Separation Performance Measurements in Distillation: Finite and Infinite Reflux Ratio Operation

The separation performance of multistage distillation processes can usually be predicted within a deviation of about ±25%, but deviations of up to ±80% may occur, especially for foaming mixtures, systems with high water content, or increased viscosity. To systematically investigate the influence of elevated viscosity, polymer-enhanced test mixtures are used. However, conventional test setups are unsuitable for such mixtures, as a bottom product purge is required, leading to partial reflux conditions, which are rarely studied. To address this, an alternative setup similar to a stripping column was developed. Using the well-known chlorobenzene/ethylbenzene test mixture, experiments under both total and partial reflux were conducted and evaluated via McCabe-Thiele analysis and the Delft mass transfer model. Results show that HETP values (Height Equivalent to a Theoretical Plate) under partial reflux are about 2.7 times higher than under total reflux, indicating a significantly reduced separation performance. The Delft model underestimates HETP values under partial reflux by up to a factor of six. This work thus provides the first systematic quantification of the substantial impact of reflux conditions on distillation performance.