Processing by a separation technique of a gas mixture formed from a product stream of a dimethyl reactor

Abstract

A method for the processing, by separation technology, of a gas mixture (k) which is formed from a product stream (d) of a reactor (4) for synthesizing dimethyl ether from synthesis gas (b), and which contains at least dimethyl ether, carbon dioxide and at least one other component which is lower-boiling than carbon dioxide, is proposed. The gas mixture (k) is cooled at a first pressure level from a first temperature level to a second temperature level and a fraction of the gas mixture (k) that remains in gaseous form at the second temperature level is washed in an absorption column (16) with a reflux (v) predominantly containing carbon dioxide. The reflux (v) predominantly containing carbon dioxide is at least partially formed from a fraction of the gas mixture (k) which is separated in liquid form during the cooling.

Claims

1. Method for the processing, by separation technology, of a gas mixture (k) which is formed from a product stream (d) of a reactor (4) for synthesising dimethyl ether from synthesis gas (b), and which contains at least dimethyl ether, carbon dioxide and at least one other component which is lower-boiling than carbon dioxide, characterised in that the gas mixture (k) at a first pressure level is cooled from a first temperature level to a second temperature level and a fraction of the gas mixture (k) that remains in gaseous form at the second temperature level is washed in an absorption column (16) with a reflux (v) predominantly containing carbon dioxide, the reflux (v) predominantly containing carbon dioxide being at least partially formed from a fraction of the gas mixture (k) which is separated in liquid form during the cooling.

2. Method according to claim 1, wherein the gas mixture (k) is cooled through several intermediate temperature levels to the second temperature level, in the course of which a plurality of condensates (l, q, r) are separated.

3. Method according to claim 1, wherein the fraction of the gas mixture (k) remaining in gaseous form at the second temperature level is washed in an absorption column (16) with the reflux (v) predominantly containing carbon dioxide, in the course of which a top product (x) and a sump product (w) are recovered, the reflux (v) which predominantly contains carbon dioxide being formed partially from the sump product (w).

4. Method according to claim 1, wherein the reflux (v) predominantly containing carbon dioxide is formed using a dimethyl ether/carbon dioxide distillation column (9).

5. Method according to claim 4, wherein the dimethyl ether/carbon dioxide distillation column (9) is operated such that at its top is formed a top gas (t) predominantly containing carbon dioxide and in its sump is formed a sump liquid rich in dimethyl ether.

6. Method according to claim 5, wherein part of a condensate which is formed from at least part of the top gas of the dimethyl ether/carbon dioxide distillation column (9) is used as the reflux (v) predominantly containing carbon dioxide.

7. Method according to claim 5, wherein at least some of the sump liquid of the dimethyl ether/carbon dioxide distillation column (9) is drawn off as a product stream (z) which has a dimethyl ether content of more than 90 mol %.

8. Method according to claim 4, wherein the dimethyl ether/carbon dioxide distillation column (9) is operated at a second pressure level which is below the first pressure level.

9. Method according to claim 1, wherein the reflux predominantly containing carbon dioxide has a dimethyl ether content of not more than 10 mol %.

10. Method according to claim 1, wherein the gas mixture (k) is formed by at least partially removing methanol and/or water from the product stream (d).

11. Method according to claim 1, wherein the first temperature level is 20 to 50? C., and wherein the second temperature level is between the melting temperature of carbon dioxide at the pressure level used and ?15? C.

12. Method according to claim 1, wherein the first pressure level is 20 to 100 bar.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an apparatus for producing dimethyl ether according to the prior art, in schematic representation,

(2) FIG. 2 shows an apparatus for producing dimethyl ether according to one embodiment of the invention, in schematic representation.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) In the Figures, elements that correspond to one another have been given identical reference numerals and are not described again, for the sake of clarity.

(4) FIG. 1 schematically shows an apparatus for producing dimethyl ether according to the prior art, which is generally designated 110.

(5) The apparatus 110 comprises a synthesis gas reactor 20, shown in highly schematic form, which can be charged with a suitable feed a, such as natural or bio gas, for example. A synthesis gas stream b can be removed from the synthesis gas reactor 20.

(6) The synthesis gas stream b can be put under increased pressure by means of a compressor 1, optionally after the addition of other streams. In this way, the pressure required for a subsequent one-step synthesis of dimethyl ether, for example a pressure of 20 to 100 bar, can be applied.

(7) A correspondingly compressed stream, now designated c, is passed through a first heat exchanger 2 which can be heated with a product stream d of a reactor 4 for synthesising dimethyl ether (see below). The correspondingly heated stream, still designated c, has a temperature of 200 to 300? C., for example, downstream of the first heat exchanger 2. The stream c is optionally passed through a second heat exchanger 3, which is also referred to as a peak heater.

(8) The stream subjected to further heating in the second heat exchanger 3, and still designated c, is fed into the reactor 4, which is configured as a tube reactor and the reaction tubes of which are filled with a suitable catalyst for the one-step synthesis of dimethyl ether. The representation in FIG. 1 is highly simplified. Typically, reactors 4 for synthesising dimethyl ether are arranged vertically, with a stream c being fed into the tube reactor 4 at the bottom. A stream d is removed from the reactor 4 at the top.

(9) As a result of the exothermic reaction in the tube reactor 4, the stream d is at an even higher temperature. The stream d is passed as a heating medium through the heat exchanger 2. As a result, it cools to a temperature which is for example about 30? C. above the temperature of the stream c downstream of the compressor 1. The correspondingly cooled stream, still designated d, is fed into a conventional separation apparatus 120. In the separation apparatus 120, a methanol stream e and a water stream f are separated from the stream d, for example with intermediate pressure release, cooling, re-compression, etc. (not shown) in one step 121. From the residue remaining, the streams g and h are formed, which may be for example a stream g enriched in carbon dioxide and a stream h enriched in dimethyl ether.

(10) The composition of the streams g and h depends, inter alia, on the composition of the stream d and the specific configuration and the operating parameters of the separation apparatus 120.

(11) FIG. 2 shows an apparatus for producing dimethyl ether according to one embodiment of the invention. This is generally designated 100.

(12) A first absorption column which is used to separate methanol and/or water is designated 6 in FIG. 2. As already explained, an absorption column 6 differs from a distillation column such as the distillation column 5 (see below) inter alia in that it does not have a sump evaporator. Vapours rising in the absorption column 6 are washed by a reflux added at the top of the absorption column 6, so that the more volatile components are concentrated at the top of the absorption column 6 and the less volatile components are concentrated in the sump of the absorption column 6.

(13) In the apparatus 100, which is shown in FIG. 2, the stream d is introduced into the absorption column 6. A top stream k is drawn off from the top of the absorption column 6 and cooled in a heat exchanger 7 against a suitable refrigerant, such as coolant water, for example. The correspondingly cooled stream k is transferred into a separation container 8, from the sump of which a liquid stream l is taken and added to the absorption column 6 by means of a pump (not marked) at least partly as a reflux.

(14) If, in addition to dimethyl ether, the stream d in the embodiment shown contains methanol, water, carbon dioxide, carbon monoxide and hydrogen (together with traces of other compounds as explained above), dimethyl ether, carbon dioxide, carbon monoxide and hydrogen pass into the top stream k from here, as a result of the backwash described. As a result of suitable cooling in the heat exchanger 7 and corresponding separation conditions in the separation container 8, a sump product is separated in the separation container 8, consisting essentially of dimethyl ether and carbon dioxide (possibly with traces of methanol).

(15) From the top of the separation container 8, a gaseous stream m, which still contains dimethyl ether in addition to carbon dioxide, carbon monoxide and hydrogen, can be drawn off. The stream m is then subjected to sequential cooling and condensation, as described hereinafter. The part of the stream l which is not added to the absorption column 6 as a liquid reflux is fed into a distillation column 9, referred to here as a dimethyl ether/carbon dioxide distillation column, like the condensates occurring in the sequential cooling and condensation of the stream m.

(16) It should be expressly pointed out that the specific preparation of the stream k, which is obtained from the product stream d, need not take place in the manner shown. Other possible ways of separating water and/or methanol may be used, provided that they lead to the production of a gas mixture at the above-mentioned first pressure level and the first temperature level and containing the stated amounts of the individual components.

(17) The invention can also be carried out without any separation of water and methanol. In this case care must be taken to ensure that, for the reasons stated above, both water and methanol are present in the gas mixture that is to be cooled, in order to make use of the frost protection effect of methanol and prevent the corrosive effect of dry methanol.

(18) A liquid stream n is taken from the sump of the absorption column 6 and fed into a distillation column 5 at a suitable height, the distillation column being operated with a sump evaporator 51 and a top condenser 52. The stream n in the embodiment shown contains the great majority of the water and methanol contained in the stream d.

(19) The sump evaporator 51 and the top condenser 52 are operated with suitable heating and cooling means, respectively, preferably contained in a corresponding apparatus. In the sump evaporator 51, a liquid stream drawn off from the sump of the distillation column 5 is partially evaporated and fed into a lower region of the distillation column 5. An unevaporated fraction can be drawn off as stream p.

(20) 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 into the distillation column 5 again in an upper region as a liquid reflux. A fraction o remaining in gaseous form is drawn off.

(21) Thus, in the distillation column 5, from the stream n which essentially still contains water, methanol, hydrogen, dimethyl ether and carbon dioxide are formed a stream (stream o) essentially containing dimethyl ether and carbon dioxide and a stream (stream p) essentially containing methanol and water. The stream o may be recycled into the separation process at a suitable point. The stream p can be used elsewhere. Any water separated off can also be fed into waste water treatment or degassing.

(22) The reflux quantity and number of plates in the absorption column 6 can be optimised so that a corresponding sump product n is obtained in as small a quantity as possible. Advantageously, the reflux which is added to the absorption column 6 as part of the stream l is adjusted so as to minimise the content of methanol and water in the stream k. The composition of the stream m thus obtained is such that in the cooling and condensation sequence to which the stream m is subjected the disadvantages mentioned previously cannot arise.

(23) The steps, mentioned several times previously, for further treatment of the stream m are generally designated 10 here. The stream m is first fed into a heat exchanger 11 and then into a separation container 12. The cooling in the heat exchanger 11 is carried out so that a first condensate q is separated in the separation container 12. A fraction remaining in gaseous form in the separation container 12 is fed into a heat exchanger 13 and then into another separation container 14. Here, too, a condensate, designated r, is obtained.

(24) The condensates q and r are fed, together with the fraction of the stream l which has not been recycled into the absorption column 6, into the dimethyl ether/carbon dioxide distillation column 9 mentioned previously, which is operated as explained hereinafter.

(25) A fraction remaining in gaseous form at the top of the separation container 14 is cooled in another heat exchanger 15. It is present downstream of the heat exchanger 15 at the second temperature level described several times hereinbefore, between the melting point of carbon dioxide (at the prevailing pressure) and ?15? C. The temperature of the stream m upstream of the heat exchanger 11 (i.e. the first temperature level) is +35? C., by contrast. The correspondingly cooled stream, here designated s, is transferred into an absorption column 16 which may be operated according to the invention.

(26) The invention may also be used in a highly simplified arrangement, for example with one-step cooling and with no separation of methanol and water. However, a fraction of the gas mixture remaining in gaseous form at the second temperature level is washed, in an absorption column 16, with a reflux predominantly containing carbon dioxide, as explained hereinafter. The reflux predominantly containing carbon dioxide is formed from a liquid fraction of the gas mixture separated during cooling.

(27) The stream s in the embodiment shown still contains dimethyl ether, carbon dioxide, carbon monoxide and hydrogen, i.e., in addition to dimethyl ether and carbon dioxide, two components which are lower-boiling than dimethyl ether. Using a liquid reflux v which is rich in carbon dioxide and is formed from a part of a condensate u which is obtained from a top stream t comprising a top gas of the dimethyl ether/carbon dioxide distillation column 9, a mixture of dimethyl ether and carbon dioxide is separated in the sump of the absorption column 16 and drawn off in the form of the sump product w. The sump product w may also be fed into the dimethyl ether/carbon dioxide distillation column 9. At the top of the absorption column 16, by contrast, a top product x is drawn off, which consists essentially of carbon dioxide, carbon monoxide and hydrogen and is poor in or preferably free from dimethyl ether. This may be used elsewhere, optionally after being suitably compressed in a compressor 17.

(28) As already mentioned, the fraction of the stream l which has not been recycled into the absorption column 6, as well as the streams q and r and the sump product w, are fed into the dimethyl ether/carbon dioxide distillation column 9. As they contain different amounts 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 (not shown) are provided.

(29) The dimethyl ether/carbon dioxide distillation column 9 is also operated with a sump 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 is added as a liquid reflux to the dimethyl ether/carbon dioxide distillation column 9. Another fraction u is used to form the reflux v and another stream y, which may be used elsewhere.

(30) From the sump of the dimethyl ether/carbon dioxide distillation column 9 is taken a liquid stream z which consists essentially of dimethyl ether in this case but is, in particular, free from or poor in carbon dioxide.