(503b) The Axial Solids Hold-up Profile in the Riser of a Cfb

There is a literature discrepancy about the hydrodynamic fluidisation mode in the riser as function of the superficial gas velocity (U) and the solid circulation flux (G), with the possible existence of three types of hold-up profiles: -a constant dilute profile in the dilute regime -an S-shaped profile in the regime often referred as ?fast fluidisation? -a constant and dense profile in the dense regime. Continuous transitions between these three regimes can be found by varying U and or G. As the research concentrates on gas/solid reactions, operating at low to moderate values of G compared with catalytic gas reactions, the dense regime is of less importance and was not experimentally investigated.

In order to define the transition between the dilute and S-shaped profile, solids hold-up profiles were measured at combined (U,G)-values. The zone of recycling bed material from the L-valve was moreover investigated using the PEPT-technique with 18F-labelled tracers.

Experimental data will be illustrated and correlated together with literature data. The transition from dilute to S-shaped profile is obtained at G > 3.9 U1.8.

The recycle loop can impose a high pressure drop on the riser, even at small G-values. In order to satisfy the pressure balance of the CFB, an S-shaped profile has to exist even at small values of G. This effect of the re-entry zone was measured by PEPT and expressed in an acceleration length, i.e. the height above the L-valve needed by the particle to achieve its stable particle velocity. Results are again Expressed as height vs. (U,G), and compared with a theoretical model to predict the acceleration zone. Experimental and model results are in very good agreement. In general, the re-entering particles will spend a time less than 5 to 7.5 % of their residence time in the riser, hence without major importance for calculating conversions in function of this residence time.

Literature data moreover enable us to define the transition between the S-shaped and dense profiles, obtained at G > 11 U1.8.

The simple equations proposed by the research prove of better accuracy than the more complicated correlations found in literature.

*Corresponding author: Professor Jan Baejens; e-mail: J.Baejens@bham.ac.uk; tel. +44 121 4145343; fax. +44 121 414 5323