In processes in which acetone is used as a solvent it is often possible to recover the greater portion of the acetone from the air of the drying stoves. For its recovery from cordite drying stoves and from various other sources an economic process has been worked out by Robertson and Rintoul.1 In the drying of the cordite, air at a temperature of 40° C. passes over the cordite. This moist air, containing a small proportion of acetone vapour, is drawn from the stoves through galvanised iron pipes to the towers, which contain a 30 per cent solution of sodium bisulphite. This solution readily absorbs the acetone, forming a compound C3H60, NaHs03. A blower or exhaust-fan leads the acetone-air mixture through the towers in series or parallel as required, the air current meeting the falling stream of bisulphite liquor. The order in which the acetone-air mixture enters the various towers and the method in which the partly saturated bisulphite liquor is circulated in the towers is so adjusted as to give the best absorption in the most economic manner. Any sodium sulphate formed by oxidisation crystallises out when the bisulphite liquors are allowed to stand. The bisulphite solution when saturated with acetone is transferred to primary stills where it is gently boiled. The absorbed acetone is then liberated, without appreciable decomposition of the sodium bisulphite solution, which may therefore be used again. The aqueous acetone solution which distils off contains small quantities of sulphur dioxide. Caustic soda is therefore added to fix the sulphur dioxide before the final distillation and rectification of the acetone. These latter stages may be carried out in the ordinary acetone stills as already described.

The acetone vapours may also be recovered by absorption in water.

Various other liquids have been suggested for the absorption of vapours such as these of acetone or "alcohol-ether." These liquids include cresol,2 glycerol,3 and oil emulsions.4 Various alcohols, including butyl alcohol,5 have been suggested for the absorption of ethyl alcohol and ether, but not expressly for acetone.

A special type of tower in which the acetone is most conveniently absorbed from the acetone-air mixture on a large scale has also been devised by Robertson and Rintoul (Fig. 98). This tower is square in section and for bisulphite solution is lined with lead. Inside the tower are erected frames on which are wound parallel strands on which the liquid is carried, these strands or threads being preferably arranged with interlacing fibres so as to present a large surface for contact with the vapours which are passed longitudinally through the tower. The tower is provided with a lid b through which passes a pipe c for drawing off the gases. The tower is closed at the bottom with the exception of an inlet pipe e for the gases and another pipe / for running off the absorbent liquid. The tower is divided by partitions into several chambers or cells, a branch d1, d2, d3, d4 from the main pipe supplying the absorbent liquid being led along the top of each cell. Immediately under each supply pipe is a dish or trough constructed of an inner portion h and an outer one i, so arranged one within the other that the strands or threads which pass between them pass through the absorbent liquid which has trickled over from the inner trough h, the edges of which are suitably lipped.

1 E.P. 25993, 25994/1901.

2 J, Bregeat, E.P. 128640, 131938/1917. See also E.P. 127309/1917.

3 E.P. 9941, 23888/1907. 4 E.P. 7098/1915. 5 D.R.P. 207554/1909.

The strands are preferably arranged as follows. A framework adapted to fit into each division or cell of the tower is built up of crossbars k, k, passing between two end plates, the topmost bars being adapted to support the troughs h, i, while the strands are threaded between the other bars. The contents of each cell of the tower, constructed, according to this arrangement, can thus conveniently be lifted out as a whole when necessary for overhauling or examination. The partitions separating the frames may be dispensed with, the fibres in adjacent frames being brought almost into contact with each other.

Fig. 98.

The strands after passing over the side of the lower trough i are carried downwards at an angle of about 75° to the side of the tower parallel to each other and at such a distance apart that the small fibres in adjacent strands somewhat interlace ; they now pass over one of the bars k, and are then carried downward at an angle to their former course until they reach the opposite side of the tower, where they pass round another bar k, and thus zigzag from side to side to the bottom of the tower, each layer making an angle of about 30° to the preceding one. As the parallel strands extend from side to side of the tower or section, a series of screens is thus formed through which the gases to be treated are made to pass.

A suitable baflle m is provided for the more uniform distribution of the gases through the tower, and to prevent the liquid from getting into the gas inlet pipe e. The method of using the tower is to allow an absorbent solution to flow or syphon from a reservoir near the top of the tower down the strands, and this solution, being distributed in its course over a multitude of interlacing fibres, presents a very large surface of active liquid to the ascending gas driven or aspirated through the apparatus, without causing any appreciable heading back of the gas current.

As the vapour pressure of acetone is small at low temperatures, it should also be possible to condense it by passing the vapours through vessels cooled by liquid ammonia or liquid air. In the case of acetone-air mixtures from cordite stoves this method would not be convenient owing to the possible presence of small quantities of "nitroglycerine "in the mixed vapours unless the traces of nitro-glycerine were first decomposed. In the closed system of Nikolsky (Eng. Pat. 3661/1906) the vapours circulate through a calcium chloride refrigerating system (cooled by liquid carbon dioxide or ammonia).

Acetone may also be removed from air by drawing the mixture over dry cellulose esters which retain the acetone by absorption (Wohl, Eng. Pat. 23995/1911).