Direct contact membrane distillation: An experimental and analytical investigation of the effect of membrane thickness upon transmembrane flux

• MD model to determine optimal thickness for different system conditions.
• Fabrication and testing of DCMD membrane as thin as 27 μm.
• Achieved DCMD flux of 60 L/h m2 with unmodified PVDF electrospun membrane.
• Introduction of structural derivation factor for electrospun membranes.

Polyvinylidene fluoride (PVDF) electrospun nanofibrous membranes (ENMs), consolidated with a heatpress process and of various thicknesses were fabricated and tested in a DCMD cell at five different operating conditions. Membranes as thin as 27 μm were sufficiently robust for evaluation and gave a transmembrane flux as high as 60 L/h m2. An analytical model was created to estimate the optimal membrane thickness for DCMD operations. It is found that the value of optimal thickness increases with reduced heat transfer coefficients; decreased feed inlet temperature; increased membrane permeability; and increased salinity. Even for 10% NaCl the predicted optimum was estimated to be 13 μm which was too thin for experimental confirmation. Based upon this analysis but with due allowance for the variation of heat transfer coefficients with temperature dependent physical properties a single Matlab model was created to fit the five sets of data. With the introduction of a structural derivation factor, which reflected the experimentally determined variation of porosity and pore size with thickness, the model was found to fit the data very well.