Gas-Liquid Mass Transfer in Pulp Suspension Mixing Operation, 1996 International Pulp Bleaching Conference Proceedings

The rate of mass transfer from the gas to water phases was measured in a commercial, high-shear, laboratory mixer under conditions typical of medium-consistency bleaching. Mass transfer was measured using the cobalt-catalyzed sulfite oxidation technique and characterized using the gas-liquid volumetric mass transfer coefficient, kL a. A fully-bleached kraft pulp was used, with gas-liquid mass transfer rates measured over a range of mixer operating conditions (N = 10.8, 20.0, and 43.2 s -l ) and suspension compositions (consistency, Cm = 0.05 and 0.10 (as a fraction); gas void fraction, Xg = 0.20, 0.30 and 0.40). Tests were also made with water and with several synthetic nylon suspensions (fibre length, 1w = 2, 5 and 7 mm; Cm = 0.02 to 0.15) to determine the extent to which fibre network properties affect the rate of mass transfer. In the absence of fibres the mass transfer rate increased with increasing rotor speed (energy dissipation) and gas void fraction, in agreement with literature data. In the presence of pulp fibre, mass transfer rates were significantly reduced over the comparable water cases, and decreased slightly with increasing pulp concentration. The same dramatic decrease in mass transfer was not observed for the nylon suspensions, although kL a did decrease with increasing suspension concentration. The mass transfer results were then compared with data obtained for ozone bleaching trials made in the same mixer and in a 3.6 t/d pilot bleach plant. By assuming that ozonation was gas-liquid mass transfer limited, it was possible to estimate a gas-liquid mass transfer coefficient for each bleaching trial. Comparison with the data measured using the sulfite oxidation technique showed good agreement, confirming that ozone bleaching at medium-consistency is gas-liquid mass transfer limited. The implications for industrial processes are discussed.