Kinetics of Carbohydrate Degradation

Few studies have been conducted to investigate carbohydrate degradation kinetics.

In order to develop a model, the experimental data as published by Matthews

(slash pine) and Smith and Williams (Loblolly pine) were employed [40,83]. In the

proposed model, the carbohydrates are divided into three components: C, GGM,

and AX. The course of the carbohydrate components during kraft pulping also

reflected the presence of three different species within each component. Thus, it

appears justified to further divide the three components into three species, similar

to the lignin model.

Unlike the situation in the lignin model, the [HS– ]showed no influence on carbohydrate

degradation. The influence on alkali concentration was, however, more

pronounced in the initial stage, where even very low concentrations led to a substantial

loss of carbohydrates. An amount of 7% (based on wood) of directly dis-

4.2 Kraft Pulping Processes 215

solved wood components, consisting of acetyl groups and low molecular-weight

hemicelluloses, was determined by Kondo and Sarkanen [80]. This value was calculated

by measuring the amount of degraded hemicelluloses of wood meal stabilized

against alkaline peeling by borohydride reduction and extrapolating to zero

time. The insertion of a rate constant k2 into the kinetics equation reflected the observation

that degradation of carbohydrates occurs easily under very mild conditions.

The proposed structure for the carbohydrate kinetics is provided in Eq. (109):

dCHij

dt _ _ kCHi _ j _ OH _ a _ k 2__ CHij _109_

for the three components, i, C, GGM, and AX, each with three species, j = 1 to 3.

Since only limited data were available for the individual carbohydrate components,

only the total carbohydrates, CH = C + GGM + AX, were considered. It has

been shown by Lindgren that the proportion of the carbohydrate species is also

dependent on the sodium hydroxide concentration [47]. The intersection of CH2

and CH3, denoted as CH*, was conducted from the log-linear extrapolation of the

experimental data in a manner similar to that described for lignin. The relationship

between CH* and reaction conditions was derived by nonlinear regression

analysis. In contrast to L*, no dependency on temperature was found; thus,

regressing CH* against [OH– ]yields Eq. (110):

CH * _ 42_3 3_65 ___ OH _ 0_05__0_54 _110_

The initial amounts for CH1,0 and CHtot,0 are known and hence the sum of the initial

amounts for species 2 and 3 CH2+3,0 = CHtot,0 – CH1,0. The initial values CH2,0

and CH3,0 depend on the cooking conditions and can be calculated as follows

using the definition of CH*:

CH 2_0 _ CH * _ Exp kCH 2 _ D t _ _ _111_

where Dt is again defined as the time interval between t (L*) and t(0).

The initial amount of CH3 is calculated using the expression in Eq. (112):

CH 3_0 _ CH 2 CH 3 _ CH 2_0 _ CH * _ Exp kCH 3 _ D t _ _ _112_

where CH2 and CH3 are calculated by using Eq. (102), where kCH2 and kCH3 are

the rate constants for the first-order reactions for species 2 and 3 in Eqs. (102) and

(103), respectively, and Dt is the time interval after which species 2 and 3 reach

the same level CH*. Summing these equations yields:

CH 23_0 _ CH * _ Exp kCH 2 _ D t _ _ Exp kCH 3 _ D t _ _ _113_

which can be solved for the time interval Dt using the nonlinear equation solver

(see Appendix).

216 4 Chemical Pulping Processes

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