Two hydrodynamic models of fluid catalytic cracking riser reactor are compared. In both of these models, the catalyst is assumed to move in clusters. In the first model, a force balance on the clusters is made ignoring the buoyancy force and pressure drop for calculating the solid and gas phase hold up along the riser height. In the second model, force balance on the clusters is made considering buoyancy force and pressure drop for calculating the phases' hold up. These models also consider the effect of volume expansion due to cracking on the phases' hold up.

A new cracking scheme based on pseudo components cracking is used with these hydrodynamic models for predicting the conversion and products'yields all along the riser height. Fifty pseudo components (lumps) are considered in the cracking scheme used in these model. This kinetic scheme is discussed in detail elsewhere. The rate constants are also predicted by using a semi-empirical function developed for this cracking scheme. The heat of cracking is estimated by taking the difference in the heats of combustion of product and reactant pseudo components. Effect of catalyst deactivation is also incorporated in the models. Since, thousands of cracking reactions are considered to be occurring in parallel in the riser reactor, for handling these reactions, the riser is divided into a number of small volume elements placed over each other in series.

The yield and velocity profiles predicted by both models compare well with the data published in literature. The incorporation of pressure drop in the model gives more realistic velocity profiles for solid and gas phases, which in turn improves the predictions of the model.