A large cylindrical vessel, of a capacity somewhat over 1,600 gallons, lined with lead which is not attacked by dilute sulphuric acid, is mounted in such a way that it may be revolved to agitate the contents. It is strongly built to resist considerable pressures. Such an instrument, whether large or small, intended for carrying out reactions under the combined influence of heat and pressure, is called an autoclave.
In Simonsen's process the autoclave is charged with 100 kilograms (220 lbs.) of sawdust and between 300 and 500 kilograms of dilute sulphuric acid (0.5 per cent. acid). Steam is blown in through openings in the axles until the whole has reached a temperature of 100° Centigrade (212° Fahr.), when the autoclave is closed. Then it is heated to about 175° Centigrade, the pressure in the interior simultaneously rising to about 135 lbs. per square inch. These conditions are maintained for about half an hour, while the contents are thoroughly stirred by rotation. The autoclave is then opened and the liquid is filtered off from the solid residue. A portion of the cellulose, under the influence of the acid, the heat and the pressure, has been converted to glucose, fermentable sugars, which are soluble and so are contained in the liquid, the filtrate. The solid residue is made up into briquettes for fuel. The acid in the filtrate is almost neutralized with lime (it is desirable to leave it feebly acid), and this necessitates another filtration, for the neutralization results in the formation of a solid precipitate of calcium sulphate which must be removed. Yeast, and a small amount of nutrient material for the yeast, are then added, and the whole is maintained at a temperature of 25° Centigrade for from three to five days. At the end of this time the fermentation is complete. The first distillation yields a 15 per cent, alcohol and a second distillation brings the concentration of the alcohol up to about 75 per cent.
Pine and fir wood give about the same quantities of alcohol, birch is better for the purpose. In a general way hard woods appear to give better results than soft woods. Seven liters of absolute alcohol from 100 kilograms of sawdust containing 20 per cent, of moisture must be considered a satisfactory yield.
Simonsen estimates that he can make 100 liters of absolute alcohol for 5.86 Marks, that is, at a cost of about 51⁄2 cents a gallon. If this estimate were strictly correct, the process could compete with those based on the direct fermentation of agricultural produce; if it were strictly correct, it is reasonable to suppose that there would be more factories making alcohol from wood than there are.
Classen's process is similar to Simonsen's, but the chemistry of it appears to be more economical. Classen runs sulphur dioxide gas (which can be easily and cheaply obtained in any of the numerous
