Bubbling bed design

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In the fast bed, heat-transfer surface usually appears only in bed walls (or in curtain walls that partially divide the bed into segments). Bubbling-bed designs commonly provide horizontal tubes embedded within the bed. Such tubes are far more subject to erosion than the fast bed s vertical surface. 

EPRI contracted an economic study on the use of Texas lignite in atmospheric fluidized bed combustion (42), and projected that the costs of AFBC would be less than those for pulverized combustion plus flue gas desulfurization. However, while AFBC appears feasible for small-scale units (less than 50 MW ), there is some doubt that large scale utility AFBC systems can operate successfully. Part of this problem stems from the inadequacy of "bubbling bed" design normally used in AFBC newer designs, such as the circulating bed, offer more promise for commercial application and are being tested presently (32). 

For the low activity FCC catalysts then available, the bubbling bed design was a decided improvement over the first CFB reactor. Until the mid-1970s, virtually all FCC units maintained a dense phase bubbling or turbulent bed in the reactor vessel. A few of the second generation bubbling bed FCC reactors are still in operation [97]. 

Fluidized-bed process incinerators have been used mostly in the petroleum and paper industries, and for processing nuclear wastes, spent cook liquor, wood chips, and sewage sludge disposal. Wastes in any physical state can be applied to a fluidized-bed process incinerator. Au.xiliary equipment includes a fuel burner system, an air supply system, and feed systems for liquid and solid wastes. The two basic bed design modes, bubbling bed and circulating bed, are distinguished by the e.xtent to which solids are entrained from the bed into the gas stream. 

Extension of the Kunii-Levenspiel bubbling-bed model for first-order reactions to complex systems is of practical significance, since most of the processes conducted in fluidized-bed reactors involve such systems. Thus, the yield or selectivity to a desired product is a primary design issue which should be considered. As described in Chapter 5, reactions may occur in series or parallel, or a combination of both. Specific examples include the production of acrylonitrile from propylene, in which other nitriles may be formed, oxidation of butadiene and butene to produce maleic anhydride and other oxidation products, and the production of phthalic anhydride from naphthalene, in which phthalic anhydride may undergo further oxidation. 

Kunii and Levenspiel(1991, pp. 294-298) extend the bubbling-bed model to networks of first-order reactions and generate rather complex algebraic relations for the net reaction rates along various pathways. As an alternative, we focus on the development of the basic design equations, which can also be adapted for nonlinear kinetics, and numerical solution of the resulting system of algebraic and ordinary differential equations (with the E-Z Solve software). This is illustrated in Example 23-4 below. 

The second reaction vessel in a catalytic cracker is called the regenerator. The solid catalyst from the reactor is combined with a compressed air stream from an air blower, and the solid and gas phases flow upward into a bed of fluidized solid catalyst. The early designs used a bubbling bed reactor in which the velocity in the bed is slightly above the minimum fluidization velocity. More recent designs use a transport fluidized-bed reactor. A typical air-to-oil weight ratio is 0.54. 

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