International Journal of Chemical Engineering

Volume 2015, Article ID 259603, 13 pages

http://dx.doi.org/10.1155/2015/259603

## Analysis of Process Variables via CFD to Evaluate the Performance of a FCC Riser

^{1}School of Chemical Engineering, University of Campinas, 500 Albert Einstein Avenida, 13083-970 Campinas, SP, Brazil^{2}PETROBRAS/AB-RE/TR/OT, 65 República do Chile Avenida, 20031-912 Rio de Janeiro, RJ, Brazil^{3}Chemical Engineering, Heriot-Watt University, Edinburgh EH144AS, UK

Received 25 August 2014; Revised 2 January 2015; Accepted 15 January 2015

Academic Editor: Deepak Kunzru

Copyright © 2015 H. C. Alvarez-Castro et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Feedstock conversion and yield products are studied through a 3D model simulating the main reactor of the fluid catalytic cracking (FCC) process. Computational fluid dynamic (CFD) is used with Eulerian-Eulerian approach to predict the fluid catalytic cracking behavior. The model considers 12 lumps with catalyst deactivation by coke and poisoning by alkaline nitrides and polycyclic aromatic adsorption to estimate the kinetic behavior which, starting from a given feedstock, produces several cracking products. Different feedstock compositions are considered. The model is compared with sampling data at industrial operation conditions. The simulation model is able to represent accurately the products behavior for the different operating conditions considered. All the conditions considered were solved using a solver ANSYS CFX 14.0. The different operation process variables and hydrodynamic effects of the industrial riser of a fluid catalytic cracking (FCC) are evaluated. Predictions from the model are shown and comparison with experimental conversion and yields products are presented; recommendations are drawn to establish the conditions to obtain higher product yields in the industrial process.

#### 1. Introduction

Since the first FCC commercial riser, many improvements have been achieved which have helped the process reliability and its capacity to transform heavier feedstock at relatively low costs; currently the FCC process remains the primary conversion process in the petrochemical industry. For a number of refineries, the fluid catalytic cracking remains the main source of profitability and the accomplishment of its operation decides the market competiveness of the cracking unit. Approximately 350 FCC units are in operation worldwide, with over 12.7 million barrels per day as total capacity. Most of the existing cracker units have been designed or modified by six major technology licensers [1].

The design of each FCC unit can be different but their common target is to upgrade low-cost hydrocarbons to more valuable products. FCC and ancillary units, such as the alkylation unit, are responsible for about 45% of the gasoline produced worldwide. Papers have flourished in recent years in the attempt to describe and simulate numerically the phenomena observed in such process. To predict the solid and gas phase behavior the Eulerian-Eulerian approach has been used due to low computational effort required [2]. In this study, the Eulerian-Eulerian approach is used, where the solid phase is treated as a continuum [3–5]. Computational fluid dynamic (CFD) was implemented to solve discretized equations; a hybrid mesh (tetrahedral mesh with refining prisms at the wall) was used as the calculation grid. The 12-lump kinetic model proposed by Wu et al. [6] with catalyst deactivation was coupled with the hydrodynamic model to evaluate the full problem. The lumping approach has been studied to describe the kinetic behavior of catalytic cracking with a large number of components where each lump is constituted by hundreds of kinds of molecules in a specific range of molecular weights. The methodology is shown to be very powerful when a large number of components are involved [7–11]. The simulation model uses a 12-lump effect that the variation of different process variables has on the conversion network which has the advantage of representing with good reliability the products and presents the option of representing the feedstock through three different lumps. The purpose of this study is to predict the yield and conversion behaviors at different operating conditions in the industrial riser of a FCC unit, with a 12-lump kinetic model. Different operational conditions have been studied, in order to estimate product yields.

#### 2. Riser Process

The riser is the main equipment of the FCC unit. Inside the riser the feedstock is fed through nozzles and mixture with the catalyst and the accelerant steam in the injection zone. The performance of the nozzles to guarantee fast vaporization of the feedstock and a good contact of the gasoil droplets with the catalyst is key to improve the FCC riser efficiency; the feedstock nozzles are positioned about 5–12 meters above the bottom of the reactor. In accordance with kind of FCC design, the number of feedstock injections can be from 1 to 15. Practically all of the riser reactions take place between 1 and 3 s. Reactions start as soon as the feed enters in contact with the hot catalyst.

The increasing velocity due to the vapor production acts as the means to carry the catalyst up in the riser. The hot solid supplies the necessary heat to vaporize the feedstock and bring its temperature to the temperature needed for cracking, compensating, also, for the reducing in temperature due to endothermic behavior of riser reactions. Standard risers are designed for an outlet velocity of 12–18 m/s. During the operation, coke deposits on the catalyst, declining the catalyst activity and thus representing a concern for the efficiency of the cracking reactions [12].

#### 3. Mathematical Model

The fluid dynamic equations and kinetic model are summarized in Section 3.1 and taken and adapted from Alvarez-Castro [13]; the catalytic cracking kinetic models are taken from Wu et al. [6] and Chang et al. [14]. In order to study the heterogeneous, kinetics, and the particle phase deactivation, (15)–(20) were implemented in the CFX code.

##### 3.1. Governing Equations for Transient Two Fluid Models

*Governing Equations*(1)Gas-solid fluid model (Eulerian-Eulerian) [15]: where is the volume fraction, is density, and is the velocity for each phase.(2)Momentum equations: where is the pressure, the viscosity, the modulus of elasticity, the acceleration of gravity, and the interphase momentum transfer: where is the solid diameter and is the drag coefficient (see [16]), where is maximum volume fraction and the packing limited about 0.65.(3)Turbulence equations:(a)The -epsilon mixture model [17]: where is the turbulence kinetic energy, is the turbulence eddy dissipation, and is constant where , , , and are constants. is the turbulence production(4)Heat transfer model:

#### 4. Simulation

The system of governing equations, twelve-lump catalytic cracking kinetic model, solid influence, and catalyst deactivation functions was solved by employing the finite volume method technique using the commercial software ANSYS CFX 14.0. The relevant results and the calculations steps are analyzed and discussed in detail in the following sections.

##### 4.1. Geometry and Grids Generation

Steam or fuel gas is often used to lift the catalyst to the feed injection. In most designs that incorporate a “Wye” section for delivering the catalyst to the feed nozzles, a lift gas distributor is used, providing sufficient gas for delivery of dense catalyst to the feed nozzles. In other designs, the lift gas rate is several magnitudes greater, with the intent of contacting the gasoil feed into a more dilute catalyst stream. In this work the geometry of the riser is considered according to industrial reactor specifications taken from Alvarez-Castro [13] as shown in Figure 1 which reports a typical riser with Wye section.