Diamine having tert-alkylamino group and primary amino group for use in gas scrubbing

Abstract

A compound of the general formula (I) ##STR00001##
in which R.sub.1, R.sub.2 and R.sub.3 are independently selected from C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.4 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.5 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; m is 2, 3, 4 or 5; n is 2, 3, 4 or 5; and o is an integer from 0 to 10. A preferred compound of the formula (I) is 2-(2-tert-butylaminoethoxy)ethylamine. Absorbents comprising a compound of the formula (I) have rapid absorption of carbon dioxide from fluid streams and are also suitable for processes for the simultaneous removal of H.sub.2S and CO.sub.2, where given H.sub.2S limits have to be observed but complete removal of CO.sub.2 is not required.

Claims

1. An absorbent for removing carbon dioxide and/or hydrogen sulfide from fluid streams, comprising an aqueous solution of a compound of the general formula (I) ##STR00005## in which R.sub.1 and R.sub.3 are each independently selected from C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.4 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.5 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; m is 2, 3, 4 or 5; n is 2, 3, 4 or 5; and o is an integer from 1 to 10.

2. The absorbent according to claim 1, with the proviso that the R.sub.5 radical on the carbon atom bonded directly to the primary amino group is hydrogen.

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

4. The absorbent according to claim 1, wherein the absorbent comprises at least one acid.

5. The absorbent according to claim 1, wherein the concentration of the compound of the formula (I) in the absorbent is 10% to 60% by weight.

6. The absorbent according to claim 1, also comprising at least one tertiary amine and/or a sterically hindered primary or secondary amine.

7. The absorbent according to claim 6, wherein the tertiary amine is methyldiethanolamine.

8. A process for removing carbon dioxide and/or hydrogen sulfide from fluid streams, the process comprising: contacting an absorbent according to claim 1 with a fluid stream, to obtain a CO.sub.2- and H.sub.2S-depleted fluid stream and a CO.sub.2- and H.sub.2S-laden absorbent.

9. The process according to claim 8, wherein the fluid stream comprises at least one hydrocarbon.

10. The process according to claim 8, wherein the fluid stream has a total pressure of at least 3.0 bar.

11. The process according to claim 8, further comprising: regenerating the CO.sub.2- and H.sub.2S-laden absorbent, wherein said regenerating comprises at least one of heating, decompressing and stripping the absorbent with an inert fluid.

12. A compound of the general formula (Ia) ##STR00006## in which R.sub.1, R.sub.2 and R.sub.3 are each independently selected from C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.4 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; each R.sub.5 is independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; and m is 2, 3, 4 or 5.

13. The compound according to claim 12, with the proviso that the R.sub.5 radical on the carbon atom bonded directly to the primary amino group is hydrogen.

14. The compound according to claim 13, which is 2-(2-tert-butylaminoethoxy)ethylamine.

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 is a schematic diagram of a twin stirred cell arrangement used to determine the relative CO.sub.2 absorption rates of absorbents.

(4) FIG. 3 shows the pH of aqueous solutions of MDEA and MDEA+TBAEEA as a function of temperature.

(5) FIG. 4 shows the cyclic capacity of aqueous MDEA solutions comprising piperazine, TBAEE and TBAEEA.

(6) According to FIG. 1, via the inlet Z, a suitably pretreated gas comprising hydrogen sulfide and/or 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/or carbon dioxide from the gas by absorption; this affords a hydrogen sulfide- and/or carbon dioxide-depleted clean gas via the offgas line 1.02.

(7) Via the absorbent line 1.03, the heat exchanger 1.04 in which the CO.sub.2- and/or 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/or H.sub.2S-laden absorbent is fed to the desorption column D and regenerated. 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/or 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 the regenerated absorbent and stripping vapor is returned to the bottom of the desorption column, 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 to the evaporator or conducted via a separate line directly from the bottom of the desorption column to the heat exchanger 1.04.

(8) The CO.sub.2- and/or 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. Condensation and phase separation also 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/or H.sub.2S-containing gas is discharged via the gas line 1.13.

(9) In FIG. 2, the following reference symbols are used: A=CO.sub.2 storage vessel, B=twin stirred cell, C=temperature regulator, D=metering valve, E=manometer. According to FIG. 2, a liquid phase of the absorbent to be tested s present in the lower part of the twin stirred cell B, and is in contact with the gas phase above it via a phase boundary. The liquid and gas phase can each be mixed with a stirrer. The twin stirred cell B is connected to the CO.sub.2 storage vessel A via a metering valve D. The pressure that exists in the twin stirred cell B can be determined by means of the manometer E. In the measurement, the volume flow rate of carbon dioxide is recorded, the volume flow rate being adjusted such that a constant pressure exists in twin stirred cell B.

EXAMPLES

(10) The following abbreviations are used:

(11) TSC: twin stirred cell

(12) HPCy.sub.2: dicyclohexylphosphine

(13) MDEA: methyldiethanolamine

(14) MeOH: methanol

(15) MTBE: methyl tert-butyl ether

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

(17) TBAEEA: 2-(2-tert-butylaminoethoxy)ethylamine

(18) THF: tetrahydrofuran

Example 1

Synthesis of 2-(2-tert-butylaminoethoxy)ethylamine (TBAEEA)

A) Synthesis of Catalyst Complex A

(19) ##STR00003##

A1) Synthesis of 4,5-bis(dicyclohexylphosphinomethyl)acridine

(20) A solution of 4,5-bis(bromomethyl)acridine (5.2 g, 14.2 mmol) and dicyclohexylphosphine (8.18 g, 36.8 mmol) in 65 mL of anhydrous degassed methanol was heated to 50 C. under an argon atmosphere for 66 h. After cooling to room temperature, triethylamine (5.72 g, 56.7 mmol) was added and the mixture was stirred for 1 h. Evaporating the solvent gave a yellow-white solid in red oil. Extraction by means of 340 mL of methyl tert-butyl ether (MTBE) and concentration of the filtrate gave a red-brown oil (.sup.1H NMR: mixture of product and HPCy.sub.2). The oil was taken up in a little warm MTBE and ice-cooled methanol was added, which caused the precipitation of a yellow microcrystalline solid. Isolation and drying under reduced pressure gave air-sensitive 4,5-bis(dicyclohexylphosphinomethyl)acridine (2.74 g, 33%) as a yellow powder.

A2) Synthesis of Catalyst Complex A

(21) 4,5-Bis(dicyclohexylphosphinomethyl)acridine (1855 mg, 3.1 mmol) and [RuHCl(CO)(PPh.sub.3).sub.3].sup.2 (2678 mg, 2.81 mmol) were heated to 70 C. in 80 mL of degassed toluene for 2 h. The resulting dark brown solution was concentrated to dryness, and the residue was suspended in 320 mL of hexane and isolated by filtration. Drying under reduced pressure gave catalyst complex A (1603 mg, 75%) as an orange-brown powder.

B) Synthesis of 2-(2-tert-butylaminoethoxy)ethylamine (TBAEEA)

(22) ##STR00004##

(23) Catalyst complex A (38.3 mg), THF (50 mL) and tert-butylaminoethoxyethanol were initially charged under an argon atmosphere in a 160 mL Parr autoclave (V4A stainless steel) with a magnetically coupled pitched blade stirrer (stirrer speed: 200-500 revolutions/minute). Ammonia (20.5 g) was precondensed at room temperature and metered in. The steel autoclave was heated electrically to 180 C. and heated (internal temperature measurement) while stirring (500 revolutions/minute) for 24 h. After heating the reaction mixture in the autoclave to 180 C., an autogenous pressure of 89 bar was evolved. After cooling to room temperature, decompression of the autoclave and outgassing of the ammonia at standard pressure, the reaction mixture was analyzed by means of GC (Rtx-5 Amine column, length 30 m, internal diameter 0.32 mm, d.sub.f 1.5 m, 60 C.-4 C./min-280 C.). At quantitative conversion, 90% of the desired 2-(2-tertbutylaminoethoxy)ethylamine are formed according to GC area % evaluation. The main by-product at 6% is the cyclized morpholine derivative depicted. The product was purified by distillation (distillation temperature 70 C. at 0.5 mbar).

Example 2

(24) For mixtures consisting of 40% by weight of MDEA and 60% by weight of water (2-1) and 30% by weight of MDEA, 15% by weight of TBAEEA and 55% by weight of water (2-2), the temperature dependence of the pH was determined. A pressure apparatus was used, in which the pH can be measured up to 120 C. The results are shown in FIG. 3. The mixture comprising TBAEEA (2-2) shows a much higher pH at 20 C. than the mixture comprising MDEA (2-1). The pH is a measure of how well CO.sub.2 or H.sub.2S can be bound. The higher the pH of the solution, the more CO.sub.2 or else H.sub.2S can be bound. In other words, at low temperatures as typically exist in absorbers, a high pH is advantageous. Overall, the mixture of TBAEEA+MDEA (2-2) shows a greater temperature dependence than the reference example comprising MDEA (2-1). For the solution (2-2) comprising TBAEEA and MDEA, the gradient is 0.027 pH unit/K, but for the MDEA solution (2-1) only 0.022 pH unit/K. For the regeneration, a maximum pH differential between higher and lower temperatures is advantageous, since the acidic components absorbed are released again with lower energy expenditure at higher temperatures and correspondingly lower pH values.

Example 3

(25) In a twin stirred cell (TSC) according to FIG. 2, the relative CO.sub.2 absorption rates of aqueous absorbents were measured.

(26) The twin stirred cell had an internal diameter of 85 mm and a volume of 509 mL. The temperature of the cell was kept at 40 C. during the measurements. In order to mix the gas and liquid phases, the cell according to FIG. 2 comprised two stirrers. Before commencement of the measurement, the twin stirred cell was evacuated. A defined volume of degassed absorbent was added to the twin stirred cell and the temperature was regulated at 40 C. The stirrers were already switched on during the heating of the unladen absorbent. The stirrer speed was selected such that a planar phase boundary formed between the liquid phase and the gas phase. Development of waves at the phase interface has to be avoided since there would otherwise be no defined phase interface. After the desired experimental temperature had been attained, carbon dioxide was introduced into the reactor by means of a metering valve. The volume flow rate was controlled such that the pressure was constant at 100 mbar abs over the entire experiment. With increasing experimental duration, the volume flow rate decreased since the absorption medium became saturated over time and the absorption rate decreased. The volume flow rate was recorded over the entire period. At the start of the experiment, the flow rates carbon dioxide flow rate was about 4 L (STP)/h. The experiment was ended as soon as the carbon dioxide flow rate was less than 0.02 L (STP)/h. The absorption medium was in an equilibrium state at the end of the experiment.

(27) The following absorbents were used:

(28) 3-1) aqueous solution of MDEA (41% by weight)

(29) 3-2) aqueous solution of MDEA (30% by weight)+TBAEE (15% by weight)

(30) 3-3) aqueous solution of MDEA (30% by weight)+TBAEEA (15% by weight)

(31) The absorption rate was determined at 20% and 50% of the loading attained at the end of the experiment (EQM). The values were normalized to the absorption rate of absorbent 3-1 at 20% and 50% EQM. The results are reported in the following table:

(32) TABLE-US-00001 Relative Relative absorption rate absorption rate Example System at 20% EQM** at 50% EQM** 3-1* MDEA (41% by wt.) 100% 100% 3-2* MDEA (30% by wt.) + 136% 128% TBAEE (15% by wt.) 3-3 MDEA (30% by wt.) + 355% 309% TBAEEA (15% by wt.) *comparative example **based on example 3-1

(33) It can be seen that, in inventive example 3-3, the absorption rate is much higher than in comparative examples 3-1 and 3-2, irrespective of the particular CO.sub.2 loading.

Example 4

(34) To estimate the cyclic capacity, a loading experiment and a subsequent stripping experiment were conducted for the following aqueous absorbents:

(35) 4-1) 30% by wt. of MDEA+8% by wt. of piperazine

(36) 4-2) 30% by wt. of MDEA+15% by wt. of TBAEE

(37) 4-3) 30% by wt. of MDEA+15% by wt. of TBAEEA

(38) The apparatus used was a thermostated glass cylinder with a reflux condenser connected above. The reflux condenser was operated at a temperature of about 5 C. and prevented water and amine from being discharged during the loading and stripping.

(39) At 40 C., 100 mL of the absorbent were introduced into the glass cylinder. Through a frit at the lower end of the glass cylinder, 8 L (STP)/h of pure CO.sub.2 were bubbled into the absorption solution for about 4 h. At the end of the experiment, the loading of CO.sub.2 in the absorbent was determined by means of measurement of the total inorganic carbon content (TIC).

(40) The laden solutions were stripped with nitrogen (8 L (STP)/h) at 80 C. in an apparatus of identical construction. Over the course of 60 min, samples of the absorbent were taken every 10 min and analyzed for the CO.sub.2 content. The stripping experiments were continued for a further 2 h, and finally the loading of CO.sub.2 in the absorbent was determined (total stripping time: 180 min). The difference between the CO.sub.2 loading attained at the end of the loading experiment and the CO.sub.2 loading determined as a function of stripping time are used to calculate the cyclic capacities of the three absorbents. The results are shown in FIG. 4. It is found that the mixture comprising TBAEEA (4-3) has the highest cyclic capacity, irrespective of the stripping time.

Example 5

(41) The volatility of the following absorbents was determined:

(42) 5-1) 41% by wt. of MDEA

(43) 5-2) 30% by wt. of MDEA+15% by wt of TBAEE

(44) 5-3) 30% by wt. of MDEA+15% by wt of TBAEEA

(45) A minimum volatility is advantageous in order to ensure a minimum discharge of amines together with the cleaned gas or together with the removed acid gas from the absorption plant.

(46) An apparatus as per the loading and stripping apparatus described in example 4 was used. The glass cylinder was heated to and kept at a temperature of 50 C., and 100 mL of the absorbent were introduced in each case. Through a frit at the lower end of the glass cylinder, 20 L (STP)/h of pure CO.sub.2 were bubbled into the absorption solution for about 8 h. In contrast to example 4, the liquids condensed out were not passed back into the glass cylinder but collected separately and analyzed for their composition after the end of the experiment. The results are shown in the following table:

(47) TABLE-US-00002 Amount Condensate composition of con- Water MDEA TBAEE TBAEEA Exam- densate [g/ [g/ [g/ [g/ ple System [g] 100 g] 100 g] 100 g] 100 g] 5-1* MDEA 15.811 99.1 0.37 5-2* MDEA + 17.284 99.2 0.39 0.37 TBAEE 5-3 MDEA + 16.949 99.2 0.42 0.27 TBAEEA *comparative example

(48) It can be seen that, in example 5-3, the volatility of the inventive compound TBAEEA is much lower than that of the compounds in comparative examples 5-1 and 5-2.