Progress in Understanding the Physical Processes inside Spinning Cone Columns

The dynamics of gas flow through spinning cone columns (SCCs) have been studied through Computational Fluid Dynamics (CFD) simulations, and the liquid flow characteristics on the rotating surfaces have been correlated through dimensional analysis. The CFD simulations have been carried out in the absence of liquid flow. Flow instabilities are predicted above a critical value of the hydraulic Reynolds number (Re, based on the minimum passage size between the cones) of about 100. Highfrequency oscillations of the velocity components and pressure are predicted to occur about their mean values. The flow regime is an unsteady laminar one, not a turbulent one, for small-scale SCCs, as shown by the discrete nature of the pulsation spectrum across the entire range of the column operation (200 < Re < 2000) irrespective of whether or not the inner cone is rotating (as in normal operation). The pulsations are synchronised with the rotor motion for normal (rotating) operation and are likely to cause mechanical vibrations as a consequence of the flow instability. The actual pressure drop through the column stage is predicted by the CFD model to within 1015%. Key parameters determining mass transfer characteristics and overall performance of SCCs include the thickness and velocity of the thin liquid films flowing up a rotating cone surface. The turbulent flow conditions of commercial-scale equipment make laminar model predictions for these devices inapplicable. Dimensionless empirical models for the average thickness and radial velocity of wavy film have been developed based on thickness measurements on a laboratory-scale cone. These measurements were made using the intensity of induced fluorescence of a flowing film illuminated by an ultraviolet light source. The film is modelled as a wavy layer on top of a laminar sub-layer attached to the disk surface, with the thickness of the film being an additive modification of the Nusselt model thickness 3 3 / 2 95 . 1 91 . 0 − − + + + η + η = δ + δ = δ wave N , where η is a normalised radial distance. The thickness of the wavy layer + δwave has been correlated with 95% confidence limits of ±12%. The proposed models, in dimensional form, express the film thickness and radial velocity as functions of cone geometrical and operating parameters. Independent velocity measurements on a rotating cone, and film thickness measurements on rotating disks, support this correlation. The normalised film thickness is predicted to be preserved for spinning cone columns of varying size scaled at constant relative capacity. This work is part of a program to develop a virtual computer model of SCCs for future design and optimization of this type of equipment.