Wood digestion

Studies on Lignin. XIV. The Composition of Douglas Fir Wood Digested... 

Pew found that the residue, mainly lignin, from unground wood digested by cellulytic enzymes was insoluble in IN sodium hydroxide while the residue from finely ground wood was freely soluble 26). Also, spruce periodate lignin, normally insoluble in methyl cellosolve and in 0. N... 

The discovery that small amounts of anthraquinone (0.05-0.25% on wood) added to alkali liquors can enhance delignification and produce kraft quality pulp has been of great interest. Although the price of anthraquinone (AQ) is high and the chemical is not recovered, only small quantities are required, and the benefits outweigh the costs sufficiently that a number of mills around the world now routinely add AQ to the kraft pulp-wood digester. 

Decay is the result of wood digestion (both lignin and cellulose) by fungi (Fig. 1). It is the major type of loss of wood which limits the use of... 

Haggin, J. Electrochemical Theories Open Up Chemistry of Wood Digestion Chem. Eng. News, 20 (15.10.1984)... 

Javor, T., Buchberger, W., and Tanzcos, I., Determination of low molecular mass phenolic and non-phenolic bgnin degradation compounds in wood digestion solution by capillary electrophoresis, Microchim. Acta, 135, 45, 2000. 

Formation in spruce lignin degradation in wood digestion with sodium bisulfide [7206]. 

Wood Digest Magazine (Woodworking professionals in cabinet manufacture, millwork, speciality wood products)... 

The above work has been considered in some detail since it vras well planned and carried out, and resulted in cutting the time to about one-half to one-fourth of that formerly necessary in the Scholler process. This achievement brings closer the practicability of wood digestion as a source of fermentable sugars. 

Electric power derived from present (ca 1992) technology via the combustion of wood and wood wastes, MSW, agricultural wastes, landfill and digester gas, and advanced digestion and turbine technology. 

Utihty production of biomass-fueled electric power is much less than nonutiUty production. In early 1985, there were only 18 faciUties having a total capacity of 245 MW, ie, nine fueled with wood (180.7 MW), five fueled with MSW (33.8 MW), two fueled with agricultural residues (22.5 MW), and two fueled with digester gas (8 MW) (112,113). The largest was the 50-MW plant in Burlington, Vermont (114). 

Determination of Lignin Content. Lignin content in plants (wood) is determined by direct or indirect methods (21). The direct method includes measurement of acid-insoluble (ie, Klason) lignin after digesting wood with 72% sulfuric acid to solubilize carbohydrates (22). The Klason lignin contents of representative lignifted materials are shown in Table 2. 

Wood Pulping. The system Mg(OH)2 S02 H20 is also used in acid bisulfite pulping. Compared to a calcium-based system which is not as amenable to regeneration of the pulping bisulfite (87), fewer technical problems are encountered in the digesters, evaporators, or recovery boiler of the Mg-based process. In the presence of excess SO2, bisulfite forms in a 43% MgSO solution, at 25°C and 101.3 kPa (1 atm) SO2 pressure, to increase MgSO solubihty. 

Chemical pulps are produced in a digester where the wood is cooked in pressurized vessels using heat and chemicals to break the intercellular stmcture of the wood and extractives. The objective is to remove the lignin from the fibers without degrading the carbohydrate content of the wood. 

Wood Residuals. Chips, sawdust, and other residuals such as planar shavings are used as a primary source of fiber for some pulp mills. Chips are screened and placed in a purchased chip pile. Sawdust and other residuals obtained from wood processing plants must be cooked separately and requite special digesters and handling equipment. These materials may also be burned as hog fuel. 

Kraft Process. The dominant chemical wood pulping process is the kraft or sulfate process. The alkaline pulping Hquor or digesting solution contains about a three-to-one ratio of sodium hydroxide, NaOH, and sodium sulfide, Na2S. The name kraft, which means strength in German,... 

Chemical charge, Hquor composition, time of heatup, and time at temperature of reaction are all functions of the wood species or species mix being digested and the intended use of the pulp. A typical set of conditions for southern pine chips in the production of bleachable-grade pulp for fine papers is active alkaH, 18% sulfidity, 25% and Hquor-to-wood ratio, 4 1. Time of heatup is 90 min to 170°C time at temperature of reaction, 90 min at 170°C. Hardwoods require less vigorous conditions primarily because of the lower initial lignin content. 

Digester Control For control purposes at constant sulfidity and alkah charge, the deligniftcation rate is treated as a homogeneous reaction, which is first order with respect to the lignin, % remaining in the wood, —dL jdt = kL. The influence of time, t, and temperature, T (Kelvin), has been incorporated into one term, called the JT-factor (33). 

By-Products. There are three stages within the pulping operation at which wood-derived chemicals can be recovered as by-products. Turpentine is obtained from the reHef of gases after an initial steaming of chips in the digester. Better yields of turpentine are obtained from batch digesters than from continuous systems. Pines and firs give the best yields. Turpentine is composed principally of unsaturated bicycHc hydrocarbons, of which ca 90% are a- and P-pinenes and 5—12% other terpenes. 

Tall oil rosin is a by-product of paper manufacturing. Raw wood chips are digested under heat and pressure with a mixture of sodium hydroxide and sodium sulfide. Soluble sodium salts of lignin, rosin, and fatty acids are formed, which are removed from the wood pulp as a dark solution. The soaps of the rosin and fatty acids float to the top of the mixture, where they are skimmed off and treated with sulfuric acid to free the rosin and fatty acids. This mixture, known as cmde tall oil (CTO), is refined further to remove color and odor bodies fractional distillation separates the tall oil rosin acids from the fatty acids (see Tall oil). 

The principle uses of Na2S04 are in the manufacture of paper, soaps, and detergents. These accounted for 65% of U.S. consumption from 1990 to 1995, representing a significant shift from 1980 when paper production alone consumed 67%. Pulp (qv) and paper consume only 25% (2). The kraft paper process uses a mixture of sodium sulfide and sodium hydroxide to digest wood chips. Both the sulfide and hydroxide are generated, starting with sodium sulfate as the raw material. 

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