Question: Besides rotary, fluidized-bed, and flash dryers, what other types of dryers can remove moisture and other volatile compounds from bulk solids?
Answer: For many applications, an efficient and cost-effective alternative is to dry the bulk solid in a silo equipped with heat exchangers or modified to permit injection of a gas. Compared to other dryers, drying silos have several advantages: their capital and operating costs are relatively low; they offer several hours of residence time; and storage or surge capacity for the dried product is provided.
Removal of moisture or volatiles is accomplished by either direct drying or indirect drying. Indirect drying silos are equipped with heat exchanger plates that are installed vertically in the straight-walled section of the silo. When direct drying is used, a drying gas, usually air or nitrogen, is injected into the solids. Some drying silos employ both direct and indirect drying. For these systems, both a sweeping gas and heat exchangers are utilized.
When a drying gas is used, the gas may flow either counter or perpendicular to the flow of solids. In a counter-flow dryer, the gas is best injected via a plenum or other device located near the hopper-cylinder interface. Unless the bulk solid is made up of very large particles, however, the allowable injection rate is often limited due to the material’s low permeability. Therefore, a countercurrent system is more commonly used as a secondary dryer to remove trace volatile components from the incoming material, unless heat exchangers are installed.
Drying efficiencies are greater in cross-flow systems since the drying gas can be injected and moisture or volatiles can be removed at several locations. In addition, greater volumes of gas can be introduced. However, the moisture content in products from cross-flow dryers may be less uniform. When heat exchangers are used together with cross-flow gas, high evaporative capacities are achievable since both the temperature of the drying gas and the driving force for moisture or volatiles removal are maintained.
Mass flow is essential for successful operation of either type of drying silo. In mass flow, the walls of the converging section of the silo are steep enough and low enough in friction to allow the bulk solids to flow along them. If the walls are too shallow or too high in friction, funnel flow (in which flow only occurs in a narrow flow channel above the silo outlet) will occur, resulting in stagnant solids, less than expected residence times, and poorer dryer performance. Solids-flow property testing is required to determine the necessary hopper angles for mass flow.
In today’s lean manufacturing environment where operational cost savings are critical, consider the use of energy-efficient gravity drying in silos.
Greg Mehos is a project engineer at Jenike & Johanson, Inc. (Tynsboro, Mass.; www.jenike.com), an engineering firm specializing in the storage and flow of powders and bulk granular solids.
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