DISTILLATION OF FATTY ACIDS.
The fatty acids obtained by various methods of saponification may be further improved by distillation.
In order to carry out this distillation, two methods may be pursued, first, the continuous method, whereby the fatty acids are continually distilled for five to six days, and, second, the two phase method, whereby the distillation continues for 16 to 20 hours, after which the residue is drawn off, treated with acid, and its distillate added to a fresh charge of fatty acids. The latter method is by far the best, since the advantages derived by thus proceeding more than compensate the necessity of cleaning the still. Better colored fatty acids are obtained; less unsaponifiable matter is contained therein; there is no accumulation of impurities; the amount of neutral fat is lessened because the treatment of the tar with acid causes a cleavage of the neutral fat and the candle tar or pitch obtained is harder and better and thus more valuable.
The stills are usually built of copper, which are heated by both direct fire and superheated steam. Distillation under vacuum is advisable. To begin the distilling operation, the still is first filled with dry hot fatty acids to the proper level. Superheated steam is then admitted and the condenser is first heated to prevent the freezing of the fatty acids, passing over into same. When the temperature reaches 230 deg. C. the distillation begins. At the beginning, the fatty acids flow from the condenser, an intense green color, due to the formation of copper soaps produced by the action of the fatty acids on the copper still. This color may easily be removed by treating with dilute acid to decompose the copper soaps.
In vacuum distillation, the operation is begun without the use of vacuum. Vacuum is introduced only when the distillation has proceeded for a time and the introduction of this must be carefully regulated, else the rapid influence of vacuum will cause the contents of the still to overflow. When distillation has begun a constant level of fatty acids is retained therein by opening the feeding valve to same, and the heat is so regulated as to produce the desired rate of distillation. As soon as the distillate flows darker and slower, the feeding valve to the still is shut off and the distillation continued until most of the contents of the still are distilled off, which is indicated by a rise in the temperature. Distillation is then discontinued, the still shut down, and in about an hour the contents are sufficiently cool to be emptied. The residue is run off into a proper receiving vessel, treated with dilute acid and used in the distillation of tar.
In the distillation of tar the same method as the above is followed, only distillation proceeds at a higher temperature. The first portion and last portion of the distillate from tar are so dark that it is necessary to add them to a fresh charge of fatty acids. By a well conducted distillation of tar about 50 per cent. of the fatty acids from the tar can be used to mix with the distilled fatty acids. The residue of this operation called stearine pitch or candle tar consists of a hard, brittle, dark substance. Elastic pitch only results where distillation has been kept constant for several days without interrupting the process, and re-distilling the tar. In a good distillation the distillation loss is 0.5 to 1.5% and loss in pitch 1.5%. Fatty acids which are not acidified deliver about 3% of pitch. Very impure fats yield even a higher percentage in spite of acidifying. For a long time it was found impossible to find any use for stearine pitch, but in recent years a use has been found for same in the electrical installation of cables.