In this paper, a thermodynamic optimization procedure for FCC riser units has been developed. The formulation uses a 2D fluid flow and kinetic model to provide the necessary information for the optimization process. The thermodynamic analysis is based on the unit entropy generation minimization, i.e., the minimization of the destroyed exergy in the system. This kind of analysis has been widely used in power generation plants, with large benefits. It was verified that for any given catalyst mass flow rate, there exists an optimum value for the catalyst to oil mass flow rate ratio, COR, for maximum mass flow rate production of gasoline, or any other desired product. Next, the objective function (net exergy production rate) was maximized through the minimization of the destroyed exergy inside the FCC unit. The optimization was conducted with respect to the catalyst to oil ratio (COR). It is important to stress that all optima are sharp, i.e., for example with H/D = 50, the variation of (E) over tilde (net) is greater than 50%, calculated from ((E) over tilde (net, max) (E) over tilde (net, min))/(E) over tilde (net, max) for 5 < COR < 25. Based on the lack of second law analysis related works for FCC plants in the technical literature and in view of the potential gains suggested by the results, the authors believe that thermodynamic optimization could bring new insight in the quest for better FCC plants. Therefore, a low computational time tool is made available for simulation, control, design and optimization of FCC units. (C) 2010 Elsevier Ltd. All rights reserved.