Chemical Applications
Non-ionic Systems

These systems, which typically contain oxygen, nitrogen, or halo-
gen derivatives of hydrocarbons such as amides, esters, or ethers,
are also similar to non-hydrocarbon systems found in petrochemical
applications. For low pressure systems, a liquid activity coefficient
method is recommended. For single liquid phase systems, the WIL
SON, NRTL, or UNIQUAC methods are equally good, provided all
interaction parameters are provided. PRO/II databanks contain
extensive parameters for NRTL and UNIQUAC, but the user must
supply interaction data for the WILSON method. The WILSON
method is the simplest, and requires the least CPU time.
For systems with two liquid phases, the NRTL or UNIQUAC meth-
ods should be used, provided that at least some interaction data is
available. The FILL option can be used to fill in any missing inter-
action data using the UNIFAC method. If no interaction data are
available, the UNIFAC method should be used since the PRO/II
databanks contain a large amount of group interaction data for both
VLE and LLE applications. For moderate pressure systems up to 10
atmospheres, a liquid activity method can still be used, provided
that the interaction parameters used are still valid in that pressure
range. For example, if the system pressure were much higher than
the pressure at which the interaction parameters were regressed, the
vapor phase fugacity may be taken into account in modeling the
phase behavior. If the PHI option is selected, the liquid-phase
Poynting correction factor is automatically selected also.
It is also important to note that all the interaction parameters in the
PRO/II databanks, except for dimerizing components such as car
boxylic acids, were regressed without including any vapor-phase
nonideality. This means that the PHI option should be used for car
boxylic acid systems at all pressures, but should only be used for
most components at high pressures. For systems containing compo
nents such as carboxylic acids that dimerize in the vapor phase, the
Hayden-O'Connell fugacity method may be used to calculate all
vapor-phase properties such as fugacity, enthalpy, and density. For
components such as hydrogen fluoride which forms hexamers in the
vapor phase, PRO/II contains an equation of state specially created
for such systems, HEXAMER. This method is recommended for
processes such as HF alkylation or the manufacture of refrigerants
such as HFC-134a. For all other components, an equation-of-state
method such as SRK or PR may be used to calculate vapor-phase
fugacities.
When supercritical gases are present in small quantities (generally
less than 5 mole %),  the Henry's Law option should be used to
compute gas solubilities. For high pressure systems, greater than 10
atmospheres, or for systems with large quantities of supercritical
gas, an equation-of-state method using an advanced mixing rule
such as SRKM or PRM should be used. The UNIWAALS equation-
of-state method uses UNIFAC structure information to predict
phase behavior. This method is useful when interaction data are not
avaialable and, unlike a liquid activity method such as UNIFAC, is
able to handle supercritical gases.

Table 3-12: Methods Recommended for Non-ionic Chemical Systems

WILSON
Recommended for single liquid phase slightly nonideal
mixtures. If all interaction data are not available use the
FILL=UNIFAC option.

NRTL/ UNIQUAC
Recommended for all nonideal mixtures. Use with the
FILL option when binary interaction parameters are not
available or with the HENRY option when <5 mole %
supercritical gases are present. For moderate
pressures use the PHI option for vapor phase
nonidealities.

SRKS/SRKM/PRM/UNIWAALS
Recommended for high pressure systems or when  > 5 mole % supercritical gases are present.

HOCV
Recommended for vapor fugacity and enthalpy and
density calculations in systems containing dimerizing
components such as carboxylic acids. Use with a
liquid activity method .

HEXAMER
Recommended for systems containing hexamerizing
components such as HF.

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