Flow characteristics of an external-loop airlift column with a tapered riser

We constructed a new type of external-loop airlift column with a reverse conical riser, and investigated flow characteristics of the column experimentally. Since this type of column is able to keep solid suspension in the riser, it will give favorable conditions for the culture system in which mycelium pellets are formed. Radial and axial distributions of gas holdup in the tapered section were measured using an electroresistivity probe method. Liquid circulation velocity was obtained by a tracer method. As a result, the radial gas holdup distribution was found to agree with a well-known second order distribution, except for the region of low gas velocity. The axial distribution of gas holdup was found to be expressed by a simple expression derived from Zuber-Findlay's equation. The gas holdup in the conical section alone coincided with the correlation of gas holdup for the standard bubble column, using an appropriate mean superficial gas velocity. The liquid circulation velocity was correlated by a semitheoretical equation derived from the pressure balance along the circulation loop.