Alkali and Hydrogen Sulfide Consumption

One prerequisite for studying the influence of [HS– ]and [OH– ]on the rate of

delignification is to use a high liquid-to-wood ratio (L/W) that ensures a constant

liquor composition. During industrial kraft cooking, where L/W ranges between 3

and 5, the concentration of cooking chemicals decreases as a result of the consumption

of chemicals by wood components. Consumption of the active cooking

chemicals must be considered when applying rate expressions to predict the

extent of delignification and carbohydrate degradation. The main alkali consump-

220 4 Chemical Pulping Processes

tion is caused by degradation reactions of carbohydrates (peeling reactions), where

an average effective alkali consumption of 1.6 mol mol–1 degraded hexose unit has

been reported [88,89]. The dissolved lignin contains approximately 0.8 phenolic

groups per monomer, and finally the released carbonic acids (acetic and uronic

acids) require a stoichiometric amount of caustic for neutralization. The alkali

consumption can thus be expressed as a linear function of the degradation rate of

the most important wood components [7,28,79,82,90]. To implement the specific

alkali consumption, values reported by Christensen into the kinetic model proved

to be most appropriate [79]. The specific effective alkali values are listed in

Tab. 4.22.

Tab. 4.22 Specific consumption of active cooking chemicals [79].