In practice, rotary tube furnaces provide notable values of throughput and efficiency for a wide range of powders, as long as the materials behave properly during the process. In some cases, the nature of the reactant material can cause flow problems. Powder adhesion to the tube can change bed mixing behavior, flow through the furnace and, in extreme cases, cause wide swings in material residence time. In other cases, material entrainment in the exhaust gas can affect throughput and flow. Sometimes changing the reactants' physical form — e.g., granulating fine powders into pellets or aggregates — can overcome these problems. Alternatively, the use of internal features within the process tube can promote desired bed behavior. When the nature of the material doesn't allow for granulation or the granules aren't sufficiently strong to retain their shape throughout the process, other means of high throughput and high efficiency processing are needed.
Recycling the process heat from the product during cooling promises to further enhance energy efficiency. Transferring that heat to incoming reactants means less energy must be inserted into the process. It also can foster more-compact designs because the equipment's heating and cooling portions may be shorter and the heating elements and support equipment may be smaller. Introducing this form of energy efficiency into bulk materials processing equipment is still in its early stages. However, equipment designs that provide this type of heat transfer for select processes have been demonstrated and additional opportunities to expand the concept to a wider range of materials are being investigated.
Empirical testing at a demonstration semi-continuous level provides a proven means to extract scaleup parameters. Scaleup data from a well-designed set of experiments on semi-continuous or continuous furnaces can serve to redefine the process recipe and identify the continuous commercial-scale device that provides the best balance between product quality, throughput, energy efficiency and operating expenses.
TOM MROZ is director of technology for Harper International, Lancaster, N.Y. ROBERT BLACKMON is vice president of integrated systems for Harper International. E-mail them at firstname.lastname@example.org and email@example.com.