Traditionally, drying of bulk solids is known to be an energy-intensive process. For many solids, a cost-effective, simple, and efficient alternative is drying in silos. Gravity-driven flow in silos can provide the extended residence time needed to dry many bulk materials. To allow removal of a volatile component, usually water, a silo is equipped with heat exchangers or modified to permit injection of a gas. Such vessels have an assortment of names, including gravity dryers, purge or conditioning columns, and silo or hopper dryers. Compared with other drying technologies, gravity flow silos have several advantages. Their capital, operating, and maintenance costs are relatively low, several hours of residence time are available, and storage or surge capacity for the dried product is provided.
Drying occurs when a solid is heated or exposed to an environment that is lean in volatiles content. In both cases, a driving force for transferring the volatile component develops since equilibrium is disrupted.
In general, two methods are used to remove moisture or other volatile components from a bulk material inside a silo: contact drying and convective drying. In contact or indirect drying vessels, heat exchanger plates are installed vertically in the straight-wall section of the silo. Steam, hot water, or heated air is passed through the plates to raise the temperature of the bulk material and evaporate its volatile components. When convective drying is used, a suitable drying gas, usually air or nitrogen, is injected into the solids. Some modified silos employ both contact and convective 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 gas injection rate is generally limited due to the material’s low permeability. Therefore, a countercurrent system is most frequently 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. 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.
Heat exchangers should only be used with relatively free-flowing materials. Cohesive powders and bulk materials made up of large particles are prone to arching between adjacent plates.
Uniform solids flow is essential for successful operation of a silo modified to allow drying. A nonuniform solids velocity profile is undesirable since the residence time of the solids will be variable. In the worse case, the flow pattern inside the silo may be funnel flow, that is, flow will only occur in an active channel above the outlet with stagnant material remaining along the periphery. Funnel flow occurs when the walls of the hopper section are not steep enough and the friction between the solids and wall surface is too high to allow flow along the surface.
The preferred flow pattern inside a modified hopper is mass flow, in which the entire bed of solids is in motion when material is discharged from the outlet. Mass flow discharge eliminates stagnant regions and provides a more uniform velocity profile. Wall friction testing is required to determine the hopper angles and wall surfaces required to allow mass flow.
Finally, significant solids stresses exist in a mass-flow vessel, especially at the cylinder-hopper junction. Therefore, a load analysis should be performed if a silo is modified to provide mass flow and to allow drying.
In today’s lean manufacturing environment where operational cost savings are critical, consider the use of energy-efficient gravity drying in silos or hoppers.
Greg Mehos is a project engineer at Jenike & Johanson Inc. (Tyngsboro, MA). Eric Maynard is a senior consultant at Jenike & Johanson and a member of Powder/Bulk Solids’ editorial advisory board.