Material Balance

The prehydrolysis-kraft process according to the Visbatch® or VisCBC operations

is divided into several steps. After a conventional chip filling procedure, prehydrolysis

(P) is effected with saturated steam, preferably at temperatures of about

170 °C. Depending on the wood species and the cellulose purity demanded, prehydrolysis

intensity is adjusted to P-factors ranging from 300 to about 1000 (equals

30 to 100 min at 170 °C, respectively). The absence of free water during steam prehydrolysis

(the liquor-to-solid ratio ranges from 0.7 to 1.2, mainly depending on

the moisture content of the wood) limits the loss of wood substance during this

stage. As an example, steam prehydrolysis of Eucalyptus urograndis with an intensity

of P-factor 300 accounts to a xylan reduction of only 38% with reference to the

total xylan removed throughout the whole process (Tab. 4.47). Nevertheless, prehydrolysis

degrades the major part of the hemicelluloses which then becomes

solubilized in the subsequent alkaline process steps. As indicated earlier, degradation

of the hydrolytically cleaved hemicelluloses is continued in the subsequent

4.2 Kraft Pulping Processes 347

neutralization (N) step. The major portion of the wood components degraded during

the whole prehydrolysis-kraft process are removed within this step. The extensive

degradation of polysaccharides requires a sufficient supply with EA to ensure

their complete extraction and to prevent recondensation and reprecipitation of

both carbohydrates and lignin compounds. The neutralization step is immediately

succeeded by the hot displacement (HD) (Visbatch®) or cooking stage (C)

(VisCBC). There, the neutralization liquor is displaced by a mixture of HWL and

HBL (which in the case of VisCBC is preset to the target [OH– ]ion and temperature).

The hot displacement step of the Visbatch process is most efficient with

regard to both delignification and final purification, with 92% of the degradable

lignin and 97% of the degradable xylan being removed at this stage (see Tab. 4.47

and Fig. 4.109). The subsequent cooking stage (C) controls the final delignification,

covering a kappa number range of about 45 (at the start) to less than 10

(before cold displacement), and ensures a rather homogeneous pulp quality due

to the leveled-out alkali profile (VisCBC). Although less than 4% of the wood substances

are removed, the cooking phase is important for process control because

the final pulp properties, viscosity and brightness, are determined. The mass balance

of the most important wood components throughout a Visbatch® process

typical for the use of eucalyptus wood are detailed in Table 4.47.

Дата добавления: 2015-10-21