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Introduction, History, and Applications

John R. Grace

University of British Columbia, Department of Chemical and Biological Engineering, 2360 East Mall, Vancouver, Canada V6T 1Z3

1.1 Definition and Origins

Fluidization occurs when solid particles are supported and allowed to move relative to each other as a result of vertical motion of a fluid (gas or liquid) in a defined and contained volume. Most commonly, the fluid is a gas blown upwards by a blower or compressor through a perforated flat plate or a series of orifices, but many other configurations are possible. Once an assembly ("bed") of particles has been actuated in this manner, it is said to be a "fluidized bed."

The origin of fluidized beds is unclear, but liquid-fluidized beds likely preceded gas-fluidized beds. For example, early fluidization has been attributed to Agricola [1] when he described and illustrated hand jigging for ore dressing. The first industrial applications of fluidized beds were likely beds of ore particles fluidized by liquids in order to classify them by size or density in an operation known as "teetering" [2].

The first widespread application of gas-fluidized beds was in the 1920s in Germany when Winkler [3] patented a novel gasifier. However, the terms "fluidization" and "fluid bed" did not emerge until about 1940 when researchers in the United States developed gas-supported beds for catalytic cracking of heavy hydrocarbons [4, 5]. A plaque commemorating the development of the fluid bed reactor at a local oil refinery was erected at the Louisiana Art and Science Museum in Baton Rouge in 1998.

The term "circulating fluidized bed" (or "CFB") has been used since the 1980s to cover configurations where there is no upper bed surface, with particles supported by fluid contained in equipment that incorporates one or more gas-solid separator (usually cyclones), as well as recirculation piping as an integral part of the system. These have become popular, mostly for calcination, energy, and metallurgical operations [6].

Commercial fluidized bed reactors are now among the largest chemical reactors in the world. For example, in China fluidized bed combustors have reached a power capacity of 660 MWe [7].

1.2 Terminology

As in other fields, specialized terminology is used by the fluidization community. Definitions of the following terms may be helpful for those new to the field:

• Agglomeration: Particles sticking together to form assemblies (agglomerates).
• Attrition: Break-up of particles due to collisions or other interactions and stresses.
• Bed expansion: Height of operating fluidized bed divided by static bed height or bed height at minimum fluidization.
• Bubbles: Voids containing few, if any, particles, rising relative to the particles above them and behaving in a somewhat analogous manner to bubbles in liquids.
• Choking: Collapse of dilute gas-solid suspension into dense phase flow when decreasing the gas velocity at constant solids flow. For different modes of choking, see [8].
• Circulating fluidized bed: Fluid and particles in relative motion in a configuration where there is no distinct upper bed surface and entrained particles are continuously separated and returned to the base of a riser.
• Cluster: Group of particles travelling together due to hydrodynamic factors.
• Dense phase: Gas-solid region where the concentration of particles is sufficiently high that there are significant particle-particle interactions and contacts.
• Dilute phase: Region where particle concentration is low enough that interparticle contacts are relatively rare.
• Downer: Vessel in which particles are contacted with a fluid while they fall downwards.
• Distributor: Horizontal plate with perforations, nozzles, or other openings or other means of introducing a fluidizing fluid to support the weight of particles and cause them to move while also supporting the dead weight of the particles when the flow of fluid is interrupted.
• Elutriation: Progressive selective removal of finer particulates by entrainment.
• Fines: Relatively small particles, typically those smaller than 37 or 44 µm in diameter.
• Fluid: Either gas or liquid, usually the former in the context of fluidization.
• Freeboard: Region extending from dense fluidized bed upper surface to top of vessel.
• Geldart powder group: See Chapter 2.
• Grid: Alternate name for gas distributor supporting the fluidized bed and assuring uniform entry of gas at its base.
• Loop seal: Common configuration (see Chapter 11) for recirculating solids to the bottom of a fluidized bed or riser without reverse flow of gas.
• Membrane walls: Containing wall consisting of vertical heat transfer tubes connected by parallel fins, commonly used in combustion applications (see Chapter 14).
• Membrane reactor: Reactor containing solid surfaces ("membranes") that are selectively permeable to one or more component of the gas mixture.
• Plenum chamber: Pressurized chamber below the distributor of a fluidization column from which fluidizing fluid is fed into the bed above the distributor.
• Riser: Tall column in which particles are carried, on average, upwards by an ascending fluid.
• Segregation: Tendency for particles of different physical characteristics (e.g. different size, density, and/or shape) to preferentially become more concentrated in different spatial regions.
• Solids: Generic term referring to solid particles.
• Superficial velocity: Volumetric flow rate of fluid divided by total column cross-sectional area.
• Voidage: Fraction of bed volume or local volume occupied by fluid.
• Windbox: Same as plenum chamber, but only when the fluidizing fluid is a gas.

Other terms are introduced and defined as needed in the text.

1.3 Applications

Gas-fluidized beds account for most of the commercial applications of fluidized beds. Relative to packed beds, gas-fluidized beds commonly offer the following advantages:

• Temperature uniformity (with variations seldom exceeding 10 °C in the dense bed and elimination of "hot spots.")
• Excellent bed-to-surface heat transfer coefficients (typically 1 order of magnitude better than in fixed beds and 2 orders of magnitude better than in empty columns.)
• Ability to add and remove particles continuously, facilitating catalyst regeneration and continuous operation.
• Relatively low pressure drops (essentially only enough to support the bed weight per unit cross-sectional area.)
• Scalable to very large sizes (e.g. there are commercial fluidized bed reactors hundreds of square metres in cross-sectional area.)
• Excellent catalyst effectiveness factors (i.e. very low intra-particle mass transfer resistances): With particles 1 order of magnitude smaller than in fixed beds, i.e. catalyst particles smaller than 100 µm, effectiveness factors usually approach 1.
• Good turndown capability: The gas flow rate can be varied over a wide range, typically by at least a factor of 2-3.
• Ability to tolerate some liquid: For example, in a number of processes, such as fluid catalytic cracking, liquids are sprayed into the column where they vaporize and then react.
• Wide particle size distributions (typically with a ratio of upper to lower decile particle diameter, dp90/dp10, of 10: 20).

These advantages must be significant enough to compensate for some significant disadvantages of gas-fluidized beds:

• Substantial vertical (axial) mixing of gas: Gas is dragged downwards by descending particles resulting in "backmixing" and large deviations from plug flow, with typical axial Peclet numbers of order 5-10.
• Substantial axial dispersion of solids: Vigorous motion of particles and their clusters results in substantial axial dispersion and backmixing of solids. As a result, in continuous processes, some particles spend very little time in the bed, while others spend much longer than the mean residence time.
• Bypassing of gas: Gas associated with a lower-density phase, e.g. rising as bubbles, passes through the bed more quickly and with less access to particles than gas associated with a denser phase in which there is better gas-solid contacting.
• Limitations on particles that can be successfully fluidized: Particles of extreme shapes (e.g. needle or flat disc shapes) or smaller than about 30 µm in mean diameter are difficult, or even impossible, to fluidize.
• Entrainment: Particles, especially fine ones, are carried upwards by the exhaust or product gas and leave the column through the exit. To minimize their losses, entrained particles must normally be continuously captured and returned to the bottom of the vessel.
• Attrition: Particles can break or be abraded when they collide/interact with each other and with fixed surfaces.
• Wear on surfaces: Particle motion tends causes erosion/wastage of fixed surfaces.
• Complexity and risk: Fluidized...