Study On Transfer Characteristics Of Slug Flow And Crystal Layer Growth In Bubbling Tubes

Melt crystallization attracts the increasing attention in separation and purification of organic chemicals in chemical industries over the past few years, due to its significant advantages such as low operation temperature, lower energy required than distillation, without using solvents, and easy to generate high-purity products. The crystallization technique by bubble column crystallizer (BCC), which was originally developed by Rutgerswerke, is one of effective techniques of layer-based melt crystallization. The technique is carried out by injecting an inert gas into melts in crystallizing tubes to produce upward slug flow, which is employed to enhance heat and mass transfer between the crystal layer and melts and to increase the separation efficiency and the crystal layer purity.In order to understand the transfer and crystallization processes in BCC, the electrochemical technique LDCT was used to study the flow and wall-liquid transfer characteristics for Taylor bubbles rising upward through the liquid column in a vertical tube under the condition of zero net liquid flow (ZNLF). Two different types of wall-liquid transfer characteristics were observed from the experimental results. For gas Froude number FrG<0.74, the direction of wall shear stress changes alternatively with the coming or leaving of Taylor bubbles. Values of wall shear stress and mass transfer coefficient increase with the falling flow of liquid film and decrease with the up-flow of liquid slugs. For Frc >0.74, however, the direction of wall shear stress is negative in the whole slug unit, which reveals that the downward near-wall liquid flow exists and the falling liquid film jets across the whole trailing liquid slug and the liquid-wall transfer is mainly controlled by the liquid film falling and jetting in this case. The transition between these two cases occurs for FrG=0.74~0.93 and the former is suitable to perform crystallization. Based on the experimental results and the analysis of hydrodynamic mechanisms, it was proposed that four different zones exist in a slug unit in the case of ZNLF, i.e. the laminar falling film zone, the turbulent falling film zone, the wake region and the remaining liquid slug zone. Correlations for wall-liquid mass transfer at these four zones were developed. By applying Chilton-Colburn analogy method, the corresponding four-zone correlation model for convective heat transfer coefficient at the crystal layer-melts interface in BCC was obtained.By analyzing heat and mass transfer in the crystallization process in BCC, mathematical models for the crystal layer growth from a pure melt and a binary melt contained in a crystallization tube of BCC were developed and their corresponding semi-analytical solutions were obtained, respectively. The predicted results by these models were compared with the experimental data conducted using the binary system of caprolactam-cyclohexanone and a good agreement was obtained. Characteristics of layer growth, distribution of impurities and migration of inclusions in crystal layer and the effects of slug flow conditions on the layer purity were also studied experimentally and theoretically. From the results, the crystal layer increases rapidly with the increase of time in all zones of a slug unit at the beginning stage of growing, and then the layer growth rate at liquid film zones gradually decreases and finally, the re-melting in the fallingliquid film zone and the crystallizing in the wake region or in the remaining liquid slug zone exist at the residual growth stage, which is benefited to enhancement of layer purity. The experimental results also show that the distribution of impurities in crystal layer is not uniform, which is mainly caused by two processes: the one is the entrapping process due to the layer growing and the other is the impurity moving out of the growing layer due to migrations of inclusions under the influence of temperature gradient across the crystal layer. Moreover, the layer purity is influenced by the slug flow parameters and its degree is rela...