Analysis and optimization of ammonia stripping using multi-fluid model

Ammonia recovery from anaerobic digestion (AD) is a notably beneficial process for removing excess nitrogen, producing low cost fertilizer and enhancing odor abatement. A process was developed for nutrients recovery by integrating a simple and effective stripping process with AD system. To design, optimize and scale up this system, multi-fluid model was developed to simulate flow, mass transfer and reactions in the ammonia stripping tower. The mass and heat transfer and dissociation reactions were introduced into the CFD framework. The air/liquid ratio in each cell of the domain was considered in heterogeneous mass transfer rate. Good agreement between CFD modeling and experiments employing a packed bed was obtained on the effect of pH and temperature upon ammonia removal. It was found that the liquid to gas mass transfer rate became slower at the lower part of the packed bed with high air/liquid ratio and short liquid resident time, decreasing ammonia removal efficiency. The predicted contours at the lower part also showed decreases in liquid volume fraction and liquid temperature. These results suggest a great potential compensation to use the multi-section feed-in and recirculation for improving reactor performance.