Page 51 - Separation process engineering

method are always the same. For example, the basic principles of distillation are always the same
whether we are separating ethanol from water, separating several hydrocarbons, or separating liquid
metals. Consequently, distillation is often called a unit operation, as are absorption, extraction, etc.

A more general idea is that design methods for related unit operations are similar. Since distillation and
absorption are both liquid-vapor contacting systems, the design is much the same for both. This similarity
is useful because it allows us to apply a very few design tools to a variety of separation methods. We will
use stage-by-stage methods where calculation is completed for one stage and then the results are used for
calculation of the next stage to develop basic understanding. Matrix solution of the mass and energy
balances will be used for detailed computer simulations.

1.2 Concept of Equilibrium

The separation processes we are studying in Chapters 1 to 14 are based on the equilibrium stage concept,
which states that streams leaving a stage are in equilibrium. What do we mean by equilibrium?
Consider a vapor and a liquid that are in contact with each other as shown in Figure 1-2. Liquid
molecules are continually vaporizing, while vapor molecules are continually condensing. If two chemical
species are present, they will, in general, condense and vaporize at different rates. When not at
equilibrium, the liquid and the vapor can be at different pressures and temperatures and be present in
different mole fractions. At equilibrium the temperatures, pressures, and fractions of the two phases cease
to change. Although molecules continue to evaporate and condense, the rate at which each species

condenses is equal to the rate at which it evaporates. Although on a molecular scale nothing has stopped,
on the macroscopic scale, where we usually observe processes, there are no further changes in
temperature, pressure, or composition.
Figure 1-2. Vapor-liquid contacting system

Equilibrium conditions can be conveniently subdivided into thermal, mechanical, and chemical potential
equilibrium. In thermal equilibrium, heat transfer stops and the temperatures of the two phases are equal.

(1-1)

In mechanical equilibrium, the forces between vapor and liquid balance. In the staged separation
processes we will study, this usually implies that the pressures are equal. Thus for the cases in this book,

(1-2)

If the interface between liquid and vapor is curved, equal forces do not imply equal pressures. In this case
the Laplace equation can be derived (e.g., see Levich, 1962).