A two‐phase model for variable‐density fluidized bed reactors with generalized nonlinear kinetics

Abstract

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.

Methanation in a fluidized bed reactor with high initial CO partial pressure: Part II— Modeling and sensitivity study

“…But, it is not clear how fast and to what extent the extra mass is transferred. Little work has been done on this subject for both volume expansion and reduction (Abba et al, 2002;Sitzmann, 1986;Böck, 1984;Irani et al, 1980;Kai and Furusaki, 1987;Shiau and Lin, 1993;Tafreshi et al, 2000;Kai et al, 2006).…”

Analysis of fluidization quality of a fluidized bed with staged gas feed for reactions involving gas‐volume reduction

“…Many industrial gas phase reactions involve a volume change that influences their conversion and selectivity. The effects of volume change on conversion in fluidized bed reactor models have been reported by some researchers 1–6. In addition, volume change can influence the hydrodynamic behavior of fluidized beds.…”

A Self-Consistent Physical Model of the Bubbles in a Gas Solid Two-Phase Flow

“…The nonlinear characteristics of the gas-solid two-phase flow and the relationship with bubbles were studied by Homsy et al [14]. Zahra researched the nonlinear dynamic characteristics of the gas-solid system using the two phase theory [15]. Komatsu and Hayakawa investigated the nonlinear dynamic characteristics of the fluidized states.…”