Study On Thermodynamic Performance Of A Single-effect Evaporation Mechanical Vapor Compression Seawater Desalination System

Mechanical vapor compression (MVC) distillation is the only thermal desalination processrun by mechanical energy. After giving a critical review of MVC, including the current status ofresearch and development of MVC abroad and in China, the characteristics and the trends andfuture prospects of MVC, this study focuses on the thermodynamic performance analyses of asingle-effect evaporation mechanical vapor compression(SEE-MVC)seawater desalinationsystem, with the main content and conclusions shown below.(1) Based on the first law of thermodynamics, the mathematical model of SEE-MVC is builtand a computer code is made, to analyze the energy utilization of SEE-MVC and the influence ofthe main parameters, including compression ratio, isentropic efficiency of compressor andevaporation temperature of seawater, on thermal performance. The results show that, lowercompression ratio, higher isentropic efficiency, and lower evaporation temperature, lead to lowerpower consumption and higher recovery rate of water production. The suitable range ofevaporation temperature is55~70℃, and that of compression ratio is1.2~1.3. Taking theminimum specific work consumption as objective function, the main parameters of several casesare given for reference.(2) Based on the second law of thermodynamics, the mathematical model of SEE-MVC isbuilt, the calculation results from energetic balance and exergetic balance are compared, and theexegetic performance of SEE-MVC, including the influence of compression ratio, isentropicefficiency and evaporation temperature, is analyzed. The results show that, the exergy loss in theevaporator/condenser is the highest (higher than50%of the power input), that of the compressorand the two preheaters follow successively, and although the heat loss of the output fresh waterand rejected brine equals to the power consumed, the exergy loss rate is very low, only about1%.Lower compression ratio is favorable to higher exergy efficiency of SEE-MVC. With theincrease of isentropic efficiency and the decrease of evaporation temperature, the exergyefficiency of SEE-MVC increases.(3) After analyzing the performance characteristics of SEE-MVC and the properties of theworking fluids, a set of estimation formulas on specific power consumption and recovery ratio ofSEE-MVC are built based on the calculation results from the computer code mentioned above. Excluding complex properties of working fluids and details of working processes in eachcomponent and the whole system, the formulas provide a simple and convenient way to estimatethe main performance indexes of SEE-MEE.