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Theoretical and Experimental Study on Micromixing-Precipitation Process of Novel Reactors and Their Application

Author: XiangYang
Tutor: ChenJianFeng
School: Beijing University of Chemical Technology
Course: Chemical Engineering
Keywords: rotating packing bed microchannel BaSO4 gemfibrozil precipitation mixing coalescence-redispersion model CFD
CLC: TQ03
Type: PhD thesis
Year: 2009
Downloads: 230
Quote: 1
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Abstract


Precipitation is a very important unit operation which is widely used in chemical industry in the production of fine solids. Typically, precipitation occurs after the mixing of two liquid streams to create supersaturation, followed by nucleation, particle growth and (very often) agglomeration. If the time scale of precipitation kinetics is much longer than that of mixing, the mixing of two liquid streams is faster than the reaction. In this case, precipitation process is often controlled by physicochemical characteristics, and not the mixing conditions. In fact, precipitation process is so fast that nucleation, growth and agglomeration have started prior to the achievement of a homogeneous supersaturation level in a reactor. The heterogeneous supersaturation field generates different driving forces, which in turn drastically affects the final product properties. Therefore, the modeling of precipitation process in chemical reactors is required by combining fluid and mixing mechanics to improve the product quality. This dissertation had a detailed research on the mixing-precipitation process of nanoparticles in a rotating packed bed and microchannel reactors. Based on the previous research results of visualization and micromixing in a rotating packed bed (RPB), a mixing-precipitation model was firstly presented to describe the flow, mixing, nucleation and growth process in RPB. The validity of this model was verified experimentally with BaSO4 precipitated from the reaction of Bal2 and Na2SO4 in RPB. Furthermore, the mixing efficiencies of various microchannel reactors were investigated by CFD technology, and exploring the effects of operational conditions on particle size distribution was experimentally carried out. Finally, on the basis of the above-mentioned researches, gemfibrozil (GEM) nanodrug was prepared in RPB. The main contents and findings are summarized as follows:1. On the basis of the previous research results of liquid flow and mixing mechanism in RPB, a coalescence-redispersion model is built to describe flow, mixing, reaction, nucleation and growth by reasonable hypothesis and combination with population balance, mass balance and crystallization kinetics for precipitation process in RPB. The mixing intensity of liquid-liquid on cages in RPB can be characterized by the coalescence probability (p), which is defined as the percentage of droplets participating in coalescence-redispersion process. Moreover, coalescence probability at different radial positions of packing was calculated by wire packing structure and mixing mechanism.2. A RPB, which is allowed sampling at radial position, is specially designed to investigate the precipitation of BaSO4 nanoparticles The important effect of inlet region of the RPB on the whole precipitation process was experimentally confirmed for the first time, which has a significant impact on the design of industrial RPB for the precipitation of sparing soluble material, especially the radial thickness (i.e.,40-50 mm in our experimental conditions). The effects of operating conditions on particle size distribution were also investigated. The results showed that the BaSO4 mean particle size and corresponding size distribution decreased with the increase of rotational speed and liquid flow rate, while increased with the increase of volumetric ratio and the decrease of reactant concentrations.3. Based on the above model, the effects of coalescence probability, supersaturation, volumetric ratio and crystallization kinetics on particle number density, supersaturation and mean particle size were explored. The predicted results indicated that mean particle size and variance decreased with the increase of coalescence probability, nucleation order, supersaturation and volumetric ratio, while increases with the increase of the nucleus size. This model has a good agreement between the experimental and predicted results, providing a theoretical base for further investigation and.optimization design of RPB.4. The mixing efficiencies (Is) of various microchannels were investigated by CFD model at different operational conditions. The calculated results showed that the mixing efficiency increased with the increase of liquid flow rate. The bigger two inlet cross angle of micrchannels is, the higher the mixing efficiency. However, the mixing efficiency only slightly increases with the increase of liquid flow rate and volumetric ratio at the higher volumetric flow rate. In addition, the precipitation of BaSO4 nanoparticles was carried out in Y-type and line-type microchannel reactors, respectively. The effects of various operational conditions on particle size distribution of precipitates were studied as well. The results showed that the mean particle size and dimensionless variance decreased with the increase of liquid flow rate, initial concentration and volumetric ratio of reactants, as well as the decrease of the microchannel size.5. On the basis of the above theoretical and experimental results in RPB, the RPB structure was further optimized for the preparation of gemfibrozil (GEM) nanodrug. The results indicated that GEM nanosized particles with a near-spherical shape, a mean particle size of about 80 nm and a narrow PSD could be successfully prepared in RPB. The as-prepared GEM powder was also characterized by XRD and BET.

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