Experimental Study On The Flow Field And Residence Time Disrribution In Multi-Stage Stirred Reactor

Continuous stirring reactors are widely used in the fields of fine chemicals?polymer chemicals and other polymerization industries.They are the core reactor equipment for chemical reactions and heat/mass transfer in the polymerization chemical industry.In recent years,a large-aspect ratio multi stages stirred reactor has begun to receive extensive attention in the production of polymer precursors.Compared with the traditional single/multi impellers continuous stirring reactor,the residence time distribution and product yield of this reactor have greater advantages.However,due to the current lack of theoretical understanding and research on the hydrodynamics and backmixing mechanism of this reactor structure,the application of this reactor with great application potential in the industry is still greatly hindered.This thesis uses computational fluid dynamics(CFD)simulation to study the flow field and residence time distribution characteristics of the multi-stage stirred reactor at different superficial velocities and different horizontal baffle opening sizes.The steady-state flow field,pressure distribution and residence time distribution curve under various variables of the multi-stage stirred reactor were obtained.At the same time,based on the fact that the flow pattern in the stirred reactor is mostly turbulence,we also used the high-resolution two-dimensional PIV method to study the turbulent single-phase flow field characteristics in the square stirred tank.The results show that:(1)The steady-state flow field and pressure distribution in the multi-stage stirred reactor exhibit the classic double loop pattern,and the change in flow pattern has nothing to do with the opening size of the horizontal baffle.The change in pressure distribution is related to the opening size of the horizontal baffle,and its internal pressure value increases as the opening size of the horizontal baffle decreases.(2)The opening size of the horizontal baffle is the key factor that determines the residence time distribution characteristics of the reactor.As its size decreases,the back-mixing between reactor stages drops significantly.Through the multi-stage series stirred tank model,the calculated equivalent number of reactors can reach a maximum of 4.2,showing a very strong plug flow trend.In addition,the superficial flow rate in the reactor is also an important factor affecting the back-mixing,and its increase can also make the stirred reactor approach plug flow.(3)The distribution of turbulent dissipation rate(?)describing the details of turbulent energy dissipation can be calculated at different analytical scales by two calculation methods:direct definition calculation and large eddy PIV.The two calculation methods have their own use scale and different values,but their final stable value is basically the same.