Process for purifying crude synthesis gas to produce an acid gas and acid gas separator

Abstract

The invention relates to a gas scrubbing process for purifying crude synthesis gas with methanol as a physical absorption medium, wherein an acid gas comprising at least hydrogen sulfide (H.sub.2S) is produced. The acid gas is produced in a hot regenerator arranged downstream of an absorption apparatus and subsequently separated from gaseous methanol in an acid gas separator by cooling and condensation. The acid gas separator has a condensation region and an absorption region, wherein both regions are separated from one another by a gas-permeable tray. This has the result that impurities such as hydrogen cyanide and/or ammonia outgassing from a first acid gas substream are not reabsorbed in the condensation region of the acid gas separator, thus avoiding an accumulation of impurities in the hot regenerator or other parts of the gas scrubbing plant. The invention further relates to an acid gas separator and to the use of the acid gas separator according to the invention in a process according to the invention.

Claims

1. A process for purifying crude synthesis gas with methanol as a physical absorption medium, wherein an acid gas comprising hydrogen sulfide (H.sub.2S) is produced and the process comprises the following process steps: a. treating crude synthesis gas comprising carbon monoxide (CO), hydrogen (H.sub.2), hydrogen sulfide (H.sub.2S) and hydrogen cyanide (HCN) and/or ammonia (NH.sub.3) with methanol in an absorption apparatus to obtain a methanol laden with at least H.sub.2S and HCN and/or NH.sub.3; b. hot-regenerating the methanol laden with H.sub.2S and HCN and/or NH.sub.3 in a hot regenerator to obtain a gas mixture comprising at least methanol, H.sub.2S, HCN and/or NH.sub.3 which is withdrawn from the hot regenerator; c. cooling the gas mixture withdrawn from the hot regenerator and transferring the cooled gas mixture into an acid gas separator, wherein the acid gas separator comprises an absorption region and a condensation region, wherein the absorption region and the condensation region are separated from one another by a gas-permeable tray; d. condensing methanol from the gas mixture in the condensation region of the acid gas separator, withdrawing the condensed methanol from the acid gas separator and transferring it to the hot regenerator; e. withdrawing a first acid gas substream comprising H.sub.2S and HCN and/or NH.sub.3 from the acid gas separator; f. passing a second acid gas substream comprising H.sub.2S and HCN and/or NH.sub.3 through the absorption region of the acid gas separator, wherein HCN and/NH.sub.3 are absorbed by cryogenic methanol supplied to the absorption region of the acid gas separator, cryogenic methanol laden with HCN and/or NH.sub.3 collects in the region of the gas-permeable tray and a second acid gas substream at least partially freed of HCN and/or NH.sub.3 is obtained; g. withdrawing the second acid gas substream at least partially freed of HCN and/or NH.sub.3 from the acid gas separator; and h. withdrawing the cryogenic methanol laden with HCN and/or NH.sub.3 from the region of the gas-permeable tray of the acid gas separator and transferring the cryogenic methanol laden with HCN and/or NH.sub.3 to the hot regenerator.

2. The process according to claim 1, wherein the acid gas separator comprises: i. an absorption region and a condensation region, wherein the absorption region and the condensation region are separated from one another by a gas-permeable tray; ii. means for supplying a gas mixture comprising at least methanol, H.sub.2S and HCN and/or NH.sub.3 to the condensation region of the acid gas separator for condensation of methanol from the gas mixture in the condensation region of the acid gas separator; iii. means for withdrawing a first acid gas substream comprising H.sub.2S and HCN and/or NH.sub.3 from the acid gas separator; iv. means for absorption of HCN and/or NH.sub.3 from a second acid gas substream comprising H.sub.2S and also HCN and/or NH.sub.3 in the absorption region of the acid gas separator; v. means for withdrawing a second acid gas substream at least partially freed of HCN and/or NH.sub.3 from the acid gas separator; vi. means for supplying cryogenic methanol to the absorption region of the acid gas separator for absorption of HCN and/or NH.sub.3 in cryogenic methanol in the absorption region of the acid gas separator; vii. means for withdrawing a cryogenic methanol laden with HCN and/or NH.sub.3 from the absorption region of the acid gas separator; and viii. means for withdrawing condensed methanol from the condensation region of the acid gas separator.

3. The process according to claim 1, wherein the cryogenic methanol has a temperature of not more than −40° C., preferably not more than −50° C., particularly preferably not more than −60° C.

4. The process according to claim 1, wherein the first acid gas substream is supplied to a Claus plant for producing sulfur.

5. The process according to claim 1, wherein the condensed methanol and the cryogenic methanol laden with HCN and/or NH.sub.3 are supplied to a mixing vessel as separate streams and after mixing in the mixing vessel are recycled to the hot regenerator.

6. The process according to claim 5, wherein the gas mixture withdrawn from the hot regenerator is supplied to the mixing vessel, combined in the mixing vessel with the condensed methanol and the cryogenic methanol laden with HCN and/or NH.sub.3, wherein methanol from the gas mixture at least partially condenses to afford a biphasic mixture and the biphasic mixture containing at least partially condensed methanol is subsequently supplied to the acid gas separator.

7. The process according to claim 5, wherein the mixing vessel has at least one filling port for supplying the cryogenic methanol laden with HCN and/or NH.sub.3 and/or the condensed methanol, wherein one end of the filling port is spaced apart from a housing wall of the mixing vessel such that the cryogenic methanol laden with HCN and/or NH.sub.3 and/or the condensed methanol do not come into direct contact with the housing wall of the mixing vessel during the filling operation.

8. The process according to claim 7, wherein the housing of the mixing vessel comprises a non-alloyed or low-alloy steel as a material of construction, preferably a non-alloyed steel.

9. The process according to claim 7, wherein the filling port comprises an acid- and rust-resistant steel as a material of construction.

10. The process according to claim 1, wherein the second acid gas substream at least partially freed of HCN and/or NH.sub.3 is supplied to a reabsorber for reabsorption of H.sub.2S present in the second acid gas substream at least partially freed of HCN and/or NH.sub.3 to obtain methanol laden with H.sub.2S in the reabsorber.

11. The process according to claim 10, wherein the methanol laden with H.sub.2S is supplied to the hot regenerator.

12. The process according to claim 10, wherein the cryogenic methanol is supplied to the absorption region of the acid gas separator from the reabsorber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features, advantages and possible applications of the invention are also apparent from the following description of a working and numerical example and from the drawings. All the features described and/or depicted, on their own or in any combination, form the subject matter of the invention, irrespective of their combination in the claims or their dependency references.

(2) In the figures

(3) FIG. 1 shows a schematic flow diagram of a prior art process and a plant for purifying crude synthesis gas to produce an acid gas; and

(4) FIG. 2 shows a schematic flow diagram of an inventive process and a plant for purifying crude synthesis gas to produce an acid gas using an inventive acid gas separator.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 shows a process/a plant 100 for purifying crude synthesis gas with methanol as a physical absorption medium, wherein an acid gas comprising hydrogen sulfide (H.sub.2S) is produced and supplied to a Claus plant for producing elemental sulfur.

(6) Via a conduit 101 an absorption apparatus A01 is supplied at a pressure of 40 bar with a crude synthesis gas which contains as desired constituents carbon monoxide (CO) and hydrogen (H.sub.2) and as undesired constituents to be removed hydrogen sulfide (H.sub.2S), carbon dioxide (CO.sub.2) and hydrogen cyanide (HCN). In the absorption apparatus A01 H.sub.2S, CO.sub.2 and HCN are removed by treatment with cold methanol as the absorption medium. Purified synthesis gas exits absorption apparatus A01 via a conduit 102. Methanol laden with H.sub.2S and CO.sub.2 is withdrawn from absorption apparatus A01 via a conduit 103 and supplied to a reabsorber R01. Furthermore, methanol laden with HCN and methanol laden with CO.sub.2 are withdrawn from the absorption apparatus as further streams. Methanol laden with HCN, also known as prewash methanol, is supplied directly to the hot regenerator H01 (not shown). After passing through one or more depressurization stages for removing CO.sub.2 (flashing) methanol laden with CO.sub.2 is likewise supplied to the hot regenerator H01 (not shown). Methanol regenerated by flashing with intrinsic vapour in hot regenerator H01 is withdrawn via a conduit 104 and via a pump P01 and a conduit 117 is supplied to the absorption apparatus A01 for renewed absorption of undesired constituents from synthesis gas.

(7) In reabsorber R01 CO.sub.2 is expelled as inert gas from methanol laden with CO.sub.2 and H.sub.2S by flashing with nitrogen. Coexpelled hydrogen sulfide is likewise reabsorbed by methanol in reabsorber R01. Thus obtained methanol laden with H.sub.2S is supplied via a conduit 118 to hot regenerator H01 for removal of H.sub.2S. In the hot regenerator H01 heating (boiling) of methanol laden with H.sub.2S and HCN affords a gas mixture which comprises methanol vapours, H.sub.2S and HCN and is withdrawn from hot regenerator H01 via a conduit 105. After cooling of the gas mixture withdrawn from hot regenerator H01 in a heat exchanger W01 to about 40° C. the mixture, now biphasic due to partial condensation of methanol, of H.sub.2S, HCN, methanol vapour and liquid methanol is supplied to a mixing vessel M01 via a conduit 106. This mixture is withdrawn from the mixing vessel M01 via a conduit 107 and after further cooling in an indirect heat exchanger W11 and an indirect heat exchanger W21 supplied via the conduits 107, 108 and 109 to an acid gas separator S01. Acid gas separator S01 has a condensation region KB01 (lower part) and an absorption region AB01 (upper region), wherein absorption region AB01 comprises a fixed bed of packing bodies to increase the internal surface area. Upon entry into acid gas separator S01 the mixture of methanol, H.sub.2S and HCN has a temperature of about −36° C. In acid gas separator S01 the gas phase composed of H.sub.2S and HCN is separated from the methanol liquid phase in the condensation region KB01. A first acid gas substream thus obtained composed of H.sub.2S and HCN is withdrawn from acid gas separator S01 via a conduit 110, used for cooling the mixture supplied from the conduit 107 in heat exchanger W11 and subsequently supplied via a conduit 111 to a Claus plant C01 for recovery of elemental sulfur. The mixture in conduit 111 has a temperature of about 25° C.

(8) A second acid gas substream is passed through absorption region AB01 of the acid gas separator S01 to absorb HCN from the second acid gas substream with cryogenic methanol. Cryogenic methanol is supplied to the absorption region AB01 of the acid gas separator S01 from reabsorber R01 via a conduit 112 and has a temperature of about −63° C. Acid gas of the second acid gas substream freed of HCN and now comprising primarily H.sub.2S is supplied via a conduit 113 to reabsorber R01, thus allowing H.sub.2S to be retained in the circuit and sent back to hot regenerator H01 with the methanol stream in conduit 104.

(9) Condensed methanol from acid gas separator S01 is supplied via a conduit 114 to mixing vessel M01 and therein combined with partially condensed methanol and gas mixture from conduit 106 and at least partially mixed, thus effecting a continuous temperature equalization between the components present in mixing vessel M01. The temperature equalization brought about in mixing vessel M01 prevents unintentional outgassing through too fast or uncontrolled heating which can cause problems during the recycling of the methanol to hot regenerator H01 via the conduits 115 and 116 and pump P11. Regenerated methanol produced in hot regenerator H01 is withdrawn as mentioned above via conduit 104 and via pump P01 and conduit 117 is supplied to the absorption apparatus A01 for renewed absorption of undesired constituents from synthesis gas.

(10) The process according to FIG. 1 has the problem that in acid gas separator S01 methanol cooled to −63° C. supplied from reabsorber R01 via conduit 112 can also absorb HCN from the first acid gas substream. However this HCN gas is destined for discharging in the direction of a Claus plant. As a result, accumulations of HCN in the hot regenerator H01 and further problems already mentioned hereinabove can disadvantageously occur.

(11) FIG. 2 shows a process 200 according to the invention for purifying crude synthesis gas with methanol as a physical absorption medium, wherein an acid gas comprising hydrogen sulfide (H.sub.2S) is produced and supplied to a Claus plant for producing elemental sulfur. FIG. 2 further shows an acid gas separator according to the invention and the use thereof in the process according to the invention.

(12) Via a conduit 201 an absorption apparatus A02 is supplied at a pressure of 40 bar with a crude synthesis gas which contains as desired constituents carbon monoxide (CO) and hydrogen (H.sub.2) and as undesired constituents to be removed hydrogen sulfide (H.sub.2S), carbon dioxide (CO.sub.2) and hydrogen cyanide (HCN). In absorption apparatus A02 H.sub.2S, CO.sub.2 and HCN are removed by treatment with cold methanol as the absorption medium. Purified synthesis gas exits absorption apparatus A02 via a conduit 202. Methanol laden with H.sub.2S and CO.sub.2 is withdrawn from absorption apparatus A02 via a conduit 203 and supplied to a reabsorber R02. Furthermore, methanol laden with HCN and methanol laden with CO.sub.2 are withdrawn from the absorption apparatus as further streams. Methanol laden with HCN, also known as prewash methanol, is supplied directly to the hot regenerator H02 (not shown). After passing through one or more depressurization stages for removing CO.sub.2 (flashing) methanol laden with CO.sub.2 is likewise supplied to the hot regenerator H02. Methanol regenerated by flashing with intrinsic vapour in hot regenerator H02 is withdrawn via a conduit 204 and via a pump P02 and a conduit 217 is supplied to the absorption apparatus A02 for renewed absorption of undesired constituents from synthesis gas.

(13) In reabsorber R02 CO.sub.2 is expelled as inert gas from methanol laden with CO.sub.2 and H.sub.2S by flashing with nitrogen. Coexpelled hydrogen sulfide is likewise reabsorbed by methanol in reabsorber R02. Thus obtained methanol laden with H.sub.2S is supplied via a conduit 219 to hot regenerator H02 for removal of H.sub.2S. In hot regenerator H02 heating (boiling) of methanol laden with H.sub.2S and HCN affords a gas mixture which comprises methanol vapours, H.sub.2S and HCN and is withdrawn from hot regenerator H02 via a conduit 205. After cooling of the gas mixture withdrawn from hot regenerator H02 in a heat exchanger W02 to about 40° C. the mixture, now biphasic due to partial condensation of methanol, of H.sub.2S, HCN, methanol vapour and liquid methanol is supplied to a mixing vessel M02 via a conduit 206. This mixture is withdrawn from mixing vessel M02 via a conduit 207 and after further cooling in an indirect heat exchanger W12 and an indirect heat exchanger W22 supplied via the conduits 207, 208 and 209 to an acid gas separator S02 according to the invention.

(14) Acid gas separator S02 has a condensation region KB02 (lower part) and an absorption region AB02 (upper region), wherein absorption region AB02 comprises a fixed bed of packing bodies to increase the internal surface area. According to the invention absorption region AB02 and condensation region KB02 are separated from one another by a gas-permeable tray GB02. Gas-permeable tray GB02 is in the form of a chimney tray in the example shown. The housing part of the absorption region AB02 of acid gas separator S02 is fabricated from a rust-resistant steel while the housing part of the condensation region KB02 of the acid gas separator S02 is fabricated from a low-alloy steel.

(15) Upon entry into acid gas separator S02 the mixture of methanol, H.sub.2S and HCN has a temperature of about −36° C. In acid gas separator S02 the gas phase composed of H.sub.2S and HCN is separated from the methanol liquid phase in the condensation region KB02. A first acid gas substream thus obtained composed of H.sub.2S and HCN is withdrawn from acid gas separator S02 via a conduit 210, used for cooling the mixture supplied from the conduit 207 in heat exchanger W12 and subsequently supplied via a conduit 211 to a Claus plant CO2 for recovery of elemental sulfur. The mixture in conduit 211 has a temperature of about 25° C.

(16) A second acid gas substream is passed from the condensation region KB02 through gas-permeable tray GB02 from bottom to top via absorption region AB02 of the acid gas separator S02 to absorb HCN from the second acid gas substream with cryogenic methanol. Cryogenic methanol laden with HCN collects as a liquid level (not shown) on gas-permeable tray GB02. Methanol laden with HCN which has collected on the gas-permeable tray GB02 in the form of a chimney tray is supplied via a conduit 218 and a filling port ST02 to mixing vessel M02. As a result of its end being spaced apart from the housing wall of the mixing vessel M02 the filling port ST02 is arranged such that cryogenic methanol laden with HCN does not come into contact with the housing wall of mixing vessel M02 during the filling operation. The housing wall of mixing vessel M02 is fabricated from a low-alloy steel while filling port ST02 is fabricated from a higher-alloy, acid- and rust-resistant steel. As a result of arrangement of the gas-permeable tray GB02 in acid gas separator S02 and the recycling of methanol laden with HCN via conduit 218 and filling port ST02 into mixing vessel M02 and then hot regenerator H02, cryogenic methanol withdrawn from the gas-permeable tray GB02 does not come into contact with HCN gas present in the condensation region KB02. This means that a reabsorption of HCN from the second acid gas stream into methanol in the condensation region KB02 of acid gas separator S02 is not possible. There is consequently no accumulation of HCN in hot regenerator H02 or other component parts of the gas scrubbing plant.

(17) Cryogenic methanol used for absorption of HCN in absorption region AB02 is supplied via a conduit 212 from reabsorber R02 to the absorption region AB02 of the acid gas separator S02. Said methanol has a temperature of about −63° C. Acid gas of the second acid gas substream freed of HCN and now comprising primarily H.sub.2S is supplied via a conduit 213 to reabsorber R02, thus allowing H.sub.2S to be retained in the circuit and sent back to hot regenerator H02 with the methanol stream in conduit 204.

(18) Condensed methanol from acid gas separator S02 is supplied via a conduit 214 to mixing vessel M02 via a filling port ST12 fabricated from rust-resistant steel and there mixed with partially condensed methanol and gas mixture from conduit 206, thus resulting in a continuous temperature equalization between the components present in mixing vessel M02. Similarly to the end of filling port ST02 the end of filling port ST12 is spaced apart from the wall of the mixing vessel M02. The temperature equalization brought about in mixing vessel M02 prevents unintentional outgassing through too fast or uncontrolled heating which can cause problems during the recycling of the methanol to hot regenerator H02 via the conduits 215 and 216 and pump P12. Regenerated methanol produced in hot regenerator H02 is withdrawn as mentioned above via conduit 204 and via pump P01 and conduit 217 is supplied to absorption apparatus A01 for renewed absorption of undesired constituents from synthesis gas.

(19) Advantages of the process according to the invention and of the acid gas separator according to the invention are further elucidated by the following numerical example. The NH.sub.3, HCN, H.sub.2S and COS content in the gas mixture leaving the acid gas separator in the direction of the Claus plant via the conduits 111 or 211 and the content of the abovementioned gases in the methanol condensed in the acid gas separator which is recycled to the mixing vessel (M01 or M02) via the conduits 114, 214 and 218 were examined. The values shown in the following table were calculated in mol % in the course of a simulation using the software “Aspen Plus” and normalized to 100% for the comparative example so that values according to the invention show the mole fraction variation based on the normalized 100% value.

(20) TABLE-US-00001 Acid gas to Claus plant Condensed methanol Example Comparative Example Comparative (invention) example (invention) example Mole fraction variation H.sub.2S 100% 100% 98% 100% COS 100% 100% 99% 100% NH.sub.3 114% 100% 57% 100% HCN 138% 100% 74% 100%

(21) The example shows that compared to the known process according to the comparative example the inventive process and the inventive acid gas separator have the result that 14% more NH.sub.3 and 38% more HCN pass into the Claus plant. Simultaneously, the proportion of these substances in the condensed methanol is advantageously reduced by 43% (NH.sub.3) and 26% (HCN).

(22) 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.

(23) While the invention has been represented and described in detail in the drawings and the preceding description, such 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 carried out by those skilled in the art of the field of the claimed invention through study of the drawings, the disclosure and the dependent claims.

(24) In the claims the word “having” or “comprising” does not exclude further elements or steps and the indefinite article “a” does not exclude a plurality. Reference numerals in the claims should not be interpreted as limiting the scope of the claims.

(25) 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.

(26) The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

(27) “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.

(28) “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.

(29) 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.

(30) 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.

(31) 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

(32) 100 Process and plant

(33) 101 to 118 Conduit

(34) A01 Absorption apparatus

(35) AB01 Absorption region

(36) C01 Claus plant

(37) H01 Hot regenerator

(38) KB01 Condensation region

(39) M01 Mixing vessel

(40) P01, P11 Pump

(41) R01 Reabsorber

(42) S01 Acid gas separator

(43) W01, W11, W21 Heat exchanger

(44) 200 Process and plant

(45) 201 to 219 Conduit

(46) A02 Absorption apparatus

(47) AB02 Absorption region

(48) C02 Claus plant

(49) GB02 Gas-permeable tray

(50) H02 Hot regenerator

(51) KB02 Condensation region

(52) M02 Mixing vessel

(53) P02, P12 Pump

(54) R02 Reabsorber

(55) S02 Acid gas separator

(56) ST02, ST12 Filling port

(57) W02, W12, W22 Heat exchanger