Methanol process

Abstract

A process for the synthesis of methanol is described comprising the steps of: (i) passing a feed gas mixture comprising a make-up gas and a recycle gas stream through a first synthesis reactor containing a water-cooled methanol synthesis catalyst to form a first product gas stream and generate a first steam stream, (ii) passing the first product gas stream, without addition of make-up gas, through a second synthesis reactor containing a water-cooled methanol synthesis catalyst to form a second product gas stream and generate a second steam stream, (iii) recovering methanol from the second product gas stream thereby forming a methanol-depleted gas mixture, and (iv) using at least part of the methanol-depleted gas mixture as the recycle gas stream, wherein the second steam stream has a lower pressure than the first steam stream.

Description

[0001] P102660

[0002] 1

[0003] Methanol

[0004] This invention relates to a process for synthesising methanol.

[0005] Methanol synthesis is generally performed by passing a synthesis gas comprising hydrogen, carbon oxides and any inert gases at an elevated temperature and pressure through one or more beds of a methanol synthesis catalyst, which is often a copper-containing composition. Methanol is generally recovered by cooling the product gas stream to below the dew point of the methanol and separating off the product as a liquid. The crude methanol is typically purified by distillation. The process is often operated in a loop: thus the remaining unreacted gas stream is usually recycled to the synthesis reactor as part of the synthesis gas via a circulator. Fresh synthesis gas, termed make-up gas, is added to the recycled unreacted gas to form the synthesis gas stream. A purge stream is often taken from the circulating gas stream to avoid the build-up of inert gases.

[0006] The process may be operated using two synthesis reactors in series, each containing a bed of methanol synthesis catalyst.

[0007] US5827901 describes a process in which methanol is produced from a synthesis gas containing hydrogen and carbon oxides on copper-containing catalysts at pressures in the range 20 to 120 bar and temperatures in the range 130 DEG to 350 DEG C. The synthesis gas is first of all passed through a first synthesis reactor, in which the catalyst is provided in tubes surrounded by water as a coolant, which is boiling at an elevated pressure. From the first reactor a first mixture containing gases and methanol vapour is withdrawn and passed without cooling through a second synthesis reactor. In the second reactor the catalyst is cooled with synthesis gas.

[0008] W02008 / 146032A1 describes a process for the synthesis of methanol comprising steps of: (a) passing a feed gas mixture comprising a loop gas and a make-up gas though a first synthesis reactor containing a methanol synthesis catalyst, said reactor cooled by boiling water under pressure, to form a mixed gas containing methanol, (b) cooling the mixed gas containing methanol, (c) passing said cooled mixed gas containing methanol through a second synthesis reactor containing a methanol synthesis catalyst in which further methanol is synthesised to form a product gas stream, (d) cooling said product gas to condense methanol, (e) recovering said methanol and returning unreacted gas as the loop gas to said first synthesis reactor, wherein the mixed gas containing methanol from the first synthesis reactor is cooled in heat exchange with either said loop gas or said make-up gas.P102660

[0009] 2

[0010] In these arrangements the second reactor is a reactor containing catalyst cooled by synthesis gas and fed with a product gas mixture containing methanol, i.e. without separation of methanol from the product gas from the first reactor. Overtime, as the catalyst activity in the first reactor declines, the temperature in the second reactor may gradually increase beyond optimum operating region. The Applicants have realised that a combination of maximising the methanol yield and steam generation over the lifetime of the catalyst is obtained when the process comprises two steam raising synthesis reactors in series.

[0011] Accordingly, the invention provides a process for the synthesis of methanol comprising the steps of:

[0012] (i) passing a feed gas mixture comprising a make-up gas and a recycle gas stream through a first synthesis reactor containing a water-cooled methanol synthesis catalyst to form a first product gas stream and generate a first steam stream, (ii) passing the first product gas stream, without addition of make-up gas, through a second synthesis reactor containing a water-cooled methanol synthesis catalyst to form a second product gas stream and generate a second steam stream, (iii) recovering methanol from the second product gas stream thereby forming a methanol-depleted gas mixture, and

[0013] (iv) using at least part of the methanol-depleted gas mixture as the recycle gas stream, wherein the second steam stream has a lower pressure than the first steam stream.

[0014] The invention overcomes the deficiencies in the prior art processes. As the catalyst in the first reactor ages and steam production decreases, the second reactor is able to take on some of the steam generation duty. Simultaneously, it is possible to achieve a low outlet temperature in the second reactor, which operates in a more equilibrium-limited zone, thereby maximizing equilibrium conversion. This effect is realized by providing that the second steam stream has a lower pressure than the first steam stream that allows for a lower temperature for saturated steam, enabling the exit temperature of the second reactor to be lower than that of the first. Advantageously, the exit temperature of the second reactor can be lower than the temperature of the first steam stream, further enabling a more favourable equilibrium-limited conversion.

[0015] The feed gas comprises a make-up gas. Make-up gas typically comprises hydrogen and carbon monoxide, and / or carbon dioxide. The make-up gas may be generated by the steam reforming of methane or naphtha using established steam reforming processes, including pre-reforming. However, the present invention is of particular effectiveness in utilising reactive synthesis gases generated by processes including a step of partial oxidation of a hydrocarbon, biomass or carbonaceous feedstock. By “reactive synthesis gases” we mean a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide in which the ratio (by volume) of carbon monoxide to carbon dioxide is typically >2:1 , preferably >5:1. Such processes include combinedP102660

[0016] 3

[0017] reforming in which a first portion of a hydrocarbon feedstock is subjected to steam reforming and a second portion is subjected to autothermal reforming; and from coal or biomass gasification. Alternatively, off-gases from refineries or other chemical processes comprising principally hydrogen and carbon oxides (mainly as carbon monoxide) may also be used.

[0018] Preferably, the make-up gas has a stoichiometry value, R, defined as R = ([H2] - [CO2]) I ([CO] + [CO2]), in the range of 1.95 to 2.05.

[0019] In some arrangements, additional hydrogen and / or carbon dioxide may be included in the feed gas mixture, for example by adding them to the make-up gas. If desired, the stoichiometry of the make-up gas may be adjusted for example by adding a hydrogen-containing gas stream, to optimise methanol synthesis. This may be particularly the case where the make-up gas contains higher amounts of carbon monoxide, for example in the range 20-35% vol or 25-35% vol. Such reactive synthesis gases may be obtained in particular by the gasification of coal or biomass, or from a hydrocarbon reforming process based on combined reforming or autothermal reforming. In these cases, the make-up gas is desirably combined with a hydrogen-containing gas stream selected from a purge gas stream or hydrogen recovered from the purge gas, for example by pressure-swing adsorption and / or by membrane separation.

[0020] The composition of mixed feed gas at the first synthesis reactor inlet is preferably; 10-30 mol% carbon monoxide, 0.5-10 mol% carbon dioxide, 55-85 mol% hydrogen and the balance one or more inert gases.

[0021] The make-up gas may be subjected to one or more steps of purification to remove sulphur and / or chlorine compounds. This step protects the methanol synthesis catalysts from poisons that reduce their activity.

[0022] The make-up gas, after optional hydrogen addition and purification, may be compressed in a syngas compressor. The make-up gas may be combined with the recycle gas upstream or downstream of a circulator to make the mixed feed gas. The mixed feed gas may then be heated upstream of the first synthesis reactor. The heating may be by electrical heating and / or by heat exchange with a process stream. In a preferred arrangement, the feed gas mixture fed to the first synthesis reactor is heated in a gas-gas heat exchanger using the second product gas stream recovered from the second synthesis reactor.

[0023] Methanol synthesis may typically be performed in the first and second synthesis reactors at elevated temperature and pressure, for example pressures in the range 20 to 120 bar abs and temperatures in the range 130°C to 350°C. The pressure of the feed gas at the first synthesisP102660

[0024] 4

[0025] reactor inlet is preferably 50-100 bar abs. The temperature of the feed gas at the first synthesis reactor inlet is preferably 200-250°C and at the outlet preferably 230-280°C.

[0026] The recycle gas stream combined with the make-up gas preferably has a recycle ratio in the range of 1 :1 to 5:1 , preferably 1:1 to 4:1, more preferably 1:1 to 3:1. By the term “recycle ratio”, we mean the molar flow ratio of the recycled gas to the make-up gas.

[0027] In the present invention, part of the methanol-depleted gas mixture is used as the recycle gas stream. A purge stream is preferably recovered from the methanol-depleted gas and the remaining methanol-depleted gas compressed in a circulating compressor and used as the recycle gas stream.

[0028] The first product gas stream, without addition of make-up gas, is passed through the second synthesis reactor. Accordingly, there is no by-pass of the make-up gas around the first synthesis reactor, so that the process is operated strictly in series. The feed to the second synthesis reactor therefore consists of the first product gas stream without addition of make-up gas.

[0029] The first product gas stream is preferably not subjected to a heating or cooling step before it is passed through the second synthesis reactor, i.e. if is preferably fed directly from the first synthesis reactor to the second synthesis reactor.

[0030] The first and second synthesis reactors are water-cooled and the exothermic methanol synthesis reaction generates steam, and so they may be described as steam-raising converters. Conventional steam recovery equipment such as steam drums, may be coupled to the first and second synthesis reactors. In the present invention, the pressure of the second steam stream is lower than the pressure of the first steam stream. Thus, the process may comprise a steam drum coupled to the second synthesis reactor operated at a lower pressure than a steam drum coupled to the first synthesis reactor. The pressure of the first steam stream may be in the range 25-45 bara and the pressure of the second steam stream may be in the range 15-35 bara. Because of the different pressures, the temperature of the steam generated by the second synthesis reactor is lower than the temperature of the steam generated by the first synthesis reactor. In addition to achieving a more favourable conversion for the exothermic synthesis reaction, the process minimises the required catalyst volume because in the second synthesis reactor, it is possible to follow a maximum rate curve further than would be achievable if the lowest attainable exit temperature were constrained by the higher steam temperature of the first reactor. The maximum rate curve represents the temperature profile at which the reaction rate is maximized, thereby minimizing the required catalyst volume.P102660

[0031] 5

[0032] In a preferred arrangement, at least a portion of the second steam stream is used to heat a water stream, such as boiler feed water (BFW), fed as a coolant to the first and / or second synthesis reactors, preferably at least the first synthesis reactor. This provides a useful increase to the higher-pressure steam production.

[0033] The first and second synthesis reactors may be axial, axial-radial, or radial-flow steam raising converters.

[0034] The first synthesis reactor may be a design with a higher heat transfer relative to the cooled catalyst volume than the second synthesis reactor. The heat transfer can be conveniently characterised by the Volumetric UA. The Volumetric UA may be defined as the multiple of the overall heat transfer coefficient, U, times the total heat transfer area A, per cubic metre of cooled catalyst in the reactor. A lower UA may be achieved through a lower U and / or a lower A. Although any converter could be used in this position, desirably the first synthesis reactor has a Volumetric UA of > 50 kW / m3 / K and more preferably > 90 kW / m3 / K. In a preferred arrangement, the second synthesis reactor has a lower heat transfer relative to the cooled catalyst volume than the first synthesis reactor. For example, the Volumetric UA may be < 40 kW / m3 / K.

[0035] In a preferred arrangement, the first synthesis reactor comprises a methanol synthesis catalyst disposed in tubes cooled by water under pressure, and the second synthesis reactor comprises a fixed bed of a methanol synthesis catalyst through which the first product gas stream passes radially.

[0036] Preferably the first synthesis reactor is an axial-flow, steam-raising converter (aSRC). In such reactors the synthesis gas typically passes axially through vertical, catalyst-containing tubes that are cooled in heat exchange with boiling water under pressure. The catalyst may be provided in pelleted form directly in the tubes or may be provided in one or more cylindrical containers that direct the flow of synthesis gas both radially and axially to enhance heat transfer. Such contained catalysts and their use in methanol synthesis are described in WO2012146904 (A1). An aSRC typically has a Volumetric UA > 100kW / m3 / K.

[0037] Preferably the second synthesis reactor is a radial-flow, steam-raising converter (rSRC). In a radial-flow steam raising converter the synthesis gas typically passes radially (inwards or preferably outwards) through a bed of particulate catalyst which is cooled by a plurality of tubes or plates through which boiling water under pressure is fed as coolant. Such reactors are known and are described for example in US4321234. A rSRC has lower heat transfer than an aSRC, but when used as the second synthesis reactor, the heat release becomes manageable. This isP102660

[0038] 6

[0039] because the first product gas stream is partially converted, resulting in lower reactant concentrations and higher product concentrations, which slows the reaction rate. Consequently, the heat release rate decreases, which can be managed by a reactor with a lower heat transfer. A rSRC typically has a Volumetric UA in the range 12-24 kW / m3 / K.

[0040] Using a rSRC as the second synthesis reactor further enhances the process as it reduces syngas consumption per ton of methanol produced, while also exporting more steam and achieving a lower loop pressure drop, resulting in a reduced power consumption.

[0041] The first and second synthesis reactors may comprise a single reactor or be configured as two or more reactors in parallel depending on the scale of the process.

[0042] The methanol synthesis catalysts are preferably copper-containing methanol synthesis catalysts, in particular the methanol synthesis catalyst in the first and second synthesis reactors is a particulate copper / zinc oxide / alumina catalyst, preferably a particulate copper / zinc oxide / alumina / silica catalyst. The latter catalyst has enhanced stability and in combination with the process of the present invention offers extended optimal operation. Particularly suitable catalysts are Mg-doped copper / zinc oxide / alumina catalysts as described in US4788175 and Si-doped catalysts described in EP3956058 A1. The same or different methanol synthesis catalysts may be used in the first and second synthesis reactors.

[0043] The product gas stream withdrawn from the second synthesis reactor may have a temperature in the range 200 to 260°C.

[0044] Temperature adjustment of the product gas may be performed using conventional heat exchange apparatus. Thus, the product gas stream from the second synthesis reactor may be cooled in one or more stages of heat exchange, e.g. with water or air cooling, to condense methanol therefrom. In a preferred arrangement, the product gas may be used to heat one or more reboilers in a downstream distillation unit. The condensed methanol may suitably be recovered using gas-liquid separators. The recovered liquid methanol stream may be passed for further processing in a distillation unit comprising one or more stages of distillation to produce a purified methanol product. Preferably upstream of the distillation unit the crude methanol is passed to a crude methanol flash drum, where it is reduced in pressure to evolve unreacted gases.

[0045] A purge gas stream is preferably recovered from the methanol depleted gas to avoid the buildup of inert gases, such as nitrogen, methane and argon in the loop. The purge gas typically comprises hydrogen and carbon oxides and may be used for hydrogen recovery, for example by pressure-swing adsorption (PSA) or by using suitable membranes, or may be subjected to oneP102660

[0046] 7

[0047] or more further processing stages including autothermal reforming, water-gas shift and methanol synthesis.

[0048] In a preferred arrangement, the recovered purge gas is washed with water to remove methanol then subjected to a stage of hydrogen separation using a membrane and / or PSA unit, with the recovered hydrogen combined with the make-up gas to optimise the stoichiometry of the makeup gas fed to the process.

[0049] The invention will be further described by reference to the figures in which;

[0050] Figure 1 depicts a process according to one embodiment of the present invention utilising an aSRC and rSRC.

[0051] 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.

[0052] In Figure 1 , a desulphurised, hydrogen-adjusted make-up gas comprising hydrogen, carbon monoxide and carbon dioxide is fed to the process via line 100. The make-up gas in line 100 is combined with compressed recycle stream 160 to form a feed gas mixture 110. The feed gas mixture 110 is fed to the shell side of gas-gas interchanger 30 where it is heated in indirect heat exchange with a product gas stream 130. The heated mixed feed gas stream forms a heated feed gas mixture, which is fed via line 115 to the inlet of the first synthesis reactor. The first synthesis reactor is an axial steam-raising converter 10, containing catalyst-filled tubes 11 through which the gas mixture is passed. The catalyst is a particulate copper / zinc oxide / alumina catalyst. The boiling water under pressure is fed to the shell side 12 of the reactor through downcomer 220 and a mixture of boiling water and steam is withdrawn and supplied to a higher-pressure steam drum 50 through riser 215. The methanol synthesis reaction takes place as the feed gas mixture gas passes axially through the catalyst-filled tubes 11 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 10 and fed via line 120 to the inlet of the second synthesis reactor. The second synthesis reactor is a radial steam-raising converter 20 containing a bed of methanol synthesis catalyst 21 , containing a plurality of heat exchange tubes 22 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 boiling water under pressure is fed to the tube side 22 of the reactor through downcomer 320 and a mixture of boiling water and steam is withdrawn and supplied to a lower-pressure steam drumP102660

[0053] 8

[0054] 60 through riser 315. The methanol synthesis reaction takes place as the synthesis gas passes radially through the bed of catalyst 21 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 20 and fed via line 130 to the tube side of gas-gas interchanger 30 where it is partially cooled. The partially cooled gas is fed via line 135 to one or more further stages of heat exchange 35 to condense methanol therefrom. The resulting two-phase mixture is passed via line 140 to a gas-liquid separator 40 and crude liquid methanol is recovered via line 170. A methanol-depleted gas mixture comprising unreacted hydrogen and carbon oxides is recovered from the separator 40 and fed by line 145 to a purge off-take line 150, which removes a portion of the gas to reduce the build-up of inert gases. The remaining methanol-depleted gas mixture forms recycle stream 155, which is compressed in circulator 50 to form compressed recycle stream 160. The crude methanol stream 170 is sent for further processing in one or more stages of distillation in a distillation unit to produce a purified methanol product. Heat from the second product gas stream is preferably used in one or more reboilers in the downstream distillation unit (not shown).

[0055] Boiler feed water streams 200 and 300 are fed to steam drums 50 and 60 respectively. Steam recovered from the lower pressure steam drum 60 via line 310 may optionally be used to heat the boiler feed water (BFW) stream 200 and / or or BFW stream 300. Steam recovered from the higher pressure steam drum 50 via line 210 may be sent for export.

[0056] The invention will now be further illustrated by reference to the following calculated Examples in which the process of Figure 1 is compared to the process depicted in Figure 1 of the aforesaid W02008146032A1. The processes were modelled to achieve the same methanol production capacity and considered the performance at the beginning of catalyst life and at the end of catalyst life.

[0057] (i) Process conditions - Beginning of life (BOL)

[0058]

[0059] P102660

[0060] 9

[0061]

[0062] (ii) Process conditions - End of life (EOL)

[0063]

[0064]

[0065] P102660

[0066] 10

[0067] (iii) steam export per tonne of product methanol

[0068] Consumption figures are based on the methanol content of crude methanol stream 170.

[0069]

[0070]

[0071] {iv) Steam import and syngas consumption per tonne of product methanol

[0072] • Import steam conditions: 116 bara, 540°C

[0073] • Syngas as H2 + CO

[0074] • consumption figures are based on the methanol content of crude methanol stream 170.

[0075]

[0076] These examples demonstrate the following features of the process:

[0077] • Higher Steam Export Rate Across Lifecycle: The invention ensures that the total steam export rate remains consistently higher than in the comparative case, from the Beginning Of Life (BOL) to the End Of Life (EOL).P102660

[0078] 11

[0079] • Enhanced Flexibility in Steam Pressure Utilization: The invention allows the two steam-raising converters to operate at different steam pressures. All steam can still be exported at the higher pressure by using the steam from the second converter to preheat boiler feed water (streams 200 and 300 in Figure 1) through condensation.

[0080] • Reduced Power Consumption: The invention achieves lower power consumption by incorporating a radial converter and utilizing only one heat exchanger (30 in Figure 1) for heat recovery, compared to two heat exchangers in the comparative case (28 and 38 in W02008146032A1 , Figure 1). This design reduces the loop pressure drop, leading to consistently lower turbine steam import requirements.

[0081] • Feedstock Efficiency: The invention maintains substantially the same syngas consumption as the comparative case from BOL to EOL, ensuring that the superior steam export and reduced power consumption are achieved without compromising feedstock efficiency.

Claims

P10266012Claims.

1. A process for the synthesis of methanol comprising the steps of:(i) passing a feed gas mixture comprising a make-up gas and a recycle gas stream through a first synthesis reactor containing a water-cooled methanol synthesis catalyst to form a first product gas stream and generate a first steam stream,(ii) passing the first product gas stream, without addition of make-up gas, through a second synthesis reactor containing a water-cooled methanol synthesis catalyst to form a second product gas stream and generate a second steam stream, (iii) recovering methanol from the second product gas stream thereby forming a methanol-depleted gas mixture, and(iv) using at least part of the methanol-depleted gas mixture as the recycle gas stream,wherein the second steam stream has a lower pressure than the first steam stream.

2. A process according to claim 1 , wherein hydrogen and / or carbon dioxide are added to the make-up gas.

3. A process according to claim 1 or claim 2, wherein the make-up gas has a stoichiometry value, R, defined as R = ([H2] - [CO2]) I ([CO] + [CO2]), in the range of 1.95 to 2.05.

4. A process according to any one of claims 1 to 3, wherein the feed gas mixture fed to the first synthesis reactor is heated in gas-gas heat exchanger using the second product gas stream recovered from the second synthesis reactor.

5. A process according to any one of claims 1 to 4, wherein there is no by-pass of the make-up gas around the first synthesis reactor.

6. A process according to any one of claims 1 to 5, wherein at least a portion of the second steam stream is used to heat a water stream fed as a coolant to the first synthesis reactor and / or the second synthesis reactor.

7. A process according to any one of claims 1 to 6, wherein the first synthesis reactor comprises a methanol synthesis catalyst disposed in tubes cooled by water under pressure, and the second synthesis reactor comprises a fixed bed of a methanol synthesis catalyst through which the first product gas stream passes radially.P102660138. A process according to any one of claims 1 to 7, wherein the first synthesis reactor is an axial flow steam-raising converter.

9. A process according to any one of claims 1 to 8, wherein the second synthesis reactor is a radial flow steam-raising converter.

10. A process according to any one of claims 1 to 9, wherein the methanol synthesis catalysts are copper-containing methanol synthesis catalysts, preferably compositions comprising copper, zinc oxide and alumina, more preferably compositions comprising copper, zinc oxide, alumina and silica.

11. A process according to any one of claims 1 to 10, wherein methanol synthesis in the first and second reactors is performed at pressures in the range 20 to 120 bar abs and temperatures in the range 130°C to 350°C.

12. A process according to any one of claims 1 to 11 , wherein the second product gas stream from the second synthesis reactor is cooled in one or more stages of heat exchange to condense methanol therefrom.

13. A process according to any one of claims 1 to 12, wherein a purge gas stream is recovered from the 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 and methanol synthesis.

14. A process according to claim 13, wherein following recovery of the purge gas stream, the remaining methanol depleted gas mixture is compressed in a circulating compressor and combined with the make-up gas.

15. A process according to any one of claims 1 to 14, wherein the methanol recovered from the second product gas stream is subjected to one or more steps of distillation in a distillation unit to produce a purified methanol product.

16. A process according to claim 15, wherein heat from the second product gas stream is used in one or more reboilers in the distillation unit.

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