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Technical & Practical

May 1, 2009

Designing and Operating Gravity Dryers

Properly designed, bulk solids bins or silos offer numerous advantages in slow, diffusion-limited drying operations

Greg J. Mehos, Jenike & Johanson

Evaporation of moisture or volatile organic compounds (VOCs) from bulk solids usually takes place in two stages: one in which the drying rate is constant, and the second in which the rate decreases over time. Devolatilization during the first stage is rapid and can be readily accomplished in flash, spray, or fluidized bed dryers. In the second stage, however, the VOC or moisture removal rate is diffusion limited, and several hours of residence time are often necessary. To meet low moisture or VOC targets, two unit operations may be required. An economical way to provide the extended residence time required for slow, diffusion-limited drying is to use a bin or silo that has been modified to allow injection of a sweeping gas and, in some cases, to provide heating. These unit operations, which have a variety of names including gravity dryers, offer numerous advantages.

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