Process for the co-production of dissolving-grade pulp and xylan

Disclosed is a process for upgrading paper-grade wood pulp to dissolving grade pulp which is suitable for use in the preparation of viscose rayon, cellulose ethers and cellulose esters such as cellulose acetate. The process utilizes a sequence of caustic extraction, xylanase treatment and caustic extraction to remove most of the xylan, which may be recovered for use in the production of xylose, xylitol, and furans.

This invention pertains to a novel process for co-producing 
dissolving-grade pulp and xylan from certain wood pulps. More 
specifically, this invention pertains to a process wherein paper-grade, 
hardwood, kraft or soda pulp is successively extracted with aqueous sodium 
hydroxide, treated with a xylanase enzyme and then aqueous sodium 
hydroxide. The dissolving-grade pulp obtained from the process is useful 
in the manufacture of viscose rayon, cellulose ethers and cellulose esters 
such as cellulose acetate, cellulose acetate propionate and cellulose 
acetate butyrate. The xylan recovered from the process extracts may be 
converted to xylose and then to xylitol, a sweetening agent, by known 
processes, or it may be converted to furfural or other derivatives. 
The production of viscose rayon and cellulose esters such as cellulose 
acetate for use in the manufacture of either film or fiber requires a 
source of high quality cellulose feedstock. The use of cotton linters, the 
source of cellulose of highest purity, has been reduced in recent years in 
favor of the more plentiful and less expensive wood pulp which also is 
used in paper manufacture. 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 hydroxide. 
(2) Kraft pulp is produced by digesting wood chips at temperatures above 
about 120.degree. 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 hydroxide. 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" to which the present invention pertains is pulp 
that 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, 
NY., 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 pulp sample 
as compared to 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, in pulp used to produce 
cellulose esters and other cellulose derivatives, xylans contribute to 
color and haze in the final product and therefore pulp for this 
application 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 
recoverable 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 produce xylose from natural products 
such as wood. The '285 patent also describes a process for purifying the 
pentosan-rich solution obtained by acid hydrolysis of xylan-containing raw 
material. The process involves first purifying the hydrolysate by ion 
exclusion and color removal, then subjecting the purified solution to 
chromatographic fractionation. The recovery of the pulp by-product is not 
disclosed. 
U.S. Pat. No. 4,087,316 describes a process for removing cellulosic fibers 
from seed hulls, such as cottonseed, and for obtaining xylose by 
hydrolysis from the remaining hull fragments in the presence of dilute 
sulfuric acid. The resulting xylose hydrolysate may be hydrogenated to 
xylitol. 
U.S. Pat. No. 4,742,814 discloses a process for obtaining xylitol and, 
optionally, cellulose and lignin from lignocellulose vegetable materials 
by treatment with a mixture of water and lower aliphatic alcohols and/or 
ketones at elevated temperature and pressure followed by separation of 
fibrous materials, organic solvents, and lignin from the treatment 
solutions. The oligosaccharides and polysaccharides remaining in solution 
from this process are hydrolyzed by dilute acid. 
U.S. Pat. No. 5,084,104 is concerned with recovery of xylose from 
hydrolysates of such natural materials as birch wood, corn cobs, cotton 
seed hulls, etc. The disclosed process involves subjecting the hydrolysate 
to a chromatographic column comprising a strong anion exchange resin, and 
eluting a xylose-rich fraction. No reference is made to the recovery of 
any cellulose remaining after extraction of the xylose. 
A more recent article (Gernot Gamerith and Hans Strutzenberger, Xylans and 
Xylanases, J. Visser et al., ed., (1992), pp. 339-348) discusses the 
recovery of xylan during viscose pulp purification. Suggested uses are as 
a raw material for such products as furfural, xylitol, xylose, etc. In the 
process disclosed, beech-wood pulp produced by magnesium bisulfite cooking 
is first bleached with alkaline peroxide and hypochlorite, which reduces 
the xylan content to about 3.6%. This pulp is then treated with "high 
concentrated" sodium hydroxide solution to reduce the xylan content 
sufficiently for the pulp to be used in viscose production. Xylan is 
recovered by acidification of the caustic solution. An unspecified amount 
of xylan remains in the final pulp which, apparently, is sufficiently pure 
for use in viscose rayon production. Although no pulp yields are given, 
the rather drastic alkali treatment suggests that the process resulted in 
a substantial loss of cellulose. 
Bleaching is another step in pulp production. Conventional bleaching 
processes involving chlorine and alkali present environmental problems as 
mentioned above, as well as substantially reducing the amount of 
dissolving-grade pulp which can be recovered from the wood. Some work has 
been done to determine whether the xylans in wood pulp can be hydrolyzed 
and removed by the action of enzymes. Most prior work has been concerned 
merely with sufficient removal of xylan to free residual lignin which is 
bound to the fibers, and aid in pulp bleaching. A number of articles and 
reviews have been published which deal with this aspect of the use of 
enzymes in pulping. A review, Enzymatic Treatment of Pulps by Thomas W. 
Jeffries in Emerging Technologies for Materials and Chemicals from 
Biomass, Roger M. Rowell, Tor P. Schultz, and Ramani Narayan, eds.; 
Advances in Chemistry Series No. 476 (1992), pp 322-327 discusses pulp 
bleaching with hemicellulases. A recent article (L. P. Christov and B. A. 
Prior, Enzyme and Microbial Technology, 18, 244-250 (1996)) describes the 
use of repeated, alternating, treatments with the hemicellulases derived 
from the yeast, Aureobasidium pullulans and alkali to enhance bleaching of 
sulfite pulps. 
The following U.S. patents disclose the use of enzymes as an aid in pulp 
bleaching. None of these patents discloses the production and/or recovery 
of chemical-grade, or dissolving-grade pulp, and none discusses the 
recovery of xylitol. 
U.S. Pat. No. 5,457,046 discloses enzymes with xylanolytic activity. 
U.S. Pat. No. 5,407,827 discloses pulp bleaching by means of 
delignification using thermostable xylanase. 
U.S. Pat. No. 5,395,765 discloses a process for treating pulp with an 
enzyme to improve pulp bleachability and reduce the amount of chlorine 
used. 
U.S. Pat. No. 5,369,024 discloses the use of xylanase for removing color 
from kraft wood pulps. 
U.S. Pat. No. 5,179,021 discloses a pulp bleaching process comprising 
oxygen delignification and xylanase enzyme treatment. 
U.S. Pat. No. 5,116,746 discloses that cellulase-free endoxylanase enzyme 
is useful in pulp delignification. 
U.S. Pat. No. 5,081,027 discloses a method for producing pulp by a 
treatment using a microorganism and its related enzymes. 
U.S. Pat. No. 2,280,307 discloses a process of manufacturing paper. 
The mechanism by which hemicellulose-degrading enzymes (xylanases and 
mannanases) assist in color removal or brightening of wood pulp is not 
completely clear and may be complex (Saake, Clark, & Puls, Holzforschung, 
49, pp 60-68 (1995)). Internal structural changes in the pulp fibers, in 
addition to surface modification by hydrolysis of reprecipitated xylan 
from the surface of kraft fibers and loosening of the bonds between the 
hemicelluloses and residual lignin may also be important. 
Christov and Prior, Biotechnology Letters 13, pp 1269-1274 (1993) describe 
the preparation of dissolving pulp, in contrast to paper-grade pulp, by 
treating bleached sulfite (not kraft) pulp with xylanases, specifically 
enzymes of Aureobasidium pullulans. They state that even with high enzyme 
loadings and 24 hour incubation periods, xylan removal was limited. The 
use of xylanases in prebleaching of bamboo kraft pulp for paper 
manufacture recently has been reported (Pratima Bajpai and Pramod K. 
Bajpai, TAPPI Journal 79(4), 225-230 (1996). 
Cellulose can exist in either of two distinctly different crystalline 
forms. Naturally occurring cellulose crystallites have a morphology known 
as cellulose I in which the individual cellulose molecules are arranged in 
a parallel, or head-to-head, fashion. The second cellulose crystalline 
morphological form is known as cellulose II. This form (which does not 
occur in nature) has the individual cellulose molecules aligned in an 
antiparallel, or head-to-tail, arrangement. Cellulose II is more stable 
than cellulose I. While cellulose I can be converted to cellulose II, the 
reversal of this process has never been achieved. 
Cellulose II fibers are stronger than cellulose I fibers. The conversion of 
cellulose I to cellulose II is practiced commercially in the process known 
as mercerization, the "mercerized" cellulose being largely cellulose II. 
The conversion of cellulose I to cellulose II (mercerization) is carried 
out by exposing the native cellulose I to high concentrations of aqueous 
sodium hydroxide, typically about 15 weight percent sodium hydroxide or 
higher. 
In the production of purified cellulose for use in cellulose ester 
production, it is important to avoid the formation of cellulose II because 
the rate of acylation of mercerized cellulose (cellulose II) is much 
slower than the rate of acylating native cellulose I. However, the 
formation of cellulose II is not objectionable if the dissolving-grade 
pulp comprising cellulose II is destined for use in the production of 
viscose rayon. For a discussion of this subject and for further 
references, see S. H. Zeronian in "Cellulose Chemistry and Its 
Applications", Ellis Harwood, Chichester, 1985, ed. T. Nevell and S. 
Zeronian, chapter 6, pages 166 ff. See especially references 27 and 28 for 
the reduced reactivity of cellulose II. 
The present invention provides a process for the co-production of 
dissolving-grade pulp and xylan from certain wood pulps. The 
dissolving-grade pulp produced may be used in the manufacture of viscose 
rayon, cellulose ethers and cellulose esters whereas the xylan recovered 
from the process may be converted to xylose and then to xylitol. The 
process of the present invention provides a process for the co-production 
of dissolving-grade pulp and xylan by the steps comprising: 
(1) intimately contacting with agitation a paper-grade, hardwood, kraft or 
soda pulp with aqueous sodium hydroxide solution at a temperature of about 
50 to 100.degree. C.; 
(2) subjecting the slurry resulting from step (1) to liquid/solid 
separation and removing sodium hydroxide from the solid material 
separated; 
(3) intimately contacting the solid material obtained from step 2 with an 
aqueous solution of a xylanase enzyme; 
(4) subjecting the slurry resulting from step (3) to liquid/solid 
separation; 
(5) intimately contacting with agitation the solid material obtained from 
step (4) with aqueous sodium hydroxide solution at a temperature of about 
50 to 100.degree. C.; 
(6) subjecting the slurry resulting from step (5) to liquid/solid 
separation and removing sodium hydroxide from the solid material separated 
to obtain dissolving-grade pulp; 
(7) recovering xylan from the xylan/sodium hydroxide solutions obtained 
from steps (2) and (6); 
wherein the paper-grade pulp comprises about 75 to 84 weight percent 
cellulose, about 15 to 22 weight percent xylans, and about 0.3-3 weight 
percent mannans; and the dissolving-grade pulp comprises about 97 to 98.5 
weight percent cellulose, about 0.5 to 3 weight percent xylans, and about 
0.1 to 0.5 weight percent mannans. 
Although our novel process is directed principally to the production of 
pulp which is suitable for use in the manufacture of cellulose esters, it 
also is suitable for the production of other grades of dissolving pulp. 
Cellulose acetate production requires an extremely pure dissolving-grade 
pulp which is very low in xylan content. Other dissolving-grade pulps, 
such as those used for the manufacture of cellulose nitrate or viscose 
rayon, while being of higher purity than paper-grade pulp, may contain a 
somewhat higher content of xylan, e.g., up to 7 weight percent xylan. 
In the first step of the process, a paper-grade, hardwood, kraft or soda 
pulp is contacted or digested with aqueous sodium hydroxide solution at a 
temperature of about 50 to 100.degree. C. To avoid cellulose II 
production, it is an important element of the present invention that the 
entire extraction/maceration constituting step (1) is performed at a 
temperature in the range of 50 to 100.degree. C. The concentration of the 
sodium hydroxide in the aqueous sodium hydroxide solution normally is 
about 8 to 12 weight percent, with a concentration of about 9 to 10 weight 
percent being preferred. The amount of paper-grade pulp typically present 
in the pulp/aqueous sodium hydroxide slurry of step (1) is in the range of 
about 3 to 15, preferably about 7 to 10, weight percent based on the total 
weight of the slurry. A particularly unique feature of the present 
invention is the use of elevated temperatures, e.g., about 50 to 
100.degree. C. during the aqueous caustic treatment of step (1). It is 
preferred to carry out step (1) at a temperature of about 60 to 80.degree. 
C. The time required for step (1) can vary substantially depending on 
various factors such as the particular pulp, sodium hydroxide 
concentration and temperature employed. Contact times of about 1 to 30 
hours are typical for step (1) although contact times in the range of 
about 0.1 to 1 hour normally are adequate. 
The second step of our novel process involves conventional liquid/solid 
separation wherein the solid material present in the step (1) mixture is 
separated, e.g., by filtration or centrifugation, from the step (1) liquid 
phase comprising a solution of sodium hydroxide, xylan and water. Residual 
sodium hydroxide present in the solid material is reduced or removed by 
washing the material with water. Normally, the material is washed, for 
example, either by washing the filter cake on the filter, by counter 
current washing or by reslurrying the solids collected in water, until the 
wash water has a pH of less than about 8, preferably a pH in the range of 
about 6 to 8. Step (2) preferably is carried out at a temperature of about 
50 to 100.degree. C., most preferably about 60 to 80.degree. C. This 
preferred embodiment produces a dissolving-grade pulp which contains 
little, if any, cellulose II and, therefore, is especially useful for use 
in the manufacture of carboxylic acid esters of cellulose. 
In step (3) of the process, the solid material collected in step (2) is 
contacted with a mixture of water and an effective amount of at least one 
xylanase enzyme. The xylanase enzymes suitable for use in the practice of 
our invention are those xylanase enzymes which are substantially free of 
cellulase activity, i. e., those which do not substantially degrade the 
cellulose content of the pulp and provide a cellulose having a Cuene IV of 
4 or greater and which afford a cellulose product sufficiently low in 
xylan content for the particular end use. See, for example, the xylanase 
enzymes described in U.S. Pat. Nos. 5,369,024, 5,395,765 and 5,407,827 and 
the references disclosed in these patents. Suitable xylanases are 
available from a number of sources and exhibit a wide range of activities 
under a variety of operating conditions. The variability of enzymes and 
the optimum conditions at which they are effective is further discussed by 
Bajpai and Bajpai, TAPPI Journal 79(4), 225-230 (1996). 
In general, the step (3) enzyme treatment is carried out at a temperature 
of between about 0 arid 80.degree. C., preferably between 20 and 
80.degree. C., and most preferably between 30.degree. C. and 70.degree. 
C., at a pH between 2 and 12 for a time between 0.1 and 10 hour, 
preferably between 0.5 and 3 hours. The pH and temperature at which an 
enzyme exhibits maximum activity vary substantially and are highly 
specific for a given enzyme. The pH and temperature at which a given 
enzyme is most effective can be determined readily by those skilled in the 
art. 
The amount of xylanase enzyme required to give satisfactory results depends 
upon the degree of xylan removal which is desired, the reaction 
conditions, and the particular enzyme used. Although xylanase assay 
typically is expressed by enzyme manufacturers as "units/mL", the units 
are measured differently by different manufacturers and, consequently, the 
"units/mL" assay is meaningful, if at all, only with respect to a specific 
enzyme supplied by a specific manufacturer. For a given enzyme type and 
source, the amount of enzyme to be used is that required to give the 
desired purity of dissolving grade pulp. The weight ratio of water to the 
step (2) solid material (dry basis) in step (3) may be about 2:1 to 
1000:1, preferably about 4:1 to 35:1. 
The fourth step of the process is a conventional liquid/solid separation 
wherein the solid pulp material present in the enzyme-treated mixture of 
step (3) is separated, e.g., by filtration or centrifugation, from the 
step (3) liquid phase comprising xylanase enzyme, water and xylan. Steps 
(5) and (6) are carried out according to the procedures described above 
relative to steps (1) and (2). As noted above, step (6) yields 
dissolving-grade pulp which comprises about 97 to 98.5 weight percent 
cellulose, 0.5 to 3 weight percent xylans, and about 0.1 to 0.5 weight 
percent mannans. 
In step (7) xylan may be recovered from the liquids of steps (2) and/or (6) 
by known procedures. A preferred method for recovering the xylan comprises 
the alcohol precipitation procedure described in U.S. Pat. No. 3,935,022. 
In this method, one or more C.sub.1 -C.sub.4 alkanols are combined with 
the liquids of steps (2) and/or (6) to precipitate the xylan from the 
liquids. Thus, step (7) preferably comprises combining the liquids of 
steps (2) and/or (6) with one or more C.sub.1 -C.sub.4 alkanols to effect 
precipitation of xylan from the liquids and subjecting the resulting 
mixture to liquid/solid separation to recover xylan. The volume of the 
alkanol(s) combined with the liquids of steps (2) and/or (6) to effect 
xylan precipitation may be in the range of about 50 to 200% of the volume 
of the liquids of steps (2) and/or (6) although alkanol volumes of about 
80 to 120% (same basis) are more typical. Methanol and ethanol are 
particularly preferred alkanols. The liquids of steps (2) and/or (6) may 
be concentrated, e.g., by vaporization or membrane separation procedures, 
prior to being combined with the alkanol(s). 
The solution comprising sodium hydroxide, alkanol(s) and water obtained 
from the liquid/solid separation of step (7) may be subjected to 
distillation to separate the alkanol(s) from the aqueous sodium hydroxide. 
Thus, both the alkanol(s) and the aqueous sodium hydroxide may be used 
repeatedly in the process. 
Alternatively, the liquids of steps (2) and/or (6) can be concentrated by 
removal of water by distillation or multiple-effect evaporation until the 
concentration of sodium hydroxide is about 40-50 weight percent. This 
concentrated solution can be treated with a C.sub.1 -C.sub.4 alkanol to 
precipitate the xylan. About 1 volume equivalent of alkanol is required. 
The precipitated xylan is recovered by filtration, centrifugation, or the 
like, and the filtrate distilled to recover the alkanol and leave a 
concentrated sodium hydroxide solution which can be diluted to the desired 
concentration for use in the xylan extraction process. 
In another variation, the liquids of steps (2) and/or (6) can be subjected 
to nanofiltration through a caustic-stable membrane which allows passage 
of water and sodium hydroxide but does not allow the passage of dissolved 
organic compounds having a molecular weight above a few hundred, e.g., 
xylan. This process variation produces a clean sodium hydroxide stream 
ready for re-use and a much smaller stream in which the xylan is highly 
concentrated in aqueous sodium hydroxide. The xylan in this organic-rich 
stream may be recovered by alkanol precipitation as described above, or by 
neutralization of the sodium hydroxide by the addition of a mineral acid 
which also precipitates the xylan. 
The dissolving-grade wood pulp produced by the present invention is useful 
for conversion into viscose or cellulose ester fibers, plastics, etc. The 
utility of the dissolving-grade wood pulp in the manufacture of cellulose 
acetate has been demonstrated by preparing cellulose acetate from both 
paper-grade pulp and the dissolving-grade pulp produced by the process of 
this invention. The roll color of cellulose acetate produced from paper 
grade pulp was 18.2-18.3 whereas the roll color of cellulose acetate 
prepared from the dissolving pulp produced in accordance with the present 
invention typically is between 11.4 and 11.9. commercially prepared 
cellulose acetate has an average roll color of 11.7. 
The xylan recovered in step (7) of the process may be converted to xylose 
and xylitol according to conventional procedures. Procedures for the 
conversion of xylan to xylose and xylitol, and recovery processes, are 
described in U.S. Pat. Nos. 4,008,356, 4,025,356, 4,075,406 and 5,084,104. 
For example, heating a slurry of xylan in water, e.g., a slurry containing 
from about 5 to 25 weight percent solids, in the presence of a mineral 
acid produces xylose. The heating normally is at a temperature in the 
range of about 70 to 150.degree. C., preferably at about 90 to 100.degree. 
C. Examples of suitable mineral acids include sulfuric acid, hydrochloric 
acid and phosphoric acid. Alternatively, the recovered xylan may be 
converted to the industrial intermediate furfural. It is possible to 
utilize the xylan present in the xylan/sodium hydroxide solutions obtained 
from steps (2) and/or (6) in the production of xylose from xylan. 
The process of the present invention is further illustrated by the 
following examples. Since they are too insoluble for direct analysis, the 
hemicelluloses (xylan and mannan) in pulp were determined by digestion 
with dilute acid followed by analysis of the hydrolysate for the resulting 
sugars (xylose and mannose) by liquid chromatography. See, for example, 
the procedures described by R. Petersen, et al., J. Chromatogr. Sci., 22 
(1984) 478-84) and K. Garleb, et al., J. Agric. Food Chrm., 37 (1989) 
1287-93. "Cuene IV", a measure (in deciliters per gram--dL/g) of the 
degree of polymerization of cellulose, was determined according to TAPPI 
procedure T230 om-89 (Revised, 1989). In this procedure, the viscosity of 
a solution of cellulose in a copper-ethylenediamine reagent is measured as 
an indicator of the molecular weight of the sample.

EXAMPLES 1-7 AND COMATIVE EXAMPLES 1-4 (C-1 TO C-4) 
These examples illustrate of treatment of aspen paper-grade kraft pulp with 
10 weight percent aqueous sodium hydroxide and relatively low levels of a 
xylanase enzyme available under the name Irgazyme 10A-X4 (4400 units of 
enzyme per mL, Genencor International, Inc.) The paper-grade pulp had a 
Cuene IV of 7.97 deciliters per cram and contained 17.80 weight percent 
xylose and 0.33 weight percent mannose. 
The paper-grade pulp (10 g) was shredded into approximately 1 inch.times.3 
inch (2.54 cm.times.7.62 cm) pieces and mixed with 200 mL of a 10 weight 
percent solution of sodium hydroxide in deionized water. The pulp and 
caustic were mixed thoroughly and shaken at different temperatures for 
different periods of time. The pulp was then transferred to a porous cloth 
bag and washed under running deionized water for 1 hour. 
The bag containing the pulp was squeezed to remove excess water, then the 
pulp was added to 200 mL of deionized water, the pH of which had been 
adjusted to pH 4.5 by addition of sodium acetate if required, and which 
contained the enzyme. This slurry was mixed well, and placed in a constant 
temperature shaker bath at 30.degree. C. for 1 hour. The mixture was 
transferred to a wash bag and washed as before for 1 hour. 
The solid material resulting from the enzyme treatment was treated with 
aqueous sodium hydroxide and washed using the same sodium hydroxide 
concentration, treatment time and temperature used in the first agueous 
sodium hydroxide treatment. After the second aqueous sodium hydroxide 
treatment and wash, the pulp was removed from the wash bag and placed in a 
temperature controlled oven overnight or until dry. Samples of the 
dissolving-grade pulp thus obtained were analyzed for Cuene IV and for 
sugars by acidic digestion to monomers followed by liquid chromatography. 
The conditions used in the aqueous sodium hydroxide extractions and the 
results achieved are shown in Table I wherein "Time" is the period of time 
(minutes) and "Temp" is the temperature (0.degree. C.) of each aqueous 
sodium hydroxide treatment; "Enzyme Conc" is the units of xylanase enzyme 
present during the enzyme treatment per g of paper-grade pulp used 
initially; "Cuene IV" is given in dL/g and has the meaning given above; 
and the values given under "Xylose" and "Mannose" are the weight 
percentages of xylose and mannose, respectively, present in the dissolving 
grade pulp obtained in each example. The comparative examples are 
characterized as C-1, C-2, etc. 
The results set forth in Table I clearly show that the sequential 
caustic/enzyme/caustic treatments are effective to purify paper-grade pulp 
and convert it to dissolving-grade pulp and that the caustic treatments at 
70.degree. C. are more effective than 30.degree. C. 
TABLE I 
______________________________________ 
Enzyme Cuene 
Time Temp Conc IV Xylose 
Mannose 
Example Min. .degree. C. u/g pulp dL/g % % 
______________________________________ 
C-1 30 30 20 7.92 2.68 0.67 
C-2 60 30 20 7.55 2.60 0.72 
C-3 60 30 60 6.14 2.40 0.65 
C-4 30 30 60 6.52 2.73 0.62 
1 60 70 60 5.93 1.85 0.75 
2 45 50 40 6.95 2.15 0.70 
3 30 70 20 7.21 2.32 0.68 
4 45 50 40 6.33 2.30 0.63 
5 30 70 60 6.46 2.23 0.66 
6 60 70 20 6.22 2.13 0.68 
7 45 50 40 6.63 2.57 0.62 
______________________________________ 
EXAMPLES 8 AND 9 AND COMATIVE EXAMPLES 5 AND 6 
The general procedure described in Examples 1-7 was repeated for Examples 
8-11 and Comparative Examples 5-8 using the same paper-grade pulp. The 
enzyme concentration used in the xylanase enzyme treatment step was 40 
units of Irgazyme 10A-X9 xylanase enzyme per g of paper-grade pulp used 
initially in each example. The enzyme treatment step was carried out at pH 
4.5 and 30.degree. C. The consistency used in these examples was 4.76 
wherein "consistency" refers to the g of paper-grade pulp initially used 
per g reaction mixture, expressed as a percentage, during the enzyme 
treatment step. The conditions used in the aqueous sodium hydroxide 
extractions and the results achieved are shown in Table II wherein "Time", 
"Temp", "Cuene IV", "Xylose" and "Mannose" have the meanings given above 
for Table I. Since the pulp lost some weight as soluble xylan and since it 
was charged as a wet solid without compensating for the diluting effect of 
the water, the actual sodium hydroxide concentration and consistency were 
somewhat lower in the second sodium hydroxide extraction than in the 
first. 
TABLE II 
______________________________________ 
Cuene 
Time Temp IV Xylose Mannose 
Example Min. .degree. C. dL/g % % 
______________________________________ 
C-5 30 30 7.66 5.31 0.70 
8 30 70 7.51 2.00 0.47 
C-6 60 30 8.15 2.98 0.50 
9 60 70 7.61 1.73 0.76 
______________________________________ 
The data presented in Table II clearly show that the lowest xylose content 
is reached when the caustic treatments are carried out at 70.degree. C. 
for 60 minutes. Although Comparative Example C-6 shows that a caustic 
extraction temperature of 30.degree. C. can produce a pulp having less 
than 3 weight percent xylan, in all cases the use of 70.degree. C. gives 
superior results when other variables are the same. We have found that, in 
general, higher concentrations of sodium hydroxide give better results 
with a concentration of about 10 weight percent being the best because 
higher concentrations will cause mercerization, even at high temperatures. 
EXAMPLES 10 AND 11 AND COMATIVE EXAMPLES 7-12 
Examples 10 and 11 and Comparative Examples 7-12 show the effect of varying 
the sequence of the aqueous sodium hydroxide treatments (designated "E") 
and the xylanase enzyme treatment(s) (designated "X") on the xylan content 
of treated pulp using two different enzymes: Irgazyme 40-X4 xylanase in 
Examples 10 and Comparative Examples 7-9 and Buzyme xylanase (available 
from Buckman Laboratories) in Example 11 and Comparative Examples 10-12. 
In these examples, each aqueous sodium hydroxide treatment was carried out 
at 70.degree. C. with 10 weight percent aqueous sodium hydroxide using the 
general procedure and the paper-grade pulp described in Examples 1-7. In 
Examples 10 and Comparative Examples 7-9 the concentration of the enzyme 
was 20 units of xylanase enzyme per g of pulp, the pH of the enzyme step 
was 6.5 and the temperature of the enzyme step was 30.degree. C. In 
Example 11 and Comparative Examples 10-12 the concentration of the enzyme 
was 60 units of xylanase enzyme per g of pulp, the pH of the enzyme step 
was 7.0 and the temperature of the enzyme step was 70.degree. C. The 
sequence of treatments used and the results achieved in each example are 
shown in Table III wherein the letters set forth below "Treatment 
Sequence" identify the order (proceeding from left to right) of the 
treatments carried out in each example and "Cuene IV", "Xylose" and 
"Mannose" have the meanings given above for Table I. The values given for 
"Weight Yield" are determined by: 
##EQU1## 
TABLE III 
______________________________________ 
Cuene Weight 
Treatment IV Xylose Mannose Yield 
Example Sequence dL/g % % % 
______________________________________ 
C-7 X-E-E 7.45 2.52 0.61 76 
C-8 X-E-X 6.61 3.14 0.68 77 
10 E-X-E 7.67 1.63 0.81 75 
C-9 E-E-X 5.76 2.21 0.69 75 
C-10 X-E-E 6.34 2.35 0.69 75 
C-11 X-E-X 7.43 3.01 0.72 77 
11 E-X-E 7.98 1.28 0.70 75 
C-12 E-E-X 6.91 1.99 0.68 76 
______________________________________ 
EXAMPLES 12-15 AND COMATIVE EXAMPLES 13-18 
The general procedure described in Examples 1-7 was repeated for Examples 
12-15 and Comparative Examples 13-18 using a eucalyptus, kraft, 
paper-grade pulp having a Cuene IV of 6.09, a xylose content of 14.49 
weight percent and a mannose content of 0.55 weight percent. The xylanase 
enzyme (Irgazyme 40-X4) treatment was carried out at pH 6.5 using a sodium 
acetate/acetic acid buffer, at 30.degree. C. for 60 minutes. The enzyme 
concentration used in the enzyme treatment step was varied from 0 to 50 
units of xylanase enzyme per g of paper-grade pulp used initially. In the 
examples in which no enzyme was used, the pulp was treated with an aqueous 
buffer solution at pH 6.5 for 30.degree. C. for 60 minutes. Each aqueous 
sodium hydroxide extraction was carried out for 60 minutes using 10 weight 
percent aqueous sodium hydroxide solution and the temperatures shown in 
Table IV. The temperatures used in the first and second aqueous sodium 
hydroxide extractions ("First Caustic" and "Second Caustic") and the 
results achieved are shown in Table IV wherein "Enzyme Conc", "Cuene IV", 
"Xylose" and "Mannose" have the meanings given above for Table I. 
TABLE IV 
______________________________________ 
First Second Enzyme Cuene 
Caustic Caustic Conc IV Xylose Mannose 
Example .degree. C. .degree. C. u/g dL/g % % 
______________________________________ 
C-13 30 30 0 6.30 5.09 0.38 
C-14 70 30 0 6.53 3.34 0.33 
12 70 70 50 4.89 2.26 0.39 
13 50 50 25 5.06 2.56 0.34 
C-15 70 30 50 5.72 2.33 0.32 
C-16 30 70 0 4.95 3.64 0.26 
14 50 50 25 5.95 2.74 0.33 
C-17 30 70 50 5.69 2.96 0.25 
C-18 70 70 0 s.49 3.19 0.36 
15 50 50 25 5.38 2.73 0.27 
______________________________________ 
The data presented in Table IV show that hot (70.degree. C.) caustic 
extraction is more effective than caustic extractions at 30.degree. C. and 
that there is no advantage in doing one extraction cold and the other hot. 
They also demonstrate that results are much poorer when the enzyme 
treatment is omitted. 
EXAMPLE 16 
Aspen kraft pulp (20 g) similar to that used in Example 1 was preheated to 
about 70.degree. C. To the pulp was added a volume of 10 weight percent 
aqueous sodium hydroxide, preheated to 70.degree. C., sufficient to give a 
suspension of 7 weight percent pulp in the aqueous sodium hydroxide. This 
mixture was maintained at 70.degree. C. for 1 hour, filtered hot, (200 ml 
of filtrate was recovered and set aside for xylan recovery) and the pulp 
washed with 70.degree. C. water until the filtrate was pH 7. The pulp then 
was diluted to 7 weight percent concentration with distilled water and 
0.166 ml of a commercial xylanase (6000 units/ml) was added to the slurry. 
This mixture was maintained at 70.degree. C. for 1 hour. The pulp again 
was separated by filtration and combined with sufficient 10 weight percent 
aqueous sodium hydroxide to give A 7 weight percent pulp suspension. After 
1 hour at 70.degree. C., the pulp was filtered hot, washed with 70.degree. 
C. water until the filtrate was neutral, and then dried in a 45.degree. C. 
forced-air oven. The dried pulp weighed 16. 1 g and contained 2.53 weight 
percent xylose and no detectable cellulose II as determined by x-ray 
diffraction analysis. 
The 200 mL of the xylan-containing, aqueous sodium hydroxide filtrate 
obtained above was stripped to approximately 100 mL and combined with 100 
mL methanol to precipitate the xylan which was collected by filtration. 
The solid xylan was washed with water and ethanol, and then dried to give 
2.1 grams of xylan product. 
When the above procedure is repeated with the exception that the pulp is 
treated with 10 weight. percent aqueous sodium hydroxide solution at 
ambient temperature (about 25.degree. C.) for about 5 minutes, then heated 
to 70.degree. C. on a steam bath for 1 hour, the pulp obtained has a 
similar xylose and mannose content, but x-ray diffraction analyses shows 
the presence of from 30 to 70 mole percent cellulose II. 
EXAMPLE 17 
This example illustrates the utility of xylan as an intermediate for the 
preparation of the industrial intermediate, furfural. To a flask was 
charged 100 g water-wet (15% solids) xylan isolated from paper-grade aspen 
pulp according to the process of the invention and one 100 mL portion of a 
1:1 mixture of concentrated hydrochloric acid and water. The mixture was 
stirred under reflux until solution of the solids had occurred; then 
distillation was commenced. There was obtained a distillate which 
separated into an upper water-rich phase and a lower organic phase. The 
organic phase was separated and shown to consist of essentially pure 
furfural by gas chromatographic comparison with an authentic sample. 
EXAMPLE 18 
This example illustrates the utility of xylan as an intermediate for the 
preparation of xylose. A mixture of 50 g of water wet xylan (equivalent to 
7.75 g dry xylan) isolated from paper-grade aspen pulp according to the 
process of the invention was mixed with 100 mL water and 3 mL sulfuric 
acid. After being stirred overnight at reflux, the initial slurry became a 
dark solution. The mix was cooled, neutralized by addition of sodium 
acetate, treated with a small amount of decolorizing charcoal, filtered, 
and freeze-dried to give 12 g of crude product comprising xylose, sodium 
sulfate and sodium acetate. Analysis by high pressure liquid 
chromatography indicated the presence of 51.7 weight percent xylose (70.4% 
of theory) and 0.6% xylobiose. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit arid scope of the 
invention.