Method and apparatus for producing methanol from synthesis gas having a high proportion of carbon dioxide

Abstract

The invention relates to a method for producing methanol and to an apparatus for producing methanol. A first fresh gas suitable for producing methanol and having a high carbon dioxide content is precompressed by a first compressor stage to obtain a second fresh gas. The second fresh gas is combined with a recycle gas stream and further compressed to synthesis pressure in a second compressor stage. Catalytic conversion of the synthesis gas stream thus obtained in a plurality of reactor stages arranged in series with intermediate condensation and separation of crude methanol reduces the amount of recycle gas in the synthesis loop to such an extent that the recycle gas can be directly recirculated to the second fresh gas stream, thus ensuring that no recycle gas compressor stage is required and the overall compressor power can be reduced.

Description

Technical Field

[0001] The invention relates to a method for producing methanol, a plant for producing methanol and the use of the plant according to the invention in the method according to the invention. Background Art

[0002] Methanol is today typically produced in a so-called low-pressure process at pressures between 60 and 120 bar from synthesis gas, which is a mixture of mainly hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2). Depending on the technology and plant capacity, a single-stage or two-stage process is used to convert the synthesis gas into methanol (with high yields in the reactor part of the process).

[0003] In the process used, the synthesis gas, which is usually produced at a pressure between 20 and 40 bar, is initially brought to the pressure required for the reaction above 60 bar via a gas compressor (synthesis gas compressor). The process is typically further carried out with a large gas loop in a so-called synthesis loop, also called a synthesis loop, in which the synthesis gas which is not converted into methanol in the first reactor pass is recycled to the reactor inlet as recycle gas. The synthesis loop is required to achieve a sufficient synthesis gas conversion and thus an overall yield (despite the low conversion per pass).

[0004] Depending on the technology, fresh gas composition and reactor system, the recycle gas amount is up to 5 times higher than the supplied fresh gas amount from the synthesis gas production. This is especially true for synthesis gas / fresh gas compositions with a high CO2 content, where significantly lower thermodynamic equilibrium conversions are observed compared to a considerably CO-rich synthesis gas. Dedicated recycle gas compressors are typically used to transport and compress the high gas loop amounts.

[0005] A disadvantage of these high recycle rates is the accumulation of inert components (i.e. components that are not convertible under the methanol synthesis conditions, such as methane and nitrogen, which must be removed from the synthesis loop as purge gas). This typically also removes hydrogen, which requires costly and inconvenient subsequent recovery (usually accomplished using a pressure swing adsorption unit).

[0006] Examples of processes with a synthesis loop are disclosed in WO 2015 / 193440 A1, WO 2017 / 167642 A1 and EP 2 116 295 A1.

[0007] The addition of additional reactor stages, including intermediate condensation of the methanol produced by each reactor stage, makes it possible to increase the conversion per pass over the total number of reactor stages and thus reduce the recycle rate.

[0008] When using a synthesis loop that delivers a large amount of unreacted synthesis gas, the various components of the relevant production equipment must have corresponding large dimensions for large recycle gas flows. This has the result that the total capital expenditure (CAPEX) cost of the equipment increases and / or for the required large capacity, the size of the reactor, in particular the outer diameter, may reach the transportation limitations of the corresponding equipment project. Recompressing the recycle gas to the reaction pressure additionally requires an additional compressor unit (recycle gas compressor), thus not only increasing the CAPEX cost, but also increasing the total compressor power required, and thus increasing the operating expenditure (OPEX) cost of the equipment.

[0009] This negative effect is additionally enhanced by the high proportion of CO2 and components that are inert under the methanol synthesis conditions, such as methane (CH4) and nitrogen (N2). The use of multiple reactor stages connected in series including intermediate condensation can reduce the proportion of recycle gas, but the number of required processing units also increases, thus typically also increasing the CAPEX costs. If the synthesis loop is completely omitted ("straight-through" process), the number of required reactors increases further and thus in turn negatively compensates for the omission of the synthesis loop.

[0010] In the course of discussions on anthropogenic climate change, synthesis gas with a high CO2 content is receiving increasing attention. Thus, for example, it is considered to use waste gases with a high carbon dioxide content to produce synthesis gas (instead of, for example, isolating these waste gases). Thus, for example, it is conceivable to produce synthesis gas from carbon dioxide from waste gas sources by combining with hydrogen from water electrolysis, wherein the current for the water electrolysis can be derived from renewable energy sources. Such synthesis gas can be considered to be climate neutral and also contains only little carbon monoxide (if any).

[0011] As mentioned above, a high carbon dioxide fraction in the synthesis gas leads to high recycle rates (based on the synthesis loop), which increases the CAPEX and OPEX costs of the plant. Compensation by introducing multiple reaction stages is only partially possible, since this approach also increases the CAPEX costs.

[0012] The synthesis of methanol from carbon dioxide and hydrogen further produces water according to the following reaction equation:

[0013]

[0014] This is not the case for the conversion of carbon monoxide with hydrogen to provide methanol according to the following reaction equation:

[0015]

[0016] The accumulation of water in the reactor leads to a continuous deactivation of the copper-based catalysts commonly used in methanol synthesis, especially when high recirculation rates are used. The use of a plurality of reactor stages arranged in series with intermediate condensation of methanol and water makes it possible to reduce this effect by reducing the water partial pressure of the catalyst bed. However, as mentioned above, the use of a plurality of reactor stages leads to an increase in the CAPEX costs of the associated equipment.

[0017] For synthesis gases containing only or predominantly carbon dioxide (as reactive carbon component) and hydrogen, a stoichiometric ratio or a slightly superstoichiometric ratio is usually set in the fresh gas. The stoichiometric number SN, defined as

[0018]

[0019] Thus 2 to 2.05. Assuming that the synthesis gas employed does not contain carbon monoxide, 3 to 3.05 mol of hydrogen (H2) / mol of carbon dioxide (CO2) are therefore required to produce 1 mol of methanol. Although this results in a high carbon conversion, a considerable proportion of the hydrogen is lost as a purge stream. Assuming that the hydrogen originates from renewable energy sources and the carbon dioxide from waste gas sources, the hydrogen has the greatest impact on the economics of the relevant process. Summary of the invention

[0020] With regard to the production of methanol from synthesis gas having a high proportion of carbon dioxide, there is therefore a need to improve existing processes.

[0021] It is an object of the present invention to provide a process for producing methanol which at least partially overcomes the disadvantages of the prior art.

[0022] Another object of the present invention is to provide a process for producing methanol which enables a saving in the energy required for the process, in particular for operating the gas compressor.

[0023] Another object of the present invention is to provide a process for producing methanol which enables savings with regard to the plant parts required for the process, in particular with regard to the number of required compressor stages.

[0024] Another object of the present invention is to provide a process for producing methanol which largely avoids hydrogen losses via the purge gas stream.

[0025] Another object of the present invention is to provide a process for producing methanol which reduces the catalyst loading and prolongs the life of the catalyst used via a high water partial pressure in the catalyst bed.

[0026] A further object of the present invention is to provide a process for producing methanol which reduces the amount of synthesis gas to be recycled in the synthesis loop to the greatest possible extent.

[0027] Another object of the present invention is to provide a plant for producing methanol, which at least partially achieves at least one of the above objects.

[0028] The independent claims provide a contribution to at least partially achieving at least one of the above objects. The dependent claims provide preferred embodiments, which contribute to at least partially achieving at least one of the objects. Where relevant, the preferred embodiments of the components of a class according to the invention are equally preferred for the same named or corresponding components of the respective other class according to the invention.

[0029] The terms "having", "comprising" or "containing" etc. do not exclude the presence of additional elements, components etc. The indefinite article "a" does not exclude the presence of a plurality.

[0030] The above objects are at least partly solved by a method for producing methanol, comprising the following method steps, wherein these do not necessarily have to be carried out in the order described:

[0031] a) providing an input gas comprising carbon oxides and hydrogen (H 2 ), wherein the proportion of carbon dioxide (CO 2 ) in the input gas is at least 80% by volume, based on the total amount of the carbon oxides;

[0032] b) introducing the input gas as a first fresh gas stream into a first compressor stage for precompressing the first fresh gas stream to obtain a second fresh gas stream;

[0033] c) introducing the recycle gas stream and the second fresh gas stream into a second compressor stage for compressing the recycle gas stream and the second fresh gas stream to a synthesis pressure to obtain a synthesis gas stream;

[0034] d) catalytically converting synthesis gas in the synthesis gas stream at a synthesis pressure in a plurality of reactor stages arranged in series to obtain a product stream in each reactor stage, the product stream comprising methanol and unreacted synthesis gas;

[0035] e) cooling the product stream obtained in each reactor stage to condense methanol and separate it from unreacted synthesis gas, and introducing the unreacted synthesis gas into the respective next stage of the reactor stages arranged in series;

[0036] f) withdrawing unreacted synthesis gas from at least one of the reactor stages as a recycle gas stream for introducing the recycle gas stream into the second compressor stage according to step c).

[0037] According to the invention, the recirculated gas is introduced into the second compressor stage together with the precompressed second fresh gas stream. The catalytic conversion of the synthesis gas in a plurality of reactor stages arranged in series reduces the amount of recirculated gas. This allows the recirculated gas stream drawn off from one or more reactor stages (together with the second fresh gas stream) to be supplied directly to the second compressor stage, as a result of which the total compressor power required for the process is surprisingly reduced.

[0038] In each reactor stage, the synthesis gas is converted into a product stream comprising methanol. This is crude methanol, which may contain not only methanol itself but also water and additional condensable by-products. The water content of the crude methanol increases with increasing concentration of carbon dioxide in the input gas.

[0039] By cooling and the condensation that occurs therewith, methanol is separated from the remaining synthesis gas stream so that unreacted synthesis gas remains mainly in the gas phase. The gas is introduced into the corresponding next stage of the reactor stage arranged in series. As is obvious to a person skilled in the art, the last of the reactor stages arranged in series is an exception. The unreacted synthesis gas extracted from the last of the reactor stages arranged in series is introduced into the second compressor stage as a recycle gas stream.

[0040] Optionally a proportion of the recycle gas stream is separated from the recycle gas stream as a purge gas stream. In this case, unreacted synthesis gas is at least partially withdrawn from at least one of the reactor stages as a recycle gas stream for introducing the recycle gas stream into the second compressor stage according to step c).

[0041] According to step e), methanol is at least partially separated from the unreacted synthesis gas by cooling and condensing from the product stream obtained. Complete separation of methanol from the unreacted synthesis gas by cooling and condensing is sought in order to shift the reaction equilibrium towards the product (methanol) side.

[0042] A preferred embodiment of the method according to the invention is characterized in that, after the withdrawal according to step f), the recycle gas stream is introduced into the second compressor stage without preceding compression.

[0043] It is preferred that the recycle gas stream is supplied to the second compressor stage directly and without using a recycle gas compressor (i.e., without using an additional compressor stage to compress the recycle gas). This not only reduces the total compressor power required for the method (reduction in OPEX). It also makes it possible to omit the acquisition of a recycle gas compressor, thus also reducing the CAPEX cost of the method. In this case, the entire methanol synthesis requires only two compressor stages, i.e., a first compressor stage (for compressing the first fresh gas to produce the second fresh gas) and a second compressor stage (for compressing the second fresh gas (together with the recycle gas) to a synthesis pressure to produce synthesis gas).

[0044] A preferred embodiment of the method according to the invention is characterized in that, according to step e), the unreacted synthesis gas is completely introduced into the respective next stage of the reactor stages arranged in series.

[0045] This involves introducing the maximum possible amount of unreacted synthesis gas (which is currently understood to mean "completely") into the respective next stage of the reactor stages arranged in series. As is known to the person skilled in the art, in the case of the present invention, a small portion of the unreacted synthesis gas always dissolves in the condensate (methanol). This small portion cannot therefore be introduced into the next reactor stage.

[0046] In this embodiment, unreacted synthesis gas is produced only in the last of the reactor stages arranged in series and the amount of recycle gas flow is reduced to a minimum.

[0047] A preferred embodiment of the method according to the invention is therefore further characterized in that, according to step f), unreacted synthesis gas is withdrawn as a recycle gas stream from the last of the plurality of reactor stages arranged in series, in particular only from the last of the plurality of reactor stages arranged in series.

[0048] A preferred embodiment of the method according to the invention is characterized in that the recycle gas flow and the second fresh gas flow are combined and introduced into the second compressor stage as a combined gas flow.

[0049] In this case, the two streams are initially mixed after merging and subsequently introduced into the second compressor stage as a combined gas stream.

[0050] A preferred embodiment of the method according to the invention is characterized in that the input gas has a stoichiometric number SN of 1.5 to 2.2, wherein

[0051]

[0052] The process according to the invention is particularly suitable for highly substoichiometric synthesis gases (with a high carbon dioxide content) to slightly substoichiometric input gases (with a high carbon dioxide content).

[0053] In this case, it is preferred that the input gas has a stoichiometric number SN of 1.5 to 1.9, preferably 1.6 to 1.8.

[0054] Investigations have surprisingly shown that compositions according to the above stoichiometric numbers allow the total compressor power required for the process according to the invention to be further reduced. When this stoichiometric number is less than 2 and when the input gas contains, for example, only carbon dioxide (based on carbon oxides), the ratio of hydrogen to carbon dioxide is less than the (ideal) stoichiometric ratio of 3 to 1. Carbon dioxide is therefore present in excess. This improves the hydrogen conversion (which also has a positive effect on the overall economics of the process). This is because it can be assumed that hydrogen is always a more expensive raw material than carbon dioxide, which is why hydrogen has a decisive influence on the economics of the process.

[0055] A preferred embodiment of the method according to the invention is characterized in that, for a recirculation rate R defined as:

[0056]

[0057] 0.15≤R≤1.25.

[0058] The volume flow is to be understood in each case as meaning the standardized volume flow (standard volume flow) based on the physical standard conditions of the respective gas at 0° C. and 1.01325 bar absolute pressure.

[0059] By using a plurality of reaction stages arranged in series with intermediate condensation, the recirculation rates in the process according to the invention are very low compared to known processes.

[0060] It is further preferred when for the recirculation rate R: 0.1≤R≤0.5.

[0061] Investigations have surprisingly shown that the recirculation rate can be further reduced while maintaining a constant high hydrogen conversion, especially when using a substoichiometric input gas, i.e. an input gas having a stoichiometric number of less than 2. When the amount of the recirculated gas flow is further reduced, in particular to a value significantly less than 1, the required total compressor power also decreases accordingly.

[0062] A preferred embodiment of the process according to the invention is characterized in that a carbon dioxide conversion of at least 80 mol % based on the input gas is achieved across the plurality of reactor stages arranged in series.

[0063] Based on the input gas, taking into account the recycle gas stream and thus taking into account the overall balance, the proportion of carbon dioxide converted over all reactor stages is preferably at least 80 mol %. Particular preference is given to carbon dioxide conversions of at least 90 mol %, more preferably at least 95 mol % and even more preferably at least 98 mol %.

[0064] Since carbon dioxide is in most cases an inexpensive feedstock, a conversion of at least 80 mol% is economically acceptable and therefore preferred, with higher conversions being more preferred since disposal of unconverted carbon dioxide then becomes less of a problem.

[0065] The hydrogen conversion is preferably at least 95 mol%, more preferably at least 98 mol% or at least 99 mol%. It is preferred to achieve the highest possible hydrogen conversion because hydrogen is in many cases a much more expensive source of feedstock than carbon dioxide.

[0066] A preferred embodiment of the process according to the invention is characterized in that the plurality of reactor stages arranged in series comprises a number of 2 to 8 reactor stages.

[0067] The optimal number of reactor stages depends on factors such as the amount of methanol to be produced, the conversion rate of each reactor stage, the composition of the input gas and in some cases additional factors. In general, CAPEX costs increase with increasing number of reactor stages and the amount of recycle gas stream decreases, thus resulting in particularly high savings on compressor power. Conversely, CAPEX costs decrease with decreasing number of reactor stages, but the amount of recycle gas stream increases, thus reducing savings on compressor power.

[0068] The plurality of reactor stages arranged in series therefore preferably comprises a number of reactor stages of 3 to 5, particularly preferably a number of reactor stages of 4. This achieves an optimum compromise with regard to savings in CAPEX costs and compressor power.

[0069] The object of the present invention is also achieved at least in part by

[0070] A plant for producing methanol is realized, the plant comprising the following plant elements arranged in fluid connection with each other:

[0071] a first compressor stage for pre-compressing a first fresh gas comprising carbon oxides and hydrogen (H2) to produce a second fresh gas;

[0072] a second compressor stage arranged downstream of the first compressor stage for compressing the second fresh gas and the recycle gas to provide synthesis gas at synthesis pressure;

[0073] a plurality of reactor units arranged in series downstream of the second compressor stage for producing a product stream comprising methanol and unreacted synthesis gas from the synthesis gas, wherein each of the reactor units comprises

[0074] - a reactor comprising a catalyst bed,

[0075] - a heat exchanger for cooling the product stream, and

[0076] - a separator for separating methanol from unreacted synthesis gas,

[0077] in

[0078] Each of the reactor units comprises an outlet for withdrawing methanol, and

[0079] Each of the reactor units comprises an outlet for supplying unreacted synthesis gas to a corresponding downstream reactor unit, and / or

[0080] for supplying unreacted synthesis gas as recycle gas to the outlet of the second compressor stage, wherein

[0081] A feed device for introducing the recycle gas into the second compressor stage is arranged between the first compressor stage and the second compressor stage.

[0082] A production unit for producing an input gas containing hydrogen (H2) and carbon oxides is arranged upstream of the device according to the invention. The input gas contains at least 80% by volume of carbon dioxide (CO2) based on the total amount of carbon oxides. The input gas is supplied to the device according to the invention as a first fresh gas.

[0083] A preferred embodiment of the apparatus according to the invention is characterized in that no compressor stage for precompressing the recycle gas before introduction into the second compressor stage is arranged between the outlet for supplying unreacted synthesis gas as recycle gas to the second compressor stage and the feed device for introducing the recycle gas into the second compressor stage.

[0084] Preference is given to when the unreacted synthesis gas is supplied directly to the second compressor stage as recycle gas after being withdrawn from the reactor unit. In this case, no additional compressor stage is provided between the outlet of the reactor unit and the feed device to the second compressor unit for compressing the recycle gas before supplying it to the second compressor stage.

[0085] A preferred embodiment of the apparatus according to the invention is characterized in that the outlet for supplying unreacted synthesis gas as recycle gas to the second compressor stage is arranged at the last, preferably only at the last, of the plurality of reactor units arranged in series.

[0086] It is preferred that all reactor units (except the last one of the reactor units arranged in series) have an outlet for supplying unreacted synthesis gas from the reactor unit to the respective next reactor unit. Only the last one of the reactor units arranged in series has an outlet for supplying unreacted synthesis gas from the reactor unit as recycle gas to the second compressor stage. This reduces the amount of recycle gas to a minimum.

[0087] Furthermore, the object of the invention is at least partially achieved by using the device according to the invention in the method according to the invention.

[0088] Input gas

[0089] The input gas comprises at least hydrogen (H2) and carbon oxides. The term "carbon oxides" includes the compounds carbon monoxide (CO) and carbon dioxide (CO2). Based on the total amount of carbon oxides, the input gas has a carbon dioxide ratio of at least 80% by volume. The input gas therefore has a high carbon dioxide content. In an example, based on carbon oxides, the input gas comprises at least 90% by volume, or at least 95% by volume, or at least 98% by volume, or at least 99% by volume of carbon dioxide. In an example, the input gas comprises only carbon dioxide (based on carbon oxides). Such an input gas does not contain any carbon monoxide or it contains only trace amounts of carbon monoxide.

[0090] The input gas may be derived from any source known to the skilled person. The carbon dioxide of the input gas is preferably derived from an exhaust gas source. The hydrogen of the input gas is preferably derived from a water electrolysis device, wherein the electric current for this device is preferably generated by a renewable energy source such as hydropower, wind power or photovoltaics.

[0091] In another example, the input gas is derived from a reforming process. This input gas comprising hydrogen and carbon oxides is produced by reforming a feedstock comprising hydrocarbons such as methane or natural gas. The reforming process may include one or more steps, such as steam reforming, partial oxidation or autothermal reforming. The reformate is preferably further converted in a water-gas shift reaction. In this case, carbon monoxide and water are converted to give carbon dioxide and hydrogen and thus an input gas with a high carbon dioxide content is obtained.

[0092] Independently of its source, the input gas can be produced at a temperature between 400° C. and 1200° C. and / or a pressure between 10 and 60 bar. In addition to the above-mentioned components, the input gas may also contain varying amounts of inert components, such as methane or nitrogen. Inert components are to be understood in particular as meaning components which are inert under the conditions of the methanol synthesis, i.e. components which are not converted into methanol or (unwanted) by-products under the conditions of the methanol synthesis.

[0093] Before the input gas is used as first fresh gas in the method according to the invention, the input gas is typically cooled to below the dew point of the steam in order to condense out water. The input gas is cooled in particular to below 100° C., preferably to below 60° C., and more preferably to 40° C. or below, in order to separate water from the input gas after condensation. The first fresh gas is thus in particular free of water or largely free of water.

[0094] Methanol

[0095] When "methanol" is mentioned in the context of the subject matter of the present invention, it is generally understood to mean crude methanol, which may contain not only methanol itself but also water and further condensable by-products. In a process or an apparatus arranged downstream of the process according to the invention or the apparatus according to the invention, this crude methanol is supplied to a purification unit to produce pure methanol.

[0096] Compressor stage

[0097] When a "compressor stage" is mentioned in the context of the subject matter of the present invention, it should be understood to mean a method step in which a gas amount is compressed in one step from a starting pressure p1 to a final pressure p2 (where p2>p1) or a device unit suitable for compressing a gas amount in one step from a starting pressure p1 to a final pressure p2 (where p2>p1). The quotient of p2 and p1 describes the compression ratio of the corresponding compressor stage.

[0098] A “compressor” or “compressor unit” may comprise a plurality of compressor stages, wherein the compression from p1 to p2 then takes place in a plurality of steps, wherein each step has a defined compression ratio.

[0099] In one example, a plurality of compressor stages may be integrated into one compressor, wherein this compressor preferably comprises intercooling after the first and / or second compressor stage. In another example, two compressors arranged in series each have only one compressor stage.

[0100] First compressor stage, first fresh gas

[0101] The first fresh gas is supplied to the first compressor stage. The first compressor stage compresses the first fresh gas to obtain a second fresh gas. The second fresh gas has a higher pressure than the first fresh gas.

[0102] In one example, the first fresh gas has a pressure corresponding at least to a large extent to the pressure of the input gas, for example a pressure of 10 to 60 bar, preferably a pressure of 25 to 45 bar, and more preferably a pressure of 20 to 40 bar.

[0103] Second compressor stage, second fresh gas

[0104] In one example, the second fresh gas has a pressure of 50 to 70 bar and preferably a pressure of 60 to 70 bar.

[0105] The second fresh gas is supplied to a second compressor stage together with the recycle gas and compressed to obtain the synthesis gas required for the catalytic conversion into methanol.Compared to the second fresh gas and / or the recycle gas, the synthesis gas has a relatively high pressure.

[0106] In one example, the synthesis gas has a pressure of 60 to 120 bar, preferably a pressure of 70 to 100 bar, and more preferably a pressure of 80 to 90 bar.

[0107] Synthetic pressure

[0108] In the second compressor stage, the second fresh gas and the recycle gas are compressed to the synthesis pressure (in particular as a combined gas stream). The synthesis pressure is the pressure required for methanol synthesis, for example the pressure that the synthesis gas should have after introduction into the first of the plurality of reactor stages. In an example, the synthesis pressure corresponds substantially to the pressure of the synthesis gas, for example from 60 to 120 bar, preferably from 70 to 100 bar and more preferably from 80 to 90 bar.

[0109] Catalytic conversion

[0110] The synthesis gas is subjected to catalytic conversion into methanol in the reactor stage.

[0111] The temperature of the synthesis catalyst is suitably from 180 to 300° C., preferably with a peak temperature not exceeding 280° C. The synthesis gas preferably enters the reactor stage at a temperature of 200 to 250° C. and preferably leaves the reactor stage again at a temperature of 220 to 270° C.

[0112] The fixed bed catalyst for methanol synthesis is preferably a copper-based catalyst. Particularly suitable are copper-based catalysts comprising compounds such as zinc oxide, aluminum oxide, chromium oxide, titanium oxide, zirconium oxide (zircon) and / or magnesium oxide.

[0113] Suitable reactor types are water-cooled reactors using boiling boiler feed water for cooling or gas-cooled reactors, wherein the cooling is carried out via the unconverted synthesis gas which is thereby heated and can be introduced into the next reactor stage in a preheated state.

[0114] Reactor stage, reactor unit

[0115] In the context of the subject matter of the present invention, a "reactor stage" is in principle understood to mean a process step and / or an apparatus suitable for carrying out a chemical reaction. In the context of the subject matter of the present invention, a "reactor unit" is understood to mean an apparatus suitable for carrying out a chemical reaction.

[0116] The reactor stage or reactor unit can include not only the actual reactor, but also, in particular, a heat exchanger arranged downstream of the reactor for cooling the product stream, in particular for condensing methanol. The reactor stage or reactor unit further includes, in particular, a separator, for example for separating condensed methanol from unconverted synthesis gas. The outlet of the reactor stage or reactor unit for drawing off unreacted synthesis gas, which is supplied to the next reactor stage or reactor unit in a further step or as recycle gas to the second compressor stage, is located in particular on the separator part of the reactor stage or reactor unit.

[0117] The product stream is preferably cooled by means of a heat exchanger to a temperature below 50°C to condense the methanol and separate it from the unreacted synthesis gas in a separator.

[0118] Product Flow

[0119] Catalytic conversion of synthesis gas over a fixed bed catalyst at elevated temperature provides a product stream comprising methanol (crude methanol, comprising methanol, water and condensable by-products) and unreacted synthesis gas. When using an input gas with a high carbon dioxide content, water is necessarily obtained as the main component in the crude methanol. Since steam causes continuous deactivation of the synthesis catalyst, especially at high partial pressures, it is separated from the unreacted synthesis gas and the methanol itself and condensable by-products by cooling and separation. Condensable by-products are, for example, dimethyl ether, methyl formate, acetone, ethanol and higher alcohols.

[0120] Unreacted synthesis gas

[0121] According to the thermodynamic equilibrium constant as a function of temperature, the synthesis gas supplied to the respective reactor stage is always only partially converted into methanol. Unconverted synthesis gas is unreacted synthesis gas. It is supplied to the next stage of the plurality of reactor stages arranged in series or as recycle gas to the second compressor stage.

[0122] Recycled gas

[0123] The proportion of the unreacted synthesis gas which, after leaving a reactor stage, is not supplied to the next stage of the plurality of reactor stages is in particular recirculated to the second compressor stage as recycle gas.

[0124] The part of recycle gas stream can be shifted out from recycle gas stream as purge stream.In methanol synthesis, the catalyst bed in the reactor stage consumes hydrogen and carbon oxides.However, the composition (such as nitrogen, argon or methane) that is inert under methanol synthesis conditions is not consumed, and may accumulate in synthesis loop in a longer time period.The accumulation of inert gas components is undesirable and therefore, particularly a part of recycle gas stream is shifted out from synthesis loop as purge stream.Synthesis loop is formed by the gas stream between the second compressor stage, a plurality of reactor stages and these elements at least.

[0125] The hydrogen is typically recovered from the purge stream, for example by a pressure swing adsorption unit or using a membrane system.For example, the hydrogen recovered from the purge stream may be supplied to the first fresh gas stream.

[0126] Preference is given to when the recycle gas is not compressed after being withdrawn from at least one reactor stage and before being supplied to the second compressor stage.In one example, the recycle gas has a pressure of 60 to 80 bar, preferably 65 to 75 bar.

[0127] Working Example

[0128] The present invention is more specifically explained below by comparative examples and examples of the present invention without limiting the subject matter of the present invention in any way. From the following description of working examples in conjunction with the accompanying drawings and numerical examples, additional features, advantages and possible applications of the present invention will become apparent.

[0129] In these figures:

[0130] Figure 1 A schematic block flow diagram showing a prior art methanol production process with a recycle gas compressor stage in the synthesis loop,

[0131] Figure 2 A schematic block flow diagram showing a prior art methanol production process with a recycle gas compressor stage in the synthesis loop,

[0132] Figure 3 A schematic block flow diagram showing a methanol production process according to the invention with a plurality of reactor stages arranged in series in a synthesis loop without a recycle gas compressor stage is shown.

[0133] Figure 1A greatly simplified schematic block flow diagram of a methanol production process comprising a recycle gas compressor stage in a synthesis loop comprising a single reactor stage and a single compressor stage for fresh gas is shown.

[0134] The first compressor stage K1 is supplied with a fresh gas stream 101 at a pressure of 34 bar. The fresh gas from the fresh gas stream 101 originates from an input gas source, which provides an input gas containing only carbon dioxide (based on carbon oxides). Due to the molar ratio of carbon dioxide to hydrogen of 3 to 1, the fresh gas from the fresh gas stream 101 has a stoichiometric number SN of 2.0. The fresh gas stream 101 is compressed by the first compressor stage K1, so a second fresh gas stream 102 is provided on the pressure side of the compressor stage K1. The second fresh gas stream 102 is merged with the recirculating gas stream 104b, thereby generating a combined gas stream referred to herein as synthesis gas stream 103. The result of compression by the compressor stage K1 is that the synthesis gas stream 102 has a pressure of 85.0 bar at the inlet entering the reactor stage R1. In the reactor stage R1, the synthesis gas from the synthesis gas stream 102 is heated to 85.0 bar at a fixed bed catalyst based on copper (with 16.8 m 3 The catalyst volume) is converted into methanol. The pressure of the reactor outlet of the reactor of R1 is 81.2 bar. The reaction is incomplete, so an unreacted synthesis gas stream 105 is provided, which is partially passed into the recycle gas compressor stage RK as a recycle gas stream 104a and compressed to provide a recycle gas stream 104b on the pressure side of the recycle gas compressor stage RK. A part of the unreacted synthesis gas 105 is removed from the synthesis loop as a purge stream 107 to avoid accumulating inert components and by-products of methanol synthesis. Reactor stage R1 not only includes the actual reactor but also includes a heat exchanger and a separator for cooling the product stream. The cooling of the product stream in the separator and the condensation of methanol provide a product stream 106 containing methanol. The methanol is in the form of crude methanol and is supplied to a suitable post-processing device (not shown) according to the required purity in order to obtain pure methanol.

[0135] exist Figure 1 In the comparative example, the carbon dioxide conversion rate was 98.5% and the hydrogen conversion rate was 98.8%.

[0136] exist Figure 1In the comparative example of , the recirculation rate R (i.e. the ratio of the amount of recirculation gas 104a or 104b to the amount of fresh gas 102) is R=3.5. The amount of recirculation gas to be compressed by the recirculation gas compressor stage thus corresponds to 3.5 times the amount of fresh gas (based in each case on the standard volume flows of recirculation gas and fresh gas). When producing 344 tons of crude methanol / day (t / d) (of which 217.5 t / d is methanol and the remainder is essentially water), Figure 1 The comparative example of requires a compressor power of 1162 kW for K1 and a compressor power of 325 kW for RK, thus corresponding to a total compressor power of 1487 kW.

[0137] Figure 2 A greatly simplified schematic block flow diagram of a methanol production process comprising a recycle gas compressor stage in a synthesis loop comprising a reactor stage and two serially arranged compressor stages for fresh gas is shown. Figure 2 The method is therefore Figure 1 The method of WO 2008 / 050444 differs only in the use of an additional compressor stage for fresh gas.

[0138] The first compressor stage K1 is supplied with a first fresh gas stream 201 at a pressure of 34 bar. The fresh gas from the fresh gas stream 201 originates from an input gas source, which provides an input gas containing only carbon dioxide (based on carbon oxides). Due to the molar ratio of carbon dioxide to hydrogen of 3 to 1, the fresh gas from the fresh gas stream 101 has a stoichiometric number SN of 2.0. The fresh gas stream 201 is compressed by the first compressor stage K1, so a second fresh gas stream 202 with a pressure of 65.0 bar is produced on the pressure side of the compressor stage K1. The second fresh gas stream 202 is compressed by the second compressor stage K2 to produce a third fresh gas stream 203 on the pressure side of K2. The third fresh gas stream 203 is merged with the recycle gas stream 206b, so a combined gas stream referred to herein as a synthesis gas stream 204 is produced. The result of compression by the compressor stage K2 is that the synthesis gas stream 204 has a pressure of 85.0 bar at the inlet entering the reactor stage R1. In reactor stage R1, synthesis gas from synthesis gas stream 204 is heated over a copper-based fixed bed catalyst (having 16.8 m 3The catalyst volume) is converted into methanol. The pressure of the reactor outlet is 81.2 bar. The reaction is incomplete, so an unreacted synthesis gas stream 205 is provided, which is partially passed into the recycle gas compressor stage RK as a recycle gas stream 206a and compressed to provide a recycle gas stream 206b on the pressure side of the recycle gas compressor stage RK. A part of the unreacted synthesis gas 205 is removed from the loop as a purge stream 208 to avoid accumulating inert components. The reactor stage R1 not only includes the actual reactor but also includes a heat exchanger and a separator for cooling the product stream. The cooling of the product stream in the separator and the condensation of methanol provide a product stream 207 containing methanol. The methanol is in the form of crude methanol and is supplied to a suitable post-processing device according to the required purity to obtain pure methanol (not shown).

[0139] exist Figure 2 In the comparative example of , the carbon dioxide conversion rate is also 98.5% and the hydrogen conversion rate is also 98.8%.

[0140] exist Figure 2 In the comparative example of , the recycle rate R (i.e., the ratio of the amount of the recycle gas 206a or 206b to the amount of the fresh gas 203) is also R = 3.5. When 344 tons of crude methanol / day (t / d) are produced (of which 217.5 t / d is methanol and the rest is substantially water), Figure 2 The comparative example of requires a compressor power of 1090 kW for K1 and K2 together and a compressor power of 325 kW for RK, thus corresponding to a total compressor power of 1415 kW. The total compressor power is thus slightly lower than Figure 1 , but for which three compressor stages K1, K2 and RK are required, thus increasing the CAPEX costs of the equipment.

[0141] Figure 3 A greatly simplified schematic block flow diagram of the inventive methanol production process is shown without a recycle gas compressor stage in a synthesis loop comprising a plurality of reactor stages arranged in series.

[0142] The first compressor stage K1 is supplied with a fresh gas stream 301 at a pressure of 34.0 bar. The fresh gas from the fresh gas stream 301 originates from an input gas source, which provides an input gas containing only carbon dioxide (based on carbon oxides). Due to the molar ratio of carbon dioxide to hydrogen of 3 to 1, the fresh gas from the fresh gas stream 301 has a stoichiometric number SN of 2.0. The first fresh gas stream 301 is compressed by the first compressor stage K1, so a second fresh gas stream 302 with a pressure of 65.0 bar is provided on the pressure side of the compressor stage K1. The second fresh gas stream 302 is merged with the recirculated gas stream 305, thereby producing a merged gas stream 303, which is introduced into the second compressor stage K2 to be compressed to a pressure of 85.0 bar. This produces a gas stream (referred to herein as synthesis gas stream 304) with a synthesis pressure on the pressure side of the second compressor stage.

[0143] The synthesis gas stream 304 is introduced into the first of a total of four reactor stages R1 to R4 arranged in series. In the first reactor stage R1, the synthesis gas 304 is heated over a copper-based fixed bed catalyst (having 4.2 m 3 of the catalyst volume) is converted into methanol.

[0144] Reactor stages R2, R3 and R4 each had 4.2 m 3 The catalyst volume thus produces 16.8m 3 The total catalyst volume (corresponding to the Figure 1 and Figure 2 The catalyst volume of a single reactor stage in the comparative example).

[0145] Incomplete conversion of the synthesis gas 304 in the reactor stage R1 provides an unreacted synthesis gas stream 306a, which is introduced into the next stage of the reactor stages arranged in series (in the case of the present invention into the reactor stage R2). The reactor stage R1 and the further reactor stages R2 to R4 comprise not only the actual reactor but also a heat exchanger for cooling the product stream and a separator. The cooling of the product stream in the separator and the condensation of the methanol provide the product stream 307a in R1.

[0146] Similar to the catalytic conversion in R1, unreacted synthesis gas 306a is converted into methanol and by-products in reactor stage R2, thereby providing additional product stream 307b. In turn, the unreacted synthesis gas in the conversion on the copper-based catalyst of R2 is introduced into the next reactor stage R3 to provide product stream 307c and unreacted synthesis gas 306c. A small proportion of unreacted synthesis gas R2 is always dissolved in the condensate (methanol) and is not available for the next reactor stage. Unreacted synthesis gas 306c is introduced into the last R4 of the reactor stage arranged in series to obtain additional product stream 307d. Product streams 307a to 307d are combined to provide total product stream 307. Total product stream 307 contains crude methanol, which is supplied to a suitable post-processing device according to the required purity to obtain pure methanol (not shown).

[0147] The pressure at the reactor outlet of reactor stage R4 is 74.9 bar. The pressure drop over all reactor stages is therefore 10.1 bar. The unreacted synthesis gas 306d in the last reactor stage is recycled as recycle gas stream 305 directly to the second fresh gas stream 302 and combined with said stream. Figure 3 In the example of , part of the unreacted synthesis gas withdrawn from the last reactor stage R4 is diverted as a purge gas stream in order to avoid accumulation of inert components and by-products in the recycle gas stream 305 .

[0148] exist Figure 3 In the example of , the carbon dioxide conversion is 98.3% and the hydrogen conversion is 98.6%. The conversions are thus comparable to those according to the comparative example ( Figure 1 and Figure 2 The conversion rates of the settings) are comparable.

[0149] exist Figure 3 In the example of the present invention, the recirculation rate R (i.e. the ratio of the amount of recirculation gas 305 to the amount of fresh gas 302) is R = 0.9. The amount of recirculation gas 305 thus corresponds to 90% of the amount of fresh gas 302 (based in each case on the standard volume flows of recirculation gas and fresh gas). At the same carbon and hydrogen conversion, the recirculation rate is thus a factor of about 3.9 smaller than in the comparative example due to the higher conversion in the plurality of reactor stages (R1 to R4) and the intermediate condensation of the products.

[0150] When producing 344 tons of crude methanol per day (t / d) (of which 217.5 t / d is methanol and the remainder is essentially water), Figure 3 The example of requires a compressor power of 783 kW for K1 and a compressor power of 601 kW for K2, which corresponds to a total compressor power of 1384 kW. The total compressor power is thus significantly lower than according to Figure 1The comparison example is slightly lower than that of Figure 2 However, Figure 2 Compared with the configuration of the comparative example, according to Figure 3 The configuration of the example requires only two compressor stages in total. Figure 1 In the comparative example of , the total compressor power is significantly higher with the same number of compressor stages.

[0151] When the input gas has a stoichiometric number SN less than 2, by Figure 3 The surprising effect achieved by the configuration of the example of is even stronger, as shown below for the stoichiometric number SN=1.7. In this second example as well, the input gas contains only carbon dioxide (based on carbon oxides). In order to achieve a relatively low stoichiometric number, the volume flow of carbon dioxide is higher than in the previous example, thus resulting in a fresh gas with an excess of carbon dioxide.

[0152] In this second example (which otherwise uses Figure 3 ), the total compressor output is 1251 kW and is therefore 133 kW lower than the first inventive example (in which SN=2.0). The substoichiometric fresh gas (lacking H2 and having an excess of carbon dioxide) surprisingly further reduces the recycle ratio to 0.41 (i.e., less than half of the recycle gas 305 relative to the fresh gas 302 is required to produce the same amount of 344 t / d of crude methanol 307 (of which 217.5 t / d is methanol and the remainder is essentially water) and has a high hydrogen conversion. The low amount of recycle gas 305 also results in a low required amount of purge gas (which in turn results in savings in hydrogen recovery). In the second example, the hydrogen conversion is 98.8% and is therefore comparable to all previous examples. The carbon conversion is slightly lower, at 89.5%, but due to the low cost of this feedstock, especially when the material originates from a waste gas source, this has no impact on the overall economics of the process.

[0153] The above results are summarized to give an overview in the following table.

[0154]

[0155] Embodiments of the invention have been described with reference to different types of subject matter. In particular, certain embodiments have been described with reference to method claims, while other embodiments have been described with reference to apparatus claims. However, from the above and below descriptions, it will be apparent to a person skilled in the art that, unless otherwise stated, any combination of features relating to different types of subject matter or different types of claims is conceivable in addition to any combination of features belonging to one type of claim. All features may be combined to achieve synergistic effects that go beyond the simple sum of the technical features.

[0156] Although the present invention has been shown and described in detail in the drawings and the preceding description, such showing and description should be considered illustrative or exemplary and not restrictive. The present invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments may be understood and performed by a person skilled in the art of the claimed invention by studying the drawings, the disclosure, and the dependent claims.

[0157] In the claims, the word "having" or "comprising" does not exclude additional elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Reference signs in the claims should not be construed as limiting the scope of the claims.

[0158] List of Reference Numerals

[0159] K1 First compressor stage

[0160] K2 Second compressor stage

[0161] RK Recirculation gas compressor stage

[0162] R1, R2, R3, R4 Reactor stage, Reactor unit

[0163] 101 First fresh gas flow

[0164] 102 Second fresh gas flow

[0165] 103 Synthesis gas flow

[0166] 104a, b Recycle gas flow

[0167] 105 Unreacted Synthesis Gas

[0168] 106 Product stream (crude methanol)

[0169] 107 Purge gas flow

[0170] 201 First fresh gas flow

[0171] 202 Second fresh gas flow

[0172] 203 Third fresh gas flow

[0173] 204 Synthesis gas flow

[0174] 205 Unreacted Synthesis Gas

[0175] 206a, b Recycle gas flow

[0176] 207 Product stream (crude methanol)

[0177] 208 Purge gas flow

[0178] 301 First fresh gas flow

[0179] 302 second fresh gas flow

[0180] 303 Combined gas streams

[0181] 304 Synthesis gas flow

[0182] 305 Recycled gas stream

[0183] 306a, b, c, d Unreacted synthesis gas

[0184] 307 Total product stream (crude methanol)

[0185] 307a, b, c, d Product stream (crude methanol)

[0186] 308 Purge gas flow

Claims

1. A method for producing methanol, comprising the following method steps: a) providing an input gas comprising carbon oxides and hydrogen (H 2 ), wherein the proportion of carbon dioxide (CO 2 ) in the input gas is at least 80% by volume, based on the total amount of the carbon oxides; b) introducing the input gas as a first fresh gas stream into a first compressor stage for precompressing the first fresh gas stream to obtain a second fresh gas stream; c) introducing the recycle gas stream and the second fresh gas stream into a second compressor stage for compressing the recycle gas stream and the second fresh gas stream to a synthesis pressure to obtain a synthesis gas stream; d) catalytically converting synthesis gas in the synthesis gas stream at a synthesis pressure in a plurality of reactor stages arranged in series to obtain a product stream in each reactor stage, the product stream comprising methanol and unreacted synthesis gas; e) cooling the product stream obtained in each reactor stage to condense methanol and separate it from unreacted synthesis gas, and introducing the unreacted synthesis gas into the respective next stage of the reactor stages arranged in series; f) withdrawing unreacted synthesis gas from at least one of the reactor stages as a recycle gas stream for introducing the recycle gas stream into the second compressor stage according to step c).

2. The method according to claim 1, characterized in that After the withdrawal in step f), the recycle gas stream is introduced into the second compressor stage without preceding compression.

3. The method according to claim 1, characterized in that According to step e), the unreacted synthesis gas is completely introduced into the respective next stage of the reactor stages arranged in series.

4. The method according to claim 2, characterized in that: According to step e), the unreacted synthesis gas is completely introduced into the respective next stage of the reactor stages arranged in series.

5. The method according to any one of claims 1 to 4, characterized in that According to step f), unreacted synthesis gas is withdrawn as a recycle gas stream from the last of the plurality of reactor stages arranged in series.

6. The method according to claim 5, characterized in that According to step f), unreacted synthesis gas is withdrawn only from the last of the plurality of reactor stages arranged in series.

7. The method according to any one of claims 1 to 4 and 6, characterized in that: The recycle gas stream and the second fresh gas stream are combined and introduced into the second compressor stage as a combined gas stream.

8. The method according to claim 5, characterized in that The recycle gas stream and the second fresh gas stream are combined and introduced into the second compressor stage as a combined gas stream.

9. The method according to any one of claims 1 to 4, 6 and 8, characterized in that: The input gas has a stoichiometric number SN of 1.5 to 2.2, wherein Wherein n is in [mol].

10. The method according to claim 7, characterized in that The input gas has a stoichiometric number SN of 1.5 to 2.2, wherein Wherein n is in [mol].

11. The method according to claim 9, characterized in that The input gas has a stoichiometric number SN of 1.5 to 1.

9.

12. The method according to claim 9, characterized in that The input gas has a stoichiometric number SN of 1.6 to 1.

8.

13. The method according to claim 10, characterized in that The input gas has a stoichiometric number SN of 1.5 to 1.

9.

14. The method according to claim 10, characterized in that The input gas has a stoichiometric number SN of 1.6 to 1.

8.

15. The method according to any one of claims 1 to 4, 6, 8, 10 to 14, characterized in that For a recirculation rate R defined as: 0.15≤R≤1.25。 16. The method according to claim 15, characterized in that For the recycling rate R: 0.1≤R≤0.

5.

17. The method according to any one of claims 1 to 4, 6, 8, 10 to 14, 16, characterized in that A carbon dioxide conversion of at least 80 mol % based on the input gas is achieved across the plurality of reactor stages arranged in series.

18. The method according to claim 15, characterized in that A carbon dioxide conversion of at least 80 mol % based on the input gas is achieved across the plurality of reactor stages arranged in series.

19. The method according to any one of claims 1 to 4, 6, 8, 10 to 14, 16, 18, characterized in that The plurality of reactor stages arranged in series comprises a number of 2 to 8 reactor stages.

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