PROCESS AND PLANT FOR PRODUCING HYDROGEN AND FOR SEPARATING CARBON DIOXIDE FROM SYNTHESIS GAS

Abstract

The present invention relates to a process for producing hydrogen and for separating carbon dioxide from synthesis gas using a physical absorption medium. The process comprises the steps where the synthesis gas and the absorption medium are cooled; carbon dioxide is removed from the cooled synthesis gas via the cooled absorption medium in a physical absorption step at elevated pressure; laden absorption medium is treated in a plurality of flash stages, wherein co-absorbed carbon monoxide, hydrogen and carbon dioxide are separately removed from the laden absorption medium; hydrogen is separated from synthesis gas freed of carbon dioxide in a physical separation step, wherein hydrogen as product gas and an offgas comprising hydrogen, carbon monoxide and carbon dioxide are obtained; product gas hydrogen and carbon dioxide are discharged from the process. The invention further relates to a plant for performing the process.

Claims

1. A process for producing hydrogen and for separating carbon dioxide from synthesis gas, comprising: (a) providing a synthesis gas, wherein the synthesis gas comprises at least hydrogen, carbon monoxide and carbon dioxide; (b) providing a physical absorption medium; (c) cooling the synthesis gas and the absorption medium; (d) removing carbon dioxide from the cooled synthesis gas via the cooled absorption medium in a physical absorption step at elevated pressure, wherein the cooled synthesis gas and the cooled absorption medium are run in countercurrent, wherein synthesis gas at least partially freed of carbon dioxide is obtained and absorption medium laden with carbon dioxide and partially co-absorbed carbon monoxide and hydrogen is obtained; (e) treating the laden absorption medium in a plurality of serially arranged flash stages, wherein co-absorbed carbon monoxide and hydrogen is removed from the laden absorption medium in at least one first flash stage and carbon dioxide is removed from the laden absorption medium in a flash stage arranged downstream of the first flash stage; (f) separating hydrogen from the synthesis gas at least partially freed of carbon dioxide in a physical separation step, wherein hydrogen as product gas and an offgas comprising hydrogen, carbon monoxide and carbon dioxide are obtained; (g) compressing to absorption pressure and at least partially recycling to step (d) the offgas obtained in step (f) and the carbon monoxide and hydrogen obtained in step (e); and (h) discharging the product gas (hydrogen) obtained in step (f) and the carbon dioxide obtained in step (e) from the process.

2. The process according to claim 1, wherein the physical separation step comprises a pressure swing adsorption.

3. The process according to claim 1, wherein offgas obtained in step (f) and carbon monoxide and hydrogen obtained in step (e) are in a common gas compressor compressed to absorption pressure and recycled to step (d).

4. The process according to claim 1, wherein the offgas obtained in step (f) is partly utilized as fuel in the production of the synthesis gas.

5. The process according to claim 1, wherein the offgas obtained in step (f) is subjected to a further absorption step with cooled absorption medium, wherein an offgas at least partially freed of carbon dioxide is obtained and a further absorption medium laden with carbon dioxide and partially co-absorbed carbon monoxide and hydrogen is obtained, and the offgas partially freed of carbon dioxide is compressed to absorption pressure and recycled to step (d).

6. The process according to claim 5, wherein the further absorption medium laden with carbon dioxide and partially co-absorbed carbon monoxide and hydrogen is treated in the plurality of serially arranged flash stages according to step (e).

7. The process according to claim 1, wherein the synthesis gas comprises water (H.sub.2O) as a further component.

8. The process according to claim 7, wherein water is removed from the absorption medium in a distillation step arranged downstream of step (e).

9. The process according to claim 7, wherein absorption medium vapours obtained in the distillation step are utilized as a stripping medium for removal of carbon dioxide in the downstream flash stage according to step (e).

10. The process according to claim 8, wherein compression heat obtained in step (g) is utilized for direct or indirect heating of the absorption medium in the distillation step.

11. The process according to claim 7, wherein the water is at least partially removed from the synthesis gas by cooling the synthesis gas below the dew point of water and separating the condensed water.

12. The process according to claim 1, wherein compression heat obtained in step (g) is utilized to heat the laden absorption medium during treatment of the laden absorption medium in the plurality of serially arranged flash stages according to step (e).

13. The process according to claim 1, wherein the physical absorption medium is hygroscopic.

14. The process according to claim 1, wherein the physical absorption medium comprises methanol or consists substantially of methanol or consists of methanol.

15. The process according to claim 1, wherein the physical absorption medium comprises N-methyl-2-pyrrolidone (NMP) or consists substantially of NMP or consists of NMP.

16. The process according to claim 1, wherein the synthesis gas is in a process step upstream of the process subjected to a water gas shift reaction to increase the hydrogen yield, thus enriching the synthesis gas with water and carbon dioxide.

17. A plant for producing hydrogen and for separating carbon dioxide from synthesis gas, comprising the following plant components: (a) a means for providing synthesis gas, wherein the synthesis gas comprises at least hydrogen), carbon monoxide and carbon dioxide; (b) a means for providing a physical absorption medium; (c) a means for cooling the synthesis gas and the absorption medium; (d) an absorption column for running the cooled synthesis gas and the cooled absorption medium in countercurrent at elevated pressure for removal of carbon dioxide from the synthesis gas, wherein synthesis gas at least partially freed of carbon dioxide is obtainable and absorption medium laden with carbon dioxide and partially co-absorbed carbon monoxide and hydrogen is obtainable; (e) a plurality of serially arranged flash stages comprising a first flash stage for removal of the co-absorbed carbon monoxide and hydrogen from the laden absorption medium and comprising a flash stage arranged downstream of the first flash stage for removal of carbon dioxide from the laden absorption medium; (f) an apparatus for separating hydrogen from the synthesis gas at least partially freed of carbon dioxide in a physical separation step, wherein hydrogen as product gas and an offgas comprising hydrogen, carbon monoxide and carbon dioxide are obtainable; (g) a compressor for compressing the carbon monoxide and hydrogen obtainable in the first flash stage and the offgas obtainable in the apparatus (f) and for recycling the carbon monoxide and hydrogen obtainable in the first flash stage and the offgas obtainable in the apparatus (f) to the absorption column (d); and (h) a means for discharging the product gas (hydrogen) obtainable in the apparatus (f) and the carbon dioxide obtainable in the downstream flash stage from the plant.

18. The plant according to claim 17, wherein the apparatus (f) comprises a unit for pressure swing adsorption.

19. The plant according to claim 17, wherein the plant comprises a further absorption column for running the offgas obtainable in the apparatus (f) and cooled absorption medium in countercurrent, wherein offgas at least partially freed of carbon dioxide is obtainable and a further absorption medium laden with carbon dioxide and partially co-absorbed carbon monoxide and hydrogen is obtainable.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0103] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0104] FIG. 1 illustrates a schematic representation of one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0105] FIG. 1 shows a simplified process flow diagram of a possible embodiment of an inventive process 100 or an inventive plant 100. According to this embodiment the absorption medium is methanol. In the process flow diagram gas streams are represented by dashed lines and liquid streams are represented by solid lines. The flow direction of the respective stream is indicated by arrows.

[0106] Synthesis gas from a reformer unit (ATR, SMR, POx, GHR or combinations thereof) which was further treated in a water gas shift unit arranged downstream of the reformer unit (both not shown) is supplied via conduit 1 and initially cooled below the dew point of water in heat exchanger HX-01 using a carbon dioxide stream from conduit 4. The cooled synthesis gas is sent on via conduit 31 and condensed water is separated from the synthesis gas in separator D-01. Synthesis gas and uncondensed water is sent on via conduit 5 and in heat exchanger HX-02 further cooled to a temperature of −10° C. against purified synthesis gas from conduit 16. To prevent the formation of ice in conduit 5 methanol is supplied via conduit 8. Synthesis gas cooled to −10° C. in heat exchanger HX-02 is sent on via conduit 32 and in separator D-02 a further amount of water is separated from the synthesis gas and supplied via conduit 34 to the conduit 33.

[0107] The synthesis gas largely freed of water and cooled is sent on via conduit 14 and supplied to absorption column T-01 in which it is subjected to a physical absorption step. Absorption column T-01 is operated at an absorption pressure of 32 bar. In the absorption column T-01 cold methanol as absorption medium from the conduits 13 and 22 is passed from top to bottom while the synthesis gas from conduit 14 is run in counter-current from bottom to top, thus resulting primarily in absorption of carbon dioxide in methanol as well as co-absorption of relatively small amounts of value gases (carbon monoxide and hydrogen) in methanol. Any residual amounts of water not condensed in the separators D-01 and D-02 are simultaneously dissolved in methanol in the absorption column T-01. Purified synthesis gas, i.e. synthesis gas largely freed of water and carbon dioxide, is withdrawn from the absorption column T-01 via conduit 16. The synthesis gas subsequently cools a sub-amount of laden methanol diverted from conduit 9 using pump P-02 from conduit 10 and sent on via conduit 11 in heat exchanger HX-06. The purified synthesis gas is subsequently sent on via conduit 15 and in heat exchanger HX-02 cools the raw synthesis gas from conduit 5. The purified synthesis gas is subsequently sent via conduit 6 to a pressure swing adsorption unit PS-01 for producing pure hydrogen.

[0108] In the sump region of the absorption column T-01 laden methanol is withdrawn via conduit 9. The laden methanol contains carbon dioxide and in relatively small amounts co-absorbed value gases (carbon monoxide and hydrogen) as absorbed gas components. A portion of the laden methanol is diverted from conduit 9 via conduit 10 using pump P-02, sent on via conduit 11 and in heat exchanger HX-06 cooled against purified synthesis gas from conduit 16. It is subsequently sent on via conduit 12 and cooled to −35° C. in refrigerant cooler HU-01. The cooled laden methanol is subsequently sent on via conduit 13 and introduced into absorption column T-01 for reabsorption of carbon dioxide.

[0109] The majority of the laden methanol is introduced into the flash column T-02 via conduit 9. The pressure is initially reduced to 18 bar via the pressure reduction valve arranged in conduit 9. In the lower portion of the flash column this pressure reduction initially brings about in a first flash step liberation of essentially co-absorbed value gases (carbon monoxide and hydrogen) which are withdrawn via conduit 26 and sent to compressor C-01. Laden methanol decompressed to 18 bar is sent on via conduit 18 and decompressed to 8 bar via the pressure reduction valve arranged in conduit 18. In the middle portion of the flash column this further pressure reduction brings about in a second flash step further liberation of essentially co-absorbed value gases (carbon monoxide and hydrogen) which are withdrawn via conduit 17 and sent to compressor C-01. Compressor C-01 compresses the value gases from the conduits 17 and 26 to absorption pressure (32 bar). The compressed value gases are subsequently recycled into the absorption column T-01 via the conduits 7, 5, 32 and 14.

[0110] The methanol withdrawn from the second flash stage via conduit 19 is in heat exchanger HX-04 heated using a portion of the raw synthesis gas from conduit 1 (not shown) and via conduit 20 sent to a third flash stage in the upper portion of the flash column. The third flash stage is a flash stage arranged downstream of the first and second flash stage. Arranged in conduit 20 is a further pressure reduction valve which decompresses the laden methanol primarily still containing carbon dioxide as absorbed gas components to 1.3 bar.

[0111] The third flash stage is supplied via conduit 25 with methanol vapours from rectification column T-03 which are used as stripping medium in the third flash stage and increase the amount of the carbon monoxide expelled (desorbed) from the absorption medium in the third flash stage compared to a pure flash arrangement. The third flash stage may thus also be considered a stripping stage, wherein methanol vapours are used as stripping medium. The heating via heat exchanger HX-04 of the laden methanol supplied via conduit 20 also has a positive effect on the yield of the separated carbon dioxide. The carbon dioxide obtained from the third flash stage is withdrawn from this flash stage via conduit 4. It has a low temperature of −36° C. and is therefore utilized for cooling this raw synthesis gas from conduit 1 in heat exchanger HX-01. The carbon dioxide product is sent on via conduit 2 and compressed by compressor C-02. The compressed carbon dioxide may subsequently be stored (CCS) or sent to a further use (CCU).

[0112] The methanol obtained in the third flash stage comprises only a low residual content of absorbed carbon dioxide. It is withdrawn from the third flash stage via conduit 21 and compressed to absorption pressure (32 bar) using pump P-01. It is subsequently sent on via conduit 22 and supplied to the top region of the absorption column T-01 for reabsorption of carbon dioxide.

[0113] Since the absorption medium methanol is permanently recirculated and undergoes a large number of absorption and desorption cycles, it is over time enriched with water absorbed from the raw synthesis gas in the absorption column T-01. Therefore, a portion of the methanol largely freed of absorbed constituents is diverted from conduit 22 and via conduit 23 initially heated against hot methanol vapours from conduit 27 in heat exchanger HX-05. The amount of the methanol withdrawn via conduit 23 is adjusted such that water cannot accumulate in the methanol in the circuit between absorption column T-01 and the flash column, i.e. a predefined threshold concentration is not exceeded. It is subsequently sent via conduit 24 to an upper region of the rectification column T-03. Arranged in conduit 24 is a pressure reduction valve through which the methanol is decompressed to 2 bar. Water is also supplied to a middle region of the rectification column T-03. This is the water separated from the raw synthesis gas in the separators D-01 and D-02. This is supplied via conduit 33 and in heat exchanger HX-03 is heated against hot water exiting the rectification column T-03 as sump product via conduit 29. Rectification column T-03 is further heated with fresh steam via a boiler (not shown). Rectification column T-03 essentially performs a separation of methanol and water, wherein methanol vapours exit the rectification column T-03 as top product via conduit 27, are slightly cooled against methanol from conduit 23 in heat exchanger HX-05 and as mentioned above are introduced into the third flash stage as stripping medium via conduit 25.

[0114] In an alternative embodiment (not shown) the methanol vapours withdrawn from conduit 27 are completely condensed and the thus obtained condensed methanol is sent to conduit 22 for further introduction into the absorption column T-01.

[0115] Water is withdrawn from rectification column T-03 as sump product via conduit 29, cooled against water from conduit 33 and withdrawn from the process.

[0116] The synthesis gas discharged from the absorption column T-01 via conduit 16 and sent on via the conduits 15 and 16 after passage through heat exchangers HX-06 and HX-02 is sent to a pressure swing adsorption unit PS-01 for producing pure hydrogen. In the pressure swing adsorption unit PS-01 hydrogen is separated from further synthesis gas constituents. Strictly speaking, the further gas constituents, for example carbon monoxide and carbon dioxide, are separated from the hydrogen, since hydrogen as a “light component” does not adsorb on the adsorbent of the fixed bed of the pressure swing adsorption unit PS-01. By contrast, the further gas constituents are separated from the hydrogen as “heavy components” by ongoing adsorption and desorption processes at the fixed bed.

[0117] The pressure swing adsorption unit PS-01 primarily separates carbon dioxide, carbon monoxide, nitrogen, methane and optionally further trace components from hydrogen by adsorption-desorption cycles, i.e. separates hydrogen therefrom. At the end of each cycle the adsorbed components are removed from the adsorbent of the fixed bed of the pressure swing adsorption unit PS-01 by pressure reduction, wherein the offgas is discharged from the corresponding vessel of the pressure swing adsorption unit PS-01. The offgas contains not only the abovementioned constituents to be separated from hydrogen but also significant amounts of hydrogen which has not been separated, i.e. was co-adsorbed on the fixed bed. The offgas is then divided into two substreams. A first portion is discharged from the process via conduit 36 and may be utilized for example as fuel for the production of the synthesis gas. The second substream is supplied to a compressor C-03 via conduit 37 and compressed from 1.2 bar to about 7.2 bar.

[0118] The compressed offgas may subsequently be sent to the value gas (hydrogen, carbon monoxide) obtained in the middle portion of the flash column T-02 in conduit 17 and together therewith compressed to absorption pressure (32 bar) in compressor C-01 (not shown). Compressor C-01 also compresses value gas (hydrogen, carbon monoxide) obtained in the first flash stage to absorption pressure (32 bar). In this embodiment the offgas from the pressure swing adsorption unit PS-01 is thus compressed to absorption pressure together with the desorbed value gases obtained in the first and second flash stage and sent to the absorption column T-01 for reabsorption.

[0119] In a further embodiment the offgas from conduit 38 is sent to a further absorption column T-04. Absorption column T-04 removes carbon dioxide from the offgas by counter-current absorption in methanol. Methanol is diverted from the regenerated methanol stream from conduit 22 and introduced through conduit 41 into the absorption column T-04. Carbon dioxide-laden absorption medium is discharged from the absorption column T-04 via conduit 40 and sent to the laden absorption medium in conduit 9. It thus then undergoes the same process steps as the laden absorption medium withdrawn from the absorption column T-01. The offgas largely freed of carbon dioxide in conduit 39 is supplied to compressor C-01 for common compression with the gases from conduits 17 and 26 to absorption pressure (32 bar). The use of an additional absorption step through absorption column T-04 further increases the total amount of carbon dioxide separated in the process.

[0120] The following table shows the advantages of the process according to the invention with partial recycling of the offgas discharged from the pressure swing adsorption unit PS-01, wherein the offgas in conduit 37 is after compression in compressor C-03 directly supplied to the flash gas in conduit 17.

[0121] In Example 1 no offgas from PS-01 is recycled, i.e. the offgas is entirely sent to a utilization as fuel for production of synthesis gas. In Example 2 50% by weight of the offgas is diverted and recycled via conduit 37; in Example 3 90% by weight of the offgas is diverted and recycled via conduit 37.

[0122] The data in the table apply to the production of 100 kNm.sup.3/h (thousand standard cubic metres per hour) of hydrogen which is subsequently compressed to a pressure of 30 bar. The data were generated by simulation with Aspen Plus (9) software.

TABLE-US-00001 Example 1 Example 2 Example 3 no offgas 50% by wt 90% by wt recycling offgas recycling offgas recycling Proportion of 83.75% 91.13% 98.07% separated carbon dioxide (CO.sub.2 capture) Purity of carbon 98.80% 98.80% 98.40% dioxide Hydrogen yield 87.73% 93.45% 98.52% (H.sub.2 recovery) Hydrogen 100.168  100.168  100.168  production/(Nm.sup.3/hr) Cooling power/kW 2.586 2.643 2.715 Steam 0.77  0.73  0.69  consumption/(t/h) Process electrical 1.783 3.662 6.422 power/kW (without CO.sub.2 compression) Cooling unit 1.378 1.409 1.447 electrical power/kW Total electrical 3.161 5.071 7.869 power/kW Cooling water/(t/h) 10    154     369    

[0123] The proportion of separated carbon dioxide is the proportion in percent of the theoretically separable carbon dioxide amount. The data show a significantly improved proportion of separated carbon dioxide when a portion of the off gas from the pressure swing adsorption unit is compressed and recycled to the absorption step as recycle gas. The proportion of separated carbon dioxide increases with the proportion of recycled offgas amount. At a recycle rate of 90% by weight more than 98% of the theoretically separable carbon dioxide is separated compared to less than 85% without recycling of the offgas. The carbon dioxide has a purity of at least 98%.

[0124] The following table shows a mass balance of the above Example 3 (90% by weight recycling of off gas from PS-01).

TABLE-US-00002 Synthesis Synthesis Hydrogen Carbon Offgas Recycled gas (con- gas (con- (conduit dioxide (conduit offgas duit 1) duit 6) 35) (conduit 3) 36, 37) (conduit 37) Temperature ° C. 40 20 20 40 20 20 Pressure bar 32 30 30 20 1 1 Quantity flow kmol/ 7366 6544 5013 1707 1531 1378 (mole flow) hr Mole fractions mol/mol Carbon 0.233 0.054 0.000 0.984 0.230 0.230 dioxide (CO.sub.2) Hydrogen (H.sub.2) 0.691 0.870 1.000 0.004 0.447 0.447 Nitrogen (N.sub.2) 0.003 0.025 0.000 0.001 0.108 0.108 Carbon 0.005 0.041 0.000 0.004 0.175 0.175 monoxide (CO) Methane (CH.sub.4) 0.002 0.009 0.000 0.005 0.040 0.040 Methanol 0.000 0.000 0.000 0.002 0.001 0.001 (CH.sub.3OH) Water (H.sub.2O) 0.068 0.000 0.000 0.000 0.000 0.000

[0125] The mass balance shows that in addition to the abovementioned advantages (high rate of separated carbon dioxide) the inventive process also affords a carbon dioxide product which is dry, i.e. contains no water as an impurity, and has a high purity of 98.4 mol %. The carbon dioxide product is thus suitable for long-term storage (CCS) or may be sent to a further use without a further purification step. For example the thus obtained carbon dioxide product as a synthesis gas constituent alongside electrolysis hydrogen would be suitable for production of methanol.

[0126] A hydrogen product free from impurities is also obtained.

[0127] The mass balance further shows that the offgas discharged from the pressure swing absorption apparatus (PSA) (PSA offgas) has a surprisingly high hydrogen content and carbon dioxide content, thus making it worthwhile to recover the hydrogen present in the PSA offgas and making the carbon dioxide present in the PSA offgas available to the carbon dioxide separation.

[0128] Embodiments of the invention are described with reference to different types of subject-matter. In particular, certain embodiments are described with reference to process claims while other embodiments are described with reference to apparatus claims. However, it will be apparent to a person skilled in the art from the description hereinabove and hereinbelow that, unless otherwise stated, in addition to any combination of features belonging to one type of claim, any combination of features relating to different types of subject-matter or types of claim may also be contemplated. Features may be combined to achieve synergistic effects which go beyond simple summation of the technical features.

[0129] While the invention has been represented and described in detail in the drawing and the preceding description, such a representation and description shall be considered elucidatory or exemplary and non-limiting. The invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments may be understood and executed by those skilled in the art of the field of the claimed invention from a study of the drawing, the disclosure and the dependent claims.

LIST OF REFERENCE SYMBOLS

[0130] 1 to 41 Conduit [0131] HX-01 to HX-06 Heat exchanger [0132] HU-01 Refrigerant cooler [0133] D-01, D-02 Separator [0134] T-01, T-04 Absorption column [0135] T-02 Flash column [0136] T-03 Rectification column [0137] P-01, P-02 Pump [0138] C-01 to C-03 Compressor [0139] PS-01 Pressure swing adsorption unit (PSA)