Process for synthesising methanol

Abstract

A process for synthesising methanol is described comprising the steps of (i) passing a feed gas comprising a make-up gas containing hydrogen and carbon dioxide to a methanol synthesis loop, (ii) recovering a product gas mixture containing methanol from the methanol synthesis loop, (iii) cooling the product gas mixture to below the dew point to condense crude methanol, (iv) separating the crude methanol from an unreacted gas mixture, (v) passing a portion of the unreacted gas mixture to the methanol synthesis loop and (vi) recovering a portion of the unreacted gas mixture as a purge gas stream, characterised by contacting the crude methanol and a portion of the purge gas in a stripping unit to strip dissolved gases from the crude methanol thereby forming a stripped crude methanol and an enriched gas mixture, and feeding at least a portion of the enriched gas mixture to the methanol synthesis loop.

Claims

1. A process for synthesising methanol comprising the steps of (i) passing a feed gas comprising a make-up gas containing hydrogen and carbon dioxide to a methanol synthesis loop, (ii) recovering a product gas mixture containing methanol from the methanol synthesis loop, (iii) cooling the product gas mixture to below the dew point to condense crude methanol, (iv) separating the crude methanol from an unreacted gas mixture, (v) passing a portion of the unreacted gas mixture to the methanol synthesis loop and (vi) recovering a portion of the unreacted gas mixture as a purge gas stream, characterised by contacting the crude methanol and a portion of the purge gas in a stripping unit to strip dissolved gases from the crude methanol thereby forming a stripped crude methanol and an enriched gas mixture, and feeding at least a portion of the enriched gas mixture to the methanol synthesis loop.

2. The process according to claim 1, wherein the make-up gas is generated by one or more steps of steam reforming, partial oxidation, autothermal reforming or gasification.

3. The process according to claim 1, wherein the make-up gas is generated by catalytic steam reforming a hydrocarbon with steam and optionally carbon dioxide in a fired steam reformer, or by combined reforming of a hydrocarbon by subjecting a first fraction of the hydrocarbon and steam to primary reforming in a primary steam reformer and secondary reforming a second fraction of the hydrocarbon, combined with the effluent of the primary reformer, with an oxygen-containing gas in an autothermal reformer.

4. The process according to claim 1, wherein a carbon dioxide gas stream is added to the make-up gas.

5. The process according to claim 1, wherein the crude methanol and a portion of the purge gas are fed to the stripping unit and contacted therein in a counter-current or co-current manner, or wherein the crude methanol is sparged with the portion of the purge gas.

6. The process according to claim 1, wherein at least a portion of the purge gas is separated into a hydrogen-rich gas stream and a hydrogen-depleted gas stream and at least a portion of the hydrogen-rich gas stream is fed to the stripping unit.

7. The process according to claim 6, wherein the hydrogen-depleted gas stream is used as a fuel, or is fed to the synthesis gas generation step to form part of the make-up gas.

8. The process according to claim 1, wherein the methanol synthesis loop comprises one, two or more methanol synthesis reactors each containing a bed of methanol synthesis catalyst, wherein the product gas mixture is recovered from at least one methanol synthesis reactor.

9. The process according to claim 1, wherein the methanol synthesis loop comprises one, two or more methanol synthesis reactors, each fed with a feed gas comprising hydrogen and carbon dioxide, each producing a product gas mixture, wherein an unreacted gas mixture separated from a product gas mixture recovered from one methanol synthesis reactor may be returned to the same or a different methanol synthesis reactor.

10. The process according to claim 8, wherein the methanol synthesis reactors are cooled by a synthesis gas or by boiling water.

11. The process according to claim 8, wherein the methanol synthesis catalyst is a copper-containing methanol synthesis catalyst.

12. The process according to claim 8, wherein methanol synthesis is effected in the methanol synthesis reactors at pressures in the range 10 to 120 bar abs, and temperatures in the range of 130° C. to 350° C.

13. The process according to claim 1, wherein the stripped crude methanol is subjected to one or more steps 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 a first embodiment of the invention; and

(3) FIG. 2 depicts the process according to FIG. 1 with additional or alternative features.

(4) 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.

(5) In FIG. 1, a natural gas stream 10 is mixed with steam from line 12 and the resulting mixture fed to a synthesis gas generation unit 14 comprising a fired steam reformer where it is catalytically reformed to form a synthesis gas stream comprising hydrogen, carbon monoxide and carbon dioxide. The synthesis gas stream is cooled and de-watered in heat exchange and separation equipment (not shown) to produce a make-up gas, which is recovered from the synthesis gas generation unit 14 via a line 16.

(6) The make-up gas 16 is mixed with a carbon dioxide stream provided by line 18 to form a feed gas 20. The composition of the feed gas 20 may be used to determine the R-value of the external feeds to the methanol synthesis loop. An enriched gas mixture from line 22 is combined with the feed gas 20 and the resulting enriched feed gas compressed in a syngas compressor (not shown) and fed to a methanol synthesis unit 24. The methanol synthesis unit comprises a methanol synthesis loop in which the feed gas is mixed with a recycled stream of unreacted gas comprising hydrogen, carbon dioxide and carbon monoxide and fed to one, two or more methanol synthesis reactors, each containing a methanol synthesis catalyst, operating in series or parallel to generate a product gas stream containing methanol. The product gas stream is cooled to condense and separate a liquid crude methanol from unreacted gas, a portion of which is compressed in a circulator and recycled to the methanol synthesis reactor.

(7) A portion of the unreacted gas is withdrawn as a purge gas stream upstream of the circulator and passed from the methanol synthesis unit 24 via line 26 to a hydrogen separation unit 28 in which the purge gas stream is separated into a hydrogen-rich stream and a hydrogen-depleted stream by passing the purge gas stream through a membrane. The hydrogen depleted stream is fed by line 30 from the separation unit 28 to the syngas generation unit 14 to be combusted as a fuel, e.g. in the fired steam reformer.

(8) The hydrogen-rich gas stream, fed by line 32 from the separation unit 28, and the crude methanol fed by line 34 from the methanol synthesis unit 24, are passed to a methanol stripping unit 36. In the methanol stripping unit 36, the crude methanol and the hydrogen-rich gas stream are contacted and the dissolved gases in the crude methanol are released into the hydrogen-rich gas to form and enriched gas mixture and a stripped crude methanol product. The enriched gas mixture is fed from the stripping unit 36 via line 22 to the suction or interstage of the syngas compressor (not shown) to form an enriched feed gas to the methanol synthesis unit 24. The stripped crude methanol is fed via line 38 from the stripping unit to a purification unit 40 comprising one, two or more distillation columns to produce a purified methanol product recovered via line 42.

(9) In FIG. 2, the process if FIG. 1 is depicted with a number of alternatives that may be used separately or in combination with each other.

(10) As an alternative to combining the enriched gas mixture 22 from the stripping unit 36 with the make-up gas, the enriched gas mixture 22 may be fed from the stripping unit 36 directly to the methanol synthesis loop via line 50, (shown as a dotted line). If the stripping unit is at a lower pressure than the loop then a compressor will be required in line 50. However, by taking the purge gas stream from downstream of the circulator and using a high pressure separation technique, such as pressure-swing adsorption, the enriched gas mixture will be at a pressure high enough that it can be fed to the loop upstream of the circulator without the need for further compression.

(11) As an alternative to combining the carbon dioxide provided by line 18 with the make-up gas 16, at least a portion of the carbon dioxide stream may be combined with the hydrocarbon-containing feed to the synthesis gas generation unit 14, via line 52, (shown as a dotted line).

(12) As an alternative to feeding the hydrogen-depleted gas from the separation unit 28 as fuel in the synthesis gas generation unit 14 via line 30, a portion of the hydrogen depleted gas may be combined with the hydrocarbon-containing feed to the synthesis gas generation unit 14, via line 54, (shown as a dotted line).

(13) As an alternative to simply using just a fired steam reformer in the syngas generation unit 14, the syngas generation is by combined reforming. Thus, the syngas generation unit 14 comprises a combination of a fired steam reformer and an oxygen-fed autothermal reformer, with a portion of the natural gas feed and the primary reformer effluent fed to the autothermal reformer to generate a crude synthesis gas. The crude synthesis gas is cooled and the condensate separated to generate the make-up gas as before.

(14) As an alternative to simply using just a fired steam reformer in the syngas generation unit 14, the syngas generation is by gasification of a carbonaceous feedstock, alone or in combination with one or more stages of steam reforming in parallel or series.

(15) The invention will be further described by reference to the following examples.

EXAMPLE 1

(16) Examples A-D were based on a process using a single fired steam reformer fed with natural gas and steam that produced make-up gas with S=300,000 Nm.sup.3/h at about 20 bar abs pressure. The R-value of the make-up gas exit the steam reformer was 2.96. The make-up gas and other gases were fed to a methanol synthesis loop comprising a single methanol synthesis reactor based on a radial-flow-steam-raising converter design as described in U.S. Pat. No. 4,321,234 containing a standard copper catalyst. In each case the R-value of the gas at the inlet of the methanol synthesis converter was 5.00. The methanol converter exit pressure was at 80 bar abs. The methanol recovery processes were the same in each case. The theoretical maximum methanol make from S=300,000 Nm.sup.3/h is 100,000 Nm.sup.3/h.

(17) Examples A, B and C are Comparative Examples and have no addition of an enriched gas from a stripping unit to the methanol synthesis loop.

(18) Comparative Example A: Carbon dioxide addition to the loop. A carbon dioxide stream was added to the make-up gas to get to R=2.09. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The process made 88,557 Nm.sup.3/h of methanol, which is a syngas efficiency of 88.56%.

(19) Comparative Example B: Carbon dioxide and purge gas hydrogen addition to the loop. A carbon dioxide stream was added to the make-up gas to get to R=2.00 before the addition of a hydrogen-rich gas extracted from the purge gas. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The hydrogen-rich gas was not used to strip dissolved gases from the crude methanol. The process made 91,948 Nm.sup.3/h of methanol, which is a syngas efficiency of 91.95%. This Example demonstrates that, for the same R-value at the inlet of the methanol converter, the syngas efficiency has improved by 3.4% compared to Example A.

(20) Comparative Example C: Carbon dioxide and purge gas hydrogen addition to the loop. The process of comparative Example B was repeated but with 70% (by mole) of the hydrogen in the purge gas recycled. A carbon dioxide stream was added to the make-up gas to get to R=1.93 before addition of the hydrogen rich gas extracted from the purge gas. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The process made 94,767 Nm.sup.3/h of methanol, which is a syngas efficiency of 94.77%. This Example demonstrates that, for the same R-value at the inlet of the methanol converter, adding more hydrogen and carbon dioxide improves the syngas efficiency.

(21) Example D according to the invention as depicted in FIG. 1: Carbon dioxide and enriched gas addition to the loop. The process of comparative Example C was repeated but with stripping of the crude methanol using the hydrogen rich gas, and addition of the resulting enriched gas to the make-up gas. A carbon dioxide stream was added to the make-up gas to get to R=2.00 before addition of the enriched gas stream from the stripping unit. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The process made 94,483 Nm.sup.3/h of methanol, which is a syngas efficiency of 94.48%. Whereas the syngas efficiency is no higher than Comparative Example C, compared to comparative Example C the process used 2,557 Nm.sup.3/h less carbon dioxide.

(22) The number of kilograms of carbon dioxide that is needed to make each extra kilogram of methanol, above the production in Example A, is almost the same for Examples B and C, at 1.23 and 1.24 respectively. The inclusion of the stripping stage in Example D therefore reduced the carbon dioxide consumption to 0.71 kg of carbon dioxide for each extra kilogram of methanol. This is a significant reduction in the marginal consumption of carbon dioxide. There is both an operating cost (in terms of increase energy consumption) and a capital cost for the equipment needed for recovery of carbon dioxide. By stripping the crude methanol to recover carbon dioxide and other dissolved gases back to the process, then there is a saving in the size for recovery of carbon dioxide from the flue gas along with an associated saving in operating cost.

(23) In both Example C and Example D, approximately 70% (by mole) of the hydrogen available in the purge gas was recovered into the hydrogen-rich gas stream. A modern membrane-based hydrogen recovery system can achieve a hydrogen recovery around 95%. With such a high hydrogen recovery, there is no disincentive to operate at R-values at the inlet of the methanol synthesis reactor of 5.00 or higher. Higher R-values than 5.00 would enable the efficiency of Example D to be increased further to supersede that in Example C. The methanol synthesis reactor used in these calculations was a radial-flow, steam-raising converter. The advantage of an R-value 5.0 at the inlet of the radial-flow steam-raising converter is that the same synthesis catalyst volume for Example A will produce over 6% more methanol when used in the arrangement of this invention.

(24) Almost identical benefits are found when some or all of the carbon dioxide was added to the natural gas/steam feed upstream of the fired steam reformer instead of to the make-up gas.

(25) It is also possible to further enhance the process by recycle of the hydrogen-depleted stream as feedstock to the fired steam reformer. Due to the low nitrogen content of the natural gas used, it is possible to recycle a large fraction of the hydrogen-depleted stream. A study of the methanol synthesis reactor inlet stream showed that the main inert gas in the methanol synthesis loop was methane and not nitrogen, so the recycle of a large fraction of the hydrogen-depleted stream had a minor impact on the syngas efficiency.

(26) The results are set out in the tables below.

(27) TABLE-US-00001 Comparative Examples Example A B C D S (H.sub.2 + CO) (Nm.sup.3/h) 300000 300000 300000 300000 Methanol in the stripped crude product (Nm.sup.3/h) 88557 91948 94767 94483 Syngas Efficiency 88.56% 91.95% 94.77% 94.48% R-value of make-up gas 2.96 2.96 2.96 2.96 R-value make-up gas with CO.sub.2 addition 2.09 2.00 1.93 2.00 R-value make-up gas with CO.sub.2 and hydrogen stream 2.09 2.11 2.13 2.15 addition R-value inlet methanol synthesis reactor including 5.00 5.00 5.00 5.00 recycle stream kg of CO.sub.2 to make one extra kg of methanol 1.23 1.24 0.71

(28) TABLE-US-00002 Comparative Examples Example Stream Component (Nm.sup.3/h) A B C D Make-up Gas 16 H.sub.2O 1514 1514 1514 1514 CH.sub.3OH 0 0 0 0 CO 50780 50780 50780 50780 CO.sub.2 24922 24922 24922 24922 H.sub.2 249220 249220 249220 249220 CH.sub.4 10448 10448 10448 10448 N.sub.2 264 264 264 264 Import CO.sub.2 18 H.sub.2O 93 106 118 106 CO.sub.2 21256 24299 26855 24298 Hydrogen-rich gas 32 H.sub.2O 0 19 35 38 CH.sub.3OH 0 20 36 39 CO 0 65 119 128 CO.sub.2 0 1529 2899 3112 H.sub.2 0 16302 30574 32853 CH.sub.4 0 408 689 754 N.sub.2 0 6 10 10 Hydrogen-rich gas to the loop H.sub.2O 0 19 35 38 CH.sub.3OH 0 20 36 39 CO 0 65 119 223 CO.sub.2 0 1529 2899 5629 H.sub.2 0 16302 30574 33102 CH.sub.4 0 408 689 1676 N.sub.2 0 6 10 18 Make-up gas + CO2 H.sub.2O 1608 1621 1632 1621 CO 50778 50779 50780 50780 CO.sub.2 46178 49221 51777 49220 H.sub.2 249222 249221 249220 249220 CH.sub.4 10447 10448 10449 10448 N.sub.2 268 266 264 264 Methanol converter inlet H.sub.2O 41903 45555 48621 48441 CH.sub.3OH 97374 100569 103243 103000 CO 63657 63021 62598 62759 CO.sub.2 134076 140152 145116 144969 H.sub.2 1122812 1156105 1183678 1183553 CH.sub.4 336555 306635 282545 287708 N.sub.2 9231 8182 7342 7053

EXAMPLE 2

(29) Examples E-H were similar to the examples A-D but used combined reforming of the natural gas using a fired steam-methane reformer (SMR) and an autothermal reformer (ATR) in place of the fired steam reformer and so were performed without imported CO.sub.2 addition to the make-up gas. The combined reforming produced make up gas with S=300,000 Nm.sup.3/h at about 35 bar abs pressure. The methanol loop and recovery were the same as in Example 1.

(30) Comparative Example E: 41.6% of the natural gas was bypassed around the SMR and 48,308 Nm.sup.3/h of oxygen was used in the ATR to provide a make-up gas with R=2.003. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The process made 97,282 Nm.sup.3/h of methanol, which is a syngas efficiency of 97.28%. This example shows the benefit of using a reactive synthesis gas such as that formed by combined reforming to the overall syngas efficiency.

(31) Comparative Example F: Purge gas hydrogen addition to the loop. 42.6% of the natural gas was bypassed around the SMR and 48,512 Nm.sup.3/h of oxygen was used in the ATR. The make-up gas had R=2.004, before addition of the hydrogen-rich gas extracted from the purge gas. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The process made 97,400 Nm.sup.3/h of methanol, which is a syngas efficiency of 97.40%. The recycle of hydrogen-rich gas is very small for this example, so the change in syngas efficiency compared to Example E is also very small.

(32) Comparative Example G: Purge gas hydrogen addition to the loop. The process of comparative Example F was repeated but with 70% (by mole) of the hydrogen in the purge gas recycled. 43.3% of the natural gas was bypassed around the SMR and 50,521 Nm.sup.3/h of oxygen is used in the ATR. The make-up gas had R=1.97 before addition of the hydrogen rich gas extracted from the purge gas. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The process made 98,553 Nm.sup.3/h of methanol, which is a syngas efficiency of 98.55%.

(33) Example H according to the invention: Enriched gas addition to the loop. The process of comparative Example G was repeated but with stripping of the crude methanol with the hydrogen rich gas and addition of the resulting enriched gas to the make-up gas. 50.2% of the natural gas is bypassed around the SMR and 48,038 Nm.sup.3/h of oxygen is used in the ATR. The make-up gas had R=2.007 before addition of the enriched gas stream from the stripping unit. Subsequent addition of the recycle gas stream gave R=5.00 at the inlet of the methanol synthesis converter. The process made 98,376 Nm.sup.3/h of methanol, which is a syngas efficiency of 98.38%.

(34) The number of kilograms of O.sub.2 that is needed to make each extra kilogram of methanol, above the production in Example E, is almost the same for Examples F and G, at 1.72 and 1.74 respectively. The inclusion of the stripping stage reduced the O.sub.2 consumption by 0.25 kg of O.sub.2 for each extra kilogram of methanol. This is a significant reduction in the marginal consumption of O.sub.2.

(35) The syngas efficiency for Example H is only slightly lower than for Example G, but the cost of oxygen on most projects means that the economics (both capital cost and operating cost) of the lower oxygen consumption for example H will be preferred in most situations.

(36) The results are set out in the tables below.

(37) TABLE-US-00003 Comparative Examples Example E F G H Bypass around the SMR (mole %) 41.6 42.6 43.3 50.2 S (H.sub.2 + CO) (Nm3/h) 300000 300000 300000 300000 Methanol in the stripped crude product (Nm3/h) 97282 97400 98553 98376 Syngas Efficiency 97.28% 97.40% 98.55% 98.38% R-value of make-up gas 2.003 2.000 1.970 2.007 R-value of combined feeds to loop 2.003 2.004 2.014 2.023 R-value inlet methanol synthesis reactor 5.00 5.00 5.00 5.00 kg of O.sub.2 to make one extra kg of methanol 1.72 1.74 −0.25

(38) TABLE-US-00004 Comparative Examples Example Stream Component (Nm.sup.3/h) E F G H Make-up Gas 16 H.sub.2O 1481 1480 1477 1481 CH.sub.3OH 0 0 0 0 CO 76708 76939 79240 76407 CO.sub.2 23188 23061 21775 23354 H.sub.2 223292 223061 220760 223593 CH.sub.4 4132 4194 4870 4053 N.sub.2 + Ar 797 795 779 724 Import O.sub.2 Ar 243 244 254 241 O.sub.2 48308 48512 50521 48038 Hydrogen-rich gas 32 H.sub.2O 0 0 4 6 CH.sub.3OH 0 5 60 69 CO 0 34 389 452 CO.sub.2 0 35 380 494 H.sub.2 0 509 5603 6909 CH.sub.4 0 196 2330 2686 N.sub.2 + Ar 0 48 458 479 Hydrogen-rich gas to the loop H.sub.2O 0 0 4 6 CH.sub.3OH 0 1 9 11 CO 0 2 27 129 CO.sub.2 0 27 285 1665 H.sub.2 0 484 5323 6840 CH.sub.4 0 18 210 1499 N.sub.2 + Ar 0 1 13 88 Methanol converter inlet H.sub.2O 1697 1696 1685 1753 CH.sub.3OH 12599 12641 13080 12424 CO 158969 159469 164570 157330 CO.sub.2 108446 108180 105391 113237 H.sub.2 1445518 1446421 1455177 1465485 CH.sub.4 471835 475899 516223 485659 N.sub.2 + Ar 118469 116782 101358 86449