CPFD simulation of solids residence time and back-mixing in CFB risers

• CPFD was applied to study solids RTD and back-mixing in the CFB riser.
• Particle residence times exhibit a non-uniform distribution.
• Solids flow deviates from plug flow and exhibits back-mixing in DPT regime.
• Severer solids back-mixing for case 2 with lower superficial gas velocity
• Solids back-mixing mainly occurs at the lower part of the riser.

A Computational Particle Fluid Dynamics (CPFD) approach was applied to investigate the solids residence time distribution (RTD) and back-mixing behavior in a circulating fluidized bed (CFB) riser. The comparison between the simulation results and the experimental data indicates that the CPFD method was capable to predict the hydrodynamics and solids mean residence time. It was found that the solids residence time exhibited a non-uniform distribution both in the axial and in the radial directions of the riser. Even in the dilute phase transport (DPT) regime, the predicted solids RTD curves had the feature of an early peak and an extended tail, indicating that the solids flow deviates from plug flow and exhibits back-mixing. The solids back-mixing mainly occurred in the lower part of the riser, which provided significant implications for the industrial applications of the CFB reactors since most of the chemical reactions and heat/mass transport processes occur at the lower part of the riser. It is important to minimize the solids back-mixing at the lower part of the riser for the industrial CFB applications like the FCC.

Solids residence time and back-mixing were studied with the CPFD method. Solids residence time exhibited a non-uniform distribution both in the axial and in the radial directions of the CFB riser. The solids back-mixing mainly occurred in the lower part of the riserFigure optionsDownload as PowerPoint slide