Flux decline induced by scaling of calcium sulfate in membrane distillation: Theoretical analysis on the role of different mechanisms

Abstract

Membrane distillation (MD) has been recognized as a valuable technique for desalination, water treatment, and other applications. However, MD processes are plagued by the scaling of sparingly soluble salts. This study proposes to investigate the roles of different mechanisms (e.g., temperature polarization, hydraulic resistance, and interfacial effects) in accounting for the flux decline induced by the scaling of calcium sulfate (CaSO 4 ) in MD. A mathematical model is developed to describe the nonlinearly coupled heat and mass transfer when correlating geometrical characteristics of the scaling layer. The modeling is combined with characterization experiments to assess the dependence of the flux decline on the cake growth. In addition to the characterization of scanning electron microscopy, the morphological evolution of the scaling layer is in-situ analyzed via an approach based on optical coherence tomography (OCT). The cake-dominated regime is identified by comparing the flux decline with the OCT-characterization results, thereby providing more reliable experimental data for the model-based analysis. It is suggested that the continuous growth of the crystals could substantially vary the geometrical characteristics of the scaling layer at the cake-membrane interface and thereby decrease the vapor flux. This study sheds light on the development of MD processes with improved performance. • A model was developed to investigate the scaling of CaSO 4 in MD. • Scaling experiments were performed to measure the flux decline in MD. • Spatiotemporal evolution of the scaling layer was characterized via OCT and SEM. • Dependence of the flux decline on the cake growth was analyzed. • Mechanisms were assessed by comparing the modeling and experimental results.