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CFD Study of an Oscillatory Flow Reactor with Conic Ring Baffles

Author: LiWei
Tutor: WuJia
School: Zhejiang University
Course: Chemical Engineering
Keywords: OFR with conic ring baffles CFD research chemical reaction process mixing efficiency axial backmixing radial mixing
CLC: TQ052
Type: Master's thesis
Year: 2010
Downloads: 74
Quote: 1
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Abstract

The oscillatory flow reactor is a new type of chemical engineering process equipment, with its great capability of enhancing the transport process. OFR possesses an excellent characteristic on the residence time distribution (RTD) when operated in a continuous state. OFR has shown a promising future of application in chemical engineering process such as chemical reacting, flocculation and crystal. OFR with conic ring baffles is utilized effectively to solve the problem of hold up of the suspended particles in the reactor. For the liquid-solid system OFR has a special advantage in application. Preliminary CFD study of OFR with conic ring baffles concentrates merely on the qualitative analysis of the velocity field, with the concentration field and the chemical reacting process not involved.In this thesis, the geometrical structure of OFR with conic ring baffles was constructed by employing the method of computational fluid dynamics executed by geometry modeling, meshing, numerical solving and analyzing, equivalent scale of that of the experimental apparatus of OFR with orificial baffles. Structure and unstructured meshing method was adopted to discretize the fourteen-cell reactor with a total mesh number of 957083. The standard K-e two-equation model was induced to model the turbulent flow while solving the time-averaged RANS equations. In the discretization of the control equations, the transient term employed a scheme of second order backward Euler, and the convective term a scheme of high resolution.Mesh deformation was utilized to simulate the oscillatory boundary and the Finite Rate Chemistry model was introduced to model the reacting process. A comprehensive study on the velocity field as well as the mixing behavior and efficiency of the concentration field was carried out both in a batch OFR and a continuous OFR, together with the characteristic of the continuous chemical reacting process of the alkaline hydrolysis of ethyl acetate. A comparative analysis was made between the simulation results in a conic-ring-baffled OFR and the experimental results in an orificial baffled OFR.For the batch process, the significant point is the vortex structure along with the axial back mixing and radial mixing. An opposition of the location of the mainstream in the up and down half of the cell was investigated. Along the total flow direction the mainstream lies in the axial region and the wall area of the second half part of the cell, due to the vortex structure. The back mixing is caused by the circumferential flow around the vortex and affected by oscillation amplitude and frequency and generally a larger amplitude and frequency results in a larger backmixing flow and the asymmetry of flow field, which caused serious bias flow. Flow field basically stays the same at a even higher amplitude and frequency. The surface-averaged concentration presented obvious difference between the up and down half part of the cell while the axial transport process of component mainly occurred in the transition area of the two half cell, from axial region in one half to the wall region in another.For the continuous process, the net flow exhibits a great influence on the flow field. For larger net flow, only in a low oscillation intensity did the net flow take the main influence. As the oscillation intensity grows, the turbulence of the velocity field is enhanced and distinct structure of vortex shows a intensified vortex intensity. For lower net flow, the transport process of component is mainly affected by the flow field while by net flow for larger net flow.For the alkaline hydrolysis of ethyl acetate, the simulated reaction conversion in a conic-ring-baffled OFR corresponds well with the experimental result previously carried out in an orificial baffled OFR. For different oscillation intensity the tendency of the two is basically the same. For small amplitude, the total reaction conversion increases as Re_o grows. A reasonable explanation is that the axial backmixing is low at small amplitude and the oscillation intensity contributes to the final mixing extent. For large amplitude, a relatively big gap between the CFD results and the experimental one exists, however, the tendency of the two still shows the same that the total reaction conversion decreases as Re_o grows. It may be concluded that with large amplitude, the mixing efficiency is enhanced with high oscillation intensity while the flow field turns out to be complicated for serious back mixing flow, which decreases the reaction extent.

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