Fatty Acid Neutralisation

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The Fatty Acid Neutralisation Process is considerably different from the saponification reactions typically undertaken in the school laboratory. This process is a two stage process in which the fats and oils are first hydrolysed to form fatty acids and then neutralised to form the fatty acid salt (soap).

Hydrolysis Step

This reaction is undertaken at elevated temperature and pressure so that water is quite soluble in the fats and oils being used (10-25%). The conditions normally used are 4-5.5 MPa and 240-270 °C, with the reaction being undertaken in tall, narrow steel tubes. A zinc oxide (ZnO) catalyst is often added to improve the rate of reaction.

The fats and oils are injected into the bottom of the column, with high pressure steam is injected into the column at various heights. The fatty acids tend to rise to the top of the reaction vessel, while the water flows to be towards the bottom of the vessel, carrying the glycerol with it. The glycerol can then be removed from the reactor pushing the equilibrium towards the products.

The fatty acids are removed from the top of the reactor and dried in a vacuum drier and distilled to reduce the impurities. The different fractions from the distillation are combined to form various mixtures of fatty acids, depending on the desired properties of the final soap.

Neutralisation Step

The fatty acid mix obtained from the hydrolysis step is neutralised by the addition of an appropriate caustic (often NaOH or KOH). Care must be taken to ensure that the reaction is mixed well and stoichiometric amounts of fatty acid and base are added so that the final mixture is neutral. This reaction is highly exothermic, so care must be taken to prevent boiling and foaming of the mixture. Some salt water is often added in this final step to control the texture of the solidified soap.

Key Differences

The key differences between the Fatty Acid Neutralisation Process and the saponification reactions performed in the school laboratory are:

  • the industrial process uses a blend of fats and oils in the reactor
  • heating and stirring is achieved using steam
  • the reaction performed is a two step process (this can also be done in the school laboratory but is not usually)
  • the hydrolysis reaction is underaken at elevated temperatures and pressures
  • the hydrolysis reaction uses water as the hydrolysing agent
  • salt is added to control the way the soap solidifies
  • glycerol is removed from the product (and used in other processes)

[Source: Bartolo, R.G., Soap, in Encyclopedia of Chemical Technology, 4th ed, J.I. Kroschwitz, Editor. 1993, Wiley Interscience. p. 297-326.]