| Mixing condition is a key index of stirred tank reactor.According to different research scales,the mixing process is usually divided into three stages:macromixing,mesomixing and micromixing.Micromixing plays an important role in the rapid reaction process(such as precipitation,crystallization and polymerization reaction)involving solid-liquid two-phase in chemical production,which directly determines the conversion rate,average particle size and particle size distribution of products.However,so far only a few scholars have explored the influence of suspended particles on micromixing,and the conclusions are not completely consistent;In addition,due to the fact that the viscosity of some complex reaction systems has not been considered before,most of the agitators used are traditional single shaft agitators suitable for clean water system,and there is little research on the micromixing performance of coaxial mixers with wide viscosity adaptability,which has been paid more and more attention in industrial production.Therefore,it is necessary to investigate the micromixing performance of this kind of mixers,which is expected to provide a new way to improve the micromixing efficiency in industrial production.In this paper,the micromixing characteristics of coaxial mixers in viscous solid-liquid system(75～300m Pa·s)were studied.With the aid of iodide-iodate characterization reaction system,the effects of feeding time and H~+concentration,solid adsorption,feeding position,rotation mode,outer impeller power,inner impeller type,inner impeller diameter,solid holdup,solid particle size and liquid viscosity on the segregation index were investigated.The results showed that the selection of H~+with a concentration of 2mol/L was more appropriate,and the interference of macromixing and mesomixing could be eliminated when the feeding time is 480s;the solid-phase glass beads had no adsorption effect on the product;feeding in the impeller area was more conducive to the micromixing;the rotation of outer impeller could significantly improve the micromixing effect,and make the solid-liquid system reach the critical bottom suspension state with low power consumption,but the effect of continuously increasing the rotation speed of outer impeller on further improving the micromixing efficiency was limited,and it was easy to cause surface aeration,which was not conducive to the characterization reaction system;the co-rotation mode was more economical and efficient than the counter-rotation mode,and it was more conducive to the suspension of particles;the six-pitched blade(45°)was more suitable as the inner impeller than the Rushton turbine;the inner impeller with a impeller-to-diameter ratio of 0.35～0.42 exhibited better micro-mixing performance;particles with diameter of 780μm could improve the micromixing performance slightly when the solid holdup was between 0.5vol%and 1vol%,however,with the solid content increasing further and forming a"solid cloud",the micromixing performance would deteriorate significantly when feeding in the clear liquid layer above the"cloud";particles with diameter of170μm showed the property of turbulence suppression in the concentration range of0.5vol%～8vol%;particles with diameter of 1013μm exhibited turbulence augmentation and decreased the segregation index slightly at a concentration of 0.5vol%to 2vol%,but when the solid holdup exceeded 2vol%,it also led to a decrease in the micromixing efficiency;the increase of liquid viscosity would result in the continuous deterioration of micromixing performance.Based on the experimental results and Buckingham’sπtheorem,the engineering calculation model of the segregation index was established by the dimensional analysis method and multiple linear regression method.The model was verified to be able to give a more reliable qualitative evaluation of micromixing performance of Anchor+PBT combination in viscous solid-liquid system,which provides a certain reference for the application of this kind of agitator in the field of micromixing. |
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