Mixture Critical Point Research Articles

Global phase behavior of imidazolium ionic liquids and compressed 1,1,1,2‐tetrafluoroethane (R‐134a)

AbstractNovel processes involving ionic liquids with refrigerant gases have recently been developed. Here, the complete global phase behavior has been measured for the refrigerant gas, 1,1,1,2‐tetrafluoroethane (R‐134a) and 1‐n‐alkyl‐3‐methyl‐imidazolium ionic liquids with the anions hexafluorophosphate [PF6], tetrafluoroborate [BF4] and bis(trifluoromethylsulfonyl)imide [Tf2N] from ∼0°C to 105°C and to 33 MPa. All of the systems studied were Type V from the classification scheme of Scott‐van Konynenburg with regions of vapor‐liquid equilibrium, miscible/critical regions, vapor‐liquid‐liquid equilibrium, and upper and lower critical endpoints (UCEP and LCEP). The effect of the alkyl chain length has been investigated, for ethyl‐([EMIm]), n‐butyl‐([BMIm]), and n‐hexyl‐([HMIm]). With increasing chain length, the temperature of the lower critical end points increases and pressure at the mixture critical points decrease. With a common cation, the temperature of the LCEP increased and the mixture critical point pressures decreased in the order of [BF4], [PF6], and [Tf2N]. © 2008 American Institute of Chemical Engineers AIChE J, 2009

Isothermal P, x, y data for the argon + carbon dioxide system at six temperatures from 233.32 to 299.21 K and pressures up to 14 MPa

Isothermal vapor–liquid equilibrium data have been measured for the argon + carbon dioxide binary system at 6 temperatures from 233.32 to 299.21 K, and at pressures from 1.5 to 14 MPa. The triple point temperature of CO 2 (216.58 K) is higher than the Ar critical temperature (150.86 K). As a consequence, working at temperatures higher than the triple point temperature of CO 2, it is not possible to define the vapor pressure of pure Ar and the vapor–liquid equilibrium isotherms will always present a mixture critical point. VLE data have been measured using the “static–analytic” method, taking advantage of two pneumatic capillary samplers (Rolsi™, Armines’s patent) designed and developed in the CEP/TEP laboratory. The isothermal P, x, y data are well represented with the Peng–Robinson equation of state combined with Mathias–Copeman alpha function and the Wong–Sandler mixing rules involving the NRTL model.

Supercritical antisolvent micronization of cyclodextrins

Micronization of α- and β-cyclodextrins solubilized in dimethylsulfoxide (DMSO) has been successfully performed using the Supercritical AntiSolvent (SAS) precipitation. We obtained sub-microparticles, microparticles and expanded microparticles of both cyclodextrins, ranging from about 0.1 to 11 μm, varying the concentration of the liquid solution from 5 to 200 mg/mL, process temperature (40–60 °C) and pressure (90–180 bar). Particularly, we observed for both materials that, increasing the concentration of the liquid solution, decreasing the pressure or increasing the temperature, the mean particle size increased and the particle size distribution enlarged. We also tried to relate the morphologies obtained to the position of the process operating point with respect to the mixture critical point (MCP) of the ternary system cyclodextrin–DMSO–CO 2.

Supercritical carbon dioxide fractionation of Lavandin essential oil: Experiments and modeling

The two main components of Lavandin essential oil are linalool and linalyl acetate. These components have different applications in the food and fragrance industry, and also for producing natural herbicides and insecticides in the case of linalool. Thus there is a commercial interest in the separation of the essential oil into linalool- and linalyl acetate-rich fractions. This work presents a study of this fractionation using supercritical CO 2 in a continuous operating high-pressure counter-current packed column. The influence of process parameters like flowrate, pressure and temperature has been determined. The separation is limited by the low gas load achieved, and it has been observed that in order to increase the load, high temperatures and CO 2/oil phase ratios were necessary with operating pressures close to the mixture critical point. Additionally, a detailed mathematical model of the fractionation column has been developed and validated with the pilot plant results. This model has been used to study the operation with conditions that exceed the capabilities of the pilot plant such as a higher CO 2/oil phase ratio or a longer fractionation column, and to propose the optimum conditions for this fractionation.

Supercritical AntiSolvent micronization of nalmefene HCl on laboratory and pilot scale

Supercritical AntiSolvent precipitation (SAS) has been used to produce micronized particles of nalmefene hydrochloride, a selective narcotic antagonist and a promising adjunctive for the treatment of several kinds of addiction. Experiments have been performed using a laboratory scale apparatus and a semi-industrial scale plant. Ethyl alcohol (EtOH) has been used as liquid solvent because of its acceptability in processing pharmaceutical compounds. The effects of SAS process parameters such as temperature, pressure, at a constant concentration of the solute in the liquid solution (1.9 wt.%) have been studied. We have related the different powder morphologies obtained to the position of the process operating point with respect to the vapor–liquid equilibria (VLEs) of the ternary system nalmefene HCl/EtOH/CO 2. Operating SAS well above the mixture critical point (MCP) of the mixture, nano-particles have been generated with a mean diameter ranging from 200 to 300 nm, near and below the MCP in the gas phase region, we have obtained submicro- and micro-particles with mean diameters ranging from 0.5 to 2 μm. Balloons larger than 10–20 μm in diameter and crystals have been also obtained. The results of interest obtained on the laboratory scale plant have been successfully reproduced on the semi-industrial scale plant. The micronized product has also been characterized using powder diffraction X-ray (XRD) and differential scanning calorimetry (DSC), to verify the influence of the micronization process on the final product properties. XRD analysis demonstrated that it is possible to obtain amorphous or crystalline precipitates, depending on the process conditions.

High-Pressure Vapor−Liquid Equilibria in the Nitrogen + n-Hexane System

In this work, new vapor−liquid equilibrium data for the N2 + C6H14 system were experimentally measured over a wide temperature range from (344 to 488) K and pressures up to 50 MPa. A static−analytical apparatus with visual sapphire windows and pneumatic capillary samplers was used during the experimental measurements. Equilibrium phase compositions and vapor−liquid equilibrium ratios are reported. The new results were compared with those reported by other authors. The comparison showed that the pressure−composition data reported in this work are in good agreement with those determined by others but they lie closer to the mixture critical point at each temperature level. The experimental data were modeled with the PR and PC-SAFT equations of state by using one-fluid mixing rules and a single temperature-independent interaction parameter. Results from the modeling effort showed that the PC-SAFT equation was superior to the PR equation in correlating the experimental data of the N2 + C6H14 system.

High-pressure vapor–liquid equilibria in the nitrogen– n-pentane system

New experimental vapor–liquid equilibrium data of the N 2– n-pentane system were measured over a wide temperature range from 344.3 to 447.9 K and pressures up to 35 MPa. A static-analytic apparatus with visual sapphire windows and pneumatic capillary samplers was used in the experimental measurements. Equilibrium phase compositions and vapor–liquid equilibrium ratios are reported. The new results were compared with those reported by other authors. The comparison showed that the pressure–composition data reported in this work are in good agreement with those determined by others but they are closer to the mixture critical point at each temperature level. The experimental data were modeled with the PR and PC-SAFT equations of state by using one-fluid mixing rules and a single temperature independent interaction parameter. Results of the modeling showed that the PC-SAFT equation fit the data satisfactorily even at the highest temperatures of study.

Supercritical antisolvent precipitation of Cephalosporins

We successfully performed the micronization of some Cephalosporinic antibiotics by Supercritical AntiSolvent (SAS) precipitation from Dimethylsulfoxide (DMSO) and evaluated the effect of temperature, pressure, concentration of the liquid solution and carbon dioxide molar fraction on the precipitation process. In particular, we varied temperature from 40 to 60 °C, pressure from 90 to 180 bar, concentration of the liquid solution from 10 to 90 mg/mL and CO 2 molar fraction from 0.5 to 0.98. We obtained different morphologies of precipitates: sub-microparticles, microparticles, balloons (micrometric empty shells) and large crystals. We tried to explain how the presence of solute modifies the vapor–liquid equilibria (VLEs) of the system DMSO–CO 2 and to relate the different morphologies observed to the position of the process operating point with respect to the ternary system mixture critical point (MCP). Particle dimensions range from 0.1 to 14 μm for spherical particles and from 3 to 50 μm for balloons.

Ternary Phase Behavior of Ionic Liquid (IL)−Organic−CO2 Systems

Recently, we have shown that gaseous or supercritical carbon dioxide can be used to induce a liquid−liquid phase split for homogeneous mixtures that consists of methanol−1-butyl-3-methylimidazolium hexafluorophosphate and aqueous/ionic liquid mixtures. [Scurto et al., J. Am. Chem. Soc. 2002, 124, 10276; Chem. Commun. 2003, 572.] Here, the factors that control the vapor−liquid−liquid equilibrium in ionic liquid−organic−carbon dioxide ternary systems at 40 °C are discussed. The lower-critical endpoint pressure (LCEP) locus, i.e., the pressure at which the liquid−liquid phase split occurs, is strongly dependent on the choice of organic, the type of ionic liquid (IL), and the concentration of IL in the organic. On the other hand, the K-point pressure, i.e., the pressure at which one of the liquid phases becomes identical with the CO2-rich phase, is dependent on the choice of organic only and it was determined to be same as the binary mixture critical point for the given organic−CO2 system. The compositions of...

05/01855 Experimental low temperature water content in gaseous methane, liquid ethane, and liquid propane in equilibrium with hydrate at cryogenic conditions: Song, K. Y. et al. Fluid Phase Equilibria, 2004, 224, (2), 271–277

It is well-known that one should be cautious with the use of composition-independent K-value correlations for phase equilibria at high pressures. The first limitation is the correlation's inability to predict convergence of the dew and bubble curves at the mixture's critical point or at a closed phase envelope. Rather, these two curves extend indefinitely beyond the critical temperatures and pressures of all the species in the mixture. However, even at low pressures, where these correlations are usually applied, results can be misleading. This paper presents some practical examples where correlations fail.In the calculation of three-phase flash equilibria one cannot use the same correlation for the K-values between the vapor phase and the liquid, hydrocarbon-rich phase, and the K-values between the vapor phase and aqueous phase. Use of the same correlation provides two linearly dependent equations, making it impossible to obtain a unique solution. The solution of this three-phase flash problem by bisection provides trivial roots, indicating different values of the aqueous and hydrocarbon-rich phase fractions but a unique value for the vapor fraction. The compositions of both liquid phases are clearly identical.A physically sound assumption for the solution of this three-phase flash equilibria problem is that hydrocarbons are insoluble in the aqueous phase. However, the solution of this case for a test ternary of n-heptane, n-decane and water (C7C10H2O) led to distinct multiple roots that satisfy the material balance constraints. Up to 67 roots were found for a system of 40% n-heptane, 20% n-decane and 40% water at 70 psia and 350 °F (a fairly low pressure). Physical properties of the phases evaluated using these roots can be substantially different; therefore, this assumption could lead to serious problems in reservoir engineering calculations.A second assumption of no-solubility of water in the hydrocarbon-rich phase also provides linearly independent equations. However, no simultaneous roots could be found for the test system under study for the same pressure and temperature and various combinations of these two variables.

05/01859 Helmholtz energy, extended corresponding states and local composition model for fluid mixtures: Estela-Uribe, J. F. et al. Fluid Phase Equilibria, 2004, 224, (1), 125–142

It is well-known that one should be cautious with the use of composition-independent K-value correlations for phase equilibria at high pressures. The first limitation is the correlation's inability to predict convergence of the dew and bubble curves at the mixture's critical point or at a closed phase envelope. Rather, these two curves extend indefinitely beyond the critical temperatures and pressures of all the species in the mixture. However, even at low pressures, where these correlations are usually applied, results can be misleading. This paper presents some practical examples where correlations fail.In the calculation of three-phase flash equilibria one cannot use the same correlation for the K-values between the vapor phase and the liquid, hydrocarbon-rich phase, and the K-values between the vapor phase and aqueous phase. Use of the same correlation provides two linearly dependent equations, making it impossible to obtain a unique solution. The solution of this three-phase flash problem by bisection provides trivial roots, indicating different values of the aqueous and hydrocarbon-rich phase fractions but a unique value for the vapor fraction. The compositions of both liquid phases are clearly identical.A physically sound assumption for the solution of this three-phase flash equilibria problem is that hydrocarbons are insoluble in the aqueous phase. However, the solution of this case for a test ternary of n-heptane, n-decane and water (C7C10H2O) led to distinct multiple roots that satisfy the material balance constraints. Up to 67 roots were found for a system of 40% n-heptane, 20% n-decane and 40% water at 70 psia and 350 °F (a fairly low pressure). Physical properties of the phases evaluated using these roots can be substantially different; therefore, this assumption could lead to serious problems in reservoir engineering calculations.A second assumption of no-solubility of water in the hydrocarbon-rich phase also provides linearly independent equations. However, no simultaneous roots could be found for the test system under study for the same pressure and temperature and various combinations of these two variables.

Binary phase behavior and aggregation of dilute methanol in supercritical carbon dioxide: a Monte Carlo simulation study.

Configurational-bias Monte Carlo simulations in the Gibbs and isobaric-isothermal ensembles using the transferable potentials for phase equilibria force field were carried out to investigate the thermophysical properties of mixtures containing supercritical carbon dioxide and methanol. The binary vapor-liquid coexistence curves were calculated at 333.15 and 353.15 K and are in excellent agreement with experimental measurements. The self-association of methanol in supercritical carbon dioxide was investigated over a range of temperatures and pressures near the mixture critical point. The temperature dependence of the equilibrium constants for the formation of hydrogen-bonded aggregates (from dimer to heptamer) allowed for the determination of the enthalpy of hydrogen bonding, DeltaHHB, in supercritical carbon dioxide with values for DeltaHHB of about 15 kJ mol(-1) falling within the range of previously proposed values. No strong pressure dependence was observed for the formation of aggregates. Apparently the decrease of the entropic penalty and of the enthalpic benefit upon increasing pressure or solvent density mostly cancel each other's effect on aggregate formation.

Solubility of carbon dioxide in dimethylsulfoxide and N-methyl-2-pyrrolidone at elevated pressure

Experimental data on the solubility of CO 2 in dimethylsulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP) are reported at temperatures of 298, 308 and 318 K and pressures approaching the mixture critical point for each binary system. The corresponding volumetric expansions of DMSO and NMP with CO 2 are also presented. The solubility data are correlated with the Peng–Robinson equation of state (PR-EOS). At a given temperature and pressure, higher solubilities of CO 2 are obtained in NMP compared with DMSO and this trend is consistent with the volumetric expansions of the solvents. A plot of the volumetric expansion data as a function of the solubility of CO 2 in the liquid phase suggests that it is unlikely, as a general rule, that the expansion isotherms for the various systems collapse onto a single expansion curve. The existence of a single expansion curve appears to hold only for a given binary system over a limited range of temperature. The use of two adjustable binary interaction parameters with the PR-EOS provides a superior correlation of the liquid phase composition in comparison with the standard PR-EOS, particularly in the vicinity of the mixture critical point.

Supercritical antisolvent micronization of Cefonicid: thermodynamic interpretation of results

In this work, the micronization of Cefonicid (a Cephalosporinic antibiotic) from Dimethylsulfoxide (DMSO) has been successfully performed using the supercritical antisolvent (SAS) precipitation. We obtained sub-microparticles or empty shells of Cefonicid ranging from about 0.2 μm to more than 50 μm simply varying the concentration in the liquid solution and/or the process temperature. The analysis of the morphologies observed has been supported by thermodynamic arguments; in particular, we tried to relate the different morphologies obtained to the position of the process operating point with respect to the mixture critical point (MCP) on the binary system and tried to describe the kind of vapor–liquid equilibria (VLE) modifications induced by the presence of Cefonicid.

Role of Phase Behavior and Atomization in the Supercritical Antisolvent Precipitation

An experimental study on supercritical antisolvent (SAS) precipitation has been performed to gain insight into the role of phase behavior and atomization in controlling morphology and dimension of precipitates. The mixture yttrium acetate/dimethyl sulfoxide has been used as the main model system and supercritical CO 2 has been used as the antisolvent. Two SAS apparatuses (laboratory and pilot scale) with two injector arrangements and operating with various injector diameters in the range 60-500 μm have been used. The results showed that operating above the mixture critical point (MCP) of the ternary mixture yttrium acetate/dimethyl sulfoxide/ carbon dioxide, sub-micrometric particles are generated nearly independently from the size of the injector and of the apparatus. We also demonstrated that it is possible to modify the particle dimension by simply changing the operating pressure and/or temperature in the vicinity of the MCP. The use of a pseudo-binary diagram pressure-molar fraction has been proposed as a base framework to explain the relationship between the particle morphology and the phase behavior of processed mixtures. Particularly, we have shown that the single-phase region in the gas-rich side of the pressure-composition diagram and below the MCP can be usefully explored to modify the particle dimensions of the precipitate.

Measurements and modeling of high-pressure phase behavior of binary CO 2–amides systems

High-pressure vapor–liquid equilibrium (VLE) data were measured for binary CO 2– N, N-dimethylformamide, CO 2– N, N-diethylformamide and CO 2– N, N-dibutylformamide systems at various temperatures (318.2–398.2 K) and pressure up to 25 MPa. These CO 2– N, N-dimethylformamide, CO 2– N, N-diethylformamide and CO 2– N, N-dibutylformamide systems exhibits type-I phase behavior, which is characterized by an uninterrupted critical mixture curve which has a maximum in pressure. The solubility of N, N-dimethylformamide, N, N-diethylformamide and N, N-dibutylformamide for the CO 2– N, N-dimethylformamide, CO 2– N, N-diethylformamide and CO 2– N, N-dibutylformamide systems increases as the temperatures increases at constant pressure. The CO 2– N, N-dimethylformamide, CO 2– N, N-diethylformamide and CO 2– N, N-dibutylformamide systems have continuous critical mixture curves that exhibit maximums in pressure at temperatures between the critical temperatures of CO 2 and N, N-dimethylformamide, N, N-diethylformamide or N, N-dibutylformamide. Also, three phases were not observed of any of the systems studied. In each systems, the mixture critical point increases as the temperatures increases, and also the mixture critical pressure does as molecular weight increases. The experimental results for CO 2– N, N-dimethylformamide, CO 2– N, N-diethylformamide and CO 2– N, N-dibutylformamide systems is modeled using the Peng–Robinson equation of state. A good fit of the calculated data is obtained with the Peng–Robinson equation of state.

Volumetric expansion and vapour–liquid equilibria of α-methylstyrene and cumene with carbon dioxide at elevated pressure

Volumetric expansion data and vapour-liquid equilibria (VLE) for α-methylstyrene (AMS) with CO 2 are reported at temperatures of 308 and 323 K and pressures approaching the mixture critical point. Similar data are reported for the mixture of carbon dioxide and cumene at a temperature of 323 K. It is shown that the volumetric expansion data for the various systems coincide when plotted as a function of the solubility of CO 2 in the liquid phase. A procedure is described for calculating the molar volume of the liquid phase from the phase compositions and volumetric expansion data. The experimental VLE data are correlated with the Peng-Robinson (PR) equation of state and the interaction parameter for each binary system is obtained by regression. The interaction parameter is subsequently used to predict the liquid molar volume for each binary system. The use of a single interaction parameter with the PR equation of state provides a satisfactory correlation of the liquid molar volume.

Evaluation of commonly used assumptions for isolated and cluster heptane drops in nitrogen at all pressures

A study is performed to assess commonly used assumptions in the modeling of drop behavior in moderate to high temperature surroundings and at all pressures. The model employed for this evaluation has been previously validated for isolated drops by using microgravity data, and is very general: it contains Soret and Dufour effects, does not assume mass transfer quasi-steadiness at the drop boundary, or necessarily the existence of a drop surface (i.e., phase discontinuity). Moreover, the numerical simulations are performed with accurate equations of state and transport properties over a wide range of thermodynamic variables. Consistent with low pressure conditions, the drop boundary is identified a posteriori of the calculations with the location of the largest density change. Simulations are here performed for isolated drops, and for monodisperse as well as binary size drop clusters. The results show that at locations arbitrarily near the boundary, the drop does not reach the mixture critical point within the wide range of conditions investigated (far-field temperatures of 470–1000 K and pressures ranging from 0.1 to 5 MPa). However, the state arbitrarily near the boundary is closer to the critical condition for smaller drops in a cluster than for the larger drops. Evaluations of the effect of the relaxation time at the drop boundary show that quasi-steadiness of the mass transfer prevails for drops of radius as small as 2 × 10−3 cm. Finally, the diameter squared exhibits a linear time variation only at atmospheric pressure. At all other pressures investigated (1–5 MPa), the diameter squared displays a negative curvature with time which never becomes linear. In agreement with existing experimental data, the drop lifetime increases monotonically with pressure at low far field temperatures (470 K), but exhibits a maximum as a function of pressure at high temperatures (1000 K). On an appropriate scale, the slope of the diameter squared versus time is shown to be independent of the drop size at all pressures.

Phase Behavior and Modeling of Supercritical Carbon Dioxide−Organic Acid Mixtures

Pressure−composition isotherms are obtained for binary mixtures of carbon dioxide with formic acid, acetic acid, butyric acid, valeric acid, caproic acid, and caprylic acid at temperatures of 35.0−120.0 °C and pressures up to 250 bar. The accuracy of the experimental apparatus was tested by comparing the measured phase equilibrium data of the carbon dioxide−acetic acid system at 40.0 and 60.0 °C with those of Laugier et al. These six carbon dioxide−polar solute systems exhibit type I phase behavior, which is characterized by an uninterrupted critical mixture curve that has a maximum in pressure. In each system, the mixture critical point increases as the temperatures increases, and also the mixture critical pressure increases as the molecular weight increases. On the contrary, the carbon dioxide−formic acid system shows a higher mixture critical pressure compared with those of the other systems. The experimental data are modeled using both the statistical associating fluid theory (SAFT) and the Peng−Robinson (P−R) equation of state. The SAFT equation of state reasonably models the pressure−composition isotherms for these six systems only if two temperature-independent mixture parameters are used for each system. The P−R equation of state calculated the phase behavior with one or two temperature-independent mixture parameters.

Numerical modeling of mass transfer in the supercritical antisolvent process: miscible conditions

A mathematical model for mass transfer between a droplet of organic solvent and a compressed antisolvent is presented for conditions such that the two phases are fully miscible. This work is complementary to our recent study, which described solvent–antisolvent mass transfer at mixture subcritical conditions. Both models are applicable to the supercritical antisolvent (SAS) method of particle formation. In this process, solute particles precipitate from an organic solution when sprayed into a compressed antisolvent continuum. The antisolvent is miscible with the organic solvent, but immiscible with the solute. The mass transfer behavior of the droplets is thought to be a key factor affecting particle morphology. At mixture supercritical conditions, the antisolvent and solvent are completely miscible, and therefore no well-defined interface exists between the droplet and its environment. By defining an effective droplet radius based on the difference in density between the solvent-rich and antisolvent-rich regions, the extent of mass transfer can be tracked, and the effect of process parameters can be determined. Calculations with toluene (solvent) and carbon dioxide (antisolvent) show that droplets swell upon interdiffusion when the solvent is denser than the antisolvent, and shrink when the antisolvent is denser. Near the mixture critical locus, droplets exhibit greater swelling, longer lifetimes, and higher sensitivity to changes in temperature and pressure, compared to conditions removed from criticality. Examination of the time scales for mass transfer indicates that droplet lifetimes are significantly shorter for miscible conditions than for operation under bulk conditions corresponding to partial miscibility. As the mixture critical point is approached, from either the subcritical or the supercritical regime, droplet swelling decreases and lifetime diverges due to the vanishing of the diffusion coefficient at the critical point.