Gas holdup, liquid circulating velocity and mass transfer properties in a mini-scale external loop airlift bubble column

The external loop airlift bubble column has been regarded as a promising type of gas–liquid or gas–liquid–solid biooreactor because of the liquid circulating flow between the riser and downcomer. A mini-scale column is useful and efficient in the process research and development for highly specialized materials such as fine chemicals, advanced bioproducts and biocatalysts utilized in two or three phase system. In this work, a mini-scale glass column of 20 ml in volume was designed and characterized. The gas holdup ε G in the riser was obtained by measuring the volume expansion through photographs taken with a digital camera. The liquid circulating velocity U L was measured by observing the time required for a tracer particle to travel a fixed distance (3.5 cm) in the downcomer through analysis of the images taken by a video camera. The gas–liquid volumetric oxygen transfer coefficient k L a and liquid–solid oxygen transfer coefficient k S were determined by our previous method in which the air oxidation of glucose was catalysed by the immobilized glucose oxidase gel beads suspended in the column to obtain a pseudo steady state concentration of the dissolved oxygen and the corresponding constant rate of glucose consumption. It was shown that even such a mini-scale external loop bubble column could be characterized in terms of gas holdup, liquid circulating velocity and mass transfer properties according to our previous correlations proposed for the bench to pilot scale column.