Saponification in the Lab and the Industry

The laboratory preparation of soap (known as the cold process) involves the following steps:

  1. Cofa (solid coconut oil) is melted in a saucepan (over a hot plate). Some Castor oil is added.
  2. Concentrated sodium hydroxide is heated to about 37oC
  3. The sodium hydroxide is then stirred into the coconut oil.
  4. This mixture is heated and stirred for up to 40 minutes. This can be stopped when the mixture is successfully traced (indicating it has reached a suitable thickness).
  5. It is then poured into a mould and allowed to solidify over time. (It is possible to add saturated salt water solution to the mixture in order to force the soap to precipitate out (this process is called 'salting out').

The industrial preparation of soap is much different. Two of the three main process for them are:

  • Kettle boiled batch process:

This process is carried out in large, heated, pressurised steel containers called kettles. The fats and oils are first mixed with concentrated sodium hydroxide in the kettles. The mixture is boiled using high-pressure and high-temperature steam (from injectors in the kettle). Additional hot brine and steam are added at the end of saponification to salt out the soap and wash it free of glycerol. The mixture settles for several days and the soap curd eventually collects at the surface. The brine, glycerol and soap curds are separated from one another and individually collected.
The soap is washed with water and then sprayed and vacuum dried. It is converted into small pellets with fragrances and colours before reblending and reprocessing into the desired form (liquid, powder or bars).

  • Fatty acid neutralisation process:

This process occurs in two steps. In the first step, high pressures (5MPa) and temperatures (250oC) are used to break down fatty esters, with steam, into fatty acids and glycerol. The reaction is carried out in long steel tubes (20m long x 1m diameter) in the presence of a zinc oxide catalyst. These are fractionally distilled and various mixtures of these are used in the second step. In the second step, the fractions are stoichiometrically neutralised with hydroxides to produce soap. The soap is recovered by salting out as usual, and processed.

Similarities

  • Fats and oils are mixed with concentrated alkali and heated.
  • The crude soap is then washed.

Differences

  • A blend of fats and oils is used in industry rather than one fat or oil used in the laboratory.
  • Concentrated brine is used to separate the soap from the aqueous phase in the industry.
  • High-pressure steam is used to heat and stir the mixture in industry; in the laboratory a glass rod and a hotplate is used to simulate these effects.
  • The glycerol is removed and purified in the industry; in the laboratory some glycerol remains in the soap.
  • In industry, settling of the soap occurs over several days; in the laboratory the soap is relatively crude.
  • No fragrances and colour were added to the soap produced in the laboratory.