REACTOR CASCADE AND METHOD FOR OPERATING A REACTOR CASCADE

Abstract

A reactor cascade for carrying out equilibrium-limited reactions, having at least two reactor units with in each case one reaction part in the form of a tubular reactor and in each case one absorption part. The reaction part has a starting product inlet and the absorption part has a starting product outlet for the discharge of excess starting products. A connecting line is provided between the starting product outlet of a first reactor unit and the starting product inlet of a second reactor unit. A pressure reduction valve for the reduction of a process pressure is provided between the first reaction unit and the second reactor unit.

Claims

1. A reactor cascade for implementing equilibrium-limited reactions, comprising: at least two reactor units each comprising a reaction section in the form of a tubular reactor and each comprising an absorption section, wherein the reaction section has a reactant inlet and the absorption section has a reactant outlet for leading off excess reactants, a connecting conduit between the reactant outlet of a first reactor unit and the reactant inlet of a second reactor unit, and a pressure reduction valve for reduction of a process pressure p between the first reactor unit and the second reactor unit.

2. The reactor cascade as claimed in claim 1, wherein the reactant inlet is provided at one end of the reaction section, and the absorption section is disposed at another end.

3. The reactor cascade as claimed in claim 1, wherein the absorption section has an absorbent outlet as well as the reactant outlet.

4. The reactor cascade as claimed in claim 3, wherein the absorbent outlet is connected to a desorption unit for unloading reaction products from an absorbent.

5. The reactor cascade as claimed in claim 1, wherein a reaction section of the first reactor unit has a higher reaction volume than a reaction section of a second reactor unit.

6. The reactor cascade as claimed in claim 1, wherein the reactor units are of identical design.

7. The reactor cascade as claimed in claim 1, further comprising: a gas filter apparatus disposed in the absorption section.

8. A process for performing an equilibrium-limited reaction, the process comprising: guiding a reactant into a reaction section of a reactor unit at least partly filled with a porous catalytic substance through which the reactant flows, wherein the reactant is at least partly converted to a reaction product at a surface of the porous catalytic substance, guiding the reaction product and excess reactant from the reaction section into an absorption section of the reactor unit, wherein the reaction product is absorbed by the absorbent and the excess reactant is separated from the reaction product by means of a gas filter apparatus, and wherein there is a pressure p1 in the reactor unit, and guiding the separated reactant through a pressure reduction apparatus and introducing the separated reactant into a second reactor unit at a pressure p2, where the pressure p2 is less than the pressure p1.

9. The process as claimed in claim 8, wherein the second reactor unit is operated at the pressure p2.

10. The process as claimed in claim 8, further comprising: a third reactor units operated at a pressure p3 lower than the pressure p2.

11. The process as claimed in claim 8, wherein a reactor cascade of at least two reactor units is provided, which are operated with a falling operating pressure pn proceeding from a first reactor unit.

12. The process as claimed in claim 11, wherein the reaction section has a tubular configuration and the reactant flows through the reaction section along its longitudinal extent.

13. The process as claimed in claim 8, wherein the reaction product comprises methanol.

14. The process as claimed in claim 8, wherein the reactant comprises carbon dioxide and hydrogen.

15. The process as claimed in claim 8, wherein the absorbent laden with the reaction product is guided through an absorbent outlet into a desorption unit, where the reaction product is unloaded therefrom.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The figures show:

[0024] FIG. 1: a reaction cascade for implementation of equilibrium-limited reactions and

[0025] FIG. 2: a schematic diagram of the processing of the absorbent.

DETAILED DESCRIPTION OF INVENTION

[0026] FIG. 1 shows a schematic diagram that serves as an example of a reaction cascade suitable for implementing equilibrium-limited reactions, represented here by the example of carbon dioxide and hydrogen, with minimum loss. In the example described, carbon dioxide and hydrogen as reactant or reactant gas is introduced into a reaction unit 4 at elevated pressure, for example of greater than 30 bar, with the aid of a compressor. More specifically, the reactant 18 is introduced into a reaction section 6 via a reactant inlet 10. A catalytic substance 30 is disposed in the reaction section 6. This catalytic substance 30, also referred to hereinafter as catalyst 30, may be in various configuration forms. In a very appropriate and simple configuration form, the catalyst 30 is in the form of a bed of powder in the reaction section 6. In principle, however, it is also possible to fit porous sintered bodies containing the catalyst 30 at least on the surface into the reaction section 6. It is thus possible to achieve a defined surface area, which, however, also entails higher expenditure in the production of the catalyst 30. The reaction section 6 here is advantageously of tubular configuration, “tubular” being understood to mean that the ratio of length to width, i.e. the aspect ratio of the reaction section 6, is greater than 1, advantageously greater than 5.

[0027] At one end of the reaction section 6, the opposite end from the reactant inlet 10, is disposed an absorption section 8, wherein the absorption section 8 and the reaction section 6 are advantageously closely connected with one another in spatial terms. More advantageously, the absorption section 8 is flanged directly onto the reaction section 6 by a flange 42. This construction of the reaction unit 4 can be configured particularly inexpensively. The absorption section 8 here advantageously has a gas filter apparatus 32 that may be configured, for example, in the form of a sintered plate or in the form of a perforated tube. Also present in this absorption section is an absorbent 14. In the diagram according to FIG. 1, the gas filter apparatus 32 is surrounded completely by the liquid absorbent 14.

[0028] There follows a description of the reaction process that takes place in the reactor unit in the individual components described, using the example of the carbon dioxide and hydrogen reactant already mentioned. The mixture of carbon dioxide and hydrogen is guided into the reaction section 6, and especially onto the catalytic substance 30 therein. The catalyst 30 has a surface that has catalytic action and converts the carbon dioxide and hydrogen to methanol. However, this reaction has an equilibrium that is established when only 20% of the methanol product has formed. In order to allow the reactions to continue, it is necessary for the product to be constantly removed from the reaction site, i.e. the surface of the catalyst 30, and new reactant to be supplied. This is achieved by the flow of the reactant through the tubular reactor section 6, in the course of which the resultant methanol product, which is liquid under the process conditions of about 30 to 50 bar and a temperature of more than 200 degrees Celsius, is formed in each case. Thus, the stream of the reactant 18 also entrains the reaction product 26, and introduces it continuously from the reaction section 6 into the absorption section 8. The reaction product 26 and the excess reactant 18 are then present together in gaseous form therein, in the form of the gas mixture of carbon dioxide and hydrogen. This gaseous mixture of reactant 18 and product 26 is guided through the gas filter apparatus 32, with absorption of the product 26, the methanol in the example specified, by the absorbent 14, generally or advantageously in the form of an ionic liquid. The gaseous reactants 18 are selectively not absorbed by the absorbent 14 and collect in a gas space 44 of the absorption section 8. From the gas space 44 of the absorption section 8, a connecting conduit 20 is provided, in which or on which a pressure reduction valve 16 is provided. The excess reactant 18, which is in the form of a pressure p1 in the reaction section, is reduced by the pressure reduction valve 16 to a pressure p2, and is introduced into a second reaction unit 200 in the form of a reactant 18′.

[0029] In the second reaction unit 200, by contrast with the first reaction unit 100, there is a reaction pressure p2 which is about 2 bar lower than the reaction pressure p1 at which the first reaction unit 100 is operated. The reduction in the process pressure p by about 2 bar, for example from 50 bar to 48 bar, leads merely to a comparatively small loss of efficiency in the performance of the equilibrium-limited reaction, as already described with regard to the first reaction unit 100. However, the effect of the pressure reduction is that it is not necessary to recompress the recovered or excess reactant 18 by an energy-intensive and technically costly compression operation. The pressure employed in the next reaction unit is merely that at which the reactant already exists in any case, and the reaction described is conducted again with slightly altered thermodynamic parameters. The result is a reaction cascade 2 having at least two reaction units 4, 100, 200, where the ultimate number n of reaction units 4 is determined by process-related boundary conditions and is set according to the conversion, total volume of the reaction units and product demand, and also according to economic considerations. It should be stated here that the configuration of the reaction unit 4 or 100 and 200 is technically relatively favorable since it is possible to dispense with moving parts, for example stirrers that have to be driven and have bearing devices. In the present configuration form according to FIG. 1, it is possible to dispense with moving parts apart from the first compressor 40 that compresses the reactant into the first reaction unit 100. The reaction cascade 2 shown in FIG. 1 has three reaction sections 4 in this case, this being a purely illustrative schematic representation. Moreover, the reaction units 4, 100, 200 and 300 are shown in equal size. They are also shown as being of the same type. This has the advantage that mass production of multiple reaction units 4 can likewise again be configured in an inexpensive manner. In principle, the reaction units 100, 200, 300 along the cascade 2 can be reduced in terms of their reaction volume. In this case, however, it is merely the volume of the reaction unit or of the reaction section and possibly also of the absorption section 8 that is reduced, but there is little change in the design thereof. The reason for the reduction in the reaction volume is that the reactant 18 is introduced only once into the cascade in the configuration envisaged. Thus, no further reactant is introduced during the progress of the reaction in the downstream reaction units 200 and 300, since this would mean further energy expenditure by compression of the base reactant 18. Thus, even within the cascade 2, the volume of the reactant 18 available decreases in the further reaction units 200 and 300, and therefore the reaction volume in the reaction section 206 and 306 of the reaction units 200 and 300 can also be reduced gradually.

[0030] FIG. 2 illustrates the circulation of the absorbent 14, specifically in the phase in which it leaves the absorption section 8 at the absorbent outlet 22. A desorption unit 24 is provided, in which the absorbent 14 laden with the reaction product 26 is freed therefrom. This “regeneration” of the absorbent 14 can be effected by lowering the pressure and/or increasing the temperature. The introduction of what is called a stripping gas for desorption may also be appropriate. The gas that has thus been freed of the absorbent 14 and contains the reaction products 26 is subsequently guided into a heat exchanger 38 in which the reaction product 26, for example methanol, is separated by condensation from the remaining gaseous constituents, especially comprising the reactant gases carbon dioxide and hydrogen. The reaction products, especially the methanol which, however, also contains water, can be removed for further processing. The reactants 18 or 18′ that have likewise been recovered therefrom can be fed back to the process, and introduced into the first reaction unit 100 via the compressor 40. The unladen absorbent, labeled 14′ here, is heated and introduced as unladen absorbent 14 via an absorbent feed 36 back into the absorption section 8.

[0031] The reaction of carbon dioxide and hydrogen to give methanol and water that proceeds over the catalytic substance 30 in the reaction section 6 is exothermic. This means that the reaction section 6 heats up. Countercurrent cooling through an outer wall of the reaction section 6 is appropriate here. The reaction section 6 here is advantageously of jacketed design in terms of its outer shell. The thermal energy obtained thereby can be used for heating in some other way, for example for heating of the reactant gas 18. It is also possible to use the energy possessed by the absorbent 14 after unloading for this purpose.

LIST OF REFERENCE NUMERALS

[0032] 2 reactor cascade [0033] 4 reactor unit [0034] 6 reaction section [0035] 8 absorption section [0036] 10 reactant inlet [0037] 12 reactant outlet [0038] 14 absorbent [0039] 16 pressure reduction valve [0040] 18 reactants [0041] 20 connecting conduit [0042] 100 first reactor unit [0043] 200 second reactor unit [0044] 210 reactant inlet to reactor unit [0045] 22 absorbent outlet [0046] 24 desorption unit [0047] 26 reaction products [0048] 28 reaction volume [0049] 106 reaction section of first reaction unit [0050] 206 reaction section of second reaction unit [0051] 30 catalytic substance [0052] 32 gas filter apparatus [0053] 34 longitudinal extent of reactor section [0054] 36 absorbent feed [0055] 38 heat exchanger [0056] 40 heat exchanger [0057] 42 flange [0058] 44 gas space of identical reactor