Effect of Change in Fluidizing Gas on Riser Hydrodynamics and Evaluation of Scaling Laws

• Published in: Industrial & engineering chemistry research 2011-04, Vol.50 (8), p.4697-4706
• Main Authors: Ellis, Naoko, Xu, Min, Lim, C. Jim, Cloete, Schalk, Amini, Shahriar
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical engineering; Exact sciences and technology; General Research; Hydrodynamics of contact apparatus
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/ie101141f

Riser hydrodynamics in a circulating fluidized bed are investigated experimentally and numerically by changing the composition of the fluidizing gas. Helium gas is added to air in the range of 0 to 96 vol % for fluidizing FCC particles 78 μm in size and density of 1560 kg/m3. Increasing He concentration decreases the fluidizing gas density and viscosity. The effect of change in gas composition is measured through the change in voidage along the riser from the pressure drop and the solids circulation rate, while analyzed by changes in slip velocity and particle Reynolds number. Numerical simulations using Computational Fluid Dynamics (CFD) have successfully predicted the experimental measurements over the entire range of fluidizing gas densities investigated. Simulations have also revealed that interphase momentum exchange in the bottom, accelerated region of the riser is dominated by cluster formation, while individual particle drag was dominant in the upper, more dilute regions. Given that CFD simulations have successfully reproduced these results, a scaling scheme is investigated whereby a hot model unit is simulated keeping either the Archimedes number or the density ratio of particle to gas constant. The results indicated better agreement between experimental and numerical voidage profiles for the density ratio scaling. Using the full set of scaling laws produced excellent prediction of the upper, fully developed region of the riser, but failed in the bottom regions. The scaling error in the bottom region was attributed to the momentum interaction in this region being dominated by cluster formation and not by the drag force on individual particles for which the scaling laws were derived. CFD has shown to be an effective tool in evaluation of scaling laws in risers.