PROCESS AND PLANT FOR PRODUCING METHANOL FROM HYDROGEN-RICH SYNTHESIS GAS

Abstract

A process for producing methanol, wherein a make-up gas stream from a reformer unit is admixed with a hydrogen-containing stream from a hydrogen recovery stage to obtain a hydrogen-rich synthesis gas, which is combined with a residual gas stream and the combined stream is passed through a bed of a methanol synthesis catalyst at elevated pressure and elevated temperature to obtain a product stream comprising methanol and the residual gas stream and wherein the product stream is cooled to remove methanol from the residual gas stream. Wherein a portion of the residual gas stream is removed as a purge gas stream and a portion of the hydrogen-rich synthesis gas stream is removed and combined with the purge gas stream to obtain a mixed synthesis gas stream and the mixed synthesis gas stream is sent to the hydrogen recovery stage to produce the hydrogen-containing stream.

Claims

1.-14. (canceled)

15. A process for producing methanol, wherein a make-up gas stream from a reformer unit comprising hydrogen and carbon oxides is admixed with a hydrogen-containing stream from a hydrogen recovery stage to obtain a hydrogen-rich synthesis gas stream having a stoichiometry number SN, defined as SN=[n(H.sub.2)−n(CO.sub.2)]/[n(CO)+n(CO.sub.2)], of not less than 2.0 and wherein the hydrogen-rich synthesis gas stream is combined with a residual gas stream and the hydrogen-rich synthesis gas stream and the residual gas stream are passed through a bed of a methanol synthesis catalyst at elevated pressure and elevated temperature to obtain a product stream comprising methanol and the residual gas stream and wherein the product stream is cooled to remove methanol from the residual gas stream, wherein: a portion of the residual gas stream is removed as a purge gas stream and a portion of the hydrogen-rich synthesis gas stream is removed and combined with the purge gas stream to obtain a mixed synthesis gas stream and the mixed synthesis gas stream is sent to the hydrogen recovery stage to produce the hydrogen-containing stream.

16. The process according to claim 15, wherein the hydrogen-rich synthesis gas stream is compressed and a portion of the compressed hydrogen-rich synthesis gas stream is removed and combined with the purge gas stream.

17. The process according to claim 16, wherein the residual gas stream is compressed and combined with the compressed hydrogen-rich synthesis gas stream and the combined streams are passed through the bed of the methanol synthesis catalyst.

18. The process according to claim 15, wherein the hydrogen-containing stream is compressed by a hydrogen compressor and the compressed hydrogen-containing stream is combined with the make-up gas stream to obtain the hydrogen-rich synthesis gas stream and a portion of the hydrogen-rich synthesis gas stream is removed and combined with the purge gas stream.

19. The process according to claim 18, wherein the hydrogen-rich synthesis gas stream and the residual gas stream are compressed and passed through the bed of the methanol synthesis catalyst together.

20. The process according to claim 15, wherein the molar flow rate proportion of the hydrogen-rich synthesis gas stream in the mixed synthesis gas stream is between 0.10 and 0.95.

21. The process according to claim 15, wherein the molar flow rate proportion of the portion removed from the hydrogen-rich synthesis gas stream based on the total molar flow rate of hydrogen-rich synthesis gas is between 0.001 and 0.999.

22. The process according to claim 15, wherein the hydrogen-rich synthesis gas stream has a stoichiometry number SN of 2.00 to 2.20.

23. The process according to claim 15, wherein the make-up gas stream has a stoichiometry number SN of less than 2.0.

24. The process according to claim 15, wherein the hydrogen recovery stage comprises a pressure swing adsorption apparatus for removing hydrogen from the mixed synthesis gas stream.

25. The process according to claim 15, wherein the hydrogen recovery stage comprises a membrane separation stage for removing hydrogen from the mixed synthesis gas stream.

26. The process according to claim 15, wherein the hydrogen-containing stream has a hydrogen proportion of at least 95% by volume.

27. A plant for producing methanol comprising the following plant components arranged in fluid connection with one another: a reformer unit for producing a make-up gas stream comprising hydrogen and carbon oxides; a hydrogen recovery stage for producing a hydrogen-containing stream, wherein the reformer unit and the hydrogen recovery stage are configured such that a hydrogen-rich synthesis gas stream having a stoichiometry number SN, defined as SN=[n(H.sub.2)−n(CO.sub.2)]/[n(CO)+n(CO.sub.2)], of not less than 2.0 is obtainable from the hydrogen-containing stream and the make-up gas stream; a reactor stage comprising a methanol synthesis catalyst bed, wherein the reactor stage is configured such that the hydrogen-rich synthesis gas stream and a residual gas stream may be passed through the methanol synthesis catalyst bed at elevated pressure and elevated temperature, the reformer unit, the hydrogen recovery stage, and the reactor stage configured to produce a product stream comprising methanol and the residual gas stream; a cooling apparatus for cooling the product stream, wherein the cooling apparatus is configured such that methanol may be removed from the residual gas stream, wherein: the plant is configured such that a portion of the residual gas stream may be removed as a purge gas stream and a portion of the synthesis gas stream may be removed and combined with the purge gas stream, thus making it possible to obtain a mixed synthesis gas stream, and the mixed synthesis gas stream may be sent to the hydrogen recovery stage to produce the hydrogen-containing stream.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention is more particularly elucidated hereinbelow by way of two inventive examples and one comparative example without in any way limiting the subject-matter of the invention. Further features, advantages and possible applications of the invention will be apparent from the following description of the working examples in connection with the drawings and the numerical examples.

[0035] FIG. 1 shows a schematic block flow diagram of a production process or a plant 100 for methanol synthesis according to a first exemplary embodiment of the invention,

[0036] FIG. 2 shows a schematic block flow diagram of a production process or a plant 200 for methanol synthesis according to a second exemplary embodiment of the invention,

[0037] FIG. 3 shows a schematic block flow diagram of a production process or a plant 300 for methanol synthesis according to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] In the process mode according to FIG. 1 a make-up gas stream 11, for example produced in a plant for autothermal reforming of natural gas (not shown), is combined with a hydrogen-containing stream 12 to produce a hydrogen-rich synthesis gas stream 13 having a stoichiometry number of not less than 2.0. The hydrogen-rich synthesis gas stream 13 is compressed to synthesis pressure by a compressor stage 30. A portion of the hydrogen-rich synthesis gas stream 13 is removed as hydrogen-rich synthesis gas substream 14 and combined with a purge gas stream 15 to afford a mixed synthesis gas stream 16. The mixed synthesis gas stream 16 is sent to the hydrogen recovery stage 31, in which by pressure swing adsorption the hydrogen-containing stream 12 is produced with a hydrogen proportion of at least 99% by volume. Offgas 17 simultaneously produced in the hydrogen recovery stage 31 and containing carbon oxides and constituents inert under the conditions of the methanol synthesis may be used for example as a fuel gas in the reformer unit arranged upstream of the methanol synthesis.

[0039] The main portion 18 of the hydrogen-rich synthesis gas stream compressed to synthesis pressure is combined with a residual gas stream 19 compressed to synthesis pressure in a compressor stage 32. The resulting combined synthesis gas stream 20 is heated in a heat exchanger 33 and as heated synthesis gas stream 21 sent to a methanol reactor 34. The methanol reactor 34 carries out the conversion of the synthesis gas from synthesis gas stream 21 over the methanol synthesis catalyst of the catalyst bed 35 to afford methanol and water. The product stream 22 resulting from the conversion in the reactor 34 which comprises not only methanol and water but also unreacted synthesis gas or residual gas is then consecutively cooled via the heat exchangers 36, 33 and 37, the product streams 23, 24 and 25 resulting downstream of the respective heat exchangers. A separator 38 subsequently carries out the separation of the cooled product stream 25 into a liquid phase comprising methanol and water and a gaseous phase comprising residual gas. The synthesis gas not converted in the reactor 34, i.e. residual gas, is withdrawn from the separator 38 as residual gas stream 26. A crude methanol stream 27 comprising methanol and water is simultaneously withdrawn from the separator 38 and sent for further workup, for example a rectification (not shown). The purge gas stream 15 is removed from the residual gas stream 26 and a remaining residual gas stream 28 is compressed to synthesis pressure in the compressor stage 32. Residual gas stream 19 compressed to synthesis pressure is in turn combined with hydrogen-rich synthesis gas stream 18 and sent back to the conversion to afford methanol in the methanol reactor 34.

[0040] FIG. 2 shows a type of process mode according to a further inventive example which is modified compared to the example of FIG. 1. In the process mode according to FIG. 2 the hydrogen-containing stream 12 produced in the hydrogen recovery stage 31 is compressed in a hydrogen compressor 40 to obtain a compressed hydrogen-containing stream 51 which is combined with the make-up gas stream 11. This affords a hydrogen-rich synthesis gas stream 13 of which the main portion 18 is sent to compressor stage 41 for compression to synthesis pressure and of which a portion is diverted as hydrogen-rich synthesis gas substream 14 and combined with the purge gas stream 15. The mixed synthesis gas stream 16 results from the streams 14 and 15. The hydrogen-rich synthesis gas stream 18 and the residual gas stream are together sent to a compressor stage 41. Compressor stage 41 has two ports on its suction side which allows simultaneous compression of the hydrogen-rich synthesis gas stream 18 and the residual gas stream 28 to obtain the combined synthesis gas stream 20 which is heated in heat exchanger 33 and sent as synthesis gas stream 21 to the methanol reactor 34.

[0041] That which is recited in connection with FIG. 1 applies correspondingly to the further elements shown in FIG. 2.

[0042] FIG. 3 shows a type of process mode known from the prior art. Here too, a mixed gas stream of synthesis gas and purge gas is sent to the hydrogen recovery stage 31 and utilized for hydrogen recovery. However, the synthesis gas proportion of the mixed gas stream is a partial make-up gas stream 60 which is diverted from the (main) make-up gas stream 11 using the throttle means 70. The partial make-up gas stream 60 and the purge gas stream 15 are recycled and as mixed synthesis gas stream 61 sent to the hydrogen recovery stage 31. In contrast to the above inventive examples the mixed synthesis gas stream 61 is thus not produced from synthesis gas already enriched with hydrogen and purge gas but rather from make-up gas and purge gas. However, as shown in the following numerical examples this type of process mode has disadvantages compared to the inventive process. One disadvantage results from the unavoidable use of the throttle means 70 required for throttling the (main) make-up gas stream 11. The reduction in pressure by the throttle means 70 must be compensated by compressor stage 30.

[0043] The advantages of the invention are hereinbelow illustrated using two numerical examples. Both examples represent simulated cases which were calculated using the simulation software “Aspen Plus”.

EXAMPLES

Example 1

[0044] Example 1 is based on the process mode according to FIG. 1 in contrast with the process mode of the prior art (FIG. 3—comparative example).

[0045] According to Example 1 and the comparative example the hydrogen-poor synthesis gas stream or make-up gas stream (11) has the following composition:

TABLE-US-00001 Component Proportion (% by vol.) Water 0.21 Carbon dioxide 8.04 Carbon monoxide 23.16 Hydrogen 65.94 Argon 0.12 Nitrogen 0.52 Methane 2.01

[0046] For the hydrogen-poor synthesis gas stream or make-up gas stream this results in a stoichiometry number SN of 1.86.

[0047] Argon, nitrogen and methane are gas constituents inert under the conditions of the methanol synthesis and are discharged from the synthesis circuit substantially via the purge gas stream (15).

[0048] According to Example 1 and the comparative example the hydrogen-rich synthesis gas stream (13, 18) has the following composition:

TABLE-US-00002 Component Proportion (% by vol.) Water 0.19 Carbon dioxide 7.54 Carbon monoxide 21.71 Hydrogen 68.07 Argon 0.11 Nitrogen 0.49 Methane 1.89

[0049] For the hydrogen-rich synthesis gas stream this results in a stoichiometry number SN of 2.07.

[0050] The molar flow rate of hydrogen-rich synthesis gas (proportion 14 of the overall stream of the hydrogen-rich synthesis gas) sent to the hydrogen recovery stage (31) is 1451.5 kmol/h. The molar flow rate of the purge gas stream (15) is 1306.3 kmol/h. Both streams together form the mixed synthesis gas stream (16) having a molar flow rate of 2757.8 kmol/h. For Example 1 this results in a molar flow rate proportion or molar proportion of the hydrogen-rich synthesis gas stream in the mixed synthesis gas stream of 0.53.

[0051] According to Example the molar flow rate proportion or molar proportion of the portion (14) removed from the hydrogen-rich synthesis gas stream based on the total molar flow rate of hydrogen-rich synthesis gas (13) is 0.059.

[0052] Compared to the process mode according to the comparative example (FIG. 3) for the same production quantity of crude methanol (crude methanol=mixture of methanol and water the following picture emerges in terms of energy consumption:

TABLE-US-00003 Comparative example Example 1 Parameter (FIG. 3) (FIG. 1) Synthesis gas 16808 15791 compressor power/ KW Mass flow natural gas 126801 126801 for make-up gas production/kg/h High-pressure steam 247190 249861 export potential/kg/h Crude methanol 209266 209266 production/kg/h Specific compressor 80.32 75.46 power for compressor stage 30/kW/MT (MT = metric ton)

[0053] The achieved energy savings in respect of the compressor power required for compression of the synthesis gas to synthesis pressure (compressor stage 30 in FIG. 1 and FIG. 3) results in an annual energy saving of 71867 GJ (71867 GJ/a). In addition the process mode according to Example (FIG. 1) provides a higher potential for production of high-pressure steam as export steam.

Example 2

[0054] Example 2 is based on the process mode according to FIG. 2 in contrast with the process mode of the prior art (FIG. 3—comparative example).

[0055] According to Example 2 and the comparative example the hydrogen-poor synthesis gas stream or make-up gas stream (11) has the following composition:

TABLE-US-00004 Component Proportion (% by vol.) Water 0.16 Carbon dioxide 7.54 Carbon monoxide 24.68 Hydrogen 65.55 Argon 0.12 Nitrogen 0.09 Methane 1.86

[0056] For the hydrogen-poor synthesis gas stream or make-up gas stream this results in a stoichiometry number SN of 1.80.

[0057] According to Example 2 and the comparative example the hydrogen-rich synthesis gas stream (13, 18) has the following composition:

TABLE-US-00005 Component Proportion (% by vol.) Water 0.14 Carbon dioxide 6.99 Carbon monoxide 22.84 Hydrogen 68.12 Argon 0.11 Nitrogen 0.08 Methane 1.72

[0058] For the hydrogen-rich synthesis gas stream this results in a stoichiometry number SN of 2.05.

[0059] The molar flow rate of hydrogen-rich synthesis gas (proportion 14 of the overall stream of the hydrogen-rich synthesis gas) sent to the hydrogen recovery stage (31) is 2280.0 kmol/hr. The molar flow rate of the purge gas stream (15) is 976.4 kmol/hr. Both streams together form the mixed synthesis gas stream (16) having a molar flow rate of 3256.4 kmol/hr. For Example 2 this results in a molar flow rate proportion or molar proportion of the hydrogen-rich synthesis gas stream in the mixed synthesis gas stream of 0.70.

[0060] According to Example 2, the molar flow rate proportion or molar proportion of the portion (14) removed from the hydrogen-rich synthesis gas stream based on the total molar flow rate of hydrogen-rich synthesis gas (13) is 0.091.

[0061] Compared to the process mode according to the comparative example (FIG. 3) for the same production quantity of crude methanol (crude methanol=mixture of methanol and water) the following picture emerges in terms of energy consumption:

TABLE-US-00006 Comparative example Example 2 Parameter (FIG. 3) (FIG. 2) Synthesis gas 10756 8797 compressor power/ KW Hydrogen n/a 84 compressor power Mass flow natural gas 131664 131147 for make-up gas production/kg/h Crude methanol 209394 209394 production/kg/h Specific compressor 51.37 42.42 power for compressor stage 30/kW/MT (MT = metric ton)

[0062] The achieved energy savings in respect of the compressor power required for compression of the synthesis gas to synthesis pressure (compressor stage 30 in FIG. 3; synthesis gas proportion compressor stage 41 and hydrogen compressor 40 in FIG. 2) results in an annual energy saving of 206548 GJ (206548 GJ/a).

LIST OF REFERENCE NUMERALS

[0063] 100, 200 Process, plant (invention) [0064] 300 Process, plant (prior art) [0065] 11 Make-up gas stream [0066] 12, 51 Hydrogen-containing stream [0067] 13, 18 Hydrogen-rich synthesis gas stream [0068] 14 Hydrogen-rich synthesis gas substream [0069] 15 Purge gas stream [0070] 16 Mixed synthesis gas stream (invention) [0071] 17 Offgas [0072] 19, 26, 28 Residual gas stream [0073] 20 Combined synthesis gas stream [0074] 21 Synthesis gas stream [0075] 22, 23, 24, 25 Product stream [0076] 26 Residual gas stream [0077] 27 Crude methanol stream [0078] 30, 32, 41 Compressor stage [0079] 31 Hydrogen recovery stage [0080] 33, 36, 37 Heat exchanger [0081] 38 Separator [0082] 40 Hydrogen compressor [0083] 60 Partial make-up gas stream [0084] 61 Mixed synthesis gas stream (prior art) [0085] 70 Throttle means

[0086] 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.