Improved enthalpy prediction of hydrocarbon fractions with a modified alpha function for the Peng-Robinson equation of state

In this paper, a pragmatic technique has been developed and validated to predict ideal gas heat capacities and enthalpies for normal alkanes/alkenes as well as hydrocarbon fractions over a wide range of temperatures and pressures. More specifically, an existing correlation has been modified to more accurately determine the ideal gas heat capacities for normal alkanes/alkenes, especially for heavy hydrocarbons. Then, a reliable and convenient enthalpy algorithm for hydrocarbon fractions has been proposed by incorporating a recently modified alpha function into the Peng-Robison equation of state (PR-EOS) together with the newly modified ideal gas heat capacity correlation. As for the new database of ideal gas heat capacities with 340 data points of normal alkanes, the newly modified correlation leads to more accurate predictions, yielding an absolute average relative deviation (AARD) of 1.24% and a maximum absolute relative deviation (MARD) of 6.21% compared with 6.96% and 10.69% from the Kesler and Lee correlation, respectively. To further validate the newly developed correlation, another database of 279 data points of normal alkenes that is excluded for correlation development shows an AARD of 2.04% and a MARD of 2.58% in comparison of 8.44% and 10.57% from the Kesler and Lee correlation, respectively. Also, an algorithm has been proposed to predict the enthalpy with AARDs of 1.25% and 2.78% and MARDs of 2.06% and 3.51% for two representative hydrocarbon fractions (i.e., the light naphtha and Alaska naphtha), respectively.