ABSORBENT FOR THE SELECTIVE REMOVAL OF HYDROGEN SULFIDE

Abstract

An absorbent for selective removal of hydrogen sulfide from a fluid stream comprises an aqueous solution comprising a) a tertiary amine; b) an aminic pH promoter selected from the compounds mentioned in the description; where the molar ratio of b) to a) is in the range from 0.05 to 1.0; and c) an acid having a pK.sub.A of less than 6 in such an amount that the pH of the aqueous solution measured at 120 C. is 7.9 to less than 8.8. Also described is a process for removing acidic gases from a fluid stream, in which the fluid stream is contacted with the absorbent. The absorbent is notable for a low regeneration energy.

Claims

1: An absorbent for selective removal of hydrogen sulfide from a fluid stream, the absorbent comprising an aqueous solution comprising: a) a tertiary amine of the general formula (I) ##STR00013## in which R.sub.1 is a C.sub.2-4-hydroxyalkyl and R.sub.2 and R.sub.3 are each independently selected from C.sub.1-3-alkyl and C.sub.2-4-hydroxyalkyl; b) an aminic pH promoter, the aminic pH promoter being selected from the group consisting of: compounds of the general formula (II): ##STR00014## in which R.sub.4 is selected from the group consisting of hydrogen; OR.sub.7 in which R.sub.7 is selected from the group consisting of hydrogen and C.sub.1-4-alkyl; and NR.sub.8R.sub.9 in which R.sub.8 and R.sub.9 are each independently selected from the group consisting of C.sub.1-5-alkyl and C.sub.2-5-hydroxyalkyl or R.sub.8 is hydrogen and R.sub.9 is selected from the group consisting of C.sub.1-5-alkyl and C.sub.2-5-hydroxyalkyl; R.sub.5 and R.sub.6 are each independently selected from the group consisting of C.sub.1-5-alkyl and C.sub.2-5-hydroxyalkyl or R.sub.5 is hydrogen and R.sub.6 is selected from the group consisting of C.sub.1-5-alkyl and C.sub.2-5-hydroxyalkyl; compounds of the general formula (III): ##STR00015## in which R.sub.10, R.sub.11, R.sub.12 and R.sub.13 are each independently selected from the group consisting of C.sub.1-5-alkyl, C.sub.1-5-aminoalkyl, C.sub.2-5-hydroxyalkyl and (di-C.sub.1-4-alkylamino)-C.sub.1-5-alkyl; and x is an integer from 2 to 4; compounds of the general formula (IV): ##STR00016## in which R.sub.14 and R.sub.15 are each independently selected from the group consisting of hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl, with the proviso that the R.sub.14 and R.sub.15 radicals on the carbon atom bonded directly to the nitrogen atom are C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl; and x is an integer from 2 to 4; and compounds of the general formula (V): ##STR00017## in which R.sub.16, R.sub.17 and R.sub.18 are each independently selected from C.sub.1-5-alkyl, C.sub.1-5-aminoalkyl, C.sub.2-5-hydroxyalkyl and (di-C.sub.1-4-alkylamino)-C.sub.1-5-alkyl; and c) an acid having a pK.sub.A of less than 6 in such an amount that a pH of the aqueous solution measured at 120 C. is 7.9 to less than 8.8, wherein a the molar ratio of b) to a) is in the range from 0.05 to 1.

2-3. (canceled)

4: The absorbent according to claim 1, wherein the total concentration of a) and b) in the aqueous solution is 10% to 60% by weight.

5: The absorbent according to claim 1, wherein the tertiary amine a) is methyldiethanolamine.

6: The absorbent according to claim 1, wherein the molar ratio of b) to a) ranges from 0.1 to 0.9.

7: The absorbent according to claim 1, wherein the acid is an acid having a pK.sub.A of less than 5.

8: The absorbent according to claim 1, wherein the acid is an inorganic acid.

9: A process for removing acidic gases from a fluid stream, the process comprising contacting the fluid stream with the absorbent according to claim 1.

10: The process according to claim 9, wherein the fluid stream comprises hydrocarbons.

11: The process according to claim 9, wherein there is a partial hydrogen sulfide pressure of at least 0.1 bar and a partial carbon dioxide pressure of at least 0.2 bar in the fluid stream.

12: The process according to claim 9, wherein the fluid stream has a total pressure of at least 3.0 bar.

13: The process according to claim 9, further comprising regenerating laden absorbent by a) heating, b) decompression, c) stripping with an inert fluid, or a combination of two or all of these measures.

Description

[0114] The invention is illustrated in detail by the appended drawing and the examples which follow.

[0115] FIG. 1 is a schematic diagram of a plant suitable for performing the process according to the invention.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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 generate 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 D, 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 D to the evaporator or conducted via a separate line directly from the bottom of the desorption column D to the heat exchanger 1.04.

[0120] 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.

EXAMPLES

[0121] In the examples, the following abbreviations are used:

[0122] MDEA: methyldiethanolamine

[0123] HMTATPA: N,N,N,N,N,N-hexamethyl-1,3,5-triazine-1,3,5(2H,4H,6H)tripropanamine

[0124] TBAEE: 2-(2-(tert-butylamino)ethoxy)ethanol

Reference Example

[0125] In this example, the correlation between the value of pH(50 C.)-pH(120 C.) and the regeneration energy required for various absorbents was examined.

[0126] The temperature dependence of the pH of aqueous amine solutions or partly neutralized amine solutions within the temperature range from 50 C. to 120 C. was determined. The Hamilton Polylite Plus 120 pH electrode was used, which is calibrated with pH 7 and pH 12 buffer solutions. A pressure apparatus with nitrogen blanketing was used, in which the pH can be measured up to 120 C.

[0127] The table which follows reports the pH (50 C.), the pH (120 C.) and the difference pH(50 C.)-pH(120 C.) for illustrative aqueous compositions.

TABLE-US-00002 pH pH pH (50 C.) Ex. Aqueous composition b/a* (50 C.) (120 C.) pH (120 C.) R1-1 40% MDEA 11.01 9.58 1.43 R1-2 40% MDEA + 0.5% H.sub.3PO.sub.4 9.76 8.29 1.47 R-1-3 30% MDEA + 15% TBAEE 0.37 11.19 9.30 1.89 R1-4 30% MDEA + 15% TBAEE + 0.8% H.sub.3PO.sub.4 0.37 10.21 8.49 1.72 R1-5 30% MDEA + 15% TBAEE + 1.6% H.sub.3PO.sub.4 0.37 9.82 8.06 1.76 R1-6 30% MDEA + 15% TBAEE + 0.6% H.sub.2SO.sub.4 0.37 10.21 8.40 1.81 R1-7 30% MDEA + 15% TBAEE + 0.8% H.sub.2SO.sub.4 0.37 9.89 8.16 1.73 R1-8 30% MDEA + 15% TBAEE + 1.2% H.sub.2SO.sub.4 0.37 9.79 8.13 1.66 R1-9 30% MDEA + 15% TBAEE + 1.6% H.sub.2SO.sub.4 0.37 9.77 7.90 1.87 R1-10 35% MDEA + 10% 4-hydroxy-2,2,6,6-tetramethylpiperidine + 0.9% H.sub.2SO.sub.4 0.22 9.87 8.21 1.66 R1-11 35% MDEA + 10% 4-hydroxy-2,2,6,6-tetramethylpiperidine + 1.2% H.sub.2SO.sub.4 0.22 9.68 8.03 1.65 *molar ratio of b/a

[0128] It is clear that there is a greater difference between the pH values at 50 C. and 120 C. in the examples in which the aqueous composition comprises a further amine in addition to MDEA and an acid.

[0129] In a pilot plant, the heating energy introduced in the course of regeneration for a defined H.sub.2S concentration of the cleaned gas was examined for aqueous absorbents.

[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 93% by volume of N.sub.2, 5% 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 60 kg/h. The temperature of the absorbent was 50 C. The regeneration energy was adjusted such that an H.sub.2S concentration of 5 ppm was attained in the cleaned gas.

[0132] The following table shows the results of these experiments:

TABLE-US-00003 Relative regeneration Ex. Aqueous composition energy* [%] R2-1 40% MDEA 100.0 R2-2 40% MDEA + 0.5% H.sub.3PO.sub.4 73.3 R2-3 30% MDEA + 15% TBAEE 91.6 R2-4 30% MDEA + 15% TBAEE + 0.8% H.sub.3PO.sub.4 57.8 R2-5 30% MDEA + 15% TBAEE + 1.6% H.sub.3PO.sub.4 56.9 R2-6 30% MDEA + 15% TBAEE + 0.6% H.sub.2SO.sub.4 65.1 R2-7 30% MDEA + 15% TBAEE + 0.8% H.sub.2SO.sub.4 64.8 R2-8 30% MDEA + 15% TBAEE + 1.2% H.sub.2SO.sub.4 64.1 R2-9 30% MDEA + 15% TBAEE + 1.6% H.sub.2SO.sub.4 62.2 R2-10 35% MDEA + 10% 4-hydroxy-2,2,6,6- 70.5 tetramethylpiperidine + 0.9% H.sub.2SO.sub.4 R2-11 35% MDEA + 10% 4-hydroxy-2,2,6,6- 64.6 tetramethylpiperidine + 1.2% H.sub.2SO.sub.4 *relative to example 2-1*

[0133] It is clear that a greater difference pH(50 C.)-pH(120 C.) in the case of compositions having comparable pH values at 120 C. results in a lower regeneration energy.

Example 1

[0134] The temperature dependence of the pH of aqueous amine solutions or partly neutralized amine solutions within the temperature range from 50 C. to 120 C. was determined as in the reference example.

[0135] The table which follows reports the pH (50 C.), the pH (120 C.) and the difference pH(50 C.)-pH(120 C.).

TABLE-US-00004 pH pH pH (50 C.) Ex. Aqueous composition b/a ** (50 C.) (120 C.) pH (120 C.) 1-1 35% MDEA + 10% cyclohexyl-N,N-dimethyl- 0.27 9.62 8.05 1.57 amine + 1.0% H.sub.2SO.sub.4 1-2 30% MDEA + 15% bis(dimethylamino)cyclo- 0.35 9.83 8.14 1.69 hexane + 2.0% H.sub.2SO.sub.4 1-3 40% MDEA + 12% HMTATPA + 5.15% H.sub.2SO.sub.4 0.10 9.92 8.09 1.83 1-4 40% MDEA + 12% HMTATPA + 6.0% H.sub.2SO.sub.4 0.10 9.71 7.90 1.81 1-5 40% MDEA + 8.5% hydroxyethylbis(dimethyl- 0.11 9.52 7.90 1.62 aminopropyl)amine + 2.0% H.sub.2SO.sub.4

[0136] It is clear that the inventive absorbents have a relatively high difference pH(50 C.)-pH(120 C.). With regard to the results of the reference example, this serves as an indication of a low regeneration energy required.