A Generalized Correlation for Characterizing the Hydrocarbon Heavy Fractions

Abstract

Abstract In the absence of detailed analytical data for the heptanes-plus fractions in a hydrocarbon system, major problems arise in predicting the thermodynamic behavior of problems arise in predicting the thermodynamic behavior of the system by equations of state. These problems result because of the difficulty in characterizing the heavy-plus fractions. However, they can be reduced by splitting the plus fractions into pseudocomponents. This paper presents a concise and accurate method for extending the molar distributions of the heavy fractions of naturally occurring hydrocarbon systems. This method originated from studying the molar behavior of thirty four hydrocarbon systems with a detailed Laboratory compositional analyses of the heavy fractions. The proposed correlation will enhance the predictive capabilities of equations of state when applied to naturally occurring hydrocarbon systems. A FORTRAN subroutine to perform the computations of the proposed method is included. proposed method is included Introduction A proper description of the physical properties of the heavy (lumped) hydrocarbon fractions is essential to adequately predict the thermodynamics and behavior of complex predict the thermodynamics and behavior of complex hydrocarbon mixtures by equations of state. These fractions, usually lumped as heptanes-plus (C7+), are difficult to characterize without an extended laboratory analysis. Several authors have shown that erroneous predictions and conclusions can result if the plus fraction is used directly as a single component in equations of state calculations. Katz and Firoozabadi demonstrated the use of the extended analysis of C7+ with the Peng-Robinson equation of state by accurately predicting the behavior of crude oil and gas condensate mixtures. Lohrenz et al proposed that the heptane- plus fraction could be divided into a mixture of normal Paraffin hydrocarbon from C7 to C40. They mathematically stated that the mole fraction of each hydrocarbon component, Z n, is determined as follows: .................(1) where n ranges from 7 to 40. The constants A and B are determined such that ...............(2) and ..................(3) Pedersen et al suggested that for naturally occurring Pedersen et al suggested that for naturally occurring hydrocarbon mixtures, an exponential relationship exists between the mole fraction, Zn, and the corresponding carbon number number, Cn, for C n 7. They expressed this relationship in the following form: .........................(4)