Experimental design and optimization of asymmetric flat-sheet membranes prepared for direct contact membrane distillation

Abstract

Flat-sheet poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, membranes have been prepared using the phase inversion technique based on fractional factorial design. Combined effects of polymer and additive (polyethylene glycol, PEG) concentrations in the casting solutions, solvent evaporation time and coagulation bath temperature on the structural characteristics of the prepared membranes as well as on their direct contact membrane distillation (DCMD) performance have been investigated using statistical approach. The morphological properties of the membranes have been studied in terms of scanning electron microscopy, atomic force microscopy and void volume fraction. The factorial linear models have been developed to describe the main effects of factors on the DCMD responses namely, pure water permeation flux, permeate flux when using salt solution and salt rejection coefficient of the prepared membranes. Analysis of variance showed that all factors have significant effects on the responses. However, the coagulation bath temperature is the least influential factor, while the PVDF-HFP concentration has the greatest effects on both the permeate flux and the salt rejection coefficient. Optimization of membrane preparation conditions has been carried out using a minimum number of experiments and applying Lagrange multipliers optimization method. Under the obtained optimum conditions, 19.1 wt.% PVDF-HFP concentration, 4.99 wt.% PEG concentration, 35 degrees C coagulation bath temperature and 102 s solvent evaporation time, the prepared membrane exhibits the highest salt rejection coefficient, 99.95%, with a permeate flux of 4.41 L/h m(2).