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Adjustment and control of moisture levels in solid materials is a critical aspect in the manufacture of many chemical products. Drying can be defined as the vaporization and removal of water or other liquids from a solution, suspension, or other solid-liquid mixture to form a dry solid. A complicated process involving simultaneous heat and mass transfer, accompanied by physicochemical transformations, industrial drying is often accomplished through one or more of four broad mechanisms, including the following: direct drying (convection); indirect or contact drying (conduction); radiant drying; and dielectric or microwave drying. This one-page reference focuses on the differences between convection and conduction drying.
Conduction (contact drying)
Contact drying involves an indirect method for removing liquid from a solid material by applying heat. In contact drying, the heat-transfer medium is separated from the material to be dried by a metal wall. Heat transfer to the product occurs predominantly by conduction through metal walls and impellers. Mixing is required to ensure contact between material surface and the heat-transfer surface. Heat-transfer fluids are often steam, hot water, or a heated oil .
Conduction drying can be particularly useful in cases where solvent needs to be removed from a solid material in a closed-cycle drying circuit, and can be a good choice for materials that are not heat-sensitive. Conduction drying could be an option when the solid material can tolerate heat-transfer-fluid temperatures well over the boiling point of the liquid.
An important benefit of conduction drying is its high thermal efficiency. Only a low flow of drying gas (air or nitrogen) is needed to sweep the evaporated liquid out of the dryer, and the air is not used as the heat source, as it is in convection drying. Smaller volumes of drying gas exiting the dryer means lower exhaust-gas enthalpy, and therefore, a higher thermal efficiency for the dryer.
Because of the higher thermal efficiency, conduction drying may offer economic and environmental advantages over convective drying approaches. Some of the possible cost benefits result from the smaller solvent recovery equipment required and the lower chilled-water requirements for the condensing operation.
Another important potential benefit for conduction drying is the minimized risk of cross-contamination. In conduction drying, the heat-transfer medium does not contact the product being dried.
Convection drying involves removing moisture from the surface of solid materials through contact with gases, usually forced air. Convective drying uses the sensible heat of the fluid that contacts the solid to provide the heat of vaporization of the liquid. Solid materials can be exposed to the heated gases via various methods, including the following:
- Gases can be blown across the surface (cross-circulation)
- Gases can be blown through a bed of solids (through-circulation; used when solids are stationary, such as wood, corn and others)
- Solids can be dropped through a slow-moving gas stream, as in a rotary dryer
- Gases can be blown through a bed of solids that fluidize the particles. In this case, the solids are moving, as in a fluidized-bed dryer
- Solids can enter a high-velocity hot-gas stream and can be conveyed pneumatically to a collector (flash dryer)
Dryer types that rely heavily on convection include fluidized-bed dryers, flash dryers, spray dryers and conveyor dryers. Fluidized-bed dryers and flash dryers are used in cases where particles have high surface-to-volume ratios. Spray dryers are typically associated with drying solids from liquids or slurries. Conveyor dryers are typically used with larger particles or friable materials because they can minimize particle breakage with a static material bed .
Determining the best drying approach for an application and selecting specific drying equipment depends greatly on the physical characteristics of the material to be dried. Table 1 compares the two drying approaches discussed here according to a set of properties .
1. Walsh, John and Whaley, M., Supplier’s Tips, Powder and Bulk Engineering, April 2014.
2. Parikh, D., Solids Drying: Basics and Applications, Chem. Eng., April 2014, pp. 42–45.
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