Deep insights on gypsum scaling in DCMD using computational fluid dynamics: Role of spacer design

The efficiency and durability of Membrane Distillation (MD) often face challenges from gypsum (CaSO₄·2H₂O) crystallization. This study uses Computational Fluid Dynamics (CFD) to investigate gypsum scaling in closed-loop Direct Contact Membrane Distillation (DCMD). A novel transient, three-dimensional CFD model integrates nucleation theory and population balance equations to analyze scaling dynamics from initial supersaturation ratio (S) to scalant growth on membrane and spacer surfaces. The efficacy of commercial and Triply Periodic Minimal Surface (TPMS)-based Gyroid spacers in mitigating gypsum scaling is evaluated. Base cases used a feed solution starting at 1900 mg. L⁻¹ of CaSO₄·2H₂O, with feed and permeate entering at 65 °C and 35 °C, respectively. The Gyroid spacer outperformed the commercial spacer with an initial flux 63 % higher, delayed scaling onset (800 min vs. 240 min), and lower scalant-mass density (0.4 mg. cm⁻² vs. 15 mg. cm⁻²). The superior performance of the Gyroid-based TPMS spacer is attributed to its intricate flow paths, which enhance micromixing and mitigate polarization effects. This study showcases CFD's pivotal role in developing novel, efficient, scale-resistant spacer designs enabled by 3D printing and in predicting optimal operating conditions, ultimately facilitating wider adoption of MD technology in water treatment applications.