Process for synthesizing methanol

Abstract

The present invention describes a process for synthesizing methanol, the process comprising the following steps: (i) forming a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide from a hydrocarbon feedstock in a reforming unit, the reforming unit comprising a heat exchange reformer and an autothermal reformer connected in series; (ii) cooling the synthesis gas in the heat exchange reformer and one or more further heat exchange stages, and recovering process condensate from the cooled synthesis gas to form a make-up gas; (iii) passing a feed gas comprising the make-up gas into a methanol synthesis loop comprising one or more methanol synthesis reaction units; (iv) recovering a product gas mixture comprising methanol from the methanol synthesis loop, cooling the product gas mixture to below the dew point to condense crude methanol, and separating the crude methanol from an unreacted gas mixture; and (v) recycling a portion of the unreacted gas mixture to the methanol synthesis loop and recovering a portion of the unreacted gas mixture as a purge gas stream, wherein a hydrogen-rich stream and a carbon-rich stream are separated from the purge gas, a portion of the hydrogen-rich stream is fed to the methanol synthesis loop, and a portion of the carbon-rich stream is fed to the reformer unit.

Description

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

[0002] Methanol synthesis is usually carried out by passing a synthesis gas comprising hydrogen and carbon monoxide and / or carbon dioxide through one or more methanol synthesis catalyst beds in a synthesis reaction unit at elevated temperature and pressure, and the methanol synthesis catalyst is usually a copper-containing composition. Crude methanol is usually recovered by cooling the product gas stream to below the dew point and separating the product in liquid form. Crude methanol is usually purified by distillation. The process is usually operated in a loop: therefore, unreacted gas can be recycled to the synthesis reaction unit via a circulation device as a part of the feed gas. Fresh synthesis gas (referred to as make-up gas) is added to the recycled unreacted gas to form a feed gas stream. A purge stream is usually proposed from the circulating gas stream to avoid the accumulation of inert gas in the loop.

[0003] Methanol synthesis can be described by the following two equations:

[0004]

[0005]

[0006] There are two stoichiometric values ​​which are commonly used to describe the ratios of reactants fed to a methanol synthesis reaction unit. These are R and Z and can be determined from the molar concentrations of the components in the synthesis gas as follows;

[0007] R=([H 2 ]-[CO 2 ]) / ([CO]+[CO 2 ])

[0008] Z=[H 2 ] / (2[CO]+3[CO 2 ])

[0009] Furthermore, for methanol synthesis, it is often useful to determine the value S; which is the H 2 (Nm 3 / h)+CO(Nm 3 / h). Then, S, Z, and R can be related by the following formula:

[0010] For Z≤1, the maximum methanol production (Nm 3 / h)=ZS / (R+1)

[0011] For Z>1, the maximum methanol production (Nm 3 / h) = S / (R + 1)

[0012] The ideal stoichiometric mixture is produced when there is enough hydrogen to convert all of the carbon oxides into methanol. This is when R = 2 and Z = 1. However, different syngas generation technologies produce different syngases with different reactant ratios.

[0013] Synthesis gas is typically generated using a fired steam reformer that reforms a mixture of hydrocarbon feedstocks such as natural gas and steam. In such plants, almost 30% of the natural gas is used for fuel purposes, with the remaining 70% used as feedstock. The combustion of the fuel produces significant amounts of carbon dioxide emissions from the process.

[0014] US 6191174 discloses a process for producing methanol from a hydrocarbon feedstock, the process comprising: contacting a steam mixture comprising the feedstock and steam in a steam reforming zone with a catalyst effective to catalyze at least one reforming reaction; recovering a synthesis gas mixture comprising carbon oxides, hydrogen and methane from the reforming zone; supplying a material of the synthesis gas mixture to a methanol synthesis zone, the methanol synthesis zone being filled with a methanol synthesis catalyst and maintained under methanol synthesis conditions; recovering a product gas mixture from the methanol synthesis zone, the product gas mixture comprising methanol and unreacted materials of the synthesis gas mixture; supplying a material of the product gas mixture to a methanol recovery zone maintained under methanol recovery conditions; recovering a crude methanol product stream and a steam stream comprising unreacted materials of the synthesis gas mixture from the methanol recovery zone; separating the material of the synthesis gas mixture into a first hydrogen-rich stream and a second carbon-rich oxide stream comprising carbon oxides and methane; supplying at least a portion of the first hydrogen-rich stream as fuel to the steam reforming zone; and recycling at least a portion of the second carbon-rich oxide stream to the steam reforming zone to form a portion of the steam mixture comprising the feedstock and steam. The steam reforming zone comprises a compact fired reforming device.

[0015] As will be apparent from this disclosure, a methanol plant also typically includes several large compressors and is traditionally driven by steam generated from heat recovered from the process and supplemented by steam from one or more fired boilers and heaters.

[0016] Applicants have realized that in the case of generating synthesis gas by a combination of a heat exchange reformer and an autothermal reformer in series, the process does not need to generate steam for power. Although synthesis gas still needs to be generated, the combustion value of the loop purge gas is more than enough for this process. Therefore, the fuel value of the carbon-rich gas recovered from the purge gas in the heat exchange reformer-autothermal reformer process can exceed the fuel demand of the combustion heater and boiler on the device, especially when an electric motor is used to drive the compressor. By eliminating the need for steam to power, the process generally uses less hydrocarbons as fuel compared to an equivalent form of the combustion reformer process, thereby reducing its carbon dioxide emissions.

[0017] Therefore, the present invention provides a process for synthesizing methanol, which process comprises the following steps: (i) forming a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide from a hydrocarbon feedstock in a reforming unit, the reforming unit comprising a heat exchange reformer and an autothermal reformer connected in series; (ii) cooling the synthesis gas in the heat exchange reformer and one or more further heat exchange stages, and recovering process condensate from the cooled synthesis gas to form a make-up gas; (iii) passing a feed gas comprising the make-up gas into a methanol synthesis loop comprising one or more methanol synthesis reaction units; (iv) recovering a product gas mixture comprising methanol from the methanol synthesis loop, cooling the product gas mixture to below the dew point to condense crude methanol, and separating the crude methanol from an unreacted gas mixture; and (v) recycling a portion of the unreacted gas mixture to the methanol synthesis loop and recovering a portion of the unreacted gas mixture as a purge gas stream, wherein a hydrogen-rich stream and a carbon-rich stream are separated from the purge gas, a portion of the hydrogen-rich stream is fed to the methanol synthesis loop, and a portion of the carbon-rich stream is fed to the reformer unit.

[0018] A "carbon-rich stream" refers to a gas stream having a higher proportion of carbon-containing compounds (carbon monoxide, carbon dioxide and methane) than the purge gas. Although individual components may have the same or even lower proportions as those in the purge gas, the total amount of all carbon-containing compounds will be higher in the carbon-rich stream.

[0019] Methane is inert in the methanol synthesis reaction but is a valuable feedstock for the reforming process for making synthesis gas, and the carbon-rich stream is used as a fuel or feedstock.

[0020] In the process of the present invention, the hydrocarbon feedstock may be any gaseous or low boiling hydrocarbon feedstock, such as natural gas, associated gas, LPG, petroleum distillates or naphtha. Preferably, it is methane, associated gas or natural gas containing a considerable proportion of methane, for example more than 85% v / v of methane. Natural gas is a particularly preferred feedstock. The feedstock is typically compressed to a pressure in the range of 10 to 100 bar absolute.

[0021] If the hydrocarbon feedstock contains sulphur compounds, the feedstock is preferably desulphurised, e.g. hydrodesulfurised using a Co or Ni catalyst, and hydrogen sulphide absorbed using a suitable absorbent such as a zinc oxide bed, before or after compression. In order to facilitate this and / or reduce the risk of carbon deposition in the reforming process, hydrogen is preferably added to the hydrocarbon feedstock. The amount of hydrogen in the resulting mixed gas stream may be in the range of 1 to 20% by volume, but is preferably in the range of 1 to 10%, more preferably in the range of 1 to 5%. In a preferred embodiment, a portion of the hydrogen-rich stream is mixed with the hydrocarbon feedstream. The hydrogen stream may be mixed with hydrocarbons upstream and / or downstream of any hydrodesulfurisation stage.

[0022] In steam reforming, a hydrocarbon feedstock is mixed with steam: the steam introduction may be carried out by direct injection of steam and / or by saturating the feedstock by contacting the feedstock with a hot water stream in a saturator. One or more saturators may be used. If desired, a portion of the hydrocarbon stream may bypass steam addition, such as the saturator. The amount of steam introduced may be such that the steam to carbon ratio is 1 to 3, preferably 1 to 2, i.e. 1 to 2 moles of steam per gram atom of hydrocarbon carbon in the feedstock. The amount of steam is preferably minimized, as this results in a lower cost, more efficient process.

[0023] The hydrocarbon / steam mixture is then advantageously preheated prior to reforming. This can be achieved by using a feed-effluent heat exchanger, wherein the mixture is heated by part of the cooled reformed gas mixture. Advantageously, the mixed stream is heated to 400°C to 500°C.

[0024] A carbon-rich stream is also fed to the reforming unit. This may be conveniently achieved by mixing a hydrocarbon or hydrocarbon and steam mixture with the carbon-rich stream using any known method. Less preferably, the carbon-rich stream may be mixed with the reformed gas mixture fed to the autothermal reformer.

[0025] Then, the resulting feedstock / steam mixture is reformed in two stages in series in a reforming unit, which may be referred to as primary steam reforming and secondary or autothermal reforming. The first stage or primary reforming is carried out using a heat exchange reformer (also referred to as a gas heated reformer (GHR)). In a preferred type of heat exchange reformer, the catalyst is disposed in a tube extending between a pair of tube sheets through a heat exchange region. Reactants are fed to the region above the upper tube sheet and passed through the tube into the region below the lower tube sheet. A heating medium is passed through the region between the two tube sheets. This type of heat exchange reformer is described in GB 1578270, WO 97 / 05947, and US2009 / 0123348.

[0026] The compressed, heated raw material / steam mixture is passed through the catalyst-filled tube in the heat exchange reformer. During the passage through these reforming catalysts, an endothermic reforming reaction occurs, wherein the heat required for the reaction is supplied by the second stage or secondary reforming gas flowing through the outer surface of the tube. The primary reforming catalyst used in the heat exchange reformer is generally nickel supported on a refractory carrier (such as a ring or porous pellet of calcium aluminate cement, alumina, titanium dioxide, zirconium oxide, etc.). Alternatively, a combination of nickel and a noble metal catalyst can be used. For example, a part of the nickel catalyst can be replaced by a noble metal catalyst such as a ruthenium-based catalyst. Alternatively or in addition thereto, a structural steam reforming catalyst can be used, wherein the carrier coating of the steam reforming catalyst is on a structural (such as monolithic) ceramic or metal carrier.

[0027] The temperature of the secondary reformed gas is preferably sufficient to cause the gas undergoing primary reforming to leave the primary reformer at a temperature in the range of 650°C to 850°C, preferably 720°C to 780°C.

[0028] The primary reformate gas produced by the heat exchange reformer, comprising methane, hydrogen, steam and carbon oxides, is fed, preferably without any dilution or heat exchange, to an autothermal or secondary reformer, wherein it is subjected to a second stage or secondary reforming.

[0029] The autothermal reforming device will generally include a burner arranged near the top of the reforming device, and the primary reforming gas and oxygen-containing gas are fed to the burner, and the flame generally extends through the combustion zone below the burner, and is located above the fixed bed of the granular steam reforming catalyst. In autothermal or secondary reforming, the heat of the endothermic steam reforming reaction is provided by the combustion of hydrocarbons and hydrogen in the feed gas. The primary reforming gas is generally fed to the top of the reforming device, and the oxygen-containing gas is fed to the burner, and mixing and combustion occur in the downstream of the burner, thereby producing a heated gas mixture, which reaches equilibrium when passing through the steam reforming catalyst. However, some steam can be added to the oxygen-containing gas, preferably without adding steam, so as to achieve a low total steam ratio for the reforming process. The secondary reforming catalyst is generally nickel supported on a refractory carrier (such as a ring or pellet of calcium aluminate cement, alumina, titanium dioxide, zirconium oxide, etc.). In a preferred embodiment, the secondary reforming catalyst comprises a layer of Ni and / or Rh catalyst supported on zirconium oxide which is more active than conventional Ni supported on alumina catalyst to reduce the volatilization of the catalyst support.

[0030] The oxygen-containing gas preferably contains ≥ 95% by volume of O 2 The oxygen-containing gas may be provided by an air separation unit (ASU) or by another oxygen source.

[0031] The amount of oxygen-containing gas required in the secondary reformer is determined by two main considerations, namely the desired composition of the product gas and the heat balance of the heat exchange reformer. In general, increasing the amount of oxygen, and thereby increasing the temperature of the reformed gas leaving the secondary reformer, results in [H 2 Alternatively, if the conditions are arranged so that the composition and temperature of the products remain constant, increasing the temperature at which the feedstock is fed to the heat exchange reformer will reduce the amount of oxygen required (at a constant oxygen feed temperature).

[0032] The amount of oxygen-containing gas added is preferably such that 40 to 60 moles of oxygen are added per 100 grams of hydrocarbon atoms in the feed to the primary reforming stage. Preferably, the amount of oxygen added is such that the secondary reformed gas leaves the secondary reformer at a temperature in the range of 800°C to 1050°C, more preferably 900°C to 1000°C.

[0033] The secondary reformed gas is then used to provide the heat required for the primary reforming step by using it as a hot gas flowing through the tubes of the heat exchange reformer. During this heat exchange, the secondary reformed gas is cooled by transferring heat to the gas undergoing primary reforming. Preferably, the secondary reformed gas is cooled by a few hundred degrees Celsius, but of course, the secondary reformed gas will leave the heat exchange reformer at a temperature slightly higher than the temperature at which the feedstock / steam mixture is fed to the catalyst-filled tubes in the heat exchange reformer. Preferably, the secondary reformed gas leaves the heat exchange reformer at a temperature in the range of 500°C to 650°C.

[0034] After leaving the heat exchange reformer, the secondary reformed gas is then further cooled in one or more steps of heat exchange. The heat recovered during this cooling can be used for reactant preheating and / or for heating water used to provide steam used in the primary reforming step. In a preferred embodiment, the secondary reformed gas mixture leaving the shell side of the heat exchange reformer is used to preheat the feedstock / steam mixture fed to the tubes in the heat exchange reformer.

[0035] The synthesis gas from the autothermal or secondary reformer is cooled to reduce the temperature below the dew point, causing the steam present in the synthesis gas to condense. The liquid process condensate can be separated from the synthesis gas, which can then be referred to as make-up gas, by conventional gas-liquid separation equipment.

[0036] The make-up gas comprises hydrogen, carbon monoxide, carbon dioxide and small amounts of unreacted methane, argon and nitrogen. The composition of the make-up gas is preferably: 10 mol % to 20 mol % carbon monoxide, 0.5 mol % to 15 mol % carbon dioxide, 55 mol % to 85 mol % hydrogen, and the remainder is one or more inert gases, including methane. The R value of the make-up gas (before adding the hydrogen-rich gas) is preferably 1.95 to 2.05, and Z is preferably 0.95 to 1.05.

[0037] The make-up gas may be compressed to the desired methanol synthesis pressure in a syngas compressor and the make-up gas mixed with the hydrogen-rich gas is then fed into the methanol synthesis loop. Preferably, the syngas compressor is electric.

[0038] Any methanol synthesis loop can be used in the process of the present invention.Methanol synthesis loop includes one or more methanol synthesis reaction units, such as the first methanol synthesis reaction unit, the second methanol synthesis reaction unit and optionally the third methanol synthesis reaction unit, each methanol synthesis reaction unit includes a methanol synthesis catalyst bed, and these methanol synthesis reaction units are arranged in series and / or in parallel to produce a product gas stream comprising methanol.Therefore, the methanol synthesis loop can include one, two or more methanol synthesis reaction units, each methanol synthesis reaction unit includes a methanol synthesis catalyst bed, and each methanol synthesis reaction unit is fed with a feed gas comprising hydrogen and carbon dioxide, and each methanol synthesis reaction unit produces a gas mixture comprising methanol. The product gas mixture comprising methanol is recovered from at least one methanol synthesis reaction unit. Methanol is recovered from one or more product gas mixtures. This can be achieved by cooling one or more methanol product gas streams to below the dew point, condensing methanol, and separating the crude liquid methanol product from the unreacted gas.

[0039] Conventional heat exchange and gas-liquid separation equipment can be used. Particularly suitable heat exchange equipment includes a gas-gas exchange device that uses the feed gas mixture to the methanol synthesis reaction unit to cool the methanol product gas stream from the reaction unit. The methanol product gas streams can be processed separately or can be combined before the crude liquid methanol product is cooled and / or separated.

[0040] The separation of the crude liquid methanol product and one or more methanol product gas streams produces an unreacted gas mixture. A portion of the unreacted gas mixture is returned to one or more methanol synthesis reaction units as a recycle or loop gas stream. The unreacted gas separated from the product gas mixture recovered from a methanol synthesis reaction unit can be returned to the same or different methanol synthesis reaction units. The unreacted gas mixture contains hydrogen, carbon monoxide and carbon dioxide, so it can be used to generate additional methanol. The recycle gas stream can be recovered from at least one methanol product gas stream and recycled to at least one methanol synthesis reaction unit. If there is more than one recycle gas stream, these recycle gas streams can be recycled to one or more methanol synthesis reaction units individually, or merged and fed to one or more methanol synthesis reaction units.

[0041] The methanol synthesis reaction unit in the methanol synthesis loop may be an uncooled adiabatic reaction unit. Alternatively, the methanol synthesis reaction unit may be cooled by heat exchange with synthesis gas, such as in a quenching reaction unit or in a reaction unit selected from a tube-cooled reformer or a gas-cooled reformer. Alternatively, the methanol synthesis reaction unit may be cooled by low-pressure boiling water, such as in an axial-flow steam-lift reformer or a radial-flow steam-lift reformer.

[0042] In an adiabatic reaction unit, synthesis gas can pass through a fixed granular methanol synthesis catalyst bed axially, radially, or axially and radially. Exothermic methanol synthesis reaction occurs, resulting in a temperature increase in the reaction gas. Therefore, it is desirable that the inlet temperature of the bed is lower than the temperature in the cooling reaction unit system to avoid overheating of the catalyst that may be detrimental to selectivity and catalyst life. Alternatively, a cooling reaction unit can be used, wherein heat exchange with a coolant in the reaction unit can be used to minimize or control temperature rise. There are a variety of usable cooling reaction unit types. In one configuration, a fixed granular catalyst bed is cooled by a tube or plate through which a coolant heat exchange medium passes. In another configuration, the catalyst is arranged in a tube around which a coolant heat exchange medium passes. The methanol synthesis reaction unit can be cooled by a feed gas or by conventional low-pressure boiling water. For example, the methanol synthesis reaction unit can be an axial steam riser, a radial steam riser, an air-cooled reformer, or a tube-cooled reformer.

[0043] In an axial flow steam lift reformer (aSRC), the synthesis gas is usually axially passed through a vertical tube containing a catalyst, which is cooled by heat exchange with low-pressure boiling water flowing outside the tube. The catalyst can be provided directly in the tube in pellet form, or can be provided in one or more cylindrical containers that radially and axially guide the synthesis gas flow to improve heat transfer. Such contained catalysts and their use in methanol synthesis are described in US8785506. A steam lift reformer in which the catalyst is present in a tube cooled by low-pressure boiling water provides a particularly useful way to remove heat from the catalyst.

[0044] In a radial steam lift reformer (rSRC), the synthesis gas is usually passed radially (inward or outward) through a bed of particulate catalyst, which is cooled by a plurality of tubes or plates by low pressure boiling water fed as a coolant. Such reaction devices are known and described, for example, in US 4321234. They provide a lower pressure drop than aSRC, but have a more complex internal construction.

[0045] In a tube-cooled reformer, the catalyst bed is cooled by the synthesis gas passing through tubes disposed in the bed, which are open-ended and discharge the heated gas into the space above the catalyst in the reactor housing. The heated gas can then pass directly through the catalyst bed without leaving the reformer. TCC can provide sufficient cooling area for many synthesis gas compositions and can be used under a wide range of conditions. As an alternative to TCC, a gas-cooled reformer (GCC) can be used to cool the catalyst bed by passing the synthesis gas through tubes or plates in a heat exchanger type arrangement. In this case, the heated synthesis gas is withdrawn from the reformer and then returned to the catalyst bed. An example of a GCC is described in US 5827901.

[0046] Alternatively, the methanol synthesis reaction unit may be a quench reaction unit in which one or more fixed particulate methanol synthesis catalyst beds are cooled by a syngas mixture injected into the reaction unit within or between the beds. Such reaction units are described in, for example, US4411877.

[0047] In a process comprising a first methanol synthesis reaction unit and a second methanol synthesis reaction unit, the first methanol synthesis reaction unit is preferably cooled by boiling water such as in an axial flow steam riser or a radial flow steam riser, more preferably in an axial flow steam riser. The second methanol synthesis reaction unit may be a radial flow steam riser. Due to the characteristics and performance of these reaction units with different feed gas mixtures, such an arrangement is particularly useful in the present invention. Alternatively, the second methanol may be cooled by synthesis gas (e.g., a gas comprising hydrogen and carbon dioxide). Therefore, the second methanol synthesis reaction unit may be a cooling reaction unit selected from a tube cooling reformer (TCC) and a gas cooling reformer (TCC). A tube cooling reformer is preferred because its design is simpler. If there is a third methanol synthesis reaction unit, it is preferably cooled by boiling water. Then, the third methanol synthesis reaction unit may be appropriately a steam riser selected from an axial flow steam riser and a radial flow steam riser, most preferably an axial flow steam riser. The first methanol synthesis reaction unit and the second methanol synthesis reaction unit can be connected in series, in which case the synthesis gas fed to the second methanol synthesis reaction unit comprises at least a portion of the methanol product gas stream recovered from the first methanol synthesis reaction unit. In such an arrangement, preferably, the synthesis gas fed to the second methanol synthesis reaction unit comprises the entire methanol product gas stream recovered from the first methanol synthesis reaction unit. Particularly preferred methanol loops are described in US 7790775, WO 2017 / 121980 and WO2017 / 121981.

[0048] The methanol synthesis catalyst in each methanol synthesis reaction unit may be the same or different. The methanol synthesis catalyst is preferably a commercially available copper-containing methanol synthesis catalyst. Specifically, the methanol synthesis catalyst is one or more granular copper / zinc oxide / aluminum oxide catalysts, which may contain one or more promoters. A particularly suitable catalyst is a magnesium-promoted copper / zinc oxide / aluminum oxide catalyst, as described in US4788175.

[0049] Methanol synthesis can be carried out in one or more methanol synthesis reaction units at a pressure in the range of 10 to 120 bar absolute pressure and a temperature in the range of 130° C. to 350° C. The pressure at the inlet of the one or more reaction units is preferably 50 to 100 bar absolute pressure, more preferably 70 to 90 bar absolute pressure. The temperature of the synthesis gas at the inlet of the one or more reaction units is preferably in the range of 200° C. to 250° C., and the temperature of the synthesis gas at the outlet of the one or more reaction units is preferably in the range of 230° C. to 280° C.

[0050] The unreacted gas mixture portion of the recycle gas stream constituting the methanol synthesis loop will generally be at a lower pressure than the make-up gas, so preferably, the recycle gas stream is compressed by one or more compressors or circulation devices. Use at least one compressor to circulate the unreacted gas stream. Preferably, the circulation compressor is electric. The resulting compressed recycle gas stream can be mixed with the make-up gas and the hydrogen-rich stream to form the feed for one or more methanol synthesis reaction units sent into the methanol synthesis loop.

[0051] The recycle ratio for forming the feed gas mixture to the one or more methanol synthesis reaction units may be in the range of 0.5: 1 to 5: 1, preferably 1: 1 to 3: 1. By the term "recycle ratio" we mean the molar flow ratio of the recycled unreacted gas stream to the make-up gas that forms the gas mixture fed to the one or more methanol synthesis reaction units.

[0052] It will be appreciated that by adding a hydrogen-rich gas stream to the make-up gas, the stoichiometric value R will increase, preferably to a value greater than 2. An R value greater than 2 indicates the presence of a stoichiometric excess of hydrogen, and the high conversion of carbon oxides to methanol in modern synthesis loops will result in R values ​​at the inlet of the methanol synthesis reactor in the range of 3 to 5, or even higher. The addition of hydrogen-rich gas that has been recovered from the circulating purge gas means that the methanol synthesis reactor can be operated at its optimal R value of 3 to 5, with the make-up gas having an R value close to the stoichiometric optimum of 2, which will maximize methanol production.

[0053] A part of the unreacted gas mixture separated from the crude liquid methanol is removed from the loop as a purge gas stream. The purge gas stream can be removed continuously or periodically to prevent the undesirable accumulation of inert gases (such as nitrogen, argon and methane) in the synthesis loop. The purge gas stream can be recovered from the separated unreacted gas before or after compression in the circulation device. Especially in the process using steam reforming as a supplementary gas source, the purge gas stream is rich in hydrogen. The purge stream preferably comprises 50-90% by volume of hydrogen and one or more of the following: carbon monoxide, carbon dioxide, nitrogen, argon and methane.

[0054] In the present invention, at least a portion of the purge gas stream is separated into a hydrogen-rich gas stream and a carbon-rich gas stream. Preferably, all purge gas streams are subjected to a separation step. A portion of the hydrogen-rich stream is fed to a methanol synthesis loop, and a portion of the carbon-rich stream is fed to a reformer unit. The separation of the hydrogen-rich and carbon-rich gas streams can be implemented using known separation equipment such as a hydrogen membrane separation device or a pressure swing adsorption unit, a cold box separation system, or any combination of these devices. Using these techniques, more than 50% of the hydrogen present in the purge gas stream can be recovered.

[0055] The hydrogen-rich gas stream recovered from the purge gas stream advantageously contains >95% by volume H 2 In addition to recycling to the methanol loop, the separated hydrogen can also be used upstream of the hydrodesulfurization of the hydrocarbon feedstock and / or for stripping dissolved gases from the crude methanol. However, in a preferred embodiment, at least 90% by volume of the separated hydrogen-rich gas stream is fed to the methanol synthesis loop.

[0056] A portion of the carbon-rich gas stream, which will typically contain carbon oxides and methane, will be fed to the synthesis gas generation step of the reforming unit to form part of the make-up gas. However, it is preferred that a portion of the carbon-rich gas stream is burned as fuel to control the accumulation of inert gases. In the case where a membrane is used to separate the hydrogen-rich stream, the carbon-rich stream will be at a pressure that enables it to be sent out to be used as part of the hydrocarbon feedstock for reforming without further compression. In the case where a pressure swing absorption system is used to separate the hydrogen-rich stream, the carbon-rich stream will be at a low pressure of typically 2 to 5 bar absolute and is therefore less preferred in the present invention.

[0057] If desired, carbon dioxide may be removed from a portion of the carbon-rich stream in a CO2 removal unit. The CO2 removal unit may be any conventional CO2 removal unit that operates by physical absorption, chemical absorption, adsorption into a porous material, or uses a membrane to selectively separate CO2 from the carbon-rich stream to form a methane-rich stream. Although for natural gas feedstocks it may not be necessary to include removal of carbon dioxide from the carbon-rich stream, for feedstocks heavier than natural gas, it may be beneficial to remove carbon dioxide from the carbon-rich stream.

[0058] The purge gas stream mixture may contain methanol, so if desired, upstream of the separation of the hydrogen-rich gas and the carbon-rich gas, a water wash may be used to recover methanol from the purge gas stream, and the recovered methanol and water sent for purification together with the crude methanol.

[0059] The crude methanol stream reclaimed from the methanol production unit comprises water, and a small amount of higher alcohols and other impurities. Crude methanol can first be fed to a flash tower, wherein dissolved gases are released and separated from the liquid crude methanol stream. Liquid crude methanol can undergo one or more purification stages, which include one or more, preferably two or three distillation stages in a methanol purification unit, which includes one, two or more distillation towers. Degassing stage and distillation stage can be heated using heat recovered from technology (e.g., in the cooling of product gas streams) or other sources. Preferably, at least a portion of crude methanol is purified by distillation to produce a purified methanol product.

[0060] The purified methanol product may undergo further processing, for example to produce derivatives such as dimethyl ether or formaldehyde. Alternatively, the methanol may be used as a fuel.

[0061] The present invention will be further described with reference to the accompanying drawings, in which:

[0062] Figure 1 A process according to one embodiment of the invention is depicted.

[0063] It will be appreciated by those skilled in the art that the drawings are illustrative and that other items of equipment may be required in a production plant, such as feedstock barrels, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, liquid level controllers, collection tanks, storage tanks, etc. The provision of such auxiliary equipment does not form part of the present invention and is consistent with conventional chemical engineering practice.

[0064] exist Figure 1 In the embodiment of the present invention, a natural gas stream 10 is mixed with a carbon-rich stream 12 and steam from a pipeline 14, and the resulting mixture is fed to a plurality of externally heated catalyst-filled tubes 18 of a heat exchange reformer 20 via a pipeline 16. Although only three tubes are depicted, there may be 10 or 100 tubes. The hydrocarbons, carbon dioxide, and steam react on the catalyst to form a primary reformed gas mixture comprising hydrogen, carbon dioxide, carbon monoxide, steam, and unreacted methane. The primary reformed gas mixture is fed directly from the heat exchange reformer 20 to an autothermal reformer 24 fed with an oxygen stream 26 via a pipeline 22. In the autothermal reformer, the primary reformed gas mixture is partially combusted with oxygen in a burner mounted near the top, and heat is generated, and the partially combusted gas is balanced by a steam reforming catalyst bed 28 disposed below the burner. The resulting secondary reforming synthesized gas stream is fed from the autothermal reformer 24 directly to the shell side of the heat exchange reformer 20 via line 30 where it heats the tubes 18 and is thereby partially cooled.

[0065] The partially cooled synthesis gas is fed from the heat exchange reformer 20 via line 32 to a heat recovery unit 34 comprising one or more heat exchangers where the synthesis gas is further cooled to below the dew point to condense the steam. A gas-liquid separation device is used in the heat recovery unit to remove process condensate from the cooled gas mixture to produce make-up gas. Make-up gas is recovered from the heat recovery unit via line 36, combined with the hydrogen-rich gas stream fed via line 38, compressed in a synthesis gas compressor 40, and fed from compressor 40 to a methanol synthesis unit 44 via line 42.

[0066] The methanol synthesis unit comprises a methanol synthesis loop, wherein a compressed mixture of make-up gas and hydrogen-rich gas is mixed with a recycle stream of unreacted gas comprising hydrogen, carbon dioxide and carbon monoxide, and fed to one, two or more methanol synthesis reaction units, each of which comprises a methanol synthesis catalyst, and these methanol synthesis reaction units are operated in series or in parallel to generate a product gas stream comprising methanol. The product gas stream is cooled to condense liquid crude methanol and separate the liquid crude methanol from the unreacted gas, and a portion of the unreacted gas is compressed and recycled to the first, second or additional methanol synthesis reaction unit in the circulation unit. Liquid crude methanol is recovered from the methanol synthesis unit 44 and fed to the methanol purification unit 48 via pipeline 46, in which the liquid crude methanol undergoes degassing and one, two or three distillation stages to produce a purified methanol product, which is recovered from the purification unit 48 via pipeline 50.

[0067] A portion of the unreacted gas is withdrawn from the methanol synthesis unit 44 upstream of the recycle device and fed as a purge gas stream from the methanol synthesis unit 44 via line 52 to a hydrogen separation unit 54, where the purge gas stream is separated into a hydrogen-rich stream and a carbon-rich stream by passing the purge gas stream through a membrane. The carbon-rich stream is recovered via line 56, a portion is withdrawn via line 58 to be used as a fuel gas in, for example, a fired heater, and the remainder is fed to the hydrocarbon feed line 10 via line 12. The hydrogen-rich gas stream is recovered from the separation unit 54 via line 38 and mixed with make-up gas in line 36 to form an enriched feed gas. The enriched feed gas is fed to the suction port or interstage of the synthesis gas compressor 40 to form a compressed enriched feed gas for the methanol synthesis unit 44.

[0068] The invention will be further described by reference to the following calculated examples made using conventional modelling software suitable for the methanol process. These examples all produce 5000 tonnes / day of refined methanol product and in each case the oxygen flow has been calculated to satisfy the heat recovery balance around the heat exchange reformer.

[0069] Example 1

[0070] Example 1 is based on Figure 1 The carbon-rich gas in this example is the retentate from the membrane separation unit, which is fed together with the purge gas from the methanol synthesis loop. 75% of the carbon-rich gas retentate is recycled as feedstock. The remaining 25% of the retentate is used as fuel for the fired heater, supplemented with natural gas.

[0071] Example 2

[0072] Example 2 is the same as Example 1, but CO 2 has been removed from the retentate recycle to show CO 2 Removed influence.

[0073] Comparative Example 3

[0074] Example 3 is a comparative example in which no carbon-rich gas is recycled as feedstock to the heat exchange reformer.

[0075] Example 1 Example 2 Comparative Example 3 Recirculation of carbon-rich gas have have none <![CDATA[CO 2 Removed from carbon-rich recycle]]> none have - Relative to the natural gas flow of Example 1 100.0％ 100.0％ 101.8％ Relative to the oxygen flowing to the ATR in Example 1 100.0％ 100.1％ 100.3％ R value of reformed gas 2.005 2.024 2.034 R-value of make-up gas (including hydrogen-rich recycle gas) 2.062 2.130 2.158

[0076] CO in carbon-rich recycle 2 The amount of CO2 removal from the carbon-rich recycle gas is relatively low (about 12 kmol / h for a 5000 t / d plant). As shown when comparing Example 1 and Example 2, for the heat exchange reactor-based flow chart used in the examples, it is not necessary to include the removal of CO2 from the carbon-rich recycle gas. 2 However, for feedstocks heavier than the light natural gas used in the examples, removal of CO from the carbon-rich recycle 2 Might be beneficial.

[0077] Examples 1 and 2 have only 25% of the retentate used as fuel, which is reduced to one-quarter the fuel stream of Comparative Example 3. Reduction to one-quarter the fuel stream is easily achieved without excessive accumulation of inert gases such as nitrogen and argon in the methanol synthesis loop. Since the combustion value of the carbon-rich stream used as fuel is usually only slightly excessive, the reduction to one-quarter the fuel stream demonstrates the beneficial effects of the present invention.

Claims

1. A process for synthesizing methanol, the process comprising the following steps: (i) forming a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide from a hydrocarbon feedstock in a reforming unit, the reforming unit comprising a heat exchange reformer and an autothermal reformer connected in series; (ii) cooling the synthesis gas in the heat exchange reformer and one or more further heat exchange stages, and recovering process condensate from the cooled synthesis gas to form a make-up gas; (iii) passing a feed gas comprising the make-up gas into a methanol synthesis loop comprising one or more methanol synthesis reaction units; (iv) recovering a product gas mixture comprising methanol from the methanol synthesis loop, cooling the product gas mixture to below the dew point to condense crude methanol, and recovering the crude methanol; (v) recycling a portion of the unreacted gas mixture to the methanol synthesis loop and recovering a portion of the unreacted gas mixture as a purge gas stream, wherein at least a portion of the purge gas stream is separated into a hydrogen-rich gas stream and a carbon-rich gas stream, a portion of the hydrogen-rich gas stream is fed to the methanol synthesis loop, and a portion of the carbon-rich gas stream is fed to the reformer unit, wherein before adding the hydrogen-rich gas stream, the stoichiometric value R of the make-up gas is in the range of 1.95 to 2.05, and after adding the hydrogen-rich gas stream, the stoichiometric value R is higher.

2. The process of claim 1, wherein the hydrocarbon feedstock is natural gas.

3. A process according to claim 2, wherein a saturator is used to provide steam for reforming the natural gas.

4. The process according to claim 1, wherein the heat exchange reformer comprises a steam reforming catalyst disposed in tubes, the tubes extending through a heat exchange zone formed between an upper tube sheet and a lower tube sheet, the synthesis gas being fed as a heat exchange medium to the heat exchange zone.

5. A process according to claim 1, wherein the autothermal reformer includes a burner arranged near the top of the reformer, the reformed gas and oxygen-containing gas from the heat exchange reformer are fed to the burner, and the flame extends through a combustion zone located below the burner and above the fixed bed of particulate steam reforming catalyst.

6. The process according to claim 5, wherein the oxygen-containing gas comprises ≥ 95% by volume of O2.

7. The process of claim 1 wherein the makeup gas has a composition of 10 to 20 mol% carbon monoxide, 0.5 to 15 mol% carbon dioxide, 55 to 85 mol% hydrogen, and the remainder being one or more inert gases, including methane.

8. The process of claim 1 wherein the make-up gas is compressed to a desired loop pressure in a syngas compressor and the syngas compressor is electric.

9. The process according to claim 1, wherein the methanol synthesis loop comprises one, two or more methanol synthesis reaction units, each methanol synthesis reaction unit comprises a methanol synthesis catalyst bed, and wherein the product gas mixture is recovered from at least one methanol synthesis reaction unit.

10. The process according to claim 9, wherein the unreacted gas mixture separated from the product gas mixture recovered from one methanol synthesis reaction unit is returned to the same or a different methanol synthesis reaction unit.

11. The process of claim 9, wherein the unreacted gas is compressed by one or more recycle compressors, and the recycle compressors are electric.

12. The process according to claim 9, wherein the methanol synthesis reaction unit is cooled by synthesis gas or by boiling water.

13. The process according to claim 9, wherein methanol synthesis is carried out in the one or more methanol synthesis reaction units at a pressure in the range of 10 to 120 bar absolute and a temperature in the range of 130°C to 350°C.

14. The process according to claim 1, wherein the separation of the hydrogen-rich gas stream and the carbon-rich gas stream is achieved using a hydrogen membrane separation device or a pressure swing adsorption unit, or a cold box separation system, or any combination of these devices.

15. The process of claim 1, wherein the crude methanol is subjected to one or more distillation steps to produce a purified methanol product.

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