Study of Gas Holdup in a Concurrent Air/Water/Fiber System, 1998 Engineering Conference Proceedings

Gas/liquid/fiber flows occur in many places in the pulp and paper industry, such as flotation deinking, bleaching with gaseous chemicals, direct contact steam heating, and air removal from stock flows. However, little is known about the dynamics of these complex flows. In this study, the gas holdup of an air/water/wood pulp suspension was characterized to determine the effects of varying pulp consistency and superficial gas (air) and liquid (pulp) velocities, where the superficial velocity is defined as the volumetric flow rate divided by the column cross-sectional area. The study focused on measuring gas holdup (percent air by volume) through pulp stock consisting of unprinted old newspaper (ONP) in a 12.7 cm diameter cocurrent bubble column. The gas holdup was measured by gamma-my densitometry, and chord-averaged gas holdup values were determined at various chord positions across specified planes at multiple column heights. Pulp consistency was specified at one of three values (0, 0.8, and 1.2%), the superficial gas velocity was varied between 0.5 and 4.0 cm/s, and the superficial liquid velocity was varied between 2.5 and 7.5 cm/s. These parameters were chosen to most accumtely resemble those of flotation deinking cells in the pulp and paper industry.

It was found that gas holdup generally increases with increasing column height, superficial gas velocity, and superficial liquid velocity, for each consistency studied. The effect of pulp consistency on gas holdup was dependent upon superficial gas and liquid velocities. Pulp fibers at 0.8% consistency caused the gas holdup to increase relative to pure water at high superficial liquid velocities. This was caused by a reduction in bubble coalescence due to the movement of the slurry and the fiber network. As pulp consistency &eased to 1.2% the gas holdup decreased below that of pure water. This decrease is thought to be due to channeling at high consistencies. Additionally, at low superficial liquid velocities, the gas holdup decreased as consistency increased, which is thought to be due to bubble coalescence and channeling.