In‐line jet mixing of liquid‐pulp‐fiber suspensions: Effect of fiber properties, flow regime, and jet penetration

Abstract

Mixing effectiveness was determined experimentally for side jet injection into pipe flow for water and pulp suspensions for a range of fiber mass concentrations (0-3.0%), mainstream velocities (0.5-5.0 m/s), and side-stream velocities (1.0-12.7 m/s). The mixing quality was measured in cross-sectional planes along the pipe using electrical resistance tomography and quantified by a modified mixing index, derived from the coefficient of variation of conductivity. Mixing depended strongly on the flow regime and jet penetration. For turbulent flow, the criteria for in-line jet mixing in water are applicable to the mixing in suspensions, with small differences likely due to differences in fiber network strength and influences of fiber-turbulence interactions in modifying turbulent structures in the bulk. When a suspension flows as a plug, however, the mixing differs greatly from that in water, depending on the fiber network strength in the core of the pipe.

Pilot-Scale Examination of Mixing Liquid into Pulp Fiber Suspensions in the Presence of an In-Line Mechanical Mixer

“…The mixing quality with the mechanical mixer for a hardwood suspension flow at C m = 0.5% was similar to that for water without fibers, whereas mixing with a tee mixer and turbulent flow was better for a dilute hardwood suspension than for water . For tee mixing alone, mixing is achieved by turbulent shear.…”

“…The mixing improved significantly when the flow regime changed, and mixing quality approached that for water when the flow was turbulent for U p ≥ 2.0 m/s. The flow regimes for the pulp suspensions in this study were characterized previously for softwood 16 and hardwood 17 pulp suspensions. Without an impeller, the suspension flowed as a plug at U p = 1.0 m/s and was turbulent for U p = 3.0 m/s.…”

“…Although a higher velocity ratio of 24.1 provided slightly better mixing than R = 8.1 for a jet penetrating to the core of the pipe, this condition may be undesirable from a practical point of view because impingement on the opposite pipe wall creates significant stress there. The design of an in-line mechanical mixer should therefore be based on the velocity ratio for the jet penetrating to the axis of the pipe, as summarized by Yenjaichon et al…”

“…The results also showed that jet mixing alone was insufficient to provide good mixing for higher suspension concentrations, even at high main-stream velocities. Yenjaichon et al extended the study to hardwood pulp suspensions and found that the jet-mixing behavior was similar to that for the softwood pulp. In the turbulent-flow regime, mixing for a dilute hardwood suspension was, however, better than that for water, whereas it was worse for a dilute softwood suspension, likely because of the influences of fiber-turbulence interactions in modifying turbulent structures in the bulk.…”

“…The mixing quality with the mechanical mixer for a hardwood suspension flow at C m = 0.5% was similar to that for water without fibers, whereas mixing with a tee mixer and turbulent flow was better for a dilute hardwood suspension than for water. 17 For tee mixing alone, mixing is achieved by turbulent shear. The shorter-and smaller-diameter fibers could alter the turbulence structure in the bulk, carrying turbulent eddies with them and colliding with each other, thereby promoting turbulent dispersion.…”