Download COSMOtherm User Manual

and the total enthalpy of sublimation are written to additional columns of the p VAP vs. T table in the
COSMOtherm table file.
The pvap=pressure [mbar] or pvap_SI=pressure [kPa] options allow for the iterative optimization of
the temperature for a given vapor pressure: The temperature of the system is varied until difference of the
COSMOtherm prediction of the total vapor pressure and the given value of “pressure” is below a certain
accuracy threshold. Thus the iterative pvap=pressure option allows the automatic calculation of a
compounds boiling point at a given pressure. The default value for the accuracy threshold of the vapor
pressure prediction is 10-4 mbar. It is possible to change this value using the thresh_pvap=p [mbar] or
thresh_pvap_SI=p [kPa] options. Alternatively it is possible to use a relative accuracy threshold
thresh_pp=pp [%], which sets the accuracy of the iterative pressure optimization to pp percent of the
given reference pressure. During the optimization of the temperature, for each temperature and for
each compound in the mixture the partial vapor pressures, the chemical potential of the compound in the
gas phase and its enthalpy of vaporization are written to the COSMOtherm output file. If convergence is
reached, i.e. if the required accuracy threshold is met, the total vapor pressure of the mixture is written to
the COSMOtherm table file in tabulated form pVAP vs. T. In addition, the total chemical potentials of the
liquid LIQUID(tot) and of the gas phase GAS(tot), as well as the heat of vaporization of the mixture HVAP are
written to the COSMOtherm table file.
A characteristic trait of COSMOtherm’s vapor pressure prediction is that the absolute quantitative
prediction of the vapor pressure at a given temperature point shows a certain error, but that the
temperature dependency of the vapor pressure, i.e. the qualitative shape of the pVAP(T) curve is predicted
very well. Thus it is possible to predict a pVAP(T) vapor pressure curve to a very high accuracy if the predicted
vapor pressure is adjusted to match a given experimental reference pressure pRef at a given reference
temperature TRef. In practice, the pressure difference between the COSMOtherm vapor pressure prediction
pSi(TRef) and the given reference pressure pRef(TRef) is used to determine a correction term to chemical
potential in the gas phase iGas,corr, which subsequently is used to scale the vapor pressure prediction,
yielding a corrected vapor pressure value piS,corr / 1 bar =exp [iGasiGas,corriS )/ RT ], which replaces the
regular prediction value piS. COSMOtherm offers several possibilities to include a reference pressure and
temperature into the pvap prediction option: it is possible to use a reference temperature TRef and pressure
pRef pair, which can be given in the mixture input line together with the pvap option. Input of a reference
temperature TRef is possible via keywords use_tref, use_tref_C, use_tref_K, or use_tref_F,
processing reference temperature values in units degree Celsius (for the first two keywords), Kelvin, and
Fahrenheit, respectively. Input of reference pressure pRef is possible via keywords use_pref, use_pref_Pa,
use_pref_kPa, use_pref_bar, or use_pref_psia, processing reference pressure values in units millibar,
Pascal, kiloPascal, bar, and psia, respectively. Further, for pure compound and mixture vapor pressure
predictions, there is the possibility to use the compound’s experimental normal boiling point temperature
as reference point(s). The compound’s boiling point temperature, as read from the compound input line, or
from the vapor pressure property file (see section 2.2) is used as reference temperature TRef = TBoil assuming
normal boiling point conditions, thus the reference pressure used is pRef(TRef) = 1 atm = 1013.25 mbar. The
usage of pure compound boiling points in a pure compounds vapor pressure prediction is toggled by the
keyword use_tboil, which either can be given in the pvap mixture line, where it is active for this mixture
only, or in the global input lines, where it will be active for all pvap computations given. The adjustment of
the vapor pressure prediction with the given boiling point data is possible for pure compound vapor
pressure computations as well as for mixtures. In the latter case COSMOtherm will perform a number of
additional computation steps in which the vapor pressures of the individual compounds are adjusted to
their given boiling points data. Of course, this adjustment is only possible if an experimental boiling point
temperature was provided for the compound, either in the compound input line, or, in its vapor pressure
property file. In addition to the use of boling points as reference pressures COSMOtherm the possibility to
use the compound’s experimental vapor pressure as reference point. The compound’s temperature
74