Selective Removal of Carbon Dioxide from Aqueous Ammonia Solutions

In this work modelling and experimental validation of an integrated process for the removal of carbon dioxide from ammonia solutions is presented (decarbonisation). In this process, carbon dioxide is stripped out from the solution simultaneously with small amounts of ammonia at ambient pressure in a packed column. Recovery of the stripped ammonia can be reached by combining absorption of ammonia and condensation of stripping steam. The interaction of the different unit operations of stripping, absorption and direct-contact condensation (DCC) can be arranged in one single packed column where stripping takes place in the lower part of the column, and the DCC and ammonia absorption in the upper part. To describe the process a rigorous model based on the film theory which considers heat and mass transfer as well as reaction rates directly has been adapted to the requirements of multicomponent reactive stripping, absorption and direct-contact condensation in packed columns. Extensive experimental investigations have been performed for both, the stripping and for the direct-contact condensation in a pilot scale packed column with diameters of 0.15 and 0.32 m. Relevant operation parameters as well as column dimensions were varied during the experiments in order to achieve a broad data base for the validation. Experimental validation of the two sub-processes and the entire decarbonisation shows good agreement between calculated and experimental values. Based on the validated model a successful optimisation of the decarbonisation process has been performed, leading to increased carbon dioxide removal and reduction of ammonia losses.