Theoretical Studies On Transmembrane Heat And Mass Transfer And Relevant Surface Adsorption Phenomena

Membrane science and technology have been widely applied in many industrialfields, where the membrane separation technology with simultaneous heat and masstransfer is becoming the future development direction and research hotspot of themembrane separation technology. In the transmembrane heat and mass transferprocess, the heat and mass transfer coupling and relevant sorption phenomena areespecially difficult to understand and study. Since non-equilibrium thermodynamicsis an effective tool to study coupling phenomena, in the present researchnon-equilibrium thermodynamics was used to analyze the typical transmembrane heatand mass coupling process, and to study the sorption phenomenon, with a specialemphasis placed on the sorption entropy and the heat of sorption. Further, the effectof the heat of sorption on the transmembrane heat transfer process was alsoinvestigated.On the basis of non-equilibrium thermodynamic theory, the coupling phenomenaof heat and mass transfer during membrane osmotic distillation were studied and therelevant physical and mathematical model was developed. The Onsager reciprocalrelation was confirmed to hold, supporting the reliability of the model. The modelwas used to analyze the relationships of the temperature and concentration differencesbetween the two sides of the membrane and the transmembrane heat and mass fluxesunder different conditions and to calculate the entropy generation rate. The resultsshow that the entropy generation rate can be reduced by lowering the membraneeffective thermal conductivity or increasing the system average temperature.The coupling phenomena of heat and mass transfer during transmembranemoisture exchange were studied and the relevant physical and mathematical modelwas established. The model was used to investigate the effects of the temperature andconcentration differences between the two sides of the membrane as well as thesystem average temperature on the transmembrane mass and heat fluxes, and tocalculate the entropy generation rate. The results show that the ratio of the mass flowinduced heat of sorption to total heat is related to not only the temperature andconcentration differences between the two sides of the membrane but also the system average temperature.Due to the existence of the sorption phenomena in the transmembrane moistureexchange process, the possibility of the existence of a steady state fornon-equilibrium sorption with a temperature difference between body gas andadsorbed gas was confirmed and the steady state was determined. The chemicalpotential difference between body gas and adsorbed gas was obtained and equationsfor evaluating the sorption entropy and the isosteric heat of sorption were derived.The changes of the sorption entropy and the isosteric heat of sorption at thenon-equilibrium steady state relative to those at the equilibrium state were calculatedand the results were compared with those obtained using the traditional equilibriumthermodynamic method. The comparison suggests that the changes of the sorptionentropy and the isosteric heat of sorption obtained using the non-equilibriumthermodynamic approach are related to not only the temperature but also theadsorptive state.The heat of sorption was obtained based on a typical sorption equation. Theinfluences of temperature, sorption capacity and sorption characteristic constant onthe heat of sorption were analyzed. The effect of the variable sorption heat on thetransmembrane heat transfer was discussed and the results were compared with thosewith constant heat of sorption. It is found that the results with variable and constantheats of sorption differ significantly, and their difference increases with increasingtransmembrane mass transfer flux.