Ideal Gas Model

The Ideal Gas equation of state is one of the earliest gas models. It was developed from experimental results under certain "ideal" conditions. It is simple and easy to use, but not the most accurate model.

Ideal Gas Equation

The Ideal Gas equation is given by PV= RuT Where

P = pressure (kPa) V = molar volume (m3/kmol) Ru = Universal gas constant T = temperature (K)

Van der Waals Model

The Van der Waals equation of state was proposed in 1873. It is one of the earliest attempts to improve the accuracy of the ideal gas law. Derived from the ideal gas equation, it accounts for two things.

1. Intermolecular forces

2. Volume occupied by gas molecules

Benedict-Webb-Rubin model In 1940, the Benedict-Webb-Rubin (BWR) equation of state was extended from the Beattie-Bridgeman equation. It contains six constants and is generally more accurate than the two-constant equations, such as the Redlich-Kwong equation or the Van der Waals equation. Some gases have more than one listing in the Gas Composition Editor and Gas Component Editor. This is because some of the listings are accurate in a limited temperature and pressure range. If they are used outside their designed validity range, accuracy may be compromised. System 1™ software does not check your selection to verify that it is being used with in its valid range.

The following table contains the temperature and pressure validity ranges for gas components of the BWR model. The names in the table are as they are listed when selecting a gas in the Gas Composition Editor and Gas Component Editor. Only the gas listings designed for use with the BWR gas model are shown.

Gas Component Minimum Temperature (K)

Maximum Temperature (K)

Maximum Pressure (MPa)

1,3-BUTADIENE(BWR) - C4H6

- - -

1-BUTENE(BWR) - C4H8

423.2 523.2 25.3

1-PENTENE(BWR) - C5H10

- - -

2-BUTENE(BWR) - C4H8

- - -

ACETYLENE(BWR) - C2H2

293.2 523.2 15.2

AMMONIA(BWR) - NH3

273.2 573.2 111.5

BUTANE(BWR) - C4H10

423.2 573.2 -

CARBON DIOXIDE -1(BWR) - CO2

283.2 423.2 70.9

CARBON DIOXIDE -2(BWR) - CO2

423.2 523.2 70.9

CARBON DIOXIDE -3(BWR) - CO2

273.2 548.2 70.9

CARBON MONOXIDE -1(BWR) - CO

133.2 248.2 101.3

CARBON MONOXIDE -2(BWR) - CO

248.2 473.2 101.3

DECANE(BWR) - C10H22

313.2 523.2 70.9

ETHANE(BWR) - C2H6 273.2 548.2 30.4

ETHYLENE(BWR) - C2H4

273.2 473.2 30.4

HEPTANE(BWR) - C7H16

548.2 623.2 -

HEXANE(BWR) - C6H14

548.2 623.2 -

HYDROGEN SULFIDE(BWR) - H2S

278.2 443.2 70.9

HYDROGEN(BWR) - H2

273.2 423.2 253.3

ISOBUTANE(BWR) - C4H10

373.2 513.2 -

ISOBUTYLENE(BWR) - C4H8

423.2 548.2 -

ISOPENTANE(BWR) - C5H12

403.2 553.2 20.3

METHANE -1(BWR) - CH4

203.2 473.2 40.5

METHANE -2(BWR) - CH4

273.2 623.2 40.5

METHYL CHLORIDE(BWR) - CH3Cl

313.2 493.2 30.4

NEOPENTANE -1(BWR) - C5H12

433.2 548.2 25.3

NEOPENTANE -2(BWR) - C5H12

303.2 473.2 7.1

NITROGEN -1(BWR) - N2

110.2 473.2 60.8

NITROGEN -2(BWR) - N2

103.2 373.2 -

NITROUS OXIDE(BWR) - N20

243.2 423.2 20.3

NONANE(BWR) - C9H20

313.2 523.2 70.9

OXYGEN(BWR) - O2 163.2 398.2 -

PENTANE(BWR) - C5H12

413.2 553.2 20.3

PROPANE(BWR) - C3H8

373.2 548.2 -

PROPENE(BWR) - C3H6

298.2 573.2 -

PROPYNE(BWR) - C3H4

323.2 473.2 30.4

SULFER DIOXIDE -1(BWR) - SO2

283.2 523.2 20.3

SULFER DIOXIDE -2(BWR) - SO2

283.2 523.2 20.3

Benedict-Webb-Rubin-Starling Model

The Benedict-Webb-Rubin-Starling equation of state is a newer modification to the Benedict-Webb-Rubin equation. The equation was developed to obtain more accurate values of the thermodynamic properties of hydrocarbons and related compounds. This equation is capable of predicting properties at reduced temperatures as low as Tr=0.3 and reduced densities as great as ρr=3.0. This equation was designed to predict thermodynamic properties in the liquid, gas, and dense fluid states.

Peng-Robinson Model

The Peng-Robinson equation of state is useful for both liquids and real gasses. The two-constant equation also includes the acentric factor when calculating gas properties. The Peng-Robinson equation is similar to the Soave-Redlich-Kwong equation and may give similar results.

Redlich-Kwong Model

The Redlich-Kwong equation of state is reasonably accurate at high pressures and is frequently used for real gases at the liquid-vapor line. Otto Redlich made an empirical modification to the Van der Waals equation. He found that the Van der Walls equation was a reasonable assessment of the repulsive forces. However, the attractive-force term needed to account for temperature dependence to be more accurate. Because of this modification, the Redlick-Kwong equation is generally more accurate than the Van der Waals equation.

Soave-Redlich-Kwong Model

The Soave-Redlich-Kwong equation is a modification of the Redlich-Kwong equation. The difference is that the constant ‘a’ was replaced with one that was dependent on temperature as well as a new constant called the acentric factor, which, in its development, was based on vapor pressure behavior. The result is an equation that is not much better in predicting volumes, but is far better in predicting vapor pressures of pure components. The Soave-Redlich-Kwong equation is similar to the Peng-Robinson equation and may give similar results.