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The hydrophobic/hydrophilic dual-layer membrane can be considered as a membrane with two different surface properties. It is also named the Janus membrane by researchers for the mythical god with two faces. Janus membranes could possibly offer the potential for membrane distillation applications with better performance. In this study, a set of Janus nanofibrous membranes were fabricated by electrospinning and initiated chemical vapor deposition. The supporting layer is the nanofibrous membrane with the intrinsically hydrophilic property. The active layer is the iCVD-treated superhydrophobic layer with the same structure and morphology. The Janus nanofibrous membranes were composed with various thickness configurations and tested in an air gap membrane distillation system. The MD performance and the heat/mass transfer through the Janus nanofibrous membranes were investigated.

It has been proposed that the air-cooled configuration for air gap membrane distillation is an effective way to simplify the system design and energy source requirement. This offers potential for the practical applications of membrane distillation on an industrial scale. In this work, membrane distillation tests were performed using a typical water-cooled membrane distillation (WCMD) configuration and an air-cooled membrane distillation (ACMD) configuration with various condensing plates and operating conditions. To increase the permeate flux of an ACMD system, the condensing plate in the permeate side should transfer heat to the atmosphere more effectively, such as using a more thermally conductive plate, adding fins, or introducing forced convection air flow. Importantly, a practical mass transfer model was proposed to describe the ACMD performance in terms of permeate flux. This model can be simplified by introducing specific correction values to the mass transfer coefficient of a WCMD process under the same conditions. The two factors relate to the capacity (B) and the efficiency (σ), which can be considered as the characteristic factors of a membrane distillation (MD) system. The experimental results are consistent with the theoretical estimations based on this model, which can be used to describe the performance of an MD process.