Experimental Study On Micromixing Characteristics In An Aerated Stirred Tank

Gas-liquid stirred tank reactor is widely used in the industry ofpetrochemistry, pharmaceutics and polymer, in which micromixing play a veryimportant role in distribution and quality of products, also safety of production.As widely used in industry, it is necessary to study micromixingcharacteristics of gas-liquid agitated reactors from which we can find the mostsuitable mixing conditions in order to improve the product properties andacquire the maximal benefit. But the literatures on micromixing of gas-liquidtwo-phase systems are limited and conflicting.An iodide-iodate competitive parallel test reaction system proposed byVillermaux and co-workers was used to investigate the influence of feedingtime, impeller speed, feeding location, backmixing, gas flow rate and impellerdiameter on micromixing with a single HEDT impeller in an N2-aerated stirredtank of300mm diameter. The results show that segregation index is stronglyinfluenced by both mechanical stirring and gas-flow agitation. The bestfeeding time is900s. Segregation index decreases with an increased averageshaft power if no backmixing occurs. Backmixing may happen within thenozzle in the impeller discharging flow, changing of nozzle diameter or pipe shape can reduce the influence of backmixing. To straight feed pipes,backmixing can be avoided if vf/vtip≥0.05.Micromixng near the liquid surfaceor vessel wall is mainly influenced by gas flow potential energy Pm, and alsostrongly influenced by bubbles bursting near the liquid surface. Agminatedbubble surface may make a bad macromixing and turbulence which plays anopposite role against Pm, the highest segregation index with apparent gasvelocity vs=0.037m/s can support this assumption. bubmade by bubblebursting also contribute to micromixing in specific limited location. Gas flowhas little impact in micromixing near the impeller tip but bad aeratedsegregation index is observed compared compared to the ungassed index. Inthe condition of same shaft power, D=100mm HEDT impeller has a better gasdispersion effect which can combine the dimensionless feed position d/D=0.1and0.2to the same turbulence intensity field when D=150mm HEDT impellerhas dispersive curves. With dimensionless feed position the same d/D,D=100mm HEDT impeller has lower segregation index curves than the largerone. Both existing PIV data and incorporation model were applied to estimatethe micromixing characteristic time tm, which has the same order ofmagnitude with the literature data. Exponential growth incorporation modelwas used to estimate the ratio of local energy dissipation and average energydissipation. Combined PIV data with exponentail growth incorporation model,Xswas simulated in the impeller discharging area without aerated.The results are of importance to the optimum design of industrial large agitated reactors.