Forms of Fluidized Beds

Throwaway processes are preferred for nonindustrial sites. Some form of fluidized bed combustion appears to be the leading candidate. 

Bubbles and Fluidized Beds. Bubbles, or gas voids, exist in most fluidized beds and their role can be important because of the impact on the rate of exchange of mass or energy between the gas and soflds in the bed. Bubbles are formed in fluidized beds from the inherent instabiUty of two-phase systems. They are formed for Group A powders when the gas velocity is sufficient to start breaking iaterparticle forces at For Group B powders, where iaterparticle forces are usually negligible, and bubbles form immediately upon fluidization. Bubbles, which are inherently... 

Over the past decades, advances have been made that reduce environmental impacts of coal burning in large plants. Some arc standard and others experimental. Limestone (mainly calcium carbonate) scrubber smokestacks react with the emitted sulfates from the combustion and contain the chemical products, thereby reducing the release of SO., into the atmosphere by a large factor (of ten or more). Pulverization of coal can also allow for the mechanical separation of some sulfur impurities, notably those in the form of pyrites, prior to combustion. Currently deployed—with more advanced versions in the development stage—are various t yies of fluidized bed reactors, which use coal fuel in a pulverized form, mixed with pulverized limestone or dolomite in a high temperature furnace. This technique reduces sulfate release considerably. There are... 

Porous property of these particles was considered to be based on crystallization mechanism of calcium phosphate, in which fine crystals formed in the bottom of fluidized bed attached to the fluidized seeds by driving force of supersaturation and fluid conditions. 

Porous or sintered plates are the ideal and are used in small-scale studies of fluidized bed behaviour (Kunii and Levenspiel, 1991) and form a highly expanded unstable gas-solid dispersion directly above the distributor which rapidly divides into a large number of small bubbles plus an emulsion phase. Bubbles grow rapidly thereafter by coalescence. Kunii and Levenspiel (1991) also suggest that other... 

One recent development in Ziegler Natta catalysts was in producing catalyst particles that expanded as the polymerization reaction occurred. In this polymer the catalyst remains dispersed throughout the polymer, retaining its activity. This led to the development of fluidized bed processes to make polyethylene and polypropylene in which a sphere of polymer formed around each initial catalyst particle, and the polymer remained sohd as the reaction proceeded, rather than requiring a liquid solution. A major class of these catalysts and fluidized bed reactor was developed by Union Carbide and by Shell Oil and called the Unipol process. In this process a very active solid catalyst is introduced into the reactor, and reaction occurs on the catalyst particles, which expand to maintain active sites on the growing polymer sphere. 

In this section, some analytical solutions of fluidized-bed models are presented. Specifically, model solutions will be given for the case of a gas-phase reactant and a single solid-catalyzed reaction of the form A —> products and bubbling fluidized bed (Type B fluidization). The same analysis holds for a reaction of the form A + B —> products, if the reaction depends only on the concentration of A. Some solutions for the cases of a single reversible reaction, for two reactions in parallel, and two reactions in series will be given as well. 

Incineration can be applied to all contaminants that have high heat content. It eliminates odor and leachate problems, which are usually associated with landfills, but metals can react with other elements in the feed stream, such as chlorine or sulfur, and form more volatile and toxic compounds. The process has high maintenance and operation costs. Straitz et al. (1995) reported that incineration is a reliable, cost-effective approach to treat VOCs and a wide range of objectionable gas. Chaouki et al. (1995) conducted experiments that examined the effect of fluidized bed combustion on the properties and characteristics of a soil contaminated with PCBs. Particle size distribution and PCB contents were determined before and after a 30-minute incineration. The incineration promoted agglomeration soil, increases in soil pH, and decreases in the PCB content to undetectable levels. 

Literature correlations to estimate heat and mass transfer coefficients are generally of the form Sh=a Re 0).ln general, they do not take into account the scale factors dp/D and dp/L which should be important, especially in the case of fluidized beds, given the complex hydrodynamics of these systems. 

The mechanism of interaction between catalyst and gas in a large fluidized bed is complex and is not well correlated with design factors. In the bed itself, large bubbles of a foot or more in diameter form and are irrigated with a rain of catalyst particles. This process occurs in parallel with a well-mixed fluidized bed. Above the bed level and before the entrained catalyst is recovered in cyclones, the reaction continues in dilute phase plug flow. Since even the physical behavior of fluidized beds is not well understood, the design of such... 

Features - particles of growing polymer form in fluidized bed - catalyst residence time 2-4 hours - morphology and psd of catalyst are important - previously (pre-1990s) restricted in range of comonomer that could be used because of emergence of "condensed" mode operation, a wide range of comonomers may now be used... 

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