Flash dryers offer an effective method of removing surface or unbound moisture from a feed product. The technology behind flash drying employs a low residence time within the actual dryer; thus, the unbound moisture is "flashed" off the feed. Flash dryers are essentially pneumatic dryers. Many variations or configurations have been developed, but all flash dryers employ the same basic concept of operation.
Flash dryers are an efficient method of drying products such as slurries, pastes and sludge (most with back mixing), friable filter cakes, powders and granules. The feed must have a relatively consistent particle size to allow for transfer without segregation and buildup. Flash dryers operate effectively on throughput rates varying from a few kilograms per hour up to several hundred tons per hour (obviously based on the bulk density of the product). Depending on throughput, they may be small, unnoticed pieces of equipment located inconspicuously in the plant, or they may require large, independent structures for support. The resulting product may have residual moisture that varies from 0% to upwards of 12% depending on operating parameters or the percentage of bound moisture contained in the feed.
Flash drying is a continuous process with the dryer being either directly or indirectly fired. They are inherently cocurrent dryers with the hottest air contacting the wettest product. They operate at inlet temperatures varying from ambient dehumidified air for sensitive products to more than 1,100oF (600oC) for robust products. Because the system has relatively low residence time and the moisture is flashed off, a significant amount of evaporative cooling takes place in the system. This allows higher inlet temperatures to be used than in many other dryers without unduly heating the product. Higher inlet temperatures also increase the overall dryer efficiency.
Unlike the descriptive name of most dryers such as rotary, spray or conveyor, flash drying refers to the mode of drying rather than the type of equipment. There is no single system configuration or component design for a flash drying system. The path of the gas stream may be a single long tube, camel back, loop design or series of cyclones, and each configuration offers unique characteristics. For example, a single, long tube will require less energy to compensate for losses and can process high throughput rates. A loop design will allow for classification and additional residence time based on moisture content, or, alternatively, may reduce the particle size by milling the material in the dryer.
Flash drying systems use a heated carrier gas (usually, air) to pneumatically convey the feed through the flash dryer (flash tube) and into a primary gas-separation device (most commonly, a cyclone or series of cyclones in series or parallel). The carrier is induced or forced into the feed throat from a hot gas generator that heats the gas to the desired operating inlet temperature. In the feed throat, the gas entrains the feed, and the moisture is evaporated quickly as the product is conveyed through the system to the primary gas/product separation device. Because the entire product is entrained in the gas stream, both primary product collection (typically, cyclones) and secondary and tertiary dust collection such as multiclones, bag houses, scrubbers and electrostatic precipitators usually are installed to meet emission limits.
Flash dryers utilize fans to provide the motive force for transferring the product. A system may include both a forced-draft and induced-draft fan, but most can operate effectively on a single induced-draft fan.
Process designs for dryer systems yield thermal energy and air requirements. The energy represents the sensible and latent heat needed to elevate the product temperature to the point at which it will allow evaporation at the ensuing pressure. The air requirement represents both the volume needed to carry this energy as well as remove and hold the moisture above the mixture's dewpoint. This requires detailed and involved calculations to obtain the correct heat and mass balance requirements. Flash dryers are not pneumatic-conveying systems with a heater in the line!
Feed is introduced (metered) into the throat of the flash dryer in a controlled fashion by means of a modulating feeding device such as a screw feeder, rotary valve, vibrating feeder or mixer. The throat is composed of a carrier introduction duct, an acceleration zone to increase feed velocity and a discharge zone where the gas-entrained product is injected into the flash tube.
Acceleration usually is accomplished by means of a venturi or mechanical accelerator that imparts kinetic energy into the feed stream, allowing it to reach the required conveying velocity. This area -- the throat -- is a critical area in the design of flash dryers. Specific and proprietary design of the mechanical accelerators allows for agglomerated feeds to be broken down into smaller particles that are conveyed and dried effectively.
Once entrained in the carrier gas, the transfer of thermal energy is quick and efficient because the carrier intimately contacts the particles and the exposed surface area is enhanced in the gas stream. If desired, residence times in these units can be altered by increasing the flash tube length, increasing holdup times in the product collection cyclone(s) or adding cyclones in series.
Flash dryers are flexible in that they can use almost any energy source. However, if inlet temperatures are restrictively low, the unit may become excessively large.
Control of these dryers depends on the complexity of the installation. They may have PLC or solid-state controllers. The system most commonly controls on the exhaust gas temperature although other control methods have been employed effectively. The control loop can modulate the energy input (inlet temperature) or the feed rate to allow for stable control and variations in input parameters.
Weigh Your ChoiceFlash dryers require little real estate relative to throughput. The flash tube of the dryer is flexible and can be routed to suit the plant constraints. As mentioned earlier, a pneumatic-conveying system is not a flash dryer, but a flash dryer can be a pneumatic-conveying system. Not only can it effectively dry products, it can elevate them, move them around a facility and preheat them for successive processes. Flash dryers have few moving parts. They can be designed to handle ex-tremely abrasive products with replaceable wear components.
But, like any dryer, flash dryers also have their limitations. Loss of power to the dryer will cause the product to fall out of suspension and build up in the dryer base and feed throat. If the product hardens under heat, this may cause a blockage that requires significant time to remove. Due to the relatively high velocities, the particle size may be reduced due to attrition and impact. High velocities also may contribute to premature component wear if the system is not designed to inhibit it.