Critical Pressures Pc Research Articles

Prediction and measurement of critical properties of gasoline surrogate fuels and biofuels

Critical properties have great significance for the optimization of the combustion process of gasoline. Mixtures of n-heptane + isooctane are often used as primary reference fuels (PRF) to replace gasoline in scientific research. The critical temperatures (Tc) and critical pressures (pc) of n-heptane + isooctane + n-propanol, n-heptane + isooctane + n-butanol and n-heptane + isooctane + ethyl propionate were measured by the flow method to study the effect of biofuels on the critical properties of traditional gasoline. The Redlich–Kister equations are used to correlate the measured data. The average absolute relative deviations for Tc and pc are 0.10% and 0.31%, respectively. The experimental data is also used to evaluate the accuracy of the prediction model based on group contribution (GC) method previously proposed by our group.

Investigating the anisotropic compression and high-pressure phase symmetry of orthorhombic RFeO3 vs RMnO3

Rare-earth orthoferrites and orthomanganites present strong couplings between their structural distortions and physical properties, namely magnetoelectricity, generating interest in predicting and realizing new phases under external parameters, such as hydrostatic pressure. In this regard, we discuss the differences and the similarities of the high-pressure behaviour of the structural distortions arising from anisotropic volume compression. This allows a better understanding of the role played by the octahedra tilt and Jahn-Teller effect as pressure accommodation mechanisms on the stabilization of different crystallographic phases after the insulator to metal structural transition occurring at the critical pressures (Pc) between 35 and 50 GPa.

Measurement of Critical Properties for Binary and Ternary Mixtures Containing n-Butanol and n-Alkane

The critical temperatures (Tc) and critical pressures (pc) of five binary mixtures of n-butanol + n-alkane (C = 6, 7, 8, 9, 10) as well as two ternary mixtures (n-butanol + hexane + heptane and n-b...

DIPPR Project 851 – Thirty Years of Vapor–Liquid Critical Point Measurements and Experimental Technique Development

Experimentally determined critical temperatures (Tc) and critical pressures (Pc) are reported for 64 compounds. In addition, the critical volume (Vc) has been experimentally determined for 14 of these compounds. The compounds in this study are of industrial interest in process design, simulation, and safety. These data also extend our understanding of and ability to predict these properties from group contribution methods.

Measurement of critical properties for binary and ternary mixtures containing potential gasoline additive diethyl carbonate (DEC)

The critical temperatures (Tc) and critical pressures (pc) of five binary mixtures (DEC + hexane, DEC + heptane, DEC + octane, DEC + cyclohexane, DEC + ethanol) and a ternary mixture (DEC + ethanol + heptane) were determined over the whole range of concentration using an accurate experimental apparatus based on flow method. The combined standard uncertainty of temperature is 0.2 K while that of pressure is 5.2 kPa. The critical temperatures of DEC + octane and DEC + cyclohexane mixtures present a non-ideal behavior. The maximum average absolute deviations of Redlich–Kister equations for Tc and pc of five binary mixtures are 0.08% and 0.41%, respectively. Cibulka's equations and Singh−Sharma's equations were used to correlate the critical parameters of ternary mixture. The critical surfaces of ternary mixture were plot using Cibulka's equations.

Measurement of the Critical Properties of the Ternary Systems Hexane + Heptane + Octane and Octane + Nonane + Decane Using a Flow Apparatus

The critical temperatures (Tc) and critical pressures (pc) of ternary mixtures of hexane + heptane + octane and octane + nonane + decane were measured by a flow apparatus. The critical points are visually determined by observing the critical opalescence. The expanded uncertainties with 0.95 level of confidence in critical temperature and pressure were estimated to be 0.4 K and 0.01 MPa, respectively. The relative expanded uncertainty with 0.95 level of confidence in mole fraction was estimated to be 0.012. The experimental data are correlated by Cibulka’s and Singh’s expressions for the ternary contribution.

18 Propagation of arterial dissection

We compute and study critical pressures (Pc) for the initiation of the propagation of an existing arterial dissection, which in general depends on the residual stress and pre-stretch of the artery, and the length and depth of the tear, using the Holzapfel-Gasser-Ogden mathematical model for the walls of the large arteries. For the simple case of a 2D tissue sample, increasing the length of the initial dissection lowers the value of Pc, but when the strip is supported by connective tissue, arrest of the dissection is obtained. A peeling test on a disc-shaped sample demonstrates that the orientation of collagen fibres plays an important role in the direction of propagation. To study the effects of residual stress on arterial dissection, we include residual stress, quantified by the opening angle when an artery is subject to a longitudinal cut. We show that Pc is approximately proportional to the opening angle, so that the observed loss of residual stress associated with ageing increases the likelihood of dissection. Finally the effects of the circumferential (arc) length and radial position of the initial tear in a cylindrical model of an artery are investigated. Pc decreases as arc length increases and as the initial dissection is placed closer to the lumen. Prolapse of the dissection into the lumen occurs when the initial tear close to the lumen and the arc length is large.

New Group Contribution Method for Predicting the Critical Pressures of Fatty Acids and Triglycerides

A new and useful method for evaluating and predicting the critical pressures (Pc) of n-fatty acids and triglycerides was developed in this study. Logarithmic equations which required critical temperature (Tc) and normal boiling temperature (Tb) were proposed for the fatty acids and their corresponding triglycerides. The method predicted with good accuracy the critical pressures (Pc) of the n-fatty acids, even without the input of the compressibility factor (ΔZb). The deviations of predicted critical pressures from experimental values were between (0-7)%, except for Caproic and Caprylic acids; where the method under predicted the critical pressures with deviations of -6.64% and -11.31% respectively. Experimental Pc values for the triglycerides were not available in the open literature for comparison.

Determination of the Critical Properties of C6–C10 n-Alkanes and Their Binary Systems Using a Flow Apparatus

A flow apparatus was established to measure the critical properties of pure components and mixtures. The critical points are visually determined by observing the critical opalescence. The measurement can be carried out up to 10 MPa for pressure and 773 K for temperature. Critical temperatures (Tc) and critical pressures (pc) of pure compounds (hexane, heptane, octane, nonane, and decane) and their binary mixtures were measured. Experimental data for hexane + heptane, hexane + octane, hexane + decane, heptane + octane, and heptane +decane are available in the open literature and compare well with our data. On the other hand, the critical properties of hexane + nonane, heptane + nonane, octane + nonane, octane + decane, and nonane + decane systems were presented for the first time in this paper. All the experimental data are correlated by the Redlich–Kister equation.

Experimental measurements and correlation of vapor–liquid equilibrium and critical data for the CO2 + R1234yf and CO2 + R1234ze(E) binary mixtures

Isothermal (P-x-y) vapor–liquid equilibrium data for the CO2 + 2,3,3,3-tetrafluoroprop-1-ene (R1234yf) binary system are reported at seven temperatures ranging from 283.15 K to 353.15 K. The experimental values were measured using a static–analytic type apparatus coupled with two electromagnetic capillary samplers (Rolsi™, Armines' patent) for repeatable and reliable equilibrium phase sampling and handling. Moreover, for the four temperatures above the critical temperature of pure CO2 (304.3 K), the mixture critical compositions (xc) and pressures (Pc) were calculated at each temperature using extended scaling laws.In a second step, the critical loci of the CO2 + R1234yf and CO2 + trans-1,3,3,3-tetrafluoroprop-1-ene [R1234ze(E)] were measured over the whole composition range using a synthetic-dynamic type apparatus. The subcritical and critical experimental data determined in this study were compared to those available in the open literature and were correlated using the Peng–Robinson equation of state with temperature-dependent binary interaction parameters.

Critical Stretching and Pores in Bolalipid Membrane from Flexible String Model

Phase diagram of a bolalipid membrane for critical stretching and pore formation is derived analytically using microscopic flexible strings model [1,2] of monopolar lipid and bolalipid [3] membranes. We use free energy density functional E of the laterally stretched membrane with pore radius R: E = E_0 + (π⊥2R⊥4/2N)∂⊥2Ft(a)/∂a⊥2 + Pπ⊥2R⊥4/Na − PπR⊥2+2ΓπR under the applied stretching pressure P and area per lipid a of the self-assembled membrane of N lipids before stretching, and pore linear tension Γ. The hydrophobic part of the lipid chain's free energy Ft was calculated previously for monopolar [1] and bolalipid [2] membranes using self-consistent microscopic model with entropic repulsion between flexible lipid chains. Finding a from the free energy minimum condition: ∂Ft/∂a+γa=0 , with surface tension γ at the hydrophobic-hydrophilic interface, and substituting it into ∂2Ft(a)/∂a2 we find analytic expression for the coefficient in front of R⊥4 in the elastic energy. Using E we derive phase diagrams for bolalipid and monopolar lipid membranes and compare their stability, i.e. critical pressures Pc of pore formation. An enhancement of Pc of the bolalipid membrane relative to monopolar lipid bilayer is a consequence of relative suppression of the midmembrane entropy of the membranespanning bolalipid chains with respect to the lipid tails entropy in bilayer midplane region [2]. Consequences of this effect for stability against pores formation of the bolalipid membrane of the archaea cell is investigated.1. Mukhin, S.I., Baoukina, S., Analytical derivation of thermodynamic characteristics of lipid bilayer from a flexible string model. Phys. Rev. E 71, 061918 (2005).2. Mukhin, S.I., Kheyfets, B.B., Analytical approach to thermodynamics of bolalipid membranes. Phys. Rev. E 82, 051901 (2010).3. Bulacu, M., Periole, X., Marrink, S.J., In Silico Design of Robust Bolalipid Membranes. Biomacromolecules 13, 196 (2011).

Critical properties of some aliphatic symmetrical ethers

The critical temperatures Tc and the critical pressures pc of dihexyl, dioctyl, and didecyl ethers have been measured. According to the measurements, the coordinates of the critical points are Tc=(665±7)K, pc=(1.44±0.04)MPa for dihexyl ether, Tc=(723±7)K, pc=(1.19±0.04)MPa for dioctyl ether, and Tc=(768±8)K, pc=(1.03±0.03)MPa for didecyl ether. All the ethers studied degrade chemically at near-critical temperatures. A pulse-heating method applicable to measuring the critical properties of thermally unstable compounds has been used. The times from the beginning of a heating pulse to the moment of reaching the critical temperature were from 0.06 to 0.46ms. The short residence times provide little decomposition of the substances in the course of the experiments. The critical properties of the ethers investigated in this work have been discussed together with those of methyl to butyl ethers. The experimental critical constants of the ethers have been compared with those estimated by the group-contribution methods of Wilson and Jasperson and Marrero and Gani. The Wilson/Jasperson method provides a better estimation of the critical temperatures and pressures of simple aliphatic ethers in comparison with the Marrero/Gani method if reliable normal boiling temperatures are used in the method of Wilson and Jasperson.

Critical temperatures and pressures of straight-chain alkanediols (C3 to C12)

The critical pressures pc and the critical temperatures Tc of diols with straight alkylchains HO(CH2)nOH and n=3, 4, 5, 6, 8, 9, 10, 12 have been measured. All the alkanediols studied decompose at near-critical temperatures. A pulse-heating method which allows measuring the critical properties of thermally unstable compounds has been used. The times of heating from the beginning of a pulse to the moment of reaching the critical temperature were from (0.03 to 0.46)ms. The short residence times provided little degradation of the substances in the course of the experiments. The uncertainties of the measurement of the critical properties of alkanediols are 0.04 pc and 0.015 Tc, where Tc is the absolute temperature. The critical properties obtained have been compared with the values measured earlier by various techniques. The experimental critical constants of alkanediols have also been compared with those calculated using the group-contribution methods of Lydersen, Wilson and Jasperson, and Marrero and Gani.

Measurement of critical temperatures and critical pressures for binary mixtures of methyl tert-butyl ether (MTBE) + alcohol and MTBE + alkane

A set of high-pressure view apparatus was designed for determining the critical properties of chemicals. In order to check the reliability of the apparatus, the critical temperatures (Tc) and critical pressures (Pc) of pure n-heptane, cyclohexane, methanol, ethanol, 1-propanol, methyl tert-butyl ether (MTBE), and binary mixture n-hexane+ethanol were measured. The experimental data were in good agreement with the literature data, which proves the reliability of the apparatus used in the work. The critical temperatures and critical pressures of five binary mixtures containing gasoline additive (MTBE+n-heptane, MTBE+cyclohexane, MTBE+methanol, MTBE+ethanol, MTBE+1-propanol) were measured using the high-pressure view cell with visual observation. The critical temperatures and critical pressures for the five binary mixtures were all reported for the first time. In addition, the critical temperatures and critical pressures of the binary mixture n-heptane+cyclohexane (two of main components in gasoline) were also measured. All the critical lines for the mixtures studied are continuous which connect the critical points of the two pure components, indicating their phase diagrams belong to type I proposed by Scott and van Konynenburg. The critical points of these systems were calculated by the Peng–Robinson equation of state with the Wong–Sandler mixing rule. This model could calculate the critical properties of the mixtures well with the binary interaction parameter kij obtained by fitting the experimental critical data. And the experimental data were all fitted well with Redlich–Kister equation. The maximum average absolute deviations were 0.06% for critical temperatures and 0.35% for critical pressures.

Validation of a New Apparatus Using the Dynamic Method for Determining the Critical Properties of Binary Gas/Gas Mixtures

In this paper, the performance and the reliability of a new apparatus to determine the critical properties of pure components and gas/gas binary mixtures (i.e., mixtures for which the two components are gaseous at ambient conditions) with a known overall composition are studied. Critical temperatures (Tc) and critical pressures (Pc) of binary mixtures composed of carbon dioxide + a light n-alkane (CO2 + n-propane, CO2 + n-butane) or of two light n-alkanes (n-propane + n-butane) are measured. For the first time, the ability of our apparatus to determine—using the dynamic mode—the critical loci of gas/gas binary systems is tested. The critical points are visually determined by observing the critical opalescence and the simultaneous disappearance and reappearance of the meniscus in the middle of the view cell which withstands operations up to 673 K and 20 MPa. The experimental critical data in this work are compared with previously measured literature data and with values predicted by the PPR78 thermodynamic...

Validation of a new apparatus using the dynamic method for determining the critical properties of binary mixtures containing CO2 and a n-alkane

In this paper the experimental setup of a new apparatus able to provide the critical properties of binary mixtures with known overall composition using a dynamic method is described. Critical temperatures (Tc) and critical pressures (Pc) of binary mixtures composed of carbon dioxide and n-alkanes (CO2+n-pentane, CO2+n-heptane and CO2+n-decane) are measured. The critical points are visually determined by observing the critical opalescence and the simultaneous disappearance and reappearance of the meniscus in the middle of the view cell which withstands operations up to 673K and 20MPa. The experimental critical data are compared with success to literature database and with their prediction from the PPR78 thermodynamic model.

Validation of a new apparatus using the dynamic and static methods for determining the critical properties of pure components and mixtures

In this paper the experimental setup of a new apparatus able to provide the critical properties of pure components and mixtures using either a dynamic or a static method is described. Critical temperatures (Tc) and critical pressures (Pc) of pure components (n-pentane, n-heptane and n-decane) and critical loci of three binary mixtures (n-pentane + n-heptane, n-pentane + n-decane and n-heptane + n-decane) are investigated using the dynamic method. For the system n-pentane + n-heptane, the results obtained with the dynamic method are compared with those obtained with the static method. The critical points are visually determined by observing the critical opalescence and the simultaneous disappearance and reappearance of the meniscus in the middle of the view cell which withstands operations up to 673 K and 20 MPa. The experimental critical data are compared with success to literature database and with their prediction from the PPR78 thermodynamic model.

Spin-state transitions in CaFeSi2O6and NaFeSi2O6under pressure

The spin-state transitions in Fe-based clinopyroxenes are predicted and investigated with the use of the band structure calculations within GGA + U approach. The transitions were found to be of first order with critical pressures Pc ∼ 40 GPa for CaFeSi2O6 and Pc ∼ 60 GPa for NaFeSi2O6. The spin-state transitions are accompanied by 4% change in the lattice volume for both compounds. The present study will motivate further experimental and theoretical investigations of the spin-state transitions in pyroxenes.

Is there a quantum critical point controlled by pressure in the superconducting borocarbides RNi2B2C (R = Y, Ho, Er, Tm)?

A decade ago, Alleno et al. [Physica C 242 169 (1995)] reported measurements under hydrostatic pressure P of the superconducting temperature Tc (P), with P<2 GPa, in the compounds RNi2B2C, where R = Y, Ho, Er, Tm. We utilise an extrapolation they propose for their data to higher pressures to address the question posed in the title. We then find critical pressures Pc ranging from ∼ 23 GPa for Y down to ∼ 12 GPa for Ho. The corresponding slopes range from ∼ −2 GPa/GPa for Er to ∼ −1 GPa/GPa for Tm. Crucial to the discussion is whether a structural phase transition is induced in RNi2B2C by pressure before this critical pressure Pc is reached. Theory presently cannot answer such a question, but the compound for say R = Er should be experimentally accessible. Though is predicted to be finite above, all important is whether Tc (P) is proportional to (Pc −P) as Pc is approached, or whether the behaviour is non-analytic, such as (Pc −P)γ , where γ is non-integer. If the former situation obtains, important consequences follow from the Ehrenfest relation for dTc/dP in terms of (a) thermal expansion, and (b) specific heat.

Method for Estimating Critical Properties of Heavy Compounds Suitable for Cubic Equations of State and Its Application to the Prediction of Vapor Pressures

Cubic equations of state (EoS) are often used for correlating and predicting phase equilibria. Before extending any EoS to mixtures, reliable vapor-pressure prediction is essential. This requires experimental, if possible, critical temperatures Tc, pressures Pc, and acentric factor ω or extensive pure-compound vapor-pressure data which, for heavy and/or complex compounds, are often not available. This work presents a method for estimating Tc, Pc, and ω values for heavy compounds (typically with MW > 130) suitable for vapor-pressure calculations with generalized cubic EoS. The proposed scheme employs a recent group-contribution method (Constantinou et al. Fluid Phase Equilib. 1995, 103 (1), 11) for estimating the acentric factor. The two critical properties are estimated via a generalized correlation for the ratio Tc/Pc (with the van der Waals surface area) and the cubic EoS at a single experimental vapor-pressure point (e.g., the normal boiling point). We have employed a modified version of the Peng−Robin...