Stock Chest Mixing & the Counterflow
Pulp and paper plants use stock chests with partial mixing and recirculation loops to reduce the magnitude of consistency fluctuations in pulp production. Minimizing these fluctuations provides a smoother, more uniform surface and significantly improves final sheet properties. This results in more first-quality tons and improved savings and revenue. The relationship between reduced fluctuations and lower costs is commonly acknowledged in the industry. It is also highly dependent on stock chest mixing/dilution.
Typical machine chest mixing is limited only to the chest bottom. This marginal mixing technique requires chest output consistency to be controlled by recirculation loops. Our solution to paper stock consistency fluctuations is to create multiple areas of zonal mixing using our patented CounterFlow Mixing System. These areas also improve residence time distribution (RTD) and prevent the short-circuiting of flow within the stock chest.
When compared to competitive product and processes, our application of the CounterFlow Impeller paired with a Raven 3800 Series mixer drive results in:
- Full chest mixing to eliminate dead zones
- Improved residence time
- Total uniformity of stock consistency (improved fiber lay)
- Maximize degassing, 50–60% reduction of entrainment air caused by Pitch Blade Turbines (PBTs)
- Reduce fiber damage from higher RPM mixing
- 33–50% reduction in mixing horsepower
Avoid the problems that often arise when using competitive product and process application techniques. Conventional turbines induce entrainment air, require significantly greater power and produce less flow.
COUNTERFLOW IMPELLER
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Advanced Impeller Technology
Philadelphia Mixing Solutions and Mixing Solutions Limited CounterFlow™ Impeller offers a higher pumping rate than conventional impellers because it efficiently operates at much higher impeller-to-tank diameter ratios. This is of greatest advantage with shear thinning fluids where stagnancy at the wall can be a problem. Conventional turbines require significantly greater power to produce flow at the walls.
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Stock Chest and Non-Newtonian Mixing
A complication that must be considered is that most viscous fluids are also non-Newtonian. This means that the viscosity that the agitator impeller “feels” is dependent on the rotational speed at which it operates. The majority of fluids exhibiting non-Newtonian properties are “shear-thinning” so, the faster the impeller rotates, the lower the apparent viscosity of the fluid. Since the viscosity changes with impeller speed, the design of an agitator (and pump) cannot be reliably accomplished using a single value of viscosity.
Incorrect understating of the effect of non-Newtonian behavior on agitation can lead to problems such as:
- Longer than anticipated batch times (longer time to achieve desired homogeneity) due to underestimating the actual viscosity of the fluid.
- Slow incorporation of added ingredients due to minimal motion at the surface of the batch.
- Stagnation / dead zones created because the impellers simply cannot overcome the viscosity of the fluid to generate motion.
- Poorer than expected heat transfer due to underestimating the apparent viscosity at the wall of the vessel.
- Philadelphia Mixing Solutions and Mixing Solutions Limited have a state-of-the-art laboratory where the appropriate properties of a non-Newtonian fluid can be measured. We also have a long and successful track record of correctly interpreting these measurements and designing equipment for these processes.
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Caverns and Dead Zones
- Caverns experience lower levels of active mixing than areas adjacent to the impellers
- Finely dispersed pigment particles can form larger agglomerates
- Agglomerates do not contribute to light scattering and reflecting properties needed in paint
- Agglomerates must be filtered and removed for disposal
- Agglomerated paint pigments must be replaced with more pigment to meet color standards
