PROCESS FOR PREPARING METHANOL FROM CARBON DIOXIDE AND HYDROGEN WITH QUANTITATIVE CARBON DIOXIDE UTILIZATION

Abstract

A process for preparing methanol from carbon dioxide and hydrogen in a methanol synthesis unit and working up the reaction mixture obtained stepwise to isolate the methanol, wherein the carbon dioxide, carbon monoxide, dimethyl ether and methane components of value from the streams separated off in the isolation of the methanol from the methanol reaction stream are combusted with an oxygenous gas, and the carbon dioxide in the resultant flue gas is separated off in a carbon dioxide recovery unit and recycled to the methanol synthesis unit.

Claims

1.-15. (canceled)

16. A process for preparing methanol by (a) converting a carbon dioxide and hydrogen containing stream (I) in a methanol synthesis unit (A) at a temperature of 150 to 300° C. and a pressure of 3 to 10 MPa abs in the presence of a methanol synthesis catalyst to a reaction mixture containing methanol, water, carbon dioxide, carbon monoxide, hydrogen, dimethyl ether and methane, condensing a methanol- and water-enriched crude methanol stream (II) out of said reaction mixture, and conducting the crude methanol stream (II) and a gaseous stream (III) comprising carbon dioxide, carbon monoxide, hydrogen and methane out of the methanol synthesis unit (A), (b) expanding the crude methanol stream (II) from stage (a) in an expansion unit (B) to a pressure of 0.1 to 2 MPa abs, and obtaining an expansion gas (IV) comprising carbon dioxide and methane and a degassed crude methanol stream (V) enriched with methanol and water, (c) separating a carbon dioxide- and dimethyl ether-comprising low boiler stream (VI) by distillation from the degassed crude methanol stream (V) from stage (b) in a distillation apparatus (C), and obtaining a methanol- and water-enriched bottom stream (VII), and (d) separating a water-containing high boiler stream (VIII) from the bottom stream (VII) from stage (c) in a further distillation apparatus (D), and obtaining methanol by distillation as stream (IX), which comprises (e) feeding the carbon dioxide, carbon monoxide, dimethyl ether and methane components of value in stream (III) and in at least one of the two streams (IV) and (VI) to a combustion unit (E) and combusting them therein with supply of an oxygenous gas (X) having an oxygen content of 30% to 100 vol.-%, forming carbon dioxide-containing flue gas (XI), (f) separating a carbon dioxide-enriched stream (XIII) from the carbon dioxide-containing flue gas (XI) from stage (e) in a carbon dioxide recovery unit (F) to form an off-gas stream (XII), (g) recycling the carbon dioxide-enriched stream (XIII) separated off in the carbon dioxide recovery unit (F) from stage (f) to the methanol synthesis unit (A) of stage (a) as a carbon dioxide containing source of stream (I), (h) supplying a hydrogen feedstock (XIV) to the methanol synthesis unit (A) of stage (a) as a hydrogen containing source of stream (I), and (i) supplying a carbon dioxide feedstock (XV) to the carbon dioxide-containing flue gas (XI) and/or to the methanol synthesis unit (A) of stage (a) as a carbon dioxide containing source of stream (I).

17. The process according to claim 16, wherein a copper- and zinc-containing heterogeneous catalyst is used as methanol synthesis catalyst in the methanol synthesis unit (A) in stage (a).

18. The process according to claim 16, wherein the methanol synthesis unit (A) in stage (a) comprises a compressor for compression of the carbon dioxide and hydrogen containing stream (I), a reactor for conversion of the compressed carbon dioxide and hydrogen containing stream (I), a condenser for condensing out the crude methanol stream (II), and a conduit for recycling of uncondensed gas to the reactor.

19. The process according to claim 16, wherein in stage (e), the carbon dioxide, carbon monoxide, dimethyl ether and methane components of value in streams (III), (IV) and (VI) are fed to the combustion unit (E).

20. The process according to claim 16, wherein the combustion unit (E) in stage (e) is supplied with an oxygenous gas (X) having an oxygen content of 90% to 100 vol.-%.

21. The process according to claim 16, wherein the combustion unit (E) in stage (e) comprises a combustion chamber and a condenser, water is condensed out of the combustion gas obtained in the combustion chamber in the condenser and conducted out of the combustion unit (E) as stream (XVI), and the remaining gaseous stream constitutes the carbon dioxide-containing flue gas (XI).

22. The process according to claim 16, wherein, in the carbon dioxide recovery unit (F) in stage (f), carbon dioxide is absorbed from the carbon dioxide-containing flue gas (XI) in an absorber in a basic solvent to form the off-gas stream (XII), the carbon dioxide-enriched stream (XIII) is released from the carbon dioxide-laden solvent in a desorber, and the carbon dioxide-depleted solvent is returned to the absorber.

23. The process according to claim 22, wherein the basic solvent used is an aqueous solution of an organic amine.

24. The process according to claim 23, wherein the organic amine used is monoethanolamine, piperazine, 2-amino-2-methyl-1-propanol, triethylenediamine, N-methyldiethanolamine or tert-butylaminoethoxyethanol.

25. The process according to claim 16, wherein the carbon dioxide-enriched stream (XIII) from stage (f) comprises oxygen, and stream (XIII), before it is recycled to the methanol synthesis unit (A), is catalytically hydrogenated to deplete the oxygen.

26. The process according to claim 16, wherein a carbon dioxide feedstock (XV) containing 5 to 95 vol.-% carbon dioxide based on the gaseous feedstock is supplied to the carbon dioxide-containing flue gas (XI).

27. The process according to claim 16, wherein a carbon dioxide feedstock (XV) containing 95 to 100 vol.-% carbon dioxide based on the gaseous feedstock is supplied to the methanol synthesis unit (A).

28. The process according to claim 16, wherein 50 to 100% of the carbon bound in the methanol (IX) is based on carbon dioxide supplied by the carbon dioxide feedstock (XV).

29. The process according to claim 16, wherein, before stream (III) is fed to the combustion unit (E), hydrogen is separated off in a hydrogen recovery unit (G) and recycled to the methanol synthesis unit (A) of stage (a).

30. The process according to claim 29, wherein the hydrogen is separated off by pressure swing adsorption in the hydrogen recovery unit (G).

Description

EXAMPLES

Interconnection 1 (Comparative Examples)

[0167] FIG. 6 shows a simplified block diagram of an interconnection for preparation of methanol according to the prior art. The labels therein have the following meanings: [0168] (A) methanol synthesis unit [0169] (B) expansion unit (low pressure expansion) [0170] (C) low boiler column [0171] (D) pure methanol column [0172] (G) pressure swing adsorption [0173] (I) carbon dioxide and hydrogen containing stream [0174] (II) methanol- and water-enriched crude methanol [0175] (IIIb) recovered hydrogen from pressure swing adsorption (G) [0176] (III) off-gas from pressure swing adsorption (G) [0177] (IV) expansion gas from expansion unit (B) [0178] (V) degassed crude methanol stream [0179] (VI) low boiler stream from low boiler column (C) [0180] (VII) bottom stream from distillation apparatus (C) [0181] (VIII) high boiler stream from pure methanol column (D) [0182] (IX) pure methanol [0183] (XIV) hydrogen feedstock (“fresh hydrogen”) [0184] (XV) carbon dioxide feedstock (“fresh carbon dioxide”)

[0185] In addition, the interconnection has the following further features: [0186] The methanol synthesis unit (A) comprises a synthesis cycle gas compressor, a reactor, a condenser and a synthesis cycle gas circuit. [0187] The hydrogen recovery rate of the pressure swing adsorption (G) is 83%.

[0188] Unless stated otherwise in the respective comparative example, hydrogen is recovered via the pressure swing adsorption (G) and recycled via stream (IIIb) to the methanol synthesis unit (A), fresh hydrogen is supplied via stream (XIV) and fresh carbon dioxide via stream (XV). Streams (III), (IV) and (VI) are each discharged from the interconnection.

Interconnection 2 (Inventive Examples)

[0189] FIG. 5 shows a simplified block diagram of an interconnection for the inventive preparation of methanol. The labels therein have the following meanings: [0190] (A) methanol synthesis unit [0191] (B) expansion unit (low pressure expansion) [0192] (C) low boiler column [0193] (D) pure methanol column [0194] (E) combustion unit [0195] (F) carbon dioxide recovery unit [0196] (G) pressure swing adsorption [0197] (I) carbon dioxide and hydrogen containing stream [0198] (II) methanol- and water-enriched crude methanol [0199] (IIIb) recovered hydrogen from pressure swing adsorption (G) [0200] (III) off-gas from pressure swing adsorption (G) [0201] (IV) expansion gas from expansion unit (B) [0202] (V) degassed crude methanol stream [0203] (VI) low boiler stream from low boiler column (C) [0204] (VII) bottom stream from distillation apparatus (C) [0205] (VIII) high boiler stream from pure methanol column (D) [0206] (IX) pure methanol [0207] (X) oxygenous gas [0208] (XI) flue gas [0209] (XII) off-gas from carbon dioxide recovery unit (F) [0210] (XIII) carbon dioxide-enriched stream from carbon dioxide recovery unit (F) [0211] (XIV) hydrogen feedstock (“fresh hydrogen”) [0212] (XV) carbon dioxide feedstock (“fresh carbon dioxide”) [0213] (XVI) condensed water from combustion unit (E)

[0214] In addition, the interconnection has the following further features: [0215] The methanol synthesis unit (A) comprises a compressor, a reactor, a condenser and a synthesis gas circuit. [0216] The combustion unit (E) comprises a combustion chamber and a condenser. [0217] The combustible components are converted to carbon dioxide and water in the combustion chamber to an extent of >99%. [0218] The carbon dioxide recovery rate of the carbon dioxide recovery unit (F) is >99%. [0219] The hydrogen recovery rate of the pressure swing adsorption (G) is 83%.

[0220] Streams (III), (IV) and (VI) are combusted in the combustion unit (E) with supply of pure oxygen, and the flue gas (XI) obtained is supplied to the carbon dioxide recovery unit (F) for recovery of carbon dioxide. Recovered carbon dioxide is recycled as stream (XIII) to the methanol synthesis unit (A). The off-gas from the carbon dioxide recovery unit (F) is discharged from the interconnection.

Example 1

Comparative

[0221] Comparative example 1 relates to methanol synthesis from carbon dioxide and hydrogen. The underlying interconnection is shown in FIG. 6 and is described in detail as “interconnection 1”. Table 1 shows the composition of the carbon dioxide feed stream (XV), which was 99.9 vol.-% of carbon dioxide and which corresponds to a typical composition for isolated carbon dioxide, and the composition of the hydrogen feed stream (XIV), which also was 99.9 vol.-% as a typical composition for hydrogen. The amounts of fresh carbon dioxide and fresh hydrogen fed into the synthesis unit as streams (XV) and (XIV) have been adjusted such that together with the recovered hydrogen (IIIb) from the pressure swing adsorption (G) and the methanol synthesis cycle gas loop inside of the methanol synthesis unit (A) a stoichiometric number S at the inlet of the methanol synthesis reactor of 3.40 was achieved. This relates to a stoichiometric number S of the streams fed to the methanol synthesis unit (A), which are the carbon dioxide feed stream (XV), the hydrogen feed stream (XIV) and the recovered hydrogen (IIIb) from the pressure swing adsorption (G), calculated as one combined stream, of 1.960. The stoichiometric number S of the mixture of the carbon dioxide feed stream (XV) and the hydrogen feed stream (XIV) was 1.869.

[0222] A stoichiometric number S of 1.960 at the inlet of the methanol synthesis is sufficient because of the high purge gas rate to assure a low inert component level in the methanol synthesis loop. Hence the amount of recovered hydrogen from the hydrogen recovery unit (G) is relatively high. A further reason is, that the carbon dioxide is much better soluble in the raw methanol coming from methanol synthesis the carbon monoxide. Hence it is removed out of the methanol synthesis loop and lost for the methanol synthesis. This causes an accumulation of the hydrogen in the methanol synthesis loop and lowers the hydrogen feedstock (XIV) amount necessary to adjust the stoichiometric demand.

[0223] The content of inerts (CH.sub.4, H.sub.2O, N.sub.2) at the reactor inlet was only 6.7 vol.-%. A low inerts concentration is beneficial to assure a high conversion of the carbon dioxide to methanol, because the activity of the catalyst is lower for the conversion of carbon dioxide in comparison to standard carbon monoxide/carbon dioxide mixtures in conventional synthesis gases.

[0224] Conversion by heterogeneous catalysis over a copper-containing methanol synthesis catalyst at 210° C. and a pressure of 7.45 MPa abs and the further workup of the reaction mixture according to the simplified block diagram of FIG. 6 results in streams (III), (IV) (after expansion to 0.6 MPa abs at 40° C.) and (VI) with the amounts and compositions specified in table 1.

[0225] Owing to the discharge of streams (III), (IV) and (VI) from the interconnection, these remain unutilized for further methanol synthesis. In processes according to the prior art, these are typically supplied solely to a thermal utilization, i.e. not physically utilized. Thus, in the present comparative example 1,752 m.sup.3 (STP) of carbon dioxide (XV) and 2158 m.sup.3 (STP) of hydrogen are required for the preparation of one metric ton of pure methanol (stream (IX)).

Example 2

Inventive

[0226] Inventive example 2 likewise relates to methanol synthesis from carbon dioxide and hydrogen. The underlying interconnection is shown in FIG. 5 and is described in detail as “interconnection 2”. Table 2 shows the composition of the carbon dioxide feed stream (XV), which was 99.9 vol.-% of carbon dioxide and which corresponds to a typical composition for isolated carbon dioxide, and the composition of the hydrogen feed stream (XIV), which also was 99.9 vol.-% as a typical composition for hydrogen, and which are identical to that from example 1. The amounts of fresh carbon dioxide and fresh hydrogen fed into the synthesis unit as streams (XV) and (XIV) have been adjusted such that together with the carbon dioxide-enriched stream (XIII) from the carbon dioxide recovery unit (F), the recovered hydrogen (IIIb) from the pressure swing adsorption (G) and the methanol synthesis recycle gas loop inside of the methanol synthesis unit (A) a stoichiometric number S at the inlet of the methanol synthesis reactor of 3.40 was achieved. This relates to a stoichiometric number S of the streams fed to the methanol synthesis unit (A), which are the carbon dioxide-enriched stream (XIII) from the carbon dioxide recovery unit (F), the carbon dioxide feed stream (XV), the hydrogen feed stream (XIV) and the recovered hydrogen (IIIb) from the pressure swing adsorption (G), calculated as one combined stream, of 1.957. The stoichiometric number S of the mixture of the carbon dioxide feed stream (XV) and the hydrogen feed stream (XIV) was 2.054.

[0227] The content of inerts (CH.sub.4, H.sub.2O, N.sub.2) at the reactor inlet was only 7.1 vol.-%. A low inerts concentration is beneficial to assure a high conversion of the carbon dioxide to methanol, because the activity of the catalyst is lower for the conversion of carbon dioxide in comparison to standard carbon monoxide/carbon dioxide mixtures in conventional synthesis gases.

[0228] Conversion by heterogeneous catalysis over a copper-containing methanol synthesis catalyst at 210° C. and a pressure of 7.45 MPa abs and the further workup of the reaction mixture according to the simplified block diagram of FIG. 5 results in streams (III), (IV) (after expansion to 0.6 MPa abs at 40° C.) and (VI) with the amounts and compositions specified in table 2. By contrast with comparative example 1, however, these are not discharged unutilized from the interconnection, but combusted in accordance with the invention in the combustion unit (E) with supply of pure oxygen (X). After condensation of 94.2% of the water present and discharge thereof as stream (XVI), the carbon dioxide-containing flue gas (XI) is fed to a carbon dioxide recovery unit (F). >99% of the carbon dioxide present in the flue gas (XI) is isolated therein as stream (XIII) and recycled to the methanol synthesis unit (A).

[0229] Only 706 m.sup.3 (STP) of carbon monoxide (XV) are thus required for the preparation of one metric ton of pure methanol (stream (IX)) in inventive example 2.

[0230] By comparison with comparative example 1, in which streams (III), (IV) and (VI) are discharged unutilized from the interconnection, the process of the invention enables, in example 2, the substantial utilization of the carbon monoxide, carbon dioxide, methane and dimethyl ether components of value present in these streams for the further synthesis of methanol. The specific consumption of hydrogen remains unchanged because the composition of the recycled carbon dioxide is nearly similar to the carbon dioxide feed to the process. Specifically, it spares the use of 46 m.sup.3 (STP) of carbon dioxide (XV) per metric ton of pure methanol (stream (IX)).

TABLE-US-00001 TABLE 1 Data for example 1 (comparative) Carbon Hydro- dioxide gen (XV) (XIV) (IIIb) (III) (IV) (VI) Amount 752 2158 68 45 10 12 [m.sup.3 (STP)/t methanol] CO.sub.2 [vol.-%] 99.9 0 0 35.01 85.66 99.26 H.sub.2 [vol.-%] 0 99.9 99.87 37.89 5.87 0.04 CO [vol.-%] 0 0 0 7.63 0.32 0 CH.sub.3OH [vol.-%] 0 0 0 0.64 3.18 0 CH.sub.4 [vol.-%] 0 0 0.07 12.15 3.58 0.25 CH.sub.3OCH.sub.3 [vol.-%] 0 0 0 0.04 0.13 0.45 H.sub.2O [vol.-%] 0 0 0 0.13 0.73 0 N.sub.2 [vol.-%] 0.1 0.1 0.07 6.44 0.29 0 O.sub.2 [vol.-%] 0 0 0 0 0 0 Ar [vol.-%] 0 0 0 0 0 0 Stoichiometric number S 1.869 (XV) + (XIV) Stoichiometric number S 1.960 (XV) + (XIV) + (IIIb)

TABLE-US-00002 TABLE 2 Data for example 2 (inventive) Carbon dioxide Hydrogen (XV) (XIV) (IIIb) (III) (IV) (VI) (XI) (XII) (XIII) Amount 706 2156 64 42 10 12 52 6 45 [m.sup.3 (STP)/t methanol] CO.sub.2 [vol.-%] 99.9 0 0 34.63 85.41 99.23 86.12 1.56 99.27 H.sub.2 [vol.-%] 0 99.9 99.86 37.52 5.89 0.04 0 0 0 CO [vol.-%] 0 0 0 7.58 0.32 0 0 0 0 CH.sub.3OH [vol.-%] 0 0 0 0.64 3.18 0 0 0 0 CH.sub.4 [vol.-%] 0 0 0.07 12.74 3.80 0.26 0 0 0 CH.sub.3OCH.sub.3 [vol.-%] 0 0 0 0.04 0.13 0.46 0 0 0 H.sub.2O [vol.-%] 0 0 0 0.13 0.73 0 3.32 3.00 0.73 N.sub.2 [vol.-%] 0.1 0.1 0.07 6.66 0.30 0 5.96 53.89 0 O.sub.2 [vol.-%] 0 0 0 0 0 0 4.50 40.68 0 Ar [vol.-%] 0 0 0 0 0 0 0.10 0.88 0 Stoichiometric number S 2.054 (XV) + (XIV) Stoichiometric number S 1.957 (XV) + (XIV) + (XIII) + (IIIb)