## 2.3. Noncubic EOS

One of the well-known noncubic EOSs is the virial equation. The virial EOS is based on theories of statistical mechanics (Mason and Spurling, 1969). The original version of virial EOS was presented by Onnes in 1901 and it may be written in a power series of molar density (pressure explicit) or pressure (volume explicit) as follows:

$Z=1+BV+CV2+DV3+…(Z=1+BρM+CρM2+DρM3+…)$

(2.41)

$Z=1+B′P+C′P2+D′P3+…$

(2.42)

in which Z is the compressibility factor, V is the molar volume, ρM is the molar density, P is the pressure and B, C, D,… are the second, ...

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