Process for preparation of dimethyl ether

Abstract

The invention relates to a process for the preparation of dimethyl ether, comprising the following steps:-introducing a methanol-containing stream (Sfeed-RD) into a reactive distillation apparatus (RD), in which-in at least one reaction zone (RZ) of the reactive distillation apparatus (RD), methanol is reacted in the presence of an acid catalyst to form dimethyl ether and water, and separating the stream by distillation into a fraction containing dimethyl ether and leaving the reactive distillation unit (RD) as an overhead stream (Soverhead-RD) and a fraction containing water and leaving the reactive distillation unit (RD) as a bottom stream (Sbottom-RD), withdrawing a methanol-containing side stream (Sside-RD) from the reactive distillation unit (RD), introducing the methanol-containing side stream (Sside-RD) into a side reactor (SR), and reacting the methanol in the presence of an acid catalyst to form dimethyl ether and water to obtain a product stream (Sproduct-SR) containing dimethyl ether, water and methanol and withdrawn from the side reactor (SR).

Description

Background Art

[0001] Dimethyl ether is an important starting material for the industrial preparation of dimethyl sulfate and is used as a propellant and refrigerant. Dimethyl ether has also attracted much attention as a synthetic fuel (e.g., as a substitute for LPG and diesel fuel).

[0002] An overview of known processes for preparing dimethyl ether can be found in the following publications:

[0003] Z. Azizi et al., "Dimethyl ether: A review of technologies and product challenges", Chemical Engineering and Processing, 2014, 82, pp. 150-172;

[0004] V. Dieterich et al., "Power-to-liquid via synthesis of methanol, DME or Fischer-Tropsch fuels: a review", Energy Environ. Sci., 2020, 13, pp. 3207-3252;

[0005] Th. Cholewa et al., "Process Intensification Strategies for Power-to-X Technologies", ChemEngineering, 2022, 6(1), 13.

[0006] On a large scale, dimethyl ether (DME) can be produced by dehydrating methanol in a reactor (e.g., a fixed bed reactor) in the presence of an acid catalyst. The dehydration reaction can be represented by the following reaction equation:

[0007]

[0008] Gaseous methanol is typically fed to a DME synthesis reactor and converted to dimethyl ether and water at a reaction temperature of about 220-400° C. The dehydration reaction occurring in the DME synthesis reactor is thermodynamically limited (equilibrium-limited reaction) and generally gives a conversion of no more than about 70-85%.

[0009] The DME synthesis reactor is usually subsequently subjected to at least two distillation steps, wherein dimethyl ether is separated from methanol and water in a first distillation step and methanol is separated from water in a second distillation step. The methanol separated by distillation is returned to the DME synthesis reactor.

[0010] The methanol fed to the DME synthesis reactor can be produced from synthesis gas in a known manner. The methanol directly obtained in this synthesis is also called crude methanol and usually contains a significant amount of water (e.g. 20-50 mol %), especially in the case of sustainable methanol synthesis using synthesis gas rich in CO2 and containing regenerated hydrogen. Therefore, a DME synthesis method in which raw methanol can be used directly (i.e. without further processing, such as water separation) as a starting material will receive much attention.

[0011] If water-containing crude methanol is added as a reactant stream to the DME synthesis reactor, this has an adverse effect on the conversion rate because the equilibrium of the dehydration reaction will shift towards the reactant (methanol). In addition, the catalytic activity and stability of some acidic solid catalysts such as γ-Al2O3 will be impaired by the presence of significant amounts of water. Therefore, it is common practice to feed substantially anhydrous methanol as a reactant stream to the DME synthesis reactor.

[0012] Therefore, the conventional process for preparing dimethyl ether is disadvantageous both in terms of energy balance and required equipment because

[0013] - water must be separated from the raw methanol to obtain essentially anhydrous methanol, which can then be fed to the DME synthesis reactor,

[0014] - methanol must be vaporized before being fed into the DME synthesis reactor, and

[0015] At least two distillation stages are connected downstream of the DME synthesis reactor, in which firstly dimethyl ether is separated from water and methanol and then methanol is separated from water by distillation.

[0016] If the conversion of methanol to dimethyl ether and water is not carried out in the gas phase but in the liquid phase, the DME synthesis reactor can in principle be operated isothermally, for example at a temperature optimized for catalytic activity. Fixed bed reactors can be used for such isothermal operation. However, even in the case of isothermal operation of the DME synthesis reactor, the hydration reaction of methanol to dimethyl ether and water is thermodynamically limited, so that conversions of more than 85% cannot usually be achieved.

[0017] It is known that the amount of equipment required in the synthesis of dimethyl ether can be reduced by using a reactive distillation unit (or reactive distillation unit). A reactive distillation unit (e.g. in the form of a reactive distillation column) comprises one or more reaction zones in which the reactants react with one another (usually in the presence of a catalyst immobilized in the reaction zone) and one or more distillative separation zones in which the reaction products and unreacted reactants (if any) are separated from one another.

[0018] US2007 / 0066855 A1 describes a process for preparing dimethyl ether, wherein a methanol-containing stream is introduced into a reactive distillation column, the methanol is converted into dimethyl ether and water in a reaction zone of the reactive distillation column in the presence of an acid catalyst, and separated by distillation to produce a head stream consisting essentially of dimethyl ether and a sump stream consisting essentially of water.

[0019] With respect to the reaction to be catalyzed, there is usually an optimum reaction temperature for the catalyst, e.g., a temperature that is high enough to achieve sufficient catalytic activity, but not too high to avoid thermal degradation of the catalyst material or to minimize undesirable side reactions. In principle, a chemical reactor can be operated isothermally (e.g., at a temperature optimized for catalytic activity). However, when operating a reactive distillation apparatus, the temperature in the reaction zone usually decreases towards the top of the tower. Therefore, the optimum reaction temperature can only be achieved in a portion of the reaction zone. As a result, the reaction zone has regions of reduced catalytic activity. The average catalytic activity of a particular catalyst that can be achieved in the reaction zone of a reactive distillation apparatus is more or less significantly lower than the maximum catalytic activity that can be achieved with this catalyst.

[0020] To compensate for this, for example, the volume of the reaction zone can be increased. This requires the insertion of additional internals or packings (with fixed catalysts) in the reaction zone, which is complex and costly.

[0021] Z. Lei et al., "Synthesis of dimethyl ether (DME) by catalytic distillation", Chemical Engineering Science, 2011, 66, pp. 3195-3203, inter alia, describes a process (referred to as "Process A" in the publication) in which (i) a stream containing methanol is first fed into a DME synthesis reactor to convert the methanol into dimethyl ether and water in the presence of an acid catalyst, and (ii) the product stream removed from the DME synthesis reactor (comprising dimethyl ether, water and unreacted methanol) is directly fed into a reactive distillation unit to convert the remaining methanol into dimethyl ether and water, and is subjected to distillative separation to produce a top stream consisting essentially of dimethyl ether and a bottom stream consisting essentially of water. Since the DME synthesis reactor is connected upstream of the reactive distillation unit and a large proportion of the methanol has been converted into dimethyl ether in this upstream DME synthesis reactor, the stream fed to the reactive distillation unit contains the desired final product (dimethyl ether) in a relatively high concentration. Therefore, only the methanol which is not converted in the DME synthesis reactor needs to be converted in the reactive distillation unit. The presence of an upstream DME synthesis reactor means that the reactive distillation unit can be smaller.

[0022] However, if crude methanol (i.e. methanol with a considerable proportion of water) is fed to a DME synthesis reactor, the "method A" described in the publication of Z. Lei et al. (i.e. using an upstream DME synthesis reactor whose product stream is fed to a reactive distillation unit) is disadvantageous. The presence of water in the reactant stream inhibits the conversion of methanol to the dehydration reaction of dimethyl ether and water in the DME synthesis reactor. In order to offset the loss of this conversion, the reaction temperature in the DME synthesis reactor or the volume of the reaction zone containing the catalyst in the DME synthesis reactor can be increased. However, these measures increase the energy input of the method. In addition, the increase in reaction temperature requires that the acid catalyst used for the dehydration reaction of methanol has a sufficiently high thermal stability. Summary of the invention

[0023] The problem addressed by the present invention is to prepare dimethyl ether by an efficient (e.g. energy-efficient) process. In particular, the process should enable efficient preparation of dimethyl ether if crude methanol (i.e. methanol with a considerable proportion of water) is used as starting material. As already mentioned above, in particular during sustainable methanol synthesis using CO2-rich synthesis gas containing regenerated hydrogen, raw methanol with a high water content is produced.

[0024] The problem is solved by a process for preparing dimethyl ether, comprising the following steps:

[0025] - The methanol-containing stream S进料-RD Feed to the reactive distillation unit RD, where

[0026] - in at least one reaction zone RZ of the reactive distillation unit RD, methanol is converted into dimethyl ether and water in the presence of an acid catalyst, and

[0027] - separation by distillation to produce a tower top stream S containing dimethyl ether 塔顶-RD The fraction leaving the reactive distillation unit RD and containing water as the bottom stream S 塔底-RD The fraction leaving the reactive distillation unit RD,

[0028] - A methanol-containing side stream S is removed from the reactive distillation unit RD 侧-RD ,

[0029] - The side stream S containing methanol 侧-RD The methanol is fed to the side reactor SR and reacted in the presence of an acid catalyst to form dimethyl ether and water to obtain a product stream S 产物-SR , which contains dimethyl ether, water and methanol and which is removed from the side reactor SR.

[0030] The process according to the invention is particularly suitable for using crude methanol, i.e. methanol with a considerable water content, as feed stream. During operation of the reactive distillation unit RD with crude methanol feed, the very highly volatile dimethyl ether is substantially distilled via the top stream S 塔顶-RD The high boiling water is removed through the bottom stream S 塔底-RD Remove, while the side flow S 侧-RD It can be withdrawn at a suitable position or height of the reactive distillation unit RD, which has a significantly higher methanol concentration or a significantly lower water concentration than the crude methanol. Due to the lower water concentration compared to the crude methanol, this side stream S removed from the reactive distillation unit RD 侧-RD A very efficient conversion of methanol to dimethyl ether can be achieved in the side reactor SR, which serves as a DME synthesis reactor. This allows the production of a product stream S having a high DME concentration in the side reactor SR. 产物-SR , for example, due to the high dimethyl ether concentration, it also has a favorable effect on the yield of dimethyl ether in the reactive distillation apparatus RD after its return to the reactive distillation apparatus RD.

[0031] In the context of the present invention, the reactive distillation unit RD is therefore used not only for the synthesis and distillative separation of dimethyl ether, but also for providing a methanol source having a lower water concentration than crude methanol, and thus enabling a higher DME yield (compared to crude methanol) to be achieved in the DME synthesis reactor.

[0032] As mentioned above, in the process according to the invention, the methanol-containing stream S 进料-RDFeed to the reactive distillation unit RD, where

[0033] In at least one reaction zone RZ of the reactive distillation unit RD, methanol is converted into dimethyl ether and water in the presence of an acid catalyst, and

[0034] The distillation separation is carried out to produce a fraction containing dimethyl ether (which is used as the top stream S 塔顶-RD Leaving the reactive distillation unit RD) and the water-containing fraction (which is the bottom stream S 塔底-RD Leaving the reactive distillation apparatus RD).

[0035] Methanol-containing stream S 进料-RD Having, for example, a methanol concentration C1 (MeOH) of at least 40 mol %, a water concentration C1 (H2O) of at most 60 mol % and a total concentration of other components (ie, components other than methanol and water) (if present) of at most 5 mol %.

[0036] As mentioned above, even if the methanol-containing stream S used as starting material 进料-RD The method according to the invention also enables efficient synthesis of dimethyl ether with a considerable amount of water. 进料-RD The water concentration C1(H2O) is 15-60 mol%, more preferably 25-50 mol%, and optionally contains components other than methanol and water in a total concentration of up to 5 mol%.

[0037] Methanol-containing stream S 进料-RD It may originate from a methanol synthesis plant, in which methanol is produced in a known manner, for example from synthesis gas, in particular synthesis gas rich in CO 2 .

[0038] Reactive distillation apparatus suitable for converting methanol into dimethyl ether and water and separating the reaction products by distillation are known to the person skilled in the art.

[0039] The reactive distillation unit RD (e.g., reactive distillation column) has one or more reaction zones RZ and one or more distillative separation zones DT. The reaction zone RZ contains one or more acid catalysts, in particular one or more acidic solid catalysts. Suitable acid catalysts for the dehydration reaction of methanol to dimethyl ether and water are known to those skilled in the art. For example, the acid catalyst is an ion exchange resin, zeolite, aluminosilicate, alumina or an acidic ionic liquid containing acidic groups (which is preferably fixed on a support). In the one or more distillative separation zones DT, distillative separation occurs to produce a DME-containing fraction (which is obtained as a top stream S 塔顶-RD Leaving the reactive distillation unit RD) and the water-containing fraction (which is the bottom stream S 塔底-RDLeaving the reactive distillation apparatus RD). The distillative separation zone DT contains, for example, internals for distillative separation, in particular trays, column packings or structured packings, as are generally known to the person skilled in the art. The fixing of the catalyst in the reaction zone RZ of the reactive distillation apparatus RD can be carried out in a manner known to the person skilled in the art, for example as random bulk packing; in the form of catalyst-filled wire mesh beads or as catalyst shaped bodies attached to the trays in the reaction zone RZ.

[0040] By using suitable internals or packings known to the person skilled in the art, the reaction zone RZ itself can already provide sufficient distillative separation of the reaction products from one another, on which the acid catalyst is present. Preferably, however, the reactive distillation apparatus RD has at least one, more preferably at least two, catalyst-free distillative separation zones DT. For example, catalyst-free distillative separation zones DT can be present in the reactive distillation column RD above and below the reaction zone RZ.

[0041] The reactive distillation apparatus RD is operated, for example, in such a way that a temperature in the range from 100 to 180° C. and / or a pressure in the range from 8 to 20 bar prevails in the reaction zone RZ.

[0042] Preferably, the methanol-containing stream S 进料-RD In principle, however, it is also possible to feed the methanol-containing stream S 进料-RD The introduction is carried out above or below the reaction zone RZ, for example into the catalyst-free distillation separation zone.

[0043] Highly volatile dimethyl ether as the top stream S 塔顶-RD Leaving the reactive distillation unit RD, water (i.e. the component with the highest boiling point) is taken as the bottom stream S 塔底-RD Leaving the reactive distillation apparatus RD.

[0044] Top flow S 塔顶-RD It may have a dimethyl ether concentration of at least 50 mol %, preferably at least 95 mol %, even more preferably at least 99 mol %.

[0045] The bottoms stream may have a water concentration of at least 50 mol %, preferably at least 90 mol %, even more preferably at least 99 mol %.

[0046] In the process according to the invention, the methanol-containing side stream S 侧-RD It is removed from the reactive distillation unit RD (e.g., from the reaction zone RZ of the reactive distillation unit RD) and fed to the side reactor SR. In the side reactor SR, methanol is converted into dimethyl ether and water in the presence of an acid catalyst to obtain a product stream S 产物-SR , which contains dimethyl ether, water and methanol, and is removed from the side reactor SR.

[0047] In the reactive distillation unit RD, dimethyl ether accumulates at the top of the column due to its high volatility, and water as the component with the highest boiling point accumulates at the bottom of the column, while fractions with a high methanol concentration can be removed as a side stream in the middle region of the column. If the reactive distillation unit RD is fed with crude methanol, these fractions that can be removed as a side stream may even have a higher methanol concentration (and therefore also a lower water concentration) than the crude methanol.

[0048] The methanol-containing side stream S removed from the reactive distillation unit RD 侧-RD There may be a water concentration C2(H2O) of up to 25 mol %, preferably of up to 10 mol %, even more preferably of up to 5 mol %. Dimethyl ether and, if present, components other than methanol, water and dimethyl ether may be present in the methanol-containing side stream S in a total concentration of up to 10 mol %. 侧-RD middle.

[0049] The appropriate position or height at which the side stream with high methanol concentration or low water concentration can be removed from the reactive distillation unit RD can be determined by a person skilled in the art based on common knowledge. 侧-RD The methanol-containing side stream S is withdrawn at a relatively high position in the reaction zone RZ, for example, in the upper third or the upper quarter of the reaction zone RZ. Thus, if the reaction zone RZ has an upper end (i.e., facing the head of the reactive distillation apparatus RD) and a lower end (i.e., facing the bottom of the reactive distillation apparatus RD) and a length L (i.e., the distance between the upper and lower ends of the reaction zone RZ), and the methanol-containing side stream S 侧-RD At position P S Move out of the reaction zone RZ, then the position P S It is possible, for example, to have a distance l from the upper end of the reaction zone RZ such that l / L≤0.33, more preferably l / L≤0.25.

[0050] As already mentioned above, the process according to the invention also enables a very efficient preparation of dimethyl ether if methanol with a high water content (crude methanol) is used as starting material. In the context of the present invention, the reactive distillation unit RD is used not only for the synthesis and distillative separation of dimethyl ether, but also for providing a methanol source with a lower water concentration than crude methanol and thus enabling a higher DME yield (compared to crude methanol) to be achieved in the downstream DME synthesis reactor.

[0051] According to an exemplary embodiment, the methanol-containing stream S fed to the reactive distillation unit RD 进料-RD The methanol-containing side stream S removed from the reactive distillation unit RD has a water concentration C1(H2O) of 15-60 mol %, more preferably 25-50 mol %, 侧-RD The water concentration C2(H2O) satisfies the following conditions:

[0052] C2(H2O)≤0.75×C1(H2O).

[0053] The following conditions preferably apply:

[0054] C2(H2O)≤0.60×C1(H2O).

[0055] For example, the methanol-containing stream S fed to the reactive distillation unit RD 进料-RD The side stream S removed from the reactive distillation unit RD may have a water concentration C1(H2O) of 15-60 mol %, more preferably 25-50 mol %, 侧-RD It may have a water concentration C2(H2O) of up to 10 mol %.

[0056] As already mentioned above, the methanol-containing stream S fed to the reactive distillation unit RD 进料-RD Components other than methanol and water may be present in a total concentration of up to 5 mol %. As also mentioned above, the methanol-containing side stream S removed from the reactive distillation unit RD 侧-RD Dimethyl ether and components other than methanol, water and dimethyl ether may be contained in a total concentration of up to 10 mol%.

[0057] The side reactor SR can be a reactor type commonly used for DME synthesis. For example, the side reactor SR is a fixed bed reactor.

[0058] For the conversion of methanol into dimethyl ether and water, the side reactor SR contains one or more acid catalysts, in particular one or more acidic solid catalysts. Suitable acid catalysts for the dehydration reaction of methanol to dimethyl ether and water are known to those skilled in the art. For example, the acid catalyst is an ion exchange resin containing acidic groups, a zeolite, an aluminosilicate, an alumina or an acidic ionic liquid (which is preferably fixed on a support).

[0059] In order to operate energy-efficiently, if the side reactor SR is fed with a methanol-containing side stream S 侧-RD and the obtained product stream S 产物-SR It may be advantageous to operate at least partially at pressures and temperatures at which the liquid phase exists.

[0060] For example, the side reactor SR is operated at a temperature in the range of 130-200°C.

[0061] The side reactor SR is preferably operated isothermally. Isothermal operation occurs when the temperature fluctuations of the reactor in the region of the acid catalyst are maximum + / -10°C, preferably + / -5°C.

[0062] The side reactor SR can be operated at a pressure of 20-150 bar.

[0063] The product stream S of the side reactor SR产物-SR Contains methanol, for example, in a concentration of not more than 50 mol %, preferably not more than 40 mol %. 产物-SR The molar ratio of dimethyl ether to water in the product stream S is, for example, in the range of 4:6 to 6:4. 产物-SR Components other than dimethyl ether, water and methanol may be present in the mixture, for example in a total concentration of up to 5 mol %.

[0064] Product Stream S 产物-SR The methanol contained in the product stream S can be at least partially returned to the reactive distillation unit RD, for example as a reactant. 产物-SR At least 20 mol %, preferably at least 50 mol %, of the methanol contained in the reaction mixture is returned to the reactive distillation apparatus RD.

[0065] In an exemplary embodiment, the product stream S removed from the side reactor SR and containing dimethyl ether, water and methanol is 产物-SR Return to the reactive distillation unit RD. For example, the product stream S 产物-SR Return to the reaction zone RZ of the reactive distillation apparatus RD.

[0066] For example, the product stream S removed from the side reactor SR may be 产物-SR Directly returned to the reactive distillation unit RD. However, in the context of the present invention, it is also possible to remove the product stream S from the side reactor SR. 产物-SR into at least two partial streams, and returning at least one of these partial streams to the reactive distillation apparatus RD. For example, all partial streams can be returned to the reactive distillation apparatus. Alternatively, it is also possible that at least one of the partial streams is not returned, for example because it contains dimethyl ether (i.e. the desired product of the process) in a high concentration, or because it contains undesirable by-products which can be removed from the process in this way.

[0067] In an exemplary embodiment, the product stream S removed from the side reactor SR 产物-SR into three or more partial streams, wherein one of these partial streams has a dimethyl ether concentration of at least 98 mol % or even consists of dimethyl ether and this dimethyl ether is not returned to the reactor distillation unit RD (since it is a desired product of the process). For example, separation by distillation (e.g. from the product stream S 产物-SR ) to obtain a partial stream consisting essentially of dimethyl ether. Optionally, one of the partial streams contains one or more undesirable by-products, and these by-products are not returned to the reactive distillation unit, but are removed from the process. Preferably, at least two of the partial streams are returned to the reactive distillation unit.

[0068] In another exemplary embodiment, the product stream S removed from the side reactor SR产物-SR The product stream S 产物-SR is separated into gaseous stream S G and liquid flow S L , the gaseous flow S G The liquid stream S contains dimethyl ether and methanol (e.g., in a total concentration of at least 80 mol %). L Contains water and methanol (e.g., in a total concentration of at least 80 mol %). To perform the separation, for example, the product stream S 产物-SR The gaseous stream S G and liquid flow S L Separately, it is fed back into the reactive distillation apparatus RD, preferably into the reaction zone of the reactive distillation apparatus RD.

[0069] By removing the product stream S from the side reactor SR 产物-SR Separation into gaseous stream S G and liquid flow S L , and these two streams S G and S L Separately returning to the reactive distillation unit RD, a further increase in the conversion of methanol to dimethyl ether can be achieved (with the product stream S 产物-SR compared to a direct return).

[0070] In the reactive distillation unit RD, the reaction zone RZ has a high water concentration and a low DME concentration in the lower region. The water concentration decreases towards the head of the reactive distillation unit, so that the upper region of the reaction zone has a very low water concentration. The product stream S removed from the side reactor SR 产物-SR contains dimethyl ether and water in relatively high concentrations (i.e., the reaction product of the methanol hydration reaction occurring in the side reactor SR). 产物-SR If the product stream S is fed directly back into the upper (i.e. very low water content) region of the reaction zone RZ, this would lead to an increase in the water concentration and thus to an inhibition of the hydration reaction of methanol in this region of the reaction zone RZ of the reactive distillation unit RD. 产物-SR Returning, for example, to the lower region of the reaction zone RZ (i.e. containing a large amount of water but very little DME), the increased concentration of low-boiling components (i.e. dimethyl ether) leads to a lower temperature in this region of the reaction zone RZ of the reactive distillation unit RD and thus to a lower conversion of the hydration reaction of methanol. By removing the product stream S from the side reactor SR 产物-SR Separation into gaseous stream S G (which contains mainly DME and methanol but very little water) and the liquid stream S L (which contains mainly water and methanol but little DME), and these two streams SG and S L Separately, the product stream S is returned to the reactive distillation unit RD at a suitable location, thereby achieving a further increase in the conversion of methanol to dimethyl ether (with the product stream S 产物-SR compared to direct recycling).

[0071] Preferably, the gaseous stream S G At position P1, it is fed into the reactive distillation unit RD (e.g., reaction zone RZ), and the liquid stream S L Feeding to the reactive distillation apparatus RD is carried out at position P2, so that position P1 is above position P2. The term "above" means that position P1 is closer to the head of the distillation apparatus than position P2.

[0072] For example, position P1 is located in the upper third (preferably the upper fifth) of the reaction zone RZ or in the catalyst-free distillation separation zone DT located above the reaction zone RZ, and position P2 is located in the lower half of the reaction zone RZ or in the catalyst-free distillation separation zone DT located below the reaction zone RZ.

[0073] Position P1 in the upper third of the reaction zone means the following: the reaction zone RZ has an upper end (i.e., facing the head of the reaction distillation apparatus RD) and a lower end (i.e., facing the bottom of the reaction distillation apparatus RD) and a length L (i.e., the distance between the upper end and the lower end of the reaction zone RZ), and the distance between position P1 and the upper end of the reaction zone RZ is l1, so that l1 / L≤0.33. For position P1 in the upper fifth of the reaction zone RZ, the following applies: l1 / L≤0.2.

[0074] Position P2 in the lower half of the reaction zone means the following: the reaction zone RZ has an upper end (i.e., the head facing the reaction distillation apparatus RD) and a lower end (i.e., the bottom facing the reaction distillation apparatus RD) and a length L (i.e., the distance between the upper end and the lower end of the reaction zone RZ), and the distance between position P2 and the lower end of the reaction zone RZ is l2, so that l2 / L≤0.5.

[0075] Suitable gas-liquid separation devices for separation into gas and liquid phases are known to those skilled in the art. For example, the separation involves subjecting the product stream S to 产物-SR The reduced pressure is subjected to in the container, so that a gas phase containing dimethyl ether and methanol and a liquid phase containing water and methanol are formed, and the gas phase and the liquid phase are separated from each other. The gas-liquid separation device SU is, for example, a flash separator. DETAILED DESCRIPTION

[0076] refer to Figure 1 An exemplary embodiment of the present invention is described in more detail:

[0077] Methanol-containing stream S 进料-RDThe methanol-containing stream S is fed to the reaction zone RZ of the reactive distillation column RD via line 1. 进料-RD An example is crude methanol, which, in addition to MeOH, also contains a considerable proportion of H2O (c1(MeOH): methanol concentration; c1(H2O): water concentration). If the reaction zone RZ already has a sufficient distillative separation effect due to the internal components used, the presence of a catalyst-free distillative separation zone can be dispensed with. Figure 1 In the embodiment shown, above and below the reaction zone RZ there are catalyst-free distillation separation zones DT.

[0078] In the reactive distillation unit RD, methanol is converted into dimethyl ether and water in the presence of an acid catalyst and separated by distillation into a column containing dimethyl ether and passing through a pipeline as a top stream S 塔顶-RD The fraction leaving the reactive distillation apparatus RD and containing water is passed via line 3 as bottom stream S 塔底-RD The fraction leaving the reactive distillation unit RD. 塔顶-RD The column bottoms stream contains essentially dimethyl ether (eg, at a concentration of at least 99 mole percent), and the column bottoms stream contains essentially water (eg, at a concentration of at least 99 mole percent).

[0079] Side stream S containing methanol 侧-RD It is removed from the reactive distillation unit RD via line 4 and fed to the side reactor SR. 侧-RD It mainly contains methanol and contains small amounts of water and dimethyl ether (c2 (MeOH): methanol concentration; c2 (H2O): water concentration; c2 (DME): dimethyl ether concentration). 进料-RD In comparison, the methanol-containing side stream S 侧-RD With higher methanol concentration and lower water concentration (i.e. c2(MeOH)>c1(MeOH); c2(H2O) <c1(H2O))。

[0080] The side reactor SR is operated isothermally (e.g. at a temperature in the range of 130-200° C.) and contains an acid catalyst for the dehydration reaction of methanol to dimethyl ether and water. In order to operate energy-efficiently, the methanol-containing side stream S fed to the side reactor SR is 侧-RD and the product stream S obtained in the side reactor SR 产物-SR The side reactor SR is operated not completely in the gas phase, but at least partially in the liquid phase.

[0081] The product stream S contains dimethyl ether, water and methanol 产物-SRFrom the side reactor SR are removed (c3(DME): dimethyl ether concentration; c3(H2O): water concentration; c3(MeOH): methanol concentration) via line 5 and returned to the reaction zone RZ of the reactive distillation unit RD. Since the methanol is converted into dimethyl ether (and water) in the side reactor SR, the product stream S 产物-SR With side flow S 侧-RD Compared with the above, it has a higher concentration of dimethyl ether and a lower concentration of methanol (c3(DME)>c2(DME); c3(MeOH) <c2(MeOH))。

[0082] The reactive distillation unit RD is used not only for the synthesis and distillation separation of dimethyl ether, but also for the methanol-containing side stream S removed via pipeline 4. 侧-RD The form of is used to provide a methanol source having a lower water concentration than the crude methanol fed to the reactive distillation unit RD via pipeline 1 and thus enabling the realization of a product stream S in the side reactor located downstream 产物-SR The DME-rich product stream S 产物-SR The recycle to the reactive distillation apparatus RD in turn has a favourable effect on the yield of dimethyl ether in the reactive distillation apparatus RD.

[0083] refer to Figure 2 Another exemplary embodiment of the method according to the present invention is described in more detail. Figure 2 The method shown in Figure 1 The differences in the methods shown in management are as follows:

[0084] The product stream S removed from the side reactor SR via line 5 产物-SR The product stream S 产物-SR is separated into gaseous stream S G and liquid flow S L , the gaseous flow S G The liquid stream S contains mainly dimethyl ether and methanol (e.g. in a total concentration of at least 80 mol %). L Contains primarily water and methanol (e.g., at least 80 mol % total concentration). G The liquid stream S is returned to the reactive distillation unit RD via line 6, and L It is returned to the reactive distillation apparatus RD via line 7.

[0085] As already described above, the product stream S removed from the side reactor SR 产物-SR Separation into gaseous stream S G and liquid flow S L , and these two streams S G and S LSeparately returned to the reactive distillation unit RD, together with the product stream S 产物-SR Compared with direct return, the conversion rate of methanol to dimethyl ether can be further improved.

[0086] about Figure 2 For all other features of the exemplary embodiments shown in Figure 1 of the above description.

Claims

1. A method for preparing dimethyl ether, comprising the following steps: - The methanol-containing stream S 进料-RD Feed to the reactive distillation unit RD, where - in at least one reaction zone RZ of the reactive distillation unit RD, methanol is converted into dimethyl ether and water in the presence of an acid catalyst, and - separation by distillation to produce a tower top stream S containing dimethyl ether 塔顶-RD The fraction leaving the reactive distillation unit RD and containing water as the bottom stream S 塔底-RD The fraction leaving the reactive distillation apparatus RD, - removing a methanol-containing side stream S from the reactive distillation unit RD 侧-RD , - the methanol-containing side stream S 侧-RD The methanol is fed to the side reactor SR and converted into dimethyl ether and water in the presence of an acid catalyst to obtain a product stream S 产物-SR , the product stream S 产物-SR Contains dimethyl ether, water and methanol and is removed from the side reactor SR.

2. The process according to claim 1, wherein the methanol-containing stream S fed to the reactive distillation unit RD 进料-RD It has a methanol concentration c1 (MeOH) of at least 40 mol %, a water concentration c1 (H2O) of at most 60 mol % and a total concentration of components other than methanol and water of at most 5 mol %.

3. The process according to claim 1 or 2, wherein the methanol-containing stream S fed to the reactive distillation unit RD 进料-RD The methanol-containing side stream S removed from the reactive distillation unit RD has a water concentration c1(H2O) of 15-60 mol %, more preferably 25-50 mol %, 侧-RD Having a water concentration c2(H2O) satisfying the following conditions: c2(H2O)≤0.75×c1(H2O).

4. The process according to claim 1 , wherein the product stream S 产物-SR The methanol contained in is at least partially returned to the reactive distillation apparatus RD.

5. The process according to any one of the preceding claims, wherein the product stream S removed from the side reactor SR 产物-SR Return to the reactive distillation apparatus RD.

6. The method according to any one of the preceding claims, further comprising the steps of: - the product stream S removed from the side reactor SR 产物-SR The product stream S 产物-SR Separation into a gaseous stream S containing dimethyl ether and methanol G and a liquid stream S containing water and methanol L , - Separately separate the gaseous stream S G and the liquid flow S L Return to the reactive distillation apparatus RD.

7. The method according to claim 6, wherein the gaseous stream S G At position P1, the liquid stream S is fed into the reactive distillation unit RD, and the liquid stream S L The feed is fed into the reactive distillation device RD at position P2, and the position P1 is located in the upper third of the reaction zone RZ or in the catalyst-free distillation separation zone DT located above the reaction zone RZ, and the position P2 is located in the lower half of the reaction zone RZ or in the catalyst-free distillation separation zone DT located below the reaction zone RZ.

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