Experiments in 3 different CFB risers have confirmed that common riser operations can be hampered in a well-defined (U, G) range where choking occurs. Geldart A-type powders were investigated. Experimental results of the choking velocity were empirically correlated, being about 30% lower than predicted by the correlation of Bi and Fan, but largely exceeding other predictions. Introducing the findings into the operation diagram presented by Mahmoudi et al. adds a region where stable riser operation is impossible. The adapted diagram enables CFB designers to better delineate the operating characteristics. 1. Introduction CFBs are widely used in the chemical, mineral, environmental, and energy process industries. Several authors stressed the need for a clear identification of the different operation regimes in the riser of a CFB, to ensure a better comprehension of the hydrodynamic context and thus correctly design the loop. First approaches to develop a “work map” of the riser operation were presented by, for example, Grace [1], Yerushalmi and Avidan [2], and Bai et al. [3]. It was further developed by Chan et al. [4] and Mahmoudi et al. [5, 6] for both Geldart A- and B-type powders: the operating gas velocity ( ) and the solids circulation flux ( ) jointly delineate different regimes, called, respectively, dilute riser flow (DRF), core-annulus flow (CAF) (possibly with a turbulent fluidized bed at the bottom of the riser (TFBB)), and dense riser upflow (DRU). For a given powder and its associated transport velocity, , the combination of and will determine the flow regime encountered. Common riser operations can, however, be hampered in a specific range where choking occurs, being understood as the phenomenon where a small change in gas or solids flow rate prompts a significant change in the pressure drop and/or solids holdup: the stable riser upflow regime is no longer maintained when values exceed a certain limit for a low-to-moderate gas velocity. Considerable efforts have been made in probing choking in CFBs, and several empirical equations have been proposed, as summarized in the following. Literature Correlations to Predict Choking Velocities, . The following equations were proposed. Leung et al. [7] Matsen [8] Yousfi and Gau [9] Bi and Fan [10] The objective of the present research hence considered (i) the delineation of the choking phenomenon in different riser geometries and using different A-type powders, (ii) the comparison of experimental results with the empirical predictions of (1)–(4), and finally (iii) the adaptation of the proposed
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