NL1007389C2 - Process for the preparation of dimethyl ether. - Google Patents

Description

Title: Process for the preparation of dimethyl ether

The invention relates to a process for the preparation of dimethyl ether (DME; CH3-O-CH1).

Prompted by growing concerns about air quality in urban areas, a good alternative to vehicles using diesel fuel has been sought for a long time. Fuels such as natural gas, LPG and methanol are a suitable alternative in terms of emissions, but they have the drawback that the energy consumption of the vehicle increases by 20 to 30%. The reason for this is that these fuels are used in Otto engines, which have a lower efficiency compared to diesel engines.

However, it has recently been found that DME combines low emissions with the low energy consumption characteristic of Diesel engines. DME can be produced from various raw materials. In addition to natural gas, coal and oil, sustainable sources such as biogas, biomass and various vegetable raw materials can also be used for production. In order to produce DME, 20 synthesis gas must first be generated, in which important components are hydrogen and CO.

Synthesis gas is characterized, among other things, by the so-called H2 / C ratio, or the molar ratio between hydrogen and carbon monoxide. In the presence of water, however, the water-gas shift reaction takes place in the syngas. This reaction is a balance and depends on the prevailing circumstances. The equation for this reaction is as follows: 30 CO + HO <=> CO; + H?

This reaction changes the H / CO ratio. In order to be able to characterize the syngas properly regardless of the possible presence of water in the mixture and the location of the water-gas equilibrium, the H, / C- (007389 ratio) is defined such that it is independent of the location of the water-gas balance.

2 HCR value - {[H; ] - [CO;]} / {[CO] + [CO;]} 5 DME is gaseous under atmospheric conditions and liquefies at a pressure of about 5 bar. It can therefore be stored in liquid form in the vehicle's fuel tank, similar to LPG. The 10 energy density (MJ / liter) of liquid DME is about half of Diesel. This means that twice as much fuel must be brought along for the same range. Another consequence is that twice as much fuel must also be injected in order to deliver the same amount of engine power.

Combustion engines on DME have significantly lower NO, particulate emissions than diesel engines. For the CCt emissions for the entire trajectory from source to wheel, DME can compete well with other fuels. 20 When DME is produced from sustainable raw materials, the CCt emission will even be significantly lower than with the fuels produced from fossil sources. Also, when DME can be produced on-site on a small scale, a large part of the CCL emissions for extraction and transport will disappear.

Currently, DME is mainly used as a propellant to replace harmful CFCs. DME's worldwide synthesis capacity is estimated at 150,000 tons / year. For comparison, the demand for methanol 30 for various applications is estimated at 25.5 million tons in 1997. DME is produced for this application by dehydration of methanol in a separate reactor or as a by-product during the methanol synthesis itself. The DME thus produced is relatively expensive, making it unsuitable for large-scale use as a fuel.

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Research has already been conducted on methods for producing so-called fuel-grade DME. German patent application 42 22 655 describes a method in which DME is prepared from synthesis gas with an H2 / C 5 ratio of about 1. In this process, the synthesis gas is fed at a pressure of about 60 bar to a reactor filled with a combination of a methanol preparation catalyst and a dehydration catalyst. Methanol is first formed in this reactor, which is then converted into DME. Since the conversion takes place only partially, the reaction mixture, which contains, in addition to unreacted synthesis gas and DME, also an amount of methanol, CO 2 and water, must be further processed. According to this method, the reaction mixture is first cooled to yield a liquid phase consisting essentially of methanol and DME, with some water, inert and part of the CO2. The residual gas phase mainly contains H 2 O, CO and CO2, as well as smaller amounts of DME, methanol and inert.

The gas is washed with methanol to remove DME and CO2. The remaining gas phase is mainly returned to the reactor. The washing liquid, which contains most of the CO2, is desorbed, after which the methanol is returned to the washing column. The gas phase of the desorption is washed in a second washing column to recover residues of methanol and DME, after which CO 2, which still contains traces of DME, is vented.

The liquid phase, which is obtained when the reaction mixture is cooled, is also fed to the second washing column. From the bottom of this column, a mixture of DME, water and methanol of variable composition is obtained.

In this method, the conversion of the synthesis gas is controlled so that, in addition to DME and methanol, CO 2 is formed as residual product.

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Another method is described in WO-A96 / 23755, in which the method is controlled in such a way that water is formed as a residual product, while CO2 is kept in circulation.

5 This method assumes an H2 / C

ratio in the synthesis gas of about two, whereby a comparatively poor purity is accepted to obtain sufficient yield.

This method is characterized, inter alia, in that the gas phase obtained after cooling the reaction mixture is for the most part recycled through the reactor. The remainder of the gas phase is then washed with methanol from the process, which washing liquid is dehydrated in a second reactor to form DME. The top product is drained. Gaseous DME is recovered from the liquid phase of the reaction mixture in a distillation column. The bottom product, which mainly consists of water and methanol, is then separated in a distillation column into a liquid residual stream consisting mainly of water and inert, and gaseous methanol which, after cooling, is used as washing liquid.

Although this method does not have the disadvantage of producing large amounts of CO; it is still far from ideal. As already indicated, low purity of the DME (<70%) is accepted. Two separate reactors are also required, while due to the large recycle of DME over the reactor, its volume is very large.

The object of the invention is to provide a method for the preparation of DME, wherein these drawbacks of the prior art do not arise. More particularly, it is an object of the invention to provide a method in which the reaction as a by-product produces water, so that no or only small amounts of CO; be produced.

A further object of the invention is to provide a method in which DME can be produced in sufficient (> 85%) 1007389 purity. It is also an object of the invention to keep the process as simple as possible, which means, among other things, that it is not necessary to allow the conversion to take place in different reactors.

The invention relates to a process for the preparation of dimethyl ether from synthesis gas with an HCR ratio of at least 1.4, wherein - the synthesis gas in a reactor is at least partly converted into a reaction mixture containing dimethyl ether, methanol and water, - the reaction mixture cools to form a first liquid phase containing methanol, dimethyl ether and water, and a first gas phase containing unreacted synthesis gas, CO2, dimethyl ether, methanol and water, - the first gas phase washes with methanol to form a second gas phase which is consisting essentially of H 2 CO 2 and CO 2 and a second liquid phase consisting mainly of methanol, dimethyl ether, water and CO 2 20 CO; recovers from the first and second liquid phases and this CO; recirculates to the reactor along with the second gas phase, and - dimethyl ether gains from the first and second liquid phases.

It has surprisingly been found that in this way dimethyl ether can be prepared with high purity. The dimethyl ether thus obtained is suitable for use as a fuel, replacing diesel, but can also be further purified.

Characteristic aspects of this process are the separation of the product stream after the reactor by cooling and separation of a liquid phase combined with the washing of the residual gas phase with a washing liquid which preferably consists mainly of methanol and the recovery and recycling to the reactor of CO2 is withdrawn from the gas phase in the separated liquid phases.

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These measures make it possible to combine good process management with high purity, while maintaining a simple process. This method also allows good reactor utilization. In addition, the process is suitable for the production of DME from synthesis gas with an HCR of 1.4 or more, preferably at least 2. DME can be produced with different HCR values. At HCR <1.4, more CO; then HjO was formed as a waste product. At HCR> 1.4, more water is formed. With HCR> = 2 it is possible to run the process without removing CO; from the process. Only DME and water are then formed. However, relatively small amounts of carbon and hydrogen compounds are discharged in various blowdowns and with the drained water (in any case H: CO, CO and methanol); the exact amounts of these losses and the exact composition of the product are in balance with the HCR of the synthesis gas used.

According to a preferred embodiment of the method according to the invention the first liquid phase in a column is further separated into a third gas phase containing methanol and dimethyl ether, and one or more liquid phases and gas phases. Preferably, at least one additional liquid phase is formed in this column, consisting essentially of water. This is removed as a residual product, possibly after further cleaning. This stream also contains liquid inert components, which are removed from the system in this way.

According to a further embodiment of the invention, a fourth liquid phase containing methanol is obtained from the latter column, which is preferably recycled to the reactor. This methanol stream is drained from one of the dishes of the distillation column. If necessary after evaporation, it is then combined with the recirculating synthesis gas. This embodiment has the advantage that an amount of methanol is already present at the inlet 1007389 7 of the reactor, so that dimethyl ether can be formed from the start. Also, recycling of methanol, an intermediate for the formation of DME, increases the total conversion, on a carbon basis, of the process.

Further variants and preferred embodiments of the invention will be further elucidated on the basis of the figure description to be given later.

The process according to the invention starts from synthesis gas, ie a gas containing H 2, CO and possibly CO. In view of the object of the invention to limit the residual products as much as possible to water, and to allow only a minimal emission of CO2, it is essential that the HCR value in the synthesis phase is at least 1.4, preferably is at least 2.0. It has been found in practice that at such a ratio in the starting gas, an equilibrium condition will eventually result from the recycle and consumption, this ratio being considerably higher in the reactor.

Synthesis gas can be prepared from practically all hydrocarbonaceous raw materials. The processes for preparation vary widely for different raw materials. Examples include (partial) processes such as coal gasification or fermentation of biomass. When the raw material is natural gas, synthesis gas can be prepared, among other things, by steam reforming: CH- + H20 => CO + 3H2 HCR = 3 In addition, much research is currently being carried out into partial oxidation: 2CH; + O; => CO + 2H :. HCR = 2 The skilled person will be able to set the HCR value on the basis of what is known about the preparation of synthesis gas. This can be done, as already indicated above 1007389 8, by combining both steam reforming and partial oxidation of natural gas, such as autothermal reforming or combined reforming.

Examples of these types of processes are ICI's Gas Heated 5 Reforming (GHR) and Kellogs' Reformer Exchanging System (KRES).

The reactor for the preparation of the dimethyl ether is preferably a fixed bed reactor, more in particular a cooled tube reactor, containing a combination of two catalysts, namely a methanol preparation catalyst and a dehydration catalyst. These catalysts can be used as a mixture or in layers. It is preferable to use a mixture. Suitable catalysts for this reaction are known and commercially available. Examples of methanol preparation catalysts are CuO and ZnO. As dehydration catalysts, use can be made, for example, of catalysts based on γ-Al 2 O 3.

The following equilibrium reactions take place in the reactor.

CO + 2H: -> CH OH 2 CH, OH CHOCH. + H; 0 The total reaction then becomes 2CO + 4H. - CHOCH .: + H; 0

From this it will be clear that the ratio H- / C

(HCR) must be at least two. During the reaction, CO: can also be formed according to the equilibrium reaction CO + H: .0 -> CO: + H:

Due to the recycle of CO; however, this reaction is as good as suppressed. In order to provide gaseous inert components, which always enter the system via the synthesis gas or are formed during the reactions as a by-product, part of the gas will have to be vented from the system. From the combination of conversions and the nature and quantity of the blowdown it can then be deduced which HCR value must be applied in the synthesis gas.

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The reaction takes place at elevated temperature and pressure, which are not very critical. The pressure is generally between 10 and 100 Bar, with a preference for the range from 25 to 75 Bar. In general, efforts will be made to keep the pressure in the system at the same level as much as possible, since this saves compression costs, however, it has been found that separation of CO2 from DME at pressures above about 57.5 Bar has been difficult. The temperature is preferably between 175 and 350 ° C, with a particular preference for 200 to 330 ° C. The maximum temperature is mainly imposed by the limited temperature resistance of the catalyst. The temperature profile in the reactor also determines the temperature range at which the reaction proceeds.

The invention will now be further elucidated with reference to the figure, in which a flow chart is given of a possible embodiment of the method according to the invention. To illustrate the method according to the invention, a possible implementation of the process 20 is illustrated on the basis of a flow sheet.

Synthesis gas, after combination with a number of flows to be indicated later, is supplied via line 1 to reactor 2. Dimethyl ether is formed in this reactor, among other things. The reaction mixture, after cooling (not shown), is fed via line 3 to separator 4. Via line 5, the first liquid phase is fed to distillation column 6. The bottom product, which mainly consists of water, is removed via line 7.

The gas phase obtained in separator 4 is fed via line 8 to washing column 9, where the gas is washed with methanol, via line 23 from distillation step 10. The remaining gas phase, mainly H 2 and CO 2, is supplied via line 11 and line 1 recycled to reactor 2.

The liquid phase from the washing column is fed via line 12 to distillation step 10. To this 1007389 10 distillation step 10, which may consist of one or more distillation columns, the gas phase and the liquid phase from distillation column 6 are also supplied, via lines 13 and 14. A liquid phase consisting essentially of methanol is discharged from distillation column 6 via line 15 and recycled to reactor 2 via line 1.

A gas phase, mainly consisting of CO-, is recycled via line 16 and line 1 to reactor 2. Dimethyl ether is finally removed via line 17. Depending on the desired purity and the intended application, it can be further purified.

Figure 2 shows a possible elaboration of the distillation step 10.

Distillation column 18 is supplied via lines 12 and 13, respectively, with a liquid and a gas phase, originating from washing column 9 and distillation column 6. From the bottom of the column, a stream consisting essentially of methanol is obtained. This is fed via line 23 to washing column 9.

The gas phase, CO-, is removed via line 21 and returned to the reactor via line 14. A liquid is supplied via line 20 to distillation column 19. The product dimethyl ether is obtained from the bottom of the column and CO is removed at the top, which is combined via line 22 with the CO2 in line 14.

Based on the flowchart described above, the process can be performed as follows. Synthesis gas 30 is fed to the process through line 1. This synthesis gas is combined with the recycle stream in lines 11, 15 and 16. After compression and heating, the stream is passed through the reactor where DME and Methanol are formed and cooled again. In the screen 4, the cooled stream is further cooled and a liquid phase is separated which mainly contains water, methanol and DME.

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The residual gas phase, which mainly contains H2, CO, CO2 and DME, is led to the washing column 9 where, using the washing liquid in the stream 23 consisting mainly of methanol, the major part of the DME is washed out. A large part of the CO2 from the gas stream 8 is also washed with the methanol.

The separated liquid phase is separated in the distillation column 6 into a) a stream 7 consisting mainly of water 10 and discharged b) a stream 14 containing as much of the DME as possible from 5 c) a gas phase 13 containing volatile components including H; , CO and CO2, which can be recycled 15 d) a liquid stream 15 which is drained from a tray, consisting mainly of methanol and recycled to the reactor via the stream 15 briefly explained below.

The liquid stream 12 from the wash column containing the DME and CO; is mixed with the methanol and DME-containing stream 14. This combined stream is heated and fed to distillation column 18. In 18 the following streams are separated: 25 a) 23 mainly containing the original washing liquid.

b) 20 containing the "crude" DME, some methanol an amount of CO? contains.

c) 21 which again contains a residual of volatile gases.

The crude DME 20 is separated in the distillation column 19 into: a) the product stream "DME", which contains about 91% pure DME.

b) as much as possible of the CO dissolved in the DME: in the stream 22 1 0 0 7 38 9 12

All separated CO2 streams are combined in stream 14, along with the methanol in stream 15, these intermediates are recycled to the reactor.

5 Furthermore, blowdown currents are tapped at two locations in the process. A purge stream is removed from the gas phase stream that is recycled after the wash column. A purge stream is also drained from the methanol wash stream so that water does not build up in the wash liquid.

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EXAMPLE

On the basis of the process according to Figures 1 and 2, dimethyl ether has been prepared by supplying synthesis gas of the following composition via line 1 to the reactor 2. (all compositions are given in Kmol / h)

Synthesis gas: CO 17.7, H? 40.9, CO? 1.8, H? 0 0.6.

At a temperature of 525-575K and a pressure of 40 bar, this synthesis gas, together with recycle of synthesis gas, methanol and CO 2, was converted into dimethyl ether and methanol.

After running through the process in Figures 1 and 2, as described above, 8.1 Kmol / h DME, 25 with a purity of 91% was obtained.

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Claims (10)

A process for the preparation of dimethyl ether from synthesis gas with an HCR value of at least 1.4, wherein - the synthesis gas in a reactor is at least partially converted into a reaction mixture containing dimethyl ether, methanol and water, - the reaction mixture is cooled to form a first liquid phase containing methanol, dimethyl ether and water, and a first gas phase containing unreacted synthesis gas, CO2, dimethyl ether, methanol and water, - washing the first gas phase with a substantially methanol-containing washing liquid to form a second gas phase consisting essentially of H 2 CO 2 and CO 2 and a second liquid phase consisting mainly of methanol, dimethyl ether, water and CO 2 CO; recovers from the first and second liquid phases and this CO; recirculates to the reactor along with the second gas phase, and dimethyl ether gains from the first and second liquid phases. The method of claim 1, wherein the first liquid phase in one or more columns is separated into at least a third liquid phase mainly containing methanol and dimethyl ether, and one or more liquid phases and gas phases. The method of claim 2, wherein at least one liquid phase is formed which consists essentially of water. 4. A method according to claim 2 or 3, wherein a third gas phase is separated, which comprises most of the H 1:, CO and CO; contains. Process according to claims 2-4, wherein a fourth liquid phase, containing methanol, is obtained from said column, which is preferably recycled to the reactor. A method according to claims 2-5, wherein the second and third liquid phase, optionally simultaneously, in a column 5 are separated into a methanol-containing liquid phase, and a dimethyl ether-containing fifth liquid phase. The method of claim 6, wherein a fourth gas phase is obtained, which CO; and is recycled to the reactor. The method of claim 7, wherein the fifth liquid phase in a column is separated into a CO; containing gas phase and a dimethyl ether-containing liquid product stream. 9. A method according to claim 7 or 8, wherein said methanol-containing liquid phase is used as a washing liquid for the first gas phase. The method of claims 1-9, wherein the HCR value in the synthesis gas is at least 2.0. 1007389