Measurement and prediction of high-pressure vapor-liquid equilibria for binary mixtures of carbon dioxide + n-octane, methanol, ethanol, and perfluorohexane

We report the measurement of high-pressure vapor-liquid equilibrium data for binary mixtures of carbon dioxide + n-octane, +methanol, and +ethanol systems at 313.14 K and carbon dioxide + perfluorohexane at 303.15-323.15 K. The experimental data were collected using a new simple apparatus for measuring high-pressure vapor-liquid equilibria and correlated using a modified SRK equation with the three-parameter conventional mixing rule proposed by Adachi and Sugie. The SAFT-VR equation of state has also been used to predict the phase behavior and found to be in good agreement with experimental data. For the carbon dioxide + methanol, carbon dioxide + ethanol and carbon dioxide + perfluorhexane systems simple Lorentz-Berthelot combining rules can be used to determine the cross interactions and predict the phase behavior. For the carbon dioxide + n-octane system cross interaction parameters fitted to experimental data are needed in order to capture the non-ideal phase behavior exhibited by this system.     

Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part I. Miscibility and foaming of poly(l-lactic acid) in carbon dioxide + acetone binary fluid mixtures

Miscibility and foaming of poly(l-lactic acid) (PLLA) in carbon dioxide + acetone mixtures have been explored over the temperature and pressure ranges from 60 to 180 °C and 14 to 61 MPa. Liquid-liquid phase boundaries were determined in a variable-volume view-cell for polymer concentrations up to 25 wt% PLLA and fluid mixtures containing 67-93 wt% CO₂ over a temperature range from 60 to 180 °C. Even though not soluble in carbon dioxide at pressures tested, the polymer could be completely solubilized in mixtures of carbon dioxide and acetone at modest pressures.Foaming experiments were carried out in different modes. Free-expansions were carried out by exposure and swelling in pure carbon dioxide in a view-cell followed by depressurization. Foaming experiments were also carried out within the confinement of specially designed molds with porous metal surfaces as boundaries to direct the fluid escape path and to generate foams with controlled overall shape and dimensions. These experiments were conducted in pure carbon dioxide and also in carbon dioxide + acetone fluid mixtures over a wide range of temperatures and pressures. Foaming in carbon dioxide + acetone mixtures was limited to 1 and 4 wt% acetone cases. Microstructures were examined using an environmental scanning electron microscope (ESEM). Depending upon the conditions employed, pore diameters ranging from 5 to 400 μm were generated. At a given temperature, smaller pores were promoted when foaming was carried out by depressurization from higher pressures. At a given pressure, smaller pores were generated from expansions at lower temperatures. Foams with larger pores were produced in mixtures of carbon dioxide with acetone.     

High pressure phase behavior of carbon dioxide + nitrobenzene binary system at 298.15, 310.45 and 322.75 K

The phase behavior of the carbon dioxide + nitrobenzene binary system has been studied in a high-pressure variable-volume view cell using an analytical method. The phase boundaries were measured at temperatures of 298.15, 310.45 and 322.75 K under pressures between 2.76 and 12.83 MPa, and it was found that three-phase equilibria existed over a temperature range from 303.60 to 313.65 K. The experimental data could be correlated with the Peng-Robinson equation of state (PR EoS) and two binary parameters. The phase behavior of the carbon dioxide + nitrobenzene system appears to belong to Type-V according to the classification of van Konynenburg and Scott.     

Interfacial tension measurements and modelling of (carbon dioxide + n-alkane) and (carbon dioxide + water) binary mixtures at elevated pressures and temperatures

Supercritical carbon dioxide (CO₂) is often used as a process fluid for enhanced oil recovery. The storage of carbon dioxide in underground formations is a potential way of mitigating climate change during a transition period to more sustainable energy sources. Combining injection with subsequent trapping of the non-wetting supercritical carbon dioxide phase in the pores of a depleted reservoir is a promising scheme for allowing the continued use of fossil fuels with minimal environmental consequences. The design of such processes is ultimately linked to the confined behaviour of the fluids in question at reservoir conditions, which is largely controlled by interfacial forces. Measurements of the relevant interfacial tensions for systems containing alkanes, carbon dioxide and water are currently limited and inconsistent while models usually fail to capture the pressure dependence of the interfacial tension. In this work, a density functional theory based on the SAFT-VR equation of state was used to predict the interfacial tension of (H₂O + CO₂ + n-alkane) binary systems over wide ranges of temperature and pressure. The comparison with a new set of reported experimental data of three (n-alkane + CO₂) systems at pressures up to the critical points, as well as with the (H₂O + CO₂) system at pressures up to 60 MPa, for a temperature range of (298-443) K, is discussed.     

High-pressure phase equilibria for the binary system carbon dioxide + benzyl alcohol

The phase equilibria of the carbon dioxide + benzyl alcohol system were measured at 298.15, 306.35 and 313.15 K, under pressures from 1.03 to 16.15 MPa. An upper critical end point (UCEP) of the binary system was identified at 307.45 K and 7.77 MPa and three-phase equilibria were observed along the liquid-liquid-vapor (LLV) equilibrium line between 279.75 and 307.45 K. The experimental data were correlated well by the Peng-Robinson equation of state with two binary parameters. According to the experimental results, the phase behavior of the carbon dioxide + benzyl alcohol system appears to belong to Type-III according to the classification of van Konynenburg and Scott.     

Volumetric properties of carbon dioxide + 2-butanol mixtures at 313.15 K

Volumetric properties were measured of carbon dioxide + 2-butanol mixtures at 313.15 K, using the vibrating tube Anton Paar DMA 512P density meter. In the present experiments, no analytical instrument was required. The saturated pressures were also measured of carbon dioxide + 2-butanol mixtures at 313.15 K by the synthetic method. The experimental data obtained were correlated with the density equation, Soave-Redlich-Kwong (SRK) equation of state, and the pseudocubic equation of state.     

Phase equilibria of carbon dioxide + 1-nonanol system at high pressures

Vapor-liquid-equilibria (VLE) and vapor-liquid-liquid equilibria (VLLE) data for the carbon dioxide + 1-nonanol system were measured at 303.15, 308.15, 313.15, 333.15, and 353.15 K. Phase behavior measurements were made in a high-pressure visual cell with variable volume, based on the static-analytic method. The pressure range under investigation was between 1.15 and 103.3 bar. The Soave-Redlich-Kwong (SRK) equation of state (EOS) coupled with both classical van der Waals and a Gibbs excess energy (G^E) mixing rules was used in semi-predictive approaches, in order to represent the complex phase behavior (critical curve, liquid-liquid-vapor (LLV) line, isothermal VLE, LLE, and VLLE) of the system. The topology of phase behavior is correctly predicted.     

Thermolysis of the polyyne C8H2 in hexane and methanol: Experimental and theoretical study

The mean lifetimes of polyyne C₈H₂ in hexane were determined at 50, 60, 80, and 100 °C and in methanol at 60 °C. The reactions are second order at all temperatures: ln k₂ = 20.5 ± 1.5-10303 ± 520T⁻¹ and the corresponding activation energy is 85.7 ± 6.3 kJ mol⁻¹ (7164 cm⁻¹). Extrapolation suggests that solutions at 1 mM concentration are significantly unstable at room temperature. Quantum chemical calculations show that polyynes C_mH₂ + C_nH₂ (m + n = 16) could be products, but these were not detected. Alternatively, C₁₆H₂ isomers could form. IR spectra of the solid residues from hexane and methanol solutions were obtained.     

Phase behavior of the poly(neopentyl methacrylate) + supercritical fluid solvents + neopentyl methacrylate system and CO2 + neopentyl methacrylate mixtures at high pressure

High-pressure phase behaviors are measured for the CO₂ + neopentyl methacrylate (NPMA) system at 40, 60, 80, 100, and 120 °C and pressure up to 160 bar. This system exhibits type-I phase behavior with a continuous mixture-critical curve. The experimental results for the CO₂ + NPMA system are modeled using the Peng-Robinson equation of state. Experimental cloud-point data up to the temperature of 180 °C and the pressure of 2000 bar are presented for ternary mixtures of poly(neopentyl methacrylate) [poly(NPMA)] + supercritical solvents + NPMA systems. Cloud-point pressures of poly(NPMA) + CO₂ + NPMA system are measured in the temperature range of 60-180 °C and to pressures as high as 2000 bar with NPMA concentration of 0.0, 5.2, 19.0, 28.1 and 40.2 wt%. It appears that adding 51.2 wt% NPMA to the poly(NPMA) + CO₂ mixture does significantly change the phase behavior. Cloud-point curves are obtained for the binary mixtures of poly(NPMA) in supercritical propane, propylene, butane, 1-butene, and dimethyl ether (DME). The impact of dimethyl ether concentration on the phase behavior of the poly(NPMA) + CO₂ + x wt% DME system is also measured at temperature of 180 °C and pressure range of 36-2000 bar. This system changes the pressure-temperature (P-T) slope of the phase behavior curves from upper critical solution temperature (UCST) region to lower critical solution temperature (LCST) region as the NPMA concentration increases.     

Processing triacetyl-β-cyclodextrin in the liquid phase using supercritical CO2

TA-β-CD has been proposed as an effective sustained release carrier for highly water soluble drugs with short biological half-lives. In this work, the possibility of preparing drug - triacetyl-β-cyclodextrin (TA-β-CD) complexes in the liquid phase by exploring its solid-liquid-gas equilibrium (S-L-G) under supercritical CO₂ atmosphere was evaluated. The S-L-G equilibrium line for the binary system TA-β-CD + CO₂ was determined by a visual method for pressures up to 25.3 MPa. At the studied conditions, a homogeneous and translucent liquid phase emerge at 307 K when exposed to CO₂ pressurized at 16 MPa and this temperature further increases with increasing pressure, up to 318 K at 25.3 MPa. The S-L-G equilibrium behaviour observed is typical of this kind of asymmetric systems, it is totally CO₂ density-dependent and opens the possibility of further processing TA-β-CD in the liquid phase. In order to investigate the possibility of preparing a drug-TA-β-CD complex in the liquid phase of TA-β-CD, a preliminary experiment was performed at 25 MPa and 308 K using flufenamic acid (FA) as the model active principle.     

Characterisation of activated nanoporous carbon for supercapacitor electrode materials

Commercial nanoporous carbon RP-20 was activated with water vapor in the temperature range from 950 °C to 1150 °C. The XRD analysis was carried out on nanoporous carbon powder samples to investigate the structural changes (graphitisation) in modified carbon that occurred at activation temperatures T ? 1150 °C. The first-order Raman spectra showed the absorption peak at 1582 cm⁻¹ and the disorder (D) peak at 1350 cm⁻¹. The low-temperature N₂ adsorption experiments were performed at −196 °C and a specific surface area up to 2240 m²g⁻¹ for carbon activated at T = 1050 °C was measured. The cell capacitance for two electrode activated nanoporous carbon system advanced up to 60 F g⁻¹ giving the specific capacitance ∼240 F g⁻¹ to one electrode nanoporous carbon ∣1.2 M (C₂H₅)₃CH₃NBF₄ + acetonitrile solution interface. A very wide region of ideal polarisability for two electrode system (∼3.2 V) was achieved. The low frequency limiting specific capacitance very weakly increases with the rise of specific area explained by the mass transfer limitations in the nanoporous carbon electrodes. The electrochemical characteristics obtained show that some of these materials under discussion can be used for compilation of high energy density and power density non-aqueous electrolyte supercapacitors with higher power densities than aqueous supercapacitors.     

Equilibrium solubilities of a p-toluenesulfonamide and sulfanilamide mixture in supercritical carbon dioxide with and without ethanol

Supercritical separation processes for a multi-component mixture of solutes are of practical interest. In this study, the experimental equilibrium solubilities of two solute mixtures, p-toluenesulfonamide (p-TSA) and sulfanilamide (SNA), in supercritical carbon dioxide (SC CO₂) were measured at temperatures of 308, 318 and 328 K and pressures in the range of 11.0-21.0 MPa using a dynamic flow method. The effect of cosolvent on the multi-component system was investigated by the addition of a 3.5 mol% ethanol. In the ternary system (p-TSA + SNA + CO₂), the solubility of SNA increased as compared to its binary system (SNA + CO₂), while the solubility of p-TSA decreased. In the quaternary system (p-TSA + SNA + ethanol + CO₂), a significant solubility enhancement was observed for both p-TSA and SNA. The selectivity, which is thought to imply the intermolecular interactions between p-TSA and SNA, was also enhanced by the presence of ethanol so that the two solutes could be separated by a max. purity of 99.4%. The influence of the hydrogen bond interaction on solubility was discussed. The equations of Chrastil, Méndez-Santiago and Teja, and their modified forms were used to correlate the experimental data.     

Fabrication of Fc-SWNTs modified glassy carbon electrode for selective and sensitive determination of dopamine in the presence of AA and UA

The electrochemistry of dopamine (DA) was investigated by cyclic voltammetry (CV) and differential pulse voltammograms (DPV) at a glassy carbon electrode modified by the hybridization adducts of Fc-SWNTs. The electro-oxidation of DA could be catalyzed by Fc/Fc⁺ couple as a mediator and had a higher electrochemical response due to the unique carbon surface of carbon nanotubes. The anodic peaks of DA, ascorbic acid (AA) and uric acid (UA) in their mixture can be well separated by the prepared electrode. Under optimum conditions linear calibration graphs were obtained over the DA concentration range 5.0 × 10⁻⁶ to 3.0 × 10⁻⁵ M with a correlation coefficient of 0.9998 and a detection limit of 5.0 × 10⁻⁸ M based on the equation C_m = 3s_b1/m. The modified electrode has been successfully applied for the assay of DA in human blood serum. This work provides a simple and easy approach to selectively detect DA in the presence of AA and UA.     

Electrochemical determination of acidity level and dissociation of formic acid/water mixtures as solvent

The autoprotolysis constant K_HS of formic acid/water mixtures as solvent has been calculated from acid-base potentiometric titration curves. A correlation of the acidity scale pK_HS of each medium versus pure water has been implemented owing to the Strehlow R⁰(H⁺) electrochemical redox function. The results show that formic acid/water mixtures are much more dissociated than pure water; such media are sufficiently dissociated to allow electrochemical measures without addition of an electrolyte. It has also been shown that for a same H⁺ concentration the activity of protons increases with formic acid concentration. For more than 80 wt.% of formic acid the acidity is sufficiently increased to locate the whole acidity scale pK_HS in the super acid medium of the generalized acidity scale pHH₂O.     

Experimental electrical resistivity of liquid carbon in the temperature range from 4800 to ∼20,000 K

Specimens of high density pyrolytic graphite (2.2 g/cm³) were placed inside thick-walled sapphire tubes and heated over several microseconds by an electric current of 68 kA. The electrical resistivity of the liquid carbon was measured in a constant volume heating process. The transition of liquid carbon from semi-metal properties (resistivity decreasing with increase of input energy) to metal-like behavior (resistivity increasing with increase of input energy) was obtained at a high input energy (25-75 kJ/g) and at a high, but not measured, pressure. The transition temperature, T, was roughly estimated through the C_V value (heat capacity under constant volume). The relationship between the density and the transition temperature is as follows: for 1.88 g/cm³ density, the transition temperature T = 6300 K, for 1.76 g/cm³, T = 10,100 K, and for 1.1 g/cm³, T = 13,500 K. The estimated temperature at the maximum input energy (75 kJ/g) for liquid metal-like carbon (just before the destruction of the sapphire tube) is 23,000 K, with a corresponding measured electrical resistivity of 3000 μΩ cm.     

Dependencies of the open-circuit voltage of Au|YSZ|Pt single-chamber fuel cell on the composition of the uniform CH4 + O2 gas mixture with solid carbon deposition

Single-chamber solid oxide fuel cell is a device where two electrodes of different materials contacting a solid oxide ionic conductor, may generate a considerable potential difference and electrical power, when supplied by a common fuel + oxidant gas mixture. The Au|YSZ|Pt system in the CH₄ + O₂ gas mixture is one of the simplest examples of such a cell. In this article the open-circuit voltage (OCV) of this cell, supplied with the gas mixture xO₂ + aCH₄ + (1 − x − a)Ar (where a = 0.01, 0.1 or 0.5), is investigated. On the basis of the obtained results, as well as those for the xCH₄ + (1 − x)(0.2O₂ + 0.8Ar) (0 ≤ x ≤ 1) gas mixture, reported in our previous work [Electrochim. Acta, 50 (2005) 2771], we postulate that the OCV of the above system arises as a result of electrode modification resulting from solid carbon deposition in the cell. After oxidation of the carbon deposit, the system, once treated by the gas mixture enabling the formation of the carbon phase, shows more and more tendency to generate the OCV. The open-circuit potential of the Au electrode depends only on the O₂ concentration in the initial gas mixture, whereas in the case of the Pt electrode it becomes dependent on chemical equilibria determining the O₂ content in the converted gas mixture. Our results reveal that the OCV achieves a reproducible limiting value of ∼650 mV, which is lower by ∼400 mV than the calculated equilibrium value.     

Formic acid electro-oxidation on carbon supported PdxPt1−x (0 ≥ x ≥ 1) nanoparticles synthesized via modified polyol method

Carbon supported nanoparticle catalysts of Pd_xPt_1−x (0 ≥ x ≥ 1) were synthesized using a modified polyol method and poly(N-vinyl-2-pyrrolidone) (PVP) as a stabilizer. Resulting nanoparticles were characterized by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperommetry (CA) study for formic acid electro-oxidation. Surface composition of the synthesized nanoparticles found from XPS revealed the Pt surface segregation even for the Pd-rich compositions. It is suggested that the surface segregation behavior in PdPt nanoparticles supported on carbon may be influenced, in addition to the difference in Pd and Pt surface energies, by particle size and particle interaction with the support. According to CA, the carbon supported Pd nanoparticles show the highest initial activity towards formic acid electro-oxidation at the potential of 0.3 V (RHE), due to the promotion of the direct dehydrogenation mechanism. However its stability is quite poor resulting in the fast deactivation of the Pd surface. Addition of Pt considerably improves the steady-state activity of Pd in 12 h CA experiment. CA measurements show that the most active catalyst is Pd_0.5Pt_0.5 of 4 nm size, which displays narrow size distribution and Pd to Pt surface atomic ratio of 27-73.     

Cathodic reduction of acids in dimethylformamide on platinum

A linear correlation was shown to exist between the acidity and the cyclic voltammetric half-potential of the reduction of acids in DMF for carboxylic and N-acids in the pK_a range of 6-16. Chlorophenols are reduced at slightly lower potentials giving a separate parallel line. Applying the obtained equation and employing the same method to literature data in DMSO, the pK_a values for conjugate aids of DMF and DMSO can be calculated, showing DMSO·H⁺ to be more acidic (pK_a = 2.9) than DMF·H⁺ (pK_a = 5.7). The analysis of cyclovoltammetric data demonstrated that a CE mechanism operates in the reduction of strong acids, including the conjugate acid of DMF. Weaker acids are reduced by direct discharge or a mixed mechanism.     

The effect of temperature on the adiabatic laminar burning velocities of CH4−air and H2−air flames

The effect of temperature on the adiabatic laminar burning velocities of CH₄ + air and H₂ + air flames was analyzed. Available measurements were interpreted using correlation S_L = S_L0 (T/T₀)^α. Particular attention was paid to the variation of the power exponent α with equivalence ratio at fixed (atmospheric) pressure. Experimental data and proposed empirical expressions for α as a function of equivalence ratio were summarized. They were compared with predictions of detailed kinetic models in methane + air and hydrogen + air flames. Unexpected non-monotonic behavior of α was found in rich methane + air flames. Modeling results are further examined using sensitivity analysis to elucidate the reason of particular dependences of the power exponent α on equivalence ratio.     

Detonation spraying of TiO2-2.5 vol.% Ag powders in a reducing atmosphere

In the present work, rutile powders containing additions of metallic silver (2.5 vol.%) were detonation sprayed in a reducing atmosphere formed by gaseous detonation products of the C₂H₂ + 1.05O₂ mixture. The initial volume of the C₂H₂ + 1.05O₂ mixture - explosive charge - used for a detonation pulse was computer-controlled as the fraction of the barrel volume filled with the mixture. Using a previously developed model of the detonation process, the particle temperatures and velocities were calculated to explain the observed phase and microstructure development in the coatings. With increasing explosive charge, the temperature of the sprayed particles increased and rutile was partially reduced to oxygen-deficient TiO_2−x and then to Ti₃O₅. When the melting temperature of rutile was not reached, the coatings were porous; semi-molten particles formed denser coatings obtained with higher spraying efficiency. Silver inclusions in the titanium oxide matrix experienced melting and substantial overheating, but remained well preserved in the coatings.