In contrast, continuous processes routinely provide a variety of options for heat recovery. For instance, counterflow gas can help cool product leaving the process and subsequently help preheat the incoming reactants. Also, gas composition at a given position in the furnace is more predictable. In cases where incoming gas chemistry or removal of gaseous products influence the reaction rate, this predictability enables minimizing gas usage and, therefore, thermal losses related to the gas.
Where the exhaust gas contains waste products having fuel value, advanced equipment design can combust the gas and use the hot stream for preheating the primary process. In some cases, such waste-gas streams can provide very substantial fractions of the total thermal requirement for a process. The result is significant energy efficiency and reduced associated costs, as well as lower costs and environmental concerns compared to alternatives for abating the waste-gas flow.
INSIGHTS FROM CARBON FIBER
Conversion of polymeric fiber materials to carbon fiber typifies how processes evolve. Large-scale production of carbon fiber began in the 1970s. Initial equipment was less than 1,000-mm wide and produced under 100 tons per year — and required an input of 150–200 kWh per kilogram of carbon fiber.
Through successive scaleup efforts, the state-of-the-art for conversion of carbon fiber now has advanced to a system width of 3,000–4,200 mm and single-facility production rates exceeding 2,500 tons per year. Through increased reuse of energy (integration of waste energy to drive the process), equipment improvements (via economies of scale and refractory advancements) and better basic feed material (polymers with higher conversion to carbon fiber), the specific energy consumption is now less than 20 kWh per kilogram of carbon fiber. Figure 1 highlights the progressive reduction of energy requirements through the scaleup process.
In many ways carbon fiber production can serve as a useful guide for improving the efficiencies of other thermal processes. Polyacrylonitirle-based carbon fiber production is performed in an entirely container-less mode; the fiber itself provides the motive force to move the product through subsequent process steps. This provides the energy efficiency of a continuous process, augmented by further gains from eliminating the need to heat ancillary support equipment. The only system inefficiency is the heat loss related to the cover gas used to maintain the appropriate atmosphere. Development efforts to reduce this atmosphere loss through improved furnace end seals are ongoing.
POWDER PROCESSING OPPORTUNITIES
Most thermal processes involve bulk materials in the form of powders or aggregated mixtures that require some type of container or construct, such as a belt conveyor or rotary tube, to assist in material transport through the process. Minimizing the energy losses associated with this aspect of the equipment design is one of the most significant opportunities for improving energy efficiency. Design optimization often involves concern for this energy efficiency in concert with considerations for enhancing reaction kinetics, gas exchange, residence time and materials of construction. Therefore, the requirements related to a specific process will drive the options for production methods and, thus, the efficiencies of that process.
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.