Separating Chemical Compounds | Methods | Unit Operations

Sometimes we have to separate a chemical compound from a mixture. We use different physical characteristics such as boiling point, polarity of compounds to separate chemical compounds from mixtures. But sometime we cannot obtain 100% pure compound after separating.


Separation of NaCl from sea water is an example for separating chemical compounds from a mixture

You know, there are a lot of salts are in the sea water such as NaCl, KCl, MgBr22 and more. But we want to separate NaCl from other chemicals (or decrease amount of other chemicals). We use EVAPORATION to remove water and then CRYSTALLIZATION to remove other salts.

Examples of separating chemical mixtures

Removing water from a solid is also an example to separating compounds. Follwing chemical industries use separation methods to prepare their products.

Separating of mixtures

Transferring of materials from one homogeneous phase to another is defined as separating of mixtures.

Petroleum Refining

Petroleum is a complex mixture of alkanes, cyclo alkane and aromatic compounds. Crude oil is not worth until it is separated into different compounds such as petrol, diesel, kerosene and many more.

Crude oil contains thousands of compounds. Composition of crude oil is classified according to their range of boiling points. These hydrocarbons are separated on the basis of their molecular mass by fractional distillation.

Advantages of separating compounds

Recover valuable materials

Valuable materials may be mixed with dirt or waste. But we need to recover or separate that valuable material. Therefore we have to follow a specific separating method to recover that material from initial mixture.

Economical advantages

Removing unwanted materials from a mixture will reduce the weight of mixtures. It is very important in transporting. So reducing weight will decrease the cost of transportation.

Separating methods of chemical compounds | unit operations

  • Distillation
  • Crystallization
  • Centrifugal Separations
  • Evaporation
  • Absorption
  • Adsorption
  • Drying
  • Chromatographic Separations
  • Liquid - liquid Extraction

These unit operations (separation methods) will be explained in detail later in this tutorial.

Mechanical separating methods of chemical compounds

Density or particle size is used when we use mechanical separating methods.

  • Cyclone separator - used in cement manufacturing process plants
  • Filter press
  • Gravity settlers

What physical properties are used in separating chemical compounds?

  • Vapor pressure
  • Solubility / Diffusivity
  • Volatility
  • Concentration differences

Boiling point

In distillation, boiling point is used to separate chemical compounds. The most industrial use of this is, petroleum refining process. Petroleum contains so many organic chemicals and they have different boiling point values.


Some compounds dissolve very well in some solvents than another solvent. In such cases, extraction is used to separate chemicals.

Explanations of unit operations

There are so many unit operations in Chemical Engineering which are helpful to separate one or more chemicals from a mixture. Now we are going to study those unit operations briefly.


Adsorption occurs, when a solid surface is exposed to fluid (a gas or a liquid). Also adsorption is defined as the enrichment of material or increase of density of the fluid in the vicinity of an interface

Adsorption occurs in two ways, physical (physisorption) and chemical (chemisorption).

Adsorption applications

  • Removal of organics from vent gases
  • Removal of SO2 from vent gases
  • Removal of H2O from air, methane, N2
  • Removal of solvent, odours from air
  • Removing NOx from N2

Absorption principle and applications

Transfer of one or more species from the gas phase to a liquid solvent is defined as absorption.

  • Species transferred to the liquid phase are referred to as solutes or absorbate.
  • No change in the chemical species present in the system.
  • Used to separate gas mixtures, remove impurities, or recover valuable chemicals.
  • The maximum amount of the gas that can be dissolved in a solvent at specific conditions (T,P) is called solubility.

Applications of absorption

  • Reduce environmental emissions

Drying principle and uses

final removal of water, or another solute, and the operation often follows evaporation, filtration, or crystallisation. Drying has more advantages than reducing weight.

  • To reduce the cost of transport due to less weight
  • For better handling of materials as, for example, with soap powders, dyestuffs and fertilisers.
  • To prevent chemical changes can be occurred when water is in the presence with product. Such as removing moisture which may cause to corrosion.

Drying is used in lot of chemical industries in the world.

Distillation and Applications

Distillation is one of the most common unit operation in the world and uses in so many industries to separate one or more chemicals from a mixture.

The separation of liquid mixtures into their various components is defined as distillation.

Distillation is the major unit operation in petroleum refining industry.

Applications of distillation in the industry

  • Crude oil distillation in the petroleum refining
  • Ethanol separation in the sugar manufacturing plant
  • In liquid air distillation to produce nitrogen and oxygen gases

Liquid - Liquid Extraction Principle and Uses

The separation of the components of a liquid mixture by treatment with a solvent in which one or more of the desired components is preferentially soluble is known as liquid–liquid extraction.

One of the uses of liquid liquid extraction is, separation of aromatics from kerosene-based fuel oils to improve their burning qualities and the separation of aromatics from paraffin and naphthenic compounds to improve the temperature-viscosity characteristics of lubricating oils.

Important factors of liquid liquid extraction

There are three steps in liquid liquid extraction

  1. Bringing the feed mixture and the solvent into intimate contact
  2. Separation of the resulting two phases, now the extracting material is in the solvent. As an example, two phases may be an aqueous phase and organic phase.
  3. Removal and recovery of the solvent from each phase.

Evaporation principle and applications

removal of water from a solution by boiling the liquor in a suitable vessel, an evaporator, and withdrawing the vapour. Heat is supplied to vaporize the solvent.

When evaporation occurs, concentration of material in the solution increases.

If solution contains solids, after evaporated, liquid will become saturated and crystals were deposited.

Applications of evaporations

  • in sugar manufacturing plant, to concentrate sugar solution, evaporation is done to remove water.

Crystallization theory and applications

Crystallization is a separation technique where a solid phase is separated from a mother liquor where the atoms or molecules are highly organized into a structure known as a crystal.

Forming a NaCl precipitation from ocean water is an example to crystallization. Also crystallization is used to large-scale productions of food additives.

Chromatography principle and uses

Chromatography is used to separation, identification and quantitative analysis of a mixture of compounds. This separation method was originally used to separate the different types of pigments in leaf extracts.

Also with new instruments chromatography can be used to identify components present in a mixture and to measure concentrations of components.

Dangerous drugs like heroin can be identified with quantitatively. Also chromotography is helpful to separate amino acids


  1. Coulson and Richardson’s, CHEMICAL ENGINEERING, VOLUME 2, FIFTH EDITION, CHAPTER 16, Drying, pg 901-970
  2. Coulson and Richardson’s, CHEMICAL ENGINEERING, VOLUME 2, FIFTH EDITION, CHAPTER 13, Liquid–Liquid Extraction, pg 721-770

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