CALCULATION OF CIRCULATING FLOWS IN BUBBLE-COLUMNS

Bubble columns are widely used as chemical reactors. The liquid phase is well mixed due to a liquid circulation present in practically all t echnical applications. The liquid circulation is shown experimentally (Lippert, 1982) to contain several circulation cells. Each cell is abo ut one bubble column diameter high and undergoes fluctuations in posit ion and circulation velocity. The liquid circulation is suppressed if the gas phase is distributed very evenly over the cross-sectional area of the bubble column as shown experimentally by Molerus and Kurtin (1 985, Chem. Engng Sci. 40, 647-652). Thus, the liquid circulation is a flow instability caused by a disturbance of gas distribution. The onse t of the liquid circulation is investigated in the present paper by ap plying an analytical method. The flow field of several consecutive cir culation cells is obtained applying a numerical method, which is speci ally adapted for this kind of flow instability. Our focus is on unders tanding the mechanism of the circulation cells. Thus, all terms of sma ller orders of magnitudes are neglected in the mass and momentum balan ces for both phases. The main terms, which cause the flow instability, are extensively discussed. We assumed a small but stationary disturba nce of gas distribution in order to simplify the calculations. The gen eral trend in two-phase modeling, even though there are some remarkabl e exceptions, is to implement more and more forces and higher sophisti cated turbulence models. But circulation cells are an example that som e flow phenomena may not be described following strictly that way. Bas ically, a very high spatial resolution and a very stable numerical met hod are required in order to predict circulation cells.