MODELLING CRYSTALLIZATION FOULING IN LIQUID-TO-AIR MEMBRANE ENERGY EXCHANGERS

Abstract

Micro-porous membranes are used in membrane-based separation processes to separate water from an aqueous solution. Liquid-to-air membrane energy exchangers (LAMEEs) use membranes to control humidity and temperature in heating, ventilation and air-conditioning (HVAC) systems. A semi-permeable hydrophobic membrane is used in LAMEEs to separate an air stream from an aqueous solution stream while allowing simultaneous heat and moisture transfer between the streams. However, the membrane of LAMEEs may be subjected to crystallization fouling under some operating conditions which impacts performance. The primary goal of this thesis is to establish a model for LAMEEs that can predict the crystallization fouling rate, decline in moisture transfer flux due to fouling, and the operating/design conditions where fouling is likely to occur. In this thesis, a semi-empirical model is developed to predict crystallization fouling of the membrane in LAMEEs. The solution concentration at solution-membrane interface is determined analytically using mass and energy balance equations, while the fouling (i.e., blockage of the membrane pores) is predicted using an empirical equation from the literature. As the fouling progresses and more of the membrane pores are blocked, moisture transfer flux through the membrane reduces. The model is validated with experimental data available in the literature and is used to determine the effect of design and operating parameters on the fouling rate. The model is also used to estimate the time when the moisture transfer flux declines to a specific value (e.g. 50% reduction) which is helpful in determining the membrane cleaning frequency. Then, the model is used to predict the conditions where crystallization fouling is likely to occur under different operating conditions for three different desiccant solutions, namely magnesium chloride (MgCl2), calcium chloride (CaCl2) and lithium chloride (LiCl). The model is expanded to determine the fouling limits (which is attainment of saturation condition at the solution-membrane interface at the LAMEE outlet) in a counter-flow LAMEE. In addition, the moisture transfer flux decline due to crystallization fouling is analyzed for MgCl2 at various locations along the LAMEE.