The role of Donnan effect in kraft cooking liquor impregnation and hot water extraction of wood

Abstract

Abstract In this article, a phenomenological model is proposed for alkali impregnation and hot water extraction of wood. Elementary reaction kinetics, ion exchange (IE) equilibrium, and mass transfer rates are considered in the model. IE is a consequence of the dissociation of the covalently bound uronic acid and phenolic groups in the wood fiber wall (FW). Due to this effect, the molality of cations is higher in the FW liquid than in the liquid external to the FW, whereas the opposite is true for anions. For the development of kinetic models under the participation of hydroxyl and hydrogen ions, the estimation of their molalities in these two liquid phases is crucial because of their ubiquitous presence in chemical reactions. This study applies a recently developed physicochemical modeling environment in which alkali impregnation and hot water extraction processes are considered. The simulations revealed that the IE effect is significant and affects the chemical reaction kinetics in both processes in focus. Furthermore, the IE effect does not remain constant but varies as a function of treatment time.