Research On The Mechanism And Process Application Of Microwave Enhanced Flash Evaporatio

As an important evaporation concentration method and vapor-liquid separation process,flash evaporation has the characteristics of fast vaporization rate and obvious liquid cooling,which is widely used in metallurgy,chemical industry,food and other related industries.However,due to the limitations of traditional flash evaporation itself,such as equipment easy to corrode and scale,redundant system composition,low efficiency of single stage devices,etc.,results in the limitation of the application of flash evaporation technology.Chemical process intensification provides a guiding idea for breaking through such problems in traditional chemical industry.And in the meantime,the use of microwave as an external field for chemical process intensification has developed into one of the research hotspots of microwave industrial applications.In view of the characteristics of microwave heating,such as fast heating speed,bulk heating,selective heating and easy control,etc.,therefore,in the present thesis,in order to improve the efficiency of flash evaporation and promote the miniaturization of equipment,（i）a new process,namely“microwave intensification flash evaporation”was firstly proposed by combining the technical advantages of microwave heating and the characteristics of the flash evaporation process,（ii）developed a microwave intensification flash evaporation experimental system,and theoretically analyzed the energy conversion characteristics of the microwave-intensification flash evaporation process,（iii）in order to reveal the process mechanism of microwave intensification flash evaporation and verify the performance of the process and system for evaporative concentration treatment of metallurgical industrial wastewater and typical low thermal conductivity,high viscosity and high corrosive liquids in chemical industry,experimental studies on fluidization evaporation,treatment of metal ion containing wastewater and preparation of polyphosphoric acid by wet process phosphoric acid concentration were carried out accordingly.Details are as follows:（1）In the development of the experimental system,the position of the waveguide supplying the microwave to the flash evaporation cavity was simulated and optimized by using the multi-physics coupling simulation software.According to the optimization results,the structure design of the flash chamber was carried out,and the microwave intensification flash evaporation chamber was developed combined with the devices and components such as feed-liquid preheating,microwave supply,vacuum supply,feed-liquid recovery,steam condensation,electrical automation and connecting pipelines and other devices,a microwave intensification flash evaporation experiment system with a flash evaporation chamber volume of 26.55 L,a microwave supply power range of 0-1.5 k W,and the chamber back pressure range of 5 k Pa–1 atm was developed.（2）The experimental study of microwave-intensification flash evaporation for fluid evaporation was carried out.the effects of technological conditions and experimental parameters on the flash evaporation rate and evaporation amount under microwave conditions were studied in detail,the flash evaporation ratio and evaporation quantity under microwave and conventional conditions were compared.The results show that the evaporation amount increases with the increase of initial liquid temperature,liquid flow rate,microwave power,and the number of porous disk layers in microwave intensification flash evaporation.The flash evaporation ratio increases with the increase of the microwave power,initial liquid temperature and the number of porous disk layers,but decreases with the increase of the liquid flow rate.In contrast,the flash evaporaton ratio of conventional flash evaporation remains within a stable range,and do not change drastically with the change of liquid flow rate and the number of porous disk layers.When other process parameters were the same,in the range of liquid flow rate of 10 L/h-45 L/h,the flash evaporation ratio of microwave intensification flash evaporation increased by 20.00%-161.33%compared with conventional flash evaporation.（3）The heat and mass transfer characteristics of microwave intensification flash evaporation process was discussed based on the measured cavity temperature and cavity pressure and the derived calculated flash evaporation process characteristic numbers of NEF（Non Equilibrium Fraction）and Ja（Jakob number）.The performance of microwave intensification flash evaporation experimental system for fluid evaporation was analyzed and evaluated by GOR（Gained Output Ratio）and specific thermal energy consumption per unit volume.The results show that in the developed experimental system,microwave-intensification flash evaporation has better flash completion degree and higher heat transfer intensity between vapor and liquid phase than conventional flash evaporation.Moreover,the GOR and volumetric heat transfer coefficients of microwave intensification flash evaporation are higher than those of conventional flash evaporation,while the specific heat consumption per unit volume is lower than that of conventional flash evaporation.Which demonstrates that the microwave is directly fed into the flash evaporation chamber to heat the feed liquid,the thermal supplemented by microwave heating Liquid strengthens the flash evaporation of the feed liquid in the chamber or promotes the evaporation of the feed liquid in the chamber,and promotes the improvement of the flash evaporation ratio and evaporation quantity.（4）The experimental study of microwave intensification flash evaporation on the treatment of metal ion containing wastewater was carried out,the influence of microwave power,ion concentration,vacuum degree and other process parameters on evaporation quantity and flash evaporation ratio was investigated,and the heat and mass transfer characteristics of microwave intensification flash evaporation under static conditions were analyzed,illustrating that microwave heating improves the flash evaporation ratio,deepens the completion of flash evaporation and intensifies the heat transfer intensity of flash evaporation under static condition.When the microwave power is 1.4 k W,the microwave intensification flash evaporation increases the flash evaporation ratio by 2.51 times compared with the conventional flash evaporation,the final temperature of the feed liquid is relatively increased by 16.4°C,and the cavity pressure is increased by 6.7 k Pa.（5）By performing depressurization operations with different depressurization values,a process of exploration experiment of microwave intensification flash evaporation and concentration of wet process phosphoric acid to prepare polyphosphoric acid was carried out under different experimental process parameters.The results illustrate that when the initial temperature of the feed liquid is 150°C,the volume of initial liquid is 500 m L,the depressurization frequency is 4-7,the final temperature is 280°C,the cavity pressure drops below 10 k Pa,and the microwave power is 1.2 k W,the wet phosphoric acid with P₂O₅content of 60.44%can be condensed into polyphosphoric acid with a P₂O₅ content of more than 80.00%.this process may provide a reference scheme for solving the problems of corrosion,time-consuming and high energy consumption in the traditional process concentrated wet phosphoric acid.