RU2795925C2 - Method for producing methanol from synthesis gas with zero emission of carbon dioxide - Google Patents

Abstract

FIELD: methanol production method.

SUBSTANCE: proposed method includes the following stages: (a) in the installation for production of synthesis gas (A) from carbon-containing raw materials (I) synthesis gas is received (II) containing carbon monoxide, carbon dioxide and hydrogen, (b) synthesis gas (II) from stage (a) is sent to the methanol synthesis unit (B) and at a temperature of 150 to 300°C and pressure from 5 to 10 MPa abs. in presence of a methanol synthesis catalyst, it is converted into a reaction mixture containing methanol, water, carbon monoxide, carbon dioxide, hydrogen, dimethyl ether and methane, from which a stream of crude methanol (III) enriched in methanol and water is condensed, and a crude methanol stream (III) alongside a gaseous stream (IV) containing carbon monoxide, carbon dioxide, hydrogen and methane are removed from the methanol synthesis unit (B), (c) a crude methanol stream (III) from step (b) is subjected to a reduced pressure ranging from 0.1 to 2 MPa abs. in a reduced pressure unit (C) and a degassed crude methanol stream (VI) enriched with methanol and water is obtained, (d) in the distillation device (D) low-boiling components (VII), containing carbon dioxide and a simple dimethyl ether, are separated from a degassed crude methanol stream (VI) with stage (c) by distillation stream and the bottom stream (VIII), enriched in methanol and water is received, and (e) water-containing stream of high-boiling components (IX) is separated and methanol is recovered by distillation as stream (X) from the bottom stream (VIII) from step (d) in an additional distillation device (E). Further, in stage (f), the valuable components of streams - carbon monoxide, carbon dioxide, dimethyl ether and methane (IV), as well as at least one of the two streams (V) and (VII), are fed into the combustion unit (F) and are burned by supplying an oxygen-containing gas (XI) which has an oxygen content of 30 to 100% by volume, and a carbon dioxide-containing flue gas (XII) is obtained in step (g) from a carbon dioxide-containing flue gas (XII) from step (f) the carbon dioxide recovery unit (G) to separate the carbon dioxide-rich stream (XIV) and form an off-gas stream (XIII), and in step (h) the carbon dioxide-rich stream (XIV ) from step (g) separated in the carbon dioxide recovery unit (G) is returned to the synthesis gas plant (A) of step (a) and/or to the methanol synthesis plant (B) of step (b).

EFFECT: method has been developed in which carbon monoxide and carbon dioxide with hydrogen contained in synthesis gas are almost completely converted into methanol and, thus, in terms of synthesis gas, it operates with zero carbon dioxide emissions.

17 cl, 6 dwg, 7 tbl, 7 ex

Description

The present invention relates to a process for producing methanol from synthesis gas, in which the carbon-containing compounds of the streams separated during the separation of methanol are converted into carbon dioxide and reused in the production of methanol, avoiding its emissions. The process is based on the continuous synthesis of methanol according to the low pressure process known to the person skilled in the art.

Methanol is one of the most important synthetic raw materials worldwide and, in addition to its use as a solvent, is used in large quantities in the synthesis of formaldehyde, acetic acid, methyl tertiary butyl ether (MTBE), dimethyl terephthalate, methyl methacrylate and methylamines.

On an industrial scale, methanol is produced from synthesis gas in a reactor in the presence of a methanol synthesis catalyst. Synthesis gas mainly contains hydrogen and carbon monoxide and, depending on the type of production and processing, corresponding amounts of carbon dioxide, water and so-called inert gases such as methane, nitrogen or argon.

According to Ullmann's Encyclopedia of Industrial Chemistry chapter "Methanol", 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, the conversion of synthesis gas to methanol usually takes place in the so-called low pressure process in the pressure range of 5 to 10 MPa abs. on copper- and zinc-containing methanol synthesis catalysts. In this process, both carbon monoxide and carbon dioxide are converted to methanol.

Based on reaction equations (1) and (2), the stoichiometric number S for methanol synthesis is calculated as

where n in each case denotes the respective molar amounts. The stoichiometric number S, equal to 2, corresponds to the calculated stoichiometry. However, since hydrogen deficiency significantly reduces selectivity to methanol, a stoichiometric S number slightly above 2 is considered optimal for methanol synthesis.

The synthesis gas used for the synthesis of methanol is usually obtained from natural gas, streams containing other hydrocarbons, and partly from coal gasification or wood gasification. As standard methods for producing synthesis gas in Ullmann's Encyclopedia of Industrial Chemistry, chapter "Methanol", 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, the following four methods are called.

a) Steam reforming, in which the original hydrocarbon gas with the addition of steam and in the absence of oxygen is endothermically split in the presence of a catalyst into hydrogen, carbon monoxide and carbon dioxide.

b) Autothermal reforming, in which the original hydrocarbon gas is exothermically oxidized with oxygen in the presence of a nickel-containing catalyst in part to hydrogen, carbon monoxide and carbon dioxide.

c) Combination of steam reforming and autothermal reforming.

d) Partial oxidation, in which the feed hydrocarbon gas is exothermically oxidized by oxygen in the absence of a catalyst, in part to hydrogen, carbon monoxide and carbon dioxide. However, due to the lack of a catalyst, partial oxidation has practical disadvantages compared to autothermal reforming.

Steam reforming typically produces synthesis gas with a stoichiometric S of about 3. Therefore, there is a significant excess of hydrogen in relation to methanol synthesis. On the contrary, in the case of autothermal reforming and partial oxidation, synthesis gas with a stoichiometric number S < 2 is formed. Therefore, in particular, the combination of steam reforming and autothermal reforming offers a technically common option that allows you to purposefully set the stoichiometric number S to a technically significant value > 2.

Since the hydrocarbon feedstock is usually not completely converted in syngas production, the syngas typically contains more or less significant amounts of unconverted hydrocarbons, such as, for example, methane. In addition, when using air as an oxidizing agent, more or less significant amounts of extraneous inert gases, such as, for example, nitrogen or argon, are introduced in the production of synthesis gas. As a rule, they are not specially separated, but together with valuable hydrogen, carbon monoxide and carbon dioxide, they are sent to the synthesis of methanol.

Even if the reaction equations (1) and (2) are equilibrium equations and a methanol synthesis catalyst is present, the equilibrium is not quantified. Therefore, immediately after the synthesis of methanol, the reaction mixture usually contains only about 5-15 wt.% methanol and considerable amounts of unreacted hydrogen, carbon monoxide, carbon dioxide and foreign gases such as methane, nitrogen or argon. In addition, the reaction mixture also contains by-products such as, for example, dimethyl ether. The prior art describes various methods for processing the resulting reaction mixture. The main steps are usually the gradual concentration of methanol, the most efficient handling of valuable gases and the prevention of the accumulation of foreign gases.

Thus, in the Lurgi MegaMethanol process described in Ullmann's Encyclopedia of Industrial Chemistry, chapter "Methanol", 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, synthesis gas is converted to methanol in the presence of a heterogeneous catalyst in a methanol synthesis reactor, and the methanol-rich crude methanol stream is first condensed from the resulting reaction mixture. The remaining gas stream contains, inter alia, unconverted hydrogen and is sent to pressure swing adsorption to separate it. The hydrogen thus liberated is returned to the methanol synthesis. The gas stream not absorbed by pressure swing adsorption is finally sent to thermal utilization. The methanol-enriched liquid stream obtained from the aforementioned condensation is then subjected to reduced pressure for degassing, and the resulting gas is also sent to thermal utilization. The remaining after degassing, further enriched with methanol, the liquid stream is then subjected to multi-stage distillation to directly extract the methanol. In this case, the flow of exhaust gases from the column of low-boiling compounds is also sent for thermal utilization.

In the case of the described Lurgi MegaMethanol process, unconverted hydrogen is separated and returned to synthesis, however, all other valuable gases after its separation are sent exclusively for thermal utilization. Therefore, unreacted carbon monoxide and carbon dioxide also remain unused for further production of methanol. Even if they remain unused for further production of methanol, their quantitative streams must be taken into account in plant design, for example, in the form of the required dimensions of the apparatus for handling quantitative streams. In addition, the handling of these quantitative streams naturally also requires energy, for example in the form of heating, compression or pumping energy, which is not used for further methanol synthesis. In addition, the thermal utilization of valuable gases leads to an additional increase in carbon dioxide emissions. Combustible gases, which also include carbon monoxide, are converted into carbon dioxide; and the already present carbon dioxide passes through thermal utilization unchanged.

In European patent EP 2228358 A1, it was found that the carbon dioxide contained in the crude methanol after its production from synthesis gas is also a valuable starting material for the further synthesis of methanol. In particular, this applies to the synthesis of methanol, in which the synthesis gas was obtained by steam reforming, since in this case there is a significant excess of hydrogen due to the stoichiometric S number of about 3. In the European patent EP 2228358 A1, it is reported that containing dioxide carbon streams from methanol synthesis can be returned to the reformer without further processing as a feed gas stream to produce synthesis gas and thus carbon dioxide can again be used as a source of carbon for methanol synthesis. In particular, figure 1 of patent EP 2228358 A1 discloses the recycling of the degassed reduced pressure gas "9" from the methanol reactor effluent and also the off-gas stream "10" of the low-boiling column as an additional feed gas stream to the reformer. . The off-gas "18" from the hydrogen recovery is fed to the reformer as fuel gas. EP 2228358 A1 also proposes to separate the carbon dioxide contained in the flue gas of the reformer as a stream "11" by means of the carbon dioxide recovery unit and feed it to the reformer as a feed gas stream and thus further use in the subsequent methanol synthesis.

The main disadvantage of the described process is that by direct recirculation of the reduced pressure degassing gas and the off-gas stream from the low-boiling column, inert gases such as nitrogen and argon are also recycled and therefore accumulate in the process. Also, if the hydrogen regeneration off-gas stream is returned to the reformer as a fuel gas, and as a result a certain accumulation of inert gases is caused, the amount of circulating gas increases, and the conversion of methanol calculated from the amount of circulating gas decreases. In addition, the reformer must also be designed to accommodate the extra gas due to the return. The recovery of carbon dioxide from the flue gas of the reformer also requires a corresponding large gas treatment plant, including its energy integration into the overall complex.

US Patent US 8,829,059 also covers the recovery and reuse of carbon dioxide from methanol synthesis. In particular, the document describes a methanol synthesis process in which the reaction mixture from the methanol synthesis reactor is fed to a condenser for condensing water-containing crude methanol, and the remaining uncondensed gas is separated into two partial streams. In this case, one partial stream is passed through the permeate side of the carbon dioxide sensitive membrane unit, and then again through the synthesis gas compressor is returned to the methanol synthesis reactor. The other partial stream is sent to the retentate side of the hydrogen sensitive membrane unit, in which a hydrogen-rich stream is generated on the permeate side for reuse in methanol synthesis. The hydrogen-depleted retentate side stream is then sent to the retentate side of the aforementioned carbon dioxide sensitive membrane unit to increase the carbon dioxide content on the permeate side and thus recycle more carbon dioxide into the methanol synthesis. The described method is shown in Figure 3 of US Pat. No. 8,829,059 as a block diagram.

The use of membrane units in plants the size of a commercial methanol synthesis plant is relatively difficult because large membrane areas are required to handle large gas flows. This is especially applicable when a high degree of separation is to be achieved. Conversely, at a low degree of separation, part of the hydrogen and carbon dioxide would respectively remain on the retentate side and be removed unused from the methanol synthesis process. In addition, there is a risk that the membrane will eventually lose its permeability due to deposits of various impurities and/or by-products of methanol synthesis, and it will need to be replaced or cleaned from time to time at great expense. In addition, US Pat. No. 8,829,059 is silent on further processing of the condenser-condensed water and methanol stream, which typically still contains significant amounts of carbon dioxide dissolved in methanol and water.

S. Reddy et al. in Energy Procedure 63 (2014) 1407-1414 also propose the use of carbon dioxide from the synthesis gas production process or respectively from the carbon dioxide containing streams from the methanol synthesis for further methanol synthesis. In particular, the publication proposes to absorb carbon dioxide from the flue gas of a syngas reformer by means of an "Econamine FG PlusSM" gas scrubber in an amine solvent, then recover it as a carbon dioxide stream, and then redirect it either together with the feed gas stream to reformer to produce synthesis gas or directly to synthesis gas. In addition, the publication states that the so-called purge gases and off-gases from processing the methanol stream can be returned directly to the compression stage before the methanol synthesis reactor and/or fed into the reformer as fuel gas. In both cases, the carbon dioxide contained in them is again returned to the synthesis of methanol. The measures described should allow for an increase in methanol production capacity of approximately 20%.

Also in the case of this process, the direct return of the purge gases and off gases to the methanol synthesis reactor is disadvantageous, since this causes the accumulation of inert gases such as nitrogen and argon, as well as methane or by-products, such as dimethyl ether, in the synthesis circulation loop. In the embodiment in which said gas streams are fed to the fuel gas in the reformer, the accumulation of inert gases and by-products is suppressed, but due to the additional amount of gas, the reformer must be designed correspondingly large. The recovery of carbon dioxide from the flue gas of the reformer also requires a corresponding large gas treatment plant, including its energy integration into the overall complex.

The object of the present invention was to find a process for producing methanol from synthesis gas which does not suffer from the aforementioned disadvantages or only to a small extent, and which almost completely converts the carbon monoxide and carbon dioxide contained in the synthesis gas with hydrogen into methanol. and thus, in terms of synthesis gas, it operates with zero carbon dioxide emissions. The process according to the invention should also be simple to implement and make as much use of the technical equipment and connections of established methanol synthesis processes. Thus, the process according to the invention should also allow easy retrofitting of existing methanol synthesis processes. For the sake of simplicity, technical interventions, modifications or additions to the synthesis gas production process should be avoided whenever possible.

A method for producing methanol was found, in which

(a) in the plant for the production of synthesis gas (A) from carbon-containing raw materials (I) receive synthesis gas (II) containing carbon monoxide, carbon dioxide and hydrogen,

(b) synthesis gas (II) from stage (a) is sent to the methanol synthesis unit (B) and at a temperature of 150 to 300 °C and a pressure of 5 to 10 MPa abs. in the presence of a methanol synthesis catalyst, it is converted into a reaction mixture containing methanol, water, carbon monoxide, carbon dioxide, hydrogen, dimethyl ether and methane, from which a stream of crude methanol (III) enriched in methanol and water is condensed and removed from the methanol synthesis unit (B ) a raw methanol stream (III) and a gaseous stream (IV) containing carbon monoxide, carbon dioxide, hydrogen and methane,

(c) the crude methanol stream (III) from step (b) is subjected to a reduced pressure in the range of 0.1 to 2 MPa abs. in an underpressure plant (C) and a degassed gas (V) containing carbon dioxide and methane, as well as a degassed raw methanol stream (VI) enriched in methanol and water, is obtained,

(d) from the degassed raw methanol stream (VI) from step (c) in the distillation device (D) the low-boiling component stream (VII) containing carbon dioxide and dimethyl ether is separated by distillation, and a bottom stream (VIII) enriched in methanol and water is obtained , And

(e) from the bottom stream (VIII) from step (d) in an additional distillation device (E) the water-containing stream of high-boiling components (IX) is separated and methanol is recovered by distillation as a stream (X),

and which is different in that

(f) valuable components - carbon monoxide, carbon dioxide, dimethyl ether and methane, streams (IV), as well as at least one of the two streams (V) and (VII), are fed into the combustion unit (F), and in it combustion by supplying an oxygen-containing gas (XI), which has an oxygen content of 30 to 100% by volume, and a carbon dioxide-containing flue gas (XII) is obtained,

(g) separating the carbon dioxide-rich flue gas (XII) from step (f) in the carbon dioxide recovery unit (G) to form an off-gas stream (XIII), and

(h) the carbon dioxide recovery unit (G) separated and the carbon dioxide-enriched stream (XIV) from step (g) is returned to the synthesis gas plant (A) of step (a) and/or the methanol synthesis unit (B) step (b).

The method according to the invention is based on the continuous synthesis of methanol according to the low pressure process known to the person skilled in the art, in which the synthesis gas at a pressure of 5 to 10 MPa abs. in the presence of a methanol synthesis catalyst, it is converted into a methanol-containing reaction mixture, and then processed step by step to isolate methanol. In this step, various streams are separated which still contain valuable components from the unconverted starting materials or by-products, such as, for example, carbon monoxide, carbon dioxide, dimethyl ether, methane or other by-products. The essence of the invention lies in the reuse of the carbon of these valuable components for the further synthesis of methanol while preventing carbon dioxide emissions. The method according to the invention is explained in more detail below.

In FIG. 1 shows a block diagram of a general embodiment of the method according to the invention, in which all three streams (IV), (V) and (VII) are fed to the combustion unit (F). On which stands

(A) Syngas production unit

(C) Methanol synthesis unit

(C) Reduced pressure setting

(D) Distillation plant

(E) Distillation plant

(F) Incineration plant

(G) Carbon dioxide recovery plant

(I) Carbonaceous raw materials

(II) Syngas

(III) Raw methanol stream

(IV) Containing carbon dioxide, carbon monoxide, hydrogen and methane gaseous stream

(V) Depressurized gas containing carbon dioxide and methane

(VI) Degassed crude methanol stream

(VII) The flow of low-boiling components from the distillation plant (D)

(VIII) Bottom stream from distillation plant (D)

(IX) Stream of high-boiling components from the distillation plant (E)

(X) Methanol

(XI) Oxygen-containing gas

(XII) Flue gas

(XIII) Off-gas flow from the carbon dioxide recovery unit (G)

(XIV) Carbon dioxide rich stream from carbon dioxide recovery unit (G)

The dotted lines in the stream (XIV) symbolize that the stream (XIV) according to the invention can be returned both to the synthesis gas plant (A) of stage (a) and to the methanol synthesis unit (B) of stage (b), or also as a split flow to both installations (A) and (B).

In the process according to the invention, a wide variety of carbon-containing feedstocks can be used as feedstock for producing synthesis gas, regardless of whether they are in solid, liquid or gaseous form, and regardless of their chemical nature. So, for example, both coal and hydrocarbons, as well as compounds containing carbon and hydrogen, can be used to produce synthesis gas. As preferred carbonaceous feedstock, mention should be made of natural gas, biogas, coal, wood, plastics, crude oil, bionaphtha or hydrocarbonaceous streams from petroleum or natural gas processing, from chemical production processes, from renewable raw materials or from plastics processing. In the case of coal or wood, the production of synthesis gas is carried out, for example, by means of a gasification process, also called coal gasification or, respectively, wood gasification. Suitable feedstocks from petroleum or natural gas processing are, for example, naphtha, LPG, gasoline, heavy oil or vacuum residue. Hydrocarbon-containing streams from chemical production processes are to be understood, for example, as hydrocarbon-containing streams that are formed as by-products and, instead of purely thermal utilization, can also be used as feedstock for the production of synthesis gas.

The use of methane-containing streams is particularly preferred, and natural gas or biogas is most preferred. In this case, it is possible and even advantageous to supply carbon dioxide also present in natural gas, and in particular in biogas, to the synthesis gas production plant (A).

In the case of the process according to the invention, in the so-called synthesis gas plant (A) from the carbonaceous raw material (I), synthesis gas (II) containing carbon monoxide, carbon dioxide and hydrogen is first produced. In the process according to the invention as natural gas (I), for example, in principle all natural gases which are suitable for producing synthesis gas as a feedstock for methanol synthesis can be used. Natural gas usually contains from 75 to 100% vol. methane. In addition to methane, homologous hydrocarbons - ethane, propane, butane, and also ethylene - should be mentioned as accompanying substances. Also in the process according to the invention, for example, in principle all biogases which are suitable for the production of synthesis gas as a feedstock for the synthesis of methanol can be used. Biogas usually contains from 40 to 75% methane by volume and as accompanying substances mainly carbon dioxide, water, nitrogen and oxygen.

Typically, the production of synthesis gas (II) is carried out using production methods commonly used on an industrial scale, and naturally the type of carbon-containing feedstock (I) also plays a role in this case. In the case of a methane-containing feedstock such as natural gas or biogas, in step (a), synthesis gas (II) is preferably produced by steam reforming, by autothermal reforming, by a combination of steam reforming and autothermal reforming, or by partial oxidation.

A particular advantage of partial oxidation is that no separate fuel gas needs to be supplied to the synthesis gas plant and therefore no flue gas containing carbon dioxide is generated. In the case of partial oxidation, the energy required to produce synthesis gas is released directly by partial oxidation from the methane-containing feedstock, and the resulting flue gases, carbon dioxide and carbon monoxide, are also used in the synthesis of methanol. Therefore, in the process according to the invention, the partial oxidation of methane-containing streams, such as, for example, natural gas or biogas, is preferred.

Commonly used connecting lines, apparatuses, process parameters, auxiliaries and treatment of the obtained raw synthesis gas are well known to the person skilled in the art and are described in detail in the prior art.

The obtained synthesis gas (II) contains carbon monoxide, carbon dioxide and hydrogen, and their total concentration is usually from 50 to 100% vol., preferably ≥ 80% vol. and especially preferably ≥ 90% vol. Possible co-substances to be mentioned are, first of all, the unconverted components of the carbonaceous feedstock used, as well as the by-products of its conversion, such as, for example, nitrogen, argon, water or methane. Typically, in addition to carbon monoxide, carbon dioxide and hydrogen, the synthesis gas (II) also contains methane due to the production process, as well as nitrogen and argon, which, for example, were introduced into the synthesis gas production process by using air.

Depending on the type of production of synthesis gas (II), it contains different amounts of carbon monoxide, carbon dioxide and hydrogen. Significant for the synthesis of methanol and a typical parameter for the characterization of synthesis gas is the stoichiometric number S, which was already mentioned at the beginning and is defined as

where n in each case denotes the respective molar amounts. The synthesis gases (II) obtained in step (a) and used in step (b) can vary over a wide range with respect to their stoichiometric number S. However, in general, synthesis gas (II) with a stoichiometric number S is introduced into the methanol synthesis unit (B) S in the range of 1 to 5, preferably ≥ 1.3, particularly preferably ≥ 1.5 and most preferably ≥ 2, and also preferably ≤ 4, particularly preferably ≤ 3 and most preferably ≤ 2.5.

The conversion of synthesis gas (II) is carried out in the so-called methanol synthesis unit (B) at a temperature of 150 to 300 °C and a pressure of 5 to 10 MPa abs. in the presence of a methanol synthesis catalyst. To do this, the synthesis gas (II) is usually compressed to the desired pressure by means of a compressor and, under the indicated conditions, subjected to conversion in a reactor.

The transformation is carried out preferably at a temperature of ≥ 170 °C and particularly preferably ≥ 190 °C, and also preferably at ≤ 280 °C and particularly preferably at ≤ 260 °C. With regard to pressure, the conversion is preferably carried out at ≥ 6 MPa abs., and also preferably at ≤ 9 MPa abs.

As reactors, in principle, all reactors can be used which are suitable for the exothermically proceeding conversion of synthesis gas into methanol under the aforementioned process conditions. Reactors for the synthesis of methanol from synthesis gas are generally known to the person skilled in the art. Examples are those specified for this purpose in Ullmann's Encyclopedia of Industrial Chemistry, Chapter "Methanol", Section 5.2.1 "Reactor Design", 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, adiabatic and quasi-isothermal reactors, Variobar reactors and the so-called double-walled superconverter.

As catalysts for the synthesis of methanol in the process according to the invention, in principle, all catalysts suitable for the conversion of synthesis gas into methanol under the aforementioned process conditions can be used. Appropriate methanol synthesis catalysts are generally known to the person skilled in the art. Examples are those specified for this purpose in Ullmann's Encyclopedia of Industrial Chemistry, chapter "Methanol", section 4.2 "Catalysts", 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, copper- and zinc-containing heterogeneous catalysts. As a rule, they also contain other elements such as, for example, aluminium, rare earth elements or chromium.

In the case of conversion of synthesis gas (II) containing carbon monoxide, carbon dioxide and hydrogen, methanol and water are formed in accordance with the reaction equations (1) and (2)

Dimethyl ether is formed as a standard by-product. In addition, the complete hydrogenation of carbon monoxide or carbon dioxide, respectively, also produces methane as an additional by-product. Therefore, the reaction mixture formed in the reactor contains methanol, water, dimethyl ether, carbon monoxide, carbon dioxide, hydrogen and methane. In addition, additional by-products, such as, for example, methyl formate, acetic acid, homologous alcohols with ≥ 2 carbon atoms, esters and ethers with ≥ 2 carbon atoms, as well as paraffins, are usually also formed under the indicated reaction conditions.

To separate the complex reaction mixture, the crude methanol (III) stream enriched in methanol and water is first condensed. For this, the reaction mixture formed in the reactor is usually fed into a condenser. As condensers, devices known to the person skilled in the art can be used, which are suitable under given conditions for obtaining a condensate enriched in methanol and water by targeted cooling. Typically, the reaction mixture is cooled to a temperature below the dew point of methanol. In accordance with the solubility and vapor pressure of the components contained in the reaction mixture, the stream of crude methanol (III), enriched in methanol and water, also contains gases dissolved in it, such as hydrogen, carbon monoxide, carbon dioxide, dimethyl ether, methane and components with boiling point higher than that of methanol. The condensed crude methanol stream (III) is then discharged from the methanol synthesis unit (B) for further processing and sent to step (c).

The uncondensed gas stream contains, in particular, unreacted starting materials carbon monoxide, carbon dioxide and hydrogen, as well as methane. In order to maintain a high partial pressure of the hydrogen, carbon monoxide and carbon dioxide components of the synthesis gas, a portion of the non-condensed gas stream is usually withdrawn. If necessary, said effluent gas stream is sent to hydrogen separation in order to increase the hydrogen partial pressure in the reactor. A higher partial pressure of hydrogen in the reactor reduces the formation of secondary components and in particular also suppresses the Fischer-Tropsch reaction. The main part of the uncondensed gas stream is returned to the methanol synthesis unit as cycle gas and passed through the methanol synthesis catalyst to achieve the best use of synthesis gas and hence high methanol yields.

Accordingly, the methanol synthesis plant (B) in step (b) advantageously comprises a compressor for compressing the synthesis gas (II), a reactor for converting the synthesis gas (II), a condenser for condensing the raw methanol stream (III) and a line for returning the uncondensed gas into the reactor.

Thus, depending on the composition of the supplied synthesis gas (II), it is already possible to set the stoichiometric number S > 2 at the reactor inlet. In the process according to the invention, a stoichiometric number S preferably ≥ 2.5 and especially preferably ≥ 2.8, and also preferably ≤ 4, and especially preferably ≤ 3.8.

The non-condensed gas stream not recycled as cycle synthesis gas is withdrawn as a gaseous stream (IV) from the methanol synthesis unit (B) and sent to stage (f) according to the invention. The amount of the output gas stream (IV) is determined by the mass balance supplied to the methanol synthesis unit (B) and discharged from it streams.

In step (c) of the process according to the invention, the crude methanol stream (III) condensed in step (b) and withdrawn from the methanol synthesis unit (B) is subjected to a reduced pressure in the range of 0.1 to 2 MPa abs. in an underpressure plant (C) and a degassed gas (V) containing carbon dioxide and methane and a degassed raw methanol stream (VI) enriched in methanol and water is obtained. The exposure to reduced pressure is usually carried out in an apparatus in which the gas phase and the liquid phase can be easily separated from each other. Typically, in the case of this apparatus, we are talking about a droplet separator. Apparatus suitable for this are known to the person skilled in the art.

Exposure to reduced pressure is usually carried out at a pressure of ≥ 0.2 MPa abs. and especially preferably ≥ 0.4 MPa abs., and also preferably ≤ 1.5 MPa abs. and especially preferably ≤ 1 MPa abs. As a rule, the temperature of the mixture subjected to reduced pressure is from 0 to 150 °C, preferably ≥ 10 °C and particularly preferably ≥ 20 °C, and also preferably ≤ 120 °C and particularly preferably ≤ 60 °C.

The degassed crude methanol stream (VI) is additionally enriched in methanol and water, however, according to the solubility and vapor pressure of the components contained in the crude methanol stream (III), also contains additional components, such as, for example, gases dissolved in it, such as hydrogen , carbon monoxide, carbon dioxide, dimethyl ether, methane, or components with a higher boiling point than methanol.

The gas (V) liberated during the reduction of pressure, containing carbon dioxide and methane, is preferably sent to stage (f) according to the invention. As an alternative, however, the gas (V) liberated on depressurization can also be withdrawn from the methanol synthesis plant and, for example, disposed of thermally or otherwise. However, its use within the methanol synthesis plant as a feed stream to the combustion unit (F) is preferred.

In step (d) of the process according to the invention, the degassed crude methanol stream (VI) obtained in step (c) is separated by distillation in a distillation device (D) into a low-boiling component stream (VII) containing carbon dioxide and dimethyl ether, and a bottom stream, enriched with methanol and water (VIII). Suitable distillation devices are, in principle, devices known to the person skilled in the art for such separation tasks, or devices that can be designed taking into account his general professional capabilities. The distillation separation in step (d) is usually carried out in one distillation column, although, of course, several distillation columns can also be used in parallel. In addition to the actual column body with built-in elements, the distillation column, as usual, also contains a dephlegmator and a bottom evaporator. The column body may be equipped with, for example, packings, bulk packings or trays. For the construction and operation of the distillation apparatus (D), the general knowledge of a person skilled in the art is sufficient.

The distillation-separated low-boiling component stream (VII) mainly contains carbon dioxide and dimethyl ether as separated low-boiling components and, depending on the composition of the degassed crude methanol (VI) stream, other low-boiling components such as methane, and also, depending on the efficiency separation and mode of operation of the distillation device, methanol or components having a boiling point higher than that of methanol, such as water. The low-boiling component stream (VII) containing carbon dioxide and dimethyl ether is preferably also sent to stage (f) according to the invention. If the depressurized gas (V) is fed into the combustion unit (F) and is thus already reused in the methanol synthesis unit, the low-boiling component stream (VII) can alternatively also be withdrawn from the methanol synthesis unit and disposed of, for example , thermally or otherwise. However, its use within the methanol synthesis plant as a feed stream to the combustion unit (F) is preferred.

The methanol-and-water-rich bottom stream (VIII) also contains additional components that boil higher than methanol, such as methanol synthesis by-products that boil higher than methanol, such as acetic acid, higher carbon alcohols, esters with a large number of carbon atoms, ethers with a large number of carbon atoms or paraffins.

To isolate the methanol, finally in step (e), the water-containing stream of high-boiling components (IX) is separated from the bottom stream (VIII) obtained in step (d) in an additional distillation device (E) and methanol is recovered by distillation as stream (X). Suitable distillation devices for step (e) are, in principle, devices known to the person skilled in the art for such separation tasks, or devices that can be designed taking into account his general professional abilities. In general, the distillation separation in step (e) may take place in a single distillation column. In addition to the actual column body with built-in elements, the distillation column, as usual, also contains a dephlegmator and a bottom evaporator. The column body may be equipped with, for example, packings, bulk packings or trays. Of course, it is also possible to use several distillation columns in parallel or to separate the methanol in stages in several distillation columns. The general knowledge of a person skilled in the art is sufficient for the construction and operation of the distillation apparatus (E).

The methanol stream (X) is usually recovered as an overhead fraction. In principle, however, it is also possible to separate the methanol in the form of a so-called side stream, and to separate the still present low-boiling components as an overhead stream.

An energetically attractive option is the so-called double pressure distillation. In this case, two distillation columns are connected in series and energetically connected to each other. The first distillation column is then operated under pressure, typically 0.5 to 1.5 MPa abs., and the methanol is separated through the top of the column as a stream of low boiling components. The first distillation column thus operates in such a way that part of the methanol remains in the cube, and it is fed into the second distillation column. The second distillation column is operated at a lower pressure, such as atmospheric pressure. The bottom of the second distillation column in this case is energetically connected with the flow of low-boiling components of the first distillation column, which means that the thermal energy released during cooling of the low-boiling stream of the first distillation column serves to heat the second distillation column. In the second distillation column, methanol is also separated through the top of the column as a low boiling point stream. High-boiling components entering the bottom of the second distillation column are separated and removed. The design and operation of dual pressure distillation, and in particular dual pressure distillation to produce methanol, is known to the person skilled in the art.

The high boiling component stream (IX) contains water as well as additional components that boil higher than methanol, such as methanol synthesis by-products that boil higher than methanol, such as acetic acid, higher carbon alcohols, esters with with more carbon atoms, ethers with more carbon atoms or paraffins. Said stream may, for example, be directed to waste water treatment.

High purity methanol ≥ 95 wt %, preferably ≥ 98 wt %, can be recovered through stream (X). and especially preferably ≥ 99% of the mass. Residual quantities of low-boiling and high-boiling components not completely separated by distillation, in particular water, and very small amounts of ethanol, esters and ethers, should be mentioned as accompanying substances.

In the case of stepwise treatment of the reaction mixture to isolate methanol as stream (X), streams (IV), (V) and (VII) are separated. However, they still contain valuable components such as, for example, carbon monoxide, carbon dioxide, methane and dimethyl ether. The essence of the invention lies in the significant reuse of the carbon of these valuable components for the further synthesis of methanol while preventing carbon dioxide emissions.

Steps (f) to (h) essential to the invention are explained below in more detail.

In the case of the method according to the invention

(f) valuable components - carbon monoxide, carbon dioxide, dimethyl ether and methane, streams (IV), as well as at least one of the two streams (V) and (VII), are fed into the combustion unit (F), and in it combustion by supplying an oxygen-containing gas (XI), which has an oxygen content of 30 to 100% by volume, and a carbon dioxide-containing flue gas (XII) is obtained,

(g) separating the carbon dioxide-rich flue gas (XII) from step (f) in the carbon dioxide recovery unit (G) to form an off-gas stream (XIII), and

(h) the carbon dioxide recovery unit (G) separated and enriched in carbon dioxide stream (XIV) from step (g) is recycled to the synthesis gas plant (A) of step (a) and/or to the methanol synthesis unit (B ) of step (b).

Preferably, in step (f), the valuable components carbon monoxide, carbon dioxide, dimethyl ether and methane of streams (IV), (V) and (VII) are fed to the combustion unit (F).

Since the valuable methane and dimethyl ether components of streams (IV), (V) and (VII), as well as other carbon-containing by-products contained therein, cannot be used directly in this form as starting materials for the synthesis of methanol, it is necessary first to chemically convert them into a suitable shape. Therefore, the solution according to the invention is to burn said streams in a combustion plant (F) to form a flue gas containing carbon dioxide. By converting to carbon dioxide, it is also possible to reuse valuable components of methane, dimethyl ether and other carbonaceous by-products in methanol synthesis as a starting material. Therefore, in the case of the method according to the invention, the streams (IV), (V) and (VII) are first fed into the combustion plant (F). In it, combustible valuable components are burned with the supply of oxygen-containing gas (XI), which has an oxygen content of 30 to 100% by volume. In this case, combustion usually takes place in a so-called combustion chamber with the formation of carbon dioxide and water. Typically, combustion takes place at atmospheric pressure. However, it is also possible to carry out combustion at a lower or higher pressure. For accuracy, the pressure range should be specified from 0.05 to 0.5 MPa abs.

As combustion chambers, in principle, all devices suitable for the highly exothermic oxidation of a respective stream containing carbon monoxide, methane and dimethyl ether with an oxygen-containing gas having an appropriate oxygen content can be used. Suitable combustion chambers are known to those skilled in the art or can accordingly be designed and operated with the general knowledge of a person skilled in the art. Examples which may be mentioned are adiabatic combustors or reactors with heat removal, such as directly heated steam generators, for example in the form of water tube boilers or fire tube or fire tube boilers.

A significant advantage proposed in the invention of the use of oxygen-containing gas (XI) with an oxygen content of from 30 to 100% vol. is a significantly higher content of carbon dioxide in the resulting combustion gas compared to the use of air containing only about 21% vol. oxygen. Accordingly, in the method according to the invention also the proportion of undesired inert gases such as nitrogen or argon is, of course, significantly lower compared to using air as oxidant.

Preferably, the combustion plant (F) is fed with an oxygen-containing gas (XI) with an oxygen content of ≥ 50% by volume, particularly preferably ≥ 80% by volume, most preferably ≥ 90% by volume, in particular ≥ 95% by volume. and in particular pure oxygen. When pure oxygen is used for combustion, one speaks of the so-called oxy-fuel method.

Usually ≥ 90%, preferably ≥ 95%, particularly preferably ≥ 98% and particularly preferably ≥ 99% of the carbon of the combustible components is converted into carbon dioxide.

Another advantage of using an oxygen-enriched gas instead of air as an oxidant is a significant reduction in thermally generated NOx. By using pure oxygen and only a very low nitrogen content in the feed streams (IV), (V) and (VII), the formation of nitrogen oxides NOx can be almost completely avoided.

In addition, combustion processes using oxygen-enriched gases as oxidizers are more energy efficient than air due to higher combustion temperatures. For example, burning pure methane with air results in a design combustion temperature of about 1800 to 2000°C, while burning pure methane with pure oxygen gives a design combustion temperature of about 4000 to 5000°C. In order to counteract the particularly high combustion temperature, primarily for material technology reasons, in the combustion according to the invention, a part of the flue gas after cooling is usually returned to the combustion chamber for temperature control.

Since when an oxygen-containing gas (XI) with a correspondingly high oxygen content is used, the resulting flue gas only has a correspondingly low nitrogen content, or even practically no nitrogen, in general, a correspondingly small separation stage is sufficient for separating nitrogen from carbon dioxide.

In order to further increase the carbon dioxide content of the resulting combustion gas, it is generally preferred to reduce the water content before leaving the combustion unit (F). This is usually done by simple condensation in a condenser. As condensers, devices known to the person skilled in the art can be used, which are suitable under given conditions for separating water in the form of condensate from a gas stream containing carbon dioxide and water by targeted cooling. Typically, the combustion gas is cooled to a temperature below the dew point of water. When the water condenses, it is removed from the combustion plant (F) as stream (XV) and the remaining water-depleted combustion gas as flue gas containing carbon dioxide (XII). Preferably, 50 to 100%, particularly preferably ≥ 80% and particularly preferably ≥ 90% of the water contained in the combustion gas is condensed.

Therefore, a method is preferred in which the combustion plant (F) in step (f) comprises a combustion chamber and a condenser, water is condensed in the condenser from the combustion gas obtained in the combustion chamber, and discharged as a stream (XV) from the combustion plant ( F), and the remaining gaseous stream forms flue gas containing carbon dioxide (XII). In FIG. 2 shows a block diagram of a general embodiment in which water is condensed from the combustion gas and removed as a stream (XV).

However, it is also possible, although not preferred, to leave the water entirely in the combustion gas and discharge it from the combustion plant (F) together with the carbon dioxide in the form of carbon dioxide-containing flue gas (XII).

An analysis of the results unequivocally shows a significant increase in the carbon dioxide content of the flue gas (XII) as a result of the use of pure oxygen as an oxidizing agent compared to air, and also as a result of the subsequent separation of water. The theoretical calculation of the combustion of pure methane with the formation of carbon dioxide and water in the case of air as an oxidant leads to a carbon dioxide content in the combustion gas of about 9% vol., and even after complete separation of water, only 12% vol. On the contrary, the use of pure oxygen as an oxidizing agent already results in a carbon dioxide content in the combustion gas of about 33% vol. By completely condensing the water, even a calculated carbon dioxide content of 100% by volume can be achieved. This corresponds to an increase in the content of carbon dioxide when using pure oxygen compared to air without separation of water by 3.7 times (9% vol. vs. 33% vol.), and even 8.3 times with separation of water (12% vol.). vs. 100% vol.). Depending on the actual composition of the gas fed to the combustion plant (F), of course, slightly different coefficients are obtained. However, the use of pure oxygen as oxidizer results in a significantly higher carbon dioxide content in the combustion gas and in particular, after water condensation, also in the effluent flue gas(XII).

In particular, due to the use of pure oxygen as oxidant, the flue gas (XII) has a significantly higher content of the valuable carbon dioxide component. In other words, as a result of using pure oxygen with the same absolute amount of carbon dioxide, significantly less flue gases are produced. Thus, in the method according to the invention, the dimensions of the combustion plant (F), and in particular the combustion chamber and condenser, as well as further devices and lines, can be correspondingly smaller.

In addition to carbon dioxide as a valuable component, the resulting flue gas (XII) usually also contains various inert gases such as nitrogen or argon, which enter the combustion plant (F) via flows (IV), (V) and (VII), and /or oxygen-containing gas (XI). Since combustion is also usually incomplete with respect to the oxygen supplied, the flue gas (XII) usually also contains unconverted oxygen, usually in the range of 2 to 20% by volume. Depending on whether water has been condensed from the flue gas and in what quantity, the flue gas (XII) also contains water formed during combustion or, respectively, the part not condensed after water condensation. Typically, the flue gas (XII) discharged from the combustion plant (F) has a carbon dioxide content of 25 to 90% by volume, preferably ≥ 50% by volume, particularly preferably ≥ 60% by volume. and most preferably ≥ 70% vol., and also preferably ≤ 85% vol. and especially preferably ≤ 80% vol.

In step (g), the carbon dioxide-containing flue gas (XII) generated in step (f) is separated in the carbon dioxide recovery unit (G) from the carbon dioxide-rich stream (XIV) to form an off-gas stream (XIII). As a carbon dioxide recovery plant (G), practically all devices and processes suitable for separating and concentrating carbon dioxide from a carbon dioxide-containing gas stream, which, in addition to carbon dioxide, also contains inert gases such as nitrogen and argon, can be used, and also, if necessary, water as additional components. Appropriate devices and methods are known to the person skilled in the art. A general overview of possible devices and methods can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, chapter "Carbon Dioxide", section 13.3 "CCS-related Separation Technologies", 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. As examples, mention should be made of so-called scrubbing processes, in which carbon dioxide is physically or chemically absorbed by a so-called scrubbing solvent and then desorbed again, as well as membrane processes, in which carbon dioxide is separated through a carbon dioxide-sensitive membrane.

In the case of physical absorption, carbon dioxide is absorbed in the absorber at elevated pressure in a suitable solvent such as methanol, N-methylpyrrolidone, or polyethylene glycol dimethyl ether, and then desorbed again in the desorber under reduced pressure. The carbon dioxide depleted solvent is usually returned to the absorber. Typically, physical absorption requires a pressure in the range of about 2 to 5 MPa abs. However, since the carbon dioxide-containing flue gas (XII) usually has a pressure of only 0.1 to 0.3 MPa abs., it must first be compressed with the application of energy.

Therefore, in the method according to the invention, chemical absorption in a solvent which chemically binds carbon dioxide is preferred. The chemical absorption generally does not require increased pressure, so that the carbon dioxide-containing flue gas (XII) does not need to be subjected to energy-intensive compression first. As chemically active solvents, basic solvents are primarily mentioned. In the case of chemical absorption in step (g) in the carbon dioxide recovery unit (G), the carbon dioxide from the carbon dioxide-containing flue gas (XII) is absorbed in the main solvent in the absorber to form an off-gas stream (XIII) and then in the desorber at a higher temperature is again desorbed. The carbon dioxide depleted solvent is usually recycled back to the absorber.

Therefore, a process is preferable in which, in step (g) in the carbon dioxide recovery unit (G), carbon dioxide from the carbon dioxide-containing flue gas (XII) is absorbed in a basic solvent in an absorber to form an off-gas stream (XIII), in a desorber from a saturated The carbon dioxide rich stream (XIV) is released with the carbon dioxide of the solvent and the carbon dioxide depleted solvent is returned to the absorber.

Basic solvents are usually aqueous solutions of basic inorganic or organic substances. Bicarbonate salts should be mentioned as the main inorganic substances, and organic amines as the main organic substances. Usually the water content in aqueous solutions is from 20 to 80% by weight, preferably ≥ 40% by weight. and especially preferably ≥ 50% of the mass.

Particularly preferably, in the process according to the invention, an aqueous solution of an organic amine is used as the main solvent. In the case of amines, we can talk about linear or cyclic, linear or branched compounds. Suitable amines are for example indicated in US patent applications US 2008/236,390, US 2010/192,770 or US 2011/094,381. Organic amines preferably have a molar mass of 50 to 500 g/mol. Especially preferred is the use of monoethanolamine, piperazine, 2-amino-2-methyl-1-propanol, triethylenediamine, N-methyldiethanolamine, t-butylaminoethoxyethanol and most preferably N-methyldiethanolamine.

Typically, in chemical absorption, the carbon dioxide-containing flue gas (XII) in the absorber is brought into contact with a basic solvent at a temperature of 10 to 100 °C, preferably ≥ 20 °C, and also preferably ≤ 60 °C, and particularly preferably ≤ 40 °C. In principle, all devices suitable for absorbing carbon dioxide from a flue gas stream containing carbon dioxide can be used as an absorber. Suitable devices for this are known to the person skilled in the art and can be developed using conventional knowledge in the art. For absorption, so-called absorption columns are usually used. For better heat and mass transfer, they preferably contain nozzles or trays. Upon contact with a basic solvent, carbon dioxide chemically binds in it. The components not absorbed in the main solvent are removed from the absorption column as an off-gas stream (XIII), the so-called purge stream. The carbon dioxide-saturated solvent is withdrawn into the absorber bottom, fed into the stripper, and then stripped with steam to release the carbon dioxide again. Preferably, the stripper is operated at a temperature that is 40-110°C higher than the temperature of the absorber. Suitable devices for this are also known to the person skilled in the art and can be developed using conventional knowledge in the art. For desorption, so-called desorption columns are usually used. For better heat and mass transfer, they also preferably contain nozzles or trays. From the desorption stream obtained at the top of the desorber, water is preferably first condensed. The condensed water can then be returned again, for example to a carbon dioxide recovery process. The carbon dioxide and water depleted solvent is then returned to the absorber.

Both absorbers and desorbers are usually operated at a pressure of 0.1 to 0.3 MPa abs; the exact pressure is typically controlled by subsequent steps in the process and can be readily determined by one skilled in the art. Alternatively, absorption can also be carried out at higher pressures. At the same time, the ability of the solvent to absorb carbon dioxide is higher than under conditions close to atmospheric pressure. As a result, regeneration in the desorber is also more energy efficient. However, in this embodiment, the carbon dioxide-containing flue gas (XII) must be compressed, which negatively affects the overall energy balance. Therefore, the absorber and desorber are preferably operated at a pressure of 0.1 to 0.3 MPa abs.

Because desorption usually occurs at a higher temperature than absorption, the solvent leaving the absorber, rich in carbon dioxide, must be heated, and the solvent leaving the stripper, depleted in carbon dioxide and water, must be cooled for reuse in the absorber. Therefore, the use of a so-called cross heat exchanger, in which the warmer solvent from the stripper heats the colder solvent from the absorber, is particularly advantageous for the energy efficiency of carbon dioxide recovery. By using additional solvent-based heat exchangers, the energy efficiency of the process can be further improved. Fine temperature control of the carbon dioxide depleted solvent for use in the absorber can then be effected, for example, by means of an air or water cooler.

In addition, it is possible to further increase the energy efficiency of carbon dioxide recovery through additional heat integration measures, such as, for example, the use of a liquid cooler at the bottom of the absorber. Solvent losses can also be reduced by additional measures. Thus, for example, it is possible, and in the case of solvents with a significant vapor pressure, it is preferable to pass the gas flow in the upper zone of the absorber through the wash bed and cool the liquid phase by direct cooling with a circulation cooler.

In order to compensate for the loss of the solvent, which usually cannot be completely avoided despite various countermeasures, the solvents or the individual components of the solvent are usually supplied in the form of a so-called make-up stream. As a result, it is possible to maintain constant levels over time in both columns. Typically, the feed is carried out at the top of the absorber.

Through the carbon dioxide recovery unit (G), carbon dioxide is selectively removed from the flue gas (XII) and recycled as a carbon dioxide-enriched stream (XIV) for further use in the methanol synthesis. Typically enriched in carbon dioxide stream (XIV), contains carbon dioxide in an amount of from 80 to 100% vol. in terms of an anhydrous basis, preferably ≥ 97% vol. in terms of an anhydrous basis, as well as small amounts of foreign gases such as nitrogen, oxygen or argon, which, however, usually amount to less than 0.3% by volume. in terms of anhydrous basis. In addition, the carbon dioxide-enriched stream (XIII) usually also contains water, which, however, has already been subtracted in the case of the % v/v data mentioned above.

Said carbon dioxide recovery processes are characterized by a relatively high degree of carbon dioxide recovery, wherein the carbon dioxide recovery rate is understood to be the ratio between the amount of carbon dioxide fed into the carbon dioxide recovery plant (G) via the carbon dioxide-containing flue gas (XII) and the amount of carbon dioxide, which is removed from the carbon dioxide recovery unit (G) via a carbon dioxide-enriched stream (XIV). With the above methods, carbon dioxide recovery rates of 90 to almost 100% can easily be achieved. The carbon dioxide recovery rate is preferably ≥ 95%, particularly preferably ≥ 98% and most preferably ≥ 99%, and also preferably ≤ 99.9%.

The off-gas stream (XIII) contains flue gas components (XII) not separated by the carbon dioxide-enriched stream (XIV), in particular inert gases such as nitrogen or argon, as well as oxygen, and not separated in the carbon dioxide recovery unit (G ) water. Therefore, the inert gases are removed from the methanol synthesis process in the first place via the off-gas stream (XIII), thereby counteracting undesirable accumulations. Thus, the off-gas stream (XIII) not only functions as an off-gas stream for the carbon dioxide recovery plant (G), but also simultaneously performs the important function of a so-called purge stream for removing inert gases from the methanol synthesis.

The carbon dioxide recovery unit (G) separated and enriched in carbon dioxide stream (XIV) is then returned to the synthesis gas unit (A) of stage (a) and/or to the methanol synthesis unit (B) of stage (b). The return according to the invention can be carried out both in the synthesis gas plant (A) of stage (a) and in the methanol synthesis plant (B) of stage (b), or also as a split stream to both plants.

If the carbon dioxide is returned to the synthesis gas production unit (A), it can, for example, be fed into the carbonaceous feedstock (I) and then used together with it to produce synthesis gas (II). Depending on the type of synthesis gas produced, carbon dioxide passes through the synthesis gas production unit (A) unchanged as carbon dioxide, or is converted to a greater or lesser extent into carbon monoxide by reaction with the generated hydrogen or by reaction with added or formed water. If the carbon dioxide is returned to the methanol synthesis unit (B), it can, for example, be fed into the synthesis gas (II) and then used together with it for the methanol synthesis. For the sake of completeness, it should be noted that in the case of a separate return of carbon dioxide, the separation ratio, of course, can vary over the entire range from 0 to 100%. However, in all the cases mentioned, the carbon dioxide carbon can be used in each case in terms of substance for the further synthesis of methanol.

Even though the stream (XIV) can be recycled without any problems to the synthesis gas production unit (A) of stage (a), for simplicity it is expedient and preferable to feed the stream (XIV) from stage (g) to the methanol synthesis unit (B) of stage (b).

Since the stream (XIV) is usually at a pressure of 0.1 to 0.3 MPa abs., and the synthesis of methanol is carried out at a pressure of 5 to 10 MPa abs., in the case of returning the stream (XIV) to the methanol synthesis unit (B) , it is preferable to direct the stream (XIV) to the synthesis gas compressor and then feed it to the reactor together with the compressed synthesis gas.

Because the carbon dioxide-enriched stream (XIV) typically still contains small amounts of oxygen, typically in the range of a few vol.-ppm to several hundred vol.-ppm, and oxygen has a negative effect on synthesis methanol from the point of view of catalyst life and productivity, it is advisable to reduce the oxygen content in the stream (XIV). In principle, various methods can be used for this, which make it possible to remove a small amount of oxygen from the carbon dioxide-containing stream. A simple way to do this is the catalytic hydrogenation of oxygen to water. Examples of catalysts suitable for this are, for example, fixed bed catalysts containing copper or noble metal. Suitable methods, their implementation and modes of operation are known to those skilled in the art. Therefore, it is preferred that the stream (XIV) be subjected to catalytic hydrogenation to reduce the oxygen content before it is returned to the synthesis gas unit (A) or to the methanol synthesis unit (B). By means of said hydrogenation, the oxygen content can easily be reduced to values < 1 ppm v/v.

In addition to the carbon dioxide recycled via stream (XIV), it is also possible in the process according to the invention to feed carbon dioxide from other sources to the synthesis gas plant (A) of stage (a) and/or to the methanol synthesis unit (B) of stage ( b). As carbon dioxide from so-called other sources, carbon dioxide from various flue gases, for example from flue gases from syngas production, from power plants or other combustion processes, should be mentioned, the carbon dioxide preferably being separated from these flue gases beforehand, for example by means of a regeneration plant. carbon dioxide.

The method for producing methanol according to the invention is carried out in a continuous mode.

Therefore, by means of the process steps according to the invention, it is possible to recycle the carbon of the valuable components in a targeted manner for further methanol synthesis, i.e. to obtain an additional valuable product, and at the same time to avoid carbon dioxide emissions from the methanol synthesis.

Thus, methanol synthesis can be carried out with zero carbon dioxide emissions.

As mentioned at the beginning, the stoichiometric number S,

equal to 2 corresponds to the calculated stoichiometry of methanol synthesis from carbon monoxide, carbon dioxide and hydrogen. Depending on the source of the synthesis gas (II), it may if necessary already have a stoichiometric number S ≥ 2, for example in the case of synthesis gas from steam reforming of methane-containing gas with a theoretical value of 3. However, from the point of view of reducing the formation of undesirable secondary components, it is desirable a stoichiometric number at the inlet to the reactor significantly greater than 2, and particularly preferably from 2.8 to 3.8. To achieve these values, a significant excess of hydrogen is required. However, it should be noted that hydrogen is removed from the methanol synthesis unit (B), for example via stream (IV). When using hydrogen-rich synthesis gas (II), such as from steam reforming, the loss of hydrogen can usually be tolerated. However, when using less hydrogen-enriched synthesis gas (II), on the contrary, it is more advantageous not to burn hydrogen from stream (IV) into water in the combustion unit (F), but to separate it and return it to methanol synthesis.

Therefore, in a preferred embodiment of the process according to the invention, hydrogen is separated in the hydrogen recovery unit (H) before the stream (IV) is fed to the combustion unit (F) and returned to the methanol synthesis unit (B) of step (b). On FIG. 3 shows a block diagram of a general embodiment in which hydrogen is separated in a hydrogen recovery unit (H) and recycled. In general, it is preferred that the separated hydrogen in the methanol synthesis plant (B) be sent to a synthesis gas compressor and then fed to the reactor together with the compressed synthesis gas.

In principle, all devices suitable for separating hydrogen from a gas stream containing carbon monoxide, carbon dioxide, hydrogen and methane can be used to separate hydrogen from stream (IV). Appropriate devices are generally known to the person skilled in the art, such as, for example, pressure swing adsorption ("Pressure Swing Adsorption") or permeability. Preferably, in the process according to the invention, hydrogen is separated in a hydrogen recovery unit (H) by means of pressure swing adsorption.

In the case of pressure swing adsorption, the gas to be separated is fed at elevated pressure, typically in the range of 0.6 to 1 MPa abs, into a reservoir filled with adsorbent. Components heavier than hydrogen are adsorbed and the hydrogen is removed from the reservoir as the light component in concentrated form. If the adsorbent bed is largely saturated with heavier components, the gas to be separated is sent for further adsorption to another container, also filled with adsorbent, and the heavier components are released from the previous container by desorption by pressure reduction and are separately removed as such. As a result, the previous reservoir is regenerated and prepared for a new cycle.

Since the complete separation of hydrogen would require particularly high costs, the separation is usually not carried out completely. Typically, when using a hydrogen recovery unit (H), only 50 to 95%, preferably ≥ 60%, and also preferably ≤ 90% of the hydrogen present in the gas stream is separated. Therefore, stream (IV) usually still contains corresponding residual amounts of hydrogen even after hydrogen has been separated.

The separated hydrogen is generally of relatively high purity. By means of preferential pressure swing adsorption, the separated hydrogen generally has a purity of 90 to 100% by volume, preferably ≥ 95% by volume, particularly preferably ≥ 99% by volume. and most preferably ≥ 99.5% vol.

Increasing the hydrogen content at the reactor inlet and, consequently, the stoichiometric number S at the reactor inlet is also possible by supplying additional hydrogen. Therefore, a method is preferred in which additional hydrogen (XVI) is supplied to the methanol synthesis unit (B) in step (b) in addition to the hydrogen supplied via the synthesis gas. On FIG. 4 shows a block diagram of a general embodiment in which additional hydrogen is supplied via stream (XVI).

Preferably, additional hydrogen is sent to the synthesis gas compressor and then fed to the reactor along with the compressed synthesis gas. However, if the additional hydrogen (XVI) is already under a correspondingly high pressure, it can, of course, also be fed separately to the methanol synthesis reactor.

In order to avoid the accumulation of non-reacting substances and in particular inert gases during the synthesis of methanol, it is desirable to supply as hydrogen (XVI) preferably hydrogen as pure as possible, or at least hydrogen with a low content of non-reacting substances. Preferably, the supplied hydrogen (XVI) has a hydrogen content of ≥ 80% by volume, particularly preferably ≥ 90% by volume, most preferably ≥ 95% by volume, in particular ≥ 99% by volume. and above all ≥ 99.5% vol.

The supplied hydrogen (XVI) can, in principle, come from a variety of sources. Examples include the supply of hydrogen from other production plants in which hydrogen is produced purposefully or as a by-product, such as from steam crackers or refineries, from syngas processing, from hydrocarbon splitting, such as methane pyrolysis and/ or hydrocarbons with a higher number of carbon atoms, or from the electrolysis of water. Other examples of plants that produce hydrogen include the conversion of 1,4-butanediol to γ-butyrolactone, the dehydrogenation of propane to propene, the dehydrogenation of methanol to formaldehyde, the dehydrogenation of cyclohexanol to cyclohexanone, and the dehydrogenation of cyclododecanol to cyclododecanone.

Especially preferred is the use of hydrogen from renewable sources. In this context, special mention should be made of the electrolysis of water using the energy of the sun, wind or water.

The amount of hydrogen fed (XVI) is usually calculated from the desired stoichiometric number S at the reactor inlet.

Even if, from the point of view of improved energy, material and environmental production of methanol, it is preferable to increase the stoichiometric number S at the reactor inlet, first of all, to use the hydrogen present in the system through the hydrogen recovery unit (H) and only if it is further necessary to supply additional hydrogen via stream (XVI), it is of course also possible to supply additional hydrogen via stream (XVI) even without using a hydrogen recovery unit. This would be interesting, for example, in the case of an excess of additional hydrogen, and would save on the provision and operation of the hydrogen recovery unit.

In FIG. 1 shows a block diagram of a general embodiment of the method according to the invention, in which all three streams (IV), (V) and (VII) are fed to the combustion unit (F). The general designations for hardware units and devices (A) - (G), as well as streams (I) - (XIV) have already been listed in the general description of the invention with reference to FIG. 1.

The following description refers to the preferred embodiment with the following characteristics.

(I) Preferably, a methane-containing gas, such as natural gas or biogas, is fed through stream (I) to the synthesis gas production unit (A).

(A) In the synthesis gas production unit (A), synthesis gas (II) is obtained from the preferably methane-containing gas (I). For this purpose, steam reforming, autothermal reforming, a combination of steam reforming and autothermal reforming, and partial oxidation are preferred.

(B) The methanol synthesis plant (B) includes a compressor for compressing synthesis gas, a methanol synthesis reactor containing a methanol synthesis catalyst for converting synthesis gas into methanol, a condenser for condensing raw methanol, and a corresponding line for recycle synthesis gas. By condensing the resulting reaction mixture, crude methanol (III) is obtained and removed. The uncondensed gas is removed as a stream (IV).

(C) Reduced pressure plant (C) includes a vessel in which separated crude methanol (III) is subjected to reduced pressure, and so-called reduced-pressure gas (V) and degassed crude methanol (VI) are formed.

(D) Degassed crude methanol (VI) is processed by distillation. To do this, in step (d), the low-boiling components (VII) are first separated, and the methanol is further enriched in the bottom stream (VIII). The distillation apparatus (D) is generally a distillation column, also referred to as a low boiling column.

(E) The so-called fine distillation of methanol is carried out in the distillation device (E). For energy reasons, the use of so-called dual-pressure distillation is particularly advantageous here. Pure methanol is isolated as stream (X) and the high boiling components are separated in the bottom as stream (IX).

(F) Combustion unit (F) turns on the combustion chamber. The separated process streams (IV), (V) and (VII) are fed into a combustion chamber and combusted with the addition of oxygen-containing gas (XI) to form carbon dioxide. Because of the advantages mentioned in the description, the use of pure oxygen as the oxidant is particularly preferred.

(G) In the carbon dioxide recovery plant (G), the flue gas (XII) which was withdrawn from the combustion plant (F) is separated from the carbon dioxide and returned via stream (XIV) to the synthesis gas production plant (A) and/or methanol synthesis unit (B). The dotted lines in the flow (XIV) symbolize the indicated options. If recycled to the methanol synthesis unit (B), stream (XIV) is preferably fed to a synthesis gas compressor.

The carbon dioxide recovery unit (G) is preferably a so-called carbon dioxide scrubbing unit, in which the carbon dioxide in the absorber is selectively washed in an aqueous organic amine solution to form an off-gas stream (XIII) and then released again in the stripper.

Fig. 2 is based on Fig. 1 and differs in that, in addition to the combustion chamber, the combustion plant (F) also contains a condenser in which water is condensed from the fuel gas and discharged as a stream (XV). As a result, by means of a relatively simple measure, the water content of the flue gas (XII) is significantly reduced and the subsequent recovery of carbon dioxide is facilitated.

Fig. 3 is based on Fig. 2 and differs in that before the stream (IV) is fed to the combustion unit (F), hydrogen is separated in the hydrogen regeneration unit (H). Preferably, hydrogen separation is carried out by pressure swing adsorption. The separated hydrogen is returned as a stream (IVb) to the methanol synthesis unit (B) and is preferably sent to the compressor there.

Fig. 4 is based on FIG. 3 and is characterized in that additional hydrogen is supplied to the methanol synthesis unit (B) via stream (XVI).

Fig. 5 is based on FIG. 4 and is characterized in that the carbon dioxide-enriched stream (XIV) is recycled to the methanol synthesis unit (B).

The method according to the invention makes it possible to obtain methanol from synthesis gas in high yield and purity, using the apparatus and connections conventional for methanol synthesis processes, but with the decisive advantage of the almost complete use of carbon-containing valuable components for methanol synthesis as a substance and the prevention of carbon dioxide emissions. gas. Through the almost complete use as a substance of carbon-containing valuable components for the synthesis of methanol, not only prevents the release of climate-damaging carbon dioxide, but also increases the yield of methanol. Due to the use for the present methanol synthesis and methanol processing of devices and connections conventional for methanol synthesis processes, the method according to the invention can also be integrated into existing methanol synthesis plants without problems. Technical interventions, modifications or additions to the synthesis gas production, even if the separated carbon dioxide is returned to the synthesis gas production, are generally not required due to the absolute amounts. The necessary arrangements and process steps for carrying out the process of the invention, whether retrofitting existing methanol synthesis plants or building new methanol synthesis plants, can be easily designed and installed by one of ordinary skill in the art. The costs of installing and operating the required components are relatively low. At the same time, the method according to the invention also very elegantly solves the problem of removing unwanted inert gases through the off-gas stream of a carbon dioxide recovery plant. The supply of climate-neutrally produced hydrogen, for example by electrolysis of water powered by solar, wind or water energy, also makes it possible to further increase the environmental sustainability of the process according to the invention.

Examples

Circuit 1 (comparative examples)

In FIG. 6 shows a simplified block diagram of a circuit for producing methanol according to the prior art. Which means:

(A) Syngas production unit

(C) Methanol synthesis unit

(C) Reduced pressure setting (low pressure vacuum)

(D) Column of light boiling components

(E) Methanol purification column

(H) Pressure swing absorption

(I) Natural gas

(II) Syngas

(IVb) Regenerated hydrogen from pressure swing adsorption (H)

(IV) Off-gas from pressure swing adsorption (H)

(V) Depressurized gas from the depressurized plant

(VII) Lights flow from the lighters column (D)

(IX) Stream of high-boiling components from the methanol purification column (E)

(X) Pure methanol

(XVI) Hydrogen ("fresh hydrogen")

In addition, the circuit has the following additional features:

- The methanol synthesis plant (B) includes a compressor, a reactor, a condenser and a synthesis gas circulation loop.

- The degree of hydrogen regeneration by pressure swing adsorption (H) is 83%.

- Fresh hydrogen (XVI) contains > 99.5% vol. hydrogen.

Unless otherwise indicated in the corresponding comparative example, hydrogen is recovered by pressure swing adsorption (H) and recycled to the methanol synthesis unit (B) via stream (IVb) and also fresh hydrogen is supplied via stream (XVI). Streams (IV), (V) and (VII) in each case are removed from the circuit.

Circuit 2 (comparative examples)

In FIG. 6 shows a simplified block diagram of a circuit for producing methanol according to the invention. Which means:

(A) Syngas production unit

(C) Methanol synthesis unit

(C) Reduced pressure setting

(D) Column of light boiling components

(E) Methanol purification column

(F) Incinerator

(G) Carbon dioxide recovery plant

(H) Pressure swing absorption

(I) Natural gas

(II) Syngas

(IVb) Regenerated hydrogen from pressure swing adsorption (H)

(IV) Off-gas from pressure swing adsorption (H)

(V) Depressurized gas from the depressurized plant

(VII) Lights flow from the lighters column (D)

(IX) Stream of high-boiling components from the methanol purification column (E)

(X) Pure methanol

(XI) Oxygen-containing gas

(XII) Flue gas

(XIII) Off-gas from the carbon dioxide recovery plant (G)

(XIV) Carbon dioxide-enriched stream from the carbon dioxide recovery unit (G)

(XV) Condensed water from the combustion plant (F)

(XVI) Hydrogen ("fresh hydrogen")

In addition, the circuit has the following additional features:

- The methanol synthesis plant (B) includes a compressor, a reactor, a condenser and a synthesis gas circulation loop.

- The combustion plant (F) includes a combustion chamber and a condenser.

- In the combustion chamber, up to > 99% of combustible components are converted into carbon dioxide and water.

- The degree of carbon dioxide recovery in the carbon dioxide recovery unit (G) is > 99%.

- The degree of hydrogen regeneration by pressure swing adsorption (H) is 83%.

- Fresh hydrogen (XVI) contains > 99.5% vol. hydrogen.

Streams (IV), (V) and (VII) are burned in a combustion unit (F) while supplying pure oxygen, and the resulting flue gas (XII) is fed to a carbon dioxide recovery unit (G) for carbon dioxide recovery. The recovered carbon dioxide is returned as stream (XIV) to the methanol synthesis unit (B). The exhaust gas from the carbon dioxide recovery unit (G) is removed from the circuit.

Example 1 (Comparative)

Comparative example 1 refers to the synthesis of methanol from synthesis gas, which is obtained by partial oxidation of natural gas (I). The circuit taken as a basis is shown in Fig. 6 and is described in more detail as "Circuit 1". Table 1 shows the composition of synthesis gas (II), which corresponds to a typical composition of synthesis gas from the partial oxidation of natural gas. Which results in a stoichiometric number S of 1.625. However, in order to set the stoichiometric number S equal to 3.40 at the inlet to the reactor, it is necessary, in addition to the hydrogen present in the synthesis gas (II), to supply the methanol synthesis unit (B) with hydrogen returned through the circulating synthesis gas, and hydrogen recycled from pressure swing adsorption (H) as well as 250.1 Nm ³ fresh hydrogen (XVI) per ton of pure methanol produced (stream (X)). The content of inert components (CH ₄ , H ₂ O, N ₂ , Ar) at the inlet to the reactor is 24.4% by volume. After a heterogeneous catalytic conversion on a copper-containing methanol synthesis catalyst at 235 °C and a pressure of 7.7 MPa abs. and further processing of the reaction mixture in accordance with the simplified block diagram in Fig.6, streams (IV), (V) (after reducing the pressure to 0.6 MPa abs. at 40 ° C) and (VII) are formed with those indicated in table 1 quantities and compositions.

Since streams (IV), (V) and (VII) are removed from the loop, they remain unused for further methanol synthesis. In the methods of the prior art, they are usually sent only for thermal disposal, i. are not used as a substance. Therefore, in the present Comparative Example 1, 1983 Nm ³ of synthesis gas (II) is required to produce one tonne of pure methanol (stream (X)) .

Example 2 (according to the invention)

Example 2 according to the invention also relates to the synthesis of methanol from synthesis gas, which is obtained by partial oxidation of natural gas (I). The outline taken as a basis is shown in Fig. 5 and is described in more detail as "Circuit 2". Table 2 shows the composition of synthesis gas (II), which corresponds to the typical composition of synthesis gas from the partial oxidation of natural gas and is identical to that of example 1. Therefore, in example 2 according to the invention, the stoichiometric number S of synthesis gas (II) is also 1.625 . Also, in order to set the stoichiometric number S equal to 3.40 at the reactor inlet in example 2, it is necessary, in addition to the hydrogen present in the synthesis gas (II), to supply the methanol synthesis unit (B) with hydrogen returned through the circulating synthesis gas , and hydrogen recovered from pressure swing adsorption (H) as well as 318.0 Nm ³ fresh hydrogen (XVI) per ton of pure methanol produced (stream (X)). The content of inert components (CH ₄ , H ₂ O, N ₂ , Ar) at the inlet to the reactor is 24.2% by volume. After a heterogeneous catalytic conversion on a copper-containing methanol synthesis catalyst at 235 °C and a pressure of 7.7 MPa abs. and further processing the reaction mixture in accordance with the simplified flowchart of FIG. 5 flows (IV), (V) (after pressure reduction to 0.6 MPa abs. at 40 °C) and (VII) are formed with the quantities and compositions indicated in Table 2. However, in contrast to comparative example 1, they are not removed from the circuit unused, but instead, according to the invention, they are burned in the combustion unit (F) with the supply of pure oxygen (XI). After condensing 88.6% of the water present and withdrawing it as a stream (XV), the carbon dioxide-containing flue gas (XII) is fed into the carbon dioxide recovery unit (G). It recovers > 99% of the carbon dioxide contained in the flue gas (XII) as a stream (XIV) and is returned to the methanol synthesis unit (B).

Therefore, to obtain one ton of pure methanol (stream (X)) in example 2 according to the invention requires only 1905 Nm ³ synthesis gas (II).

Compared to comparative example 1, in which streams (IV), (V) and (VII) are removed from the circuit unused, the method according to the invention in example 2 makes it possible to use the valuable components contained in these streams, carbon monoxide, carbon dioxide, methane and dimethyl ether, for further methanol synthesis. In particular, this leads to an additional consumption of 67.9 Nm ³ of fresh hydrogen per ton of pure methanol (stream (X)), but, on the other hand, saves the use of 78 Nm ³ of synthesis gas (II) per ton of pure methanol (stream ( X)) and thus a corresponding amount of natural gas, and also reduces the emission of additional carbon dioxide, which would otherwise be formed during the purely thermal utilization of streams (IV), (V) and (VII) in the circuit according to the prior art.

By separating and recirculating the carbon dioxide according to the invention via stream (XIV), the content of carbon dioxide at the inlet to the reactor is increased from 3.0% vol. in comparative example 1 to 3.9% vol. in example 2 according to the invention, however, is still in the range for which the commonly used methanol synthesis catalysts are designed.

Example 3 (Comparative)

Comparative Example 3 relates to the synthesis of methanol from synthesis gas, which is obtained from the autothermal reforming of natural gas. The outline taken as a basis is shown in Fig. 6 and is described in more detail as "Circuit 1". Table 3 shows the composition of synthesis gas (II), which corresponds to a typical composition of synthesis gas from autothermal reforming of natural gas. Which results in a stoichiometric number S of 1.765. However, in order to set the stoichiometric number S equal to 3.40 at the inlet to the reactor, it is necessary, in addition to the hydrogen present in the synthesis gas (II), to supply the methanol synthesis unit (B) with hydrogen returned through the circulating synthesis gas, and hydrogen recycled from pressure swing adsorption (H) as well as 128.7 Nm ³ fresh hydrogen (XVI) per ton of pure methanol produced (stream (X)). The content of inert components (CH ₄ , H ₂ O, N ₂ , Ar) at the inlet to the reactor is 24.8% by volume. After a heterogeneous catalytic conversion on a copper-containing methanol synthesis catalyst at 235 °C and a pressure of 7.7 MPa abs. and further processing the reaction mixture in accordance with the simplified flowchart of FIG. 6 flows (IV), (V) (after pressure reduction to 0.6 MPa abs. at 40 °C) and (VII) are formed with the quantities and compositions indicated in Table 3.

Since streams (IV), (V) and (VII) are removed from the loop, they remain unused for further methanol synthesis. In the methods of the prior art, they are usually sent only for thermal disposal, i. are not used as a substance. Therefore, in the present Comparative Example 3, 2255 Nm ³ of synthesis gas (II) is required to produce one tonne of pure methanol (stream (X)) .

Example 4 (according to the invention)

Example 4 according to the invention also relates to the synthesis of methanol from synthesis gas, which is obtained from the autothermal reforming of natural gas. The outline taken as a basis is shown in Fig. 5 and is described in more detail as "Circuit 2". Table 4 shows the composition of synthesis gas (II), which corresponds to the typical composition of synthesis gas from autothermal reforming of natural gas and is identical to that of example 3. Therefore, in example 4 according to the invention, the stoichiometric number S of synthesis gas (II) is also 1.765 . Also, in order to set the stoichiometric number S equal to 3.40 at the reactor inlet in example 4, it is necessary, in addition to the hydrogen present in the synthesis gas (II), to supply the methanol synthesis unit (B) with hydrogen returned through the circulating synthesis gas , and hydrogen recovered from pressure swing adsorption (H) as well as 273.1 Nm ³ fresh hydrogen (XVI) per ton of pure methanol produced (stream (X)). The content of inert components (CH ₄ , H ₂ O, N ₂ , Ar) at the inlet to the reactor is 23.7% by volume. After a heterogeneous catalytic conversion on a copper-containing methanol synthesis catalyst at 235 °C and a pressure of 7.7 MPa abs. and further processing the reaction mixture in accordance with the simplified flowchart of FIG. 5 flows (IV), (V) (after pressure reduction to 0.6 MPa abs. at 40 °C) and (VII) are formed with the quantities and compositions indicated in Table 4. However, in contrast to comparative example 3, they are not removed from the circuit unused, but instead, according to the invention, they are burned in the combustion unit (F) with pure oxygen (XI). After condensing 93.5% of the water present and withdrawing it as a stream (XV), the carbon dioxide-containing flue gas (XII) is fed into the carbon dioxide recovery unit (G). It recovers > 99% of the carbon dioxide contained in the flue gas (XII) as a stream (XIV) and is returned to the methanol synthesis unit (B).

Therefore, to obtain one ton of pure methanol (stream (X)) in example 4 according to the invention requires only 2093 Nm ³ synthesis gas (II).

Compared to comparative example 3, in which streams (IV), (V) and (VII) are removed from the circuit unused, the method according to the invention in example 4 makes it possible to use the valuable components contained in these streams, carbon monoxide, carbon dioxide, methane and dimethyl ether, for further methanol synthesis. In particular, this leads to an additional consumption of 144.4 Nm ³ of fresh hydrogen per ton of pure methanol (stream (X)), but, on the other hand, saves the use of 162 Nm ³ of synthesis gas (II) per ton of pure methanol (stream ( X)) and thus a corresponding amount of natural gas, and also reduces the emission of additional carbon dioxide, which would otherwise be formed during the purely thermal utilization of streams (IV), (V) and (VII) in the circuit according to the prior art.

By separating and recirculating the carbon dioxide according to the invention via stream (XIV), the carbon dioxide content at the inlet to the reactor is increased from 6.6% by volume. in comparative example 3 to 7.4% vol. in example 4 according to the invention, however, is still in the range for which the commonly used methanol synthesis catalysts are designed.

Example 5 (Comparative)

Comparative Example 5 relates to the synthesis of methanol from synthesis gas, which is obtained from a combination of steam reforming and autothermal reforming of natural gas. Table 5 shows the composition of synthesis gas (II), which corresponds to a typical composition of synthesis gas from a combination of steam reforming and autothermal reforming of natural gas. Which results in a stoichiometric number S of 2.007. Since the hydrogen present in the synthesis gas (II) together with the hydrogen recycled through the circulating synthesis gas is already sufficient to set the reactor inlet stoichiometric S number to 3.40, in this Comparative Example 5 there is no need to regenerate the hydrogen. by means of a hydrogen recovery plant with the return of regenerated hydrogen, nor in the supply of fresh hydrogen. Therefore, the circuit taken as a basis in comparative example 5 is also based on the circuit shown in Fig. 6 and described in more detail as "Circuit 1", but has neither a hydrogen recovery unit (H) with subsequent hydrogen return, nor a fresh hydrogen supply ( XVI). The content of inert components (CH ₄ , H ₂ O, N ₂ , Ar) at the inlet to the reactor is 24.4% by volume. After a heterogeneous catalytic conversion on a copper-containing methanol synthesis catalyst at 235 °C and a pressure of 7.7 MPa abs. and further processing the reaction mixture in accordance with the simplified flowchart of FIG. 6 flows (IV), (V) (after pressure reduction to 0.6 MPa abs. at 40 °C) and (VII) are formed with the amounts and compositions indicated in Table 5.

Since streams (IV), (V) and (VII) are removed from the loop, they remain unused for further methanol synthesis. In the methods of the prior art, they are usually sent only for thermal disposal, i. are not used as a substance. Therefore, in the present Comparative Example 5, 2478 Nm ³ of synthesis gas (II) is required to produce one tonne of pure methanol (stream (X)) .

Example 6 (according to the invention)

Example 6 according to the invention also relates to the synthesis of methanol from synthesis gas, which is obtained by a combination of steam reforming and autothermal reforming of natural gas. The outline taken as a basis is shown in Fig. 5 and is described in more detail as "Circuit 2". Table 6 shows the composition of synthesis gas (II) which corresponds to a typical composition of synthesis gas from a combination of steam reforming and autothermal reforming of natural gas and is identical to that of example 5. Therefore, in example 6 according to the invention, the stoichiometric number S of synthesis gas (II ) is also 2.007. In contrast to comparative example 5, in example 6 according to the invention, in order to set the stoichiometric number S equal to 3.40 at the reactor inlet, it is necessary, in addition to the hydrogen present in the synthesis gas (II) and the hydrogen returned through the circulating synthesis gas to the methanol synthesis plant (B), feed more hydrogen from pressure swing adsorption (H) as well as 156.1 Nm ³ fresh hydrogen (XVI) per ton of pure methanol produced (stream (X)). The content of inert components (CH ₄ , H ₂ O, N ₂ , Ar) at the inlet to the reactor is 24.9% by volume. After a heterogeneous catalytic conversion on a copper-containing methanol synthesis catalyst at 235 °C and a pressure of 7.7 MPa abs. and further processing the reaction mixture in accordance with the simplified flowchart of FIG. 5 flows (IV), (V) (after pressure reduction to 0.6 MPa abs. at 40 °C) and (VII) are formed with the quantities and compositions indicated in Table 6. However, unlike comparative example 5, they are not removed from the circuit unused, but instead, according to the invention, they are burned in the combustion unit (F) with pure oxygen (XI). After condensing 94.2% of the water present and withdrawing it as a stream (XV), the carbon dioxide-containing flue gas (XII) is fed into the carbon dioxide recovery unit (G). It recovers >99% of the carbon dioxide contained in the flue gas (XII) as a stream (XIV) and is returned to the methanol synthesis unit (B).

Therefore, to obtain one ton of pure methanol (stream (X)) in example 6 according to the invention requires only 2238 Nm ³ synthesis gas (II).

When using a flue gas scrubber as a carbon dioxide recovery unit (G) with an absorption column and a desorption column that are connected to each other via a cross heat exchanger and that operate with an amine absorbent as an absorption liquid, said carbon dioxide recovery of > 99% requires a circulating an absorbent stream of 2.71 tons of absorbent per ton of pure methanol (stream (X)), as well as a regeneration energy of 0.087 MW per tonne of pure methanol (stream (X)).

Compared to comparative example 5, in which streams (IV), (V) and (VII) are removed from the circuit unused, the method according to the invention in example 2 makes it possible to use the valuable components contained in these streams, carbon monoxide, carbon dioxide, methane and dimethyl ether, for further methanol synthesis. In particular, this leads to an additional consumption of 156.1 Nm ³ of fresh hydrogen per ton of pure methanol (stream (X)), but, on the other hand, saves the use of 240 Nm ³ of synthesis gas (II) per ton of pure methanol (stream ( X)) and thus a corresponding amount of natural gas, and also reduces the emission of additional carbon dioxide, which would otherwise be formed during the purely thermal utilization of streams (IV), (V) and (VII) in the circuit according to the prior art.

By separating and recirculating the carbon dioxide according to the invention via stream (XIV), the carbon dioxide content at the reactor inlet is increased from 6.8% by volume. in comparative example 5 to 7.7% vol. in example 6 according to the invention, however, is still in the range for which the commonly used methanol synthesis catalysts are designed.

Example 7 (Comparative)

Comparative example 7 corresponds to example 6 according to the invention, but differs in that the streams (IV), (V) and (VII) are burned in the combustion unit (F) with air supply (stream (XI)) instead of pure oxygen, and then condense 61 .2% of the water present. As a result, the amount of flue gas (XII) generated increases significantly from about 92 Nm ³ (Example 7) to about 649 Nm ³ per tonne of pure methanol (stream (X)), and the carbon dioxide content of the flue gas (XII) decreases markedly from about 77% vol. up to about 11% vol. Accordingly, the absorption surface required in the absorption column is also increased by about 3.8 times the area. In addition, the absorbent circulating flow increases from 2.71 t to 4.93 t per ton of pure methanol (stream (X)), and the required regeneration energy from 0.087 MW to 0.12 MW per ton of pure methanol (stream (X)) . An overview of these results is presented in Table 7.

By using pure oxygen as the oxygen-containing gas (XI) in the combustion plant (F), compared to using air, the design and operation of the flue gas scrubber can be significantly more efficient.

Table 1 Data from example 1 (comparative) synthesis gas
(II) fresh hydrogen
(XVI) (iv) (V) (VII) Quantity
[Nm ³ /t methanol] 1983 250.1 16.8 10.9 10.7 CO [% vol.] 33.91 4.50 2.77 0.11 CH ₃ OH [% vol.] 0 1.15 5.85 6.76 H ₂ [% vol.] 62.20 > 99.5 16.37 19.02 0.43 CH ₄ [% vol.] 0.44 27.86 30.13 7.07 CH ₃ OCH ₃ [% vol.] 0 0 0.02 0.30 CO ₂ [% vol.] 2.70 6.27 21.66 44.86 H ₂ O [% vol.] 0.29 0.04 0.14 12.11 N ₂ [% vol.] 0.38 38.13 10.22 0.42 About ₂ [% vol.] 0 0 0 0 Ar [% vol.] 0.08 5.68 4.48 0.68 Stoichiometric number S 1625

table 2 Data from example 2 (according to the invention) synthesis gas
(II) fresh hydrogen
(XVI) (iv) (V) (VII) (XII) (XIII) (XIV) Quantity
[Nm ³ /t methanol] 1905 318.0 16.2 11.6 11.6 43.5 12.6 30.8 CO [% vol.] 33.91 3.87 2.27 0.08 0 0 0 CH ₃ OH [% vol.] 0 1.09 5.68 6.05 0 0 0 H ₂ [% vol.] 62.20 > 99.5 16.69 18.15 0.36 0 0 0 CH ₄ [% vol.] 0.44 27.48 28.01 5.78 0 0 0 CH ₃ OCH ₃ [% vol.] 0 0 0.02 0.28 0 0 0 CO ₂ [% vol.] 2.70 7.49 26.36 49.39 64.32 0.44 90.51 H ₂ O [% vol.] 0.29 0.05 0.20 12.47 7.47 2.57 9.48 N ₂ [% vol.] 0.38 37.75 9.36 0.34 17.49 60.14 0.01 About ₂ [% vol.] 0 0 0 0 7.21 24.77 0.004 Ar [% vol.] 0.08 5.59 4.17 0.56 3.51 12.08 0.001 Stoichiometric number S 1625

Table 3 Data from example 3 (comparative) synthesis gas
(II) fresh hydrogen
(XVI) (iv) (V) (VII) Quantity
[Nm ³ /t methanol] 2255 128.7 40.3 15.1 12.6 CO [% vol.] 24.13 3.28 1.02 0.03 CH ₃ OH [% vol.] 0 1.04 5.10 6.87 H ₂ [% vol.] 65.71 > 99.5 16.99 11.98 0.18 CH ₄ [% vol.] 1.22 52.43 36.95 5.65 CH ₃ OCH ₃ [% vol.] 0 0 0.03 0.24 CO ₂ [% vol.] 8.36 10.80 34.24 53.55 H ₂ O [% vol.] 0.28 0.09 0.38 12.17 N ₂ [% vol.] 0.19 9.98 1.57 0.04 About ₂ [% vol.] 0 0 0 0 Ar [% vol.] 0.12 5.39 2.86 0.30 Stoichiometric number S 1765

Table 4 Data from example 4 (according to the invention) synthesis gas
(II) fresh hydrogen
(XVI) (IV) (V) (VII) (XII) (XIII) (XIV) Quantity
[Nm ³ /t methanol] 2093 273.1 38.8 14.6 13.0 70.4 12.9 57.5 CO [% vol.] 24.13 3.18 0.89 0.02 0 0 0 CH ₃ OH [% vol.] 0 1.02 4.91 7.38 0 0 0 H ₂ [% vol.] 65.71 > 99.5 17.76 11.79 0.16 0 0 0 CH ₄ [% vol.] 1.22 51.15 34.17 4.71 0 0 0 CH ₃ OCH ₃ [% vol.] 0 0 0.03 0.23 0 0 0 CO ₂ [% vol.] 8.36 11.90 37.48 55.09 75.72 0.83 92.50 H ₂ O [% vol.] 0.28 0.10 0.44 11.93 6.80 3.71 7.49 N ₂ [% vol.] 0.19 9.68 1.41 0.03 6.69 36.54 0.002 About ₂ [% vol.] 0 0 0 0 7.11 38.82 0.003 Ar [% vol.] 0.12 5.21 2.65 0.26 3.68 20.09 0.001 Stoichiometric number S 1765

Table 5 Data from example 5 (comparative) synthesis gas
(II) fresh hydrogen
(XVI) (iv) (V) (VII) Quantity
[Nm ³ /t methanol] 2478 0 119.1 15.4 12.6 CO [% vol.] 21.52 1.76 0.95 0.02 CH ₃ OH [% vol.] 0 0.57 5.04 6.39 H ₂ [% vol.] 67.94 55.27 11.64 0.17 CH ₄ [% vol.] 1.79 31.29 38.83 5.81 CH ₃ OCH ₃ [% vol.] 0 0 0.03 0.24 CO ₂ [% vol.] 8.23 6.06 34.80 53.97 H ₂ O [% vol.] 0.26 0.05 0.40 12.38 N ₂ [% vol.] 0.17 3.45 0.95 0.02 About ₂ [% vol.] 0 0 0 0 Ar [% vol.] 0.09 1.56 1.46 0.15 Stoichiometric number S 2007

Table 6 Data from example 6 (according to the invention) synthesis gas
(II) fresh hydrogen
(XVI) (IV) (V) (VII) (XII) (XIII) (XIV) Quantity
[Nm ³ /t methanol] 2238 156.1 58.7 15.0 12.8 92.2 15.8 76.4 CO [% vol.] 21.52 3.09 0.77 0.02 0 0 0 CH ₃ OH [% vol.] 0 1.01 4.81 6.96 0 0 0 H ₂ [% vol.] 67.94 > 99.5 17.07 10.76 0.14 0 0 0 CH ₄ [% vol.] 1.79 57.57 37.05 4.97 0 0 0 CH ₃ OCH ₃ [% vol.] 0 0 0.03 0.23 0 0 0 CO ₂ [% vol.] 8.23 11.92 37.51 55.06 76.85 0.89 92.60 H ₂ O [% vol.] 0.26 0.10 0.47 12.13 6.80 3.96 7.39 N ₂ [% vol.] 0.17 6.37 0.88 0.02 5.38 31.30 0.002 About ₂ [% vol.] 0 0 0 0 8.66 50.41 0.004 Ar [% vol.] 0.09 2.87 1.42 0.14 2.31 13.44 0.001 Stoichiometric number S 2007

Table 7 Comparison of examples 6 and 7 Example 6
(according to the invention) Example 7
(comparative) Flow (XII) [Nm ³ /t methanol] 92 649 CO ₂ [% vol.] 77 eleven Circulating absorbent flow
[t absorbent/t methanol] 2.71 4.93 Regeneration energy
[MW/t methanol] 0087 0:12

Claims (26)

1. A method for producing methanol, in which (a) in the plant for the production of synthesis gas (A) from the carbon-containing feedstock (I) receive synthesis gas (II) containing carbon monoxide, carbon dioxide and hydrogen, (b) the synthesis gas (II) from step (a) is sent to the methanol synthesis unit (B) and at a temperature of 150 to 300° C. and a pressure of 5 to 10 MPa abs. in the presence of a methanol synthesis catalyst, it is converted into a reaction mixture containing methanol, water, carbon monoxide, carbon dioxide, hydrogen, dimethyl ether and methane, from which a stream of crude methanol (III) enriched in methanol and water is condensed and removed from the methanol synthesis unit ( B) a crude methanol stream (III) and a gaseous stream (IV) containing carbon monoxide, carbon dioxide, hydrogen and methane, (c) the crude methanol stream (III) from step (b) is subjected to a reduced pressure in the range of 0.1 to 2 MPa abs. in an underpressure plant (C) and a depressurized gas (V) is obtained containing carbon dioxide and methane, as well as a degassed raw methanol stream (VI) enriched in methanol and water, (d) from the degassed raw methanol stream (VI) from step (c) in the distillation device (D), the low-boiling component stream (VII) containing carbon dioxide and dimethyl ether is separated by distillation, and a bottom stream (VIII) enriched in methanol is obtained and water, and (e) from the bottom stream (VIII) from step (d) in an additional distillation device (E) the water-containing stream of high-boiling components (IX) is separated and methanol is recovered by distillation as a stream (X), characterized in that (f) valuable components - carbon monoxide, carbon dioxide, dimethyl ether and methane streams (IV), as well as at least one of the two streams (V) and (VII), are fed into the combustion unit (F) and burned in it by supplying an oxygen-containing gas (XI), which has an oxygen content of 30 to 100% by volume, and a carbon dioxide-containing flue gas (XII) is obtained, (g) the carbon dioxide-containing flue gas (XII) from step (f) in the carbon dioxide recovery unit (G) separates the carbon dioxide-enriched stream (XIV) to form an off-gas stream (XIII), and (h) the carbon dioxide recovery unit (G) separated and the carbon dioxide-enriched stream (XIV) from step (g) is returned to the synthesis gas plant (A) of step (a) and/or the methanol synthesis unit (B) step (b). 2. The method according to p. 1, characterized in that in stage (a) natural gas, biogas, coal, wood, plastics, oil, bionaphtha or hydrocarbon-containing streams from oil or natural gas processing, from chemical production processes, from renewable raw materials or from plastics processing. 3. The method according to claim 1, characterized in that in step (a) a methane-containing feedstock is used, and synthesis gas (II) is obtained by steam reforming, by autothermal reforming, by a combination of steam reforming and autothermal reforming, or by partial oxidation. 4. The method according to p. 1, characterized in that the synthesis gas (II) contains methane. 5. The method according to claim 1, characterized in that in step (b) in the methanol synthesis unit (B), a copper- and zinc-containing heterogeneous catalyst is used as a methanol synthesis catalyst. 6. Method according to claim 1, characterized in that the methanol synthesis unit (B) in step (b) comprises a compressor for compressing synthesis gas (II), a reactor for converting synthesis gas (II), a condenser for condensing the raw methanol stream (III) and a line for returning the non-condensed gas to the reactor. 7. The method according to p. 1, characterized in that in stage (f) valuable components - carbon monoxide, carbon dioxide, dimethyl ether and methane streams (IV), (V) and (VII) are fed into the combustion unit (F) . 8. The method according to p. 1, characterized in that the combustion plant (F) at stage (f) serves oxygen-containing gas (XI) with an oxygen content of 90 to 100% vol. 9. Method according to claim 1, characterized in that the combustion plant (F) in step (f) comprises a combustion chamber and a condenser, water is condensed in the condenser from the combustion gas produced in the combustion chamber and discharged as a stream (XV ) from the combustion plant (F) and the remaining gaseous stream forms a carbon dioxide-containing flue gas (XII). 10. The method according to claim 1, characterized in that in step (g) in the carbon dioxide recovery unit (G), carbon dioxide from the carbon dioxide-containing flue gas (XII) is absorbed in a basic solvent in an absorber to form an off-gas stream (XIII) , the carbon dioxide-rich solvent is released in the stripper from the carbon dioxide-rich solvent (XIV) and the carbon dioxide-depleted solvent is returned to the absorber. 11. The method according to claim 10, characterized in that an aqueous solution of an organic amine is used as the main solvent. 12. The method according to claim 11, characterized in that monoethanolamine, piperazine, 2-amino-2-methyl-1-propanol, triethylenediamine, N-methyldiethanolamine or tert-butylaminoethoxyethanol are used as the organic amine. 13. Process according to claim 1, characterized in that in step (h) the stream (XIV) from step (g) is returned to the methanol synthesis unit (B) of step (b). 14. The method according to claim 1, characterized in that the carbon dioxide-enriched stream (XIV) from step (g) contains oxygen and stream (XIV) before it is returned to the synthesis gas production unit (A) or, respectively, to the methanol synthesis unit ( B) subjected to catalytic hydrogenation to reduce the oxygen content. 15. Process according to claim 1, characterized in that before the stream (IV) is fed to the combustion unit (F), hydrogen is separated in the hydrogen recovery unit (H) and returned to the methanol synthesis unit (B) of step (b). 16. Method according to claim 15, characterized in that hydrogen is separated in the hydrogen recovery unit (H) by pressure swing adsorption. 17. Process according to claims 1 to 16, characterized in that additional hydrogen (XVI) is supplied to the methanol synthesis unit (B) in step (b) in addition to the hydrogen supplied via the synthesis gas.

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