Modeling and evaluation of boundary layer resistance at feed in the permeation of VOC/N2 mixtures through PDMS membrane

By using a phenomenological approach, model equations incorporating the resistance-in-series concept were established to evaluate concentration polarization in the boundary layer in feed adjacent to the membrane surface in the vapor permeation and separation of volatile organic compound (VOC)/N2 mixtures through poly(dimethylsiloxane) (PDMS). The vapor permeation experiments were carried out at various VOC concentrations in feed, operating temperatures and feed flow rates. Chlorinated hydrocarbons, such as methylene chloride, chloroform, 1,2-dichloroethane and 1,1,2-trichloroethane were used as organic vapor. With decreasing feed flow rate, the permeability coefficients of VOC component were observed to decrease drastically due to concentration polarization while the nitrogen permeability increased slightly in all of the permeations. From the permeation data of the mixtures, the overall mass transfer coefficient, mass transfer coefficient of VOC component across the boundary layer and concentration polarization modulus as a measure of the extent of concentration polarization were determined quantitatively with help of the model equations. From the analysis on the determined model parameters, the boundary layer resistance due to the concentration polarization of VOCs component was found to be more significant when VOC condensability was greater, VOC content in the feed mixture was higher, or operating temperature was lower. This study seeks to emphasize the importance of the boundary resistance on the vapor permeation of the vapor/gas mixtures with high permeability and high selectivity towards the minor component VOC.