Mass Transfer: Distillation Process

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. 

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 (p_A = x_APv) and vapor phase pressure is calculated by Dalton’s law (p_A = y_AP_T); 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.

Fluid Flow: Types of Fluids

Every fluid behaves differently according to particle size, shape, distribution, particle volume fraction, particle-particle interaction. There are some external parameters also that affect the behaviour like type, rate, and time of deformation. The fluids are categorized in time dependent (thixotropic and rheopectic) and time independent (Newtonian and non-Newtonian fluid).  

1. Thixotropic: Viscosity decreases with time of deformation, that does not get thicken with time like paint should be thixotropic

2. Rheopectic: Viscosity increases with time of deformation

3. Newtonian Fluid: Viscosity remains constant with time and rate of deformation

4. non-Newtonian Fluid:

a) Dilatant: Viscosity of fluid increases with rate of shear that doesn't get diluted i.e. shear thickening. These type of fluids can be used to make bullet proof jacket i.e. when force is applied it will behave like a solid. One more application can be in the knee joint, where we need a material that acts like a solid when we rest and act like a lubricant when we move.

b) Pseudoplastic: Viscosity of fluid decreases with rate of shear i.e. shear thinning. Like toothpaste, when we hold it in upside down, it will not come out, but when we apply a force, it starts coming. i.e. viscosity of the paste decreases.