Process for safe production of phosgene

Abstract

In a process for safe production of phosgene (COCI.sub.2) from carbon monoxide and chlorine according to the reaction scheme CO (g)+Cl.sub.2 (g)>COCI.sub.2 (g) in a plant with a capacity of phosgene below 10 t/hr, the CO is produced on site from a feed stock based mainly on CO.sub.2. The plant preferably comprises a solid oxide electrolysis cell (SOEC) stack system producing CO for use together with chlorine in the phosgene synthesis. This way of producing phosgene is based on using primary raw materials for which escape concentrations above 1000 ppm or even above 10000 ppm or 10% will not result in any health risk.

Claims

1. A process for the production of phosgene (COCl.sub.2) from carbon monoxide and chlorine using a catalyst according to the reaction scheme
CO(g)+Cl.sub.2(g)->COCl.sub.2(g)(1) wherein the CO is produced electrolytically on site from CO.sub.2 in a solid oxide electrolysis cell (SOEC) stack system; and wherein the concentration of the CO is increased through a separation process prior to feeding to the production of phosgene.

2. Process according to claim 1, wherein the SOEC stack system producing CO is accommodated to turndown ratios below 30%.

3. Process according to claim 1, wherein oxygen from the SOEC stack system is used wholly or in part for the production of chlorine.

4. Process according to claim 1, wherein part of the SOEC unit is used to make electrical energy from the hydrogen produced in a chlorine plant when full capacity on the CO production is not needed.

5. Process according to claim 1, wherein the SOEC stack system producing CO is accommodated to turndown ratios below 10%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the process of the present invention for the safe production of phosgene; and

(2) FIG. 2 shows the conversion of CO.sub.2 to CO being carried out electrolytically in an SOEC cell stack.

DETAILED DESCRIPTION OF THE INVENTION

(3) The present invention thus relates to a novel concept for the production of phosgene, which is safe also when carried out in smaller plants. In the present context, a smaller plant is a plant producing less than 10 tons of phosgene per hour, preferably less than 1 ton of phosgene per hour. An example of this novel production concept is shown in FIG. 1, where the chlorine is produced electrolytically from salt (NaCl) and water in the reactor A according to equation (2) above. Carbon dioxide (CO.sub.2) which, unlike CO, is a relatively harmless gas, is converted locally to carbon monoxide in the reactor B according to the reaction:
2CO.sub.2->2CO+O.sub.2(6)

(4) Then the phosgene synthesis is carried out in the reactor C by reacting CO with Cl.sub.2 according to equation (1) above.

(5) This novel concept for the production of phosgene is based on using primary raw materials for which escape concentrations above 1000 ppm or even above 10000 ppm or 10% will not result in any health risk.

(6) The carbon dioxide, which is necessary for producing the requisite carbon monoxide, can be produced locally, e.g. from natural gas or various other hydrocarbons by reforming as mentioned above in combination with the water gas shift reaction. Carbon monoxide can also be captured from fermentation of effluent gas, from power plants or engine flue gas, removed from synthesis gas or captured from natural underground CO.sub.2 sources.

(7) Well established technologies are available for this purpose and are typically based on various scrubbing technologies, where the CO.sub.2 is captured in liquid phase containing for example an amine and subsequently released to the atmosphere or utilized in various processes. It may also be produced from carbon dioxide contained in atmospheric air.

(8) For small to medium scale carbon monoxide production the CO.sub.2 is typically captured at a source such as those mentioned above, purified to meet technical or food grade quality and then transported on trucks in liquid form. The truck delivers the CO.sub.2 to a local storage tank where the CO.sub.2 is stored in liquid form. The tank unit is equipped with an evaporator, and the CO.sub.2 is delivered to the carbon monoxide generating plant from the storage tank.

(9) As mentioned above, the conversion of CO.sub.2 to CO is preferably carried out electrolytically in an SOEC stack system as shown in FIG. 2:

(10) Carbon dioxide is fed to the fuel side of an SOEC system with an applied current to convert CO.sub.2 to CO and transport the oxygen surplus to the oxygen side of the SOEC system. Air, nitrogen or CO.sub.2 may be used to flush the oxygen side. Flushing the oxygen side of the SOEC system has two advantages: reducing the oxygen concentration and related corrosive effects, and providing means for feeding energy into the SOEC system, thereby operating it endothermically.

(11) The product stream from the SOEC system contains mixed CO and CO.sub.2. This can be fed directly to the phosgene production, or the CO concentration can be increased in a separation process, such as pressure swing adsorption (PSA), temperature swing adsorption (TSA), membrane separation, cryogenic separation or liquid scrubber technology, e.g. wash with N-methyl diethanolamine (MDEA).

(12) Two important advantages of using an SOEC system to provide CO in relation to phosgene production are that: The oxygen by-product passes a membrane and hence there will be no oxygen in the CO product stream. Remaining contents of CO.sub.2 in the product stream are practically inert in the phosgene synthesis process and will not lead to the production of undesired by-products in the phosgene synthesis process.

(13) The electrolysis process in the SOEC requires an operating temperature between 650 and 850 C. Depending on the specific operating conditions, the stack configuration and the integrity of the stack, the overall operation can consume heat (i.e. be endothermic), it can be thermoneutral or it can generate heat (i.e. be exothermic). Any operation carried out at such high temperatures also leads to a significant heat loss. Therefore it will typically require external heating to reach and maintain the desired operating temperature.

(14) By producing CO locally from carbon dioxide, it becomes possible to produce phosgene without storage of larger amounts of poisonous chemicals being necessary, without transportation of poisonous chemicals into or away from the phosgene plant, and without the need for continuous exchange of containers or tanks with poisonous chemicals.

(15) Producing CO locally from carbon dioxide has also some key advantages compared to CO production from natural gas reforming or methanol cracking. It is very important that the CO feedstock for phosgene production is free of methane, as any methane present will form the detrimental impurity CCl.sub.4 (tetrachloromethane). This impurity is notoriously difficult to avoid and remove, and it will cause an optical deterioration of the finished product, especially polycarbonate products. The CO obtained by local production from CO.sub.2 will be free of methane, as the commercially available CO.sub.2 feedstock does not contain methane and no methane can be formed during the conversion. It is also very important to secure that no H.sub.2 or H.sub.2O is present in the feedstock, as this will lead to formation of HCl causing corrosion problems. CO produced by either natural gas reforming or methanol cracking will contain these impurities, whereas locally produced CO from CO.sub.2 avoids these impurities, whereby the corrosion risk decreases and the process safety is increased. The product quality typically required is CCl.sub.4<20-80 ppm. Minimum requirements for CO feedstock are CH.sub.4<0.1 vol % and H.sub.2<0.5 vol %, although they can be stricter depending on the actual product quality requirement. The limitation with respect to oxygen is more of an operations issue as any oxygen in the CO feed to the phosgene reactor. Any oxygen in the CO feed oxidises the activated carbon catalyst situated in the phosgene reactor, thus consuming the phosgene catalyst and forming CO.sub.2.

(16) To avoid CH.sub.4 and H.sub.2 in the CO product gas from an SOEC unit it is important to feed the SOEC unit with a sufficiently pure CO.sub.2 feedstock. It is particularly important to avoid H.sub.2 and H.sub.2O in the CO.sub.2 fed to the SOEC, as H.sub.2O will be converted into H.sub.2 which in turn may combine with the CO.sub.2 to form CH.sub.4. Consequently, the H.sub.2 and H.sub.2O contents of the feedstock should both be well below 0.5%. This requirement can for example be fulfilled with food grade CO.sub.2 as defined in EIGA (European Industrial Gases Association) standard 70/08/E which sets very low limits for CO.sub.2 with respect to the contents of moisture (water), ammonia, oxygen, NO.sub.x, volatile hydrocarbons, acetaldehyde, benzene, carbon monoxide, methanol, hydrogen cyanide and total sulfur.

(17) One special feature of the SOEC carbon monoxide generator is that very low turndown ratios can be accommodated, so that the CO production in each case can be matched to the required raw material for the phosgene production. Especially small and medium-scale producers require high turndown ratios down to 10%. The SOEC plant can accommodate this, and even in a way that preserves the stack lifetime in an optimal way. Turndown is accomplished by operating only a subset of the stacks, thereby preserving the lifetime of stacks which are not in operation.

(18) In addition it will be possible to exploit by-products from the SOEC system in the production of chlorine and vice versa, i.e. exploit by-products from the chlorine production in the SOEC system, viz.: Chlorine can be obtained from HCl and oxygen:
4HCl+O.sub.2->2Cl.sub.2+2 H.sub.2O (7)

(19) where the oxygen from the SOEC system may be used, and SOEC stacks may also be used in reverse mode as fuel cell stacks. Whenever full capacity on the CO plant is not needed, it is thus possible to use part of the SOEC unit to make electrical energy from the hydrogen produced in the chlorine plant.