Table 119

100 tons of crude glycerine at 80 per cent glycerol = 80 tons glycerol yield on one distillation.

The foots on treatment and subsequent distillation will yield two-thirds of their content of glycerine, giving a final yield of strongs 92 per cent, weaks 5 per cent, and a loss of 3 per cent.

In considering the heat efficiency of the various types of glycerine stills it is not sufficient to take into account only the heat in the steam used for heating the glycerine up to distilling temperature and the heat in the free steam used in the distillation, but also account must be taken of the heat required to drive the various pumps and engines included in the plant. The yield of strong glycerine, steam required for concentrating the weaks, for the treatment and recovery of glycerine from the foots also must be considered.

There is an obvious saving of heat in the Garrigue and Wood processes over the Van Ruymbeke, as the two former utilise a portion of the latent heat of the glycerine vapour in condensing for subsequent operations and they do not condense the whole of the steam arising from the stills. The saving is approximately equal to the heat required for the evaporation of the weaks or in other words, to the heat in the free steam required for the distillation. The claims for economy of heat in the Wood system as compared with the Garrigue, by reason of the utilisation of the same flow of steam in a series of stills, must be discounted by the fact that there must be a loss of heat after passing through each condenser, as the temperature of the steam cannot be higher than the temperature of the boiling water in the condenser, and therefore it must absorb heat from the crude glycerine pumped into the next still, which is fresh heat taken up in the glycerine heaters from the steam supplied direct from the boiler. There is also an additional consumption of heat by the pumps required to circulate the crude glycerine through the heaters and stills. The yield of strong glycerine on the first distillation is higher in the Garrigue and Wood systems than in the Van Ruymbeke by reason of the more efficient condensers, but the Van Ruymbeke system has the advantage of leaving at the end of the distillation considerably less glycerine as foots than either of the other two.

Comparisons of fuel consumption must be taken over long periods of working, where all factors are taken into account, and not on short test runs, and should be based on the amount of fuel required to produce a given weight of the final product and not on the amount required to distil a given weight of the crude material.

As all the above systems cannot be found working together under the same conditions in any one factory, comparisons are very difficult to make, but from figures supplied from different sources, and after making due allowances for the varying conditions, a careful computation gives the following result.

With steam of 180 lb. per sq. inch pressure at the plant, and assuming an evaporation of 1 to 8, for the production of 1 ton of once distilled glycerine, the Van Ruymbeke system requires 16 cwt. of coal, the Garrigue 14 cwt., and the Wood 13 cwt. These figures are capable of considerable reduction, and it ought to be possible to produce one ton of once distilled glycerine with 10 cwt. of coal.

A patent has recently been taken out by the Societe Francaise des

Glycerines (English Patent Specification 125574, June 10, 1919) for a system of distilling glycerine without the use of free steam

The process consists in atomising glycerine, previously heated to about 180° C, by passing it through suitable jets from one vessel to another at a lower pressure, the difference in pressure between the two vessels being relied upon to effect the atomisation. The atomised glycerine is vaporised at the reduced pressure of the second vessel and is collected in condensers, the non-volatile residue being drawn out through a cock at the bottom of the vessel.

The apparatus (Figs. 187 and 188) consists of a cylindrical vessel A containing the glycerine, in the interior of which is mounted a second concentric cylinder b, in which the atomising takes place.

Fig. 187. Fig. 188.

Glycerine distillation plant. Sociele' Francaise des Glycerines.

Steam coils c provide sufficient heat to the vessels a and B to maintain a temperature of 180° C.

The glycerine is kept at a constant level in vessel a by means of a float valve d. The pressure in the vessel is reduced to 26 cm. of mercury, and any impurities in the glycerine volatile below 180° C. escape through the vapour pipe e, which is connected to a water condenser, and this in turn to the vacuum pump.

A drain cock f and a pressure gauge l are fitted, and a pipe G terminating in a rose supplies the jets or atomisers h with glycerine.

The cylinder b or atomising vessel has a drain cock J connected to a drum for collecting the residue, which is produced as the distillation proceeds and which must be discharged without having to stop the plant.

A vapour pipe S leads to a series of condensers where the glycerine vapours condense, and finally to the vacuum pump. The pressure in this second vessel B is indicated by the gauge K and is kept at about 4 cm. of mercury, so that there is a pressure difference of 22 cm. between the two vessels, which is said to be adequate to effect the atomisation. The atomisers H, three in number, are arranged symmetrically around the top of the vessel b with their jets directed downwards. The vessels A and b may be independent of each other instead of one being contained in the other.

It is claimed that the process is applicable to glycerine of all kinds, but no information is available concerning its application on a commercial scale, and it can be imagined that difficulties might arise due to the choking of the jets when crude glycerine is the product undergoing distillation; it would also be a difficult matter to remove the practically solid residue from the second vessel b during the course of the distillation.

The method, however, is most ingenious, and provided that a sufficiently high vacuum and temperature can be maintained the principle of distilling without the aid of free steam and thus getting very little sweet water, and the economy effected in the use of steam and power by utilising the difference in pressure between the two vessels, is one that must commend itself to every one.