Chlorination of kraft black spruce pulp was studied in a fixed bed reactor. Breakthrough curves of the reactant chlorine and the products TOC, chloride and methanol were measured. The flow residence time distribution was determined by performing a stimulus response tracer experiment preceding chlorination of each pulp pad. The chlorine and methanol break-through curves were well described by a parallel plug flow model and instantaneous chlorine-lignin and first order chlorine-carbohydrates reactions. The representation of the residence time distribution by plug flow through segregated channels rather than by axial dispersed plug flow was confirmed by the effect of operating variables on the residence time distribution and by comparison with theory and numerical predictions. The good representation of the actual flow by the parallel plug flow model can be explained by poor radial mixing and a relatively large scale of variation in radial velocity. The latter is a direct consequence of the large aspect ratio of pulp fibers and their associated tendency to flocculate. This also explains why the dispersion in pulp pads is larger than in beds of regular shaped particles which have the same equivalent diameter as pulp fibers. Chlorine-lignin and methanol-lignin stoichiometries were determined as a function of operating conditions. The chlorine-lignin stoichiometry is a function of mean residence time and temperature, but not of chlorine feed concentration. The values of the chlorine-lignin stoichiometry are lower than found for corresponding batch chlorination of softwood pulps. The methanol-lignin stoichiometry is not influenced by any of the operating variables. The methanol concentration in the effluent can be used as an indicator for the completion of the chlorine-lignin reaction. Pulp properties such as lignin content, kappa number and viscosity were measured at the end of an experiment. Maximum delignification in any channel is obtained at chlorine breakthrough. Recycling of reconcentrated spent filtrate did not hinder delignification rates. The reciprocal intrinsic viscosity is proportional to the chlorine charge on pulp. Higher pulp viscosities are obtained at lower temperatures and mean residence times and at higher chlorine feed concentrations for the same chlorine charge on pulp. Viscosity protection agents like chlorine dioxide are effective in dynamic chlorination.

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