METHOD FOR THE PRODUCTION OF METHANOL FROM SYNTHESIS GAS WITH A HIGH PROPORTION OF INERTIC GAS COMPONENTS

Description

Technical field of the invention

[0001] The invention relates to a process for producing methanol from synthesis gas, in particular from synthesis gas with a high proportion of inert gas components. State of the art

[0002] Methanol is produced on an industrial scale from synthesis gas, a mixture of carbon oxides and hydrogen. Synthesis gas is typically generated from fossil feed gases, such as natural gas, through processes like steam reforming (SMR) or autothermal reforming (ATR). These are established industrial processes that produce a synthesis gas mixture consisting almost entirely of carbon monoxide, carbon dioxide, and hydrogen. Inert components such as methane or nitrogen make up only a minor proportion of the synthesis gas mixture.

[0003] Methanol can be produced from feed gases rich in carbon dioxide, using high recycling rates, or, in multi-stage syntheses with intermediate condensation of the crude methanol, even from virtually carbon monoxide-free feed gases. In the latter case, the actual synthesis gas converted to methanol is therefore a mixture of carbon dioxide and hydrogen.

[0004] To reduce industrially produced carbon dioxide emissions, carbon dioxide can either be captured and sequestered (carbon capture and storage, CCS) or, after capture, used for further processing (carbon capture and utilization, CCU). In the case of CCU, methanol synthesis using carbon dioxide-rich feed gases, such as industrial exhaust gases, is a viable option. At the very least, such feed gases can be blended with "conventional" synthesis gas, as they are not always directly suitable for methanol synthesis, even after the addition of hydrogen.

[0005] The aforementioned feed gases, however, typically contain a high proportion of inert components, more precisely, a high proportion of gas components that are inert under methanol synthesis conditions, meaning they do not react and therefore do not contribute to product formation. This poses a problem when using high recycling rates in conventional single-stage methanol syntheses, as inert components accumulate at the reactor inlet, significantly increasing the reactor load, causing a high pressure drop across the reactor, and reducing the carbon conversion yield. This also increases the load on the synthesis gas and recycled gas compressors, leading to higher operating costs (opex). Furthermore, the pipeline cross-sections must be correspondingly larger, increasing capital expenditures (capex).

[0006] The aforementioned problems also partially apply to multi-stage methanol syntheses with intermediate condensation of the crude methanol. These are typically designed so that almost all the carbon in the synthesis gas is converted to methanol in a single pass through all reactor stages. Therefore, recycling the unreacted synthesis gas is often unnecessary.

[0007] In order to utilize feed gases with high proportions of inert components for methanol synthesis, a purification of the feed gas, i.e., a complex separation of the inert components, is usually necessary for the aforementioned reasons before they can be made available for synthesis.

[0008] GB 2 142 331 A discloses the production of methanol in which a purge gas is extracted from the methanol synthesis cycle, from which hydrogen is separated using a pressure swing adsorption device. The hydrogen stream thus obtained is adiabatically depressurized via an expander, and the energy thus obtained is used to drive an adiabatic compressor.

[0009] WO 2017 / 021245 A1 discloses a process for the chemical synthesis of methanol from compressed gaseous reactants. Energy is extracted from the resulting product mixture by reducing the pressure and is used to compress the reactants. This is preferably done using gaseous components of the product mixture.

[0010] WO 2021 / 043560 A1 discloses the catalytic conversion of a synthesis gas stream to a product stream containing alkanes, alkenes, and alcohols. The product stream, under increased pressure, is expanded in a turbine, generating electrical energy to meet the process's power requirements. Description of the invention

[0011] In many cases, it must be assumed that the complex separation of inert gas components from the respective feedstock is not economically viable. Therefore, an alternative approach must be found to enable technically and economically feasible methanol production even with a high proportion of inert components in the feedstock gas.

[0012] A key role is played by the synthesis gas compressor, that is, the compressor that compresses the synthesis gas mixture to the synthesis pressure required for methanol synthesis. This is usually a machine that requires a significant amount of electrical energy to operate.

[0013] The aim should be to reduce or maintain the amount of externally generated electrical energy required for the synthesis gas compressor when operating with a feed gas containing a high proportion of inert components, compared to operating the synthesis gas compressor with a feed gas containing no inert components. Such a result can be achieved, for example, through improved process integration, by utilizing energy generated within the process in such a way as to reduce the need for externally generated energy.

[0014] EP 2 228 357 A1 discloses a process for the production of methanol in which a purge gas stream is depressurized using an expander. The mechanical work generated in this process is used internally, for example to drive a recycled gas compressor.

[0015] The disadvantage is that the purge gas stream within a methanol synthesis with recirculation of the unreacted synthesis gas represents only a relatively small partial volume flow compared to the total volume flow of the residual gas not reacted in the reactor, and the energy recovery is accordingly limited to this partial volume flow.

[0016] When using feed gases with a high proportion of inert gas components, it would be desirable to utilize as large a proportion of the gas stream as possible for in-process energy recovery, or to generate additional opportunities for in-process energy recovery. In any case, the goal is to minimize the energy input for the synthesis gas compressor.

[0017] One object of the present invention is therefore to overcome at least some of the aforementioned disadvantages of the prior art.

[0018] In particular, one object of the present invention is to expand or improve the possibilities of using energy within the process.

[0019] Furthermore, an object of the present invention is to further improve the process integration of methanol production with regard to external energy requirements, so that methanol production can also be realized in a technically and economically viable way with feed gases which have a high proportion of inert components, without the need for prior separation of the inert components from the respective feed gas.

[0020] The independent claims contribute to at least partially fulfilling at least one of the above objectives. The dependent claims provide preferred embodiments that contribute to at least partially fulfilling at least one of the objectives. Preferred embodiments of components of one category according to the invention are, where applicable, also preferred for identically named or corresponding components of another category according to the invention.

[0021] The terms "indicating," "comprehensive," or "containing," etc., do not preclude the possibility of the presence of further elements, ingredients, etc. The indefinite article "a" does not preclude the possibility of a plurality.

[0022] According to a first aspect of the invention, a process for the production of methanol is proposed, comprising the process steps a) Providing a synthesis gas stream comprising at least a carbon oxide, hydrogen, and an inert gas component; b) Compressing the synthesis gas stream to synthesis pressure in a synthesis gas compression device; c) Converting the compressed synthesis gas stream to a product stream on a methanol synthesis catalyst, wherein the product stream comprises at least methanol, water, unreacted synthesis gas, and the inert gas component, wherein the conversion of the compressed synthesis gas stream takes place in a reactor arrangement, the reactor arrangement comprising at least one reactor stage and a phase separation device; d) Separating the product stream in the phase separation device into a liquid crude methanol stream comprising at least methanol and water, and a residual gas stream comprising at least unreacted synthesis gas and the inert gas component;e) Reducing the pressure of at least a portion of the crude methanol stream in a liquid expansion device to a pressure lower than the synthesis pressure, the liquid expansion device performing mechanical work; f) Transferring at least a portion of the mechanical work performed by the liquid expansion device to the synthesis gas compression device to drive the synthesis gas compression device, or converting at least a portion of the mechanical work performed by the liquid expansion device into electrical energy and using the electrical energy to drive the synthesis gas compression device.

[0023] One reactor stage of the reactor assembly contains the methanol synthesis catalyst. This can be any catalyst suitable for methanol synthesis known to those skilled in the art, for example, a copper-based catalyst. The catalyst can be configured, for example, as a fixed bed in the form of pellets or as a structured packing.

[0024] A phase separation device is configured to separate the product stream from the residual gas stream. For example, the phase separation device includes a cooling stage in the form of a heat exchanger for condensing the product stream, and a separation stage for separating the condensed (liquid) phase from the gas phase.

[0025] According to the invention, at least a portion of the crude methanol stream is expanded to a pressure in a liquid expansion device, and the mechanical work performed by the expansion device is utilized within the process. This utilization can be direct, by mechanically driving the synthesis gas compression device, or indirect, by first converting the mechanical work performed by the expansion device into electrical energy. This electrical energy is then used to drive the synthesis gas compression device.

[0026] Optionally, the residual gas stream can be recompressed to synthesis pressure in a residual gas compression device and combined with the compressed residual gas stream. According to this optional embodiment, the residual gas stream is thus returned to the inlet of the reactor stage and reacted with the compressed synthesis gas to form the crude methanol stream. Optionally, the mechanical work performed by the liquid expansion device can also be used to drive the residual gas compression device, or the mechanical work performed by the liquid expansion device can be converted into electrical energy, and the electrical energy used to drive the residual gas compression device.

[0027] The liquid expansion device is, for example, an expansion machine that operates according to the positive displacement principle (e.g., piston expansion machine) or the fluid flow principle (e.g., turbo expansion machine, expansion turbine). The selection of the liquid expansion device depends on the volumetric flow rate of the crude methanol stream to be expanded and the prevailing pressures.

[0028] The expansion pressure is preferably lower than the synthesis pressure by a factor of at least 5, or at least 10, or at least 15, or at least 20, or at least 25, or at least 50. For example, the synthesis pressure is in the range of 60 bar to 90 bar, and the expansion pressure is in the range of atmospheric pressure (approximately 1 bar absolute) to 5 bar.

[0029] If the work performed by the liquid expansion device is first converted into electrical energy, this can be done, for example, by a generator. The resulting electric current can then, for example, drive an electric motor, which in turn drives the synthesis gas compression device.

[0030] The synthesis gas compression device consists of one or more compressors connected in parallel and / or series, which in turn operate either according to the displacement principle (for example, piston compressor) or according to the flow principle (for example, turbo compressor).

[0031] The synthesis gas stream contains at least one carbon oxide, hydrogen, and one inert gas component. "Carbon oxide" refers to carbon monoxide (CO) or carbon dioxide (CO₂). The inert gas component can be a single gas (for example, exclusively nitrogen) or a mixture of gases (for example, nitrogen, argon, and methane). "Inert" means that the gas component does not react under the conditions of methanol synthesis; that is, it is neither converted into the product nor into an undesired byproduct.

[0032] According to one embodiment, particularly in a methanol synthesis with only one or with two reactor stages, at least a portion of the residual gas stream is recycled to the inlet of one or more of the reactor stages. A purge stream can be diverted from a portion of this recycled stream to prevent the accumulation of inert components in the synthesis loop. The purge stream can be used, with the aid of a gas expansion device as known from EP 2 228 357 A1, to perform mechanical work and thus generate energy within the process.

[0033] One embodiment of the method is characterized in that the reactor arrangement comprises a plurality n of reactor stages arranged in series and a plurality of p phase separation devices, wherein each of the reactor stages is assigned a phase separation device, wherein the phase separation device is arranged downstream of the assigned reactor stage, and in each of the phase separation devices a liquid crude methanol partial stream and a residual gas partial stream are generated, and wherein the residual gas partial stream generated in a phase separation device is at least partially introduced into the reactor stage subsequently arranged, and wherein the residual gas partial stream generated in the last phase separation device is discharged from the reactor arrangement.

[0034] According to this embodiment, the reactor arrangement comprises at least two reactor stages and at least two phase separation devices. Each reactor stage is assigned a phase separation device, which is arranged downstream of the respective reactor stage. The product stream discharged from a reactor stage is separated in the downstream phase separation device into a crude methanol stream and a residual gas stream. The crude methanol stream is discharged from the reactor arrangement as a crude product stream and subjected to further processing. The residual gas stream withdrawn from a phase separation device is preferably fed entirely into the next reactor stage in series. Alternatively, only a portion can be fed into the next reactor stage, and the remainder is used for another purpose.

[0035] The last of the phase separation stages in series is an exception. The residual gas stream extracted from this last phase separation stage is removed from the reactor assembly and reused.

[0036] The configuration of the reactor stages and phase separation devices within the reactor arrangement according to this embodiment can also be referred to as a multi-stage reactor arrangement with intermediate condensation.

[0037] One embodiment of the method is characterized in that the method includes the following further process steps: g) Expanding at least a part of the residual gas stream discharged from the reactor arrangement in a gas expansion device to an expansion pressure lower than the synthesis pressure, the gas expansion device performing mechanical work; h) Transferring at least a part of the mechanical work performed by the gas expansion device to the synthesis gas compression device to drive the synthesis gas compression device, or converting at least a part of the mechanical work performed by the gas expansion device into electrical energy, and using the electrical energy to drive the synthesis gas compression device.

[0038] This embodiment relates to the configuration of the reactor arrangement as a multi-stage reactor arrangement. Advantageously, according to this embodiment, the residual gas partial flow discharged from the reactor arrangement is also used to perform mechanical work by expanding this residual gas partial flow to an expansion pressure in a gas expansion device.

[0039] When using a number of reactor stages n that results in complete or near-complete carbon conversion across all stages, the residual gas stream discharged from the last phase separation stage contains practically no valuable gases, but consists primarily of the gases of the inert gas component. Therefore, it is particularly advantageous to utilize this residual gas stream for the in-process generation of mechanical or electrical energy to drive the synthesis gas compressor. This is especially beneficial when using a synthesis gas stream containing an inert gas component with a proportion of at least 1% by volume.

[0040] In contrast to the method known from EP 2 228 357 A1, a portion is not diverted from the residual gas stream as a purge stream, but the entire residual gas stream can be used for the internal generation of mechanical or electrical energy.

[0041] The expansion pressure of the expanded residual gas partial stream is preferably lower than the synthesis pressure by a factor of at least 5, or at least 10, or at least 15, or at least 20, or at least 25, or at least 50. For example, the synthesis pressure is in the range of 60 bar to 90 bar, and the expansion pressure is in the range of atmospheric pressure (approximately 1 bar absolute) to 5 bar.

[0042] The gas expansion device is, for example, an expansion machine that operates according to the positive displacement principle (e.g., piston expansion machine) or the fluid flow principle (e.g., turbo expansion machine, expansion turbine). The selection of the gas expansion device depends on the volumetric flow rate of the crude methanol stream to be expanded and the prevailing pressures.

[0043] Depending on the composition of the residual gas partial stream, it may be advantageous to feed the depressurized residual gas partial stream to a hydrogen recovery device and to return the recovered hydrogen to methanol synthesis.

[0044] One embodiment of the method is characterized in that the entire residual gas partial stream discharged from the reactor arrangement is depressurized in the gas expansion device.

[0045] The use of the entire residual gas stream discharged from the reactor arrangement for the internal generation of mechanical or electrical energy is particularly useful when the number n of reactor stages is so high that a complete or substantially complete carbon conversion is achieved across all reactor stages n of the reactor arrangement.

[0046] Alternatively, it is also possible that a portion of the residual gas partial stream discharged from the reactor arrangement is depressurized in the gas expansion device, and a further portion of the residual gas partial stream discharged from the reactor arrangement is compressed to synthesis pressure in a residual gas compression device, and the compressed residual gas partial stream is returned to at least one of the majority of reactor stages arranged in series, preferably to the first of the majority of reactor stages arranged in series.

[0047] This is particularly advantageous if the residual gas stream discharged from the last phase separation device still contains a usable proportion of carbon oxides and hydrogen. In this case, only a portion of the residual gas stream is used for the internal generation of mechanical or electrical energy; the other portion is recompressed to synthesis pressure and preferably returned to the first of the multiple reactor stages arranged in series.

[0048] The mechanical or electrical energy gained internally through the expansion of the residual gas partial flow can optionally also be used to drive the residual gas compression device.

[0049] According to one embodiment, a portion of the residual gas partial flow, which has been depressurized to relaxation pressure, is recompressed to synthesis pressure in the synthesis gas compression device, and the compressed residual gas partial flow is fed to the reactor arrangement.

[0050] If the residual gas stream has a suitable composition, it is advantageous to return it to the synthesis gas compression device after pressure reduction. Here, the residual gas stream is compressed together with the synthesis gas stream to synthesis pressure and then fed to the reactor assembly.

[0051] One embodiment of the process is characterized in that the proportion of the inert gas component in the synthesis gas stream is at least 1 vol.%, or at least 5 vol.%, or at least 10 vol.%, or at least 20 vol.%.

[0052] In particular, the proportion of the inert gas component in the synthesis gas stream is between 0.5 vol% and 30 vol%, or between 1 vol% and 20 vol%, or between 1 vol% and 10 vol%.

[0053] One embodiment of the method is characterized in that the inert gas component comprises nitrogen and / or methane.

[0054] One embodiment of the method is characterized in that the synthesis pressure is at least 70 bar, or at least 75 bar, or at least 80 bar, or at least 85 bar.

[0055] One embodiment of the method is characterized in that the crude methanol stream in the liquid expansion device is expanded to a pressure of 1 bar to 5 bar, preferably to a pressure of 1 bar to 3 bar. According to a preferred embodiment, the crude methanol stream in the liquid expansion device is expanded to a pressure corresponding to atmospheric pressure, i.e., approximately 1 bar (absolute).

[0056] A particularly preferred embodiment of the method is characterized in that the crude methanol stream is fed to a downstream thermal separation device for separating the crude methanol into methanol and water, wherein the thermal separation device is operated at a predetermined pressure, and the crude methanol stream is depressurized in the liquid expansion device to a pressure which corresponds to the predetermined pressure in the thermal separation device.

[0057] In particular, the crude methanol stream is depressurized in the liquid expansion device to a pressure that essentially corresponds to the predetermined pressure in the thermal separation device.

[0058] This measure allows the pressure-reduced crude methanol stream to be fed directly into the downstream thermal separation device.

[0059] According to one embodiment, a low-pressure phase separation device is connected upstream of the thermal separation device. In this configuration, the phase separation device immediately downstream of the reactor stage is configured as a high-pressure phase separation device, or the phase separation devices downstream of the reactor stages are configured as high-pressure phase separation devices. The liquid expansion device is then arranged between the high-pressure phase separation device and the low-pressure phase separation device, or between the high-pressure phase separation devices and the low-pressure phase separation device.

[0060] According to one example, the thermal separation device is a rectification column, or an arrangement of several rectification columns.

[0061] One embodiment of the method is characterized in that the residual gas partial flow in the liquid expansion device is expanded to a pressure of 1 bar to 5 bar, preferably to a pressure of 1 bar to 3 bar. According to a preferred embodiment, the residual gas partial flow in the liquid expansion device is expanded to a pressure corresponding to atmospheric pressure, i.e., approximately 1 bar (absolute).

[0062] One embodiment of the method is characterized in that the following applies to the plurality n of reactor stages arranged in series and the plurality p of phase separation devices. n = 3 to 6, preferably n = 4, and p = 3 to 6, preferably p = 4, and where n = p.

[0063] According to a second aspect of the invention, a process for the production of methanol is proposed, comprising the process steps a) Providing a synthesis gas stream comprising at least a carbon oxide, hydrogen, and an inert gas component; b) Compressing the synthesis gas stream to synthesis pressure in a synthesis gas compression device; c) Converting the compressed synthesis gas stream to a product stream on a methanol synthesis catalyst, wherein the product stream comprises at least methanol, water, unreacted synthesis gas, and the inert gas component, wherein the conversion of the compressed synthesis gas stream takes place in a reactor arrangement, the reactor arrangement comprising at least one reactor stage and a phase separation device; d) Separating the product stream in the phase separation device into a liquid crude methanol stream comprising at least methanol and water, and a residual gas stream.comprising at least unreacted synthesis gas and the inert gas component; wherein the reactor arrangement comprises a plurality n of reactor stages arranged in series and a plurality of p phase separation devices, wherein each of the reactor stages is assigned a phase separation device, wherein the phase separation device is arranged downstream of the assigned reactor stage, and in each of the phase separation devices a liquid crude methanol partial stream and a residual gas partial stream are generated, and wherein the residual gas partial stream generated in a phase separation device is at least partially fed into the reactor stage subsequently arranged, and wherein the residual gas partial stream generated in the last phase separation device is discharged from the reactor arrangement,and further comprising the process steps e) depressurizing at least a portion of the residual gas partial stream discharged from the reactor arrangement in a gas depressurization device to a depressurization pressure which is lower than the synthesis pressure, wherein the gas depressurization device performs mechanical work; f) transferring at least a portion of the mechanical work performed by the gas depressurization device to the synthesis gas compression device, for driving the synthesis gas compression device, or converting at least a portion of the mechanical work performed by the gas depressurization device into electrical energy, and using the electrical energy to drive the synthesis gas compression device.

[0064] Advantageously, according to the second aspect of the method, the residual gas partial stream discharged from the reactor arrangement is used to perform mechanical work by expanding this residual gas partial stream to an expansion pressure in a gas expansion device.

[0065] When using a number of reactor stages n that results in complete or near-complete carbon conversion across all stages, the residual gas stream discharged from the last phase separation stage contains practically no valuable gases, but consists primarily of the gases of the inert gas component. Therefore, it is particularly advantageous to utilize this residual gas stream for the in-process generation of mechanical or electrical energy to drive the synthesis gas compressor. This is especially beneficial when using a synthesis gas stream containing an inert gas component with a proportion of at least 1% by volume.

[0066] In contrast to the method known from EP 2 228 357 A1, a portion is not diverted from the residual gas stream as a purge stream, but the entire residual gas stream can be used for the internal generation of mechanical or electrical energy.

[0067] According to one embodiment of the second aspect of the method, the entire residual gas partial stream discharged from the reactor arrangement is decompressed in the gas expansion device.

[0068] According to one embodiment of the second aspect of the method, a portion of the residual gas partial stream discharged from the reactor arrangement is expanded in the gas expansion device, and another portion of the residual gas partial stream discharged from the reactor arrangement is compressed to synthesis pressure in a residual gas compression device, and the compressed residual gas partial stream is returned to at least one of the plurality of reactor stages arranged in series, preferably to the first of the plurality of reactor stages arranged in series.

[0069] According to one embodiment of the second aspect of the method, a portion of the residual gas partial flow, which has been depressurized to expansion pressure, is recompressed to synthesis pressure in the synthesis gas compression device, and the compressed residual gas partial flow is fed to the reactor arrangement.

[0070] According to one embodiment of the second aspect of the process, the proportion of the inert gas component in the synthesis gas stream is at least 1 vol.%, or at least 5 vol.%, or at least 10 vol.%, or at least 20 vol.%.

[0071] According to one embodiment of the second aspect of the method, the inert gas component contains nitrogen and / or methane.

[0072] According to one embodiment of the second aspect of the method, the synthesis pressure is at least 70 bar, or at least 75 bar, or at least 80 bar, or at least 85 bar.

[0073] According to one embodiment of the second aspect of the method, the residual gas partial flow is expanded in the gas expansion device to a pressure of 1 bar to 3 bar.

[0074] According to one embodiment of the second aspect of the method, the following applies to the plurality n of reactor stages arranged in series and the plurality p of phase separation devices. n = 3 to 6, preferably n = 4, as well as p = 3 to 6, preferably p = 4, and n = p. Example of implementation

[0075] The invention is explained in more detail below by means of exemplary embodiments. The following detailed description refers to the accompanying drawings, which form part of the exemplary embodiments and in which specific embodiments of the invention are illustrated.

[0076] In the following description and in the drawings, identical elements are designated with the same reference symbols. Gas flows are represented as dashed lines in the figures, while liquid flows or two-phase flows are represented as solid lines. The flow direction of the respective flows is indicated by arrows.

[0077] They show Figure 1 shows a process 1 according to a first example of the invention with a one-stage methanol synthesis, Figure 2 shows a process 2 according to a second example of the invention with a four-stage methanol synthesis, Figure 3 shows a process 3 according to a third example of the invention with a four-stage methanol synthesis, and Figure 4 shows a process 4 according to a fourth example of the invention with a four-stage methanol synthesis.

[0078] Figure 1 Figure 1 shows a process 1 according to a first example of the invention, in which the synthesis of methanol takes place in a single-stage reactor arrangement.

[0079] A synthesis gas stream 10, containing hydrogen, carbon monoxide, carbon dioxide, and nitrogen and methane as an inert gas component, is supplied and compressed in a synthesis gas compression device 12, here a synthesis gas compressor, to a synthesis pressure suitable for methanol synthesis. The synthesis pressure can be 90 bar, as shown in one example. A resulting compressed synthesis gas stream 11 is fed into a reactor stage 13, in which the compressed synthesis gas stream 11 is converted to a product stream 14, initially gaseous and, after cooling, two-phase. This product stream contains at least methanol, water, unreacted synthesis gas (hydrogen, carbon monoxide, and carbon dioxide), and the inert gas component. The reactor stage 13 contains a suitable copper-based methanol synthesis catalyst (not shown).Since the reaction of methanol formation from synthesis gas is exothermic, reactor stage 13 is cooled using boiling boiler feedwater. The steam generated in this process can be exported or used for an upstream process, such as a steam reformer. Reactor stage 13 forms part of a reactor assembly that includes at least reactor stage 13 and a phase separation device 15 downstream of the reactor stage. The phase separation device has at least one heat exchanger for cooling and condensing the initially gaseous crude methanol into the product stream 14, and a separator downstream of the heat exchanger for separating the liquid crude methanol phase from the gaseous phase. The heat exchanger and the separator are not assigned dedicated reference numerals.

[0080] A liquid stream of crude methanol 16, containing methanol, water, and any condensed byproducts of methanol synthesis, is drawn off from the phase separation device 15. The separator of the phase separation device 15 is a high-pressure separator, so the crude methanol stream 16 has a pressure that differs from the synthesis pressure mainly due to unavoidable pressure losses across the reactor assembly. The pressure of the crude methanol stream 16 is sufficiently high to be used for the in-process generation of mechanical and / or electrical energy. For this purpose, the crude methanol stream 16 is fed to a liquid expansion device 19, which could, for example, be an expansion turbine. In the liquid expansion device 19, the crude methanol stream 16 is expanded to a pressure of approximately 2 bar.This pressure corresponds to the pressure prevailing in a downstream thermal separation unit for the production of pure methanol (not shown). The depressurized crude methanol stream 18 is first fed to a further separator, in this case a low-pressure separator 30. There, a further separation of gases takes place, which, at higher pressures in the high-pressure separator, remain absorbed in the liquid phase of the crude methanol. The gases desorbed in the low-pressure separator are discharged from the process as exhaust gas stream 31. The crude methanol stream 29, now further free of gases, is fed to a subsequent distillation unit for the thermal separation of the crude methanol into methanol, water, and byproducts (not shown).

[0081] Mechanical work is performed in the liquid expansion device 19, which is converted into electrical energy by means of a generator (not shown). For this purpose, the liquid expansion device 19 is operatively connected to a motor 24, which in turn is operatively connected to the synthesis gas compression device 12, which is driven by the motor. According to this embodiment, the liquid expansion device is indirect operative connection with the synthesis gas compression device 12. Alternatively, the liquid expansion device 19 can also be in direct operative connection with the synthesis gas compression device 12, for example via a direct mechanical coupling.

[0082] A residual gas stream 17 is drawn off from the high-pressure separator of the phase separation device 15 as the gaseous phase of the separation. This residual gas stream consists mainly of unreacted synthesis gas (hydrogen, carbon monoxide, carbon dioxide) and the inert gas component (nitrogen, methane). This residual gas stream 17 is recompressed to synthesis pressure using a residual gas compression device 27, in this case a recycled gas compressor, and combined with the compressed synthesis gas stream. Since continuous recirculation would cause inert components to concentrate in the synthesis gas stream supplied to reactor stage 13, a purge gas stream 20 is continuously diverted from the residual gas stream 17. This purge gas stream 20 is fed to a gas expansion device 22, in this case a gas expander, where it is expanded to atmospheric pressure.The depressurized purge gas stream 21 is either sent to the flare (not shown) or, if it has a sufficiently high content of valuable gases, fed to a hydrogen recovery plant (not shown). The gas depressurization device 19, as described above for the liquid depressurization device 21, is also in direct or indirect operative connection with the synthesis gas compression device 12.

[0083] The following numerical example demonstrates, based on simulation data, the advantage of the method according to the invention for a configuration such as for Figure 1 described.

[0084] In a pilot plant, methanol is produced from a nitrogen-rich synthesis gas stream containing 12.1 vol% carbon dioxide, 12.9 vol% carbon monoxide, 49.9 vol% hydrogen, and 25.1 vol% nitrogen. The synthesis gas stream is fed into the reactor at a mass flow rate of 8.5 kg / h and compressed from an initial pressure of 4 bar to a synthesis pressure of 90 bar using the synthesis gas compressor. This yields a crude methanol stream with a mass flow rate of 3.3 kg / h, which is then expanded from approximately 90 bar to 2.1 bar via an expansion turbine. At this pressure, the expanded crude methanol stream can be directly fed to the subsequent distillation stage. Utilizing the work performed by the expansion turbine results in a saving of 0.011 kW of compressor power, corresponding to 0.6% of the total compressor power.

[0085] Figure 2Figure 2 shows a method according to a second example of the invention, in which the synthesis of methanol takes place in a four-stage reactor arrangement.

[0086] The reactor arrangement according to Figure 2 has four reactor stages 13a to 13d connected in series, each reactor stage having a downstream phase separation device assigned to it, the phase separation devices being designated accordingly as 15a to 15d.

[0087] The product partial stream 14a generated in reactor stage 13a is processed in the phase separation device 15a analogously to the procedure according to Figure 1The crude methanol is separated into a liquid phase (16a) and a gaseous phase (17a) using a heat exchanger and a high-pressure separator. The crude methanol stream 16a is combined with further crude methanol streams 16b, 16c, and 16d to produce the crude methanol stream 16 (or total crude methanol stream). The gaseous phase 17a, drawn from the separator, is fed to the subsequent reactor stage 13b for further conversion of the remaining synthesis gas from the gaseous phase 17a. In the phase separation device 15b, which is associated with and downstream of reactor stage 13b, a crude methanol stream 16b and a gaseous phase 17b are again generated. Analogous processes take place in the subsequent reactor stages 13c and 13d, as well as in the phase separation devices 15c and 15d.Due to the serial arrangement of reactor stages 13a to 13d and the (intermediate) condensation of the crude methanol in the phase separation devices 15a to 15d, a complete or at least nearly complete carbon conversion can be achieved across the entire reactor arrangement, provided the synthesis gas is appropriately designed and composed. The residual gas stream 17d drawn from the fourth phase separation device 15d therefore contains only negligible amounts of carbon oxides and hydrogen, and thus consists primarily of the inert gas component.

[0088] The crude methanol stream 16 is fed to the liquid expansion device 19 for the in-process generation of mechanical or electrical energy. Regarding the configuration and the functional connections of the liquid expansion device 19, the gas expansion device 22, the motor 24, and the synthesis gas compression device 12, the specifications apply as described for Figure 1 described.

[0089] In contrast to procedure 1 according to Figure 1 will be carried out in accordance with procedure 2 of the Figure 2 Not the purge stream, but the entire residual gas stream, here residual gas partial stream 17d, is fed to the gas expansion device 22. This results in a pressure-reduced residual gas partial stream 23, which is sent, for example, to the flare (not shown).

[0090] The following numerical example demonstrates, based on simulation data, the advantage of the method according to the invention for a configuration such as for Figure 2 described.

[0091] On an industrial scale, methanol is produced from a nitrogen-rich synthesis gas stream containing only carbon dioxide as its carbon monoxide component. Since the synthesis gas stream contains no carbon monoxide, a low equilibrium conversion to the product is expected in a single reactor stage. Therefore, a four-stage methanol synthesis with intermediate condensation of the product is chosen for this synthesis gas composition. The synthesis gas stream has a mass flow rate of 15,240.1 kg / h with respect to carbon dioxide, 2,095.5 kg / h with respect to hydrogen, and 294.1 kg / h with respect to nitrogen. The synthesis gas stream is compressed from 10 bar to a pressure of 81 bar using the synthesis gas compressor.

[0092] A crude methanol stream is obtained with a mass flow rate of 11,258 kg / h with respect to the methanol content and a mass flow rate of 6,111 kg / h with respect to the water content. The crude methanol stream also contains 328 kg / h of dissolved carbon dioxide, which was not removed in the high-pressure separators of the phase separation devices 15a to 15d.

[0093] The residual gas stream drawn from the last of the serially arranged phase separation devices contains nitrogen at a mass flow rate of 643 kg / h, carbon dioxide at a mass flow rate of 573 kg / h, methanol at a mass flow rate of 78 kg / h, carbon monoxide at a mass flow rate of 23 kg / h, hydrogen at a mass flow rate of 85 kg / h, and water at a mass flow rate of 3 kg / h. The main components of the residual gas stream are therefore nitrogen, as an inert gas component, and carbon dioxide, which remains unreacted due to its inertness.

[0094] The crude methanol stream is expanded from 81 bar to 2.1 bar in an expansion turbine to be fed to the subsequent distillation process. The residual gas stream is expanded from 81 bar to 1 bar in an expander.

[0095] The synthesis gas compressor has an electrical energy requirement of 7704 kW. Due to the energy used internally by the expansion turbine, this requirement can be reduced by 116.3 kW, corresponding to an energy saving of 1.5%. This represents a significant saving, particularly in the case of an industrial process, which more than compensates for the additional equipment costs in the long term. At the same time, a comparison with the example above makes it clear that the energy savings achieved are proportional to the scale of the process (comparison of pilot scale and industrial scale).

[0096] Furthermore, the electrical energy required for the synthesis gas compressor can be reduced by an additional 53.4 kW due to the energy used internally by the expander, resulting in a further energy saving of 0.7%. This achieves a total saving of 2.2% in electrical energy.

[0097] Figure 3 shows a process 3 according to a third example of the invention, in which the synthesis of methanol is carried out analogously to process 2 in a four-stage reactor arrangement.

[0098] As shown in the previous numerical example, the residual gas stream 17d may still contain valuable gases, i.e., gases convertible to methanol such as hydrogen, carbon monoxide, and carbon dioxide. Depending on the proportion of these valuable gases, it may be worthwhile to divert a portion of the residual gas stream 17d and return it to the first reactor stage of the four-stage reactor arrangement, in particular, using a residual gas compression device. This is described in process 3 according to Figure 3 realized. A partial stream 25 of the residual gas partial stream 17d is recompressed to synthesis pressure in a residual gas compression device 27, here a recycled gas compressor, and combined with the already compressed synthesis gas stream 11 as a compressed partial stream 26 of the residual gas partial stream 17d.

[0099] Figure 4 shows a process 4 according to a fourth example of the invention, in which the synthesis of methanol is carried out analogously to process 2 in a four-stage reactor arrangement.

[0100] The procedure according to Figure 4 This represents an alternative to process 3 regarding the utilization of valuable gases contained in the residual gas partial stream 17d. According to process 4, a portion of the already depressurized residual gas partial stream 23 is diverted as partial stream 28 and combined with the still uncompressed synthesis gas stream 10. Compression then takes place in the synthesis gas compression device 12. The configuration according to Figure 4 indicates compared to the configuration according to Figure 3 The advantage is that no residual gas compression device 27 is required. The disadvantage is that the recompression of the partial stream 28 requires additional electrical energy in the synthesis gas compression device 12. Reference symbol list

[0101] 1, 2, 3, 4 Process according to the invention 10 Synthesis gas stream 11 Compressed synthesis gas stream 12 Synthesis gas compression device (synthesis gas compressor) 13 Reactor stage 13a, 13b, 13c, 13d Reactor stage 14 Product stream 14a, 14b, 14c, 14d Partial product stream 15 Phase separation device 15a, 15b, 15c, 15d Phase separation device 16 Crude ethanol stream 16a, 16b, 16c, 16d Partial crude ethanol stream 17 Residual gas stream 17a, 17b, 17c, 17d Partial residual gas stream 18 Pressure-reduced crude ethanol stream 19 Liquid expansion device (expansion turbine) 20 Purge gas stream 21 Depressurized purge gas stream 22 Gas expansion device (expander) 23 Depressurized residual gas partial stream 24 Motor 25 Partial stream of residual gas partial stream 17d 26 Compressed partial stream of residual gas partial stream 17d 27 Residual gas compression device (recycle gas compressor) 28 Partial stream of depressurized residual gas partial stream 23 29 Crude ethanol stream 30 Low-pressure separator 31 Exhaust gas stream

Claims

1. A method (1,2,3,4) for producing methanol, comprising the method steps a) Providing a synthesis gas stream (10), comprising at least one carbon oxide, hydrogen, and an inert gas component; b) Compressing the synthesis gas stream to a synthesis pressure in a synthesis gas compression device (12); c) Reacting the compressed synthesis gas stream (11) on a methanol synthesis catalyst to form a product stream (14), wherein the product stream (14) comprises at least methanol, water, unreacted synthesis gas and the inert gas component, wherein the reaction of the compressed synthesis gas stream (11) takes place in a reactor arrangement, wherein the reactor arrangement comprises at least one reactor stage (13) and a phase separation device (15); d) Separating the product stream (14) in the phase separation device (15) into a liquid crude methanol stream (16), comprising at least methanol and water, and a residual gas stream (17), comprising at least unreacted synthesis gas and the inert gas component; e) Expanding at least a part of the crude methanol stream (16) in a liquid expansion device (19) to an expansion pressure which is lower than the synthesis pressure, wherein the liquid expansion device (19) performs mechanical work; f) Transferring at least a part of the mechanical work performed by the liquid expansion device (19) to the synthesis gas compression device (12), for driving the synthesis gas compression device (12), or Converting at least a part of the mechanical work performed by the liquid expansion device (19) into electrical energy, and using the electrical energy to drive the synthesis gas compression device (12).

2. The method according to claim 1, wherein the reactor arrangement comprises a plurality n of series-connected reactor stages (13a, 13b, 13c, 13d) and a plurality p of phase separation devices (15a, 15b, 15c, 15d), wherein each of the reactor stages is assigned a phase separation device, wherein the phase separation device is arranged downstream of the assigned reactor stage in each case, and in each of the phase separation devices (15a, 15b, 15c, 15d) a liquid crude methanol partial stream (14a, 14b, 14c, 14d) and a residual gas partial stream (17a, 17b, 17c, 17d) is produced, and wherein the residual gas partial stream produced in a phase separation device is at least partially fed into the respectively subsequent reactor stage, and wherein the residual gas partial stream (17d) produced in the last phase separation device (15d) is discharged from the reactor arrangement.

3. The method according to claim 2, comprising the method steps g) Expanding at least a part of the residual gas partial stream (17d) discharged from the reactor arrangement in a gas expansion device (22) to an expansion pressure which is lower than the synthesis pressure, wherein the gas expansion device (22) performs mechanical work; h) Transferring at least a part of the mechanical work performed by the gas expansion device (22) to the synthesis gas compression device (12), for driving the synthesis gas compression device (12), or Converting at least a part of the mechanical work performed by the gas expansion device (22) into electrical energy, and using the electrical energy to drive the synthesis gas compression device (12).

4. The method according to claim 3, wherein the entire residual gas partial stream (17d) discharged from the reactor arrangement is expanded (22) in the gas expansion device.

5. The method according to one of claims 3 or 4, wherein a portion of the residual gas partial stream (17d) discharged from the reactor arrangement is expanded in the gas expansion device (22), and a further portion (25) of the residual gas partial stream discharged from the reactor arrangement is compressed to synthesis pressure in a residual gas compression device (27), and the compressed residual gas partial stream (26) is recycled to at least one of the plurality of series-connected reactor stages (13a, 13b, 13c, 13d), preferably to the first of the plurality of series-connected reactor stages.

6. The method according to one of claims 3 to 5, wherein at least a part (28) of the residual gas partial stream (23) expanded to the expansion pressure is recompressed to synthesis pressure in the synthesis gas compression device (12), and the compressed residual gas partial stream is fed to the reactor arrangement.

7. The method according to one of the preceding claims, wherein the proportion of the inert gas component in the synthesis gas stream is at least 1% by volume, or at least 5% by volume, or at least 10% by volume, or at least 20% by volume.

8. The method according to one of the preceding claims, wherein the inert gas component comprises nitrogen and / or methane.

9. The method according to one of the preceding claims, wherein the synthesis pressure is at least 70 bar, or at least 75 bar, or at least 80 bar, or at least 85 bar.

10. The method according to one of the preceding claims, wherein the crude methanol stream is expanded in the liquid expansion device (19) to a pressure of 1 bar to 3 bar.

11. The method according to one of the preceding claims, wherein the crude methanol stream (16) is fed to a downstream-arranged thermal separation device for separating the crude methanol into methanol and water, wherein the thermal separation device is operated at a given pressure, and the crude methanol stream (16) is expanded in the liquid expansion device to a pressure which corresponds to the given pressure in the thermal separation device.

12. The method according to one of claims 2 to 11, wherein the residual gas partial stream (17d) is expanded in the gas expansion device (22) to a pressure of 1 bar to 3 bar.

13. The method according to one of claims 2 to 12, wherein for the plurality n of series-connected reactor stages (13a, 13b, 13c, 13d) and the plurality p of phase separation devices (15a, 15b, 15c, 15d) the following applies - n = 3 to 6, preferably n = 4, and - p = 3 to 6, preferably p = 4, and wherein it holds that - n = p.