Glycerine is one of the constituents of fixed oils and solid fats, and though discovered by Scheele over a century ago, it has only lately come into general use. Few things in the history of chemical industry are more wonderful than the enormous development in the use of this substance, which a few years ago was thrown away as a waste product, but which now finds so many useful applications in the arts and 'sciences. The researches of Chevreul, which demonstrated the constitution of fats, showed that glycerine exists in nearly all neutral fats in a combined state, and small traces of it have lately been discovered uncombined in palm oil. It is formed, as Pasteur has shown, in the process of fermentation, 100 parts cane-sugar forming 3'5 parts glycerine. For practical purposes, however, glycerine is always obtained from the bye-products of candle and, quite lately, of soap-factories. Cap worked out the first process for preparing it on a commercial scale from the waste liquor of saponification of tallow by lime in the first stage of stearic acid making.
Early in 1854 Tilghman produced it by pumping an emulsion of 2 parts tallow and 1 part water through a coil of pipe heated to 612° F. (322° C), after which the emulsion separated into 2 layers, the upper one of fatty acids, and the lower of glycerine and water. Several modifications of this were afterwards patented, but the only one worked on a large scale was that of G. F. Wilson and G. Payne, dated July 24th, 1854, under which enormous quantities of glycerine have been made by Price's Candle Co. In this process, neutral fats are put into a still provided with a fine steam-worm, and a fractional condensing apparatus of large surface similar to that used in candle-making; they are then heated to between 550° and 600° F. (288° to 315° C), and plenty of superheated steam is injected; mixed vapours of fatty acids, glycerine, and water are carried over to the condense, where the divisions nearest the still collect only fatty acids, while those farthest from it yield mixtures of fatty acids with glycerine and water in various stages of concentration.
Glycerine so made can be concentrated in a vacuum-pan. Care must be taken that the temperature does not exceed 600° F. (315° C.) and that plenty of steam is present, else some of the glycerine is decomposed, and acrolein, a compound most irritating to the eyes, is formed -
Glycerine = Water + Acrolein. C3H803 = 2H2O + C3H40 Raw glycerine is also prepared from the water employed to wash the fatty acids after acidification of the neutral fats. The acid liquid is neutralized by carbonate of lime or of baryta, either of which may be added until effervescence ceases; it is then concentrated to 28° B. in an open, shallow, cast-iron pan. Of late, however, glycerine has become sufficiently valuable to cause candle manufacturers to adopt that method of preparing fatty acids which gives them the greatest yield of glycerine from neutral fats. This process, called the autoclave, as patented by De Milly on Nov. 19, 1856, is now very extensively used for glycerine-making, both on the continent of Europe and in England, and is thus conducted: - About 1 ton of fat, usually mixed tallow and palm-oil, is heated with 2 per cent, lime and 1/2 the fat-volume of water, in an upright Papin's digester, under 8 atmos. pressure for 4 hours. The whole is then blown out into a tank, and the " sweet water " is run off. The lime-soap is decomposed in the usual way with sulphuric acid, and the resulting fatty acids are either pressed or acidified and distilled for stearic acid.
It is then concentrated in a modification of the "Wetzel" evapo-rating-pan (originally introduced for sugar-boiling), constructed by Chenail-lier, Paris. This evaporateur universel, as he terms it, which is very economical and effective, consists essentially of pairs of saucers set edge to edge upon a hollow central revolving shaft, through which steam passes to the interior of the saucers (the waste steam from a high-pressure engine will do); the lower edges of the saucers dip in a jacketed trough of the liquid to be evaporated, and when they are revolved, layers of this are brought up and speedily concentrated on their surface. It may also be worked in a vacuum. Evaporation is continued to 26° B., when the glycerine is of a brownish colour and known as " raw,"in which state it is sold for many purposes. At Price's Candle Co.'s works, the further purification is conducted as follows: - The raw glycerine, sp. gr. 1.245 to 1.250, is heated in a jacketed pan with that kind of animal charcoal known as ivory black, and is then distilled; this alternate treatment is repeated as often as may be necessary.
The distillation is performed with superheated steam in a copper still provided with copper fractional condensers, the still being also heated externally; the operation is performed at as low a temperature as is consistent with distillation, usually about 440° F. (227° C). The number of distillations depends upon the quality of the raw glycerine and the purity of the product demanded. Of the 6 runs, Nos. 1, 2, and 3 usually give pure glycerine, while the dilute condense-products from Nos. 4, 5, and 6 are generally returned to the still, though occasionally concentrated in an evaporateur universel, or in a vacuum-pan. Some stills hold as much as 3 tons, but they are usually smaller, and in all cases the process is conducted very slowly. A form of still and condenser much used on the continent of Europe is outlined in Fig. 13. External heat and injected superheated steam are used to effect distillation. The still A has an unusually large bead B, and the gooseneck C is provided with a catch-box D, in case the still-contents should, as sometimes happens, boil over; the fractional condensers E are upright cylinders with longitudinal partitions F running nearly their whole length; the condensed products run out through G into receptacles H. The whole apparatus is of iron, and usually made to distil 1/2 ton at a time; in some cases the process is conducted continuously, with a properly-arranged feed.