Effect of Temperature. The process of delignification during kraft pulping can be divided into three

The process of delignification during kraft pulping can be divided into three

phases depending on the rate of lignin dissolution [15]. It is commonly accepted

that, in all three phases, the delignification rate is of apparent first order with

respect to the remaining lignin content. The temperature dependence of the rate

constants follows the Arrhenius expression according to Eq. (75). It appears reasonable

to assign the degradation reactions of the different lignin structural units

(e.g., alkyl-aryl ether cleavage) with different phases of the technical pulping process

[48]. It has been shown by extensive studies using model compounds that the

degradation of lignin during kraft pulping may be primarily ascribed to the cleavage

of alkyl aryl-ether linkages [49]. The alkyl-aryl ether bond types can be classified

into phenolic a-aryl ether linkages, b-aryl ether bonds in phenolic and nonphenolic

units (see Chapter 2.1.1.3.2). Thus, the facile cleavage of phenolic a-aryl

ether bonds has been shown to dominate the initial phase of delignification [50].

The initial period is characterized by rapid delignification, significant hemicellulose

degradation, and major alkali consumption. The activation energy of the degradation

reaction of a model substrate representing the p -hydroxy-phenylcumaran

structures in the lignin (a-aryl ether bond) to form o, p ′-dihydroxystilbene in an

aqueous alkaline solution of 1 M sodium hydroxide was found to be 77.5 kJ mol–1

[51]. Kinetic studies using wood chips or wood meal as a substrate revealed activation

energies for the initial phase of delignification in the range between 40 and

86 kJ mol–1 (see Tab. 4.19). The low activation energy values indicate that delignification

in the initial phase is mainly a diffusion-controlled process that is independent

of the alkali concentration, as long as it is above a minimum level. The initial

phase delignification is expressed in general as Eq. (98):

dL

dt _ _ Ai _ __ T _

_ Exp _

Ea

R _ T _ __ L _98_

where L represents the percentage of lignin in the wood with respect to the initial

composition, T is the reaction temperature (in K), and E a is the activation energy

(in kJ mol–1).

According to the published literature, an activation energy of 50–55 kJ mol–1 can

be assumed for the initial phase delignification (see Tab. 4.19). The existing database

does not allow any influence to be assumed of the composition of the cooking

liquor (Kraft versus Soda), additives (AQ) and wood species on the activation

energy of the initial phase delignification.

Kondo and Sarkanen proposed that the initial delignification in kraft pulping

consists of two kinetically distinguishable periods, ID1 and ID2, resulting in the

dissolution of 13% and 11% of the initial lignin, respectively. ID1 is characterized

as a rapid phase of indeterminate kinetic order and an estimated activation energy

of 50 kJ mol–1, whereas ID2 is designated as subsequent slower phase, conforming

with first-order kinetics and a determined activation energy of 73 kJ mol–1.

The bulk phase delignification is associated with the cleavage of b-aryl ether

bonds in nonphenolic arylpropane units which could be expected to constitute the

rate-determining reaction [48]. Miksche investigated the alkaline degradation of

200 4 Chemical Pulping Processes

erythro-veratrylglycerin-b-guaiacyl ether by determining the formation of guaiacol

in an aqueous alkaline solution of 1 M sodium hydroxide [52]. The fragmentation

reaction follows first-order kinetics with respect to the substrate and the hydroxide

ion concentration. The activation energy, Ea, for this degradation reaction has

been determined as 130.6 kJ mol–1. The data in Tab. 4.19 show that most published

activation energies of bulk phase delignification are reasonably close to this

value, and are largely independent of the wood species, the presence of hydrosulfide

ions (Kraft versus soda), and additives (AQ, PS).

Tab. 4.19 Comparison of literature data an activation energies for delignification.

Phase/

Period

wood source Process I:s ratio EA

[KJ mol–1]

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