Heat Capacities and Enthalpies of Normal Alkanes in an Ideal Gas State

Nikolai Kuznetsov,Sergey Frolov

doi:10.3390/en14092641

Abstract

The reference literature contains data on the ideal gas-phase heat capacities and enthalpies of substances obtained experimentally. In this brief report, the highly accurate and simple unified analytical dependences of the heat capacity and enthalpy of normal alkanes larger than propane in the ideal gas state on temperature and the number of carbon atoms in a molecule are derived based on the analysis of the structure of chemical groups in the molecules and on the reference values of heat capacity and enthalpy for any two selected normal alkanes at one temperature. The dependences include a single set of coefficients for all normal alkanes and can be applied even to normal alkanes with the number of carbon atoms larger than 20, for which there is no data available. The dependences are thoroughly checked against two different sets of experimental data and two different sets of computational data obtained by low- and high-level theoretical methods: PM6 and G4. The error in calculating the heat capacity and enthalpy in the temperature range 300–1500 K is about 0.1% or less, which is comparable with the error of reference experimental data. Due to the generality of these dependences, one can treat the arising deviations as the indication of possible errors in the sets of reference data used for analysis. Such dependences are useful for estimating the heat capacities and enthalpies of vapors of large n-alkanes with scarce or lacking data. In addition, they can be readily used in computational fluid dynamics applications and provide the accurate interpolation/extrapolation of the tabulated experimental data without using semi-empirical methods of chemical thermodynamics and high-order polynomials.

Highlights

The real gas enthalpy H ( T, P) is equal to the sum of the ideal gas enthalpy H 0 ( T )and the residual enthalpy: Publisher’s Note: MDPI stays neutral Hres ( T, P) =with regard to jurisdictional claims in Z P ∂V V−T ∂T dP, (1)P published maps and institutional affiliations.where T is the temperature, V is the volume, and P is the pressure
H 0 ( T ) for large n-alkanes, Cn H2n+2, with different numbers of carbon atoms, n, in the molecule. Knowledge of this dependence allows accurate estimates of the heat capacities and enthalpies of vapors of large n-alkanes with scarce or lacking data, providing the accurate interpolation/extrapolation of available experimental data without applying various semiempirical methods. They can be readily used in computational fluid dynamics (CFD) applications instead of high-order polynomials with specific sets of coefficients for each compound
The dependences include a single set of coefficients for all normal alkanes and can be applied even to normal alkanes with the number of carbon atoms n > 20, for which there is no data available

Summary

Introduction

The real gas enthalpy H ( T, P) is equal to the sum of the ideal gas enthalpy H 0 ( T ). H 0 ( T ) for large n-alkanes, Cn H2n+2 , with different numbers of carbon atoms, n, in the molecule Knowledge of this dependence allows accurate estimates of the heat capacities and enthalpies of vapors of large n-alkanes with scarce or lacking data, providing the accurate interpolation/extrapolation of available experimental data without applying various semiempirical methods. They can be readily used in CFD applications instead of high-order polynomials with specific sets of coefficients for each compound

Heat Capacity

Enthalpy

Findings

Conclusions