Bubble Size Control to Improve Oxygen-Based Bleaching: Characterization of Flow Regimes in Pulp-Water-Gas Three-Phase Flows

Abstract

SUMMARYThis document reports on the results of a research program aimed at the elucidation of various hydrodynamic aspects of low consistency paper pulp-water-gas three-phase flows, conducted jointly by the G.W. Woodruff School of Mechanical Engineering, Georgia Tech, and the Institute of Paper Science and Technology.Hydrodynamic flow characteristics of solid-liquid-gas slurry made by intimately mixing fibrous paper pulp with water and air were first investigated in a short, vertical circular column.The pulp consistency (weight fraction of pulp in the pulp-water mixture) was varied in the low consistency range of 0.0 ~ 1.5%. The test section was 1.8 m long, with 5.08 cm inner diameter.Mixing of the slurry prior to entering the test section was done using a patented mixer with controlled cavitation that generated finely dispersed micro-bubbles. Flow structures, gas holdup (void fraction), and the geometric and population characteristics of gas bubbles in the gas-pulpliquid three-phase flow were experimentally investigated, using visual observation, Gamma-ray densitometry, and flash X-ray photography. Superficial velocities of the gas and liquid/pulp mixture covered the ranges 0~26 cm/s and 21~51 cm/s, respectively.Five distinct flow regimes could be visually identified. These included dispersed bubbly, characterized by isolated micro-bubbles entrapped in fiber networks; layered bubbly, characterized by bubbles rising in a low consistency annular zone near the channel wall; plug; churn-turbulent; and slug. The dispersed and layered bubbly regimes could be maintained only at very low gas superficial velocities or gas holdups. Flow regime maps were constructed using phasic superficial velocities as coordinates, and the regime transition lines were found to be sensitive to consistency.The cross section-average gas holdup data showed that both the dispersed and the layered bubbly regimes could best be represented by the homogeneous mixture model. The drift flux model could best be applied to the reminder of the data when the plug and churn-turbulent flow regimes were treated together, and the slug flow was treated separately. The drift flux parameters depended on the pulp consistency.The feasibility of using artificial neural networks (ANNs) for the identification of the flow regimes in the aforementioned three phase gas/liquid/pulp fiber systems, using the statistical characteristics of pressure fluctuations measured by a single pressure sensor, was then examined.The aforementioned experimental test facility was used for this purpose. Experiments were carried out in the transparent, vertical circular column. The pulp consistency (weight fraction of 2 pulp in the pulp-water mixture) was varied in the low consistency range of 0.0 ~ 1.5%, and flow structures and other hydrodynamic characteristics were experimentally investigated. Local pressure fluctuations at a station 1.2 m above the test section inlet were also recorded using a highsensitivity pressure transducer that was installed in a manner not to cause flow distu...