The mass transfer and hydrodynamics of a gas-liquid centrifugal de-oxygenator

Abstract

The mass transfer and hydrodynamic characteristics of a packed rotary contactor with a continuous liquid phase for the de-oxygenation of water using a stripping gas has been investigated. The primary purpose of this research was to gain a clearer understanding of the physical processes that occur within packings between the gas and liquid phases in an increased gravitational environment. The eventual aim is to design and develop a more efficient and cost effective industrial piece of equipment for the removal of dissolved oxygen from river or sea water. The mass transfer between two phases is directly related to the interfacial area which, in turn, is dependant on the gas bubble size. The use of centrifugal acceleration to generate increased gravitational environments leads to smaller bubbles being produced, with subsequent improvements in the mass transfer. In order to produce this increased gravitational environment, a one metre diameter rotor filled with a torus shaped packing was rotated between 200 and 400 rpm, with the gas phase dispersed in the liquid phase and passing through counter-currently. An examination of the overall gas and liquid flow through the packing in the rotor using visual and tracer techniques has been made which shows that the gas nozzle design and liquid flowrate are the two dominant parameters in achieving an effective and uniform distribution throughout. The gas bubble sizes produced have been visually analysed throughout the packing, and found to range from between 0.4 - 1.0 mm. in diameter. The mass transfer achieved in the rotor showed general trends of increasing with the gas flowrate and rotational speed, whilst falling as the liquid flowrate increased. For the entire range of rotor opemting conditions, the number of mass transfer units achieved was found to be in the range 1.5 - 4.5, and the corresponding height of a transfer unit between 4.5 - 12 cm.