CFD-DEM analysis of gas-solid-solid three-phase flow in a moving bed pyrolysis reactor

Abstract

Pyrolysis is an effective method for the clean and resourceful utilization of oil sludge. A solid heat carrier moving bed pyrolysis process can efficiently pyrolyze oil sludge, and the introduction of carrier gas can reduce secondary cracking, resulting in high-quality pyrolysis products. The flow and mixing uniformity of oil sludge particles with the heat carriers, as well as the uniformity of carrier gas flow, directly impact the efficiency of oil sludge particle pyrolysis. In this study, the flow and mixing characteristics of gas-solid-solid three-phase fluid in a cold moving bed were investigated using the coupled discrete element method (DEM) with computational fluid dynamics (CFD) based on the Euler-Euler-Lagrange multiphase flow model. The results indicated that a higher the mass ratio of heat carriers to oil sludge particles leads to better mixing uniformity of the mixture, while a lower mass ratio results in better flow uniformity of the particles. An increase in the volume of carrier gas initially improves the uniformity of particle flow but subsequently deteriorates it; conversely, a smaller volume of carrier gas enhances the mixing uniformity of the mixture. A smaller mass ratio of the heat carriers and a smaller volume of carrier gas lead to a more uniform distribution of carrier gas flow within the moving bed. The optimal conditions were found to be a mass ratio of oil sludge particles to heat carriers of 1:1 and a carrier gas flow rate of 7.5 L/min, which yielded the best flow mixing effect for oil sludge particles.