CHARACTERIZING THE PLUS FRACTION FOR EOS' APPLICATIONS

To use any EOS; the Peng‑Robinson EOS as an example, one must be able to provide the critical pressure, the critical temperature, and the acentric factor for each component in the mixture. Nearly all naturally occurring petroleum fluids contain a quantity of heavy fractions that are not well defined. These heavy fractions are often lumped together as heptanes‑plus fraction. The problem of how to adequately characterize the C7+ fractions in terms of their critical properties and acentric factors has been long recognized in the petroleum industry. Changing the characterization of C7+ fractions present in even small amounts can have a profound effect on the PVT properties and the phase Equilibria of a hydrocarbon system as predicted by the Peng and Robinson equation of state. Recognizing that the inadequacy of the predictive capability of the PR EOS lies with the improper procedure for calculating the parameters a, b, and C_ of the equation for the C7+ fraction; this paper presents an approach for determining these parameters from the following two readily measured physical properties of C7+:

‑ molecular weight M7+

‑ specific gravity C_7+

The approach is based on generating 49 density values for the C7+ by applying the Riazi and Daubert correlation. These values were subsequently subjected to 10 temperatures and 10 pressure values in the range of 60aF‑300aF and 14.7‑7,000 psia, respectively. The Peng‑Robinson EOS was then applied to match the 4,900 generated density values by optimizing the parameters a, b, and C_ using a non‑linear regression model. The optimized parameters for the heptanes‑plus fraction are given by mathematical expressions. The paper presents results of applying the proposed EOS; without tuning, to numerous experimental data to illustrate the capability of the modified EOS and to demonstrate its accuracy.