Characterization of Fouling with Hygroscopic and Non-hygroscopic Aerosols in Composite Polymer Membranes for Water Vapor Transport Applications Engarnevis, Amin; Huizing, Ryan; Green, Sheldon; Rogak, Steven
Composite membranes using a thin vapor-permeable polymer layer over a structural substrate are used in gas dehydration, food-packaging, and humidity control of indoor spaces. The impact of exposure to air pollution on the water vapor permeability and selectivity is investigated to develop an understating of potential air-side particulate fouling mechanisms and resulted performance degradation during membrane lifetime in the field. Samples of commercial membrane media were loaded with hygroscopic NaCl, and non-hygroscopic spark-generated graphite (SGG) aerosol particles. The effects of particle charge distribution and number concentration, air flowrate, temperature-gradient across membrane (Thermophoresis), and membrane surface on the rate of particle deposition were investigated using a Scanning Mobility Particle Sizer (SMPS). It was found that particle charge distribution and air flowrate had the largest impact on the rate of particle deposition. The results of permeability measurements showed that deposition of SGG and NaCl particles under a dry loading condition (RH<20%) had minimal influence on the membrane. However, when membranes loaded with hygroscopic particles in dry condition were exposed to an elevated humidity (RH>70%) leading to surface condensation, the membrane permeability reduced by up to 30%. This is hypothesized to be caused by increased resistance of microporous membrane substrate due to a pore-narrowing process. Scanning electron microscopy (SEM) combined with EDX analysis was used to examine the morphology and chemical composition of the fouled membrane surface. Analysis of SEM images showed a significant reduction in the average pore diameter of degraded samples, proportional to the fouling degree. It was also found that cleaning of fouled samples can reverse their permeability back to nearly the initial value. The reversibility of the loaded membrane permeability along with the EDX analyses imply that re-crystallization of salt ions, entrained into the pores of membrane substrate in aqueous form, is a potential explanation for the changes observed.
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