METHOD OF SEPARATING PHOSGENE AND HYDROGEN CHLORIDE

Abstract

A process of separating phosgene and hydrogen chloride, comprises: conveying a mixed stream containing hydrogen chloride and phosgene into a distillation column; withdrawing from the distillation column a bottom stream containing phosgene; withdrawing a top stream containing hydrogen chloride; compressing at least a portion of the top stream and at least partially condensing the compressed top stream to form a liquid stream, decompressing at least a portion of the liquid stream to form a cooled liquid stream and a cooled gas stream; and recycling the cooled liquid stream to the top of the distillation column as a reflux; the process additionally comprising temporarily introducing an absorbing solvent into the distillation column, in particular during starting-up and/or shutting-down of the process. The process allows for safe operation even when hydrogen chloride production only gradually begins or decreases, without the necessity of storing hydrogen chloride.

Claims

1.-11. (canceled)

12. A process of separating phosgene and hydrogen chloride, comprising: conveying a mixed stream containing hydrogen chloride and phosgene into a distillation column; withdrawing from the distillation column a bottom stream containing phosgene; withdrawing a top stream containing hydrogen chloride; compressing at least a portion of the top stream and at least partially condensing the compressed top stream to form a liquid stream, decompressing at least a portion of the liquid stream to form a cooled liquid stream and a cooled gas stream; and recycling the cooled liquid stream to the top of the distillation column as a reflux; the process additionally comprising temporarily introducing an absorbing solvent into the distillation column, wherein the absorbing solvent is introduced at the top of the distillation column and/or into the reflux liquid.

13. The process according to claim 12, wherein the absorbing solvent is introduced during starting-up and/or shutting-down of the process.

14. The process according to claim 12, wherein the absorbing solvent is selected from monochlorobenzene and dichlorobenzene.

15. The process according to claim 12, wherein the absorbing solvent is mixed with the liquid stream before decompressing and is introduced into the distillation column with the recycled cooled liquid stream.

16. The process according to claim 12, wherein the pressure drop during decompressing is at least 5 bar.

17. The process according to claim 12, wherein the mixed stream is conveyed into the distillation column as a side feed, the distillation column having a rectifying section above the side feed and a stripping section below the side feed.

18. The process according to claim 12, wherein the compressed top stream is directed to a rectifying column and a stream obtained at the bottom of the rectifying column is recycled to the top of the distillation column.

19. The method according to claim 12, wherein the liquid stream is heat-exchanged in a heat exchanger with the top stream before the top stream is compressed.

20. The method according to claim 12, wherein an uncondensed portion of the top stream is heat-exchanged in a heat exchanger with the compressed top stream that is to be condensed.

21. The method according to claim 12, comprising withdrawing a gaseous side stream from the distillation column via a side takeoff, at least partially condensing the side stream in a first cooler, recycling a liquid fraction into the distillation column and supplying a gaseous portion to a second cooler, wherein in the second cooler the gaseous portion is heat-exchanged with the top stream.

22. The method according to claim 21, wherein the top stream, before being fed to the second cooler, is heat-exchanged with the condensed top stream.

Description

[0056] The invention is illustrated by the accompanying drawing and the examples that follow.

[0057] FIG. 1 shows an embodiment of the method according to the invention.

[0058] According to FIG. 1, a mixed stream containing hydrogen chloride and phosgene is supplied to lower region 3 of distillation column 1 via feed 5. In the distillation column, the mixed stream is separated by distillation into a phosgene-comprising bottoms stream withdrawn via bottom takeoff 7 and an essentially hydrogen chloride-comprising stream withdrawn at top takeoff 9.

[0059] The mixed stream containing hydrogen chloride and phosgene preferably derives from isocyanate production. In one embodiment the pressure at which the distillation is performed is equal to the pressure at which the phosgene- and hydrogen chloride-comprising stream was withdrawn from isocyanate production. In an alternative embodiment the distillation is performed at a pressure below the pressure at which the mixed stream containing hydrogen chloride and phosgene was withdrawn from isocyanate production.

[0060] To aid distillation, distillation column 1 comprises internals 11, for example a structured packing or a random packing. Distillation column 1 may alternatively be a tray column.

[0061] In accordance with the invention the top stream containing hydrogen chloride is withdrawn via top takeoff 9 and is compressed to a higher pressure in compressor 13. The pressure to which the top stream is compressed is preferably in the range of from 5 to 25 bar.

[0062] After compression, the compressed top stream is passed into top condenser 15. The compressed top stream is partially condensed in the top condenser to form a liquid stream. Due to the high pressure, this partial condensation does not necessitate cooling to temperatures that would be necessary had the condensation been carried out at the pressure prevailing at the top of the distillation column. The liquid stream is decompressed in decompression means 17, for example a throttle or a valve, to the pressure prevailing at the top of distillation column 1. This stream thus cools down, while undergoing partial evaporation, to its boiling temperature at column pressure and is recycled into the top of distillation column 1 via return 19. The uncondensed portion of the compressed top stream is withdrawn from top condenser 15 via gas takeoff 21.

[0063] In accordance with the invention, an absorbing solvent such as monochlorobenzene is introduced into and mixed with the liquid stream before decompressing via line 23 during the starting-up of the process. Addition of the absorbing solvent prevents phosgene from ascending into the column top as long as there is no or insufficient hydrogen chloride in the system to provide decompression cooling and sufficient reflux of liquefied hydrogen chloride. The stream of absorbing solvent is reduced and finally discontinued as hydrogen chloride is gradually produced and the system can be filled so that the temperature at the column top decreases.

COMPARATIVE EXAMPLE

[0064] A gaseous stream of 35 t/h containing 28.6 wt % HCl, 62.8 wt % phosgene and 8.6 wt % chlorobenzene is taken from the reaction section of an isocyanate plant and subjected to separation in a plant according to FIG. 1.

[0065] The gaseous stream emerges from the isocyanate plant at a temperature of 90? C. and a pressure of 2.4 barg. The stream is partially condensed in several steps to obtain a liquid/gas mixed stream at ?13.5? C. and 2.3 barg. This stream (5) is fed to the sump of a distillation column (1) operated at a top pressure of 2.2 barg. The packing (11) realizes 20 theoretical stages.

[0066] During startup, liquid hydrogen chloride from a storage container (not shown in FIG. 1) is supplied via line 23 and valve 17. The gaseous product at the top (9) mainly consisting of hydrogen chloride is compressed to a pressure of 12.2 barg (13) and partially condensed in condenser (15) at about ?23? C. to obtain a liquid stream of 2624 kg/h. The liquid stream is flashed via valve 17 to the top of the distillation column to realize a low-temperature reflux.

[0067] The non-condensed gas flow 21 (about 8150 kg/h) from condenser mainly contains HCl with 0.1 wt % phosgene. At the bottom of the distillation column 1, a stream 7 of 26850 kg/h is withdrawn, containing about 7 wt % hydrogen chloride, 81.8 wt % phosgene and 11.2 wt % chlorobenzene.

[0068] The storage container holding liquid hydrogen chloride required for startup holds a volume of 5 m.sup.3 and is normally filled with liquid HCl at 12.2 barg and kept at temperature below ?23? C. The investment for equipment including a pressure-resistant vessel and heat-insulating layers is very high. Also, in case of leakage there is a considerable risk of releasing a large amount of toxic liquid hydrogen into environment.

EXAMPLE ACCORDING TO THE INVENTION

[0069] The comparative example described above is repeated. However no liquid hydrogen chloride from the storage container is supplied. A stream of chlorobenzene of 11640 kg/h at ?25? C. is supplied via line 23 and valve 17 to the top of column 1 to obtain a predetermined maximum concentration of 0.1 wt % phosgene in stream 21 (see step 1 in table below).

[0070] After a while, enough hydrogen chloride is available and about 500 kg/h of condensate is taken from condenser 15 and fed back together with chlorobenzene via valve 17 to the column in step 2. The feed of chlorobenzene can be reduced to 9684 kg/h to keep the phosgene concentration in stream 21 at 0.1 wt %. After several steps of increasing condensate flow and decreasing chlorobenzene flow, steady-state distillation is reached at step 6.

TABLE-US-00001 MCB Liquid stream from Gas stream 9 Bottoms stream 7 23 (15) wt % Wt % Wt % Wt % Stage kg /h kg/h ? C. Phosgene Kg/h ? C. HCl Phosgene MCB 1 11640 0 ?5.0 0.1 38750 ?0.5 5.5 56.7 37.8 2 9684 500 ?10.6 0.1 36780 ?2.8 5.7 59.7 34.6 3 7620 1000 ?17.2 0.1 34680 ?5.3 6.0 63.3 30.7 4 5489 1500 ?24.9 0.1 32510 ?7.9 6.3 67.6 26.1 5 3311 2000 ?34.5 0.1 30280 ?10.6 6.6 72.6 20.8 6 0 2624 ?52.5 0.1 26850 ?13.7 7.0 81.8 11.2 (MCB = monochlorobenzene)

[0071] This procedure dispenses with the necessity of storing toxic hydrogen chloride and avoids the related investment.