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)

$Z_c=\frac{p_c{v}_c}{R{T}_c}=\frac{8}{27}$

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):

$a_c=Z_c\frac{343}{216}\frac{R^2{T}_c^2}{p_c}$

$b_c=Z_c\frac{R{T}_c}{4p_c}$

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)