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In evaluating design alternatives to rotary furnaces, the following considerations should be made:
- Heating and cooling should be limited to only the process material. Containment should remain at temperature and the process material should move through it.
- The opportunity for product sticking or gas-phase entrainment during processing should be minimized.
- Product uniformity and reaction time should be maximized by minimizing or eliminating bulk bed effects.
- Heat and mass exchange should be provided between products and reactants for improved efficiency.
- Personnel efficiency and return on investment should be maximized.
One means of providing these advantages is under evaluation in the form of a vertical conveyor furnace (see Figure 1).1 In rotary furnaces, material is typically conveyed into the rotating tube using a screw feed device, and the rotation and angle of the container tube facilitates the movement of the material for the remainder of the process. In the vertical conveyor furnace, screw feeders provide all of the motive force to push the reactants into the hot zone. Material exits initially by force of gravity, and ultimately by a second screw feeder.
In the feeding portion, the reactant material is pushed up against gravity through a gradually tapering cone, part of which resides inside the hot zone of the furnace. The material is preheated during this conveyance through the walls of the conveyor and cone. At the top of the cone, the material at the very surface is exposed to direct radiation heating and the gas environment in the furnace chamber. Because of this direct exposure, it is anticipated that reactions occur uniformly and quickly over this surface layer of material, and that off-gassed material exits the process upward and away from the product.
Continued feeding of material from below causes this now-reacted layer to spill over the top of the cone and fall by force of gravity into the larger containment tube surrounding the cone. The initial movement of material from the top of the cone facilitates a very sharp drop in process temperature, which limits the potential for overheating problems such as melting/sublimation, sintering or excessive grain growth. The material continues to fall through the outer tube to a cooled portion of tube under the furnace hot zone. At the bottom of the exit tube is a second screw feeder, which removes the material from the apparatus and transports it to a product hopper.
By carefully controlling the rate of material removal from the outer tube, a bed of cooling material can be intentionally built up within the apparatus. By means of the design, this bed is in direct contact with the inner tube conveying the reactant material into the furnace. This contact allows for direct heat transfer from the product to the reactants, assisting in the preheating of the incoming material while at the same time cooling the exiting material. Thus, in addition to the container-less nature of the design, the thermal transfer between product and reactants significantly improves the power efficiency of the unit (see Figure 2).
Given the nature of this design, it is anticipated that a wide range of product feedstocks may prove suitable for processing in this equipment. The plug flow of material in the entry cone should minimize the sticking problems observed in rotary tubes, and should not necessarily require only materials that exhibit low angles of repose. Product mixing concerns during reaction are minimized, given that the peak process temperature is encountered at the immediate top of the reaction bed. This reaction zone is continuously removed and replenished in a fountain-like manner, ensuring that the entire bulk of material is exposed to radiative heating and solid-gas interchange in a nearly-uniform manner. Because the material is not continuously agitated, lower levels of fine powder entrainment are also expected.
A pilot-scale model of this design is currently being tested for high-temperature calcining applications for phase conversion. It is anticipated that it will provide significant advantages for the efficient processing of materials requiring high-temperature solid-solid and solid-gas reactions, such as carbides, nitrides, borides, and refractory metal powders.
1. Miller et al., “Vertical Conveyor Apparatus for High Temperature Continuous Processing of Materials,” U.S. Patent Number 6,910,882, issued June 28, 2005.