Key Difference – Distillation vs Extraction
Although distillation and extraction are two of the most commonly used physical separation methods having an equal importance in the industry to obtain pure chemicals for many applications, there exists a difference between distillation and extraction based on their procedures. The key difference between distillation and extraction is that distillation follows heating of a liquid mixture and collecting the vapor of the liquid at their boiling point and condensing the vapor to get the pure substance whereas, in extraction, a suitable solvent is used for the separation process.
What is Distillation?
Distillation is one of the oldest, but still most frequently used method for separation of liquid mixtures, based on the differences in their boiling points. It includes heating a liquid mixture gradually to reach the boiling points of the liquids in the mixture, to get their vapor at different boiling points and is followed by condensing the vapor to obtain the pure substance in liquid form.
The liquids with lower boiling points (the most volatile substances) are boiled off first as the mixture is heated while less volatile substances remain in the mixture until the temperature in the mixture reaches to their boiling points. A specially design set of apparatus is used for the distillation process.
What is Extraction?
The process of extraction involves withdrawing of an active agent or a waste substance from a solid or a liquid mixture, using a proper solvent. The solvent is neither fully nor partially miscible with the solid or the liquid, but it is miscible with the active agent. The active agent transfers from the solid or liquid mixture to the solvent by intensive contact with the solid or the liquid. The mixed phases in the solvent are separated by centrifuging or gravity separation methods.
What is the difference between Distillation and Extraction?
Distillation and Extraction Methods
Consider a liquid mixture with four liquids, A, B, C and D.
Boiling points: Bpliquid A (TA) > Bpliquid B (TB) > Bpliquid C(TC) > Bpliquid D(TD)
(Least volatile compound) (Most volatile compound)
Temperature of the mixture = Tm
Upon heating the liquid mixture, the most volatile liquid (D) leaves the mixture first, when temperature of the mixture is equal to its boiling point (Tm = TD) while other liquids remain in the mixture. The vapour of liquid D is collected and condensed to get pure liquid D.
As the liquid is heated further, the other liquids also boil off at their boiling points. As the distillation process continues, the temperature of the mixture increases.
Consider an active substance A is in liquid B and they are completely miscible. The solvent C is used to separate A from B. Liquid B and liquid C are not miscible.
1: Substance A is dissolved in liquid A
2: After adding solvent C, some of the molecules in liquid A go to solvent C
3: As the time passes more molecules go to the solvent C. (Solubility of A in the solvent is greater than that of in liquid A)
4: Solvent C is separated from liquid A since they are immiscible. Another method is used to isolate A from the solvent.
Multiple extractions are done to completely separated A from solvent B. Temperature is constant in this process.
Distillation and Extraction Types
Distillation: The most commonly used distillation methods are “simple distillation” and “fractional distillation.” Simple distillation is used when the liquids to be separated have quite different boiling points. Fractional distillation is used when the two liquids to be separated possesses nearly the same boiling points.
Extraction: The most commonly available extraction types are “solid – liquid extraction” and “liquid – liquid extraction.” Solid – liquid extraction involves separating a substance from a solid using a solvent. Liquid – liquid extraction involves isolating a substance from a liquid using a solvent.
Applications of Distillation and Extraction
Distillation: This separation method is used in the fractional distillation of crude oil production, chemical and petroleum industry. For example, to separate benzene from toluene, ethanol or methanol from water and acetic acid from acetone.
Extraction: It is used to isolate organic compounds such as phenol, aniline and nitrated aromatic compounds from water. It is also useful to extract essential oils, pharmaceuticals, flavors, fragrances and food products.
Image Courtesy: “The extraction of Oil using steam” by Micov at English Wikipedia. (CC BY-SA 3.0) via Commons