Absorbent and process for selectively removing hydrogen sulfide

Abstract

An absorbent for the selective removal of hydrogen sulfide from a fluid stream comprising carbon dioxide and hydrogen sulfide, wherein the absorbent contains an aqueous solution, comprising: a) an amine or a mixture of amines of the general formula (I) wherein R.sup.1 is C.sub.1-C.sub.5-alkyl; R.sup.2 is C.sub.1-C.sub.5-alkyl; R.sup.3 is selected from hydrogen and C.sub.1-C.sub.5-alkyl; x is an integer from 2 to 10; and b) an ether or a mixture of ethers of the general formula (II): R.sup.4—[O—CH.sub.2—CH.sub.2].sub.y—OH; wherein R.sup.4 is C.sub.1-C.sub.5-alkyl; and y is an integer from 2 to 10; wherein R.sup.1 and R.sup.4 are identical; wherein the mass ratio of b) to a) is from 0.08 to 0.5. The absorbent is suitable for the selective removal of hydrogen sulfide from a fluid stream comprising carbon dioxide and hydrogen sulfide. 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 is easily obtainable. ##STR00001##

Claims

1. An absorbent for the selective removal of hydrogen sulfide from a fluid stream comprising carbon dioxide and hydrogen sulfide, wherein the absorbent contains an aqueous solution, comprising: a) an amine or a mixture of amines of the general formula (I) ##STR00009## wherein R.sup.1 is C.sub.1-C.sub.5-alkyl; R.sup.2 is C.sub.1-C.sub.5-alkyl; R.sup.3 is selected from hydrogen and C.sub.1-C.sub.5-alkyl; x is an integer from 2 to 10; and b) an ether or a mixture of ethers of the general formula (II); ##STR00010## wherein R.sup.4 is C.sub.1-C.sub.5-alkyl; and y is an integer from 2 to 10; wherein R.sup.1 and R.sup.4 are identical; wherein the mass ratio of b) to a) is from 0.08 to 0.5.

2. The absorbent according to claim 1, wherein the number average of x and the number average of y do not differ from each other by more than 1.0.

3. The absorbent according to claim 2, wherein x and y are identical.

4. The absorbent according to claim 3, wherein x and y are 3.

5. The absorbent according to claim 1, wherein the amine a) is selected from (2-(2-tert-butylaminoethoxy)ethyl)methyl ether, (2-(2-isopropyl-aminoethoxy)ethyl)methyl ether, (2-(2-(2-tert-butylaminoethoxy)ethoxy)ethyl)methyl ether, (2-(2-(2-isopropylaminoethoxy)ethoxy)ethyl)methyl ether, (2-(2-(2-(2-tert-butylaminoethoxy)-ethoxy)ethoxy)ethyl)methyl ether, and (2-(2-(2-(2-Isopropylaminoethoxy)ethoxy)ethoxy)-ethyl)methyl ether; and the ether b) is selected from 2-(2-methoxyethoxy)ethanol, 2-(2-(2-methoxyethoxy)ethoxy)-ethanol, and 2-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)ethanol.

6. The absorbent according to claim 5, wherein the amine a) is (2-(2-(2-tert-butyl-aminoethoxy)ethoxy)ethyl)methyl ether and the ether b) is 2-(2-(2-methoxyethoxy)-ethoxy)ethanol.

7. The absorbent according to claim 1, wherein the mass ratio of b) to a) is from 0.15 to 0.35.

8. The absorbent according to claim 1, wherein the absorbent comprises an acid c).

9. A process for the production of absorbent according to claim 1, wherein an ether of formula (II) is reacted with a primary amine of the general formula (III) ##STR00011## wherein R.sup.2 is C.sub.1-C.sub.5-alkyl and R.sup.3 is selected from hydrogen and C.sub.1-C.sub.5-alkyl; to form an amine of formula (I), wherein the ether of formula (II) is not completely consumed in the reaction and the ether of formula (II) is not fully separated from the amine of formula (I).

10. The process according to claim 9, wherein the molar amount of the primary amine of formula (III) exceeds the molar amount of the ether of formula (II) during the reaction.

11. The process according to claim 9, wherein the reaction is carried out in the presence of a hydrogenation/dehydrogenation catalyst.

12. A process for the selective removal of hydrogen sulfide from a fluid stream comprising carbon dioxide and hydrogen sulfide, in which the fluid stream is contacted with the absorbent according to claim 1, wherein a laden absorbent and a treated fluid stream are obtained.

13. The process according to claim 12, wherein the laden absorbent is regenerated by means of at least one of the measures of heating, decompressing and stripping with an inert fluid.

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 of the invention.

(3) FIG. 2 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading of an aqueous solution of methoxyethoxyethoxyethyl-tert-butylamine (M3ETB, 30 weight-%) and an aqueous solution of M3ETB (30 weight-%) and methyltriethylene glycol (MTEG, 10 weight-%).

(4) FIG. 3 is a plot of the acid gas loading over time of an aqueous solution of M3ETB (30 weight-%) and an aqueous solution of M3ETB (30 weight-%) and MTEG (10 weight-%).

(5) FIG. 4 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading of an aqueous solution of M3ETB (30 weight-%), an aqueous solution of M3ETB (30 weight-%) and H.sub.2SO.sub.4 (2 weight-%), and an aqueous solution of M3ETB (30 weight-%), MTEG (10 weight-%) and H.sub.2SO.sub.4 (2 weight-%).

(6) FIG. 5 is a plot of the acid gas loading over time of an aqueous solution of an aqueous solution of M3ETB (30 weight-%), an aqueous solution of M3ETB (30 weight-%) and H.sub.2SO.sub.4 (2 weight-%), and an aqueous solution of M3ETB (30 weight-%), MTEG (10 weight-%) and H.sub.2SO.sub.4 (2 weight-%).

(7) FIG. 6 is a plot of the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading of an aqueous solution of M3ETB (30 weight-%), an aqueous solution of M3ETB (30 weight-%) and MTEG (10 weight-%), and an aqueous solution of M3ETB (30 weight-%) and sulfolane (10 weight-%).

(8) FIG. 7 is a plot of the acid gas loading over time of an aqueous solution of an aqueous solution of M3ETB (30 weight-%), an aqueous solution of M3ETB (30 weight-%) and MTEG (10 weight-%), and an aqueous solution of M3ETB (30 weight-%) and sulfolane (10 weight-%).

(9) 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.

(10) 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.

(11) 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.

(12) 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.

(13) 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.

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

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

(16) MTEG: 2-(2-(2-methoxyethoxy)ethoxy)ethanol

(17) TBA: tert-butylamine

EXAMPLE 1: PREPARATION OF AN ABSORBENT COMPRISING (2-(2-(2-TERT-BUTYLAMINOETHOXY)-ETHOXY)ETHYL)METHYL ETHER (M3ETB)

(18) A 1.4 L high pressure tubular reactor (length 2,000 mm, diameter 30 mm, equipped with an axially placed temperature probe with a diameter of 5 mm) with oil heating mantle, connected to a high pressure inlet for H.sub.2 and N.sub.2, as well as liquid MTEG and TBA feed lines equipped with high pressure pumps, and a high pressure and low pressure phase separator and sampling station downstream to the reactor, was filled stepwise with 500 mL ceramic balls, 500 mL amination catalyst (containing Ni, Co, Cu, Sn on Al.sub.2O.sub.3 and obtained according to WO 2011/067199, example 5) and 400 mL ceramic balls. The reactor was closed and air was displaced by N.sub.2. Subsequently, the catalyst was activated by passing 400 standard L/h H.sub.2 at 280° C. and ambient pressure for 24 h. After 24 h, the reactor was cooled to 50° C. and the H.sub.2-pressure was increased to 200 bar with a flow rate of 100 standard L/h H.sub.2. Then, the temperature was raised to 100° C. and dosage of TBA was started. The TBA flow rate was increased step-wise to 600 g/h. When the TBA load was stable, MTEG was initially pumped into the reactor at a flow rate of 51 g/L. Gradually over several days, the catalyst load was increased to 0.3 kg/(L.Math.h) MTEG, while TBA load was adjusted to 0.4 kg/(L.Math.h), which corresponds to a molar ratio of 6:1 TBA:MTEG. Temperature was set to 225° C. The reaction output was analyzed by means of gas chromatography (column: 30 m Rtx-5 Amine by Restek, internal diameter: 0.32 mm, d.sub.f: 1.5 μm, temperature program 60° C. to 280° C. in steps of 5° C./min). The following analysis values are reported in GC area percent (for retention times t.sub.R cf. table). Conversion was 99.2%, and the sample contained 96.3% M3ETB (calculated as TBA-free) which corresponds to a selectivity of 97%. An aliquot of the collected reaction mixture was collected for distillation.

(19) TBA was removed to a large extent under ambient pressure using a rotary evaporator at 90° C. bath temperature. 3.5 kg of the remaining crude product comprising 13.26 vol.-% TBA, 1.01 vol.-% MTEG, 82.62 vol.-% M3ETB and 3.11 vol.-% of other compounds were charged into a 4 L glass vessel and distilled over a column of 1,000 mm length with a diameter of 40 mm filled with Pall rings. 16 fractions were taken. The first fractions consisted of TBA, as determined from their boiling point. Fractions 7 to 16 (1,900 g) were combined. The purity of this sample was 99.8%.

(20) TABLE-US-00001 No. of boiling pressure mass TBA MTEG M3ETB fraction point [° C.] [mbar] [g] (t.sub.R 3.45 min)* (t.sub.R 25.88 min)* (t.sub.R 31.65 min)* Other 1 20-90 10 265 no GC 2  90-104 10 282 no GC 3 104-108 5 132 0.00 4.22 95.78 0.0 4 108 1 72 0.00 2.70 97.30 0.0 5 108 1 54 0.00 2.06 97.94 0.0 6 108 1 62 0.00 3.17 96.76 0.1 7 108 1 48 0.00 1.86 98.14 0.0 8 108 1 125 0.00 0.88 99.12 0.0 9 109 1 78 0.00 0.46 99.54 0.0 10 108 1 65 0.00 0.32 99.63 0.0 11 109 1 82 0.00 0.26 99.74 0.0 12 109 1 73 0.00 0.04 99.96 0.0 13 109 1 265 0.00 0.02 99.96 0.0 14 109 1 406 0.00 0.05 99.95 0.0 15 109 1 433 0.00 0.00 99.99 0.0 16 109 1 325 0.00 0.00 99.94 0.1 Sump 90 0.00 0.00 87.74 12.3 *t.sub.R = retention time

(21) For examples 2 to 4, the following procedures were used.

(22) 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.

(23) 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.

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

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

(26) Gas flow rate: 154 SCCM

(27) Temperature: 40.8° C.

(28) Pressure: 1 bar

(29) Volume: 15 mL (i=0.1 min)

(30) Stirring rate: 200 rpm

(31) The experiments of examples 2 to 4 were performed by flowing gas mixtures as specified above through the autoclave. The autoclave was previously loaded with the respective aqueous amine solution, as specified below. 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.

(32) 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.

(33) M3ETB with a purity of 99% was used to obtain the different aqueous solutions described below.

(34) 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.

EXAMPLE 2

(35) An aqueous solution of M3ETB (30 weight-%) was compared to an aqueous solution comprising M3ETB (30 weight-%) and MTEG (10 weight-%). The acid gas loading over time was determined, as well as the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading. The results are shown in FIGS. 2 and 3.

(36) Aqueous M3ETB has a maximum selectivity of about 11 at a loading of about 0.35 moles. The selectivity declines at higher H.sub.2S and CO.sub.2 loadings. It was shown that the presence of MTEG has only a minor impact on selectivity, with a maximum selectivity of about 10 and the maximum not shifted to lower loadings.

(37) The acid gas loading of aqueous M3ETB over time shows a maximum CO.sub.2 loading of about 0.62 moles of CO.sub.2 per mole of amine after about 600 minutes, while the H.sub.2S loading rises to a maximum of about 0.25 moles of H.sub.2S per mole of amine after about 150 minutes, afterwards falling to about 0.15 moles of H.sub.2S per mole of amine at about 400 minutes and remaining essentially steady afterwards. Probably, bound H.sub.2S is displaced by CO.sub.2 at higher loadings. The presence of MTEG has only a minor impact on acid gas loading over time, following the same trend with a slightly lower H.sub.2S loading from about 150 minutes onwards.

EXAMPLE 3

(38) An aqueous solution of M3ETB (30 weight-%) was compared to an aqueous solution of M3ETB (30 weight-%) and H.sub.2SO.sub.4 (2 weight-%), and an aqueous solution of M3ETB (30 weight-%), MTEG (10 weight-%) and H.sub.2SO.sub.4 (2 weight-%). The acid gas loading over time was determined, as well as the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading. The results are shown in FIGS. 4 and 5.

(39) As discussed in example 2, aqueous M3ETB has a maximum selectivity of about 11 at a loading of about 0.35 moles, which declines at higher H.sub.2S and CO.sub.2 loadings. Addition of H.sub.2SO.sub.4 leads to a decrease of the maximum selectivity to about 10 and a shift of the maximum towards lower loadings of about 0.30 moles. The presence of H.sub.2SO.sub.4 and MTEG leads to an increase of selectivity to about 13 and a slight shift of the maximum towards lower loadings of about 0.25 moles.

(40) FIG. 5 shows that the addition of H.sub.2SO.sub.4 to an aqueous solution of M3ETB decreases acid gas capacity. In the case of CO.sub.2, the maximum loading is about 0.47 moles of CO.sub.2 per mole of amine as compared to 0.62 moles of CO.sub.2 per mole of amine without H.sub.2SO.sub.4. In the case of H.sub.2S, the final H.sub.2S loading is about 0.10 moles of H.sub.2S per mole of amine at about 250 minutes as compared to about 0.15 moles of H.sub.2S per mole of amine at about 400 minutes without H.sub.2SO.sub.4.

(41) An aqueous solution of M3ETB, MTEG and H.sub.2SO.sub.4 displays an acid gas loading between that of an aqueous solution of M3ETB and that of an aqueous solution of M3ETB and H.sub.2SO.sub.4.

EXAMPLE 4

(42) An aqueous solution of M3ETB (30 weight-%) and MTEG (10 weight-%) was compared to an aqueous solution of M3ETB (30 weight-%) and sulfolane (10 weight-%). The acid gas loading over time was determined, as well as the selectivity of H.sub.2S over CO.sub.2 as a function of acid gas loading. The results are shown in FIGS. 6 and 7.

(43) The aqueous solution of M3ETB and MTEG has a maximum selectivity of about 10. The aqueous solution of M3ETB and sulfolane has a maximum selectivity of about 8.5.

EXAMPLE 5

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

(45) The unit comprised a sight class vessel equipped with a pressure gauge and a thermocouple. A heater was used to supply heat to the vessel. Measurements are 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 mixtures with the amine M3ETB.

(46) TABLE-US-00002 Critical Solution Aqueous Solutions Temperature 36 wt.-% M3ETB 107° C. 30 wt.-% M3ETB + 5 wt.-% MTEG 128 to 130° C. 30 wt.-% M3ETB + 5 wt.-% MTEG + 124° C. 1.6 wt.-% H.sub.3PO.sub.4 30 wt.-% M3ETB + 10 wt.-% MTEG + —* 1.6 wt.-% H.sub.3PO.sub.4 30 wt.-% M3ETB + 10 wt.-% MTEG + —* 2 wt.-% H.sub.2SO.sub.4 30 wt.-% M3ETB + 10 wt.-% MTEG + —* 4 wt.-% H.sub.2SO.sub.4 25 wt.-% M3ETB + 8.3 wt.-% MTEG + —* 2 wt.-% H.sub.2SO.sub.4 35 wt.-% M3ETB + 11.7 wt.-% MTEG + —* 2 wt.-% H.sub.2SO.sub.4 30 wt.-% M3ETB + 10 wt.-% Sulfolane + —* 2 wt.-% H.sub.2SO.sub.4 30 wt.-% M3ETB + 5 wt.-% Sulfolane + 137 to 139° C. 2 wt.-% H.sub.2SO.sub.4 30 wt.-% M3ETB + 10 wt.-% MDEA + 129° C. 2 wt.-% H.sub.2SO.sub.4 *no phase separation observed at up to 140° C.