DIRECT SYNTHESIS OF DME AT EQUILIBRIUM

Abstract

The invention relates to a process for production of DME (17) comprising the steps of: generating a syngas (12) containing CO and H2 in a device (11) for syngas generation, passing the syngas (12) into a DME reactor (13) for direct synthesis of DME by catalysed conversion of the syngas (12) by means of a catalyst to form a product stream (14) containing DME, CO2, H2O, CH3OH and unconverted syngas (12). The invention provides that the direct synthesis is carried out at or close to the chemical equilibrium.

Description

The invention relates to a process for production of DME (dimethyl ether) as classified in the preamble of claim 1.

A process of this type includes at least the steps of: generating a syngas containing CO and H₂ in a device for syngas generation (e.g. in a syngas reactor), such a device being any device suitable for syngas generation, and passing the syngas into a DME reactor for direct synthesis of DME by catalysed conversion of the syngas by means of a catalyst to form a product stream (reactor effluent) containing DME, CO₂, H₂O, CH₃OH and unconverted syngas (CO and H₂).

The syngas mixture of CO and H₂ can be produced from natural gas, for example by steam reforming:

CH₄+H₂O —>CO+3H₂.

It is also possible to convert natural gas into syngas by partial oxidation:

2CH₄+O₂—>2CO+4H₂.

Syngas can further also be generated through what is known as autothermal reforming (combination of steam reforming and partial oxidation in one apparatus). The two processes are combined with each other such that the advantage of the oxidation (provision of thermal energy) and the advantage of steam reforming (higher yield of hydrogen) complement each other to advantage.

Finally, syngas is also obtainable through what is known as combined reforming (combination of steam reforming and partial oxidation in separate apparatuses).

The direct synthesis of DME is effected in a known manner from syngas in accordance with the following empirical equation:

3H₂+3CO—>DME+CO₂

In accordance with the stoichiometry, the use of a syngas having an H2/CO ratio of about 1:1 is preferred.

The mechanism is currently believed to involve the following reaction pathway via intermediate methanol:

2H2+CO—>CH3OH

2CH3OH—>DME+H₂O

H2O+CO—>CO2+H2

(water gas shift reaction).

A process of this type is known from U.S. Pat. No. 6,458,856 B1 for example.

What has been found to be problematical with this is that the DME reactor effluent also includes a large amount of unconverted syngas owing to the typically low rate of conversion. Gas separation is accordingly costly and inconvenient, since unconverted syngas is to be recycled into the DME reactor, whereas the CO₂ formed has to be recycled into the syngas part (syngas generation) in the natural gas-based embodiment of the process described at the beginning.

Against this background, the problem addressed by the present invention is therefore that of providing a process that permits a comparatively simple gas separation.

This problem is solved by a process having the features of claim 1.

This process provides that the direct synthesis is carried out at or close to the chemical equilibrium in order that the concentration of syngas in the said product stream may be reduced distinctly. It is preferable to use for this purpose a Cu-based catalyst which has an acidic functionality for a corresponding high activity and selectivity for DME (bifunctional catalyst). The functionality referred to favours in particular the elimination of water as per 2CH₃OH—>DME+H₂O.

The equilibrium concentration of dimethyl ether is present when the reaction of carbon monoxide and hydrogen to form dimethyl ether and carbon dioxide is in chemical equilibrium. The chemical equilibrium of the reaction is reached when the rate of the forward reaction (3H₂+3CO→DME+CO₂) is equal to the rate of the reverse reaction (DME+CO₂→3H₂+3CO).

Increasing the conversion rate at or close to the chemical equilibrium does away with the need to recycle the syngas into the DME reactor, so the unconverted syngas and CO₂ can be conjointly recycled into the syngas part.

Owing to the properties of earlier catalyst systems and engineering limitations, operating close to the chemical equilibrium of direct DME synthesis was hitherto out of the question. However, the preferred use herein of high-activity and selective catalysts/catalyst systems has made it possible to operate the DME reactor at or close to the chemical equilibrium, that is to say preferably at at least 70% conversion, preferably at least 80% conversion, more preferably 85% conversion, most preferably at least 90% conversion based on the CO fraction of the syngas.

By comparison, conversion in the prior art is significantly lower (e.g. 50% conversion in demonstration plant as per the presentation: New Clean Fuel DME, Y. Ohno (JFE Holdings, Inc., Japan), DeWitt Global Methanol&MTBE Conference, Bangkok, March 12-14, 2007).

It is preferably further provided in this regard that the direct synthesis in the DME reactor is at least carried out until achievement of a DME concentration in the reactor effluent amounting to at least 70%, at least 80%, at least 85% or at least 90% of the chemical equilibrium concentration of DME.

Furthermore, the DME reactor product stream containing CO, H₂, CO₂, DME, H₂O and methanol is cooled to separate the remaining (in particular low-concentrated) syngas and also CO2 from a DME-rich (liquid) phase, so CO2, H2 and CO can be withdrawn for example overhead from a column as gaseous phase and DME can be withdrawn as bottom product in a liquid phase.

The gaseous phase is then preferably recycled into the syngas part (syngas generation).

Depending on the intended use for the DME (propellant, solvent, LPG admixture, power fuel, feedstuff for olefin synthesis, . . . ), further separation steps may optionally be carried out to remove CO₂, methanol or water from the said liquid phase.

A further aspect of the present invention is a process for production of DME comprising the steps of:

• • generating a syngas containing CO and H₂ in a device for syngas generation,
  • passing the syngas into a DME reactor for direct synthesis of DME by catalysed conversion of the syngas by means of a catalyst to form a product stream containing DME, CO₂, H₂O, CH₃OH and unconverted syngas,
  • wherein the catalyst is designed to hasten especially the synthesis of methanol and also the elimination of water as per 2CH₃OH—>DME+H₂O, wherein more particularly the catalyst is a Cu-based catalyst having an acidic functionality, and
  • wherein carbon dioxide formed in the direct synthesis and/or syngas not converted in the direct synthesis is returned for syngas generation.

In a further possibility, the syngas to be introduced into the DME reactor is produced by dry reforming wherein methane and carbon dioxide are converted into carbon monoxide and hydrogen.

This dry reforming is advantageously carried out in the presence of a modified, soot-resistant nickel-based catalyst of the type also used in similar fashion in steam-reforming processes. The dry-reforming process is advantageously carried out at a temperature between 750° C. and 950° C.

Methane for the purposes of the invention also comprehends methane-containing gases such as natural gas. Dry reforming for the purposes of the invention is to be understood as meaning the conversion of methane or natural gas and CO₂ by heating in the absence of water into a syngas having a stoichiometric ratio of about 1:1 for H₂ and CO. Dry reforming for the purposes of the invention more particularly also comprehends the conversion of CH₄ or natural gas and CO₂ in the presence of water vapour, although water is only present in a stoichiometric ratio of 1:2, 1:3, 1:4, 1:5, 1:10 or 1:20 in relation to methane or natural gas. In general, dry reforming for the purposes of this invention more particularly requires the molar ratio of water to carbon in the feed to be less than 2:1 and preferably less than 1:1.

DME can further be converted in an olefin synthesis step into a product comprising olefins, especially ethylene and/or propylene, in which case the product stream is fed directly to the olefin synthesis step. Alternatively, merely CO₂ can be separated from the product stream and then the product stream be fed to the olefin synthesis step.

It is particularly advantageous to carry out the direct synthesis in the DME reactor and/or the generation of syngas at a pressure of 20 bar to 70 bar, preferably of 30 to 50 bar. It is more particularly advantageous when direct synthesis in the DME reactor and the generation of syngas is effected at the same pressure or at a pressure that differs by not more than 3 bar but preferably by not more than 1 bar. The pressure difference between the direct synthesis in the DME reactor and the generation of syngas is not more than 3 bar and preferably not more than 1 bar at the point of exit from the syngas generation and at the point of entry into the DME reactor.

Illustrative embodiments will now be more particularly described with reference to the figures and the related description to elucidate further details and advantages of the invention.

FIG. 1 shows a block diagram of a process according to the invention.

FIG. 1 shows a block diagram of an inventive process for production of DME by direct synthesis from syngas (CO and H₂).

Syngas is generated in a syngas step (syngas generation) 11 from natural gas 10, for example by steam reforming. The syngas 12 is passed into a DME reactor and reacted in the presence of an appropriately designed Cu-based catalyst (see above) into DME while the reactor is operated at or close to the chemical equilibrium:

3H₂+3CO—>DME+CO₂.

As a result, comparatively little syngas is left unconverted, which appreciably facilitates the subsequent separation tasks, since it is now no longer necessary to separate the syngas from CO₂.

The reactor effluent stream (product stream) 14 containing CO, H2, CO2, DME, H2O and methanol is fed from the DME reactor 13 into a separation step 15 in which the reactor effluent is cooled to obtain a gaseous phase containing CO, H2 and CO2, which is recycled into the synthesis part 14, and a liquid phase 17, which is a DME-enriched state.

List of reference signs:
10 Natural gas
11 Syngas generation
12 Syngas (CO + H2)
13 DME reactor for direct synthesis
14 Reactor effluent (product stream)
15 Cooling at gas-liquid separation
16 Gaseous phase (CO2, CO, H2)
17 Liquid phase with DME

Claims

1. Process for production of DME (17) comprising the steps of:

generating a syngas (12) containing CO and H2 in a device (11) for syngas generation,

passing the syngas (12) into a DME reactor (13) for direct synthesis of DME by catalysed conversion of the syngas (12) by means of a catalyst to form a product stream (14) containing DME, CO2, H2O, CH3OH and unconverted syngas (12),

characterized in that

the direct synthesis is carried out at the chemical equilibrium or in that the direct synthesis in the DME reactor (13) is at least carried out until achievement of a DME concentration in product stream (14) amounting to 70%, 80%, 85% or 90% of the chemical equilibrium concentration of DME.

2. Process according to claim 1, characterized in that the catalyst is a Cu-based catalyst having an acidic functionality which favours in particular the elimination of water as per 2CH3OH—>DME+H2O.

3. Process according to claim 1, characterized in that the product stream (14) containing CO, H2, CO2, DME, H2O and methanol is cooled to generate a gaseous phase (16) containing CO2, H2 and CO and a liquid phase (17) containing DME.

4. Process according to claim 3, characterized in that the gaseous phase (16) is recycled into the device (11) for syngas generation.

5. Process according to claim 1, characterized in that the syngas (12) is generated by dry reforming wherein methane and carbon dioxide are converted into carbon monoxide and hydrogen.

6. Process according to claim 1, characterized in that DME is converted in an olefin synthesis step into a product comprising olefins, especially ethylene and/or propylene, wherein the product stream is fed directly to the olefin synthesis step, or in that merely CO2 is separated from the product stream and then the product stream is fed to the olefin synthesis step.

7. Process for production of DME (17) comprising the steps of:

generating a syngas (12) containing CO and H2 in a device (11) for syngas generation,

passing the syngas (12) into a DME reactor (13) for direct synthesis of DME by catalysed conversion of the syngas (12) by means of a catalyst to form a product stream (14) containing DME, CO2, H2O, CH3OH and unconverted syngas (12),

wherein the catalyst is designed to hasten the elimination of water as per 2CH3OH—>DME+H2O, and

wherein carbon dioxide (16) formed in the direct synthesis and/or syngas (12) not converted in the direct synthesis is returned for syngas generation.

8. Process according to claim 7, characterized in that the catalyst is a Cu-based catalyst having an acidic functionality.

9. Process according to claim 7, characterized in that the syngas (12) is generated by dry reforming wherein methane and carbon dioxide are converted into carbon monoxide and hydrogen.

10. Process according to claim 7, characterized in that DME is converted in an olefin synthesis step into a product comprising olefins, especially ethylene and/or propylene, wherein the product stream is fed directly to the olefin synthesis step, or in that merely CO2 is separated from the product stream and then the product stream is fed to the olefin synthesis step.

11. Process according to claim 1, characterized in that the direct synthesis in the DME reactor (13) and/or the generation of syngas (12) is carried out at a pressure of 20 bar to 70 bar, preferably of 30 to 50 bar.

12. Process according to claim 1, characterized in that the direct synthesis in the DME reactor (13) is carried out at a temperature between 150° C. and 400° C., preferably between 200° C. and 320° C.