Lower Critical End Point Research Articles

T–x diagrams for the CO2–H2O system: The importance of water content in CO2 transportation for carbon capture and storage

AbstractWhen water is present in a CO2 pipeline, corrosion or plugging can occur due to the formation of liquid water or gas hydrate, respectively. Understanding how corrosion and hydrate plugging can be avoided is important for enhanced oil recovery and carbon dioxide capture and storage processes. If the CO2 is sufficiently dried prior to transportation, the formation of these problematic free‐water phases can be avoided. In this work, isobaric T–x diagrams were developed from the P–T diagram for the CO2–H2O binary system. Pressures ranging from just below the lower quadruple point to above the lower critical end point were studied. These diagrams are meant to give the reader a better conceptual understanding of how the water composition in CO2–H2O mixtures will influence phases that form at different temperatures and pressures. The diagrams are analyzed from the perspective of flow assurance in CO2 transportation.

Experimental determination and modelling of high-pressure phase behavior for the binary system CO2 + cyclooctane

Using a synthetic method, the phase behavior of the CO2 (1) + cyclooctane (2) binary system was for the first time experimentally studied between 292.95 K and 373.55 K. In this temperature range, the bubble- and dew-point pressures, ranging from 13.2 to 183.4 bar were measured for carbon dioxide mole fractions between 0.1014 and 0.9701. A total of 96 experimental data points were acquired. The experimental data were obtained using a high-pressure cell by visual observation of phase transitions. These measurements made it possible to conclude that the studied binary system probably exhibits a type V phase behavior in the classification scheme of Van Konynenburg and Scott. In particular, the experimental temperature of the suspected lower critical end point (LCEP) was found to be 303 K. Despite the complexity of the phase behavior, the experimental data could be accurately correlated with the Peng-Robinson equation of state combined with sophisticated mixing rules that either involve temperature-dependent binary interaction parameters (kij(T)) or that embed the residual part of the Wilson activity coefficient model.

Mass Transfer on Multiphase Transitions in Low-Temperature Carbon-Dioxide Floods

Summary Mixtures of reservoir oil and carbon dioxide (CO2) can exhibit complex multiphase behavior at temperatures typically less than 120°F, in which a third CO2-rich liquid (L2) phase can coexist with the oleic (L1) and gaseous (V) phases. The three-phase behavior is bounded by two types of critical endpoints (CEPs) in composition space. The lower CEP (LCEP) is a tie line in which the two liquid phases merge in the presence of the V phase, and the upper CEP (UCEP) is a tie line in which the L2 and V phases merge in the presence of the L1 phase. Slimtube tests reported in the literature show that low-temperature oil displacement by CO2 can result in the high displacement efficiency of more than 90% when three phases are present during the displacement. The nearly piston-like displacements can be quantitatively reproduced in numerical simulations when the CEP behavior is properly considered. However, it is uncertain how multicontact miscibility (MCM) is developed through the interaction of flow and three-hydrocarbon-phase behavior. This research presents a detailed analysis of mass conservation on multiphase transitions between two and three phases for the limiting three-phase flow, where the L1 phase is completely displaced by the L2 phase on the LCEP. The analysis indicates that interphase mass transfer on multiphase transitions occurs in the most-efficient way for MCM development. Simple analytical conditions derived for MCM through three phases are applied to 1D fine-scale simulations of CO2 floods by use of four and more components. Results show that the MCM conditions are nearly satisfied when the effect of numerical dispersion is small. MCM is likely developed through three hydrocarbon phases on the LCEP in the cases studied. This is consistent with analytical solutions of water and gas injection presented in the literature, in which MCM is developed on a CEP for the aqueous, V, and L1 phases. For MCM cases in this research, the L2-V two phases are present upstream of the miscible front if the composition path does not go through the UCEP tie line. However, they also can be miscible on the non-L1 edge of the UCEP tie line if the MCM composition path goes through it. Three-phase flow gradually changes to two-phase flow with varying pressure in the presence of numerical dispersion. It is shown that interphase mass transfer on multiphase transitions becomes less efficient during the change until the three-phase region completely disappears.

Mechanisms for High Displacement Efficiency of Low-Temperature CO2 Floods

Summary CO2 floods at temperatures typically below 120°F can involve complex phase behavior, where a third CO2-rich liquid (L2) phase coexists with the oleic (L1) and gaseous (V) phases. Results of slimtube measurements in the literature show that an oil displacement by CO2 can achieve high displacement efficiency of more than 90% when three hydrocarbon phases coexist during the displacement. However, the mechanism for the high-displacement efficiency is uncertain because the complex interaction of phase behavior with flow during the displacement is not fully understood. In this paper, we present the first detailed study of three-phase behavior predictions and displacement efficiency for low-temperature CO2 floods. Four-component EOS models are initially used to investigate systematically the effects of pressure, temperature, and oil properties on development of three-phase regions and displacement efficiency. Multicomponent oil displacements by CO2 are then considered. We use a compositional reservoir simulator capable of robust three-phase equilibrium calculations. Results show that high displacement efficiency of low-temperature CO2 floods is a consequence of both condensing and vaporizing behavior. The L2 phase serves as a buffer between the immiscible V and L1 phases within the three-phase region. Components in the L1 phase first transfer efficiently to the L2 phase near a lower critical endpoint (LCEP). These oil components then transfer to the V phase near an upper critical endpoint (UCEP) at the trailing edge of the three-phase region. The CEPs are defined where two of the three coexisting phases merge in the presence of the other immiscible phase. Unlike two-phase displacements, condensation and vaporization of intermediate components occur simultaneously within the three-phase region. The simultaneous condensing/vaporizing behavior involving the CEPs is also confirmed for simulations of several west Texas oil displacements. Quaternary fluid models can predict qualitatively the complex displacements because four is the minimum number of components to develop CEP behavior in composition space at a fixed temperature and pressure.

The phase behavior of two-dimensional symmetrical mixtures

Using Monte Carlo simulation methods in the grand canonical and semigrand canonical ensembles, we study the phase behavior of two-dimensional symmetrical binary mixtures of Lennard-Jones particles. We discuss the interplay between the demixing transition in a liquid and the freezing in detail. Phase diagrams for several systems characterized by different parameters describing interactions in the system are presented. It is explicitly demonstrated that different scenarios involving demixing and freezing transitions, described in our earlier paper [A. Patrykiejew and S. Sokołowski, Phys. Rev. E, 81, 012501 (2010)], are possible. In one class of systems, the λ-line representing a continuous demixing transition in a liquid phase starts at the liquid side of either the vapor-liquid or liquid-solid coexistence. The second class involves the systems in which the λ-line begins at the liquid side of the vapor-liquid coexistence, in the lower critical end point, and then terminates at the liquid side of the liquid-solid coexistence, in the upper critical end point. It is also shown that in such systems the solid phase may undergo a demixing transition at the temperature above the upper critical end point.

Interplay between demixing and freezing in two-dimensional symmetrical mixtures

The interplay between demixing and freezing in two-dimensional symmetrical binary mixtures of Lennard-Jones particles is studied using Monte Carlo simulation. It is demonstrated that different scenarios are possible. For example, the line of continuous liquid demixing transition can start at the liquid side of the vapor-liquid coexistence at the lower critical end point and then it can terminate at the liquid side of the liquid-demixed solid coexistence at the upper critical end point. Other situations are also possible. We distinguish four different scenarios depending on the interactions between unlike particles.

Evaluation of the Multiphase Behavior in Binaries Using a New Technique for Describing Solid Phases Based on the Soave−Redlich−Kwong Equation of State

In the petroleum industry, knowledge of phase behavior is essential to solve many problems, such as the design of the catalytic cracking of heavy oil. The catalyst tends to lose activity as a result of the formation of the solid phase, so the calculation of multiphase behavior of systems containing solid phases will serve to solve the problem. The Soave−Redlich−Kwong equation of state is widely employed to evaluate multiphase behavior, but cubic equations of state are incapable of calculating the properties of solid phases and thus cannot evaluate the multiphase behavior of systems containing solid phases. In this research, a new technique for describing the solid phase has been developed. The multiphase behavior of propane binaries with ployaromatic hydrocarbons was then explored. To evaluate the critical end point, which is difficult to calculate, an algorithm combining the method of Heidemann and Khalil (AIChE J. 1980, 26, 769−780) to compute the critical point and the tangent-plane criterion was developed previously (Yang et al. Ind. Eng. Chem. Res. 2009, 48, 6877−6881). Setting the initial guesses with the values at the obtained critical end points, the three-phase loci were then computed successfully. The calculation results show that the three-phase loci terminate at a lower critical end point (LCEP), which is wrong according to the experimental data. To correct this mistake, the newly developed technique was employed, and the quadruple points where the three-phase loci really end were evaluated successfully.

High-pressure phase equilibria for the synthesis of ionic liquids in compressed CO 2 for 1-hexyl-3-methylimidazolium bromide with 1-bromohexane and 1-methylimidazole

The use of carbon dioxide in the synthesis of ionic liquids (ILs) has many advantages over conventional solvents. Here, the high-pressure phase equilibria (including CO 2 solubility, volume expansion, and mixture critical points) are measured and modeled for the system involved in the synthesis of a model imidazolium ionic liquid 1-hexyl-3-methylimidazolium bromide ([HMIm][Br]) from 1-bromohexane and 1-methylimidazole. The global phase behavior of 1-methylimidazole was investigated and found to be a Type V system (or potentially IV) from the classification of Scott and van Konynenburg with regions of vapor–liquid equilibrium, vapor–liquid–liquid equilibrium, liquid–liquid equilibrium, an upper and lower critical endpoint and mixture critical points. The solubility and volume expansion of CO 2 in 1-methylimidazole, 1-bromohexane, a 1:1 mixture of 1-methylimidazole and 1-bromohexane and [HMIm][Br] was determined at 313.15 K and 333.15 K for pressures ranging from 10 to 160 bar. The solubility of CO 2 and the volume expansion increases in the order of [HMIm][Br] ≪ 1-methylimidazole < 1:1 mixture of reactants < 1-bromohexane. The Peng–Robinson equation of state with van der Waals 2-parameter mixing rules was used with estimated critical properties to well correlate the vapor–liquid equilibrium. The results have important ramifications on the kinetics and process constraints of an actual IL synthesis with CO 2.

Solubility of carbon dioxide in pentaerythritol tetraoctanoate

In this paper, solubility measurements of CO 2 in pure pentaerythritol tetraoctanoate (PEC8) between 273 and 343 K are presented. The experiments were performed according to the static, synthetic method. The data are represented using the Peng–Robinson equation of state with the Huron–Vidal mixing rules and the UNIQUAC equation for the excess Gibbs Energy ( g E ) at infinite pressure. This system shows immiscibility in liquid phase, with lower critical end point (LCEP) at T = 268 ± 0.1 K and x C O 2 = 0.98 ± 0.001 and upper critical end point (UCEP) at the critical point of pure CO 2.

Experimental Determination and Correlation of Phase Equilibria for the Ethane + 1-Propanol and Propane + 1-Propanol Systems

In this work, isothermal vapor−liquid equilibrium (VLE) for the ethane + 1-propanol and propane + 1-propanol systems were measured in an apparatus based on the static-analytic method. VLE data and the critical point for the ethane + 1-propanol system were measured at (317.59, 324.51, and 348.68) K. Upper critical end point, lower critical end point, and liquid−liquid−vapor locus were also obtained. For the propane + 1-propanol system, equilibrium measurements were performed at (318.30, 324.31, and 348.93) K. Experimental VLE data were correlated with the Peng−Robinson equation of state using the classical and the Wong−Sandler mixing rules. A better representation of the phase behavior was obtained using the Wong−Sandler mixing rule.

Phase behavior of the vanillin–CO 2 system at high pressures

The high-pressure phase behavior of the vanillin–CO 2 is experimentally determined with emphasis on the determination of the two solid–liquid–gas (SLG) boundaries for this system. The SLG line starting at the melting point of vanillin exhibits a temperature minimum of ∼65 °C at 150 bar and ends at the upper critical end point at 73.6 °C, 1347 bar, at a vanillin concentration of 15.8 mol%. The low-temperature branch of the SLG line virtually superposes onto the vapor pressure curve of CO 2 and ends at the lower critical end point at 31.6 °C, 74.5 bar, at a vanillin concentration of 0.72 mol%. Solid–fluid phase behavior exists at pressure–temperature ( P– T) conditions located between the two SLG lines. The P– T trace of low-pressure region of the SLG line starting at the melting point of vanillin differs from that reported previously in the literature probably due to the differences in the experimental techniques used to determine the SLG line.

Phase equilibria of near critical CO 2 + propane mixtures with fixed oils in the LV, LL and LLV region

Solvents like propane and carbon dioxide have been studied in previous works at supercritical and subcritical conditions and also as high-pressure liquid solvents for the extraction of lipids and oils. Propane is completely miscible with vegetable oils at room temperature but has liquid–liquid phase equilibrium with typical vegetable oils at temperatures between the lower critical end point (LCEP) and the upper critical end point (UCEP)—around 340 K and 370 K. In this temperature range, the propane solvent power for triglycerides is drastically reduced unless higher pressures are used to recover the single phase region. The use of propane + carbon dioxide mixtures can offer special properties regarding solvent power, selectivity and safety. In the present work experimental data on phase equilibria of sunflower oil + propane + carbon dioxide are measured in a windowed variable volume cell. The CO 2 concentration effect on the emergence of the liquid–liquid equilibrium phenomena in ternary mixtures with propane and oil is studied.

Phase and interface behaviors in type-I and type-V Lennard-Jones mixtures: Theory and simulations

Density gradient theory (DGT) and molecular-dynamics (MD) simulations have been used to predict subcritical phase and interface behaviors in type-I and type-V equal-size Lennard-Jones mixtures. Type-I mixtures exhibit a continuum critical line connecting their pure critical components, which implies that their subcritical phase equilibria are gas liquid. Type-V mixtures are characterized by two critical lines and a heteroazeotropic line. One of the two critical lines begins at the more volatile pure component critical point up to an upper critical end point and the other one comes from the less volatile pure component critical point ending at a lower critical end point. The heteroazeotropic line connects both critical end points and is characterized by gas-liquid-liquid equilibria. Therefore, subcritical states of this type exhibit gas-liquid and gas-liquid-liquid equilibria. In order to obtain a correct characterization of the phase and interface behaviors of these types of mixtures and to directly compare DGT and MD results, the global phase diagram of equal-size Lennard-Jones mixtures has been used to define the molecular parameters of these mixtures. According to our results, DGT and MD are two complementary methodologies able to obtain a complete and simultaneous prediction of phase equilibria and their interfacial properties. For the type of mixtures analyzed here, both approaches have shown excellent agreement in their phase equilibrium and interface properties in the full concentration range.

Rapid expansion of cellulose triacetate from ethyl acetate solutions

AbstractRapid‐expansion‐of‐supercritical‐solution (RESS) experiments were performed for solutions of cellulose triacetate [CTA; weight‐average molecular weight = 145,700, polydisperity = 2.07] in ethyl acetate [EA] over a range of concentrations and conditions of polymer–solvent phase behavior. Solid‐solubility experiments were carried out beforehand and identified hot and compressed liquid EA at approximately 175°C as a good solvent for CTA. Cloud‐point measurements were then used to locate the region of liquid–liquid equilibrium (LLE) for this system. The RESS results indicate that the phase state of the pre‐expansion mixture determines the product size, and the overall concentration of the pre‐expansion mixture determines the product morphology. However, we have also discovered a new constraint: for the production of well‐formed fibers and particles, the rapid‐expansion path must include penetration into a region of LLE, which must exist over a sufficient pressure range so that a separate, polymer‐rich liquid phase has time to develop before the onset of vapor–liquid equilibrium (VLE). If rapid expansions are carried out at temperatures near or below the lower critical end point, the expansion path leads directly into a region of VLE, and a disruptive vapor expansion occurs within the continuous liquid phase. In this case, hollow particles, hollow structures, and even foams are produced. By the proper choice of operating conditions, we were able to produce continuous CTA fiber from a 5 wt % solution in EA at 250°C and 69 bar. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 290–299, 2005

Phase behavior of the binary system ethane+linalool

As part of a study to assess the capacity of supercritical fluids in deterpenating citrus essential oils, high-pressure vapor–liquid equilibria of the binary system ethane+linalool were determined experimentally. Bubble and dew points were measured at ethane mole fractions ranging from 0.2 to 0.9998 and within temperature and pressure ranges of 278–368 K and 2.6–11.4 MPa, respectively. This binary system also exhibited a region of liquid–liquid two-phase split, resulting in the presence of a three-phase liquid–liquid–vapor equilibrium. Experimental values of the upper and lower critical endpoints, in addition to normal critical points are presented. From the experimental results it could be concluded that the system ethane+linalool shows type-V fluid phase behavior in the classification of Van Konynenburg and Scott. The experimental results also show the interesting phenomenon of double retrograde vaporization, in which the dew point curve has a double-domed shape. In this limited region, increasing the pressure at fixed concentration results in triple- or quadruple-valued dew points. The experimental results were correlated by the Stryjek–Vera modified version of the Peng–Robinson equation of state using the Mathias–Klotz–Prausnitz mixing rule.

Phase equilibria of a near-critical ionic system. Critical exponent of the order parameter

The density ρ of coexisting phases of the ternary system water+1,4-dioxane+potassium chloride was investigated along the liquid–liquid–solid coexistence curve near the critical end-point using a vibrating tube densimeter. By visual determination, this lower critical end-point was located at 311.026±0.010 K with a mole ratio dioxane (D) to water (W) rc=nD/nW=0.418±0.004. Density measurements were carried out in the range 0.01 K&amp;lt;(T−Tc)&amp;lt;31.5 K (i.e., 3×10−5&amp;lt;t=(T−Tc)/Tc&amp;lt;0.1). The obtained coexistence curve displays an apparent sharp crossover at reduced temperatures t≃10−2 from a nonclassical (Ising) to a classical (mean field) exponent. In ancillary experiments, the critical salt concentration was determined. Reduced critical values were calculated and compared with those predicted by the simplest ionic model (RPM, Restricted Primitive Model). The relation between critical behavior and reduced critical parameters in the present and other non-Coulombic systems is discussed.

Phase equilibria of polypropylene samples with hydrocarbon solvents at high pressures

AbstractThe objective of this work is to provide high‐pressure phase equilibrium data for polypropylene (PP) samples (two commercial and one metallocenic grades) with some hydrocarbon solvents (propane, propylene, n‐butane and 1‐butene). The experiments were carried out in a high‐pressure variable‐volume view cell in the temperature range of 75–150°C, for polymer compositions ranging from 0.3 to 10 wt %. Phase transitions were recorded visually as cloud points and identified as liquid–liquid and vapor–liquid transitions. When pressure transition data obtained for olefins and paraffins of similar chain length and at similar temperatures and polymer compositions are compared, it may be observed that paraffins lead to the decrease of the pressure transition values. Therefore, the one‐phase region is larger for paraffins than for olefins. In the range of commercial interest analyzed, it is also observed that the influence of polymer average molecular weight upon the cloud points is negligible. Experimental data are then used to build the P‐T diagrams for these systems, allowing the identification of the LCST (Lower Critical Solution Temperatures) and LCEP (Lower Critical End Points) curves. Finally, the original SAFT equation of state (SAFT‐EOS) is used to model the experimental data. It is shown that the SAFT‐EOS provides an excellent description of the solution behavior in the whole experimental range analyzed. © 2001 John Wiley &amp; Sons, Inc. J Appl Polym Sci 81: 3044–3055, 2001

High Pressure Phase Equilibria in the Systems Ammonia+Potassium Iodide, +Sodium Iodide, +Sodium Bromide, and +Sodium Thiocyanate: Solid–Liquid–Vapor and Liquid–Liquid–Vapor Equilibria

Solid salt liquid vapor equilibria and liquid liquid vapor equilibria were determined experimentally for the binary system NH 3 + KI in the temperature range 333 to 673 K and at pressures up to 80 MPa. It is found that the system NH 3 + KI belongs to Type V of fluid phase behavior according to the classification of van Konynenburg and Scott. The pressure of the three-phase curve solid salt liquid vapor is monotonically increasing in the temperature range investigated and reaches a value of 76 MPa at 670 K. The three-phase liquid liquid vapor curve starts in a lower critical end point at 407.3 K and 11.59 MPa and ends in an upper critical end point at 408.8 K and 11.92 MPa. For the systems NH 3 + NaI, NH 3 + NaBr, and NH 3 + NaSCN, Type V fluid phase behavior is also found, In the system NH 3 + NaI, the lower critical end point is found at 406.8 K and 11.52 MPa and the upper critical end point at 408.5 K and 11.86 MPa. For the system NH 3 + NaBr, these coordinates are 404.6 K. 11.03 MPa and 408.8 K, 11.90 MPa, respectively, and, for the system NH 3 + NaSCN, 409.2 K, 11.99 MPa and 409.6 K, 12.06 MPa.

Phase Behavior of Poly(l-lactide) in Supercritical Mixtures of Carbon Dioxide and Chlorodifluoromethane

Phase behavior data are presented for poly(L-lactide) (L-PLA: MW = 2000 g.mol -1 ), a biodegradable polymer, in supercritical solvent mixtures of carbon dioxide (CO 2 ) and chlorodifluoromethane (HCFC-22). Experimental cloud point curves, which were the phase boundaries between single and liquid-liquid phases, were measured using a variable-volume view cell apparatus for various CO 2 compositions up to about 82 mass % (on a polymer-free basis) and for temperatures up to about 393.15 K. The location of the phase transition boundaries between liquid (L) and liquid-vapor (LV) and between liquid-liquid (LL) and liquid-liquid-vapor (LLV) was also investigated as a function of solvent composition for the L-PLA dissolved in mixed solvents of CO 2 and HCFC-22. The cloud point curves exhibited a lower critical solution temperature phase behavior. As the CO 2 content in the solvent mixture increased, the cloud point pressure at a fixed temperature increased significantly. Addition of CO 2 to HCFC-22 caused a lowering of dissolving power of the mixed solvent, A lower critical end point (LCEP) was estimated from the cloud point curve and the L-to-LV and LL-to-LLV phase transition curves. The LCEPs were shifted to lower temperatures as the CO 2 content in the mixed solvent increased.

The metamorphosis from LCEP to UCEP in the metastable three-phase region in type V phase-diagrams

The three-phase line in a type V phase diagram starts with a lower critical end point (LCEP), where the two liquid phases merge, and ends with a upper critical end point (UCEP), where the vapor and liquid phases merge. Along the phase line no indication is seen as to why the situation is so different at the end points. If however one considers the complete set of V( x) or p( x) plots, i.e., including all the unstable and metastable binodal lines, it can be seen that a `dramatic' rearrangement takes place somewhere between the ends of the three phase line at the `plait point' transformation. I unearthed many figures showing this transformation and although one obtains the impression that van der Waals and Korteweg were aware of this behavior, no explanation was given in their work. Korteweg described the general rules for special points on the binodals, but no thorough calculation of the plait point transformation seems to exist. This paper is confined to the description of this transformation; the full mathematical treatment will be given in a later publication.