Influence of size and shape effects on the high-pressure solubility of n-alkanes: Experimental data, correlation and prediction☆
Introduction
The (solid + liquid) phase equilibria (SLE) of n-alkanes systems have gained increasing interest in the recent decade. For the petroleum products the mixtures of n-hydrocarbons with branch chain hydrocarbons, or an aromatic hydrocarbons, or cyclic hydrocarbons seems to be very important, especially in wide range of temperature and pressure. Phase equilibrium data of n-alkane systems with these solvents are of importance for the safe and efficient operation of chemical plants. They are necessary for high-pressure polymerization processes and for the design of oil-recovery processes. New high-pressure SLE data of n-alkanes systems offers also new perspectives for modern technologies. Besides its importance for technological processes such as crystallization and purification at high pressure, SLE provides a good tool for examining the thermodynamic nature of many systems. The ultimate aim of this work is to investigate the influence of the solvent – C6 – hydrocarbon structure and chemical character on the solubility of long chain hydrocarbons at normal and high pressure. The conclusions arrived at are based on sets of our solubility data and discussion, published earlier [1], [2], [3], [4], [5] and the subsequent discussion has been deliberately applied to n-alkane mixtures, because of the importance of conformational effects in the alkane thermodynamics. n-Alkanes are highly ordered in the solid state, existing in many crystalline phases which depend on the chain length [6]. In the liquid state they still retain much of their order, as was proved in the literature [7]. Using a light scattering technique it was showed that globular or spherical solvents destroy the conformational order in liquid long-chain hydrocarbons, giving high positive values for the enthalpy and entropy of mixing [8]. According to an early study of Patterson et al. [9], the thermodynamic properties of alkane mixtures are made up of combinatorial, free volume and interactional (field forces and order destruction) contributions, as was shown when they considered the importance of each contribution to the activity coefficients at infinite dilution of many short globular alkanes dissolved in long-chain n-alkanes. Because of the high melting temperatures and low volatilities of long-chain hydrocarbons, heat of mixing or (vapour + liquid) equilibria for these systems are very scarce. Thus, in this work the SLE data have been used to discuss the conformational effects in binary mixtures of hydrocarbons.
From a historical perspective, the very first (solid + liquid) phase equilibria measurements of organic liquids at high-pressure condition were presented by Baranowski and co-workers [10], [11], [12], [13]. Simultaneously, results for different organic mixtures were presented by Nagaoka and Makita [14], [15], [16], Nagaoka et al. [17] and Tanaka and Kawakami [18]. Recently, Yang et al. [19], [20] presented results involving n-alkane mixtures with alcohols. The pressure effect on phase behaviour of binary mixtures of fatty acids up to 200 MPa was measured and well described by Inoue et al. [21].
Modelling of (solid + liquid) equilibrium under high pressures is presented by different authors using the Chain Delta Lattice Parameter Model [22], the Sako-Wu-Prausnitz EOS (SWP), [23], [24] or the van der Waals EOS [25]. Prediction of solid–fluid phase diagrams of light gases-heavy hydrocarbons systems up to 200 MPa using an equation of state, GE model is developed and presented by Pauly et al. [26], [27].
The eutectic mixtures become usually richer in the component with the highest slope dp/dT of the melting curve and the eutectic temperature rises with increasing pressure; also different compositions of the eutectic point are observed [17], [18]. The first results of binary mixtures of n-alkanes were presented for the eutectic mixtures of (n-tetradecane, or n-hexadecane + cyclohexane, or benzene) at high pressures up to 120 MPa by Tanaka and Kawakami [18]. The SLE coexistence curve and the eutectic point shifts to higher temperatures with an increase of pressure. Changing the pressure from (0.1 to 120) MPa increases the temperature of eutectic point for (n-tetradecane, or n-hexadecane + benzene) binary mixtures by ΔT = (25 and 30) K, respectively. For the (n-tetradecane, or n-hexadecane + cyclohexane) it is ΔT = (30 and 35) K. The composition of the eutectic point shifts to higher concentration of the n-alkane [18].
In this paper, the high-pressure SLE results, obtained with piston-cylinder apparatus, described in details in our previous work [4], [5], [28], [29] were presented. New data were measured for (n-hexadecane, or n-octadecane + 3-methylpentane, or 2,2-dimethylbutane, or benzene) and were compared with two additional C6 hydrocarbons: cyclohexane [4] and n-hexane [5], which were presented previously. The data were measured at very high pressures, up to 1.0 GPa, and in wide temperature range, from T = (293 to 363) K.
The experimental data will be described using the predictive method proposed by Pauly et al. [27], corrected for improved description of the phase behaviour at very high pressures. The performance of the modified model was also tested on our data for (n-alkanes + 3-methylpentane, or 2,2-dimethylbutane, or 1-hexyne, or benzene, or n-hexane, or cyclohexane) published previously [4], [5], [28], [29]. The (solid + liquid) coexistence curves of these systems were also correlated by the equation proposed by Yang et al. [19], [20].
Section snippets
Materials
The origins of the chemicals and its purity are: n-hexadecane (International Enzymes Limited, UK, 0.99 mass fraction purity), n-octadecane (Koch-Light Lab., 0.99 mass fraction purity), 3-methylpentane (Aldrich, 0.99 mass fraction purity), 2,2-dimethylbutane (Aldrich, 0.99 mass fraction purity), benzene (Aldrich, 0.99 mass fraction purity). n-Alkanes have been used directly, without purification. The benzene was distilled in a column of 25-plate lab. column; the other solvents were purified by
Results and discussion
(Solid + liquid) phase equilibria at high pressure of the (n-hexadecane, or n-octadecane + 3-methylpentane, or 2,2-dimethylbutane, or benzene) binary systems is a continuation of our previous work, presented earlier [4], [5], [28], [29]. The pressure dependence of the melting temperatures for crystals with simple structures was described with the semi-empirical equation of Simon [33], as we made in our previous works. The behaviour of pure substances can be also purely predicted with very good
Modelling
The condition of equilibrium between the solid and liquid phases is given by the equality of the fugacities in both phases for each individual component, iFor this particular system the solid phase will be a pure compound while the liquid phase is a binary mixture.
The general (solid + liquid) phase equilibrium equation relates, for the crystallizing compound, the fugacities of both phases in the standard state, fi(Po), with the pure component thermophysical properties
Conclusions
(Solid + liquid) phase equilibria for six systems of binary (n-hexadecane, or n-octadecane + 3-methylpentane, or 2,2-dimethylbutane, benzene) mixtures have been investigated under the high pressure up to 1.0 GPa. With increasing pressure the freezing curves shift to the higher temperatures monotonously.
At the pressure up to 400 MPa the solubilies of solute in tested solvents are of the same order with a slightly lower solubility in benzene, especially at lower concentration of n-alkane. These
Acknowledgements
The authors gratefully acknowledge the Warsaw University of Technology for the financial support and Prof. J.A.P. Coutinho for the co-operation and software of the high-pressure prediction.
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Presented at the 18th IUPAC International Conference on Chemical Thermodynamics, August 17–21 2004, Beijing, China.