CLEARFLOW™ Feedwell and Bustle pipes systems for green and liquor clarifiers have been developed by McFarlen Eng using 3D fluid dynamic modeling technology to improve the removal of kinetic energy in the feedstrem to the clarifier, which allows dregs and lime mud particle to settle efficiently into the clarifier resulting in Improved clarity of green and white liquor. This leads to improve filter ability of the lime mud and reduce scaling in the digester, reducing chemical bleaching cost.
McFarlen Engineering initiated CLEARFLOW™ Feedwell design project because older industrial clarifiers’ designs were inadequate in reducing kinetic energy,reduction of turbidity in the liquor from green, white and mud washing industrial clarifiers improves the operation of the Recaust Plant.
Many pulp mills have increased the loading on their clarifiers in order to meet greater demands for liquor in the digesters. In addition, hydrodynamic modeling shows 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. Furthermore, during the past three decades, design criteria have changed to satisfy the requirements for cleaner liquors and reduced dead load and energy consumption in the recausticizing system.
Consequently, the feedwell and bustle pipe systems in many older clarifiers are not optimally designed and therefore do not perform as desired. Convection currents are introduced unnecessarily and overflow velocities are much higher and more localized than desired, causing high turbidity levels.
An improvement, especially in green liquor turbidity, can lead to substantial savings. McFarlen Engineering Ltd. is confident that these savings can be achieved through the application of ClearFlow™ technology, which optimizes feedwell and bustle pipe hydrodynamic design to maximize dregs retention, minimize flow velocities and eliminate short circuiting of all flow streamlines. The areas for potential cost savings are illustrated below.
Typical Improvements by Retrofitting with ClearFlow™ Technology
Flow modeling showing the results of injecting two tangential streams into an older design
Improved Feedwell Design Many clarifiers contain two feed pipes entering the feedwell to introduce liquor into the tank, an internal “Y” type bustle pipe to collect clarified liquor, and a rotating rake to draw particulate into the bottom discharge well. Flow is introduced into the feedwell via two tangential, counter rotating feed pipes at different elevations. A feedwell is supposed to reduce swirl and flow energy entering the tank, promote flocculation and diffuse the flow over the entire feedwell to reduce flow disturbances and promote longer retention. In practice, most older design feedwells transmit too much energy into the tank and have no effective mechanism to diffuse the flow uniformly across the feedwell exit resulting in the inefficient usage of the tanks large size for settling.
Figure 1 shows a computational fluid dynamic (CFD) model of the flow behavior of a typical feedwell with injection of liquor via two tangential nozzles (CW = clockwise & CCW = counter clockwise). The flow forms two jets that spread out along the inner walls creating a fluid layer that is thin and wide. The two counter-rotating streams collide, causing most of the liquor to flow downward along one side of the feedwell. This ineffective use of the feedwell creates excessive, localized momentum entering the clarifier promoting mixing, reducing the time available for particles to settle, thus increasing turbidity. In addition, the tangential injection causes a residual swirl imparting unnecessary radial momentum at the feedwell exit. The effective area of the feedwell is reduced and outlet velocity is increased 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. Higher turbidity and poorer liquor quality result.
Figure 1: Flow modeling showing the results of injecting two tangential streams into an older design circular feedwell. The jets spread along the inner wall, collide, and only a small portion of the feedwell is used resulting in excess momentum in the clarifier.
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 your mill, 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 your arrangement, is illustrated in Figure 2. 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, which will use proprietary criteria to design a new bustle pipe that will collect liquor uniformly throughout the tank, provide high quality clean liquor and be much less prone to fouling over time.
Figure 2: Typical bustle pipe segment showing unequal flow through uniformly distributed openings.
Design of a ClearFlow™ Feedwell and Bustle Pipe System
To significantly improve clarifier performance, the following is undertaken to determine the best feedwell and bustle pipe system:
Perform a 3-D CFD numerical model to design a new feedwell having an entrance that will minimize kinetic energy of incoming liquor, stop radial momentum, minimize flow velocities, maximize solids retention in the feedwell for flocculation and eliminate short-circuiting of flow streamlines. Several iterations in design will be necessary to optimize the dimensions of this feedwell to ensure that flow is diffused uniformly and effectively from the feedwell skirt.
A bustle pipe complete with hanger supports will be similarly engineered, and manufactured for your clarifier. These proprietary feedwell and bustle pipe designs have demonstrated excellent performance in various liquor applications in other pulp mills. References and contacts are available upon request. McFarlen Engineering Ltd. is confident that similar results are attainable in your mill.
Other Factors Affecting Industrial Clarifier Performance
The influence of rake movement, liquor density, flow variations and thermal gradients can also affect the performance of a clarifier. McFarlen Engineering Ltd. will provide a list of suggestions for the mill to consider to minimize the effects of these issues.