Comparison of scale formation between inside-out and outside-in hollow fiber membrane distillation (MD) modules when concentrating hyper saline feed solutions

Abstract

This study focused on the comparison of inside-out or outside-in membrane distillation (MD) modules under the conditions where fouling due to scale formation occurs. Experiments were car­ried out using a laboratory-scale direct contact MD setup. Synthetic feed water containing NaCl of 200,000 mg/L and CaSO4 of 2,000 mg/L was used for accelerated fouling tests. In-house MD mini modules were fabricated and operated either in the inside-out or outside-in modes. Results showed that the initial flux of the inside-out MD module was slightly higher than the outside-in MD mod­ule under no fouling conditions. The temperature difference was maintained at 40°C (feed inlet: 60°C; distillate: 20°C). The flow rate of the feed was the same as that of the distillate, which was 0.4 L/min. The average flux values of the inside-out and outside-in modes were, approximately, 1.56 and 1.42 kg/m2h, respectively. However, a more rapid decline of flux due to scale formation occurred in the inside-out MD modules compared with the outside-in MD modules under the same operating condition. The blocking coefficient was initially very low and suddenly increased after the volume of concentration factor (VCF) of 1.35 in the case of the inside-out mode. The blocking coefficient started to increase after the VCF of 1.75 in the case of the outside-in mode. Different mechanisms for scale formation in the inside-out and outside-in modes were suggested to elucidate the results of fouling tests, which were also supported by the scanning electron microscope analysis.