MIXING GASES AND PULP SUSPENSIONS IN A CONTINUOUS LABORATORY MIXER

The ability to mix gases and pulp suspensions was studied with a labor atory rotary mixer adapted for continuous operation. The laboratory mi xer had a concentric-cylinder geometry consisting of a vaned rotor and baffled housing, similar in design to some commercial mill-scale and laboratory pup mixers. Air was mixed into kraft pulp suspensions havin g mass concentrations up to 4% to give suspensions with gas contents u p to 50% by volume Pulp flows to the mixer were varied from 75 to 125 L/min. This gave nominal residence times varying from 0.8 to 2.7 s dep ending on the volume of gas added. Mixing effectiveness was evaluated based on the flow pattern observed in the mixer and the power drawn by the rotor: The Now pattern and power consumed in mixing were affected by suspension composition (pulp concentration (p.c.) and gas content) ; rotor speed, and flow rate through the mixer. Changes in flow rate c ould result in a step change in the power drawn by the rotor and flow pattern produced. The gas capacity of the mixer was reduced over the c ase of batch operation due to gas hold-up within the mixer: The gas ho ld-up was reduced as the flow velocity through the mixer increased, as the rotor speed was decreased, and as the suspension pc. was reduced. The ability to sustain effective mixing in this particular device was limited to gas contents of below 10% once the suspension p.c. reached 3%.