Summary
Continuous thermodynamics, in which a fluid's composition is regarded as a continuous function rather than as a set of discrete mole fractions, has been applied to petroleum fluids in a number of forms. The method provides a practical alternative to pseudocomponent methods in phase-equilibrium calculation with an equation of state (EOS). Current continuous-thermodynamic methods are limited by the functional form assumed for the composition; they are unable to retain a continuous description of all phases in the calculation while preserving material balance.
A new functional representation of continuous mixtures based on an orthonormal expansion is proposed that preserves material balance between phases and provides a versatile composition description. With such a representation, the conditions for phase equilibrium are shown to be of a simple form. Because these conditions are similar to those for a discrete composition, phase equilibrium can be calculated in much the same way. The method's accuracy approaches that of a calculation in which the composition is fully represented by a large number of discrete components. The computer time requirement, however, is far less because only a small number of terms must be retained in the expansion series. Use of this new method is demonstrated for petroleum mixtures with a modified Soave EOS.
