Investigation of the surface tension of methane and n-alkane mixtures by perturbed-chain statistical associating fluid theory combined with density-gradient theory

The perturbed-chain statistical associating fluid theory (PC-SAFT) and density-gradient theory are used to construct an equation of state to describe the phase behavior of binary methane–n-alkane mixtures. With the molecular parameters and influence parameters regressed from bulk properties and surface tensions of pure fluids, respectively as input, both the bulk and interfacial properties are investigated. The surface tension of the binary systems methane–propane, methane–pentane, methane–heptane and methane–decane are predicted, and the results are satisfactory compared with the experimental data. Our results show that PC-SAFT combined with density-gradient theory is able to describe the interfacial properties of binary methane–n-alkane mixtures in wide temperature and pressure ranges, and illustrate the influence of the equilibrium bulk properties and chain length of n-alkane molecule on the interfacial properties.     

Viscosity of heavy n-alkanes and diffusion of gases therein based on molecular dynamics simulations and empirical correlations

The viscosity of pure n-alkanes and n-alkane mixtures was studied by molecular dynamics (MD) simulations using the Green–Kubo method. n-Alkane molecules were modeled based on the Transferable Potential for Phase Equilibria (TraPPE) united atom force field. MD simulations at constant number of molecules or particles, volume and temperature (NVT) were performed for n-C₈ up to n-C₉₆ at different temperatures as well as for binary and six-component n-alkane mixtures which are considered as prototypes for the hydrocarbon wax produced during the Gas-To-Liquid (GTL) Fischer–Tropsch process. For the pure n-alkanes, good agreement between our simulated viscosities and existing experimental data was observed. In the case of the n-alkane mixtures, the composition dependence of viscosity was examined. The simulated viscosity results were compared with literature empirical correlations. Moreover, a new macroscopic empirical correlation for the calculation of self-diffusion coefficients of hydrogen, carbon monoxide, and water in n-alkanes and mixtures of n-alkanes was developed by combining viscosity and self-diffusion coefficient values in n-alkanes. The correlation was compared with the simulation data and an average absolute deviation (AAD) of 11.3% for pure n-alkanes and 14.3% for n-alkane mixtures was obtained.     

Experimental measurements and prediction of liquid densities for n-alkane mixtures

We present experimental liquid densities for n-pentane, n-hexane and n-heptane and their binary mixtures from (273.15 to 363.15) K over the entire composition range (for the mixtures) at atmospheric pressure. A vibrating tube densimeter produces the experimental densities. Also, we present a generalized correlation to predict the liquid densities of n-alkanes and their mixtures. We have combined the principle of congruence with the Tait equation to obtain an equation that uses as variables: temperature, pressure and the equivalent carbon number of the mixture. Also, we present a generalized correlation for the atmospheric liquid densities of n-alkanes. The average absolute percentage deviation of this equation from the literature experimental density values is 0.26%. The Tait equation has an average percentage deviation of 0.15% from experimental density measurements.     

One parameter friction theory models for viscosity

In this work the friction theory (f-theory) for viscosity modeling is used in conjunction with the SRK, PR and PRSV cubic equations of state in order to develop three one parameter general models for viscosity prediction. The models are considered one parameter models because they only require a characteristic critical velocity, which is a parameter normally not tabulated. The models use these rather simple cubic equations of state as a basis to obtain accurate modeling of the viscosity of fluids for wide ranges of temperature and pressure. The general models presented in this work are based on the viscosity behavior of n-alkanes from methane to n-octadecane. Although best performance is obtained for the considered n-alkanes, a good model performance is also obtained for other systems. Thus, recommended characteristic critical viscosity values for several systems are also reported in this work. However, in the case of n-alkanes, an empirical equation for the characteristic critical viscosity is provided so that no additional parameters are required. In addition, with the use of simple mixing rules, the viscosity of several binary to quaternary n-alkane mixtures can also be predicted with a satisfactory accuracy.     

Prediction of activity coefficients and Henry's constants at infinite dilution for mixtures of n-alkanes

Trejo Rodríguez, A. and Patterson, D., 1984. Prediction of activity coefficients and Henry's constants at infinite dilution for mixtures of n-alkanes. Fluid Phase Equilibria, 17: 265–279.The corresponding-states principle (CSP) for chain molecules has been used to predict activity coefficients γ_i^∞ at infinite dilution and Henry's constants H_i,j for mixtures of n-alkanes. The mixtures for which predictions of γ_i^∞ are made include n-C₄ through n-C₁₀ as the solute in several pure higher n-alkanes, at several temperatures from 30 to 90°C. Predictions of H_i,j are made for mixtures where the solute is C₂ through n-C₈ and the solvent any n-alkane from n-C₄ through n-C₂₂, also at several temperatures, from –15 to 177°C. The predicted values are compared with experimental data from the literature, and in all cases the agreement is remarkably good. The temperature dependence of H_i,j for some mixtures is used to derive heats of solution ΔH_s, and comparison is again carried out with available experimental values.     

Signal processing of GC-MS data of complex environmental samples: characterization of homologous series

Identification and characterization of homologous series by GC-MS analysis provide very relevant information on organic compounds in complex mixtures. A chemometric approach, based on the study of the autocovariance function, EACVF_tot, is described as a suitable tool for extracting molecular-structural information from the GC signal, in particular for identifying the presence of homologous series and quantifying the number of their terms. A data pre-processing procedure is introduced to transform the time axis in order to display a strictly homogenous retention pattern: n-alkanes are used as external standard to stretch or shrink the original chromatogram in order to build up a linear GC retention scale. This addition can be regarded as a further step in the direction of a signal processing procedure for achieving a systematic characterization of complex mixture from experimental chromatograms. The EACVF_tot was computed on the linearized chromatogram: if the sample presents terms of homologous series, the EACVF_tot plot shows well-defined deterministic peaks at repeated constant interdistances. By comparison with standard references, the presence of such peaks is diagnostic for the presence of the ordered series, their position can be related to the chemical structure of the compounds, their height is the basis for estimating the number of terms in the series. The power of the procedure can be magnified by studying SIM chromatograms acquired at specific m/z values characteristic of the compounds of interest: the EACVF_tot on these selective signals makes it possible to confirm the results obtained from an unknown mixture and check their reliability.The procedure was validated on standard mixtures of known composition and applied to an unknown gas oil sample. In particular, the paper focuses on the study of two specific classes of compounds: n-alkanes and oxygen-containing compounds, since their identification provides information useful for characterizing the chemical composition of many samples of different origin. The robustness of the method was tested in experimental chromatograms obtained under unfavorable conditions: chromatograms acquired in non-optimal temperature program conditions and chromatographic data affected by signal noise.     

Second-order thermodynamic derivative properties of selected mixtures by the soft-SAFT equation of state

We have discussed the capability of the soft-SAFT equation of state (EoS) to predict second order thermodynamic derivative properties of pure fluids in a recent paper [F. Llovell, L.F. Vega, J. Phys. Chem. B 110 (2006) 11427–11437]. The goal of this work is to extend these calculations to selected binary mixtures. The equation was applied in a semi-predictive manner: the pure component molecular parameters needed to apply soft-SAFT to experimental systems were obtained by fitting vapor–liquid equilibrium data and used, without further fitting, to calculate isochoric and isobaric heat capacities of selected alkane + n-alkane and n-alkane + 1-alkanol binary mixtures; isentropic compressibility coefficients and the speed of sound of selected n-alkane + 1-alkanol mixtures were calculated following the same procedure. We have used the crossover soft-SAFT equation which explicitly incorporates a renormalization group term in order to take into account the long range fluctuations appearing in the near critical region. Soft-SAFT was able to capture the qualitative behavior of the mixture properties studied, for a wide range of conditions, showing quantitative agreement with experimental data in some of the cases. As a further test to the equation, we have also calculated excess properties. The equation was able to capture the non-ideal behavior upon mixing experienced by these properties. This work shows the robustness of the molecular parameters and the equation to calculate properties not included in the fitting procedure, in a predictive manner.     

Selective Separation of Oxy-PAH from n-Alkanes and PAH in Complex Organic Mixtures Extracted from Airborne PM2.5

A fast and simple fractionation method was optimized to selectively separate oxy-PAH from polycyclic aromatic hydrocarbons (PAH) and n-alkanes contained in solvent extracted organic matter (SEOM) from atmospheric particles with an aerodynamic diameter ≤2.5 μm (PM_2.5). Samples were collected in Mexico City. Multivariate parameters were adjusted on a standard mixture, and on SEOM spiked with pure standard mixture solutions: type and amount of phase; packing densities; type, proportion and amount of solvents, and elution flow rates were tested under several elution schemes. Cyanopropylsilyl-bonded phase material was the selected stationary phase. The separation method was applied to real samples of SEOM (2.6, 5.6 and 8.5 mg) spiked with n-alkanes, PAH and oxy-PAH. n-Alkanes overlapped with PAH due to an excess of n-alkanes in real samples overloading the capacity of the stationary phase. Oxy-PAH was separated totally from n-alkanes and PAH. Mean recoveries ± confidence intervals (95%) for n-alkanes ranged from 53 ± 17% (n-tetracontane) to 101 ± 11% (n-hexacosane); for PAH from 58 ± 5% (phenanthrene) to 85 ± 9% (benzo[k]fluoranthene); and for oxy-PAH from 68 ± 12% (9,10-dihydrobenzo[a]pyren-7(8H)one) to 108 ± 9% (1,2-benzopyrone). This method is an efficient fractionation procedure to be applied to oxy-PAH, PAH and n-alkanes in complex organic mixtures extracted from PM_2.5.     

Correlation of the molar volumes of n-alkanes and their mixtures

Molar volume data are analyzed for n-alkanes and their mixtures over the range 10 to 126°C and n = 6 to n = 62. At a given temperature the logarithm of the molar volume of pure n-alkane liquids divided by the carbon number is equal to a constant plus a term proportional to the reciprocal of the carbon number. For solutions, deviations from the Brønsted principle of congruence are small, and for any given solution the deviations are proprotional to the product of the volume fractions of the individual components.     

Vapour pressure measurements and prediction for heavy esters

In order to describe the phase behaviour of drilling fluids with gases in down hole conditions, the pure component properties must be well known. The new formulations are often based on heavy esters. This paper focuses on the experimental measurements of the vapour pressure of a number of selected esters. The results are similar to those of n -alkanes with identical molecular weight.Using the results collected, several predictive methods are tested in order to evaluate what method is most adapted to the type of components used.It appears that the best method of all is that proposed by Jensen et al. [7] If a cubic equation is used with the Somayajulu method for predicting the critical parameters, acceptable results are obtained as well.     

Correlation for prediction of interfacial tensions of pure alkane,naphthenic and aromatic compounds between their freezing and critical points

A simple equation has been developed for predicting the interfacial tensions of pure alkanes, aliphatic and aromatic hydrocarbons between their freezing and critical points. The equation has the form

and represents the interfacial tension using a generalized correlation developed by Sivaraman et al. (1984) for predicting the latent heats of vaporization of normal fluids and coal-liquid model compounds. Here σ^* is the reduced dimensionless interfacial tension, and L^*₍₀₎ and L^*₍₁₎ are the reduced dimensionless latent heats of vaporization; A and N are system-independent constants. Subsequent tests of this correlation in predicting interfacial tension over a broad domain of reduced temperatures for a large number of different types of compounds have confirmed the validity of our approach. The percentage deviations in interfacial tension are in the range? 3.87 to 5.99 in the range of reduced temperatures 0.03 < ? = (T_c ? T)/T_c < 0.55.     

A novel approach to the modelling of the fluid multiphase behaviour of ternary mixtures of carbon dioxide + n-alkane + 1-alkanol

In this work, modelling of the phase behaviour in ternary systems composed of CO₂ + n-alkanes + 1-alkanols is described. The model assumes that the ternary system is congruent to binary systems composed of CO₂ + n-alkanes and that the phase behaviour of the ternary system can be related to the average molecular size of the solute molecules. The average molecular size of the solute molecules is calculated taking into account alkanol aggregation. Although some crude assumptions have been made, the model is able to describe experimental results qualitatively. Values for model parameters like the degree of aggregation and the equilibrium constant of the aggregation reaction correspond very well to values for these parameters obtained from IR-spectroscopy.     

Development of a urea-adduct process for recovery of n-alkane based diluent from spent PUREX/UREX solvent

Urea-adduct process is commercially used to selectively separate n-alkanes from industrial hydrocarbon mixtures. Authors have explored application of this method for recovery of n-alkane based diluents from spent PUREX/UREX solvent. Traditionally this separation is performed by vacuum distillation, an energy-intensive process. The proposed method is simple and does not involve either exotic chemicals or complex processing steps. Application of urea-adduct process for recovery of diluent from spent solvent is reported here possibly first time in literature. Physical properties such as densities, viscosities and vapour pressure for irradiated organic solutions were also measured and reported.     

Prediction of molecular weight and density of n-alkanes (C6-C44)

Heavy n-alkanes and their mixtures were characterized by high temperature-simulated distillation using gas chromatography with a capillary column. In this work, the atmospheric boiling point is determined by the HT-SimDis GC method. In this study, molecular weights and density of n-alkanes were evaluated with this method by using retention times and normal boiling points as input data. ASTM D2887 calibration mixture containing 17 n-alkanes in the C₆-C₄₄ range were used for qualitative analyses. Retention times (t_R) of n-alkanes were measured with this method. The other input data that normal boiling points (T_b) and molecular weight (M) had been taken in the literature. Experimental densities (at 20 °C) of n-alkanes were obtained from API Research Projects. Empirical molecular weight and density correlations were developed by using the nonlinear and multiple regressions with correlation coefficients. The results of calculations were compared with experimental data. Normal boiling point predictions were obtained as an average absolute deviation of 1.07%. Molecular weight and density results were evaluated as average absolute deviations of 0.68% and 0.21%, respectively.     

Vapor–liquid critical properties of multi-component mixtures containing carbon dioxide and n-alkanes

Vapor–liquid critical temperatures, pressures and densities of multi-component mixtures containing CO₂ and n-alkanes (C4–C7) were measured in a high-pressure view cell by direct visual observations. The molar ratio of alkanes was fixed during the experiment while the composition of CO₂ was varied over the whole range. The critical loci show type I fluid phase behavior if the n-alkanes mixture is treated as a pseudo-continuous component, while correspondingly, CO₂ is a discrete component. The critical properties were calculated by Redlich–Kwong–Soave equation of state (SRK) combined with a renormalization group correction (RG). The predictions of critical properties by SRK + RG are in good agreement with the experimental results.     

Density and surface tension variation with temperature for n-nonane + 1-hexanol

New experimental densities and surface tensions for n-nonane + 1-hexanol at 288.15, 298.15 and 308.15 K are reported. Densities were measured with an Anton Paar DMA 4500 densimeter, and surface tensions using a Lauda TVT2 automated tensiometer, which uses the principle of the pending drop volume. The experimental data of pure liquids and mixtures have been used to calculate excess molar volumes and surface tension deviations of n-nonane + 1-hexanol as a function of mole fractions. A comparative study of these properties together with those available in the literature for the n-alkane + 1-alkanol mixtures has been performed. In addition, the magnitude of these experimental quantities is discussed in terms of the nature and type of intermolecular interactions in binary mixtures.     

Surface tension of chain molecules through a combination of the gradient theory with the CPA EoS

Despite the interest in systems containing non-associating compounds such as alkanes and fluoroalkanes or associating compounds like alkanols, their vapor–liquid interfaces have received little quantitative attention. Aiming at modeling the interfacial tensions of several families of chain molecules, a combination of the density gradient theory of fluid interfaces with the Cubic-Plus-Association (CPA) equation of state was developed. The density gradient theory is based on the phase equilibria of the fluid phases separated by the interface, for what an adequate equation of state is required.     

Easy prediction of the refractive index for binary mixtures of ionic liquids with water or ethanol

In this paper, we demonstrate that it is possible to know the refractive index, n_D, of every given mixture of 1-alkyl-3methyl imidazolium tetrafluoroborate with water and ethanol just from the knowledge of the refractive index and density of pure components. To do that, we measured n_D for seven different mixtures in all range of existing concentrations and, independently, we deduce n_D theoretically. Both sets of values differ less than a 0.2% on average. The theoretical deduction takes into account that these mixtures are quasi-ideal from the molar volume point of view, as recently published, and so density for any composition of the mixture can be obtained with a precision better than 0.5% from the pure compounds value. Now we simply apply Newton or Gladstone–Dale models, which relate the refractive index of a binary mixture with its density from the value of both pure components, without any fitting parameter. Both models are very similar in form and in the values they deduce (less than a 0.2% of difference), but while that of Newton performs slightly better for ethanol mixtures, the model of Gladstone–Dale gives some better results for aqueous mixtures. We think that these results can be extended to the majority of ionic liquid plus solvent systems.