Vapor-Phase Nonideality: When to Worry

Should you account for vapor property nonidealities in chemical process simulations? The answer will depend on the degree of uncertainty allowed in your system.

Distillation, supercritical extraction, gas separation, and reactions are all common chemical processes that involve gases and vapors. Designing facilities to handle such operations requires accurate properties such as enthalpy (h) and entropy (s) for heat effects and chemical potential or fugacity coefficient (ϕ_i) for phase and reaction equilibria. These depend on the component mole fraction (x or y) and one or more state conditions of temperature (T), pressure (P), volume (v), and density (ρ). Chemical engineers typically obtain these properties from equations of state (EoS), the simplest being the ideal gas (IG) equation, which applies to the air around us but which can be inaccurate under many process conditions. Vapor nonideality at moderate pressures is rigorously described by the virial expansion in pressure or density (1–3). This model is reliable and accurate for many situations when combined with upper limits for P or ρ (1).

This article examines how the effects of vapor nonideality can be reliably estimated for common process conditions. It focuses on vapor and gas properties used in process design and simulation for common conditions, i.e., low to moderate pressures for substances below their critical temperatures and high pressures for supercritical components. The discussion centers on the virial EoS truncated at the second coefficient (2V), since it provides the initial departure from the ideal gas law (1–3)...