Strong Acid Processes for Wood Saccharification

Strong Acid Processes for Wood Saccharification

	Strong Acid Processes for Wood Saccharification These methods are characterised by the use of large amounts of concentrated acid which bring about extreme swelling of the cellulose. This serves to break the bonds which hold cellulose in the crystalline state and make it highly resistant to ordinary dilute acid hydrolysis. After extreme swelling and partial hydrolysis, the hydrolysis must be completed in dilute acid solution.
The very early work on cellulose saccharification made use of sulphuric acid. Wood is given a prehydrolysis with dilute acid to remove the hemicellulose. The dried residue is treated with sulphuric acid in an edge runner. This mixture is then diluted with the prehydrolysis liquor, and heated to complete the hydrolysis. Advantages 1. High yield of alcohol. 2. Recovery of furfural and other eventual by-products such as acetic acid and ethereal oils. 3. Possibility of working with sawdust as well as with chips. Disadvantages 1. High consumption of acid. 2. Necessity of solving the problem of placing large quantities of calcium sulphate.
Wood is hogged to chips not more than 1 cm in the long dimension, and conducted by a pneumatic conveyor to a rotary drier. Waste stack gas and the wood move parallel through the drier, and the moisture content is lowered to 6 %. The wood is then loaded into 50-cubic-metre digesters lined with rubber and acid resistant brick and extracted with 50% (by volume) hydrochloric acid. There are two parallel batteries of 14 extractors, half of each being extracted with concentrated acid, and the other half with water.
The total time cycle per digester is 55 hours. As a result of the countercurrent extraction, with acid, a syrup is obtained consisting of water with 32% sugar, and 28% hydrochloric acid. This syrup goes to an evaporator system operating at 30 to 44 mm at 40° C where the sugar concentration is raised 60 to 63 % and the acid concentration lowered to 2 to 5%. These evaporators have separate heaters in which syrup circulates inside the porcelain tubes. The head of the heater is of rubber-lined steel. Steam is then injected into the syrup to reduce the acid concentration. The carbohydrate in solution at this point consists primarily of oligosaccharides, and in order to convert it to monosaccharides it is 'inverted' by diluting and boiling. The residual acid is sufficient to catalyse the hydrolysis. The product is neutralised with lime and used for the production of yeast. The substances in solution consist of 70% glucose, 10% pentose, and 20% calcium chloride.
	The recovered acid and the dilute acid washings pass to an acid recovery system where water is removed. This is accomplished by the addition of calcium chloride which increases the concentration of HCl in the gas phase. Water is continuously removed from the calcium chloride in the same apparatus. Conditions in this piece of equipment are extremely corrosive, and the operation is troublesome. The temperature is 145°C. In order to resist such conditions-high temperature, high acid concentration, and th presence of calcium chloride - the apparatus is lined with rubber and a double layer of brick. The heating tubes are of copper, plated with gold or platinum.

Clostridium thermoacellum which feeds on cellulose and Thermoanerobacter ethanolicus which feeds on starch saccharify the starch into fermentable sugar and may have the potential benefits in fuel alcohol production.

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