METHOD FOR THE SELECTIVE REMOVAL OF HYDROGEN SULFIDE

Abstract

An absorbent for selective removal of hydrogen sulfide from a fluid stream comprising carbon dioxide and hydrogen sulfide comprises a) an amine compound of the formula (I)

##STR00001##

in which X, R.sup.1 to R.sup.7, x, y and z are as defined in the description; and b) a nonaqueous solvent; where the absorbent comprises less than 20% by weight of water. 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 high load capacity, high cyclic capacity, good regeneration capacity and low viscosity.

Claims

1: An absorbent for selective removal of hydrogen sulfide over carbon dioxide from a fluid stream, which comprises: a) an amine compound of the formula (II) ##STR00008## wherein R.sub.9 and R.sub.10 are independently alkyl; R.sub.11 is hydrogen or alkyl; R.sub.12, R.sub.13 and R.sub.14 are independently selected from the group consisting of hydrogen and C.sub.1-C.sub.5-alkyl; R.sub.15 and R.sub.16 are independently C.sub.1-C.sub.5-alkyl; x and y are integers from 2 to 4 and z is an integer from 1 to 3; or an amine compound of the formula (III) ##STR00009## wherein R.sub.17 and R.sub.18 are independently C.sub.1-C.sub.5-alkyl; R.sub.19, R.sub.20 and R.sub.22 are independently selected from the group consisting of hydrogen and C.sub.1-C.sub.5-alkyl; R.sub.21 is C.sub.1-C.sub.5-alkyl; R.sub.23 and R.sub.24 are independently C.sub.1-C.sub.5-alkyl; x and y are integers from 2 to 4 and z is an integer from 1 to 3; and b) a nonaqueous solvent that is a glycol or a polyalkylene glycol; wherein the absorbent comprises less than 20% by weight of water.

2. (canceled)

3: The absorbent according to claim 1, wherein the amine compound is a compound of the formula (II), selected from the group consisting of 2-(2-tert-butylaminoethoxy)ethyl-N,N-dimethylamine, 2-(2-tert-butylaminoethoxy)ethyl-N,N-diethylamine, 2-(2-tert-butylaminoethoxy)ethyl-N,N-dipropylamine, 2-(2-isopropylaminoethoxy)ethyl-N,N-dimethylamine, 2-(2-isopropylaminoethoxy)ethyl-N,N-diethylamine, 2-(2-isopropylaminoethoxy)ethyl-N,N-dipropylamine, 2-(2-(2-tert-butylaminoethoxy)ethoxy)ethyl-N,N-dimethylamine, 2-(2-(2-tert-butylaminoethoxy)ethoxy)ethyl-N,N-diethylamine, 2-(2-(2-tert-butylaminoethoxy)ethoxy)ethyl-N,N-dipropylamine, and 2-(2-tert-amylaminoethoxy)ethyl-N,N-dimethylamine.

4. (canceled)

5: The absorbent according to claim 1, wherein the amine compound is a compound of the formula (III), selected from the group consisting of pentamethyldiethylenetriamine, pentaethyldiethylenetriamine, pentamethyldipropylenetriamine, pentamethyldibutylenetriamine, hexamethylenetriethylenetetramine, hexaethylenetriethylenetetramine, hexamethylenetripropylenetetramine and hexaethylenetripropylenetetramine.

6-9. (canceled)

10: The absorbent according to claim 1, wherein the absorbent further comprises a tertiary amine or highly sterically hindered amine other than the compounds of the formula (I) and (II), wherein high steric hindrance means a tertiary carbon atom directly adjacent to a primary or secondary nitrogen atom.

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

12: The process according to claim 11, wherein the laden absorbent is regenerated by at least one measure selected from the group consisting of heating, decompressing and stripping with an inert fluid.

Description

[0109] The invention is illustrated in detail by the appended drawing and the examples which follow.

[0110] FIG. 1 is a schematic diagram of a plant suitable for performing the process of the invention.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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 of 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

[0116] The invention is illustrated in detail by the examples which follow.

[0117] The following abbreviations were used:

[0118] AEPD: 2-amino-2-ethylpropane-1,3-diol

[0119] BDMAEE: bis(2-(N,N-dimethylamino)ethyl) ether

[0120] EG: ethylene glycol

[0121] MDEA: methyldiethanolamine

[0122] PMDETA: pentamethyldiethylenetriamine

[0123] TBAEE: 2-(2-tert-butylaminoethoxy)ethanol

[0124] TBAAEDA: 2-(2-tert-butylaminoethoxy)ethyl-N,N-dimethylamine

[0125] TDG: thiodiglycol

[0126] TEG: triethylene glycol

Example 1: Preparation of 2-(2-tert-butylaminoethoxy)ethyl-N,N-dimethylamine (TBAEEDA)

[0127] An oil-heated glass reactor having a length of 0.9 m and an internal diameter of 28 mm was charged with quartz wool. The reactor was charged with 200 mL of V2A mesh rings (diameter 5 mm), above that 100 mL of a copper catalyst (support: alumina) and finally 600 mL of V2A mesh rings (diameter 5 mm).

[0128] Subsequently, the catalyst was activated as follows: Over a period of 2 h, at 160 C., a gas mixture consisting of H.sub.2 (5% by volume) and N.sub.2 (95% by volume) was passed over the catalyst at 100 L/h. Thereafter, the catalyst was kept at a temperature of 180 C. for a further 2 h. Subsequently, at 200 C. over a period of 1 h, a gas mixture consisting of H.sub.2 (10% by volume) and N.sub.2 (90% by volume) was passed over the catalyst, then, at 200 C. over a period of 30 min, a gas mixture consisting of H.sub.2 (30% by volume) and N.sub.2 (70% by volume) and finally, at 200 C. over a period of 1 h, H.sub.2.

[0129] 50 g/h of a mixture of tert-butylamine (TBA) and 2-[dimethylamino(ethoxy)]ethan-1-ol (DMAEE, CAS 1704-62-7, Sigma-Aldrich) in a TBA:DMAEE weight ratio=4:1 were passed over the catalyst at 200 C. together with hydrogen (40 L/h). The reaction output was condensed by means of a jacketed coil condenser and analyzed by means of gas chromatography (column: 30 m Rtx-5 Amine from Restek, internal diameter: 0.32 mm, d.sub.f: 1.5 m, temperature program 60 C. to 280 C. in steps of 4 C./min). The following analysis values are reported in GC area percent.

[0130] The GC analysis shows a conversion of 96% based on DMAEE used, and 2-(2-tert-butylaminoethoxy)ethyl-N,N-dimethylamine (TBAEEDA) was obtained in a selectivity of 73%. The crude product was purified by distillation. After the removal of excess tert-butylamine under standard pressure, the target product was isolated at a bottom temperature of 95 C. and a distillation temperature of 84 C. at 8 mbar in a purity of >97%.

Example 2: pK.SUB.A .Values and Temperature Dependence of the pK.SUB.A .Values

[0131] The pKa values of various amine compounds were determined at concentrations of 0.01 mol/kg at 20 C. or 120 C. by determining the pH at the point of half-equivalence of the dissociation stage under consideration by means of addition of hydrochloric acid (1st dissociation stage 0.005 mol/kg; 2nd dissociation stage: 0.015 mol/kg; 3rd dissociation stage: 0.025 mol/kg). Measurement was accomplished using a thermostated closed jacketed vessel in which the liquid was blanketed with nitrogen. The Hamilton Polylite Plus 120 pH electrode was used, which was calibrated with pH 7 and pH 12 buffer solutions.

[0132] The pK.sub.A of the tertiary amine MDEA is reported for comparison. The results are shown in the following table:

TABLE-US-00001 Amine pK.sub.A1 pK.sub.A2 pK.sub.A3 pK.sub.A1 (120-20 C.) TBAEEDA 10.4 8.4 2.4 BDMAEE 9.7 8.2 * PMDETA 10.3 8.8 6.5 * MDEA 8.7 1.8 *not determined

[0133] The result of a marked temperature dependence of the pKa is that, at relatively lower temperatures as exist in the absorption step, the higher pK.sub.A promotes efficient acid gas absorption, whereas, at relatively higher temperatures as exist in the desorption step, the lower pK.sub.A supports the release of the absorbed acid gases. It is expected that a great pK.sub.A differential for an amine between absorption and desorption temperature will result in a comparatively small regeneration energy.

Example 3: Loading Capacity, Cyclic Capacity and H.SUB.2.S:CO.SUB.2 .Loading Capacity Ratio

[0134] A loading experiment and then a stripping experiment were conducted.

[0135] A glass condenser, which was operated at 5 C., was attached to a glass cylinder with a thermostated jacket. This prevented distortion of the test results by partial evaporation of the absorbent. The glass cylinder was initially charged with about 100 mL of unladen absorbent (30% by weight of amine in water). To determine the absorption capacity, at ambient pressure and 40 C., 8 L (STP)/h of CO.sub.2 or H.sub.2S were passed through the absorption liquid via a frit over a period of about 4 h. Subsequently, the loading of CO.sub.2 or H.sub.2S was determined as follows:

[0136] 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, the 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).

[0137] The laden solution was stripped by heating an identical apparatus setup to 80 C., introducing the laden absorbent and stripping it by means of an N.sub.2 stream (8 L (STP)/h). After 60 min, a sample was taken and the CO.sub.2 or H.sub.2S loading of the absorbent was determined as described above.

[0138] The difference in the loading at the end of the loading experiment and the loading at the end of the stripping experiment gives the respective cyclic capacities. 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. The product of cyclic H.sub.2S capacity and H.sub.2S:CO.sub.2 loading capacity ratio is referred to as the efficiency factor .

[0139] The H.sub.2S:CO.sub.2 loading capacity ratio serves as an indication of the expected H.sub.2S selectivity. The efficiency factor can be used in order to assess absorbents in terms of their suitability for the selective H.sub.2S removal from a fluid stream, taking account of the H.sub.2S:CO.sub.2 loading capacity ratio and the H.sub.2S capacity. The results are shown in Table 1.

TABLE-US-00002 TABLE 1 CO.sub.2 loading H.sub.2S loading H.sub.2S:CO.sub.2 [m.sup.3 (STP)/t] Cyclic [m.sub.3 (STP)/t] Cyclic loading Efficiency Absorbent after after CO.sub.2 capacity after after H.sub.2S capacity capacity factor # Amine Solvent loading stripping [m.sup.3 (STP)/t] loading stripping [m.sup.3 (STP)/t] ratio 1* 10% by wt. 90% by wt. of 22.2 4.7 17.5 22.0 3.2 18.8 1.0 of water TBAEEDA 2 10% by wt. 90% by wt. of 14.9 1.3 13.6 17.0 2.5 14.5 1.1 of EG TBAEEDA 3 10% by wt. 90% by wt. of 5.3 0.7 4.6 17.0 3.0 14.0 3.2 of TEG TBAEEDA 4 10% by wt. 90% by wt. of 1.4 1.3 1.1 9.2 1.7 7.5 6.6 of sulfolane TBAEEDA 5* 30% by wt. 70% by wt. of 70.1 7.4 62.7 58.8 7.4 51.4 0.8 41.1 of water BDMAEE 6 30% by wt. 70% by wt. of 18.9 1.5 17.4 46.7 7.4 39.3 2.5 98.3 of EG BDMAEE 7 30% by wt. 70% by wt. of 2.2 0.2 2.0 23.9 3.2 20.7 10.8 223.6 of TEG BDMAEE 8 30% by wt. 70% by wt. of 11.3 0.8 10.5 30.5 1.3 29.2 2.7 78.8 of TDG BDMAEE 9 30% by wt. 70% by wt. of 0.4 0.1 0.3 18.4 2.3 16.1 46 740.6 of sulfolane BDMAEE 10* 30% by wt. 70% by wt. of 68.7 9.2 59.5 60.0 9.6 50.4 0.9 45.4 of water PMDETA 11 30% by wt. 70% by wt. of 23.8 1.4 22.4 50.3 2.5 47.8 2.1 100.4 of EG PMDETA 12 30% by wt. 70% by wt. of 1.0 0.3 0.7 26.4 0.8 25.6 26.4 675.8 of TEG PMDETA 13* 40% by wt. 60% by wt. of 56.1 4.6 51.5 51.4 1.4 50.0 0.9 45.0 of water MDEA 14* 30% by wt. 70% by wt. of 15.5 0.2 15.3 34.2 2.6 31.6 2.2 69.5 of EG MDEA 15* 30% by wt. 70% by wt. of 4.4 0.1 4.3 26.5 0.2 26.3 6.0 157.8 of TEG MDEA 16* 30% by wt. 70% by wt. of 3.3 0.1 3.2 18.2 0.1 18.1 5.5 99.6 of MDEA sulfolane *comparative example

[0140] It is clear from the examples in table 1 that aqueous absorbents have high cyclic H.sub.2S capacity but a lower efficiency factor . Nonaqueous absorbents of the invention (for a given amine component) exhibit higher efficiency factors .

Example 5: Thermal Stability

[0141] A Hastelloy cylinder (10 mL) was initially charged with the absorbent (30% by weight amine solution, 8 mL) and the cylinder was closed. The cylinder was heated to 160 C. for 125 h. The acid gas loading of the solutions was 20 m.sup.3 (STP)/t.sub.solvent of CO.sub.2 and 20 m.sup.3 (STP)/t.sub.solvent of H.sub.2S. The decomposition level of the amines was calculated from the amine concentration measured by gas chromatography before and after the experiment. The results are shown in the following table:

TABLE-US-00003 Decomposition Absorbent level 30% by wt. of MDEA + 70% by wt. of water 15% 30% by wt. of TBAEEDA + 70% by wt. of water 9%

[0142] It is clear that TBAEEDA has a higher thermal stability than MDEA.

Example 6: Viscosity

[0143] The dynamic viscosities of various compounds were measured in a viscometer (Anton Paar Stabinger SVM3000 viscometer).

[0144] The results are shown in the following table:

TABLE-US-00004 Amine Dynamic viscosity [mPa .Math. s] MDEA* 34.1 TBAEE* 16.9 AEPD* 1844 BDMAEE 0.9 PMDETA 1.0 TBAEEDA 1.5 *comparative compound

[0145] In addition, the dynamic viscosities of various absorbents (without acid gas loading) were measured in the same instrument.

[0146] The results are shown in the following table:

TABLE-US-00005 Absorbent Dynamic viscosity Amine (30% by wt.) Solvent (70% by wt.) [mPa .Math. s] MDEA* EG 15.7 MDEA* sulfolane 8.2 MDEA* TEG 22.7 TBAEE* EG 17.2 AEPD* EG 25.3 BDMAEE EG 12.3 BDMAEE sulfolane 3.6 PMDETA TEG 15.3 TBAEEDA sulfolane 5.5 *comparative example

[0147] It is clear that the dynamic viscosity of the inventive absorbents is much lower than that of the comparative examples.