Approach flow stock mixing.

The behaviour of a thick stock jet was examined to elucidate the important factors for the dispersion of medium consistency pulp suspensions into a dilute white water stream. The model system employed an axially adjustable, thickstock injection lance jetting into a dilute secondary stream. For various velocity ratios the jet break up was characterised by simultaneous radial sampling and by stepwise axial sampling. The effect of jet velocity and the secondary stream velocity on the mixing behaviour was studied for; two similar fluids, a long fibre pulp and water, and a short. fibre pulp and water. A combined axial and radial concentration profile for the jet breakup was obtained to characterise the mixing behaviour. The criteria for 'good mixing' was defined as the distance from injection where the range in radial samples was less than 5% of the mean. The mixing behaviour of pulp jets was observed to differ significantly from the case where two similar fluids are mixed. With pulp a longer mixing zone is required for 'good mixing'. The extra resistance to dispersion exhibited by the pulp was attributed to the additional energy required to disperse the flocs. The results were consistent with an earlier qualitative study but were more rigorous with respect to the characterisation of mixing. Criteria for the design and operation of co-axial pulp jet mixers were established.