Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/24—Halogens or compounds thereof
  • C25B1/26—Chlorine; Compounds thereof
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/02—Process control or regulation
  • C25B15/023—Measuring, analysing or testing during electrolytic production
  • C25B15/025—Measuring, analysing or testing during electrolytic production of electrolyte parameters
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/70—Assemblies comprising two or more cells
  • C25B9/73—Assemblies comprising two or more cells of the filter-press type
  • C25B9/77—Assemblies comprising two or more cells of the filter-press type having diaphragms

Definitions

• the invention relates to a method for electrolysis of aqueous solutions of hydrogen chloride for the production of chlorine by means of a gas diffusion electrode
• hydrochloric acid Aqueous solutions of hydrogen chloride, hereinafter referred to as hydrochloric acid, arise as a waste product in many processes in which organic hydrocarbon compounds are chlorinated with chlorine in an oxidizing manner.
• the recovery of chlorine from these hydrochloric acids is economically interesting. The recovery can take place electrolytically with the use of gas diffusion electrodes which consume oxygen in the cathode compartment (oxygen distillation cathode).
• a corresponding ner driving is known from US-A-5 770 035. Accordingly, the electrolysis takes place in an electrolytic cell with an anode compartment with a suitable anode, e.g. a noble metal coated or doped titanium electrode, which is filled with the aqueous solution of hydrogen chloride.
• a suitable anode e.g. a noble metal coated or doped titanium electrode
• the chlorine formed on the anode escapes from the anode compartment and is fed to a suitable treatment.
• the anode compartment is separated from a cathode compartment by a commercially available cation exchange membrane.
• a gas diffusion electrode rests on the cation exchange membrane.
• a power distributor is located behind the gas diffusion electrode.
• An oxygen-containing gas or pure oxygen is usually introduced into the cathode compartment.
• the type of start-up and operation of an electrolysis cell has an influence on the service life of the anodes or the anode half-element and thus on the economy of the process.
• Oxidizing agents for example iron (LTJ) or copper (II) as corrosion protection added. These additives must then be removed from the hydrochloric acid again using an additional apparatus. In addition, they represent a contamination of the hydrochloric acid, which may impair the action of the ion exchange membrane or lead to crystallization.
• US-A-5 770 035 does not disclose any conditions for starting the cell.
• the object of the present invention was to provide a process for the electrolysis of aqueous solutions of hydrogen chloride with optimized operating parameters.
• the invention relates to a process for the electrolysis of aqueous solutions of hydrogen chloride for the production of chlorine, the following process parameters being observed for operation:
• the anode half element is filled with a 5 to 20 wt .-% hydrochloric acid.
• the concentration of hydrochloric acid during operation is more than 5% by weight.
• the volume flow of the hydrochloric acid through the anode half-element is set so that at the start of the electrolysis the hydrochloric acid has a speed in the anode compartment of 0.05 cm / s to 0J5 cm / s.
• the electrolysis is started with a current density of 0.5 to 2 kA m 2 the current density is then increased continuously or discontinuously until the target current density is reached.
• the optimum concentration of hydrochloric acid for starting up, starting up and operating is around 13% by weight.
• Above a concentration of 20% by weight the stress also increases and the corrosion increases.
• the anode coating can be damaged, for example, by a 25% by weight hydrochloric acid at 80 ° C. Therefore, the concentration of hydrochloric acid must also be at least 5% by weight for commissioning.
• commissioning is understood to mean the operating period from the start of electrolysis until the target current density is reached.
• a noble metal coated or noble metal doped titanium electrode is preferably used as the anode.
• Chlorine serves as corrosion protection for the anode metal and the
• Metal forming anode space e.g. Titanium.
• Hydrochloric acid that has penetrated through micropores of the anode coating can attack the anode metal, for example titanium. As the corrosion of the anode metal progresses, the coating may flake off. It is therefore important to ensure that sufficient chlorine, but at least 1 mg / 1, preferably at least 50 mg / 1, particularly preferably 300 mg / 1, of free chlorine is contained in the hydrochloric acid during commissioning, when the system is at a standstill and when filling. This condition is practically always fulfilled when the set current density is reached.
• the hydrochloric acid is pumped through the anode half-element and kept in circulation.
• the electrolysis cell must be operated with a volume flow in the range from 0.05 cm / s to 0.15 cm s in order to achieve optimal electrolysis efficiency. In particular, meaningful operation is not possible with a lower volume flow.
• the temperature of the hydrochloric acid is too high Start preferably between 30 and 50 ° C, during the operation of the electrolysis in the range of 50 to 70 ° C.
• the electrolysis cell is put into operation with a current density of 0.5 to 2 kA m 2 , preferably 1 to 2 kA / m 2 , very particularly preferably 1.5 kA / m 2 , in any case with a lower than the desired current density to be achieved later.
• a current density 0.5 to 2 kA m 2 , preferably 1 to 2 kA / m 2 , very particularly preferably 1.5 kA / m 2 , in any case with a lower than the desired current density to be achieved later.
• the target current density should be above 1 kA / m 2 , but preferably in the range from 2 to 8 kA / m 2 . The exact value depends on the amount of chlorine to be produced. A too low target current density leads to insufficient chlorine gas development.
• the current density should not increase by less than 0.5 kA / m 2 within 25 minutes and not more than 1.5 kA / m 2 within 5 minutes.
• a faster start-up that is to say a faster increase in the current density from the start-up to reaching the desired current density, can lead to overheating of the electrolysis cell, which is associated with a risk to the mechanical and chemical stability of the titanium.
• the electrolyte can also strike back from the standpipe into the anode compartment when the vehicle is started up quickly.
• the increase can preferably be carried out discontinuously, with the current density being increased by 0.5 to 1.5 kA / m 2 , preferably by 1 kA / m 2 , at intervals of 5 to 25 minutes.
• the current density can be increased continuously until the target current density is reached.
• the pressure difference between the anode compartment and the cathode compartment is greater than 50 mbar during commissioning until the target current density is reached, and then preferably greater than 100 mbar during operation. In this way, additional contact resistances and a higher electrolysis voltage are avoided, which occur if the pressure is too low, since the gas diffusion electrode has to be pressed onto the cathodic current collector by the higher pressure in the anode space.
• the anolyte In operation, the anolyte is more compressible due to its chlorine content, the density of the anolyte decreases as the chlorine content increases. Therefore, the pressure difference between the anode space and the cathode space in operation after reaching the target current density is preferably greater than 100 mbar.
• the volume flow of the hydrochloric acid can preferably be set so that the hydrochloric acid in the anode half-element has a speed of 0.2 cm / s to 0.4 cm / s. In this way, lifting off via the standpipes and an uneven supply of liquid to the half-elements are avoided.
• the method according to the invention can also be optimized in that the temperature difference between the entry of the hydrochloric acid into the anode half-element (anolyte inlet) and the outlet of the hydrochloric acid from the anode half-element (anolyte
• Outlet is less than 15 ° C. This enables a uniform, low temperature distribution in the anolyte, which in particular avoids temperature peaks of over 60 ° C.
• the method according to the invention is preferably to be used when as
• Electrolysis cell an electrolyzer is used, in which the electrolyte and the chlorine formed are led out of the anode half-element via a standpipe.
• the electrolyzer for carrying out the method according to the invention usually consists of several electrochemical cells, anode and Cathode half elements are arranged alternately.
• the anode half element is formed from the anode space and the anode, the cathode half element from the cathode space and the gas diffusion electrode and a current distributor.
• the anode and cathode half elements are separated by a cation exchange membrane.
• the anode frame for forming the anode half-element, the cathode frame for forming the cathode half-element and the anode consist of resistant materials such as, for example, noble metal-coated or -doped titanium or titanium alloys. Commercial membranes such as the membrane from DuPont, National 324, can be used as the cation exchange membrane.
• In the cathode compartment is oxygen or an oxygen-rich
• Gas introduced can be carried out using commercially available gas diffusion electrodes, for example from E-TEK (USA), with 30% platinum on Vulcan® XC-72 (activated carbon), with a noble metal coating of the electrode of 1.2 mg Pt / cm 2 , As described in EP-A-785 294, the gas diffusion electrode is pressed onto the current distributor by the cation exchange membrane due to a higher pressure in the anode compartment than in the cathode compartment. Sufficient electrical contact is thus established.
• E-TEK USA
• platinum on Vulcan® XC-72 activated carbon
• the examples described below were carried out with an electrolytic cell consisting of anode half cell and cathode half cell.
• the anode used consisted of expanded titanium, which was activated with a ruthenium oxide layer.
• a carbon-based gas diffusion electrode with a noble metal coating from E-TEK (USA) was used as the cathode.
• the gas diffusion electrode was connected to a current collector.
• the current collector also consisted of activated titanium expanded metal.
• Example 1 (hydrochloric acid with chlorine; serves as a comparison to example 2 with regard to the HCl concentration, and as comparison with comparison example 1 and example 3 with regard to the chlorine content)
• the electrolytic cell was filled with 9% by weight hydrochloric acid, which contained 780 mg / 1 free chlorine. Then the oxygen supply to the cathode half-element was opened and the oxygen was supplied at a volume flow of 1.25 m 3 / h. The volume flow of the hydrochloric acid was set so that the speed of the
• Hydrochloric acid at the start of the electrolysis was 0.1 cm / s.
• the current density was 1 kA / m at the beginning of the electrolysis and was increased by 1 kA / m 3 in intervals of 15 minutes until the desired value of the current density (desired current density). of 4 kA / m 3 was reached.
• the volume flow of hydrochloric acid was increased so that its speed was 0.3 cm / s.
• the concentration of hydrochloric acid never fell below 5% by weight during commissioning.
• the hydrochloric acid concentration of 9% by weight was maintained by continuously adding fresh concentrated hydrochloric acid (32% by weight), while dilute hydrochloric acid and chlorine were continuously removed.
• the temperature of the hydrochloric acid was initially
• the electrolytic cell was filled with 13% by weight hydrochloric acid, which contained no chlorine. Then the oxygen supply to the half-element of the cathode was opened and the oxygen was supplied at a volume flow of 1.25 m 3 / h.
• the volume flow of the hydrochloric acid was set so that the speed of the hydrochloric acid at the start of the electrolysis was 0J cm / s.
• the current density was at the beginning of the
• Electrolysis 1 kA / m 2 and was increased by 1 kA m 2 at intervals of 15 minutes until the target value of the current density (target current density) of 4 kA / m 2 was reached. After reaching the target flow density, the volume flow of hydrochloric acid was increased so that the speed was 0.3 cm / s. The concentration of hydrochloric acid never fell below 5% by weight during commissioning. During the
• the hydrochloric acid concentration of 13% by weight was maintained by continuously supplying fresh concentrated hydrochloric acid (32% by weight), while dilute hydrochloric acid and chlorine were continuously removed.
• the temperature of the hydrochloric acid was initially 40 ° C (at 1 kA / m 2 ) and was increased to 60 ° C. The was between the inlet and outlet of the hydrochloric acid
• Example 2 Influence of the HO concentration on voltage when the desired current density is reached; a voltage minimum is 13% by weight
• the electrolytic cell was filled with 17% hydrochloric acid, which contained 1280 mg / 1 free chlorine. Then the oxygen supply to the cathode half-element was opened and the oxygen was supplied at a volume flow of 1.25 m 3 / h.
• the volume flow of the hydrochloric acid was set so that the speed of the hydrochloric acid at the start of the electrolysis was 0.1 cm / s.
• the current density was at the beginning of the electrolysis 1 kA / m 2 and was dissolved in 15 minute intervals by 1 kA / m 2 increased until the desired value of the current density (target current density) of 4 kA / m 2 was reached.
• the volume flow of hydrochloric acid was increased so that its speed was 0.3 cm / s.
• the concentration of hydrochloric acid never fell below 5% by weight during commissioning.
• the hydrochloric acid concentration of 17% by weight was maintained by continuously supplying fresh concentrated hydrochloric acid (32% by weight), while dilute hydrochloric acid and chlorine were continuously removed.
• the temperature of the hydrochloric acid was initially 40 ° C (at 1 kA / m 2 ) and was increased to 60 ° C.
• the electrolysis voltage was 1.47 V at a target current density of 4 kA / m 2 . No traces of corrosion were observed on the anode and anode half-element at the end of the test.
• the procedure according to Comparative Example 1 was that the hydrochloric acid was additionally mixed with chlorine: the electrolytic cell was filled with 13% by weight hydrochloric acid which contained 200 mg / l free chlorine. Then the oxygen supply to the cathode half-element was opened and the oxygen was supplied at a volume flow of 1.25 niVh. The volume flow of the hydrochloric acid was set so that the speed of the hydrochloric acid at the start of the electrolysis was 0J cm s. The current density at the start of the electrolysis was 1 kA m 2 and was increased by 1 kA / m 2 at intervals of 15 minutes until the setpoint of Current density (target current density) of 4 kA / m 2 was reached.
• the volume flow of hydrochloric acid was increased so that the speed was 0.3 cm / s.
• the concentration of hydrochloric acid never fell below 5% by weight during commissioning.
• the hydrochloric acid concentration of 13% by weight was maintained by continuously supplying fresh concentrated hydrochloric acid (32% by weight), while dilute hydrochloric acid and chlorine were continuously removed.
• the temperature of the hydrochloric acid was initially 40 ° C (at 1 kA / m 2 ) and was increased to 60 ° C.
• the temperature difference between the inlet and outlet of the hydrochloric acid was always less than 15 ° C.
• the electrolysis voltage was 1.43 V at a target current density of 4 kA / m 2 . No traces of corrosion were observed in the anode half-element even after an operating time of 2400 h.
• the electrolytic cell was filled with 13% by weight hydrochloric acid, which contained 200 mg / 1 free chlorine. Then the oxygen supply to the cathode half-element was opened and the oxygen was supplied at a volume flow of 1.25 m 3 / h.
• the volume flow of the hydrochloric acid was set so that the speed of the hydrochloric acid at the start of the electrolysis was 0.2 cm / s.
• the temperature of the hydrochloric acid was set at 40 ° C. The commissioning could not take place because strong pressure pulsations developed which led to safety shutdowns.
• the safety circuit is intended primarily to prevent damage to the cation exchange membrane and the gas diffusion electrode as well as the electrolysis half elements as a whole.
• Electrolysis could only be started when the flow velocity was reduced to 0.14 cm / s.
• the current density was at the beginning of the electrolysis 1 kA / m 2 and was dissolved in 15 minute intervals by 1 kA / m 2 increased until the desired value of the current density (target current density) of 4 kA / m 2 was reached. After reaching the target current density, the flow rate was increased to 0.3 cm / s for continuous operation.
• hydrochloric acid Concentration of hydrochloric acid at no time below 5% by weight.
• the hydrochloric acid concentration of 13% by weight was maintained by continuously supplying fresh concentrated hydrochloric acid (32% by weight), while dilute hydrochloric acid and chlorine were continuously removed.
• the temperature of the hydrochloric acid was increased from initially 40 ° C (at 1 kA / m 2 ) to 60 ° C.
• the temperature difference between the inlet and outlet of the hydrochloric acid was always less than 15 ° C.
• the electrolysis voltage was 1.43 V at the target current density.

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• 2001
  • 2001-10-23 DE DE10152275A patent/DE10152275A1/de not_active Withdrawn
• 2002
  • 2002-10-16 US US10/493,484 patent/US7128824B2/en not_active Expired - Fee Related
  • 2002-10-16 JP JP2003538432A patent/JP2005506454A/ja active Pending
  • 2002-10-16 WO PCT/EP2002/011560 patent/WO2003035938A2/de active Application Filing
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