Model Structure. Kinetic studies are important in providing essential evidence as to the mechanisms

Kinetic studies are important in providing essential evidence as to the mechanisms

of chemical processes. The chemical reactions occurring during the kraft

pulping process in the solid-interface of wood and pulping liquor are very complex

and thus not known in detail. The pulping processes therefore cannot be treated

as having reaction kinetics similar to those of homogeneous reactions in solution.

The present picture of the reaction mechanisms is that the swollen lignin in the

188 4 Chemical Pulping Processes

wood chip is degraded into fragments at the solid–liquid interface by the hydroxyl

and hydrosulfide ions present in the pulping liquor. Assuming different lignin

species, the delignification reaction of each can be approximated to a first-order

reaction. The rate constant can be calculated from the simple expression:

_

dLj

dt _ kj _ Lj _74_

The influence of the pulping temperature on the rate of delignification can be

expressed quantitatively according to the Arrhenius equation [9]:

k _ A _ Exp _

EA

R _

T _ _ _75_

The validity of this simple approach has, however, been questioned on the

grounds that the rate-determining reactions are unknown [10]. Nevertheless, the

temperature dependence of the delignification reaction using the Arrhenius equation

can be applied, provided that the reaction does not change over the temperature

range studied [11].

The major objective of the development of kinetic models is to improve the control

of digester operation. During the past 45 years, many studies of the kinetics

and transport behavior of the kraft pulping process have been carried out. Based

on the experimental results of these studies, kraft pulping models of varying complexity

have been developed for control and design purposes. Basically, all models

are data-driven and, according to Michelsen, can be categorized as either empirical

(black box) and pseudo first-principle models (partly mechanistic) [12].