The hot potassium carbonate (K2CO3) process uses hot potassium carbonate to remove both CO2 and H2S. It works best on a gas with CO2 partial pressures in the range of 30-90 psi. The main reactions involved in this process are:
It can be seen from Equation 7-12 that H2S alone cannot be removed unless there is sufficient CO2 present to provide KHCO3, which is needed to regenerate potassium carbonate. Since these equations are driven by partial pressures, it is difficult to treat H2S to the very low requirements usually demanded (1/4 grain per 100 scf). Thus, final polishing to H2S treatment may be required.
The reactions are reversible based on the partial pressures of the acid gases. Potassium carbonate also reacts reversibly with COS and CS2.
Figure 7-5 shows a typical hot carbonate system for gas sweetening. The sour gas enters the bottom of the absorber and flows counter-current to the potassium carbonate. The sweet gas then exits the top of the absorber. The absorber is typically operated at 230°F; therefore, a sour/sweet gas exchanger may be included to recover sensible heat and decrease the system heat requirements.
The acid-rich potassium carbonate solution from the bottom of the absorber is flashed to a flash drum, where much of the acid gas is removed. The solution then proceeds to the stripping column, which operates at approximately 245°F and near-atmospheric pressure. The low pressure, combined with a small amount of heat input, drives off the remaining acid gases. The lean potassium carbonate from the stripper is pumped back to the absorber. The lean solution may or may not be cooled slightly before entering the absorber. The heat of reaction from the absorption of the acid gases causes a slight temperature rise in the absorber.
The solution concentration for a potassium carbonate system is limited by the solubility of the potassium bicarbonate (KHCO3) in the rich stream. The high temperature of the system increases the solubility of KHCC>3, but the reaction with CO2 produces two moles of KHCO3 per mole of K2CO3 reacted. For this reason the KHCO3 in the rich stream limits the lean solution K2CO3 concentration to 20-35% by weight.
The entire system is operated at high temperatures to increase the solubility of potassium carbonate. Therefore, the designer must be careful to avoid dead spots in the system where the solution could cool and precipitate solids. If solids do precipitate, the system may suffer from plugging, erosion, or foaming.
The hot potassium carbonate solutions are extremely corrosive. All carbon steel must be stress-relieved to limit corrosion. A variety of corrosion inhibitors are available to decrease corrosion.