Table 118

Weight of crude glycerine charged to still, 149.26 cwt. at 82.8 per cent glycerol

= 123.5 cwt. glycerol.

Duration of test 42 hours.

Vacuum . . 1.6 in. abs. pressure.

Steam pressure 180 lb. per square inch.

Another type of glycerine distillation plant is the "Garrigue." This plant aims at the production of the maximum amount of strong glycerine, and effects economy in the use of steam by employing the vapour from the evaporation of the sweet water as the free steam for the distillation; the still, condensers, strong glycerine concentrator, and sweet-water evaporator being one self-contained unit.

The following illustration (Fig. 183) shows the general arrangement of the plant, and it is operated as now described.

Fig. 183. - Glycerine distillation plant. Garrigue system.

Crude glycerine is fed into the still A continuously, keeping the level a little above the free steam jets, a close coil supplying heat to the glycerine undergoing distillation.

The evaporator G contains sweet water from a previous distillation, or water in the case of starting the plant for the first time, and this is heated by the steam exhausting from the close coil in the still. When the sweet water boils its vapours pass through the tubes in the superheater c, and issue from the jets below the surface of the glycerine in the still.

The glycerine and water vapours issuing from the still pass along the pipe A1 and through the catchall b, where any entrainment from the still is deposited and returned to the still through the pipe Bl, then along the pipe b2 through the superheater c on the outside of the tubes. The glycerine condensing on these tubes superheats the steam in the tubes from the evaporator, and the condensed glycerine passes down the pipe C1 to the receiver C2 and thence to the concentrator f.

The vapours uncondensed in the superheater pass along the pipe c3 to the cooler d, through the tubes of which a very little water is flowing so as to maintain a temperature at which most of the glycerine and very little of the water vapour will condense, this condensate flows down the pipe d1 and collects in the receiver d2 and also flows to the concentrator F.

Steam is admitted to the close coil in the concentrator at such a rate as to produce dynamite glycerine of specific gravity 1.262 at the end of the distillation, the control being by means of a thermometer with its bulb immersed in the liquid, the water vapour passing along the pipe f1 to the condenser e.

The vapour passing through the cooler travels along the pipe d3 to the condenser e, through the tubes of which cold water is flowing in sufficient volume to condense all the vapour. The condensed sweet water runs down the pipe e1 and collects in the sweet-water receiver e2, and runs from there to the sweet-water evaporator G, in which it is being continuously concentrated as already described.

The vacuum pump draws from the top of the sweet-water receiver through the pipe e3.

It is advisable to regulate the water in the cooler D so that some glycerine passes on to the condenser, and so to the evaporator. This glycerine will contain volatile impurities from the crude glycerine, such as the lower fatty acids, caproic and caprylic. and it is to keep these out of the strong glycerine that some glycerine is allowed to pass to the cooler.

Usually at the end of a week's run the sweet water from the evaporator is concentrated to about 80 per cent glycerine. The drain pipe from the sweet-water receiver e2 is closed, and the water collected in the receiver is run to waste, as it should contain no glycerine but is highly contaminated with volatile impurities. The concentrated sweet water is fed to the still at the beginning of the next charge.

The residue or "foots " from the still is run out periodically, dissolved in water, or in partly concentrated sweet water from the evaporator, treated to remove impurities, concentrated, and redistilled.

With crude glycerine of ordinary good quality 90 per cent of the glycerine contained in the crude should be recovered as dynamite glycerine, specific gravity 1.260 to 1.262, while approximately 3 per cent of the glycerine will be contained in the sweet water, and 7 per cent in the foots.

The dynamite glycerine is subjected to a further distillation in an exactly similar plant to produce chemically pure glycerine. The Garrigue plant gives good results with crude glycerine of good quality, but with poor crudes with a high residue at 160° C. the results obtained are undoubtedly poorer as regards the amount of glycerine left in the foots than those obtained with the Van Ruymbeke plant.

This result might be expected from a consideration of the working of the plant, as the free steam supply obviously tends to become less and to be at a lower temperature towards the end of the distillation, with the effect that the distillation slackens just at the point when a large volume of free steam at a high temperature is required to drive off the last traces of glycerine from the viscous residue in the still.

This, however, is not a source of loss of glycerine, as the foots can be treated and the glycerine subsequently recovered, but it means a lower yield of strong glycerine on the first distillation, and the subsequent treatment and redistillation involves expenditure on chemicals, labour, and steam.

Great care has to be taken in regulating the condensation of the weaks and strong glycerine, and careful control of the temperature of the cooler and of the cold water supply to the condenser is necessary. Good results are usually obtained when the specific gravity of the sweet water in the sweet-water receiver is between 1.002 and 1.003.

The fuel consumption of the Garrigue plant is estimated by competent observers to be about 0.5 lb. of coal per 1 lb. of crude glycerine distilled, and this includes the concentration of the weaks, while that of the Van Ruymbeke plant - not including the concentration of the weaks - is also about 0.5 lb. of coal per 1 lb. of crude, and if it is assumed that the weight of the weaks is equal to the weight of glycerine fed to the still, and that 0.4 lb. of steam, equal to 0.05 lb. of coal, is required in a double effect evaporator to concentrate 1 lb. of this sweet water to 80 per cent glycerine, the total fuel consumption of the Van Ruymbeke plant is 0.55 lb. of coal per 1 lb. of crude glycerine distilled.

The above quoted figures include the whole of the steam required for the distillation, including the steam for the vacuum pumps and the free and close steam for the still and dynamite glycerine concentrator, and are based on long periods of working and not on short test runs.

Another type of glycerine distilling apparatus is that of Frank J. Wood, U.S. Pat. 1098543, June 2, 1914, Eng. Pat. 24920, 1913, in which a novel method of bringing the free steam into intimate contact with the crude glycerine is employed, and it is claimed that economy in the use of steam is effected by using a single current of steam to distil the glycerine in a number of stills arranged in series. Each unit of the whole plant consists of a tubular feed heater for the crude glycerine, from which it is pumped into the upper portion of the still, where it meets a current of superheated steam, the vapours passing to a condenser designed in such a way that the glycerine is condensed but the water vapour passes on and supplies the free steam for the next still. A double effect evaporator for concentrating the sweet water condensed in the last still of the series forms an integral part of the whole plant, and supplies the free steam for the distillation of the glycerine in the first still.

If an objection is raised to the use of steam from the sweet water on the grounds that it is contaminated with volatile impurities from the glycerine, an additional evaporator fed with pure water may be provided.

Fig. 184 shows a still in perspective with the upper part broken away; Fig. 185 is a sectional elevation of one unit; Fig. 186 represents a complete plant of three stills in series.

Fig. 185. - Sectional elevation of one of the units.

The boiler "A" supplies high pressure steam to the preheaters F1, f2, f3. Crude glycerine is fed to the still d1 at 1, and the centrifugal pump E1 connected to the outlet pipe 2 forces the glycerine through the preheater f1 and into the upper portion of the still d1, through the curved pipe 3, so that the glycerine is directed tangentially against the cylindrical wall of the still and thus caused to circulate within the still above the flat ring 4, the inner edge of which is serrated. Steam from the evaporator b2 passes into the still at 5 below this ring and is caused to circulate round the still by means of the deflector plate 6. The glycerine falling over the edge of the ring forms a curtain through which the steam has to pass, and mingling with it carries away with it glycerine vapour, the combined vapours passing on to the condenser

Fig. 186. - Glycerine distillation plant. Wood system.

G1 which consists of two portions, the upper being a tubular hot water system 7 and the lower a non-condensing separator or catchall 8, arranged within an air-cooled condenser 9.

The arrows show the direction of the vapours through the condenser, and the direction of the condensed glycerine to the receivers J1 and K1

The catchall 8, being surrounded by glycerine vapours, is kept hot, so that very little if any glycerine condenses therein, but it separates any entrained glycerine which collects in the receiver H1 and is returned to the still.

Strong glycerine of first quality is condensed in the air-cooled condenser 9, and collects in the receiver J1

The water in the tubular condenser is kept at such a temperature that most of the glycerine in the vapours is condensed and collected in the receiver k1 This glycerine is not of such good quality as that collected in the receiver J1 being contaminated with volatile impurities - mostly fatty acids of low molecular weight. Very little water vapour is condensed, the greater part passing on and supplying the free steam to the next still d2, which operates in exactly the same manner as the first. The vapours from the last condenser g3 pass to a cold-water tubular condenser l from which the condensed sweet water flows to the evaporator b1 the uncondensed vapour being carried away through the pipe 10 connected with a vacuum pump m.

From points above the water level in the condensers Gl,G2, and G3 pipes 11a, 116. and 11c lead to the pipe 12, connected through a pressure regulator N with the pipe 13 leading to both evaporators, b2 and b1 with valves at the junction of the pipe with the evaporators to control the admission of steam to their heating systems.

The vacuum pump m is operated to maintain a pressure of about 2 in. of mercury in the evaporator b2, the stills Dl, d2, and d3, the condensers g1, g2, and g3, and the cold-water condenser L. The pressure regulator n is adjusted so that the pressure in the water spaces of the condensers is kept at 14 in. of mercury absolute, at which pressure the water will boil at about 80° C.; this temperature is sufficient to condense the glycerine while maintaining the water in a state of vapour.

The method of mixing the steam and glycerine in the stills by spraying is very effective, and overcomes the difficulty often experienced with certain crude glycerines of foaming and priming when steam is used in the form of jets below the surface of the glycerine.

The usual method of feeding the stills is to supply the first still with fresh crude glycerine, the second still is fed from the residue from the first, and so on until the last still of the series is reached where the foots are accumulated and pumped away for treatment periodically. The illustration (Fig. 186) shows three stills in series, but as many as six have been operated successfully.

Table 119 gives the result of the distillation of 100 tons of crude glycerine.