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2.. CIRCULATING FLUIDIZED BEDSIRCULATING FLUIDIZED BEDS

Solid particles are often of great interest in the chemical process industry, mineral

 processing, pharmaceutical production, energy related processes, etc. In some cases the

 particles serve as catalysts for reacting gases and/or liquids. In other cases, as in ore

 processing, the particles must be chemically converted. In still other processes the

 particles must undergo physical transformation, as in drying of particulate solids.

A number of possible configurations are available for carrying out such reactions and

contacting operations. For example, combustion processes, there have long been

competing technologies based on fixed beds (or moving paced beds !here the particles

travel slo!ly do!n!ard in contact !ith each other", fluidi#ed beds (!here the particles

are supported by gas or liquid introduced through a distributor at the bottom of a vessel"

and dilute phase transport systems (!here the particles are conveyed through a duct or

 pipe". $he circulating fluidi#ed bed has come to prominence in the past t!o decades in

terms of ma%or applications. A typical configuration for a &F' reactor is sho!n

schematically in Figure .). *equired are a tall vessel, a means of introducing particles(often simply called +solids" usually near the bottom, a sufficient up!ard flo! of fluid

(generally a gas, but a liquid or a gas-liquid mixture is also possible" to cause substantial

entrainment of particles from the top of the vessel, and a means of capturing a substantial

ma%ority of these particles and returning them continuously to the bottom. $he term

circulating   signifies that the particle separation and return systems are integral and

essential components of the overall reactor configuration. $he !ords fluidized bed  denote

the fact that the particles are supported by the fluid, !hile there is a substantial

suspension density. ote that there is unliely to be a true +bed in the normal sense in

 particular, most circulating fluidi#ed beds operate in the so-called  fast fluidization

hydrodynamic regime !here there is no distinct or recogni#able upper bed surface.

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2.1 Distinguishing Characters

A gas-solids t!o-phase vertical flo! system !ithout mechanical restraint can be operated

in three modes1 co-current upflo!, co-current do!nflo! and counter-current flo! !ith

gas flo!ing up!ard as sho!n in Figure .. A circulating fluidi#ed bed sho!n in Figure

.) resembles a bottom restraint system in !hich solids are prevented from escaping from

the bottom. At lo! solids feed rates, all in%ected particles are carried up!ard giving co-

current up!ard flo!. 2hen the solids feed rate is increased to such an extent that the

up!ard flo! collapses due to saturation of solids entrainment, excess particles fall

do!n!ard and a dense region forms in the bottom section of the riser. A circulating

fluidi#ed bed can thus be operated in either a co-current up!ard flo! mode or fast

fluidi#ation mode, depending on the gas velocity and solids circulation rate.

Figure 2.1  $ypical configuration for circulating fluidi#ed bed system

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