Method and absorption medium for absorbing CO2 from a gas mixture

Abstract

The absorption of CO.sub.2 from a gas mixture by contacting the gas mixture with an absorption medium that comprises water and 5 to 50 wt % of amino acid salts of formula R.sup.1R.sup.2CHNHCH.sub.2COOK, in which R.sup.1 and R.sup.2 are n-alkyl radicals and the radicals R.sup.1 and R.sup.2 together have 2 to 4 carbon atoms, affords a high CO.sub.2 absorption capacity per unit weight in the cyclical operation of absorption and desorption.

Claims

1. An absorption medium for absorbing CO.sub.2 from a gas mixture, comprising water and 5 to 50 wt % of amino acid salts of formula (I):
R.sup.1R.sup.2CHNHCH.sub.2COOK(I) in which R.sup.1 and R.sup.2 are n-alkyl radicals, and the radicals R.sup.1 and R.sup.2 together have 2 to 4 carbon atoms; which further comprises potassium ions in the form of potassium carbonate or potassium hydrogen carbonate, wherein the molar ratio of potassium ions in the form of potassium carbonate or potassium hydrogen carbonate to amino acid salts of formula I is in the range from 0.01 to 0.1; and wherein: more than 90 wt % of the amino acid salts of formula (I) are potassium N-isopropylglycinate.

2. The absorption medium of claim 1, further comprising a corrosion inhibitor, a wetting promoter and/or a defoamer.

3. The absorption medium of claim 1, comprising 10 to 48 wt % of amino acid salts of formula (I).

4. The absorption medium of claim 1, comprising 35 to 45 wt % of amino acid salts of formula (I).

5. The absorption medium of claim 1, comprising more than 40 wt % of water.

6. A process for absorbing CO.sub.2 from a gas mixture by contacting the gas mixture with the absorption medium of claim 1.

7. The process of claim 6, wherein the absorption medium comprises 10 to 48 wt % of amino acid salts of formula (I).

8. The process of claim 6, wherein the absorption medium comprises 35 to 45 wt % of amino acid salts of formula (I).

9. The process of claim 6, wherein the absorption medium comprises at least 40 wt % of water.

10. The process of claim 6, wherein the gas mixture is a combustion off-gas, a natural gas or a biogas.

11. The process of claim 6, wherein CO.sub.2 absorbed in the absorption medium is desorbed again by increasing the temperature, reducing the pressure or both and, after this desorption of CO.sub.2, the absorption medium is used again for absorbing CO.sub.2.

12. The process of claim 11, wherein the absorption is carried out at a temperature in the range from 0 to 80 C. and the desorption is carried out at a higher temperature in the range from 50 to 200 C.

13. The process of claim 11, wherein the absorption is carried out at a pressure in the range from 0.8 to 50 bar and the desorption is carried out at a lower pressure in the range from 0.01 to 10 bar.

14. The process of claim 12, wherein the absorption is carried out at a temperature in the range from 20 to 70 C. and the desorption is carried out at a higher temperature in the range from 80 to 150 C.

15. The process of claim 12, wherein desorption is at a temperature at least 30 C. above the temperature during desorption.

16. The process of claim 13, wherein the absorption is carried out at a pressure in the range from 0.9 to 30 bar and the desorption is carried out at a lower pressure in the range from 0.01 to 10 bar.

17. The process of claim 5, wherein the gas mixture further comprises SO.sub.2 and, before absorbing CO.sub.2 from the gas mixture, the SO.sub.2 is depleted using a desulfurization process.

18. The process of claim 6, wherein, before being brought in contact with said absorption medium, the gas mixture has a CO.sub.2 content of 0.1-50% by volume.

19. The process of claim 6, wherein, before being brought in contact with said absorption medium, the gas mixture has a CO.sub.2 content of 8-20% by volume.

Description

EXAMPLES

Example 1

Preparation of an Aqueous Solution of Potassium N-isopropylglycinate

(1) 37.52 g of glycine were dissolved in a mixture of 500 ml of water and 147 ml of acetone. The pH of the solution was then adjusted to a value of 8.5 by addition of 1.49 g of 85 wt % potassium hydroxide. Following addition of 10 g of 5 wt % palladium on activated carbon (50 wt % water-moist), hydrogen was injected to 6 bar, and the mixture was stirred at 55 C. for 14 hours under a constant hydrogen pressure of 6 bar. The catalyst was subsequently removed by vacuum filtration. A .sup.1H NMR spectrum of the resulting solution showed N-isopropylglycine and N,N-diisopropylglycine as reaction products of glycine, in a molar ratio of 50:1. The solution was concentrated on a rotary evaporator to approximately 100 ml. Then 31.6 g of 85 wt % potassium hydroxide were added, and the mixture was made up to 222 g with water.

Example 2

Preparation of an Aqueous Solution of Potassium N-(sec-butyl)glycinate

(2) 75.1 g of glycine were dissolved in 450 ml of water, and the pH of the solution was adjusted to a value of 8.5 by addition of 3.3 g of 85 wt % potassium hydroxide. Following addition of 108.2 g of 2-butanone and 7.5 g of 5 wt % palladium on activated carbon (50 wt % water-moist), hydrogen was injected to 5 bar, and the mixture was stirred at 55 C. for 48 hours under a constant hydrogen pressure of 5 bar. The catalyst was then removed by vacuum filtration. A .sup.1H NMR spectrum of the resulting solution showed N-sec-butylylglycine as reaction product of glycine, and unreacted glycine, in a molar ratio of 50:1. The solution was concentrated on a rotary evaporator to approximately 200 ml. Then 60 g of 85 wt % potassium hydroxide were added, and the mixture was made up to 500 g with water.

Examples 3 to 17

Determination of the CO2 Absorption Capacity

(3) To determine the CO.sub.2 loading and the CO.sub.2 uptake, 150 g of aqueous absorption medium, containing the proportions of amino acid and potassium hydroxide indicated in table 1, were charged to a thermostatable container with a top-mounted reflux condenser cooled at 3 C. After heating to 40 C. or 100 C., a gas mixture of 14% CO.sub.2, 80% nitrogen and 6% oxygen by volume was passed at a flow rate of 59 l/h through the absorption medium, via a frit at the bottom of the container, and the CO.sub.2 concentration in the gas stream exiting the reflux condenser was determined by IR absorption using a CO.sub.2 analyser. The difference between the CO.sub.2 content in the gas stream introduced and in the exiting gas stream was integrated to give the amount of CO.sub.2 taken up, and the equilibrium CO.sub.2 loading of the absorption medium was calculated. The CO.sub.2 uptake was calculated as the difference in the amounts of CO.sub.2 taken up at 40 C. and at 100 C. The equilibrium loadings determined in this way at 40 C. and 100 C., in mol CO.sub.2/kg absorption medium, and the CO.sub.2 uptake in mol CO.sub.2/kg absorption medium are given in Table 1.

(4) TABLE-US-00001 TABLE 1 Loading Loading CO.sub.2 at 40 at 100 uptake Exam- C. in C. in in ple Amino acid KOH mol/kg mol/kg mol/kg 3* 30.0 g glycine 26.5 g 1.74 1.33 0.41 4* 30.0 g N-methylglycine 22.5 g 1.74 1.16 0.58 5* 30.0 g ethylglycine 19.5 g 1.92 1.08 0.84 6* 30.0 g N,N- 19.5 g 1.78 0.67 1.11 dimethylglycine 7* 30.0 g N-propylglycine 17.1 g 1.54 0.93 0.61 8 30.0 g N- 17.1 g 1.81 0.82 0.99 isopropylglycine 9 30.0 g N-(sec-butyl) 14.3 g 1.44 0.63 0.81 glycine 10 30.0 g N-(3-pentyl) 13.6 g 1.32 0.54 0.78 glycine 11 37.6 g N- 21.4 g 2.16 0.84 1.32 isopropylglycine 12* 45.0 g glycine 33.4 g 2.91 2.23 0.68 13* 45.0 g N-methylglycine 33.5 g 2.43 1.65 0.78 14* 45.0 g N-methylalanine 30.0 g 2.71 1.51 1.20 15* none 25.7 g 2.57 2.18 0.39 16* 7.5 g N-methylalanine 25.7 g 2.55 1.87 0.68 17* 45.0 g N-methylalanine 25.7 g 2.47 1.33 1.14 *not according to the invention