Characterizing convective heat transfer coefficients in membrane distillation cassettes

Accurate characterization of conducive heat transfer in membrane distillation (MD) is essential for determining membrane permeability, calculating thermal efficiency, and describing the effects of process parameters on permeate flux. This work demonstrates the effect of convective heat transfer on common MD performance metrics, reviews existing methods to model and measure convective heat transfer in MD cassettes, and proposes a novel experimental method to validate and or correct estimated convective heat transfer coefficients. This simple method compares trends in experimental permeability to those of theoretical permeability over a series of driving forces and/or flow conditions. We first validate this method using experimental data collected from two cassettes using a single membrane. After correcting the heat transfer coefficients for each cassette, the permeability of a single membrane is consistent across two different cassettes. We then validate this method for a single cassette using two different membranes, finding that the experimentally determined cassette correction factor is consistent within the cassette. Consistent and accurate reporting of cassette heat transfer coefficients will promote direct comparison of membrane and module performance across different MD systems.