Energy, fuels, and chemicals 



3231 



LIVESTOCK FOOD 

 HEMICELLULOSES ► xTlOsI'!' XYLITOL, 



HARDWOOD ► I PREHYDROLYSIS [ ► ■+■ 



FURFURAL 



PULP, 



CELLULOSIC RESIDUE ► FIBERBOARD, 



FUEL 



Figure 26-40. — Prehydrolysis of pine-site hardwoods for chemical, animal food supple- 

 ment, and fiber or fuel use. 



Prehydrolysis can be carried out by hot water extractions. Acids in the wood 

 and those released by hydrolysis cause extensive depolymerization of hemicellu- 

 loses. The resulting water soluble carbohydrates are mostly low molecular 

 weight polysaccharides with smaller amounts of monomeric sugars. Yield and 

 composition depend on temperature and time of extraction. Typically, water 

 temperatures of 100° to 170°C and extraction time of 30 minutes are sufficient. 

 The polysaccharides containing xylose units (xylans) are more easily hydrolzyed 

 than the corresponding glucans; thus, prehydrolysis recovers xylose from hard- 

 woods at lower temperatures than needed to remove hemicellulose from 

 softwoods. Typically the hot water prehydrolyzate of hardwood will contain 

 polymers of xylose (30 percent) and glucose (25 percent) (Herrick and Hergert 

 1977). 



Hemicellulose extracts have been marketed since 1965 as Masonex, an ingre- 

 dient of livestock feed (Galloway 1975). The fibrous residue or pulp is used to 

 manufacture hardboard. Only steam and pressure are used to pulp the wood 

 chips and so the hemicellulose fractions removed are not contaminated by added 

 chemicals. The hot water extract is separated from the pulp by vacuum filtration 

 and concentrated by evaporation to 65-percent solids. The resulting product 

 resembles molasses and can be added in cattle feed to 10 percent by weight. 

 Reportedly, the product contains 890 kilocalories of metabolizable energy per 

 gallon (10.4 lb). 



Funk (1975) described a prehydrolysis process that enables recovery of up to 

 80 percent of the hermicelluloses, mainly as the monomeric sugar xylose. The 

 process does not seriously affect the fiber residue. The degree of polymerization 

 of the residual cellulose is on the order of 900 DP with viscosity between 30 and 

 40 centipoise. 



In the process, organic acids (acetic and formic) are vaporized and injected 

 with steam into the digester where the wood chips are. Reaction time ranges 

 from 15 minutes to 1 hour and the temperature is kept below 135°C. Above 

 135°C, xylose degrades and fibers deteriorate. More acids are generated in this 

 process than are injected, so acid recovery by vacuum distillation is feasible. 

 Xylose can be separated from glucose and other sugars by selective crystalliza- 

 tion or microbial conversion. An alternative is to reduce the product mixture and 

 separate the sugar alcohols. 



When furfural is desired as the product from a hardwood prehydrolysis proc- 

 ess, it may be advantageous to add some dilute sulfuric acid. The dilute mineral 

 acid helps to produce furfural by removing and hydrolzying the hemicelluloses 



