SPLIT-CYLINDER AIRLIFT REACTORS: HYDRAULICS AND HYDRODYNAMICS OF A NEW MODE OF OPERATION

Abstract

Abstract An unusual mode of operation of internal-loop airlift reactors is described as being of potential use in extending the range of practicable operating volumes or turndown ratios in wastewater treatment applications. Unlike normal operation, the new mode employs an inital static liquid level that is lower than the upper edge of the draft-tube or the splitting baffle. Hydrodynamics of the operation are analyzed using fundamental principles and an equation is developed for predicting the minimum required gas flow for attaining recirculation of the liquid. The equation is proven experimentally using data for air-water in split-cylinder reactors (aspect ratio = 7; volume = 0.08 m3; riser-to-downcomer cross-sectional area ratio = 2.44; − 0.03 < hc ≤ 0.12, where hc is the dimensionless clearance between the upper edge of the baffle and the static height of gas-free liquid), Effects of the clearance hc on gas holdup in the riser and downcomer, mixing behavior and liquid circulation rate are presented. In terms of mixing characteristics, the riser behaves as a bubble column until the gas-liquid dispersion spills into the downcomer. Further increase in gas flow rate gradually increases recirculation of liquid and the characteristic oscillatory tracer response pattern of airlift reactors is obtained eventually. Gas holdup in the riser is consistently affected by the hc value although the effect is small. In contrast, the hc -value strongly affects the downcomer gas holdup. The commonly observed linear relationship between the riser and the downcomer gas holdup that applies to the normal mode or operation, breaks down for the configuration tested. The downcomer remains gas-free until a certain critical gas holdup (dependent on Ac-value) has been attained in the riser. Further increase causes a rapid increase in the downcomer holdup. Similarly, the variation of induced liquid circulation rate with gas flow velocity is different from the characteristic patterns reported for the normal mode of operation. In summary, for all the main hydrodynamic parameters tested, the behaviour of the new mode of operation differed from the previously reported characteristic behaviour of internal-loop airlift devices.