Experimental And Theoretical Study On Micromixing Of Novel Chemical Reactors And Their Application

Before a chemical reaction can occur between two or more reactants, the reactants have to be mixed on a molecular scale. Micromixing (i.e., mixing at the molecular scale) is the last stage of turbulent mixing and consists of the viscous-convective deformation of fluid elements, followed by molecular diffusion. It is believed to play a very important role in the chemical industry when the time scale of the chemical reaction involved is at the same magnitude or smaller than the time scale of the mixing process. Industrial processes, such as crystallization, precipitation, polymerization and so on are greatly influenced by micromixing. In these-processes, the reactions may have occurred or been completed before the reactants accomplish homogeneous mixing at the molecular scale (i.e. rapidness of the reactions). Consequently, conversion, selectivity, particle size distribution and molecular weight distribution, etc., are significantly influenced by micromixing. So, in order to improve the product properties and acquire the maximal benefit, it is necessary to explore the micromixing performance of chemical reactors by virtue of experimental and theoretical method. In this study, with the help of the new iodide-iodate parallel-competing test system, micromixing efficiency of three chemical reactors-tubular (packed) reactor, rotating packed bed (RPB) and microchannel reactor-were characterized. Effects of operational conditions on the micromixing efficiency (indicated by segregation index Xs) were also investigated. Based on experimental results and previous researches, the theoretical study on the micromixing was executed and the characteristic micromixing time scale of RPB was acquired firstly, which give some directions of the application of the reactors. Finally, on the basis of the above research of micromixing, study on the scale-up of preparation of nano-copper lubricating oil additive by high gravity technology (RPB as the core reactive equipment) was carried out.Some major findings obtained in this work are as follows:1. Through a special configuration of RPB which can achieve radial sampling, the importance of inlet region of packing on the intensification of micromixing process was experimentally confirm firstly, which provide an important proof for the optimization of packing thickness in the industrial process.2. The experimental results of the tubular (packed) reactor indicated that the segregation of reactants in the reactor mainly appeared in the initial contacting region and was improved with the increase of reactants flow rates. The insert of wired packing can intensify the micromixing process of tubular reactor greatly. The experimental results of RPB show that Xs decreases with an increase in rotational speed and a decrease in reactant concentration. A slight decrease is also observed as the liquid flow rate increases. The experimental results of the microchannel reactors show that the microchannel reactors possess a pretty micromixing efficiency (small Xs) and micromixing efficiency can be improved by increasing reactants flow rates, that is to say, high micromixing performance and high throughput can be achieved simultaneously.3. Based on previous visual study and appropriate assumptions on liquid flows in RPB, the energy dissipation rate of RPB was estimated firstly and reached about103W·kg-1, which great larger than that of CSTR. Combining with the estimation of energy dissipation rate, the classical engulfment model was adopted to study the micromixing of rotating packed bed (RPB). The results given by engulfment model fit pretty well with the experimental values and can reflect the effects of different operational conditions on segregation index accurately. Moreover, on the basis of the previous visual study on mixing microelements with high speed stroboscopic microscopic photography, a slab-shrinking micromixing model was proposed to investigate the effect of flow rates on the segregation index of microchannel reactor. The agreement between model predictions and experimental data prove that the slab-shrinking model can predict the effect of volume flow rates on the micromixing efficiency correctly. All the above works give some theoretical directions for further applications of the reactors in the nanoparticle preparation and related domains.4. In order to further understand the mixing mechanism of PRB and provide some theoretical directions for the development of new technology, it is urgent to know the characteristic micromixing time scale of RPB. On the base of the experimental study, through both Kolmogorov micromixing theory with appropriate assumptions on energy dissipation rate of RPB and an incorporation model, tm is approximately evaluated to be as low as about10-4s. Hence, this proves that RPB possesses far superior micromixing performance than many other mixing devices.5. Both the pilot scale and the industrial scale production indicated that qualified nano-copper lubricating oil additive can be produced by means of the high gravity technology and good economic benefits can be achieved.Without scale-up effects and good batch repeatability prove that RPB possess a good micromixing performance.