Phase equilibrium for clathrate hydrates formed from an ozone + oxygen gas mixture coexisting with carbon tetrachloride or 1,1-dichloro-1-fluoroethane

This paper reports an attempt at acquiring phase-equilibrium pressure (p) versus temperature (T) data for ozone-containing clathrate hydrates formed from an ozone + oxygen gas mixture, a hydrophobic hydrate-forming liquid, and water in the liquid state. For dealing with ozone (O₃), a chemically unstable material continuously decaying to oxygen (O₂) in the gas phase, we devised a new method, i.e., a modified pressure-search method, to determine the equilibrium p-T conditions while maintaining the ozone concentration in the gas phase nearly constant by repeatedly replacing the contents of the gas phase with a freshly generated O₃ + O₂ mixture. Using carbon tetrachloride (CCl₄) as the hydrophobic hydrate-forming liquid, we obtained equilibrium p-T data in the range of 0.167 MPa ≤ p ≤ 0.361 MPa and 275.6 K ≤ T ≤ 277.3 K in the presence of a gas phase containing O₃ at the molar concentration of 6.9 ± 0.8%. We also obtained, for comparison, the corresponding p-T data, using pure O₂ gas, instead of the O₃ + O₂ mixture, and the conventional pressure-search method. The two data groups obtained from the O₃-containing and O₃-free systems, respectively, show simple, mutually consistent p-T relations each well fitted by the Clausius-Clapeyron equation assuming a constant enthalpy of hydrate dissociation. The paper also describes our additional attempt at obtaining equilibrium p-T data using 1,1-dichloro-1-fluoroethane (R141b) as a substitute for CCl₄. Because of the partial decomposition of R141b due to the coexistence of O₃ and water, however, we obtained only limited data which are tentative in nature.     

Enthalpies of dissociation of clathrate hydrates of carbon dioxide,nitrogen, (carbon dioxide + nitrogen), and (carbon dioxide + nitrogen + tetrahydrofuran)

A calorimetric technique is described for measuring the enthalpy of dissociation liberated from solid hydrates. In this study, the enthalpies of dissociation were determined at T =  273.65 K andp =  0.1 MPa for simple and mixed hydrates of carbon dioxide, nitrogen, (carbon dioxide  +  nitrogen), and (carbon dioxide  +  nitrogen  +  tetrahydrofuran) using an isothermal microcalorimeter. The addition of tetrahydrofuran (THF) promoted hydrate stability and increased the number of guest molecules encaged in the small and large cavities of the hydrate lattice, resulting in lower enthalpy of dissociation, compared with structure II hydrate. The composition ratio of guest molecules did not affect the enthalpy of dissociation, which was found to be nearly constant for the same mixture.     

Quaternary isobaric (vapor + liquid + liquid) equilibrium and (vapor + liquid) equilibrium for the system (water + ethanol + cyclohexane + heptane) at 101.3 kPa

Experimental isobaric (vapor + liquid + liquid) and (vapor + liquid) equilibrium data for the ternary system {water (1) + cyclohexane (2) + heptane (3)} and the quaternary system {water (1) + ethanol (2) + cyclohexane (3) + heptane (4)} were measured at 101.3 kPa. An all-glass, dynamic recirculating still equipped with an ultrasonic homogenizer was used to determine the VLLE. The results obtained show that the system does not present quaternary azeotropes. The point-by-point method by Wisniak for testing the thermodynamic consistency of isobaric measurements was used to test the equilibrium data.     

Phase equilibrium measurements for semi-clathrate hydrates of the (CO2 + N2 + tetra-n-butylammonium bromide) aqueous solution system

The application of semi-clathrate hydrate formation technology for gas separation purposes has gained much attention in recent years. Consequently, there is a demand for experimental data for relevant semi-clathrate hydrate phase equilibria. In this work, semi-clathrate hydrate dissociation conditions for the system comprising mixtures of {CO₂ (0.151/0.399 mole fraction) + N₂ (0.849/0.601 mole fraction) + 0.05, 0.15, and 0.30 mass fraction tetra-n-butylammonium bromide (TBAB)} aqueous solutions have been measured and are reported. An experimental apparatus which was designed and built in-house was used for the measurements using the isochoric pressure-search method. The range of conditions for the measurements was from 277.1 K to 293.2 K for temperature and pressures up to 16.21 MPa. The phase equilibrium data measured demonstrate the high hydrate promotion effects of TBAB aqueous solutions.     

Isobaric (vapour + liquid + liquid) equilibrium data for (di-n-propyl ether + n-propyl alcohol + water) and (diisopropyl ether + isopropyl alcohol + water) systems at 100 kPa

Isobaric (vapour + liquid + liquid) equilibria were measured for the (di-n-propyl ether + n-propyl alcohol + water) and (diisopropyl ether + isopropyl alcohol + water) system at 100 kPa.The apparatus used for the determination of (vapour + liquid + liquid) equilibrium data was an all-glass dynamic recirculating still with an ultrasonic homogenizer couple to the boiling flask.The experimental data demonstrated the existence of a heterogeneous ternary azeotrope for both ternary systems. The (vapour + liquid + liquid) equilibria data were found to be thermodynamically consistent for both systems.The experimental data were compared with the estimation using UNIQUAC and NRTL models and the prediction of UNIFAC model.     

(Vapour + liquid) equilibrium in (N,N-dimethylacetamide + ethanol + water) at the temperature 313.15 K

Total vapour pressures, measured at the temperature 313.15 K, are reported for the ternary mixture (N,N-dimethylacetamide + ethanol + water), and for binary constituent (N,N-dimethylacetamide + ethanol). The present results are also compared with previously obtained data for (amide + ethanol) binary mixtures, where amide = N-methylformamide, N,N-dimethylformamide, N-methylacetamide, 2-pyrrolidinone, and N-methylpyrrolidinone. We found that excess Gibbs free energy of mixing for binary (amide + ethanol) mixtures varies roughly linearly with the molar volume of amide.     

Phase equilibrium measurements and modelling of ternary system (carbon dioxide + ethanol + palmitic acid)

This work reports phase equilibrium measurements for the ternary system (palmitic acid + ethanol + CO₂). The motivation of this research relies on the fact that palmitic acid is the major compound of several vegetable oils. Besides, equilibrium data for palmitic acid in carbon dioxide using ethanol as co-solvent are scarce in the literature. Phase equilibrium experiments were performed using a high-pressure variable-volume view cell over the temperature range of (303 to 343) K and pressures up to 20 MPa and mole fraction of palmitic acid from 0.0199 to 0.2930. Vapour–liquid and solid–fluid transitions were visually observed for the system studied. The Peng–Robinson equation of state, with the classical van der Waals quadratic mixing rule was employed for thermodynamic modelling of the system investigated with a satisfactory agreement between experimental and calculated values.     

Phase equilibria for H2 + CO2 + H2O system containing gas hydrates

Isothermal phase equilibrium (pressure–composition in the gas phase) for the ternary system of H₂ + CO₂ + H₂O has been investigated in the presence of gas hydrate phase. Three-phase equilibrium pressure increases with the H₂ composition of gas phase. The Raman spectra suggest that H₂ is not enclathrated in the hydrate-cages and behaves only like the diluent gas toward the formation of CO₂ hydrate. This fact is also supported by the thermodynamic analysis using Soave–Redlich–Kwong equation of state.     

Hydrate phase equilibrium and structure for (methane + ethane + tetrahydrofuran + water) system

The separation of methane and ethane through forming hydrate is a possible choice in natural gas, oil processing, or ethylene producing. The hydrate formation conditions of five groups of (methane + ethane) binary gas mixtures in the presence of 0.06 mole fraction tetrahydrofuran (THF) in water were obtained at temperatures ranging from (277.7 to 288.2) K. In most cases, the presence of THF in water can lower the hydrate formation pressure of (methane + ethane) remarkably. However, when the composition of ethane is as high as 0.832, it is more difficult to form hydrate than without THF system. Phase equilibrium model for hydrates containing THF was developed based on a two-step hydrate formation mechanism. The structure of hydrates formed from (methane + ethane + THF + water) system was also determined by Raman spectroscopy. When THF concentration in initial aqueous solution was only 0.06 mole fraction, the coexistence of structure I hydrate dominated by ethane and structure II hydrate dominated by THF in the hydrate sample was clearly demonstrated by Raman spectroscopic data. On the contrary, only structure II hydrate existed in the hydrate sample formed from (methane + ethane + THF + water) system when THF concentration in initial aqueous solution was increased to 0.10 mole fraction. It indicated that higher THF concentration inhibited the formation of structure I hydrate dominated by ethane and therefore lowered the trapping of ethane in hydrate. It implies a very promising method to increase the separation efficiency of methane and ethane.     

Phase equilibrium data for the ternary system (propane + chloroform + oryzanol)

The compound oryzanol available in the rice bran (oriza sativa) is well known for its antioxidant activity. Phase equilibrium data involving oryzanol in compressed fluids, hardly found in the literature, are important to provide the basis for the extraction and fractionation processes. In this sense, the aim of this work is to report phase equilibrium measurements for the system (γ-oryzanol + chloroform) in compressed propane. Phase equilibrium experiments were performed using the static synthetic method (cloud points transition data) in a high-pressure variable-volume view cell in the temperature range of 303 K to 353 K, pressures up to 17 MPa, for oryzanol overall mass fractions of 2 wt%, 5 wt% and 10 wt% in (propane + chloroform) mixtures. A complex phase behaviour comprising vapour–liquid, liquid–liquid, vapour–liquid–liquid, solid–liquid, solid–liquid–liquid, solid–liquid–liquid–vapour transitions were visually observed for the system studied.     

Hydrate phase equilibrium for the (hydrogen + tert-butylamine + water) system

The three-phase equilibrium conditions of ternary (hydrogen + tert-butylamine + water) system were first measured under high-pressure in a “full view” sapphire cell. The tert-butylamine–hydrogen binary hydrate phase transition points were obtained through determining the points of intersection of three phases (H–L_w–V) to two phases (L_w–V) experimentally. Measurements were made using an isochoric method. Firstly, (tetrahydrofuran + hydrogen) binary hydrate phase equilibrium data were determined with this method and compared with the corresponding experimental data reported in the literatures and the acceptable agreements demonstrated the reliability of the experimental method used in this work. The experimental investigation on (tert-butylamine + hydrogen) binary hydrate phase equilibrium was then carried out within the temperature range of (268.4 to 274.7) K and in the pressure range of (9.54 to 29.95) MPa at (0.0556, 0.0886, 0.0975, and 0.13) mole fraction of tert-butylamine. The three-phase equilibrium curve (H + L_w + V) was found to be dependent on the concentration of tert-butylamine solution. Dissociation experimental results showed that tert-butylamine as a hydrate former shifted hydrate stability region to lower pressure and higher temperature.     

Phase equilibrium measurements for clathrate hydrates of flue gas (CO2 + N2 + O2) in the presence of tetra-n-butyl ammonium bromide or tri-n-butylphosphine oxide

This paper reports the measured hydrate phase equilibria of simulated flue gas (12.6 vol% CO₂, 80.5 vol% N₂, 6.9 vol% O₂) in the presence of tetra-n-butyl ammonium bromide (TBAB) or tri-n-butylphosphine oxide (TBPO), at (0, 5 and 26) wt%, respectively. The measurements of the phase boundary between (hydrate + liquid + vapor) (H + L + V) phases and (liquid + vapor) (L + V) phases were performed within the temperature range (275.97 to 293.99) K and pressure range (1.56 to 18.78) MPa with using the isochoric step-heating pressure search method. It was found that addition of TBAB or TBPO allowed the incipient equilibrium hydrate formation conditions for the flue gas to become milder. Compared to TBAB, TBPO was largely more effective in reducing the phase equilibrium pressure.     

Liquid–liquid equilibria of ternary mixture (propargyl alcohol + diisopropyl ether + water)

The liquid–liquid equilibria (LLE) of ternary mixture (propargyl alcohol + diisopropyl ether + water) were measured under atmospheric pressure and at different temperatures of 297.25, 304.35, 313.15, and 323.25 K. It was found that the end points of tie-lines at the four temperatures were located almost on a common solubility curve but the tie-lines possessed different slopes that described different equilibrium relations. A comparison of the predicted values through use of UNIFAC method with the measured LLE data was carried out but the predicted values showed remarkable deviations from the experimental data.     

Experimental (vapour + liquid) equilibrium data of (methanol + water), (water + glycerol) and (methanol + glycerol) systems at atmospheric and sub-atmospheric pressures

Experimental (vapour + liquid) equilibrium results for the binary systems, (methanol + water) at the local atmospheric pressure of 95.3 kPa and at sub-atmospheric pressures of (15.19, 29.38, 42.66, 56.03, and 67.38) kPa, (water + glycerol) system at pressures (14.19, 29.38, 41.54, 54.72, 63.84, and 95.3) kPa and the (methanol + glycerol) system at pressures (32.02 and 45.3) kPa were obtained over the entire composition range using a Sweitoslwasky-type ebulliometer. The relationship of the liquid composition (x₁) as a function of temperature (T) was found to be well represented by the Wilson model. Computed vapour phase mole fractions, activity coefficients and the measured values along with optimum Wilson parameters are presented.     

Phase equilibria of hydrogen sulfide and carbon dioxide simple hydrates in the presence of methanol, (methanol + NaCl) and (ethylene glycol + NaCl) aqueous solutions

This work aims at reporting the dissociation pressures of hydrogen sulfide and carbon dioxide simple hydrates in the presence of methanol, (methanol + NaCl) and (ethylene glycol + NaCl) aqueous solutions at different temperatures and various concentrations of inhibitor in aqueous solution. The equilibrium results were generated using an isochoric pressure-search method. These values are compared with some selected experimental data from the literature on the dissociation conditions of hydrogen sulfide and carbon dioxide simple hydrates in the presence of pure water to show the inhibition effects of the above mentioned aqueous solutions. Comparisons are finally made between our experimental values and the corresponding literature data. Some disagreements among the literature data and our data are found.     

Phase equilibria for liquid mixtures of (benzonitrile + a carboxylic acid + water) atT = 298.15 K

(Liquid  +  liquid) equilibrium data are presented for mixtures of {benzonitrile(1)  +  acetic acid or propanoic acid or butanoic acid or 2-methylpropanoic acid or pentanoic acid or 3-methylbutanoic acid(2)  +  water(3)} atT =  298.15 K. The relative mutual solubility of each of the carboxylic acids is higher in the benzonitrile layer than in the aqueous layer. The influence of 3-methylbutanoic acid, pentanoic acid, 2-methylpropanoic acid, and butanoic acid on the solubility of the hydrocarbons in benzonitrile is greater than that of the acetic and propanoic acids. Three three-parameter equations have been fitted to the binodal curve data. These equations are compared and discussed in terms of statistical consistency. The NRTL and UNIQUAC models were used to correlate the experimental tie lines and to calculate the phase compositions of the ternary systems. The NRTL equation fitted the experimental data far better than the UNIQUAC equation. Selectivity values for solvent separation efficiency were derived from the tie line data.     

(Liquid + liquid) equilibrium for the system {ethyl stearate(1) + ethanol(2) + glycerol(3)}

This paper reports the results of a new experimental study on the (liquid + liquid) equilibrium of the system {ethyl stearate(1) + ethanol(2) + glycerol(3)} at atmospheric pressure and at T = (313.15 and 323.15) K. The equilibrium compositions were measured by gas chromatography. Ternary diagrams were obtained for each temperature and the equilibrium data were compared to the system in the presence of salt (NaCl) at T = 323.15 K. The experimentally determined (liquid + liquid) equilibrium data were satisfactorily correlated with NRTL and UNIQUAC equations. A comparative analysis was performed using the UNIFAC-LLE group contribution method. From the results presented herein good predictions were obtained for this ternary system.     

Temperature dependence on mutual solubility of binary (methanol + limonene) mixture and (liquid + liquid) equilibria of ternary (methanol + ethanol + limonene) mixture

Mutual solubility data of the binary (methanol + limonene) mixture at the temperatures ranging from 288.15 K close to upper critical solution temperature, and ternary (liquid + liquid) equilibrium (tie-lines) of the (methanol + ethanol + limonene) mixture at the temperatures (288.15, 298.15, and 308.15) K have been obtained. The experimental results have been represented accurately in terms of the extended and modified UNIQUAC models with binary parameters, compared with the UNIQUAC model. The temperature dependence of binary and ternary (liquid + liquid) equilibrium for the binary (methanol + limonene) and ternary (methanol + ethanol + limonene) mixtures could be calculated successfully using the extended and modified UNIQUAC model.     

High-pressure phase equilibrium data for the (carbon dioxide + l-lactide + ethanol) system

Experimental phase equilibrium values (cloud points) for the ternary system involving carbon dioxide, l-lactide and ethanol have been measured in order to provide fundamental values to conduct the polymerization reaction in supercritical carbon dioxide medium. The experiments were performed using a variable-volume view cell over the temperature range from 323 K to 353 K, system pressure between 9 MPa and 25.0 MPa and different mole ratios of ethanol to l-lactide (0.5:1, 1:1 and 1.5:1). Phase transitions of vapour-liquid types were observed. The experimental results were modelled using the Peng–Robinson (PR) equation of state with the Wong–Sandler (PR–WS) mixing rule, providing a good representation of the experimental phase equilibrium values.