VERTICAL SLUG FLOWS OF GAS-SOLID TRANSPORT IN CIRCULATING FLUIDIZED BEDS

Abstract

In gas-solid slug flows in circulating fluidized beds, two different slugs can be distinguished: round-nosed and square slugs. Separated mechanistic models are proposed to describe these different slugs. Based on the models round-nosed slugs are shown most likely to behave like fast bubbles and square slugs to behave like slow bubbles. New criteria are established to predict the type of slug flow under which a circulating system will operate. Experiments of slugging transport bed were conducted using 1050 μm sand in 6.35 and 10.16 cm columns. Predictions of the present criteria satisfactorily agree with the data obtained in this study and the observations of the dense bed in Battel le's Multisolid Circulating Fluidized Bed Combustor being operated commercially. The criteria are verified to be more general than Stewart's criterion (1965) for the demarcation of different types of slug flows. The present models are also more general than Chen and Fan's (1987) by considering a more general case of continuous solid flow in the circulating system. The rise velocity of any slug can be described by a single equation by the models, US = k1▪ + k2 (gD)1/2, where k1 is greater than 1 for the round-nosed slug flow and is less than 1 for the square slug flow. The slower rise velocity of square slugs is attributed to the significant wall friction of non-fluidized particles.