Modelling mass transport through a porous partition: Effect of pore size distribution

Abstract

Direct contact membrane distillation process has been studied using microporous polytetrafluoroethylene and polyvinylidene fluoride membranes. The membranes were characterized in terms of their non-wettability, pore size distribution and porosity. The mean pore sizes and pore size distributions were obtained by means of wet/ dry flow method. The mean pore size and the e¤ective porosity of the membranes were also determined from the gas permeation test. A theoretical model that considers the pore size distribution together with the gas transport mechanisms through the membrane pores was developed for this process. The contribution of each mass transport mechanism was analyzed. It was found that both membranes have pore size distributions in the Knudsen region and in the transition between Knudsen and ordinary di¤usion region. The transition region was the major contribution to mass transport. The predicted water vapor permeability of the membranes were compared with the experimental ones. The e¤ect of considering pore size distribution instead of mean pore size to predict the water vapor permeability of the membranes was investigated.

Flux Prediction in Direct Contact Membrane Distillation

“…In other word, when the ratio of pore radius to the mean free path is lower than 0.05 ( < 0.05), then Knudsen kind of flow model is employed. The membrane permeability ( ) in Knudsen region is given as [2], [7], [15], [16];…”

“…In this case, the expression below may be adopted to evaluate the membrane permeability [2], [16]; (10) when the membrane pores is assumed to have a uniform size, then the average pore radius is used and it's given as [2], [16]; (11) where P a is the air pressure in the membrane, P is the total pressure inside the membrane pore and is the diffusion coefficient which can be determined from the expressions below [2], [4], [17], [18] where P is in Pa.m 2 /s Transition region occurs when we have 0.01 < K n < 10 (0.1 < d p < 100 ). In this case, the molecules of liquid water collides with each other and diffuse through the air molecules.…”

“…For transition region, combined Knudsenordinary molecular diffusion type of flow is responsible for the mass transfer. The membrane permeability for this case is determined using the relationship below [2], [16];…”

The effects of iCVD film thickness and conformality on the permeability and wetting of MD membranes

“…This means that the Knudsen numbers, Kn = λ/2r, is within the transition regime (0.2 < Kn < 1.0) in which both continuum and discrete effects are important. Although it has been met with some criticism [18], the permeability (often referred to as the membrane coefficient) for MD membranes is often described using the dusty gas model [19]. In the dusty gas model, the total resistance to diffusion is the sum of the resistances to molecular and Knudsen diffusion.…”

“…where P is the total pressure inside the pores (Pa), D is the water diffusion coefficient with air (m 2 /s) and pa is the air pressure within the pores (Pa) [19]. According to Eq.…”