Pjg. Huttenhuis et al., THE EFFECT OF GAS-PHASE DENSITY ON BUBBLE FORMATION AT A SINGLE ORIFICE IN A 2-DIMENSIONAL GAS-FLUIDIZED BED, Chemical Engineering Science, 51(24), 1996, pp. 5273-5288
In this study the effect of the gas-phase density on the process of bu
bble formation at a single orifice in a two-dimensional gas-fluidized
bed has been studied experimentally and theoretically. Specifically, a
detailed comparison between experimentally observed and theoretically
calculated bubble growth curves has been made in the case where the d
ensity of the gas injected through the orifice (He and SF6) differs si
gnificantly from the density of the primary fluidizing agent (air). Th
e calculations have been carried out using an earlier developed, first
principles hydrodynamic model of gas-fluidized beds which has been ex
tended with a species conservation equation to calculate the compositi
on of the fluidizing gas in the vicinity of the evolving bubbles. Besi
des, the present experimental and theoretical results were compared wi
th predictions obtained from adapted versions of approximate bubble fo
rmation models previously reported in the literature. The advanced hyd
rodynamic model appears to predict the experimentally observed diamete
rs satisfactorily. In addition, the model correctly predicts the effec
t of the gas-phase density on the experimentally observed bubble growt
h. This effect can be explained satisfactorily in terms of the depende
nce of the interphase momentum transfer coefficient on gas-phase densi
ty. Finally, calculations with a three-dimensional version of our hydr
odynamic model have been carried out to account for the effect of the
front and back wall of the pseudo two-dimensional gas-fluidized bed us
ed in our experiments. Our preliminary computational results indicate
that the magnitude of the wall effect strongly depends on the boundary
condition enforced for the gas-solid dispersion at these walls. In th
e case that the no-slip boundary condition was enforced in the calcula
tions for the solid phase, the wall effect was significant and a consi
derable deviation between computed and experimentally observed bubble
growth curves was found. However, when a more realistic partial slip b
oundary condition for the solid phase was implemented the agreement be
tween theory and experiment could be improved by altering the slip par
ameter in the partial slip boundary condition expression. Copyright (C
) 1996 Elsevier Science Ltd