Pulsating fluidized beds (PFBs) are reactors with periodic or continuous gas flow that cycles bed material expansion and contraction. Unlike typical fluidized beds, PFBs have pulsations because gas flow alternates between high and low flow rates. PFBs are useful for combustion, gasification, chemical reactions, and drying because of their dynamic behavior, which improves mixing and heat transfer. Pulsation improves particle circulation, heat and mass transfer, and reactor efficiency. PFBs have greater residence time control, lower particle attrition, and higher response rates than conventional fluidized beds.

Thanks to the reference paper entitled “  Mixing and segregation behaviors of a binary mixture in a pulsating fluidized bed ”, a 2D CFD simulation of the pulsating fluidized bed is performed. This study has many challenges in its own way and can be very helpful to mechanical and chemical engineers.

• Reference [1]: Lim, Leonard Jing Jie, and Eldin Wee Chuan Lim. “Mixing and segregation behaviors of a binary mixture in a pulsating fluidized bed.” Powder Technology345 (2019): 311-328.
• Reference [2]: Liu, G-Q., et al. “Experimental studies of particle flow dynamics in a two-dimensional spouted bed.” Chemical Engineering Science4 (2008): 1131-1141.
• Reference [3]: Zhang, Kai, et al. “Prediction of segregation behavior of binary mixture in a pulsed fluidized bed.” Advanced Powder Technology11 (2019): 2659-2665.

Figure 1: . Snapshots of segregation process – Pulsating Fluidized Bed

Simulation Process

The easiest part of the project is dedicated to geometry and mesh generation. A simple rectangular domain and a properly structured grid is generated. In order to do so, Design Modeler and ANSYS Meshing are employed. In total, 5695 elements are used.

There are numerous challenges in the solver. Firstly, three phases are included in the simulation. Two of them have a granular nature with different diameters. So, the Eulerian multiphase model is utilized. Granular properties, including granular viscosity, conductivity, frictional viscosity and other vital parameters, are set based on the reference paper. Indeed, the unsteady incoming phases are entering in a pulsating manner. We need to write a user-defined function (UDF) to apply it.

Post-processing

The volume fraction contours reveal distinct segregation behavior between the two granular phases (flotsam and jetsam) in the pulsating fluidized bed. The flotsam phase shows volume fractions ranging from 0 to 0.122, with the highest concentrations appearing in the lower portion of the bed. The distribution pattern demonstrates clear stratification, with a gradual transition from high concentration zones to lower concentration regions  moving upward. This segregation pattern is characteristic of the pulsating nature of the flow, where the cyclic gas flow creates alternating expansion and contraction of the bed material.

Figure 2: Granular volume fraction in Pulsating Fluidized Bed

The jetsam phase follows a complementary distribution pattern, with volume fractions ranging from 0 to 0.352 and greater maximum concentrations than the flotsam phase. The contours show a more consistent mixing pattern in the center of the bed, with distinct concentration bands appearing at various heights. The dynamic interfaces between different concentration zones, especially in the transition regions, indicate the usefulness of the pulsing action in boosting particle circulation. The animation sequence confirms the cyclic nature of the mixing process by demonstrating how the pulsating flow alternately promotes mixing and enables partial segregation, resulting in a dynamic equilibrium that improves total heat and mass transfer compared to conventional fluidized beds.

Figure 3: Jetsam volume fraction in Pulsating Fluidized Bed