ABSTRACT
Chapter 6 laid the groundwork for the topic to be taken up here by examining in considerable detail the various phase equilibria that enter into the formulation of an
equilibrium
stage
. The equilibrium stage, also termed an
ideal stage
or
theoretical stage
, plays a central role in the analysis of an important class of mass transfer operations termed
staged
processes
. In these operations, two phases are brought into intimate contact in a stirred tank or its equivalent, and the desired mass transfer process is allowed to take place. The two phases are then separated, and the process is either repeated (multistage contact) or brought to a halt (single-stage contact). While there is only one mode of single-stage contact, multistage operations can be arranged in a variety of geometrical configurations. In
co-current operations
, the two phases move through the stages and cascade parallel to each other and in the same direction. This mode of contact is rarely used in practice. When the phases move in opposite directions but are still parallel, we speak of a
countercurrent operation
. Finally, when the two phases move at right angles to each other, the process is termed a
crosscurrent operation
. These modes are sketched in Figure 7.1a through Figure 7.1d.
