1020_Development of simultaneous membrane distillation?crystallization (SMDC) technology...

ESEL Paper Review_20141020
By Hong Guo
Mail: hongguo@gist.ac.kr
Phone: (+82) (0)10 82276568
1, Title and Author
Title: Development of simultaneous membrane distillation?crystallization (SMDC) technology for treatment of saturated brine
Journal: Chemical Engineering Science
FeliniaEdwie, Tai-ShungChung*
Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore
2. Summary of Paper
? The effect of feed temperature variation from 40 c to 0 c on the SMDC performance in terms of membrane flux and kinetics of NaCl crystallization have been investigated. Increasing feed temperature increases membrane flux but the flux declines rapidly with time at higher feed temperaures (60 °C and 70°C) due to the occurrences of membrane scaling and wetting facilitated by salt oversaturation at the boundary layer.
? In order to prevent salt oversaturation, we have calculated the critical fluxes at different Reynolds numbers and crystallizer temperatures. For instance, the critical fluxes when the feed temperature is 70 °C increase from 5kgm?2 h?1 to 20kgm?2 h?1 for the laminar and turbulent flows, respectively.
? By keeping the membrane flux lower than the critical flux, a stable membrane performance during a continuous SMDC operation over the period of 5000 min has been achieved.
? Increasing feed temperature also increases the yield of NaCl crystals from 7.5 kg per m3 solution to 34 kg per m3 for feed temperatures of 40 C and 70C after 200 min operation, respectively.
? However, the average crystal sizes decrease from 87.4 um to 48.82 um with increasing feed temperature from 40 °C to 70 °C due to a higher nucleation rate at a higher degree of super saturation.
? Regardless of the feed temperature, the NaCl crystals are in a uniform cubical shape with the coefficient of variations which are in the range of 30-38% that implies a narrow dispersion
3. Results
? Effect of feed temperature on kinetics of crystal nucleation and growth
? The production of salt crystals conforms to the distillate production exhibiting an increasing trend with increasing feed temperature and operation duration.
? The highest yield of 34 kg NaCl per 1m3 of feed solution is achieved when the feed temperature is 70 °C after a 200-min operation.
? In between there exists a critical size at which the system free energy reduces for both cases of the nucleus dissolution and the nucleus growth. The nucleation is controlled by the critical size because a smaller critical size increases the probability for formation of a stable nucleus.
? Critical size is smaller for a solution which has a higher degree of super saturation. Therefore, the crystallization at higher feed temperatures is dominated by nucleation that results in the formation of smaller size crystals. The nucleation and growth rate increases while the growth rate decreases with increasing feed temperature.
? The nucleation rate for the NaCl crystal was in the order of 109-1011 crystals per m3 of the crystallizing solution.
? With increasing number of crystals in the crystallizer, the probability for the secondary nucleation increases because of a higher chance for crystal collision.
? Modeling of the critical flux and continuous performance of SMDC
? It is important in the SMDC operation to ensure the salt concentration near the membrane surface lower than the saturated concentration in order to prevent scaling and membrane wetting.
? The salt concentration near the membrane surface is governed by the membrane flux and the salt mass transfer coefficient.
? The critical flux at which the salt concentration can be estimated. Thus, it is recommended to choose a membrane with a flux lower than the critical flux to prevent salt oversaturation in the membrane module.
? The critical flux increases with increasing Reynolds number, for instance when the Reynolds number is 500, the critical fluxes are estimated to be around 5kg m-2h-1 and 2kgm-2h-1 for the feed temperatures of 70 °C and 50 °C.
? A higher Reynolds number provides a better mixing and hence, increases the heat and mass transfer coefficients at the boundary layer.
? As a result, the membrane operated under a turbulent flow can tolerate a higher water evaporation point of view, it is preferable to operate the SMDC process under a turbulent flow as it can enhance the production rates of fresh water and salt crystals. However, the turbulent flow may increase the probability for the crystal collision and breakage.
? Thus, is recommended to operate SMDC under a laminar flow if the aims are to produce crystals with a uniform shape and a narrow size distribution.
? Operation the crystallizer at a lower temperature is another option to lower the salt concentration near the membrane surface. This is because the operating temperature of crystallizer determines the inlet concentration of feed solution o the membrane module.
? The solution leaving the crystallizer is saturated with respect to the crystallizer temperature. Therefore, by lowering the crystallizer temperature, the concentration at the module inlet is lower and hence, it can tolerate a higher water evaporation rate and its subsequent higher concentration polarization.
? The effect of the crystallizer temperature on the critical flux is less than that of Reynolds number because of the minor effect of solution temperature on the solubility of NaCl
? Fig.11 shows that the membrane flux is stable over 5000 min of the continuous SMDC operation withthte distillate conductivity ranging from 0.9uS cm-1 to 2.6 uS cm-1 when feed temperature is fixed at 40 C.
4. Contribution:
This research studies the simultaneous membrane distillation-crystalliation hybrid desalination technology for the concurrent productions of pure water and salt crystal from the saturated brine solution. Additionally Modeling of heat and mass transfers reveals that a higher rate of water evaporation induces higher concentration and temperature polarizations generating oversaturation in the module. This oversaturation facilitates both scaling and membrane wetting that deteriorate the membrane flux by three different mechanisms: suppressing the driving force for the transport of water vapor, narrowing the liquid?vapor interface, and blocking the diffusion path way of water vapor. It would be greatly helpful for the modeling simulation research.
5. Contact (Mail address): chencts@nus.edu.sg (T.-S.Chung).

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