Chapter 4 described the extent of separation that can be achieved in a closed vessel under three basic categories of separation: phase equilibrium based separations; external force based separations; membrane based separations. Beginning with Chapter 6, we focus on separation achieved in an open vessel: fluid streams and/or solid streams may flow into and/or out of the vessel. Thus, we have bulk flow/s in and/or out of this device. A broad variety of bulk flow patterns can exist in a separation vessel. We will, however, mostly study separations under three general categories of bulk flow configurations defined with respect to the direction of the force which is the source of the basic separation phenomenon. The three general categories of bulk flow–force combinations are:

In the bulk flow–force combination of (c), there can be cases where, instead of two fluid phases, one can have one fluid phase and another solid phase. Categories (b) and (c) provide a broader and more useful framework than the category of bulk flow perpendicular to the force direction illustrated by Giddings (1991) using a few examples.