The Importance of Reducing Turbidity in Clarifiers for Pulp Mills

Reducing turbidity in the liquor from green, white, and mud washing clarifiers is crucial for improving the operation of the Recaust Plant. Many pulp mills have increased the loading on their clarifiers to meet greater demands for liquor in the digesters. However, hydrodynamic modeling has shown that older technology for feedwell and bustle pipe design is inadequate to sustain flows above critical levels, where product clarity decreases dramatically. As a result, low turbidity may not be achieved even when injecting polymer. Consequently, the feedwell and bustle pipe systems in many older clarifiers are not optimally designed and therefore do not perform as desired. Unnecessary convection currents and higher localized overflow velocities contribute to high turbidity levels.

The Benefits of ClearFlow™ Technology

McFarlen Engineering Ltd. has developed ClearFlow™ technology to address these issues. This technology optimizes feedwell and bustle pipe hydrodynamic design to maximize dregs retention, minimize flow velocities, and eliminate short-circuiting of all flow streamlines. The potential cost savings from implementing ClearFlow™ technology are significant.

Typical Improvements by Retrofitting with ClearFlow™ Technology

Causticizing Efficiency: Reducing 400 ppm dregs in the clarified green liquor can increase causticizing efficiency by as much as 2%.

Kiln Operation: If dregs in the clarified green liquor can be reduced from 400-500 ppm to less than 75 ppm, dry solids from the lime mud precoat filter (LMPF) may be increased by as much as 8-10%. This translates into energy savings in the kiln and more available kiln capacity. Balling and ringing issues may also be lessened or eliminated. It may no longer be necessary to buy more lime to keep up with liquor demand and then purge excess calcium carbonate, thus reducing or eliminating the costs for extra make-up lime.

Polymer: Improved WLC and GLC operations should reduce or eliminate polymer requirements.

Dead Load: Dead load in the system decreases as dregs are brought under control, effectively increasing white liquor capacity and clarity.

White Liquor Filtration/Clarification: Mills with white liquor pressure filters may experience reduced acid washing and sock change frequency. Secondary or indirect failures such as tube bending may also be avoided. In mills with gravity white liquor clarification, dirt in the pulp due to dregs can be greatly reduced, and lower lime carry-over reduces heat transfer surface fouling and chemical consumption in the bleach plant.

Recaust System Performance: Installing high-performance feedwells and bustle pipes in green, white, and mud washing clarifiers can effectively de-bottleneck the system by allowing design criteria to be increased without sacrificing performance. This means that existing equipment can handle higher flow rates and still deliver current or better quality liquor. Further, the capacity of the whole system may be increased.

Result: Improved operation and economics, with payback potentially achieved in just a matter of months.

Old Feedwell Design

Incoming flow forms two jets that spread out along the inner walls, creating a thin and wide fluid layer. The two counter-rotating streams collide, causing most of the liquor to flow downward along one side of the feedwell. In addition, the tangential injection causes a residual swirl, imparting unnecessary radial momentum at the feedwell exit. This reduces the effective area of the feedwell and increases outlet velocity on one side. The low target feedwell exit velocity is not achieved, and higher localized velocities disrupt the settling of solids flowing toward the discharge well, resulting in higher turbidity and poorer liquor quality.

Old Bustle Pipe Design

Clarified liquor is typically collected through a horizontal, square, triangular, “V” or “Y” type internal bustle pipe, which has multiple openings. Because the bustle pipe diameter is constant, the holes are uniform in diameter, and the pipes are significantly long, most of the bustle pipe flow is through the opening nearest the pump suction. In practice, it is found that with these arrangements, the holes farthest from the suction are not used effectively and tend to plug more quickly. Drawing flow equally from multiple holes over relatively long pipe sections with only one suction point is not a trivial task in fluid mechanics. Biased bustle pipes, like the ones installed at many mills, draw too much fluid from one side of the tank and use a much smaller portion of the tank for clarification, resulting in poor liquor quality.

Although most designers treat this as a simple rule of thumb, 3-D simulations of bustle pipes show that great care during design must be given to ensure that liquor is drawn off equally across the entire clarifier. A typical bustle pipe segment with evenly spaced openings, corresponding to one side of a typical arrangement. Only the first few holes are able to draw off significant flow, and over time, scaling further compounds the problem. A rigorous design approach was not available to the original equipment manufacturer; however, this can be overcome through the application of ClearFlow™ technology. This technology uses proprietary criteria to design a new bustle pipe that collects liquor uniformly throughout the tank, provides high-quality clean liquor, and is much less prone to fouling over time.

Conclusion

The reduction of turbidity in the liquor from green, white, and mud washing clarifiers is essential for improving the operation of the Recaust Plant. Many pulp mills have increased clarifier loading to meet greater demands for liquor in the digesters, revealing that older feedwell and bustle pipe designs cannot handle these flows effectively. ClearFlow™ technology by McFarlen Engineering Ltd. offers a solution by optimizing hydrodynamic design, leading to significant cost savings and improved system performance. With advancements in feedwell and bustle pipe design, mills can achieve lower turbidity levels, higher causticizing efficiency, and better overall plant performance. Investing in modernizing clarifier technology is a strategic move that can yield substantial economic and operational benefits.