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Fluid Catalytic Cracking (FCC) Process Modeling, Simulation, and Control4 Carla I. C. Pinheiro,*,† Joana L. Fernandes,‡ Luís Domingues,† Alexandre J. S. Chambel,† In^es Grac-a,† Nuno M. C. Oliveira,§ Henrique S. Cerqueira,# and Fernando Ram^oa Ribeiro† †
Institute for Biotechnology and Bioengineering (IBB), Department of Chemical Engineering, Instituto Superior Tecnico/Universidade Tecnica de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal ‡ Process Design and Modeling Division, IFP Energies Nouvelles Lyon, Rond-point de l’echangeur de Solaize, B.P. 3, 69360 Solaize, France § Centre for Chemical Processes Engineering and Forest Products (CIEPQPF), Department of Chemical Engineering, Universidade de Coimbra, R. Sílvio Lima Polo II, 3030-790 Coimbra, Portugal # ATP Engenharia, Rua S~ao Jose 90/2201-C, 20010-020 Centro, Rio de Janeiro, RJ, Brazil ABSTRACT: This paper focuses on the fluid catalytic cracking (FCC) process and reviews recent developments in its modeling, monitoring, control, and optimization. This challenging process exhibits complex behavior, requiring detailed models to express the nonlinear effects and extensive interactions between input and control variables that are observed in industrial practice. The FCC models currently available differ enormously in terms of their scope, level of detail, modeling hypothesis, and solution approaches used. Nevertheless, significant benefits from their effective use in various routine tasks are starting to be widely recognized by the industry. To help improve the existing modeling approaches, this review describes and compares the different mathematical frameworks that have been applied in the modeling, simulation, control, and optimization of this key downstream unit. Given the effects that perturbations in the feedstock quality and other unit disturbances might have, especially when associated with frequent changes in market demand, this paper also demonstrates the importance of understanding the nonlinear behavior of the FCC process. The incentives associated with the use of advanced model-based supervision strategies, such as nonlinear model predictive control and real-time optimization techniques, are also presented and discussed.
1. INTRODUCTION Growing demand for refinery products combined with the decreasing quality of crude oils and tighter product specifications due to environmental constraints is forcing refiners to make significant investments. In fact, during the past decades, new hydroprocessing units have been built, together with the revamping of old refining processes to meet market demands.1 In this context, the use of advanced process engineering tools has become essential for refiners, not only for design but also in the tasks of process control, optimization, scheduling, and planning. Besides the application to specific process units, these techniques are also being used for the entire refinery supply chain.2 Fluid catalytic cracking (FCC) remains a key unit in many refineries; it consists of a three-step process: reaction, product separation, and regeneration. In this cyclic process, gas oils from vacuum distillation towers and/or residues from atmospheric distillation towers are converted into lighter and more valuable products. One of the most desired products is cracked naphtha, which is the major constituent of the gasoline pool.3 Operating conditions comprise high reaction temperatures in the range of 750800 K and pressures close to atmospheric conditions. FCC is able to process a wide variety of feedstocks and is suitable to operate in special campaigns46 that may also soon include coprocessing of renewable feedstocks.79 Nowadays, more than 400 FCC units are operated worldwide.10 A multicomponent catalyst that usually contains an acid USHY zeolite, an active alumina matrix, an inert matrix (kaolin), and a r 2011 American Chemical Society
binder is responsible for the very large number of reactions involved in the FCC process.11,12 Different additives may also be added to the FCC catalyst with the purpose of modifying the FCC yields and/or reducing pollutant emissions.13,14 The most common are combustion promoters,15 octane and light olefins booster,13,1618 SOx and NOx reducers,19,20 and metal traps.21 Like all industrial processes involving heterogeneous catalysis, the FCC process also deals with catalyst deactivation.22 Actually, a significant fraction of the FCC feedstock (usually ∼6 wt % from a typical vacuum gas oil plus residue feedstock23) is converted into a mixture of compounds (called coke) that remain retained in the catalyst structure after stripping. These compounds quickly deactivate the acid sites of the catalyst, resulting in a significant activity loss.23,24 For that reason, the FCC unit was designed to allow a continuous recirculation of catalyst between the reactor and the regenerator, where coke is removed from the catalyst by combustion at high temperatures (typically 9501030 K). One advantage of this continuous catalyst recirculation is that FCC units operate in heat balance; that is, the heat released during the burning of the coke deposited on the spent catalyst is utilized to heat the regenerator flue gas, to vaporize and heat the feed to the reaction temperature, and to heat the steam and the additional quench Received: April 7, 2011 Accepted: November 17, 2011 Revised: October 21, 2011 Published: November 17, 2011 1
dx.doi.org/10.1021/ie200743c | Ind. Eng. Chem. Res. 2012, 51, 1–29
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Figure 1. Scheme of some FCC unit designs: (a) UOP stacked unit; (b) model IV; (c) Exxon Flexicracking unit; (d) R2R residue unit (adapted from Montgomery32).
streams to outlet temperatures while providing enough heat for the endothermic cracking reactions and remaining heat losses. During the successive reactionseparationregeneration cycles, the FCC catalyst particles may break, producing fines that will result in particulate emissions. To compensate for this loss (and hence to maintain the catalyst activity), addition of fresh
catalyst is frequently required. Moreover, for FCC units processing feedstocks with high levels of metals,2527 it is also common to replace a portion of the inventory by fresh catalyst, to keep the amount of contaminant metals at an acceptable level. This regular addition of fresh catalyst makes the FCC process one of the most important markets for catalysts.12,21 As a result, the 2
dx.doi.org/10.1021/ie200743c |Ind. Eng. Chem. Res. 2012, 51, 1–29
Industrial & Engineering Chemistry Research
REVIEW
so-called equilibrium catalyst (e-cat) circulating in the FCC unit is a heterogeneous mixture, ranging from young particles (fresh, highactivity catalyst with very low metal concentrations) to old particles (aged, low-activity catalyst with high metal concentrations). Since the first FCC unit started operation in 1942, several design improvements have been made.28 Indeed, almost all of the components of the FCC unit have been modified to improve performance.29 The first unit in operation was model I from Standard Oil Development Co. (SOD), now ExxonMobil. This unit was composed of multiple small vessels and had a catalyst up-flow configuration in both the reactor and regenerator vessels. The regenerator operated at low pressures, and external cyclones were used. In 1947, UOP built the first unit that used the concept of spent catalyst stripping: the stacked FCCU (Figure 1a). This unit had smaller and improved regenerators, where the regenerated catalyst was lifted to a bed cracking reactor by vaporized feed and the spent catalyst flown by gravity to the regenerator.30,31 In 1951, M. W. Kellogg introduced the Orthoflow unit, composed by a low elevation regenerator and a high reactor with an internal stripper. In this model, the catalyst flow was made through internal vertical straight tubes, a standpipe, and a lift line, controlled by plug valves. Another FCC configuration, called model IV, was introduced by SOD in 1952. This unit presented smaller vessels arranged side by side (Figure 1b) and was operated at higher pressures and internal velocities; catalyst flow control was done by changes in the differential pressure between the reactor and regenerator (U-bend concept) and by changes in the aeration in the spent catalyst entrance to the regenerator. The riser cracking unit was first proposed by Shell in 1957, which, together with the introduction of high-activity zeolite catalysts in the 1960s, definitively established this configuration. Since then, all new FCC unit designs have included riser cracking reactors.28 The improvement of FCC catalysts (e.g., through addition of combustion promoters) allowed further developments in the FCCs’ regeneration systems, which made possible the reduction of coke on the regenerated catalyst to
