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The surface properties of TiO2-PVDF nanocomposite membranes were investigated by incorporating different chemically modified TiO2 nanoparticles into the poly (vinylidene fluoride) (PVDF) matrix. The nanocomposite membranes were prepared via dual coagulation bath diffusion and the induced phase inversion method. The membrane surface morphologies were investigated by using SEM and AFM and related to the membrane surface energy via contact angle goniometry. The results showed that the average membrane surface pore sizes were increased with the addition of TiO2 nanoparticles. Nonetheless, the contact angle measurements demonstrated that the hydrophobicity of nanocomposite membranes can be maintained even with the addition of hydrophilic TiO2. This observation could be rationalized as surface roughness enhancement. The experimental results demonstrated that the initial flux of acid treated TiO2 has both higher initial flux and high COD removal due to their induced surface roughness. The TiO2-PVDF membranes were found to possess the significant bactericidal effect on B. Subtilis compared to the neat membrane even without the presence of UV light.
In this study, the influence of the salts as an additive on the performance of the membrane was investigated and an extensive work was performed to optimize PVDF hollow fiber membranes through a response surface methodology (RSM). The prepared membranes were characterized by SEM, contact angle and LEP measurement. Then, the RSM was used for the optimization of surface pore size, porosity and hydrophobicity of the synthesized hollow fiber at different conditions (polymer concentration, salt concentrations, and air gap). Under MD conditions (feed concentration, 100 mg/l; feed temperature 80 °C, and cooling temperature 15 °C), the optimum membrane was compared with the virgin one in the same condition. In addition, the influence of distillate flux at different feed concentrations and temperatures was evaluated. The results show that the optimum hollow fiber membrane was fabricated in the polymer concentration of 22 %w/w, BaCl2 concentration of 2.9 %w/w and an air gap of 34.5 cm. Consequently, the optimum fiber was examined for the desalination of water with 35, 50 and 70 g/l salt concentration by DC and AG membrane distillation. Our findings show that the distillate flux with the salt rejection of 99.9% was increased to 46% and 31% for DCMD and AGMD, respectively.
Membrane distillation has the potential to concentrate solutions to their saturation level, thus offering the possibility to recover valuable salts from the solutions. The process performance and stability, however, is strongly dependent upon the features of membranes applied. In addition, several other parameters, membrane thickness and thermal conductivity significantly affect the process performance. These parameters are of fundamental importance in the selection of optimum module length due to their influence on temperature and flux profiles along the fiber. In the current study, the experimental data from a lab-scale membrane distillation plant has been modeled to analyze the interrelated effect of membrane thickness, thermal conductivity and module length on process performance. It has been observed that flux initially improves by decreasing the membrane thickness followed by a decrease and ultimately negative value. For any given fiber length and thickness, the flux can be greatly improved by decreasing the membrane-conductivity. The length that corresponds to the highest flux and the maximum fiber length ensuring a positive flux have been identified as a function of membrane thickness and thermal conductivity.
Direct contact membrane distillation (DCMD) which emerges as an alternative separation technology can effectively perform a colloidal separation process under thermal driven force. DCMD is capable of extracting pure water from aqueous solutions containing non-volatile nanoparticles through the hydrophobic microporous membrane when a vapour pressure difference was established across the membrane. This work aims to study the efficiency of the MD process in separating TiO2 nanoparticles. It was interesting to find out that below 1.0 g/L TiO2 concentration, no sign of flux reduction was noticed. It is indicated that the pore blocking phenomenon was not significant. However, as concentration exceeding 1.0 g/L, the flux started to decline due to the resistance of the gelation layer which impeded water from flowing through the membrane. The blocking law analysis showed that the cake layer was developed within 3 hours of operation. At higher feed velocity, the flux declination problem could be solved due to the surface scouring effect.
The possibility of applying membrane distillation to support the fermentation process was investigated. The capillary polypropylene membranes were assembled in the membrane modules. The studies were carried out using the standard solutions containing the compounds frequently occurring in the broths such as ethanol, citric, acetic and lactic acids, glycerol and 1,3-propanediol. The performance of membrane bioreactor, in which glycerol was fermented by the use of Citrobacter freundii bacteria was also examined. The separation of particular components of broths was investigated in a long-term application of membrane distillation and a good resistance to wetting of the used polypropylene membrane was demonstrated during the two year period.
Fresh juices of colourful wild berries: cornelian cherry, blackthorn, white beam and elderberry are considered as valuable, highly nutritive beverages and characterized by the high level of vitamins and antioxidant capacity. The concentration process of these juices by membrane osmotic distillation was studied, where only water vapour is eliminated, while the heat sensitive, valuable compounds can be preserved. To shorten the length of the concentration period, cascade model systems with 2, 3 and 4 stages were examined, using model sucrose solutions and real fruit juices. 60 °Brix of juice concentration was possible to reach, with a flux of 0.3-2.4 L m-2 h-1. Furthermore, as a result of cascade system experiments, the length of the separation could be shortened, significantly.
The energy requirement of vacuum membrane distillation (VMD) with or without recirculation was modelled using both experimental results and theoretical data. The trends are generally consistent between the theoretical and experimental data. Thermal energy contributes the most to the total energy required for the VMD process. To lower the thermal energy cost, waste heat resource and heat recovery of latent heat from the permeate vapour are needed. The electrical energy consumption for VMD is slightly higher than brackish water reverse osmosis (RO) but lower than sea water RO. It is generally more energy efficient to operate the VMD in recirculation mode than single pass mode. Process engineering modelling results indicate that VMD may not be able to compete with RO directly but could be used as a complimentary process to RO, such as for brine concentrate treatment.