Biomedical Engineering Reference
In-Depth Information
EDED, LCEHH and LCEDED, of which L, D, C, E, and H represent Lac, chlorine
dioxide, chloride, alkali treatment, and sodium hypochlorite treatment, respectively.
The enzyme treatment is generally arranged before the conventional bleaching
processes, thereby saving subsequent chemical bleaching agents. The development
of the traditional chemical bleaching process is going toward the nonpolluting
bleaching direction, so enzyme treatment can be used as a pretreatment for the
chemical bleaching process.
(2) Enzyme treatment is after aerobic delignification.
In the study of the bleaching process containing oxygen bleaching (O), ozone
bleaching (Z), and peroxide bleaching (P), to achieve the bleaching effect in
the aerobic delignification process, the L step can be arranged after the aerobic
delignification to use the xylanase, such as OLDED OLZED, OL (DC) ED OLDP
and OLPDP. The L step is prior to the Z step. Yang et al. proposed a new enzyme
treatment process that was totally chlorine-free (TCF) bleaching, such as OLZP,
sometimes called the EnZone method.
8.3.5
Bioassisted Bleaching Technology
The parameters of the xylanase-involved biological process include the optimum
pH value of enzyme, a temperature of 30-60
ı
C, a pulp concentration of 5-10 %,
an enzyme dosage of 1-10 IU
g
1
, and a reaction time of 1-3 h. Usually, the pH
value of xylanase produced by bacteria is about 6-9, which is higher than that of
fungi. The parameters of enzyme treatment must be appropriately selected to obtain
the best treatment effect.
8.3.6
Development of and Prospects for Xylanase Technology
A large amount of xylan from the pulp and papermaking process is discharged into
rivers, causing environmental and ecological pollution. There is also a considerable
amount of xylan in agricultural residues. The transformation of xylan into a useful
product is the future development direction of xylanase, and xylanase will have
broad prospects in the process of lignocellulosic biomass as well as the development
of renewable resources and energy.
Xylanase has made some progress in biochemistry, synthesis regulation, and
molecular biology research. To make the xylanase applied in the industrial produc-
tion on a large scale, a xylanase that is effective in a variety of operating conditions
must be found. So far, the use of xylanase in biobleaching has experienced three
generations: acidic enzyme, neutral enzyme, and alkaline enzyme. Currently, the
research and development of the third-generation xylanase (alkaline enzyme) is at
its peak. With protein engineering identifying the xylanase activity site residues by
chemical modification means, X-ray crystallography, and site-directed mutagenesis,
