Hydrodynamic behaviour of a gas—solid counter-current packed column at trickle flow

Trickle flow of a more or less fluidized catalyst through a packed column is a promising new gas—solid counter-current operation. The hydrodynamic, behaviour of such a column, filled with dumped PALL rings, has been investigated, while some results have been obtained with RASCHIG rings and cylindrical screens as packing. The solid used was a microspherical catalyst carrier. Pressure drop, hold-up, loading and flooding were evaluated and compared with literature data for gas—liquid systems. The behaviour is analogous although the absolute magnitude is different.\ud \ud Pressure drop is low, up to 50% of the solid being carried by the packing. A correlation for the pressure drop, which is mainly caused by suspended particles, has been derived. At low gas velocities particle velocity is constant, whilst near flooding the slip velocity between gas and solid reaches a constant value. Using empirical values for particle velocity and slip velocity, hold-up, loading and flooding can be predicted. Scaling-up problems still need to be investigated. Results on mass transfer, axial dispersion of both phases and solid spread factors will be published later.\u

Mass transfer in a gas-solid packed column at trickle flow

The height of an overall transfer unit has been evaluated in a gas—solid packed column at trickle flow by measuring column performance during steady state adsorption experiments. Results have been interpreted with an extraction model: mass transfer and axial dispersion in both phases. Using Bodenstein numbers for the gas and solid phases from a previous investigation the height of a true transfer unit has been calculated.\ud \ud The column was filled with dumped Pall rings, the solid phase was a freely flowing catalyst carrier, and the gas phase was air at ambient conditions containing freon-12 as adsorbing component.\ud \ud At low gas velocities column performance is entirely determined by axial dispersion but at higher gas velocities mass transfer limitations become important. For conditions of practical importance the height of a true transfer unit corresponds to 4 – 9 Pall ring layers

Axial dispersion of gas and solid phases in a gas—solid packed column at trickle flow

Axial dispersion of gas and solid phases in a gas—solid packed column at trickle flow, a promising new countercurrent operation, was evaluated using residence time distribution (RTD) experiments. The column was packed with dumped Pall rings, the gas phase was air at ambient conditions and the solid was a porous catalyst carrier.\ud \ud The RTD experiments for the solid phase were carried out using the “perfect pulse method”, while for the gas phase the “imperfect pulse method” was used. The model parameters were calculated by the methods of moments and various parameter optimization methods.\ud \ud At a given solid flow rate axial dispersion of the gas phase decreases with increasing gas velocity and is strongly dependent upon solid mass flux. Axial dispersion of the solid phase is approximately independent of the gas velocity and it is reduced if the solid mass flux is increased. For conditions of practical importance, 2 – 5 and 5 – 15 Pall ring layers correspond to the height of a mixing unit in the gas and solid phase, respectively