Study On Exhausts Reduction And Energy Saving For Cleaner Production Of Ammonium Phosphate

Based on the principle of green chemistry and recycling economy, Programming and developing cleanly producing technology of phosphor resource had been brought into the medium-long term technological developing programme in China. Aim at the phosphate actuality of the producing process in China, Taking example for the exhausting gases discharged from a 600(kt/a) diammonium phosphate(DAP) production line that introduced into Norsk Hydro process in a certain plant.Thermodynamic analysis was made to the mechanism of P-NH3 polluted droplets formation in the exhausting gases discharged from a 600 (kt/a) diammonium phosphate (DAP) production line. The key factor is due to the inevitable steam condensation which takes place anywhere in the stack once the dew point temperature of the high humidity tail gas is over the local temperature.So,the cleaner process for monoammonium phosphate production combining with recycling of exhaust gases from diammonium phosphate production was developed. A radical measurement is to reduce the steam content of the tail gas below a critical value of 0.114 kg(H₂O)/kg(Air) thereby its dew point temperature below 55℃. A clean process was proposed for a 240 (kt/a) monoammonium phosphate (MAP) production line to use the energy released from dehumidifying of DAP tail gas to substitute as much as 45% of the total energy consumption for MAP production. To improve the efficiency of this secondary energy transport, an unique technique was developed to make a full use of the limited driving force for heat transfer by integrating the processes of partial condensation of tail gas with falling film evaporation of raw phosphoric acid. The feasibility was demonstrated by the pilot experiments in which the total heat transfer coefficient reached the industrial equipment level of 500～600 W·m^-2·K^-1. Some other advantages were also significant such as lower evaporation temperature(< 60℃), lower acid cycling amount and especially more than one magnitude expanding of evaporation surface areas compared with the traditional phosphoric acid concentration technique.Modeling and simulations were made to the processes of condensation dehumidifying of high humidity industrial exhaust gases which widely exist in process industry. An allocation factor,α, is introduced in the model to quantitatively express the effect of condensation manner on dehumidifying characteristics when the two mechanisms of fogging and film condensation simultaneously play their role in the process. From the pilot experiment of DAP tail gas cooling and dehumidifying in the region of 34～57% reduction of steam fraction, the considerably consistent experimental results were obtained to verify the simulation results for the allocation factorα=0.2.In experiment, The relation between the average heat transfer coefficient, the mass transfer coefficient of phosphate acid falling film and Re number were revealed. The average heat transfer coefficient of phosphate acid falling film is linear with Re number primitively, more increased afterwards, minished finally. As the Re number advanced, So didn't the mass transfer coefficient of phosphate acid falling film and along with the flow of phosphate acid falling film increasing, the mass transfer coefficient of phosphate acid falling film minished quickly.Influence of phosphate acid subcooling fall film on evaporation of phosphate acid falling film had studied by experiment. The experimental results displayed that the increased subcooling redounded to heat transfer of phosphate acid falling film.but went against the vaporing intensity of phosphate acid falling film. The experimental results displayed a very promising future application for subcooling falling film evaporation to be used for cleaner process of phosphoric acid concentration. Data of phosphate acid concentration (P₂O₅%)in the range of 21.1—41.7% had obtained by spot experiment. It displayed the probability of phosphoric acid concentration in temperature below 50°C and the phenomena of phosphate acid subcooling fall film evaporation.Experiment results that study on defluorination in vacuum evaporation of phosphoric acid falling film indicated that the operational temperature had great influence on defluorination in vacuum evaporation of phosphoric acid falling film. In a definite vacuum, the phosphate acid temperature was determined by phosphoric acid concentration. The more high phosphate acid concentration was, the more great influence on defluorination in vacuum evaporation of phosphoric acid falling film.In the process of ammonium phosphate concentration , the control level of liquid surface and measurement of the density were highly important to equipment running and quality of grainy product. The contact manometer was widely applied to industrial production for on-line measurement of the density and level of liquid surface. But its transducer couldn't induce the main body pressure of liquid so as to be invalid because the viscous slurries adhered to the transducer surface of manometer contacted by viscous slurries.A measurement that liquid pressure signal was transferred by air or inert gases was presented. Based on the principles of thermodynamics and hydromechanics, an equation was deduced to understand the phenomena of interface fluctuation corresponding to the change of temperature and pressure inside the measuring tube, demonstrative calculations were made to water and air system within the range of 160℃and 0.7MPa. An analytic criterion for gas escape which eventually leads to invalidation of measurement was obtained. The region of parameters allowing reliable measurement was determined from the calculation results. Furthermore a mass transfer model was proposed to analyse the negative effect of mass transfer of gas molecules through the interface to the flowing liquid, which may cause invalidation of measurement if the majority of gas loses from the measuring tube. It was demonstrated by the calculation that this process of mass transfer is very slow for the slightly dissolved gas-liquid system. Taking air-water as an trial system, the experiments were made to verify the model prediction. Under a constant pressure of 0.2 MPa and the temperature of 50℃and 80℃respectively, the change of interface position inside the measuring tube due to mass transfer was less than 5% during a period of 90 days, well consistent with the model prediction. It was indicated that the method was fitted well with the engineering requirement for reliable application.Ultrasonic was an important physical means of green reaction. Based on the fundamental themodynamics theory, vapor-liquid phase equilibrium thermo-dynamics in ultrasonic field had been studied, and phase equilibrium conditions are proposed. Through introduction of ultrasonic force potential, the thermodynamics model for phase equilibrium in the exert ultrasonic field was theoretically deduced. Effect of the external ultrasonic field on vapor-liquid phase equilibrium of phosphoric acid concentration was theoretically proved. It was theoretically indicated that ultrasonic could completely change the original condition of equilibrium system and a new condition of equilibrium system was built up accordingly.Furthermore, based on the characteristic of fluid flow surrounding ultrasonic gas cavity and the penetrate theory of mass transfer, combining the principles of hydrokinetics, a model was built up and deduced for mass transfer at phase interface evolving ultrasonic gas cavity. The model could describe mass transfer characteristic induced by ultrasonic gas cavity in liquid. The model was checked with literature date. It shows that the model could reflect the results of experimentation, It was reasonable that the model described the behavior of mass transfer induced by ultrasonic gas cavity in liquid at phase interface. The theory was provided for enhancing mass transfer process with ultrasonic by the model.