FCC Riser: Gas-Solid Separation System

FCC Riser Gas-Solid Separation System, CFD Simulation, ANSYS Fluent Training

Description

This project simulates gas-solid flow within a fluid catalytic cracking (FCC) system using ANSYS Fluent. FCC systems are widely used in chemical process units to convert high-boiling-point oil fractions into lighter, higher-value products. Within an FCC reactor, two functionally distinct zones govern the process: a dilute upper zone called the disengager, responsible for separating oil vapor from catalyst particles, and a denser lower zone called the stripper.

Since this system involves distinct interacting phases, the Eulerian multiphase model was used, solving separate momentum and conservation equations for each phase. Oil vapor, defined as the primary phase, carries a density of 3.471 kg/m³, specific heat capacity of 1006.43 J/kg·K, and thermal conductivity of 0.0242 W/m·K; the catalyst material, defined as the secondary phase, has a density of 1500 kg/m³, with its specific heat capacity and thermal conductivity derived from kinetic theory.

Oil vapor enters the reactor through a dedicated steam inlet at 5.5 m/s and 300 K, while a combined catalyst-and-oil-vapor stream enters through a separate feed injection inlet at 18.57 m/s and 300 K. The simulation was run using an unsteady solver over 20 seconds, with a time step of 1 second.

Geometry & Mesh

The 3D geometry was designed in Design Modeler as a cylindrical reactor, modeling only a quarter of the geometry given its symmetrical structure, reducing computational cost. The upper portion of the model corresponds to the disengager, with the lower portion representing the stripper. The domain was meshed in ANSYS Meshing using an unstructured grid totaling 2,488,759 elements.

Methodology

Several assumptions were applied: a pressure-based solver was used, the simulation was run as unsteady, and gravitational effects were included at -9.81 m/s² along the z-axis.

Key simulation settings included:

• Viscous model: Standard k-epsilon with standard wall functions; energy equation enabled

• Multiphase model: Eulerian, implicit formulation, with two phases (oil vapor and catalyst)

• Boundary conditions: Steam inlet with oil vapor at 5.5 m/s and catalyst at 0 m/s, both at 300 K; feed injection inlet with both oil vapor and catalyst at 18.57 m/s and 300 K; product outlet at 0 Pa gauge pressure; inner walls stationary with coupled thermal condition; baffles, disengager, and stripper walls stationary with zero heat flux

• Solution methods: Phase Coupled SIMPLE for pressure-velocity coupling, PRESTO! for pressure discretization, second-order upwind for momentum, turbulent kinetic energy, turbulent dissipation rate, and energy, with QUICK scheme for volume fraction

• Initialization: Standard method, with 0 Pa gauge pressure, z-velocity of 18.57 m/s for both oil vapor and catalyst, 300 K for both phases, and a catalyst volume fraction of 0.6

Conclusion

Results include 3D contours of pressure, velocity, and volume fraction for each phase, captured at the final second (20 s) of the simulation. These results characterize how the oil vapor and catalyst phases distribute and separate within the reactor's disengager and stripper zones, offering insight directly relevant to optimizing FCC reactor design for efficient catalyst-product separation in real-world petrochemical refining operations.