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Hydrophobic alumina hollow fiber membranes were first used for sucrose concentration in a vacuum membrane distillation (VMD) system. The alumina hollow fiber membranes were prepared using the combined phase inversion and sintering method and then grafted with FAS to impart hydrophobicity. The as-prepared hydrophobic alumina hollow fiber membrane had a high mechanical strength of 191 MPa, a high water contact angle of 140° and a high liquid entry pressure of water of 5.0 bar. Fifty-two hydrophobic alumina hollow fibers were packed into a shell-and-tube module. To evaluate the sucrose concentration performance of the prepared membrane module, the effects of various parameters on the permeate flux, such as the feed flow rate, temperature, vacuum pressure and concentration, were investigated in the VMD system. In the batch sucrose concentration tests, an 8 L 10°Brix sucrose solution was successfully concentrated to 50°Brix by removing 6.4 L of water using the VMD system. At a feed flow rate of 5 L/min and temperature of 70 °C, the permeate flux remained as high as 35.1 and 19.0 kg/m2 h when the sucrose solution was concentrated to 10°Brix and 50°Brix. Furthermore, the permeate flux could be recovered in a new concentration cycle after the membrane module was washed with pure water. The results revealed that the hydrophobic alumina hollow fiber membranes are promising and feasible for concentration processes in a VMD system.

The present investigation revealed how the morphology and crystalline forms of a poly(vinylidene fluoride) (PVDF) membrane, prepared by using the vapor-induced phase separation (VIPS) method, were affected by the temperature at which PVDF was dissolved to form the casting solution. The results showed that there existed an important transition dissolution temperature, being referred to as the “critical dissolution temperature”, across which the morphology and crystalline forms of membranes drastically changed. With a dissolution temperature above the critical dissolution temperature, the prepared membranes were composed of nodules and the size of nodules decreased as the dissolution temperature decreased. And, with a dissolution temperature below the critical, membranes with lacy (bi-continuous) structure were obtained. In addition, the α-crystalline form of PVDF grew faster than the β-form when the dissolution temperature was below the critical, and the β-form became faster and dominant as the dissolution temperature increased to be above the critical. The existence of the critical dissolution temperature was observed for all the three solvents used in the present study, N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), and N,M-dimethylformamide (DMF), indicating that the phenomena are general and not limited to a specific solvent. Also, it was observed that the two PVDF/NMP solutions, prepared with dissolution temperatures above and below the critical temperature, responded in different ways when water was added into the solutions. Though both solutions gelled, the solution with higher dissolution temperature started the gelation with a crystallization-initiation gelling process, while the other with a non-crystallization-initiation gelling. We propose that the competition between the two gelling processes play an important role in determining the PVDF membrane morphology and crystalline polymorphs, and the dissolution temperature can change the competition. And the critical dissolution temperature can be interpreted as a dissolution temperature across which the dominant gelling process switched.

A systematic study investigating the effect of membrane material and morphology on the filtration-cycle based on performance of three microfiltration membranes employed for filtering feed solution of Bovine Serum Albumin with yeast suspension has been carried out in dead-end filtration mode. The effects of operating conditions, like feed solution pH, transmembrane pressure (TMP), and membrane cleaning methods have been studied to establish a possible relationship with membrane fouling and to reduce the fouling that occurs during filtration. It was found that filtration resistance increases for the selected membranes with an increase in pH of the feed solution. An increase in TMP increases the permeate flux; however, the effect is not linear due to cake compression at higher TMP. The performance based on the filtration cycle is evaluated, and potential cleaning solutions to restore the permeate flux of filtration after several filtration cycles are identified. Scanning electron microscopy images demonstrate the experimental findings. The expanded polytetrafluoroethylene membrane flushed with caustic soda solution is found to be the most suitable and efficient membrane for microfiltration without significant reduction in flux and protein transmission, even after several filtration cycles.