Removal Of N-Butyl Alcohol From The Rectifying Column

The n-butyl alcohol (azeotropic) mixture with water is run off from the rectifying column by one of the tapped pipes p. The n-butyl alcohol and water are first cooled by passing through a coil Q, immersed in water, and then flow into a separator r. The lower layer from the separator, consisting of a saturated solution of n-butyl alcohol in water and traces of acetone, is returned to the boiling column by pipe x. The upper layer from the separator, consisting of a saturated solution of water in n-butyl alcohol, is run off at the observation glass. The still runs very evenly for about 48 hours, but after that time the temperature of the vapour entering the dephlegmator tends to rise above 56° C. and it is difficult to keep it down to this normal level. The reason for this appears to be that ethyl alcohol gradually accumulates in the rectifying column above the level at which n-butyl alcohol is drawn off. To overcome this difficulty the supply of wash to the still must be replaced by water. The distillation is then continued till the temperature at the base of the rectifying column rises to 100° C. By this means the rectifying column is cleared of the accumulation of ethyl alcohol. The distillate collected during this process, and containing some ethyl alcohol, is treated with the rest of the crude acetone in the final distillation. A liquid rich in ethyl alcohol might also be run off from the rectifying column at a suitable level, but this has not yet been attempted.

Control Of The Still

The flow of wash is controlled by the regulator described under alcohol distillation, page 310, Fig. 117. The supply of steam to the boiling column is automatically regulated by the apparatus described under alcohol distillation, p. 310, Fig. 118; and the flow of acetone and n-butyl alcohol from the still is regulated by test glasses similar to that described on p. 311. The temperature at the base of the rectifying column and of the vapour entering the dephlegmator must be kept very steady. If acetone is not run off fast enough from the still, the temperature at the base of the rectifying column will tend to

R fall, and the separation of the two layers of n-butyl alcohol-water mixture in the separator will no longer take place properly, owing to the presence of too much acetone. If acetone is run off too rapidly, the temperature of the vapour entering the dephlegmator will very soon rise above 56° C. It is important to keep the pressure in the rectifying column very steady. For this purpose pressure gauges are fixed at the base and near the top of the rectifying column.

Acetone of over 99 per cent purity can be obtained with this still, but the presence of traces of impurities make it unfit for immediate use in the manufacture of cordite. To remove these impurities the acetone is redistilled, after the addition of small quantities of caustic soda, in a discontinuous still.

In practice the acetone was generally run off from the still at a lower strength, as this somewhat facilitates the efficient removal of n-butyl alcohol from the column. But this procedure is not necessary.

Final Purification Of Acetone Produced By The Fermentation Pro--Cess

Two types of still used for this purpose are shown in Fig. 91 1 and Fig. 92. The still (Fig. 91 or 92) is charged with about 4800 lb. of crude acetone from the primary still. The charge is slowly heated by live steam introduced into the liquid through the perforations of a ring placed in the base of the still. By this means most of the gas dissolved in the crude acetone is driven off before any condensate is formed in the condenser. As soon as the evolution of gas ceases, steam is shut off, and the necessary quantity of caustic soda solution, containing about 8 lb. of sodium hydroxide, is added, and the heating continued. The crude acetone produced from Coffey's still contains so little carbon dioxide in solution that the preliminary heating is unnecessary, and the caustic soda can be added as soon as the still is charged.

The distillate is collected in three fractions :1. The first runnings, consisting of almost pure acetone, amount to about 125 lb., or 2.6 per cent by weight of the charge; it contains, however, too much carbon dioxide to pass the specification and has to be submitted to a second alkaline distillation.

2. The second fraction consists of good acetone which needs no further treatment. This fraction amounts to about 74 per cent by weight of the charge.

3. The third fraction with specific gravity over 0.802 amounts to about 23.3 per cent by weight of the charge. The collection of this fraction ceases when the specific gravity of the distillate rises to 1.0. This fraction is redistilled after the addition of caustic soda and permanganate.

Redistillation of Third or "High Gravity" Fraction. - The redistillation of the third fraction referred to above is carried out in the same type of still as that already described.

Three fractions are collected :1. Acetone specific gravity <0.802, forming nearly 13 per cent by weight of the charge.

1 A still similar to that shown on p. 299 for the rectification of ethyl alcohol might be used.

2. "Crude Ketone," forming about 70 per cent by weight of the charge. This fraction is redistilled.

3. n-Butyl alcohol and water, forming nearly 3.0 per cent by weight of the charge. The remainder consists of water.

Fig. 91. - Acetone Rectifying Still made by Messrs. J. Dore & Co., London.

A, Still; B, Steam coils; C, Vapour chamber (not water jacketed); D1. D2, D3, Water-jacketed section of the column; E, Cold water supply to jackets D1, D2, D3; F, Syphon from D3 to D2; F,, Syphon from D2 to D,; G, Vent pipes from water jackets; H, Outlet pipe for water in jackets; K, Supplementary water jacket; L, Vapour pipe; M, Condenser; N, Still watcher.

Redistillation of the Second or "Crude Ketone " Fraction.

The redistillation of this fraction yields three fractions: 1. Acetone, 3.5 per cent by weight of the charge.

2. "Finished Ketone," 87.0 per cent of the charge.

3. n-Butyl alcohol and water, 9.5 per cent of the charge.

Fig. 92. - Acetone Rectifying Still, made by Messrs. J. Miller & Co., Glasgow.

A, Still; B, Steam coils; C, Lower section of still-head, not water cooled; D, Section of column provided with cooling coils E above each plate; F, Water bath; Q, Vapour pipe; H, Condenser; K, Still watcher.