Process equipment involves complicated geometries, heterogeneous conditions, and complex interactions. As such, the use of mass transfer coefficients is limited. For those cases we use an approach based on the concept of an ideal stage. This is a device in which entering streams not at equilibrium are brought to that condition before they exit. The mass transfer driving forces are the differences in composition between the entering and exiting equilibrium values.

Mass transfer operations such as distillation, absorption, extraction, leaching, and crystallization can be analyzed by the ideal stage concept. Solutions for these mass transfer processes or the design of devices to carry out such operations require calculations of (1) a material or mass balance, (2) an enthalpy or energy balance, and (3) equilibrium data. The first two items are the conservation of mass and conservation of energy. The third item represents the appropriate relationship between the phases present, for example, vapor-liquid equilibrium data for distillation. Table 5-3 shows the type of data required for a given process.