Distillation and Distillation Column

How does mass transfer occur in a distillation column and how the concept of distillation have become so important in purification and separation process are the interesting topics for the chemical engineers. Designing a distillation column should been known to all the fresh engineers after completion of their course study. Ok, how do we define a distillation operation? Simple definition is “Distribution of substances or components between a gas or vapor and liquid phase”. This evolves the objective of adjusting the phases.

 One thumb rule is to be memorized is that distillation means the heat content (heat relation of substances) i.e. energy property related to the mass percentage. The advantage of distillation operation is that it is the ultimate purification process for chemical products. That is why it dominates all mass transfer operations. The latest research around the world is focused on the packing structure, material of construction and plates for stage contacts. Application of heat plays an important role in distillation process that is why thermodynamic properties data are required (which include the heat capacity data at particular temperatures) and can be obtained for the research papers and standard data packages and materials.

 Before learning distillation operation one should gain knowledge on the concept of EQUILIBRIA. From physical chemistry, this concept is explained in details which include the equilibria which exist between liquid and vapor phases, properties of phases of each compound. It explains about the graphs that show the relation between temperature, vapor pressure, and concentration of the compounds. This information is sufficient to the design equation of equipment used for distillation of required components. By apply simple material and energy balance over the process equipment they optimize the amount of flow rates, heat to supplies, condenser capacity, and number of stages required. McCabe-Thiele methods and Ponchon-Savarit method are frequently used for estimation of calculating number of theoretical plates of a distillation column is based on both material and enthalpy balances.

If Rm is the minimum reflux ratio the optimum reflux ratio is 1.25 to 1.5 Rm. Rayleigh equation applies to differential distillation. If feed to a distillation column is introduced onto a plate one plate below the feed plate, the total number of theoretical plates required for a desired separation will increase. There is a relationship between the reflux ratio used in a distillation column and the column diameter as the reflux ratio increases, the column diameter increases. Azeotropic distillation is a special case of multicomponent distillation for azeotropic separation of acetic acid-water mixture the entrainer that is employed is n-butyl acetate and for azeotropic separation of ethanol-water mixture, the entrainer that is used isobenzene.

Types of Distillation Operations

  1. Flash Distillation
  2. Steam Distillation
  3. Simple or differential Distillation
  4. Continuous Distillation
  5. Azeotropic Distillation
  6. Extraction Distillation
  7. Vacuum Distillation
  8. Distillation with Pervaporation  

Distillation at reduced pressure is practiced for separation of heat sensitive materials only and components having low volatilities at normal pressures. The minimum number of plates required for a desired separation by distillation does not depend on the thermal condition of the feed. 
To formulate calculations describing to distillation operation, a knowledge of vapour –liquid equilibria for the concerned system is a key requirement. Literature provides vapour – liquid equilibria data for various systems. In absence of this, the equilibrium data is computed from relative volatility ‘α’ as under
By definition
ALPHA(α) = Ya(1 - Xa)/Xa(1-Ya)
Where A refers to more volatile component, from this a relation between Ya and Xa is obtained as
Ya = α × Xa/ [1+ (α -1 )Xa]
In the absence of data, the value of ALPHA can be computed from vapour pressure data of the components as,
α ab = Pa/ Pb
 a and b are two components in the mixture.

FLASH DISTILLATION:  

In this case a definite-fraction of liquid is vaporised in such a way that the evolved vapour and residual liquid are in equilibrium with each other. The process is described by
Ya= - [1-f/f]Xa + X Fa/ f
Where f= molar fraction of feed vaporised.
Differential (simple) distillation: In this case the vapour is removed as soon as it is formed without appreciable condensation. The course of a differential distillation process is given by Rayleigh equation