Absorbent and process for selectively removing hydrogen sulfide

Abstract

An absorbent for the selective removal of hydrogen sulfide over carbon dioxide from a fluid stream, wherein the absorbent contains an aqueous solution, comprising an amine of formula (I) and/or an amine of formula (II) wherein U—V—W is CH—O—CHR.sup.5, N—CO—CHR.sup.5 or N—CO—NR.sup.5; U′—V′—W is C—O—CR.sup.5; R.sup.1 is independently C.sub.1-C.sub.5-alkyl; R.sup.2 is selected from hydrogen and C.sub.1-C.sub.5-alkyl; R.sup.3 is independently selected from hydrogen and C.sub.1-C.sub.5-alkyl; R.sup.4 is independently selected from hydrogen and C.sub.1-C.sub.5-alkyl; R.sup.5 is selected from hydrogen, C.sub.1-C.sub.5-alkyl, (C.sub.1-C.sub.5-alkoxy)-C.sub.1-C.sub.5-alkyl, and hydroxy-C.sub.1-C.sub.5-alkyl; and x is an integer from 1 to 10. The absorbent has a reduced tendency for phase separation at temperatures falling within the usual range of regeneration temperatures for the aqueous amine mixtures and a low volatility in aqueous solvents. ##STR00001##

Claims

1. A process for the selective removal of hydrogen sulfide over carbon dioxide from a fluid stream, comprising contacting the fluid stream with an absorbent containing an aqueous solution, comprising an amine of formula (I) ##STR00009## wherein U—V—W is N—CO—CHR.sup.5; R.sup.1 is independently C.sub.1-C.sub.5-alkyl; R.sup.2 is selected from hydrogen and C.sub.1-C.sub.5-alkyl; R.sup.3 is independently selected from hydrogen and C.sub.1-C.sub.5-alkyl; R.sup.4 is independently selected from hydrogen and C.sub.1-C.sub.5-alkyl; R.sup.5 is selected from hydrogen, C.sub.1-C.sub.5-alkyl, (C.sub.1-C.sub.5-alkoxy)-C.sub.1-C.sub.5-alkyl, and hydroxy-C.sub.1-C.sub.5-alkyl; and x is an integer from 1 to 10, wherein the absorbent comprises a total amount of 15% to 70% by weight of amine of formula (I), based on the total weight of the absorbent, and obtaining a laden absorbent and a treated fluid stream.

2. The process according to claim 1, wherein the amine is selected from 1[2-(tert-butylamino)ethyl]pyrrolidin-2-one, 1-[2-(tert-butylamino)propyl]pyrrolidin-2-one, 1-[2-(isopropylamino)ethyl]pyrrolidin-2-one, and 1-[2-(isopropylamino)propyl]pyrrolidin-2-one.

3. The process according to claim 2, wherein the amine is selected from 1-[2-(tert-butylamino)ethyl]pyrrolidin-2-one, and 1-[2-(isopropylamino)ethyl]pyrrolidin-2-one.

4. The absorbent according to claim 3, wherein the amine is 1-[2-(tert-butylamino)-ethyl]pyrrolidin-2-one.

5. The process according to claim 1, wherein the absorbent additionally comprises an acid.

6. The process according to claim 1, wherein the absorbent additionally comprises a tertiary amine and/or severely sterically hindered 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.

7. The process according to claim 1, wherein the absorbent comprises a non-aqueous organic solvent.

8. The process according to claim 7, wherein the non-aqueous organic solvent is selected from C.sub.4-10 alcohols, ketones, esters, lactones, amides, lactams, sulfones, sulfoxides, glycols, polyalkylene glycols, di- or mono(C.sub.1-C.sub.4-alkyl ether) glycols, di- or mono(C.sub.1-C.sub.4-alkyl ether) polyalkylene glycols, cyclic ureas, thioalkanols and mixtures thereof.

9. The process according to claim 1, further comprising regenerating the laden absorbent by heating, decompressing or stripping with an inert fluid.

Description

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

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

(3) FIG. 2 shows the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading.

(4) According to FIG. 1, via the inlet Z, a suitably pre-treated 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, 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.

(7) 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 desorption column D, where the phase separation between the vapour 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 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 vapour. 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 desorption column D, and a CO.sub.2- and H.sub.2S-containing gas is discharged via the gas line 1.13.

(9) In the description of the examples, the following abbreviations were used:

(10) HEP: hydroxyethylpyrrolidone

(11) M3ETB: (2-(2-(2-tert-butylaminoethoxy)ethoxy)ethyl) methyl ether

(12) MDEA: methyldiethanolamine

(13) TBA: tert-butylamine

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

(15) TBAEEM: tert-butylaminoethoxyethylmorpholine

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

(17) TBAEM: tert-butylaminoethylmorpholine

Example 1: Synthesis of 1-[2-(tert-butylamino)ethyl]pyrrolidin-2-one (TBAEPY)

(18) The synthesis of 1-[2-(tert-butylamino)ethyl]pyrrolidin-2-one (TBAEPY) was carried out by reacting hydroxyethylpyrrolidone (HEP) and tert-butylamine (TBA) using a continuous laboratory high pressure plant. The plant consisted of two oil-heated reactors of 100 cm length and an inner diameter of 12 mm. The reactors were placed in series and connected to high pressure feed pumps for two liquid feeds and a high pressure supply for hydrogen. At the outlet of the reactors, a RECO valve was placed and set to the desired pressure to be kept in the reactors. The crude reaction mixture was collected in a low-pressure phase separator and connected off-gas to a fume-hood.

(19) Each of the reactors was filled with reduced-passivated catalyst grit containing Ni, Co, Cu, Sn on Al.sub.2O.sub.3 (obtained according to WO 2011/067199, example 5). This catalyst was activated as follows: 10 L of hydrogen/L of catalyst were passed over the reactors at 260° C. for 20 h at ambient pressure. Then, the pressure was raised to 70 bar and at a hydrogen flow of 10 standard liters/L catalyst and hour and at a temperature of 188° C., TBA and hydroxyethylpyrrolidone in a molar ratio of 3:1 were pumped into the reactors. The reactor load relative to hydroxyethylpyrrolidone was 0.2 kg/L/h catalyst. About 1,000 g of HEP were converted in such a manner using 1,694 g of TBA. The crude reaction product was analyzed by gas chromatography (GC). The product was identified by GC-MS (molecule mass peak at 184 amu) using electronic ionization and ionic ionization.

(20) GC-method: the column used was of the 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 till 280° C., then 35 min at 280° C.

(21) The product was analyzed by GC-MS, and the mass spectrum was recorded using electronic and chemical ionization (EI and CI respectively). Conditions for the EI: mass range: 25-785 amu; ionization energy: 70 eV. Conditions for CI: mass range: 50-810 amu.

(22) The conversion of HEP was 73.6% and the selectivity for TBAEPY was 96.8%. Excess TBA was first removed in a rotary evaporator and then the product was isolated by distillation over a 30 cm column at 1 mbar. The boiling point was 146° C. at 1 mbar. Purity by GC after distillation was >99%.

(23) Peaks are listed in the following 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. m/z=184 (<1%, M.sup.+); 169 (20%); 127 (15%); 112 (100%, M-HN—C.sub.3H.sub.9); 99 (58%, M-C.sub.4H.sub.6NO+H); 98 (25%); 86 (72%); 84 (37%, M-CH.sub.2CH.sub.2—NC.sub.3H.sub.9); 70 (17%, M-NCH.sub.2CH.sub.2NHC.sub.4H.sub.9); 69 (20%); 57 (40%, C.sub.4H.sub.9.sup.+); 56 (13%); 44 (17%); 43 (17%); 42 (25%); 41 (36%); 30 (96%).

(24) The expected molar peak M.sup.+ was found. The structure was confirmed by the fragmentation pattern.

Example 2: pK.SUB.A .Values

(25) The pKa values of MDEA and TBAPD were determined by means of titration. An aqueous solution with an amine concentration of 0.005 mol/L of amine was used and titrated with aqueous hydrochloric acid (0.1 mol/L) at 20° C.

(26) The results are shown in the following table:

(27) TABLE-US-00001 MDEA TBAEPY pK.sub.A (20° C.) 8.7 9.6

(28) TBAEPY exhibits a higher the pK.sub.A value than MDEA. It is believed that a high pK.sub.A value at relatively low temperatures (such as 20° C.) as exist in the absorption step promotes efficient acid gas absorption.

Example 3: Loading Capacity and Cyclic Capacity

(29) A loading experiment and then a stripping experiment were conducted.

(30) 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:

(31) 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, 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).

(32) 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.

(33) 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.

(34) The results are shown in table 1. Aqueous TBAEPY has CO.sub.2 and H.sub.2S loading capacities similar to that of known suitable H.sub.2S-selective absorbents.

(35) TABLE-US-00002 TABLE 1 CO.sub.2 loading capacity H.sub.2S loading capacity [mol.sub.CO2/mol.sub.amine] [mol.sub.H2S/mol.sub.amine] Absorbent after after cyclic CO.sub.2 capacity after after cyclic H.sub.2S capacity # Composition loading stripping [mol.sub.CO2/mol.sub.amine] loading stripping [mol.sub.H2S/mol.sub.amine] 1* 30% wt.-% MDEA + 0.77 0.05 0.72 0.68 0.11 0.57 70 wt.-% H.sub.2O 2* 30% wt.-% TBAEE + 0.97 0.24 0.73 0.92 0.25 0.67 70% wt.-% H.sub.2O 3  30% wt.-% TBAEPY + 1.02 0.13 0.89 0.85 0.24 0.61 70% wt.-% H.sub.2O *comparative example

Example 4: Volatility

(36) The volatility of several amines in 30% by weight aqueous solutions was examined.

(37) The same apparatus as in example 3 was used, except that the condensate obtained in the glass condenser was not returned to the glass condenser but was separated and analyzed for its composition after the experiment had ended via gas chromatography and Karl Fischer titration. The glass cylinder was regulated to 50° C., and 200 mL of the absorbent were introduced in each case. Over an experimental duration of 8 h, 30 L (STP)/h of N.sub.2 were passed through the absorbent at ambient pressure.

(38) The experiments were repeated three times. The obtained average values are shown in the following table:

(39) TABLE-US-00003 Condensate Water Amine Solution [mL] [wt.-%] [wt.-%] 30 wt.-% M3ETB + 70 wt.-% H.sub.2O* 30.1 99.2 0.7 30 wt.-% TBAEE + 70 wt.-% H.sub.2O* 30.0 99.3 0.7 30 wt.-% MDEA + 70 wt.-% H.sub.2O* 27.1 99.4 0.7 30 wt.-% TBAEEM + 70 wt.-% H.sub.2O* 30.5 99.6 0.2 30 wt.-% TBAEM + 70 wt.-% H.sub.2O* 34.8 98.0 1.6 30 wt.-% TBAEPY + 70 wt.-% H.sub.2O 28.8 99.5 0.2 *comparative example

(40) It is clear that TBAEPY has a lower volatility compared to M3ETB, TBAEE, MDEA, and TBAEM, comparable to that of TBAEEM.

Example 5: Phase Separation

(41) To determine the critical solution temperature, a miscibility unit was used. This unit allows the measurement of the temperature at which phase separation occurs (“critical solution temperature”).

(42) The miscibility unit comprised a sight glass vessel equipped with a pressure gauge and a thermocouple. A heater was used to supply heat to the vessel. Measurements were possible up to a temperature of 140° C. The possible phase separation could be visually observed via the sight glass. In the following table, the measured critical solution temperatures are shown for different aqueous amine mixtures.

(43) TABLE-US-00004 Aqueous Solution Critical Solution Temperature 36 wt.-% M3ETB* 107° C. 36 wt.-% TBAEEM* 131° C. 30 wt.-% TBAEM* 115 to 120° C. 36 wt.-% TBAEPY —** *comparative example **no phase separation observed at up to 140° C.

(44) It is clear that an aqueous solution comprising TBAEPY has a higher critical solution temperature than the comparative examples.

Example 6: Selectivity for H.SUB.2.S Over CO.SUB.2

(45) The experiments were carried out in an absorption unit (semi-batch system), comprising a stirred autoclave to which gas could be fed in up-flow mode, and a condenser. The autoclave was equipped with a pressure gauge and a type J thermocouple. A safety rupture disc was attached to the autoclave head. A high wattage ceramic fiber heater was used to supply heat to the autoclave. The gas flows were regulated by mass flow controllers (from Brooks Instrument) and the temperature of the condenser was maintained by a chiller. The maximum working pressure and temperature were 1000 psi (69 bar) and 350° C., respectively.

(46) During runs at atmospheric pressure, the pH of the solution was monitored in situ by using a pH probe (from Cole-Parmer), which was installed in the bottom of the autoclave. This pH probe was limited by a maximum temperature and pressure of 135° C. and 100 psi, respectively. Therefore, before carrying out experiments at a pressure above atmospheric pressure (“higher pressure”), the pH probe was removed and the autoclave was capped. In both cases (atmospheric pressure and higher pressure), liquid samples were collected by directly attaching a vial (atmospheric pressure) or a stainless steel cylinder filled with caustic (higher pressure) to the sampling system. A specifically designed LabVIEW program was used to control the absorption unit operation and to acquire experimental data like temperature, pressure, stirrer speed, pH (at atmospheric pressure), gas flow rate and off-gas concentration.

(47) The gas mixture used in the examples had the following properties:

(48) Gas feed composition: 10 mol-% CO.sub.2, 1 mol-% H.sub.2S, 89 mol-% N.sub.2

(49) Gas flow rate: 154 SCCM

(50) Temperature: 40.8° C.

(51) Pressure: 1 bar

(52) Volume: 15 mL (T=0.1 min)

(53) Stirring rate: 200 rpm

(54) The experiments were performed by flowing gas mixtures as specified above through the autoclave. The autoclave was previously loaded with the respective aqueous amine solution (amine concentration: 2.17 M). The acid gas mixture was fed to the bottom of the reactor. The gases leaving the autoclave were passed through the condenser, which was kept at 10° C., in order to remove any entrained liquids. A slip-stream of the off-gas leaving the condenser was fed to a micro-gas-phase chromatograph (micro-GC, from Inficon) for analysis while the main gas flow passed through a scrubber. After reaching breakthrough, nitrogen was used to purge the system.

(55) The slip-stream of the off-gas was analyzed using a custom-built micro-GC. The micro-GC was configured as a refinery gas analyzer and included 4 columns (Mole Sieve, PLOT U, OV-1, PLOT Q) [by Aglient] and 4 thermal conductivity detectors. A portion of the off-gas was injected into the micro-GC approximately every 2 min. A small internal vacuum pump was used to transfer the sample into the micro-GC. The nominal pump rate was approximately 20 mL/min in order to achieve 10× the volume of line flushes between the sample tee and the micro GC. The actual amount of gas injected into the GC was approximately 1 μL. The PLOT U column was used to separate and identify H.sub.2S and CO.sub.2, and the micro-TCD was used to quantify them.

(56) The selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading was determined. The results are shown in FIG. 2.

(57) The term “acid gas loading” as used herein stands for the concentration of the H.sub.2S and CO.sub.2 gases physically dissolved and chemically combined in the absorbent solution as expressed in moles of gas per moles of the amine.

(58) Aqueous TBAEPY (2.17 M) has a maximum selectivity of about 13.0 at a loading of about 0.15 moles, which declines at higher H.sub.2S and CO.sub.2 loadings. This value is significantly higher than the maximum selectivity of aqueous TBAEEM, aqueous M3ETB, aqueous TBAEE and aqueous MDEA.