A two hollow-fiber-set membrane module for air gap membrane distillation with high thermal efficiency

An air gap membrane distillation (AGMD) module was developed by incorporating a poly(etheretherketone) (PEEK) hollow fiber membrane (HFM) having a nonporous wall. This PEEK HFM was placed inside a polyvinylidene fluoride (PVDF) hydrophobic porous wall HFM with a larger bore diameter. The outside diameter (OD) of PVDF HFM is 925 μm, small enough to be capable of achieving a high surface area packing density of 1297 m²/m³. The air gap thickness was very small, 121 μm. Hot brine flowed on the outside of the PVDF HFM; the colder liquid was passed through the lumen of the PEEK-based condenser hollow fibers. Water vapor condensed in the air gap formed between the inner surface of the porous PVDF HFM and the outer surface of the nonporous condenser PEEK fiber. With 85^o C hot brine flowing at 40 mL∙min⁻¹ and 5^o C coolant flowing at 8 mL∙min⁻¹, the water vapor flux was 9.05 kg/m²∙h with a salt rejection of 98.7 %. Simulation by COMSOL Multiphysics predicted water flux and interfacial temperature of HFM, which supported the experimental observations. Moreover, the influence of module geometry, membrane characteristics and internal flow configuration on permeate flux, thermal efficiency, gained output ratio (GOR), and temperature and concentration polarization were evaluated. Principal component analysis (PCA) was used to illustrate the interconnections among various parameters and their respective contributions to water flux and other performance indicators. Air gap thickness had the strongest influence on temperature polarization.