Numerical Analysis of Gas Distribution in Fluidized Beds
Objective: The objectives of this project are to improve the performance of fluidized bed drying using different ideas such as new designs of the distribution plate and gas chamber, by modifying the gas injection system or by using intermittency. The goal is to carry out a numerical study to understand the effect of various operating parameters and geometric changes. The numerical simulations will be carried out using ANSYS Fluent V18.2.
Methodology: The Gas distribution of fluidized bed column is simulated in stages. First, the gas chamber and the gas distributor are simulated together for single phase i.e., air as an inlet fluid.
The single and multi-phase flow theories used for the current simulations using ANSYS Fluent V18.2, Transient flow, drag model applied, group B particle of 275-micron diameter is used, grid independence test is carried out to understand the effects of grid sizing.
Outcome/Conclusion: In this research, a number of gas distribution systems with various gas distributor designs were proposed. Their performance in terms of the uniformity of gas distribution at the exit of orifice holes of the gas distributor was examined with the use of computational fluid dynamic analysis. The simulations were carried out in ANSYS FLUENT v18.1 and 18.2 using single and multiphase models. The base case design of gas distributor with uniform percentage open area showed the non-uniform distribution of gas. Hence, the distributor geometries with different percentage open area (for the circular pattern and triangular pitch arrangement), type of gas entry were used to understand if the quality of fluidization can be improved. It was observed that the non-uniformity of gas distribution of circular pattern increases as the percentage open area is increased from 15 to 20; however, the gas distribution again improved for 25% open area, we would like to check this behavior again. On the other hand, for the triangular pitch arrangement of the orifice holes (which is the most commonly used arrangement in industries), the non-uniformity increases as the percentage open area is increased. The comparison of two patterns of orifice arrangement for the lower open area showed that the triangular pitch arrangement provides a better air distribution. The results also revealed that the non-uniformity in air distribution occurs mainly in central and middle part of gas distributor for the lower open area, while, for the plates with higher percentage open area, the non-uniformity is prominent near the edges of the gas distributor plate. An attempt is made to further improve the uniformity using variable open area in different regions of the plate. The simulation results of the variable opening area proved that the new design can generate a better gas distribution with more uniform velocity pattern than the designs discussed earlier, at least for the bottom entry of the gas nozzle. The simulation results also show that the gas distribution is severely affected by gas nozzle entry position. The results show that the bottom entry position of nozzle provides uniform distribution, while the side entry results in severe non-uniformity in gas distribution.
The two-phase fluidized bed simulations were also carried out to analyze the particle behavior in the presence of gas distributor with varying percentage open area and different gas inlet entry. The Eulerian-Eulerian approach is incorporated in the two-phase simulation with constant volume fraction. The simulation results showed that the particles gradually start fluidizing at lower flow time, as the flow time increases the bed expands, and the fast fluidization is observed, eventually, the particle falls back in the bed. The higher percentage open area showed a turbulent regime. For the fluidized bed with side entry, the results showed that the particles start fluidizing on the side of the chamber opposite to the entry position. In general, lower percentage open area and bottom entry of the gas nozzle should be preferred. The other parameters used were the optimized parameters from the previous work. However, a more detailed two-phase simulation should be carried out to further analyze the use of the variable open area for uniform fluidization.