Computational fluid dynamics(CFD): Studies on Centrifugal contactors

Abstract

Annular centrifugal contactors find application in liquid–liquid extraction processes in various industries such as reprocessing of spent nuclear fuel, hydrometallurgy, petrochemicals, pharmaceuticals, etc. The special features of this device-high efficiency, low retention volumes, small size, and ease of maintenance-have made it a better extraction device compared to traditional devices like mixer-settlers. An annular centrifugal contactor consists of two concentric cylinders: the inner one rotating and the outer one stationary. The inner cylinder is provided with an opening at the bottom. The two phases are fed into the annular section between the cylinders, where one phase becomes dispersed in the other due to the high shear rate present there. The dispersion travels to the rotor bottom under the combined and opposing effects of gravitational and centrifugal acceleration and enters the rotor through the opening at the bottom. The high centrifugal acceleration in the rotor zone pumps up the dispersion and induces coalescence of drops among themselves and with their home phase. The heavier phase moves outward, and the lighter phase moves inward. The separated phases are collected from the top of the rotor using an elaborate arrangement. Thus, in one combined unit, three processes take place: mixing, pumping, and separation. In this thesis, an effort is made to improve the understanding of the hydrodynamics of flow in the rotor and the mixer regions using a commercially available computational fluid dynamics software. Full?scale simulations are carried out for three different multiphase problems using the VOF (Volume of Fluid) method for tracking the interface and the geo-reconstruct method to construct the interface. The flow field in a partially filled rotating cylinder, closed at the bottom and open at the top, is investigated under no?net?upward?flow conditions. The widely known solution to this problem, in which no interaction is permitted between the liquid and the gas phases, leads to a parabolic shape of the interface and solid?body?like rotation of the liquid at steady state. When interaction is permitted between the two phases in full?scale simulation, we find that gas is sucked into the cylinder from the inner region of the open top. It travels downward, changes direction, moves upward along the gas–liquid interface, and is expelled from the outer region of the open top. The gas moving upward along the interface drags liquid with it. The liquid present near the solid wall also climbs upward. Both liquid streams climbing upward meet at the top and move downward along the interface. The 2D axisymmetric simulations thus predict the presence of two circulation loops in the liquid, and gas entering from the inner region of the open top and escaping from the peripheral region. Simulations were also carried out for a three?phase system consisting of carbon tetrachloride–water–air. The air–water interface behaved in the same way as in the previous case. The simulations were next carried out with liquid entering from the bottom and leaving from the top, as in the pumping mode. Simulations show that the angular momentum of the incoming liquid significantly influences the shape of the gas–liquid interface and the maximum pumping capacity of a rotor. Traces of recirculation loops observed in the liquid phase in the no?upflow case are also seen in the net?upflow case. The effect of the height of baffles located below the rotor on the hydrodynamics of flow through the annular zone-investigated experimentally and computationally earlier in the group-was re?examined. In these experiments, critical rotational speed was measured for various baffle heights, annular gaps, and gaps below the rotor for no?net?flow and a fixed steady?state liquid height in the annular zone. The simulations carried out earlier, with the assumption of a flat interface in the annular region, captured the dependence of critical angular speed on various parameters except baffle height. These simulations were repeated in this work using the VOF method so that the interface shape is computed as part of the simulations. The predictions showed significant improvement in agreement for one baffle height but not for another case.