The Significance of Liquor-to-Wood Ratio on the Reaction Kinetics of Spruce Sulphate Pulping

Abstract: In 1957 Vroom presented an article that dealt with the kinetics of the sulphate cook. He showed that the lignin dissolution exhibited a temperature/time dependency that could be explained by the Arrhenius equation and that the reaction was of first order with respect to lignin. However, even before Vroom introduced the H-factor all wood components were assumed to react according to a first order reaction. In recent years progresses in this area have been made. Lignin for example is nowadays considered to dissolve during three parallel first order reactions, all with differences in activation energies. When the kinetics are evaluated, several cooking series at different temperatures and concentrations of active cooking chemicals are needed. The data points obtained are then fitted into some equation. If the concentration of the active cooking chemicals is constant, the activation energies and the chemical dependency for the dissolution of wood components can easily be found. In order to simplify the evaluations of the kinetics, very high liquor-to-wood ratios are sometimes used, often as high as 50:1 or even 75:1. In this manner, the chemical concentrations are almost constant during the cook. The problem is that in the normal industrial cook where the liquor-to-wood ratio is about 4:1, the chemical concentration is not constant. This is due mostly to the alkali consumption that takes place in the cook for example when neutralising the acidic groups in the hemicelluloses. A disadvantage with high liquor-to-wood ratios is the high dilution of the dissolved organic matter. A high concentration of dissolved lignin boosts the dissolution of the remaining lignin in the wood residue and xylan can redeposit on the fibres when its concentration in the cooking liquor is high. The aim of this project was to describe how different liquor-to-wood ratios influence the kinetics during sulphate cooking of spruce.