(483d) The Residence Time Distribution of the Gas Phase in the Riser of a Circulating Fluidised Bed

An increasing interest arises in new applications of the circulating fluidised bed for gas catalytic reactions, in addition to the well-established catalytic cracking (FFC). Gas mixing in the riser of the CFB becomes an important parameter. The present paper focuses on the riser of the reactor, where reactions take place. A literature review on gas mixing is presented, together with experimental results obtained in a 0.1 m I.D. riser. A propane tracer is used. The superficial gas velocity was varied between 2 and 10 m/s for gas circulation fluxes up to 180 kg/m².s.

The behaviour is different when the bed material is inactive, such as sand (90 µm), or active, such as catalyst (68 µm). In the inactive case, tests at different working conditions of gas velocity and solids circulating rate showed that only a minor amount of gas mixing is detected in the riser and that the residence time can be calculated assuming plug flow, at specific combinations of gas velocity and solids circulation flux. When adsorption of the gas on the catalyst takes place, there is a significant amount of gas back mixing (adsorbed on the reflux particles near the wall), and the plug flow approach no longer holds.

Experimental results will prove this different behaviour and will be used (i) to assess common RTD-models such as plug flow, perfect mixing, plug flow with dispersion, fractional tubularity etc., (ii) to develop empirical equations that will describe the average residence time and the RTD-curve, and finally (iii) to develop a model for gas mixing in the riser of a CFB.

At certain combined values of gas velocity and solids flux, the core-annulus regime prevails, whereas the CFB operates in a near-plug flow mode at high velocities and/or low solids fluxes.

Illustration of the design approaches will be given for the case of the acrylonitrile and aniline production in a CFB.

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