On the basis of non-equilibrium thermodynamic theory,the coupling phenomena of heat and mass transfer during the process of moisture exchange across a membrane were studied and the relevant physical and mathematical models were established.Formulae for calculating the four characteristic parameters included in the non-equilibrium thermodynamic model were derived,and the dependences of these parameters on the temperatures and concentrations on the two sides of the membrane were analyzed,providing a basis for calculating the heat and mass fluxes.The effects of temperature and concentration differences between the two sides of membrane and the membrane average temperature on the transmembrane mass and heat fluxes were investigated.The results show that for a given membrane average temperature,a larger concentration difference or a smaller temperature difference leads to a higher mass flux.For fixed concentration and temperature differences and with the mass flux predominantly caused by the concentration difference,a higher membrane average temperature yields a higher mass flux.The ratio of the heat of sorption induced by mass flow to total heat relates not only to the temperature and concentration differences between the two sides of membrane but also to the membrane average temperature and the ratio increases when the temperature difference is reduced.
A theoretical study was carried out into membrane transport phenomena.Formulae for calculating the membrane transport resistance and transmembrane mass flux were given,variations in membrane resistance and moisture flux with the membrane sorption constant(C)under various humidity conditions were analyzed,and the value of C corresponding to the minimum membrane resistance or the maximum moisture flux was obtained.The results show that the membrane resistance and moisture flux relate not only to C but also to the relative humidities on both sides of the membrane.As C increases,membrane resistance initially decreases but then increases,i.e.,a minimum occurs,while the moisture flux first decreases and then increases,i.e.,a maximum occurs.The membrane resistance and moisture flux reach their extrema at the same value of C,which is determined by the relative humidities on both sides of the membrane.To reduce the membrane resistance,the value of C should be chosen based on the humidity conditions.
By using the non-equilibrium thermodynamic approach,the possibility of the existence of a steady state for non-equilibrium adsorption with a temperature difference between body gas and adsorbed gas was confirmed and the steady state was determined.The chemical potential difference between body gas and adsorbed gas was obtained and equations for evaluating the adsorption entropy and the isosteric heat of adsorption were derived.The changes of the adsorption entropy and the isosteric heat of adsorption at the non-equilibrium steady state relative to those at the equilibrium state were calculated and the results were compared with those obtained using the traditional equilibrium thermodynamic method.The comparison suggests that the changes of the adsorption entropy and the isosteric heat of adsorption obtained using the non-equilibrium thermodynamic approach are related with not only temperature but also adsorptive state,while those obtained using the equilibrium thermodynamic method are only a function of temperature.The main reason is that the present study treats the adsorption and gas temperature change as an integrated process and considers their interaction,whereas the equilibrium thermodynamic approach separates the adsorption and gas temperature change as two independent processes and neglects their interaction.
