Blend Prediction Model for the Freeze Point of Jet Fuel Range Hydrocarbons

It has long been
understood that phase changes are one way to accomplish
separations of mixtures, and for liquid mixtures, the process of freezing
followed by a thaw results in a high degree of purification for the
final component in the mixture to melt. This thaw, into a liquid mixture,
occurs at a lower temperature than the freeze/melting point of the
highest-freeze-point component. As such, the freeze point is dependent
on the mixture, and therefore, it is generally unknown for arbitrary
mixtures along with associated phase change properties such as the
enthalpy and entropy of fusion. In this work, forty-one freeze points
of mixtures containing either bicyclohexyl or n-tridecane
are measured and discussed. These molecules, neat have freeze points
of 6.4 and −3.6 °C, respectively, which are well above
the allowable maximum freeze point of jet-a fuel, but in lower concentration,
these molecules may help to achieve compliance with all the requirements
of ASTM D7566 fuel specification for sustainable aviation fuel. A
predictive freeze point model for hydrocarbon mixtures is developed
here using a Hess cycle, 1st law, and 2nd law analysis to determine
the enthalpy and entropy of fusion, and thus, the freeze point of
the mixture through the Gibbs free energy equilibrium condition for
the solid–liquid phase changes. The first-principle-developed
model is validated against the experimental freeze point measurements.
It captures the nonlinearity with the mole fraction of the highest-freeze-point
component (bicyclohexyl or n-tridecane) and provides
a conservative estimate for the freeze point at mole fractions between
0.065 and 0.25where the freeze point varies considerably with
changes in mole fraction.