A Simple Model to Predict the Pressure Drop in Three Phase Inverse Fluidized Bed

A simple model to predict the pressure drop in three phase inverse fluidized bed

R. Trivedi*, T. Renganathan, K. Krishnaiah

Department of Chemical Engg., IIT Madras – 600036, India

Abstract

Three phase inverse fluidized beds are contacting patterns in which continuous downflowing liquid phase and upflowing dispersed gas phase maintain a bed of suspended particles with density lower than the liquid. Compared to classical cocurrent three phase fluidization, higher mass transfer rates are achieved in inverse fluidized beds, due to the low inertia of particles used in the system. Owing to this advantage IFBs are used in wastewater treatment and biochemical processes. Successful design and operation of IFB requires information on hydrodynamics, transport rates, kinetics and contacting. Hydrodynamic characteristics include flow regime, pressure drop and phase holdups. Several studies have been conducted on the hydrodynamics of three phase IFB [1, 2]. However, no attempt has been made to predict pressure drop in terms of operating parameters. Hence the objective of the present work is to predict pressure drop in three phase IFB using gas holdup in bubble column and weight of particles.

Experiments are conducted for bubble columns and three phase IFB in an acrylic column of 89 mm I.D. and 186 cm height. Measured flowrates of water and air are allowed countercurrently into the column through distributors at the top and bottom respectively. Spherical polypropylene particles are used as solid phase. L-tube manometers and conductivity probes were used to measure pressure drop and holdups respectively. Experiments were conducted in bubble column for different liquid and gas flowrates. Data were taken on the height of gas liquid dispersion and pressure drop using which gas holdup was calculated. Experiments were conducted in IFB with particles at the same gas and liquid flowrates as those used for bubble column experiments. Data were taken on pressure drop, bed height, and conductivity.

In bubble column, the pressure drop and the gas holdup increase with gas flowrate. While homogeneous bubbling regime was observed at low gas flowrates, coalescence of bubbles occurred at higher gas flowrates reducing the rate of increase of pressure drop and gas holdup. Liquid flowrate has a marginal effect on pressure drop and gas holdup in the countercurrent bubble column. In three phase IFB experiments, it is observed that for particles of given characteristics and bed height, pressure drop increases with increase in gas and liquid flowrates.

It is hypothesized in the present work that pressure drop in three phase IFB at a particular gas and liquid velocities can be considered to be the sum of pressure drop in bubble column at the same velocities and the net buoyant force of the bed of particles per unit cross-sectional area of the column, assuming pseudo fluid (gas liquid mixture) [3]. Accordingly, the pressure drop in IFB is predicted by adding the pressure drop measured in bubble column and net buoyant force per unit area of the bed of particles. The predicted values compared satisfactorily with the experimentally measured pressure drop in IFB with an RMS error of 9%.

To make the calculation of pressure drop in IFB completely predictive, the pressure drop in bubble column was correlated by proposing a new empirical equation for gas holdup in countercurrent bubble column [4]. Using the predicted value of pressure drop for bubble column instead of experimental value, the pressure drop in three phase IFB could be predicted with an RMS error of 11%.  Given the complex interactions between the phases and the simplicity of the model, the error seems to be satisfactory.

References:

1.      Krishnaiah, K., S. Guru and V. Sekar, “Hydrodynamic Studies on Inverse Gas-liquid-solid Fluidization”, Chem. Eng. J., 51, 109-112 (1993). 

2.      Renganathan, T. and K. Krishnaiah, “Prediction of Minimum Fluidization Velocity in 2- and 3- phase Inverse Fluidized Beds”, Can. J. Chem. Eng., 81, 853-860 (2003).

3.      Felice R. D., “The pseudo-fluid model applied to three-phase fluidisation,” Chem. Eng. Sci., 55, 3899-3906 (2000).

4.      Koide K., S Morooka, K. Ueyama and A. Matsuura, “Behaviour of bubble swarm in large scale bubble column,” J. Chem. Eng. Jpn., 12, 98-104 (1979).