A model for predicting the effect of crystallization fouling on moisture transfer in membrane energy exchangers

Abstract

Crystallization fouling is an important phenomenon that impacts the design and operation of membrane-based separation processes. Recently, membranes have been applied to separate aqueous salt solutions and air streams for moisture control in buildings using liquid-to-air membrane energy exchangers (LAMEEs). LAMEEs are efficient energy exchangers that use a semi-permeable hydrophobic membrane to separate air and aqueous salt solutions, while allowing simultaneous heat and moisture exchange between the fluids. However, in some design and operating conditions, crystallization fouling occurs which hinders the performance of LAMEEs. The main objective of this paper is to develop a model which predicts the fouling rate for membranes in LAMEEs. The semi-empirical model estimates the decline in moisture transfer rate by taking into account the surface blockage of the membrane due to crystallization fouling. The model is validated with experimental data available in the literature. The model can then be used to predict the theoretical crystallization fouling limits and the effect of design and operational parameters on fouling rates. For example, the initial fouling rate is four times higher for a supersaturated solution (3% above saturation) than for a saturated solution. The main contribution of this study is that the developed semi-empirical model can help the designers predict the fouling rate for long-term periods and determine the cleaning intervals of the LAMEE.