Heat Transfer from a Single Tube to the Flowing Gas-Solid Suspension in a CFB Riser

Abstract

The first published studies concerning heat transfer between flowing gas-solid suspensions (pneumatic conveying) and pipe walls appeared in the literature in the early 1960s. More recently, experimental investigations have been extended to circulating fluidized beds. Despite the relatively large number of studies undertaken, mostly in CFB-risers of limited dimensions, the variation of the CFB heat transfer coefficient with operating gas velocity and solids' loading could not be predicted accurately. In his review paper [1], Grace summarized the situation as follows: “no existing correlations give consistent agreement with the available data.” The situation in a circulating fluidized bed is indeed a complex one, with a core/annulus flow and clusters of particulates being formed, leading to a non-uniform distribution of the solids over the cross-sectional area of the riser. Existing correlations largely ignore these flow characteristics. Experimental investigations using a FCC catalyst were carried out in a 100 mm ID riser, heated from an embedded heat transfer probe. Superficial velocities up to 8 m/s and solid circulation rates between 10 and 50 kg/m2s were used. Experimental results for the operation at a constant gas flow rate reveal an initial nearly constant heat transfer coefficient for low solids' loading, followed by a subsequent significantly increasing heat transfer coefficient with increasing solids' loading. Increasing the gas flow rate at a constant solids' loading significantly reduced the heat transfer coefficient. The experimental results were correlated with predictions from the theoretical approach of Molodtsof and Muzyka [2, 3]. A good agreement was obtained.