ossible if the number of

For separation processes, a design solution is possible if t independent equations equals the number of unknowns,

N,= Nv - Nc where Nv is the total number of variables (unknowns) involved in the process under consideration, Nc is the number of restricting relationships among the unknowns (independent equations), and N, is termed the number of design variables. In the analogous phase-rule analysis, N, is usually referred to as the degrees of freedom or variance. It is the number of variables that the designer must specify to define one unique operation (solution) of the process.

The variables Ni with which the designer of a separation process must be concerned are:

1. Stream concentrations (e.g., mole fractions) Temperatures Pressures Stream flow rates Repetition variables Nr The first three are intensive variables. The fourth is an extensive variable that is not considered in the usual phase-rule analysis. The fifth is neither an intensive nor an extensive variable but is a single degree of freedom that the designer utilizes in specifying how often a particular element is repeated in a unit. For example, a distillation-column section is composed of a series of equilibrium stages, and when the designer specifies the number of stages that the section contains, he

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