Twu-Sim-Tassone equation of state

Twu, Sim and Tassone presented a cubic equation of state for accurate representation of hydrocarbons that has become known as the Twu-Sim-Tassone or TST equation of state. With a critical compressibility factor of (Eq. 5)

$Z_{c}={\frac {p_{c}v_{c}}{RT_{c}}}={\frac {8}{27}}$

it better represents the compressibility than many of than the Redlich-Kwong equation of states, including the Soave modified version, and the Peng and Robinson equation of state.

The equation follows the general cubic form resulting in the equation (Eq. 2):

$p={\frac {RT}{V_{m}-b}}-{\frac {a}{(V_{m}-0.5b)(V_{m}+3b)}}$

where $V_{m}$ is the molar volume, $R$ is the molar gas constant, $T$ is the temperature, $p$ is the pressure, and $a$ and $b$ are the attractive and repulsive parameters akin to those of the Van der Waals equation of state. Relations exists between $a$ and $b$ and the critical parameters $T_{c}$ and $p_{c}$ in the forms (Eqs. 3 and 4):

$a_{c}=Z_{c}{\frac {343}{216}}{\frac {R^{2}T_{c}^{2}}{p_{c}}}$

$b_{c}=Z_{c}{\frac {RT_{c}}{4p_{c}}}$

References

Related reading

• Chorng H. Twu, Wayne D. Sim and Vince Tassone "Getting a Handle on Advanced Cubic Equation of State", Chemical Engineering Progress November pp. 58-65 (2002)