REMOVAL OF CARBON DIOXIDE FROM A FLUID FLOW

Abstract

An absorbent for removing carbon dioxide from a fluid stream, comprising an aqueous solution a) of an amine of the general formula (I)

##STR00001##

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.1-4alkyl 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; X is OH or NH(CR.sub.1R.sub.2R.sub.3); m is 2, 3, 4 or 5; n is 2, 3, 4 or 5; and o is 0 or 1; and b) at least one activator selected from b1) a sterically unhindered primary amine and/or a sterically unhindered secondary amine; and a carboanhydrase. The absorbent allows rapid absorption of carbon dioxide from fluid streams.

Claims

1. An absorbent for removing carbon dioxide from a fluid stream, comprising an aqueous solution comprising: a) an amine of formula (I) ##STR00004## in which R.sub.1, R.sub.2 and R.sub.3 are each independently C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl, each R.sub.4 is independently hydrogen, C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl; each R.sub.5 is independently hydrogen, C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl; X is OH or NH(CR.sub.1R.sub.2R.sub.3); m is 2, 3, 4 or 5; n is 2, 3, 4 or 5; and o is 0 or 1; and b) at least one activator, which is a 5-, 6- or 7-membered saturated heterocycle comprising at least one NH group optionally one or two further heteroatoms, which are nitrogen or oxygen, in the ring.

2. The absorbent according to claim 1, wherein a molar ratio of b) to a) is in the range of from 0.05 to 1.0.

3. The absorbent according to claim 1, wherein a total amount of a) and b) in the aqueous solution is 10% to 60% by weight.

4. The absorbent according to claim 1, wherein the amine a) is selected from the group consisting of 2-(2-tert-butylaminoethoxy)ethanol, 3-(tert-butylamino)propanol, 4-(tert-butylamino)butanol, 2-(2-tert-amylaminoethoxy)ethanol, 2-(2-(1-methyl-1-ethylpropylamino)ethoxy)ethanol, 2-(tert-butylamino)ethanol and (2-(tert-butylamino)ethyl)methylamine.

5. The absorbent according to claim 1, wherein the at least one activator is selected from the group consisting of piperazine, 2-methylpiperazine, N-methylpiperazine, N-ethylpiperazine, N-(2-hydroxyethyl)piperazine, N-(2-aminoethyl)piperazine, homopiperazine, piperidine and morpholine.

6. The absorbent according to claim 1, wherein the aqueous solution further comprises at least one organic solvent.

7. The absorbent according to claim 1, wherein the aqueous solution further comprises at least one acid.

8. A process for removing carbon dioxide from a fluid stream, the processing comprising: contacting the fluid stream with the absorbent according to claim 1, thereby obtaining a treated fluid stream.

9. The process according to claim 8, wherein a partial carbon dioxide pressure in the fluid stream is from 0.01 to less than 3.0 bar.

10. The process according to claim 8, wherein a partial carbon dioxide pressure in the treated fluid stream is less than 0.05 bar.

11. The process according to claim 8, wherein the fluid stream is a hydrocarbonaceous fluid stream.

12. The process according to claim 8, wherein the fluid stream is an oxygenous fluid stream.

13. The process according to claim 8, wherein the absorbent is regenerated by at least one measure selected from the group consisting of i. heating, ii. decompression, and iii. stripping with an inert fluid.

Description

[0056] The invention is illustrated in detail by the appended drawings and the examples which follow.

[0057] FIG. 1 is a schematic diagram of a plant suitable for performing the process according to the invention.

[0058] FIG. 2 is a schematic diagram of a twin stirred cell arrangement used to determine the relative CO.sub.2 absorption rates of absorbents.

[0059] FIG. 3 shows the stability of MEA, MDEA and TBAEE in aqueous absorbents in the presence of oxygen.

[0060] 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.

[0061] 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. 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 raise 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.

[0062] 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. 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.

[0063] 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 is 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.

EXAMPLE 1

[0064] In a twin stirred cell (TSC) according to FIG. 2, the relative CO.sub.2 absorption rates of aqueous absorbents were measured.

[0065] 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 50° 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 50° 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 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 50 mbar abs over the entire experiment. With increasing experimental duration, the volume flow rate decreased since the absorbent became saturated over time and the absorption rate decreased. The volume flow rate was recorded over the entire period. The experiment was ended as soon as no further carbon dioxide flowed into the twin stirred cell. The absorbent was in an equilibrium state at the end of the experiment. The reported absorption rate was determined at a loading of 10 m.sup.3 (STP) of (CO.sub.2)/t (absorbent).

[0066] The following absorbents were examined: (1-1) aqueous solution of methyldiethanolamine (MDEA) (2.2 M) and piperazine (1.5 M); (1-2) aqueous solution of 2-(2-tert-butylaminoethoxy)ethanol (TBAEE) (2.2 M) and piperazine (1.5 M); and (1-3) aqueous solution of 2-(2-tert-butylaminoethoxy)ethanol (TBAEE) (2.2 M) and monoethanolamine (MEA) (1.5 M). The results are reported in the following table:

TABLE-US-00001 Example System Relative absorption rate**  1-1*  MDEA + piperazine 100% 1-2 TBAEE + piperazine 311% 1-3 TBAEE + MEA .sup.   120% *comparative example **based on example 1-1*

[0067] It can be seen that, in inventive example 1-2, the absorption rate is much higher than in examples 1-1 and 1-3.

EXAMPLE 2

[0068] In this example, in a pilot plant according to FIG. 1, the CO.sub.2 removal from a gas stream consisting of 91% N.sub.2 and 9% CO.sub.2 was examined. The gas stream was at a pressure of 1.05 bar at 40° C. The mass flow rate was 42 kg/h. The particular absorbent had a temperature of 40° C. Structured packings were used in the absorber (height 4.2 m, diameter 0.1 m, pressure 60 bar). Structured packings were likewise used in the desorber (height 2.0 m, diameter 0.085 m, pressure 1.8 bar).

[0069] The absorbents used were [0070] (2-1) a 30% by weight aqueous solution of monoethanolamine (MEA); [0071] (2-2) an aqueous solution of methyldiethanolamine (MDEA) (2.2 M) and piperazine (1.5 M); and [0072] (2-3) an aqueous solution of 2-(2-tert-butylaminoethoxy)ethanol (TBAEE) (2.2 M) and piperazine (1.5 M).

[0073] The absorbent circulation rate and heating energy were varied such that 70% of the carbon dioxide was removed from the flue gas. The absorbent circulation rates specified in the table below correspond to the absorbent circulation rate with the minimum energy consumption. The relative values specified in the table below are based on the 30% MEA reference system.

TABLE-US-00002 Exam- Relative absorbent Relative energy** ple System circulation rate** [%] 2-1* MEA 100%  100%  2-2*  MDEA + piperazine 81% 95% 2-3  TBAEE + piperazine 63% 76% *comparative example **based on example 2-1*

[0074] It can be seen that the required CO.sub.2 removal level of 70% is achieved in example 2-3 with a lower absorbent circulation rate and a lower relative energy consumption compared to comparative examples 2-1 and 2-2.

EXAMPLE 3

[0075] In this example, the stability of aqueous absorbents to oxygenous fluid streams was examined.

[0076] The absorbents used were: [0077] (3-1) aqueous solution of monoethanolamine (MEA, 30% by weight); [0078] (3-2) aqueous solution of methyldiethanolamine (MDEA, 25% by weight) and piperazine (15% by weight); and [0079] (3-3) aqueous solution of 2-(2-tert-butylaminoethoxy)ethanol (TBAEE, 37% by weight) and piperazine (13% by weight).

[0080] A glass pressure reactor with a reflux condenser connected above and a stirrer was initially charged with the absorbent (about 120 g). Through a metal frit with a mean pore size 10 μm, a gas mixture of 8% by volume of O.sub.2, 28% by volume of CO.sub.2 and 64% by volume of N.sub.2 was passed continuously into the absorbent. The volume flow rate of gas was 12.5 L (STP)/h. The glass reactor was heated up to 100° C. and the pressure in the reactor was adjusted to 2 bar by means of a pressure-regulating valve. Under these conditions, the experiments were run over 400 to 500 h. The water discharged together with the gas was recycled by means of the reflux condenser connected above the reactor, which was operated at about 5° C. At regular intervals, samples were taken from the liquid phase and analyzed by means of GC for the amine content (TBAEE and/or MDEA and/or MEA). The amine contents are reported as normalized area proportions in the GC analysis.

[0081] FIG. 3 shows the plot of the amine content against time for MEA, MDEA and TBAEE. The normalized area proportion for TBAEE was still nearly 100% after 500 hours of the experiment; thus, no significant breakdown of TBAEE was detectable. In contrast, only about 75% of the amount of MEA used was found after about 400 hours of the experiment; only about 90% of the amount of MDEA used was found.