Pantopikou, Constantinia; (1992) Hydrodynamics of a circulating fluidised bed. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The principle of circulation of solids in a gas-solid system had led to the development of a number of processes. These processes in the form of circulating fluidised beds (CFB) are usually large-scale operations, with the fluid catalytic cracking process (FCC) and combustion as the major examples. In this work pressure drop and solid circulation rate measurements have been conducted during the operation of a circulating fluidised bed (CFB) consisting of a riser, a solid collecting system (cyclones), a slow bed, a ball valve and a riser distributor. Most of the experiments have been conducted with FCC powder of average particle diameter of 64 μm and particle density of 884 kg/m+. Some further experiments were conducted with either pure Poralox powder of particle diameter of 127 μm and density of 1850 kg/m+ or a 50/50 v/v mixture of Poralox and FCC. The gas velocities were varied between 1.5 and 4 m/s. The three riser distributor plates had the same free percentage of open area, divided into 1 nozzle, 7 and 13 holes. The pressure profiles around the circulating fluidised system should be such that the higher pressure drop values should be located above and below the ball valve for safe operation of the riser type reactors. Increasing the slow bed inventory from 21 to 60 cm resulted in an increase in the riser pressure drop and hence in the axial solids fraction present in the riser. The design of the distributor plate affects the performance of the CFB. For the same operating conditions using the 7 and 13 hole distributors much higher solid circulation rates were observed compared with the 1 nozzle distributor. Increasing the density and the size of particles from FCC to Poralox leads to higher solid circulation rates but also higher pressure drops. Operating the CFB at high opening areas of the ball valve will normally leads to a downflow of solids in an annulus adjacent to the riser wall. If the ball valve is fully open then the CFB cannot be operated without instabilities. The Dense Annulus Lean Core model (DALC) is proposed which relates the pressure drop along the axis of the riser to particle and fluid properties. The DALC model is shown to give good predictions, both qualitatively and quantitatively when the CFB is operated so as to achieve high solid circulation rates. The DALC model is then applied to the case in which a first order catalytic reaction takes place in the riser. By increasing the reaction rate constant to 15s-1 the model indicates a conversion of 55 % in the downflow region within a distance of less than 3m above the distributor. By increasing the gas velocity in the riser the model predicts that the conversion of a first order reaction decreases.

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