Vapor–liquid–liquid Equilibrium Research Articles

Experimental screening towards developing ionic liquid-based extractive distillation in the dearomatization of refinery streams

Abstract Ionic liquids (ILs) are potential neoteric solvents to design new advanced separation processes. Among several separation cases studied so far, the good performance of ILs regarding the dearomatization of liquid fuels, i.e. pyrolysis and reformer gasolines, has received especially attention. Indeed, a wide number of works has been done to characterize the phase equilibria for {aliphatic + aromatic + ILs} systems as well as the IL thermophysical properties, concluding in the development of a liquid-liquid extraction process. However, this technology seems not to be enough to fulfill current aromatic commercial standards nor potential incoming restrictions for aromatic content in liquid fuels as a result of its low separation effectiveness for extreme aliphatic and aromatic purification. Extractive distillation with ILs stands as a new process configuration to overcome these limitations by enhancing the aliphatic/aromatic relative volatilities. In this work, an IL experimental screening in the n-heptane/toluene separation was done to further develop this new IL-based technology. Nine ILs were tested as mass agents in a wide range of conditions, i.e. solvent to feed (S/F) ratios from 1 to 10 and temperatures from 323.2 to 403.2 K. The required vapor-liquid-liquid equilibria (VLLE) data were obtained by an experimental procedure based on headspace gas chromatography (HS-GC) developed in the framework of this work. Although all pre-selected ILs have shown good performance, tricyanomethanide-based ILs have been the most promising mass agents.

The capability of tetra alkyl ammonium bromides for aqueous biphasic systems formation with both polymers and electrolytes in aqueous solutions

It was found that the tetra alkyl ammonium halides can solute-out the water soluble polymers and solute out by electrolyte in aqueous solutions and form the aqueous biphasic systems (ABS). To investigate these new kinds of ABS, vapor – liquid equilibria (VLE), vapor – liquid - liquid equilibria (VLLE) and liquid – liquid equilibria (LLE) measurements were carried out for ternary systems containing tetra alkyl ammonium bromides (TAAB): tetra methyl ammonium bromide (TMAB), tetra ethyl ammonium bromide (TEAB), tetra propyl ammonium bromide (TPAB) and tetra butyl ammonium bromide (TBAB); electrolytes: NaCl, NaNO3, Na2CO3 and Na3Cit; and polymers: polypropylene glycol400 (PPG400) and polyethylene glycol400 (PEG400) at different temperatures. It was found that in these systems the soluting-out effect and then the tendency to ABS formation increases by increasing the difference between the hydrophilicity of two components. In the case of TAAB + polymer aqueous systems, the polymer is solute out by the more hydrophilic component TAAB and therefore ABS are formed for aqueous solutions containing PPG (which is more hydrophobic than PEG) and TMAB/TEMB (which are more hydrophilic than TPAB and TBAB). In these types of ABS, the soluting-out effects increase with an increase in temperature. On the other hand, in the case of TAAB + salt aqueous systems, the TAAB is solute out by salt which is the more hydrophilic than TAAB and therefore aqueous solutions containing (TBAB + NaNO3, Na2CO3 and Na3Cit), (TPAB and TEAB + Na2CO3 and Na3Cit) and (TMAB + Na2CO3) are undergoing phase separation and their soluting-out effects increase with decreasing temperature. The isopiestic measurements of the investigated systems show that the constant water activity lines of aqueous PPG + TAAB systems show the positive and the negative deviation from the semi-ideal behavior respectively in biphasic and monophasic area, but those of aqueous salt + TAAB systems show the negative deviation in both biphasic and monophasic area. However, the constant water activity lines of the systems with soluting-in effect, show the positive deviation from the semi-ideal behavior.

On the volatility of aromatic hydrocarbons in ionic liquids: Vapor-liquid equilibrium measurements and theoretical analysis

The use of ionic liquids (ILs) as solvent in the liquid-liquid extraction of aromatic compounds is one of their most studied applications. Nevertheless, the recovery of the extracted hydrocarbons has been much less investigated, being a required task to complete the global separation process. Taking into account the negligible vapor pressure of the ILs, this step could be easily carried out by flash distillation, which requires the study of vapor-liquid equilibrium (VLE). In order to study this topic deeper, in this work a systematic analysis of the VLE and vapor-liquid-liquid equilibrium (VLLE) data for {aromatic hydrocarbon+IL} binary mixtures was carried out, from both an experimental and computational point of view. For that, new experimental VLE and VLLE data of 24 {toluene+IL} binary mixtures were measured at 323.15K using a technique based on the static headspace gas chromatography (HS-GC), providing relevant information on the toluene retained in the liquid depending on the cation/anion structure of the IL in the mixture. Furthermore, the quantum chemical Conductor-like Screening Model for Real Solvents (COSMO-RS) method was applied to better understand the structure-property relationship determining the phase behavior of {aromatic hydrocarbon+IL} binary systems. First, the suitability of COSMO-RS to predict VLE and VLLE data of {toluene+IL} binary mixtures was evaluated by comparison to 225 experimental data at 323.15K, including 24 different ILs over the whole composition range. Valuable conclusions were achieved respect to the molecular model of IL needed to adequately predict VLE and VLLE data of the {aromatic hydrocarbon+IL} binary mixtures. Once the computational approach was stated, COSMO-RS methodology was used to analyze the influence of the intermolecular interactions between the toluene and the IL component on the phase behavior of their mixtures. As a result, COSMO-RS was demonstrated as a useful tool for the rational design of ILs with optimized properties for the separation of aromatic+aliphatic hydrocarbon binary mixtures, considering both liquid-liquid extraction and solvent regeneration steps.

Calculation of Multiphase Chemical Equilibrium by the Modified RAND Method

A robust and efficient algorithm for simultaneous chemical and phase equilibrium calculations is proposed. It combines two individual nonstoichiometric solving procedures: a nested-loop method with successive substitution for the first steps and final convergence with the second-order modified RAND method. The modified RAND extends the classical RAND method from single-phase chemical reaction equilibrium of ideal systems to multiphase chemical equilibrium of nonideal systems. All components in all phases are treated in the same manner and the system Gibbs energy can be used to monitor convergence. This is the first time that modified RAND was applied to multiphase chemical equilibrium systems. The combined algorithm was tested using nine examples covering vapor–liquid (VLE) and vapor–liquid–liquid equilibria (VLLE) of ideal and nonideal reaction systems. Successive substitution provided good initial estimates for the accelerated computation with modified RAND, to ultimately converge to the equilibrium sol...

Experimental studies and thermodynamic analysis of isobaric vapor-liquid-liquid equilibria of 2-propanol + water system using n-propyl acetate and isopropyl acetate as entrainers

Heterogeneous azeotropic distillation is a widely used technique in chemical industries for the purification of alcohols from their aqueous mixtures. This study aims at exploring the possibility of using n-propyl acetate and isopropyl acetate as entrainers for heterogeneous azeotropic distillation to effect the separation of 2-propanol + water azeotropic mixture. A novel yet simple still was constructed to measure the vapor-liquid equilibrium (VLE) and vapor-liquid-liquid equilibrium (VLLE) data for the binary and ternary systems. The isobaric VLE data for the binary systems 2-propanol + water, 2-propanol + n-propyl acetate and 2-propanol + isopropyl acetate and isobaric VLLE data for the ternary systems 2-propanol + water + n-propyl acetate and 2-propanol + water + isopropyl acetate were measured at atmospheric pressure. Subsequently, the experimental data was compared with predictions by thermodynamic models such as UNIFAC, UNIQUAC and NRTL. The feasibility of the entrainers n-propyl acetate and isopropyl acetate in separating the azeotropic mixture 2-propanol + water was studied using the Residue Curve Map (RCM). The experimental data showed the existence of a ternary homogeneous azeotrope in the 2-propanol + water + n-propyl acetate system and a heterogeneous ternary azeotrope in the 2-propanol + water + isopropyl acetate system. Since the heterogeneous azeotropic distillation process depends on the presence of ternary heterogeneous azeotrope, n-propyl acetate is not considered a suitable entrainer for the separation of 2-propanol + water mixture. Finally, a separation sequence for the heterogeneous azeotropic distillation of 2-propanol + water system using isopropyl acetate as entrainer has been proposed.

Calculation of simultaneous chemical and phase equilibrium by the method of Lagrange multipliers

The purpose of this work is to develop a general, reliable and efficient algorithm, which is able to deal with multiple reactions in multiphase systems. We selected the method of Lagrange multipliers to minimize the Gibbs energy of the system, under material balance constraints. Lagrange multipliers and phase amounts are the independent variables, whose initialization is performed by solving a subset of the working equations. This initialization is the unconstrained minimization of a convex function and it is bound to converge. The whole solution procedure employs a nested loop with Newton iteration in the inner loop and non-ideality updated in the outer loop, thus giving an overall linear convergence rate. Stability analysis is used to introduce additional phases sequentially so as to obtain the final multiphase solution. The procedure was successfully tested on vapor-liquid equilibrium (VLE) and vapor-liquid-liquid equilibrium (VLLE) of reaction systems.

A rigorous method to evaluate the consistency of experimental data in phase equilibria. Application to VLE and VLLE

This work forms part of a broader study that describes a methodology to validate experimental data of phase equilibria for multicomponent systems from a thermodynamic‐mathematical perspective. The goal of this article is to present and justify this method and to study its application to vapor–liquid equilibria (VLE) and vapor–liquid–liquid equilibria (VLLE), obtained under isobaric/isothermal conditions. A procedure based on the Gibbs‐Duhem equation is established which presents two independent calculation paths for its resolution: (a) an integral method and (b) a differential method. Functions are generated for both cases that establish the verification or consistency of data, δψ for the integral test and δζ for the differential approach, which are statistically evaluated by their corresponding average values [ , ], and the standard deviations [ , ]. The evaluation of these parameters for application to real cases is carried out using a set of hypothetical systems (with data generated artificially), for which the values are adequately changed to determine their influence on the method. In this way, the requirements of the proposed method for the data are evaluated and their behavior in response to any disruption in the canonical variables (p,T, phase compositions). The conditions for thermodynamic consistency of data are: , , , and . In systems with VLLE, in addition to the previous criteria, must occur that: and . The new proposed method has been tested with a set of 300 experimental binary systems, biphasic and triphasic, obtained from published bibliography, and the results are compared with those of other tests commonly used for testing thermodynamic consistency. The results show that the greater rigor of the proposed method is mainly due to the simultaneous verification of various independent variables. As a result, the conditions for the new test are verified for fewer systems than using other tests mentioned in the literature (i.e., Fredenslund‐test and direct of Van Ness). Its unique application is sufficient to ensure the consistency of experimental data, without using other tests. © 2017 American Institute of Chemical Engineers AIChE J, 2017

Proposal of Isobutyl Alcohol as Entrainer To Separate Mixtures Formed by Ethanol and Water and 1-Propanol and Water

Isobutyl alcohol (IBA) has been proposed as a solvent to carry out the dehydration of ethanol and 1-propanol by means of a nonconventional distillation process. In this way, isobaric vapor–liquid equilibrium (VLE) and vapor–liquid–liquid equilibrium (VLLE) data at atmospheric pressure have been obtained for the ternary systems ethanol (1a) + water (2) + IBA (3) and 1-propanol (1b) + water (2) + IBA (3). Then, these data have been correlated to obtain a set of parameters capable of estimating VLE and VLLE. According to the results achieved, distillation sequences to separate water and ethanol or 1-propanol have been proposed. Finally, a study about the minimum amount of entrainer required to break the azeotrope of the mixture ethanol/1-propanol + water is carried out.

Isobaric Vapor–Liquid–Liquid Phase Equilibria Measurements of Three Ternary Water + 2-Butanone + Aliphatic Alcohol (Ethanol, 1-Propanol, 2-Propanol) Systems at 101.3 kPa

Vapor–liquid–liquid equilibrium (VLLE) data were measured for the ternary systems water + 2-butanone + alcohol at 101.3 kPa by means of a Gillespie type still equipped with an ultrasonic homogenizer. VLLE were observed in the temperature ranges of 346.65–346.79 K, 346.74–352.70 K, and 346.52–346.75 K for the water + 2-butanone + ethanol, water + 2-butanone + 1-propanol, and water + 2-butanone + 2-propanol systems, respectively. For all three systems, a region of liquid–liquid immiscibility was observed, and the vapor phase compositions were outside the liquid–liquid phase envelope. The absence of ternary heterogeneous azeotropes was thus established. The nonrandom two-liquid (NRTL), universal quasichemical (UNIQUAC), and universal functional (UNIFAC) activity coefficient models were used for comparison with experimental results. The parameters were fitted to the binary subsystems vapor–liquid and liquid–liquid equilibrium experimental data. While correct estimations of experimental vapor phase trends were...

Heterogeneous azeotropic distillation for the separation of n-propanol + water mixture using n-propyl acetate as entrainer

Heterogeneous azeotropic distillation is a widely used process in the chemical process industries for the separation of binary azeotropic mixtures into their constituent pure components. It involves the addition of an entrainer which is partially miscible and forms a heterogeneous azeotrope preferably with one of the original components. For the design of heterogeneous azeotropic distillation columns, a detailed study of the heterogeneous region is crucial and this requires experimental vapor-liquid-liquid equilibrium (VLLE) data. The present study focuses on the study of n-propyl acetate as entrainer to effect the separation of industrially available azeotropic mixture n-propanol-water. A new and simple still was constructed to measure the VLLE data for the ternary system. The isobaric VLE data for the two binary systems n-propanol - water and n-propanol - n-propyl acetate and isobaric VLLE data for the ternary system n-propanol - water - n-propyl acetate were measured at atmospheric pressure. The experimental results have been compared with various thermodynamic models such as NRTL, UNIQUAC, UNIFAC (VLE) and UNIFAC (LLE). The set of parameters obtained from the above models has been used to construct the residue curve map (RCM) for the ternary system and the effect of n-propyl acetate to accomplish the separation of the azeotropic mixture has been discussed.

Simulation of N-Propanol Dehydration Process Via Heterogeneous Azeotropic Distillation Using the NRTL Equation

Abstract Numerical values of the NRTL equation parameters for calculation of the vapour - liquid - liquid equilibria (VLLE) at atmospheric pressures have been presented for 5 ternary mixtures. These values were fitted to the experimental VLLE and vapour - liquid equilibrium (VLE) data to describe simultaneously, as accurately as possible, the VLE and the liquid - liquid equilibria (LLE). The coefficients of this model called further NRTL-VLL were used for simulations of n-propanol dehydration via heterogeneous azeotropic distillation. The calculations performed by a ChemCAD simulator were done for 4 mixtures using hydrocarbons, ether and ester as an entrainer. In majority simulations the top streams of the azeotropic column had composition and temperature similar to the corresponding experimental values of ternary azeotropes. The agreement between the concentrations of both liquid phases formed in a decanter and the experimental values of the LLE was good for all four simulations. The energy requirements were the most advantageous for the simulation with di-npropyl ether (DNPE) and isooctane. Simulations were performed also for one mixture using the NRTL equation coefficients taken from the ChemCAD database. In that case the compositions of the liquid organic phases leaving the decanter differed significantly from the experimental LLE data.

A New PC-SAFT Model for Pure Water, Water–Hydrocarbons, and Water–Oxygenates Systems and Subsequent Modeling of VLE, VLLE, and LLE

Accurate analytic thermodynamic modeling of water and its mixtures with hydrocarbon and oxygenates is difficult even with new and advanced equations of state such as the perturbed-chain statistical associating fluid theory (PC-SAFT). Several attempts have been made in the past by various authors to solve this issue. However, current models generally fail to describe simultaneously and accurately pure water properties (especially its liquid density) and liquid–liquid equilibria for mixtures involving water, hydrocarbons, and oxygenates. In the current work, this problem is dealt with by modification in the fundamental structure of the model. It was established that the temperature dependent diameter d(T) does not behave in the same way for water as it inscribed in the original model. Hence, a modification was proposed for d(T) of water in order to correctly represent the phase behavior of pure water and its mixtures with hydrocarbons and oxygenates. The deviations in saturated liquid densities and vapor pressure for pure water were reduced to 0.6% and 2.2%, respectively, in a large temperature range. The results for liquid–liquid equilibrium (LLE), vapor–liquid equilibrium (VLE) and vapor–liquid–liquid equilibrium (VLLE) of various water–hydrocarbons and oxygenates show the accuracy of this new model and its predictive capability when coupled with a group contribution approach. For certain oxygenated mixtures such as water with aldehydes, ketones, ethers, and esters a new contribution to the Helmholtz energy, known as the “non-additive hard sphere” contribution, was used. The cross-interaction parameters obtained for mixtures were validated qualitatively by calculating octanol/water partition coefficients and the Gibbs free energy of hydrogen bonding (ΔGHB). Results are found in good agreement with experimental data.

Application of the Aldolization Reaction in Separating the Mixture of Ethylene Glycol and 1,2-Butanediol: Thermodynamics and New Separation Process

The separation of ethylene glycol (EG) and 1,2-butanediol (1,2-BDO) azeotrope is a key technical problem in the synthesis process of EG via dimethyl oxalate (DMO) from syngas. On the basis of systematic investigation, aldolization is expected to be the solution to this industrial problem. Thus, the essential thermodynamics data were determined and correlated well by the corresponding thermodynamic equation, including the vapor pressure of 2-methyl-1,3-dioxolane (2MD) and 4-ethyl-2-methyl-1,3-dioxolane (4EMD), vapor–liquid equilibrium (VLE) data of binary mixture 2MD–4EMD at 101.3 kPa, and liquid–liquid equilibrium (LLE) data of binary system 4EMD–water at atmospheric conditions. The vapor–liquid–liquid equilibrium (VLLE) of binary system 4EMD–water has been successful predicted by LLE experimental data. Finally, the thermodynamics parameters were provided as a reference to design a separation process of the EG and 1,2-BDO mixture. The simulation and optimization results indicate that EG and 1,2-BDO could ...

Measurement and prediction of phase equilibria of ethylene + vinyl acetate + methanol + poly(ethylene-co-vinyl acetate) systems

This work investigated the solubilities of ethylene in the 10 wt% poly (ethylene-co-vinyl acetate) (EVA) solutions including vinyl acetate (VA) and methanol which are related to production of EVA. The bubble point pressure (BP) for vapor-liquid equilibrium (VLE) and the cloud point pressure (CP) for liquid-liquid equilibrium (LLE) of ethylene + VA + EVA ternary and ethylene + VA + EVA + methanol quaternary systems were measured at temperatures of 313 K, 343 K, 373 K and at pressures up to 20 MPa with a synthetic method. The experimental results revealed that the BP and CP increased with increasing temperature and ethylene weight fraction, especially the influence of ethylene weight fraction on CP was found to be drastic. The Sanchez-Lacombe equation of state (SL EOS) could correlate the BP data of ethylene + VA + EVA ternary and ethylene + VA + EVA + methanol quaternary systems within 3.6% an average relative deviation. The phase equilibria of the EVA + VA + ethylene ternary system could be predicted over wide polymer concentration ranges by the SL EOS with the determined parameters in the correlations. With increasing pressure, phase behavior changed from VLE at low pressure to vapor-liquid-liquid equilibrium (VLLE) and LLE at high pressures.

Isothermal vapour-liquid equilibrium data for the binary systems of (CHF3 or C2F6) and n-heptane

Isothermal vapour-liquid equilibrium (VLE) values for two binary systems; trifluoromethane and n-heptane at temperatures between T=(272.9 and 313.2)K, and hexafluoroethane and n-heptane at temperatures between T=(293.0 and 313.2)K were measured with a static-analytic apparatus. Bubble pressures at temperatures between T=(293.0 and 313.2)K, at several compositions, were also measured with a variable-volume static-synthetic apparatus. Vapour-liquid-liquid equilibrium (VLLE) was found to occur for certain isotherms for both of the systems. The PR EOS, with the Mathias-Copeman (MC) alpha function, combined with either the classical mixing rule or the Wong-Sandler (WS) mixing rule was used to correlate the experimental results. Either the NRTL or the UNIQUAC activity coefficient model was used within the WS mixing rule.The indirect extended scaling laws of Ungerer et al. were used to extrapolate critical loci from the experimental coexistence data, and the calculation procedure of Heidemann and Khalil was employed to calculate the mixture critical locus curves at temperatures close to the refrigerant critical temperatures. At lower temperatures on the mixture critical curve, gas-liquid critical points occurred, whereas, at higher temperatures, the critical points occurred along a liquid-liquid locus curve. The two systems were categorised according to the van Konynenburg and Scott classification.

Phase behavior of the carbon dioxide + 1-dodecanol system at high pressures

Vapor–liquid (VLE), liquid–liquid (LLE), and vapor–liquid–liquid equilibria (VLLE) data for the carbon dioxide + 1-dodecanol system at 293.15, 303.15, 313.15, 333.15, and 353.15 K up to 9.36 MPa and phase compositions of the two liquid phases and vapor phase as a function of temperature along the liquid–liquid–vapor (LLV) line are reported. Phase behavior measurements were made in a high-pressure visual cell with variable volume, based on the static-analytic method. The new experimental results are discussed and compared with available literature data. New measured and literature data for carbon dioxide + 1-dodecanol system were modeled with several cubic equations of state (EoS), namely the General Equation of State (GEOS), Peng–Robinson (PR), and Soave–Redlich–Kwong (SRK), using classical van der Waals (two-parameter conventional mixing rule, 2PCMR) mixing rules. A non-cubic hard-sphere perturbed equation of state, CSBM GEOS, was also considered, in order to test its capability to describe the equilibrium phase behavior in systems with molecular asymmetry.

Modeling of vapor–liquid–liquid equilibria in binary mixtures

Vapor compression and Joule–Thomson (JT) cycles provide cooling power at the boiling temperatures of the refrigerants. Maintaining a fixed pressure in the evaporator allows for a stable cooling temperature at the boiling point of a pure refrigerant. In these coolers enhanced cooling power can be achieved by using mixed refrigerants. However, gas mixtures usually do not change their phase at a constant temperature, therefore, the cooling temperature has to be actively controlled. An exception to this rule holds for binary mixtures that can form a vapor–liquid–liquid equilibrium (VLLE).Phase equilibria in binary mixtures are usually modeled based on experimental results only. In the present study only the vapor pressures of the pure mixture components are required. The calculated results of nitrogen–ethane, nitrogen–ethylene, and nitrogen–propane mixtures are compared with experimental data presented in literature showing deviations of less than 1%.

Simultaneous VLLE data correlation for ternary systems: Modification of the NRTL equation for improved calculations

Simultaneous correlation of vapour–liquid–liquid equilibrium (VLLE) data is hardly ever attempted in literature with one common set of parameters for all the equilibrium regions present in the systems. It is common practice to obtain different sets of parameters for VLE, LLE and VLLE regions when experimental equilibrium data for ternary systems are fitted with a given model (e.g. an activity coefficient model for the liquid phase). Besides, when dealing with the correlation of VLE for ternary systems, it is quite frequent to obtain different sets of parameters for each one of the three binary subsystems from those obtained when exclusively binary VLE data are correlated. In the present work, the simultaneous correlation of all the equilibrium regions in the ternary VLL system is attempted. The objective function and the restrictions of the system are clearly presented. In addition, a simple modification of the NRTL equation for the Gibbs energy of mixing (GM) is proposed including a ternary term and a binary correction based on the Wohl equation. This modification has significantly improved the quality of the results obtained in the simultaneous correlation of different equilibrium regions.

Isobaric Vapor–Liquid–Liquid Equilibrium for Water + Cyclohexane + Acrylic Acid at 200 mmHg

Isobaric vapor–liquid–liquid equilibrium (VLLE) data were measured for the ternary system water + cyclohexane + acrylic acid at 200 mmHg (absolute pressure). The apparatus are a modified dynamic recirculating still equipped with mixing section for experimental test and a gas chromatography (Agilent GC7890A) for analyzing, respectively. The VLLE data have been used to verify the dependability of the NRTL, NRTL-HOC, and UNIQUAC-HOC models prediction results for the water + cyclohexane + acrylic acid system. The parameters for these models were obtained using experimental data regression. Comparing the predictions and experimental data, the HOC equations have a good effect in vapor phase prediction with a strong self-polymerization of compounds system, and the NRTL-HOC model has a more acceptable prediction of the VLLE data than the UNIQUAC-HOC model.

Isothermal (vapour + liquid) equilibrium data for binary systems of (n-hexane + CO2 or CHF3)

The (vapour+liquid) equilibrium (VLE) was measured for the (carbon dioxide+n-hexane) binary system at temperatures between T=(303.1 and 323.1)K. In addition, VLE and (vapour+liquid+liquid) equilibria (VLLE) were determined for the (trifluoromethane+n-hexane) binary system at temperatures between T=(272.9 and 313.3)K and pressures in the range of P=(1.0 to 5.7)MPa. Measurements were undertaken in a static-analytic apparatus, with verification of experimental values undertaken using a static-synthetic equilibrium cell to measure bubble point pressures at several compositions.The phase equilibrium results were modelled with the Peng–Robinson equation of state with the Mathias–Copeman alpha function, coupled with the Wong–Sandler mixing rules. Regression of the data was performed with the NRTL and the UNIQUAC activity coefficient models with the Wong–Sandler mixing rules, and the performance of the models was compared. Critical loci for both systems were estimated, using the calculation procedures of Ungerer et al. and Heidemann and Khalil.For the (trifluoromethane+n-hexane) system, liquid–liquid immiscibility was experienced at the lowest temperature measured (T=272.9K). At higher temperatures, no immiscibility was visible during the measurements; however, the models continued to predict a miscibility gap.