A model for the pressure balance of a low density circulating fluidized bed

The pressure balance along the solid circulation loop of a circulating fluidized bed equipped with a solid flux regulating device has been modelled and the influence of the pressure balance on the riser behaviour has been predicted. The solid circulation loop has been divided into many sections, where the pressure drop was calculated independently: riser, cyclone, standpipe, control device and return duct. A new theoretical model, that is able to predict the pressure losses in the return path of the solid from the standpipe to the riser, has been built. A new correlation for cyclone pressure loss with very high solid loads has been found on the basis of experimental data. The pressure loss in the riser has been calculated by imposing the closure of the pressure balance, (sigma)(delta)P = 0. Once the riser pressure drop had been calculated, the holdup distribution along the riser was obtained by imposing a particular shape of the profile, according to the different fluid-dynamics regimes (fast fluidization or pneumatic transport). In the first case, an exponential decay was imposed and the bottom holdup was adjusted to fit the total pressure drop, in the second case, the height of the dense zone was instead varied. The experimental data was used to develop the sub-models for the various loop sections have been obtained in a 100mm i.d. riser, 6m high, CFB. The solid was made of Geldart B group alumina particles. The tests were carried out with a gas velocity that ranged between 2 and 4 m/s and a solid flux that ranged between 20 and 170 kg/m2s. A good agreement was found between the model and experimental data.