Methanol method

Abstract

A methanol synthesis process is described, comprising the steps of: (i) passing a first synthesis gas mixture comprising a make-up gas through a first synthesis reactor comprising a 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 loop recycle gas stream to form a second synthesis gas mixture; (iv) passing the second synthesis gas mixture through a second synthesis reactor comprising a 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) applying at least a portion of the second methanol-depleted gas mixture as a loop recycle gas stream; wherein the first synthesis reactor has a higher heat transfer per cubic meter of catalyst than the second synthesis reactor, no loop recycle gas stream is fed to the first synthesis gas mixture, and the recycle ratio of the loop recycle gas stream forming the second synthesis gas mixture is 1.1:1-6:1.

Description

[0001] The present invention relates to a method for synthesizing methanol.

[0002] Methanol synthesis is typically carried out by passing a synthesis gas containing hydrogen, carbon oxides and any inert gases through a bed of one or more methanol synthesis catalysts (which are typically copper-containing compositions) at high temperature and high pressure. Methanol is typically recovered by cooling the product gas stream to a dew point below the methanol and separating it from the product as a liquid. Crude methanol is typically purified by distillation. The method typically operates in a circulation loop, so the remaining unreacted gas stream is typically circulated back to the synthesis reactor as a portion of the synthesis gas through a circulator. Fresh synthesis gas, also known as make-up gas, is added to the circulating unreacted gas to form a synthesis gas stream. To avoid accumulation of inert gases, an exhaust stream is often extracted from the circulating gas stream.

[0003] The process may be operated using two synthesis reactors, wherein each synthesis reactor comprises a bed of methanol synthesis catalyst.

[0004] US 7,790,775 discloses a method for balancing an exothermic gas phase reaction, comprising the steps of: (a) providing a recycle stream to which make-up gas is added to form a feed gas stream; (b) heating the feed gas stream; (c) passing the heated feed gas stream through a first reactor containing a catalyst for the exothermic gas phase reaction under conditions suitable for the reaction; (d) removing a product stream containing product and unreacted gas from the first reactor; (e) cooling and partially condensing the product stream to form a gas phase and a liquid phase; (f) separating the liquid phase containing the desired product from the product stream and removing the liquid phase; (g) separating the gas phase from the product stream to form a gas stream; (h) optionally subjecting the product stream to a reaction. (g) is a process according to claim 1, wherein the gaseous stream of the gaseous stream is mixed with additional make-up gas; (i) heating the gaseous stream; (j) passing the heated gaseous stream through a final reactor containing a catalyst for an exothermic gas phase reaction under conditions suitable for the reaction; (k) removing a final product stream containing product and unreacted gas from the final reactor; (l) cooling and partially condensing the final product stream to form a final gas phase and a final liquid phase; (m) separating a final liquid phase containing the desired product from the final product stream and removing the final liquid phase; and (n) separating a gaseous phase from the final product stream and recycling the gas to step (a); and wherein the gaseous stream from step (g) is compressed before heating in step (i).

[0005] US 8,536,235 discloses a methanol synthesis method, comprising the following steps: (a) passing a synthesis gas mixture comprising a recycle gas and a make-up gas through a first synthesis reactor comprising a methanol synthesis catalyst, wherein the reactor is cooled by boiling water under pressure to form a mixed gas comprising methanol, (b) cooling the methanol-containing mixed gas, (c) passing the cooled methanol-containing mixed gas through a second synthesis reactor comprising a methanol synthesis catalyst, wherein methanol is further synthesized therein to form a product gas stream, (d) cooling the product gas to condense methanol, and (e) recovering methanol and returning unreacted gas to the first synthesis reactor as recycle gas, wherein the methanol-containing mixed gas from the first synthesis reactor is cooled by heat exchange with the recycle gas or make-up gas.

[0006] US Pat. No. 5,827,901 describes a process for producing methanol from synthesis gas containing hydrogen and carbon oxides over a copper-containing catalyst at a pressure of 20-120 bar and a temperature of 130-350°C. The synthesis gas is first passed through a first synthesis reactor, wherein the catalyst is provided within tubes surrounded by water as a coolant, the water being boiled under high pressure. A first mixture containing gas and methanol vapor is withdrawn from the first reactor and passed uncooled through a second synthesis reactor. In the second reactor, the catalyst is cooled using the synthesis gas to which make-up gas has been added.

[0007] US Pat. No. 8,629,191 describes a method for producing methanol from synthesis gas containing hydrogen and carbon oxides. The synthesis gas is passed through a first water-cooled reactor, where a portion of the carbon oxides is catalytically converted to methanol. The resulting mixture, comprising synthesis gas and methanol vapor, is supplied to a second gas-cooled reactor, where another portion of the carbon oxides is converted to methanol. The methanol is then separated from the synthesis gas, and the synthesis gas is recycled back to the first reactor. Cooling gas is passed through the second reactor concurrently with the mixture withdrawn from the first reactor.

[0008] US Pat. No. 5,631,302 describes a process for producing methanol from a synthesis gas containing hydrogen and carbon oxides at a pressure of 20-20 bar and a temperature of 200-350° C. over a copper-containing catalyst. The synthesis gas flows through a first synthesis reactor consisting of an axial reactor containing a fixed bed of the copper-containing catalyst. The reaction in the axial reactor is carried out adiabatically without recirculation of the synthesis gas. The unreacted gas mixture from the first synthesis reactor, along with recycle gas, flows through a second synthesis reactor containing a copper-containing catalyst, which is placed in a tube and cooled indirectly by boiling water.

[0009] US 2014 / 0031438 A1 describes a method for producing methanol from inert gas-rich synthesis gas. A catalytic prereactor is installed upstream of a single- or multi-stage synthesis loop, where a first portion of the synthesis gas is converted into methanol in the prereactor. Additionally, an inert gas separation stage, such as a pressure swing adsorption system or a membrane system, is connected downstream of the synthesis loop to allow the hydrogen-rich synthesis gas stream to be returned to the synthesis loop. In the treatment of methane-rich synthesis gas, the inert gas separation stage may also include an autothermal reformer, in which methane is converted into carbon oxides and hydrogen, which can also be returned to the synthesis loop.

[0010] WO 2014 / 012601 A1 describes a method for producing methanol, comprising the following steps: (a) providing fresh methanol synthesis gas containing hydrogen, carbon monoxide and carbon dioxide; (b) providing a recycle gas stream containing unconverted methanol synthesis gas and mixing part of the recycle stream with the fresh synthesis gas to form a process gas stream; (c) introducing the process gas stream into a first methanol reaction unit and reacting it in the presence of a methanol catalyst, and obtaining a first effluent stream containing methanol and part of the unconverted synthesis gas contained in the recycle stream; and (d) introducing at least another part of the recycle gas stream into a second methanol reaction unit and reacting it in the presence of a methanol catalyst, and obtaining a second effluent stream containing methanol and another part of the unconverted synthesis gas contained in the recycle stream, wherein the recycle stream is pressurized by a common circulator.

[0011] WO2014 / 206635 A1 describes a method for preparing methanol in parallel reactors, comprising the following steps: (a) reacting carbon oxides and hydrogen in the presence of a methanol catalyst in a first methanol reactor to obtain a first effluent containing methanol, (b) introducing unconverted synthesis gas into a second methanol reactor in the presence of a methanol catalyst and reacting it to obtain a second effluent containing methanol, the first methanol reactor and the second methanol reactor being operated in parallel, (c) combining the first and second effluents, and (d) cooling and separating the combined and cooled effluent into a liquid phase containing methanol and unconverted synthesis gas, wherein the methanol catalyst in the first methanol reactor is indirectly cooled with boiling water, and the methanol catalyst in the second methanol reactor is directly or indirectly cooled with unconverted synthesis gas, and the unconverted synthesis gas is subsequently converted into the second effluent stream.

[0012] DE 3518362 A1 describes a process for producing methanol, which starts with a conventional methanol synthesis process, wherein unreacted synthesis gas is recycled to the reactor inlet, and a methanol synthesis reactor operated without recycling is arranged upstream of the recycling process.

[0013] We realized that the efficiency of multistage methanol synthesis can be improved by applying different recycle ratios to different types of reactors.

[0014] Therefore, the present invention provides a method for synthesizing methanol, comprising the following steps:

[0015] (i) passing a first synthesis gas mixture comprising a make-up gas through a first synthesis reactor comprising a cooled methanol synthesis catalyst to form a first product gas stream;

[0016] (ii) recovering methanol from the first product gas stream to form a first methanol-depleted gas mixture;

[0017] (iii) combining the first methanol-lean gas mixture with a loop recycle gas stream to form a second synthesis gas mixture;

[0018] (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;

[0019] (v) recovering methanol from the second product gas stream, thereby forming a second methanol-depleted gas mixture; and

[0020] (vi) using at least a portion of the second methanol-depleted gas mixture as a loop recycle gas stream;

[0021] The first synthesis reactor has a higher heat transfer capacity per cubic meter of catalyst than the second synthesis reactor, no loop recycle gas stream is fed to the first synthesis gas mixture, and the recycle ratio of the loop recycle gas stream forming the second synthesis gas mixture is 1.1:1-6:1.

[0022] The present invention utilizes the advantages of each type of reactor, and thus it has a zero recycle ratio zone for the first synthesis reactor and a high recycle ratio zone for the second synthesis reactor.

[0023] The term "recycle ratio" refers to the molar flow ratio of the recycle loop gas to the make-up gas that forms the synthesis gas mixture fed to the second synthesis reactor. Thus, the recycle ratio of the second synthesis gas is derived from the proportion of the loop gas combined with the first methanol-lean gas mixture, both relative to the make-up gas.

[0024] The circulation ratio of the loop recycle gas stream forming the second synthesis gas mixture may be 1.1:1-6:1, preferably 1.5:1-6:1, and more preferably 2:1-6:1.

[0025] The first synthesis gas comprises a make-up gas. The make-up gas typically comprises hydrogen, carbon monoxide and / or carbon dioxide. The make-up gas can be produced by steam reforming of methane or naphtha using existing steam reforming methods (including pre-reforming). However, the present invention is particularly effective when a reactive synthesis gas produced by a method comprising a partial oxidation step of a hydrocarbon, biomass or carbonaceous feedstock is applied. "Reactive synthesis gas" refers to a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide, with a ratio (volume ratio) of carbon monoxide to carbon dioxide typically ≥2:1, preferably ≥5:1. This method includes combined reforming, in which the first part of the hydrocarbon feedstock is steam reformed and the second part is autothermal reforming; and from coal or biomass gasification. Alternatively, tail gases mainly comprising hydrogen and carbon oxides (mainly carbon monoxide) discharged from refineries or other chemical processes can also be used.

[0026] The use of more reactive syngas results in the use of smaller amounts of catalyst, and the net heat of reaction is greater, resulting in more than twice the heat released per unit volume of catalyst compared to processes based solely on steam reforming. Therefore, as the carbon monoxide to carbon dioxide ratio in the syngas increases, it becomes important to provide effective cooling of the catalyst.

[0027] The make-up gas can be sent directly to the first methanol synthesis reactor without being diluted with other gases. This can be done when the make-up gas contains an appropriate amount of carbon monoxide, such as 10-20 vol% CO. This synthesis gas can be obtained by conventional steam reforming of hydrocarbons. However, if necessary, the stoichiometric amount of the first synthesis gas can be adjusted, for example, by adding a hydrogen-containing gas stream to optimize the methanol synthesis in the first synthesis reactor. This can be especially the case when the synthesis gas contains a higher amount of carbon monoxide, such as 20-35 vol% or 25-35 vol%. This reactive synthesis gas can be obtained specifically by gasification of coal or biomass, or by a hydrocarbon reforming process based on combined reforming or autothermal reforming. In these cases, the first synthesis gas is ideally diluted with a hydrogen-containing gas stream selected from the following: an exhaust gas stream from other methanol processes or a hydrogen stream obtained from a suitable hydrogen-containing gas mixture, for example, by pressure swing adsorption or membrane separation.

[0028] The composition of the first synthesis gas at the inlet of the first synthesis reactor is preferably as follows: 15-30 mol% carbon monoxide, 0.5-10 mol% carbon dioxide, 55-85 mol% hydrogen, and the balance being one or more inert gases. The pressure of the first synthesis gas at the inlet of the first synthesis reactor is preferably 50-100 bar (absolute pressure). The temperature of the first synthesis gas at the inlet of the first synthesis reactor is preferably 200-250° C. and at its outlet is preferably 230-280° C.

[0029] The composition of the second synthesis gas at the inlet of the second synthesis reactor is preferably as follows: 3-10 mol% carbon monoxide, 0.5-10 mol% carbon dioxide, 65-95 mol% hydrogen, and the balance being one or more inert gases. The pressure of the second synthesis gas at the inlet of the second synthesis reactor is preferably 50-100 bar (absolute pressure). The temperature of the second synthesis gas at the inlet of the second synthesis reactor is preferably 215-250° C. and at its outlet is preferably 250-300° C.

[0030] A single circulator may be used to feed the combined loop cycle gas and the first methanol-lean gas mixture to the second synthesis reactor.

[0031] In the present invention, at least a portion of the second methanol-depleted gas mixture is used as a loop recycle gas stream. Thus, the second methanol-depleted gas is a source of the loop recycle gas stream. A vent stream can be recovered from the second methanol-depleted gas and / or the loop recycle gas stream.

[0032] For example, if necessary, if the transport diameter is a limitation, in order to adjust the load and the relative sizes of the first and second synthesis reactors, a portion of the make-up gas can be bypassed through the first synthesis reactor and fed as the second feed to the high recycle ratio loop. Thus, a portion of 0-70 vol% of the make-up gas can be fed to the second synthesis reactor. However, for efficiency reasons, it is preferred that this portion be ≤10 vol% of the make-up gas and more preferably 0 vol%, i.e., no bypass, so that the process is operated in series.

[0033] The first synthesis reactor is preferably designed to have a high heat transfer relative to the volume of the cooled catalyst. The heat transfer can be conveniently characterized by volume UA. Volume UA can be defined as the total heat transfer coefficient U multiplied by the total heat transfer area A per cubic meter of cooled catalyst in the reactor. Although any converter can be used in this position, the first synthesis reactor ideally has a heat transfer capacity of ≥ 50 kW / m 3 / K volume UA and more preferably ≥90kW / m 3 / K. Such converters include those in which the catalyst is placed within a plurality of tubes that are cooled by a heat exchange medium.

[0034] Compared to the first synthesis reactor, the second synthesis reactor has a lower heat transfer relative to the cooling catalyst volume. For example, the volume UA can be ≤ 40 kW / m 3 / K. The second synthesis reactor can be of any type, but high overall conversion of carbon oxides to methanol is associated with high recycle flow rates or low converter outlet temperatures. Several converter types can be used, including: (i) converters featuring one or more adiabatic beds and no heat exchange surfaces in contact with the catalyst; (ii) converters with gas cooling, such as tubular-cooled converters, isothermal methanol converters, and gas-cooled converters; and (iii) water-cooled converters with radial flow.

[0035] The first and second synthesis reactors may comprise one or more reactors.

[0036] In a preferred design, the first synthesis reactor comprises a methanol synthesis catalyst placed in a tube that is water-cooled under pressure, and the second synthesis reactor comprises a fixed bed of methanol synthesis catalyst that is cooled by heat exchange under pressure with water or a synthesis gas mixture selected from the first synthesis gas mixture and the second synthesis gas mixture.

[0037] The first synthesis reactor is preferably an axial flow steam reformer (aSRC). In this reactor, the synthesis gas usually flows axially through a vertical catalyst-containing tube, which is cooled by heat exchange with boiling water under pressure. The catalyst can be provided directly in the tube in granular form or in one or more cylindrical containers, wherein the container guides the radial and axial flow of the synthesis gas to enhance heat transfer. The catalysts included and their use in methanol synthesis are described in WO2012146904 (A1). aSRC typically has a capacity of ≥100kW / m 3 A steam-generating reformer, in which the catalyst resides in tubes cooled under pressure with boiling water, provides a useful means of removing heat from the catalyst. However, while aSRC offers the highest cooling factor, it utilizes the reactor volume poorly, resulting in a large reactor shell relative to the amount of catalyst held. Furthermore, aSRC can generate high pressure drops. By having zero recirculation in the first synthesis reactor, the advantages of aSRC are enhanced and the disadvantages are mitigated.

[0038] The second synthesis reactor can be a radial flow steam reformer, a gas-cooled reformer, or a tubular-cooled reformer. In each of these, the particulate catalyst bed is cooled by tubes or plates through which a coolant heat exchange medium flows. Alternatively, the second synthesis reactor can be a quench reactor in which one or more beds of particulate catalyst are cooled by a syngas mixture injected into or between the reactor beds.

[0039] In a radial flow steam reformer (rSRC), the synthesis gas typically flows radially (inward or outward) through a bed of particulate catalyst, which is cooled by a plurality of tubes or plates fed with pressurized boiling water as a coolant. Such reactors are known and are described, for example, in US Pat. No. 4,321,234. The heat transfer of an rSRC is poorer than that of an aSRC, but the pressure drop is very low, thus facilitating operation at high recycle ratios. An rSRC typically has a power output of 12-24 kW / m 3 / K's volume UA.

[0040] In a tubular cooled reformer (TCC), the catalyst bed is cooled by the feed syngas flowing through open-ended tubes within the bed, which discharge the heated gas onto the catalyst. Thus, a TCC can provide sufficient cooling area for more reactive syngas, such as those from combined reforming or coal gasification, but the increased heat of reaction means that the recirculation loop gas flow will be insufficient to remove the heat of reaction unless the recirculation ratio is high. TCCs typically have a cooling capacity of 6-15 kW / m². 3 As an alternative to TCC, a gas-cooled converter (GCC) can be used to cool the catalyst bed by passing the synthesis gas through tubes arranged in a heat exchanger configuration. For example, GCC is described in the aforementioned US Pat. No. 5,827,901. TCC is superior to GCC because it uses open-top tubes, eliminates the top header, and eliminates all differential expansion problems associated with gas-cooled converters, making it simpler and cheaper to produce. Thus, TCC has the advantages of low equipment investment and low outlet temperature, which is more conducive to the equilibrium of the synthesis reaction, but its heat transfer is lower than that of aSRC and its pressure drop is higher than that of rSRC.

[0041] In a quench reactor, one or more beds of particulate catalyst are cooled by a syngas mixture injected into or between the reactor beds. Therefore, the volume UA of the quench reactor is 0 kW / m 3 / K. Such reactors are described, for example, in US3458289, US3475136 and US4411877.

[0042] Alternative converter designs, such as the Linde Variobar converter, which contains a methanol synthesis catalyst bed cooled by heat exchange with boiling water flowing through a helical tube heat exchanger within the bed, typically have a power of 30-40 kW / m 3 / K. Such a reformer can be used as the second synthesis reactor, for example, in combination with an axial flow steam-generating reformer, or as the first synthesis reactor, in combination with a quench reactor, a tubular cooled reformer or even a radial flow steam-generating reformer.

[0043] The methanol synthesis catalyst is preferably a copper-containing methanol synthesis catalyst. In particular, the methanol synthesis catalyst in the first and second synthesis reactors is a granular copper / zinc oxide / aluminum oxide catalyst. A particularly suitable catalyst is the Mg-doped copper / zinc oxide / aluminum oxide catalyst described in US Pat. No. 4,788,175. The first and second synthesis reactors may use the same or different methanol synthesis catalysts.

[0044] Methanol synthesis can be carried out in the first and second synthesis reactors, typically at high temperature and high pressure, for example at a pressure of 20-120 bar absolute and a temperature of 130-350° C. When the circulating loop gas of the first and second synthesis reactors is subjected to two-stage or separate circulation, they can be operated at the same or different pressures. Therefore, the first reactor can be operated at a higher pressure, the same pressure, or a lower pressure than the second reactor. This can provide advantages in terms of methanol recovery. In a preferred embodiment, the pressure of the second synthesis reactor is higher than that of the first synthesis reactor. The pressure difference between the reactors can be ≥4 bar. The circulator can be a conventional compressor suitable for processing the circulating loop gas at the desired pressure.

[0045] The temperature of the product gas stream withdrawn from the second synthesis reactor is typically in the range 180-250°C.

[0046] The ratio of methanol produced in the first and second reactors may be from 30:70 to 70:30, such as from 40:60 to 60:40 or 50:50.

[0047] The gas mixture fed to the first and second synthesis reactors can be heated before being fed to the reactors. The heating can be implemented by conventional heat exchange methods using suitable heat exchange equipment. Preferably, the first and second synthesis gases are heated using product gases from the reactors in a gas-gas heat exchanger. Other temperature adjustments for the feedstock or product gases can be implemented using conventional heat exchange equipment. Therefore, the product gas streams from the first and second synthesis reactors can be cooled through one or more stages of heat exchange, for example, with water or air, to condense methanol therefrom, which can be appropriately recovered using a gas-liquid separator. Cooling can be implemented to condense methanol from the first and second product gas streams in whole or in part. Preferably, all methanol is condensed from the second product gas stream. The recovered liquid methanol streams can be processed separately, but are preferably combined and sent for further processing, for example, in one or more, preferably two or three, distillation stages to produce a purified methanol product.

[0048] The exhaust gas stream is preferably recovered from the circulation loop to avoid accumulation of inert gases such as nitrogen, methane and argon in the loop. The exhaust gas typically comprises hydrogen and carbon oxides and can be used to recover the hydrogen, for example by pressure swing adsorption or application of a suitable membrane, or can be subjected to one or more other processing stages including autothermal reforming, water gas shift and methanol synthesis. The exhaust gas can be recovered from the first methanol-lean gas or the second methanol-lean gas, depending on whether the stoichiometry of the make-up gas is hydrogen-rich or carbon-rich. Preferably, the exhaust gas is recovered from the second methanol-lean gas mixture, and the remaining methanol-lean gas mixture is used as the circulation loop gas mixture.

[0049] The present invention is further described with reference to the following drawings, in which:

[0050] Figure 1 A method according to one embodiment of the present invention using aSRC and rSRC is described.

[0051] Those skilled in the art will understand that the drawings are exemplary and that further equipment elements may be required in an industrial plant, such as feed tanks, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, liquid level controllers, receiving tanks, storage tanks, etc. The provision of such auxiliary equipment does not form part of the present invention and is carried out in accordance with conventional chemical engineering practice.

[0052] exist Figure 1In the process, the make-up gas in line 10, comprising hydrogen, carbon monoxide, and carbon dioxide, is optionally combined with the hydrogen-containing gas fed via the dot-dash line 12, and the resulting first synthesis gas mixture is passed through line 14 to a gas-to-gas heat exchanger 16, where it is heated by indirect heat exchange with a first product gas stream 24. The heated first synthesis gas mixture is fed via line 18 to the inlet of an axial flow steam generator reformer 20, which comprises catalyst-filled tubes 22 through which the synthesis gas mixture flows. The tubes are cooled with boiling water under pressure. The catalyst is a granular copper / zinc oxide / aluminum oxide catalyst. Pressurized boiling water is fed to the shell side of the reactor, and a mixture of boiling water and steam is withdrawn and sent to a steam drum (not shown). As the synthesis gas flows axially through the catalyst-filled tubes 22, a methanol synthesis reaction occurs, thereby forming a first product gas stream comprising methanol vapor. The first product gas stream is recovered from the outlet of the first synthesis reactor 20 and fed via line 24 to an intermediate heat exchanger 16, where it is partially cooled. The partially cooled gas is fed via line 26 to one or more heat exchangers 28 to condense methanol therefrom. The resulting gas-liquid mixture is fed 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 separator 30 and fed to a circulation loop via line 34, where it is combined with a portion of the second methanol-depleted gas fed via line 36 to form a second synthesis gas mixture. The second synthesis gas mixture is fed via line 38 to a circulator 40. The circulator compresses the second synthesis gas mixture, which is then fed from the circulator via line 42 to a gas-to-gas heat exchanger 44, where it is heated by indirect heat exchange with a second product gas stream 52. The heated second synthesis gas is fed via line 46 to the inlet of a radial flow steam reformer 48, which contains a bed 50 of methanol synthesis catalyst having a plurality of heat exchange tubes through which pressurized boiling water flows as a coolant. Although only tubes are depicted, alternative heat exchange equipment such as plates through which a coolant can flow may also be used. The catalyst is a granular copper / zinc oxide / aluminum oxide catalyst. Pressurized boiling water is fed to the reactor tube side, and a mixture of boiling water and steam is withdrawn and provided to a steam drum (not shown). As the synthesis gas flows radially through the catalyst bed 50, a methanol synthesis reaction occurs, thereby forming a second product gas stream containing methanol vapor. The second product gas stream is recovered from the outlet of the second synthesis reactor 48 and fed to the intermediate heat exchanger 44 via pipeline 52, where it is partially cooled. The partially cooled gas is fed to one or more heat exchangers 56 via pipeline 54 to condense methanol therefrom. The resulting gas-liquid mixture is sent to a gas-liquid separator 58, and liquid methanol is recovered via pipeline 64. The second methanol-depleted gas mixture is recovered in the separator 58 and fed to the exhaust gas discharge line 62 via pipeline 60, where a portion of the gas is removed to reduce the accumulation of inert gases.The remaining second methanol-depleted gas mixture is fed to recycle loop line 36 where it is combined with the unreacted gas fed via line 34. Crude methanol streams 32 and 64 are combined and sent via line 66 for further processing, such as through one or more stages of distillation, to produce a purified methanol product.

[0053] The present invention is further described with reference to the following examples.

[0054] Simulate the process to describe Figure 1 The compositions and flow rates of the various gas streams for the process shown, in which the radial flow steam reformer 48 is replaced by a tubular cooled reformer, are given in the table below.

[0055] logistics 10 12 14 18 24 32 34 Pressure MPa (absolute pressure) 8.5 8.5 8.5 8.2 8.0 7.7 7.7 Temperature 150 40 132 230 258 50 50 <![CDATA[Flow rate kNm 3 / hr (steam)]]> 465 91 556 556 427 358 Flow rate t / hr (liquid) 96.6 Composition Mol% <![CDATA[H2O]]> 0.5 0.1 0.4 0.4 0.9 5.7 0.0 <![CDATA[H2]]> 65.8 82.0 68.5 68.5 58.5 0.4 69.7 CO 22.7 3.1 19.4 19.4 10.6 0.4 12.5 <![CDATA[CO2]]> 8.7 3.2 7.8 7.8 9.7 3.2 11.0 <![CDATA[CH3OH]]> 0 0.4 0.1 0.1 15.2 89.8 0.9 Inert 2.4 11.3 3.8 3.8 5.0 0.3 5.9 logistics 36 42 46 52 60 62 64 Pressure MPa (absolute pressure) 7.6 8.3 8.2 8.0 7.7 7.6 7.7 Temperature 50 59 153 241 50 50 50 <![CDATA[Flow rate kNm 3 / hr (steam)]]> 1838 2196 2196 2040 1921 83 Flow rate t / hr (liquid) 146.5 Composition Mol% <![CDATA[H2O]]> 0.1 0.1 0.1 1.8 0.1 0.1 29.8 <![CDATA[H2]]> 69.3 69.3 69.3 65.3 69.3 69.3 0.2 CO 2.3 3.9 3.9 2.1 2.3 2.3 0.0 <![CDATA[CO2]]> 3.8 5.0 5.0 3.7 3.8 3.8 0.8 <![CDATA[CH3OH]]> 0.7 0.7 0.7 4.6 0.7 0.7 68.1 Inert 23.8 20.9 20.9 22.5 23.8 23.8 0.9

[0056] like Figure 1 As shown, the first synthesis reactor (axial flow steam generator converter 20) has a higher heat transfer per cubic meter of catalyst than the second synthesis reactor (tubular cold converter), no loop circulating gas flow is fed to the first synthesis gas mixture, and the circulation ratio of the loop circulating gas flow to form the second synthesis gas mixture is 1.1:1-6:1.

Claims

1. A method for synthesizing methanol, comprising the following steps: (i) passing a first synthesis gas mixture consisting of a make-up gas and an optional diluent hydrogen-containing 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 to form a first methanol-depleted gas mixture; (iii) combining the first methanol-lean gas mixture with a 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) using at least a portion of the second methanol-depleted gas mixture as a loop recycle gas stream; in: The first synthesis reactor is an axial flow steam reformer containing a methanol synthesis catalyst placed in a water-cooled tube under pressure. The second synthesis reactor is a radial flow steam reformer, which contains a fixed bed of methanol synthesis catalyst, and the fixed bed of methanol synthesis catalyst is cooled by heat exchange with water under pressure, or the second synthesis reactor is selected from a tubular cooled reformer, a gas cooled reformer or a quench reactor, which contains a fixed bed of methanol synthesis catalyst, and the fixed bed of methanol synthesis catalyst is cooled by heat exchange with a synthesis gas mixture selected from the first synthesis gas mixture and the second synthesis gas mixture; The first synthesis reactor has a higher heat transfer per cubic meter of catalyst than the second synthesis reactor; The make-up gas comprises hydrogen, carbon monoxide, and carbon dioxide, and the first syngas mixture is a reactive syngas having a volume ratio of carbon monoxide to carbon dioxide of ≥ 2:1 and having the following composition: 15-30 mol% carbon monoxide, 0.5-10 mol% carbon dioxide, 55-85 mol% hydrogen, and the balance being one or more inert gases; and No loop recycle gas stream is fed to the first synthesis gas mixture, and the recycle ratio of the loop recycle gas stream forming the second synthesis gas mixture fed to the second synthesis reactor is from 2:1 to 6:1, wherein the recycle ratio for the second synthesis gas mixture is derived from the proportion of the loop recycle gas stream combined with the first methanol-lean gas mixture, relative to the make-up gas.

2. The process of claim 1, wherein the make-up gas comprises 10-20 vol% of carbon monoxide, and is fed directly to the first methanol synthesis reactor without being diluted with other gases.

3. The process of claim 1, wherein the make-up gas comprises 20-35 vol% carbon monoxide and is diluted with a hydrogen-containing gas stream selected from a vent gas stream or a hydrogen stream from another methanol process.

4. The process of any one of claims 1 to 3, wherein a single circulator is used to feed the combined loop cycle gas stream and the first methanol-lean gas mixture to the second synthesis reactor.

5. The method according to any one of claims 1 to 3, wherein the methanol synthesis catalyst is a copper-containing methanol synthesis catalyst.

6. The method of any one of claims 1 to 3, wherein the methanol synthesis catalyst is a composition comprising copper, zinc oxide and aluminum oxide.

7. The process according to any one of claims 1 to 3, wherein the methanol synthesis is carried out in the first and second reactors at a pressure of 20 to 120 bar absolute and a temperature of 130 to 350°C.

8. The process according to any one of claims 1 to 3, wherein the gas mixture fed to the first and second synthesis reactors is heated in a gas-gas heat exchanger with product gas from said reactors.

9. The process of any one of claims 1 to 3, wherein the product gas streams from the first and second synthesis reactors are cooled in one or more heat exchange stages to condense methanol therefrom.

10. The process of any one of claims 1 to 3, wherein a purge gas stream is recovered from the second methanol-depleted gas mixture and used to recover hydrogen or subjected to one or more further processing steps including autothermal reforming, water gas shift and methanol synthesis.

11. The process of claim 9, wherein the condensed methanol is recovered using a gas-liquid separator, combined and sent to one or more distillation stages for further processing to produce a purified methanol product.

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