Radial Hydrodynamics in Risers

Abstract

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.

Drift flux modelling of CFB risers

“…Agreement with the FCC data was poor although the general trends were reasonable. The predicted solids mass flux at the centre line was approximately a factor of two greater than the measured value and solids downflow along the wall was over-predicted (Godefroy et al, 1999). However, by simply changing the exponent, , from 0.4 to 0.27, the fit between the experimental data and model is excellent, as shown in Figure 7.…”

“…There are few pressure taps to cover the entire 27 m but the data would suggest that the entrance and exit effects are minimal: the suspension density decreases linearly with height and the decrease is proportional to the increase in gas velocity. Void fraction data collected by Knowlton (1995) for the CFB Workshop are reproduced in Figure 5 (Godefroy et al, 1999) together with pilot plant data at similar operating conditions. FCC catalyst with a density of 1712 kg/m 3 and an average particle diameter of 76 m was used in the study and the diameter of the riser was 0.2 m. The experiment was conducted at ambient conditions with air.…”

“…The data were generated in two risers with a diameter of 0.2 and 0.4 m and three types of particles. Knowlton measured the longitudinal pressure drop as well as the radial profile of the axial mass flux and void fraction at gas velocities ranging from 2.4 and 11 m/s and mass fluxes as high as 800 kg/m 2 s. The comparison between the experimental data and the mathematical modelling was generally poor: some models could account for some of the reported trends in the data but more work was required to capture entrance region affects and the affect of particle diameter on solids hold-up (Godefroy et al, 1999). Experimental studies continue to swell the scientific literature with an ever increasing number of riser conditions, geometries, applications and powder properties.…”

“…Godefroy et al (1999) reported an expression for ␥ for a value of ␤ equal to 6 and = 0.4. A more general form for the equation is:…”

“…Progress has been made on computational fluid dynamic modelling but it would appear that empirical correlations are more practical and at least as accurate at characterizing the radial and axial solids hold-up (Godefroy et al, 1999). A more ambitious effort has been undertaken to develop a generic fluidized bed reactor model that spans the flow regimes (Abba et al, 2003).…”