distillation of the condensed hydrogen chloride, the purified hydrogen chloride is removed as the overhead product of the distillation, distillation taking place at standard conditions of pressure, temperature etc for such a distillation known to the skilled person.
The hydrogen chloride separated off and optionally purified according to the process described above can then be fed to HC1 oxidation with oxygen. This HC1 oxidation takes place as described preferably according to the Deacon process.
As already described above, the catalytic process known as the Deacon process is preferably used. Hydrogen chloride is herein oxidised with oxygen in an exothermic equilibrium reaction to chlorine, water vapour being produced. The reaction temperature is normally 150 to 500°C, the normal reaction pressure is 1 to 25 bar. Since this is an equilibrium reaction, it is expedient to work at the lowest temperatures at which the catalyst still has sufficient activity. Since there is no fear of selectivity losses, it can be economically advantageous to work at relatively high pressure and correspondingly at a longer dwell time compared to standard pressure.
Suitable preferred catalysts for the Deacon process contain ruthenium oxide, ruthenium chloride or other ruthenium compounds on silicon dioxide, aluminium oxide, titanium dioxide or zirconium dioxide as support. Suitable catalysts can for example be obtained by applying ruthenium chloride to the support and subsequent drying or drying and calcining. Suitable catalysts can also contain in addition to or in place of a ruthenium compound, compounds of other noble metals, for example
gold, palladium, platinum, osmium, iridium, silver, copper or rhenium. Suitable catalysts can furthermore contain chromium(III) oxide.
The catalytic hydrogen chloride oxidation can be carried out adiabatically or preferably isothermally or approximately isothermally, discontinuously, but preferably however continuously as a fluid-bed or fixed-bed process, preferably as a fixed-bed process, particularly preferably in multi-tube reactors on heterogeneous catalysts at a reactor temperature of 180 to 500°C, preferably 200 to 400°C, particularly preferably 220 to 350°C and a pressure of 1 to 25 bar (1000 to 25000 hPa), preferably 1.2 to 20 bar, particularly preferably 1.5 to 17 bar and in particular 2.0 to 15 bar.
Normal reaction apparatus in which the catalytic hydrogen chloride oxidation is carried out are fixed-bed or fluid-bed reactors. The catalytic hydrogen chloride oxidation can preferably also be carried out in multiple stages.
Several, thus 2 to 10, preferably 2 to 6, particularly preferably 2 to 5, in particular 2 to 3, series-connected reactors with additional intermediate cooling can be used in the isothermal or approximately isothermal procedure. The oxygen can be added in full together with the hydrogen chloride before the first reactor or distributed over the various reactors. This series connection of individual reactors can also be combined in one apparatus.
A further preferred embodiment of a device suitable for the process consists of using a structured catalyst bed in which the catalyst activity increases in the direction of flow. Such a structuring of the catalyst bed can take place by different impregnation of the catalyst supports with activated material or by different dilution of the catalyst with an inert material. Rings, cylinders or pellets of titanium dioxide, zirconium dioxide or mixtures thereof, aluminium oxide, steatite, ceramic, glass, graphite or noble steel for example can be used as inert material. With the preferred use of catalyst shaped bodies, the inert material should have preferably similar external dimensions.
Shaped bodies with any shapes are suitable as catalyst shaped bodies; tablets, rings, cylinders, stars, cartwheels or pellets are preferred as the shape, particularly preferred are rings, cylinders or star strands.
In particular, ruthenium compounds or copper compounds on support materials which can also be doped, are suitable as heterogeneous catalysts; optionally doped ruthenium catalysts are preferred. Silicon dioxide, graphite, and titanium dioxide with a rutile or anatase structure, zirconium dioxide, aluminium oxide or mixtures thereof, preferably titanium dioxide, zirconium dioxide, aluminium oxide or mixtures thereof, particularly preferably y- or 8-aluminium oxide or mixtures thereof, are suitable as support materials.
The copper or ruthenium support catalysts can be obtained for example by impregnating the support material with aqueous solutions of CuC12 or RuCl3 and optionally a promoter for doping, preferably in the form of their chlorides. The shaping of the catalyst can take place after or preferably before impregnation of the support material.
Alkali metals such as lithium, sodium, potassium, rubidium and caesium, preferably lithium, sodium and potassium, particularly preferably potassium, alkaline-earth metals such as magnesium, calcium, strontium and barium, preferably magnesium and calcium, particularly preferably magnesium, rare earth metals such as scandium, yttrium, lanthanum, cerium, praseodymium and neodymium, preferably scandium, yttrium, lanthanum and cerium, particularly preferably lanthanum and cerium, or mixtures thereof, are suitable as promoters for doping the catalysts.
The shaped bodies can then be dried and optionally calcined at a temperature of 100 to 400°C, preferably 100 to 300°C for example under a nitrogen, argon or air atmosphere. The shaped bodies are preferably first of all dried at 100 to 150°C and then calcined at 200 to 400°C.
The reaction to hydrogen chloride in the single pass can be limited preferably to 15 to 90%, preferably 40 to 85%, particularly preferably 50 to 70%. Unreacted hydrogen chloride can be returned after separation partially or completely to the catalytic hydrogen chloride oxidation.
The heat of reaction of the catalytic hydrogen chloride oxidation can be used in an advantageous way to produce high-pressure water vapour. This can be used to operate a phosgenation reactor and or distillation columns, in particular isocyanate distillation columns.
In a next step, the chlorine formed is separated off. The separation step normally comprises several stages, namely the separation and optionally return of unreacted hydrogen chloride from the product gas stream of the catalytic hydrogen chloride oxidation, the drying of the stream obtained containing substantially chlorine and oxygen and the separation of chlorine from the dried stream.
The separation of unreacted hydrogen chloride and of water vapour formed can take place by condensing out aqueous hydrochloric acid from the product gas stream of the hydrogen chloride oxidation by cooling. Hydrogen chloride can be absorbed even in dilute hydrochloric acid or water.
The process according to the invention is illustrated for example in greater detail in the following by means of Fig. 1
Fig. 1 shows a preferred embodiment of the new process here in combination with the production of isocyanates.
Examples:
Fig. 1 shows an example for the use of the process as a supplement and part of an isocyanate production.
In a first stage 1 of the isocyanate production, chlorine 11 is reacted with carbon monoxide 10 to phosgene 13. In the following stage 2, phosgene 13 from stage 1 is reacted with an amine 14 (e.g. toluene diamine) to a mixture 15 of isocyanate (e.g. toluene diisocyanate, TDI) and hydrogen chloride, the isocyanate 16 is separated off (in stage 3) and utilised and the HC1 gas 17 subjected to a purification 4. The purified HC1 gas 18 is reacted in the HC1 oxidation process 5 with oxygen, here in a Deacon process by means of catalyst.
The reaction mixture 19 from stage 5 is cooled (step 6). Aqueous hydrochloric acid 26 which thereby occurs mixed optionally with water or dilute hydrochloric acid, is channelled out.
The gas mixture 20 thus obtained, consisting at least of chlorine, oxygen and secondary constituents such as nitrogen, carbon dioxide etc and [sic] is dried with cone, sulfuric acid 21 (96%) (step 7).
In a purification stage 8, chlorine 11 is separated from the gas mixture 22 from stage 7. The residual stream 23 having oxygen and secondary constituents is fed to electrolysis stage 9. The electrolysis stage is an ODC electrolysis in which on the cathode side oxygen is used as reactant.
The chlorine gas obtained from the purification stage 8 can be used again directly in the phosgene synthesis 1. The oxygen-containing stream 23 occurring in this step is used in step 5 (HC1 oxidation) and step 9 (electrolysis cell).
The concentration of the hydrochloric acid 27 fed to the electrolysis cell 9 is 14 to 15 wt.% HC1, that of the hydrochloric acid 28 discharging from electrolysis 9
is 11 to 13 wt.% HC1. Concentrated hydrochloric acid 26 from the separation stage 6 is added to the hydrochloric acid stream 28 and again fed to cell 9.
The oxygen consumed in step 5 and step 9 is replaced by oxygen from an external source 24. The oxygen 25 not consumed in the cathode chamber of the electrolysis cell is passed in the cycle and fresh oxygen from an external source 24 added.
The approx. 2 wt.% hydrochloric acid stream 29 likewise occurring in the cathode chamber is fed to the hydrochloric acid separation 6 and serves there as absorption agent for excess gaseous hydrogen chloride.
The chlorine 30 produced according to step 9 is combined with the chlorine-containing gas stream 20.
The combined gas stream 30, 20 is cooled in a single- or multi-stage cooling by means of a cooling unit, e.g. a tubular heat exchanger, and dried. Drying 7 can take place for example by means of a suitable drying agent in an absorption column fitted with material exchange elements. A suitable drying agent can, as described e.g. in DE 10 235 476 A, be, in addition to molecular sieves or hygroscopic adsorbents, e.g. sulfuric acid. Drying can take place in a single or multiple stages. Drying preferably takes place in two stages, by the chlorine to be dried being brought into contact in a first stage with a sulfuric acid of lower concentration, preferably 70 to 80%), particularly preferably 75 to 80%. In a second stage, the residual moisture is removed from the chlorine by means of a higher-concentrated sulfuric acid of preferably 88 to 96%, particularly preferably 92-96%. The chlorine (22) dried in this way with a residual moisture of preferably maximum 100 ppm, particularly preferably maximum 20 ppm, can be passed through a droplet separator in order to optionally remove sulfuric acid droplets still contained therein.
The dried chlorine gas stream 22 is then subjected to a chlorine purification 8.
Example 1
A stream of 35.9 t/h purified hydrogen chloride from an isocyanate plant is divided into two partial streams. 29.5 t/h are fed to a HCl oxidation and 6.4 t/h to a HCl absorption. 29.5 t/h HCl are fed with 12.9 t/h oxygen (content greater than 99%) of a catalytic HCl oxidation. Oxidation takes place at 333°C and 3.4 bar. The HCl conversion in the reactor is 85%. The gas mixture leaving the reactor is cooled to 100°C, the HCl condensed with the reaction water in a HCl absorption. A partial stream of the hydrochloric acid-depleted anolytic acid stream 28 coming from electrolysis is introduced into the hydrochloric acid 26 from the HCl absorption. 32.1 t/h of the hydrochloric acid-depleted analytic acid 28 with a HCl concentration of 12.2 wt.% is fed to HCl absorption. The cooled process gas stream (4.4 t/h HCl, 7.4 t/h oxygen, 24.4 t/h chlorine, 6.18 t/h water) is passed with 6.4 t/h purified hydrogen chloride into the HCl absorption. In this HCl absorption unit, a 30 wt.% hydrochloric acid 26 which is combined with the residue of the depleted anolytic acid 28 and again fed to the electrolysis cell, is produced. 2.96 t/h of the depleted anolytic acid 28 are channelled from the anolytic acid cycle (not shown).
Electrolysis is operated with a current density of 5 kA/m2 at 55°C and a voltage of 1.39 V. A palladium-stabilised titanium is used as the anode and cathode material. 10.1 t/h chlorine is developed on the ruthenium oxide-coated anode manufactured by DENORA, Germany. The anode and cathode half-shells are separated by an ion exchange membrane, type Nation 324, manufactured by DUPONT. An oxygen absorption cathode manufactured by ETEK which contains a rhodium sulfide-supported catalyst is used as the cathode. Oxygen is fed to the cathode half-element with 100%o excess, i.e. at 9.17 t/h. The oxygen is recycled in the electrolysis, a purge stream of 1% of the feed quantity is led off behind electrolysis 9 (not shown) or used in HCl oxidation 5. The pressure in the anode half-cell is higher than that in the cathode half-cell. The differential pressure is 200 mbar. A condensate stream of 8.8 t/h is removed from the cathode half-cell.
The electrolysis unit 9 consists of 615 electrolysis cells, one element consisting of an anode half-shell with anode, an ion exchange membrane and a cathode half-shell with oxygen absorption cathode.
Example 2
As in Example 1, a purified HCl gas stream is fed at 35.9 t/h to the HCl oxidation with 100% oxygen excess, i.e. 15.7 t/h oxygen. The conversion is 85%, so that 5.4 t/h HCl, 9.0 t/h oxygen, 7.5 t/h water and 29.7 t/h chlorine leave the reactor. This process gas 19 is fed to a HCl absorption which is operated with a first partial stream of 177.8 t/h of a NaCl-depleted NaCl-containing solution (18.3 wt.% NaCl) coming from NaCl electrolysis. The water and hydrogen chloride of the process gas 19 is absorbed in this NaCl-containing solution. The stream leaving the absorption is composed as follows : 152.8 t/h water, 32.5 t/h NaCl, 5.4 t/h hydrogen chloride. This stream is then combined with the second partial stream of the NaCl-containing solution of 118.2 t/h, 26.4 t/h solid NaCl are added and it is again fed to the NaCl electrolysis. NaCl electrolysis consists of 1475 biopolar electrolysis elements per 2.71 m2 membrane surface. NaCl electrolysis is operated with anode half-shells made of titanium which has a noble metal-coated titanium anode. The cathode half-shell consists of nickel and supports a noble metal oxide-coated nickel cathode. The anode and cathode half-shells are separated by an ion exchange membrane from Nafion 982 manufactured by DUPONT. 21.2 t/h chlorine are produced anodically, 74.8 t/h sodium hydroxide solution with a concentration of 32 wt.% and 0.6 t/h hydrogen cathodically. The NaCl-depleted anolyte is partially fed again to the HCl absorption.

Claims
1. Process for the production of chlorine by means of multi-stage oxidation by
a) thermal reaction of hydrogen chloride with oxygen using catalysts or by
non-thermal activated reaction of hydrogen chloride with oxygen,
in which the gas mixture forming during reaction a), consisting of the target products chlorine and water, unreacted hydrogen chloride and oxygen, and other secondary constituents, in particular carbon dioxide and nitrogen,
b) is cooled to condense hydrochloric acid and
c) the aqueous solution of hydrogen chloride (hydrochloric acid) forming is separated off from the gas mixture, characterised in that
d) the hydrochloric acid separated off is fed at least partially to an electrochemical oxidation in which at least part of the aqueous hydrochloric acid is oxidised to chlorine,
e) the chlorine gas produced in step d) is optionally added to the gas mixture occurring in step c),
f) the residues of water present in the gas mixture from steps c) and e), in particular by washing with sulfuric acid, are removed,
g) the chlorine-rich gas mixture forming is freed of oxygen and optionally of secondary constituents.
2. Process according to Claim 1, characterised in that the electrochemical
oxidation of the aqueous hydrogen chloride solution takes place according to
step d) by means of diaphragm electrolysis.
3. Process according to Claim 1, characterised in that the electrochemical oxidation of the aqueous hydrogen chloride solution according to step d) takes place by means of diaphragm electrolysis.
4. Process according to Claim 3, characterised in that electrodes based on graphite are used as electrodes for the anode and/or the cathode.
5. Process according to Claim 3 or 4, characterised in that cathodes based on graphite which have a coating containing iridium, are used
6. Process according to at least one of Claims 1 to 5, characterised in that hydrogen chloride is separated off completely from the gas mixture as hydrochloric acid according to steps b) and c).
7. Process according to at least one of Claims 1 to 6, characterised in that the separation of the hydrogen chloride from the gas mixture according to step b) and c) takes place by additional washing of the gas mixture with water or dilute hydrochloric acid, in particular with water.
8. Process according to at least one of Claims 1, 2 and 4 to 7, characterised in that a gas diffusion electrode which is operated as an oxygen absorption cathode is used as the cathode for the electrolysis according to step d).

9. Process according to Claim 8, characterised in that the gas diffusion electrode is based at least on an electrically conductive fabric, woven, knitted fabric, net or nonwoven of carbon which is arranged between a carbon-containing catalyst layer and a gas diffusion layer.
10. Process according to Claim 9, characterised in that the catalyst layer contains rhodium and/or a rhodium sulfide.
11. Process according to at least one of Claims 1, 2 and 4 to 10, characterised in that a titanium-containing electrode which has a coating of at least one noble metal oxide, preferably of ruthenium oxide, is used as the anode.
12. Process according to at least one of Claims 1, 2 and 4 to 11, characterised in that titanium and/or a titanium alloy is used as the material for the electrolysis cell.
13. Process according to one of Claims 1 to 12, characterised in that the hydrogen chloride used as the starting material for the first oxidation a) is the product of a production process of isocyanates and that the purified chlorine gas of step g) freed of oxygen and optionally of secondary constituents is reused in the production of isocyanates, in particular as part of a material cycle.
14. Process according to at least one of Claims 1, 2 and 6 to 13, characterised in that the hydrochloric acid in step d) is oxidised together with alkali chlorides, in particular with sodium chloride in an anodic alkali chloride electrolysis.
15. Process according to at least one of Claims 1, 2 and 6 to 14, characterised in that the first oxidation a) takes place by thermal catalysed oxidation (Deacon process).
16. Process according to Claim 14 or 15, characterised in that a depleted alkali chloride solution coming from the alkali chloride electrolysis is used as alkali chloride solution.
17. Process according to at least one of Claims 1, 2 and 6 to 16, characterised in
that part of the hydrogen chloride provided for the first HCl oxidation is
branched, subjected to an absorption with water or dilute hydrochloric acid,
in particular dilute hydrochloric acid, and the concentrated hydrochloric acid
forming combined with the hydrochloric acid from step c) and fed to
electrolysis according to step d).
18. Process according to at least one of Claims 1 to 17, characterised in that the first HCl oxidation a) is operated with a conversion of 50 to 98%, in particular with a conversion of 60 to 95%.

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Patent Number

268428

Indian Patent Application Number

9432/DELNP/2008

PG Journal Number

36/2015

Publication Date

04-Sep-2015

Grant Date

30-Aug-2015

Date of Filing

11-Nov-2008

Name of Patentee

BAYER MATERIALSCIENCE AG

Applicant Address

51368 LEVERKUSEN, GERMANY

Inventors:

#	Inventor's Name	Inventor's Address
1	RAINER WEBER	FORSTSTR.15 A, 51519 ODENTHAL, GERMANY
2	JURGEN KINTRUP	MAX-LIEBERMANN-STR. 11, 48282 EMSDETTEN, GERMANY
3	ANDREAS BULAN	ALT LANGENFELD 15, 40764 LANGENFELD, GERMANY
4	FRIEDHELM KAMPER	BIRMESSTR. 65A, 47807 KREFELD, GERMANY

PCT International Classification Number

C01B 7/04

PCT International Application Number

PCT/EP2007/004147

PCT International Filing date

2007-05-10

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	10 2006 023 261.5	2006-05-18	Germany