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Investigation Of Transport Phenomena And Operation In A Bubble Column Crystallizer

Posted on:2000-10-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:P W MingFull Text:PDF
GTID:1101360155453726Subject:Chemical Engineering
Abstract/Summary:PDF Full Text Request
Melt crystallization is used as an effective separation technique that will require less energy and produce less pollution. The bubble column crytallizer is a new distinctive solid layer melt crystallization equipment with high yield and efficiency, in which the inert gas plugs arc rising one by one. But the investigation of its transport phenomena and operation is not plenitudinous for scale-up and wide application.In this dissertation, a review is made for the melt crystallization, the gas-liquid slug flow in vertical tubes and its measurement technique — electrochemical method. Combining the limiting diffusion current technique and the synchronous data record method, the hydrodynamics and the transfer of several flow regimes that may occur in this crystallizer is investigated at first. The experiment shows that the homogeneous flow model is not suitable for the bubbling flow, and the correlation of experimental data shows that the nature convection plays an important role in the transfer of the bubbling flow.Applying the same experimental method, the hydrodynamics and the transfer of the plug-rising flow regime is investigated and discussed in detail. The experiment shows the wall shear stress and the Sherwood number follows an analogous law. Their minimum appears at the nose of gas plug, while their maximum appears at the bottom of gas plug. The liquid flow direction on the wall is always downward, but it does not reverse in comparison with the developed slug flow. In this case, since the falling film around short gas plug does not reach the force-equilibrium, it is accelerated to fail down continuous, while its jet thrills through the following short liquid slug. Consequently the falling film and its jet control the transfer process. In this dissertation, a model for mass transfer in gas plug-rising regime is presented. This model agrees well with the results from experiments.From the viewpoint of time-progression, a model for the crystal layer growth in bubble column crystallization is presented based on the analysis of the heat-balance in crystal layer and the transfer model mentioned above. This model also agrees well with experiments of the crystallization at both constant-temperature and reducing-temperature conditions. Subsequently, the effects on the crystal growth rate of superficial gas velocity and operating temperatures are discussed. It is found that the periodical arrival of gas plugs may cause the crystallization and partial re-melt in the interface between crystal layer and melt In the steady state, the separation effect of this crystallizer is analyzed at different operating conditions when the time-progression crystallization and partial re-melt...
Keywords/Search Tags:melt crystallization, slug flow, bubble column crystallizer, electrochemical method, sweating, crystal layer growth
PDF Full Text Request
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