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Fundamental Study on the Preparation of Inorganic Salt Crystal by Membrane Crystallization

Author: NiWei
Tutor: DingZhongWei
School: Beijing University of Chemical Technology
Course: Chemical Engineering
Keywords: membrane crystallization heat transfer coefficient CSD coefficient of variation growth rate
CLC: TQ028.8
Type: Master's thesis
Year: 2007
Downloads: 121
Quote: 0
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Abstract


Membrane distillation （MD） is a new membrane separation technique,which can handle aqueous solution with extremely high concentration,because, unlike the other membrane processes, the solution concentrationhas little effect on the performance of MD. The evaporation of solvent andthe crystallization of solute can be separated, if MD was used to the remoteof solvent. The gentle operation condition and well remote control makeit possible that the crystallization occurs on the right super-saturation, andthus, crystal product with good CSD might be produced.Membrane crystallization combines membrane distillation withcrystallization. How to avoid crystallization on the membrane surface is ofmost importance in this technique. So the heat transfer coefficient ofthermal boundary layer for membrane distillation has to be determined inorder to control the temperature of the membrane surface accurately. Bymeasuring the trans-membrane mass flux and feed concentration incondensing the aqueous solution of NaCl and KCl, the critical values ofthem and the corresponding feed temperature at the membrane surface canbe found thereon crystallizing occurs on the membrane surface, so the heattransfer coefficient can be determined. The experiments were conducted at different feed velocities, and the obtained results were used to build acorrelation of heat transfer. The correlation was utilized to calculate themembrane distillation coefficient and to predict the mass flux of pure waterMD. And the results showed good agreement with the experimental ones.Both the batch type and the continuous type were used in this work.The feed tank in the continuous membrane crystallization apparatus stopscrystal drag to the suction pipe, avoiding blockage of the membrane poreseffectively. Experiments were made to study the crystallization kinetics atdifferent retention times and agitation speeds. The median size of theobtained KCl crystals at different conditions was around 75μm. Thecoefficient of variation was about 35%, which was relatively low comparedto the product produced by ideal MSMPR crystallizers. The growth rate ofKCl crystal was in the range of 2.5×10-7 to 4.5×10-7m/s, higher than thevalues reported. All of above confirmed the feasibility and advancement ofthe membrane crystallization.

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CLC: > Industrial Technology > Chemical Industry > General issues > Chemical processes ( physical processes and physical and chemical processes ) > Separation process > New separation method
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