Control of the solids retention time by multi-staging a fluidized bed reactor

Abstract

Fluidized bed reactors are often operated with particles of different size and density present simultaneously (e.g. inert bed material and fuel particles in fluidized bed combustion, or catalyst particles and polymer particles in fluidized bed polymerization). This paper presents a general concept to separate a single fluidized bed reactor into two reaction zones by introducing a primary chamber in the bed where flotsam particles are fed and spend a certain initial residence time. The feasibility of the concept is experimentally proven in a fluid-dynamically down-scaled unit, both in terms of functionality (ability to maintain two separate reaction regions and to avoid backflow from the secondary to the primary region of the solids fed) and of operability (ability to control the residence time distribution of the solids fed in the primary chamber through operational parameters such as pressure and bed height). Numerical simulations show that the residence time distribution is sensitive to the degree of segregation in the primary chamber, and that the heat transfer between the two reaction regions by means of bulk solids mixing is still sufficient to sustain an endothermic process in one zone by an exothermic process in the other.