Diffusion of lignin macromolecules within the fibre walls of kraft pulp. Part I: Determination of the diffusion coefficient under alkaline conditions

Abstract

The intrinsic rate of diffusion of soluble lignin from fibre walls to bulk liquor has never been determined previously because of experimental difficulties; for example, the diffusion rate determined in a stirred cell is affected by the mechanical action of stirring. In our work, the intrinsic rate of diffusion of lignin macromolecules from the fibre walls of a softwood kraft pulp was determined under alkaline conditions using a displacement cell which eliminated external heat and mass transfer resistances and pulp fibre disturbances. The effects of such experimental conditions as pulp bed height and liquid flow rate were studied. The diffusion rate can be described by a diffusion model for a hollow cylinder with a very wide range of diffusion coefficients. The diffusion rate increased with increasing pH. Our results provide a new understanding of the lignin diffusion process in fibre walls, which is affected by the size of lignin molecules and the pores, and by the electrostatic interactions between intrafibre pore walls and lignin.La vitesse de diffusion intrinseque de la lignine soluble de la paroi des fibres vers le coeur de la liqueur n'a jamais ete determinee anterieurement a cause de difficultes experimentales; par exemple, la vitesse de diffusion determinee dans une cellule agitee est influencee par I'action mecanique de I'agitatioddu brassage. Dans ce travail, nous avons determine la vitesse de diffirsion intrinseque des macromolecules de lignine de la paroi des fibres d'une plte kraft de bois tendre dans des conditions alcalines, au moyen d'une cellule de deplacement qui elimine les resistances extemes au transfert de chaleur et de matiere et les perturbations des fibres de plte. Les effets de ces conditions experimentales, comme la hauteur du lit de plte et le debit liquide, ont ete etudies. La vitesse de diffusion peut &re decrite par un modele de diffusion pour un cylindre creux dans une tres grande g a m e de coefficients de diffusion. La vitesse de diffusion augmente avec le pH. Nos resultats permettent une nouvelle interpretation du processus de diffusion de la lignine dans la paroi des fibres, lequel est influence par la taille des molecules de lignine et des pores et par les interactions electrostatiques entre les parois de pores intrafibres et la lignine.

Modeling intra-particle convection in heterogeneous non-catalytic reacting systems

“…The chemical degradation and dissolution of the solid structure of the wood is responsible for the increase of the porosity of the chips. This has been ignored in previous attempts to describe the intra-particle process heterogeneity (Johnsson, 1970;Tyler, 1982;Gustafson et al, 1983;Gustafson, 1988;Agarwal, 1993;Agarwal, Gustafson, & Arasakesari, 1994;Agarwal & Gustafson, 1997;Li, Phoenix, & Macleod, 1997). In these works a considerable discrepancy between the experimental values and the predicted space averaged proÿles does exist and the range of values anticipated for non-uniformity is far from reality.…”

Mass Transport of Lignin in Confined Pores

“…After the main delignification reactions, liberated lignin fragments need to diffuse out toward the free liquor: the rate of delignification can therefore be controlled by either chemical kinetics of the main reactions or diffusion events during pulping [ 4 ]. So far, studies published on the kraft process have focused primarily on the diffusion of chemicals and cooking reagents into the wood chips [ 2 , 5 , 6 , 7 ] or on the reaction kinetics [ 2 , 4 , 8 , 9 , 10 , 11 , 12 ]. In 1999, Li et al studied lignin diffusion in kraft delignification kinetics by investigating the effect of temperature and compared it to a calculated value based on reaction kinetics.…”

Gas‐Liquid Mass Transfer in Pulp Retention Towers

“…Paterson and Kerekes (1984) showed that diffusion of dissolved chlorine through an unmixed pulp suspension controlled progress of reaction through the suspension. Li et al (1996) showed that the diffusion constants of reacted lignin fragments spanned a huge range, with diffusion times for the largest fragments being on the order of hours. Measurements of pulp and gas phase residence times have been reported for a number of industrial towers.…”

“…Paterson and Kerekes (1984) showed that diffusion of dissolved chlorine through an unmixed pulp suspension controlled progress of reaction through the suspension. Li et al (1996) showed that the diffusion constants of reacted lignin *Author to whom correspondence may be addressed. E-mail address: cpjb@chml.ubc.ca.…”