Journal of Supercritical Fluids最新文献

SC-CO₂ fracturing technology is a feasible method for green extraction of unconventional oil and gas reservoirs. However, due to a series of problems such as sand transportation during SC-CO₂ fracturing, CO₂ thickening has become the focus of current research. Polymer thickeners have been widely used in oilfields. Due to the macroscopic nature and limitations of laboratory experiments, we have not been able to elucidate the microscopic mechanism of polymer thickening CO₂ and the viscosity change of the CO₂ fracturing fluid system downhole after polymer addition. In this study, we used all-atom modeling and molecular simulation of the COMPASSII force field. We selected nine polymers to study their microscopic properties and the factors affecting their effects. Finally, we simulated the viscosity change of the CO₂ system after the addition of polymer tackifiers in the wellbore under the CO₂ fracturing construction conditions by combining the data from one well in the oilfield.

In this study, strontium titanate (SrTiO₃) nanoparticles were obtained utilizing a one-step supercritical continuous solvothermal synthesis process involving acetylacetonate precursors for both strontium and titanium cations instead of the historical alkoxide ones. These precursors are expensive, difficult to access (especially for strontium isopropoxyde) and inadequately stable, forcing the use of a glove box and controlled atmospheres, which is not the case for acetylacetonates. Pure SrTiO₃ nanoparticles with a crystallite size of roughly 20 nm were successfully synthesized. In addition to the cubic structure of SrTiO₃, FTIR revealed surface functions that are typical of "wet" processes, while Raman spectroscopy showed the activation of non-centrosymmetric modes brought on by non-linear contributions. The nanoparticles show a faceted shape and are stable at elevated temperatures (up to 800 °C), according to in-situ high temperature XRD measurements. However, due to a chemical deficiency in strontium, titanium dioxide (TiO₂) phases are formed at higher temperatures. In-situ high temperature HRTEM investigations showed the existence of two populations of particles, with a better stability for the bigger-sized particles after thermal treatment as well as the sintering and restructuring of the smallest ones. Also, the microscopy results suggest the possibility of a chemical inhomogeneity within the crystallites. Overall, this study offers important new knowledge on the physicochemical characteristics of the synthesized SrTiO₃ nanoparticles and their thermal stability using a novel supercritical continuous solvothermal approach based on the use of acetylacetonate precursors.

Hydrothermal extraction of green coffee beans was examined by using a semi-batch type apparatus and an optical cell for extracting beneficial compounds. Extraction curves were evaluated across varying temperatures (373–523 K) and pressures (0.6–2.1 MPa). At 473 K, increasing pressure enhanced the extraction of caffeoylquinic acids (CQAs), total phenolic content (TPC), antioxidant capacity (AOC), and caffeine. At 2.1 MPa, TPC remained constant with temperature, while AOC, CQAs, and caffeine initially increased then decreased at higher temperatures. Direct observations confirmed high temperature promoted extraction. An extraction model was proposed to account for decomposition of components at high temperatures, effectively reproducing experimental curves. This model estimated equilibrium values and rate constants representing initial extraction rate. The obtained results indicate that adjusting temperature, extraction time, and pressure can control extract composition during hydrothermal extraction of green coffee beans.

In this work, we simulated the heat transfer in a two-vessel (1-m³, length-to-diameter ratio of 4) industrial plant to assess the effect of the temperature gradients formed during the reconditioning stage on the extraction curves. We simulated the extraction of 1-mm particles using 5 mm/s of CO₂ at 48 MPa and 40 °C (case with an imposed temperature gradient) or 60 °C (case with temperature gradients from the reconditioning stage), with the service fluid at 60 °C. The results of these non-isothermal extractions were compared with those obtained in representative isothermal cases. The temperature gradients slightly affected the cumulative extraction curves in non-isothermal cases. We considered the presence of a basket containing the solid substrate. We also changed the superficial CO₂ velocity to 3 or 10 mm/s and the particle size to 0.50 or 1.25 mm to compare the extraction curves. The effects of the basket and the changes in superficial CO₂ velocity and particle size were minor. We simulated a limit case with higher temperature and pressure (80 °C and 70 MPa), where the extraction time was extremely short (10 min) and more significant temperature gradients were formed during the reconditioning stage. We observed more significant differences at this extreme extraction condition than when using an isothermal process at the required extraction temperature.

Flexible silicone rubber (SR) foams with tunable cellular morphologies were fabricated via supercritical CO₂ foaming. Conductive carbon black (CB) modified at a low cost and with high complex viscosity was preferentially selected. The effects of the pore size and void fraction on the electrical conductivity and dielectric permittivity of the SR/CB composite foams were carefully investigated. The pore size of the foams affected the specific shielding effectiveness (SSE) and absorption coefficient (A), whereas the variation in the void fraction did not generate evident changes. In comparison with its solid counterpart, a foam with a pore diameter of 59.9 μm showed a 50 % decrease in density, 31.6 % increase in absorptivity, and 95.7 % increase in SSE, demonstrating a considerable electromagnetic interference shielding effectiveness (EMI SE). Decreases in the pore size and void fraction of the foams improved compression modulus and strength. In addition, sample preparation process was simplified, making industrial production easier.

Rosmarinus officinalis L. is one of the most widely exploited medicinal plants for its bioactive compounds, including rosmarinic acid (RA), carnosic acid (CA) and carnosol (CAR). This study aims to selectively extract these compounds, using supercritical fluids extraction with carbon dioxide and ethanol as co-solvents, based on the Hansen solubility parameters approach. This approach predicted that CA and CAR are highly soluble in the supercritical fluid. However, a weak interaction was predicted between RA and this fluid. The experimental findings indicated the highest amounts of RA and CA were observed at 150 bar and 80°C, resulting in yields of 3.43 mg/g dry weight (DW), and 18.9 mg/g DW, respectively. Favourable extraction conditions for CAR were recorded at 250 bar and 40°C, with a content of 19.1 mg/g DW. Evaluation of antioxidant activity using DPPH tests revealed that extracts obtained at 250 bar at 40 and 80°C rich in CAR have a stronger antioxidant power.

This study investigates the estimation of solute solubility in supercritical carbon dioxide (SC-CO₂) within a pressure and temperature range of 80 bar to 490.29 bar and 308 K to 423 K. We propose a novel empirical model that establishes a correlation between relevant parameters and the targeted solubility. A feature importance algorithm facilitated the development of this empirical model. The model’s accuracy is comprehensively evaluated using 40 published experimental datasets, with an average absolute relative deviation (AARD) of 9.9 %. It demonstrates superior performance compared to 12 previously established models. Furthermore, a fine-tuned artificial neural network (ANN) is developed to harness the unique capabilities of machine learning techniques. The ANN outperforms the proposed model, achieving a significantly lower AARD% of 4.38. This outcome emphasizes the potential of machine learning techniques, particularly ANNs, for achieving superior accuracy.

This study investigates the hydrothermal synthesis of KNbO₃-NaNbO₃ particles, which contains compositions of interest as lead-free piezoelectric materials. Precursor solutions containing Nb₂O₅, NaOH/KOH were hydrothermally reacted at 190 ⁰C for 15 hours using either a conventional reactor or a reactor with stirring. For 50 % KOH or lower, the product was cubic or elongated cuboid/belt-shaped NaNbO₃, whereas solely KOH resulted in hexagonal plate-like KNbO₃. When 75 % KOH was used, the product was a mixture of large K₅Na₃Nb₆O₁₉·9 H₂O hexagonal plates and smaller NaNbO₃ cubes. We hypothesized this phase segregation was induced by highly concentrated microenvironments, and stirring would prevent that. Accordingly, stirring hydrothermal treatment of 75 % KOH solution produced only K₅Na₃Nb₆O₁₉·H₂O hexagonal plates, which could be transformed to K_0.7Na_0.3NbO₃ by calcination. By reducing the hydrothermal treatment time to 1 – 6 hours we isolated reaction intermediates, and based on them propose a kinetics-controlled mechanism for hydrothermal reactions of Nb₂O₅ and KOH/NaOH.

Amphoteric oxides can be challenging to produce in phase pure form due to their sensitivity to the pH of the reaction mixture. The high-pressure, high-temperature reaction conditions of hydrothermal processes make the pH difficult to control. Here we report a simple approach to obtain nanoparticles of amphoteric oxides, where cheap metal nitrate precursors are reacted in alcohol (2-propanol) water mixtures (25 vol%:75 vol%) to maintain a neutral pH under solvothermal conditions. Phase pure SnO₂ nanoparticles were synthesized in a continuous flow solvothermal reactor at 250 °C, 300 °C, 350 °C and 400 °C with sizes ranging from 3 nm to 7 nm. The nanoparticles were characterized using powder X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy, ¹¹⁹Sn MAS NMR spectroscopy, Brunauer-Emmett-Teller nitrogen adsorption, UV–VIS, and as anode material in CR2032 Li-ion battery cells. To demonstrate generality, amphoteric oxides ZnO, Bi₂O₃, Cr₂O₃ and γ-AlOOH were also synthesized.

The electrochemical carbon dioxide reduction reaction (CO2RR) is of increasing importance for the development of a closed carbon cycle. Here, electrolytes are often used to increase the electrical conductivity and thus the current density. In addition, the use of compressed CO₂ and electrolyte-containing organic solvents suppresses hydrogen evolution. However, the electrical conductivity of CO₂-saturated electrolyte solutions is mostly unknown. Therefore, their electrical conductivity was investigated in this work at different electrolyte concentrations, pressures (up to 150 bar) and temperatures (298.15 K and 343.15 K). In addition, the simplified Mean Spherical Approximation (MSA-simple) model, supported by ePC-SAFT for density modeling, was used to successfully model the conductivity. The results show that CO₂-saturated electrolyte solutions generally exhibit lower conductivities than CO₂-free electrolyte solutions. However, at very low electrolyte and CO₂ concentrations, the conductivities were higher in CO₂-saturated electrolyte solutions than in CO₂-free electrolyte solutions, which is a new finding in the literature.