Influence of temperature and properties of solids on the size and growth of bubbles in gas fluidized beds

Abstract

In the present paper, models are developed for the description of coalescence and splitting of bubbles in gas fluidized beds. The distinction between slow and fast bubbles, which originates from the Davidson model of gas flow in and around a rising bubble, was found to be decisive for the modelling of coalescence processes. On the basis of the respective models, it is shown that a change in bed temperature does influence the mechanisms of both bubble coalescence and splitting. In cases of both slow and fast bubbles undergoing splitting, the theory predicts a decrease of bubble size with temperature whereas in the case of a nonsplitting system with fast bubbles, typically encountered with Geldart type B particles, no significant temperature effect on bubble size is expected. The theoretical predictions are shown to be in agreement with own measurements as well as with the results of other workers.

Investigation of the bubble behaviour at the free surface of a large three-dimensional gas fluidized bed

“…The bubble fraction which is also called the bubble hold-up by Hilligardt and Werther (1987) or the bubble density by Johnsson et al (1991) may be determined from several different equations given in the literature.…”

“…Theories related to the absolute rise velocity of a bubble (Davidson and Harrison, 1963;Gera and Gautam, 1995), superficial gas velocity and minimum fluidisation velocity (Ergun, 1952;Kunii and Levenspiel, 1969;Thonglimp et al, 1984) exist in the literature. Also, numerous correlations have been developed for predicting the size of a bubble as a function of fluidised conditions (Werther, 1983;Hilligardt and Werther, 1987;Lim et al, 1990;Cai et al, 1994).…”

“…In general, the techniques used can be divided into two groups on the nature of the sensors contacting the bubble phase (Lim et al, 1990) except for direct visual observation (Hatate et al, 1985). Miniatere capacitance/electroresistivity probes (Lanneau, 1960;Toei et al, 1965;Rigby et al, 1970;Werther and Molerus, 1973;Tomita and Adachi, 1973;Cranfield and Geldart, 1974;Yutani et al, 1983;Hilligardt and Werther, 1987), optical probes (Glicksman et al, 1987;Mainland and Welty, 1995) and static pressure probes (Weimer and Quarderer, 1985) are classified as internal sensors. Such sensors make direct contact with bubbles.…”

Mass Transfer Coefficient for Drying of Moist Particulate in a Bubbling Fluidized Bed

“…Among the many available correlations, the following equations have been found to provide good matches with the experimental measurements: [17] and Hilligardt and Werther [18].…”

Fluidized-Bed Reactors

“…For more exact calculations, a differential equation for bubble growth should be used in the case of Geldart group A and B solids [47], [48]:…”