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

An unusual mode of operation of internal-loop airlift reactors is desc ribed as being of potential use in extending the range of practicable operating volumes or turndown ratios in wastewater treatment applicati ons. Unlike normal operation, the new mode employs an inital static li quid level that is lower than the upper edge of the draft-tube or the splitting baffle. Hydrodynamics of the operation are analyzed using fu ndamental principles and an equation is developed for predicting the m inimum required gas flow for attaining recirculation of the liquid. Th e equation is proven experimentally using data for air-water in split- cylinder reactors (aspect ratio = 7; volume = 0.08 m(3); riser-to-down comer cross-sectional area ratio = 2.44; -0.03 < h(c) less than or equ al to 0.12, where h(c) is the dimensionless clearance between the uppe r edge of the baffle and the static height of gas-free liquid). Effect s of the clearance h(c) on gas holdup in the riser and downcomer, mixi ng behavior and liquid circulation rate are presented. In terms of mix ing characteristics, the riser behaves as a bubble column until the ga s-liquid dispersion spills into the downcomer. further increase in gas flow rate gradually increases recirculation of liquid and the charact eristic oscillatory tracer response pattern of airlift reactors is obt ained eventually. Gas holdup in the riser is consistently affected by the h(c)-value although the effect is small. In contrast, the h(c)-val ue strongly affects the downcomer gas holdup. The commonly observed li near relationship between the riser and the downcomer gas holdup that applies to the normal mode of operation, breaks down for the configura tion tested. The downcomer remains gas-free until a certain critical g as holdup (dependent on h;value) has been attained in the riser. Furth er increase causes a rapid increase in the downcomer holdup. Similarly , the variation of induced liquid circulation rate with gas flow veloc ity is different from the characteristic patterns reported for the nor mal mode of operation. In summary, for all the main hydrodynamic param eters tested, the behaviour of the new mode of operation differed from the previously reported characteristic behaviour of internal-loop air lift devices.