Abstract
Electrospun membranes are gaining interest for use in membrane distillation (MD) due to their high porosity and interconnected pore structure; however, they are still susceptible to wetting during MD operation because of their relatively low liquid entry pressure (LEP). In this study, post-treatment had been applied to improve the LEP, as well as its permeation and salt rejection efficiency. The post-treatment included two continuous procedures: heat-pressing and annealing. In this study, annealing was applied on the membranes that had been heat-pressed. It was found that annealing improved the MD performance as the average flux reached 35 L/m2·h or LMH (>10% improvement of the ones without annealing) while still maintaining 99.99% salt rejection. Further tests on LEP, contact angle, and pore size distribution explain the improvement due to annealing well. Fourier transform infrared spectroscopy and X-ray diffraction analyses of the membranes showed that there was an increase in the crystallinity of the polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) membrane; also, peaks indicating the α phase of polyvinylidene fluoride (PVDF) became noticeable after annealing, indicating some β and amorphous states of polymer were converted into the α phase. The changes were favorable for membrane distillation as the non-polar α phase of PVDF reduces the dipolar attraction force between the membrane and water molecules, and the increase in crystallinity would result in higher thermal stability. The present results indicate the positive effect of the heat-press followed by an annealing post-treatment on the membrane characteristics and MD performance.
Highlights
The shortage of water is one of the biggest concerns for future society as the human population is increasing steadily
A polypropylene (PP) filter layer purchased from Ahlstrom (Helsinki, Finland) was applied as support layer in all the membrane distillation (MD) tests except when commercial membrane was in use
The wetting of electrospun membranes could be judged based on the steady increase in the flux and the conductivity electrospun membranes could be judged based on the steady increase in the flux and the conductivity of the the permeate permeate
Summary
The shortage of water is one of the biggest concerns for future society as the human population is increasing steadily. Desalination, a good option for coastal areas short of fresh water, has been becoming the major approach for potable water as the supply of seawater can be considered unlimited. Reverse osmosis (RO) has reached the state of the art and has become the dominant technology because of its higher energy efficiency and stability compared to conventional thermal-based processes [1]. There are two main issues in RO: high energy consumption and brine treatment, which are becoming great challenges for future human society [2]. Continuous efforts for finding new technologies that can provide lower energy consumption while still obtaining high process and production efficiencies are being sought out [3]. Membrane distillation (MD) is one of the most promising emerging technologies [4]
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