Page 447 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN013A-619 July 26, 2001 19:32

252 Pulp and Paper

as a byproduct of lumber or plywood manufacturing. In region of the United States, growth routinely exceeds har-
recent years the by-product source of wood has become vest. This situation is aided by short rotation ages of
increasingly important, virtually displacing all cordwood pulpable southern species from loblolly pine to Ameri-
in Pacific Coast pulp mills. Today, over 40% of all wood can sycamore. Loblolly pine can be grown in 15- to 30-
utilized by U.S. pulp mills comes from such chips. This year rotations, while American sycamore can be grown in
development resulted both from technologies to produce 5- to 10-year rotations.
and to utilize chips from sawmill slabs and green clippings Pulpwood also is plentiful in such countries as Canada
from the plywood mill. and the Russia; and abundant tropical forests exist in
Although wood is the dominant raw material for pulp such countries as Brazil. Adequate wood supplies ex-
and paper in the developed world, a wide range of fibers ist in Scandanavia as well. Silvicultural practices in the
are utilized for papermaking in other parts of the world. Scandanavian region, coupled with intensive utilization
In many countries pulp production is based entirely on of harvested materials, have prevented undue scarcity in
agro-based fibers and over 25 countries depend on agro- that geographic area.
based fibers for over 50% of their pulp production. The
leading countries for production of pulp and paper from
B. Wood Quality
agro-based fibers are China and India, with China hav-
ing over 73% of the world’s agro-based pulping capacity. Issues of quality include anatomical, physical, and chemi-
China mainly utilizes straw for papermaking while India cal properties of various types of furnish. Anatomical con-
and Mexico utilize large quantities of sugar cane bagasse cerns focus upon wood fiber length, because fiber length
(fiber waste from sugar production). India also incorpo- influences a variety of paper properties from strength to
rates some jute fiber and large quantities of bamboo, al- printability. Physical properties of consideration include
though the supply of bamboo is not sufficient to meet de- various measures of strength. Measures of strength can be
mands for paper production. There has been considerable inferred from fiber length and specific gravity. Chemical
interest in the use of kenaf as an alternate fiber source in propertiesofconcernincludepercentagecomposition,cel-
the U.S. and a number of successful press runs of kenaf lulose, the hemicelluloses, and lignin. Cellulose content
based paper (82–95%) were carried out in the pressrooms largely determines yield of chemical pulping. Lignin con-
of the Bakersfield Californian, the Houston Chronicle, the tent determines the higher heating value of spent pulping
Dallas Morning News and the St. Petersburg Times. liquor. The extractives content determines the economic
Practically any natural plant can be utilized as a source value of byproduct production of naval stores from Kraft
of papermaking fibers, but there is considerable varia- pulp mills. Such mills are the dominant sources of rosin,
tion in the quality of paper realized from alternate plant distilled tall oil, and turpentine in the current forest prod-
sources. Factors such as fiber length, content of non- ucts industry.
fibrous components such as parenchyma tissue, contam- Typical properties of selected wood species are shown
inants such as silica, etc. greatly influence the quality of in Table II. Note that the clear distinctions between the
the final sheet. Procurement of sufficient quantities of the softwoods and hardwoods include fiber length, hence re-
raw material and seasonal fluctuations in supply can also sulting pulp strength. Softwoods are clearly superior from
pose problems. It is also necessary to use alternate pulping a strength perspective. Note, also the higher cellulose con-
equipment to handle the plant materials since the material tent of hardwoods—implying that such species as trem-
tends to mat down in the digester making it difficult to get bling aspen will have higher chemical pulp yields than
uniform circulation of the cooking chemicals. coniferous woods. In general hardwoods have 45% cel-
lulose, 30% hemicelluloses, and 20% lignin, while soft-
woods will have 42% cellulose, 27% hemicelluloses, and
A. Wood Availability
28% lignin. It is useful to note that properties of wood
The U.S. has over 200 million hectares (490 million acres change as trees age. For example, Bendston has shown
or 770,000 square miles) of commercial forest land, a that an 11-year-old loblolly pine has a tracheid length of
resource base that routinely produces more cubic me- 2.98 mm and a cell wall thickness of 3.88 µm. A 39-year
ters of timber than is harvested annually. Of the timber old tree of the same species will have a tracheid length
producing regions of the United States, only the Pacific of 4.28 mm and a cell wall thickness of 8.04 µm. More
Coast witnesses more harvest than growth. The anomaly mature trees will yield higher strength fibers.
of the Pacific Coast results from the large inventory of Given the general properties of wood furnish as identi-
old growth Douglas-fir. As second growth stands become fied above, it is now important to examine specific chemi-
more prominent, this harvest/growth deficit will be re- cal and mechanical pulping, bleaching, and papermaking
versed. In the south, the major pulp and paper producing technologies.