PROCESS FOR REMOVAL OF ACID GASES FROM A FLUID STREAM WITH A LIQUID ABSORBENT COMPRISING A PIPERAZINE RING

Abstract

A process for removal of acid gases from fluid stream, wherein the fluid stream is brought into contact with an absorbent to obtain a treated fluid stream and a laden absorbent, the absorbent comprising a diluent and a compound of the general formula (I) wherein R.sup.1 is selected from C.sub.1-C.sub.8-alkyl and C.sub.2-C.sub.8-hydroxyalkyl; R.sup.2 is selected from hydrogen and C.sub.1-C.sub.8-alkyl; R.sup.3 is selected from hydrogen and C.sub.1-C.sub.8-alkyl; R.sup.4 is selected from hydrogen and C.sub.1-C.sub.8-alkyl; R.sup.5 is C.sub.1-C.sub.8-alkyl; with the proviso that at least one of the following conditions (i) and (ii) is met: (i) R.sup.5 is C.sub.3-C.sub.8-alkyl bound to the nitrogen atom via a secondary or tertiary carbon atom; (ii) when R.sup.4 is hydrogen, R.sup.3 is C.sub.1-C.sub.8-alkyl; or when R.sup.4 is C.sub.1-C.sub.8-alkyl, at least one of R.sup.2 and R.sup.3 is C.sub.1-C.sub.8-alkyl; and n is an integer from 0 to 6. Further provided is an absorbent for the absorption of acid gases from a fluid stream, comprising a diluent and a compound of the general formula (I) as defined above, as well as the use of a compound of the general formula (I) as defined above for removal of acid gases from a fluid stream. The absorbents are useful for the selective removal of hydrogen sulfide from fluid streams and have high acid gas loading capacity, high stability, and low volatility.

##STR00001##

Claims

1.-10. (canceled)

11. A process for removal of acid gases from fluid stream, wherein the fluid stream is brought into contact with an absorbent to obtain a treated fluid stream and a laden absorbent, the absorbent comprising a diluent and a compound of the general formula (I) ##STR00008## wherein R.sup.1 is C.sub.1-C.sub.5-alkyl; R.sup.2 is hydrogen; R.sup.3 is hydrogen; R.sup.4 is hydrogen or C.sub.1-C.sub.5-alkyl; R.sup.5 is isopropyl, tert-butyl or tert-pentyl; and n is 0 or 1, wherein the absorbent comprises a total amount of 10% to 70% by weight of the compound of the general formula (I), based on the total weight of the absorbent.

12. The process according to claim 11, wherein the compound of the general formula (I) is selected from the group consisting of 3-(4-methylpiperazin-1-yl)propyl-tert-butylamine; and 2-(4-methylpiperazin-1-yl)ethyl-tert-butylamine.

13. The process according to claim 11, wherein the diluent is selected from the group consisting of water, organic solvents, and combinations thereof.

14. The process according to claim 11, wherein the absorbent comprises a tertiary amine or severely sterically hindered primary amine or severely sterically hindered secondary amine other than the compounds of the general formula (I), wherein severe steric hindrance is understood to mean a tertiary carbon atom directly adjacent to a primary or secondary nitrogen atom.

15. The process according to claim 11 for selective removal of hydrogen sulfide over carbon dioxide from a fluid stream.

16. The process according to claim 11, wherein the laden absorbent is regenerated by means of at least one of the measures of heating, decompressing and stripping.

17. An absorbent for the absorption of acid gases from a fluid stream, comprising a diluent and a compound of the general formula (I) ##STR00009## wherein R.sup.1 is C.sub.1-C.sub.5-alkyl; R.sup.2 is hydrogen; R.sup.3 is hydrogen; R.sup.4 is hydrogen or C.sub.1-C.sub.5-alkyl; R.sup.5 is isopropyl, tert-butyl or tert-pentyl; and n is 0 or 1, wherein the absorbent comprises a total amount of 10% to 70% by weight of the compound of the general formula (I), based on the total weight of the absorbent.

18. The absorbent according to claim 17, wherein the compound of the general formula (I) is selected from the group consisting of 3-(4-methylpiperazin-1-yl)propyl-tert-butylamine; and 2-(4-methylpiperazin-1-yl)ethyl-tert-butylamine.

19. The absorbent according to claim 17, wherein the diluent is selected from the group consisting of water, organic solvents, and combinations thereof.

Description

[0214] FIG. 1 is a schematic diagram of a plant suitable for performing the process of the invention.

[0215] FIG. 2 shows the mass spectrum of 3-(4-methylpiperazin-1-yl)propyl-tert-butylamine (TBAP-MPIP).

[0216] FIG. 3 shows the mass spectrum of 2-(4-methylpiperazin-1-yl)ethyl-tert-butylamine (TBAE-MPIP).

[0217] According to FIG. 1, a suitably pre-treated gas comprising hydrogen sulfide and carbon dioxide is fed into absorber A1 via inlet Z and contacted in countercurrent with regenerated absorbent which is fed into absorber A1 via absorbent line 1.01. The absorbent removes hydrogen sulfide and carbon dioxide from the gas by absorption, which affords a hydrogen sulfide- and carbon dioxide-depleted gas via offgas line 1.02.

[0218] Via absorbent line 1.03, heat exchanger 1.04, in which the CO.sub.2— and H.sub.2S-laden absorbent is heated up with heat from the regenerated absorbent conducted through the absorbent line 1.05, and absorbent line 1.06, the CO.sub.2— and H.sub.2S-laden absorbent is fed to desorption column D and regenerated.

[0219] Between 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, e.g., 3 to 15 bar.

[0220] From the lower part of desorption column D, the absorbent is conducted into boiler 1.07, where it is heated. The resulting steam is recycled into desorption column D, while the regenerated absorbent is fed back to absorber A1 via absorbent line 1.05, 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, absorbent line 1.08, cooler 1.09 and absorbent line 1.01.

[0221] Instead of the depicted boiler, it is also possible to use other heat exchanger types for introducing energy, 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 desorption column D, where the phase separation between the vapor and the absorbent takes place. The regenerated absorbent fed to heat exchanger 1.04 is either drawn off from the circulation stream conducted from the bottom of desorption column D to the evaporator, or conducted via a separate line directly from the bottom of the desorption column D to heat exchanger 1.04.

[0222] The CO.sub.2— and H.sub.2S-containing gas released in desorption column D leaves the desorption column D via offgas line 1.10. It is fed 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 conducted separately from one another. Subsequently, the condensate is conducted through absorbent line 1.12 into the upper region of desorption column D, and a CO.sub.2— and H.sub.2S-containing gas is discharged via gas line 1.13.

[0223] The following abbreviations are used:

[0224] TBA: tert-butylamine

[0225] MDEA: methyldiethanolamine

[0226] TBAEE: 2-(2-tert-butylaminoethoxy)ethanol

[0227] M3ETB: (2-(2-(2-tert-butylaminoethoxy)ethoxy)ethyl)methyl ether

[0228] TBAEPY: 1-[2-(tert-butylamino)ethyl]pyrrolidin-2-one

[0229] TBAP-MPIP: 3-(4-methylpiperazin-1-yl)propyl-tert-butylamine

[0230] TBAE-MPIP: 2-(4-methylpiperazin-1-yl)ethyl-tert-butylamine

[0231] DEAE-EPIP: 2-(4-ethylpiperazin-1-yl)ethyl-diethylamine

EXAMPLE 1: SYNTHESIS OF 3-(4-METHYLPIPERAZIN-1-YL)PROPYL-TERT-BUTYLAMINE (TBAP-MPIP)

[0232] ##STR00006##

[0233] Synthesis of 3-(4-methylpiperazin-1-yl)propyl-tert-butylamine was carried out starting from 3-(4-methylpiperazin-1-yl)propan-1-ol using a high pressure autoclave with a volume of 2.5 L. The autoclave was equipped with a basket made from metal meshing for shaped catalyst bodies, a mechanical stirrer, baffles, an electrical heating mantle and an inlet for H.sub.2 and N.sub.2. 100 g of 3×3 mm pellets of a reduced-passivated catalyst containing Ni, Co, Cu, Sn on Al.sub.2O.sub.3 (obtained according to WO 2011/067199 A1, example 5) were filled into the basket, and a mixture of 730 g (10.0 mol) tert-butylamine and 200 g (1.264 mol) 3-(4-methylpiperazin-1-yl)propan-1-ol was introduced into the autoclave.

[0234] The autoclave was closed, and a test was performed to make sure the autoclave was sealed tightly at 200 bar by pressurizing with N.sub.2. The autoclave was then purged three times with N.sub.2 by pressurizing to 5 bar and releasing the pressure to 1 bar. Then, it was pressurized to 50 bar with hydrogen and under stirring, the contents were heated to 180° C. When this temperature was reached, the pressure was adjusted to 200 bar. After stirring for 15 hours under these conditions, the autoclave was allowed to cool down and depressurized. The liquid mixture was taken out and filtered.

[0235] The crude reaction product was analyzed by GC. The product was identified by GC-MS (molecule mass peak at 213 u) using electronic ionization and ionic ionization (GC-method: the column used was of type RTX5 Amin, length 30 m; diameter 0.32 mm; layer thickness 1.5 μm; temperature program: injection at 60° C., then directly temperature gradient 4° C./min until 280° C., then 35 min at 280° C.)

[0236] The crude product mixture contained 7.4% starting material, 43.0% product, 16.7% 1,3-bis(4-methyl-piperazin-1-yl)propane (GC area-%). Light boilers were removed at 80° C. and 20 mbar in a rotary evaporator, then the remaining crude product was distilled over a column filled with rings with a length of 30 cm, and one fraction (13.8 g) with a purity of 95% was retained while the rest (133.5 g) was redistilled over a smaller column with a length of 20 cm, yielding 48.6 g of product with a purity of 87%. The balance was mostly starting material which was considered not relevant for the desired testing.

[0237] The product was analyzed by GC-MS, and the mass spectrum was recorded using electronic ionization (EI) (conditions: mass range: 25-785 amu; ionization energy: 70 eV). Selected peaks are listed as follows with the exact mass divided by charge and the intensity relative to the most intense signal in parentheses. Additionally, molecular fragments are assigned to the peaks where possible. [0238] m/z=213 (<1%, M.sup.+); 198 (2%, M.sup.+-CH.sub.3); 183 (2%, M.sup.+-2 CH.sub.3); 182 (8%, M.sup.+-CH.sub.3—H); 169 (12%); 143 (25%, M.sup.+-NC.sub.4H.sub.9); 127 (14%); 113 (63%, C.sub.6H.sub.13N.sub.2.sup.+, fragment N,N′-dimethylpiperazinyl.sup.+), 101 (5%, N-methylpiperazine+H.sup.+); 100 (20%, N-methyl-piperazine.sup.+); 99 (10%); 98 (35%); 97 (6%); 96 (7%); 72 (8%) 71 (27%); 70 (100%), 69 (2%); 58 (18%, C.sub.4H.sub.9.sup.+); 57 (24%); 56 (19%); 55 (5%); 54 (3%).

[0239] The expected molar peak M.sup.+ was found. The structure was confirmed by analysis of the fragmentation pattern. The mass spectrum of TBAP-MPIP is shown in FIG. 2.

EXAMPLE 2: SYNTHESIS OF 2-(4-METHYLPIPERAZIN-1-YL)ETHYL-TERT-BUTYLAMINE (TBAE-MPIP)

[0240] ##STR00007##

[0241] Using the same setup and equipment as in example 1, 200 g (1.387 mol) 1-(hydroxyethyl)-4-methylpiperazine were reacted with 1014 g (13.87 mol) tert-butylamine over 100 g of the catalyst described in example 1 at 120 bar and 180° C. After 8 h the reactor was allowed to cool to room temperature, depressurized, and a sample taken that was analyzed as described before. This showed a conversion of only 35% as calculated from the area-% of starting material and various products.

[0242] The crude reaction mixture was reacted for a further 12 h at 190° C. and 120 bar total pressure. A sample of the crude mixture as analyzed by gas chromatography showed a conversion of 90% and 66% product. Excess TBA and water were removed by evaporation under reduced pressure (60° C., 50 mbar) and the residue was purified by fractional distillation over a 15 cm column. At about 1 mbar the product distilled at 54° C. head temperature. A fraction of 90 g product with a purity of 93% was collected and used for further testing. The remainder was mostly starting material and not considered relevant for the evaluation.

[0243] The product was analyzed by GC-MS, and the mass spectrum was recorded using electronic ionization (EI) (conditions: mass range: 25-785 amu; ionization energy: 70 eV). Selected peaks are listed as follows with the exact mass divided by charge and the intensity relative to the most intense signal in parentheses. Additionally, molecular fragments are assigned to the peaks where possible. [0244] m/z=199 (1%, M.sup.+); 184 (1%, M.sup.+-CH.sub.3); 168 (1%, M.sup.+—H, −2 CH.sub.3); 114 (15%, C.sub.6H.sub.14N.sub.2.sup.+, N,N′-dimethylpiperazine.sup.+); 113 (69%, C.sub.6H.sub.13N.sub.2.sup.+, fragment N,N′-dimethylpiperazinyl.sup.+), 101 (5%, N-methylpiperazine+H.sup.+); 100 (20%, N-methylpiperazine.sup.+); 99 (10%); 98 (11%); 71 (16%); 70 (100%), 69 (1%); 58 (8%, C.sub.4H.sub.9.sup.+); 57 (11%); 56 (10%); 55 (3%); 54 (2%).

[0245] The expected molar peak M.sup.+ was found. The structure was confirmed by analysis of the fragmentation pattern. The mass spectrum of TBAE-MPIP is shown in FIG. 3.

EXAMPLE 3: RELATIVE VOLATILITY

[0246] The volatility of amines TBAP-MPIP and TBAE-MPIP in 30 wt.-% aqueous solutions was compared to that of amines M3ETB, MDEA and TBAEPY in 30% wt.-% aqueous solutions.

[0247] A glass condenser, which was operated at 5° C., was attached to a glass cylinder with a thermostated jacket. The glass cylinder was thermostated to 50° C., and 200 mL of the absorbent were introduced for each test. Over an experimental duration of 8 h, 30 L (STP)/h of N.sub.2 were passed through the absorbent at ambient pressure. For each amine, the test was conducted three times. The condensate was analyzed by means of GC and Karl Fischer titration. The results are shown in the following table.

TABLE-US-00001 Amine Condensate [ml] Water [wt %] Amine [wt %] M3ETB* 30.1 99.2 0.7 MDEA* 27.1 99.4 0.7 TBAEPY* 28.8 99.5 0.2 TBAP-MPIP 30.3 99.7 0.3 TBAE-MPIP 29.28 99.3 0.7 *reference example

[0248] It is evident that TBAP-MPIP and TBAE-MPIP have a suitably low volatility.

EXAMPLE 4: ACID GAS LOADING CAPACITY

[0249] A loading experiment and subsequently a stripping experiment were conducted. A glass condenser, which was operated at 5° C., was attached to a glass cylinder with a thermostated jacket. Condensate obtained in the course of the experiment was returned to the glass cylinder so as to prevent distortion of the test results by partial evaporation of the absorbent.

[0250] 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:

[0251] 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. 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).

[0252] 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.

[0253] 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.

[0254] The results are shown in the following table.

TABLE-US-00002 CO.sub.2 loading H.sub.2S loading Amine [mol CO.sub.2/mol.sub.amine] Cyclic [mol H.sub.2S/mol.sub.amine] Cyclic (30 wt.-% after after CO.sub.2 capacity after after H.sub.2S capacity in water) loading stripping [mol CO.sub.2/mol.sub.amine] loading stripping [mol H.sub.2S/mol.sub.amine] TBAEE* 0.97 0.24 0.73 n.d.** n.d.** n.d.** MDEA* 0.77 0.05 0.72 0.68 0.11 0.61 TBAP-MPIP 1.41 0.70 0.71 1.48 0.33 1.15 TBAE-MPIP 1.46 0.22 1.24 1.62 0.35 1.27 DEAE-EPIP* 1.48 0.13 1.35 0.98 0.31 0.67 *comparative example **n.d. = not determined

[0255] It is evident that absorbents based on TBAP-MPIP and TBAE-MPIP have significantly higher acid gas loading capacities than absorbents based on TBAEE and MDEA at comparable or even higher cyclic acid gas capacities. DEAE-EPIP displays a similar cyclic CO.sub.2 capacity as TBAE-MPIP, but the cyclic H.sub.2S capacity of DEAE-EPIP is significantly lower than the capacity of both TBAP-MPIP and TBAE-MPIP.

EXAMPLE 5: pK.SUB.A .VALUE

[0256] The pKa values of the amino groups of MDEA, TBAP-MPIP, TBAE-MPIP and DEAE-EPIP were each determined by means of titration of an aqueous solution comprising 0.005 mol of amine per liter with hydrochloric acid (0.1 mol/L) at 20° C. The results are shown in the following table:

TABLE-US-00003 # Amine pK.sub.A 1 pK.sub.A 2 pK.sub.A 3 1* MDEA 8.7 — — 2 TBAP-MPIP 10.74 7.68 5.80 3 TBAE-MPIP 10.01 8.82 5.47 4* DEAE-EPIP 10.32 8.61 6.2 *comparative example

[0257] It is evident that TBAP-MPIP and TBAE-MPIP have high first pK.sub.A values at relatively low temperatures, as prevail in the absorption step. A high pK.sub.A value at relatively low temperatures promotes efficient acid gas absorption. Additionally, the second pK.sub.A value is close to the pKa value of MDEA, which is in line with the higher CO.sub.2 and H.sub.2S loading capacity per mole amine as measured in Example 4.

EXAMPLE 6: THERMAL STABILITY

[0258] The thermal stability of TBAP-MPIP (30 wt.-% in water) and TBAE-MPIP (25 wt.-% in water) was compared to both MDEA (40 wt.-% MDEA in water) and DEAE-EPIP (30 wt.-% DEAE-EPIP in water) with and without acid gas loading.

[0259] A cylinder (10 mL) was initially charged with the respective solution (8 mL) and the cylinder was closed. The cylinder was heated to 150° C. for 125 h. In the experiments conducted under acid gas loading, 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-00004 Ratio of Degradation Without Acid With Acid Aqueous Solution Gas Loading Gas Loading 40 wt.-% MDEA + 0.98 0.89 60 wt.-% H.sub.2O* 30 wt.-% TBAP-MPIP + 0.99 0.91 70 wt.-% H.sub.2O 25 wt.-% TBAE-MPIP + 1.00 0.88 75 wt.-% H.sub.2O 30 wt.-% DEAE-EPIP + 1.00 0.77 70 wt.-% H.sub.2O* *comparative example

[0260] It is evident that TBAP-MPIP and TBAE-MPIP, respectively, have a stability in aqueous solutions comparable to MDEA, and higher than DEAE-EPIP.