Heat Transfer In Membrane Distillation Research Articles

Analyzing scaling behavior of calcium sulfate in membrane distillation via optical coherence tomography

Deepening the understanding of scaling processes would facilitate the improvement of membrane distillation (MD) as a promising technique for sustainable development. This study investigated the scaling of calcium sulfate in MD via an approach based on optical coherence tomography (OCT). The OCT-based characterization enabled an analysis that correlated the flux decline with the morphological evolution of the scaling layer. It was revealed by this analysis that the reduction in the evaporation rate could be dominated by different mechanisms as the crystalline particles grew and deposited on the membrane surface; the striping phenomenon visualized by mapping the local growth rates provided evidence for the hydrodynamic instability induced by the coupled mass and heat transfer in MD. Moreover, the OCT-based characterization was exploited to unravel the interplay between the crystallization and the porous structure by quantifying the membrane deformation as a function of time; the varied precipitation kinetics in the boundary layer was confirmed by comparing the temporal variations in the OCT signals at different depths. All these results shed light on mechanisms underlying complex scaling processes, which are the basis for optimizing the design of MD.

Impact of module design on heat transfer in membrane distillation

Nusselt correlations originally developed for estimating heat transfer rates in heat exchangers poorly describe heat transfer in membrane distillation (MD) processes. In this work, we assess the impact of module design in bench-scale experiments, simplified treatment of heat transfer rates in MD models, and the effect of permeate flux on temperature polarization as sources of error in Nusselt correlation estimates of heat transfer rates. To test these effects, we systematically vary membrane structure, module sizes, temperatures, and Reynolds numbers to generate a large dataset (n = 240) of MD experiments. We apply this dataset to estimate the heat transfer rate for each unique membrane/module combination and compare our predictions to the classical Sieder-Tate Nusselt correlation (Nus-t). Our results show that heat transfer rates in small modules can be up to five times higher than predicted by Nus-t. The heat transfer rate decreases with increasing module size, with heat transfer in large modules adequately described by the Sieder-Tate correlation. We demonstrate that this high heat transfer rate in small modules is a result of an entrance effect, which increases fluid mixing over the membrane area. These results validate the use of Nu correlation in large membrane modules while highlighting issues with their application in small scale systems. This work also emphasizes the importance of bench-scale module design in materials evaluation and process characterization. Finally, it highlights the need for direct measurement techniques that better characterize interfacial processes in membrane systems with modeled driving forces.

Influence of the boundary conditions on heat and mass transfer in spacer-filled channels

The purpose of this study is to discuss some problems which arise in heat or mass transfer in complex channels, with special reference to the spacer-filled channels adopted in membrane processes. Among the issues addressed are the consistent definition of local and mean heat or mass transfer coefficients; the influence of the wall boundary conditions; the influence of one-side versus two-side heat/mass transfer. Most of the results discussed were obtained by finite volume CFD simulations concerning heat transfer in Membrane Distillation or mass transfer in Electrodialysis and Reverse Electrodialysis, but many of the conclusions apply also to different processes involving geometrically complex channels

Characterizing convective heat transfer coefficients in membrane distillation cassettes

Accurate characterization of conductive 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.