A METHOD FOR PRODUCING SYNGAS USING CATALYTIC REVERSE WATER GAS SHIFT

Abstract

A method for producing syngas is provided, which comprises providing a feed stream comprising H.sub.2 and CO.sub.2; heating the feed stream in a first heat exchanger to provide a first heated feed stream, which is introduced into a first RWGS reactor and subjected to a first catalytic RWGS reaction in the presence of a non-methanation promoting catalyst, thereby obtaining a first syngas containing stream, which is cooled in the first heat exchanger against the feed stream, thereby obtaining a first cooled syngas stream, which is separated in a first gas/liquid separator thereby obtaining a first water-enriched stream and a first water-depleted syngas stream; heating the first water-depleted syngas stream in a second heat exchanger thereby obtaining a heated first water-depleted syngas stream, which is introduced into a second RWGS reactor and subjected to a second catalytic RWGS reaction in the presence of a non-methanation promoting catalyst.

Claims

1-11. (canceled)

12. A method for producing syngas using a catalytic reverse water gas shift (RWGS) reaction, the method at least comprising the steps of: (a) providing a feed stream comprising at least hydrogen (H.sub.2) and carbon dioxide (CO.sub.2); (b) heating the feed stream in a first heat exchanger, thereby obtaining a first heated feed stream; (c) introducing the first heated feed stream into a first RWGS reactor and subjecting it to a first catalytic RWGS reaction in the presence of a non-methanation promoting catalyst wherein the pressure as used in the first RWGS reactor is above 20 bara and wherein the temperature as used in the first RWGS reactor is from 450 to 600 C., thereby obtaining a first syngas containing stream, wherein the first syngas containing stream comprises at most 1.0 vol. % methane (CH.sub.4); (d) cooling the first syngas containing stream in the first heat exchanger against the feed stream, thereby obtaining a first cooled syngas stream; (e) separating the first cooled syngas stream in a first gas/liquid separator thereby obtaining a first water-enriched stream and a first water-depleted syngas stream; (f) heating the first water-depleted syngas stream in a second heat exchanger thereby obtaining a heated first water-depleted syngas stream; (g) introducing the heated first water-depleted syngas stream into a second RWGS reactor and subjecting it to a second catalytic RWGS reaction in the presence of a non-methanation promoting catalyst wherein the pressure as used in the second RWGS reactor is above 20 bara and wherein the temperature as used in the second RWGS reactor is from 450 to 600 C., thereby obtaining a second syngas containing stream; (h) cooling the second syngas containing stream in the second heat exchanger against the first water-depleted syngas stream, thereby obtaining a second cooled syngas stream; (i) separating the second cooled syngas stream in a second gas/liquid separator thereby obtaining a second water-enriched stream and a second water-depleted syngas stream; (j) separating the second water-depleted syngas stream in a CO.sub.2 removal unit thereby obtaining a CO.sub.2-enriched stream and a CO.sub.2-depleted syngas stream, wherein the CO.sub.2-depleted syngas stream has a hydrogen to carbon monoxide (H.sub.2/CO) volume ratio in the range of from 1.5 to 2.5; (k) combining the CO.sub.2-enriched stream with the feed stream and/or the first water-depleted syngas stream; wherein the temperature of the first syngas containing stream and the second syngas containing stream is kept below 600 C.; and wherein the first and the second RWGS reactors each comprise a multi-tubular reactor heated by molten salt circulating around the tubes of the multi-tubular reactor.

13. The method according to claim 12, wherein the first heated feed stream comprises a hydrogen to carbon dioxide (H.sub.2/CO.sub.2) volume ratio of between 1.2 and 3.0.

14. The method according to claim 12, wherein the molten salt used for heating the multi-tubular reactors of the first and the second RWGS reactors is coming from a shared molten salt circulation system.

15. The method according to claim 12, wherein the pressure as used in the first and second RWGS reactors is from 20 to 200 bara.

16. The method according to claim 12, wherein the temperature as used in the first and second RWGS reactors is below 550 C.

17. The method according to claim 12, wherein the catalyst as used in the catalytic RWGS reaction in step (c) comprises cerium oxide, zirconium oxide or a combination thereof.

18. The method according to claim 12, wherein the first syngas containing stream comprises at most 0.1 vol. % methane.

19. The method according to claim 12, wherein the temperature of the second syngas containing stream is at most 20 C. higher than the temperature of the first syngas containing stream.

20. The method according to claim 12, wherein the heated first water-depleted syngas stream comprises a hydrogen to carbon dioxide (H.sub.2/CO.sub.2) volume ratio in the range of from 1.5 to 3.5.

21. The method according to claim 12, wherein at least a part of the CO.sub.2-enriched stream is combined with at least the feed stream, thereby obtaining a combined stream.

22. The method according to claim 12, wherein the CO.sub.2-depleted syngas stream comprises at most 10 vol. % CO.sub.2.

23. The method according to claim 12, wherein the temperature of the first syngas containing stream and the second syngas containing stream is kept below 550 C.

24. The method according to claim 13, wherein the first heated feed stream comprises a hydrogen to carbon dioxide (H.sub.2/CO.sub.2) volume ratio above 1.5 and below 2.0.

25. The method according to claim 12, wherein the first syngas containing stream comprises at most 0.01 vol. % methane.

26. The method according to claim 12, wherein the temperature of the second syngas containing stream is at most 10 C. higher than the temperature of the first syngas containing stream.

27. The method according to claim 12, wherein the temperature of the second syngas containing stream is not higher than the temperature of the first syngas containing stream.

28. The method according to claim 12, wherein the heated first water-depleted syngas stream comprises a hydrogen to carbon dioxide (H.sub.2/CO.sub.2) volume ratio in the range of from 1.8 to 2.5.

29. The method according to claim 12, wherein the CO.sub.2-depleted syngas stream comprises at most 5 vol. % CO.sub.2.

30. The method according to claim 12, wherein the CO.sub.2-depleted syngas stream comprises at most 2 vol. % CO.sub.2.

31. An apparatus suitable for performing the method for producing syngas according to claim 12, the apparatus at least comprising: a first heat exchanger for heat exchanging the feed stream against the first syngas containing stream obtained in the first RWGS reactor, to obtain a first heated feed stream and a first cooled syngas stream; a first RWGS reactor for subjecting the first heated feed stream to a catalytic RWGS reaction to obtain a first syngas containing stream; a first gas/liquid separator for separating the first cooled syngas stream to obtain a first water-enriched stream and a first water-depleted syngas stream; a second heat exchanger for heat exchanging the first water-depleted syngas and the second syngas containing stream obtained in the second RWGS reactor, to obtain a heated first water-depleted syngas stream and a second cooled syngas product stream; a second RWGS reactor for subjecting the heated first water-depleted syngas stream to a catalytic RWGS reaction to obtain a second syngas containing stream; a second gas/liquid separator for separating the second cooled syngas stream to obtain a second water-enriched stream and a second water-depleted syngas stream; a CO.sub.2 removal unit for separating the second water-depleted syngas stream to obtain a CO.sub.2-enriched stream and a CO.sub.2-depleted syngas stream; wherein the apparatus is configured to combine the CO.sub.2-enriched stream obtained in the CO.sub.2 removal unit with the feed stream and/or the first water-depleted syngas stream; wherein the first and the second RWGS reactors each comprise a multi-tubular reactor that can be heated by molten salt circulating around the tubes of the multi-tubular reactor; and wherein the apparatus further comprises a molten salt circulation system for heating the multi-tubular reactors of both the first and the second RWGS reactors.

Description

[0094] Hereinafter the present invention will be further illustrated by the following non-limiting drawings.

[0095] Herein shows:

[0096] FIG. 1 schematically an embodiment of a process line-up suitable for performing the method for producing syngas using a catalytic RWGS reaction according to the present invention;

[0097] FIG. 2 schematically a first comparative line-up (not according to the present invention), wherein the line-up also has a CO.sub.2-recycle but (different to the present invention) only one RWGS reactor; and

[0098] FIG. 3 schematically a second comparative line-up (not according to the present invention), wherein the line-up has two RWGS reactors but (different to the present invention) no CO.sub.2-recycle.

[0099] For the purpose of this description, same reference numbers refer to same or similar components.

[0100] The process line-up (or apparatus) of FIG. 1, generally referred to with reference number 1, comprises a first RWGS reactor 2 and a second RWGS reactor 12; a first heat exchanger 3, a second heat exchanger 13 and further heat exchangers 4, 5, 14 and 15; a first gas/liquid separator 6 and a second gas/liquid separator 16; and a CO.sub.2 removal unit 8.

[0101] Each of the RWGS reactors 2 and 12 comprise a catalyst bed and is provided with external heating 7, 17, 27 (heated by a molten salt heater). The catalyst bed comprises a non-methanation promoting catalyst (such as cerium oxide, zirconium oxide, or a combination thereof).

[0102] During use, a feed stream 10 is provided, which feed stream comprises at least hydrogen (H.sub.2) and carbon dioxide (CO.sub.2).

[0103] The feed stream is heated in the first heat exchanger 3 thereby obtaining a first heated feed stream 20. As shown in the embodiment of FIG. 1, the heated feed stream 20 may be further heated in a further heat exchanger 4. This further heat exchanger 4 may form part of the first RWGS reactor 2.

[0104] The first heated feed stream 20 is introduced into the first RWGS reactor 2 and subjected to a first catalytic RWGS reaction, thereby obtaining a first syngas containing stream, which is removed as stream 30 from the first RWGS reactor 2.

[0105] Then, the first syngas containing stream 30 is cooled in the first heat exchanger 3 by indirect heat exchange against the feed stream 10, thereby obtaining a first cooled syngas stream 40. As shown in the embodiment of FIG. 1, the cooled syngas stream 40 may be further cooled in the further heat exchanger 5.

[0106] Subsequently, the first cooled syngas stream 40 is separated in the first gas/liquid separator 6 thereby obtaining a first water-enriched stream 60 and a first water-depleted syngas stream 50.

[0107] The first water-depleted syngas stream 50 is then heated in the second heat exchanger 13 thereby obtaining a heated first water-depleted syngas stream 70. This heated first water-depleted syngas stream 70 is then introduced into the second RWGS reactor 12 and subjected to a second catalytic RWGS reaction, thereby obtaining a second syngas containing stream which is removed from the second RWGS reactor 12 as stream 80.

[0108] This second syngas containing stream 80 is cooled in the second heat exchanger 13 by indirect heat exchange against the water-depleted syngas stream 50, thereby obtaining a second cooled syngas stream 90.

[0109] This second cooled syngas stream 90 is (in the embodiment of FIG. 1 after heat exchanging in further heat exchanger 15) subjected to separating in second gas/liquid separator 16 thereby obtaining a second water-enriched stream 110 and a second water-depleted syngas stream 100. This second water-depleted syngas stream 100 is separated in the CO.sub.2 removal unit 8 thereby obtaining a CO.sub.2-enriched stream 120 and a CO.sub.2-depleted syngas stream 130.

[0110] As can be seen in the embodiment of FIG. 1, the CO.sub.2-enriched stream 120 is combined (in part) with the feed stream 10 and (in part) with the first water-depleted syngas stream 50. Although the CO.sub.2-enriched stream 120 may according to the present invention is combined with only the first water-depleted syngas stream 50, it is preferred that the combination of the CO.sub.2-enriched stream 120 with the feed stream 10 is always present (the combination with the first water-depleted syngas stream 50 being optional). For sake of clarity, the combined stream (of the CO.sub.2-enriched stream 120 with the feed stream 10) is referred to with stream 19.

[0111] According to the present invention, the first and the second RWGS reactors 2,3 each comprise a multi-tubular reactor (shown in FIG. 2) that can be heated by molten salt circulating around the tubes of the multi-tubular reactor.

[0112] Preferably, the apparatus 1 comprises a molten salt circulation system (not shown) for heating the multi-tubular reactors of both the first and the second RWGS reactors 2,3. In this way, the molten salt circulation system is a shared system in the sense that the same molten salt flows around the tubes of the multi-tubular reactors of both the first and second RWGS reactors 2,3. Preferably, the molten salt flow inside the shell of the multi-tubular reactor is counter-currently when compared to the flow of the gas inside the tubes. As shown, the molten salt may be heated by separate external heating, preferably an e-heater. Preferably, there is a common circuit for the molten salt for the two (or more) RWGS reactors.

[0113] The heat exchangers 4 and 14 may be integrated with each other.

[0114] FIG. 2 shows schematically a first comparative line-up (not according to the present invention), wherein the line-up also has a CO.sub.2-recycle but (different to the present invention) only one RWGS reactor.

[0115] FIG. 3 shows schematically a second comparative line-up (not according to the present invention), wherein the line-up has two RWGS reactors but (different to the present invention) no CO.sub.2-recycle.

EXAMPLES

Example 1. Recycle of Stream 120 Only to Stream 10

[0116] The apparatus of FIG. 1 with recycle of the CO.sub.2 enriched stream 120 to only the feed stream 10 (and not to the first water-depleted syngas stream 50) was used for illustrating an exemplary method according to the present invention. The compositions and conditions of the streams in the various flow lines are provided in Table 1 below.

[0117] The values in Table 1 were calculated using a model generated with commercially available UniSim software, whilst using an equilibrium reactor with settings such that only the (R)WGS reactions are allowed to occur and whilst arranging the settings such that no methanation occurred (hence 0 vol % CH.sub.4 in all streams). Thus, the standard Gibbs model was not used, which model would predict excess methanation (which does not occur or is at least minimized according to the present invention). Further, for the CO.sub.2 removal unit 8 a CO.sub.2 removal efficiency of 98% was assumed (this value of 98% is realistic; there are CO.sub.2 removal units that come close to an efficiency of 100%).

[0118] As can be seen from Table 1 below, an overall CO.sub.2 conversion of 98.9% was achieved, whilst aiming for a H.sub.2/CO ratio of 1.9, which ratio is e.g. suitable for subsequent Fischer-Tropsch reaction.

TABLE-US-00001 TABLE 1 Recycle of stream 120 to only stream 10. stream 10 19 30 50 60 80 100 110 120 130 T [ C.] 20 20 550 40 40 550 40 40 40 40 CO.sub.2 [vol. %] 26 35 19 23 <1 15 16 <1 >99 <1 H.sub.2 [vol. %] 74 65 48 58 <1 50 54 <1 <1 64 CO [vol. %] 0 <1 16 19 <1 27 29 <1 <1 35 H.sub.2O [vol. %] 0 <1 16 <1 >99 8 <1 >99 <1 <1 H.sub.2/CO.sub.2 2.8 1.8 2.5 2.5 3.3 3.3 [mol/mol] H.sub.2/CO 3.0 3.0 1.8 1.8 1.8 [mol/mol] XCO.sub.2.sup.1[%] 46.1 64.3 98.9 .sup.1XCO.sub.2 = % overall conversion of CO.sub.2, based on feed stream 10 and product stream 130.

Example 2 ComparativeOne RWGS Reactor with CO.SUB.2 .Recycle

[0119] For comparison with Example 1 according to the present invention, a further set of calculations (whilst using the same UniSim software as used in Example 1) was performed for the line-up of FIG. 2, i.e. with still a CO.sub.2-recycle but only one RWGS reactor. The compositions and conditions of the streams in the various flow lines are provided in Table 2 below.

[0120] Please note that the temperature, pressure and feed streams and resulting H.sub.2/CO ratios were kept essentially the same; hence the difference between Example 1 and 2 is the number of RWGS stages (two for Example 1 according to the present invention and one for the comparative Example 2).

TABLE-US-00002 TABLE 2 Comparative. Single-stage RWGS with recycle of stream 120. stream 10 19 30 60 100 120 130 T [ C.] 20 20 550 40 40 40 40 CO.sub.2 [vol. %] 26 49 32 <1 38 >99 <1 H.sub.2 [vol. %] 74 51 33 <1 40 0 65 CO [vol. %] 0 <1 18 <1 21 0 34 H.sub.2O [vol. %] 0 <1 18 >99 0 0 0 H.sub.2/CO.sub.2 2.8 1.0 1.1 1.1 [mol/mol] H.sub.2/CO 1.9 1.9 1.9 [mol/mol] XCO.sub.2.sup.1[%] 35.6 96.5 .sup.1XCO.sub.2 = % overall conversion of CO.sub.2, based on feed stream 10 and product stream 130.

[0121] To further explain the benefits of the present invention, the single-stage RWGS of (comparative) Example 2 versus two-stage RWGS line-up according to the present invention was compared in terms of the effects of reactor temperature (Table 3) and CO.sub.2 conversion levels (Table 4) whilst the same composition of the feed streams was used.

[0122] Further, a comparison was made (see Table 5) showing the effect of the CO.sub.2 recycle on the overall CO.sub.2 conversion for a two-stage RWGS system.

[0123] Table 3 shows that according to the present invention (using two-stage RWGS) the CO.sub.2 recycle flow rate can be reduced by a factor 3 compared to a single-stage RWGS system (also using a CO.sub.2 recycle). This implies that also the whole separation section and recycle compressorand associated costscan be reduced by roughly a similar factor 3.

With per pass CO.sub.2 conversion is meant the CO.sub.2 conversion based on the CO.sub.2 mass flow rate in the inlet to the first RWGS reactor, i.e. stream 20, and CO.sub.2 mass flow rate in the outlet of the last reactor, i.e. stream 80 for the two-stage system and stream 30 for the single stage.

TABLE-US-00003 TABLE 3 Per pass CO.sub.2 conversions - comparison at 3 different temperatures. T of the reactors (single and two stage) [ C.] 520 550 590 Per pass CO.sub.2 conversion 31 36 41 [%] single stage (comparative Example 2) Per pass CO.sub.2 conversion 59 64 71 [%] two stage (Example 1) Relative increase in per 88 80 72 pass CO.sub.2 conversion [%] Overall CO.sub.2 conversion 99 99 99 CO.sub.2-recycle flow vs CO.sub.2 2.1 1.7 1.4 feed flow single stage CO.sub.2-recycle flow vs CO.sub.2 0.7 0.5 0.4 feed flow two stage [kg/kg] Relative CO.sub.2 recycle 67 68 70 decrease

[0124] Table 3 shows that for the comparative embodiment using single RWGS stage with recycle (see FIG. 2) the per pass CO.sub.2 conversions are only 31, 36 and 41% for the temperatures 520 C., 550 C. and 590 C. For Example 1 according to the present invention these per pass CO.sub.2 conversions at the same temperatures are respectively 59, 64 and 71%. This represents a relative increase between 72-88%.

[0125] Table 4 below shows that to achieve the same per pass CO.sub.2 conversions in a single stage as for the line-up of FIG. 1, the temperatures required would need to be in the range of 745-900 C. In this respect it is noted that temperatures above 600 C. result in severe material challenges. Hence the present invention allows for achieving the same high conversion at 225-310 C. lower than for a single stage RWGS system.

TABLE-US-00004 TABLE 4 Comparison single versus two-stage for three per pass CO.sub.2 conversion levels. T reactor two stage [ C.] 520 550 590 CO.sub.2 conversion per pass [%] 59 64 71 T reactor needed to reach 745 800 900 same conversion per pass for single stage [ C.]

Example 3 ComparativeTwo-Stage RWGS without CO.SUB.2 .Recycle

[0126] For yet another comparison with Example 1 according to the present invention, a further set of calculations (whilst using the same UniSim software as used in Example 1) was performed for the line-up of FIG. 3, i.e. with two RWGS reactor stages, but without a CO.sub.2 recycle. Again the conditions were kept essentially the same as in Example 1.

[0127] Table 5 below compares the effect of the use of the CO.sub.2 recycle stream (120 in FIG. 1; recycled to stream 10 thereby obtaining stream 19) on the total CO.sub.2 conversion of a two-stage RWGS system.

[0128] As can be seen from Table 5, Comparative A (2-stage RWGS but no CO.sub.2 recycle as in FIG. 3) shows that even for a two stage RWGS system (but no CO.sub.2 recycle) the overall CO.sub.2 conversion at 550 C. is only 74% compared to close to 99% for the present invention (Example 1, having two-stage RWGS with CO.sub.2 recycle).

[0129] As Comparative B shows, to achieve the same overall CO.sub.2 conversion in a two-stage RWGS system without CO.sub.2 recycle, the temperatures required would need to be above 1000 C. because even at 1000 C. the achievable CO.sub.2 conversion in a 2-stage RWGS system without CO.sub.2 recycle is only 95% versus 99% for Example 1 at a temperature of only 550 C. Please note in this respect that temperatures above 600 C. already result in severe material challenges, and temperatures above 1000 C. are prohibitive for conventional reactor technologies.

TABLE-US-00005 TABLE 5 Effect of the use of the CO.sub.2 recycle stream (120 in FIG. 1) on the total CO.sub.2 conversion of a two-stage RWGS system T of two-stage RWGS reactors [ C.] 550 550 1000 (Example 1, (Comparative (Comparative with CO.sub.2 A, no CO.sub.2 B, no CO.sub.2 recycle) recycle) recycle) Overall CO.sub.2 98.9 74.2 95.4 conversion [%] Feed H.sub.2/CO.sub.2 2.8 2.8 2.8 ratio (stream 10) [mol/mol] Syngas 1.8 2.8 1.9 H.sub.2/CO ratio (stream 100) [mol/mol]

DISCUSSION

[0130] As can be seen from the above Examples, the method according to the present invention allows for an effective way of producing syngas using a catalytic RWGS reaction, whilst maintaining the temperature in the RWGS reactors below 600 C. and whilst still achieving desirable per pass CO.sub.2 conversions (per pass CO.sub.2 conversion in the range of 59-71), and hence relatively small CO.sub.2 recycles, with just 2 RGWS stages.

[0131] The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention.