METHOD FOR PRODUCING DIMETHYL ETHER (DME) FROM SYNTHESEGAS

Description

Field of invention

[0001] The invention relates to a process for producing dimethyl ether (DME) from a synthesis gas containing hydrogen and carbon oxides. State of the art

[0002] The catalytic production of dimethyl ether (DME) from methanol by catalytic dehydration has been known for many years. For example, US patent US 2014408 describes a process for producing DME from methanol using catalysts such as aluminum oxide, titanium oxide, and barium oxide, with temperatures of 350 to 400°C being preferred.

[0003] An article by Yingying Zhu et al., Fuel Processing Technology, Volume 91, Issue 4, 2010, pages 424-429, describes a two-stage synthesis for the production of DME, comprising methanol synthesis and methanol dehydration in a fixed-bed reactor.

[0004] Further information on the state of the art and current practice in the production of dimethyl ether can be found in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, entry "Dimethyl Ether". In particular, Chapter 3 "Production" explains that the catalytic conversion of pure, gaseous methanol is carried out in a fixed-bed reactor.

[0005] From a reaction engineering perspective, fixed-bed reactors are preferred for the catalytic dehydration of methanol to DME in the gas phase due to their simple design. For example, German patent application DE 3817816 describes a process integrated into a methanol synthesis plant for the production of dimethyl ether by catalytic dehydration of methanol without prior separation of the synthesis gas that remains unreacted in the methanol reactor. A simple fixed-bed reactor is used as the dehydration reactor. If this reactor is designed without additional temperature control measures, but is merely surrounded by external insulation to prevent heat loss, it is also referred to as an adiabatic fixed-bed reactor.

[0006] The dehydration of methanol to dimethyl ether according to the reaction equation 2 CH₃OH = (CH₃)₂O + H₂O is an exothermic equilibrium reaction; therefore, from a thermodynamic perspective, high conversion rates are achieved at the lowest possible reaction temperatures. On the other hand, from a reaction kinetic perspective, a minimum reaction temperature is required to ensure sufficient reaction rates and thus acceptable methanol conversions. A disadvantage of the adiabatic fixed-bed reactors used to date is the inability to ensure optimal temperature control to guarantee high conversion rates and minimize the formation of byproducts.

[0007] The formation of byproducts, such as carbon monoxide (CO), carbon dioxide (CO₂), hydrogen (H₂), and methane (CH₄), occurs preferentially at higher temperatures. The formation of the first three byproducts is thought to be due to the steam decomposition of methanol in the feed stream or of already formed DME with water vapor, which is produced as a reaction byproduct. Methane can be formed, for example, by a subsequent reaction of the carbon oxides formed with hydrogen. The formation of these byproducts is undesirable because it impairs the purity of the reaction product and reduces the selectivity of the reaction to DME.

[0008] The theoretical study "Modeling and Optimization of MeOH to DME Isothermal Fixed-bed Reactor", Farsi et al., International Journal of Chemical Reactor Engineering, Volume 8, 2010, Article A79, describes the optimal reactor temperature profile for the catalytic dehydration of methanol to dimethyl ether in a quasi- (or largely) isothermal fixed-bed reactor. In this reactor, the solid catalyst is arranged in tubes surrounded on the shell side by partially evaporating water as a cooling medium. Using a genetic algorithm that considers thermodynamic and kinetic aspects of the dehydration reaction, an exponentially decreasing temperature profile from reactor inlet to reactor outlet is calculated as the optimum, with the reactor inlet temperature being approximately 800 K and the reactor outlet temperature approximately 560 K.Based on this axial temperature profile, a methanol conversion of approximately 86% is calculated for the optimized isothermal reactor, while it is only about 82% for the adiabatic reactor. However, the aforementioned work provides no information on how to design an optimized fixed-bed reactor for DME production from methanol. Furthermore, only the methanol conversion, and not the formation of any byproducts, is used as an optimization criterion.

[0009] A disadvantage of the previously discussed methods for producing DME is that methanol must first be produced and isolated in a complex process. In a second step, the methanol is then converted to the target product, DME. Overall, two-stage DME synthesis processes have the drawbacks that the required synthesis plant comprises two separate reactors, necessitating more equipment and resulting in considerable equipment costs. Furthermore, the methanol produced in the first synthesis step must be separated, purified, and cooled.

[0010] Therefore, processes for so-called direct DME synthesis have already been discussed in the prior art, in which the intermediate product methanol does not occur and / or is not isolated, but the synthesis proceeds directly from synthesis gas to the target product DME. For example, US patent 10501394 B2 discloses a process for producing DME from synthesis gas, in which at least one stream of synthesis gas is subjected to at least one synthesis step in which the components present in the feed stream are at least partially converted into dimethyl ether (DME), yielding at least one crude product stream containing at least DME and the unreacted components of the feed stream. The at least one synthesis step is carried out under isothermal conditions.

[0011] However, such one-step synthesis or DME direct synthesis methods have the following disadvantages: In single-stage synthesis, many byproducts are generated, such as methanol, methane, carbon dioxide, small amounts of C2+ hydrocarbons, and unreacted synthesis gas components. More unreacted compounds must be recycled back into the synthesis reactor to ensure a high DME yield. The selectivity of single-stage synthesis for DME is generally poor. The H2 / CO ratio of the synthesis gas has a significant impact on selectivity. The catalyst lifetime is often too short. Description of the invention

[0012] It is therefore the object of the present invention to propose a method for producing DME that avoids the aforementioned disadvantages of single-stage and two-stage production processes known from the prior art. This object is achieved in a first aspect of the invention by a method with the features of claim 1. Further aspects of the invention are set forth in the dependent method claims.

[0013] The methanol synthesis conditions required for the conversion of synthesis gas to methanol and the DME synthesis conditions required for the conversion of methanol to DME are known to those skilled in the art from the prior art, for example, from the publications discussed at the outset. These are the physicochemical conditions under which a measurable, preferably a technically relevant, conversion of synthesis gas to methanol or of methanol to DME is achieved. Those skilled in the art will make any necessary adjustments to these conditions to the respective operational requirements based on routine experiments. Any disclosed, specific reaction conditions may serve as a guide in this regard, but they are not to be understood as limiting with respect to the scope of the invention.

[0014] The aforementioned states of matter—solid, liquid, and gaseous or vaporous—are always to be understood in relation to the local physical conditions prevailing during the respective process step or in the respective part of the plant, unless otherwise specified. For the purposes of this application, the states of matter—gaseous and vaporous—are to be considered synonymous.

[0015] Thermal separation processes within the meaning of the present invention are all separation processes that are based on the establishment of a thermodynamic phase equilibrium. Preferably, these are distillation or rectification with multiple establishments of vapor-liquid equilibrium, or the separation of a gas-liquid mixture or vapor-liquid mixture in a phase separation device with a single establishment of vapor-liquid equilibrium. However, the use of other thermal separation processes is also conceivable, for example, absorption, extraction, or extractive distillation.

[0016] In the context of the present invention, the division or separation of a material stream is understood to mean the generation of at least two partial streams from the original material stream, wherein the division or separation is associated with an intended change in the material composition of the resulting partial streams with respect to the original material stream, for example by applying a thermal separation process or at least a thermal separation step to the original material stream. In contrast, the division of the original material stream generally does not involve a change in the material composition of the resulting partial streams.

[0017] A thermoplate according to the invention consists of two sheets (thermoplates) welded together at their edges. A multitude of spot welds, which also connect the sheets, are distributed across their surface. Such thermoplates can be manufactured automatically by robots and thus at very low cost. After welding, the sheets are expanded by hydraulic forming, typically by injecting a fluid under high pressure, creating cushion-like channels between the sheets through which a cooling or heating fluid can freely flow. The thermoplates can be oriented parallel to each other on the horizontal axis or perpendicular to the horizontal axis in the synthesis reactor. The space between two thermoplates can be filled with a bed of solid, granular catalyst.With a suitable arrangement of three or more thermal plates, a sandwich-like structure results with good accessibility of the catalyst for incoming and outgoing material flows, for example a feed gas and a product gas, as well as very good heat transfer between the catalyst beds and the cooling fluid or heating fluid.

[0018] The invention relates to carrying out the two-stage DME synthesis without isolating the intermediate methanol in a common synthesis reactor, in order to retain as many of the advantages of both synthesis routes as possible and to minimize their disadvantages. For this purpose, a first reaction zone, in which synthesis gas is converted to methanol, is combined with a second reaction zone, in which methanol reacts further to form DME. Both reaction zones are arranged in a common, pressure-bearing reactor jacket (jacket tube), and the intermediately produced methanol is neither isolated nor purified.

[0019] Both reaction steps take place on packed beds of a solid, particulate catalyst specific to each partial reaction. The invention relates to the arrangement of the catalyst beds between two adjacent thermal plates, allowing the respective feed gas to flow through them. The thermal plates are permeable to a fluid cooling medium. Due to the sandwich-like arrangement of catalyst and cooling layers, very good temperature control is achieved within the catalyst bed, so that isothermal conditions are reached or at least approximated in the individual catalyst beds. In particular, so-called hot spots in the catalyst beds, i.e., zones of localized high temperature, are avoided or reduced.Hot spots negatively affect the possible operating time of the catalysts and, in exothermic equilibrium reactions, reduce the integral conversion of the reactant components achievable via a catalyst bed.

[0020] The product gas stream containing DME exiting the synthesis reactor is fed to a separation device operating according to at least one thermal separation process; this can be, for example, a single-stage or multi-stage distillation. In the separation device, the DME-containing product gas stream is separated into a DME end-product stream, a gas by-product stream containing unreacted carbon oxides and hydrogen, a methanol by-product stream, and a wastewater stream. At least part of the gas by-product stream is recycled back to the reactor inlet to increase the overall DME yield.

[0021] Since controlling reaction temperatures and pressure is important to achieve maximum conversion, various embodiments of the invention deal with the use of different material streams as coolants and their guidance through the synthesis reactor. Special heat transfer fluids, as well as the reactant and / or product streams of the reaction zones, can be used as coolants.

[0022] The pressure in the synthesis reactor according to the invention is 40 to 90 bar absolute, preferably 60 to 80 bar absolute. A synthesis gas with a higher CO concentration than its CO₂ concentration is preferred, as this results in less water formation and better selectivity for DME. However, other synthesis gases can also be used, for example, mixtures of CO₂ and hydrogen with only a small amount or no CO admixture.

[0023] Preferably, the temperature in the first reaction zone is between 180 and 350 °C, most preferably between 200 and 280 °C.

[0024] Preferably, the temperature in the second reaction zone is between 220 and 350 °C, more preferably between 240 and 320 °C, and most preferably between 260 and 300 °C. Investigations have shown that high yields of DME and low yields of by-products are obtained in these temperature ranges.

[0025] Special heat transfer fluids, as well as the reactant and / or product streams from the reaction zones, can be used as coolants. In one example, water is used as the cooling medium; this could be fresh water, water generated during the reaction, or mixtures of both. In a second example, cold synthesis gas is used as the cooling medium, which is itself preheated, thus reducing the process's heating energy requirement. In a third example, a methanol byproduct stream from the separation unit is used as the cooling medium. This reduces the energy requirement for the downstream production of pure methanol.

[0026] An advantage of the process according to the invention is its rapid start-up capability: For this purpose, the second reaction zone (DME synthesis) is first put into operation with methanol supplied from the outside and synthesis gas is passed through the second reaction zone as a cooling medium in order to reach the desired temperature and then the first reaction zone (methanol synthesis) can be put into operation.

[0027] An advantage of the process according to the invention is that the synthesis reactor enables the minimization of byproducts while simultaneously achieving near-equilibrium conversion (40% DME, 40% H₂O, 20% unreacted methanol). The intermediate products (methanol) and end products (DME, CO, CO₂, hydrogen, and methane) are not catalyst poisons for the commercially available catalysts used.

[0028] A maximum reaction temperature of 400 °C should not be exceeded in the second reaction zone in order to keep the extent of methanization low. Further aspects of the invention

[0029] A second aspect of the process according to the invention is characterized in that a first part of the gas by-product stream is recycled to the DME synthesis reactor and introduced into the DME synthesis reactor together with the first feed gas stream. This increases the conversion of reactant components and the DME yield.

[0030] A third aspect of the process according to the invention is characterized in that a second part of the gas by-product stream is discharged from the process as a purge stream. In this way, an accumulation of any inert components in the sense of the two partial reactions, for example methane, is prevented.

[0031] A fourth aspect of the process according to the invention is characterized in that at least a portion of the methanol by-product stream is recycled to the DME synthesis reactor and introduced into the interstitial space and / or into the catalyst beds in the second reaction zone. In this way, the yield of the second partial reaction, the DME synthesis, can be increased in particular.

[0032] A fifth aspect of the method according to the invention is characterized in that a first cooling water stream is used as the first fluid cooling medium and a second cooling water stream is used as the second fluid cooling medium. This makes it easy to set the optimal temperatures in the first and second reaction zones.

[0033] A sixth aspect of the method according to the invention is characterized in that a common cooling water stream is used as both the first and second fluid cooling medium. This aspect allows for resource-efficient use of cooling water.

[0034] A seventh aspect of the process according to the invention is characterized in that the common cooling water flow is first passed through one reaction zone and then, after optional cooling, through the other reaction zone. This aspect allows for resource-efficient use of cooling water and simultaneously offers more possibilities for setting optimal temperatures.

[0035] An eighth aspect of the process according to the invention is characterized in that the common cooling water flow is first passed through the second reaction zone and then, after optional cooling, through the first reaction zone. This aspect allows for resource-efficient use of cooling water and simultaneously offers more possibilities for setting optimal temperatures. The common cooling water flow moves countercurrently to the reactant and product gases through the synthesis reactor.

[0036] A ninth aspect of the process according to the invention is characterized in that the common cooling water flow is first passed through the first reaction zone and then, after optional cooling, through the second reaction zone. This aspect allows for resource-efficient use of cooling water and simultaneously offers more possibilities for setting optimal temperatures. The common cooling water flow moves in the same direction as the reactant and product gases through the synthesis reactor.

[0037] A tenth aspect of the method according to the invention is characterized in that the first cooling water flow and / or the second cooling water flow and / or the common cooling water flow are guided in cocurrent flow through the first reaction zone and / or the second reaction zone, relative to the gas flow through the first reaction zone and / or the second reaction zone. This aspect relates to the flow direction of the cooling medium within a reaction zone.

[0038] An eleventh aspect of the method according to the invention is characterized in that the first cooling water flow and / or the second cooling water flow and / or the combined cooling water flow are guided countercurrently through the first reaction zone and / or the second reaction zone, relative to the gas flow through the first reaction zone and / or the second reaction zone. This aspect relates to the flow direction of the cooling medium within a reaction zone.

[0039] A twelfth aspect of the method according to the invention is characterized in that at least a portion of the wastewater stream, after optional cooling, is used as the first cooling water stream and / or second cooling water stream and / or a combined cooling water stream. This aspect allows for a particularly resource-efficient use of cooling water.

[0040] A thirteenth aspect of the process according to the invention is characterized in that at least a portion of the first cooling water stream and / or the second cooling water stream and / or the common cooling water stream is at least partially evaporated as it passes through the first reaction zone and / or the second reaction zone and is discharged as vapor or a vapor-liquid two-phase mixture. Due to the phase transition and the associated, particularly large enthalpy change, a particularly large cooling effect is achieved.

[0041] A fourteenth aspect of the process according to the invention is characterized in that at least a portion of the first feed gas stream and / or at least a portion of the gas by-product stream recycled to the DME synthesis reactor is used as the first fluid cooling medium and / or as the second fluid cooling medium before the at least a portion of the first feed gas stream and / or the at least a portion of the gas by-product stream recycled to the DME synthesis reactor is introduced into the DME synthesis reactor. In this way, cooling medium, for example cooling water or heat transfer oil, is saved and the corresponding gas streams are pre-tempered.

[0042] A fifteenth aspect of the process according to the invention is characterized in that at least a portion of the methanol by-product stream is introduced into the first reaction zone as a first fluid cooling medium, the methanol by-product stream being heated and then introduced into the interstitial space and / or into the catalyst beds in the second reaction zone. In this way, the yield of the second partial reaction, the DME synthesis, can be increased. At the same time, cooling medium, for example cooling water or heat transfer oil, is saved, and the methanol by-product stream is pre-tempered.

[0043] A sixteenth aspect of the process according to the invention is characterized in that the second reaction zone (DME synthesis) is first started up with externally supplied methanol, and synthesis gas is passed through the second reaction zone as a cooling medium to reach the desired temperature, and then the first reaction zone (methanol synthesis) is started up. In this way, the process according to the invention can be started up particularly quickly.

[0044] A seventeenth aspect of the process according to the invention is characterized in that a maximum reaction temperature of 400 °C is not exceeded in the second reaction zone. This allows the extent of methanization to be kept low.

[0045] An eighteenth aspect of the process according to the invention is characterized in that the temperature in the first reaction zone is between 180 and 350 °C, more preferably between 200 and 280 °C, and that the temperature in the second reaction zone is between 220 and 350 °C, more preferably between 240 and 320 °C, more preferably between 260 and 300 °C. Investigations have shown that high yields of DME and low yields of by-products are obtained in these temperature ranges.

[0046] A nineteenth aspect of the process according to the invention is characterized in that the operating pressure of the DME synthesis reactor is equal to or greater than 90 bar, absolute, and the minimum operating temperature of both reaction zones is equal to or greater than 250 °C, preferably equal to or greater than 260 °C. In this way, the DME synthesis reactor can be operated without condensation occurring inside the reactor, particularly within the first and second reaction zones. Examples of implementation

[0047] Further features, advantages, and applications of the invention will become apparent from the following description of embodiments, numerical examples, and drawings. All features described and / or illustrated, individually or in any combination, constitute the subject matter of the invention, irrespective of their compilation in the claims or their cross-references.

[0048] They show: Fig. 1 a schematic representation of the process according to the invention in an exemplary embodiment of a DME synthesis reactor, Fig. 2 a detailed view of a first reaction zone of the DME synthesis reactor according to the invention, Fig. 3 a constructive detail of the thermal plates (schematic).

[0049] In the following, the statement "not shown" should be understood to mean that an element in the discussed figure is not graphically represented, but is nevertheless present according to the description.

[0050] Figure 1Figure 1 shows a schematic representation of the process according to the invention in an exemplary embodiment of a DME synthesis reactor 1. A first reaction zone 11 and a second reaction zone 12 are arranged one above the other in a common, pressure-bearing jacket tube 10, which is perpendicular to its longitudinal axis. Synthesis gas is introduced into the DME synthesis reactor 1 as a feed stream via a line 13 and flows through it from top to bottom; this is indicated by dashed flow arrows. The feed stream enters the first reaction zone 11 via a first inlet (not shown), the catalyst zones of which are filled with a commercially available, particulate catalyst active for methanol synthesis.

[0051] In the first reaction zone 11, the feed stream is at least partially converted into methanol under methanol synthesis conditions. The feed stream, containing at least some of the methanol, is discharged from the first reaction zone 11 via a first outlet (not shown) as the first methanol-containing product gas stream and then introduced into a second reaction zone 12 via a second inlet (not shown) for the first product gas.

[0052] In the second reaction zone 12, the first product gas stream containing methanol is at least partially converted into DME under DME synthesis conditions. A second product gas stream containing DME is obtained, which is discharged from the second reaction zone via a second outlet (not shown) and via a line 15 (product outlet) on the jacket tube 10 from the DME synthesis reactor 1 and fed to a separation device (not shown) operating according to at least one thermal separation process.

[0053] In the separation device, the product gas stream containing DME is separated into a DME end-product stream, a gas by-product stream containing unreacted carbon oxides and hydrogen, a methanol by-product stream, and a wastewater stream. In one example, the DME end-product stream is fed to DME storage, DME purification, and / or further DME processing (all not shown). The gas by-product stream is preferably at least partially recycled to the DME synthesis reactor 1 and introduced into it via line 13. The methanol by-product stream is preferably at least partially recycled to the DME synthesis reactor 1, introduced into it via line 14, and fed directly to the second reaction zone. In one example, the wastewater stream is discharged from the process. In another example, the wastewater stream is returned to the DME synthesis reactor 1 as a cooling medium after optional cooling.

[0054] A first fluid cooling medium is introduced into the first reaction zone 11 via a line 16 and an inlet (not shown). This first fluid cooling medium absorbs at least some of the heat of reaction from the exothermic methanol synthesis and is itself heated in the process. The heated first fluid cooling medium is then discharged from the first reaction zone 11 via an outlet (not shown) and a line 17.

[0055] A second fluid cooling medium is introduced into the second reaction zone 12 via a line 18 and an inlet (not shown). This second fluid cooling medium absorbs at least some of the heat of reaction from the exothermic DME synthesis from methanol and is itself heated in the process. The heated second fluid cooling medium is then discharged from the second reaction zone 12 via an outlet (not shown) and a line 19.

[0056] In one example, the pressure in the synthesis reactor according to the invention is 40 to 90 bar absolute, preferably 60 to 80 bar absolute. Preferably, a synthesis gas is introduced into the DME synthesis reactor 1 whose CO concentration is higher than its CO₂ concentration, since this results in less water being produced and better selectivity for DME. However, other synthesis gases can also be used, for example, mixtures of CO₂ and hydrogen with only a small amount or no admixture of CO.

[0057] In one example, the temperature in the first reaction zone 11 is preferably between 180 and 350 °C, more preferably between 200 and 280 °C. In another example, the temperature in the second reaction zone 12 is between 220 and 350 °C, more preferably between 240 and 320 °C, more preferably between 260 and 300 °C. Investigations have shown that high yields of DME and low yields of byproducts are obtained in these temperature ranges.

[0058] Special heat transfer fluids, as well as the reactant and / or product streams from the reaction zones, can be used as coolants. In one example, water is used as the cooling medium, with fresh water, water generated during the reaction, or mixtures of both being suitable. In another example, cold synthesis gas is used as the cooling medium, which is itself preheated, thus reducing the process's heating energy requirement. In yet another example, a methanol byproduct stream from the separation unit is used as the cooling medium. This reduces the energy demand for the downstream production of pure methanol.

[0059] Fig. 2Figure 1 shows a detailed view of a first reaction zone 11 of the DME synthesis reactor 1 according to the invention. The second reaction zone 12 has basically the same structure, however, the catalyst zones are filled with a catalyst active for the DME synthesis from methanol.

[0060] In the first reaction zone 11, thermal plates 30 are preferably arranged parallel and equally spaced. The detailed structure of the thermal plates 30 is described below in connection with Fig. 3The arrangement of the thermal plates creates spaces 20 which are filled with packed beds of a solid, particulate catalyst active for methanol synthesis. Both ends of the spaces are permeable to gas flows. The lower end of the spaces, and preferably also the upper end, have supports or retaining devices (not shown) for the catalyst beds, for example, sieve trays, perforated plates, or wire mesh. The retaining device, preferably located at the inlet side of the gas flow into the first reaction zone 11, can advantageously also serve to homogenize and distribute the feed gas flow onto the individual catalyst beds.

[0061] A first fluid cooling medium is introduced into the thermal plates 30 in the first reaction zone 11 via line 16 and an inlet (not shown). This first fluid cooling medium absorbs at least some of the heat of reaction from the exothermic methanol synthesis and is itself heated in the process. The heated first fluid cooling medium is then discharged from the first reaction zone 11 via an outlet (not shown) and line 17 (not shown). The introduction and discharge of the first fluid cooling medium into and out of the thermal plates 30 is accomplished via a distribution system (not shown). This is indicated by lines 16 and 17, represented by arrows.

[0062] Figure 3aFigure 1 shows an xy-view of a thermal plate 30 across the surface of a sheet metal part that forms one side of the thermal plate 30. Points 311 to 319 represent the so-called spot welds, which connect the sheet metal part to the sheet metal part on the opposite side (not shown) by means of an additional spot weld. Points 311 to 313, 313 to 316, and 316 to 319 each lie on a straight line, with the points of every other line alternating between lines in the other dimension. The lines are parallel to each other and are spaced d5 apart.

[0063] By welding the thermal plate not only at the edges of two superimposed sheets, but also having additional weld points 31 1 to 31 9 on it, the xz view results in a thermal plate which is in Figure 3bThe section shown is along the line A - A'. Channels 321 and 322 are formed between the individual spot welds 311 to 319, which are generally produced by pressure forming, particularly preferably by internal high-pressure forming. The diameter of such a channel 321 or 322, d2, describes the distance between the two sheets 30a and 30b at the maximum channel size, while the diameter d4 denotes the thickness of the spot weld 311 to 319. The distance between two spot welds 311 to 319, which corresponds exactly to the distance between two spot welds 311 to 319 on a straight line, is d3. Preferably, d3 > d2 > d4. Numerical examples

[0064] The following table lists exemplary operating conditions for the DME synthesis reactor according to the invention, under which reactor operation without falling below the dew point and thus without condensation in the reactor is possible. This is important because it does not impair the service life and potential operating time of the catalyst. Tabel: Exemplary operating conditions for the DME synthesis reactor Percentage / mol% p / bar Dew point / °C p / bar Dew point / °C Total pressure 90,0 60,0 DME 40,0 36,0 24,0 Water 40,0 36,0 250 24,0 225 Methanol 20,0 18,0 12,0

[0065] As a result, it is advantageous to select the operating pressure of the DME synthesis reactor according to the invention as equal to or greater than 90 bar, absolute, and the minimum operating temperature of both reaction zones as equal to or greater than 250 °C, preferably as equal to or greater than 260 °C. Reference symbol list

[0066] 1DME synthesis reactor 10Curb 11First reaction zone 12Second reaction zone 13Feed gas line 14Methanol side feed line 15Product gas line 16First fluid coolant inlet line 17First fluid coolant outlet line 18Second fluid coolant inlet line 19Second fluid coolant outlet line 20Catalytic packings in the spaces between thermal plates (hatched) 30Thermal plates 31Spot welds 32Free interior

Claims

1. A method for producing dimethyl ether (DME) from a synthesis gas containing carbon oxides and hydrogen, comprising the following steps: (a) providing a DME synthesis reactor, comprising the following components: (a1) a first reaction zone with a first inlet for synthesis gas as a first feed gas and a first outlet for a first, methanol-containing product gas, wherein the first reaction zone comprises a plurality of thermo-plates, which are arranged such that: - between every two adjacent thermo-plates, a bed of a solid, particulate catalyst active for methanol synthesis from the synthesis gas is arranged, through which the synthesis gas can flow, - the thermo-plates have an inlet for a first fluid cooling medium and an outlet for the first fluid cooling medium and are traversable in their interior by the first fluid cooling medium; (a2) a second reaction zone with a second inlet for the first product gas and a second outlet for a second, DME-containing product gas, wherein the second reaction zone comprises a plurality of thermo-plates, which are arranged such that: - between every two adjacent thermo-plates, a bed of a solid, particulate catalyst active for DME synthesis from methanol is arranged, through which the first product gas can flow, - the thermo-plates have an inlet for a second fluid cooling medium and an outlet for the second fluid cooling medium and are traversable in their interior by the second fluid cooling medium; (a3) an outer, pressure-bearing shell tube, in the interior of which the first reaction zone and, spaced therefrom by an intermediate space, the second reaction zone are arranged, wherein the shell tube has at the end adjacent to the first reaction zone a reactant inlet for introducing synthesis gas as a first feed gas and at the end adjacent to the second reaction zone a product outlet for discharging the second, DME-containing product gas; (b) introducing a synthesis gas stream as a first feed gas stream into the DME synthesis reactor via the reactant inlet on the shell tube and via the first inlet into the first reaction zone; (c) reacting the first feed gas stream in the first reaction zone under methanol synthesis conditions; (d) discharging a first, methanol-containing product gas stream from the first reaction zone; (e) introducing the first, methanol-containing product gas stream into the second reaction zone; (f) reacting the first product gas stream in the second reaction zone under DME synthesis conditions; (g) discharging a second, DME-containing product gas stream from the second reaction zone via the second outlet and via the product outlet on the shell tube from the DME synthesis reactor; (h) feeding the DME-containing product gas stream to a separation device operating according to at least one thermal separation method, separating the DME-containing product gas stream in the separation device into a DME final product stream, a gas by-product stream containing unreacted carbon oxides and hydrogen, a methanol by-product stream, and a wastewater stream.

2. The method according to claim 1, characterized in that a first part of the gas by-product stream is recycled to the DME synthesis reactor and is introduced together with the first feed gas stream into the DME synthesis reactor.

3. The method according to claim 1 or 2, characterized in that a second part of the gas by-product stream is discharged from the process as a purge stream.

4. The method according to one of the preceding claims, characterized in that at least a part of the methanol by-product stream is recycled to the DME synthesis reactor and is introduced into the intermediate space and / or into the catalyst beds in the second reaction zone.

5. The method according to one of the preceding claims, characterized in that a first cooling water stream is used as a first fluid cooling medium and a second cooling water stream is used as a second fluid cooling medium.

6. The method according to one of the preceding claims, characterized in that a common cooling water stream is used as the first fluid cooling medium and as the second fluid cooling medium.

7. The method according to claim 6, characterized in that the common cooling water stream is first passed through the one reaction zone and then, after optional cooling, through the other reaction zone.

8. The method according to claim 7, characterized in that the common cooling water stream is first passed through the second reaction zone and then, after optional cooling, through the first reaction zone.

9. The method according to claim 7, characterized in that the common cooling water stream is first passed through the first reaction zone and then, after optional cooling, through the second reaction zone.

10. The method according to one of claims 5 to 9, characterized in that the first cooling water stream and / or the second cooling water stream and / or the common cooling water stream are passed in cocurrent flow through the first reaction zone and / or the second reaction zone, relative to the gas flow through the first reaction zone and / or the second reaction zone.

11. The method according to one of claims 5 to 9, characterized in that the first cooling water stream and / or the second cooling water stream and / or the common cooling water stream are passed in countercurrent flow through the first reaction zone and / or the second reaction zone, relative to the gas flow through the first reaction zone and / or the second reaction zone.

12. The method according to one of the preceding claims, characterized in that at least a part of the wastewater stream is used, after optional cooling, as a first cooling water stream and / or a second cooling water stream and / or a common cooling water stream.

13. The method according to one of the preceding claims, characterized in that at least a part of the first cooling water stream and / or the second cooling water stream and / or the common cooling water stream is at least partially vaporized when passing through the first reaction zone and / or the second reaction zone and is discharged as steam or a steam-liquid two-phase mixture.

14. The method according to one of the preceding claims, characterized in that at least a part of the first feed gas stream and / or at least a part of the gas by-product stream recycled to the DME synthesis reactor is used as a first fluid cooling medium and / or as a second fluid cooling medium, before the at least one part of the first feed gas stream and / or the at least one part of the gas by-product stream recycled to the DME synthesis reactor is introduced into the DME synthesis reactor.

15. The method according to one of the preceding claims, characterized in that at least a part of the methanol by-product stream is introduced as a first fluid cooling medium into the first reaction zone, wherein the methanol by-product stream is heated and is then introduced into the intermediate space and / or into the catalyst beds in the second reaction zone.

16. The method according to one of the preceding claims, characterized in that first the second reaction zone (DME synthesis) is put into operation with externally supplied methanol and that synthesis gas is passed as a cooling medium through the second reaction zone to achieve the desired temperature and then the first reaction zone (methanol synthesis) is put into operation.

17. The method according to one of the preceding claims, characterized in that a maximum reaction temperature of 400 °C is not exceeded in the second reaction zone.

18. The method according to one of the preceding claims, characterized in that the temperature in the first reaction zone is between 180 and 350 °C, mostly preferably between 200 and 280 °C, and that the temperature in the second reaction zone is between 220 and 350 °C, more preferably between 240 and 320 °C, most preferably between 260 and 300 °C.

19. The method according to one of the preceding claims, characterized in that the operating pressure of the DME synthesis reactor is equal to or greater than 90 bar, absolute, and the minimum operating temperature of both reaction zones is equal to or greater than 250 °C, preferably equal to or greater than 260 °C.