KINETICS, CATALYSIS, AND REACTION ENGINEERING Radial Hydrodynamics in Risers Larin Godfroy, † Gregory S. Patience,* ,‡ and Jamal Chaouki † Department of Chemical Engineering, E Ä cole Polytechnique de Montre ´ al, C.P. 6079, Succursale Centre-Ville, Montre ´ al, Que ´ bec, Canada H3C 3A7, and E.I. du Pont de Nemours & Company, Wilmington, Delaware 19880-0262 On the basis of the benchmark modeling exercise at Fluidization VIII, predicting riser hydrodynamics continues to be more of an art than a science. Ten different hydrodynamic models were compared with a set of experimental data that covered a wide range of operating conditions and showed reasonable to poor overall agreement. Herein, we describe the model that gave the best overall agreement with the experimental data. Density is calculated by a correlation based on slip factor, and the radial voidage profile depends solely on the cross-sectional average void fraction. Both the gas and velocity profile follows a power law type expression; the gas velocity at the wall is zero. The model predictions agree well with experiments conducted with sand but not as well as those conducted with fluidized catalytic cracking catalyst. 1. Introduction Circulating fluid bed (CFB) hydrodynamic models are useful for understanding gas-solids mixing, scale-up, plant optimization, and control. Hydrodynamics impact reactor performance: conversion, selectivity, and heat transfer. Furthermore, CFB riser operating conditions affect the efficiency of downstream equipment such as cyclones, filters, standpipes, and so forth. Hydrodynamic modeling is useful for understanding and optimizing plant conditions, but they do not offer the level of confidence required to design, a priori, a new com- mercial plant. Rather, new commercial facilities are designed on the basis of extensive piloting and conser- vative extrapolations of pilot-plant basic data. Pilot plants, at a sufficiently large scale, minimize the risk of projecting performance to commercial scale and provide the means with which to test alternative designs rapidly and economically. Three examples of this ap- proach include the NUCLA power generation facility (1), Mobil’s short contact time catalytic cracker (2), and DuPont’s butane to maleic anhydride process (3). In the last two decades, significant advances have been made in experimental measurements of riser hydrodynamics and a number of models have emerged to characterize these data. However, most models are developed on the basis of a limited data set and their extrapolation to conditions outside the range is not well- documented. For this reason J. Chen proposed a “bench- mark modelling exercise” to compare model predictions against unpublished experimental data that cover a wide range of operating conditions. T. Knowlton pre- pared the exercise and invited modelers to predict the axial pressure drop, radial void fraction, and mass flux in two different risers. He disclosed the CFB geometry, particle characteristic, and operating conditions. Ten teams accepted this challenge and T. Knowlton, D. Geldart, and J. Matsen presented the results of the exercise at Fluidization VIII. In this paper, we discuss the benchmark modeling database and describe in detail the model proposed by Chaouki, Godfroy, and Patience. Throughout this dis- cussion, we highlight some difficulties in measuring experimental data and the strengths and weaknesses of our model. 2. Design Considerations Operational flexibility is of particular importance in many CFB applications. In both combustion and fluid catalytic cracking (FCC), operators often require the ability to treat a variety of feedstocks. Flexibility is an advantage of CFB technology but, at the design stage, this flexibility often translates into uncertainty. The largest uncertainty relates to predicting the solid volu- metric fractionssolid holdup or inventorysas a function of geometry and operating conditions. Holdup increases with an increasing solid circulation rate and decreases with an increasing gas velocity. The solid holdup not only affects the riser pressure drop but may also affect reactor performance: for example, in FCC units, higher solid holdup, resulting from increasing the solid circula- tion rate, may alter the temperature profile and, thus, the hydrocarbon product distribution. Together with an increasing temperature, an increased inventory affects the specific reaction rates. Figure 1 is a simplified schematic of the principle interactions between reaction kinetics and hydrody- namics at the design stage. To meet economic objectives, the process equipment size should be minimized and process yields maximized (conversion, X, and selectivity, S). Process equipment sizing depends on both the overall catalyst inventory and gas volumetric flow rates. There- fore, process design is an exercise in minimizing the * To whom correspondence may be addressed. † E Ä cole Polytechnique de Montre ´al. ‡ E.I. du Pont de Nemours & Co. 81 Ind. Eng. Chem. Res. 1999, 38, 81-89 10.1021/ie960784i CCC: $18.00 © 1999 American Chemical Society Published on Web 12/03/1998