Distillation is a very important mass transfer operation which is used to separate components on the basis of difference in boiling points. It is mainly used in refineries to distil crude oil into useful petroleum products.
As in distillation, all the components are volatile, so all components will be there in liquid and vapour phase, but the more volatile component (lesser boiling point), has more tendency to be in vapour phase as compared to liquid phase at any temperature.
As in distillation, all the components are volatile, so all components will be there in liquid and vapour phase, but the more volatile component (lesser boiling point), has more tendency to be in vapour phase as compared to liquid phase at any temperature.
Distillation column is divided into 2 sections depending on concentration of more volatile component. The two sections stripping section and rectifying section or extraction and enriching section. These sections are divided based on the location of feed plate.
Above the feed plate, where the more volatile component has higher concentration as compared to other component gets enriched or rectified is rectification/enriching section.
And below the feed plate, where the more volatile component generally has lower concentration as compared to other component gets extracted or stripped from the solution is stripping/extraction section.
In distillation of binary mixture let say benzene (x in liquid, y in vapor) and toluene ;
Let at any plate x = 0.3 and y = 0.6; then how can benzene be transferred from the liquid to vapour phase, that means low concentration (0.3) to high concentration (0.6)? The answer is equilibrium concentration of benzene in liquid and vapor phases.
The concentration can be expressed as pressure of benzene. The saturation pressure of benzene in the liquid phase (ideal) is calculated through Raoult’s law (pA = xAPv) and vapor phase pressure is calculated by Dalton’s law (pA = yAPT); if the saturation pressure in liquid phase is higher, then the component will go in vapor phase increases till the pressure in vapor phase becomes equal to liquid phase saturation pressure. The equilibrium composition changes with temperature, therefore the temperature increases as we go down the column.
When vapour and liquid streams at different temperatures (not in equilibrium) are brought in contact, more volatile in the liquid phase will vaporise and less volatile from the vapour phase will condense. So, vapour phase will be enriched with more volatile and liquid phase will be enriched with less volatile components.
Let at any plate x = 0.3 and y = 0.6; then how can benzene be transferred from the liquid to vapour phase, that means low concentration (0.3) to high concentration (0.6)? The answer is equilibrium concentration of benzene in liquid and vapor phases.
The concentration can be expressed as pressure of benzene. The saturation pressure of benzene in the liquid phase (ideal) is calculated through Raoult’s law (pA = xAPv) and vapor phase pressure is calculated by Dalton’s law (pA = yAPT); if the saturation pressure in liquid phase is higher, then the component will go in vapor phase increases till the pressure in vapor phase becomes equal to liquid phase saturation pressure. The equilibrium composition changes with temperature, therefore the temperature increases as we go down the column.
When vapour and liquid streams at different temperatures (not in equilibrium) are brought in contact, more volatile in the liquid phase will vaporise and less volatile from the vapour phase will condense. So, vapour phase will be enriched with more volatile and liquid phase will be enriched with less volatile components.
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