Concentric Mixing of Hardwood Pulp and Water

Concentric mixing experiments with velocity ratios of up to 6 have been completed using hardwood pulp of 1.0, 1.9, and 2.9% consistency and water. Qualitative observations reveal that by increasing the velocity ratio, the inner jet spread angle is found to be larger (i.e., the jet spreads faster) and the downstream mixing region is more uniform. Furthermore, local consistency measurements show a flattening of the concentration profile with increasing velocity ratio, confirming mixing improves as velocity ratio increases. The mixing of hardwood pulp fiber with water is different from water-water mixing when the velocity ratio R„ (ratio of inner:outer velocity) is small. For the fiber stock tested, the mixing is found to be significantly dependent on the stock consistency when the velocity ratio is small (12. E. 1). This result indicates that shear stress and turbulence required to fully dislodge the fiber network are not delivered by the fluid streams when the velocity ratio is small. Thus, mixing is due to hydrodynamic instabilities and macroscale variations, which leads to downstream nonuniformities. When the velocity ratio is high, the mixing efficiency is found to be more strongly dictated by the velocity ratio than the consistency. This is evidence that once the fiber network strength is overcome by shear stress and turbulence, the mixture behaves as a conventional Newtonian fluid in turbulent flow. The mixing at high velocity ratio is due to microscale turbulence that leads to a relatively uniform downstream mixture. In addition, when the flow is turbulent, the mixing process is not affected significantly by the stock consistency. Turbulent hardwood pulp and water concentric mixing is modeled with the standard k-e turbulence model. Numerical studies show that the closure constants in the turbulence model can influence the predicted mixing effects. Constant values of C u = 1.88 and Cu = 2.4 for the standard k-e model are used to obtain reasonable qualitative agreement with the experimental data when the velocity ratio is high. INTRODUCTION The concentric mixing process appears to be simple, but complex flow phenomena are required to thoroughly mix the two fluid streams. When the two fluid streams enter the mixing region at different velocities, a high shear region forms at the interface between the two fluid streams. Instabilities at this interface cause vortex pairing, intertwining, and rollup. As the vortices evolve downstream, the annular stream cascades toward the center while the center jet disintegrates radially, enabling mixing. Depending on the mixer and mixing fluid behavior, the two streams may or may not mix completely or uniformly. The degree of mixing in a concentric mixer can depend on the following [1]: the ratio of inner-to-outer pipe diameter; the ratio of inner-to-outer pipe flow rate or velocity; the ratio of specific gravity between the two fluid streams; the inner and outer pipe Reynolds numbers; the pipe surface roughness; and any secondary pipe flows. When one of the constituents is a fiber suspension, additional parameters related to the fiber characteristics (e.g., fiber length, coarseness, flexibility, etc.) also affect the mixing process [2-6]. The process is further complicated due to the tendency of fiber flocculation. The mixing of a relatively thick fiber stream with a dilute fiber stream or white water is common in the pulp and paper industry. Concentric mixing occurs both in chemical mixing and in the approach flow area immediately ahead of the fan pump, where thick stock is mixed with thin stock to dilute the fiber suspension to the proper headbox consistency. Thick stock is supplied through a basis weight valve and supplies the inner pipe in a pipewithin-a-pipe design. The outer pipe can be supplied with clear accepts, secondary and tertiary screen accepts, deaeration overflow, etc. [7]. This mixing, if not done properly, can significantly affect the spatial and temporal t Corresponding author: [email protected]