Case Study: Dryer Recovers High-Value Clay Product
An integrated dryer-dust collector recovers fine clay at Southern Clay Products.
Southern Clay Products, Inc. in Gonzales, Texas, has increased the capacity of its drying process, strengthened the quality and consistency of its bentonite clay products, and reduced costs by recovering material that would otherwise be lost as waste. The improvements for the global supplier of synthetic and clay-based rheology modifiers are due to the addition of a vibrating fluid bed dryer by the Witte Co. that integrates drying and dust collection into a single unit. Since 1938, the family-owned and operated Witte has been designing and manufacturing vibrating fluid bed dryers, coolers, de-waterers, screeners and other process equipment for companies such as Kraft, Nestle, Morton, Merck, DuPont, and ChevronTexaco.
Southern Clay purchased the integrated dryer-dust collector to boost production as part of a plant expansion completed to meet rising customer demand, according to Clyde Bates, the company’s engineering manager. Bates is responsible for managing plant maintenance and construction in seven processing plants, as well as a variety of storage and warehousing facilities on a sprawling 88-acre site. A division of Rockwood Specialties, Inc., Southern Clay Products develops and manufactures a range of specialty rheology modifiers and additives used to modify viscosity, thickness, and flow characteristics.
Already running a rotary dryer, drum dryer, flash dryer and a Witte fluid bed dryer since he joined the company in the 1980s, Bates determined that a larger fluid bed dryer would meet the increased production goals, while its gentle vibrating motion would protect the sensitive products from any damage that would alter their rheological properties. A fluidized bed dryer supplies heated drying air to lift and suspend particles for highly efficient heat transfer and moisture removal, along with gentle handling to safeguard particle integrity.
Adding linear vibration, the dryer moves the product from inlet to discharge in a first-in, first-out process. Bates set up a test at the Witte’s testing laboratory using his bentonite clay to determine the dryer specifications needed to meet goals for production and quality. The Witte lab makes full-sized dryers, coolers, conveyors and other process equipment available (instead of pilot-sized equipment) to accurately simulate process conditions and eliminate concerns about scale-up.
Once Witte engineers began to work with the extremely fine particles, they realized a conventional fluid bed dryer with a remote dust collector similar to the dryer operating for decades in the Southern Clay plant would not provide the most efficient approach. According to Larry Stoma, design engineer for Witte, “The fineness of the clay would require the dryer to be run at a very low air velocity to prevent the product from being entrained in the air stream, requiring a much larger dryer.” Stoma recommended an alternative design the company had proven in a number of installations where very fine, high-value particles were dried in an efficient manner.
A Different Design
The Witte drying system places an integral baghouse dust collector on top of the vibrating fluid bed dryer, directly above the vertical airflow. The dust collector captures 99.9% of clay particles entrained in the process air stream, continues the drying and automatically returns the particles into the process, thereby eliminating any product loss and converting it into saleable, dry product. By recovering the fine particles, the dryer is able to operate at the higher airflows needed to achieve high drying capacities while permitting a footprint less than half the size of a conventional dryer design. Conventional fluid bed dryers, by contrast, could entrain and remove up to 20% or more.
“We looked at several types of dryers from different companies, and the nature of the Witte design was by far the best,” says Bates. “The beauty of the integral dust collector is you don’t lose any product.”
Eliminating the remote dust collector also eliminates the need for screw conveyors, rotary valves, drums, ductwork and other ancillary equipment needed to manage the lost material, along with maintenance chores and associated line downtime when they plug. “It’s easy to pump liquids around a plant without much downtime, but moving powders is a different animal,” says Bates. “This system has definitely improved our material handling and boosted our uptime.”
Understanding the Process
The clay products enter the dryer in-feed as a fine filter cake from a filter press with a moisture content ranging from 30-70%, depending on the material. In the dryer, gas-fired process air reduces the moisture content to just 2% as the vibrating motion advances the material toward discharge. Then it continues through a screener and on to milling, grinding and packaging in a variety of 50-lb bags, sacks, and flexible intermediate bulk containers.
To accommodate the requirements and characteristics of different clay products run on the same dryer, the drying system includes pneumatically operated radius gates called autoweirs. These autoweirs automatically rotate on a cycle timer to allow the bed depth and retention time to be adjusted at any time. This allows the flow rate through the drying section to be slowed down or accelerated based on the desired specifications for moisture content, particle shape and other parameters. The process air flow rate and temperature can also be adjusted in real time; these adjustments are controlled on an Allen-Bradley PLC-HMI system that is directly linked into the company’s SCADA system.
Tracking performance not only of the drying process but also of the entire operation, Bates and his team monitor the temperature at the dryer in-feed and outlet, the pressure differential in each drying section and across the dust collector, and pay close attention to uncovering trends by analyzing historical data. The ability to see every step of the process in real time and compare years of data has helped stabilize the process and improve run-to-run consistency, which has led directly to improvements in product quality.
“We used to produce about 1-5% off-spec product, but with the Witte system, we’ve been able to reduce that to 1-2%,” says Bates, who installed similar tracking controls on all of his machinery and equipment throughout his multiple facilities. “Once you experience the power of this technology, you can’t go back.”
This integrated drying system also includes an automated explosion suppression system. In addition, for easy access to the inside of the dryer, the entire drying section is mounted on rails and may be rolled out by one person for cleaning. This is typically done monthly in advance of processing a different product.
“Witte understands the importance of cleanability, they know how the process is supposed to run, and they have been great to work with,” says Bates. “The Witte drying system has helped us keep our costs low and make us more competitive.”
For additional information, visit www.witte.com.