The rotary kiln system is used for material processing and consists of unitary operating equipment and other components that are added together. Perhaps the most important is the rotary reactor, which is considered the heart of the process and therefore it is important to pay special attention to it. The rotary reactor is usually a long horizontal cylinder inclined on its axis. In most rotary kiln process applications, the objective is to drive specific bed reactors, which, for kinetic or thermodynamic reasons, frequently require high temperatures.
There are several rotary kiln designs, each specific to the process required. Although most kilns consist of straight cylindrical structures, bell-shaped designs due to varying drum sizes can bring benefits to the process application. With respect to the internal fittings of the kiln, most direct-fired kilns are lined with refractory materials for several reasons, but the main purposes are to insulate and protect the kiln shell from thermal damage and to save energy. Kilns may also be equipped with dams to increase material residence time or with lifters to assist axial flow of materials. We do offer an improved design of the lifters system, extending the length and diameter of the current one to recover and guide as much material as possible back into the process.
Depending on the rotational speed of the kiln, bed motion in the transverse plane may be characterized as sliding, sinking, rolling, cascading, cataract, and centrifugal. Centrifugal bed motion occurs at critical and high speeds. This is an extreme condition where all the bed material rotates with the drum wall. Cascading motion occurs at relatively high rotational speeds and is a condition where the height of the leading edge of the material is raised above the bed surface and particles cascade or rain down onto the free surface. However, operating the rotary kiln under any of these conditions is uncommon due to wear and dusting problems.
Drying applications utilize the high exposure of particles to the heat transfer fluid associated with the cascade mode and the separation effect is caused by the centrifugal force component. For example, starting at the other extreme, i.e., at very low rotational speeds and progressively moving to higher speeds, the bed typically transitions from sliding, whereby most of the bed material, in bulk, slides against the wall, to sinking, whereby a segment of the bulk material in the cutting wedge becomes unstable, yields, and empties down the slope, to rolling, which involves a constant discharge onto the bed surface. In sinking mode, the dynamic angle of repose varies cyclically, while in rolling mode the angle of repose remains constant.
In rolling mode, where mixing in the rotary drum is maximized, two distinct regions can be discerned, the shear region, called the active layer, formed by particles near the free surface, and the passive or plug flow region at the bottom where the shear rate is zero. The mode chosen for an operation depends on the intent of the application. Due to the low thermal efficiency of earlier long kilns and the need for fuel efficiency, most designs aim to maximize mixing and heat transfer. To achieve this, kilns are often equipped with heat recuperators, such as preheaters, in which some of the energy is recovered from the exhaust gases to preheat the feed before it enters the kiln. Although coolers are frequently used to cool down the product for safe material handling, they are also used to recover energy, which would otherwise be wasted, as in older kilns, to preheat the combustion air and/or to meet other energy needs.
Other pertinent features include internal elements, such as constriction dams and lifters, which affect residence time. Since the cylinder is partially filled and rotates on its horizontal axis, the clearance or open space above the bed depends on the depth of the bed and therefore the kiln load.
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