Hydrodynamics of an external-loop three-phase airlift (TPAL) reactor

This study reports a theoretical and epxerimental investigation into the performance of an external-loop three-phase gas—liguid—solid airlift reactor. The reactor, 301 in volume, consisted of 2 m long riser and downcomer sections of 0.08 m diameter. The three phases were air, water and glass beads. Experimental data on the gas and solid holdups in the riser and downcomer have been obtained over a range of solids concentrations (40–100 kg m −3). This was obtained from direct measurements of phase volumes using a series of quick-closing valves placed in the flow circuit. The data on the phase holdups in the riser have been correlated using a three-phase drift-flux expression. A further correlation has been formulated relating the downcomer gas holdup to the riser gas holdup. An analysis of the pressure drops in the system is presented. The head losses in the flow path have been estimated from static pressure measurements made using a set of manometers placed in the recirculating flow path, and estimates of these head loss coefficients in three-phase flow have also been calculated using an extension of existing correlations for single-phase and two-phase flow. A hydrodynamic model of the reactor has been used to predict the liquid recirculation velocity and reactor stalling point. This builds on previously published work, and the new experimental data obtained in this paper are used to validate the model by verifying the (model predicted) differential holdup of solids between the riser and downcomer regions. Overall, the model predicts the various system parameters satisfactorily, providing a firm basis for the development of a design methodology for three-phase airlift (TPAL) reactors.