SELECTIVE ABSORPTION OF HYDROGENE SULFIDE FROM GASES WHICH ALSO CONTAIN CARBON DIOXIDE

Abstract: A substantially anhydrous solution of a tertiary amine and/or a hindered amine is used for the selective absorption of hydrogen sulfide from a gaz stream (10) which also contains carbon dioxide. A physical solvent may be used as a diluent for the amine. The hydrogen sulfide-containing absorption solution is passed to the stripping column (24) and the stripped absorption solution is recycled back to the absorber (10).

Description: SELECTIVE ABSORPTION OF HYDROGEN SULFIDE FROM GASES WHICH ALSO CONTAIN CARBON DIOXIDE Background of the Invention The present invention is directed to an improved process for the selective absorption of hydrogen sulfide (H2S) from gas streams, such as natural or synthesis gas streams, which also contain carbon dioxide (CO2). It is known that hydrogen sulfide and carbon dioxide can be absorbed by the use of aqueous solutions of alkanolamines and sterically hindered amines. Use of tertiary alkanolamines and sterically hindered amines such as triethanolamine (TEA) and methyldiethanolamine (MDEA) , as well as certain sterically hindered amines, renders the system partially selective for hydrogen sulfide in the presence of carbon dioxide. Partial selectivity is obtained because tertiary amines are unable to form carbamates by direct an fast reaction with carbon dioxide, and sterically hindered amines form unstable carbamates. However, both types of amines form carbonates and bicarbonates in aqueous solutions, although at a slow rate, as carbon dioxide has to react first with water, forming carbonic acid, before reaction with the amines can take place. Hydrogen sulfide, on the other hand, reacts directly, at a fast rate with the amines, forming sulfides and hydrosulfides. The actual selectivity for hydrogen sulfide of such aqeuous absorp¬ tion solutions is therefore based on the difference in rate of reaction between hydrogen sulfide with the amine and carbon dioxide with the amine. While the rate of reaction of the amine with hydrogen sulfide is faster than its effective rate of reaction with carbon dioxide, there are still formed considerable amounts of the reaction products of carbonates and bicarbonates with the amine, reducing its effectiveness as a selective absorbent for hydrogen sulfide, and requiring an expensive measure to break the formed chemical bonds to regenerate the amine. It would be desirable to provide an effective process for removal of hydrogen sulfide from gas streams containing carbon dioxide that does not involve chemical reaction of carbon dioxide, thereby increasing the effectiveness of the amine to remove hydrogen sulfide from the gas stream. Summary of the Invention According to the present invention, there is provided a process for removal of hydrogen sulfide from gas streams comprising hydrogen sulfide and carbon dioxide by contacting the gas stream with a substantially anhydrous absorption solution comprising at least one amine compound selected from the group consisting of tertiary amines and hindered amines, present in an amount sufficient to absorb the hydrogen sulfide contained in the gas stream at a temperature above the dew point of water in the gas stream, to selectively remove hydrogen sulfide substantially to the essential exclusion of carbon dioxide, and stripping the absorbed hydrogen sulfide from the substantially anhydrous absorption solution to regenerate the amine for recycle. Preferably, the process is carried out at ambient temperature and atmospheric pressure or above, with the temperature preferably ranging from ambient to about 15ϋ°F. If operated at elevated pressure, physically absorbed carbon dioxide can be desorbed effectively by pressure reduction after the absorption stage. While the absorption solution may be 100% amine, it is convenient to provide a solution diluted with a low-cost diluent for the amine, which diluent has only physical absorptivity for carbon dioxide and other constituents of the gas such as COS, CS2 and mercaptans, and does not chemically bind carbon dioxide. The physical absorptivity of the diluent is preferably about 5 volumes or less carbon dioxide per volume of diluent at a temperature of 77°F and a partial pressure of CO2 of one atmosphere. However, if more removal of CO2 from the feed gas is desired, solvents of higher absorptivity for CO2 may be employed. Moreover, the ratio of the amount of physical solvent in the mixture to the amount of amine in the mixture can be adjusted so that the required amount of CO2 and H2S are absorbed simultaneously from the gas undergoing treatment. The pressure of the treating solution is then reduced and the absorbed C0 2 and other physically absorbed gases are flashed from the absorbant solution. The absorbed H2 which is chemically bonded to the amine or other H2 reactant, is not desorbed until the solution is stripped in the heated stripping column. The H2 and CO2 are thereby effectively separated for more economical processing of the H2S in a Claus plant. The absorption solution can contain up to about 5% by weight water, for, at this concentration the rate of absorption of carbon dioxide by the water, for forming carbonates and bicarbonates, is very low. On a positive basis, the small amount of water may be beneficially used as a stripping vapor in the stripping of hydrogen sulfide from the absorption solution. In the process of the instant invention, the amine chemically absorbs hydrogen sufide. Any carbon dioxide absorbed is, for all practical purposes, absorbed physically. The spent solution may first be formed to a pressure-reduction zone, where carbon dioxide and other physically absorbed constituents are removed by flashing, and then to a recovery zone, where the hydrogen sulfide is stripped from the solution by application of heat, with or without prior elimination of carbon dioxide. The stripping carrier gas may comprise the small amount of. water present in the solution, which water is condensed and recycled to maintain water balance in the solution. The hydrogen sulfide is extracted in a concentrated form for passage through a sulfur-recovery unit, such as a Claus plant. Brief Description of the Drawings The attached drawing schematically illustrates a system for carrying out the process of the instant invention. Detailed Description With reference now to the Drawing, there is shown a basic flow scheme for the processing of hydrogen sulfide and carbon dioxide containing gases, in accordance with the instant invention. As will be understood, the object is to remove hydrogen sulfide substantially to the exclusion of carbon dioxide, as the former is highly toxic and the latter is innocuous. There is fed to absorption tower iθ a sour feed gas 12 containing hydrogen sulfide as the most undesirable impurity, but also containing carbon dioxide and other minor constituents such as COS, CS2 and mercap- tans. Absorber 10 is normally a multi-plate absorption column where, as shown, the absorption solution is intro¬ duced by line 14 countercurrent to the flow of gas. Purified gas leaves by line 16 at the top of the column, and spent absorbent leaves by line 18 at the base of the column. Spent absorbent, after being heated in 'exchanger 20 with return stripped absorbent from heated stripping column 24, is passed by line 22 to the top of heated stripping column 24. Heat is applied at the base of the stripping column, conveniently through reboiler 26, to furnish the heat required to desorb the absorbed hydrogen sulfide and to vaporize water to act as carrier vapor for hydrogen sulfide. Stripped hydrogen sulfide and water vapor leave the top of the stripping column and flow to condenser 28, where the water is condensed from the stream then sepa¬ rated from the gas in knock-out column 29 and recycled to the stripping column for recombination with the absorp¬ tion solution. Pressure-reduction flash zone 30 may be employed ahead of heated stripping column 24, in cases where it is desired to release physically absorbed carbon dioxide and other physically dissolved constituents. Regenerated absorption solution from heated strip¬ ping column 24 is returned by pump 36 through cooler 38 to absorption tower 10, and condensate from knock-out drum 29 is returned by pump 32 to heated stripping column 24. A concentrated hydrogen sulfide stream is passed by ' line 34 to a sulfur-recovery system, such as a Claus plant. One criterion desired for the effective practice of the instant invention is that the absorption solution be water-free, or substantially water-free, with a water concentration of up to about 5% by volume being tolerable. Another desired criterion is that the amine employed be a tertiary amine, a hindered amine, or mixtures thereof, and, if a diluent for the amine is employed, such diluent is no more than a physical absorbent for carbon dioxide, preferably a physical absorbent wherein the capacity is not more than about 5 volumes of gaseous carbon dioxide, at 77°F and one atmosphere partial pressure of carbon dioxide per liquid volume of physical absorbent. Solu¬ tions meeting such criteria are effectively precluded from enabling significant reaction of carbon dioxide with the amine. , Illustrative but not limiting of tertiary amines, there may be mentioned triethanolamine, methyldiethanol- amine, and the like. As used herein, and as defined in Industrial Engineering Chemistry Fundamentals, 4th Ed. , 1983, Sartori and Savage, 22, 239 (incorporated herein by reference), a "sterically hindered amine" is a primary amine in which the amino group is attached to a tertiary carbon atom, or a secondary amine in which the amino group is attached to a secondary or tertiary carbon atom. The amine acts as a chemical absorbent for hydrogen sulfide, and strongly bonds hydrogen sulfide to the amine following the formation of hydrosulfides. Reaction is very rapid. The flow and concentration rates of the a ine are adjusted, in terms of the hydrogen sulfide con¬ centration of the stream to be purified, to essentially enable absorption equilibrium to be reached. This is accomplished by sufficient contact time with a solution sufficiently lean along the length of the column to substantially absorb all the hydrogen sulfide present in the sour gas stream, thereby yielding a sweet gas for further processing. Although an absorption solution of 100% amine may be employed, it is convenient to dilute the amine with a non- chemical absorbent for carbon dioxide. There may be employed, for instance, physical absorbents for carbon dioxide which, at the normal temperatures of operation, i.e., ambient or above, have a relatively low solubility for carbon dioxide, e.g., about 5 or less volumes (gas) per volume liquid. Lower-than-ambient temperatures may also be employed to increase the amount of carbon dioxide taken up by the physical absorbent that will become part of the hydrogen sulfide separated in the stripper 24. However, the CO2 can be separated from the H2S by flash¬ ing zone 30. While there may be employed many compounds which are solvents for tertiary and hindered amines, and which display the desired low physical absorptivity for carbon dioxide, it is convenient for economic considerations to employ low-cost solvents. Exemplary of the physical solvents are glycols such as diethylene glycol, tri¬ ethylene glycol, and ethylene glycol; glycol esters; and glycol ethers such as monoalkyl and dialkyl ethers of diethylene glycol, triethylene glycol ethers, polyethylene glycol ethers and the like. As specific physical absorbents there may be mentioned N-methyl pyrrolidone, ethylene glycol diacetate, diethylene glycol diacetate, triethyl phosphate, butyl carbitol acetate, methyl cellusolve acetate, propylene glycol methyl ether. tributyl phosphate, dipropylene glycol methyl ether, tripropylene glycol methyl ether, carbitol, diethyl¬ ene glycol diethyl ether, propylene carbonate, dimethyl sulfolane, dimethoxy tetraglycol, mixtures of ethylene carbonate, benzyl alcohol, and the like. The amount of diluent solvent utilized may be up to about 90% by weight, based on the weight of the diluent solvent and the amine, and provide a combined solution which has a sufficiently low viscosity to enable flow through the absorption column at and below ambient temperatures. It may be desirable that the feed gas be pretreated with a dessicant to remove as much water vapor as possible. This is because it is desirable to operate the absorption column under substantially anhydrous conditions, and it is nor- mally inevitable that some moisture will be taken up by the solution, as the diluent for the amine will normally be hygroscopic, or some water condensation or retention may occur. To minimize the amount of water in the system, it is preferred to operate the absorption column at a tem¬ perature above the dew point of water in the gas stream undergoing purification, to prevent condensation. This limits water take-up to that which may result in conse¬ quence of the hygroscopicity of the solution employed. In operating the process, maintaining a water-content level of about 5% by volume of absorption solution or less, can be beneficial. This will essentially eliminate the formation of carbonates and bicarbonates, because the rate of reaction is low, yet there is a sufficient amount of water to enable its effective use in stripping hydrogen sulfide from the absorption solution. The advantage of the practice of the process of the instant invention is a materially sharper selectivity of hydrogen sulfide to the exclusion of chemically bound carbon dioxide. In addition, hydrogen sulfide, beyond that which chemically bonds with the amine, is absorbed in the diluent solvent by physical solubility and may be recovered in the stripping column. The sweetened gas stream may, if desired, be separately processed for carbon dioxide removal using chemical solvents. What is unique, however, is the achievement of separation at extremely low cost using conventional absorbents and relying on substantially anhydrous conditions to achieve the desired result. Without limitation, the following Example illustrates the instant invention. Example There is processed a sour gas stream containing 5% by volume hydrogen sulfide and 5% by volume carbon dioxide. Normal gas pressure is somewhat above atmospheric. The lean absorption solution contains 23.8% methyldiethanola- mine, the balance ' being anhydrous diethylene glycol. The absorption solution temperature is 40°C. The process gas stream is passed countercurrent to the absorption solution. A 90% approach to equilibrium is realized, and the amount of hydrogen sulfide absorbed is about 4.8 volumes per volume of absorption solution. By contrast, the amount of carbon dioxide physically absorbed in the diethylene glycol is only about 0.4 volumes per volume of treating solution. While the ratio of hydrogen sulfide to carbon dioxide in the gas to be treated is one-to-one, a hydrogen sulfide-to-carbon dioxide ratio of about twelve-to-one is obtained in the acid gas stream leaving the stripping zone. In essence, about 91% carbon dioxide passes through the absorption column without take-up. The absorption solution is then passed to a flash zone, where the physically absorbed carbon dioxide is released through a reduction in pressure; the balance of the absorption solution is then passed through a stripping zone, where the absorbed hydrogen sulfide is released by heating as a concentrated hydrogen sulfide stream for processing to sulfur; and the lean absorption solution is recycled back to the absorption column.

Claims: WHAT IS CLAIMED IS: 1. A process for the selective separation of hydrogen sulfide from a gas stream comprising hydrogen sulfide and carbon dioxide, which process comprises: a) contacting the gas stream in an absorption zone with a substantially anhydrous liquid absorption solution comprising an amine compound selected from the group consisting of tertiary amines, hindered amines, and mixtures thereof, to selectively separate hydrogen sulfide from the gas stream, to the substantial exclusion of carbon dioxide; b) passing the hydrogen sulfide-containing absorption solution to a stripping zone and stripping the absorbed hydrogen sulfide from the absorption solution; and c) recycling the stripped absorption solution to the absorption zone. 2. A process as claimed in claim 1 in which the gas stream contains water and in which the gas stream is contacted with the absorption solution at a temperature above the dew point of water in the gas stream. 3. A process as claimed in claim 1 in which the absorption solution contains a diluent for the amine compound, which diluent is a physical absorbent for carbon dioxide which does not chemically bind carbon dioxide and which is present in an amount of up to about 90% by weight based on the weight of the diluent and the amine compound. 4. A process as claimed in claim 1 in which the amine compound is selected from the group consisting of methyldiethanolamine, triethanolamine and mixtures thereof. 5. A process as claimed in claim 3 in which the amine compound is selected from the group consisting of methyldiethanolamine and triethanolamine. 6. A process as claimed in claim 3 in which the physical absorbent is selected from the group consisting of glycols, glycol esters, glycol ethers, N-methylpyrrolidone, and mixtures thereof. 7. A process as claimed in claim 5 in which the physical absorbent is selected from the group consisting of glycols, glycol esters, glycol ethers, N-methylpyrrolidone, and mixtures thereof. 8. A process for the selective separation of hydrogen sulfide from a gas stream containing water, hydrogen sulfide and carbon dioxide, which process comprises: a) at a temperature above the dew point of water contained in the gas stream, contacting the gas stream in an absorption zone with a liquid absorption solution containing up to about 5% by weight water and comprising an amine compound selected from the group consisting of tertiary amines, hindered amines, and mixtures thereof, to selectively separate hydrogen sulfide from the gas stream, to the substantial exclusion of carbon dioxide; Claim 8, continued. . . b) passing the hydrogen sulfide-containing absorption solution to a stripping zone and heating the absorption solution to strip the absorbed hydrogen sulfide from the absorption solution, the stripped hydrogen sulfide containing water; c) condensing water from the stripped hydrogen sulfide; d) combining the water with the stripped 0 absorption solution; and e) recycling the stripped absorption solution to the absorption zone. 9. A process as claimed in claim 8 in which the • ° absorption solution is maintained at a temperature of up to about 150°F. 10. A process as claimed in claim 8 in which the absorption solution contains a diluent for the amine 0 compound, which diluent is a physical absorbent for carbon dioxide, has a solubility of about 5 volumes or less of carbon dioxide at 77°F and one atmosphere partial pressure of carbon dioxide per volume of diluent, and is present in an amount of up to about 90% by weight 5 based on the weight of the diluent and the amine compound. 11. A process as claimed in claim 8 in which the amine compound is selected from the group consisting of methyldiethanalomine and triethanolamine. 12. A process as claimed in claim 10 in which the amine compound is selected from the group consisting of methyldiethanalomine and triethanolamine. 13. A process as claimed in claim 10 in which the phsyical absorbent is selected from the group consisting of glycols, glycol esters, glycol ethers, N-methylpyrrolidone, and mixtures thereof. 14. A process as claimed in claim 12 in which the physical absorbent is selected from the group consisting of glycols, glycol esters, glycol ethers, N-methylpyrrolidone, and mixtures thereof. 15. A process for the selective separation of hydrogen sulfide from a gas stream comprising hydrogen sulfide and carbon dioxide, which process comprises: a) contacting the gas stream in an absorption zone with a substantially anhydrous liquid absorption solution comprising an amine compound selected from the group consisting of tertiary amines, hindered amines, and mixtures thereof, to selectively separate hydrogen sulfide and at least a portion of the carbon dioxide from the gas stream; b) separating the carbon dioxide from the absorption solution to leave a hydrogen sulfide- containing absorption solution that is substantially free of carbon dioxide; c) passing the hydrogen sulfide-containing absorption solution to a stripping zone and stripping the absorbed hydrogen sulfide from the absorption solution; and d) recycling the stripped absorption solution to the absorption zone. 16. A process as claimed in claim 15 in which the gas stream contains water and n which the gas stream is contacted with the absorption solution at a temperature above the dew point of water in the gas stream. 17. A process as claimed in claim 15 in which the carbon dioxide is flash-separated from the absorption solution. 18. A process as claimed in claim 15 in which the absorption solution includes a diluent for the amine compound, which diluent is a physical absorbent for carbon dioxide which does not chemically bind carbon dioxide and which is present in an amount of up to about 90% by weight based on the weight of the diluent and the amine compound. 19. A process as claimed in claim 17 in which the absorption solution includes a diluent for the amine compound, which diluent is a physical absorbent for carbon dioxide which does not chemically bind carbon dioxide and which is present in an amount of up to about 90% by weight based on the weight of the diluent and the amine compound. 20. A process as claimed in claim 15 in which the amine compound is selected from the group consisting of methyldiethanolamine, triethanolamine and mixtures thereof. 21. A process as claimed in claim 19 in which the amine compound is selected from the group consisting of methyldiethanolamine and triethanolamine. 22. A process as claimed in claim 21 in which the physical absorbent is selected from the group consisting of glycols, glycol esters, glycol ethers, N-methylpyrrolidone, and mixtures thereof. 23. A process for the selective separation of hydrogen sulfide from a gas stream containing water, hydrogen sulfide and carbon dioxide, which process comprises: a) at a temperature above the dew point of water contained in the gas stream, contacting the gas stream in an absorption zone with a liquid absorption solution containing up to about 5% by weight water and comprising an amine compound selected from the group consisting of tertiary amines, hindered amines, and mixtures thereof, to separate hydrogen sulfide and at least a portion of the carbon dioxide and watp- from the gas stream, to the substantial exclusion of carbon dioxide; b) flashing the absorbed carbon dioxide from the absorption solution by a reduction in solution pressure to leave a water-diluted hydrogen sulfide- containing absorption solution; c) passing the water-diluted hydrogen sulfide- containing absorption solution to a stripping zone and heating the absorption solution to strip the absorbed hydrogen sulfide from the absorption solution, the stripped hydrogen sulfide containing water; d) condensing water from the stripped hydrogen sulfide; e) combining the water with the stripped absorption solution; and f) recycling the stripped absorption solution to the absorption zone. 24. A process as claimed in claim 23 in which the absorption solution is maintained at a temperature of up to about 150°F. 25. A process as claimed in claim 23 in which the absorption solution contains a diluent for the amine compound, which diluent is a physical absorbent for carbon dioxide, has a solubility of about 5 volumes or less of carbon dioxide at 77°F and one atmosphere partial pressure of carbon dioxide per volume of diluent, and is present in an amount of up to about 90% by weight based on the weight of the diluent and the amine compound 26. A process as claimed in claim 23 in which the amine compound is selected from the group consisting of methyldiethanalomine and triethanolamine. 27. A proce.ss as claimed in claim 25 in which the amine compound is selected from the group consisting of methyldiethanalomine and triethanolamine. 28. A process as claimed in claim 25 in which the phsyical absorbent is selected from the group consisting of glycols, glycol esters, glycol ethers, N-methylpyrrolidone, and mixtures thereof. 29. A process as claimed in claim 25 in which the physical absorbent is selected from the group consisting of glycols, glycol esters, glycol ethers, N-methylpyrrolidone, and mixtures thereof.

Inventors: RIESENFELD FRED CHARLES, RIESENFELD, FRED, CHARLES
Applicants: PARSONS CO RALPH M, THE RALPH M. PARSONS COMPANY
Application year: 1986
Main classification: C01B17
IPC codes: B01D53, C01B17