Gas—solid fluidization

Gas—solids fluidization is the levitation of a bed of solid particles by a gas. Intense soflds mixing and good gas—soflds contact create an isothermal system having good mass transfer (qv). The gas-fluidized bed is ideal for many chemical reactions, drying (qv), mixing, and heat-transfer appHcations. Soflds can also be fluidized by a Hquid or by gas and Hquid combined. Liquid and gas—Hquid fluidization appHcations are growing in number, but gas—soHds fluidization appHcations dominate the fluidization field. This article discusses gas—soHds fluidization. 

Fluidized-Bed Combustion The principles of gas-solid fluidization and their application to the chemical process industry are treated in Sec tion 17. Their general application to combustion is reviewed briefly here, and their more specific application to fluidized-bed boilers is discussed later in this section. 

Cheremisinoff, N. P. and P. N. Cheremisinoff, Hydrodynamics of Gas-Solids Fluidization, Gulf Publishers, Houston, TX, 1984. 

Marmo, L., Rovero, G., and Baldi, G., Modeling of catalytic gas-solid fluidized bed reactors, Catal. Today, 52, 235-247 (1999). 

Bi, H. T., Grace, J. R., and Zhu. J., Propagation of Pressure Waves and Forced Oscillations in Gas-solid Fluidized Beds and their Influence on Diagnostics of Local Hydrodynamics, Powder Technol., 82 239 (1995)... 

Van Swaaij, W. P. M., and Zuiderweg, F. J., The Design of Gas-Solids Fluidized Beds - Prediction of Chemical Conversion, Proc. Int. Symp. on Fluidization and itsAppl., Ste Chimie Industrielle, Toulouse (1973)... 

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Source Adapted from various sources M. A. Elliot, Ed., Chemistry of Coal Utilization, 2nd Supplementary Volume, Wiley-Interscience, 1981 Cheremis-inoff, N, and P. Cheremisinoff, Hydrodynamics of Gas-Solid Fluidization, Gulf Publishing Co., 1984 Joaquin, R. H., Kinetic and Hydrodynamic Study of Waste Wood Pyrolysis for its Gasification in Fluidized Bed Reactor, European Foundation for Power Engineering, 2002 (http //www.efpe.org/papers.html). 

In considering heat transfer in gas-solid fluidization it is important to distinguish between, on the one hand, heat transfer between the bed and a heat transfer surface (be it heated bed walls or heat transfer coils in the bed) and, on the other hand, heat transfer between particles and the fluidizing gas. Much of the fluidization literature is concerned with the former because of its relevance to the use of fluidized beds as heterogeneous chemical reactors. Gas-particle heat transfer is rather more relevant to the food processing applications of fluidization such as drying, where the transfer of heat from the inlet gas to the wet food particle is crucial. 

For ethanol production a substantial improvement over the packed bed reactor is offered by the gas-solid fluidized bed fermenter which uses a liquid instead of a solid substrate and replaces the packed bed of substrate or micro-organisms with a fluidized bed of yeast pellets (Smith et al, 1997). This system can be viewed as a standard fermentation against which the performance of different bioreactors may be assessed. 

A theoretical model (Beck and Bauer, 1989), based on ethanol inhibition alone as the limiting factor in gas-solid fluidized bed fermenters run with recirculating inert gas, suggested that the potential of this technique has not been fully explored. Hayes (1998) suggested significant improvements to the model and provided experimental confirmation of its validity. 

A summary of the factors which are known to influence ethanol production from glucose in a gas-solid fluidized bed fermenter, or which may have an influence based on observations with submerged fermentations, is shown in Figure 6.1. In anaerobic beds, the key factors are the fermentation temperature and ethanol inhibition, both of which have a dramatic effect on the specific rafe of ethanol production. Bed dehydration and its influence on yeast pellet moisture content is also important, since a failure of fermentation may occur if the pellets become too dry (Bauer, 1986). 

Figure 6.1 Factors affecting ethanol production from glucose using baker s yeast (S. cerevisiae) in a gas-solid fluidized bed fermenter. Reproduced from Hayes (1998) with permission.

The two inert gases used in anaerobic gas-solid fluidized bed fermenters are carbon dioxide (Moebus and Teuber, 1982a) and nitrogen (Rottenbacher, 1985). The former found that the addition of air at the beginning of the run substantially increased carbon dioxide production compared to a bed run under pure carbon dioxide. An identical bed operated under strict anaerobiosis appeared to ferment glucose... 

Figure 6.2 is a schematic diagram of the anaerobic gas-solid fluidized bed fermenter used by Hayes and co-workers (Hayes, 1998 Smith... 

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