The influence of gas and liquid physical properties on entrainment inside a sieve tray column

[摘要] ENGLISH ABSTRACT: Distillation column design and operation require understanding of both the hydrodynamicand thermodynamic behaviour and limitations. One of the hydrodynamic aspects thatnegatively influence separation efficiency in the distillation column is entrainment of theliquid with the rising vapour or gas. Inaccurate entrainment predictions will lead to poorseparation efficiencies in the column and consequently over design of the column diameterand/or height has to be incorporated. This has a significant impact on the capital cost due tothe size and scale of industrial columns. Therefore, small improvements in entrainmentprediction will lead to large savings in capital investment.Previous research published in the open literature focused primarily on the influence of gasand liquid flow rates and, tray geometry on entrainment for the air/water system.Consequently the non-air/water database is small and consists of data obtained fromvarious tray and column geometries. As a result the accuracy of current entrainmentprediction models is questionable for systems other than air/water. Therefore, the firstobjective of this work was to investigate whether current prediction models perform wellfor systems other than air/water. To prove this air/water, air/ethylene glycol and air/siliconoil data were measured and compared with current prediction correlations. It was foundthat current prediction models perform poorly for the air/ethylene glycol and air/silicone oilsystems. At the same time a new observation was made with regard to froth developmentand behaviour inside the column. The observation shows that liquid flow rate has a nonmonotonicinfluence on entrainment, caused by the short (475mm) tray flow path.The second objective was to examine the influence of gas physical properties onentrainment. New entrainment data were measured by individually contacting air, CO2 andSF6 with water and ethylene glycol, while n-butanol was contacted with CO2 and SF6. Thedata was compared with current prediction models which performed poorly for SF6 results.This shows the inability of these models to predict entrainment for gas systems with highdensities. Modified Reynolds and Froude numbers were developed to show the influence ofgas physical properties on entrainment. Low modified Reynolds numbers and large modifiedFroude numbers resulted in high entrainment.The third objective was to determine the influence of liquid physical properties onentrainment. New entrainment data were measured using CO2 with Isopar G, n-butanol,water, silicone oil and ethylene glycol. Current prediction models compared poorly to thedata and did not include the influence of liquid viscosity on entrainment. It was found thatviscosity had an intricate non-monotonic influence on entrainment.The fourth and final objective was to correlate the influence of gas and liquid properties onentrainment as determined by the previous two objectives. To make the dataset morecomplete, entrainment was measured for four tray spacings using CO2/Isopar, CO2/nbutanol,air/ethylene glycol, CO2/ethylene glycol, air/silicone oil and CO2/silicone oil (over1700 data points). Two new correlations are presented to predict the fraction of liquidentraining with the rising gas (L'/G with R2 = 85%) and the fraction of liquid entering the traythat entrains (L'/L with R2 = 92%). The performance of the L'/G correlation (R2 = 85%) isvastly superior to two other prominent correlations (R2 = 61% and 23%). This correlation canbe implemented to predict entrainment successfully for different tray geometries bycombining the predicted influence of tray geometry, by Kister and Haas (1988), with resultsfrom the newly developed correlation. All four objectives are presented as manuscripts forjournal publication and serve as alone standing documents.

[发布机构] Stellenbosch University