N-alkane Mixtures Research Articles

Solid–fluid equilibrium for organic molecules: understanding the link between molecular structure and phase diagrams

We consider the problem of using molecular modeling and simulation in understanding trends in solid-fluid and solid-solid phase behavior of organic molecules as we change molecular structure. Our focus is on using molecular models that are simple enough to be studied over a wide range of conditions, but realistic enough to reveal important insights into the molecular level behavior. As examples, we review some recent work on the phase behavior of molecular models of n-alkanes and aromatic molecules. For the alkanes we discuss the molecular origins of the chain length dependence of the melting points as well as the role of rotator phases in the phase diagrams of longer chain molecules. We also consider solid–fluid and solid–solid phase equilibrium in binary n-alkane mixtures. The solid–fluid equilibrium in aromatic molecules reflects a competition between the molecular shape effects and electrostatic interactions. We discuss this in the context of the solid phases of benzene and also in the context of the stability of solid phase compounds of aromatic mixtures.

Modeling the Cloud Curves and the Solubility of Gases in Amorphous and Semicrystalline Polyethylene with the SAFT-VR Approach and Flory Theory of Crystallization

The statistical associating fluid theory for potentials of variable range SAFT-VR [Gil-Villegas, A.; et al. J. Chem. Phys. 1997, 106, 4168] is used to examine the fluid-phase behavior of mixtures of n-alkanes, alk-1-enes (R-olefins), and nitrogen with polyethylene. The molecules are modeled as flexible chains of tangent spherical segments, with segment-segment dispersive interactions treated via square-well potentials. The parameters of the polyethylene polymer are determined from those of the n-alkanes by using simple extrapolations with molecular weight. As a test of the extrapolated parameters for polyethylene, the absorption (vapor-liquid equilibria) and cloud curves (liquid-liquid equilibria) of n-pentane-polyethylene systems are predicted without adjustable binary parameters, and the effect of the polymer parameters on the phase behavior of the mixture is discussed. The liquid-liquid immiscibility curve is found to be very sensitive to the intermolecular potential parameters used to describe the polymer. The change in the lower critical solution temperature (LCST) for mixtures of n-alkanes of increasing chain length in polyethylene is also predicted. Good agreement with experimental data is also obtained for the absorption of small gases in amorphous polyethylene. The predictions of the SAFT-VR approach confirm experimental findings that the solubility of nitrogen increases with temperature while the solubility of heavier molecules such as ethene and but-1-ene decreases. Strong synergies can be observed when several gases absorb in the same polyethylene polymer. The co-absorption effects are explained in terms of the interactions between gas and polyethylene molecules. When the temperature of interest is below the melting temperature of polyethylene, the polymer exists in a semicrystalline state. In this case, the crystallinity of polyethylene has to be taken into account in order to predict the solubility of the various gases. We present an accurate theory to predict the crystallinity of polyethylene as a function of temperature. The approach, which is based on Flory’s theory of copolymer crystallinity [Flory, P. J. Trans. Faraday Soc. 1955, 51, 848], is accurate for a large variety of polyethylene samples, and requires only the experimental crystallinity or polymer density at 25 °C as an input parameter. On combining the Flory and SAFT-VR approaches, we can predict the solubility of various gases in semicrystalline polyethylene samples, by assuming that the molecules of gas only absorb in the amorphous regions of the polymer.

Characterization of urban and rural organic particulate in the Lower Fraser Valley using two Aerodyne Aerosol Mass Spectrometers

Two Aerodyne AerosolMass Spectrometers (AMS) were depl oyed at three sites representing urban, semi-ruraland rural areas during the Pacific 2001 experiment in the Lower Fraser Valley (LFV), British Columbia, Canada in August 2001. The AMS provides on-line quantitative measurements of the size and chemical composition of the non-refractory fraction of submicron aerosol particles. A significant accumulation mode with a peak around 400–500 nm was observed at all sites that was principally composed of sulphate, organics, ammonium and some nitrate. Another significant mode with a peak below 200 nm was also observed at the urban site and when urban plumes affected the other sites. This paper focuses on the variability of the organic particulate composition and size distribution as a function of location and photochemical activity with a particular emphasis on the urban and rural areas. The small organic mode at the urban site was well correlated with gas phase CO, 1,3-butadiene, benzene and toluene with Pearson’s r values of 0.76, 0.71, 0.79 and 0.69, respectively, suggesting that combustion-related emissions are likely to be the main source of the small organic mode at this site. The mass spectra of the urban organic particulate are similar to those of internal combustion engine lubricating oil, and of diesel exhaust aerosol particles, implying that they were composed of a mixture of n-alkanes, branched alkanes, cycloalkanes, and aromatics. In contrast, organic particulate at the rural site was dominated by shorter chain oxidized organic compounds. Correlations between the two organic modes and gas phase compounds at the ruralsite indicated that a significant part of the smal lmode originated from combustion sources, while the large accumulation organic mode appeared to be the result of photochemical processing. Processing

Nanofiltration of organic solvents

Transport mechanisms and process limitations are relatively well understood for aqueous nanofiltration systems. Much work has also been done on the use of membranes for the removal of suspended matter from organic solvents. The removal of organic solute compounds from organic solvents using membrane technology has been addressed by very few workers, and little is known of the fundamental transport and separation mechanisms. The work aims to enhance the understanding of non-aqueous nanofiltration by focusing on the flux performance of organic solvents through a dense 2 μm polydimethylsiloxane composite membrane. The flux of alcohols, n-alkanes, i-alkanes and cyclic compounds were studied in deadend mode, at pressures of 10–900 kPa. Fluxes of 10–80 l/m2 h were obtained for alkanes and cyclic compounds, whereas alcohol flux was around two orders of magnitude lower. The results suggest that the solvent flux through polydimethylsiloxane takes place via two distinct mechanisms – namely hydraulic and chemical transport. Hydraulic transport appears to dominate at pressures above 300 kPa, whereas chemical transport becomes more apparent at lower pressures. Comparison of the hydraulic transport data with a Hagen-Poisuelle model gives good agreement for similar solvents. Swelling effects caused by solvent-membrane interactions are identified as playing a major role in solvent flux behaviour, and compressibility effects are also thought to account for deviations from the Hagen-Poisuelle model. Viscous flow was verified by a nonseparation of mixtures of n-alkane and cyclic compounds, which suggests that the polydimethylsiloxane layer cannot sustain a dense structure when used in organic solvent nanofiltration applications.

Crystallization of Mixed Paraffin from Model Waxy Oils and the Influence of Micro-crystalline Poly(ethylene-butene) Random Copolymers

We report rheological, microscopic, and calorimetric studies of the crystallization of long chain n-paraffins and their mixtures from model waxy oils and the effect of microcrystalline poly(ethylene-butene) (PEB) random copolymers. Optical micrographs and differential scanning calorimetry (DSC) reveal that the crystals formed from decane solutions of binary mixtures of C36 + C32 and of C32 + C28 are of mixed composition, whereas solutions of C28 + C24, C36 + C28, C32 + C24, and C36 + C24 form separate crystal phases. There is no miscibility when the difference in carbon number between two long chain n-paraffins Δnc > 4. These findings agree with Kravchenko's prediction for crystallization of molten binary n-alkane mixtures. However, the crystallization of long chain n-paraffins from decane solution gives a stable crystal structure directly, while from the melt it tends to pass through a metastable rotator phase. Since PEB can either self-assemble into a needlelike structure or cocrystallize with long chai...

In-Situ SAXS/WAXS and Turbidity Studies of the Structure and Composition of Multihomologous n-Alkane Waxes Crystallized in the Absence and Presence of Flow Improving Additive Species

Wax deposition on engineering components can cause severe operational problems in oil recovery. These problems are examined through a study focused on understanding the mediating role played by the chemical inhibitors that are commonly used to prevent or delay the wax crystallization process. Solution crystallization data from homologous mixtures of n-alkanes, as measured via optical turbidometric methods and in-situ combined small and wide-angle X-ray scattering (SAXS/WAXS) techniques, reveal a direct correlation between the type and concentration of polymeric additive used and the resultant crystallization behavior. The behavior of additives, such as polar macromolecules with nonpolar alkyl chains protruding from the backbone, was consistent with their binding within the basal plane of the wax crystal structure, associated with intermolecular (alkane/additive) interactions of an epitaxial nature. Overall, the results were consistent with a number of structurally related factors influencing the additive-...

Inability of unpacked gooseneck liners to stop the sample liquid after injection with band formation (fast autosampler) into hot GC injectors.

With the introduction of the fast autosampler, the rules on how to perform splitless injection should have been revisited: the fast autosampler releases the sample liquid as a band that is no longer nebulized by solvent vapors as in previous injection techniques. In empty gooseneck liners, the sample liquid is shot to the bottom of the injector and jumps around in a largely uncontrolled manner. Visual experiments showed that the liquid is partially rejected. Another portion passes by the column inlet into the column attachment region, while the last part may directly enter the column. The impact on chromatography was investigated by using a mixture of n-alkanes: the higher boiling components passing by the column entrance into the zone of the column attachment were largely lost, i.e., were discriminated against the volatile components. It was concluded that empty gooseneck liners are not suitable for injection by fast autosamplers.

Solid–fluid and solid–solid phase equilibrium in a model of n-alkane mixtures

Solid-fluid and solid-solid phase equilibrium for binary mixtures of hard sphere chains modeling n-hexane, n-heptane, and n-octane has been calculated using Monte Carlo computer simulations. Thermodynamic integration was used to calculate the Gibbs free energy and chemical potentials in the solid and fluid phases from pure component reference values. A multiple stage free energy perturbation method was used to calculate the composition derivative of the Gibbs free energy. Equation of state and free energy data for the fluid phase indicate ideal solution behavior. Nonideality is much more significant in the solid phase with only partial solubility of shorter chains in the longer chains and essentially no solubility at the other end of the composition range. The miscibility decreases with increasing chain length difference between the components. For the model of n-hexane/n-octane mixtures solid--solid phase separation has been observed directly in some of the simulations, with the components segregating between the layers of the solid structure. The behavior is similar to that seen in some binary n-alkane mixtures with longer chain lengths but comparable chain length ratios between the components. Such phase separation, although indicated thermodynamically, is not seen directly in the simulations of the n-heptane/n-octane mixture due to the difference in the pure component crystal structures.

Effects of biosurfactants produced by Candida antarctica on the biodegradation of petroleum compounds

The effects of biosurfactants on the biodegradation of petroleum compounds were investigated. Candida antarctica T-34 could produce extracellular biosurfactant mannosylerythritol lipids (MELs) when it was cultured in vegetable oil. In addition, in our previous study, it was found that this strain could also produce a new type of biosurfactant while it grew on n-undecane (C11H24), and the biosurfactant was named as BS-UC. In flask culture of Candida antarctica, the addition of BS-UC could improve the biodegradation rate of some n-alkanes (e.g. 90.2% for n-decane, 90.2% for n-undecane, 89.0% for dodecane), a mixture of n-alkanes (82.3%) and kerosene (72.5%). By comparing the effects of the biosurfactants BS-UC and MEL and chemical surfactants on the biodegradation of crude oil, it was found that biosurfactants could be used to enhance the degradation of petroleum compounds instead of chemical surfactants. In a laboratory scale immobilized bioreactor, the addition of biosurfactant improved not only the emulsification of kerosene in simulated wastewater but also its biodegradation rate. The highest degradation rate of kerosene by addition of MEL and BS-UC reached 87 and 90% at 15 h, respectively. The results showed that the biosurfactant BS-UC was highly promising for work on biodegradation of hydrophobic contaminants.

An overview of developments related to the characterization and significance of high molecular weight paraffins/hydrocarbons (>C 40 ) in crude oils

Abstract Over the past 10 years, investigations into the characteristics of the high molecular weight hydrocarbon (HMWHC) fraction in crude oils and, to a lesser extent, source rock extracts have continued to reveal novel information concerning the distribution of hydrocarbons >C 40 . The major impetus for this work has come from the fact that HMWHCs can cause significant production problems in certain geographical regions and particularly deepwater frontier areas. Since these HMWHCs appear to be ubiquitous in crude oils, the primary questions that need to be addressed are: what are these compounds, where do they come from, and how do they affect physical properties of oils? Here, we review our work over the past decade and discuss the significance of these results and their potential application to reservoir and production problems involving paraffins and asphaltenes. It was commonly believed for many years that only oils derived from source rocks containing higher plant source material would have a high paraffin content. However, it is now abundantly clear that oils derived from lacustrine and marine source rocks also contain relatively high concentrations of higher molecular weight hydrocarbons. In addition to developing methods for the qualitative and quantitative separation of HMWHCs from asphaltenes, progress has been made in identifying individual components of the high molecular weight fraction. This fraction is not a simple mixture of n -alkanes but a complex mixture of seven or eight homologous hydrocarbon series, each with significantly different physical properties. A knowledge of these structures is important in predicting crude oil properties such as cloud point and pour point. Series identified to date include alkylcyclopentanes, alkylcyclohexanes, alkylbenzenes and various branched hydrocarbons. In summary, since the 1970s most of the geochemical research emphasis has been placed on compounds below C 40 . Whilst compounds above C 40 may not have the same degree of structural specificity as the traditional biomarkers, the amount of information available from these compounds could be extremely beneficial in the long term, particularly for reservoir characterization and production purposes and other problems involving high molecular weight hydrocarbons.

High pressure phase equilibria and critical phenomena of water + iso-butane and water + n-butane systems to 695 K and 306 MPa

The equilibrium surface ( x, p, T, V m) for water+ iso-butane was measured and additional equilibrium data for water+ n-butane are reported. Mole fractions of water for water+ iso-butane from 0.5 to 0.98, pressures up to 306 MPa and temperatures up to 695 K were studied. Critical curves, represented as the envelope of the isopleths starting at the critical point of water (647 K), went through a temperature minimum and then increased at higher pressures and higher temperatures, giving evidence for (gas+gas) equilibria of the third type. The results are compared with the phase equilibrium of previously measured (water+ n-alkane) mixtures. The excess molar volumes, V E, and excess molar Gibbs energies, G E, for water+ n-butane on the phase equilibrium surfaces have been calculated for 600 and 620 K from 40 to 200 MPa. All the V E and G E values were found to be positive.

Modeling Some Alcohol/Alkane Binary Systems Using the SAFT Equation of State with a Semipredictive Approach

An approach proposed in an earlier work for modeling n-alkane + n-alkane systems (Benzaghou, S.; Passarello, J. P.; Tobaly, P. Fluid Phase Equilib. 2001, 180, 1) is extended to the VLE (vapor-liquid equilibrium; vapor pressure and liquid-phase molar volume) representation of n-alkane + 1-alcohol binary systems using the SAFT equation. The original approach was based on molecular assumptions that consider all n-alkanes to be composed of identical segments. These assumptions are here extended to the 1-alkanol chemical series. The thermodynamic representation of the chemical series of pure 1-alkanols thus requires only four adjustable parameters. The values of the corresponding parameters were determined by fitting the data of the series of light 1-alkanols (1-propanol to 1-octanol). The extrapolation capability of this method was then tested. The VLE thermodynamic properties of pure heavy 1-alcohols (up to 1-octadecanol) and 1-alcohol + n-alkane mixtures were predicted without any additional binary parameters. The model predicts the experimental data within a few percent.

Measurement and modeling of surface tensions of asymmetric systems: heptane, eicosane, docosane, tetracosane and their mixtures

To extend the surface tension database for heavy or asymmetric n-alkane mixtures, measurements were performed using the Wilhelmy plate method. Measured systems included the binary mixtures heptane+eicosane, heptane+docosane and heptane+tetracosane and the ternary mixture heptane+eicosane+tetracosane at temperatures from 313.15 K (or above the melting point of the mixture) up to 343.15 K. All the measurements were performed at atmospheric pressure. Using these data, along with data previously measured by us and collected from the literature, a recently proposed corresponding states model was assessed. It is shown that using a new generalized combining rule for the critical temperature, the data can be described with deviations of about 1% that is within the experimental uncertainty of the measurements.

Liquid–solid phase equilibrium study of tetradecane and hexadecane binary mixtures as phase change materials (PCMs) for comfort cooling storage

Phase diagrams (equilibrium diagrams) are a convenient way of depicting the concentration–temperature–pressure relationships of a chemical system at equilibrium. They are invaluable to PCMs thermal storage researches and developers. In the present paper, the liquid–solid phase equilibrium of binary mixture system of tetradedcane and hexadecane has been studied. For theoretical evaluation of the thermodynamic equilibrium between the liquid phase and solid phase of tetradecane and hexadecane, several methods were compared: the UNIFAC group-contribution method, the model of Won, and Pedersen’s model. The temperature–composition phase diagram of the binary system has been obtained by the calculation. The diagram illustrates a binary, isomorphous system with a temperature minimum. Differential scanning calorimetry (DSC) was used to study the phase transformation of the binary system. DSC was run at different ramp rates and results indicate that these mixtures melt and freeze over a temperature range and the temperature range depends on the DSC ramp rate, and in a short mixture concentration interval appears low temperature solid–solid peaks. From the DSC results presented, it is concluded that the behaviour of binary mixtures of n-alkanes is far more complicated than considered in earlier studies. The phase diagram and all information from DSC are very important for designing PCM storage system.

Phase equilibria and pVT predictions for alkyl carbonate + n-alkane systems using equations of state

In this paper experimental data available in the literature on vapor–liquid equilibria, densities and excess molar volumes of dialkyl carbonate+ n-alkane mixtures on broad temperature and pressure ranges have been used in order to test four equations of state (EoS): Soave–Redlich–Kwong (SRK), Peng–Robinson (PR), Patel–Teja (PT) and Dohrn–Prausnitz (DP). For the pure components, when the critical parameters were not available in the literature, the group contribution method of Klincewicz and Reid was used to estimate the critical temperature and pressure. For dialkyl carbonate+ n-alkane mixtures we have determined the binary interaction parameter, k ij , using experimental vapor–liquid equilibria data and then with these parameters the pVTx values were predicted. The best correlations for VLE and predictions for the volumetric behavior were obtained with PR and PT equations.

New Dieterici-type equations of state for fluid phase equilibria

The vapour–liquid critical properties of binary mixtures are calculated using conformal solution theory, the one-fluid model, the van der Waals and Carnahan–Starling–van der Waals equations, and a new Dieterici-type equation of state reported recently [J. Chem. Phys. 115 (2001) 371]. Comparisons with experimental data are reported for the critical properties of n-alkane mixtures ranging from methane to n-octane. A common feature of these comparisons is that the new Dieterici-type equation of state yields good prediction of the pressure–temperature critical locus without the need to use arbitrary adjustable parameters in the combining rules for mixture properties. In particular, the Dieterici-type equation correctly predicts the maximum in the pressure–temperature curve. In contrast, the van der Waals-type equations are inaccurate. Dieterici-type equations of state may have a useful rule in the prediction of fluid phase equilibria by reducing the need for adjustable parameters in the combining rules.

The solubility of high molecular weight n-alkanes in supercritical carbon dioxide at pressures up to 50 MPa

Solubilities of n-tetracosane, n-pentacosane, n-hexacosane, n-heptacosane and n-nonacosane in supercritical carbon dioxide were measured in the pressure range 10–50 MPa and 313 K. The solubility data were found to be internally self-consistent according to the theory of dilute solutions. Krichevskii parameters were calculated from the data and exhibited regular trends with the carbon number of the n-alkane. The Krichevskii parameter vs carbon number behavior was also consistent with that found in our earlier work for mixtures of low and high molecular weight n-alkanes in carbon dioxide.

Prediction of the Salting-Out Effect of Strong Electrolytes on Water + Alkane Solutions

In this work we investigate the salting out of n-alkanes in water by strong electrolytes using an extension of the statistical associating fluid theory for attractive potentials of variable range which incorporates ionic interactions (SAFT-VRE) [Galindo, A.; Gil-Villegas, A.; Jackson, G.; Burgess, A. N. J. Phys. Chem. B 1999, 103, 10272]. The systems are treated as water (1) + n-alkane (2) + cation (3) + anion (4) four-component mixtures. The water molecules are modeled as spherical with four associating sites to mediate hydrogen bonding, while the n-alkane molecules are modeled as chains of tangentially bonded spherical segments interacting via square-well potentials. The phase behavior of the binary water + n-alkane mixtures is well-described by the theoretical approach using two unlike adjustable parameters which are transferable for different alkane molecules. The salt is incorporated in the model as fully dissociated and assumed to be present only in the water-rich phase. In this way, the anion and c...

Morphology and chemistry of Dufour glands in four ectoparasitoids: Cephalonomia tarsalis, C. waterstoni (Hymenoptera: Bethylidae), Anisopteromalus calandrae, and Pteromalus cerealellae (Hymenoptera: Pteromalidae)

The venom apparatus of four hymenopterous parasitoids, including two bethylids, C. tarsalis (Ashmead) and C. waterstoni (Gahan), and two pteromalids, A. calandrae (Howard) and P. cerealellae (Ashmead), were removed and the associated Dufour glands characterized with respect to their external morphology and chemistry. Dufour glands in all four species have a characteristic translucent appearance that apparently results from their lipid content. The stalked Dufour glands of C. tarsalis and C. waterstoni are pear-shaped and have overall lengths of approximately 0.2 and 0.15 mm, respectively. The thin venom glands are bifurcate and insert through a fine duct into the transparent ovoid- to pear-shaped venom reservoir in these bethylids. In A. calandrae and P. cerealellae the Dufour glands are elongated, tubular structures of ca. 0.35 and 0.8 mm in length, respectively, that constrict to a short stalk that empties into the common oviduct. The venom glands in these pteromalids are simple elongated structures that insert into the sac-like venom reservoir through a fine duct. The chemistry of the volatile contents of the Dufour gland in these four species differs considerably. C. tarsalis Dufour glands contain the same hydrocarbon components as found on the cuticle of this species (Ann. Entomol. Soc. Am. 91:101–112 (1998)), and no other chemicals. The Dufour glands of C. waterstoni also contain only hydrocarbons, most of which are the same as the cuticular hydrocarbons (Ann. Entomol. Soc. Am. 85:317–325 (1992)), but in addition the Dufour gland contains ca. 3% of a mixture of 2,17- and 2,19-dimethyl C 23. A. calandrae Dufour gland chemistry is somewhat more complex than that of either of the two bethylids, but like the bethylids, only hydrocarbons are present. The carbon number range is from C 30 to C 39 and consists of a mixture of n-alkanes (C 30-C 38); 3-, 5-, 7-, 9-, 11-, 12-, 13-, 14-, 15- and 17-methyl alkanes; 3,7- and 3,11-dimethyl alkanes; 5,9- and 5,17-dimethyl alkanes; 7,11-, 9,13-, 13,17-, 14,18- and 15,19-dimethyl alkanes; 3,7,11- and 3, 9,15-trimethyl alkanes; and 3,7,11,15-tetramethyl alkanes. The cuticular hydrocarbons of this species have not been previously reported, but they are the same as the Dufour gland hydrocarbons. The Dufour glands of P. cerealellae contain both hydrocarbons and two long-chain aldehydes. Most of the hydrocarbons are identical to those found on the cuticle of this species (Ann. Entomol. Soc. Am. 94:152–158 (2001)), but in addition, 5,9-dimethyl C 27, 5,13-, 5,17- and 5,19-dimethyl C 35, 12- and 14-methyl C 36, 12,16- and 13,17-dimethyl C 36, 13-methyl C 37 and 13,17-dimethyl C 37 are present. The two aldehydes detected in glands from P. cerealellae are n-tetracosanal (C 23CHO) and n-hexacosanal (C 25CHO).

Phase equilibrium modeling of mixtures of long-chain and short-chain alkanes using Peng–Robinson and SAFT

Peng–Robinson and selected implementations of the Statistical Associating Fluid Theory (SAFT) equation of state (EoS) were evaluated for their ability to correlate and predict vapor–liquid equilibrium for highly asymmetric mixtures of n-alkanes. Peng–Robinson is surprisingly accurate for fitting such mixtures, but only if the pure component parameters are regressed with vapor pressure and liquid density data, as is normally done with SAFT. Furthermore, these parameters were found to be well-behaved and to increase monotonically with carbon number. In spite of its deficiencies in modeling the near-critical and critical regions, PC-SAFT is recommended for predicting the phase behavior of asymmetric mixtures of n-alkanes for which no data are available. For example, with a binary interaction parameter of zero, PC-SAFT accurately predicts liquid-phase compositions for n-alkane mixtures of C 6+C 16, C 6+C 24 and C 6+C 36 over a wide range of temperatures and pressures.