Question

When calculating multicomponent distillation, why is it best to list the components in order of decreasing volatility?

Answer 1

Multicomponent Distillation

1.Need more than one distillation tower, for n components, n-1fractionators are required

2. Specification Limitations

The following are established at the beginning

1. Temperature, pressure, composition, and rate of the feed.

2. Pressure of the distillation

3. Feed to be introduced on that tray which will result in the least total number of trays (optimum feed-tray location)

4. Heat loses (even if assumed to be zero)

Under these circumstances only three additional items of the following list need to be specified,any other items must be calculated.

1. Total number of trays.

2. Reflux ratio

3. Reboil ratio (ratio of vapor produced by the reboiler to residue withdrawn)

4. Concentration of one component in one product (a maximum of two may be chosen)

5. Ratio of flow of one component in the distillate to the flow of the same component in the

residue, or split of the component (a maximum of two may be chosen)

6. Ratio of total distillate to total residue

Key components

The feed components are listed in order of their relative volatility. The more volatile components are called light, the less volatile are heavy. The components separated in a tower are called light key (more volatile) and heavy key(less volatile). Any components lighter than the light key are called light components , while those heavier than the heavy key are called heavy components.

In hydrocarbon systems, because of nonidealities, the equilibrium data are often represented by

yA= KAxA, yB= KBxB, yC= KCxC, yD= KDxD

where KA is the vapor-liquid equilibrium constant or distribution coefficient for component A.

K is a function of temperature and pressure, but is assumed to be independent of composition, which is sufficiently accurate for most engineering calculations. For an ideal system,

KA= PA*/PT= vapor pressure/total pressure.

The relative volatility in multicomponent distillation is always computed with respect to the heavy key

a_j =K_j/K_hk= (y_je/x_je)/(y_hke/x_hke)

The value of a is greater than 1 for any component lighter than the heavy key and less than 1 for any heavier component.