Effect of pressure on heat and mass transfer performance in spray drying tower with low inlet temperature

The spray drying tower is an important device of the air circulation evaporation-separation electroplating wastewater treatment system based on Brayton refrigeration cycle, which can acquire the optimum system performance by controlling the operational pressure. In order to explore the optimum pressure in the tower, the heat and mass transfer performances of the spray drying tower under different pressures are investigated. The experimental results show that the evaporation rate increases with the decrease of the pressure. The effect of pressure on tower height and inlet air temperature required for droplet to be completely dried is simulated to use the model verified by experimental data. A three-stage heat and mass transfer model for single-droplet is developed in the simulation. The simulation results show that there is an optimal pressure corresponding to the lowest required tower height. The optimal pressure is increased with the increase of inlet air temperature. When the tower height is fixed, the inlet air temperature required for drying droplet firstly decreases and then increases with the increase of pressure. In addition, through analyzing heat and mass transfer, it is found that the decrease of pressure results in the increase of not only the mass transfer driving force but also the air velocity, thus the droplet residence time is decreased, which leads to the existence of optimal pressure. It is feasible to achieve complete drying in spray drying process by reducing the pressure in the tower with low inlet temperature.