Resin storage vessels such as bins, silos, and tanks often double as purge vessels, in which products are purged of volatiles or polymerization reactions are controlled. Processing is achieved by gas flowing among resin particles or pellets. Problems associated with these purge vessels include typical bin flow problems such as now stoppages due to arching or ratholing, erratic discharge, particle segregation, and flooding or flushing. In addition, purging may be non-uniform or incomplete, and cross contamination between grades may occur. These problems are often the result of either a funnel flow pattern, where some of the material in the vessel moves while the rest remains stationary, or a non-uniform velocity profile, where some parts of the vessel flow much faster than others. Also, the location and the manner in which gas is injected into the vessel contents affect the uniformity of exposure of polymer particles to the gas. This paper describes methods of designing purge vessels to eliminate stagnant regions. Also, velocity gradients of solids in mass flow vessels (where all of the material is in motion whenever any is withdrawn) are discussed. Considerations of the gas introduction schemes that do not disturb the resin now pattern are also described.