Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon dioxide or hydrogen sulfide component
Reexamination Certificate
1999-07-13
2001-11-06
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon dioxide or hydrogen sulfide component
C210S649000, C210S650000, C210S651000, C210S652000, C210S653000
Reexamination Certificate
active
06312655
ABSTRACT:
SUMMARY OF THE INVENTION
THIS INVENTION relates to a method for the removal of carbon dioxide from a process gas, and to the treatment of an aqueous solution suitable for use in said removal of carbon dioxide. More particularly,the invention relates to a method for the removal of carbon dioxide from a process gas by contacting the process gas with an aqueous solution containing potassium carbonate dissolved therein, and to the treatment of such solution.
According to one aspect of the invention in the 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:
K
2
CO
3
+CO
2
+H
2
O→2KHCO
3
,
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:
2KHCO
3
→K
2
CO
3
+CO
2
+H
2
O,
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, there is provided 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 remainder of the side stream, after it has passed over the membrane, being returned to the circuit, 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, potassium carbonate make-up and diethanol amine make-up being added to the circuit, continuously or intermittently as required, to maintain them in the aqueous solution at the necessary concentrations.
The organic acids may be derived from the process gas. Instead or in addition, they may arise in the circuit.
The semi-permeable membrane may be selected from, but is not limited to, nanofiltration membranes, ultrafiltration membranes and reverse-osmosis membranes.
In the circuit, downstream of the desorption and upstream of the absorption there may be a potassium carbonate concentration of 200-250 g/l, a diethanol amine concentration of 15-20g/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, although, naturally, these values can vary from one circuit to another, depending on practical and economic considerations.
The absorption may take place at a temperature of 94-107° 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, Diluting the side stream may be such that the side stream, after dilution thereof, has a concentration of organic acid salts of potassium of less than 16 g/l.
The passing of the side stream over the membrane may be at a temperature of 30-60° C. Accordingly, the method may include cooling the side stream, eg after said dilution thereof, to said temperature of 30-60° C. at which it is passed over the membrane. The method may include filtering the side stream before it is passed over the membrane. In this case the method may include cooling the side stream after the diluting of the side stream and before the filtering of the side stream, the cooling being to a temperature of at most 60° C.
More particularly, the side stream may be 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; and in this case the method may include 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° C., and the filtering of the side stream being to remove all particles larger than 5 &mgr;m from the side stream.
According to another aspect of the invention there is provided a method of treating an aqueous solution which contains, dissolved therein, potassium carbonate, potassium bicarbonate, dicthanol 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 rest of said diluted part, after it has passed over the membrane, being returned to the remainder of the solution, the membrane and the pressure drop across the membrane being selected such that the 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.
In this aspect of the invention the diluting of the withdrawn part of the solution may be such that it, after dilution thereof, has a concentration of organic acid salts of potassium of less than 16 g/l; and the passing of the diluted solution over the membrane may be at a temperature of 30-60° C. The method may Include filtering the diluted solution before it is passed over the membrane; and when the filtered solution is at a temperature above 60° C., the method may include cooling the diluted solution after the diluting thereof and before the filtering thereof, being to a temperature of at most 60° C.
The diluted solution may be 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; and when the diluted solution is at a temperature above 60° C., the method may include 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° C., and the filtering of the side stream being to remove all particles larger than 5 &mgr;m from the side stream.
The methods of the invention are expected to find particular application in the treatment of Benfield solutions or any other process solutions containing potassium carbonate, potassium bicarbonate, diethanol amine and potassium vanadate and, as impurities, organic acids salts of potassium, of the type used for removing carbon dioxide from process gas streams. Typically, the aqueous solution treated is thus in the form of a side stream from a carbon dioxide-scrubbing circuit which employs a said potassium carbonate solu
du Toit Francois Jacobus
Hesse Horst Joachim Franz August
Smit Mathys Johannes
(Sasol Technology (Proprietary) Limited)
Griffin Steven P.
Ladas & Parry
Vanoy Timothy C
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