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A curious insight into removal and recovery of ammonia from struvite thermal decomposition unit outlet by acidic adsorption Basiri, Neshat


In this study investigated the recovery of ammonia from the hot and humid gas outlet of a struvite thermal decomposition unit. Based on the project objectives and the initial studies, adsorption of the ammonia by oxalic acid dihydrate was selected as the ammonia removal method. Oxalic acid’s solid form not only would reduce the energy loss through adsorption, but also could be a potential carrier for the final product that could be used as a nitrogen fertilizer. This research presents experimental stages specifically designed for recovering the ammonia and energy of this stream. Experiments started in ambient temperatures and humidity to gain a good understanding of the process. Later, it evolved to simulate the actual conditions of the struvite thermal decomposition unit outlet. During the investigation phase, factors such as flow rate, adsorbent crystal size, and contact time were recognized as the most influential parameters for this process. The efficiency of ammonia removal could be optimized to stay above 90% by controlling these factors in a two-hour period. In the next phase, as the struvite thermal decomposition unit had output conditions of 80°C and relative humidity of 80%, experiments were continued by accommodating these conditions. At this stage, difficulties were observed in the stability of initial adsorbent. This led to a substitution of this adsorbent with anhydrous oxalic acid and potassium tetraoxalate dehydrate. The results indicated a high capacity and potential for ammonia adsorption. Despite the adverse effects of high temperature on adsorption processes, ammonia removal efficiency could be kept above 93% at all times, with modifications in the reactor for setting a good range of contact times for the adsorption. Although ammonia removal from the stream could achieve high efficiencies the ammonia capture in the adsorbents never exceeded 10-15% of the maximum theoretical adsorption. This suggested that the adsorption is only occurring in the surface layer of the adsorbents and the ammonia molecules are not penetrating and reacting with the total acid mass. Therefore, it is recommended that further research be continued, to gain new insight into this problem.

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