PROCESS FOR REMOVAL OF ACIDIC GAS CONSTITUENTS FROM SYNTHESIS GAS AND APPARATUS FOR PRODUCING RECYCLE GAS

Abstract

The invention relates to a process and a plant for removal of acidic gas constituents from synthesis gas by absorption in a physical scrubbing medium. A first scrubbing medium laden with at least carbon dioxide (CO.sub.2) is supplied to a decompression vessel for depressurization from a first absorption apparatus via a first feed conduit. Furthermore, a second scrubbing medium laden with at least carbon dioxide (CO.sub.2) is supplied to the decompression vessel for depressurization from a second absorption apparatus via a second feed conduit. The carbon dioxide concentration in the first laden scrubbing medium is higher than the carbon dioxide concentration in the second laden scrubbing medium. According to the invention it is provided that an at least partial commixing of the first and the second laden scrubbing medium is effected in the decompression vessel, wherein the commixing and depressurization affords a partially regenerated scrubbing medium and the first feed conduit and the second feed conduit are arranged such that carbon dioxide desorbed from the first laden scrubbing medium by the depressurization is at least partially absorbed by the second laden scrubbing medium.

Claims

1. A process for removal of acidic gas constituents from synthesis gas by absorption of the acidic gas constituents in a physical scrubbing medium, the process comprising the steps of: supplying a first scrubbing medium laden with at least carbon dioxide (CO.sub.2) to a decompression vessel for depressurization from a first absorption apparatus via a first feed conduit; supplying a second scrubbing medium laden with at least carbon dioxide (CO.sub.2) to the decompression vessel for depressurization from a second absorption apparatus via a second feed conduit, wherein the carbon dioxide concentration in the first laden scrubbing medium is higher than the carbon dioxide concentration in the second laden scrubbing medium, wherein: an at least partial commixing of the first and the second laden scrubbing medium is effected in the decompression vessel, wherein the commixing and depressurization affords a partially regenerated scrubbing medium, and the first feed conduit and the second feed conduit are arranged such that carbon dioxide desorbed from the first laden scrubbing medium by the depressurization is at least partially absorbed by the second laden scrubbing medium.

2. The process according to claim 1, wherein a first synthesis gas stream is supplied to the first absorption apparatus and a second synthesis gas stream is supplied to the second absorption apparatus, wherein the carbon dioxide content in the first synthesis gas stream is higher than the carbon dioxide content in the second synthesis gas stream.

3. The process according to claim 2, wherein the first synthesis gas stream has been subjected to a water-gas shift reaction and the second synthesis gas stream has not been subjected to a water-gas shift reaction.

4. The process according to claim 2, wherein the first synthesis gas stream has been partially subjected to a water-gas shift reaction and the second synthesis gas stream has not been subjected to a water-gas shift reaction.

5. The process according to claim 2, wherein the first synthesis gas stream has been subjected to a water-gas shift reaction and the second synthesis gas stream has been partially subjected to a water-gas shift reaction.

6. The process according to claim 1, wherein gases desorbed in the decompression vessel are recompressed and after compression are recycled to the first and/or the second synthesis gas stream as recycle gases.

7. The process according to claim 6, wherein the carbon dioxide content in the recycle gas is not more than 65 mol %, preferably not more than 50 mol %, particularly preferably not more than 35 mol % and more preferably not more than 20 mol %.

8. The process according to claim 1, wherein the first feed conduit is arranged below the second feed conduit.

9. The process according to claim 1, wherein the carbon dioxide concentration in the first laden scrubbing medium is at least 5 mol % higher than in the second laden scrubbing medium, preferably at least 10 mol % higher, particularly preferably at least 15 mol % higher and more preferably at least 20 mol % higher.

10. The process according to claim 1, wherein the partially regenerated scrubbing medium is withdrawn from the decompression vessel and sent to at least one further depressurization stage.

11. The process according to claim 10, wherein a decompression gas withdrawn from the at least one further decompression stage is passed to the decompression vessel via a third feed conduit, wherein the third feed conduit is arranged such that carbon dioxide present in the decompression gas is at least partially absorbed by the second laden scrubbing medium.

12. The process according to claim 11, wherein the third feed conduit is arranged below the second feed conduit.

13. The process according to claim 11, wherein the third feed conduit is arranged below the first and second feed conduit.

14. The process according to claim 1, wherein the physical scrubbing medium is selected from at least one element of the group comprising methanol, 1-methyl-2-pyrrolidone (NMP), an aqueous solution of 1-methyl-2-pyrrolidone (NMP), a mixture of dimethyl ethers of polyethylene glycol, and propylene carbonate (4-methyl-1,3-dioxolane-2-one).

15. The process according to claim 14, wherein the physical scrubbing medium is methanol.

16. An apparatus for producing a recycle gas from a scrubbing medium laden with at least carbon dioxide (CO.sub.2) comprising a decompression vessel for depressurization of a first and second scrubbing medium laden with at least carbon dioxide (CO.sub.2); means for discharging recycle gases producible in the depressurization from the decompression vessel; means for discharging the partially regenerated scrubbing medium producible in the depressurization from the decompression vessel; a first feed conduit comprising a first pressure reduction element for supplying the first laden scrubbing medium to the decompression vessel; a second feed conduit comprising a second pressure reduction element for feeding the second laden scrubbing medium to the decompression vessel; characterized in that the first feed conduit and the second feed conduit are arranged such that in the decompression vessel on account of a difference in the carbon dioxide concentration between the first and the second laden scrubbing medium carbon dioxide desorbable from the first laden scrubbing medium is absorbable by the second laden scrubbing medium.

17. The apparatus according to claim 16, wherein the first feed conduit is arranged below the second feed conduit.

18. The apparatus according to claim 16, wherein the carbon dioxide concentration in the first laden scrubbing medium is higher than the carbon dioxide concentration in the second laden scrubbing medium.

19. The apparatus according to claim 16, wherein the apparatus has a third feed conduit for supplying a decompression gas from an at least one further decompression stage to the decompression vessel, wherein the third feed conduit is arranged such that the carbon dioxide present in the decompression gas from the at least one further decompression stage is at least partially absorbable by the second laden scrubbing medium.

20. A plant for removal of acidic gas constituents from synthesis gas by absorption of the acidic gas constituents in a physical scrubbing medium containing a first absorption apparatus and a second absorption apparatus, each for removal of acidic gas constituents from synthesis gas, and an apparatus for producing a recycle gas according to claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The invention is more particularly elucidated herein below by way of an example without in any way limiting the subject matter of the invention. Further features, advantages and possible applications of the invention will be apparent from the following description of the working example in connection with the drawing and a numerical example.

[0075] The Figure shows a schematic diagram of an exemplary embodiment 100 of the process according to the invention/of the apparatus according to the invention as part of a plant according to the invention with methanol as the scrubbing medium.

DETAILED DESCRIPTION OF THE INVENTION

[0076] Converted synthesis gas (crude synthesis gas), i.e. synthesis gas reacted in a water-gas shift reaction, enters via conduit 101 into the indirect heat exchanger E01, is cooled therein and is sent via conduit 102 to the absorption apparatus T01. In absorption apparatus T01 the synthesis gas is scrubbed at high pressure (about 55 bar) by the methanol entering via the conduits 103, 104 and 127 and respectively connected liquid distributors. This affords a purified synthesis gas which exits absorption apparatus T01 via the conduits 105 and 106 while intermediately being warmed in the indirect heat exchanger E01. The absorption apparatus T01 has dedicated, i.e. separate, regions for removal of different impurities in the synthesis gas. The individual regions are separated from one another by chimney trays in T01. In a lower prescrubbing region of T01 hydrogen cyanide (HCN) and other trace impurities are removed from the synthesis gas by the methanol entering via conduit 128. Laden methanol resulting therefrom is withdrawn in the bottom region of the absorption apparatus T01 and via conduit 107 sent to an apparatus H02. H02 serves for hot regeneration of laden methanol and removal of water from hot-regenerated methanol. Primarily taking place in the middle region of the absorption apparatus T01 above the lower chimney tray is a desulfurization effected by the methanol supplied via conduit 108 and the liquid distributor connected thereto. The desulfurization removes hydrogen sulfide (H.sub.2S) and carbonyl sulfide (COS) from the synthesis gas. The methanol supplied via conduit 108 contains carbon dioxide (CO.sub.2) since it has already been used for removal of carbon dioxide in the upper region of the absorption apparatus above the upper chimney tray. Methanol resulting therefrom laden primarily with hydrogen sulfide and carbon dioxide is withdrawn from absorption apparatus T01 via conduit 109 and sent as the first laden scrubbing medium to the decompression vessel T03 via a depressurization valve (not shown). Conduit 109 is therefore a first feed conduit in the context of the invention. Decompression vessel T03 is operated at intermediate pressure (15 to 40 bar) and valuable gases absorbed in T01 (carbon monoxide, hydrogen) are desorbed by the pressure reduction in decompression vessel T03. In the upper region of the absorption apparatus T01, carbon dioxide is removed by scrubbing with methanol entering into the upper region of the absorption apparatus T01 via the conduits 103, 104 and 127 and respectively connected liquid distributors. The methanol laden with carbon dioxide subsequently passes via conduit 110 into the depressurization apparatus H01 which is operated at a lower pressure than T03 and in which carbon dioxide is removed from the laden methanol by depressurization (flashing). H01 comprises a plurality of depressurization stages, i.e. a plurality of decompression vessels connected in series, the pressure decreasing from vessel to vessel.

[0077] Operated simultaneously with the absorption apparatus T01 processing converted synthesis gas is absorption apparatus T02 in which unconverted (unshifted) synthesis gas is subjected to a gas scrubbing. Unconverted synthesis gas (crude synthesis gas) enters via conduit 111 into the indirect heat exchanger E02, is cooled therein and is sent via conduit 112 to absorption apparatus T02. The pressure in the absorption apparatus T02 is somewhat higher (57 bar) than the pressure in the absorption apparatus T01 (56 bar). Trace constituents such as hydrogen cyanide are removed in the lower part of the absorption apparatus T02 by methanol entering via conduit 113 and the liquid distributor connected thereto. Methanol laden with trace constituents is withdrawn via conduit 114 and sent via conduit 107 to the apparatus H02 for hot regeneration of the methanol. Synthesis gas in the absorption apparatus T02 then ascends further through the lower chimney tray upwards into the middle part of the absorption apparatus T02 in order therein to be subjected to a desulfurization. The desulfurization removes hydrogen sulfide and carbonyl sulfide from the synthesis gas. To this end the middle region of T02 is supplied via conduit 115 with methanol that has previously been laden with carbon dioxide in the upper region of T02. The methanol laden with sulfur components and carbon dioxide is subsequently sent via conduit 116 and a depressurization valve (not shown) to the upper region of the decompression vessel T03. Conduit 116 is therefore a second feed conduit in the context of the invention. Decompression vessel T03 is operated at intermediate pressure (15 to 40 bar) and valuable gases unintentionally absorbed in absorption apparatus T02 (carbon monoxide, hydrogen) are desorbed by the pressure reduction in decompression vessel T03. Laden methanol supplied via conduit 116 has a lower carbon dioxide concentration than laden methanol supplied via conduit 109.

[0078] Synthesis gas from the middle region of the absorption apparatus T02 ascends further through the upper chimney tray upwards into the upper region of the absorption apparatus T02. As described hereinabove it is essentially carbon dioxide that is removed therein. This is effected by the methanol supplied via conduit 117 which after depressurization in depressurization apparatus H01 is laden with only a little carbon dioxide. This is also effected by the methanol recovered by hot regeneration in apparatus H02 which is free from carbon dioxide residues. Hot-regenerated methanol is sent to the absorption apparatus T02 via conduit 118.

[0079] Methanol laden with hydrogen sulfide and carbon dioxide from absorption apparatus T02 is less heavily laden with carbon dioxide, i.e. has a lower carbon dioxide concentration, than methanol laden with hydrogen sulfide and carbon dioxide from absorption apparatus T01. This is because converted synthesis gas, which from the outset has a significantly higher carbon dioxide content than unconverted synthesis gas, is scrubbed in the absorption apparatus T01. The higher carbon dioxide content has a corresponding effect on the carbon dioxide concentration in the respective laden scrubbing medium (methanol).

[0080] Methanol laden with hydrogen sulfide and carbon dioxide from absorption apparatus T02 (unconverted synthesis gas) passes via conduit 116 into the upper part of decompression vessel T03. Methanol laden with hydrogen sulfide and carbon dioxide from absorption apparatus T01 passes via conduit 109 into the lower part of decompression vessel T03. The port for conduit 116 (the second feed conduit) is arranged above the port for conduit 109 (the first feed conduit) (ports not shown). An at least partial commixing of the laden scrubbing medium streams takes place in decompression vessel T03 and the depressurization to intermediate pressure desorbs valuable gases (carbon monoxide, hydrogen) and a portion of the absorbed carbon dioxide as decompression gases. Since the scrubbing medium stream from absorption apparatus T02 has a lower carbon dioxide concentration than the scrubbing medium stream from the absorption apparatus T01 the scrubbing medium stream from absorption apparatus T02 is supplied above the scrubbing medium stream from absorption apparatus T01 and carbon dioxide desorbed from the laden methanol supplied via conduit 109 can therefore be absorbed by the laden methanol supplied via conduit 116. Carbon dioxide desorbed in the lower part of the decompression vessel T03 is thus rescrubbed by methanol supplied in the upper region of the decompression vessel.

[0081] At the same time methanol laden with carbon dioxide is sent via conduit 110 to the decompression apparatus H01. The desorbed gases obtained in a first decompression stage of H01 and also containing co-absorbed valuable gases are sent via conduit 119 to the middle part of the decompression vessel T03. Carbon dioxide is additionally absorbed, i.e. rescrubbed, by the methanol supplied via conduit 116. The valuable gases altogether desorbed in T03, together with non-rescrubbed carbon dioxide, pass via conduit 120 into the recycle gas compressor K01 and therein are compressed to the pressure prevailing in absorption apparatus T02. After compression in K01 the recycle gases pass via conduit 121 to the synthesis gas stream in conduit 111, are cooled in the indirect heat exchanger E02 and sent via conduit 112 to the absorption apparatus T02.

[0082] In an alternative to the described example the recycle gases may also be recycled to the absorption apparatus T01 depending on the gas composition ultimately desired.

[0083] Further decompression stages of the depressurization apparatus H01 produce a high-purity carbon dioxide product stream and a carbon dioxide-containing offgas withdrawn via conduits 122 and 123.

[0084] The methanol laden with hydrogen sulfide and carbon dioxide withdrawn in the bottom region of T03 is sent via conduit 127 to depressurization apparatus H01. Methanol laden primarily with sulfur constituents obtained at low pressure in T03 is sent via conduit 124 to apparatus H02 for hot regeneration to effect desulfurization. In apparatus H02 a gas containing primarily hydrogen sulfide is produced and water is removed from methanol. Hydrogen sulfide is withdrawn from apparatus H02 via conduit 125 and water via conduit 126. The gas comprising primarily hydrogen sulfide, also known as acid gas, may after condensative removal of the methanol present therein be sent to a Claus plant for sulfur synthesis (not shown).

[0085] The advantages of the invention are more particularly elucidated in the comparative example which follows. The table which follows shows recycle gas compositions from a gas scrubbing process with methanol in which a substantially completely converted synthesis gas (shifted gas) and an unconverted (unshifted) synthesis gas have been subjected to a gas scrubbing in dedicated absorption apparatuses according to the above example. The example according to the invention compares the obtained recycle gas composition to a comparative example in which the scrubbing medium streams from the absorption apparatuses have been initially completely commixed and subsequently subjected to a customary depressurization (flashing).

TABLE-US-00001 Example Comparative (invention) example Difference CO.sub.2 content in 31.0 mol % 57.5 mol % 26.5 mol % recycle gas CO content in 27.2 mol % 15.8 mol % +11.4 mol % recycle gas H.sub.2 content 40.7 mol % 26.1 mol % +14.6 mol % in recycle gas H.sub.2 + CO 99.701% 99.673% +0.03% recovery for overall plant Recycle gas 309 kW 424 kW 115 kW (27%) compressor power consumption Cooling power 8400 kW 8578 kW 178 kW (2%) consumption

[0086] The inventive example specifies a recycle gas having a content of 31.0 mol % of carbon dioxide which corresponds to an absolute improvement of 26.5 mol % compared to the value for the comparative example (57.5 mol %). In relative terms the process according to the invention reduces the carbon dioxide content in the recycle gas by about 46% and thus represents a significant improvement. This advantageously reduces the power consumption of the recycle gas compressor by 115 kW or in relative terms 27%. Simultaneously the power consumption of the cooling unit is advantageously reduced by 115 kW or in relative terms by 2% since the absorption apparatuses have a reduced amount of recycled gases to absorb and accordingly a reduced heat of absorption is generated. Simultaneously, the lower carbon dioxide content in the recycle gas means that the content of the valuable gases carbon monoxide and hydrogen advantageously increases by 11.4 mol % and by 14.6 mol % respectively.

[0087] The following table shows the individual concentrations of the components in the laden methanol obtained for the example and the comparative example. The left-hand column shows values for laden methanol from the gas scrubbing of the unconverted synthesis gas while the right-hand column shows values for laden methanol from the gas scrubbing of the substantially completely converted synthesis gas. The pressure during the gas scrubbing in the absorption apparatus was 56.7 bar in the case of the unconverted synthesis gas while the pressure was 55.3 bar in the case of the converted synthesis gas.

TABLE-US-00002 Laden methanol from Laden methanol from absorption apparatus absorption apparatus for unconverted for converted synthesis gas synthesis gas CO.sub.2 concentration 21.86 mol % 40.22 mol % COconcentration 1.27 mol % 0.05 mol % H.sub.2 concentration 0.21 mol % 0.53 mol % H.sub.2S concentration 0.49 mol % 0.27 mol % COSconcentration 0.00 mol % 0.00 mol %

[0088] The laden methanol having the composition of the left-hand column is an example of a second laden scrubbing medium in the context of the invention. The laden methanol having the composition of the right-hand column is an example of a first laden scrubbing medium in the context of the invention. The laden methanol according to the right-hand column was obtained from a second absorption apparatus in the context of the invention and has a markedly higher carbon dioxide concentration than the laden methanol according to the left-hand column which was obtained from a first absorption apparatus in the context of the invention.

[0089] 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 claim type, any combination of features relating to different types of subject matter or claim types may also be contemplated. All features may be combined to achieve synergistic effects which go beyond simple summation of the technical features.

[0090] While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

[0091] The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

[0092] Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

[0093] Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

[0094] Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

[0095] Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

[0096] All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.

LIST OF REFERENCE NUMERALS

[0097] 100 Process or plant according to the invention

[0098] 101 to 127 Conduit

[0099] E01 Indirect heat exchanger

[0100] E02 Indirect heat exchanger

[0101] H01 Depressurization apparatus

[0102] H02 Apparatus for hot regeneration

[0103] K01 Recycle gas compressor

[0104] T01 Absorption apparatus

[0105] T02 Absorption apparatus

[0106] T03 Decompression vessel