| At present,desalination of seawater is an important measure to solve the problem of water shortage.However,at present,desalination technologies generally have the problem of high energy consumption and difficulty in handling high-concentration brine.Membrane distillation technology is a new type of membrane separation technology with an operating characteristics of low temperature and normal pressure.It has great advantages in the treatment of high concentration of salt water.The external supply of heat and cold energy is needed in the process of membrane distillation.In such a case,a seawater desalination system combined with liquid gap membrane distillation and heat pump is proposed in this paper,and the improved design and thermodynamic analysis of the system are also carried out.First of all,based on the conventional design idea of heat pump membrane distillation,a conventional liquid gap heat pump membrane distillation system is designed.According to the characteristics of liquid gap membrane distillation,a new single-effect regenerative heat pump membrane distillation system and a double-effect regenerative heat pump membrane distillation system were proposed.Physical and mathematical models were constructed respectively,and programming was done by using Matlab and Refprop software.Secondly,the variation of heat pump membrane distillation system heat thermal efficiency,COP,GOR and tenergy consumption per ton of water with the hot side feed temperature,feed flow rate,membrane encapsulation rate and thermal conductivity of condenser tube were studied,the results show that the two kinds of system changes with the conditions are basically the same.In the regenerative heat pump membrane distillation system,when the hot side feed liquid inlet temperature increased,the membrane flux and the thermal efficiency increased,GOR and COP decreased,energy consumption per ton of water increased;when the liquid flow rate increased,COP and membrane flux increased and GOR decreased,energy consumption per ton of water increased;when the membrane encapsulation rate is increased,the membrane flux is reduced,the change of COP is very small,the thermal efficiency and GOR increased,energy consumption per ton of water decreased;within a certain range,the membrane flux,thermal efficiency and GOR increase with the increase of the thermal conductivity of the condensing tube,COP and tons of water consumption decreased,while the thermal conductivity is greater than a certain value,the change rate of each performance index is close to 0.It is also found that besides the inlet temperature of hot side material,the influence of the heat pump coefficient on the heat pump coefficient of the single effect system is relatively large.Other parameters have little effect on the COP,but the effect is very significant in the double-effect system.Then,The performance of the conventional single-effect heat pump membrane distillation system and the single-effect regenerative heat pump membrane distillation system are compared..The results show that under the same operating conditions,The GOR and thermal efficiency of the single-effect regenerative system is 60.5% and 52.8% higher than that of the conventional system,and energy consumption per ton of water of the system is reduced by 35.4%,but the COP is relatively low.The performance of the single-effect regenerative heat pump membrane distillation system and the doubleeffect regenerative heat pump membrane distillation system are also compared.The results show that the thermal efficiency of double-effect system is 57.80% higher than that of single effect system,and GOR is increased by 41.62%,while the energy consumption per ton of water is reduced by 28.55%,but the heat pump coefficient of heat pump is lower.Finally,in order to improve the temperature matching between material and heat pump working fluid in heat transfer process,using R22/R141 b mixed refrigerant as heat pump working fluid,the results show that mixing compressor can reduce compressor pressure ratio,improve GOR and COP. |
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