Modifying your storage vessel for trouble-free continuous purging or conditioning — Part II
This two-part article explains how a conventional storage silo or bin can be modified to provide continuous purging or conditioning of dry bulk materials. Information covers the six criteria your plant's design engineer or a consulting bulk solids handling engineer can use in designing the vessel for successful operation. Part I (November) covered the first three vessel design criteria; Part II covers the final three.
In a purge or conditioning vessel, mass transfer occurs as volatiles are transferred from the material to the gas. The driving force for mass transfer is the difference in the volatiles content between the gas and the material. However, the driving force for mass transfer isn't constant in purging and conditioning applications, because as the gas flows upward -- countercurrent to the material's downward flow -- the composition of both the gas and material streams varies along the vessel's height. To ensure that this driving force doesn't disappear entirely in one part of the vessel when the vessel is operating at your specified material feedrate, you must inject gas into the vessel at a certain minimum rate. If the gas injection rate drops below this minimum level and causes the driving force in one part of the vessel to vanish, you won't be able to achieve the devolatilization or conditioning level you require.