Search

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 (RH70%) 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.

We report on an experimental study of the effects of relative humidity and temperature on the transport of water vapor and CO² through a series of standard polymeric materials to determine their potential for use as membrane media in energy recovery ventilators (ERVs). Results are reported for four polymers of two major types (glassy and rubbery). The selectivity of water vapor over CO² is also evaluated from permeation experiments. Permeability results show that rubbery membrane samples, with glass transition temperatures well below the temperature range of experiments (30°C to 50°C), have a higher water vapor permeability and a much higher CO² permeability compared to glassy membrane samples. This is hypothesized to be mainly due to the higher diffusivity of water vapor and CO² in rubbery polymers with higher chain flexibility leading to a much lower selectivity for rubbery membrane samples. In all polymer samples, water vapor permeability increases with relative humidity (up to an order of magnitude) and decreases with temperature. This is attributed to the negative enthalpy of water vapor sorption dominating its activation energy of permeation, which is associated with a lower solubility at higher temperatures. In contrast, CO² permeability increases with temperature because of the positive activation energy of CO² permeation. CO² permeability decreases slightly with increasing relative humidity (up to 30%), which is hypothesized to be due to the competitive sorption between water vapor and CO² at higher humidity levels. Therefore, the selectivity of membrane samples for water vapor over CO² decreases with temperature and increases with relative humidity, and these trends are dominated by water vapor permeability variations. In general, the permeability results reported here suggest that ERV exchangers using polymer membranes can achieve high latent effectiveness (i.e. very high water vapor permeability) over a wide range of operating temperature and relative humidity while maintaining very low CO² permeability and crossover rates (