Method For The Removal Of Carbon Dioxide From Gas Flows With Low Carbon Dioxide Partial Pressures

Abstract

The invention relates to a method for the removal of carbon dioxide from a gas flow with a carbon dioxide partial pressure in the gas flow of less than 200 mbar, whereby the gas flow is brought into contact with a liquid absorption agent, selected from an aqueous solution (A) of an amino compound with at least two tertiary amino groups in the molecule and (B) an activator, selected from the primary and secondary amines, or (A) a tertiary aliphatic amine, the reaction enthalpy ΔRH for the protonation of which is greater than that for methyldiethanolamine and (B) an activator, selected from 3-methylaminopropylamine, piperazine, 2-methylpiperazine, N-methylpiperazine, homopiperazine, piperidine and morpholine. The method is particularly suitable for the treatment of flue gases and also relates to an absorption agent.

Description

The invention is described in more detail on the basis of the accompanying figure.

FIG. 1 is a diagrammatic representation of a plant suitable for carrying out the inventive process.

According to FIG. 1, a suitably pretreated combustion gas which comprises carbon dioxide is brought into contact in counter-current flow in an absorber 3 with the regenerated absorption medium which is fed by the absorption medium line 5. The absorption medium removes carbon dioxide from the combustion gas by absorption; in the process a clean gas which is low in carbon dioxide is produced via an off-gas line 7. The absorber 3 can have (which is not shown), above the absorption medium inlet, backwash trays or backwash sections which are preferably equipped with packings, where entrained absorption medium is separated off from the CO₂-depleted gas using water or condensate. The liquid on the backwash tray is recycled in a suitable manner via an external cooler.

Via an absorption medium line 9 and a throttle valve 11, the carbon-dioxide-loaded absorption medium is passed through a desorption column 13. In the lower part of the desorption column 13 the loaded absorption medium is heated and regenerated by means of a heater (which is not shown). The resultant carbon dioxide which is released leaves the desorption column 13 via the off-gas line 15. The desorption column 13 can have (which is not shown), above the absorption medium inlet, backwash trays or backwash sections which are preferably equipped with packings, where entrained absorption medium is separated off from the released CO₂ using water or condensate. In line 15, a heat exchanger having a top distributor or condenser can be provided. The regenerated absorption medium is then fed back to the absorption column 3 by means of a pump 17 via a heat exchanger 19. To prevent the accumulation of absorbed substances which are not expelled, or are expelled only incompletely in the regeneration, or of decomposition products in the absorption medium, a substream of the absorption medium taken off from the desorption column 13 can be fed to an evaporator in which low-volatile byproducts and decomposition products arise as residue and the pure absorption medium is taken off as vapors. The condensed vapors are recirculated to the absorption medium circuit. Expediently, a base, such as potassium hydroxide, can be added to the substream, which base forms, for example together with sulfate or chloride ions, low-volatile salts, which are taken off from the system together with the evaporator residue.

EXAMPLES

• In the examples hereinafter, the following abbreviations are used:
• DMEA: N,N-dimethylethanolamine
• DEEA: N,N-diethylethanolamine
• TMPDA: N,N,N′,N′-tetramethylpropanediamine
• MDEA: N-methyldiethanolamine
• MAPA: 3-methylaminopropylamine
• Niax: 1-dimethylamino-2-dimethylaminoethoxyethane

All percentages are percentages by weight.

Example 1

CO₂ Mass Transfer Rate

The mass transfer rate was determined in a laminar jet chamber using water vapor-saturated CO₂ at 1 bar and 50° C. and 70° C., jet chamber diameter 0.94 mm, jet length 1 to 8 cm, volumetric flow rate of the absorption medium 1.8 mils and is reported as gas volume in cubic meters under standard conditions per unit surface area of the absorption medium, pressure and time (Nm³/m²/bar/h). p The results are summarized in the following table 1. The CO₂ mass transfer rate reported in the table is related to the CO₂ mass transfer rate of a comparison absorption medium, which comprises 35% by weight MDEA and 5% by weight piperazine.

TABLE 1
Amine	Activator	Temperature	Relative CO2 mass
[35% by weight]	[5% by weight]	[° C.]	transfer rate [%]
DEEA	Piperazine	50	121.60
DEEA	Piperazine	70	117.24
TMPDA	Piperazine	50	157.60
TMPDA	Piperazine	70	145.98

Example 2

CO₂ Uptake Capacity and Regeneration Energy Requirement

To determine the capacity of various absorption media for the uptake of CO₂ and to estimate the energy consumption in the regeneration of the absorption media, firstly measured values were determined for the CO₂ loading at 40 and 120° C. under equilibrium conditions. These measurements were carried out for the systems CO₂/Niax/MAPA/water; CO₂/TMPDA/MAPA/water; CO₂/DEEA/MAPA/water; CO₂/DMEA/MAPA/water; CO₂/Niax/piperazine/water; CO₂/TMPDA/piperazine/water in a glass pressure vessel (volume=110 cm³ or 230 cm³), in which a defined amount of the absorption medium had been charged, evacuated and, at constant temperature, carbon dioxide was added stepwise via a defined gas volume. The amount of carbon dioxide dissolved in the liquid phase was calculated after gas space correction of the gas phase. The equilibrium measurements for the system CO₂/MDEA/MAPA/water were performed in the pressure range>1 bar using a high pressure equilibrium cell; in the pressure range<1 bar, the measurements were carried out using headspace chromatography. The equilibrium data for the system CO₂/MDEA/piperazine/water were calculated according to the electrolyte approach of Pitzer (Kenneth S. Pitzer, Activity Coefficients in Electrolyte Solutions 2nd ed., CRC-Press, 1991, Chapt. 3, Ion Interaction Approach: Theory and Data Correlation; the parameters were matched to measured data).

To estimate the absorption medium capacity, the following assumptions were made:

1. The absorber is exposed at a total pressure of one bar to a CO₂-comprising flue gas of 0.13 bar CO₂ partial pressure (=13% CO₂ content).

2, In the absorber bottom, a temperature of 40° C. prevails.

3. During the regeneration, a temperature of 120° C. prevails in the desorber bottom.

4. In the absorber bottom, an equilibrium state is achieved, that is the equilibrium partial pressure is equal to the feed gas partial pressure of 13 kPa.

5. During the desorption, a CO₂ partial pressure of 5 kPa prevails in the desorber bottom (the desorption is typically operated at 200 kPa. At 120° C. pure water has a partial pressure of about 198 kPa. In an amine solution the partial pressure of water is somewhat lower, therefore a CO₂ partial pressure of 5 kPa is assumed).

6. During the desorption, an equilibrium state is achieved.

The capacity of the absorption medium was determined from (i) the loading (mole of CO₂ per kg of solution) at the intersection of the 40° C. equilibrium curve with the line of constant feed gas CO₂ partial pressure of 13 kPa (loaded solution at the absorber bottom in equilibrium); and (ii) from the intersection of the 120° C. equilibrium curve with the line of constant CO₂ partial pressure of 5 kPa (regenerated solution at the desorber bottom in equilibrium). The difference between the two loadings is the circulation capacity of the respective solvent. A high capacity means that less solvent need be circulated and thus the apparatuses such as, for example, pumps, heat exchangers, but also the piping, can be dimensioned so as to be smaller. In addition, the circulation rate also influences the energy required for regeneration.

A further measure of the service properties of an absorption medium is the gradient of the working lines in the McCabe-Thiele diagram (or p-X diagram) of the desorber. For the conditions in the bottom of the desorber, the working line is generally very close to the equilibrium line, so that the gradient of the equilibrium curve to an approximation can be equated to the gradient of the working line. At a constant liquid loading, for the regeneration of an absorption medium having a high gradient of equilibrium curve, a smaller amount of stripping steam is required. The energy requirement to generate the stripping steam makes an important contribution to the total energy requirement of the CO₂ absorption process.

Expediently, the reciprocal of the gradient is reported, since this is directly proportional to the amount of steam required per kilogram of absorption medium. If the reciprocal is divided by the capacity of the absorption medium, this gives a comparative value which directly enables a relative statement on the amount of steam required per absorbed amount of CO₂.

In table 2, the values of the absorption medium capacity and the steam requirement are standardized to the mixture of MDEA/piperazine. In table 3, the values of the absorption medium capacity and the steam requirement are standardized to the mixture of MDEA/MAPA.

It can be seen that absorption media having a tertiary amine whose reaction enthalpy Δ_RH of the protonation reaction is greater than that of methyidiethanolamine have a higher capacity and require a lower amount of steam for regeneration.

TABLE 2
	Relative	Relative required
Absorption medium	capacity [%]	amount of steam [%]
Niax (37%)/piperazine (3%)	162	52
TMPDA (37%)/piperazine (3%)	186	57
MDEA (37%)/MAPA (3%)	100	100

TABLE 3
	Relative	Relative required
Absorption medium	capacity [%]	amount of steam [%]
Niax (37%)/MAPA (3%)	162	43
MDEA (37%)/MAPA (3%)*	100	100
TMPDA (37%)/MAPA (3%)	180	69
DMEA (37%)/MAPA (3%)	174	70
DEEA (37%)/MAPA (3%)	180	72
*comparative example

Claims

1. A process for removing carbon dioxide from a gas stream in which the partial pressure of the carbon dioxide in the gas stream is less than 200 mbar, which comprises bringing the gas stream into contact with a liquid absorption medium which comprises an aqueous solution of (A) an amine compound containing at least two tertiary amino groups in the molecule and(B) an activator which is selected from primary and secondary amines.

2. A process for removing carbon dioxide from a gas stream in which the partial pressure of the carbon dioxide in the gas stream is less than 200 mbar, which comprises bringing the gas stream into contact with a liquid absorption medium which comprises an aqueous solution of (A) a tertiary aliphatic amine and(B) an activator which is selected from 3-methylaminopropylamine, piperazine, 2-methylpiperazine, N-methylpiperazine, homopiperazine, piperidine and morpholine, wherein the tertiary aliphatic amine A is characterized by a reaction enthalpy ΔRH of the protonation reaction A+H+→AH+which is greater than that of methyldiethanolamine.

3. The process according to claim 1, wherein the amine compound has the general formula RaRbN—X—NRa′Rb′,where Ra, Rb, Ra′ and Rb′ independently of one another are selected from C1-C6-alkyl groups, C2-C6-hydroxyalkyl groups or C1-C6-alkoxy-C2-C6-alkyl groups and X is a C2-C6-alkylene group, —X′—NR—X2— or —X1O—X2—, where X1 and X2 independently of one another are C2-C6-alkylene groups and R is a C1-C6-alkyl group.

4. The process according to claim 3, wherein the amine compound is selected from N,N,N′,N′-tetramethylethylenediamine, N,N-diethyl-N′,N′-dimethylethylene-diamine, N,N,N′,N′-tetraethylethylenediamine, N,N,N′,N′-tetramethyl-1,3-propanediamine N,N,N′,N′-tetraethyl-1,3-propanediamine and bis(dimethylaminoethyl) ether.

5. The process according to claim 1, wherein the activator is selected from a) 5- or 6-membered saturated heterocycles containing at least one NH group in the ring orb) compounds of the formula R1—NH—R2—NH2, where R1 is C1-C6-alkyl and R2 is C2-C6-alkylene.

6. The process according to claim 5, wherein the activator is selected from piperazine, 2-methylpiperazine, N-methylpiperazine, homopiperazine, piperidine and morpholine.

7. The process according to claim 5, wherein the activator is 3-methylaminopropylamine.

8. The process according to claim 1, wherein the concentration of the amine compound is from 20 to 60% by weight and the concentration of the activator is from 1 to 10% by weight, based on the total weight of the absorption medium.

9. The process according to claim 1, wherein the gas stream results from a) the oxidation of organic substances,b) the composting or storage of waste material containing organic substances, orc) the bacterial decomposition of organic substances.

10. The process according to claim 1, wherein the loaded absorption medium is regenerated by a) heating,b) expansion,c) stripping with an inert fluidor a combination of two or all of these measures.

11. The process according to claim 10, wherein the loaded absorption medium is regenerated by heating at a pressure of from 2 to 10 bar.

12. An absorption medium for removing carbon dioxide from a gas stream comprising an aqueous solution of (A) an amine compound of the formula RaRbN—X—NRaRb,where Ra, Rb, Ra′ and Rb′ independently of one another are selected from C1-C6-alkyl groups, C2-C6-hydroxyalkyl groups or C2-C6-alkoxy-C2-C6-alkyl groups and X is a C2-C6-alkylene group, —X1—NR—X2— or —X—O—X2—, where X1 and X2 independently of one another are C2-C6-alkylene groups and R is a C1-C6-alkyl group,(B) an activator which is selected from primary and secondary amines.

13. An absorption medium for removing carbon dioxide from a gas stream, comprising an aqueous solution of (A) a tertiary aliphatic amine and(B) an activator which is selected from 3-methylaminopropylamine, piperazine, 2-methylpiperazine, N-methylpiperazine, homopiperazine, piperidine and morpholine, wherein the tertiary aliphatic amine A is characterized by a reaction enthalpy ΔRH of the protonation reaction A+H+→AH+which is greater than that of methyldiethanolamine.

14. The absorption medium according to claim 13, wherein the tertiary aliphatic amine is selected from N,N-diethylethanolamine, N,N-dimethylethanolamine, 2-diisopropylaminoethanol, N,N,N′,N′-tetramethylethylenediamine, N,N-diethyl-N′,N′-dimethylethylenediamine, N,N,N′,N′-tetraethylethylenediamine, N,N,N′,N′-tetramethyl-1,3-propanediamine, N,N,N′,N′-tetraethyl-1,3-propanediamine and bis(dimethylaminoethyl) ether.

15. The process according to claim 2, wherein the amine compound has the general formula RaRbN—X—NRa′Rb′where Ra, Rb, Ra′ and Rb′ independently of one another are selected from C1-C6-alkyl groups, C2-C6-hydroxyalkyl groups or C1-C6-alkoxy-C2-C6-alkyl groups and X is a C2-C6-alkylene group, —X1—NR—X2— or —X1—O—X2—, where X1 and X2 independently of one another are C2-C6-alkylene groups and R is a C1-C6-alkyl group.

16. The process according to claim 2, wherein the activator is selected from a) 5- or 6-membered saturated heterocycles containing at least one NH group in the ring orb) compounds of the formula R1—NH—R2—NH2, where R1 is C1-C6-alkyl and R2 is C2-C6-alkylene.

17. The process according to claim 3, wherein the activator is selected from a) 5- or 6-membered saturated heterocycles containing at least one NH group in the ring orb) compounds of the formula R1—NH—R2—NH2, where R1 is C1-C6-alkyl and R2 is C2-C6-alkylene.

18. The process according to claim 4, wherein the activator is selected from a) 5- or 6-membered saturated heterocycles containing at least one NH group in the ring orb) compounds of the formula R1—NH—R2—NH2, where R1 is C1-C6-alkyl and R2 is C2-C6-alkylene.

19. The process according to claim 2, wherein the concentration of the amine compound is from 20 to 60% by weight and the concentration of the activator is from 1 to 10% by weight, based on the total weight of the absorption medium.

20. The process according to claim 3, wherein the concentration of the amine compound is from 20 to 60% by weight and the concentration of the activator is from 1 to 10% by weight, based on the total weight of the absorption medium.