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Comment PDF Separation Processes

Facts at your Fingertips: Distillation

By Scott Jenkins |

Computer-generated simulations of distillation processes are often validated experimentally. This reference discusses three concepts that influence comparisons between experimental results and simulation predictions: vapor-liquid equilibrium (VLE) of the components; relative liquid-to-vapor flowrates (L/V ratios in the column); and vapor-liquid contactor (tray) efficiency. Understanding these concepts can help explain discrepancies between experimental data and simulation results.

Vapor-liquid equilibrium

The relative volatility of an ideal binary chemical system is equal to the ratio of the two pure-component vapor pressures. If the components’ interactions are close to ideal and the components have sufficient relative volatility (>2), then small errors in VLE predictions may not significantly affect the separation. However, if the relative volatility is small (<1.5), deviations between predicted and actual VLE will lead to large differences between the actual and predicted number of stages required to accomplish the separation.

Non-ideal systems often have a pinch at the upper or lower end of the binary VLE curve (vapor and liquid compositions of both components are nearly equal). The pinch is usually at the upper (right-hand) end of the VLE curve. Here, the actual vapor pressure of the heavy-boiling component is nearly equal to that of the light-boiling component, due to the increased liquid-phase activity coefficient of the heavy-boiling component at infinite dilution. With sufficient non-ideality, the activity coefficient of the heavy-boiling component will become even greater, causing the heavy-boiling component’s actual vapor pressure to exceed that of the light-boiling component at infinite dilution of the heavy-boiling component. This is the case for minimum-boiling azeotropes, and at the azeotrope composition, the actual mixture vapor pressures (pure-component vapor pressure × activity coefficient at the component’s liquid concentration) of the two components will be equal. The closer the composition is to a pinch zone or azeotrope, the greater the effect of errors in the VLE prediction.

Figure 1.  Murphree efficiency represents how close the actual vapor composition comes to reaching equilibrium with the liquid on an actual distillation tray. Non-attainment of equilibrium is shown as a pseudo-equilibrium line on a McCabe-Thiele diagram

Figure 1. Murphree efficiency represents how close the actual vapor composition comes to reaching equilibrium with the liquid on an actual distillation tray. Non-attainment of equilibrium is shown as a pseudo-equilibrium line on a McCabe-Thiele diagram

Liquid/vapor ratios

The relative vapor-to-liquid flowrates within the column are not only determined by the feedrate, reflux ratio, boilup ratio, and distillate-to-bottoms ratio, but also by heat effects. Excessive heat loss from a high-temperature column with many stages can significantly increase the internal reflux. This heat loss reduces both the liquid and vapor flowrates as one progresses up the column. The L/V ratio will be the lowest at the top of the column, where it can be experimentally determined from the reflux and distillate flowrates.

However, the L/V ratio will progressively increase as one moves down the column, due to condensation of the vapor. This effect can be thought of as an increase in the effective reflux ratio going down the column. This increase in L/V ratios due to heat loss will cause the column to have a better separation than predicted by a computer simulation, but at the expense of higher energy costs and lower capacity than expected. Without understanding that the actual L/V ratios are different than expected, the VLE curve would appear to be more open (higher relative volatility or less pinched) than is really the case.

Efficiencies

Process simulators generally use theoretical stages to represent trays in a distillation column, but in real columns, the vapor leaving a tray is seldom in equilibrium with the liquid from that tray. The difference between the actual and equilibrium vapor compositions can be expressed as a Murphree vapor efficiency. Values tend to range from 50–75% for lab- and pilot-scale columns, but can lie outside this range. The Murphree vapor efficiency is an indication of how closely the vapor composition in an actual distillation tray approaches equilibrium with the liquid leaving that tray (Figure 1).

At 100% efficiency, the number of actual trays required for a separation would equal the number of theoretical trays. At efficiencies less than 100%, the required number of actual trays will be greater than this theoretical number. Therefore, if the actual tray efficiencies are lower than predicted, more actual trays will be required to accomplish a desired separation than would be anticipated based on the required number of theoretical trays and the predicted efficiency. Without properly accounting for the reduced efficiency, the separation will appear to be more difficult than predicted.

The efficiency of packed columns is routinely expressed as HETP (height equivalent to a theoretical plate). If the actual HETP is greater than predicted, the column will contain less theoretical stages than predicted, producing similar results to a column with trays that have lower efficiencies. n

Editor’s note: This column was adapted from Graham, G., Pednekar, P. and Bunning, D., Experimental Validation of Column Simulations, Chem. Eng., February 2018, pp. 30–37.

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