Methanol process

Abstract

A process is described for the synthesis of methanol comprising the steps of: (i) passing a first synthesis gas mixture comprising a make-up gas and a first loop recycle gas stream through a first synthesis reactor containing a cooled methanol synthesis catalyst to form a first product gas stream, (ii) recovering methanol from the first product gas stream thereby forming a first methanol-depleted gas mixture, (iii) combining the first methanol-depleted gas mixture with a second loop recycle gas stream to form a second synthesis gas mixture, (iv) passing the second synthesis gas mixture through a second synthesis reactor containing a cooled methanol synthesis catalyst to form a second product gas stream, (v) recovering methanol from the second product gas stream thereby forming a second methanol-depleted gas mixture, and (vi) forming the first and second loop recycle gas streams from the second methanol-depleted gas mixture, wherein the first synthesis reactor has a higher heat transfer per cubic metre of catalyst than the second synthesis reactor and the recycle ratio of the first loop recycle gas stream to form the first synthesis gas mixture is in the range 0.1 to 1:1, and the recycle ratio of the second loop recycle gas stream to form the second synthesis gas mixture is in the range 1.1:1 to 6:1.

Claims

1. A process for synthesizing methanol comprising the steps of: (i) passing a first synthesis gas mixture comprising a make-up gas and a first loop recycle gas stream through a first synthesis reactor containing a first cooled methanol synthesis catalyst to form a first product gas stream, (ii) recovering methanol from the first product gas stream to form a first methanol-depleted gas mixture, (iii) combining the first methanol-depleted gas mixture with a second loop recycle gas stream to form a second synthesis gas mixture, (iv) passing the second synthesis gas mixture through a second synthesis reactor containing a second cooled methanol synthesis catalyst to form a second product gas stream, (v) recovering methanol from the second product gas stream to form a second methanol-depleted gas mixture, and (vi) forming the first and second loop recycle gas streams from the second methanol-depleted gas mixture, wherein the first synthesis reactor has a higher heat transfer per cubic metre of catalyst than the second synthesis reactor and the recycle ratio of the first loop recycle gas stream to form the first synthesis gas mixture is in the range of from 0.1 to 1:1, and the recycle ratio of the second loop recycle gas stream to form the second synthesis gas mixture is in the range of from 1.1:1 to 6:1.

2. The process according to claim 1 wherein the recycle ratio of the loop recycle gas stream to form the second synthesis gas mixture is in the range of from 1.5:1 to 6:1.

3. The process according to claim 1 wherein the make-up gas contains carbon monoxide in the range of from 20 to 35% vol.

4. The process according to claim 1 wherein the loop recycle gas streams are circulated by means of a two-stage circulator comprising a first stage and a second stage, or two separate circulators.

5. The process according to claim 4 wherein a circulator comprises a first stage and a second stage, the first stage is fed with the first methanol-depleted gas mixture and the second stage is fed with the second recycle loop gas stream, and the first methanol-depleted gas mixture and the second recycle loop gas stream are combined in the circulator to provide the second synthesis gas.

6. The process according to claim 5 wherein the methanol-depleted gases fed to the circulator are enriched by a portion of the make-up gas.

7. The process according to claim 4 wherein a circulator comprises a first stage and a second stage, the first stage is fed with the second methanol-depleted gas stream minus any purge gas and produces two separate loop recycle gas streams; one from the circulator first stage, which is combined with the first methanol-depleted gas mixture and fed to the second synthesis reactor, and one from the circulator second stage, which is fed to the first synthesis reactor.

8. The process according to claim 7 wherein the first methanol-depleted gas mixture contains a portion of the make-up gas.

9. The process according to claim 4 wherein two separate circulators are used, a first circulator is fed with the first methanol-depleted gas mixture and a second circulator is fed with the second methanol-depleted gas stream, minus any purge stream, the second circulator product is divided into the first and second recycle loop gas streams, and the second recycle loop gas stream is mixed with the first methanol-depleted gas stream product of the first circulator.

10. The process according to claim 9 where the first methanol-depleted gas stream product of the first circulator is diluted with a portion of the make-up gas.

11. The process according to claim 1 wherein the first synthesis reactor comprises the first methanol synthesis catalyst disposed in tubes that are cooled by water under pressure, and the second synthesis reactor comprises a fixed bed of the second methanol synthesis catalyst that is cooled in heat exchange with either water under pressure or a synthesis gas mixture that is the first synthesis gas mixture or the second synthesis gas mixture.

12. The process according to claim 1 wherein the first synthesis reactor is an axial flow steam-raising converter.

13. The process according to claim 1 wherein the second synthesis reactor is a radial flow steam-raising converter, a tube-cooled converter, a gas-cooled converter or a quench reactor.

14. The process according to claim 1 wherein the first, second, or first and second cooled methanol synthesis catalysts are copper-containing methanol synthesis catalysts.

15. The process according to claim 1 wherein methanol synthesis in the first and second reactors is performed at pressures in the range of from 20 to 120 bar abs and temperatures in the range of from 130 C. to 350 C.

16. The process according to claim 15 wherein the pressure in the second synthesis reactor is higher than the pressure in the first synthesis reactor.

17. The process according to claim 1 wherein the gas mixtures fed to the first and/or second synthesis reactors are heated in gas-gas heat exchangers using the product gases from the reactors.

18. The process according to claim 1 wherein the product gas streams from the first and second synthesis reactors are cooled in one or more stages of heat exchange to condense methanol therefrom.

19. The process according to claim 1 wherein a purge gas stream is recovered from the second methanol depleted gas mixture and is used for hydrogen recovery, or is subjected to one or more further processing stages including autothermal reforming, water-gas shift, or methanol synthesis.

20. The process according to claim 1 wherein the recycle ratio of the loop recycle gas stream to form the second synthesis gas mixture is in the range of from 2:1 to 6:1.

21. The process according to claim 1 wherein the loop recycle gas streams are circulated by means of two circulators arranged either in a single casing having one inlet and two outlets at different pressures, a single casing having two inlets at different pressures and one outlet, or in two separate casings.

22. The process according to claim 1 wherein the first, second, or first and second cooled methanol synthesis catalysts are compositions comprising copper, zinc oxide and alumina.

23. The process according to claim 18, wherein the condensed methanol is recovered and subjected to further processing by one or more stages of distillation to produce a purified methanol product.

Description

(1) The invention will be further described by reference to the figures in which;

(2) FIG. 1 depicts a process according to an embodiment of the present invention utilising an aSRC and rSRC where the second reactor is at a higher pressure than the first reactor;

(3) FIG. 2 depicts a process according to an embodiment of the present invention utilising an aSRC and rSRC where the second reactor is at a lower pressure than the first reactor; and

(4) FIG. 3 depicts a process according to an embodiment of the present invention utilising an aSRC and rSRC where both reactors are at a similar pressure.

(5) It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks and the like may be required in a commercial plant. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

(6) In FIG. 1, a make-up gas in line 10 comprising hydrogen, carbon monoxide and carbon dioxide is combined with a first recycle loop gas stream 70 and the resulting first synthesis gas mixture passed via line 14 to a gas-gas interchanger 16 where it is heated in indirect heat exchange with a first product gas stream 24. The heated first synthesis gas mixture is fed by line 18 to the inlet of an axial steam-raising converter 20, containing catalyst-filled tubes 22 through which the synthesis gas mixture is passed. The tubes are cooled by boiling water under pressure. The catalyst is a particulate copper/zinc oxide/alumina catalyst. The boiling water under pressure is fed to the shell side of the reactor and a mixture of boiling water and steam is withdrawn and supplied to a steam drum (not shown). The methanol synthesis reaction takes place as the synthesis gas passes axially through the catalyst-filled tubes 22 to form a first product gas stream containing methanol vapour. The first product gas stream is recovered from the outlet of the first synthesis reactor 20 and fed via line 24 to the interchanger 16 where it is partially cooled. The partially cooled gas is fed via line 26 to one or more further stages of heat exchange 28 to condense methanol therefrom. The resulting gas-liquid mixture is passed to a gas-liquid separator 30 and liquid methanol is recovered via line 32. A first methanol-depleted gas mixture comprising unreacted hydrogen and carbon oxides is recovered from the separator 30 and fed by line 34 to the first stage of a two-stage circulator 72. The second stage of the two-stage circulator 72 is fed with a second recycle loop gas fed by line 74. The first methanol-depleted gas 34 and the second recycle loop gas 74 are combined in the circulator 72 to form a second synthesis gas mixture. The circulator compresses the second synthesis gas mixture which is fed from the circulator by line 42 to a gas-gas interchanger 44 where it is heated in indirect heat exchange with a second product gas stream 52. The heated second synthesis gas is fed by line 46 to the inlet of a radial steam-raising converter 48 containing a bed of methanol synthesis catalyst 50, containing a plurality of heat exchange tubes though which boiling water under pressure is passed as coolant. Whereas tubes are depicted, alternative heat exchange devices such as plates through which the coolant may be passed, may also be used. The catalyst is a particulate copper/zinc oxide/alumina catalyst. The boiling water under pressure is fed to the tube side of the reactor and a mixture of boiling water and steam is withdrawn and supplied to a steam drum (not shown). The methanol synthesis reaction takes place as the synthesis gas passes radially through the bed of catalyst 50 to form a second product gas stream containing methanol vapour. The second product gas stream is recovered from the outlet of the second synthesis reactor 48 and fed via line 52 to the interchanger 44 where it is partially cooled. The partially cooled gas is fed via line 54 to one or more further stages of heat exchange 56 to condense methanol therefrom. The resulting gas liquid mixture is passed to a gas-liquid separator 58 and liquid methanol is recovered via line 64. A second methanol-depleted gas mixture is recovered in the separator 58 and fed by line 60 to a purge off-take line 62, which removes a portion of the gas to reduce the build-up of inert gases. The remaining second methanol-depleted gas mixture in line 36 is divided to form the first recycle loop gas 70 and the second recycle loop gas 74. The crude methanol streams 32 and 64 are combined and send by line 66 for further processing such as one or more stages of distillation to produce a purified methanol product.

(7) A portion of the make-up gas may be diverted from line 10 by dotted-line 76 to line 34 and/or line 74 fed to the circulator 72. This may be done in order to adjust the duty and so the relative sizes of the first and second synthesis reactors, or may also be used to enrich the second synthesis gas. Where the portion of make-up gas 76 is fed to line 74, a suitable control valve on line 10 may be used to control the flow.

(8) In FIG. 2, the two-stage circulator is configured to produce separate recycle loop gas streams. Such an arrangement provides the flexibility of switching the pressure of first and second synthesis reactors. Thus in FIG. 2 a make-up gas fed via line 10 is mixed with a first recycle loop gas containing hydrogen, fed by line 80, to form the first synthesis gas mixture 14. The first synthesis gas mixture is converted in the first synthesis reactor 20 in the same manner as depicted in FIG. 1. The first methanol depleted gas mixture 34 is mixed with a second recycle loop gas 82 to form the second synthesis gas mixture 42. The second synthesis gas mixture is converted in the second synthesis reactor in the same manner as depicted in FIG. 1. The second methanol-depleted gas recovered from the gas-liquid separator 58 is fed by line 60 to a purge off-take line 62, which removes a portion of the gas to reduce the build-up of inert gases. The remaining second methanol-depleted gas mixture is fed to the first stage of the circulator 72 by line 36. The first stage of the circulator provides the second recycle loop gas 82 used to form the second synthesis gas mixture 42. The second stage of the circulator 72 provides the first recycle loop gas stream 80. The crude methanol streams 32 and 64 are again combined and send by line 66 for further processing such as one or more stages of distillation to produce a purified methanol product.

(9) A portion of the make-up gas may be diverted from line 10 by dotted-line 76 to line 34 used to prepare the second synthesis gas mixture 42. This may be done in order to adjust the duty and so the relative sizes of the first and second synthesis reactors, and/or may also be used to enrich the second synthesis gas to enhance overall process efficiency.

(10) In FIG. 3, two separate circulators are used to produce separate recycle loop gas streams. Such an arrangement provides the ability to operate the first and second synthesis reactors at the same pressure. Thus in FIG. 3 a make-up gas fed via line 10 is mixed with a first recycle loop gas containing hydrogen, fed by line 90, to form the first synthesis gas mixture 14. The first synthesis gas mixture is converted in the first synthesis reactor 20 in the same manner as depicted in FIG. 1. The first methanol-depleted gas mixture is passed by line 34 to a first circulator 92 where it is compressed and passed by line 94 to be mixed with a second recycle loop gas in line 96 to form the second synthesis gas mixture 42. The second synthesis gas mixture is converted in the second synthesis reactor in the same manner as depicted in FIG. 1. The second methanol-depleted gas recovered from the gas-liquid separator 58 is fed by line 60 to a purge off-take line 62, which removes a portion of the gas to reduce the build-up of inert gases. The remaining second methanol-depleted gas mixture is fed by line 36 to a second circulator 98. The second circulator provides a compressed recycle loop gas stream 100 which is divided to provide the first recycle loop gas 90 use to prepare the first synthesis gas mixture 14, and the second recycle loop gas 96 use to prepare the second synthesis gas mixture 42. The crude methanol streams 32 and 64 are again combined and send by line 66 for further processing such as one or more stages of distillation to produce a purified methanol product.

(11) A portion of the make-up gas may be diverted from line 10 by dotted-line 76 to line 94 used to prepare the second synthesis gas mixture 42. This may be done in order to adjust the duty and so the relative sizes of the first and second synthesis reactors, and/or may also be used to enrich the second synthesis gas to enhance overall process efficiency.

(12) The same processes as depicted in FIGS. 1-3 may be performed replacing the radial steam raising converter 48 with a tube-cooled converter in which the catalyst bed is cooled in direct heat exchange with the second synthesis gas. Thus in each case, the second synthesis gas may be fed from heat exchanger 44 via line 46 to the bottom of a tube cooled converter and passed upwards through a plurality of tubes disposed within the catalyst bed. The gas is heated as it passes upwards through tubes. The heated gas exits the tubes within the reactor above the bed and then passes down through the bed where it reacts to form a gas mixture containing methanol vapour. The product gas may be collected and fed via line 52 to heat exchanger 44 where it is cooled.

(13) The Invention is further illustrated by reference to the following Example.

EXAMPLE 1

(14) A computer model of a process according to the present invention was developed and compared against a process where there is a single common circulator operating at either a high or low recycle ratio. The benefits of operating the process in which the recycle ratios are different for the different reactors are as follow: 1. When compared to the process operating at a high recycle ratio, the present invention achieves the same catalyst and feed efficiency and benefits from higher energy efficiency and smaller piping and loop equipment. 2. When compared to the process operating at a low recycle ratio, the present invention achieves higher catalyst and feed efficiency. It also benefits from operating at lower pressure to achieve the same feed efficiency.

(15) If the process of U.S. Pat. No. 7,790,775, operating with two radial-flow steam-raising converters at a molar recycle ratio of 3.38, is compared with the process of FIG. 1 operating with a molar recycle ratio of 0.55 for the axial-flow steam raising converter and 2.50 for the radial-flow steam-raising converter, the following figures are obtained;

(16) TABLE-US-00001 Configuration Comparative Example FIG. 1 Capacity, MTPD 5500 5500 Methanol production 50% rSRC and 50% rSRC 60% aSRC and 40% rSRC Total catalyst, m.sup.3 182.8 159.6 Reactor pressure, bara 76.0/80.0 76.0/80.0 Total Converter heat 148.05 167.53 removed as steam, MW Circulator Power, MW 10.05 5.61 (2 stage total) Syngas Comp Power, MW 9.21 9.21 Total Compressor Power, 19.26 14.82 MW

(17) The benefit of the present invention is that a lower catalyst volume is required, higher steam production is achieved and lower compression power is required. Accordingly, the present invention is also able to provide significant equipment cost and energy savings.

(18) The compositions, temperatures and pressures of for the streams depicted in FIG. 1 are set out in the following tables.

(19) TABLE-US-00002 Stream 10 14 18 24 32 34 42 46 Pressure 7.7 7.7 7.6 7.4 7.1 7.1 8.2 8.1 MPa(abs) Temper- 120 93 225 255 45 45 55 240 ature C. Flow 520 811 811 613 512 1521 1521 kNm.sup.3/hr (vapour) Flow 142.8 Tonne/hr (liquid) Compo- sition Mole % H.sub.2O 0.5 0.3 0.3 0.3 1.8 0.0 0.0 0.0 H.sub.2 67.6 73.4 73.4 65.0 0.4 77.7 81.8 81.8 CO 29.4 19.7 19.7 9.7 0.3 11.6 5.5 5.5 CO.sub.2 1.9 1.6 1.6 2.3 0.8 2.6 1.5 1.5 CH.sub.3OH 0.0 0.2 0.2 16.4 96.4 0.7 0.6 0.6 Inerts 0.7 4.8 4.8 6.3 0.2 7.5 10.5 10.5

(20) TABLE-US-00003 Stream 52 60 62 64 70 74 76 Pressure 8.0 7.7 7.7 7.7 7.7 7.7 7.7 MPa(abs) Temperature 270 45 45 45 45 45 120 C. Flow 1399 1326 26 291 1009 0 kNm.sup.3/hr (vapour) Flow 98.0 Tonne/hr (liquid) Composition Mole % H.sub.2O 0.7 0.0 0.0 13.1 0.0 0.0 0.5 H.sub.2 79.5 83.9 83.9 0.4 83.9 83.9 67.6 CO 2.3 2.5 2.5 0.1 2.5 2.5 29.4 CO.sub.2 1.0 1.0 1.0 0.3 1.0 1.0 1.9 CH.sub.3OH 5.0 0.6 0.6 85.9 0.6 0.6 0.0 Inerts 11.4 12.0 12.0 0.3 12.0 12.0 0.7