METHOD AND PLANT FOR PREPARING DIMETHYL ETHER

Abstract

A method for preparing dimethyl ether (DME) from synthesis gas, wherein an input, which is formed using shifted and/or non-shifted synthesis gas, undergoes a catalytic conversion, thereby forming a product stream. The product stream undergoes a first separation, wherein a gas mixture is formed by at least partial separation of methanol and/or water from the product stream, and the gas mixture is partially condensed at a first pressure level by means of cooling from a first to a second temperature level. A portion of the gas mixture remaining in gaseous form at the second temperature level is washed in an absorption column with a return predominantly containing dimethyl ether, wherein a dimethyl ether product is formed using the portion of the gas mixture condensed during cooling.

Claims

1. A method for preparing dimethyl ether (DME) from synthesis gas, wherein an input, which is formed using shifted and/or non-shifted synthesis gas, undergoes a catalytic conversion, thereby forming a product stream, wherein the product stream undergoes a first separation, wherein a gas mixture is formed by at least partial separation of methanol and/or water from the product stream, and the gas mixture is partially condensed at a first pressure level by means of cooling from a first to a second temperature level, wherein a portion of the gas mixture remaining in gaseous form at the second temperature level is washed in an absorption column with a return predominantly containing dimethyl ether, wherein the return predominantly containing dimethyl ether is formed at least partially from a portion of the gas mixture condensed during cooling, and wherein the gaseous portion of the gas mixture not washed out in the absorption column is at least partially transferred into the input, and a dimethyl ether product is formed using the portion of the gas mixture condensed during cooling.

2. The method according to claim 1, in which the gas mixture is cooled to the second temperature level via multiple intermediate temperature levels, wherein multiple condensates are deposited.

3. The method according to claim 1, in which the portion of the gas mixture remaining in gaseous form at the second temperature level is washed in the absorption column with the return predominantly containing dimethyl ether, thereby obtaining a top product and a bottom product, wherein the return predominantly containing dimethyl ether is formed at least partially from the bottom product and/or partially from the top product.

4. The method according to claim 1, in which the return predominantly containing dimethyl ether is formed using a dimethyl ether carbon dioxide distillation column.

5. The method according to claim 4, in which the dimethyl ether carbon dioxide distillation column is operated in such a way that a top gas predominantly containing carbon dioxide forms at the top, and a bottom liquid enriched with dimethyl ether forms at the bottom.

6. The method according to claim 5, in which some of the dimethyl ether product which is formed from at least some of the bottom liquid of the dimethyl ether carbon dioxide distillation column is used as the return predominantly containing dimethyl ether.

7. The method according to claim 5, in which at least some of the bottom liquid of the dimethyl ether carbon dioxide distillation column is drawn off as a dimethyl ether product having a dimethyl ether content of more than 90 mole percent, in particular more than 95 mole percent, in particular more than 98.5 mole percent.

8. The method according to claim 4, in which the dimethyl ether carbon dioxide distillation column is operated at a second pressure level below the first pressure level.

9. The method according to claim 1, in which the return predominantly containing dimethyl ether has a carbon dioxide content of at most 5 percent by weight, in particular at most 2 percent by weight, at most 1 percent by weight, at most 0.5 percent by weight or at most 0.1 percent by weight.

10. The method according to claim 1, wherein a portion, not transferred into the input, of the gaseous portion of the gas mixture not washed out is discharged in part from the method as a purge stream, wherein the purge stream is subjected to a wash with at least partially liquid carbon dioxide, which is provided in particular using the dimethyl ether carbon dioxide distillation column, thereby obtaining a wash stream, and the wash stream is at least partially conveyed back into the dimethyl ether carbon dioxide distillation column.

11. The method according to claim 1, in which the first temperature level is 10 to 50° C., in particular 20 to 40° C., and/or in which the second temperature level is 0.5 to 20° C., in particular 1 to 10° C. above the melting temperature of carbon dioxide at the first pressure level, and/or in which the first pressure level is 20 to 100 bar, in particular 30 to 80 bar.

12. The method according to claim 1, wherein the synthesis gas is transferred into the input via the absorption column together with the gaseous portion of the gas mixture not washed out.

13. The method according to claim 1, wherein a top gas of the absorption column is at least partially condensed and the condensed portion of the top gas x′ is at least partially conveyed back into the top region of the absorption column.

14. A separation plant which is configured for the separation processing of a gas mixture which can be formed from a product stream of a reactor for the synthesis of dimethyl ether from synthesis gas, and which contains at least dimethyl ether, carbon dioxide and at least one further component which boils more easily than carbon dioxide, with means which are configured to provide the gas mixture while at least partially separating methanol and/or water from the product stream and to cool it at a first pressure level from a first to a second temperature level and to wash a portion of the gas mixture remaining in gaseous form at the second temperature level in an absorption column with a return predominantly containing dimethyl ether, wherein means are further provided which are configured to form the return predominantly containing dimethyl ether at least partially from a portion of the gas mixture condensed during cooling.

15. A plant for preparing dimethyl ether, comprising at least one reactor configured for the synthesis of dimethyl ether from synthesis gas and a separation plant according to claim 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] FIG. 1 is a schematic representation of a plant for producing dimethyl ether according to the prior art

[0053] FIG. 2 is a schematic representation of a plant for producing dimethyl ether according to an embodiment of the invention.

[0054] In the figures, elements corresponding to one another are indicated by identical reference signs and are not explained repeatedly for the sake of clarity.

DETAILED DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 schematically illustrates a plant for producing dimethyl ether according to the prior art, which is denoted as a whole by 110.

[0056] The plant 110 comprises a synthesis gas reactor 20, which is shown in a highly schematic manner here and which can be charged with a suitable input a, for example natural gas or biogas. A synthesis gas stream b can be withdrawn from the synthesis gas reactor 20.

[0057] The synthesis gas stream b can be pressurized, optionally after adding further streams, by means of a compressor 1. As a result, a pressure required for a subsequent single-stage dimethyl ether synthesis, for example a pressure of 20 to 80 bar, can be set.

[0058] A correspondingly compressed stream, now denoted by c, is conducted through a first heat exchanger 2 which can be heated with a product stream d of a reactor 4 for the synthesis of dimethyl ether (see below). The correspondingly heated stream, still denoted by c, has, for example, a temperature of 200 to 300° C. downstream of the first heat exchanger 2. The stream c is optionally conducted through a second heat exchanger 3, which is also referred to as a peak heater.

[0059] The stream further heated in the second heat exchanger 3, also referred to here as c, is fed into the reactor 4, which is formed as a tubular reactor, the reaction tubes of which are filled with a suitable catalyst for single-stage synthesis of dimethyl ether. The illustration in FIG. 1 is greatly simplified.

[0060] Typically, reactors 4 are arranged upright for the synthesis of dimethyl ether, a stream c being fed into the tube reactor 4 at the bottom. A stream d is withdrawn from the top of the reactor 4.

[0061] Due to the exothermic reaction in the tubular reactor 4, the stream d is at a still higher temperature. The stream d is conducted as a heating medium through the heat exchanger 2. As a result, it cools down to a temperature which is, for example, approximately 30° C. above the temperature of the stream c downstream of the compressor 1. The correspondingly cooled stream, still denoted by d, is fed to a conventional separation plant 120. In the separation plant 120, a methanol stream e and a water stream f are separated from the stream d, for example with intermediate expansion, cooling, recompression, etc. (not shown) in a step 121. The remaining residue is used to form the streams g and h, which may be, for example, a stream g enriched with carbon dioxide and a stream h enriched with dimethyl ether.

[0062] The composition of the streams g and h depends, inter alia, on the composition of the stream d and the specific configuration and operating parameters of the separation plant 120.

[0063] FIG. 2 illustrates a plant for producing dimethyl ether according to an embodiment of the invention. This is denoted as a whole by 100.

[0064] A first absorption column, which is used for separating out methanol and/or water, is denoted by 6 in FIG. 2. As already explained, an absorption column 6 differs from a distillation column such as the distillation column 5 in that it does not have a bottom evaporator, among other things. Vapors rising in the absorption column 6 are washed by a return provided at the top of the absorption column, so that the more volatile components are enriched at the top of the absorption column and the less volatile components are enriched at the bottom of the absorption column.

[0065] In the plant 100 shown in FIG. 2, the stream d is introduced into the absorption column 6. From the top of the absorption column 6, a top stream k is withdrawn and cooled in a heat exchanger 7 against a suitable refrigerant, for example cooling water. The correspondingly cooled stream k is transferred into a separator container 8, from the bottom of which a liquid stream l is withdrawn and at least partially fed to the absorption column 6 as a return by means of a pump (without designation).

[0066] If the stream d in the example shown also contains methanol, water, carbon dioxide, carbon monoxide and hydrogen (as well as traces of other compounds as explained above) in addition to dimethyl ether, then dimethyl ether, carbon dioxide, carbon monoxide and hydrogen pass therefrom into the top stream k, due to the backwash explained. As a result of suitable cooling in the heat exchanger 7 and corresponding deposition conditions in the separator container 8, a bottom product is deposited in the separator container 8, which bottom product consists substantially of dimethyl ether and carbon dioxide (possibly with traces of methanol).

[0067] From the top of the separator container 8, a stream m can be drawn off in gaseous form, which also contains dimethyl ether in addition to carbon dioxide, carbon monoxide and hydrogen. The stream m subsequently undergoes sequential cooling and condensation, as explained below. The portion of the stream l that is not fed as a liquid return to the absorption column 6 is fed, like the condensates obtained during the sequential cooling and condensation of the stream m, into a distillation column 9, referred to here as dimethyl ether carbon dioxide distillation column 9.

[0068] It is expressly emphasized that the specific provision of the stream k obtained from the product stream d does not have to take place in the manner shown. Other possibilities for separating out water and/or methanol can also be used, as long as they lead to the production of a gas mixture at the aforementioned first pressure level and the first temperature level and with the stated contents of the individual components.

[0069] It is also possible, for example, to dispense with the distillation column 5 and to separate water from the stream n in another way. In particular, the water is thus fed into the dimethyl ether carbon dioxide distillation column 9. In such cases, the water may be separated from the dimethyl ether product z in a plant component downstream of the dimethyl ether carbon dioxide distillation column 9.

[0070] A liquid stream n is withdrawn from the bottom of the absorption column 6 and fed at a suitable level into the distillation column 5, which is operated with a bottom evaporator 51 and a top condenser 52. In the example shown, the stream n contains the predominant portion of the water and methanol contained in the stream d.

[0071] The bottom evaporator 51 and the top condenser 52 are operated with suitable heating or cooling media, preferably present in a corresponding plant. In the bottom evaporator 51, a liquid stream withdrawn from the bottom of the distillation column 5 is partially evaporated and fed into a lower region of the distillation column 5. A non-evaporated portion can be drawn off as stream p.

[0072] From the top of the distillation column 5, a gaseous stream is drawn off, partially liquefied in the top condenser 52 of the distillation column 5, and fed back into an upper region of the distillation column 5 as a liquid return. A portion o remaining in gaseous form is drawn off.

[0073] In the distillation column 5, a stream (stream o) containing substantially dimethyl ether and carbon dioxide and a stream (stream p) containing substantially methanol and water are thus formed from the stream n, which substantially still contains water, methanol, hydrogen, dimethyl ether and carbon dioxide. The stream o can be conveyed back into the separation process at a suitable point. The stream p can be used elsewhere. Deposited water can also be conducted to a wastewater treatment or a degassing process.

[0074] The distillation column 5 can also be operated in such a way that substantially no water, but methanol, passes into the top product o in a non-negligible amount. This is advantageous, for example, when the dimethyl ether is to be used for purposes in which methanol does not interfere with use. Thus, a higher yield can be achieved in relation to the synthesis gas used.

[0075] The return quantity and bottom number of the absorption column 6 can be optimized in such a way that the smallest possible amount of a corresponding bottom product n is produced.

[0076] Advantageously, the return which is applied as a portion of the stream l to the absorption column 6 is set such that the methanol and/or water content in the stream k is minimized. The composition of the stream m resulting in this way is such that the disadvantages explained at the outset can no longer arise in the cooling and condensation sequence to which the stream m is subjected.

[0077] The steps already mentioned several times for further treatment of the stream m are indicated here as a whole with 10. The stream m is firstly fed to a heat exchanger 11 and then fed into a separator container 12. The cooling in the heat exchanger 11 is carried out in such a way that a first condensate q is deposited in the separator container 12. A portion remaining in gaseous form in the separator container 12 is fed to a heat exchanger 13 and then fed into a further separator container 14. A condensate, referred to here as r, is also obtained there.

[0078] The condensates q and r are fed together with the portion of the stream l not conveyed back to the absorption column 6 into the above-mentioned dimethyl ether carbon dioxide distillation column 9, which is operated as explained below. A portion remaining in gaseous form at the top of the separator container 14 is cooled in a further heat exchanger 15. This stream, referred to here as s, at the “second” temperature level explained several times just above the melting point of carbon dioxide (at the prevailing pressure) downstream of the heat exchanger 15. In contrast, the temperature of the stream m upstream of the heat exchanger 11 (i.e., the “first” temperature level) is for example 35° C. The correspondingly cooled stream s is transferred into an absorption column 16, which can be operated according to the invention.

[0079] The invention can also be used in a highly simplified arrangement, for example in a single-stage cooling system which is connected downstream of the separation of methanol and water. However, a portion of the gas mixture remaining in gaseous form at the second temperature level is always washed in an absorption column 16 with a return predominantly containing dimethyl ether, as explained below. The return predominantly containing dimethyl ether is here formed from a liquid portion of the gas mixture deposited during cooling.

[0080] In the example shown, the stream s also contains dimethyl ether, carbon dioxide, carbon monoxide and hydrogen, i.e., in addition to dimethyl ether and carbon dioxide here two components which boil more easily than carbon dioxide. Using a liquid return v which is rich in dimethyl ether and is formed from some of a condensate z obtained from a bottom stream from a bottom liquid of the dimethyl ether carbon dioxide distillation column 9, a mixture of dimethyl ether and carbon dioxide is deposited in the bottom of the absorption column 16 and drawn off in the form of the bottom product w. The bottom product w can likewise be fed into the dimethyl ether carbon dioxide distillation column 9. At the top of the absorption column 16, however, a top product x is drawn off, which consists substantially of carbon monoxide and hydrogen and is low in or preferably free of carbon dioxide. This is, if appropriate after appropriate compression in a compressor 17, added at least in part as a recycling stream j to the stream b.

[0081] A further part of the top product x depleted of carbon dioxide from the absorption column 16 can be discharged from the method as a purge stream i.

[0082] The purge stream i can be subjected to an additional wash with liquid carbon dioxide in order to convey the largest possible portions of the dimethyl ether contained therein back into the dimethyl ether carbon dioxide distillation column 9. Some of the condensed top gas t from the dimethyl ether carbon dioxide distillation column 9 can advantageously be used for this wash. This makes it possible to reduce the losses of dimethyl ether, which otherwise would result via the discharge of the purge stream i. However, since only the purge stream i is subjected to this additional wash, no additional carbon dioxide is introduced into the recycling stream j as a result. Dimethyl ether contained in the recycling stream j is thus conveyed back through the reactor 4 into the separation cycle and thus remains largely retained.

[0083] In some embodiments, the top gas x′ of the absorption column 16 can be condensed in an optional top condenser 162 and used with the stream v as a return to the absorption column 16. This reduces the portion of dimethyl ether in the top product x, which leads to an improved reaction equilibrium and reduced loss via the purge stream i.

[0084] As mentioned, the portion of the stream l not conveyed back into the absorption column 6 and the streams q and r and the bottom product w are fed into the dimethyl ether carbon dioxide distillation column 9. Since these have different contents of dimethyl ether and carbon dioxide (traces of carbon monoxide and hydrogen are also present in dissolved form), they are fed into the dimethyl ether carbon dioxide distillation column 9 at different heights, for which purpose suitable valves (without designation) are provided.

[0085] The dimethyl ether carbon dioxide distillation column 9 is also operated with a bottom evaporator 91 and a top condenser 92. A top stream t formed from a top gas of the dimethyl ether carbon dioxide distillation column 9 is at least partially liquefied in the top condenser 92 using a heat exchanger operated with a suitable refrigerant and fed as a liquid return to the dimethyl ether carbon dioxide distillation column 9. A further portion is used to form a further stream y, which can be used elsewhere.

[0086] A liquid stream z is withdrawn from the bottom of the dimethyl ether carbon dioxide distillation column 9, which here consists substantially of dimethyl ether, but is in particular free of or low in carbon dioxide. Some of this bottom stream z is used to form the return v for the absorption column 16. In addition, the dimethyl ether product is formed from the bottom stream z.

[0087] The condensed top stream o of the distillation column 5 is also suitable as the return v, since it is substantially free of carbon dioxide and rich in dimethyl ether and possibly methanol.

[0088] If water is separated out only downstream of the dimethyl ether carbon dioxide distillation column 9, as described above with respect to an embodiment, it is expedient to use a substantially water-free side stream of the dimethyl ether carbon dioxide distillation column 9 instead of its bottom product z as the return v for the absorption column 16. In this case, the side stream is selected such that it contains less than 10 mass percent, in particular less than 5 mass percent carbon dioxide.

[0089] The return v is cooled before being fed to the absorption column 16, typically to a temperature level which is somewhat above the freezing point of carbon dioxide at the selected conditions, for example to a temperature level in the range from −30° C. to −49° C. For this purpose, a heat exchanger 164 can be provided, for example.