Riser reactors are extensively used for fluid catalytic cracking, combustion of coal, calcination of cement raw meal etc. In riser reactors, particles are introduced into a flowing fluid at the bottom of a vertical column. Particles are accelerated by fluid drag (termed as the acceleration region) along the riser height to a limiting value (termed as the terminal region). Even in the terminal region, gas to particle slip velocities are observed to be far greater than the terminal velocity of a single particle and this is attributed to particles moving as aggregates or clusters.

In the present model, particles entering at the bottom of the riser are assumed to aggregate as clusters and accelerate upwards due to drag force exerted by gas till they attain a constant final velocity ("Accelerating Region"). They further move upwards with this constant velocity ("Terminal Region"). Cluster size is assumed to be a constant along the height of the riser. As cluster velocity increases from bottom to the start of terminal region, axial solid hold up decreases with height. As clusters attain a constant velocity in the Terminal region, axial solid hold up does not change with the height of the riser. Voidage in the Terminal region can be obtained considering that slip velocity is equal to cluster terminal velocity. The model compares well with the literature data