REMOVAL OF HYDROGEN SULPHIDE AND CARBON DIOXIDE FROM A STREAM OF FLUID

Abstract

A process for removing hydrogen sulfide and carbon dioxide from a fluid stream comprises a) an absorption step in which the fluid stream is contacted with an absorbent comprising an aqueous solution (i) of an amine of the general formula (I)

##STR00001##

in which R.sub.1, R.sub.2 and R.sub.3 are each independently selected from C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.4 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.5 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; X is OH or NH(CR.sub.1R.sub.2R.sub.3); m is 2, 3, 4 or 5; n is 2, 3, 4 or 5; and o is 0 or 1; and optionally (ii) at least one tertiary amine, where the molar ratio of (i) to (ii) is greater than 0.05; wherein at least 90% of the hydrogen sulfide is removed from the fluid stream and selectivity for hydrogen sulfide over carbon dioxide is not greater than 8, wherein a CO.sub.2- and H.sub.2S-laden absorbent is obtained; b) a regeneration step in which at least a substream of the CO.sub.2- and H.sub.2S-laden absorbent is regenerated and a regenerated absorbent is obtained; and c) a recycling step in which at least a substream of the regenerated absorbent is recycled into the absorption step a). The process allows a high level of hydrogen sulfide removal with a simultaneously high coabsorption of carbon dioxide.

Claims

1. A process for removing hydrogen sulfide and carbon dioxide from a fluid stream, the process comprising a) contacting the fluid stream with an absorbent comprising an aqueous solution of (i) an amine of formula (I) ##STR00004## in which R.sub.1, R.sub.2 and R.sub.3 are each independently C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl; each R.sub.4 is independently hydrogen, C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl; each R.sub.5 is independently hydrogen, C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl, X is OH or NH(CR.sub.1R.sub.2R.sub.3); m is 2, 3, 4 or 5; n is 2, 3, 4 or 5; and o is 0 or 1; and optionally (ii) at least one tertiary amine, where a molar ratio of (i) to (ii) is greater than 0.05; wherein there is a partial hydrogen sulfide pressure of at least 0.1 bar and a partial carbon dioxide pressure of at least 1 bar in the fluid stream; wherein the fluid stream has a total pressure of at least 20 bar, wherein at least 90% of the hydrogen sulfide is removed from the fluid stream; wherein a CO.sub.2- and H.sub.2S-laden absorbent and a treated fluid stream are obtained; and wherein a selectivity S for hydrogen sulfide over carbon dioxide calculated as follows is not greater than 8; $S=\frac{\frac{{y\left(H_2.Math.S\right)}_{\mathrm{feed}}-{y\left(H_2.Math.S\right)}_{\mathrm{treat}}}{{y\left(H_2.Math.S\right)}_{\mathrm{feed}}}}{\frac{{y\left({\mathrm{CO}}_2\right)}_{\mathrm{feed}}-{y\left({\mathrm{CO}}_2\right)}_{\mathrm{treat}}}{{y\left({\mathrm{CO}}_2\right)}_{\mathrm{feed}}}},$ in which y(H.sub.2S).sub.feed is a molar proportion of H.sub.2S in the fluid stream, y(H.sub.2S).sub.treat is a molar proportion in the treated fluid stream, y(CO.sub.2).sub.feed is a molar proportion of CO.sub.2 in the fluid stream and y(CO.sub.2).sub.treat is a molar proportion of CO.sub.2 in the treated fluid stream; b) regenerating at least a substream of the CO.sub.2- and H.sub.2S-laden absorbent to obtain a regenerated absorbent; and c) recycling at least a substream of the regenerated absorbent into a).

2. The process according to claim 1, wherein a total concentration of (i) and (ii) in the aqueous solution is 10% to 60% by weight.

3. The process according to claim 1, wherein the amine (i) is 2-(2-tert-butylaminoethoxy)ethanol.

4. The process according to claim 1, wherein the aqueous solution comprises the at least one tertiary amine (ii), which is methyldiethanolamine.

5. The process according to claim 1, wherein the absorbent does not comprise any sterically unhindered primary or secondary amines.

6. The process according to claim 1, wherein the absorbent comprises at least one organic solvent.

7. The process according to claim 1, wherein the fluid stream comprises at least one hydrocarbon.

8-9. (canceled)

10. The process according to claim 1, wherein the CO.sub.2- and H.sub.2S-laden absorbent is regenerated b) to an H.sub.2S loading corresponding to an equilibrium loading for an H.sub.2S content of less than 90% of a H.sub.2S content of the treated fluid stream.

11. The process according to claim 1, wherein a cumulated CO.sub.2 and H.sub.2S loading of the CO.sub.2- and H.sub.2S-laden absorbent is at least 0.25 mol/mol and a cumulated CO.sub.2 and H.sub.2S loading of the regenerated absorbent is less than 0.20 mol/mol.

12. The process according to claim 1, wherein the regenerating b) comprises at least one measure of heating, decompressing and stripping with an inert fluid.

13. The process according to claim 1, further comprising: d) passing at least a substream of the CO.sub.2- and H.sub.2S-containing gas stream released in b) into a Claus plant to obtain a Claus tail gas, which is hydrogenated to obtain a hydrogenated Claus tail gas; e) treating the hydrogenated Claus tail gas with the regenerated absorbent to obtain a first H.sub.2S-laden absorbent; and f) passing the first H.sub.2S-laden absorbent into b) and/or into a).

14. The process according to claim 13, wherein the treating e) is effected at a lower pressure than a pressure in a).

15. The process according to claim 1, further comprising: d′) treating a sub stream of a CO.sub.2- and H.sub.2S-containing gas stream released in b) with the regenerated absorbent to obtain a second H.sub.2S-laden absorbent; and e′) passing the second H.sub.2S-laden absorbent into b).

16. The process according to claim 15, wherein the substream of the CO.sub.2- and H.sub.2S-laden absorbent is passed into d′).

17. The process according to claim 15, further comprising: f′) passing the substream of the CO.sub.2- and H.sub.2S-containing gas stream released in b) into a Claus plant to obtain a Claus tail gas, which is hydrogenated to obtain a hydrogenated Claus tail gas; g′) treating the hydrogenated Claus tail gas with the regenerated absorbent to obtain a first H.sub.2S-laden absorbent; and h′) passing the first H.sub.2S-laden absorbent into b) and/or into a).

18. The process according to claim 1, further comprising i″) recycling and passing into a) a substream of the CO.sub.2- and H.sub.2S-containing gas stream released in b).

19. The process according to claim 1, wherein b) comprises: b1) decompressing the CO.sub.2- and H.sub.2S-laden absorbent is to obtain a CO.sub.2-containing gas stream and a partly regenerated absorbent; and b2) heating and/or stripping the partly regenerated absorbent to obtain the regenerated absorbent; and wherein the process further comprises: d″) treating the CO.sub.2-containing gas stream with the regenerated absorbent to obtain a third H.sub.2S-laden absorbent; and e″) passing the third H.sub.2S-laden absorbent into b).

20. The process according to claim 19, further comprising: f″) decompressing the CO.sub.2- and H.sub.2S-laden absorbent to a pressure between a pressure in a) and a pressure in b1), in order to release essentially dissolved gas constituents other than carbon dioxide and hydrogen sulfide from the CO.sub.2- and H.sub.2S-laden absorbent.

Description

[0090] The invention is illustrated in detail by the appended drawings and the examples which follow. FIGS. 1 to 8 use the same reference symbols for elements of the same function. Plant components not required for understanding, such as pumps, are not shown in the figures for the sake of clarity.

[0091] FIG. 1 is a schematic diagram of a plant suitable for performing the process according to the invention.

[0092] FIG. 2 is a schematic diagram of a further plant suitable for performing the process according to the invention.

[0093] FIG. 3 is a schematic diagram of a further plant suitable for performing the process according to the invention.

[0094] FIG. 4 is a schematic diagram of a further plant suitable for performing the process according to the invention.

[0095] FIG. 5 is a schematic diagram of a further plant suitable for performing the process according to the invention.

[0096] FIG. 6 is a schematic diagram of a further plant suitable for performing the process according to the invention.

[0097] FIG. 7 is a schematic diagram of a further plant suitable for performing the process according to the invention.

[0098] FIG. 8 is a schematic diagram of a further plant suitable for performing the process according to the invention.

[0099] FIG. 9 shows the H.sub.2S selectivity of TBAEE and MDEA as a function of the loading at low partial H.sub.2S pressure.

[0100] FIG. 10 shows the H.sub.2S selectivity of TBAEE, MDEA and a TBAEE/MDEA mixture as a function of the loading at high partial H.sub.2S pressure.

[0101] FIG. 11 shows the H.sub.2S selectivity of MDEA and a TBAEE/MDEA mixture as a function of the absorbent circulation rate with constant reboiler output.

[0102] FIG. 12 shows the H.sub.2S selectivity of 1,2-bis(tert-butylamino)ethane (bis-TBAE) and MDEA as a function of the loading at low partial H.sub.2S pressure.

[0103] FIG. 13 shows the H.sub.2S selectivity of 1,2-bis(tert-butylamino)ethane (bis-TBAE) and MDEA as a function of the loading at high partial H.sub.2S pressure.

[0104] According to FIG. 1, via the inlet Z, a suitably pretreated gas comprising hydrogen sulfide and carbon dioxide is contacted in countercurrent, in an absorber A1, with regenerated absorbent which is fed in via the absorbent line 1.01. The absorbent removes hydrogen sulfide and carbon dioxide from the gas by absorption; this affords a hydrogen sulfide- and carbon dioxide-depleted clean gas via the offgas line 1.02.

[0105] Via the absorbent line 1.03, the heat exchanger 1.04 in which the CO.sub.2- and H.sub.2S-laden absorbent is heated up with the heat from the regenerated absorbent conducted through the absorbent line 1.05, and the absorbent line 1.06, the CO.sub.2- and H.sub.2S-laden absorbent is fed to the desorption column D and regenerated.

[0106] Between the absorber A1 and heat exchanger 1.04, a flash vessel may be provided (not shown in FIG. 1), in which the CO.sub.2- and H.sub.2S-laden absorbent is decompressed to, for example, 3 to 15 bar.

[0107] From the lower part of the desorption column D, the absorbent is conducted into the boiler 1.07, where it is heated. The mainly water-containing vapor is recycled into the desorption column D, while the regenerated absorbent is fed back to the absorber A1 via the absorbent line 1.05, the heat exchanger 1.04 in which the regenerated absorbent heats up the CO.sub.2- and H.sub.2S-laden absorbent and at the same time cools down itself, the absorbent line 1.08, the cooler 1.09 and the absorbent line 1.01. Instead of the boiler shown, it is also possible to use other heat exchanger types to raise the stripping vapor, such as a natural circulation evaporator, forced circulation evaporator or forced circulation flash evaporator. In the case of these evaporator types, a mixed-phase stream of regenerated absorbent and stripping vapor is returned to the bottom of the desorption column, where the phase separation between the vapor and the absorbent takes place. The regenerated absorbent to the heat exchanger 1.04 is either drawn off from the circulation stream from the bottom of the desorption column to the evaporator or conducted via a separate line directly from the bottom of the desorption column to the heat exchanger 1.04.

[0108] The CO.sub.2- and H.sub.2S-containing gas released in the desorption column D leaves the desorption column D via the offgas line 1.10. It is conducted into a condenser with integrated phase separation 1.11, where it is separated from entrained absorbent vapor. In this and all the other plants suitable for performance of the process according to the invention, condensation and phase separation may also be present separately from one another. Subsequently, a liquid consisting mainly of water is conducted through the absorbent line 1.12 into the upper region of the desorption column D, and a CO.sub.2- and H.sub.2S-containing gas is discharged via the gas line 1.13.

[0109] According to FIG. 2, via an inlet Z, a suitably pretreated gas comprising CO.sub.2 and H.sub.2S, preferably natural gas, is contacted in countercurrent, in an absorber A1, with regenerated absorbent which is fed in via the absorbent line 2.01. The absorbent removes CO.sub.2 and H.sub.2S by absorption from the gas; at the same time, a CO.sub.2- and H.sub.2S-depleted gas is obtained via an offgas line 2.02. Via an absorbent line 2.03, the CO.sub.2- and H.sub.2S-laden absorbent is passed into a decompression vessel HPF and decompressed (for example from about 70 bar to from 3 to 15 bar, preferably 5 to 10 bar), the temperature being essentially equal to the temperature of the laden absorbent. Typically, the temperature differential is less than 10° C., preferably less than 5° C. Under these conditions, essentially all the hydrocarbons present in the laden absorbent are released as gas and can be removed via line 2.04.

[0110] Via absorbent line 2.05, 2.07, heat exchanger 2.08 in which the laden absorbent is heated up with the heat from the regenerated absorbent discharged from the lower region of the boiler 2.10 via the absorbent line 2.09, and absorbent line 2.11, the laden absorbent is fed to a desorption column D, where it is regenerated. The regenerated absorbent is conducted into the boiler 4.09, where it is heated. The mainly water-containing vapor is recycled into the desorption column D, while the regenerated absorbent is removed via the absorbent line 2.09, the heat exchanger 2.08, absorbent line 2.12, cooler 2.13 and absorbent line 2.14, and divided into two substreams 2.01 and 2.15 and fed to the absorbers A1 and/or TGA. The relative volume flow rates of streams 2.01 and 2.15 can be varied in order to achieve the desired specifications of the offgas even in the case, for example, of varying H.sub.2S content of the fluid stream to be treated.

[0111] The gas which comprises CO.sub.2 and H.sub.2S and is obtained in the desorption column D leaves the desorption column D via the gas line 2.16 and is separated from entrained vapor in the condenser with integrated phase separation 2.17, and then a liquid consisting mainly of water is recycled via the absorbent line 2.18 into the upper region of the desorption column D. The gas comprising CO.sub.2 and H.sub.2S is fed via the gas line 2.19 to a Claus plant CL, the offgas from which is fed to a hydrogenation plant HY. The hydrogenated Claus tail gas is fed into the tail gas absorber TGA, where it is contacted in countercurrent with the regenerated absorbent fed in via the absorbent line 2.15. Via a gas line 2.20, CO.sub.2-enriched gas is removed from the tail gas absorber TGA. Via an absorbent line 2.06, the H.sub.2S-laden absorbent is combined with the laden absorbent conducted in line 2.05 and fed via the absorbent line 2.07 to the desorption column D.

[0112] The plant shown in schematic form in FIG. 3 corresponds to the plant of FIG. 2, except that the H.sub.2S-laden absorbent from the tail gas absorber TGA is fed via the absorbent line 2.06 into the middle region of the absorber A1.

[0113] According to FIG. 4, via an inlet Z, a suitably pretreated gas comprising CO.sub.2 and H.sub.2S is contacted in countercurrent, in an absorber A1, with regenerated absorbent which is fed in via the absorbent line 4.01. The absorbent removes CO.sub.2 and H.sub.2S by absorption from the gas; at the same time, a CO.sub.2- and H.sub.2S-depleted gas is removed via the gas line 4.02.

[0114] Via an absorbent line 4.04, absorbent line 4.06, heat exchanger 4.07 in which the CO.sub.2- and H.sub.2S-laden absorbent is heated up with the heat from the regenerated absorbent discharged from the lower region of the boiler 4.09 via the absorbent line 4.08, and absorbent line 4.10, the CO.sub.2- and H.sub.2S-laden absorbent is fed to the desorption column D, where it is regenerated. The absorbent is conducted into the boiler 4.09, where it is heated. The mainly water-containing vapor is recycled into the desorption column D, while the regenerated absorbent is removed via the absorbent line 4.08, the heat exchanger 4.07, the absorbent line 4.11, the cooler 4.12 and the absorbent line 4.13. The regenerated absorbent is divided into the substreams 4.01 and 4.14 and fed to the upper region of the absorbers A1 and A2. The relative volume flow rates in the absorbent lines 4.01 and 4.14 can be varied in order to achieve the desired specifications of the offgas even in the case of a varying H.sub.2S content.

[0115] The CO.sub.2- and H.sub.2S-enriched gas obtained in the desorption column D leaves the desorption column D via the gas line 4.15 and is separated from entrained vapor in the condenser with integrated phase separation 4.16, and then a liquid consisting mainly of water is recycled via the absorbent line 4.17 into the upper region of the desorption column D. The CO.sub.2- and H.sub.2S-enriched gas is removed via the gas line 4.18. A substream is sent to a further treatment via the gas line 4.19, and a substream is fed into the lower region of the absorber A2 via the gas line 4.20.

[0116] In the absorber A2, the CO.sub.2- and H.sub.2S-enriched gas from the line 4.20 is contacted in countercurrent with the regenerated absorbent fed in via the absorbent line 4.14. Via a gas line 4.21, the acid gas-depleted absorbent is removed from the absorber A2 and discharged from the plant. Via an absorbent line 4.05, the H.sub.2S-laden absorbent from the absorber A2 is combined with the CO.sub.2- and H.sub.2S-laden absorbent conducted in line 4.04 and fed to the desorption column D via absorbent line 4.06.

[0117] The plant shown in schematic form in FIG. 5 corresponds to the plant of FIG. 4, except that a substream of the CO.sub.2- and H.sub.2S-laden absorbent is passed via the absorbent line 4.23, cooler 4.24 and absorbent line 4.25 into the middle section of the absorber A2.

[0118] According to FIG. 6, via an inlet Z, a suitably pretreated gas comprising CO.sub.2 and H.sub.2S is contacted in countercurrent, in an absorber A1, with regenerated absorbent fed in in the upper region via the absorbent line 6.01 and the partly laden absorbent fed in in the middle region via the absorbent line 6.02. The absorbent removes acid gases by absorption out of the gas; this involves removal of an acid gas-depleted gas via the gas line 6.03 and discharge from the plant.

[0119] Via absorbent line 6.05, the CO.sub.2- and H.sub.2S-laden absorbent is drawn off and divided into substreams 6.06 and 6.07. A portion of the laden absorbent is fed via the absorbent line 6.07 into the middle section of the absorber A2. The remaining absorption capacity of the absorbent from absorber A1 can thus be utilized.

[0120] The other portion of the CO.sub.2- and H.sub.2S-laden absorbent is fed via the absorbent line 6.06, absorbent line 6.09, heat exchanger 6.10 in which the CO.sub.2- and H.sub.2S-laden absorbent is heated up with the heat from the regenerated absorbent discharged from the lower region of the boiler 6.12 via the absorbent line 6.11, and absorbent line 6.13, to a desorption column D, where it is regenerated. The regenerated absorbent is conducted into the boiler 6.12, where it is heated. The mainly water-containing vapor is recycled into the desorption column D, while the regenerated absorbent is conducted onward via the absorbent line 6.11, the heat exchanger 6.10, absorbent line 6.14, cooler 6.15 and absorbent line 6.16, and divided into the substreams 6.01, 6.17 and 6.18. A portion of the regenerated absorbent is conducted via the absorbent line 6.01 into the upper section of the absorber A1, another portion of the regenerated absorbent is conducted via the absorbent line 6.17 into the upper section of the absorber TGA, and a further portion of the regenerated absorbent is conducted via the absorbent line 6.18 into the upper section of the absorber A2. The relative volume flow rates in the absorbent lines 6.01, 6.17 and 6.18 can be varied in order to achieve the desired specifications of the offgas even in the case of a varying H.sub.2S content.

[0121] The gas which comprises CO.sub.2 and H.sub.2S and is obtained in the desorption column D leaves the desorption column D via the gas line 6.19 and is separated from entrained vapor in the condenser with integrated phase separation 6.20, and then a liquid consisting mainly of water is recycled via the absorbent line 6.21 into the upper region of the desorption column D. The gas comprising CO.sub.2 and H.sub.2S is partly fed via the gas line 6.23 into the lower region of the absorber A2.

[0122] The other substream of the gas comprising CO.sub.2 and H.sub.2S is fed via the gas line 6.24 to a Claus plant CL, the offgas from which is hydrogenated in a hydrogenation plant HY. The hydrogenated Claus tail gas is fed into the tail gas absorber TGA, where it is contacted in countercurrent with the regenerated absorbent fed in via the absorbent line 6.17. Via the absorbent line 6.02, the H.sub.2S-laden absorbent from the tail gas absorber TGA is fed into the middle section of the absorber A1. The remaining absorption capacity of the absorbent from tail gas absorber TGA can thus be utilized. Via a gas line 6.25, the H.sub.2S-depleted or CO.sub.2-enriched gas is removed from the tail gas absorber TGA, combined with the gas stream 6.26 and discharged via gas line 6.27.

[0123] In the absorber A2, the gas comprising CO.sub.2 and H.sub.2S from gas line 6.23 is contacted in countercurrent with the regenerated absorbent fed in via the absorbent line 6.18 in the upper region and the CO.sub.2- and H.sub.2S-laden absorbent from absorber A1 fed in via the absorbent line 6.07 in the middle region. Via a gas line 6.26, the acid gas-depleted absorbent is removed from the absorber A2. Via an absorbent line 6.08, an H.sub.2S-laden absorbent from the absorber A2 is combined with the laden absorbent conducted in line 6.06 and conducted onward to the desorption column D via absorbent line 6.09.

[0124] The plant shown in schematic form in FIG. 7 corresponds to the plant of FIG. 1, except that a substream of the gas 1.13 comprising CO.sub.2 and H.sub.2S is recycled to the inlet Z via the line 1.14. The line 1.14 may comprise a compressor (not shown), which is necessary in plants in which the inlet pressure of the inlet Z is greater than the outlet pressure of the condenser with integrated phase separation 1.11.

[0125] According to FIG. 8, via an inlet Z, a suitably pretreated gas comprising CO.sub.2 and H.sub.2S is contacted in countercurrent, in an absorber A1, with regenerated absorbent which is fed in via the absorbent line 8.01. The absorbent removes CO.sub.2 and H.sub.2S by absorption from the gas; at the same time, a CO.sub.2- and H.sub.2S-depleted gas is obtained via a gas line 8.02. Via an absorbent line 8.03, the CO.sub.2- and H.sub.2S-laden absorbent is passed into a decompression vessel HPF and decompressed (for example from about 70 bar to from 3 to 15 bar, preferably 5 to 10 bar), the temperature being essentially equal to the temperature of the laden absorbent. Typically, the temperature differential is less than 10° C., preferably less than 5° C. Under these conditions, essentially all the hydrocarbons present in the laden absorbent are released as gas and can be discharged via line 8.04.

[0126] Via an absorbent line 8.05, a heat exchanger 8.06 in which the CO.sub.2- and H.sub.2S-laden absorbent is heated up with the heat from the regenerated absorbent discharged from the lower region of the boiler 8.08 via the absorbent line 8.07, and an absorbent line 8.09, the laden absorbent is passed into a decompression vessel LPF and decompressed (to less than about 5 bar, preferably less than about 3 bar). Under these conditions, significant portions of the carbon dioxide present in the laden absorbent are released as gas and can be removed via the gas line 8.10 to obtain a partly regenerated absorbent. The CO.sub.2 gas here comprises considerable amounts of H.sub.2S, which has to be removed before the CO.sub.2 can be discharged. For this purpose, the CO.sub.2 gas is fed via a cooler 8.11 and the gas line 8.12 into the absorber LPA, where it is contacted in countercurrent with the regenerated absorbent fed in via the absorbent line 8.13. This affords a CO.sub.2-enriched gas which is conducted out of the plant via a gas line 8.14.

[0127] The partly regenerated absorbent discharged from the lower region of the decompression vessel LPF and the H.sub.2S-laden absorbent discharged from the lower region of the absorber LPA is fed via the absorbent lines 8.15 and 8.16 into the upper region of the desorption column D, where it is regenerated. The regenerated absorbent is conducted into the boiler 8.08, where it is heated. The mainly water-containing vapor that results therefrom is recycled into the desorption column D, while the regenerated absorbent is removed via absorbent line 8.07, heat exchanger 8.06, absorbent line 8.17, cooler 8.18 and absorbent line 8.19, and divided into two substreams 8.01 and 8.13 and fed to the absorbers A1 and/or LPA.

[0128] The acid gas-enriched gas obtained in the desorption column D leaves the desorption column D via the gas line 8.20 and is fed to the condenser with integrated phase separation 8.21. In the condenser with integrated phase separation 8.21, the gas stream is separated from entrained vapor, and then a liquid consisting mainly of water is conducted via the absorbent line 8.22 into the upper region of the desorption column D, and an acid gas-enriched gas is discharged via the gas line 8.23.

EXAMPLE 1

[0129] In a pilot plant, the H.sub.2S selectivity of TBAEE compared to MDEA or TBAEE+MDEA was examined at various absorbent circulation rates.

[0130] The pilot plant corresponded to FIG. 1. In the absorber, a structured packing was used. The pressure was 60 bar. The packing height in the absorber was 3.2 m with a column diameter of 0.0531 m. In the desorber, a structured packing was used. The pressure was 1.8 bar. The packing height in the desorber was 6.0 m with a diameter of 0.085 m.

[0131] A gas mixture of 96% by volume of N.sub.2, 2% by volume of CO.sub.2 and 2% by volume of H.sub.2S was conducted into the absorber at a mass flow rate of 47 kg/h and a temperature of 40° C. In the absorber, the absorbent circulation rate was varied from 30 to 100 kg/h. The temperature of the absorbent was 50° C. H.sub.2S was removed to less than 80 ppm. The following table shows the results of these experiments:

TABLE-US-00001 Absorbent circulation rate Example System [kg/h] Selectivity 1-1* TBAEE 30 —** 1-2  TBAEE 42 1.14 1-3  TBAEE 60 1.11 1-4* MDEA 60 1.35 1-5  MDEA + TBAEE 60 1.11 *comparative examples **H.sub.2S specification not attained

[0132] At the low absorbent circulation rate in comparative example 1-1, the exothermicity of the absorption in the TBAEE-based absorbent was too high, and so it was not possible to achieve a specification of less than 80 ppm of H.sub.2S in the treated fluid stream. At a somewhat higher circulation rate (example 1-2), the separation problem is solved. It is apparent that the selectivity of TBAEE at the same absorbent circulation rate (example 1-3) is lower than that of MDEA (comparative example 1-4). The combination of MDEA+TBAEE (example 1-5) also has a lower selectivity than pure MDEA.

EXAMPLE 2

[0133] In an absorption unit according to example 13 of EP 0 084 943 A2, absorption experiments were conducted with various absorbents.

[0134] In a first experiment, a gas mixture of 10% by volume of CO.sub.2 (partial CO.sub.2 pressure 0.1 bar), 1% by volume of H.sub.2S (partial H.sub.2S pressure 0.01 bar) and 89% by volume of N.sub.2 was passed through 100 mL of aqueous absorbent in a glass cylinder at a rate of 216 L (STP)/h and at a temperature of 40° C. The absorbent comprised 3 M MDEA or 3 M TBAEE. Aliquots of the absorbent were drawn off periodically, and the H.sub.2S and CO.sub.2 content was determined. The results are shown in FIG. 9. The H.sub.2S selectivity is shown as a function of the loading in mol(CO.sub.2+H.sub.2S) per mole of amine. It is apparent that both MDEA and TBAEE have a high selectivity at low loadings and low partial pressures. With rising loading, the selectivity of MDEA decreases, while TBAEE still has a high H.sub.2S selectivity.

[0135] In a second experiment, a gas mixture of 90% by volume of CO.sub.2 (partial CO.sub.2 pressure 0.9 bar) and 10% by volume of H.sub.2S (partial H.sub.2S pressure 0.1 bar) was passed through 150 mL of aqueous absorbent in a glass cylinder at a rate of 10 L (STP)/h and at a temperature of 40° C. The absorbent comprised 1.9 M MDEA, 1.9 M TBAEE or 1.4 M MDEA+0.5 M TBAEE. Aliquots of the absorbent were drawn off periodically, and the H.sub.2S and CO.sub.2 content was determined. The results are shown in FIG. 10. The H.sub.2S selectivity is shown as a function of the loading in mol(CO.sub.2+H.sub.2S) per mole of amine. It was found that, at the greater partial pressures and the higher absorbent circulation rate, the H.sub.2S selectivity rises with increasing loading for all absorbents until a plateau is reached. The H.sub.2S selectivity of MDEA is higher than that of TBAEE, the H.sub.2S selectivity of TBAEE+MDEA being between MDEA and TBAEE.

EXAMPLE 3

[0136] Absorption experiments were conducted in a pilot plant. The pilot plant was constructed as in example 1.

[0137] The H.sub.2S selectivity of an aqueous absorbent which comprised 40% by weight of MDEA and of an aqueous absorbent which comprised 30% by weight of MDEA and 15% by weight of TBAEE was studied in natural gas at various absorption circulation rates. Concentrations of 5% CO.sub.2 and 2% H.sub.2S were present in the natural gas stream. H.sub.2S was removed to less than 10 ppm. The pressure was 60 bar. The energy required to regenerate the absorbent (reboiler output) was kept constant and the resulting H.sub.2S selectivity of the absorbents was examined. FIG. 11 shows the measurement data.

[0138] It is apparent that the H.sub.2S selectivity is higher at a low absorbent circulation rate. Here, the selectivities of the absorbents comprising MDEA and MDEA+TBAEE are still close to one another, the selectivity of the MDEA+TBAEE mixture always being lower. In both cases, the selectivity decreases when the absorbent circulation rate is increased. However, from about 50 kg/h upward, the selectivity of the MDEA absorbent is relatively constant, while the selectivity of the MDEA+TBAEE mixture decreases further. Thus, the higher the absorbent circulation rate, the more favorable it is to use TBAEE with MDEA compared to pure MDEA if not only a high level of H.sub.2S removal but also a high carbon dioxide coabsorption is to be achieved while maintaining defined minimum amounts.

EXAMPLE 4

[0139] In an absorption unit according to example 13 of EP 0 084 943 A2, absorption experiments were conducted with various absorbents.

[0140] In a first experiment, a gas mixture of 10% by volume of CO.sub.2 (partial CO.sub.2 pressure 0.1 bar), 1% by volume of H.sub.2S (partial H.sub.2S pressure 0.01 bar) and 89% by volume of N.sub.2 was passed through 100 mL of aqueous absorbent in a glass cylinder at a rate of 216 L (STP)/h and at a temperature of 40° C. The absorbent comprised 0.64 M MDEA or 0.64 M 1,2-bis(tert-butylamino)ethane (bis-TBAE). Aliquots of the absorbent were drawn off periodically, and the H.sub.2S and CO.sub.2 content was determined. The results are shown in FIG. 12. The H.sub.2S selectivity is shown as a function of the loading in mol(CO.sub.2+H.sub.2S) per mole of amine. It is apparent that, at low loadings and low partial pressures, MDEA and bis-TBAE have relatively similar selectivity. With rising loading, the selectivity of MDEA decreases much more rapidly than the H.sub.2S selectivity of bis-TBAE.

[0141] In a second experiment, a gas mixture of 90% by volume of CO.sub.2 (partial CO.sub.2 pressure 0.9 bar) and 10% by volume of H.sub.2S (partial H.sub.2S pressure 0.1 bar) was passed through 150 mL of aqueous absorbent in a glass cylinder at a rate of 10 L (STP)/h and at a temperature of 40° C. The absorbent comprised 0.64 M MDEA or 0.64 M 1,2-bis(tert-butylamino)ethane (bis-TBAE). Aliquots of the absorbent were drawn off periodically, and the H.sub.2S and CO.sub.2 content was determined. The results are shown in FIG. 13. The H.sub.2S selectivity is shown as a function of the loading in mol(CO.sub.2+H.sub.2S) per mole of amine. It was found that, at the greater partial pressures and the higher absorbent circulation rate, the H.sub.2S selectivity of MDEA is higher than that of bis-TBAE.