In pulp bleaching processes, pre-distribution of chemicals in suspensions ahead of tower reactors is essential to ensure efficient lignin removal and optimal use of the chemicals. In-line mixers, combined with chemical injectors, are commonly used to achieve this goal. In spite of its importance, in-line mixing of pulp suspensions is not well understood. In this thesis, liquid distribution and gas dispersion were investigated downstream of in-line mixers, including jet and mechanical mixers, to provide better understanding and guidance for mixer design and process optimization. In the present work, non-intrusive electrical resistance tomography (ERT) was used to quantify mixing based on two novel mixing indices, derived from the standard deviation of image pixel values. This technique was also implemented as a real-time mixing assessment tool in industrial pulp bleaching, with success in monitoring mixing quality as a function of process operating conditions. Liquid jet mixing was found to depend strongly on the flow regime and jet penetration. For turbulent flow, the criteria for in-line jet mixing in water apply also to suspensions. When a suspension flows as a plug, mixing differs greatly from that in water, depending on the fibre network strength in the core of the pipe. With an impeller present, mixing improved substantially, primarily in the high-shear zone around the impeller, with rapid reflocculation downstream. Gas mixing depended on the flow regime and buoyancy in a complex manner. When buoyancy was not significant, impeller operation enhanced mixing since bubbles dispersed throughout the pipe cross-section, whereas without the impeller, the bubbles congregated near the wall due to robust fibre networks in the core of the pipe. For buoyancy-dominated flow, the impeller worsened mixing since it disrupted the fibre networks and delivered gas to the top of the pipe, whereas the networks caused liquid/pulp slugs to flow at the top for a tee alone.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat
