Rotary kilns have been used in various industrial applications (e.g., oil shale retorting, tar sands coking, incineration, cement production, etc.). The rotation of a cylinder-shaped vessel positioned longitudinally approximately 30° of the horizontal position ensures a continuous motion of catalyst between the entrance and exit of the kiln. With regard to the spent catalyst regeneration, the description of rotary kilns was given by Ellingham and Garrett [451]. There are two types of rotary kilns, i.e., direct fire and indirect fire.

The direct fire is a single shell vessel with rings added inside to slow the catalyst as it tumbles from the inlet (elevated part) towards outlet (lower part). The oxidation medium flows countercurrent to catalyst movement. The O2 concentration in the medium will decrease in the same direction because of its consumption. Therefore, the zone in the vessel located near the inlet may function as a stripper of volatile components of coke. The kiln is fired by gas burners directly against the outer shell of the vessel. The temperature inside the kiln is controlled by adjusting the burner heat, varying concentration of O2 in the oxidizing medium and its flow. The indirect fire kiln comprises a double-shell cylinder vessel. The inner shell is similar as that of the direct fire kiln. The space between the shells is heated either by combustion gas or steam. In some cases, the inner cylinder shell is ebullated allowing hot gases or steam to enter and contact the tumbled catalyst. The catalyst temperatures are controlled by monitoring the temperatures of the inlet and outlet gases. It is believed that Eurocat process evolved from a rotary kiln process by be improving the control of operating parameters such as temperature, gas flow, speed of rotation, etc.