| Due to large contact area,high heat and mass transfer efficiency and excellent operating flexibility,stirred tank reactors are widely used in industrial processes,such as food,pharmaceutical,metallurgical and polymer industries.Since impeller is the core component of stirred tank reactors,the most desirable and economical approach to improve mixing is through configurational optimization of stirring impeller.In recent years,researchers have developed some new impellers suitable for different operating environments,such as punched impeller,fractal impeller,flexible impeller,and two kinds of new impellers(centripetal turbine and zigzag punched impeller)invented by our research group.Preliminary results showed that centripetal propeller and zigzag punched impeller have good performance.However,the micro-mixing performance of these two new impellers in non-Newtonian fluid systems is not clarified.In view of these,using hydroxyethyl cellulose as a non-Newtonian viscous additive and a parallel competitive phosphate-iodide-iodate reaction system,the micro-mixing characteristics of the new impellers and combinations was investigated in a 0.282 m diameter cylindrical oval-bottom baffled stirred tank.Firstly,standard six straight blade turbine(DT),45° down-pumping pitched blade turbine(PBTD),centripetal turbine(CT),and ten dual-impeller combinations were investigated on their micro-mixing performance.The effect of feeding time on micro-mixing was tested.The power numbers and micro-mixing performance were compared among the targeted impellers and their combinations.It was found that the power numbers of DT,CT and PBTD remained constant with the increase of Reynolds number,and the power numbers of DT was roughly twice that of CT and four times of PBTD.With an increase of power consumption per unit volume,the segregation index of the three propellers gradually decreased,and keeps the relative level of DT<CT<PBTD.For the dual-impellers systems,the power consumption was mainly affected by the impeller with large power numbers,and independent of the installation sequence of upper and lower impellers.The segregation index was mainly determined by the upper impeller when the feeding location was at the liquid surface.The combination of PBTD and DT with both strong shear and intensive circulation was more advantageous among the dual-impeller systems.The dual CTs but with the lower one functioning as radial impeller enhanced the micro-mixing.Contrast between single and double impellers at the same power consumption per unit volume,most of dual-impeller systems can improve the micro-mixing due to complementary flow pattern.Then,by using the same micro-mixing model reaction,the power numbers and micro-mixing characteristics of the novel zigzag punched impeller(ZPI)were tested systematically.The effects of feed position,holes size on blade,single section length of a blade and the zigzag angle of neighboring sections on micro-mixing were also investigated.The configuration optimization of the ZPI was carried out by experiment.ZPI can not only reduce power consumption,but also produce efficient impinging streams to disturb the regular circular motion of the fluid and thus improve micro-mixing performance.Single section length of zigzag blade had a significant effect on the segregation index when the hole diameter was small.The ZPI paddles with large zigzag angle,large single section length,and large hole diameter had good micro-mixing performance at low power consumption,while ZPI with large zigzag angle,low single section length and small hole diameter provided good conditions for micro-mixing at high power consumption.When the feed position was near the tip of blade,the micro-mixing efficiency of DT was the highest;when the feeding point was near the liquid surface,the ZPI was the best.ZPI can be selected for reactor mixing when it is necessary to control the homogeneity of product quality,or when surface feeding is needed considering safety requirement and construction complexity. |
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