Comparative study of Transport Disengaging Height (TDH) correlations in gas–solid fluidization

Abstract

Transport Disengaging Height (TDH), defined as the freeboard height whereby the entrainment rate does not change appreciably [1–6], is an important parameter in the design of gas–solid fluidized bed systems to minimize particle loss. Unfortunately, despite the initiation and subsequent development of TDH correlations since 1958 [4], poor agreement between predicted and experimental values persists [1] due to the reliance on empirical data-fitting in the absence of a fundamental understanding of the TDH phenomenon. Accordingly, this work aims to provide a comprehensive review of the available TDH correlations.TDH values predicted by 25 correlations were evaluated over a range of superficial gas velocities, particle sizes, particle size distributions, and column diameters. Four observations are worth highlighting: (i) Discrepancies of up to five orders of magnitude were found among TDH values predicted by the various correlations, (ii) Unphysical phenomena predicted include negative TDH values, (iii) Geldart Group B correlations perform better for Geldart Group B particles than Geldart Group A correlations for Geldart Group A particles, and (iv) Prediction often fails in the transition from the freely bubbling to the slugging regime.The ad hoc inclusion and/or exclusion of parameters, and the use of empirical constants to improve empirical data-fitting in the generation of empirical TDH correlations are not useful in either improving predictions of TDH values or advancing the understanding of the TDH phenomenon. Correlations empirically derived do not perform well beyond the narrow scope of experimental condition tested, while semi-empirical or theoretical models available fall short. The lack of predictive capability of the available TDH correlations appears to stem from a deficiency in an understanding of the impact of inter-particle (e.g., cohesion or clustering effects) and inter-species interactions (e.g., collisional momentum transfer effects).