PROCESS AND PLANT FOR PRODUCING METHANOL

Abstract

The invention relates to a process and a plant for producing methanol in which a compressed make-up gas stream which contains at least one carbon oxide and hydrogen is combined with a residual gas to afford a synthesis gas stream and reacted to afford methanol. According to the invention the residual gas stream and the make-up gas stream are combined using a jet pump, wherein the compressed make-up gas stream is supplied to the jet pump as motive medium via its motive media connection at a pressure p.sub.1 and the residual gas stream is supplied to the jet pump as suction medium via its suction port at a pressure p.sub.3 and wherein the synthesis gas stream is discharged from the jet pump via its pressure port at a pressure p.sub.2 and subsequently supplied to the synthesis stage and wherein p.sub.1>p.sub.2>p.sub.3.

Claims

1. A process for producing methanol, comprising: a) providing and compressing a make-up gas stream, wherein the make-up gas stream comprises at least one carbon oxide and hydrogen; b) reacting a synthesis gas stream in a synthesis stage over a solid methanol synthesis catalyst, wherein raw methanol and a residual gas stream are obtained and wherein the raw methanol after cooling and condensation as liquid reaction product and the residual gas stream are discharged from the synthesis stage; c) combining the residual gas stream with the make-up gas stream to obtain the synthesis gas stream, wherein the residual gas stream and the make-up gas stream are combined using a jet pump, wherein the compressed make-up gas stream is supplied to the jet pump as motive medium via its motive media connection at a pressure p.sub.1 and the residual gas stream is supplied to the jet pump as suction medium via its suction port at a pressure p.sub.3 and wherein the synthesis gas stream is discharged from the jet pump via its pressure port at a pressure p.sub.2 and subsequently supplied to the synthesis stage and wherein p.sub.1>p.sub.2>p.sub.3.

2. The process according to claim 1, wherein for a recirculation rate R defined as $R=\frac{\mathrm{Volume}\mathrm{flow}\left(\mathrm{RG}\right)}{\mathrm{Volume}\mathrm{flow}\left(\mathrm{MG}\right)},$ 0.3≤R≤2.0.

3. The process according to claim 1, wherein the process comprises a plurality N of serially arranged synthesis stages, wherein the raw methanol after cooling and condensation as liquid reaction product is discharged from each of the synthesis stages and the residual gas stream is discharged from the last of the N synthesis stages.

4. The process according to claim 3, wherein N≥2.

5. The process according to claim 1, wherein the proportion of carbon dioxide in the carbon oxides in the make-up gas stream MG is at least 50% by volume.

6. A plant for producing methanol, comprising the following components in operative connection with one another: a) a compressor for compressing a make-up gas stream, wherein the make-up gas stream comprises at least one carbon oxide and hydrogen; b) a reaction apparatus for reacting a synthesis gas stream in a reactor stage over a solid methanol synthesis catalyst, wherein raw methanol and a residual gas stream are obtainable by the reaction over the methanol synthesis catalyst and wherein the reactor stage comprises means for cooling and condensing the raw methanol and means for discharging the raw methanol as liquid reaction product and the residual gas stream from the reactor stage; c) means for combining the residual gas stream and the make-up gas stream by which the synthesis gas stream is obtainable, wherein the plant comprises a jet pump as a means for combining the residual gas stream and the make-up gas stream and the plant comprises means for supplying the compressed make-up gas stream to a motive media connection of the jet pump at a pressure p.sub.1, thus making the make-up gas stream employable as the motive medium of the jet pump, and the plant comprises means for supplying the residual gas stream to a suction media connection of the jet pump at a pressure p.sub.3 and the synthesis gas stream is dischargeable from the jet pump and suppliable to the reaction apparatus via a pressure port of the jet pump at a pressure p.sub.2 and wherein p.sub.1>p.sub.2>p.sub.3.

7. The plant according to claim 6, wherein the reaction apparatus of the plant comprises a plurality P of serially arranged reactor stages, wherein the raw methanol after cooling and condensation as liquid reaction product is dischargeable from each of the reactor stages and the residual gas stream is dischargeable from the last of the P reactor stages.

8. The plant according to claim 7, wherein P≥2.

9. The plant according to claim 6, wherein the plant is configured such that it is operable at a recirculation rate R defined as $R=\frac{\mathrm{Volume}\mathrm{flow}\left(\mathrm{RG}\right)}{\mathrm{Volume}\mathrm{flow}\left(\mathrm{MG}\right)},$ and 0.3≤R≤2.0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The invention is more particularly elucidated hereinbelow by way of two inventive examples without in any way limiting the subject-matter of the invention.

[0060] Further features, advantages and possible applications of the invention will be apparent from the following description of the exemplary embodiments in connection with the drawings.

[0061] FIG. 1 shows a block flow diagram of an inventive process or an inventive plant according to a first example of the invention, wherein the process comprises a single synthesis stage, and

[0062] FIG. 2 shows a block flow diagram of an inventive process or an inventive plant according to a second example of the invention, wherein the process comprises three synthesis stages.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0063] FIG. 1 shows a highly simplified block flow diagram of an inventive process (or an inventive plant) 1 according to a first example of the invention. The process according to FIG. 1 comprises only a single synthesis stage (or reactor stage) 22. The process comprises an apparatus for synthesis gas production 10, for example a steam reformer. The apparatus 10 produces a synthesis gas which comprises at least carbon monoxide, carbon dioxide and hydrogen and as make-up gas stream 11 at a pressure of 30 bar is compressed in compressor 12 to 95 bar. The compressed make-up gas stream MG 13 is subsequently supplied to an inventive jet pump 14 via its motive media connection 15. The jet pump 14 further comprises a suction port 16 and a pressure port 17. After entry of the compressed make-up gas stream MG 13 via the motive media connection 15 of the jet pump 14 said stream is accelerated by a motive nozzle (not shown) present in the jet pump 14, as a result of which a region of relatively low pressure is generated in the head region of the jet pump 14 and a residual gas stream 19 supplied via the suction port 16 is thus entrained. Mixing of the streams 13 and 19 inside the jet pump 14 thus produces a synthesis gas stream 18 which exits the jet pump 14 at the pressure port 17. The synthesis gas stream 18 has a pressure which is lower than the pressure of the compressed make-up gas stream MG 13 and higher than the pressure of the residual gas stream RG 19. In any case the synthesis gas stream SG 18 has a pressure which is at least as high as the synthesis pressure required for the methanol synthesis which is necessary for the conversion of the synthesis gas into methanol in the synthesis stage 22.

[0064] The process further comprises three measurement and control sites for detecting and adjusting the pressures of the streams 13, 19 and 18 which are supplied to the jet pump 14 or discharged therefrom. Any required control valves or other measurement and control means are not shown. The pressure p.sub.1 is the motive media pressure, i.e. the pressure of the compressed make-up gas stream MG 13 upstream of the motive media connection 15 of the jet pump 14. The pressure p.sub.3 is the suction pressure, i.e. the pressure of the residual gas stream RG 19 upstream of the suction port 16 of the jet pump 14. Finally, the pressure p.sub.2 is the counterpressure, i.e. the pressure of the synthesis gas stream 18 immediately downstream of the outlet, i.e.

[0065] pressure port 18, of the jet pump 14.

[0066] The synthesis gas stream SG 18 exiting via the pressure port 17 of the jet pump 14 is supplied to the synthesis stage (or reactor stage) 22 and therein converted into raw methanol, a mixture of methanol and water and any undesired byproducts, over a solid methanol synthesis catalyst. The synthesis stage 22 comprises means for discharging the reaction heat of the exothermic methanol formation reaction and for cooling and condensing the raw methanol which is subsequently separated from the residual gas stream RG 19 as raw methanol stream 21 by a separation step and discharged from the synthesis stage 22 as liquid reaction product. The residual gas stream RG 19 from the synthesis stage 22 is also discharged and recycled to the suction port of the jet pump 14 and thus combined with the make-up gas stream MG 13 supplied via the motive media connection 15. To prevent accumulation of reactant or product components inert under the conditions of methanol synthesis in the synthesis loop a stream is diverted from a portion of the residual gas stream RG 19 as purge gas stream 20 and supplied to a further use,

[0067] The raw methanol stream 21 is supplied to a further workup to obtain pure methanol (not shown).

[0068] In the case of the example according to FIG. 1 the recirculation rate R=4.6.

[0069] FIG. 2 shows a highly simplified block flow diagram of an inventive process (or an inventive plant) 2 according to a second example of the invention. The process according to FIG. 2 comprises three synthesis stages (or reactor stages) 22a, 22b and 22c.

[0070] The example according to FIG. 2 differs from the example according to FIG. 1 in that instead of a single synthesis stage 22 the process instead comprises three serially arranged synthesis stages 22a, 22b and 22c. Furthermore, the make-up gas stream 11 from the compressor 12 is only compressed to 90 bar relative to the pressure of the compressed make-up gas stream MG 13. Due to the presence of three serially arranged synthesis stages and the resulting higher carbon conversion (conversion of carbon bound in the synthesis gas to carbon bound in methanol) the “per pass” recirculation rate R is only 1.0.

[0071] Each of the synthesis stages 22a, 22b and 22c comprises means for cooling, condensing and separating raw methanol. This is accordingly discharged from the corresponding synthesis stages 22a, 22b und 22c as three separate raw methanol streams 21a, 21b and 21c. The three raw methanol substreams 21a, 21b und 21c are subsequently combined to afford an overall raw methanol stream 21. The raw methanol stream 21 is supplied to a further workup to obtain pure methanol (not shown). A stream of unconverted synthesis gas is also separated from the respective raw methanol stream per synthesis stage. In the case of the first synthesis stage 22a the unconverted synthesis gas is supplied to the second synthesis stage 22b as intermediate gas stream 1G 23a. The synthesis gas not converted in the second synthesis stage 22b is supplied to the third and last synthesis stage 22c as intermediate gas stream IG 23b. Finally, synthesis gas not converted into methanol and water in the third synthesis stage 22c is recycled to the suction port 16 of the jet pump 14 as residual gas stream RG 19 and therein in turn combined with the make-up gas stream MG 13 supplied via the motive media connection 15.

[0072] Depending on the process mode it is also possible for substreams to be diverted from the respective intermediate gas streams IG 23a and/or 23b and recycled not into the subsequent synthesis stage but rather to the motive media connection 15 of the jet pump 14.

[0073] To prevent accumulation of reactant or product components inert under the conditions of methanol synthesis in the synthesis loop, in the case of the example according to FIG. 2 too, a purge gas stream 20 is diverted from the residual gas stream RG 19 and supplied to a further use.

[0074] In the case of the example according to FIG. 2 the recirculation rate R=1.0.

[0075] The following numerical examples serve to further elucidate the invention. The numerical examples were generated using Aspen Plus® simulation software. The examples assume a “worst-case scenario” namely the case of a synthesis gas comprising exclusively carbon dioxide as the carbon oxide which is very unfavourable for methanol formation. In such a case the required recirculation rate is particularly high since the thermodynamic equilibrium is markedly on the reactant side and the carbon conversion (conversion of carbon in the synthesis gas into methanol) is comparatively low.

[0076] The following table shows a comparison between two configurations, according to the prior art and the invention, for a process having a single synthesis stage. Comparative Example 1 corresponds to a configuration having a dedicated residual gas compressor while Example 1 corresponds to a configuration having a jet pump as shown in FIG. 1.

TABLE-US-00001 Comparative Example 1 Example 1 (residual gas compressor) (jet pump) Pressure at compressor inlet 30   30   (stream 11)/bar Recirculation rate  4.6  4.6 Pressure drop over synthesis 2  2  stage/bar Pressure at compressor outlet 80   95   (stream 13)/bar Normalized power demand 100%   120%   for compression work of compressor 12

[0077] The omission of the residual gas compressor according to Comparative Example 1 and replacement of this residual gas compressor by a jet pump according to Example 1 increases the power demand of the compressor 12 by 20%. Especially in the case of plants on a small or intermediate scale the associated higher operating costs can however be overcompensated by the simultaneously falling capital and maintenance costs.

[0078] The following table shows a comparison between two configurations, according to the prior art and the invention, for a process having three serially arranged synthesis stages. Comparative Example 2 corresponds to a configuration having a dedicated residual gas compressor while Example 2 corresponds to a configuration having a jet pump as shown in FIG. 2.

TABLE-US-00002 Comparative Example 2 Example 2 (residual gas compressor) (jet pump) Pressure at compressor inlet 30   30   (stream 11)/bar Recirculation rate  1.0  1.0 Pressure drop over synthesis 6  6  stage/bar Pressure at compressor outlet 80   90   (stream 13)/bar Normalized power demand 100%   112%   for compression work of compressor 12

[0079] Since Comparative Example 2 and Example 2 relate to a three-stage synthesis with intermediate condensation of the raw methanol the carbon conversion “per pass” is higher and the recirculation rate is correspondingly several times lower than according to Comparative Example 1 and Example 1. The pressure drop over the synthesis stages is 6 bar instead of 2 bar due to the presence of three times as many synthesis stages.

[0080] The omission of the residual gas compressor according to Comparative Example 2 and replacement of this residual gas compressor by a jet pump according to Example 2 increases the power demand of the compressor 12 by only 12%. This is a surprising finding in view of the fact that the pressure drop in the case of the multi-stage synthesis according to FIG. 2 is 3 times as high as the single-stage synthesis according to FIG. 1 and must be compensated by compression. In this case the low recirculation rate of 1.0 thus has an overcompensating effect in terms of the required compressor power demand despite the higher pressure drop in the synthesis loop.

[0081] Synthesis gases having a high carbon monoxide content result, even in single-stage configurations, in a high “per pass” carbon conversion, as a result of which correspondingly lower recirculation rates are required. The abovementioned effect should therefore come into play especially also in the case of single-stage or two-stage processes having low recirculation rates which exhibit a low pressure drop over the synthesis stages.

LIST OF REFERENCE SYMBOLS

[0082] 1, 2 Process, Plant

[0083] 10 Apparatus for synthesis gas production

[0084] 11 Make-up gas stream

[0085] 12 Compressor

[0086] 13 Compressed make-up gas stream MG

[0087] 14 Jet pump

[0088] 15 Motive media connection

[0089] 16 Suction port

[0090] 17 Pressure port

[0091] 18 Synthesis gas stream SG

[0092] 19 Residual gas stream RG

[0093] 20 Purge gas stream

[0094] 21, 21a, 21b, 21c Raw methanol stream

[0095] 22, 22a, 22b, 22c Synthesis stage or reactor stage

[0096] 23a, 23b Intermediate gas stream IG

[0097] p.sub.1, p.sub.2, p.sub.3 Measurement and control sites for pressure

[0098] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.