A New Thermodynamic Model for Paraffin Precipitation in Highly Asymmetric Systems at High Pressure Conditions

Abstract

The predictions of the crystallization temperature and the amount of precipitates of paraffin waxes at high pressure conditions may be inaccurate using existing thermodynamic models. This is mainly due to the lack of experimental data on the molar volume of solid paraffins at high pressures. This inaccuracy is even more pronounced for mixtures of high asymmetry. The present work provides a new accurate modeling approach for solid–fluid equilibrium (SFE) at high pressure conditions, more specifically, for highly asymmetric systems. In contrast to the conventional methods for high pressure SFE modeling which define a Poynting molar volume correction term to calculate the paraffin solid phase nonideality at high pressures, the new method exploits the values of thermophysical properties of importance in SFE modeling (temperatures and enthalpies of fusion and solid–solid transition) evaluated at the high pressure condition using a new insight into the well-known Clausius–Clapeyron equation. These modified parameters are then used for evaluation of the fugacity in the solid phase at higher pressure using the fugacity of pure liquid at the same pressure and applying the well-established formulation of the Gibbs energy change during melting. Therefore, the devised approach does not require a Poynting correction term. The devised approach coupled with the well-tested UNIQUAC activity coefficient model is used to describe the nonideality of the solid phase. For the fluid phases, the fugacities are obtained with the SRK EoS with binary interaction parameters calculated with a group contribution scheme. The model is applied to highly asymmetric systems with SFE experimental data over a wide range of pressures. It is first used to predict crystallization temperature in binary systems at high pressures and then verified by applying it on multicomponent mixtures resembling intermediate oil and natural gas condensates.

Phase boundaries for synthetic light oils and gas condensate mixtures using predictive equations of state with a solid solution model

“…The knowledge of wax formation phenomena has been improved both experimentally and theoretically . While literature is rich in reporting wax appearance temperature (WAT), scientists argue that wax disappearance temperature (WDT) is closer to the wax equilibrium condition .…”

“…A range of values from 0.85 to 0.92 can be deemed for a better model accuracy. Nevertheless, Coutinho's seminal work paved the way for other investigators, exemplified by Zuo et al, Ji et al, Ghanaei et al, Nasrifar and Fani Kheshti, Ameri Mahabadian et al, Shaahmadi et al, Yang et al and Bagherinia et al Ameri Mahabadian et al further extended the calculations to the systems prone to gas hydrates formation using cubic plus association (CPA) EoS …”

Development and application of computational algorithms and thermodynamic models for multiphase, multicomponent chemical systems

“…Most research studies that were published after Pauly et al [104] and dealt with the thermodynamic modeling of wax formation at high pressures aimed at developing a more accurate representation of the Poynting correction [108][109][110][111]. A comprehensive review and analysis of the different modifications applied to mixtures of n-alkanes is presented by Ghanaei et al [111] and Ameri-Mahabadian et al [112]. The referenced research studies evaluated their high pressure models against SLE experimental data of binary, ternary and multicomponent mixtures of paraffins.…”

“…To this end, Ji et al [114] proposed a solid-phase model, in which the reference state pressure is the actual pressure of the system, but only the melting temperature is calculated at the working conditions, with the use of a correlation the authors proposed (linear extrapolation based on the slope of the melting line of pure paraffins). Ameri-Mahabadian et al [112] proposed two new solid-phase models, based on calculating the solid-phase fugacity at high pressures. The first one is a modification of the Ji et al model, in which except for the melting temperature, the SS transition temperature is also calculated at the pressure of the system, using a similar linear extrapolation scheme.…”

“…As a result, the use of a different fluid-phase EoS necessitates refitting of these parameters. Ameri-Mahabadian et al [112] used the SRK EoS combined with the group contribution scheme for evaluating temperature-dependent parameters, as presented by Jaubert and Privat [115].…”