fluidization and fluidized beds

Fluidization & Fluidized Beds

Aijaz Ali Mooro

What is Fluidization?

The operation by which fine solids are transformed into a fluidlike state through

contact with a gas or liquid.

Some Key Terminology

• Attrition: breakdown of particles• Choking: collapse of a dilute-phase

suspension into a dense-phase flow as the gas velocity is reduced at constant solids flow

• Circulating fluidized bed: configuration intended to send particles around in a loop continuously, with no upper interface within the bed


• Downer: column where particles are made to fall through under gravity, usually with cocurrent gas flow

• Distributor or Grid: support plate at bottom which introduce the gas to the bottom of the bed and supports the weight of the bed when it is shut down

• Elutriation: tendency for fine particles to be preferentially entrained from the reactor


• Fast fluidization: flow regime whereby there is a relatively dense suspension, but no distinct upper surface, and a superficial velocity generally at least 3 m/s

• Fines: generally particles smaller than 37 µm in diameter (smallest regular sieve size)

• Freeboard: region extending from top of bed surface to top of reactor vessel


• Interstitial gas: gas between the particles in dense suspension

• Porosity: fraction of gas in bed/given region as a whole or only inside the particles; sometimes used interchangeably with voidage

• Riser: column where particles are carried upwards by the gas, with no distinct bed surface


• Segregation: tendency for particles to gather in different zones according to their size and/or density

• Solids: used synonymously with particles• Superficial velocity: gas flow rate divided

by total column surface area


• Transport disengaging zone: region in freeboard beginning at bed surface in which particle flux decreases with height and above which the entrainment is independent of height

• Voidage (or void fraction): fraction by volume of suspension or bed which is occupied by the fluid

Elements of Fluidized Bed Reactors

Contacting Methods

• Batch, cocurrent, backmix, crossflow, countercurrent

• Solids may often be represented by backmix flow• By using proper baffling and staging of units, and

with negligible entrainment of solids, the contacting in fluidized beds can be made to approach closely the usually desirable extreme of cuntercurrent plug flow

• For good design, proper contacting of phases is essential

Advantages of Fluidized Beds

• The smooth, liquid-like flow of particles allows continuous automatically controlled operations with ease of handling.

• The rapid mixing of solids leads to nearly isothermal conditions throughout the reactor, hence the operation can be controlled simply and reliably.

• It is suited to large-scale operations.

Advantages of Fluidized Beds

• The circulation of solids between two fluidized beds makes it possible to transport the vast quantities of heat produced or needed in large reactors.

• Heat and mass transfer rates between gas and particles are high when compared with other modes of contacting.

• The rate of heat transfer between a fluidized bed and an immersed object is high, hence heat exchangers within fluidized beds require relatively small surface areas.

Disadvantages of Fluidized Beds

• The difficult-to-describe flow of gas, with its large deviation from plug flow and the bypassing of solids by bubbles, represents an inefficient contacting system.

• The rapid mixing of solids in the bed leads to nonuniform residence times of solids in the reactor.

• Friable solids are pulverized and entrained by the gas.• Erosion of pipes and vessels from abrasion by

particles.• For noncatalytic operations at high temperature the

agglomeration and sintering of fine particles can necessitate a lowering in temperature of operation, reducing the reaction rate.

Commercial Applications

• Solid-Catalysed Gas-Phase Reactions:– Fluid catalytic cracking, reforming– Fischer-Tropsch synthesis– Phthalic and maleic anhydride– Acrylonitrile and aniline– Chlirination and bromination of hydrocarbons– Polyethylene and polypropylene– Oxidation of SO2 to SO3


• Gas-Solid Reactions:– Roasting or ores (ZnS, Cu2S, nickel sulphides,

etc.)– Combustion and incineration– Gasification, coking and pyrolysis/carbonization– Calcination (limestone, phosphates, aluminium

hydroxide)– Flurination of uranium oxide– Fluid coking– Reduction of iron oxide– Catalyst regeneration


• Gas-Phase Non-Catalytic Reactions:– Natural gas combustion

• Gas-Liquid-Solid:– Hydrotreating, hydroprocessing– Biochemical processes


• Physical Processes:– Drying of particles– Coating of surfaces– Granulation (growing particles)– Heat treatment (e.g. annealing, quenching)– Medical beds– Filtration– Back-purging of filters– Blending– Classification

Flow Regimes for Upward Flow of Gas through Solid Particulate

Materials

Various Kinds of Contacting of a Batch of Solids by Fluid

Classification of Fluidized Beds

Classification of Dense-Phase Fludized Beds

Industrial Applications of Fluidized Beds

Winkler Gas Generator

Large Scale Fluid Bed Catalytic Cracking Pilot Plant

Two-Stage Fluidized Salt Dryer

Pilot Plant for Fluidized Drying of Air with Adsorbent

The drying of air by circulation of large (3.2 to 4.8 mm) silica gel beads of multistage fluidized adsorption.

To reduce the humidity from 0.00191 to 0.0015 kg/kg pilot plant uses a five-stage fluidized absorber 1.2 m square in cross section, 6.1 m high, a pressure drop of 127 cm H2O.

A perforated plate distributor with rubber flaps at the lower end of the downcomers to assure steady flow of particles from stage to stage.

Typical Flow Regimes Observed

Qualitative Fluidization Map for Fine Solids

Solids Mixing by a Single Rising Bubble in a Bed of Small Particles

Solids Mixing by a Single Rising Bubble in a Bed of Large Particles

fluidization and fluidized beds

Engineering