(87a) Effect of Cohesion on Gas Residence Time Distribution in Fluidized Beds

Effect of Cohesion on Gas Residence Time Distribution in Fluidized Beds
J. Kolehmainen¹, A. Ozel², Y. Jiang¹, S. Sundaresan¹

¹Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08540, USA

²School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK

Gas residence time distribution (RTD) is an important consideration in industrial fluidized bed reactors, owing to its influence on conversion and product selectivity [1]. Inhomogeneous flow behavior, which is a concern in fluidized beds, affects the RTD appreciably. Although inhomogeneities arise in fluidization of both cohesive and non-cohesive particles, the present study focuses on the effect of inter-particle cohesion on gas RTD in a model system.

In industrial applications such as fluid cokers [2,3], cohesion would come about through liquid bridge formed between particles [4]. Dissolution of small gaseous molecules in this liquid would affect their RTDs. In the present study, we have employed, for the sake of simplicity of numerical computations, inter-particle cohesion through van der Waals force as a simpler proxy for the more complex liquid bridge force. The strength of cohesion is characterized by the Bond number (Bo) representing the ratio of maximum cohesive force between two particles and the weight of a particle. In this study, we performed CFD-DEM simulations of a small fluidized bed. Changing Bo changes the flow behavior. In addition to the flow, we tracked the evolution of the concentration of an absorbing tracer. The rate of mass transfer between the particles and the gas is quantified through Gunn’s correlation [5]. The partitioning of the tracer between the gas and the solid is quantified through a linear isotherm. By studying outlet concentration following a step change in feed concentration of the tracer, we determined the gas phase RTD. Through simulation campaign, we have stablished the effect of Bo and equilibrium constant on the RTD.

The mean residence time is essentially independent of the cohesion, but increases linearly with the equilibrium constant, which is consistent with one-dimensional models. For weakly absorbing particles, cohesion has only a mild effect on the higher moments of the residence time distributions. However, for moderately absorbing particles there is an appreciable increase in the higher moments when cohesion is increased. This increase in the higher moment results from a longer tail in the RTD, which is attributed to clustering that hinders the mass transfer between the particles and the gas.

[1] Gao, Yijie, Fernando J. Muzzio, and Marianthi G. Ierapetritou. "A review of the Residence Time Distribution (RTD) applications in solid unit operations." Powder technology 228 (2012): 416-423.

[2] Cui, H. P., et al. "Gas and solids mixing in a dynamically scaled fluid coker stripper." Chemical engineering science 61.2 (2006): 388-396.

[3] Song, Xuqi, et al. "Gas mixing in the reactor section of fluid cokers." Industrial & engineering chemistry research 44.16 (2005): 6067-6074.

[4] Boyce, C. M., et al. "Analysis of the Effect of Small Amounts of Liquid on Gas‐Solid Fluidization using CFD‐DEM Simulations." AIChE Journal (2017).

[5] Gunn, D. J. "Transfer of heat or mass to particles in fixed and fluidised beds." International Journal of Heat and Mass Transfer 21.4 (1978): 467-476.