(673a) Changing the Mass Flow Limit - What Bin Designs Will Minimize Segregation and How Do We Change the Mass Flow Limit to Optimize Segregation Prevention

Mass flow is often needed in hoppers to prevent stable rathole formation, mitigate segregation, control residence time, and enhance blending in process equipment. Traditional theory assumes that stresses in the hopper section are a linear function of the span of the hopper. However, there are two common situations when this assumption proves untrue: when the hopper is connected to a vertical section or with a stand-alone hopper, when the area of concern is simply flow near the top of the hopper. In these situations, the traditional mass flow analysis fails to predict reality.

Today’s process engineer has come to look at mass flow as the cure-all for all flow problems. By definition, mass flow means flow at the walls of the vessel as material is discharged. There are many velocity profiles that will induce flow along equipment walls. But, this flow profile may not be what is required by the process. For example, to minimize changeover times it is necessary minimize the velocity profile across the process equipment during discharge. Typical mass flow limit maintains a 3 to 1 velocity ratio across the hopper. This may not be enough to minimize changeover of product from the bin. In this case, the real parameter to design to is the sharpness of the residence time distribution in the process equipment. Further, a material sensitive to caking requires sufficient inter-particle movement within the critical time constant of caking to break bonds between all of the particles. Applying a plug flow (uniform velocity design) in this material would be a mistake. In this case, it is necessary to maximize the velocity profile without generating regions in the bin where the velocity profile becomes flat. Mass flow is best, but this must be a mass flow condition that has some optimal velocity profile. Alternately, when a layered segregation pattern exists in a bin, then mass flow is desirable. But, the mass flow must induce enough velocity profile to remix the layers upon discharge. If the segregation pattern is purely radial, then mass flow with a nearly uniform velocity profile is required to mix radial concentration distributions upon exit of the material. Therefore, proper hopper design is not so much a question of mass flow as it is a question of getting the right residence time distribution or velocity profile for the task at hand.

Current design charts always give just the mass flow limit. Some give the velocity ratio in the hopper. None present the mass flow limit to obtain the proper residence time distribution and none express the mass flow limit as a means to minimize segregation. This paper will look at redefining the mass flow limit for the process specific case, and providing guidance to assist design engineers in selecting the right hopper for the task at hand - or inducing the right gas or stress gradients through external means to utilize an existing bin and change the mass flow limit.