Process for the preparation of methanol

Abstract

The present invention relates to a method for preparing methanol, wherein a synthesis gas comprising carbon oxides and hydrogen is provided, the synthesis gas is passed through a catalyst bed of a methanol synthesis catalyst at high pressure and high temperature for converting the synthesis gas into methanol to obtain a product stream comprising crude methanol and unreacted synthesis gas, and the product stream is cooled for condensing and removing the crude methanol comprising at least methanol and water. The unreacted synthesis gas is recycled to the catalyst bed inlet and combined with the synthesis gas to produce a mixed synthesis gas that passes through the catalyst bed at high pressure and high temperature. According to the present invention, it is envisaged that the mixed synthesis gas has a stoichiometric number SN of ≥0.80 at the catalyst bed inlet, the catalyst bed has a maximum catalyst bed temperature of ≤280°C in the conversion of the mixed synthesis gas to methanol, and the mixed synthesis gas has a carbon monoxide concentration of ≤20% by volume at the catalyst bed inlet. The combination of these parameters effectively suppresses the formation of by-products.

Description

Technical Field

[0001] The present invention relates to a method for preparing methanol, an apparatus configured to carry out the method for preparing methanol according to the present invention, and the use of the apparatus in the method for preparing methanol according to the present invention. Prior art

[0002] Synthesis gas containing at least carbon oxides (carbon monoxide and carbon dioxide) and hydrogen and which can be prepared from any hydrocarbon source can be converted into methanol over a suitable catalyst according to reactions (1) and (2).

[0003]

[0004]

[0005] The premise is that the catalyst poisons have been removed from the syngas to a tolerable threshold and the composition of the syngas has a suitable stoichiometric number SN defined as follows

[0006] Where n is in [mol],

[0007] For synthesis gas used for methanol synthesis, an SN value of more than 2.0, or more than 2.5, or even more than 3.0 is often required. An SN value of more than 2.0 indicates an excess of hydrogen, and an SN value of less than 2.0 indicates a lack of hydrogen.

[0008] The general opinion among those skilled in the art is that the use of synthesis gas compositions having a stoichiometric ratio of only slightly above 2.0 or even below 2.0 in the methanol synthesis leads to intolerable by-product formation.

[0009] A high degree of by-product formation indicates a low selectivity with respect to the methanol target product and thus leads to an undesirably low methanol yield.

[0010] If large amounts of by-products are formed, it may be the case that these by-products cannot be removed from the crude methanol obtained as the main product by a thermal separation process following the methanol production (e.g. by distillation). In addition, by-products which are difficult to separate from the methanol target product due to similar physical properties (e.g. boiling point, vapor pressure) lead to increased energy consumption and / or increased methanol losses in the thermal separation process used. It is a general view among professionals in the field that, as the stoichiometric number of the synthesis gas used decreases, the formation of by-products becomes so high that post-treatment of the crude methanol by a thermal separation process following the methanol production will not give sufficiently pure methanol, which may be the case, for example, when the by-product concentration in the crude product is greater than 10 000 ppm (1% by weight).

[0011] Therefore, there is a need to improve existing methods. Summary of the invention

[0012] One problem addressed by the present invention is to provide a process for producing methanol which at least partially overcomes the disadvantages of the prior art.

[0013] Another problem addressed by the present invention is to provide a process for the preparation of methanol which is characterized by reduced by-product formation.

[0014] Another problem addressed by the present invention is to provide a process for the preparation of methanol which enables the use of low-stoichiometric synthesis gas for the methanol synthesis and which at the same time features reduced by-product formation.

[0015] Another problem solved by the present invention is to provide a plant for preparing methanol, which at least partially solves at least one of the aforementioned problems.

[0016] The independent claims contribute to at least partially solving at least one of the aforementioned problems. The dependent claims provide preferred embodiments that contribute to at least partially solving at least one of these problems. Preferred embodiments of components according to one class of the invention are, where relevant, also preferred for the same named or corresponding components of the corresponding other class of the invention.

[0017] The aforementioned problems are at least partially solved by a process for preparing methanol, wherein the process comprises the following process steps, which steps do not necessarily have to be carried out in a specified order:

[0018] a. Providing a synthesis gas comprising carbon oxides and hydrogen;

[0019] b. passing the synthesis gas through a catalyst bed of a methanol synthesis reactor at high pressure and temperature for converting the synthesis gas into methanol to obtain a product stream comprising crude methanol and unreacted synthesis gas;

[0020] c. cooling the product stream for condensing and separating crude methanol comprising at least methanol and water from the cooled product stream;

[0021] d. recycling at least a portion of the unreacted synthesis gas to the catalyst bed inlet, wherein the unreacted synthesis gas is combined with the synthesis gas to obtain a mixed synthesis gas, and the mixed synthesis gas is passed through a catalyst bed of a methanol synthesis catalyst at high pressure and temperature for converting the mixed synthesis gas into methanol,

[0022] It is characterized in that

[0023] The mixed synthesis gas has a stoichiometric number SN of ≥ 0.80 at the catalyst bed inlet, wherein

[0024] Where n is in [mol],

[0025] The catalyst bed has a maximum catalyst bed temperature of ≤280° C. in the conversion of the mixed synthesis gas into methanol, and the mixed synthesis gas has a carbon monoxide concentration of ≤20% by volume at the catalyst bed inlet.

[0026] It has been surprisingly found that the formation of by-products can be suppressed when

[0027] - limiting the maximum temperature in the catalyst bed, i.e. the maximum catalyst bed temperature, to a maximum value of 280° C.,

[0028] - the stoichiometric number of the mixed synthesis gas at the catalyst bed inlet is at least 0.80 and

[0029] - The mixed synthesis gas has a carbon monoxide concentration of not more than 20 percent by volume at the catalyst bed inlet.

[0030] Detailed studies have shown that, when the parameters defined according to the invention are observed, the crude methanol obtained always has a concentration of by-products of less than 10000 ppm. The by-product content reported in ppm here relates to the total mass of by-products formed relative to the mass of crude methanol separated from the product mixture by cooling, which crude methanol consists of methanol (CH3OH), water (H2O) and unavoidable by-products. For example, a concentration of by-products of 6500 ppm means that 6500 mg of by-products / kg of crude methanol have been formed.

[0031] The method according to the present invention is configured as a so-called methanol synthesis loop, meaning that a part of the unconverted synthesis gas (unreacted synthesis gas) in the catalyst bed is separated from the condensed crude methanol phase by cooling and the phase separation caused thereby, and returned, i.e., recycled to the catalyst bed inlet. The recycled synthesis gas is merged with the synthesis gas to obtain a mixed synthesis gas. Therefore, what is used to convert the synthesis gas into methanol is a mixed synthesis gas, which is passed through the catalyst bed at high pressure and high temperature to thereby obtain a product stream comprising crude methanol and unreacted synthesis gas. Synthesis gas can also be referred to as "fresh" synthesis gas, fresh gas or make-up gas. Recycled synthesis gas can also be referred to as return gas or recycle gas. Unreacted synthesis gas is returned to the catalyst bed inlet in whole or in part and merged with synthesis gas. The usual situation is to recycle only partially unreacted synthesis gas, because a part of the unreacted synthesis gas is typically separated from the unreacted synthesis gas as a purge gas. This is intended to prevent inert components such as methane or nitrogen from accumulating in the methanol synthesis loop under methanol synthesis conditions. Furthermore, the purge gas can be sent, for example, to a pressure swing adsorption (PSA) in order to separate the hydrogen from the other components of the purge gas. The hydrogen thus obtained can be fed, for example, to the synthesis gas in order to adjust its stoichiometry to the desired value.

[0032] It has surprisingly been found that for the formation of by-products to a small extent, the stoichiometric number of the mixed synthesis gas at the catalyst bed inlet needs to have a relatively low minimum value of only 0.80, as described above in terms of quantity in conjunction with the further parameters defined.

[0033] The stoichiometric number of the mixed synthesis gas at the inlet of the catalyst bed should here be strictly distinguished from the stoichiometric number of the synthesis gas or fresh gas. Depending on the production method, the synthesis gas produced as the main product has a stoichiometric number of about 1.7 to 2.2. The mixing of synthesis gas stream and recycled synthesis gas stream and the optional supply of internally or externally generated hydrogen can change the stoichiometric number of the mixed synthesis gas at the inlet of the catalyst bed within a wider range.

[0034] The relatively low stoichiometric number of 0.80 means that the mixed synthesis gas is low in hydrogen and rich in carbon oxides (carbon monoxide and carbon dioxide). This opens up the possibility of using unmodified synthesis gas (i.e. synthesis gas which has not been enriched with hydrogen by internal or external sources) in the process according to the invention. This is the case at least when the concentration of carbon monoxide in the mixed synthesis gas does not exceed a concentration of 20% by volume and at the same time a maximum catalyst bed temperature of 280° C. is observed.

[0035] A preferred embodiment of the process according to the invention is characterized in that the catalyst bed has a maximum catalyst bed temperature of ≤ 265° C. in the conversion of the mixed synthesis gas to methanol. The formation of undesirable by-products is further suppressed if the maximum catalyst bed temperature is controlled in such a way that a temperature of 265° C. is not exceeded. Studies have shown that if the maximum catalyst bed temperature is limited to 265° C., the amount of undesirable by-products is reduced to 5000 ppm or less.

[0036] Further preferably, the catalyst bed has a maximum catalyst bed temperature of ≤ 250° C. in the conversion of the mixed synthesis gas to methanol. As studies have shown, if the maximum catalyst bed temperature is limited to 250° C., the concentration of undesirable by-products drops to 3500 ppm or less.

[0037] A preferred embodiment of the process according to the invention is characterized in that the catalyst bed has a maximum catalyst bed temperature of 205° C. to 280° C. in the conversion of the mixed synthesis gas to methanol.

[0038] A further preferred embodiment of the process according to the invention is characterized in that the catalyst bed has a maximum catalyst bed temperature of 205° C. to 265° C. in the conversion of the mixed synthesis gas into methanol.

[0039] A preferred embodiment of the process according to the invention is characterized in that the mixed synthesis gas has a stoichiometric number SN of ≥ 2.0 at the catalyst bed inlet. If the stoichiometric number of the mixed synthesis gas is adjusted so that it assumes a value of 2.0 or more for the mixed synthesis gas at the catalyst bed inlet, the formation of undesirable by-products in the crude methanol can be further suppressed. Studies have shown that the concentration of by-products in the crude methanol in this case is always 5000 ppm or less.

[0040] A preferred embodiment of the process according to the invention is characterized in that the mixed synthesis gas has a stoichiometric number SN of 0.80 to 10.0 at the catalyst bed inlet.

[0041] Another preferred embodiment of the method according to the present invention is characterized in that the mixed synthesis gas has a stoichiometric number of 0.80 to 2.20 at the catalyst bed inlet. It has been unexpectedly found that even when the stoichiometric number of the mixed synthesis gas at the catalyst bed inlet is limited to 2.20 and the further conditions of the minimum stoichiometric number and the maximum carbon monoxide concentration in the mixed synthesis gas at the catalyst inlet and the highest catalyst bed temperature are met according to the present invention, less than 10000ppm of by-products are formed. In view of this, it is further preferred that the mixed synthesis gas has a carbon monoxide concentration of 9.0% to 13.0% by volume at the catalyst bed inlet. This simultaneously achieves a high hydrogen conversion rate of 80% or more and even 90% or more when observing other boundary parameters. Hydrogen is the "most valuable" gas in the synthesis gas mixture, especially in the case of obtaining synthesis gas by autothermal reforming or by partial oxidation. This also applies to the case of converting synthesis gas rich in carbon dioxide into methanol. The latter technology is gaining greater significance. This is because, in view of discussions on anthropogenic climate change and CO2 pricing, both environmental and economic benefits in terms of CO2 valorization have increased. Therefore, the aforementioned technologies always have the goal of high hydrogen conversion in methanol production.

[0042] The preferred embodiment of the method according to the invention is characterized in that the synthesis gas has a stoichiometric number SN of 1.0 to 2.85, preferably a stoichiometric number SN of 1.0 to 2.30. The method according to the invention is also suitable for synthesis gases with low stoichiometric numbers, especially with stoichiometric numbers of 2.0 or less. The feature of such synthesis gases is that their hydrogen is low and / or rich in carbon dioxide compared to carbon monoxide. The method according to the invention is therefore also suitable for unmodified synthesis gases that are independent of internal or external hydrogen sources, and for synthesis gases that are mainly or even only contain carbon dioxide with respect to carbon oxides.

[0043] In one embodiment of the method according to the present invention, the ratio of unreacted recycled synthesis gas to synthesis gas in the mixed synthesis gas (defined as the recycle ratio RR) is 2.0 to 4.5, wherein

[0044]

[0045] In this case, the volume flow rate of the recirculated synthesis gas is at least two to four and a half times the volume flow rate of the (fresh) synthesis gas.

[0046] The preferred embodiment of the method according to the present invention is characterized in that, at the catalyst bed inlet, the mixed synthesis gas has a carbon dioxide concentration of ≥20.0% by volume. It has been unexpectedly found that the synthesis gas with a very high carbon dioxide content of 20.0 percentages or more by volume leads to the formation of very low levels of undesirable by-products, provided that other conditions according to the present invention are observed. Studies have shown that in this case, the concentration of undesirable by-products in crude methanol is always lower than 1000ppm. Therefore, the method according to the present invention is particularly suitable for synthesis gas rich in carbon dioxide and low in carbon monoxide. In this embodiment, the mixed synthesis gas preferably has a carbon monoxide concentration of less than 5% by volume, or less than 3% by volume, or less than 1% by volume. For example, this can be a synthesis gas that has been mixed with a relatively large amount of waste gas from a combustion device.

[0047] The feature of the preferred embodiment of the method according to the present invention is that the catalyst bed is divided into a plurality of catalyst bed stages arranged in series, wherein step c) is carried out downstream of each of these catalyst bed stages. In this embodiment (also referred to as the multi-reactor concept or multi-reactor stage concept), the condensation of the crude methanol formed in each of these catalyst bed stages occurs thereafter, and correspondingly crude methanol is discharged from the process at a plurality of points. The more reactor stages or catalyst bed stages used, the less unreacted synthesis gas that has to be recycled to the inlet of the first catalyst bed stage. Carbon yield can be improved by a plurality of catalyst bed stages.

[0048] A preferred embodiment of the process according to the invention is characterized in that step b) is carried out at a pressure of 30 to 120 bar, preferably at a pressure of 40 to 90 bar. The specified pressure range corresponds to the conventional pressures used in the preparation of methanol in modern low-pressure processes.

[0049] A preferred embodiment of the method according to the invention is characterized in that step b) is carried out at 2000 to 16000 m 3 (STP) / (m 3 The specified space velocity corresponds to the residence time of the reactants in the catalyst bed which results in a particularly high carbon conversion.

[0050] A preferred embodiment of the method according to the invention is characterized in that a portion of the unreacted synthesis gas is removed as purge gas. This prevents any significant accumulation of components that are inert under the conditions of methanol synthesis in the methanol synthesis loop.

[0051] A preferred embodiment of the process according to the invention is characterized in that the synthesis gas is converted into methanol in the catalyst bed at a cooling temperature of the cooling medium used of 190° C. to 250° C. The choice of an appropriate temperature of the cooling medium or coolant, typically pressurized boiling water, can be used to set the maximum catalyst bed temperature accordingly.

[0052] The problem solved by the present invention is also at least partly solved by a plant for preparing methanol configured to carry out the process according to the invention in one of the aforementioned embodiments.

[0053] The problem addressed by the present invention is also at least partly solved by using the apparatus according to the present invention in a method according to any of the preceding embodiments for the preparation of methanol.

[0054] Catalyst, catalyst bed

[0055] The catalyst bed is a fixed bed of a methanol synthesis catalyst known to those skilled in the art. In one example, the fixed bed of the catalyst bed is configured as a bed of loose particles (e.g., pellets), such as in the form of tablets or cylinders. In another example, the fixed bed of the catalyst bed is configured as a structured catalyst, such as having a porous monolithic structure.

[0056] In connection with the subject matter of the present invention, the catalyst bed inlet is understood to mean a region upstream of the catalyst bed and in which the conversion of synthesis gas and / or mixed synthesis gas into crude methanol has not yet occurred. Preferably, the catalyst bed inlet is understood to mean a region immediately upstream of the catalyst bed. In other words, the synthesis gas enters the catalyst bed immediately downstream of the catalyst bed inlet.

[0057] The methanol synthesis catalyst can be any catalyst known to those skilled in the art. In one example, it is a catalyst based on copper as a catalytically active substance. Examples of other components (especially copper-based catalysts) are zinc oxide, aluminum oxide, chromium oxide, titanium oxide, zirconium oxide (zircon) and magnesium oxide. An example of a frequently used catalyst is a catalyst comprising at least copper, ZnO and Al2O3. Copper-based catalysts are available, for example, in a temperature range from 180°C to 300°C.

[0058] Maximum catalyst bed temperature

[0059] If the synthesis gas mixture enters a cooled methanol synthesis reactor, the temperature of the synthesis gas is generally initially lower than the temperature of the coolant used.

[0060] The coolant used is a gaseous or liquid coolant. An example of a gaseous coolant is the synthesis gas and / or recycle gas used, which is preheated by cooling the process gas. An example of a liquid coolant is boiling water under high pressure, which is evaporated by cooling the reaction mixture and can subsequently be used as export steam or as heating steam or process steam in the process.

[0061] The first part of the catalyst bed is used to heat the synthesis gas, transferring heat from the coolant to the synthesis gas and the catalyst. In this process, the reaction to form methanol gradually begins, wherein, due to the exothermic nature of the reaction, heat is generated and the temperature of the catalyst and the gas mixture (synthesis gas and gaseous methanol / water, as well as unreacted synthesis gas) increases. As the reaction proceeds further, the temperature of the catalyst bed and the gas mixture substantially corresponds to the coolant temperature.

[0062] In the second part of the catalyst bed, the reaction continues, further generating heat and further heating the catalyst bed and the gas mixture. The rate of generating heat in this second part of the catalyst bed is faster than the heat transferred from the coolant, so that the temperature increase of the gas mixture and the catalyst bed is higher than the temperature of the coolant. The heat generated in the reaction first heats the solid catalyst. Subsequently, the heat is transferred from the catalyst to the gas mixture to cool the catalyst. Subsequently, the gas mixture transfers heat to the coolant used in the reactor. Another type of heat transfer is heat convection from the solid catalyst to the inside of the reactor. The temperature increase in this part of the catalyst bed is much higher than the temperature of the coolant. During the reaction, the consumption of reactants continues, and more and more crude methanol is produced. Since catalytic methanol synthesis is an equilibrium reaction, the reaction rate and therefore the rate of heat generation are close to reaching the limit of the equilibrium concentration of reactants and products.

[0063] In the third portion of the catalyst bed, the rate of heat generation slows down as the reaction approaches equilibrium conditions. Heat transfer from the catalyst to the gas mixture and ultimately to the cooling system continues and enables further reduction of the catalyst bed temperature.

[0064] In the last, fourth section of the catalyst bed, the reaction is at equilibrium without significant heat generation. In this section of the catalyst bed, the temperature drops further in the direction of the coolant temperature.

[0065] As described above, the highest catalyst bed temperature occurs between the second and third portions of the catalyst bed, respectively. At this temperature maximum, the rate of formation of heat of reaction is substantially balanced with the rate of heat transfer, so that the temperature in the catalyst bed neither rises nor falls.

[0066] In practice, the maximum catalyst bed temperature can be measured directly by known methods. For example, on a laboratory or pilot plant scale, a thermowell can be placed in the catalyst bed and the thermocouples can be manually moved to different positions in the thermowell in order to measure the temperature in the longitudinal direction along the catalyst bed. In this way, a curve of the catalyst bed temperature can be determined in the reactor tube, in which case the turning point of the curve corresponds to the maximum catalyst bed temperature.

[0067] For example, on an industrial scale, multiple point thermocouples can be used to simultaneously monitor the temperature at multiple measurement locations along the catalyst bed. Another alternative used on an industrial scale is to use multiple thermocouples placed in multiple reactor channels and at different heights within the catalyst bed. In this way, a complete picture of the temperature distribution in the catalyst bed throughout the reactor can be generated.

[0068] It is expensive and inconvenient to use such measuring devices in industrial reactors to directly measure the maximum catalyst bed temperature. Therefore, in the design phase of the equipment, but also as a conventional reactor monitoring tool, a simulation of the reactor conditions under operating conditions can be used to establish a reaction rate model based on the measured reaction kinetics and a given gas composition. Many references related to methanol reaction kinetics are available to those skilled in the art. Examples are given in the following table:

[0069]

[0070] Figure 1 Computer simulations ("Calculated") compared to experimentally measured data ("Data") from a commercial tubular reactor for the production of methanol are shown. The simulated and measured catalyst bed temperatures are plotted against the normalized length of the tubular reactor. Also shown is the coolant temperature ("Tcool"), which is 232°C in the case shown. Figure 1 Also evident from the image are four temperature zones of the catalyst bed, corresponding to the above explanation. The highest catalyst bed temperature in this example is about 254° C. This example also shows that the actual conditions in the reactor can be preserved with very high accuracy on the basis of computer simulations.

[0071] In addition, according to the model concept as described above, models can be established for heat and mass transfer within the catalyst bed, heat and mass transfer from the catalyst bed to the gas phase, and finally heat transfer to the cooling surface in the reactor. The following table contains some references to typical models and correlations for the above-mentioned processes. Such models can be created by those skilled in the art and require some additional known or easily measurable parameters, such as physical properties of the catalyst, pressure drop correlations, and the state equation of the gas mixture.

[0072]

[0073] The maximum catalyst bed temperature can be influenced and monitored in various ways in order to adjust the operating point of the reactor so that it is within a predetermined process window.

[0074] During the design phase of the reactor, the maximum catalyst bed temperature can be predicted by simulation, as shown above. In order to affect the maximum catalyst bed temperature, many reactor characteristics known to those skilled in the art can be adjusted. For example, the coolant temperature can be changed to increase or reduce the maximum catalyst bed temperature. The size of the catalyst bed can also be changed to improve the heat transfer characteristics. An example thereof is the use of multiple tubes with relatively small diameters in a tubular reactor for improving heat transfer, which reduces the maximum catalyst bed temperature. Alternatively, the distance between the cooling plates can be reduced to reduce the maximum catalyst bed temperature. In addition, the gas volume flow rate can be increased to reduce the maximum catalyst bed temperature. In addition, the gas composition can be changed so as to reduce reactivity, and the maximum catalyst bed temperature decreases accordingly. This can be achieved by synthesis gas composition or by adding steam and / or methanol. Another option in the design phase is to reformulate the catalyst to adjust the catalyst activity. This can be achieved by changing the physical properties of the catalyst, for example, by using catalyst pellets with different sizes of the same composition, or by diluting the active catalyst material with different amounts of inert carrier materials. The catalyst activity can also be chemically changed by using more or less amounts of active catalyst materials known to those skilled in the art.

[0075] The methanol reactor is part of a synthesis loop in which unreacted synthesis gas is at least partially recycled. In this way, the maximum catalyst bed temperature can also be controlled by the recirculation rate RR. In particular, with the increase of the stoichiometric number SN, a greater recirculation rate leads to a reduction in the maximum catalyst temperature, because the gas mixture includes less reactive gas that ensures improved heat transfer. In addition, in order to control the maximum catalyst temperature, the coolant temperature can be adjusted within a narrow range by adjusting the pressure in the coolant vapor drum. In the case where specific limitations prevent the establishment of a maximum catalyst temperature, the catalyst can still be replaced with one or more catalysts with different activity curves during the shutdown of the equipment, which allows the maximum catalyst bed temperature to be adjusted as a function of the catalyst activity.

[0076] By-products

[0077] The crude methanol formed in the catalytic reaction of synthesis gas and / or mixed synthesis gas to methanol contains water and additional unavoidable by-products. The most frequently occurring by-products are

[0078] - hydrocarbons, which are also often called waxes, such as hexane, heptane,

[0079] - ethers, especially dimethyl ether and ethers with longer carbon chains,

[0080] - esters, such as methyl formate and ethyl formate,

[0081] - ketones, such as acetone, methyl ethyl ketone, and

[0082] - Higher alcohols, such as ethanol.

[0083] The total amount of by-products in the crude methanol is, for example, the total amount of all individual groups mentioned above.

[0084] A detailed discussion of the types of by-products in the production of methanol can be found in GC Hinchen et al., Appl. Catal. 36 (1988) 1-65.

[0085] high pressure

[0086] For the catalytic reaction to obtain methanol, the synthesis gas is passed through the catalyst bed at high pressure (also referred to as reaction pressure). The reaction pressure is the main and required pressure for the catalytic reaction of the components of the synthesis gas and / or mixed synthesis gas to obtain methanol, so as to convert the synthesis gas and / or mixed synthesis gas into methanol. In an example, the reaction pressure in the catalyst bed is 30 to 120 bar, preferably 40 to 90 bar, more preferably 75 to 90 bar and further preferably 75 to 85 bar.

[0087] Syngas

[0088] Synthesis gas comprises at least hydrogen (H2) and carbon oxides. The term "carbon oxides" encompasses the compounds carbon monoxide (CO) and carbon dioxide (CO2). Synthesis gas preferably has a carbon monoxide content of at least 20% by volume, based on the total volume of carbon oxides. Synthesis gas preferably has a high carbon monoxide content. In one example, the synthesis gas contains at least 50% by volume, or at least 70% by volume, or at least 90% by volume, or at least 95% by volume, or at least 99% by volume of carbon monoxide relative to carbon oxides. In one example, the synthesis gas actually contains only carbon monoxide relative to carbon oxides, in which case carbon dioxide is present only in trace amounts. Such synthesis gas is, for example, obtainable by treating crude synthesis gas in methanol scrubbing. In methanol scrubbing or other suitable gas scrubbing processes, carbon dioxide can be virtually completely removed. Selectivity The process is one for which it is particularly suited.

[0089] The process according to the invention is furthermore suitable for use with synthesis gas having a high carbon dioxide content, which contains a carbon dioxide content of at least 50% by volume, or at least 75% by volume, or at least 90% by volume of carbon dioxide relative to carbon oxides. This means that carbon can also be obtained from a carbon dioxide source for methanol synthesis, which is gaining more and more significance in the context of discussions on anthropogenic climate change.

[0090] Synthesis gas can be derived from any source known to those skilled in the art. Examples are steam reforming, partial oxidation or autothermal reforming of natural gas or other suitable carbon sources, and gasification of coal or other solid fuels such as biomass or public waste. The carbon dioxide in the synthesis gas can also be derived from waste gas sources, such as garbage incineration equipment. The hydrogen in the synthesis gas can also be derived from a hydrogen electrolysis device, in which case the electricity used for this device is preferably generated by renewable energy sources such as hydropower, wind power or photovoltaics.

[0091] Irrespective of its origin, the synthesis gas can be produced at temperatures between 400° C. and 1200° C. and / or pressures between 10 and 60 bar. In addition to the above-mentioned components, the synthesis gas may also contain varying amounts of inert components such as methane or nitrogen. Inert components are to be understood in particular as meaning components which are inert under the conditions of the methanol synthesis, i.e. components which are not converted into methanol or (unwanted) by-products under the conditions of the methanol synthesis.

[0092] Before the synthesis gas is used in the process according to the invention, the synthesis gas is typically cooled to below the dew point at which the steam condenses water. The synthesis gas is cooled in particular to below 100° C., preferably to below 60° C. and further preferably to 40° C. or below, in order to separate water from the synthesis gas after condensation. The synthesis gas is thus in particular free of water or largely free of water.

[0093] Hydrogen conversion rate, carbon conversion rate

[0094] The hydrogen conversion rate and the carbon conversion rate are respectively the proportion of hydrogen present in the fresh synthesis gas and the proportion of carbon present in carbon monoxide or carbon dioxide that are finally converted into crude methanol. The sum of the carbon converted from carbon monoxide and carbon dioxide is the total carbon conversion rate. The conversion rate is reduced by, for example, the amount of purge gas separated or the gas dissolved in the crude methanol. The dissolved gases are those components of the synthesis gas that remain dissolved in the crude methanol when the crude methanol is condensed. For example, in the case of a two-stage condensation using high-pressure and low-pressure separators, they can be degassed from the crude methanol in the low-pressure separator. According to this example, the final formula for calculating the conversion rate is

[0095]

[0096] The conversion rate of component i is X iIt is measured in mol / mol, and the molar amounts of the corresponding components (hydrogen, carbon monoxide or carbon dioxide) in the purge gas (n(purge gas)), dissolved gas (n(dissolved gas)) and fresh gas (n(fresh gas)) are measured in mol.

[0097] Methanol synthesis loop, recirculation rate

[0098] Since the formation of methanol from carbon oxides and hydrogen is an equilibrium reaction, the unreacted synthesis gas is returned to the catalyst bed inlet as recycle gas in order to achieve the maximum carbon and hydrogen conversion. In contrast to the once-through process, this situation is called a synthesis loop. In contrast to conventional catalysts based on copper / zinc oxide / aluminum oxide, carbon conversions of 99% or more can therefore be achieved under optimal conditions, meaning that 99% or more of the carbon used (whether in the form of carbon monoxide or carbon dioxide) is ultimately recovered in the form of binding in methanol. The ratio of recycled unreacted synthesis gas (recycle gas) to freshly used synthesis gas is also called the recycle ratio RR and is defined as

[0099]

[0100] It is not uncommon for it to have values ​​of up to 4. This means that the amount of unreacted synthesis gas that is recycled can be up to 4 times the amount of (fresh) synthesis gas that is used.

[0101] Working Example

[0102] The present invention is explained in more detail below by means of examples, but the subject matter of the present invention is not limited in any way.

[0103] The accompanying drawings show:

[0104] Figure 1 The temperature profile of the catalyst bed over the length of the tubular methanol reactor determined by measurements and by simulations, which indicates the maximum catalyst bed temperature,

[0105] Figure 2 according to Figure 3a and 3b A simplified schematic process flow diagram of a pilot plant for carrying out the method according to the invention is shown in FIG.

[0106] Figure 3a and 3b Use according to Figure 2 Tabular compilation of results obtained with the pilot plant.

[0107] Figure 1 A typical temperature profile along the catalyst bed of a methanol synthesis reactor as set out above is shown.

[0108] Figure 2The method according to the invention is characterized and used to determine the Figure 3a and 3b Tabular compilation of the results of the process scheme for the pilot plant 1 for methanol synthesis.

[0109] In the mixing station 20, a steam-preheated synthesis gas consisting of hydrogen, carbon monoxide and carbon dioxide (heating not shown) is produced from the corresponding pure gases provided in industrial quality and introduced under high pressure (p in barg) via conduits 10 and 11 into a water-cooled reactor 21.

[0110] The composition of the synthesis gas is based on Examples 1 to 43 and Non-Inventive Examples 101 to 105 (see Figure 3a and 3b ) is varied in such a way as to result in a stoichiometric number (SN_MUG) of the fresh synthesis gas in the duct 10 being between 0.97 and 2.17.

[0111] The water-cooled reactor 21 is cooled by high-pressure boiling water through a heat exchanger 22 and a water circuit 12 coupled to a steam generator (not shown). The cooling water flows around the reaction tube 23 of the reactor 21 in a cooling jacket 24. The reaction tube 23 (outer diameter × wall thickness = 33.7 mm × 4.05 mm; volume = 3 dm 3 ) has a catalyst bed 25 filled with cylindrical catalyst pellets based on Cu / ZnO / Al2O3 (Clariant Megamax 800, 6×4 mm). The catalyst bed height is 501 cm. The cooling jacket temperature (T(cooling)), i.e. the temperature of the preheated synthesis gas, is determined according to Figure 3a and 3b The temperature profile in the catalyst bed 25 (which also includes the maximum catalyst bed temperature) is determined according to the method described above with the aid of a thermowell and a multi-point thermocouple (not shown) in order to detect the temperature at different locations in the catalyst bed 25.

[0112] The crude methanol containing methanol, water and inevitable impurities produced in the reaction tube 23 of the reactor 21 is discharged through the conduit 12, pre-cooled in the heat exchanger 26 and fed to the high-pressure separator 27 through the conduit 13. In the high-pressure separator 27, the phase is separated into a liquid methanol-water phase (crude methanol) and a gas phase substantially including unreacted synthesis gas. The unreacted synthesis gas is discharged from the high-pressure separator 27 as a recycle gas stream through the conduit 14 and fed to the compressor 28 (recycle gas compressor), wherein the recycle gas is compressed to the reaction pressure. Through the conduit 15, the recycle gas stream is combined with the synthesis gas stream from the conduit 10 in the conduit 11, which obtains a mixed synthesis gas as a combined stream in the conduit 11. The composition of the mixed synthesis gas is generated by the ratio of the fresh synthesis gas stream in the conduit 10 to the recycle gas stream in the conduit 15. The mixed synthesis gas has a stoichiometric number (SN_in (SN_in)) different from the stoichiometric number (SN_MUG) of the fresh synthesis gas. As Figure 2 The stoichiometric number of the mixed synthesis gas at the catalyst bed inlet is determined by gas chromatography analysis of the composition of the mixed synthesis gas as shown in (gas chromatography-GC). The ratio of the recycle gas flow to the synthesis gas flow (recycle ratio (RR)) is determined according to Figure 3a and 3b Numerical examples of vary in the range from 0.194 to 4.44.

[0113] The purge gas is separated from the recycle gas in conduit 14 via conduit 16 and discharged from the process (not shown) via an intermediate vessel 29. The separation of the purge gas prevents the accumulation of inert components within the methanol synthesis loop.

[0114] The crude methanol is discharged as a liquid phase from the high-pressure separator 27 via conduit 17 and fed to a low-pressure separator 30. Up to this process step, further gaseous components remaining dissolved in the crude methanol are separated from the crude methanol in the low-pressure separator 30, and these leave the low-pressure separator 30 via conduit 18 and are discharged from the process via an intermediate container 31 (not shown).

[0115] The condensed crude methanol is discharged from the low pressure separator 30 through the conduit 19, collected in the collecting vessel 32 and subjected to gas chromatography analysis (GC) to determine the by-products formed. The results are detailed in Figure 3a and 3b in the compilation of tables.

[0116] exist Figure 2Additional sampling points for gas chromatography analysis are marked accordingly with "GC". Samples are taken at regular intervals (e.g., every hour) in order to monitor the conversion to methanol and the selectivity of the reaction. The gas chromatography method used is derived from the method of the International Methanol Producers & Consumers Association (IMPCA), which is described, for example, at http: / / www.methanol.org / wp-content / uploads / 2016 / 07 / IMPCA-Ref-Spec-08-December-2015.pdf.

[0117] Figure 3a and 3b The table compilation shows the use of the above description and the Figure 2 The experimental results obtained in the pilot plant shown in . The examples listed are examples 1 to 43 of the present invention and comparative examples 101 to 105 of non-inventive embodiments. The details shown in the columns from left to right are as follows:

[0118]

[0119] The cooling temperature Tcool of the cooling medium varies in the range from about 200° C. to about 250° C. in order to establish a corresponding maximum catalyst bed temperature Tmax. The fresh synthesis gas or fresh gas has a stoichiometric number SN_MUG between 0.97 and 2.17. Depending on the composition (stoichiometric number) SN_MUG of the fresh synthesis gas and the desired stoichiometric number SN_INLET of the mixed synthesis gas at the catalyst bed inlet, the recirculation rate RR varies between about 0.2 and about 4.5. The gas hourly space velocity is between about 2200 and 16000 m / s. 3 (STP) / (m 3 h) varies.

[0120] All numbers in ppm are on a mass basis (mg / kg).

[0121] With the settings mentioned, carbon dioxide conversions XCO2 of up to 97.0%, carbon monoxide conversions XCO of up to 99.9% and total carbon conversions XCO2 (cumulative carbon dioxide and carbon monoxide) of up to 99.6% are achieved.

[0122] The hydrogen proportion yH2_inlet at the catalyst bed inlet is calculated from the stoichiometric number SN_inlet and also yCO2_inlet and yCO_inlet.

[0123] In accordance with Figure 3bIn the non-inventive examples No. 101 to 105 (comparative examples), the impurities found in each case had a total concentration far above 10 000 ppm, i.e. between 17 900 and 31 000 ppm. In all five comparative examples, the stoichiometric number of the mixed synthesis gas at the catalyst bed inlet was below 0.80 and the carbon monoxide concentration in the mixed synthesis gas was far above 20% by volume.

[0124] If the stoichiometric number of the mixed synthesis gas at the catalyst bed inlet is increased to 0.80 or more and at the same time the carbon monoxide concentration is reduced to 20% by volume or less, a reduction in by-product formation is observed according to Examples 1 to 43, which is always below 10 000 ppm with respect to the by-products as a whole. At the same time, the maximum catalyst bed temperature is limited to 280° C. or less. In Examples 1 to 43, the maximum catalyst bed temperature has a range of 205° C. to 277° C.

[0125] As shown by Examples 1, 2, 6-10, 13-16, 19-24, 28, and 33-43, if the maximum catalyst bed temperature is limited to 265°C or less, the byproduct concentration does drop to 5000 ppm or less.

[0126] As shown by Examples 8, 9, 14-16, 19-23, 35-38, and 41-43, if the maximum catalyst bed temperature is limited to 250°C or less, the byproduct concentration is further reduced to 3500 ppm or less.

[0127] As shown by Examples 9-23 and 34-43, even relatively low stoichiometric numbers (SN_inlet) of 0.80 to 2.20 of the mixed synthesis gas at the catalyst bed inlet lead to impurities of less than 10000 ppm when the conditions according to the invention are observed. In view of this, as shown by Examples 9-12, 14-17, 19-21 and 23, it is particularly advantageous when the proportion of CO in the mixed synthesis gas is 9.0% to 13.0% by volume, because in this case, despite the low stoichiometric number, a hydrogen conversion of far more than 80%, here from 86.8% to 98.7%, is still achieved.

[0128] As shown by Examples 1-9, 15, 16, and 24-36, if the stoichiometric number of the mixed syngas at the catalyst bed inlet is 2.0 or greater, the concentration of impurities is indeed 5000 ppm or less.

[0129] The method according to the invention is particularly suitable for synthesis gas with a high carbon dioxide content. As shown by Examples 34-43, if the carbon dioxide content in the mixed synthesis gas is 25% by volume or more, less than 1000 ppm of by-products are indeed formed.

[0130] Embodiments of the invention have been described with reference to different types of subject matter. In particular, certain embodiments have been described with reference to method claims, while other embodiments have been described with reference to apparatus claims. However, from the above and below descriptions, it will be apparent to a person skilled in the art that, unless otherwise stated, any combination of features relating to different types of subject matter or different types of claims is contemplated in addition to any combination of features belonging to one type of claim. Features may be combined to achieve synergistic effects that go beyond the simple sum of the technical features.

[0131] Although the present invention has been shown and described in detail in the drawings and the preceding description, such showing and description should be considered illustrative or exemplary and not restrictive. The present invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments may be understood and performed by a person skilled in the art of the claimed invention by studying the drawings, the disclosure, and the dependent claims.

[0132] In the claims, the word "having" or "comprising" does not exclude additional elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Reference signs in the claims should not be construed as limiting the scope of the claims.

[0133] List of Reference Numerals

[0134] 1 Methods, pilot equipment 10-19 catheter 20 Mixing Station 21 Reactor 22，26，28 Heat Exchanger 23 Reaction tube 24 Cooling Jacket 25 Catalyst bed 27 High pressure separator 29，31 Intermediate container 30 Low pressure separator 32 Collection container

Claims

1. A method for producing methanol, wherein: The method comprises the following method steps: a. Providing a synthesis gas comprising carbon oxides and hydrogen; b. passing the synthesis gas through a catalyst bed of a methanol synthesis reactor at high pressure and temperature for converting the synthesis gas into methanol to obtain a product stream comprising crude methanol and unreacted synthesis gas; c. cooling the product stream for condensing and separating crude methanol comprising at least methanol and water from the cooled product stream; d. recycling at least a portion of the unreacted synthesis gas to the catalyst bed inlet, wherein the unreacted synthesis gas is combined with the synthesis gas to obtain a mixed synthesis gas, and the mixed synthesis gas is passed through a catalyst bed of a methanol synthesis catalyst at high pressure and temperature for converting the mixed synthesis gas into methanol, It is characterized in that The mixed synthesis gas has a stoichiometric number SN of 0.80 to 2.20 at the catalyst bed inlet, wherein Where n is in [mol], The catalyst bed has a maximum catalyst bed temperature of ≤280° C. in the conversion of the mixed synthesis gas into methanol, and the mixed synthesis gas has a carbon monoxide concentration of 9.0 to 13.0% by volume at the catalyst bed inlet.

2. The method according to claim 1, characterized in that The catalyst bed has a maximum catalyst bed temperature of ≤ 265°C during conversion of the mixed synthesis gas to methanol.

3. The method according to claim 1, characterized in that The catalyst bed has a maximum catalyst bed temperature of 205°C to 280°C during conversion of the mixed synthesis gas to methanol.

4. The method according to claim 1, characterized in that The catalyst bed has a maximum catalyst bed temperature of 205°C to 265°C during conversion of the mixed synthesis gas to methanol.

5. The method according to claim 2, characterized in that: The catalyst bed has a maximum catalyst bed temperature of 205°C to 265°C during conversion of the mixed synthesis gas to methanol.

6. The method according to any one of claims 1 to 5, characterized in that The synthesis gas has a stoichiometric number SN of 1.0 to 2.

85.

7. The method according to claim 6, characterized in that The synthesis gas has a stoichiometric number SN of 1.0 to 2.

30.

8. The method according to any one of claims 1 to 5 and 7, characterized in that: The mixed synthesis gas has a carbon dioxide concentration of ≥ 20.0% by volume at the catalyst bed inlet.

9. The method according to any one of claims 1 to 5 and 7, characterized in that: The catalyst bed is divided into a plurality of catalyst bed stages arranged in series, wherein step c) is carried out downstream of each of these catalyst bed stages.

10. The method according to any one of claims 1 to 5 and 7, characterized in that: The synthesis gas is converted into methanol in the catalyst bed at a cooling temperature of the cooling medium used of 190° C. to 250° C.

Images