I met with Senior Applications Engineer of the Chemical Pulping Group at Kadant Black Clawson, Chris Knollman, to discuss some of the available options currently employed in kraft mills for the clarification of liquors. For today’s discussion we have selected to compare the pros and cons of the top three most common systems – Clarifiers, Pressure Disk Filters, and Pressure Tube Filters.
We’ll be looking at the relative cost, footprint, and general performance of each system, as well as other important considerations such as maintenance and energy consumption.
Clarifiers
Most recognized as the large cylindrical storage tanks common to many chemical production facilities, clarifiers can be considered the option with the largest footprint and apparently greatest initial capital expenditure. This cost, however, is greatly mitigated by the fact that most mills have suitable storage tanks already onsite. Additionally, current upgrade and retrofit technologies are available that can vastly improve performance in existing systems which give this option an attractive edge over filtration solutions.
Chemical recovery from clarifiers provides a low green to white ratio and high solids underflow. These units run slow and steady, reliably delivering 38 to 40 percent solids for conventional systems and up to 55 percent or more with high-torque rake systems.
That said, energy use is the real advantage found with clarifiers. Current systems can run efficiently and reliably with the low power draw of a 5-horsepower motor, while the other solutions demand the vastly greater energy draw of 500-horsepower vacuum pumps.
With well-run clarifier systems, maintenance is low and there are few spare parts costs. There is an additional integral benefit as all volume above the bustle pipe is considered as liquor storage removing the need for any post-filtration White Liquor storage tanks.
Pressure Tube Filtration
These once popular systems were valued for their relatively small footprint and low upfront cost. However, these undersized units proved inconsistent performers and required frequent maintenance, and an ongoing annual replacement cost of the filtrate “socks” worn from frequent mud build up.
They deliver a high ratio of green to white liquor and suffer from low underflow solids, 38 to 40 percent maximum with no options to increase. They also require lime mud washing in the post-filter stage, more causticizer retention time than clarifiers, and provide no liquor storage. As a result, these units are outdated and have had few if any new installations.
Pressure Disk Filtration
Disk filters provide excellent chemical recovery, delivering a low green to white liquor ratio and solids up to 75 to 80 percent. Additionally, lime mud washing is not required.
Unfortunately, this is where the good news ends. High maintenance and frequent disk segment replacements add to a large initial capital investment and generally high operating cost. As discussed earlier, high energy use is the real cost accelerator, with demanding high horsepower vacuum pumps at the heart of the system.
Like tube filtration, disk filters demand more causticizer retention time than clarifiers and a dedicated liquor storage tank is required.
|