Enhancement Of Liquid-liquid Mixing In A Helical Tube Reactor And Its Application

Mixing plays an important role in chemical industry, involving homogeneous and heterogeneous mixing processes. The reactor's mixing performance has a significant effect on product distribution, quality, and so on.Mixing is one of the most important topics for the process intensification.As the typical passive mixing equipment, a helical tube reactor ?HTR? has the advantages of compact structure, high pressure resistance, less axial backmixing, ease of temperature control, and so on. Due to these advantages,HTRs are used in nitration reaction, oxidation reaction, halogenating reaction,crystallization, emulsion polymerization, biodiesel preparation, liquid-liquid extraction as well as nanoparticles preparation. Among these applications, it is noteworthy that the fast complicated reaction and liquid-liquid heterogeneous mixing processes were involved. However, little attention was paid to the micromixing and liquid-liquid dispersion performance of HTRs, which are very important for the above two kinds of processes.In view of the great effect of premixing performance on reaction process,a novel HTR equipped with a pre-mixer was designed for the fast complicated reaction processes. A preferred selection of pre-mixer type was conducted by using both CFD simulation and experimental methods. Then the micromixing performance of HTR equipped with the preferred pre-mixer was characterized experimentally. Furthermore, the micromixing performance of the HTR was regulated by premixing optimization. The relationship between premixing performance increasing and the corresponding micromixing efficiency increasing of the HTR was explored preliminarily. In view of the great effect of initial dispersion size on liquid-liquid dispersion performance, another type of HTR equipped with a pre-dispersion unit was designed when in the case of the liquid-liquid heterogeneous mixing processes, and the pre-dispersion unit was used to adjust the mean droplet diameter of the dispersed phase. The pressure drop and liquid-liquid dispersion performance were investigated experimentally by using a emulsification system. Finally, the HTR equipped with the pre-dispersion unit was developed for the continuous solvent extraction of wet-process phosphoric acid. Main work is as following:1. The novel HTR for the fast complicated processes was composed of a pre-mixer and a helical tube, and the pre-mixer was used to change the premixing behavior between the involved reactants. Depending on the difference in contacting ways of two liquid streams, the pre-mixer can be classified as either a co-current flow pre-mixer ?CCM? or a cross-flow pre-mixer ?CRM?. CFD results revealed that the CRM has better pre-mixing performance than the CCM, suggesting that the predicted micromixing efficiency of the HTR consisting of the CRM ?CRM-HTR? better than that comprising the CCM ?CCM-HTR?. The CRM-HTR was preferably selected by a consistency discussion between the results of the CFD simulation and micromixing experiments. Then the micromixing performance of CRM-HTR was characterized experimentally by using a iodide-iodate reaction. Results show that superior micromixing efficiency was obtained in the CRM-HTR as compared to that in a straight tube reactor ?CRM-TR?.2. Concerning the CRM-HTR, two methods, named as tangential-feeding and spiral disturbed way, were applied to modify the structure of the CRM with the aim of optimizing premixing behavior of CRM-HTR. CFD simulation results indicated that both methods can change the fluid flow, enhance the turbulence kinetic energy, and improve the premixing performance in the pre-mixers. Furthermore, the micromixing performance was investigated in the CRM-HTRs with optimal pre-mixers by a parallel competing reaction. Then the CFD predicated results were confirmed experimentally. Based on the results of CFD simulation and micromixing experiments, a new dimensionless parameter RPM that used to reveal the relationship between the premixing performance increasing and the corresponding micromixing efficiency increasing of the HTR was defined. The value of the RPM was in the range of 0.3-0.5 for the different structures of CRM-HTR. The micromixing time in various CRM-HTRs was calculated using an incorporation model.3. The novel HTR for the liquid-liquid heterogeneous mixing processes was comprised of a pre-dispersion unit and a helical tube. The pre-dispersion unit was used to adjust the initial dispersion size of dispersed phase for the HTR. Effects of the number and pore diameter of nickel foam elements in the pre-dispersion unit, volume flow ratio, and curvature ratio on the pressure drop and liquid-liquid dispersion performance were experimentally investigated by using a water/Tween 80/cyclohexane system. Values of emulsion Sauter mean diameter ?SMD? were between 30-65 ?m under the number of pre-dispersion number 2-6, volume flow ratio 6-15, and the total volume flow rate 960-1730 ml/min. The drop size distribution was narrow.Corresponding empirical correlation d₃₂/D_m = 0.80We_c^-0.32De^-0.24n_e^-0.15?^0.68 for the SMD in function of the relevant properties of the system, respectively. The experimentally obtained values of SMD were in agreement with model predictions.4. Continuous solvent extraction of wet-process phosphoric acid were carried out in the HTR equipped with the pre-dispersion unit. Effects of phase ratio, phosphoric acid mass friction, and helical structure parameters on the extraction efficiency were experimentally investigated. Values of extraction efficiency were between 33%-59% under the phase ratio 1-3, phosphoric acid mass friction 30%-70%, curvature ratio 7.5-15, number of turns 25-50, pitch 10 mm. Compared with the other equipment, the extraction performance of the HTR equipped with the pre-dispersion unit was equal to the rotating disc column.