Liquid phase mixing in a slurry bubble column with draft tube

In this work we characterize liquid phase mixing in a 150 L bubble column with a draft tube (internal airlift configuration) for a water-air-sand system at high solid concentrations. Liquid mixing is assessed by measuring the evolution of sodium chloride concentration after a pulse of concentrated NaCl solution is injected. Tracer concentrations were measured by means of electrical conductivity probes. The experimental set up consists of a 0.29 m internal diameter, 3 m length Plexiglas column with a conical bottom (cone apex angle of 60 degrees) and a concentric draft tube with 0.14 m internal diameter and 2 m length. The gas superficial velocity based on the cross section of the column vaired from 0.057 to 0.22 m/s. Sand particles of 280 mu m in average size were used, with slurry concentrations ranging from 120 to 500 kg/m(3) From the tracer outputs, circulation time (time between peaks of the response curve) and mixing time (time required to achieve a 95% homogeneous solution) were determined after the pulse had been injected. The experimental data were analyzed by means of a tank in series model with recirculation. The circulation and mixing times were found to increase with solids concentration, and to decrease as the riser gas velocity was increased. The circulation times were larger in continuous operation than in semibatch mode. The results show that the system has equivalent dispersion coefficients that are one order of magnitude lower than those found in a conventional bubble column.