Biomedical Engineering Reference
In-Depth Information
In 1986, at the International Paper Process Biotechnology Seminar, the Finnish
scientist Viikari first reported that the xylanase pretreatment of pine and birch
kraft pulp (KP) can reduce the consumption of bleaching chlorine and improve
pulp brightness [
28
]. In 1989, the xylanase bleaching KP industrial test was first
performed in Finland [
29
]. Use of the xylanase preparations as bleaching aids
can reduce the bleaching waste pollution lignose, with significant benefits and
easy industrialization. Currently, it has been applied by more than 30 large-scale
paper mills in Europe and North America, which is the most successful example of
biotechnology in biopulping [
30
,
31
].
Biobleaching is described from the following three aspects: First, microorgan-
isms play a direct role in pulp biobleaching. Second, hemicellulase is involved in
pulp biobleaching. Third, biobleaching is conducted by ligninolytic enzymes.
8.3.1
Hemicellulose in the Raw Materials and Pulp
(1) Lignin-carbohydrate complex. The main components of plant materials are
cellulose, hemicellulose, and lignin. Cellulose is the skeleton of substances that
makes up plant cell walls, and the mixture of lignin and hemicellulose fills
among the microfibers. It has been proven that there are chemical connections
between the lignin and carbohydrates, forming the LCC.
(2) Hemicellulose in pulp and its reabsorption. In the cooking process, part of
the xylan is dissolved in the cooking liquid. With the decrease of the liquid
concentration, the dissolved xylan is reabsorbed on the fibers to form the xylan
deposition. The mass fractions of galactoglucomannan and xylan in softwood
and spruce are 22.9 % and 10.4 %, respectively. However, when the yield
of the cooking pulp gained by the KP method was 46.6 %, their contents
were reduced to 8.3 % and 6.9 %, respectively. The loss of hemicellulose in
xylan is minor, mainly because uronic acid exists at the xylan branches chain,
reducing the rate of the peeling reaction, and xylan has higher alkali stability
than galactoglucomannan.
The dissolved xylan is reabsorbed to the fibers again in a later stage of
cooking. The ratio of polygalactose glucose mannose reabsorption is less than
xylan. In samples of pulp, the reabsorbed xylan was estimated to be from 4 to
12 %. Reabsorbed xylan in the pine pulp was estimated as 50 % of the total
amount. Many hemicelluloses are reabsorbed and deposited on the fiber surface
to reduce the permeability of the fiber surface and hinder the dissolution of
residual lignin in pulp washing and bleaching.
Part of the reabsorbed hemicelluloses and residual lignins can form the LCC,
which makes it more difficult to remove the residual lignin in the subsequent
pulp washing and bleaching process. This part of the LCC has been analyzed
by the enzymolysis method, chromatographic separation and mass spectrometry
(MS), ultraviolet, and so on. During the continuous cooking process, black
liquor was continuously removed and replaced by new cooking liquid; the
