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Multicomponent Distillation Column

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Consider a distillation column operating at 101.325 kPa. This column contains 10 stages, a partial reboiler, and a total condenser. A multicomponent mixture composed of 40 mole% benzene, 30 mole% toluene and 30 mole% ‐xylene is to be separated by this column. The feed is a saturated liquid with a flow rate equal to 100 kmole/hr. Feed enters at stage 5 counting from the top. At normal operating conditions, the reflux and reboil ratios are both set to 3. If one assumes a uniform tray spacing equal to 24 inches and an operating vapor-phase velocity equal to 80% of the value of the flooding velocity, then the column diameter can be calculated and is equal to 1.35 m. The active area is set to 80% of the total cross-sectional area of the column.

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The momentum balance for each tray is neglected. The Francis weir formula is assumed and provides the additional equations used in the Demonstration to compute molar hold-up of the trays. The weir height is set equal to 5 cm. In addition, condenser and reboiler volumes are taken to be equal to 2.5 .

A step in either the reflux or reboil ratio is applied at time 10 minutes. For every stage, the Demonstration shows plots of the composition and the temperature profiles as well as the molar hold-up (all variables are plotted versus time in minutes) for user set values of the percent step.

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Contributed by: Housam Binous and Ahmed Bellagi (January 2013)
Open content licensed under CC BY-NC-SA

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The Francis weir formula is given by:

,

where is the molar hold-up at stage in kmol, is the weir height in meters, is the molar flow rate in kmol/min, is the gravitational acceleration, is the active area, is the weir length (calculated from the knowledge of the active area and from purely geometrical considerations), and is the liquid density at stage .

Expressions for pure component molar liquid densities and vapor and liquid enthalpies were adapted from Aspen HYSYS.

The mixture is assumed to obey Raoult's law since it is composed of only aromatic compounds.

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