An Overview of Reactions During Kraft Pulping

The course of dissolution of lignin and carbohydrates reveals three distinct phases

of a kraft cook: initial, bulk, and residual delignification which affect the single

wood components as summarized in Scheme 4.24.

_ Initial phase: the initial stage is characterized by losses in the carbohydrate

fraction, which is more pronounced for hardwoods as

compared to softwoods [133]. The hemicelluloses undergo deacetylation

and physical dissolution, and peeling reactions also start.

Cellulose degradation by peeling is negligible in terms of yield

loss. Reactive phenolic lignin units, such as a-O-4-ethers, are

cleaved as early as the initial phase.

4.2 Kraft Pulping Processes 183

_ Bulk phase: The core delignification occurs in the bulk phase and,

importantly, both phenolic and nonphenoplic b- O –4-ether bonds

are cleaved. About 70% of the lignin is removed. The reactions of

the carbohydrates are characterized by secondary peeling (i.e.,

alkaline cleavage of the glycosidic bonds), but also by stopping

reactions, which are favored at elevated temperature. Methanol is

liberated from 4- O -methylglucuronic acid side chains, and hexenuronic

units are formed.

_ Residual phase: the residual phase begins at a delignification rate

of about 90%. Delignification has slowed down considerably due

to depletion of reactive lignin units. It is believed that the chemical

nature of the residual lignin hampers further degradation

reactions. A slow delignification is accompanied by rapid carbohydrate

degradation, causing disproportionate carbohydrate losses.

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