Operating Characteristics And Evaluation Of Nacl Solution Crystallization Driven By High Temperature Heat Pump

Evaporative crystallization is a basic chemical unit,which is very common in chemical production.The conventional evaporative crystallization system consumes a large amount of raw steam and produces secondary steam with low pressure and low grade.Because it can not meet the application conditions,this part of secondary steam is directly discharged into the atmosphere or condensed by the cooling system,resulting in a waste of energy.Heat pump technology can improve low energy to high grade energy by consuming electric energy,and in the field of industrial waste heat recovery it has been widely used.Therefore,this paper proposes a new form of evaporative crystallization system which uses secondary steam as low temperature heat source and generates high grade heat energy through heat pump to supply energy to crystallization equipment.Because the heat source required by the crystallizer is generally 85℃ and above,the heat pump in the system belongs to the category of high temperature heat pump.Therefore,this system form is called High temperature heat pump evaporative crystallization(HPC)system.The HPC system coupled the heat pump subsystem with the Draft Tube Baffle(DTB)crystallization subsystem to form a closed cycle of secondary steam latent heat,which resulted in new operating characteristics of the two subsystems.In conventional DTB crystallization system,the secondary steam parameters are determined by the internal parameters of the crystalzer,while in HPC system,the secondary steam can affect the internal state of the crystalzer through the heat pump system.Similarly,evaporation temperature and condensation temperature in conventional heat pump systems are only correlated through refrigerants,while evaporation temperature and condensation temperature in HPC systems can also interact through secondary steam.Due to the above characteristics,the operation characteristics of the coupled HPC system are more complex,and the conventional evaluation indexes in the field of heat pump and evaporation crystallization are not able to comprehensively evaluate the performance of HPC system.In this paper,sodium chloride solution was used as the raw material for crystallization,and the following four aspects were studied in view of the operation characteristics and evaluation of HPC system:(1)Mathematical model analysis and application of DTB crystallization subsystem.The mathematical model of DTB crystallization subsystem was deeply analyzed and established.The effective volume modeling of the crystallizer was completed by using the particle number balance model.At the same time,the structural model of the crystallizer was improved by combining the literature research results.According to the mathematical description of the DTB crystallization subsystem,the solution program was completed,and the application case of the model of the DTB crystallization subsystem was built according to the known conditions.(2)Mathematical modeling and verification of HPC system.Based on the conservation of mass and energy,the thermal model of heat pump subsystem was built according to the refrigerant pressure enthalpy diagram,and the heat transfer performance was studied.The shell and tube heat exchanger structure is selected and the structure optimization model is established.After the modeling of heat pump subsystem and DTB crystallization subsystem,the coupling model is constructed.The solution method of HPC system was obtained.(3)Research on HPC system construction and operation characteristics.In order to master the influence of different design parameters on design results of HPC system,the influence law was analyzed by using the solver program.Based on analysis results and known conditions,an application case of HPC system with evaporation capacity of 300kg/h was built,and the COP of the case reached5.85.In order to simulate the operating characteristics of HPC system under different working conditions,a simulation program was designed by using iterative method,and the influence rules of four operating parameters were analyzed.(4)Technical and economic evaluation method and application of HPC system.Combined with common indexes in the field of heat pump and crystallization,the evaluation indexes of HPC system in the four aspects were established.In this paper,HPC system is proposed,which enriches the choice of energy-saving technology in the field of evaporative crystallization.At the same time,the influence law of key design parameters on the design results during the construction process of HPC system and the characteristic changes during the operation process are studied.And a comprehensive evaluation method is established.This paper provides theoretical and data reference for the optimization design,engineering scheme comparison and commissioning of HPC system.