CFD modelling of an ammonium nitrate fluidised bed: effect of distribution plate geometry

Abstract

Fluidisation, which is a process governed by the suspension of particles in a closed area by blowing air or another medium through the bed of particles, is gaining popularity in industries like pharmaceuticals and mining, to name a few. Contributing researchers developed theory and conducted experiments in an attempt to understand this concept of fluidisation. A wide range of investigations have been conducted, but according to current knowledge, no information was reported on the fluidisation of porous granular ammonium nitrate (PGAN). According to Geldart classification, PGAN falls under Group B particles therefore the behaviour of the particles are expected to be in agreement with that was observed for sand-like particles. The aim of this study is to investigate the effect of different distributor plates on bed hydrodynamics related to an ammonium nitrate fluidised bed in laboratory-scale and pilot-scale setup by: (i) obtaining the characteristics of different suggested distributor plates for use in the fluidisation of PGAN for modelling purposes in a CFD environment, (ii) generating a CFD model for PGAN granules fluidisation that can effectively reproduce the bed pressure drop versus superficial gas velocity curve for different distributor plates using PGAN granules with different particle sizes, and (iii) devising, using the CFD model, means to determine bed density and bed expansion for the (PGAN) fluidisation operation. The plate pressure drop against superficial gas velocity curve was plotted to calculate Darcy- Forchheimer coefficients which were used in the CFD model environment to characterise the distributor plates. The plate pressure drop was found to increase with increasing superficial gas velocity. In the laboratory-scale investigation, the behaviour of the distributor plates was investigated for both a straight duct configuration and bend duct configuration below the distributor plate, while cone-shaped and column-shaped ducts were used in the pilot-scale setup. The model provided a good representation of the experimental observations for both laboratory and pilot-scale fluidisation setups. The velocity profiles obtained from employing the porousSimpleFoam solver with porous media specified by the Darcy-Forchheimer coefficients were used to generate a polynomial function which was employed to specify the boundary conditions of air in the two-phase system. The bed pressure drop versus superficial gas velocity curves resulting from modelled data follow the trend proposed in the literature, therefore the model can be used to predict the bed pressure drop of a fluidised bed system. Fluidisation initialises quicker in the predictive model than in the experimental setups. The model overestimates the bed pressure drop for particles with small average particle diameters and underestimates the bed pressure drop for particles with higher average particle diameters. The model predicts the bed pressure drop accurately at low superficial gas velocities and the error increases with an increase in superficial gas velocity. The range of minimum fluidisation velocity for all particle sizes in the laboratory scale fluidised bed was found to be 0.3-0.6 m/s. Flow patterns were discussed for particles with an average particle diameter of 1.8 mm when fluidising with a superficial gas velocity of 1.1 m/s in a model for 2 seconds. The particles were specified as alpha.particles with a void fraction of 55%. Jetting was observed in a system with a straight duct configuration below the distributor plate. It was concluded that a preferential flow resulted when employing straight ducting configurations as well as bend ducting configurations in the air inlet duct below the distributor plates, both resulting in a high final particle content towards the walls of the duct after a running time of 2s. The fluidised bed expands in general quicker in the model compared to the experiments, but little to no correspondence in behaviour was observed for a bend ducting configuration system due to preferential flow patterns observed in the system. The bed expansion ratio was found to be directly proportional to superficial gas velocity up to a certain (maximum) extent. The bed pressure drop measured along the horizontal axis of the fluidised bed in the direction of the position of the air blower at three different probe positions above the perforated plate was used to calculate bed density across the bed. There was no significant correspondence observed between the experimental bed density and the modelled bed density using plates associated with either a straight duct configuration or a bend duct configuration. A higher bed density was observed when measuring directly above the plate due to high particle content at that position. For all the plates in the pilot-scale setup, the superficial gas velocity and probe position have influenced the bed density. An increase in e.g. the height of measurement above the plate resulted in a decrease in the measured fluidised bed density.