| There are about 5 billion people in the world where freshwater security may be threatened,and climate change,population growth and human activities will further deepen this situation.Therefore,one of the major environmental problems in the 21st century may be the sustainable management of global water resources.Membrane distillation(MD)has the advantages of low energy cost,low driving pressure and temperature,which can be effectively used in seawater desalination,brackish water treatment and other aspects,and has become one of the technical methods to solve the global water resources security problems.However,at present,membrane distillation technology still has some problems,such as high cost,small membrane flux,unstable operation and so on.Therefore,systematic research on membrane distillation technology plays an important role in promoting the application of membrane distillation technology in practical engineering.Taking the hollow fiber liquid gap membrane module as an example,the mathematical calculation model is established and the accuracy of the model is verified.The effects of different parameters on the performance of liquid gap membrane distillation(LGMD)were simulated and analyzed.The results show that within the scope of this study,the feed liquid inlet temperature feed liquid inlet flow qfi,cold side inlet temperature Tli,thermal conductivity of coolant λg,the average porosity of the membrane ε has obvious influence on the characteristic parameters of LGMD,and the inlet temperature of feed liquid Tfi is the most important factor.When the inlet temperature of feed liquid Tfi increases from 40℃to 80℃,the membrane flux increases from 1.32 kg/(m2·h)to 11.38 kg/(m2·h),increases by 761%,the Gained Output Ration(GOR)increases from 0.025 to 0.079,increases by 212%,the thermal efficiency(η)increases from 0.34 to 0.68,increases by 97%,and the Specific Thermal Energy Consumption(STEC)decreases from 27.3 kwh/kg to 9.1 kwh/kg,decreases by 67%.However the coolant inlet flow qli,penetration gap width dg and thickness of condensing tube dp has little effect on the characteristic parameters of LGMD.In addition,because the membrane area of the membrane module in this study is only 0.0273 m2,the GOR of LGMD is low,and the maximum is only 0.21.Taking advantage of the characteristics that absorption heat pump can produce cold and hot ends at the same time and can be driven by waste heat,the absorption heat pump LGMD system was built by coupling LGMD with absorption heat pump.The effects of inlet temperature of cold and hot side of membrane distillation and feed liquid flow rate on COP,Specific Thermal Energy Consumption of system(STECs)and thermal efficiency of the system(ηs)were studied by using control variates method.It turned out that with the increase of the hot side inlet temperature of the membrane module t13,the COP and STECs decrease,while the system ηs increases.With the increase of the inlet temperature of the cold side of the membrane module t18,the COP and ηs increase,but the STECs decreases.With the increase of feed flow rate qfi,the COP and STECs increase,the system ηs decreases.If the operation parameters of the system are properly selected,the comprehensive performance of the system will reach the best state.In this paper,the optimum operation parameters of the absorption heat pump LGMD seawater system are obtained by optimizing the parameters of several groups of data obtained by the discrete method.The results show that when the inlet temperature of hot side of membrane distillation t13 is 80℃,cold side inlet temperature t18 is 45℃ and feed liquid flow rate of single membrane module qfi is 150L/h,the membrane flux is 5.90kg/(m2·h),the COP is 1.68,the STECs is only 0.85 kwh/kg,and the systemηs can reach 50.74%.The research results of this thesis provides a reference for the characteristic analysis of LGMD and the design and operation optimization of absorption heat pump desalination system. |
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