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F luidized bed reactors (FBRs) have a broad range of applications in the processing industry due to their well-known fluidodynamic and transport characteristics. Hence, the modelling and simulation of these reactors have attracted considerable research effort (Puchyr et al., 1996; Grace and Berruti, 1995; Berruti et al., 1995; Sun and Grace, 1992). In general, in a bubbling fluidized bed, actual conversion is limited by gas-bypassing in the bubble phase, gas back-mixing in the emulsion phase and kinetic factors. Different assumptions and approaches have been chosen in the literature to simplify the complexities introduced by these factors.The two-phase theory of fluidization was originally proposed by Toomey and Johnstone (1 952). According to this theory, the bed consists of a bubble (dilute) phase and an emulsion (dense) phase and above the minimum fluidization conditions, the excess gas passes through the bed as solid-free bubbles. This proposition was then used by Furthermore, Shiau and Lin (1 993) investigated the applicability of this model for reactions with significant volume changes. In most applications, attention has been confined to first order reactions in a constant density reacting medium. However, many industrially important processes such as hydrocracking, polymerization, condensation, hydrogenation and halogenation reactions, experience volume changes as a result of the reaction stoichiometry. lrani et al.A model based on the classical two-phase concept is developed for the simulation of variable-density reaction with generalized nonlinear kinetics in a bubbling fluidized bed. The influence of reaction density parameter on the fluidodynamics and performance of the reactor for four general types of reactions was explored. The results show that the expansion factor has a significant effect on both fluidodynamic characteristics and reaction conversion. In all types of reactions, higher values of hydrodynamic variables were obtained when E ? 0. Reaction conversion, however, dropped as the expansion factor increased. This trend was more pronounced for reaction orders higher than unity. This suggests that bubbling fluidized operations are probably not optimal and applicable for certain types of reactions. Comparative analysis between reaction type and implications for optimum fluidized bed reactor are discussed.

The methane oxidation activities of Pt-NiO and Co-NiO bimetallic catalysts have been investigated as part of a larger research program on the autothermal reforming of methane (combined methane oxidation and steam reforming) in a fluidized bed reactor. Experiments at atmospheric pressure and 783-1023 K for both catalysts showed that the reaction was more selective towards 1-12 production at CH4:O2 ratios greater than unity. Light-off temperature increased with decreasing CI-I4:O2 ratios, but increase in gas velocity (beyond minimum fluidization) increased the light-off temperature. Co-NiO was as promising as the more expensive Pt-NiO catalyst for the oxidation.