Ever industry. Along the zero contour defined by where

Ever since the appearance of the van der Waals
equation in 18731,
many authors have proposed variations in the semiempirical relationship. One of
the most successful modifications was that made by Redlich and Kwong 1949.
Since that time, numerous modified Redlich- Kwong (RK) equations have been
proposed 2-5. Some of them have introduced
deviation functions to fit pure substance PVT data. One of the more
recent modifications of the RK equation is that proposed by Soave 1972. The
Soave-Redlich-Kwong (SRK) equation has rapidly gained acceptance by the
hydrocarbon processing industry. Along the zero contour defined by

the compressibility factor is the same as for an ideal gas6,
the density of many fluids has been found to be nearly a linear function of
temperature. Lately, in 1975 Peng and Robinson7
proposed another modification of van der Waals equation that was semiempirical
one. Recently, regularity for fluids also gases was derived which has some
theoretical basis: Parsafar and Mason 8
have introduced a regularity based on intermolecular potential energy which

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linearly with ?2. This relation was originally suggested on the
basis of a simple lattice-type model applied to a Lennard–Jones (12, 6) fluid. Goharshadi
et al.9
presented new regularities for liquids based on average potential energy in

 changes linearly with ? for each
isotherm of any liquid. In this derivation Lennard–Jones (9, 6) potential
energy is used. Baniasadi et.al10
proposed a non-linearity relationship exists between


, where

 is the molar density for all isotherms of
gases. In derivation of the regularity, Lennard–Jones (9, 6, 3) potential
energy is used., but it’s complicated with four coefficients.

The major of this paper is to introduce a new
regularity and also derive a new equation of state
for gases. The equation of state, derived accordingly, is presented as a linear
isotherm. The isotherm by this modeling is applied to
calculate densities of gases for which PVT data are available. Density is an important
physical property which is usually measured and other thermodynamics properties
can be determined from it 11, 12


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