Thermal Performance Analysis And Optimization Of Low Temperature Multi-effect Evaporation Desalination System

With the rapid growth of population, the development of industrialization and urbanization and the intensification of environmental pollution, the shortage of fresh water resources has become a significant factor that restricts the economic development. Desalination is an effective method to solve the scarcity of fresh water resources. Many researchers have focused on the low temperature multi-effect evaporation (LT-MEE) desalination, because it has the advantages of low energy consumption, good quality of produced water, less chemical consumption and excellent anti-scale and anti-corrosion performance, et al. Intensive analysis and exact grasp of the thermal process is the base for developing the optimal design of MEE desalination plant. In this thesis, a comprehensive mathematical model of LT-MEE desalination system is developed, and an in-depth study is carried out for the thermal performance analysis and optimization of LT-MEE based on the experimental data, to promote the formation of thermal analysis theory and the development of advanced LT-MEE desalination technology with complete scientific connotation.Based on the first and second laws of thermodynamics, a comprehensive mathematical model is developed to represent the LT-MEE desalination system, which considers the thermodynamic losses caused by boiling point elevation (BPE) of brine and flow resistance caused by the steam flowing across tube bundle, demister, steam channel and the condensation process inside tube. The flow resistance and heat transfer coefficient are obtained from the database established by the targeted experiments. The effectiveness and advancement of mathematical model developed in this thesis is verified by comparing with the actual operating data and the results of other existing models from the references.The distributions of generated steam, salinity and temperature of brine, heat transfer coefficient and thermodynamic losses in each evaporator and their variations with the design variables are obtained by simulation. The thermodynamic process of LT-MEE desalination system is deeply analyzed. The effects of feed arrangements, preheat modes and the combination with thermal vapor compressor (TVC) on the performance parameters of LT-MEE desalination system have been further discussed. In addition, a comparative analysis of thermal performance and exergy destruction is performed between LT-MEE and LT-MEE-TVC. The results indicate that the thermodynamic losses caused by the flow resistances and BPE have important influences on the performance of LT-MEE desalination system. According to the analysis on thermodynamic process, the characteristics of thermal process in LT-MEE desalination plant are summarized as small temperature difference, low flow resistance, saturated state and high sensitivity. In the case of generated steam as the preheating heat source, the location of preheater is shifted backward, and the temperature rise of preheater gets raised, both of which are beneficial to the improvement of thermal performance of LT-MEE desalination system. The MEE-TVC desalination system can obtain higher gained output ratio(GOR), lower specific heat transfer area and specific cooling seawater than the MEE desalination system.The method which combines the accurate model, response surface approximation model and multi-objective genetic algorithm is applied to optimize LT-MEE desalination system. Based on the method of total revenue requirement, the economic model of LT-MEE desalination system has been established as the detailed calculation foundation of cost per unit mass of fresh water Cp,tot. To get the maximization of GOR and the minimizations of specific exergy consumption ed and Cp,tot, the response surface approximate models of performance parameters have been established first, and then an improved multi-objective genetic algorithm (NSGA-?) is adopted to obtain the Pareto optimal solution set. The comparison and analysis between the optimal solutions are fulfilled from these aspects of objective functions, design variables, exergy destruction and composition of cost, and the design features of optimal solutions are obtained based on various preferences. The optimization results show that compared with the initial plan of example for the MEE-TVC desalination system, the optimal solution preferring the thermodynamic performances raises GOR by 44.6% and reduces ed by 26.7% respectively, and the optimal solution preferring the economic performances reduces Cp,tot by 13.7%. It indicates that the optimization method adopted in the thesis is effective for the optimization research of MEE/MEE-TVC desalination system.