Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Reexamination Certificate
1999-12-08
2003-01-28
Barts, Samuel (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
C127S037000, C127S030000, C127S042000, C127S058000, C435S105000, C435S200000, C435S158000
Reexamination Certificate
active
06512110
ABSTRACT:
The present invention relates to a process for the production of xylose from a paper-grade, hardwood pulp. More specifically, this invention relates to a process wherein the xylan contained in said pulp is extracted using an aqueous solution of a xylanase enzyme. Optionally, the process also comprises one or two alkalic treatments. Xylose is obtained by a hydrolysis of the xylan exctracted from the pulp. The paper-grade hardwood pulp used as raw material is preferably soda pulp or kraft pulp. In one embodiment of the present invention, dissolving-grade pulp of high purity is co-produced with xylan.
Xylose is a valuable raw material in the sweets and flavouring industries, for example, and particularly as a starting material in the production of xylitol. Xylose is formed in the hydrolysis of xylan containing hemicellulose. Vegetable materials rich in xylan include wood material from wood species, particularly hardwood, such as birch, aspen and beech, various parts of grain (such as straw and husks, particularly corn and barley husks and corn cobs), bagasse, coconut shells, cottonseed skins, etc.
The dissolving-grade pulp obtained is useful in the manufacture of viscose rayon, cellulose esters such as cellulose acetate, cellulose acetate propionate and cellulose acetate butyrate. The production of these cellulose derivatives requires a source of high quality cellulose feedstock. Wood pulp, however, requires extensive purification before it is suitable for viscose or cellulose ester manufacture. The additional purification, which involves treatment with alkali to remove and destroy hemicelluloses and bleaching to remove and destroy lignin reduces the yield and increases the cost of “dissolving-grade” cellulose derived from wood pulp.
“Pulp” is an aggregation of random cellulosic fibers obtained from plant fibers. As used herein, the term “pulp” refers to the cellulosic raw material used in the production of paper, paperboard, fiberboard, and similar manufactured products. Pulp is obtained principally from wood which has been broken down by mechanical and/or chemical action into individual fibers. Pulp may be made from either hardwoods (angiosperms) or softwoods (conifers or gymnosperms). Hardwood and softwood pulps differ in both the amount and the chemical composition of the hemicelluloses which they contain. In hardwoods, the principal hemicellulose (25-35%) is glucuronoxylan while softwoods contain chiefly glucomannan (25-30%) (Douglas W. Reeve, Pulp and Paper Manufacture, Vol. 5, pp. 393-396).
There are three general types of chemical pulps:
(1) Soda pulp is produced by digesting wood chips at elevated temperatures with aqueous sodium carbonate.
(2) Kraft pulp is produced by digesting wood chips at temperatures above about 120° C. with a solution of sodium hydroxide and sodium sulfide. Some kraft pulping is also done in which the sodium sulfide is augmented by oxygen or anthraquinone. Although kraft pulping removes most of the lignin originally present in the wood, enough remains that a bleaching step is required to give pulp of acceptable color. As compared with soda pulping, kraft pulping is particularly useful for pulping of softwoods, which contain a higher percentage of lignin than do hardwoods.
(3) Sulfite pulp is produced by digesting wood with sulfur dioxide and an alkali such as calcium, magnesium, or sodium base. The process operates in the presence of a good deal of free sulfur dioxide, at low pH. Although this process, like kraft pulping, separates most of the lignin from the cellulose fibers, considerable color remains.
“Dissolving-grade pulp” is pulp which has been purified sufficiently for use in the production of viscose rayon, cellulose ethers, or cellulose esters with organic or inorganic acids. It may be produced from either kraft, soda, or sulfite pulp by bleaching and other treatments which will be discussed herein. Historically, dissolving-grade pulp (in contrast to paper-grade pulp) referred to pulp which reacted with carbon disulfide to afford a solution of cellulose xanthate which then could be spun into fibers (viscose rayon) with evolution of carbon disulfide and regeneration of cellulose. Dissolving-grade pulp now refers to pulp which is used to manufacture various cellulose derivatives such as inorganic and organic esters, ether, rayon and the like.
“Bleaching” is the removal of color from pulp, primarily the removal of traces of lignin which remains bound to the fiber after the primary pulping operation. Bleaching usually involves treatment with oxidizing agents, such as oxygen, peroxide, chlorine, or chlorine dioxide. Classically, the pulp is treated with chlorine, then extracted with caustic, and finally treated with hypochlorite. The alkaline extraction may be with either hot or cold caustic. The relative merits of extraction with cold, versus hot, caustic are discussed at length by M. Weyman in
The Bleaching of Pulp,
W. Howard Rapson, editor,
TAPPI Monograph Series
No. 27 (1963), Technical Association of the Pulp and Paper Industry, New York, N.Y., Chapter 5, pp. 67-103. Weyman concludes that cold caustic extraction is the superior method for xylan removal from pulp.
While the chief purpose of the chlorine and caustic treatments is to render the residual lignin in the pulp soluble and extractable, the caustic also degrades and dissolves not only a substantial amount of the hemicellulose, but it also attacks the cellulose itself, with resulting decreases in degree of polymerization and pulp yield. The low molecular weight of some of the hemicellulose fragments makes them hard to isolate, while in some cases (prehydrolysis kraft), the harsh conditions convert the hemicelluloses to decomposition products. In conventional operation, therefore, no attempt is made to recover useful products from the hemicellulose. Chlorine bleaches lead to undesired impurities and make recycle of caustic very difficult. The use of chlorine as the bleaching agent also inevitably produces traces of extremely toxic chlorinated dioxins.
One measure of the effectiveness of bleaching is the brightness of the resulting pulp. Brightness is defined as the reflectivity of a specified standard surface using blue light with a peak wavelength at 457 nm.
Hardwood pulp produced by the kraft process contains a significant amount of hemicelluloses, chiefly xylans. The xylans, in moderate amounts, are desirable in paper manufacture because they help maintain a random dispersion of fiber in the furnish, resulting in more uniform and mechanically stronger paper webs. However, when pulp is used in the production of cellulose esters and other cellulose derivatives, the pulp normally must contain a very low level of xylan. Hardwood kraft pulp for paper manufacture generally contains about 80 to 84% cellulose, about 15 to 20% xylans, and about 0.3-3% mannans. In contrast, dissolving-grade pulp suitable for cellulose ester manufacture for fiber and film applications should contain about 97 to 98.5 weight percent cellulose, not more than about 3 weight percent, e.g., 0.5 to 3 weight percent, xylans, and not more than about 0.5 weight percent, e.g., 0.1 to 0.5 weight percent, mannans. This requirement for higher purity necessitates more drastic treatment with alkali, with resulting decrease in pulp yield. Since the hemicelluloses removed normally are not recovered from such treatments, they are used, if at all, as fuel and have negligible value. The manufacture of dissolving pulps is discussed in detail by J. F. Hinck et al., Chapter VIII, Dissolving Pulp Manufacture in Volume 4, Sulfite Science & Technology of
Pulp and Paper Manufacture,
Third Edition, O. V. Ingruber, M. J. Kocurek, and A. Wong, ed., published by the Technical Section, Canadian Pulp and Paper Association Montreal, QC, Canada, pp. 213-243. Although the relative amounts of impurities vary somewhat between kraft and sulfite pulps, both contain significant amounts of lignin and hemicelluloses which must be reduced.
U.S. Pat. No. 4,008,285 (and related U.S. Pat. No. 4,075,406) contains a brief review of early attempts to
Heikkila Heikki
Lindroos Mirja
Ravanko Vili
Sarkki Marja-Leena
Barts Samuel
Khare Devesh
Scully Scott Murphy & Presser
Xyrofin Oy
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