Numerical investigation and optimization of flow and heat transfer in a multi-tubular fixed bed reactor
Li Hao, Bin Jiang, Luhong Zhang
Abstract:Multi-tubular fixed-bed reactor is one of the key equipment to propylene oxidation process and the reactor performance is closely related to heat carrier flow, heat transfer and temperature distribution. Reasonable reactor structure and suitable flow and temperature field distribution can effectively control hot spot temperature and improve the efficiency of the reactor. In this paper, a numerical study of fluid flow and heat transfer performance in the shell side of an industrial-scale cross-flow multi-tubular fixed bed reactor for propylene oxidation reaction was presented using computational fluid dynamics (CFD) method. Detailed information of fluid field and heat transfer in the shell-side of the industrial scale multi-tubular fixed-bed reactor was obtained by using three steps CFD simulation analysis method to control hot spot temperature. Firstly, to simulate flow behaviors, catalytic oxidation reaction, heat and mass transfer adopting porous medium model on tube side to achieve the temperature distribution. Secondly, based on the conclusions of tube-side, a novel configuration multi-tubular fixed-bed reactor was proposed and their performance of fluid flow and heat transfer was analyzed to ensure the uniformity condition on shell-side. There were circulation channels in the shell-side and there was no tube in the central region. Finally, the effects of operating conditions and structural parameters to control the hot spots were investigated, including inlet molten salt flow rate, flow direction, baffles number, baffles cut area, non-tube central region and flow circulation channels. And the optimization parameters were obtained. The simulation results showed that the velocity and temperature distribution varied along axial and radial direction, and there were stagnation regions before and behind the baffles, in which the heat transfer coefficient was low, but high heat transfer was observed in cross flow zones and baffle cut zones. The central region area had great influence on temperature distribution at the hot spot position. The radial temperature difference at hot spot position decreased significantly with the central region area increased. The radial temperature difference increased with the increase in the number of circulation channels. The validity of the CFD results was confirmed by comparing its results of outlet temperature, total pressure drop and average heat transfer coefficient with empirical method results under different heat flux and inlet velocity conditions. The calculating results had a good agreement with the results of empirical methods. And this study had significant theoretical and application meanings for the design of multi-tubular fixed-bed reactors.
Keywords: Numerical analysis; multi-tubular fixed-bed reactor; structural optimization; heat transfer; hot spot control
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