CFD analysis of spinning cone columns: prediction of unsteady gas flow and pressure drop in a dry column

Abstract

Computational fluid dynamics (CFD) simulations have been carried out on the gas flow patterns and pressure distribution in a laboratory-scale (0.15 m diameter) spinning cone distillation column (SCC) in the absence of liquid flow. These simulations show that the flow becomes unstable at a hydraulic Reynolds number ( Re hyd, based on the minimum passage size between the cones) above a critical value of about 100. The instability leads to high-frequency oscillations of the velocity components and pressure about their mean values. The discrete nature of the pulsation spectrum indicates that the flow regime is an unsteady laminar one for the small-scale SCC considered here, not a turbulent one. This unsteady regime occurs across the entire range of the column operation (200< Re hyd<2000). The instability develops regardless of whether the inner cone is rotating (as in normal operation) or not. For normal (rotating) operation, the pulsations are synchronised with the rotor motion. Comparison of time-average pressure values with experiment shows that the actual pressure drop through the column stage is predicted by the CFD model to within 10–15%. The pressure pulsations are likely to cause mechanical vibrations and are a consequence of the flow instability.