Alcohol from wood, - In obtaining alcohol from sawdust or other waste-wood material, the so-called " cellulose " of the wood is hydrolysed by treatment with acids under pressure. By this means it is in part converted into dextrose and other fermentable sugars. The acid is then neutralised, or nearly so, with lime or alkali, and the resulting liquid is fermented and distilled.

There is some doubt as to what is the "cellulose" actually hydrolysed. The cellulose of cotton does not respond to the treatment. Also the material left by sawdust which has been once treated still contains cellulose, but a repetition of the process yields very little more fermentable sugar. The portion which is transformed is sometimes distinguished as the "easily-attacked ' cellulose. It probably consists of polysaccharides of pentoses and hexoses.

Fig. 27.   French column apparatus for rectification of alcohol.

Fig. 27. - French column apparatus for rectification of alcohol.

Formerly much used abroad. A, boiling vessel, heated by steam; B, rectifier; C, condenser; D, refrigerator.

According to Willstatter and Zechmeister,1 cellulose can be completely converted into dextrose at the ordinary temperature by treatment with a strong solution of hydrochloric acid (40 to their observation does not seem to have been utilised industrially.

A, back for crude spirit; b, back for water. B, B', purifying and concentrating columns for foreshots; C, rectifying column; D, D', final purifying columns; J, K, L, condensers; J', K', L', 0, refrigerators (Egrot and Grange, Paris).

1 Ber., 1913, 46, 2401.

41 per cent, of HC1), which dissolves it in a few seconds, and gives complete hydrolysis in twenty-four to forty-eight hours. Their

Fig. 28.   modern still, for continuous rectification.

Fig. 28. - modern still, for continuous rectification.

Statement, however, appears to be erroneous.1 In any case,

1 M. Cunningham, Trans. Chem. Soc, 1918, 113, 173.

As far back as 1819, Braconot showed that when wood waste was heated with sulphuric acid a product was obtained which contained sugar. Simonsen, however, some eighty years later, appears to have been the first to attempt the manufacture of sugar from sawdust on any large scale.1 He used diluted sulphuric or hydrochloric acid, at a strength of 0 3 to 0 7 per cent., and heated the mixture of sawdust and acid under a pressure of 7 to 8 atmospheres. Ekstrom attempted to solve the same problem by using strong sulphuric acid.2

Classen, in 1900, showed that aqueous sulphurous acid converted wood celluloses into dextrose when the mixture was heated under pressure to a temperature of 120-140°. There is an advantage in using a volatile acid, such as sulphurous or hydrochloric, instead of sulphuric, inasmuch as a better penetration of the wood is obtained.

After laboratory experiments at Aachen, the method was tried in an experimental plant near Chicago, and afterwards an installation on a commercial scale was erected at Hattiesburg, Missouri. It consisted of an apparatus for preparing the sulphur dioxide; a converter in which the wood was treated with the acid; an extraction battery; neutralisation vats; fermentation vessels, and distillation apparatus. The sawdust was treated in the converter with a saturated aqueous solution of sulphur dioxide, the temperature being gradually raised to 143-149° and the action allowed to proceed during from four to six hours. Then the sulphur dioxide was blown off by steam and collected for re-use, the woody residue treated with water in the extraction-vessels to remove the sugar, and the solution thus obtained neutralised, fermented, and distilled. One hundred kilos, of sawdust gave from 7 1/2 to 8 1/2 litres of alcohol.

As thus worked, the process fell just short of success. The reasons given for the failure were: (1) Length of time required for the conversion (four to six hours for two tons); (2) production of gummy matters and caramelisation due to the prolonged action; (3) the large quantity of acid required; and (4) difficulties due to the action of the acid on the lead lining of the converter - a cylinder 30 ft. long, and 3 ft. in diameter.

Two chemical engineers, Messrs. Ewen and Tomlinson, improved the process by using a much shorter and wider converter (12 ft. by 8 ft.), and lining it with firebrick instead of with lead 1 Sulphur dioxide gas to the extent of 1 per cent, of the weight of wood treated is introduced into the converter, and steam passed in until a pressure of 100 lb. is obtained. The steam is then turned off and the cylindrical converter slowly revolved for forty-forty-five minutes, the temperature and pressure being kept constant: the total time required for conversion is about an hour. The temperature is raised as quickly as possible to the "critical" point, between 135° and 163°, above which there is an excessive destruction of sugars and production of unfermentable substances.

1 W. P. Cohoe, J. Soc. Chem. Ind., 1912, 31, 513.

2 D. R.-P., 193112.

When the extraction is completed, the solution obtained has a total acidity of 064 per cent, (calculated as H1SO4), and contains about 5 1/2 per cent, of reducing sugars, with small amounts of phenols, tannins, and furfural. It is neutralised, fermented, and distilled as usual, and gives about 38 1/2 gallons (U.S.) of alcohol per ton of dry wood, or between 75 and 80 per cent, of the theoretical yield.2

More recent particulars have been given by G. H. Tomlinson.3 The digesters or converters now employed are standard 14 ft. globular rotatory bleaching boilers, protected on the inside with an acid-proof tile lining. This type allows of easy filling and emptying. Separation of the sugar from the woody residue is effected in a standard beet-sugar diffusion battery provided with a similar lining to that of the digester. From 80 to 100 tons of material per day can be treated in this apparatus. Per ton of dry wood, the maximum yield is 35 gallons of alcohol (95 per cent.), but on a large scale the actual average yields have hardly exceeded half of this amount.