Bulk Molecular Characterization Approach for the Simulation of FCC Feedstocks

Abstract

Feedstock quality affects the fluid catalytic cracking (FCC) process to such an extent that even the operating variables or the catalyst selection seem to be of secondary importance. This paper focuses on the way that the bulk properties of an FCC feedstock can explain the extent of catalytic reactions (crackability) and coke production (coking tendency). The final goal of this effort is to develop a model for the characterization of fluid catalytic cracking feedstocks on the basis of standard analytical procedures accessible to the average refinery. Using different approaches to the characterization of petroleum fractions, a model was developed for the prediction of the behavior of fluid catalytic cracking feedstocks under real FCC conditions. A large database of experiments, performed in the FCC pilot plant of the Chemical Process Engineering Research Institute (Thessaloniki, Greece), was used for the development of the feedstock characterization procedure. The aromatic carbon, the average carbon number, and the total nitrogen and sulfur contents were appropriately combined to predict the effect of feedstock quality on the conversion and coke yield of the fluid catalytic cracking process. The simulation results reveal the ability of this approach to predict accurately the crackability and coking tendency of an FCC feedstock from easily measured properties and to explain the impact of these properties on the FCC outcome. IntroductionThe modeling of fluid catalytic cracking (FCC) units is of pivotal interest, because of their complexity and associated economic incentives. The effective simulation of FCC units requires a thorough understanding of the combined kinetics and hydrodynamics of the two interconnected reactors, the riser and the regenerator. Furthermore, understanding the interacting effects of the two leading factors in the catalytic reactions, the feedstock and the catalyst, is crucial. Although numerous efforts have been made to add knowledge through the simulation of FCC operations and to develop advanced catalytic systems, a relative gap appears in the area of the simulation of the effect of the feed on the FCC process. In fluid catalytic cracking, feed quality is especially critical because it affects the heat balance and the ultimate cracking severity, in addition to its fundamental effects on the inherent crackability of the molecular structures. FCC feeds include straight-run distillates, vacuum gas-oils, and atmospheric and vacuum residua, which are unidentified complex hydrocarbon mixtures, thus increasing the degree of complexity of FCC modeling. The number of components and hydrocarbon types in FCC feeds increases with boiling range, which causes increased difficulties in the degree of possible analytical identification. The theoretically possible number of paraffin isomers gives an idea of the complexity of the problem: 75 isomers with 10 carbon atoms can exist, so at a carbon number of 20, about 3.66 × 10 5 paraffin isomers can be found, whereas at a carbon number of 100, the number of pos...

Modeling and Simulation of Fluidized Catalytic Cracking Riser Reactor Using Pseudo Reaction Kinetics : A Review

“…This was attributed to the high aromatic contents in the feeds containing LCO .The study has found an optimum content of LCO in the FCC was to be 7 vol % ,which maximizes both gasoline production and gasoline RON. [5] in their research studies on bulk molecular characterization approach for the simulation of FCC feedstocks. In their study they had presented a bulk molecular characterization approach for the simulation of FCC feedstock regarding their relative potential to enhance catalytic cracking and coke formation .The final models for the prediction of crackability and coking tendency incorporated the specific gravity, the refractive index, and the TBP distillation of the feedstocks, along with the sulfur ,nitrogen, and carbon contents.…”

Integrated FCC riser—regenerator dynamics studied in a fluid catalytic cracking pilot plant

“…The pseudo-steady state and dynamic sub-models that constitute the dynamic simulator, as developed in CPERI, have been presented in the literature (Bollas et al, 2002;Bollas et al, 2004;Faltsi-Saravelou, Vasalos & Dimogiorgas, 1991) and will be briefly adduced in this section. A kinetic-hydrodynamic model was developed for the simulation of the pilot riser reactor in pseudo-steady state conditions (Bollas et al, 2002).…”

“…The prediction of gas-oil conversion and coke yield are the only two lumps of the riser sub-model, essential for inclusion in the complete simulator, which reduces the necessity of using a more detailed lumped model. The effect of feedstock properties on gas-oil conversion and coke yield was expressed through semi-empirical correlations developed on the basis of experiments performed with constant catalyst and a variety of feedstocks (Bollas et al, 2004). The effect of catalyst type was expressed through a "catalyst index" (Bollas et al, 2004).…”

“…The effect of feedstock properties on gas-oil conversion and coke yield was expressed through semi-empirical correlations developed on the basis of experiments performed with constant catalyst and a variety of feedstocks (Bollas et al, 2004). The effect of catalyst type was expressed through a "catalyst index" (Bollas et al, 2004). The model of the regenerator is based on the two-phase theory (Davidson, Clift & Harrison, 1985), in which the gas-solids flow is assumed to follow the bubbling bed regime, consisting of two zones: a dense zone at the regenerator bottom comprised by a bubble and an emulsion phase, and a dilute zone at the regenerator top, called the freeboard.…”

“…(1) and (2), was simulated with a "bulk molecular characterization approach" (Bollas et al, 2004), that is the calculation of the relative (compared to a reference feedstock) potential of an FCC feedstock to enhance catalytic cracking conversion (crackability) and coke formation (coking tendency). The proposed models focus on the needs of industry; hence they include the bulk feedstock properties of Fig.…”