EP0866890B1 - Process for direct electrochemical gaseous phase phosgene synthesis - Google Patents

Description

The invention relates to a process for the electrochemical conversion of hydrogen chloride to phosgene. According to the usual methods, phosgene becomes catalytic generated from free chlorine. The chlorine is either generated generically from one NaCl electrolysis provided, e.g. from isocyanate production originating HCl gas is processed in the form of hydrochloric acid or as Recycle chlorine recovered from the electrolysis of aqueous hydrochloric acid.

US 5 411 641 describes an electrochemical process for the production of chlorine described in which a dry direct oxidation in the electrochemical cell from HCl to chlorine and protons. The process runs even with cathode side aqueous electrolyte combined with hydrogen production operating voltages significantly cheaper than classic electrolysis aqueous Hydrochloric acid.

The invention is based on the object, starting from gaseous hydrogen chloride, according to claim 1, to produce phosgene directly by electrochemical means.

This object is achieved in that the anode with a proton-conducting membrane equipped electrochemical cell as Educts of dry HCl gas and dry CO gas are supplied and the at the anodic oxidation of HCl gas by chlorine radicals with the CO gas immediately react to phosgene while the protons formed simultaneously migrate through the membrane to the cathode and there when operated with aqueous HCl can be reduced to hydrogen or in the presence of oxygen to water.

anodisch zu Phosgen oxidiert.The chlorine radicals are modeled on the anode with CO gas according to the reaction equations

anodized to phosgene.

The process is preferably carried out in such a way that, in addition to the electrochemical anodic oxidation, an exothermic catalytic conversion of molecular chlorine with CO gas to phosgene takes place in the carbon-containing carrier material of the activated diffusion anode in accordance with the reaction equation CO + Cl ₂ ⇒ COCl ₂ .

The anodic overvoltage can be caused by the phosgene radicals that occur be lowered by 0.2 V to 0.6 V.

The method is advantageously carried out in such a way that to lower the Operating voltage of the electrochemical cell of oxygen at the cathode (3) is reduced and with the protons diffusing through the membrane to water reacted.

Alternatively, the method can also be carried out so that the cathode (3) operated in aqueous hydrochloric acid, with hydrogen as a by-product is produced.

The membrane is advantageously used to adjust its proton conductivity Supply of moist oxygen which leads to the cathode with the starting gas is additionally moistened.

According to a preferred embodiment, the electrochemical reactions take place at the cathode and anode at a pressure of 2 bar to 6 bar.

A further development of the method according to the invention is that the Phosgene current drawn off on the anode side under the operating pressure in a recuperator cooled and liquefied and the liquefied phosgene on the secondary side in the recuperator is expanded and evaporated, the one required for liquefaction Generates cooling capacity and the primary liquefied phosgene from HCI and CO induct gas is exempted. These educt gas fractions can then be returned to the electrochemical cell.

The electrochemical cell is expediently used in a closed system, which also includes the recuperator at a pressure of 2 bar to 10 bar, preferably 2 bar to 6 bar, operated such that the Differential pressure between the closed system and the electrochemical Cell is approximately zero, so that the electrochemical cell even when operating below higher pressures can be operated virtually without pressure.

• Der trockene Chlorwasserstoff kann unter Zugabe entsprechender CO-Mengen in der Gasphase elektrochemisch direkt zu Phosgen umgesetzt werden.
• Bei entsprechender Einstellung der Zusammensetzung des Eduktgasgemischs kann der Anteil von freiem Chlor im Produktgas bis auf vernachlässigbar kleine Werte zurückgedrängt werden. Das Produktgas kann aber selbst für den Fall, daß noch geringe HCl- und CO-Anteile vorhanden sind, direkt für bestimmte chemische Prozesse, z.B. die Isocyanat- oder Polycarbonatherstellung genutzt werden, da diese Restgasanteile in diesem Fall passiv durch den Prozess mitlaufen und sich dann mit dem bei der Isocyanat- bzw. Polykarbonatbildung freiwerdenden HCl-Strom vereinigen, der als Eduktgas wieder der elektrochemischen Phosgenerzeugung zugeführt werden kann. Nicht abreagierte Phosgenreste stören hierbei die elektrochemische Reaktion nicht. Allenfalls wirken sie sich bei nennenswerten Konzentrationen als Diffusionsballast an der Gasdiffusionsanode aus.
• Der apparative verfahrenstechnische Aufwand kann aufgrund der relativ einfach aufgebauten Elektrolyseapparatur im Vergleich zu den bei der klassischen Phosgenherstellung erforderlichen Vielzahl von aufeinanderfolgenden Verfahrensstufen beträchtlich reduziert werden (niedrigere Investitionskosten).
• Die vielen Verfahrenschritte bei der konventionellen Phosgenherstellung, bei der bereits mit der dabei eingesetzten wässrigen Salsäureelektrolyse ein Energiebedarf von ca. 180 kWh/100 kg Chlor erforderlich ist, haben aufgrund der Vielzahl der erforderlichen Pumpen bzw. Kompressoren und aufgrund der benötigten Kühlmittel (Fremdkälte) einen weitaus größeren Energieverbrauch zur Folge. Das erfindungsgemäße Verfahren arbeitet in dieser Hinsicht mit erheblich günstigeren Betriebskosten.
• Unter rein thermodynamischen Gesichtspunkten wäre bereits die elektrochemische Umsetzung von HCl-Gas mit Sauerstoff bei ca. 0,18 Volt exotherm. In der Praxis verschlechtert jedoch die Sauerstoffüberspannung von 300 - 400 mV und der elektrische Widerstand der Ionenaustauschermembran die Energiebilanz.
• Die unmittelbare CO- bzw. COCI-Radikal-Teilnahme am elektrochemischen Prozess beeinflußt durch deren Exothermie die Elektrolysepotentiale positiv. Es läßt sich eine Absenkung von ca. 200 bis 600 mV erreichen.

The following advantages are achieved compared to conventional phosgene production processes:

• The dry hydrogen chloride can be converted electrochemically directly into phosgene by adding appropriate amounts of CO in the gas phase.
• If the composition of the educt gas mixture is set accordingly, the proportion of free chlorine in the product gas can be reduced to negligibly small values. However, the product gas can even be used directly for certain chemical processes, for example isocyanate or polycarbonate production, in the event that small amounts of HCl and CO are still present, since in this case these residual gas portions pass through the process passively and then combine with the HCl stream liberated in the formation of isocyanate or polycarbonate, which can be fed back to the electrochemical phosgene production as the starting gas. Unreacted phosgene residues do not interfere with the electrochemical reaction. At most, at significant concentrations, they act as diffusion ballast on the gas diffusion anode.
• Due to the relatively simple design of the electrolysis apparatus, the outlay in terms of apparatus technology can be considerably reduced in comparison to the large number of successive process stages required in classic phosgene production (lower investment costs).
• The many process steps in conventional phosgene production, in which an energy requirement of approx. 180 kWh / 100 kg chlorine is already required with the aqueous salsic acid electrolysis used, have one due to the large number of pumps or compressors required and due to the required coolants (external cooling) far greater energy consumption. In this respect, the method according to the invention works with considerably lower operating costs.
• From a purely thermodynamic point of view, the electrochemical conversion of HCl gas with oxygen would be exothermic at approx. 0.18 volts. In practice, however, the oxygen overvoltage of 300 - 400 mV and the electrical resistance of the ion exchange membrane worsen the energy balance.
• The direct CO or COCI radical participation in the electrochemical process has a positive influence on the electrolysis potential due to its exothermic nature. A reduction of approx. 200 to 600 mV can be achieved.

Fig.1

schematisch den Aufbau einer Elektrolysezelle für die direkte elektrochemische Phosgenerzeugung und

Fig. 2

den grundsätzlichen Aufbau einer Phosgen-Elektrolyseanlage in einem druckfesten System unter Verwendung eines Phosgen-Rekuperators

The invention is explained in more detail below with reference to drawings and exemplary embodiments. Show it:

Fig. 1

schematically the structure of an electrolysis cell for the direct electrochemical phosgene production and

Fig. 2

the basic structure of a phosgene electrolysis system in a pressure-resistant system using a phosgene recuperator

First, the general reaction mechanisms of the cathode and Anode-running electrochemical processes can be described as a model.

1. Cathode process

A catalytic oxygen reduction (catalyst, for example Pt, Ir, or Pd) of the supplied oxygen takes place at the cathode at the interface with the proton-conducting membrane located between the two electrodes. The oxygen or the supplied oxygen-containing gas mixture (feed gas) is moistened with water, similar to a PEM fuel cell, to the saturation point. The reaction follows the equation: (1) 1/2 O ₂ + 2e ^- + 2H ⁺ ⇒ H ₂ O (g)

The water balance of the proton-conducting membrane is pre-moistened of the feed gas taking into account the formation of water of reaction in accordance with Equation (1) controlled.

2. Electrolyte

Analogous to the PEM fuel cell, a single-layer proton-conducting membrane is used from fluoropolymers with protonated sulfonic acid groups in the ion transport channels as a solid electrolyte between cathode and anode. The proton conductivity is, as described above, improved by moistening the cathode side.

3. Anode process

The basic process is the direct oxidation of dry HCl gas to chlorine and protons, which are fed into the membrane serving as electrolyte, according to the following reaction

The oxidation takes place catalytically (Pt, Ir, Rh, or Pd catalyst) at the interface between the anode and the proton-conducting membrane. The HCl direct oxidation delivers dry chlorine without the presence of other reactants, which immediately reacts further with the dry CO gas offered at the same time. Two reaction paths are possible, both of which are exothermic:

3.1 Immediate influence on direct HCl oxidation

CO reacts with the anodically formed chlorine radical to form the COCl radical, which in turn reacts with another chlorine radical to form COCl ₂ and diffuses out of the field of electrocatalytic analysis. The reaction mechanism at the anode then looks like this:

The hydrogen chloride oxidation is thus in both reaction steps by the CO influenced directly or indirectly. The exothermic nature of the reaction steps becomes at least in part in a lowering of the activation energy of the electrochemical HCl direct oxidation implemented with the consequence of a decrease the cell voltage.

3.2 Indirect process

The chlorine radicals that have not reacted with CO or COCl radicals recombine to form Cl ₂ . The usual carrier material for electrochemically active catalysts integrated in the electrodes is carbon in the form of vulcanized carbon black or acetylene black, this microporous carrier layer being passed through by the product gases Cl ₂ and COCl ₂ coming from the electrolysis. This layer acts as an activated carbon surface, which, at the usual cell temperatures of approx. 80 ° C, is the non-electrochemical, but exothermic reaction (5) CO + Cl ₂ ⇒ COCl ₂ catalyzed. A dry anodic product gas with the following composition is then obtained: COCl ₂ + unreacted HCl gas + unreacted CO + possibly traces of Cl ₂ .

The following is an electrochemical cell for realizing the above Reactions described.

The electrochemical cell 1 acc. 1 essentially consists of the gas diffusion anode 2, the gas diffusion cathode 3 and the one arranged between the electrodes, acting as an electrolyte proton-conducting membrane 4. Such membrane electrolytes are commercially available for electrochemical fuel cells. The anode 2 consists of a porous, catalytically activated activated carbon matrix 5, which is connected on the inside to the membrane 3 and on the outside with one from a conductive gas distributor 6, which is connected to a anodic current distributor 7 is contacted. The analog cathode 3 consists of the catalytic activated carbon matrix 8, the conductive gas distributor 9 and the power distributor 10. Primarily come as catalytic material Platinum, iridium, rhodium and palladium in question. Such gas diffusion anodes or cathodes are also commercially available (e.g. electrodes of the type ELAT from GDE Gasdiffusionselektroden GmbH. Frankfurt a. Main).

The anode 2 is in an anode gas space 11, the cathode 3 in a cathode gas space 12 arranged. The two gas spaces 11 and 12 are except for the inlet and Drain pipe closed. The anode gas space becomes via the feed connector 13 11 a dry educt gas mixture of HCI and CO and over the feed pipe 14 the gaseous educt gas mixture from the cathode gas space 12 Oxygen and saturated water vapor supplied. The cathodic one Reduction of the resulting water vapor together with that caused by the educt gas supplied steam for sufficient moistening of the membrane 4 so that it cannot dry out. Along with unreacted oxygen can over the outlet stub 16 excess water vapor are derived.

At the gas diffusion anode _2, phosgene (COCl ₂ ) is generated according to the reaction mechanism described above, which is discharged via the product nozzle 15. The electrochemical reactions at the anode and cathode are carried out at temperatures from 40 ° C to 80 ° C, at a cell voltage of 0.8 to 1.2 volts and at cell current densities of approx. 3 kA / m ² . However, the method can also be carried out with higher current densities. The starting materials are fed in according to the above reaction equations in a stoichiometric ratio. However, CO gas can also be supplied to the anode in a stoichiometric manner in order to suppress the formation of free chlorine.

In the further developed electrolyzer shown in FIG. 2, there is a large number of electrochemical cells 1 constructed analogously to FIG. 1 as a bipolar in series or parallel-connected cell stack 17 installed in a housing 18.

The enclosed pressure chamber 19 forms a gas-tight, pressure-tight, closed system, which is designed for pressures up to a maximum of 10 bar, the differential pressure from the actual process pressure being compensated for almost zero. The dry educt gas mixture HCl + CO is fed to the anodes via the educt gas line 20 and the compressor 21. The feed of O ₂ + H ₂ O on the cathode side as feed gas takes place through the feed gas line 22 and the compressor 23. With the aid of the compressors 21 and 23, the feed gas mixtures can be compressed to about 6 bar.

The product line 24 attached to the exit of the cell stack 17 is connected to a Phosgene recuperator 25 connected in which the phosgene generated in the cell stack 17 is liquefied by cooling condensation on the heat exchanger tube bundle 26. The liquid phosgene flows through line 27 into a storage container 28. The cooling capacity required for liquefaction is achieved by releasing liquid Phosgene generated from the reservoir 28 in the recuperator 25. To this Purpose is the heat exchanger tube 26 via a riser 29 to the reservoir 28 connected. The liquid flows directly in front of the recuperator 25 Phosgene through a relaxation throttle 31 in the riser 29. During relaxation evaporates the liquid phosgene. The phosgene thus serves in this case as a refrigerant to supply the product gas consisting essentially of phosgene condense. Due to the condensation and re-evaporation, the product gas freed from unreacted HCI and CO feed gas fractions. That on this Gaseous phosgene purified in this manner is discharged through the removal line 32. The relaxation takes place from the educt gas excess pressure prevailing in the cell stack 17 to about normal pressure or to that for the following ones Reactions necessary low form, so that from the electrolyser withdrawn line 32 no pressure-resistant fittings are required. The enriched in the head part of the recuperator 25, consisting of HCl and CO Residual gases are recycled through the return line 33 to the anode input. The The cathode-side exit of the cell stack 17 is connected to an exhaust gas line 34 Removal of excess oxygen and water vapor connected. The Pressure chamber 19 is supplied with an inert gas, e.g. nitrogen pressurized and maintained at about the same pressure as that with the Compressors 21 and 23 generated reactant gas pressure corresponds. Otherwise a pressure-proof design of the electrochemical cells would be required. With this encapsulation, an inerting of the reaction part is possible at the same time be monitored for starting material or product gas leakage with simple means can.

Claims (7)

1. Process for the electrochemical conversion of hydrogen chloride and carbon monoxide to phosgene in the form of a dry anodic product gas, characterised in that dry HCl gas and dry CO gas are supplied as educts to the anode (2) of an electrochemical cell (1) equipped with a proton-conducting membrane (4) and the chlorine radicals formed from the anodic oxidation of HCl gas react directly with the CO gas to yield phosgene, while the simultaneously formed protons migrate through the membrane (4) to the cathode (3) and are there reduced to hydrogen or, in the presence of oxygen, to water.
2. Process according to claim 1, characterised in that the CO gas is supplied in stoichiometric excess.
3. Process according to claim 1 or 2, characterised in that the cathode (3) is operated in aqueous hydrochloric acid and hydrogen is produced as a secondary product.
4. Process according to claims 1 to 3, characterised in that, in order to adjust its electrical conductivity, the membrane (4) is additionally moistened by supplying moist oxygen to the cathode (3).
5. Process according to claims 1 to 4, characterised in that the electrochemical reactions at the cathode (3) and anode (2) proceed at a pressure of 2 bar to 10 bar.
6. Process according to claims 1 to 5, characterised in that the stream of phosgene drawn off from the anode side is cooled under pressure in a recuperator (25) and liquefied and the liquefied phosgene is depressurised and vaporised in the recuperator (25), wherein the refrigeration capacity required for liquefaction is created and any HCl and CO educt gas fractions present in the phosgene are simultaneously removed.
7. Process according to claims 5 or 6, characterised in that the electrochemical cell is operated in a closed system (19), which also includes the recuperator (25), at a pressure of 2 bar to 10 bar, preferably of 2 bar to 6 bar, such that only a slight pressure differential prevails relative to the components in which the reaction proceeds.

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