Enhanced Analysis Of Heat And Mass Transfer And Comparative Study On Characteristics Of Membrane Distillation

In the fields of seawater desalination,industrial wastewater treatment,and concentration and separation of chemical substances,typical membrane distillation technology has been applied.However,low membrane flux and high energy consumption hinder the industrial development of this technology.Membrane distillation is a process that crosses heat and mass.At present,there is no unified research on the internal mechanism of this process.Therefore,this dissertation analyzes the heat and mass transfer process by applying the divergence effect of field synergy,the entropy dissipation theory,and the thermodynamic coupling mechanism,revealing the influencing factors of the enhanced membrane flux from a new perspective,and takes these factors as the main variables.Calculated and compared and analyzed the influence of these factors on the characteristics of typical membrane distillation.Firstly,applying the field synergy dispersion theory and the entropy dissipation theory,the field synergy effect in the boundary layer of the membrane distillation phase change system and the entropy dissipation process of the transmembrane heat transfer process are respectively analyzed.The results show that changing the surface structure of the membrane,reducing the acute angle between the flow rate of the hot liquid and the temperature gradient,and strengthening the convective heat source term on the hot liquid side can weaken the polarization effect;while ensuring the strength and hydrophobicity of the membrane,reduces the thickness of the hydrophobicity membrane and the increase of the number of micropores,and the increase of the thermal conductivity of the membrane skeleton,which can reduce the entropy dissipation resistance of heat transfer across the membrane and strengthen the membrane flux.Secondly,for the phase change process and countercurrent heat transfer process of the membrane distillation system and the diffusion process of steam molecules,the thermodynamic coupling mechanism and the theory of thermal and mass diffusion of steam molecules in the pores are applied to qualitatively analyze the process.The results show that the evaporation process is a process in which the liquid molecules contained in the high chemical potential liquid phase flow to the low chemical potential vapor phase,and the spontaneous mass transfer process drives the non-spontaneous process in which the liquid phase absorbs the latent heat of phase change.When the temperature gradient of the evaporation heat transfer process of the hot material liquid side of the membrane distillation system increases,the absolute value of the negative entropy production rate in the heat transfer process increases,and the entropy production rate of the isolated system in the phase change region is getting closer and closer to zero,and the evaporation process The irreversibility is reduced,the mass transfer process on the membrane surface is strengthened,and the membrane flux of the system is increased.Then,with the feed liquid temperature,permeate side temperature,membrane thickness,membrane pore diameter,and bending factor as independent variables,the characteristics of typical membrane distillation systems were calculated and compared through Matlab programming.The calculation error of the existing empirical formula is compared and verified.The results show that under the same porosity conditions,the properties of the membrane modules prepared by the stretching method are better than those of the phase-inversion method,and this advantage is better for direct contact membrane distillation,vacuum membrane distillation,and air gap membrane distillation.Membrane distillation has a more significant impact on membrane flux,and has the greatest impact on the thermal efficiency of a direct contact membrane distillation system;as the temperature of the feed liquid increases or the temperature of the permeate side decreases,the chemical potential gradient of the vapor on both sides of the membrane increases,the membrane flux is strengthened;the thickness of the membrane module increases,the membrane flux decreases,and the thermal efficiency of vacuum membrane distillation,air gap membrane distillation and air sweep membrane distillation decreases,but the thermal efficiency of direct contact membrane distillation does not change;Taking the temperature of the feed liquid,the temperature of the permeate and the thickness of the hydrophobic porous membrane as the independent variables,based on the relationship of membrane flux of the heat resistance of the entropy dissipation of transmembrane heat transfer,the relative error is small compared with the empirical formula,which verifies the reliability of the relationship.Finally,in view of the maximum transmembrane heat dissipation of the direct contact membrane distillation system,the system is coupled with the vapor compression heat pump system and the characteristics of the coupled system are analyzed.The results show that when the temperature of the permeate is constant,as the temperature of the feed liquid increases,the heating capacity and heating coefficient of the coupled membrane distillation system decrease,while the water production ratio of the coupled membrane distillation system increases;When the temperature of the permeate rises,the characteristics of the coupled membrane distillation system are just the opposite;with the increase of the compressor displacement of the heat pump system,the heating capacity of the heat pump system increases.Under the condition that the temperature on both sides of the membrane does not change,the increase of the power consumption of the heat pump system leads to the decrease of the water production ratio of the coupled membrane distillation system.