Gas-Liquid Mass Transfer in Pulp Retention Towers, 2002 International Pulp Bleaching Conference Proceedings

V.B. Rewatkar and C.P.J. Bennington

The volumetric gas-liquid mass transfer rate, kLa, was measured under batch operating conditions in a 0.28 m diameter laboratory-scale retention column. Tests on water and on unbleached kraft (UBK) pulp suspensions having mass concentrations of Cm = 0.013 to 0.09 (as fractions) were conducted under batch operating conditions. Air, or nitrogen, was sparged through the column at superficial gas velocities between 0.0015 to 0.05 m/s. In water, and in low consistency pulp suspensions (Cm < 0.03),="" the="" dissolved="" oxygen="" concentration="" was="" measured="" in="" situ="" using="" a="" dissolved="" oxygen="" probe.="" as="" suspension="" concentration="" increased,="" liquid="" samples="" were="" withdrawn="" from="" the="" column="" to="" measure="" the="" dissolved="" oxygen="" content.="" above="">Cm = 0.09, it was difficult to obtain liquid samples from the column.

The gas-liquid mass transfer rate varied with suspension mass concentration and superficial gas velocity. kLa initially decreased with increasing mass concentration, reaching a minimum between Cm = 0.03 and 0.06. Further increases in Cm resulted in an increase in kLa. The minimum in kLa coincided with a change in gas hydrodynamics within the column, from bubble column behaviour below Cm = 0.03, to porous solid behaviour above Cm = 0.06.