- When the system is in equilibrium the total number of variables that may be independently fixed in a system is the difference between the total number of variables that characterize the intensive state of the system and the number of independent equations that can be written by connecting the variables.
- The intensive state of the system having N number of chemical components and Π number of phases in equilibrium is described by the intensive variables, pressure P, temperature T, and N-1 mole fractions for each phase.
- The total number of phase rule variables
2 + (N-1) (Π)
- Mass of the phases can not include because they are not intensive state variables.
- The total number of independent phase equilibrium equations can be written by connecting intensive state variables for N components for each pair of phases present in the system are
(Π – 1) (N)
- From the definition of phase rule, the difference between the total number of phase rule variables and the total number of independent equations is called the degrees of freedom (F).
F = 2+ (N-1) (Π) - (Π – 1) (N)
F= N - Π + 2
Where: N= Number of chemical components in the system
Π= Total number of phases involved in the system
F= Degree of freedom (total number of variables required to define the state of the system completely)
Basic Concepts of Chemical Engineering Thermodynamics
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Syllabus for GATE - 2022 Chemical Engineering and GATE Question Papers