ESEL Paper Review_20141104

By Hong Guo

Mail: hongguo@gist.ac.kr

Phone: (+82) (0)10 82276568

1, Title and Author

Title: Concentration of NaCl solution by membrane distillation integrated with crystallization

Journal: Separation Science and Technology

Authors:

M.Gryta

Department of Water Technology and Environmental Engineering, Institute of Chemical Technology and Environmental Engineering, Technical University of Szczecin, PL-70 322 Szczecin

2. Summary of Paper

? Concentration of NaCl solutions by direct contact membrane distillation integrated with salt crystallization has been studied. The salt crystallization was carried out in a batch mode or continuously.

? The influence of process parameters (flow rates, temperatures, and concentrations) on permeate flux has been investigate.

? To evaluate the polarization phenomena, a simple model was used. This model showed good agreement with the experimental results. Two types of membrane distillation MD capillary modules, with the membranes arranged in a form of braided capillaries or helically wounded, were tested.

? The membrane wettability during long-term testing was investigated. A slow decline of the module efficiency was observed since the membranes were partially wetted during the process.

3. Results

? The observed decline of lux was caused by the reduction of the driving force for the mass transfer as a result of a decrease of the partial pressure of water vapor over the salt solution. The increase in solute concentration also affects the temperature and concentration polarization phenomena resulting in a decline of permeate flux.

? Generally, the efficiency of MD module increases with an increase of the flow rate. This effect is particularly significant for the feed velocity and is slightly smaller for the distillate velocity. Moreover, an increase in the module efficiency caused by variation of the flow rate is limited. Therefore, for each type of MD module an optimum flow rate should be determined.

? Firstly, the values of the heat transfer coefficient a rise along with an increase of the flow rate, thus a negative influence of temperature polarization decreases. Secondly, an

enhancement of the flow rate caused that the outlet temperatures of streams were closer to their temperatures at the module entrance, which also increases the driving force for mass transfer

? This driving force decreases with the module length, as a consequence of heat and mass transfer through the membranes.

? The module efficiency is also affected by the concentration polarization. This phenomenon may be reduced by an increase in the flow rate, which improves the mixing conditions inside the module

? Increasing of distillate temperature causes the reverse effect, that is, declines of permeate flux. However, the changes of permeate flux are not such pronounced as in the case of feed temperature variation. The dependence of the vapor pressure on temperature is represented by exponential function. Therefore, considerably large changes of pressure change as a function of temperature were achieved for feed (high temperature) than those for distillate (low temperature).

? A rapid increase of hydrodynamic pressure was observed at the inlet of MD module on the feed side, in the case of higher salt concentration, as a result of salt crystallization inside the capillary membrane.

? The crystallization of salt in the module is disadvantage since it leads to a rapid wettability of the membrane.

? MD process with continuous crystallization should be carried out at the distillate temperature above 318K. In the case of lower temperatures, a slow increase in the hydraulic pressure at the inlet of MD module on the feed side was observed. It was associated with the crystallization of salt proceeded inside the capillary membranes.

? Firstly, the helicoidal flow improves the conditions of mixing in the layer adjacent to the membrane. This results in the reduction of disadvantageous phenomena of the temperature and concentration polarization. Moreover, such conditions make the nucleation of salt more difficult, and consequently the helicoidal flow prevents the formation of salt deposit onto the membrane surface.

? Secondly, a smaller permeate flux was achieved in M3 module, as a result of lower flow rates of feed and distillate streams. A decrease in the amounts of water evaporated from the membrane surface results in the reduction of the concentration polarization

? A decline of permeate flux was found along with enhancement of salt concentration, and for the saturated NaCl solution, the flux was lower by 50% in comparison with that for distilled water. The polarization phenomena have a considerable influence on the MD performance during the concentration of NaCl solutions. Therefore, the flow rate of streams alongside the membrane surface has a great influence on the module efficiency

? A rapid crystallization of salt on the membrane surface was observed during the concentration of saturated solutions. The maximum bulk concentration of salt in the feed is dependent on both the used process parameters and module design

4. Contribution:

As the kinds of review paper for the MCr process by using the NaCl, this paper showed the big picture for the research. However, future research requires more detail explanation for the modeling of the crystallization phenomeana.

5. Contact (Mail address): margryta@mailbox.tuniv.Szczecin.pl