Improving green liquor clarifier performance through the addition of a packed bed of dendrite fibers

by Campbell, Brian K.

As pulp mills add incremental capacity and move closer to closed-cycle operations, the amount of chemicals and non-process elements processed in unit operations increases, sometimes beyond the optimum levels of equipment design. One piece of equipment where this is very likely to occur is the green liquor clarifier. If the solids removal efficiency of the green liquor clarifier is reduced, then the increased load of solids (referred to as dregs) will contaminate the lime cycle, reduce causticizing efficiency, increase dead load, and increase the need for makeup chemicals. A master’s level project showed that it is possible to increase the solids removal efficiency of a bench-scale clarifier through the addition of a packed bed of small “dendrite” fibers to the clarifier’s interior. The current research continued this work, and had two goals: first, to determine the effects the bed of fibers has on bulk flow patterns in the clarifier; and second, to determine what if any effects exist due to filtration. Residence time distribution (RTD) experiments were conducted on bench-scale and pilot scale clarifiers. Flow rate and bed position were varied on the pilot-scale clarifier, while flow rate and bed thickness were varied on the bench-scale clarifier. The tracer system was potassium chloride in water. The filtration studies were conducted on the bench-scale clarifier and a small settling column. Green liquor and dregs were used in most of the filtration studies, while water and calcium carbonate was used to study the effect of pH on the filtration system. This research showed that green liquor clarifiers produce residence time distributions similar to their waste treatment counterparts despite their design differences, and that their RTDs revealed these clarifiers to be very close to mixed flow. Because the vii data showed the use of plug flow based models in the literature to be impossible, a new model based on the Weibull distribution was discovered and used here because of its mathematical similarity to the RTD for a mixed flow vessel. In regard to the stated objectives, this research showed that the dendrite fibers do indeed alter bulk flow patterns in the clarifier, causing them to act more like a plug flow vessel. This change in flow patterns accounted for the majority of the increase in solids removal observed in the filtration studies and in previous work. Filtration efficiency was shown to be quite low for the green liquor and dregs system and decreased with time. The mechanism of filtration is believed to be dominated by surface chemistry, because smaller particles were captured by the bed and the efficiency decreased over the time of the experiments. viii