New analytic apparatus for experimental determination of vapor–liquid equilibria and saturation densities

A new experimental apparatus for performing simultaneous determination of high-pressure vapor–liquid equilibria (VLE) and saturated densities was developed in this work. The experimental methodology was verified by measuring these properties for the carbon dioxide + 1-propanol and carbon dioxide + 2-propanol systems from 313 to 363 K. The apparatus is based on the static-analytic method for VLE determinations and was slightly modified by coupling a vibrating U-tube densitometer to obtain saturated densities for both vapor and liquid phases. VLE measurements agreed with previous literature data and were correlated with the Peng–Robinson equation of state coupled to the Wong–Sandler mixing rules. Saturation densities at temperatures above 313 K have not been published up to now.     

Isothermal vapour–liquid equilibrium data for the binary propan-2-one+chloroalkanes mixtures at temperatures from 298.15 K to 313.15 K

Isothermal vapour–liquid equilibrium data are reported for binary mixtures of propan-2-one with 1,1,2,2-tetrachloroethane (I) and 1,4-dichlorobutane (II) within the 298.15–313.15 K temperature range. Total vapour pressures were measured by a static method, the mixtures beeing prepared by weight and degased directly into the working cell. The experimental data were correlated by Barker's method and different expressions for excess Gibbs energy were tested. The results indicate negative deviation from ideality for mixture (I), while mixture (II) behaves almost ideally, with slightly positive deviations.     

Isobaric vapour–liquid equilibria for the binary systems of fuel oxygenates with aromatic hydrocarbons

Isobaric vapour–liquid equilibrium data at 720?mm?Hg for the binary systems of diisopropyl ether with o-xylene and m-xylene and dimethoxymethane with benzene and toluene are determined. A Swietoslawski type ebulliometer is used for the measurements. The experimental T?x data are used to estimate Wilson parameters and the parameters in turn are used to calculate the vapour compositions and activity coefficients. The activity coefficients are used to calculate molar excess Gibbs free energy (G ^E). All the systems studied here do not exhibit azeotropes. Excess Gibbs free energy values are positive over the entire range of composition for all the systems.     

Onsager heat of transport at the aniline liquid–vapour interface

The heat of transport for passage of matter through a gas–liquid interface has been determined for the aniline liquid–vapour system, by measuring stationary-state pressure differences produced by known temperature differences over a distance of 2 mm on the vapour side of the interface. For the range of pressures used, 2 mm is between 6 and 34 mean free paths. Coupling of the heat and matter fluxes is significant over the whole of this range. At the higher pressures the heat of transport is more than 20% of the heat of condensation; at the lower pressures it is more than 50%.     

New approach to simplify the equation for the excess Gibbs free energy of aqueous solutions of electrolytes applied to the modelling of the NH3–CO2–H2O vapour–liquid equilibria

A new, simple, empirical equation for G^E (excess Gibbs free energy) of electrolyte solutions is proposed in which, contrary to the commonly used Pitzer equation, binary and ternary interaction parameters relate to the interactions of electrolytes in a solution rather than to the interactions of real species in a solution (i.e., anions, cations and nondissociated molecules). Such an approach radically reduces the number of parameters in the new equation for G^E as compared with the Pitzer equation and consequently significantly simplifies their calculation. The efficiency of the new approach is demonstrated on the example of modelling the vapour–liquid equilibria of the industrially important and widely investigated NH₃–CO₂–H₂O system.     

High pressure vapour–liquid equilibria for the mixture dl-γ-tocopherol/methanol

dl-γ-Tocopherol is one of the four homologues compounds also known as vitamin E. Tocopherols are present in all four forms (α-, β-, γ-, δ-tocopherol) in many natural sources and extraction design of purification processes used for their enrichment needs detailed knowledge of the phase equilibrium behaviour. Experimental vapour–liquid equilibrium data (pressure–composition isotherms) for the binary system dl-γ-tocopherol/methanol at four temperatures 473 K, 498 K, 523 K, 543 K and pressures up to 12 MPa are presented. The measurements were carried out in a static constant volume high-pressure apparatus. Different cubic equations of state and mixing rules were used to model the data. A modified Redlich–Kwong–Soave EOS gives the better correlation. On the basis of the new experimental data and those for the system dl-α-tocopherol/methanol and reasonable agreement between the two systems, using for both the same thermodynamic model, behaviour of the ternary system dl-α-tocopherol/dl-γ-tocopherol/methanol has been predicted.     

Modelling of high-pressure phase equilibria using the Sako–Wu–Prausnitz equation of state: II. Vapour–liquid equilibria and liquid–liquid equilibria in polyolefin systems

Phase equilibria in binary and ternary polyolefin systems are calculated using the cubic equation of state proposed by Sako–Wu–Prausnitz (SWP). Calculations were done for high-pressure phase equilibria in ethylene/polyethylene (LDPE) systems and for liquid–liquid equilibria (LLE) in systems containing either high-density polyethylene or poly(ethylene-co-propylene). The calculations for the copolymer/solvent systems are compared with those using the SAFT EOS. The two equations of state can describe UCST, LCST as well as U-LCST behaviour with similar accuracy. Whereas, the binary interaction parameter is temperature-independent for SAFT, it is found to be a function of temperature for the SWP model. Moreover, the influence of an inert gas on the LCST of the polyethylene/hexane system is investigated using the SWP EOS. The polydispersity of the different polyethylenes is considered in the phase equilibrium calculations using pseudocomponents chosen by the moments of the experimental molecular weight distributions.     

Vapour–liquid equilibria of butyl acetate with aromatic hydrocarbons at 298.15 K

Vapour pressures of butyl acetate?+?benzene or toluene or o- or m- or p-xylene were measured by static method at 298.15?±?0.01?K over the entire composition range. The activity coefficients and excess molar Gibb's free energies of mixing (G ^E) for these binary mixtures were calculated by fitting vapour pressure data to the Redlich–Kister equation using Barker's method of minimizing the residual pressure. The G ^E values for the binary mixtures containing benzene are positive; while these are negative for toluene, ortho, meta and para xylene system over the whole composition range. The G ^E values of an equimolar mixture for these systems vary in the order: benzene?>?m-xylene?>?o-xylene?>?p-xylene?>?toluene     

Detection of haemoglobin using an adsorption approach at a liquid–liquid microinterface array

The behaviour of haemoglobin (Hb) at the interface between two immiscible electrolyte solutions (ITIES) has been examined for analytical purposes. When Hb is fully protonated under acidic conditions (pH <pI) in the aqueous phase, it undergoes a potential-dependent adsorption and complexation, at the interface, with the anions of the organic phase electrolyte. When utilised as a simple and fast preconcentration step, consisting of adsorbing the protein at the interface, in conjunction with voltammetric desorption, this opens up the ITIES to the adsorptive stripping voltammetry approach. Utilising a 60 s adsorption step and linear sweep voltammetry, a linear response to Hb concentration in aqueous solution over the range 0.01–0.5 μM was achieved. The equation of the best-fit straight line was I _p ? =?7.46 C???0.109, R?=?0.996, where I _p is the peak current (in nanoampere) and C is haemoglobin concentration (in micromolar). The calculated detection limit (3σ) was 48 nM for a 60 s preconcentration period, while the relative standard deviation was 13.3 % for six successive measurements at 0.1 μM Hb. These results illustrate the prospects for simple, portable and rapid label-free detection of biomacromolecules offered by electrochemistry at arrays of liquid–liquid microinterfaces.     

Solid–liquid equilibria in the NaCl–SrCl2–H2O system at 288.15 K

The phase equilibria in the ternary system NaCl–SrCl₂–H₂O at 288.15 K were studied with the isothermal equilibrium solution method. The phase diagram and refractive index diagram were plotted for this system at 288.15 K. The phase diagram contains one invariant solubility point, two univariant solubility curves, and two crystallization fields of NaCl and SrCl₂ · 6H₂O. The refractive indices of the equilibrium solution change regularly with w(NaCl) increase. The calculated refractive index data are in good agreement with the experimental data. Combining the experimental solubility data of the ternary system, the Pitzer binary parameters for NaCl at 288.15 K and SrCl₂ at 298.15 K, the Pitzer mixing parameters θ_{Na, Sr}, Ψ_{Na, Sr, Cl} and the solubility equilibrium constants Ksp of solid phases existing in the ternary system at 288.15 K were fitted using the Pitzer and Harvie-Weare (HW) models. The mean activity coefficients of sodium chloride and strontium chloride, and the solubilities for the ternary system at 288.15 K were presented. A comparison between the calculated and measured solubilities shows that the predicted data agree well with the experimental results.     

Potentiometric study of acid–base equilibria in the {KCl + LiCl} eutectic melt at temperatures in the range (873 to 1073) K

Some heterogeneous reactions of oxide ion exchange (carbonate ion dissociation and magnesium oxide dissolution) in the molten {KCl + LiCl} eutectic at temperatures of (873, 973 and 1073) K were studied using an electrochemical cell with an oxygen membrane electrode Pt(O₂)|ZrO₂(Y₂O₃). The dissociation constant of the CO₃²⁻ was found to increase with increasing temperature: pK (873 K)=(2.39 ± 0.05); pK (973 K)=(1.81 ± 0.09); pK (1073 K)=(1.53 ± 0.08). Removal of CO₂ from the gas above the melt allows the complete transformation of CO₃²⁻ to O²⁻. pP_MgO values decrease more from (6.99 ± 0.08) to (5.41 ± 0.04). The oxobasicity indices, pI_(KCl+LiCl), were calculated from the solubility data to be 3.2 at 873 K, 3.4 at 973 K, and 3.6 at 1073 K. This trend suggests an increase in acidity with increasing temperature of {KCl + LiCl}.     

Isothermal vapor–liquid equilibria for the 1,1,1-trichloroethane+tetrahydrofuran+propan-2-ol system at 298.15 K

Isothermal vapor–liquid equilibria (VLE) for mixtures containing 1,1,1-trichloroethane+tetrahydrofuran+propan-2-ol have been measured using a modified version of a Boublik–Benson still at 298.15 K. A test of thermodynamic consistency, like McDermott–Ellis method was applied to the activity coefficients. Excess molar Gibbs free energies were calculated over the entire range composition. Different expressions existing in the literature were used to predict the activity coefficients.     

Molecular simulation of the phase behaviour of ternary fluid mixtures: the effect of a third component on vapour–liquid and liquid–liquid coexistence

The Gibbs ensemble algorithm is implemented to determine the vapour–liquid and liquid–liquid phase coexistence of dilute ternary fluid mixtures interacting via a Lennard–Jones potential. Calculations are reported for mixtures with a third component characterised by different intermolecular potential energy parameters. Comparison with binary mixture data indicates that the choice of energy parameter for the third component affects the composition range of vapour–liquid substantially. The addition of a third component lowers the energy of liquid phase while slightly increasing the energy of the vapour phase.     

Development of a new equation of state for mixed salt and mixed solvent systems,and application to vapour–liquid and solid (hydrate)–vapour–liquid equilibrium calculations

An equation of state that can be used for phase equilibrium and other thermodynamic property calculations at high pressures is developed for systems that contain aqueous solutions of strong electrolytes and molecular species. The proposed equation of state is based upon contributions to the Helmholtz free energy from a non-electrolyte term and three electrolyte terms. The non-electrolyte term comes from the Trebble–Bishnoi equation of state and the electrolyte terms consist of a Born energy term, a mean spherical approximation term and a newly developed hydration term. The application of the proposed equation of state to aqueous systems containing mixed salts and mixed solvents is illustrated by calculating the vapour–liquid equilibrium (VLE) and solid (Clathrate hydrate)–vapour–liquid equilibrium (SVLE) conditions for several systems. The solubility of CO₂ in salt water systems is examined at elevated pressures. As well, the new equation of state is used in conjunction with the model of van der Waals and Platteeuw to predict the SVLE conditions for gas hydrate forming systems in the presence of single salts, mixed salts and a mixture of aqueous salts and methanol. It is found that the new equation of state is able to accurately represent the experimental data over a wide range of pressure, temperature and salt concentration.     

Liquid–liquid equilibria for butyl tert-butyl ether + (methanol or ethanol) + water at several temperatures

Liquid–liquid equilibrium (LLE) data for butyl tert-butyl ether + (methanol or ethanol) + water were measured experimentally at 298.15, 308.15 and 318.15 K. The experimental data were correlated with the NRTL and UNIQUAC equations. The equations were used to perform the correlation of each temperature data set and for the three temperatures data set simultaneously. The best results were found with UNIQUAC and NRTL (α = 0.1), respectively. Data prediction was carried out using the UNIFAC method, however the results found were not quantitative.     

Development of a dispersive liquid–liquid microextraction method for the determination of fluoroquinolones in chicken liver by high performance liquid chromatography

A simple and cost effective sample pre-treatment method, dispersive liquid–liquid microextraction (DLLME), has been developed for the extraction of six fluoroquinolones (FQs) from chicken liver samples. Clean DLLME extracts were analyzed for fluoroquinolones using liquid chromatography with diode array detection (LC-DAD). Parameters such as type and volume of disperser solvent, type and volume of extraction solvent, concentration and composition of phosphoric acid in the disperser solvent and pH were optimized. Linearity in the concentration range of 30–500 μg kg⁻¹ was obtained with regression coefficients ranging from 0.9945 to 0.9974. Intra-day repeatability expressed as % RSD was between 4 and 7%. The recoveries determined in spiked blank chicken livers at three concentration levels (i.e. 50, 100 and 300 μg kg⁻¹) ranged from 83 to 102%. LODs were between 5 and 19 μg kg⁻¹ while LOQs ranged between 23 and 62 μg kg⁻¹. All of the eight chicken liver samples obtained from the local supermarkets were found to contain at least one type of fluoroquinolone with enrofloxacin being the most commonly detected. Only one sample had four fluoroquinolone antibiotics (ciprofloxacin, difloxacin, enrofloxacin, norfloxacin). Norfloxacin which is unlicensed for use in South Africa was also detected in three of the eight chicken liver samples analyzed. The concentration levels of all FQs antibiotics in eight samples ranged from 8.8 to 35.3 μg kg⁻¹, values which are lower than the South African stipulated maximum residue limits (MRL).     

Modeling of three-phase vapor–liquid–liquid equilibria for a natural-gas system rich in nitrogen with the SRK and PC-SAFT EoS

In this work, we present the modeling of three-phase vapor–liquid–liquid equilibria for a mixture of natural gas (Hogback gas) containing high concentrations in nitrogen (51.8 mol%) with the SRK and PC-SAFT equations of state. The interest of studying this mixture is due to the experimental evidence of the occurrence of multiple equilibrium liquid phases for this mixture over certain ranges of temperature and pressure. The calculation of the multiphase equilibria was carried out by using an efficient numerical procedure based on the minimization of the system Gibbs energy and thermodynamic stability tests to find the most stable state of the system. The results of the calculated vapor–liquid–liquid equilibria (VLLE) show that the PC-SAFT equation of state predicts satisfactorily the phase behavior that experimentally exhibits this mixture, whereas the SRK equation of state predicts a three-phase region wider than the experimentally observed. The two-phase boundary for this mixture was also calculated through flash calculations, and the results showed that this mixture does not present any gas-liquid critical point.     

Liquid—liquid equilibria of the system water + acetic acid + methyl ethyl ketone at several temperatures

This article reports the solubilities and equilibrium data of the ternary mixture water + acetic acid + methyl ethyl ketone at 25, 35 and 45°C, and the equilibrium data of the binary system water + methyl ethyl ketone over the range 20–55°C. NRTL and UNIQUAC equations have been fitted to the experimental data for the ternary system, which are also compared with the values predicted by the UNIFAC method.