(139d) Kinetics Study and Its Implementation in a First Principles Model for the Riser of a Resid Fluid Catalytic Cracking Unit

Fluid catalytic cracking is one of the most important conversion processes in modern refining operations, converting heavy petroleum feedstock into valuable lighter products like LPG and cracked naphtha, and is a major contributor to the commercial gasoline market. However, coke is also deposited on the catalysts as a considerable by-product of cracking reactions, causing rapid catalyst deactivation. The spent catalysts are transported to a regenerating section, where the activity is restored, and the hot catalysts transported back to the reactor section, thereby also supplying the heat requirement to the otherwise endothermic cracking process. When a high CCR (Conradson Carbon Residue) atmospheric residue is introduced as feed to the catalytic cracking unit, high amounts of coke are deposited on the spent catalyst. In such a case, the catalyst regeneration by coke combustion needs to be conducted in a 2-stage regenerator to avoid hydrothermal deactivation of the catalyst in the regenerator.

Since the RFCC (Resid Fluid Catalytic Cracking) unit is the heart of the refinery, optimization of the operating conditions to maximize the production of valuable products can generate significant additional revenue for the refinery. This necessitates availability of robust predictive models, developed from first principles for the two risers and the two regenerators, and effectively integrating them.

Several experimental runs using an equilibrated commercial catalyst were conducted in a bench-scale, bubbling fluidized bed, multi receiver ACE unit, and kinetics of the RFCC process describing the cracking reactions and coke formation derived using 10 product lumps. Experiments were designed to accurately estimate the kinetics of the intermediate steps. To achieve this, intermediate liquid products, were also injected as feed in some experiments. The two-phase fluidization model is composed of a two-phase bed section, with a dense phase modelled as a CSTR and a bubble phase modelled as a PFR, and a freeboard section modelled as a PFR. The extracted kinetics has been further utilized for developing a detailed reactor model for the riser of the RFCC unit, which was modeled as a fast fluidized bed reactor. It considers the heat and mass transfer between the gas and solids, and additional hydrodynamic parameters (e.g. slip velocity) have been captured in detail.

The hydrodynamics considerations and reactor model details for both the ACE unit and the commercial riser will be presented in detail in the presentation. The models have been executed using an Equation-Oriented modeling platform gPROMS, with properties of the different lumps estimated simultaneously during model solution using Multiflash property estimation package which enhances the fidelity of the model solutions. Parametric runs were executed for varying riser temperature, cat-to-oil ratio, catalyst circulation rate, and the metal and additive contents in the equilibrium catalyst, to examine their effects on selectivities to the valuable products and coke formation on the catalyst. This reactor model, currently developed using design data, will be further tuned with plant operating data, and will be utilized to account for the pressure balance and heat balance of the unit, which are key for proper operation of the unit, as well as for operational optimization of the RFCC unit.