With the advent of readily-available high-performance computing hardware, the use of computational fluid dynamics (CFD) in the study, design, and scale-up of gas-solid flow reactors and process equipment has become commonplace. In research-oriented studies, reactor sizes are generally limited to smaller-scale reactors facilitating comparison with experimental data and computational tractability owing to smaller mesh sizes. In contrast, the industrial practitioner is most interested in simulating industrial-scale equipment either to understand full-scale performance in the context of new process development or to troubleshoot and remediate under-performing production assets. Agrawal et al(2000) have indicated that the mesh resolution used in simulating rapid gas-solid flows should not exceed ten particle diameters if standard interphase drag laws are used and solution accuracy is to be preserved. For the industrial practitioner seeking to simulate full-scale process equipment such as a 30 ft diameter x 25 ft tall FCC regenerator bed with 75 mm Sauter mean FCC catalyst, the recommended 750 µm mesh size poses a formidable simulation challenge. For this case, simulating even 60 seconds of real time at the required mesh resolutions would require impractically long computing times using today’s state-of-the-art hardware.
Filtered gas-solid flow models can offer relief from the large computational loads required for accurate simulation of commercial-scale equipment. As discussed by Igci et al(2008), the filtered model seeks to replace the traditional micro-scale drag laws with drag laws better suited for the larger cell sizes required for timely commercial-scale simulations. The development of such meso-scale drag laws is done by performing a number of simulations in a periodic domain and filtering the results using filters of various sizes. These filtered results are consolidated into a Richardson-Zaki-type relation that can be used across various cell sizes without compromising accuracy.
In this presentation, industrial experience involving the use of a filtered two-fluid model for simulating reacting gas-solid flow in a commercial-scale FCC regenerator will be discussed. The discussion will emphasize hardware modifications conceived with the aid of CFD analysis and before-and-after comparisons.
Agrawal, K., P. N. Loezos, M. Syamlal, and S. Sundaresan, “The Role of Meso-Scale Structures in Rapid Gas-Solid Flows”, J. Fluid Mech., 445, 151 (2001).
Igci, Y., A. T. Andrews, S. Pannala, T. O’Briens and S. Sundaresan, “Filtered two-fluid models for fluidized gas-particle suspensions” AIChE J., 54, 1431-1448 (2008).
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