Modeling study of the heat of absorption and solid precipitation for CO capture by chilled ammonia.

The contribution of individual reactions to the overall heat of CO absorption, as well as conditions for solid NHHCO(s) formation in a chilled ammonia process (CAP) were studied using Aspen Plus at temperatures between 2 and 40 °C. The overall heat of absorption in the CAP first decreased and then increased with increasing CO loading. The increase in overall heat of absorption at high CO loading was found to be caused mostly by the prominent heat release from the formation of NHHCO(s). It was found that NHHCO(s) precipitation was promoted for conditions of CO loading above 0.7 mol CO/mol NH and temperatures less than 20 °C, which at the same time can dramatically increase the heat of CO absorption. As such, the CO loading is recommended to be around 0.6-0.7 mol CO/mol NH at temperatures below 20 °C, so that the overall absorption heat is at a low state (less than 60 kJ mol CO). It was also found that the overall heat of CO absorption did not change much with temperature when CO loading was less than 0.5 mol CO/mol NH, while, when the CO loading exceeded 0.7 mol CO/mol NH, the heat of absorption increased with decreasing temperature.