Yang Fluidization, Solids Handling, and Processing

mFixed bed pressure drop exponent, dimensionless

nFluidized bed expansion exponent, dimensionless

578 Fluidization, Solids Handling, and Processing

θTime, sec

ΘDimensionless time in particle-fluid heat transfer; dimensionless subsidence time in bed collapsing test

µViscosity, gm/cm·sec

ρ= ρs - ρf, effective density

Φo Dimensionless pressure drop in accelerative particle motion

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580 Fluidization, Solids Handling, and Processing

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Three-Phase Fluidization Systems 583

The term three-phase fluidization, in this chapter, is taken as a system consisting of a gas, liquid, and solid phase, wherein the solid phase is in a non-stationary state, and includes three-phase slurry bubble columns, three-phase fluidized beds, and three-phase flotation columns, but excludes three-phase fixed bed systems. The individual phases in threephase fluidization systems can be reactants, products, catalysts, or inert. For example, in the hydrotreating of light gas oils, the solid phase is catalyst, and the liquid and gas phases are either reactants or products; in the bleaching of paper pulp, the solid phase is both reactant and product, and the gas phase is a reactant while the liquid phase is inert; in anaerobic fermentation, the gas phase results from the biological activity, the liquid phase is product, and the solid is either a biological carrier or the microorganism itself.

The important inherent differences in the operating characteristics between fixed bed systems and fluidization systems as given by Fan (1989) are summarized here. Fixed bed systems produce high reactant conversions for reaction kinetics favoring plug flow patterns because of the small axial dispersion of phases and low macromixing present in such reactors. Fixed bed systems offer the advantage of high controllability over product selectivity for complex reactions and low solids attrition and consumption, permitting the use of precious metals as catalysts. In fluidization systems, high macromixing with large axial dispersion is prevalent which produces high reactant conversions for reaction kinetics favoring completely mixed flow patterns. The high mixing in fluidization systems also yields uniformity in the temperature throughout the system and ease in temperature control. The non-stationary nature of the particles in a fluidization system allow for ease in continuous catalyst replacement and, hence, minimum flow maldistribution.

Fan (1989) provided a detailed historical development of threephase fluidization systems in reactor applications. Only a brief review of the significant accomplishments and the economic factors affecting the development of three-phase reactors will be provided here. Table 1 provides the important contributions in the application of three-phase fluidization systems for the past several decades. The direct liquefaction of coal to produce liquid fuels was the first commercial reactor application of three-phase fluidization systems, with development having occurred from the mid-1920’s throughout the 1940’s. A large effort was put forth at this time in Europe for the production of liquid fuels from coal as a direct

584 Fluidization, Solids Handling, and Processing

result of the need for such fuels during World War I and II. The commercial production of liquid fuels from direct coal liquefaction peaked at an average annual production of 4.2 million tons of primarily aviation fuel (Donath, 1963). At the end of the war, the direct production of liquid fuels from coal was phased out. Another important process which was developed during and prior to the 1940’s is the application of a three-phase reactor for the reactions known as Fischer-Tropsch (F-T) synthesis. These reactions produce liquid fuels via indirect coal liquefaction involving hydrogen and synthesis gas derived from coal gasification, in the presence of a catalyst. The peak production for the liquid phase F-T synthesis occurred in the early 1950’s at a daily production rate of 11.5 tons of liquid fuels (Kolbel and Ralek, 1980). Fischer-Tropsch synthesis ceased in the mid-1960’s because of the availability of relatively cheap crude oil. However, renewed interest in F-T synthesis in the early 1970’s, caused by the oil shortages of the time, lead to further studies in the commercial scale production of liquid fuels via F-T synthesis in slurry bubble column reactors. This development has continued throughout the 1980’s and into the 1990’s.

Table 1. Industrial Development and Application of Three-Phase

Fluidization Systems