Numerical Analysis of a Reacting Gas/Solid Flow in the Riser Section of an Industrial Fluid Catalytic Cracking Unit

The Fluid Catalytic Cracking (FCC) is an important process in a refinery used mainly to produce higher value gasoline from heavy hydrocarbons. An FCC unit can crack different type of feeds and can use different types of catalysts during its lifetime, which makes this process very flexible and profitable. In this paper, an industrial unit located in Mexico was analyzed and the computational results were compared with available design data. The FCC process involves complex reaction kinetics along with multiphase flow hydrodynamics and heat transfer. This makes it nearly impossible to model every physical phenomenon that occurs in this process. However, an interesting model should be able to predict reasonably well the yields of the cracking reactions. Cracking reactions are catalytic, and the catalyst distribution in the riser needs to be known a priori. Therefore, we modeled the physical process as a gas/solid multiphase flow with reaction. A transient Eulerian approach plus a simple 3-lumps reaction kinetic scheme (Nace et al., 1971) describe the multiphase flow model. The gas density was assumed to follow an ideal gas law, and can, thus, change with the number of moles in the system. Conservation of mass, momentum, and species (heavy oil, gasoline and gas+coke) were solved. The gas/solid hydrodynamics and the yields of the reacting species were predicted in the 2-D Riser section of the FCC unit. The results showed a dramatic effect of the change in the number of moles of the chemical species on the hydrodynamics. The increase in the number of moles due to the cracking reactions resulted in a significant increase in the gas flow rate, which changed significantly the flow regime. The reaction yields compared reasonably well with the design data for this specific unit.