New insights into the flow of pulp suspensions.

Application: Pulp suspensions behave increasingly like a homogenous liquid as consistency is reduced. The results of this empirical study of pulp flow suggest that increasing the liquid viscosity has the same effect as reducing pulp consistency.

Fiber suspensions have different flow characteristics than other solid-liquid systems. Fibers, floes, floc fragments, and networks can exist either separately or simultaneously within the suspension, with each showing a different response to shear. Increasing the viscosity of the suspending liquid impedes floc formation while reducing the strength of network structures,

Study objectives

This paper investigates the effect of liquid viscosity on pulp flow. The results provide new insights into the shear behavior of fiber suspensions and suggest the potential of improving papermaking processes through control of stock viscosity.

Experimental

Stock flow was investigated in the laboratory using a range of liquids, viscosities, pulp types, and fiber concentrations. Pipe frictional pressure losses and velocity profiles were measured in a flow loop with a transparent section to observe the flow. A high-resolution video camera was mounted over a rectangular Perspex channel to record changes in flocculation, which were quantified by optically scanning and then image processing the flow. A grid fitted inside the channel facilitated the study of floc breakup and reformation. Pulp viscosity was adjusted using either carboxymethyl cellulose (CMC) or, in some cases, dissolved sugar solutions.

Results

Friction-loss curves for viscous fiber-CMC suspensions are significantly different from the equivalent fiber-water curves, as seen in Fig. 1. The various flow regimes normally observed for pulp-water suspensions (plug flow, transition flow, and turbulent flow) are not apparent in the viscous suspensions. Turbulence-damping drag reduction--at and beyond the point where the pulp and liquid curves intersect--is observed in both water and viscous suspensions, but its onset occurs at higher velocities for viscous suspensions at a given fiber consistency.

[FIGURE 1 OMITTED]

Measurement of velocity profiles in viscous fiber-liquid suspensions suggests that there is still a plug in the center of the pipe at low velocities, but that the plug diameter decreases with increasing viscosity. The fact that the velocity ratio--the ratio of velocity at the centerline to the bulk velocity--increases with increasing viscosity demonstrates that the fiber-liquid suspension behaves more like a viscous liquid as viscosity increases. Indeed, the laboratory results revealed that the strength of the fiber network decreases with increasing liquid viscosity.

Photographic analysis of flow in a rectangular channel showed that floc size was reduced by either decreasing the bulk consistency or increasing the liquid viscosity. The presence of smaller floes translates to a reduction in the suspension's disruptive shear stress, the point where the network undergoes permanent disruption.

Various flow mechanisms for viscous fiber suspensions were explored and compared with the established flow mechanisms in conventional fiber-water flows. The lack of solid-solid plug-wall interaction and the absence of rolling floes both suggest that viscous liquids effectively weaken the network structure, even at low viscosities. Theoretical analysis confirmed that the network periphery becomes less coherent as the liquid viscosity increases.

Conclusion

Increasing the viscosity of a fiber suspension has the same effect as reducing the pulp consistency. The higher the viscosity of the suspending liquid, the more the suspension tends to behave like a homogenous liquid or continuum. With additional development, these concepts could prove helpful in improving the control of pulp flow.

Paul is a former postgraduate student; Duffy is a professor; and Chen is an associate professor in the Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand. Paul is now with PAPRO NZ, Forest Research, Private Bag 3020, Rotorua, New Zealand. Address correspondence to Duffy by email at [e-mail]