Estimation of physical properties:

Estimation of physical properties:

Material properties are determined by the internal structure of the molecules and the intermolecular interaction forces.

1. Correlations: to reproduce a number of measured values as accurately as possible, to reliably interpolate between the measured values and to extrapolate well over the range covered by measured values. Many correlations have proven themselves very well.
2. Estimation methods: to predict substance properties with as little preliminary information as possible, e.g. Of course, less value is placed on accuracy; it is much more important that as few systems as possible occur with very high deviations.

Estimation Methods:

All of the calculation methods described are essentially based on the determination of the internal structure of the molecules using group contribution methods and the intermolecular interactions using the extended correspondence principle using critical temperature (Tc), critical pressure (pc) and acentric factor.

The determination of the critical data in particular is associated with a high level of experimental effort, so that these quantities are often not available. Many substances also decompose before the critical temperature is reached.

Estimation of critical temperature, critical pressure and critical volume based on group contribution method:

• Group contribution methods (Joback). Joback’s method only requires the structural formula as starting information.

• The structure group contributions for DeltaT, DeltaP and DeltaV can be found in Table 1. nA is the number of atoms in the molecule.

• Joback does one also need the normal boiling point (TNBP)
• Poling et al. give an average error of 1.1% for the Joback method in relation to the absolute temperature if the standard boiling point is known. Only 1% of the test substances showed an error > 5%.

Estimation of normal boiling point based on group contribution method::

The normal boiling point is one of the most easily accessible material properties and is known for a large number of substances.

In the event that no information is available, the normal boiling point (abbreviated: NBP for normal boiling point) can be calculated according to Joback.

Estimation of melting point and enthalpy based on group contribution method::

The application of group-contribution methods in the estimation of the melting point is severely limited, since information about the molecular symmetry is usually lost during the incrementation. Although Joback gives group contributions for the estimation of melting points, the mean error is more than 20 K even for simple molecules, which is unacceptable for practical applications.

Estimation of standard enthalpy of formation and free standard enthalpy of formation based on group contribution method::

The standard enthalpy of formation of a substance is the enthalpy change that occurs when 1 mole of the substance is formed from its constituent elements in their standard states. A pure element in its standard state has a standard enthalpy of formation of zero.

Standard enthalpy of formation Dh0B and standard free enthalpy of formation Dg0B are important in the calculation of reaction enthalpies and chemical equilibria. They are relevant for thermal engineering calculations insofar as they are decisive for the maximum possible product temperature or the heat to be dissipated by cooling in chemical reactions. The standard state is defined as T = 298.15 K and p = 1 atm in the ideal gas state. Reaction enthalpies can be calculated from standard enthalpies of formation. They usually serve as a reference point for the calculation of enthalpies:

Analogous to the estimation of the critical data, the method of Joback is recommended.

Estimation of liquid density based on equation of state (correlation):

Simple equations of state are only able to do this to a limited extent:

• Liquids are incompressible to a first approximation. If the pressure dependency plays a role in the calculation, a highly accurate equation of state +should be used if possible.
• The density of mixtures should be determined via the linear averaging of the specific volumes:

Estimation of gas density based on equation of state:

• At moderate pressures of up to about 5 bar, the ideal gas law can be used
• For substances at higher pressures, cubic equations of state such as those of Peng-Robinson (PR) or Soave-Redlich-Kwong (SRK) [33] are particularly suitable:

• These equations are a cubic function of volume. They provide three real solutions for the specific volume in the subcritical range at a given temperature and pressure. However, the cubic equations of state are not suitable for calculating liquid densities.