Cyclic amine for selectively removing hydrogen sulphide

Abstract

The use of an amine of the formula (I) ##STR00001##
in which the R.sup.1 to R.sup.5 radicals are each as defined in the description, and an absorbent and a process for removing acidic gases from a fluid stream, especially for selectively removing hydrogen sulfide over carbon dioxide. The invention also relates to particular amines suitable for selective removal of hydrogen sulfide. Absorbents based on amines of the formula (I) have high selectivity, high loading capacity and good regeneration capacity.

Claims

1. 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 an absorbent comprising an amine of formula (I): ##STR00007## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently selected from C.sub.1-C.sub.5-alkyl and C.sub.2-C.sub.5-hydroxyalkyl; R.sup.5 is NR.sup.6R.sup.7; and R.sup.6 is selected from hydrogen, C.sub.1-C.sub.5-alkyl and C.sub.2-C.sub.5-hydroxyalkyl and R.sup.7 is selected from C.sub.1-C.sub.5-alkyl and C.sub.2-C.sub.5-hydroxyalkyl, with the proviso that, when R.sup.6 is hydrogen, R.sup.7 is C.sub.3-C.sub.5-alkyl bonded to the nitrogen atom via a secondary or tertiary carbon atom; to obtain a treated fluid stream and a laden absorbent, wherein the following expression is satisfied:
(mol(H.sub.2S)/(mol(CO.sub.2) in the liquid phase/(mol(H.sub.2S)/(mol(CO.sub.2)) in the gas phase≥1.

2. The process according to claim 1, in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are methyl.

3. The process according to claim 1, wherein the absorbent is an aqueous solution.

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

5. The process according to claim 1, wherein the absorbent comprises an acid having a pK.sub.A of less than 6.

6. The process according to claim 1, wherein the absorbent comprises a tertiary amine or highly sterically hindered amine.

7. The process according to claim 1, wherein a residual carbon dioxide content in the treated fluid stream is at least 0.5% by volume.

8. The process according to claim 1, wherein the laden absorbent is regenerated by at least one selected from the group consisting of heating, decompressing and stripping with an inert fluid.

9. The process according to claim 1, wherein R.sup.7 is C.sub.2-C.sub.5-hydroxyalkyl.

10. The process according to claim 1, wherein R.sup.5 is NR.sup.6R.sup.7; R.sup.6 is selected from hydrogen and C.sub.1-C.sub.5-alkyl and R.sup.7 is C.sub.1-C.sub.5-alkyl, with the proviso that, when R.sup.6 is hydrogen, R.sup.7 is C.sub.3-C.sub.5-alkyl bonded to the nitrogen atom via a secondary or tertiary carbon atom.

11. The process according to claim 10, wherein the amine of formula (I) is selected from the group consisting of: 4-(N,N-dimethylamino)-2,2,6,6-tetramethylpiperidine, 4-(N,N-diethylamino)-2,2,6,6-tetramethylpiperidine, 4-isopropylamino-2,2,6,6-tetramethylpiperidine, and 4-(tert-butylamino)-2,2,6,6-tetramethylpiperidine.

Description

(1) The invention is illustrated in detail by the appended drawings and the examples which follow.

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

(3) FIG. 2 shows the H.sub.2S selectivity of 4-butylamino-2,2,6,6-tetramethylpiperidine (butyl-TAD), 4-dimethylamino-2,2,6,6-tetramethylpiperidine (DATP), methyldiethanolamine (MDEA), 4-amino-2,2,6,6-tetramethylpiperidine (TAD), and a mixture of 4-hydroxy-2,2,6,6-tetramethylpiperidine and MDEA (TAAol+MDEA) and at various acid gas loadings.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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

(9) The following abbreviations were used:

(10) Butyl-TAD: 4-butylamino-2,2,6,6-tetramethylpiperidine

(11) DATP: 4-dimethylamino-2,2,6,6-tetramethylpiperidine

(12) MDEA: methyldiethanolamine

(13) TAAol: 4-hydroxy-2,2,6,6-tetramethylpiperidine

(14) TAD: 4-amino-2,2,6,6-tetramethylpiperidine

(15) TBATP: 4-(tert-butylamino)-2,2,6,6-tetramethylpiperidine

(16) Figures in percent are generally % by weight.

Example 1—pK.SUB.A .Values

(17) The pK.sub.A values of various amines were determined by means of the half-equivalence method. For this purpose, the amines were dissolved in water with a concentration of 0.01 to 0.5 mol/L and partly neutralized with half the molar amount of hydrochloric acid (0.005 to 0.025 mol/L). The mass of the amine solutions was 250 g. The measured pH corresponded to the pKa. The measurements were conducted at 20 and 120° C. The pH electrode used was the “Hamilton Polylite Plus 120” model, which is calibrated with pH 7 and pH 12 buffer solutions. The measurement was effected in a thermostated closed jacketed vessel with nitrogen blanketing.

(18) TABLE-US-00001 pK.sub.A at pK.sub.A at ΔpK.sub.a Amine 20° C. 120° C. (120-20° C.) MDEA*  8.7 7.0 1.7 DATP.sup.+ 10.7 8.1 2.6 TAAol 10.2 7.6 2.6 *comparative compound .sup.+in the case of DATP, the first pK.sub.A was reported

(19) It is expected that the great pK.sub.A differential for DATP and TAAol between absorption and desorption temperature will result in a comparatively small regeneration energy.

Example 2—Selectivity

(20) A glass reactor with a thermostated jacket and stirrer (stirrer speed=200 rpm) was initially charged with about 200 mL of unladen aqueous absorbent (TAAol+MDEA: TAAol: 0.77 M: MDEA: 0.63 M; residual absorbent: 1.4 M). At the top of the glass cylinder, a glass condenser was attached, which was operated at 5° C. This prevented distortion of the test results by partial evaporation of the absorbent. To determine the absorption capacity, at ambient pressure and 40° C., 216 L (STP)/h of acid gas (1.0% by volume of H.sub.2S, 10% by volume of CO.sub.2 and 89% by volume of N.sub.2) were passed through the absorption liquid via an immersed tube. Samples were taken from the glass reactor at regular time intervals and the loading of CO.sub.2 and H.sub.2S was determined as follows:

(21) 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).

(22) The selectivity was calculated as

(23) $\frac{\frac{\mathrm{mol}\left(H_{2}S\right)}{\mathrm{mol}\left({\mathrm{CO}}_2\right)}\mathrm{in}\mathrm{the}\mathrm{liquid}\mathrm{phase}}{\frac{\mathrm{mol}\left(H_{2}S\right)}{\mathrm{mol}\left({\mathrm{CO}}_2\right)}\mathrm{in}\mathrm{the}\mathrm{gas}\mathrm{phase}}$

(24) The results are shown in FIG. 2. It is clear that the absorbent based on TAAol+MDEA and DATP has a higher selectivity than the comparative examples, especially at higher acid gas loadings.

Example 3—Loading and Stripping Experiment

(25) A glass cylinder with a thermostated jacket was initially charged with about 100 mL of unladen absorbent (30% by weight). At the top of the glass cylinder, a glass condenser was attached, which was operated at 5° C. This prevented distortion of the test results by partial evaporation of the absorbent. To determine the absorption capacity, at ambient pressure and 40° C., 8 L (STP)/h of acid gas H.sub.2S or CO.sub.2 were passed through the absorption liquid via a frit. Subsequently, the loading of CO.sub.2 or H.sub.2S was determined as in example 2.

(26) 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 in example 2.

(27) 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 capacity. The results are shown in table 1.

(28) TABLE-US-00002 TABLE 1 CO.sub.2 loading H.sub.2S loading [m.sup.3 (STP)/t] Cyclic [m.sup.3 (STP)/t] Cyclic H.sub.2S:CO.sub.2 Absorbent after after CO.sub.2 capacity after after H.sub.2S capacity loading # Composition loading stripping [m.sup.3 (STP)/t] loading stripping [m.sup.3 (STP)/t] capacity ratio 3-1* 30% by wt. of MDEA + 43.4 2.7 40.7 38.7 6.7 32.0 0.79 70% by wt. of water 3-2 30% by wt. of DATP + 55.2 12.2 43.0 55.5 12.5 43.0 1.0 70% by wt. of water 3-3 30% by wt. of DATP + 28.8 4.6 24.2 39.9 7.0 32.9 1.36 70% by wt. of ethylene glycol 3-4 30% by wt. of DATP + 4.8 0.7 4.1 34.5 5.0 29.5 4.10 70% by wt. of triethylene glycol *comparative example

(29) It is clear from the comparison of examples 3-1 and 3-2 that DATP has both a higher CO.sub.2 loading capacity and a higher H.sub.2S loading capacity, and higher cyclic CO.sub.2 and H.sub.2S capacities. An elevated H.sub.2S:CO.sub.2 loading capacity ratio is also apparent.

(30) It is also clear that nonaqueous solvents result in reduced CO.sub.2 and H.sub.2S loading capacity and lower cyclic CO.sub.2 and H.sub.2S capacities, but cause a higher H.sub.2S selectivity.

Example 4—pH Gradient/Regeneration Energy

(31) The temperature dependence of the pH of aqueous amine solutions or partly neutralized amine solutions was determined in the temperature range from 50° C. to 120° C. 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.

(32) Table 2 reports the pH (50° C.), the pH (120° C.) and the differential pH (50° C.)-pH (120° C.) for aqueous compositions. 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 4-hydroxy-2,2,6,6-tetramethylpiperidine.

(33) 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. 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.

(34) 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. The results are shown in table 3.

(35) TABLE-US-00003 TABLE 2 TAAol: pH pH pH(50° C.) - Ex. Aqueous composition MDEA ** (50° C.) (120° C.) pH(120° C.) 4-1* 40% MDEA — 11.01 9.58 1.43 4-2* 40% MDEA + 0.5% H.sub.3PO.sub.4 —  9.76 8.29 1.47 4-3 35% MDEA + 10% 4-hydroxy-2,2,6,6-tetramethylpiperidine + 0.4% H.sub.2SO.sub.4 0.22 10.23 8.62 1.61 4-4 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 4-5 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 *comparative example ** molar ratio of 4-hydroxy-2,2,6,6-tetramethylpiperidine to MDEA

(36) TABLE-US-00004 TABLE 3 Ex. Aqueous composition Relative regeneration energy** [%] 4-6* 40% MDEA 100.0 4-7* 40% MDEA + 0.5% H.sub.3PO.sub.4  73.3 4-8 35% MDEA + 10% 4-hydroxy-2,2,6,6-tetramethylpiperidine + 0.9% H.sub.2SO.sub.4  70.5 4-9 35% MDEA + 10% 4-hydroxy-2,2,6,6-tetramethylpiperidine + 1.2% H.sub.2SO.sub.4  64.6 *comparative example **relative to example 4-7*

(37) It is clear that the aqueous compositions comprising 4-hydroxy-2,2,6,6-tetramethylpiperidine have a lower regeneration energy requirement,