Mass Transfer Distillation


Distillation is an operation in which the constituents of a liquid mixture are separated using thermal energy. The difference in the vapor pressure is responsible for a separation. Distillation, particularly important in petroleum refining operations. Separation of components by distillation is not possible for α = 1 which says that the ratio of vapor pressures of  high volatile matter to that of low volatile matter in the mixture, so this ratio which is called as relative volatility should always be greater than 1 which is determined from the lab experiments. The relative volatility decreases with increasing pressure in the system. An example of an ideal solution is a mixture of benzene, toluene and xylene which is preferred for understanding the concept of relative volatility. simple distillation set is enough to determine the relative volatility of the mixture.

In a distillation operation, separation is accomplished by application of heat. Raoults law is applicable to ideal solutions, which explains for ideal solutions relative volatility 'α', is related to the vapor pressure of components in a binary mixture according to the relations: α = PA /PB. Minimum boiling azeotrope occurs when the total pressure curve at a constant temperature passes through a maximum. One of the examples of minimum boiling azeotropes is a mixture of ethanol and water. Steam distillation is usually employed for insoluble liquids and a component which is heat sensitive. A system comprising of two insoluble liquids, A and B, will boil when the total pressure Pt is equal to PAsat + PBsat . In azeotropic mixture, the equilibrium vapor composition at the azeotropic point is equal to the liquid composition. Positive deviation from Raoult's law is exhibited by a mixture whose total pressure is greater than that computed for ideality. One of the examples of maximum boiling azeotropes is a mixture of hydrochloric acid and water and acetone and chloroform. In batch distillation with constant reflux, the overhead product purity decreases with time.

In flash vaporization, the product vapor may be in equilibrium with the liquid leaving the flash chamber and the extent of equilibration depends on the vapor – liquid contact time in the chamber. In order to obtain an overhead product of constant composition from a batch distillation column, the reflux ratio should be increased gradually. In a continuous distillation column as the reflux ratio is increased, then overhead product purity increases. At total reflux in continuous distillation the numbers of plates are maximum. At total reflux both the enriching and the exhausting section operating lines merge with the diagonal.
Fenske's equation is used to calculate number of plates in a distillation column at total reflux and for system having constant relative volatility. Minimum reflux ratio corresponds to that situation when infinite numbers of theoretical stages are required for the separation desired when minimum reboiler heat load is required when minimum condenser cooling load is required. The driving force for separation by distillation is the highest at total reflux. At total reflux the reflux ratio is infinite. As the reflux ratio increases the slope of the enriching section operating line increases.

If xD is the mole fraction of the more volatile component in the overhead product and R is the reflux ratio, the intercept and slope of the enriching section operating line in a McCabe –Thiele diagram is xD/(R+1) and R/(R+1). When the feed to a distillation column is a saturated liquid, the slope of the feed line is infinity. For example 100 moles of vapor per hour is leaving the top plate of a continuous distillation column and overhead product at the rate of 50moles/hr is being withdrawn. Then the reflux ration is 1.0. The distillate flow rate from a distillation column is 100 Kmole/hr and the reflux ratio is 2. The flow rate of vapor from the top plate is 300 Kmole/h. In the enriching section of a continuous distillation column vapor is enriched with respect to the more volatile component. A partial condenser is considered as a theoretical plate when vapor and liquid leaving the condenser are in equilibrium.

When open is employed for heating in continuous distillation column, the stripping section operating line in McCabe-Thiele diagram passes through [x w' o] xW is the mol fraction of the more volatile component in bottom product. When saturated vapor is fed to a distillation column, the slope of the feed line is zero. The ratio of "slope of the enriching section operating line corresponding to minimum reflux" to "slope of the operating line corresponding to reflux large than minimum" is less than one.
When feed to a distillation column is a mixture of 50% vapor and 50% liquid, the slope of the feed line is 0.5.