Patent EA-001543-B1 (EA)
Title: A method for the continuos removal of carbon dioxide from a process gas
Inventor: HESSE HORST JOCHIM FRANZ AUGUS
IPC: B01D53/96
Filed: 1997-12-12
Abstract: 1. A method for a continuous removal of carbon dioxide from a process gas by contacting the process gas under pressure in an absorption stage with an aqueous solution containing, dissolved therein, potassium carbonate as a reagent, potassium bicarbonate as a reaction product, diethanol amine as a catalyst and potassium vanadate as a corrosion inhibitor, the carbon dioxide in the process gas being absorbed by the solution and reacting therein with the potassium carbonate reagent according to the reaction: K2CO3 + CO2 + H2O -> 2KHCO3 followed by an increase in the temperature of the solution and a decrease in the pressure exerted thereon to cause desorption, in a desorption stage separate from the absorption stage, of the carbon dioxide from the solution according to the reverse reaction: 2 KHCO3 -> K2CO3 + CO2 + H2O, the aqueous solution continuously being recycled from the desorption stage to the absorption stage for further removal of carbon dioxide from the process gas so that the aqueous solution circulates around a circuit comprising said absorption stage and said desorption stage, the aqueous solution containing organic acids dissolved therein in the form of organic acid salts of potassium, and the removal of carbon dioxide from the process gas being associated with a continuous increase in the concentration of said organic acids, in the aqueous solution, the method which comprises: the withdrawal, as a side stream, of part of the aqueous solution circulating around the circuit, diluting the side stream with an aqueous diluent, and passing the side stream over a semi-permeable membrane across which there is a pressure drop, to cause an aqueous solution of the organic acid salts of potassium to pass through the membrane, the membrane and the pressure drop across the membrane being selected such that vanadate anions are essentially prevented from passing through the membrane, and such that acceptably small proportions of potassium carbonate, potassium bicarbonate and diethanol amine pass through the membrane,  the remainder of the side stream, after it has passed over the membrane, being returned to the circuit, potassium carbonate make-up and diethanol amine make-up being added to the circuit, as required, to maintain them in the aqueous solution at the necessary concentrations. 2. A method as claimed in claim 1, in which, in the circuit downstream of the desorption and upstream of the absorption, there is a potassium carbonate concentration of 200 - 250 g/l, a diethanol amine concentration of 15 - 20 g/l, a potassium vanadate concentration of 16 - 18 g/l, a concentration of organic acid salts of potassium of up to 160 g/l and a concentration of potassium bicarbonate of 150 - 250 g/l. 3. A method as claimed in claim 1 or claim 2, in which the absorption takes place at a temperature of 94 - 107 degree C and at a pressure of 2500 - 3000 kPa, the side stream being withdrawn from the circuit after the absorption and before the pressure decrease which causes the desorption. 4. A method as claimed in any one of the preceding claims, in which the diluting of the side stream is such that the side stream, after dilution thereof, has a concentration of organic acid salts of potassium of less than 16 g/l. 5. A method as claimed in any one of the preceding claims, in which the passing of the side stream over the membrane is at a temperature of 30 60 degree C. 6. A method as claimed in any one of the preceding claims, which includes filtering the side stream before the side stream is passed over the membrane  7. A method as claimed in claim 6, which includes cooling the side stream after the diluting of the side stream and before the filtering of the side stream, being to a temperature of at most 60 degree C. 8. A method as claimed in any one of the preceding claims , in which the side stream is passed over a said membrane which is a nanofiltration membrane, so that the solution of organic acid salts of potassium is separated from the remainder of the side stream by nanofiltration. 9. A method as claimed in claim 8, which includes both cooling the side stream and filtering the side stream before the side stream is passed over the nanofiltration membrane, the cooling of the side stream taking place before the filtering of the side stream and being to a temperature of at most 60 degree C, and the filtering of the side stream being to remove all particles larger than 5 μm from the side stream. 10. A method of treating an aqueous solution which contains, dissolved therein, potassium carbonate, potassium bicarbonate, diethanol amine, potassium vanadate and organic acid salts of potassium, so as to remove said organic salts from the solution while retaining the potassium vanadate in the solution, the method comprising: withdrawing part of the solution from the remainder thereof, diluting said part of the solution with an aqueous diluent, and passing the diluted part of the solution over a semi-permeable membrane across which there is a pressure drop, to cause an aqueous solution of the organic acid salts of potassium to pass through the membrane, the membrane and the pressure drop across the membrane being selected such that the vanadate cations are essentially prevented from passing through the membrane, and such that acceptably small proportions of potassium carbonate, potassium bicarbonate and diethanol amine pass through the membrane, the rest of said diluted part, after it has passed over the membrane, being returned to the remainder of the solution. 11. A method as claimed in claim 10, in which the diluting of the withdrawn part of the solution is such that it, after dilution thereof, has a concentration of organic acid salts of potassium of less than 16 g/l. 12. A method as claimed in claim 10 or claim 11, in which the passing of the diluted solution over the membrane is at a temperature of 30 - 60 degree C. 13. A method as claimed in any one of claims 10 - 12, which includes filtering the diluted solution before it is passed over the membrane. 14. A method as claimed in claim 13, in which the filtered solution is at a temperature above 60 degree C, the method including cooling the diluted solution stream after the diluting thereof and before the filtering thereof, being to a temperature of at most 60 degree C. 15. A method as claimed in any one of the claims 10 - 14, in which the diluted solution is passed over a said membrane which is a nanofiltration membrane, so that the solution of the organic acid salts of potassium is withdrawn from the rest of said diluted part due to the nanofiltration. 16. A method as claimed in claim 15, in which the diluted solution is at a temperature above 60 degree C, the method including both cooling the diluted solution and filtering it before it is passed over the nanofiltration membrane, the cooling of the solution taking place before the filtering of the side stream and being to a temperature of at most 60 degree C, and the filtering of the side stream being to remove all particles larger than 5 μm from the side stream.