PROCESS AND REACTOR FOR PRODUCING PHOSGENE

Abstract

The invention relates to a process for producing phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst in a reactor that comprises a plurality of contact tubes arranged parallel to one another, which contact tubes are filled with the catalyst and around which at least one fluid heat transfer medium flows, a feed stream of a mixture of a chlorine input stream and a carbon monoxide input stream being conducted into the contact tubes and reacted to form a phosgene-containing product gas mixture, characterised in that the product gas mixture is discharged from the contact tubes at an outlet end of the contact tubes. The method according to the invention is characterised in that the gas phase reaction is carried out in the reactor such that the position of the highest temperature in a contact tube (hot spot) moves along the longitudinal axis of the contact tube at a predetermined rate of migration, the hot spot having a rate of migration in the longitudinal direction of the contact tubes which is in the range of 1 to 50 mm per day. The invention also relates to a reactor for carrying out the process.

Claims

1.-14. (canceled)

15. A process for producing phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst in a reactor comprising a multitude of catalyst tubes arranged parallel to one another that are filled with the catalyst and around which at least one fluid heat carrier flows, in which a feed stream of a mixture of a chlorine feed stream and a carbon monoxide feed stream is guided into the catalyst tubes at an inlet end of the catalyst tubes and is allowed to react in the catalyst tubes to give a phosgene-comprising product gas mixture, and the product gas mixture is removed from the catalyst tubes at an outlet end of the catalyst tubes, which comprises performing the gas phase reaction in the reactor in such a way that the position of the highest temperature in a catalyst tube (hotspot) moves along the longitudinal axis of the catalyst tube at a defined speed of migration, where the hotspot has a speed of migration in longitudinal direction of the catalyst tubes in the range from 1 to 50 mm per day.

16. The process according to claim 15, wherein the position of the hotspot moves continuously in the direction of the outlet end of the catalyst tubes.

17. The process according to claim 15, wherein continuous movement of the hotspot is brought about by controlled variation of the operating conditions.

18. The process according to claim 17, wherein the operating conditions are brought about via partial recycling of the product gas mixture into the feed stream.

19. The process according to claim 16, wherein continuous movement of the hotspot is brought about by controlled deactivation of the catalyst in the catalyst tubes.

20. The process according to claim 19, wherein a catalyst subject to controlled deactivation under the operating conditions is used.

21. The process according to claim 20, wherein the catalyst is continuously chemically deactivated, especially by addition of oxygen to the feed stream.

22. The process according to claim 15, wherein the feed stream has a stoichiometric excess of carbon monoxide to chlorine of 0.1 to 50 mol %.

23. The process according to claim 15, wherein the feed stream is fed in at an absolute pressure in the range from 0.5 to 20 bar.

24. The process according to claim 15, wherein the at least one fluid heat carrier flows around the catalyst tubes in separate cooling zones.

25. A reactor (10) for production of phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst comprising a multitude of catalyst tubes (12) arranged parallel to one another that are filled with the catalyst and are welded into one tube sheet (13, 14) at each end, with supply of the reactants at an inlet end (21) of the catalyst tubes (12) and discharge of the gaseous reaction mixture at an outlet end (22) of the catalyst tubes (12), in each case via a hood (15, 17), and with feed and drain devices (26, 29) for a fluid heat carrier into a shell space (25) between the catalyst tubes (12), wherein the reactor (10) has a control device (30) for monitoring the speed of migration of the position of the highest temperature in the catalyst tubes (hotspot).

26. The reactor according to claim 25, wherein the control device (30) has at least one temperature measurement probe (31) for determining the temperature in at least one catalyst tube (12) at at least two measurement points (Tx1, Tx2, Tx3, Tx4) spaced apart along the longitudinal axis of the catalyst tube and an evaluation unit (35).

27. The reactor according to claim 25, wherein the control device, for monitoring of the speed of migration, also has control means for varying the operating conditions of the reactor.

28. The reactor according to claim 26, wherein the control means controls the addition of a catalyst-deactivating component to the feed stream and/or controls the addition of oxygen or chlorine oxides to the feed stream.

Description

[0063] The figures show:

[0064] FIG. 1 a schematic diagram of a reactor suitable for performance of the process of the invention;

[0065] FIG. 2 a detail view of the region of the reactor identified by II in FIG. 1;

[0066] FIG. 3 a temperature profile over the cross section of a catalyst tube of the reactor of FIG. 1 along a line II-Ill in FIG. 2;

[0067] FIG. 4 a temperature profile along the longitudinal axis of a catalyst tube of the reactor of FIG. 1 at different times; and

[0068] FIG. 5 a temperature profile measured at location x as a function of time.

[0069] FIG. 1 shows a phosgene reactor 10 suitable for performance of the process of the invention, having an essentially cylindrical reactor shell 11. The reactor 10 shown in longitudinal section in FIG. 1 has a bundle of catalyst tubes 12 that are secured with sealing parallel to one another in longitudinal direction of the reactor 10 in upper and lower tube sheets 13, 14. At the two ends of the reactor are respectively provided an upper hood 15 with an inlet stub 16, and a lower hood 17 with an outlet stub 18. In the upper hood 15 is disposed a gas distributor 19 for homogenization and distribution of the gas flows over the reactor cross section. The catalyst tubes 12 open into the upper hood 15 via inlet ends 21, and into the lower hood 17 via outlet ends 22.

[0070] The reactant mixture is introduced via the inlet stub 16 and distributed via the gas distributor 19 and between the inlet ends 21 of the catalyst tubes 12. The catalyst tubes 12, in the example shown, consist of 1.4462 duplex steel and have a typical length L of about 6 m, corresponding essentially to the bed height of the catalyst present in the catalyst tubes. The catalyst tubes each have an internal diameter D of 39.3 mm and are filled with cylindrical activated carbon catalyst particles 23 (cf. FIG. 2) of diameter 4 mm and length 5 mm. After flowing through the catalyst tubes 12, the reaction mixture flows out of the outlet ends 22 into the lower hood 17 and is discharged via the outlet stub 18.

[0071] Between the catalyst tubes 12 themselves, and between the catalyst tubes 12 and an inner wall 24 of the reactor is provided a shell space 25 through which a liquid heat exchange medium can flow. For this purpose, a fluid heat carrier (not shown) is introduced in countercurrent to the gas flow of the reaction gases at the lower end of the reactor 10 via an entry stub 26. The heat carrier is guided in a meandering flow through the reactor by means of baffle plates 27 disposed at right angles to the longitudinal direction of the reactor, each of which has clear alternating passage openings 28 in the edge region of the reactor, and exits again from the shelf space 25 of the reactor 10 via an exit stub 29. The reactor 10 lacks tubes in the regions of the passage openings 28 since only inadequate cooling of the catalyst tubes would be possible in these regions as a result of the transition of the coolant flow from a transverse flow to a longitudinal flow.

[0072] In the example shown, the reactor 10 has one cooling zone. In alternative embodiments, the reactor may alternatively have two or more, for example two, separate cooling zones that are separated from one another by intermediate plates. In this case, the cooling zones may be cooled with different heat carriers. However, preference is given to using the same heat carrier adjacent cooling zones since the openings in the intermediate plates can be fully sealed only with very great difficulty in respect of the passage through the catalyst tubes. Even when the same heat carrier is used, however, it is possible to use different cooling schemes. For example, it is possible to use a liquid coolant that removes heat by means of evaporative cooling in the first cooling zone, while the heat is removed in the second cooling zone by pure liquid cooling.

[0073] FIG. 1 also shows a schematic of a control device 30 for monitoring the speed of migration of the position of the hotspot. The control device 30 comprises at least one temperature measurement probe 31 which is introduced into a catalyst tube 12 via an upper stub 32 or a lower stub 33. The temperature measurement probe may also be executed in a divided manner, such that one part of the probe is introduced into the catalyst tube 12 from the bottom and one part of the probe from the top.

[0074] FIG. 2 shows an enlarged detail of a region of the reactor cross section of FIG. 1, identified by II in FIG. 1. Sections of three catalyst tubes 12 alongside one another are apparent, surrounded by a shell space 25 through which the fluid heat carrier flows. The temperature measurement probe 31 is in the middle catalyst tube 12. What are shown are, apart from the baffle plates 2 and, for orientation, the direction arrows that indicate the longitudinal axis of the catalyst tubes 12 (direction arrow x) and a direction in the cross section of the catalyst tubes at right angles to longitudinal direction (direction arrow y). As can be seen in the detail diagram of FIG. 2 in particular, the temperature measurement probe 31 is preferably executed as a multiple thermocouple, with numerous measuring elements 34 disposed along the longitudinal axis of the temperature measurement probe 31 in order to ascertain a temperature profile in the catalyst tube. Typically, the distances between the individual measuring elements are in the region of 50-100 mm, preference being given to a tighter arrangement of the measuring elements particularly in the region of the hotspot, in order to increase the accuracy of the closed-loop control of the speed of migration of the hotspot. Each measuring element 34 gives a temperature value Tx.sub.i for a point x along the longitudinal axis of the catalyst tube that corresponds to the distance of the respective measuring element from the inlet end 21 of the catalyst tube 12 (indicated schematically in FIGS. 1 and 2 by the arrow x). In FIG. 2, a temperature value Tx.sub.2 is assigned by way of example to a measuring element 34′. Where reference is made hereinafter to temperature values Tx.sub.1, Tx.sub.2, Tx.sub.3, Tx.sub.4, this does not necessarily mean that these are temperature values from immediately successive measuring elements 34, but it is the case that a temperature value Tx.sub.i+1 is measured by a measuring element further removed from the inlet end 21 than a measuring element that measures the temperature value Tx.sub.i.

[0075] As indicated schematically in FIG. 1 by the data link 36, the temperature measurement probe 31 transmits the temperature data Tx.sub.1, Tx.sub.2, Tx.sub.3, Tx.sub.4, . . . to the control device 30. The control unit 30 has an evaluation unit 35 that uses the data supplied from the temperature measurement probe, the known distance of the measuring elements 34 of the temperature measurement probe from the inlet end 21 and hence also the distance of the measuring elements from one another, and an internal or external clock of the evaluation unit 35 to ascertain the speed of migration of the hotspot in the catalyst tube 12. When the speed of migration ascertained varies from a defined target speed of migration, the evaluation unit 35 may manipulate suitable manipulated variables in order to adjust the speed of migration of the hotspot according to the setpoint.

[0076] FIG. 1 shows a schematic of manipulations of different manipulated variables by arrows 37 and 38. Arrow 37 is supposed to symbolize that the control device 30 can manipulate the properties of the feed stream 39 (for example the temperature and composition thereof) and/or any optional addition of an added feed stream 40 comprising components that at least partly deactivate the catalyst. Arrow 38 symbolizes that the control device 30 manipulates the properties of the coolant stream 41, for example the temperature thereof and/or, via control of a coolant pump 42, the volume flow rate thereof.

[0077] FIG. 3 shows a temperature profile along a diameter line III-III, shown in FIG. 2, of a cross section through a catalyst tube 12 at right angles to the longitudinal axis of the catalyst tube. What is apparent is an essentially parabolic progression of the temperature profile within the catalyst tube 12 with diameter D, with the highest temperature attained in the center of the catalyst tube. The temperature then drops toward the cooled tube wall of the catalyst tube. Within the tube wall, the temperature drops essentially in a linear manner to the temperature of the heat carrier interface at the outer wall of the catalyst tube 12. Within the coolant interface, there is a further, essentially linear temperature drop caused by the external heat transfer to the bulk temperature of the fluid heat carrier.

[0078] FIG. 4 shows the migration of a typical temperature profile of a catalyst tube from industrial phosgene synthesis, which is achieved, for example, by controlled deactivation of the catalyst in the catalyst tube. What are shown are two temperature profiles Tt.sub.1 and Tt.sub.2 that have been recorded at different times t.sub.1 and t.sub.2. It can be seen that the shape of the temperature profile does not change significantly at different times, but is primarily shifted along the longitudinal axis x of the catalyst tube. Therefore, the speed of migration of the hotspot HS need not necessarily be measured at the maximum of the temperature profile. If the same temperature T is measured at different times at different measurement points, the speed of migration of the hotspot is found, as already elucidated above, via the formula W=Δx/Δt, with Δx=x.sub.3−x.sub.2 and Δt=t.sub.2−t.sub.1.

[0079] FIG. 5 shows the progression of the temperature against time at an individual measurement point (here the Mac measurement point x.sub.3 from FIG. 4). The temperature profile against time, with known corrosion rate K.sub.R(T), can be used to determine the integral removal of material at this point.