Process For Producing Methanol And Ammonia

Abstract

A process for the co-production of methanol and ammonia is described comprising the steps of: (a) forming a first synthesis gas stream by reacting a first portion of a hydrocarbon feedstock and steam in a steam reformer, (b) forming a second synthesis gas stream in parallel to the first synthesis gas stream by reacting a second portion of the hydrocarbon feedstock with an oxygen-containing gas and steam in an autothermal reformer, (c) synthesising methanol from a first process gas comprising the first synthesis gas stream, and (d) synthesising ammonia from a second process gas prepared from the second synthesis gas stream, wherein a purge stream containing hydrogen is recovered from the methanol synthesis step (c) and a portion of the purge gas stream is fed to the autothermal reformer and/or the second synthesis gas in step (b).

Claims

1. A system for co-producing methanol and ammonia according to the process of claim 1, comprising (i) a source of a hydrocarbon feedstock, (ii) a steam reformer configured to reform a first portion of the hydrocarbon feedstock and steam to form a first synthesis gas stream, (iii) an autothermal reformer configured to reform a second portion of the hydrocarbon feedstock with an oxygen-containing gas and steam to form a second synthesis gas stream, (iv) a methanol production unit to synthesis methanol from a first process gas comprising the first synthesis gas stream, (v) an ammonia production unit to synthesise ammonia from a second process gas prepared from the second synthesis gas stream, and (vi) a methanol purge gas recovery conduit that provides a portion of a methanol purge gas from the methanol production unit to the autothermal reformer and/or the second synthesis gas stream.

2. The system according to claim 1, wherein the methanol purge gas is separated into a hydrogen-rich gas stream and a hydrogen-depleted gas stream and the hydrogen-rich gas stream is combined with the second synthesis gas stream.

3. The system according to claim 2, wherein the hydrogen-depleted gas is fed to the autothermal reformer.

4. The system according to claim 1, wherein the hydrocarbon feedstock is natural gas.

5. The system according to claim 1, wherein the hydrocarbon feedstock is desulphurised and then divided into the first portion and the second portion.

6. The system according to claim 1, wherein one or more process streams selected from the hydrocarbon feedstock, steam and oxygen-containing gas are preheated in a flue gas duct of the steam reformer

7. The system according to claim 1, wherein the oxygen containing gas is selected from air or an oxygen-enriched air.

8. The system according to claim 1, wherein the autothermal reformer is operated at a pressure in the range 80 to 200 bar abs.

9. The system according to claim 1, wherein the methanol synthesis step is operated at a pressure equal or greater than the operating pressure of the autothermal reformer.

10. The system according to claim 1, wherein the methanol is synthesised in a methanol production unit by steps comprising passing the first process gas through a bed of methanol synthesis catalyst disposed in a methanol synthesis reactor to form a methanol-containing product gas, recovering methanol and an unreacted gas stream from the methanol-containing product gas, returning a portion of the unreacted gas stream to the methanol synthesis reactor in a synthesis loop and recovering the purge stream containing hydrogen from the synthesis loop.

11. The system according to claim 1, wherein at least a portion of the synthesised methanol is purified by distillation.

12. The system according to claim 1, wherein at least a portion of the methanol is reacted with air over an oxidation catalyst in a formaldehyde production unit to form formaldehyde.

13. The system according to claim 1, wherein the second process gas is prepared by steps comprising passing the second synthesis gas through one or more stages of water-gas shift to form a shifted gas stream, separating carbon dioxide from the shifted gas stream in a carbon dioxide removal unit and converting any remaining carbon oxides present in the second process stream to methane in a methanation unit downstream of the carbon dioxide removal unit.

14. The system according to claim 1, wherein the ammonia is synthesised in an ammonia production unit by passing the second process gas through a bed of ammonia synthesis catalyst disposed in an ammonia synthesis reactor to form an ammonia-containing product gas, recovering ammonia and an unreacted gas stream from the ammonia-containing product gas, returning a portion of the unreacted gas stream to the ammonia synthesis reactor in a synthesis loop and recovering a purge stream containing methane from the synthesis loop.

15. The system according to claim 1, wherein at least a portion of the ammonia is reacted with carbon dioxide in a urea production unit to from urea.

16. The system according to claim 15, wherein at least a portion of the carbon dioxide is recovered from the purge gas stream recovered from the methanol loop.

17. The system according to claim 12, wherein at least a portion of the ammonia is reacted with carbon dioxide in a urea production unit to from urea, at least a portion of the formaldehyde is reacted with the urea to form a urea-formaldehyde concentrate, and at least a portion of the urea-formaldehyde concentrate is reacted with urea to form a stabilised urea product.

Description

[0057] The invention will now be described by reference to the accompanying drawings in which;

[0058] FIG. 1 is a schematic representation of a process according to one aspect of the present invention utilizing an untreated methanol purge gas,

[0059] FIG. 2 is a schematic representation of a process according to a further aspect of the present invention utilizing a hydrogen-rich gas separated from a methanol purge gas, and

[0060] FIG. 3 is a schematic representation of a process according to a further aspect of the present invention in which the process is integrated with urea and formaldehyde production units.

[0061] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, 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. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0062] In FIG. 1, a natural gas stream 10 is fed to a desulphurisation unit 12 where it is mixed with 2-5% vol. hydrogen and passed over a hydrodesulphurisation catalyst and then a bed of hydrogen sulphide adsorbent. The desulphurised natural gas stream 14 is divided to form a first portion 16 and a second portion 18. The first portion 16 is combined with steam fed via line 20 and the resulting mixture fed to a fired steam reformer 22 where it is steam reformed in externally-heated catalyst-filled tubes to generate a first synthesis gas stream. The first synthesis gas stream recovered from the steam reformer 22 is cooled to below the dew point by one or more stages of heat exchange and the resulting condensate separated by a gas-liquid separator (not shown).

[0063] The de-watered synthesis gas is then fed via line 24 to a methanol production unit 26 where it is combined with a portion of a methanol loop gas to form the first process gas, heated and fed to a methanol synthesis reactor containing a bed of methanol synthesis catalyst. Methanol is synthesised over the catalyst and the product gas from the reactor is cooled to below the dew point in one or more stages of heat exchange to condense methanol from the unreacted gas stream. Crude methanol is recovered using a gas liquid separator (not shown) and fed via line 28 to a purification unit 30 where it is subjected to one or more stages of distillation to produce a purified methanol product 32. A water by-product stream is recovered from the purification unit 30 via line 34. The unreacted gas stream recovered from the gas-liquid separator is compressed and returned to the methanol synthesis reactor as the loop gas. Prior to compression, a portion of the unreacted gas is recovered from the methanol production unit 26 as a purge gas stream 36.

[0064] The second portion of desulphurised natural gas 18 is combined with steam fed via line 38 and the resulting mixture fed to an autothermal reformer 40 where it is partially combusted with air fed via line 42 and the partially combusted gas subjected to steam reforming in a bed of steam reforming catalyst in the autothermal reformer downstream of the burner. The resulting second synthesis gas stream is cooled and fed via line 44 to one or more stages of water gas shift in a water gas shift unit 46, where the hydrogen content of the second synthesis gas is increased and the carbon monoxide converted to carbon dioxide by reaction with steam over one or more beds of water-gas shift catalyst.

[0065] In this embodiment, the purge gas stream is fed without treatment via line 36 to the autothermal reformer 40 and/or the second synthesis gas stream 44. The purge gas may be added directly to the autothermal reformer 40 or may be mixed with the second portion of desulphurised natural gas 18, or the mixture of the second portion of desulphurised natural gas 18 and steam 38.

[0066] A shifted gas is recovered from the water-gas shift unit 46, cooled and fed via line 48 to a carbon dioxide removal unit 50 where it is subjected to carbon dioxide removal using a chemical or physical absorbent liquid. The carbon dioxide removal unit also removes residual water from the shifted gas. The carbon dioxide is separated from the absorbent liquid and recovered via line 52.

[0067] A carbon dioxide-depleted gas is recovered from the carbon dioxide removal unit 50, heated and fed via line 54 to a methanation unit 56 comprising a methanator containing a bed of methanation catalyst. Residual carbon oxides in the carbon-dioxide-depleted gas are methanated to form methane. The methanated gas stream is cooled to below the dew point by one or more stages of heat exchange and the resulting condensate separated by a gas-liquid separator (not shown). The condensate is recovered from the methanation unit via line 58.

[0068] A de-watered methanated gas recovered from the methanation unit 56 is compressed, heated and fed via line 60 to an ammonia production unit 62 comprising an ammonia synthesis reactor containing a bed of ammonia synthesis catalyst, where it is mixed with an ammonia loop gas to form the second process gas. The second process gas is passed over the ammonia synthesis catalyst. Ammonia is synthesised. The product gas from the ammonia synthesis reactor is cooled to below the dew point in one or more stages of heat exchange to condense ammonia from the unreacted gas stream which is recirculated to the ammonia synthesis reactor. The condensed ammonia is recovered using a gas liquid separator (not shown) to provide a purified ammonia product 64.

[0069] In FIG. 2, the process is the same as that depicted in FIG. 1 except the purge gas stream 36 is fed to a hydrogen separation unit 66 where it is washed with water in a scrubbing unit and passed over a hydrogen separation membrane in a membrane separator to generate a hydrogen-rich gas stream 68 and a hydrogen-depleted gas stream 70. The hydrogen-rich gas stream 68 is fed to the second synthesis gas stream 44 where it supplements the hydrogen content of the gas fed to the water-gas shift stage 46. The hydrogen-depleted gas steam, which comprises carbon oxides and methane, is fed via line 70 to the autothermal reformer 40. The hydrogen-depleted gas may be added directly the autothermal reformer 40 or may be mixed with the second portion of desulphurised natural gas 18, or the mixture of the second portion of desulphurised natural gas 18 and steam 38.

[0070] In FIG. 3, the process is the same as that depicted in FIG. 2 except a portion of the crude methanol stream 28 or, as depicted, a portion of the purified methanol product 32, is fed to a formaldehyde stabiliser production unit 72 where it is oxidised with air fed via line 74 and the formaldehyde absorbed using an aqueous urea stream fed via line 76 to form a urea-formaldehyde concentrate (UFC). In addition, a portion of the purified ammonia product from the ammonia production unit 62 is fed via line 78 to a urea production unit 80 where it is reacted with the carbon dioxide stream 52 recovered from the carbon dioxide separation unit 50 to form a urea product. The urea product is fed via line 84 to a urea stabilisation unit 86 where it is reacted with the UFC fed via line 88 from the formaldehyde stabiliser production unit 72. The resulting UFC-stabilised urea is recovered from the stabilisation unit 86 via line 90.

[0071] The present invention will now be further described by reference to the following Example.

[0072] A process according to FIG. 2 was modelled using conventional modelling software on a 11330 kmol/h natural gas feed and produces 6325 mt/day methanol and 4164 mt/day ammonia using an air-fed autothermal reformer without an air separation unit.

[0073] The compositions, temperatures and pressures for the streams are set out in the following tables.

TABLE-US-00001 Stream 10 14 16 18 20 24 28 36 Pressure (bara) 30.0 29.0 29.0 29.0 30.0 24.0 78.0 78.0 Temp (° C.) 30 250 250 250 250 880 40 40 Flow (kmol/h) 11330 11601 10239 1362 28087 55733 10979 11290 Mole fraction (%) Water 0.00 0.00 0.00 100.00 30.01 23.60 0.05 Hydrogen 1.95 1.95 1.95 51.64 0.29 83.70 Carbon Monoxide 0.05 0.05 0.05 10.86 0.05 2.33 Carbon Dioxide 1.40 1.41 1.41 1.41 5.03 0.55 2.00 Methane 96.81 94.81 94.81 94.81 2.43 0.51 11.27 Nitrogen 0.20 0.20 0.20 0.20 0.04 0.00 0.17 Ethane 1.39 1.36 1.36 1.36 Propane 0.12 0.12 0.12 0.12 Butane 0.04 0.04 0.04 0.04 Pentane 0.04 0.04 0.04 0.04 Methanol 0.01 0.01 0.01 75.01 0.48

TABLE-US-00002 Stream 38 42 44 48 54 60 68 70 Pressure (barea 30.0 30.0 25.0 21.0 19.0 18.0 40.0 77.0 Temp (° C.) 250 650 883 203 250 40 70 70 Flow (kmol/h) 7946 6530 22640 30369 21755 21646 7764 3488 Mole fraction (%) Water 100.00 32.27 19.17 0.25 0.21 0.15 Hydrogen 32.7 53.52 74.71 74.33 94.93 59.60 Carbon Monoxide 6.53 0.18 0.25 0.88 5.58 Carbon Dioxide 5.09 9.20 1.94 2.13 Methane 0.50 0.88 1.23 1.48 1.97 32.10 Nitrogen 79.00 22.90 17.06 23.82 23.94 0.07 0.41 Oxygen 21.00 0.00 Methanol 0.00 0.03