Absorbent for selective removal of hydrogen sulfide from a fluid stream

Abstract

An absorbent for selective removal of hydrogen sulfide over carbon dioxide from a fluid stream comprises an aqueous solution of a) a tertiary amine, b) a sterically hindered secondary amine of the general formula (I) ##STR00001##
in which R.sub.1 and R.sub.2 are each independently selected from C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are each independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl, with the proviso that at least one R.sub.4 and/or R.sub.5 radical on the carbon atom bonded directly to the nitrogen atom is C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl when R.sub.3 is hydrogen; x and y are integers from 2 to 4 and z is an integer from 1 to 4; where the molar ratio of b) to a) is in the range from 0.05 to 1.0, and c) an acid in an amount, calculated as neutralization equivalent relative to the protonatable nitrogen atoms in a) and b), of 0.05 to 15.0%. One preferred amine of the formula I is 2-(2-tert-butylaminoethoxy)ethanol. The absorbent allows a defined H.sub.2S selectivity to be set at pressures of the kind typical in natural gas processing.

Claims

1. A process for the selective removal of hydrogen sulfide over carbon dioxide from a fluid stream, said process comprising: contacting the fluid stream which is selected from gases and which has a total pressure of at least 3.0 bar, with an absorbent, comprising an aqueous solution comprising: a) a tertiary amine; b) a sterically hindered secondary amine of the general formula (I) ##STR00005## in which R.sub.1 and R.sub.2 are each independently selected from C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are each independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl, with the proviso that at least one R.sub.4 and/or R.sub.5 radical on the carbon atom bonded directly to the nitrogen atom is C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl when R.sub.3 is hydrogen; x and y are integers from 2 to 4 and z is an integer from 1 to 4; where the molar ratio of b) to a) is in the range from 0.3 to 0.7, and c) an acid in an amount, calculated as neutralization equivalent relative to the protonatable nitrogen atoms in a) and b), of 1.0 to 9.0%; wherein the absorbent does not comprise a sterically unhindered primary or secondary amine, there is a partial hydrogen sulfide pressure of at least 0.1 bar and/or a partial carbon dioxide pressure of at least 0.2 bar in the fluid stream; and wherein the selectivity for hydrogen sulfide over carbon dioxide is less than 1.6.

2. The process according to claim 1, wherein the fluid stream comprises a hydrocarbon.

3. The process according to claim 1, wherein there is a partial hydrogen sulfide pressure of at least 0.1 bar and a partial carbon dioxide pressure of at least 1.0 bar in the fluid stream.

4. The process according to claim 1, wherein the laden absorbent is regenerated by a) heating, b) decompression, c) stripping with an inert fluid or a combination of two or all of these measures.

Description

(1) FIG. 1 is a schematic diagram of a plant suitable for performing the process according to the invention.

(2) 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.

(3) 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.

(4) 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.

(5) 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.

(6) 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, 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.

(7) 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

(8) In the examples, the following abbreviations are used:

(9) MDEA: methyldiethanolamine

(10) TBAEE: 2-(2-tert-butylaminoethoxy)ethanol

Example 1

(11) The temperature dependence of the pH of aqueous amine solutions or partly neutralized amine solutions was determined in the temperature range from 20° C. to 120° C. A pressure apparatus was used, in which the pH can be measured up to 120° C.

(12) The table which follows reports the pH (50° C.), the pH (120° C.) and the difference pH (50° C.)−pH (120° C.).

(13) TABLE-US-00001 DN*** pH pH pH(50° C.) − Ex. Composition b/a ** [%] (50° C.) (120° C.) pH(120° C.) 1-1* 40% MDEA — — 11.01 9.58 1.43 1-2* 40% MDEA + 0.5% H.sub.3PO.sub.4 — 5.02 9.76 8.29 1.47 1-3 40% MDEA + 6.2% TBAEE + 0.118% H.sub.2SO.sub.4 0.11 0.64 10.87 9.2 1.67 1-4 40% MDEA + 6.2% TBAEE + 1.89% H.sub.2SO.sub.4 0.11 10.30 9.45 7.9 1.55 1-5 30% MDEA + 15% TBAEE + 0.3% H.sub.2SO.sub.4 0.37 1.77 10.57 8.83 1.74 1-6 30% MDEA + 15% TBAEE + 0.6% H.sub.2SO.sub.4 0.37 3.55 10.21 8.4 1.81 1-7 30% MDEA + 15% TBAEE + 0.8% H.sub.2SO.sub.4 0.37 4.73 9.89 8.16 1.73 1-8 30% MDEA + 15% TBAEE + 1.2% H.sub.2SO.sub.4 0.37 7.10 9.79 8.13 1.66 1-9 30% MDEA + 15% TBAEE + 1.6% H.sub.2SO.sub.4 0.37 9.46 9.77 7.9 1.87 1-10 30% MDEA + 15% TBAEE + 0.3% H.sub.3PO.sub.4 0.37 2.66 10.56 8.81 1.75 1-11 30% MDEA + 15% TBAEE + 0.8% H.sub.3PO.sub.4 0.37 7.10 10.21 8.49 1.72 1-12 30% MDEA + 15% TBAEE + 1.6% H.sub.3PO.sub.4 0.37 14.21 9.82 8.06 1.76 *comparative example ** molar ratio of b/a ***degree of neutralization (based on TBAEE + MDEA)

(14) It is clear that there is a greater difference between the pH values at 50° C. and 120° C. in the inventive examples. Since the absorption is effected in the region of 50° C. and desorption or regeneration in the region of 120° C., the greater pH differential is a pointer to an energetically improved regeneration.

Example 2

(15) In a pilot plant, the CO.sub.2 absorption and the heating energy introduced in the course of regeneration for a defined H.sub.2S concentration of the cleaned gas were examined for aqueous absorbents.

(16) 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.

(17) 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.

(18) The following table shows the results of these experiments:

(19) TABLE-US-00002 Relative y(CO.sub.2) at regeneration absorber outlet energy** Ex. Aqueous composition [% by vol.] [%] 2-1* 40% MDEA 1.87 100.0 2-2* 40% MDEA + 0.5% H.sub.3PO.sub.4 1.89 73.3 2-3* 30% MDEA + 15% TBAEE 0.91 91.6 2-4 30% MDEA + 15% TBAEE + 0.99 57.8 0.8% H.sub.3PO.sub.4 2-5* 30% MDEA + 15% TBAEE + 1.15 56.9 1.6% H.sub.3PO.sub.4 2-6 30% MDEA + 15% TBAEE + 1.54 65.1 0.6% H.sub.2SO.sub.4 2-7 30% MDEA + 15% TBAEE + 1.47 64.8 0.8% H.sub.2SO.sub.4 2-8 30% MDEA + 15% TBAEE + 1.50 64.1 1.2% H.sub.2SO.sub.4 2-9 30% MDEA + 15% TBAEE + 1.55 62.2 1.6% H.sub.2SO.sub.4 *comparative example **with regard to example 2-1*

(20) In a comparison of comparative example 2-2* with examples 2-4 to 2-9, it is clear that the additional use of TBAEE brings about an increased CO.sub.2 absorption (lower CO.sub.2 concentration y(CO.sub.2) at the absorber outlet) for the same H.sub.2S absorption. At the same time, the heating energy introduced in the regeneration remains approximately the same or falls. The comparison of comparative example 2-3* with examples 2-6 to 2-9 shows that the addition of acid significantly lowers the heating energy introduced in the course of the regeneration. Since the H.sub.2S concentration in the purified gas was always 5 ppm, the examples show how varying the compositions within the limits according to the invention permits the setting of a defined H.sub.2S selectivity.

Example 3

(21) The stability of various aqueous absorbents was investigated.

(22) Aqueous solutions having an MDEA and TBAEE content in accordance with the table below, and a loading in each case of 20 m.sup.3 (STP)/t(absorbent) CO.sub.2 and H.sub.2S were held in a closed vessel at a temperature of 160° C. for 125 hours. Subsequently the amount of undecomposed MDEA was determined, and the fraction of decomposed MDEA was calculated.

(23) The results are listed in the table below.

(24) TABLE-US-00003 MDEA TBAEE [% by [% by DN** Decomposed Ex. wt.] wt.] Acid [%] MDEA [%] 3-1* 35.7 12.1 3.7% by wt. H.sub.2SO.sub.4 20.1 20 3-2 35.7 12.1 2.0% by wt. H.sub.2SO.sub.4 10.9 19 3-3 35.7 12.1 1.0% by wt. H.sub.2SO.sub.4 5.4 14 3-4 35.7 12.1 0.5% by wt. H.sub.2SO.sub.4 2.7 8 3-5* 35.7 12.1 — — 2.5 *comparative example **degree of neutralization (based on TBAEE + MDEA).

(25) It is evident that the presence of acid accelerates the decomposition of MDEA. The degree of decomposition is dependent on the amount of acid, which is why a relatively small amount of acid as in the compositions according to the invention is advantageous.