Parameters Units First stage Second stage

Consistency % 12 >10.5

NaOH-charge kg adt–1 25 0

Oxygen charge kg adt–1 18–25 low charge

Retention time min 30 60

Temperature °C 80–85 90–100

Pressure (top) MPa 0.8–1.0 0.4

is injected to raise the temperature to the desired level for the second stage. A

booster pump ensures flow through the second high-shear mixer and the second

reactor to the blow tank, where gas and pulp slurry are separated. Finally, the pulp

is pumped to post-oxygen washing.

Fig. 7.55 Typical OxyTrac process flowsheet [130].

Implementation of the OxyTrac system in full scale confirms the superior

delignification performance as compared to single-stage oxygen delignification

systems [127,130]. It is reported that adoption of the OxyTrac system resulted in a

significant increase (from 39% to >60%) in the degree of pulp delignification for

the Arauco mill in Chile using radiata pine kraft pulp, while preserving strength

properties. The ability to level out variations of the incoming pulp is another

important feature of a two-reactor oxygen delignification system.

A second commercial solution to the two-reactor oxygen delignification system

is the Dualox™ system provided by Kvaerner. The major differences from the

OxyTrac system comprise the short retention time in the first reactor of only about

5 min, and the split addition of oxygen. The short retention time in the first reac-

730 7Pulp Bleaching

tor is chosen by following results from kinetic investigations [14]. The first reactor

was built as a long and thick tube, enabling a very space-saving design.

A typical Dualox process flowsheet is shown in Fig. 7.56. Caustic is added to the

standpipe before the first medium-consistency pump. The pulp slurry is then

pumped through the first high-shear mixer and the pipe-type prereactor and

further on, by a booster pump, through the second high-shear mixer and the

upflow reactor to the blow tank. Oxygen and steam are added both to the first and

the second high-shear mixers. In the situation when a greater temperature

increase is required, a dedicated static steam mixer is added to the system before

the second pump [131].

Fig. 7.56 Typical Dualox™ process flowsheet [131].

A characteristic feature of the Dualox™ system is that both reactors are run at

the highest possible pressures in order to extend delignification as much as possible.

In contrast to the OxyTrac system, the oxygen charge to the first reactor is

kept low, while the main part of oxygen is charged to the second reactor.

According to current experiences from mill-scale operations, the Dualox™ tworeactor

system provides delignification performances that are comparable to those

reported for the OxyTrac system [132]. In the case of delignifying softwood kraft

pulp, the kappa number reduction approaches values of about 65%, which typically

ensures kappa number values of 8–12 entering the bleaching plant. The conversion

of a conventional oxygen delignification system to the Dualox™ process in a

hardwood kraft pulp production line was reported to improve the viscosity, despite

extending delignification by almost 5 kappa number points [133].

Дата добавления: 2015-10-21