The hydrodynamic flow structure of fluid catalytic cracking particles in laboratory scale cocurrent downflow reactors is investigated. The flow characteristics and the typical densification in the radial solids concentration profiles experimentally observed close to the walls are modeled using two different approaches. First, an empirical correlation, incorporating the operating conditions of solids circulation rate and superficial gas velocity is proposed which adequately correlates published data of solids hold-up away from entrance and exit effects (fully developed flow). This correlation describes the suspension homogeneity and hence, degree of gas-solid contact efficiency, according to the operating conditions. Second, the concept of cellular automata is used to simulate the downflow reactor hydrodynamics by envisioning the gas-solids suspension as a set of tractable individual particles. The cellular automaton is based on simple rules governing the particle-particle and particle-wall interactions as well as on the particle velocity profile. The rules for these interactions are derived based on experimental evidence using high speed cinematography. The cellular automata is able to describe the mechanism by which the densification in the radial solids hold-up occurs.
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