METHOD FOR PRODUCING PHOSGENE

Abstract

The present invention relates to a process for preparing phosgene by reacting chlorine with carbon monoxide over an activated carbon catalyst, wherein the content of chlorine oxides in the chlorine feed stream is low, to an apparatus for preparation of phosgene and to the use of the phosgene prepared by the process of the invention.

Claims

1.-15. (canceled)

16. A process for preparing phosgene comprising a) providing a chlorine feed stream having a content of chlorine oxides of not more than 130 ppm by volume, wherein i) chlorine is prepared by electrolyzing an aqueous solution of sodium chloride under conditions under which chlorine is obtained with a chlorine oxide content of not more than 130 ppm by volume, or ii) chlorine having a chlorine oxide of more than 130 ppm by volume is subjected to a workup by which the content of chlorine oxides is reduced to a value of not more than 130 ppm by volume, or iii) a first chlorine gas stream having a chlorine oxide content of more than 130 ppm by volume is mixed with a second chlorine gas stream having a chlorine oxide content of less than 130 ppm by volume in such a ratio as to result in a chlorine feed stream having a content of chlorine oxides of not more than 130 ppm by volume, or iv) the chlorine is prepared by using a process other than the electrolysis of an aqueous solution of sodium chloride in which chlorine with a chlorine oxide content of not more than 130 ppm by volume is obtained, b) subjecting the chlorine feed stream provided in step a) to a catalytic reaction with carbon monoxide over an activated carbon catalyst in at least one reactor.

17. The process according to claim 16, wherein, in step i), i1) an aqueous stream having a reduced sodium chloride content and an elevated sodium chlorate content compared to the aqueous sodium chloride solution supplied to the anode chamber is withdrawn from the anode chamber of the electrolysis cell used for electrolysis of the aqueous sodium chloride solution, i2) the aqueous stream withdrawn from the anode chamber is partly or fully discharged, or at least a portion of the sodium chlorate present is removed from the aqueous stream withdrawn from the anode chamber, i3) the sodium chloride content of the portion of the aqueous stream withdrawn from the anode chamber that was not discharged in step i2) is increased and it is recycled into the anode chamber.

18. The process according to claim 17, wherein, in step i2), at least a portion of the sodium chlorate present in the aqueous stream withdrawn from the anode chamber is removed by admixing the stream with acid.

19. The process according to claim 17, wherein at least a portion of the aqueous stream withdrawn from the anode chamber is subjected to a treatment to reduce the content of chlorine oxides present therein by UV irradiation.

20. The process according to claim 16, wherein, in step ii), the chlorine, for workup, is subjected to a thermal, chemical catalytic, or photochemical treatment, to obtain a chlorine feed stream having a content of chlorine oxides of not more than 130 ppm by volume.

21. The process according to claim 16, wherein chlorine prepared by HCl electrolysis, Deacon process or the KEL chlorine process is used as second chlorine gas stream in step iii), or wherein chlorine prepared by HCl electrolysis, Deacon process or the KEL chlorine process is used in step iv).

22. The process according to claim 16, wherein the chlorine oxide comprises chlorine dioxide or consists of chlorine dioxide.

23. The process according to claim 16, wherein the phosgene synthesis is conducted in at least one shell and tube reactor with activated carbon catalyst present in the tubes thereof.

24. The process according to claim 16, wherein the phosgene synthesis is effected in at least one shell and tube reactor, and the reactor tubes are cooled by contacting with a liquid heat carrier or by means of evaporative cooling.

25. The process according to claim 24, wherein cooling is effected using a liquid heat carrier which is guided in cocurrent or in countercurrent to the flow direction of the gases reacting in the reactor tubes.

26. The process according to claim 24, wherein the shell and tube reactor has a shell space through which the heat carrier flows, and the shell space is divided into at least two zones that are supplied separately with liquid or boiling heat carrier for cooling.

27. The process according to claim 16, wherein the activated carbon used is prepared by pyrolysis of a natural raw material, wherein the activated carbon catalyst is in the form of spheres, cylindrical strands, platelets or rings.

28. An apparatus for preparation of phosgene, comprising: a unit for reducing the chlorine oxide content in a chlorine oxide-containing chlorine feed stream having an inlet for the chlorine feed stream and an outlet for the chlorine oxide-depleted chlorine feed stream, and a shell and tube reactor with reactor tubes containing an activated carbon catalyst bed, wherein the shell and tube reactor has an inlet for the chlorine oxide-depleted chlorine feed stream and an outlet for a phosgene-containing product stream, wherein the chlorine oxide-depleted chlorine feed stream is mixed with a carbon monoxide feed stream before entering the shell and tube reactor.

29. A process for preparing phosgene as defined in claim 16, using an apparatus for preparation of phosgene, comprising: a unit for reducing the chlorine oxide content in a chlorine oxide-containing chlorine feed stream having an inlet for the chlorine feed stream and an outlet for the chlorine oxide-depleted chlorine feed stream, and a shell and tube reactor with reactor tubes containing an activated carbon catalyst bed, wherein the shell and tube reactor has an inlet for the chlorine oxide-depleted chlorine feed stream and an outlet for a phosgene-containing product stream, wherein the chlorine oxide-depleted chlorine feed stream is mixed with a carbon monoxide feed stream before entering the shell and tube reactor; wherein the chlorine oxide content in the chlorine feed stream is determined and the temperature of the chlorine feed stream is controlled as a function of the chlorine oxide content such that the content of chlorine oxides at the inlet to the phosgene reactor is not more than 130 ppm by volume.

30. The use of the phosgene prepared by the process according to claim 16 for preparation of isocyanates.

Description

DESCRIPTION OF FIGURES

[0152] FIG. 1 shows a schematic of the reactor used for phosgene synthesis in the examples (laboratory monoliner reactor).

[0153] FIG. 2 shows the progression of the CO concentration at the reactor outlet against time in example 1, where the ClO.sub.2 content is 685 ppm by volume based on the chlorine feed stream.

[0154] FIG. 3 shows the progression of the CO concentration at the reactor outlet against time in example 2, where the O.sub.2 content is 685 ppm by volume based on the chlorine feed stream.

[0155] FIG. 4 shows the rate at which the CO concentration rises in the gas stream at the reactor outlet (product gas) as a function of the ClO.sub.2 content in the chlorine feed stream.

[0156] FIG. 5 shows the rate at which the CO.sub.2 concentration rises in the gas stream at the reactor outlet (product gas) as a function of the ClO.sub.2 content in the chlorine feed stream.

EXAMPLES

[0157] Experimental Setup:

[0158] In the examples that follow, the influence of chlorine oxides on the reaction to form phosgene from CO and Cl.sub.2 was examined. The chlorine oxide used in the examples was ClO.sub.2 (chlorine dioxide), which was prepared by a known process by passage of chlorine gas over sodium chlorite (Derby et al., Inorganic syntheses, vol. IV. p. 152, 1953).

[0159] The influence of ClO.sub.2 on the heterogeneously catalyzed phosgene synthesis in the presence of activated carbon catalyst was examined in a laboratory monoliner reactor (see FIG. 1) with a reaction tube having an internal diameter of 5.4 mm. The reaction tube is positioned in a copper block and its temperature is controlled thereby. The activated carbon catalyst used was commercially available activated carbon of the Donaucarbon ED47 type, which was in the form of extrudates (average diameter 4 mm, length 5 to 20 mm) and was positioned in the reaction tube with individual extrudates separated by inert glass beads. The total mass of catalyst was 0.8 g. The reaction tube was kept at a temperature of 100° C. and the pressure in the reactor was controlled at 5 bar absolute. The standard feed metered into the reactor was 15.9 l (STP)/h of CO and 14.6 l (STP)/h of Cl.sub.2 from pressurized gas bottles via mass flow controllers. The gases flowed through the reactor from the bottom upward and were analyzed for their CO content by means of IR spectroscopy at the exit after expansion to an ambient pressure. The CO content in the gas stream at the reactant outlet (product gas) based on the CO content in the feed is a measure of CO conversion by formation of phosgene from the reaction with chlorine.

[0160] A further stream of 5 l (STP)/h is added in each case to the abovementioned standard feed, which is pure nitrogen (=reference feed 1) or a ClO.sub.2-containing feed (=ClO.sub.2 feed) or an O.sub.2-containing feed (=reference feed 2).

[0161] In example 1, the ClO.sub.2 feed was provided by guiding 5 l (STP)/h of a test gas (0.1% by volume of Cl.sub.2, 99.9% N.sub.2) through a preliminary reactor filled with NaClO.sub.2, and in this way a ClO.sub.2-containing feed having a maximum of 0.2% by volume of ClO.sub.2 (in the case of full conversion) was produced by the method of Derby and Hutchinson. The ClO.sub.2 content was 685 ppm by volume based on the chlorine feed stream.

[0162] In order to show the particular deactivating effect of ClO.sub.2 compared to O.sub.2, in example 2, an O.sub.2-containing feed (reference feed 2) of 5 l (STP)/h of a test gas (0.2% by volume of O.sub.2, 99.8% by volume of N.sub.2) was metered into the experimental reactor. The oxygen content of the reference feed 2 corresponds to the ClO.sub.2 content of the ClO.sub.2-containing feed.

Example 1

[0163] The above-described plant was first started up with the standard feed and additionally reference feed 1, and temperature and pressure were adjusted. The CO concentration at the reactor outlet (see FIG. 2) was 12.7% by volume at first, corresponding to a conversion of about 82%. After a little more than 1 hour, the gas feed stream in the reactor was switched to standard feed and additionally the ClO.sub.2-containing feed. There is a clear continuous rise in the CO concentration to up to 13.6% by volume, corresponding to a decline in the conversion from 82% to 80.6%. The presence of ClO.sub.2 leads to deactivation of the catalyst. This deactivation is found to be irreversible since switching back to standard feed and additionally reference feed 1 did not bring any recovery in the CO conversion after about 3 hours.

Example 2

[0164] This experiment was conducted analogously to example 1, except that reference feed 1 was replaced by reference feed 2 (O.sub.2-containing feed). In addition, an already partly deactivated catalyst was used here. The intention here was to show that the deactivating effect observed in example 1 does indeed result from ClO.sub.2 and not from oxygen released from the cleavage of the ClO.sub.2. The intention was also to show that this effect also occurs in the case of an already aged catalyst. The plant was first started up with the standard feed and additionally reference feed 2, and temperature and pressure were adjusted. The CO concentration at the outlet was about 27.6% by volume of CO, corresponding to a CO conversion of 53%. Compared to example 1 with more than 80% conversion, the partial deactivation of the catalyst was shown. After about 2.5 hours, the gas feed stream in the reactor was switched to standard feed and additionally the ClO.sub.2-containing feed. There was first a sharp rise here in the CO concentration in the product gas stream, followed by a continuous rise to up to 29.5% by volume of CO after a run time of about two hours. The rise in concentration corresponds to a decrease in the CO conversion to about 48.4%. After changing over again to standard feed and additionally reference feed 2, there was a slight recovery in the CO conversion, but the level before the reaction with ClO.sub.2 was not reached again and it settled out at 29% by volume of CO, corresponding to a conversion of 49.6%.

[0165] Result:

[0166] Even at a ClO.sub.2 content of 685 ppm by volume, based on the chlorine feed stream at a temperature of 100° C., a distinct decrease in catalyst activity and a decline in conversion was manifested within a very short time.

[0167] Comparative experiments with an oxygen content of 685 ppm by volume, based on the chlorine feed stream, did not show any deactivation of the activated carbon catalyst.

Example 3

[0168] The above-described plant was first started up with the standard feed and additionally reference feed 1 of 5 l (STP) of nitrogen/h, and temperature and pressure were adjusted.

[0169] Subsequently, the content of ClO.sub.2 in the chlorine feed stream was increased stepwise. For this purpose, nitrogen in the reference feed was partly replaced by ClO.sub.2-containing feed. FIG. 4 shows the rate at which the CO concentration rises in the gas stream at the reactor outlet (product gas) as a function of the ClO.sub.2 content at the inlet, calculated for the chlorine feed stream. Over and above a ClO.sub.2 content in the feed of more than 130 ppm, a much faster decrease in the CO conversion (and hence a faster rise in CO in the product gas) was observed.

[0170] In the experiments, the CO.sub.2 concentration in the gas stream was additionally measured by means of calibrated IR at the reactor outlet (product gas). As can be inferred from FIG. 5, the values show a distinct rise in CO.sub.2 formation with rising ClO.sub.2 content in the chlorine feed stream. In comparative experiments without catalyst, it was not possible to detect CO.sub.2 formation, and so a possible carbon source for CO.sub.2 formation is the activated carbon used as catalyst. Therefore, an increasing amount of ClO.sub.2 in the chlorine feed stream leads to a distinct increase in CO.sub.2 formation by reaction of the activated oxygen with the activated carbon. One possible explanation for the decrease in the CO conversion would thus be the decreasing activity of the activated catalyst.