Waste Not, Want Not
Located in Riverside, Calif., Sierra Aluminum is a prospering company that deals in the fabrication, anodizing, painting and extruding of aluminum. Many of the company's processes involve acid cleaning and etching of aluminum. The resulting waste solution is neutralized and precipitated out as aluminum hydroxide. After squeezing out the excess moisture in a filter press, what remains is a filter cake with approximately 70 percent free moisture.
For years, the company viewed and treated this stream as a waste product, simply giving it away to another company to avoid incurring the expense of having it hauled away.
A few years ago, however, Sierra decided to test the potential market and determined that this product could prove a valuable source of revenue -- if they could find a way to continuously dry the aluminum hydroxide. This was easier said than done.
Because the filter cake was quite sticky and difficult to handle, continuous drying was sure to be a daunting task. The company's experience with the aluminum hydroxide material had already shown that it would stick to or plug up virtually any piece of process equipment with moving parts. The continuous drying system they needed seemed out of reach -- that is, until the company tested a Turbo-Dryer from Wyssmont Co., Fort Lee, N.J.
A rotary tray dryer that operates at atmospheric pressure using the principle of forced hot-air convection, the Turbo-Dryer consists of a vertical housing that contains a stack of toroidal shelves that rotate around a set of sirocco fans on a vertical shaft. The shelves are divided into tray segments by radial slots that allow the material to be transferred down to successive shelves and ultimately discharged through the dryer bottom (figure 1).
As wet material enters the dryer through a flanged chute in the roof, it forms a pile on each tray segment as it passes under the feed chute. These piles are leveled to a predetermined depth by a stationary blade in order to increase the exposed surface area. Each rotating pile of material is transferred down to the next shelf when it meets a stationary wiper blade after almost a full revolution. This scraping action of the wipers enables the dryer to handle sticky pastes without buildup that would shut down the dryer. Because it is an essentially plug-flow operation, the retention time can be set and controlled precisely by a variable-speed drive on the rotating tray structure.
The hot-air drying medium enters at various levels through a side-mounted vertical manifold. This hot air is well mixed with the air already inside the dryer by the internal fans. This method of air circulation serves to create a horizontal air velocity that increases the drying rate while allowing for precise temperature control of the air in contact with the product. Multiple hot-air inlets allow for different temperature zones from top to bottom inside the dryer.
The moisture-laden hot air is ultimately exhausted out of a stack at the top of the dryer in this semi-countercurrent operation.
In Action at SierraAt Sierra Aluminum, the 32-shelf rotary tray dryer is fed 70 percent wet filter cake at the top of the dryer and discharges a primarily granular product at uniform product moisture below 10 percent at its discharge. The complete drying system consists of a 12' dia., 18' tall dryer; a filter-cake-breaking feeder; discharge rotary airlock; external natural gas-fired heating system; exhaust cyclone and fan; and interconnecting ductwork. An energy-saving recirculation loop is part of the system, which also contains temperature controls and an inclined, cleated belt conveyor that delivers the filter cake from the filter press to the dryer feed chute.
The features offered by the rotary-tray system made the choice of a dryer an easy one for the management at Sierra Aluminum, says Edward Weisselberg, president of Wyssmont.
First, the material-handling issue had to be addressed. The only way this drying operation could have been cost-effective was if it was done continuously. “The gravity-utilizing, vertical nature of the Turbo-Dryer, combined with the gentle piling and leveling action, allows for the aluminum hydroxide to be handled with virtually no shear,” Weisselberg explained. The product does not stick to the trays, and the stationary wipers provide an essentially self-cleaning unit.
Secondly, the dryer's vertical design helps minimize dust loss. The dryer's gentle handling of the product creates very little fines during drying. “In addition, most of the air velocity is horizontal as opposed to vertical,” Weisselberg added. “Because the vertical velocities are so low almost no fines leave the dryer.” This results in an increased product yield through the dryer bottom discharge chute and enables use of a cyclone rather than a baghouse dust collector.
Finally, Sierra found that Wyssmont was able to provide a drying system that could be efficient in terms of operating costs while meeting the stringent emission requirements of Southern California. The heating system was required to have an ultra-low NOX gas burner (20 ppm maximum NOX emission) with less than 2 million BTU/hr capacity.
“The Wyssmont design team was able to meet this requirement by implementing an exhaust recycle loop that increased the efficiency of the system through heat recovery,” Weisselberg said. “Most of the hot exhaust stream from the dryer is recycled back across the burner and into the manifold, in effect reducing the amount of heat that must be delivered by the burner.” The result is a drying operation with efficiency in the range of 1,200 to 1,300 BTU per pound of water evaporated.
The Turbo-Dryer installation has been in operation for less than two years, but according to Weisselberg, it has already proven to be an invaluable piece of equipment for Sierra Aluminum Co. It continues to be a reliable unit that operates 24 hours a day, 7 days a week while requiring little maintenance or operator attention and consistently delivering on-spec product. PH
Wyssmont Co., Fort Lee, N.J., custom engineers and manufactures circular tray drying systems for process applications. For more information from Wyssmont, call (201) 947-4600; e-mail email@example.com; or visit www.wyssmont.com.