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Hydrodynamics of gas spouting at high temperature Wu, Stanley W. M.


The spouted bed technique was developed for handling solids which were too coarse to fluidize well. In its early stages, it was primarily used for drying wheat. It was later found that spouting has potential application in high temperature operations such as coal combustion and gasification. However, literature review will show that there are very few reports on the hydrodynamics of spouted beds at high temperature and/or pressure. Most existing correlations or expressions are based upon experiments done at room conditions; they have not been tested with data from higher temperatures. The goal of this study was to obtain experimental data at high temperatures, to examine the validity of existing equations and to modify the latter where appropriate. Spouting of sand particles (Ottawa sand) with preheated air, ranging from 20 to 420 °C, was conducted in a 156 mm stainless steel half-column, equipped with a glass panel. The transparent surface allowed one to measure spout diameter, fountain height, annulus height and other important parameters which otherwise are difficult to obtain in a full stainless steel column. In addition to air, helium and methane, at room conditions, were also used as spouting gases. With these two gases, it became possible to investigate the effect of changing gas density at constant gas viscosity and the effect of changing gas viscosity at constant gas density. The main experimental measurements were of minimum spouting velocities, spout diameters, maximum spoutable heights and bed pressure drops. For selected runs, additional measurements, such as of flow regime maps, particle circulation rates, radial and longitudinal pressure profiles, fountain heights and annular fluid velocities, were also obtained. In general, it was found that the range of stable spouting decreased with decreasing gas density and increasing gas viscosity, hence with increasing air temperature. Some of the existing equations were found to be inadequate. The Mathur and Gishler (1955) equation was unsatisfactory when tested against the experimental values of Ums. The expression of Epstein and Levine (1978) gave good prediction of the overall bed pressure drop for room conditions but overestimated the effect of temperature. The McNab (1972) equation for estimating spout diameter worked reasonably well for air spouting at room temperature but it underpredicted at higher temperatures. These equations were empirically modified to fit the new data obtained.

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