Fluidized Bed Reactor

This chapter covers key features of ˜uidized bed reactors for catalytic and noncatalytic reactions involving solid particles and one or more ˜uids, most commonly a gas, and also liquids and gas-liquid mixtures. Fluidized beds ”nd wide application in several industries-chemical processes, petrochemicals, polymers, mineral processing, pharmaceuticals, and food processing-because of their unique features that are advantageous in a number of applications. Enough background is provided for the reader to understand the

CONTENTS

Introduction ................................................................................................................................. 199 Key Features of Gas-Solid-Fluidized Bed Reactors ................................................................200 Fluidized Bed Catalytic Reactor Processes .............................................................................. 203

Fluid Catalytic Cracking ....................................................................................................... 203 Partial Oxidation Reactions .................................................................................................. 203 Fischer-Tropsch Synthesis ..................................................................................................... 203 Oxychlorination of Ethylene ................................................................................................. 204 Propylene Ammoxidation ..................................................................................................... 204 Polymerization ........................................................................................................................ 204 Catalytic Reforming ............................................................................................................... 205

Gas-Solid Reactions .................................................................................................................... 205 Combustion ............................................................................................................................. 207 Gasi”cation .............................................................................................................................. 208 Pyrolysis .................................................................................................................................. 209 Calcination .............................................................................................................................. 210 Roasting ................................................................................................................................... 210 Other Processes ....................................................................................................................... 211

Ultrapure Silicon and Iron Ore Reduction ..................................................................... 211 Chlorination and Fluorination of Metal Oxides ........................................................... 211 Nano-and Ultra”ne Particles .......................................................................................... 211

Liquid-Solid Reactions in Fluidized Beds ............................................................................... 211 Gas-Liquid-Solid (Three-Phase) Chemical Processes in Fluidized Beds ............................ 212

Biochemical Processes ........................................................................................................... 212 Hydrocarbon Processing ....................................................................................................... 213

Fluidized Bed Reactor Modeling .............................................................................................. 213 Conclusions .................................................................................................................................. 214 References ..................................................................................................................................... 214

major advantages and disadvantages of ˜uidized bed reactors, the most important design challenges and considerations, and the principal applications. Emphasis is placed on gassolid-˜uidized beds, but liquid-solid and gas-liquid-solid (three-phase) ˜uid bed reactors are also treated in brief. For more extensive coverage, the reader should consult standard reference works on ˜uidization.[1-6]

The advantages of gas-solid-˜uidized bed reactors relative to packed bed reactors are:

• Greatly improved bed-to-wall and bed-to-immersed-surface heat transfer • Reduced axial and lateral temperature gradients, minimizing the probability of

hot spots, catalyst sintering, and unwanted side reactions • Ability to add or remove particles continuously or intermittently, without shutting

down the process • Reduced pressure drops (The pressure drop across the bed, once ˜uidized, essen-

tially remains equal to only that required to support the weight of the bed.) • Smaller catalyst particles, leading to improved catalyst effectiveness factors • Ability to introduce (usually as a spray) modest quantities of liquid reactants that

vaporize before reacting or yield solid products upon reaction inside the bed

Fluidized beds, however, have some important disadvantages relative to packed (”xed or moving) bed reactors:

• Substantial axial gas mixing, causing much larger deviations from plug ˜ow than for packed bed reactors, thereby adversely affecting conversions and selectivities

• For reactions where the particles themselves react, substantial particle mixing, greatly broadening solid residence time distributions relative to moving beds

• Particle attrition because of particles colliding with each other and with ”xed surfaces

• Wear on immersed tubes and other interior surfaces because of the particle impingement

• Entrainment of particles, causing loss of catalyst and/or solid product, contributing to air pollution and requiring gas-solid separation equipment

• Increased risk because of complex hydrodynamics and dif”culties in characterizing and predicting reactor performance

Gas-solid contacting and axial dispersion of both gas and particles depend on the particle properties, operating conditions, column geometry, and scale, all of which affect the motion of gas and solids, commonly referred to as hydrodynamics. For extensive coverage of hydrodynamics[2-6] may be consulted. Here we summarize the key features that affect the performance of gas-solid-˜uidized beds as chemical reactors.