The Kettle Boiled Batch Process
has many similarities to the saponification reactions that you
can perform in the school laboratory: it uses a direct saponification reaction.
A lot about this process can actually be determined from the name:
- A kettle is a large, open, steel tank. The kettles used for saponification
can hold up to 130,000 kg of material.
- The reaction mixture is kept boiling through the injection of high
temperature, high pressure steam. This also acts to keep the reaction
mixture well-stirred. Some soap from previous reactions is usually left
in the kettle to help the water and oil mix by forming an emulsion (see
emulsification later in this module).
- Batch Process
- A batch process is a reaction where everything is added at the beginning,
the reaction proceeds until it reaches completion and then the products
are removed. (This is in contrast with a semi-batch reaction where more
reactants are added as the reaction proceeds, or a continuous process
where reactants are added and products removed constantly throughout the
reaction.) In this case, the fats and oils, caustic soda (sodium hydroxide),
salt and water are added at the beginning of the reaction.
At the conclusion of the saponification reaction, additional salt is added to
the mixture (thus changing the way the soap solidifies). The mixture
is washed with more steam and allowed to settle, removing the glycerol. The
washing and settling process usually takes several days to complete.
The key differences between the Kettle Boiled Batch 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
- salt is added to control the way the soap solidifies
- glycerol is removed from the product (and used in other processes)
Bartolo, R.G., Soap, in Encyclopedia of Chemical Technology, 4th ed,
J.I. Kroschwitz, Editor. 1993, Wiley Interscience. p. 297-326.]