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Numerical simulation and analysis of the flow and temperature fields in the axial fixed bed reactor

Author: WeiGang
Tutor: WangHua
School: Kunming University of Science and Technology
Course: Chemical processes
Keywords: Axial fixed bed reactor FLUENT Numerical simulation Flow field Temperature field
CLC: TQ018
Type: Master's thesis
Year: 2010
Downloads: 111
Quote: 0
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Abstract

Energy crisis and environmental deterioration are increasingly intensified on a global extent. Therefore, natural gas, a kind of high quality clean energy and chemical raw material with rich sources, is being widely focused and studied. As a new method arisen in recent years, partial oxidation of methane to syngas using lattice oxygen has been increasingly attached attentions. As axial fixed bed reactor is applied as the main reactor for the method, therefore, it is essential to make a systemic research on flow and temperature field in axial fixed bed reactor.This paper briefs the applications of CFD technologies on chemical engineering; and reviews the general aspects of studies on CFD numerical simulation of the fixed bed reactor. For the characteristic of axial fixed bed reactor, FLUENT, large-sized commercial CFD software is selected to make related numerical simulations. Based on the concept of CSP, this paper makes geometric constructions on physical model of the axial fixed bed reactor. Mathematical model is established to the model features according to fluid motion characteristics. Non-structural mesh generation is applied according to the geometric features of the model.Three turbulent flow models (RNG k-εmodel, Realizable k-εmodel and Reynolds stress model) are applied to make numerical simulations on three-dimensional turbulent flow in study. Through comparation and analysis, it was found that RNGk-εturbulent flow model was the best one for this study, also RNGk-εturbulent flow model was selected for the further simulation and calculation. Findings are made that the pressure drop in the axial fixed bed reactor is bearing close relationship with catalyst arrangement methods in the bed; and different catalyst arrangement methods should be applied for different types of chemical reactions to obtain the best reaction results. For partial oxidation of methane to syngas using lattice oxygen, the reaction is most advantageous withθ=0°. The contacting time of the catalyst surface and fluid in the bed area is obtained in velocity diagram. And judgment is made accordingly whether the reaction if fully made.This paper makes simulations on the temperature field distributions of axial fixed bed reactor under three catalyst arrangements (θ=0°,15°,30°), three gas inlet velocities (V=0.1m/s,1 m/s,10 m/s) and three gas inlet temperatures (300K,473K,673K). According to studies on effects of catalyst arrangement manners on the temperature fields, heat transfer in the axial fixed bed reactor is closely related to the arrangement manner of the catalyst. Meanwhile, gases flowing across the lower stream of the fixed bed are more evenly with the increment of the 8 value. For partial oxidation of methane to syngas using lattice oxygen, the reaction zone in the bed will be increased accordingly when theθvalue is increased. However, variations of catalyst arrangement manner will affect the pressure fields in the axial fixed bed reactor. Therefore, in selection of catalyst arrangement manner, comprehensive considerations are required on pressure field and temperature field factors. According to studies of effects of gas inlet velocity on the temperature fields, a reduction of air inlet velocity is favorable to increase the evenness of the bed temperature and to avoid local hot spots in the bed. However, a reduction of gas inlet velocity will also reduce the virgin gas charged into the reactor. Therefore, a balance point is required between the reaction conversion and the gas flow rate to obtain the maximum yield of the syngas. According to studies of effects of gas inlet velocity on the temperature fields, an increment of air inlet velocity is favorable to reduce the temperature difference in the reactor bed. However, this requires preparatory heating of gases before charging, leading to additional expenses on energy consumption and preheating apparatuses.

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