Abstract
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.
Highlights
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
Wang et al [1] claimed that the heat and mass transfer in membrane distillation is a complicated, interactive process; Phattaranawik and Jiraratananon [2,3] studied the heat and mass transfer in a direct contact membrane distillation; Alves and Coelhoso [4] analyzed the heat and mass transfer in a pervaporation process; Soowhan and Mench [5] experimentally investigated the thermo-osmosis phenomena using various membranes; and Niu and Zhang [6] discussed the impacts of various factors on the heat and mass transfer in a trans
The phenomenological coefficients or characteristic parameters involved in the non-equilibrium thermodynamic model are generally assigned empirical values obtained through experiments under certain conditions
Summary
Consider the process of water vapor transfer through a nonporous membrane, as shown in Figure 1, where the water vapor concentrations on the two sides of membrane are C1 and C2, and the temperatures are T1 and T2, respectively, with C1>C2 and T1>T2. Consider the process of water vapor transfer through a nonporous membrane, as shown, where the water vapor concentrations on the two sides of membrane are C1 and C2, and the temperatures are T1 and T2, respectively, with C1>C2 and T1>T2. According to the solution-diffusion theory, water vapor will transfer from the side with higher humidity to that with lower humidity due to the concentration and temperature gradients. To highlight the process of moisture transfer across membrane itself, the convective heat and mass transfer resistances on the two sides of membrane are neglected, that is, the concentration and temperature in the space on each side of membrane are considered to be constant (i.e., there are neither concentration nor temperature boundary layers)
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