The Catalysis Of Potassium Carbonate Solutions In Carbon Dioxide Absorption
Authors: Eickmeyer A.G. 
Published: December 2007

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The activity of hot potassium carbonate solutions has been found to be a function of contaminants, having a deleterious effect, and catalysts, which activate the solution. A small amount of "Catacarb" catalyst cancels the effect of contamination and larger amounts give a much more active solution than pure potassium carbonate. In commercial plant tests in existing carbonate plants, the Catacarb Process resulted in increased capacity, reduced steam usage, and less residual CO2. In new plant construction, less equipment and utility requirements of the Catacarb process permit savings in capital investment and operating costs.

Introduction

The hot potassium carbonate process, developed by the Bureau of Mines, has been applied extensively in recent years to CO2 removal in the purification of hydrogen and natural gas. Most of tee hot carbonate units are in ammonia plants, but use of the process for treating natural gas, including sour gas, is increasing. The equipment involves the familiar absorber and regenerator. The usual process cycles have been previously described. As compared with amine scrubbing the main advantages of hot carbonate are the elimination of heat exchange equipment and the lower steam and cooling water requirements. The disadvantage of a hot carbonate system is the inability to secure a satisfactory degree of clean up in the scrubbed gas except with very large towers, a more complicated process cycle or even a second absorber operated at high pressure. This paper deals with "Catacarb", the catalytic process for CO2 removal. In the Catacarb Process a catalyst is used to activate a carbonate solution in the absorption and desorption of carbon dioxide, thus overcoming the above disadvantage of carbonate scrubbing.

THEORY AND DEFINITIONS

The absorption of an acid gas in any alkaline solution is classed as chemi-sorption, that is, absorption accompanied by chemical reaction. The rate of chemical reaction determines the size of the towers required by the process. Unlike caustic, a carbonate solution has very few hydroxyl ions to react directly with CO2. Therefore, it is believed that CO2 must first become hydrated, or react with water to form carbonic acid, which in turn reacts with a carbonate ion to form two bicarbonate ions.

CO2 + H2O = H2CO3 (1)

H2CO3 + CO3 = 2 HCO-3 (2)

Confirmation of the reaction with water is the finding, both in commercial plant operations and laboratory experiments, that dilute solutions which have a higher concentration of free water, are more active than concentrated carbonate solutions. The term "free water" takes into account the well known fact that potassium ions are highly hydrated, although the exact ratio of combined water to potassium is uncertain and probably varies with concentration. As in any practical chemical process, we are concerned with equilibrium and with kinetics. Equilibrium for the potassium carbonate-bicarbonate-CO2-water system has been reported. We now focus our attention on heretofore neglected kinetics or solution activity, which is the relative rate of approach to equilibrium.