ABSORPTION AGENT AND A METHOD FOR SELECTIVELY REMOVING HYDROGEN SULPHIDE

Abstract

An absorbent for selective removal of hydrogen sulfide from a fluid stream comprising carbon dioxide and hydrogen sulfide, which comprises a) 10% to 70% by weight of at least one sterically hindered secondary amine having at least one ether group and/or at least one hydroxyl group in the molecule; b) at least one nonaqueous solvent having at least two functional groups selected from ether groups and hydroxyl groups in the molecule; and c) optionally a cosolvent; where the hydroxyl group density of the absorbent .sub.abs is in the range from 8.5 to 35 mol(OH)/kg. Also described is a process for selectively removing hydrogen sulfide from a fluid stream comprising carbon dioxide and hydrogen sulfide, wherein the fluid stream is contacted with the absorbent. The absorbent features good regeneration capacity and high cyclic acid gas capacity.

Claims

1. An absorbent for selective removal of hydrogen sulfide over carbon dioxide from a fluid stream, which comprises: a) 10% to 70% by weight of at least one sterically hindered secondary amine having at least one ether group and at least one hydroxyl group in the molecule; b) at least one nonaqueous solvent having at least two functional groups selected from the group consisting of ether groups and hydroxyl groups in the molecule; and c) optionally a cosolvent; where a hydroxyl group density of the absorbent .sub.abs is in a range from 8.5 to 35 mol(OH)/kg.

2. The absorbent according to claim 1, wherein a contribution .sub.a of the sterically hindered secondary amine a) to .sub.abs is in a range from 0 to 6 mol(OH)/kg and a contribution .sub.b of the nonaqueous solvent b) to .sub.abs is in a range from 2.5 to 35 mol(OH)/kg.

3. The absorbent according to claim 1, wherein the sterically hindered secondary amine a) comprises an isopropylamino group, a tert-butylamino group or a 2,2,6,6-tetramethylpiperidinyl group.

4. The absorbent according to claim 1, wherein the sterically hindered secondary amine a) is selected from the group consisting of 2-(2-tert-butylaminoethoxy)ethanol, 2-(2-isopropylaminoethoxy)ethanol, 2-(2-(2-tert-butylaminoethoxy)ethoxy)ethanol, 2-(2-(2-isopropylaminoethoxy)ethoxy)ethanol, 4-(3-hydroxypropoxy)-2,2,6,6-tetramethylpiperidine and 4-(4-hydroxybutoxy)-2,2,6,6-tetramethylpiperidine.

5. The absorbent according to claim 1, wherein the nonaqueous solvent b) at a temperature of 293.15 K and a pressure of 1.0133.Math.10.sup.5 Pa has a relative dielectric constant c of at least 7.

6. The absorbent according to claim 1, wherein the absorbent comprises the nonaqueous solvent b) and a cosolvent c) in such proportions by mass that a mixture of the nonaqueous solvent b) and a cosolvent c) in a ratio of these proportions by mass at a temperature of 293.15 K and a pressure of 1.0133.Math.10.sup.5 Pa has a relative dielectric constant 8 of at least 7.

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

8. The absorbent according to claim 1, wherein the nonaqueous solvent b) is selected from the group consisting of C.sub.2-C.sub.8 diols, poly(C.sub.2-C.sub.4-alkylene glycols), poly(C.sub.2-C.sub.4-alkylene glycol) monoalkyl ethers and poly(C.sub.2-C.sub.4-alkylene glycol) dialkyl ethers.

9. The absorbent according to claim 8, wherein the nonaqueous solvent b) is selected from the group consisting of ethane-1,2-diol, propane-1,2-diol, propane-1,3-diol, butane-1,4-diol, diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether and tetraethylene glycol monomethyl ether.

10. The absorbent according to claim 1, wherein the cosolvent c) is present, and is selected from the group consisting of water, C.sub.4-C.sub.10 alcohols, esters, lactones, amides, lactams, sulfones and cyclic ureas.

11. The absorbent according to claim 10, wherein the cosolvent c) is selected from the group consisting of n-butanol, n-pentanol, n-hexanol, sulfolane, N-methyl-2-pyrrolidone, dimethylpropyleneurea and -butyrolactone.

12. The absorbent according to claim 1, wherein the absorbent comprises 20% to 60% by weight of the sterically hindered secondary amine a), 20% to 80% by weight of the nonaqueous solvent b) and 10% to 60% by weight of the cosolvent c), where the cosolvent c) comprises not more than 20% by weight, based on the weight of the absorbent, of water.

13. A process for selectively removing hydrogen sulfide over carbon dioxide from a fluid stream, comprising contacting the fluid stream with the absorbent according to claim 1 to obtain a laden absorbent and a treated fluid stream.

14. The process according to claim 13, further comprising regenerating the laden absorbent by at least one of the measures of heating, decompressing and stripping with an inert fluid.

Description

[0074] The invention is illustrated in detail by the appended drawing and the examples which follow.

[0075] FIG. 1 is a schematic diagram of a plant suitable for performing the process according to the invention.

[0076] 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.

[0077] 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.

[0078] Between the absorber A1 and heat exchanger 1.04, one or more flash vessels 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.

[0079] From the lower part of the desorption column D, the absorbent is conducted into the boiler 1.07, where it is heated. The steam that arises 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 for energy introduction, 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 steam 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.

[0080] 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, the condensate 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

[0081] The following table shows the hydroxyl group density p of selected compounds:

TABLE-US-00001 Number Molar of OH mass [mol(OH)/ Compound groups [g/mol] kg] Methanol 1 32.04 31.21 n-Butanol 1 74.12 13.49 n-Pentanol 1 88.15 11.34 n-Hexanol 1 102.18 9.79 Ethane-1,2-diol (ethylene glycol, EG) 2 62.07 32.22 Propane-1,3-diol 2 76.09 26.28 Butane-1,4-diol 2 90.12 22.19 Diethylene glycol (DEG) 2 106.12 18.85 Triethylene glycol (TEG) 2 150.18 13.32 Tetraethylene glycol 2 194.23 10.30 Pentaethylene glycol 2 238.30 8.39 Diethylene glycol monomethyl ether 1 120.15 8.32 Diethylene glycol monoethyl ether 1 134.18 7.45 Diethylene glycol monopropyl ether 1 148.20 6.75 Triethylene glycol monomethyl ether 1 164.20 6.09 Triethylene glycol monoethyl ether 1 178.20 5.61 Triethylene glycol monopropyl ether 1 192.25 5.20 Tetraethylene glycol monomethyl ether 1 208.26 4.80 Polyethylene glycol dimethyl ether 0 250.00* 0.00 (PEGDME) Dimethylethanolamine (DMAE) 1 89.14 11.22 Methyldiethanolamine (MDEA) 2 119.16 16.78 2-(Isopropylamino)ethanol (IPAE) 1 103.16 9.69 2-Isopropylamino-1-propanol (IPAP) 1 117.19 8.53 2-(2-Isopropylaminoethoxy)ethanol 1 147.00 6.80 (IPAEE) tert-Butylaminoethanol (TBAE) 1 117.19 8.53 2-(2-tert-Butylaminoethoxy)ethanol 1 161.00 6.21 (TBAEE) Dibutylaminoethanol (DBAE) 1 173.3 5.77 Triethanolamine (TEA) 3 149.2 20.11 Sulfolane 0 120.17 0.00 Water 2 18.02 110.99 *mean molar mass

Example 1

[0082] A thermostated jacketed glass cylinder was initially charged with about 250 mL of unladen absorbent according to table 1. In order to prevent any loss of absorbent during the experiment, a glass condenser which was operated at 5 C. was connected at the top of the glass cylinder. To determine the absorption capacity, at ambient pressure and 40 C., 8 L (STP)/h of H.sub.2S or CO.sub.2 were passed through the absorption liquid via a frit. After the experiment had run for 4 h, the maximum loading had been attained. This was verified by sampling after 1, 2 and 3 h. The loading of CO.sub.2 or H.sub.2S was determined as follows:

[0083] The determination of H.sub.2S was effected by titration with silver nitrate solution. For this purpose, the sample to be analyzed was weighed into an aqueous solution together with about 2% by weight of sodium acetate and about 3% by weight of ammonia. Subsequently, the H.sub.2S content was determined by a potentiometric turning point titration by means of silver nitrate solution. At the turning point, H.sub.2S is fully bound as Ag.sub.2S. The CO.sub.2 content was determined as total inorganic carbon (TOC-V Series Shimadzu).

[0084] The loading of CO.sub.2 and H.sub.2S was identical within the measurement accuracy after an experiment duration of 3 h and 4 h. The H.sub.2S:CO.sub.2 loading capacity ratio was calculated as the quotient of the H.sub.2S loading divided by the CO.sub.2 loading.

[0085] The laden solution was stripped by heating the apparatus to 80 C., introducing the laden absorbent and stripping it by means of a nitrogen stream (8 L (STP)/h) at ambient pressure. After 30 min, a sample was taken and the CO.sub.2 or H.sub.2S loading of the absorbent was determined as described above.

[0086] The results are shown in table 1.

TABLE-US-00002 TABLE 1 .sub.abs CO.sub.2 loading H.sub.2S loading H.sub.2S:CO.sub.2 Absorbent [mol(OH)/ [mol(CO.sub.2)/mol(amine)] [mol(H.sub.2S)/mol(amine)] loading # Composition kg] after loading after stripping after loading after stripping capacity ratio 1-1* 40% by wt. of MDEA + 73.31 0.683 0.019 0.744 0.062 1.09 60% by wt. of water 1-2* 30% by wt. of MDEA + 27.59 0.275 0.015 0.605 0.046 2.2 70% by wt. of EG 1-3* 30% by wt. of MDEA + 14.36 0.078 0.001 0.468 0.003 6 70% by wt. of TEG 1-4* 30% by wt. of MDEA + 5.04 0.058 0.001 0.323 0.001 5.6 70% by wt. of sulfolane 1-5* 30% by wt. of TBAEE + 79.55 0.972 0.236 0.922 0.250 0.95 70% by wt. of water 1-6 30% by wt. of TBAEE + 24.42 0.795 0.007 1.101 0.154 1.38 70% by wt. of EG 1-7 30% by wt. of TBAEE + 11.19 0.280 0.001 1.192 0.006 4.25 70% by wt. of TEG 1-8* 30% by wt. of TBAEE + 1.86 0.060 0.00 0.837 0.002 13.95 70% by wt. of sulfolane 1-9 30% by wt. of TBAEE + 11.5 0.467 0.004 0.907 0.010 1.94 30% by wt. of EG + 40% by wt. of sulfolane 1-10* 30% by wt. of TBAEE + 5.8 0.132 0.001 0.780 0.005 5.9 30% by wt. of TEG + 40% by wt. of sulfolane 1-11 30% by wt. of TBAE + 25.1 0.828 0.019 ** ** ** 70% by wt. of EG 1-12 30% by wt. of TBAE + 11.9 0.369 0.002 ** ** ** 70% by wt. of TEG 1-13 30% by wt. of IPAEE + 24.6 0.707 0.034 ** ** ** 70% by wt. of EG 1-14 30% by wt. of IPAE + 25.5 0.636 0.027 ** ** ** 70% by wt. of EG 1-15* 30% by wt. of DBAE + 24.3 0.340 0.002 ** ** ** 70% by wt. of EG 1-16* 30% by wt. of TEA + 28.6 0.137 0.002 ** ** ** 70% by wt. of EG 1-17* 30% by wt. of MDEA + 5.04 0.029 0.001 0.218 0.001 7.5 70% by wt. of PEGDME 1-18* 30% by wt. of TBAEE + 1.86 0.030 0.001 0.396 0.001 13.2 70% by wt. of PEGDME *comparative example **not determined

[0087] Examples 1-1 to 1-4 and 1-5 to 1-8 show that the H.sub.2S:CO.sub.2 loading capacity ratio increases with decreasing hydroxyl group density .sub.abs. A decreasing hydroxyl group density .sub.abs likewise results in improved regeneration, apparent from low residual H.sub.2S and CO.sub.2 loadings after stripping. Too low a hydroxyl group density .sub.abs results in reduced CO.sub.2 and H.sub.2S loading capacities, as apparent from examples 1-8, 1-9, 1-10, 1-17 and 1-18.

[0088] It is clear from the comparison of examples 1-6 and 1-7 with comparative examples 1-2 and 1-3 that the sterically hindered secondary amine TBAEE, as compared with the tertiary amine MDEA, allows elevated CO.sub.2 and H.sub.2S loading combined with comparable H.sub.2S:CO.sub.2 loading capacity ratio and similarly good regeneration.