| The volume content of water vapor in ammonium phosphate exhausts which discharged through chimney is about 12%-20%, and the temperature is about 60℃-80℃, which is very close to the thermodynamic critical temperature of occurring fogging. It is prone to treat the abundant dust particles as nucleus, condense and fog because of local super-saturation, and further generate droplets around the chimney, forming "white smoke". The formed "white smoke" not only causes serious environmental pollution and affects the corporate impression, but also leads to a waste of resources.In this paper, the reason why the ammonium phosphate exhausts form "white smoke" was analyzed, the spray condensing method that used to recover water vapor and heat to solve the "white smoke" problem around the chimney was proposed. Through energy balance calculation, the simulation of spray system in condensing tower were carried out, the optimization of structure parameters and operation parameters in condensing tower, finally the design method of condensing tower was proposed. The main contents and conclusions were as follows:(1)The material balance and energy balance calculation of the spray condensing process were carried out, and the formulas which were used to calculate the required cooling water flow rate were also obtained.(2)The nozzle atomization characteristics were studied. Base on the Fluent software, the atomization model combined with discrete phase model(DPM) was used to simulate the spray system in spray condensing tower, and the spatial distribution of the spray droplet diameter was obtained:with the increasing of axial distance, the droplet diameter was decreased at first and then increased; the further the radial distance, the bigger the droplet diameter. The impact of spray pressure, spray angle and gas flow rate on atomization characteristics were also analyzed. The results showed that:with the increasing of axial distance, the droplet diameter was decreased and then increased; with the increasing of atomization pressure, the droplet size decreased, but the droplet diameter distribution was more uniform at the low pressure; with the diffusion angle was increasing, the droplet diameter and relative falling velocity of droplet were decreased; the larger the gas velocity, the larger the droplets diameter being taken out of the tower, and the droplet diameter change extent along the radial distance was big.(3) Based on the FLUENT software, the two-phase flow field in spray condensing tower was simulated, and the effect of structure parameters (gas inlet angle and nozzle arrangement manner) and operating parameters (superficial gas velocity and spray pressure) on gas-liquid two-phase flow field were discussed. The results showed that the optimized parameters were:gas inlet angle was 12°-15°, the nozzle arrangement manner was equilateral triangle arrangement, superficial gas velocity was about 3m/s, spray pressure was 5-6 bar.(4) According to the mechanism that water vapor condenses and occurs to phase change on the surface of droplet, the formula which used to calculate the mass transfer rate of water vapor condensation on the moving droplets was deduced. Based on the Merkel enthalpy difference equation, the heat transfer model of droplet swarm and gas phase in tower was obtained. Based on the above theories, the space incremental method that was used to calculate the height of condensing tower was proposed, and the result was validated by using tower height calculation method of cooling tower, and it was turned out to be feasible. Through tracking parameters of each infinitesimal segment in the tower, the variation laws of the related parameters of heat and mass transfer with tower height were also obtained. |
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