Quantitative Structure-Flux Relationships of Membrane Distillation Materials for Water Desalination
Membrane distillation (MD) is an emergent water desalination technology with potential for scalable, sustainable production of fresh water from highly concentrated brines. Wider adoption of MD technology depends upon improvements to process efficiency. In recent years, researchers have published a number of experimental papers seeking to improve mass and heat transport properties of MD membranes. However, an imperfect understanding of how intrinsic membrane geometry affects MD performance limits efforts to optimize membrane structure. The objective of this dissertation is to help elucidate effects of membrane structure on MD flux, permeability, and thermal performance, with a focus on novel fibrous membranes. Mechanistic and empirical modeling methods were employed to relate the structural characteristics of bacterial nanocellulose and electrospun polymeric membranes to experimentally-measured MD performance. Through these experimental and modeling studies, three conclusions are reached. First, the MD community can hasten the search for optimal membrane structures by improving the quality and reproducibility of reported experimental data. Review of published and newly-collected MD data shows that feed and permeate stream channel geometry and flow non-idealities can substantially affect measured performance metrics for MD membranes. If these factors are accounted for by careful characterization of convective heat transfer coefficients, membrane permeability and thermal efficiency can be definitively deduced. A new methodology is presented for determining convective heat transfer coefficient using experimentally-validated Nusselt correlations. Accurate reporting of cassette heat transfer metrics will facilitate inter-study experimental reproducibility and comparison. Second, use of dimensional analysis to empirically model MD transport is effective for predicting vapor flux in fibrous membranes. Advantages of the model include its use of easily-measurable structural parameters tailored specifically for fibrous membranes and the incorporation of all relevant vapor, membrane, and system characteristics into a mathematically simple, yet theoretically sound, regression model. The new model predicts MD flux more accurately than the mechanistic Dusty Gas Model or previously published empirical MD models. Dimensional-analysis-based transport models may be generalizable for a variety of novel membrane types, lead to a more rigorous understanding of structural influences on vapor transport processes, and guide the development of high-performance membrane structures. Finally, MD process efficiency can benefit by development of highly porous, scalable membrane materials. Bacterial nanocellulose aerogel membranes exhibit substantial improvements in intrinsic permeability and thermal efficiency as compared to traditional phase-inversion membranes, suggesting that there is an opportunity to advance MD process viability through improved membrane design. By mimicking the porosity and pore-interconnectivity of nanocellulose aerogels, novel membrane materials can achieve high thermal efficiency and low mass transport resistance. This dissertation contributes experimental data and modeling techniques to improve knowledge of membrane structural effects on MD performance. These contributions have implications for the wider adoption of MD technology through better reproducibility of published experimental results, enhanced transport modeling to optimize membrane structure, and demonstrated thermal efficiency of a highly porous materials.