(74r) Characterization of Critical Properties and Acentric Factors for High-Molecular-Weight-Fluids, Petroleum Fractions, Coal and Natural Products

Characterizing methods reported by Walas (1985), Poling et al. (2001) and Ihmels-Ambrose (2010) for pure substances are based on structural formula and therefore not suitable for pseudo-component properties (or poorly characterized mixtures encountered with petroleum, coal and natural products, which are usually based on measured properties such as average molecular weight, boiling point and specific gravity (at 20^oC)¹ rather than molecular structure). The Paraffinic-Naphthenic-Aromatic (PNA) reported by Peng- Robinson (1978) or the group contribution methods by Lydersen et al. (1955) (or the methods based on carbon number reported by Whitson (1983) and Panuganti-Chapman (2013) are not considered in this report because the intention here is not to gain molecular insights into crude-oil properties but rather to treat pseudo-component properties as single-lumped component for the sole purpose of modeling by using cubic equations of state. Also, the trusted Database as tabulated by the NIST Groups for the critical properties and acentric factors do not include n-tetracontane (nC₄₀H₈₂), even though specific volumes at several temperatures and pressures for n-tetracontane have been reported by Arthur Doolittle since 1964 (J. Chem. Eng. Data, 9 (2), 275, 1964).

Consequently, thirty-six methods of characterizing critical properties and acentric factors for the parameters of cubic equations of state used in the process and reservoir simulation are evaluated for ease of implementation and use for high-molecular-weight-fluids, petroleum fractions, coal tar liquids and natural products. Analyses of results shows the methods are not adequate for poorly characterized pseudo-components of high aromatic, naphthenic and cycloalkanes; and beyond n-Octadecane hydrocarbons, some critical property parameters (critical pressure and volume, critical-Z factor and acentric factor: which is greater than 1 and therefore, is physically meaningless) are usually based or trial-and trial error techniques: thus, lacking consistency with lighter hydrocarbons in the petroleum fractions assay. While most of the estimation methods provide satisfactory predictions of the critical temperatures, they all underestimated critical pressures of the pseudo-components.

By using the API Database as reported by the University of Pennsylvania Group and gathering data from Hydrocarbon Processing and other relevant sources (theses and dissertations reports), we developed a Paraffinic-Naphthenic-Aromatic Characterization Factor into the correlation for critical temperature in terms of average molecular weight (Mw), boiling point (T_b) and specific gravity (S_g) and we use several cross-plotting of the ratio of boiling point to critical temperature (T_b/T_c) versus molecular weight, critical properties (P_c, V_c) and acentric factor (ω) to generate high-correlation coefficients; correlating groups, such as (T_b/T_c), (T_c/P_c), (V_c/Z_c), (M_w/P_c), (M_w/T_c) and Z_c versus molecular weight or acentric factor show a high-correlation coefficients. While we claim no theoretical basis (apart from the high-correlation coefficients and the statistically derived predictive groups), the principles of dimensional similitude provide the convenient basis in the selected groupings. The grouping parameters reduce the number of tailored-fitting parameters and thus preclude the need for extensive experimentally measured data for the unstable petroleum fractions and pseudo-components.

Hundred petroleum fractions reported by Roess (1936), Lenoir-Hipkin enthalpy (1973) and petroleum fraction data are used to validate the pseudo-component critical properties developed in this reported. In comparison with available correlative methods, the current approach is better in accuracy and ease of implementation in cubic equations of state modeling. The current approach is also validated for predicting viscosity of coal liquids (Baltatu, 1985; 1995) as well as predicting densities of natural products.  While the current approach may not be convenient for the type of work in the petroleum refining operations, the methodology is superior for empirical characterization of pseudo-component critical properties and acentric factors as lumped-single component and it thus offers another technique to the traditional pseudo-component, or hypothetical-component modeling approach for process and reservoir simulation models. It is therefore recommended for high-molecular-weight components for which experimental critical properties are typically unavailable.

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