| The mechanism of thermodynamics process of P-NH3 polluted droplets formation during the exhaust gas discharge from a 600 kt·a-1 diammonium phosphate (DAP) production line was analyzed. One of its key factors was the high humidity of tail gas, which caused condensation due to local super-saturation. Based on the principles of clean processes, solving question from its reason and reducing mission, a recirculation technique, coupled the DAP tail gas dehumidification with the process of MAP production, was proposed. The wet section tail gas dehumidification was employed in the raw processing of MAP. At the same time, the energy gained from the condensation of wet section tail gas was used to raw process of MAP. Hence, it can not only reduce pollution but also save energy consumption of MAP. This technique benefits both environment and economy. The experimental results indicate that water content in the tail gas must be less than (0.065 kg H2O)/(kg air) so that the discharged DAP tail gas far below the thermal saturation (i.e., no condensation). The clean process was proposed including phosphoric acid pre-evaporation with tail gas partial condensation and ammonia evaporation combined with tail gas further condensation. Utilizing low temperature character of ammonia evaporation during the tail gas'second further condensation can gain great thermal degree difference and hence high equipment produce-capability. The main water (over 90%) was, however, dehumidified during the phosphoric acid pre-evaporation and partial condensation of tail gas. On the one hand, the tail gas has relatively low temperature (80oC) and small steam pressure compared with steam, respectively. On the other hand, phosphoric acid concentration equipments need high thermal degree difference. Therefore, the main plot of the cleaner process is to develop equipment, which fit tail gas partial condensation and phosphoric acid concentration. In the thesis a pilot experiment of phosphoric acid falling film evaporation was conducted. Firstly, numerical simulation of phosphoric acid laminar film flow in the pipe was made. Then its heat-and mass transfer coefficient was calculated. The parameter k, film thickness factor, was used to correlate the difference between falling film flow and full pipe flow. Finally, the heat-and mass transfer correlations for laminar film flow were deduced. They fit the experimental results well. The industrial experiment of DAP tail gas partial condensation combine with phosphoric acid falling film flow evaporation was carried out. All data were collected under long-term work conditions. The experimental results demonstrated the credibility of the above-mentioned numerical approach. The experiment showed that the total heat transfer coefficient of the combined process can keep 500-600 W·m-2·K-1,which is 75% of that of traditional heat exchanger, while the amount of recycling phosphoric acid is less than 1/30 of that of traditional heat exchanger. We obtained the data of falling film evaporation of phosphoric acids with different concentrations in the range of 27.1% and 41.7%. The feasibility of concentrating phosphoric acid at low temperature of 55℃was investigated. We also found the phenomenon of subcooled boiling and studied the effect of the subcooling on heat and mass transfer during phosphoric acid concentration. The study of this thesis presented theory analysis and numerical expression ofthe cleaning technique of DAP wet section tail gas partial condensation combine with MAP process, which were proved by experimental results. The data gained from industrial experiment supply the basis for further industrial equipment design.
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