Real Gases
Equations of State for Non
Ideal Gases
Real Gas Relationships
◼In this section, we discuss three methods
of getting or predicting real gas
properties in lieu of having experimental
data:
1. Compressibility Charts
2. Equations of State
3. Estimated Properties
Introduction
◼The ideal gas is the basis of the simplest
and most convenient equation of state.
◼Its shortcoming is that it can be seriously
inaccurate.
◼At a sufficiently low temperature and/or a
sufficiently high pressure, a value of V
predicted with the ideal gas equation could
be off by a factor or two or three or more in
either direction.
Critical Temperature and Pressure
◼In general, the highest temperature at
which a species can con-exist in two
phases (liquid and vapor) is the critical
temperature of that species, Tc, and the
corresponding pressure is the critical
pressure Pc.
◼A substance at Tcand Pcis said to be at its
critical state.
Critical state (point) mean?
◼The critical state for the gas-liquid
transition is the set of physical conditions
at which the density and other properties
of the liquid and vapor becomes identical.
(Himmelbleau)
Vapor or Gas????
◼A vapor is a gaseous species below its
critical temperature
◼A gas is a species above its critical
temperature at a pressure low enough for
the species to be more like a vapor than a
liquid.
◼Substances at temperatures above Tcand
pressures above Pcare referred to as
supercritical fluids.
Supercritical Fluids ????
◼A supercritical fluid, that is, a compound in
