Page 450 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Pulp and Paper 255

aqueous ethanol and 85% aqueous acetic acid, were estab- liquor. This process is summarized in Fig. 2, a flowsheet
lishedinGermanyintheearly1990s,however,neithermill of Kraft pulping.
was successful and both were shut down for a variety of The digester is the heart of the Kraft mill. It may be a
reasons. continuous digester, such as the unit at Leaf River, Missis-
With pulping, the inter-fiber lignin bond is broken down sippi, shown in Fig. 3. Alternatively batch digesters may
by mechanical and/or chemical treatments to free the cel- be used. The continuous digester offers somewhat higher
lulose fibers for papermaking. In the forest, white rot yields and reduced energy requirements than the batch di-
fungi perform a similar task on wood left behind. The gester. However, the batch digester offers greater product
enzymes of the fungi do the work of lignin degradation. flexibility.
This is the basis of new biopulping approaches that have Kraft pulping requires the consumption of 14–20
been under development for over 10 years. Wood chips GJ/tonne of pulp in the form of heat energy (3–10 atm
or agricultural materials are treated with a white rot fun- steam); and 900–1000 kW h/tonne of pulp either as elec-
gus and nutrients for about two weeks which breaks down tricity or shaft power. Variation results as much from
and alters the lignin gluing substance in the lignocellu- local economic conditions as from severity of pulping
losic material. The biomass then can be much more easily conditions associated with product requirements. The un-
disintegrated by mechanical treatment in a disk refiner. bleached Kraft pulp mill can generate virtually all of its
Since some mechanical treatment is required the method energy internally with the exception of the 2 GJ/tonne re-
ismoreproperlytermedbiomechanicalpulping.Investiga- quired as oil or gas for the lime kiln. Even there progress
torsattheU.S.DepartmentofAgriculture,ForestProducts is being made in commercializing wood-fired lime kilns.
Laboratory in Madison, WI, evaluated hundreds of fungi Yields of 50% and reduced energy consumption have
for this purpose and found that treatment with the white been achieved by a history of innovation. Such innovation
rot fungus, Ceriporiopsis subvermispora, resulted in the has included the Tomlinson black liquor recovery boiler,
greatest reduction in energy requirements for mechani- the Kamyr continuous digester and associated diffusion
cal disintegration and the best strength properties from washer,multiple-effectevaporators,andlow-odorconcen-
the resulting paper. Pilot level trails with biomechanical trators. Economic advantages also have been gained by the
pulping have demonstrated the viability of the process, development of systems for recovering extractives such as
which is nearing commercial application. All of these tall oil, fatty acids, and resin from the pulping liquor for
new approaches have been reviewed by Young and Akhtar sale as naval stores. Future innovations may focus on the
(1998). lime kiln and other related systems.
Chemical pulping systems other than the Kraft process
described earlier also have, at their center, the digester
C. Process Considerations
and the recovery system. The major process differences
Chemical pulping, as performed in the Kraft process, is es- between the Kraft and sulfite pulping methods, from a
sentially a closed process. Wood in log form is debarked process perspective, are in the chemical and energy recov-
and chipped. Pulp chips are screened and then sent to ery area. Aho (1983) has pointed out that sodium-based
continuous or batch digesters. Cooking occurs in the di- systems require highly complex recovery systems such
gestor where the wood reacts with pulping (white) liquor as the Tampella Recovery Process, the Stora Process, the
containing NaOH and Na 2 S at elevated temperatures and CE Silvola process, and the SCA-Billenid Process. Mag-
pressures; following cooking, the chips are “blown” to nesium based systems permit both energy and chemical
produce fibers, washed to achieve pulp-liquor separation, recovery; however calcium-based liquor incineration re-
and then transported as pulp either to the bleach plant sults in a loss of base, a loss of sulfur, and serious scaling
or pulp dryer. The spent pulping (black) liquor is passed problems.
through evaporators and concentrators until its moisture Ammonia-based liquors, when incinerated, result in a
content is reduced to about 40%. The black liquor, a mix- loss of nitrogen as N 2 in the flue gas. This difficulty is
ture of pulping chemicals and dissolved lignin, is then highly responsible for the domination of Kraft pulping.
burned in the recovery boiler to achieve energy and chem- Magnesium-based liquor incineration is most easily ac-
ical recovery. Energy is recovered as high-pressure stream. complished, and can be achieved either in a Tomlinson
Chemical recovery is accomplished with sodium carbon- furnace or a Copeland fluidized bed system.
ate (Na 2 CO 3 ) and sodium sulfide (Na 2 S) being tapped While sulfite pulping is less popular than Kraft pulp-
from the bottom of the boiler. The smelt is dissolved in ing, it is more prevalent in the production of dissolving
water, reacted with calcium oxide from the lime kiln to pulps. Further, sulfite pulping permits recovery of ethanol
convert Na 2 CO 3 to NaOH, and then returned to the white from the spent pulping liquor before incineration, as is