Online measurement of the bubble size distribution in medium-consistency oxygen delignification

Progress of a new imaging-based measurement of oxygen bubble size was continued by developing an automated method for bubble detection from image data. The imaging application was found to be able to measure oxygen bubble size in mill conditions when using imaging apparatus connected to the sampling valve after the first oxygen stage mixer in a Finnish softwood kraft pulp mill. This appears to be the first time that bubble size distribution has been measured in a mill-based oxygen delignification process. Mixer rotor speed had a clearly measurable effect on the oxygen bubble size distribution in the pulp suspension at the window of the imaging assembly. The volumetric average bubble size with mixer rotor speed 890 rpm was 0.064 mm while with mixer rotor speed 1380 rpm it was 0.037 mm. The automated method estimated bubble volumes reasonably well, enabling further development of the online measurement application.