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

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/m. 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.

[1]  Z. Yang,et al.  Particle motion in the CFB riser with special emphasis on PEPT-imaging of the bottom section , 2009 .

[2]  R. Jackson,et al.  Frictional–collisional constitutive relations for granular materials, with application to plane shearing , 1987, Journal of Fluid Mechanics.

[3]  O. Levenspiel,et al.  Drag coefficient and terminal velocity of spherical and nonspherical particles , 1989 .

[4]  S. Ogawa,et al.  On the equations of fully fluidized granular materials , 1980 .

[5]  Weixing Huang,et al.  Hydrodynamic similarity in circulating fluidized bed risers , 2008 .

[6]  Michael S. Detamore,et al.  A kinetic-theory analysis of the scale-up of circulating fluidized beds , 2001 .

[7]  Leon R. Glicksman,et al.  Scaling relationships for fluidized beds , 1984 .

[8]  Jamal Chaouki,et al.  Scaling considerations for circulating fluidized bed risers , 1992 .

[9]  C. Wen.  Mechanics of Fluidization , 1966 .

[10]  Jamal Chaouki,et al.  Effective drag coefficient investigation in the acceleration zone of an upward gas–solid flow , 2007 .

[11]  D. Jeffrey,et al.  Kinetic theories for granular flow: inelastic particles in Couette flow and slightly inelastic particles in a general flowfield , 1984, Journal of Fluid Mechanics.

[12]  David G. Schaeffer,et al.  Instability in the evolution equations describing incompressible granular flow , 1987 .

[13]  Jamal Chaouki,et al.  Hydrodynamics of circulating fluidized bed risers: A review , 1995 .

[14]  L. R. Glicksman,et al.  Scaling relationship for fluidized beds , 1984 .