The DIPPR database [9] provides parameters of a specific equation form, for instance equation 105, that are used to fit the liquid molar volume data.

(2–93)

(2–94)

There are four parameters from the DIPPR database: a, b, c, and d; and is the molecular weight of the liquid species.

The Rackett density model [11] is an empirical model that has been shown to yield accurate saturated liquid molar volume for a wide range of species.

(2–95)

and are the critical temperature and the critical pressure, respectively. is the reduced temperature. is a unique constant for each species and the values of selected species are listed in Table 3-10 in [11]. If the value of a species is not available, it can be estimated from the acentric factor as

(2–96)

Note that the Rackett density model is not accurate near the critical point unless ( is the compressibility at the critical point).

By default, the liquid is treated as an ideal mixture; that is, and the mixture molar volume is computed as

(2–97)

Or

(2–98)

is the molar volume of the liquid species i and can be obtained either from the DIPPR model or from the Rackett model. and are the mole fraction and the molecular weight of the liquid species i and the mixture mean molecular weight is computed as

(2–99)

When all liquid species have the required data for the Rackett density model, that is, , , , and, the Rackett model will be used to compute the molar volume of the liquid mixture.

(2–100)

(2–101)

where is the reduced temperature of the liquid mixture

(2–102)

and the Chueh-Prausnitz rules [11] are used to compute the critical temperature of the liquid mixture

(2–103)

(2–104)

(2–105)

(2–106)

The liquid density models described earlier are for the liquid density at the saturation pressure of the given temperature. Therefore, these density models depend on the liquid temperature only. To compute the liquid density at a pressure much greater than the vapor pressure corresponding to the given temperature, the Tait compressibility correction is applied to both the individual species and the mixture (ideal and Rackett) density calculations [11].

The liquid molar volume at a pressure different from the saturated vapor pressure at the given temperature can be computed from the saturated liquid molar volume as

(2–107)

or for the liquid density:

(2–108)

The compressibility K is a function of pressure and the vapor pressure, which itself depends on temperature,

(2–109)

and

(2–110)

(2–111)

Where is the critical pressure, is the reduced temperature, and is the acentric factor optimized for the Soave-Redlich-Kwong (SRK) cubic EOS.

The parameter is computed by

(2–112)

The values of the model constants a, b, d, … can be found in [11].

For a liquid mixture, the vapor pressure is estimated by

(2–113)

is computed from the generalized Riedel vapor pressure equation [11]

(2–114)

and

(2–115)

(2–116)

(2–117)

where the mixture acentric factor is

(2–118)

and the mixture reduced temperature and the mixture critical temperature is computed according to the mixture rule.

(2–119)

where represents the mass of the liquid species .

Liquid mole fraction

(2–120)

where and are the number of moles and the molecular weight of liquid species , respectively.

Mole fraction to Mass fraction (same as the gas phase)

(2–121)

Mass fraction to Mole fraction (same as the gas phase)

(2–122)

(2–123)

is the total liquid volume. is the mixture-averaged density of species and is different from the liquid species density .

Mass fraction to molar concentration

(2–124)

where is the density of the liquid mixture.

(2–125)

Mole fraction to molar concentration

(2–126)