Characterisation of gas mixing in water and pulp-suspension flow based on electrical resistance tomography

Abstract

Gas dispersion in horizontal developing pipe flow downstream of a 90°-tee mixer and an in-line mechanical mixer was investigated by means of electrical resistance tomography (ERT) for both water and softwood kraft pulp suspensions over a range of fibre mass concentrations (0–3.0%), superficial liquid/pulp velocities (0.5–5.0m/s) and superficial gas velocities (0.11–0.44m/s). A gas mixing index, derived from the standard deviation of local gas holdup in each image pixel, quantified the uniformity of gas in cross-sectional planes along the pipe for various flow patterns. The distribution of the gas phase in a cross-section was determined from the vertical gas holdup profiles, and the relative size of gaseous entities for different flow conditions was evaluated based on the scale of segregation. For air–water flow, the gas uniformity and size of gaseous entities depended strongly on the flow pattern. For pulp fibre suspensions, the gas flow varied significantly depending on the flow regime. Mixing was similar to that in water when the flow was turbulent for dilute suspensions, but differed greatly for higher mass concentrations, likely due to robust fibre networks of a plug in the core of the pipe causing bubbles to concentrate near the wall and accelerating coalescence. The impeller disrupted the plug and distributed gas throughout the cross-section, leading to significantly improved gas uniformity in the high-shear zone around the impeller, but decaying turbulence and re-establishment of fibre networks caused bubbles to coalesce at the top of the pipe and worse mixing downstream.