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CO2 conversion via hydrogenation over iron‐based catalysts on non‐carbon supports produces mainly CO or methane by the Sabatier reaction, while the formation of C2+ hydrocarbons is of greatest interest. CxHy production from CO2 may be considered as a two‐step process with the initial formation of carbon monoxide by the reverse water gas shift reaction followed by the Fischer‐Tropsch synthesis (FTS). In the present work CO2 hydrogenation over iron‐based catalysts (Fe, FeCr, FeK) deposited on a carbon carrier has been studied. The catalyst structure has been investigated by XRD, TEM, XPS, Mössbauer spectroscopy and in situ magnetometry. Spinel‐type oxide phases (magnetite Fe3O4; maggemite γ‐Fe2O3, and, in the case of FeCr/C catalyst, iron chromite Fe1+xCr2‐xO4) are formed on the catalysts, and they contribute exclusively to the CO production. Iron carbides, active in FTS, are formed on Fe‐ and FeK‐catalysts during pre‐activation in reducing environment and then during the reaction. The reaction over the 20Fe1K/C catalyst in supercritical high‐density CO2/H2 substrate (400 °C, 8.5 MPa) leads to 72 % selectivity for C1–C12+ hydrocarbons (alkanes and alkenes). Under the same conditions, iron carbides do not form on the FeCr/C catalysts, and CO2 hydrogenation results in the CO formation with the selectivity of 90–100 %. [ABSTRACT FROM AUTHOR]

Conversion of isopropanol under supercritical conditions (400 °C, 11.0 MPa) on calcium and magnesium stannates MSnO3 (M = Ca, Mg) was studied. The main direction of the process is dehydrogenation of isopropanol to acetone, which then enters into the aldol condensation with selective formation of isomeric mesityl oxides. Products of hydrogenation of the C=C and C=0 bonds in mesityl oxide as well as products of isopropanol dehydration were also obtained. Under the experimental conditions, isopropanol is both a supercritical solvent and a source of hydrogen for hydrogenation of the multiple bonds. According to the results of diffuse reflectance infrared Fourier transform spectroscopy study using deuterated chloroform as a probe molecule, basic oxygen sites of the catalysts are of medium strength. During the conversion of isopropanol the more active catalyst MgSnO3 completely decomposes into the oxides MgO and SnO2 and the latter is then reduced to metallic tin, while the less active catalyst CaSnO3 is reduced slightly. [ABSTRACT FROM AUTHOR]

[Display omitted] • Supercritical toluene was used to depolymerize e-waste thermoplastics. • STL temperature and time on depolymerization of PA, PC, POM, and PEEK were studied. • STL conversion reached 100% for PEEK but only reached 32.23% for POM. • Depolymerized PA and PC resulted in significant nitrogen containing chemicals. Solvothermal liquefaction (STL) is a thermochemical conversion technique that employs solvents other than water to transform waste plastics into valuable compounds. The objective of this study was to explore the potential use of supercritical toluene, a nonpolar solvent, for the depolymerization of four electrical waste (e-waste) thermoplastics, namely polyamide (PA), polycarbonate (PC), polyoxymethylene (POM), and polyether ether ketone (PEEK), into liquid products. Depolymerization experiments were carried out in batch reactors at three reaction temperatures (325, 350, and 375 °C), and three residence times (1, 3, and 6 h). The findings revealed that increasing STL temperature and extending the reaction time enhances the depolymerization of e-waste thermoplastics. The highest STL conversation (100 %) was observed for POM, and the lowest STL conversation (32.23 %) was observed for PEEK. Additionally, the ultimate analysis showed that the liquid product obtained from STL at 375 °C and 6 h exhibited higher heating values (HHV) within the range of 31.43 to 35.31 MJ/kg. Thermogravimetric analysis (TGA) demonstrated that the boiling point distributions of liquid products are highly dependent on thermoplastic type. Finally, the reaction mechanisms of STL for PA, PC, POM, and PEEK were proposed based on gas chromatography-mass spectrometry (GCMS) analysis. [ABSTRACT FROM AUTHOR]

Rice bran oil fatty acid distillate (RBOFAD), a by-product from rice bran oil production, was used as feedstock for the production of biodiesel. The hydrolysis–esterification–transesterification of RBOFAD with ethanol was performed in a microreactor under supercritical conditions. The biodiesel product in the form of fatty acid ethyl ester (FAEE) was obtained. The operating conditions, including the sulfuric acid content (0.2–0.6%), water content (0–7.5%), ethanol-to-oil molar ratio (5:1–16:1), and reaction temperature (260–320 °C), were optimized. The water content could affect the reaction system via both hydrolysis of triglycerides and reverse esterification of fatty acids. Biodiesel with the FAEE content of 93% was obtained at the optimal conditions (sulfuric acid content of 0.4%, water content of 2.5%, ethanol-to-oil molar ratio of 7:1, residence time of 19 min, and reaction temperature of 280 °C). The properties of the biodiesel, including density, viscosity, acidity, oxidation stability, freezing point, and pour point, were compared with standards. The properties such as viscosity, acid value, heating value, and oxidation stability were suitable as biodiesel according to EN14213, except for the contents of ester, monoglyceride, and diglyceride.

he escalating carbon dioxide (CO2) emissions have precipitated climate change, underscoring the urgent need for innovative solutions. Carbon Capture and Storage (CCS) represents a groundbreaking technology to mitigate carbon emissions by capturing CO2 and depositing it deep within reservoirs. This study focuses on the storage aspect of CCS by scrutinising reservoir properties. A pivotal factor influencing the geological storage of CO2 is the wettability of the rock/CO2/brine system, which is intricately linked to the contact angle. However, previous research has indicated that pH can significantly influence the wettability of this system. Acetic acid, a prevalent organic acid in the petroleum industry, is commonly encountered and serves as a means to modulate pH. This experiment investigates pH variations, ranging from 3 to 7, through the introduction of acetic acid, while maintaining a salinity level of 30,000 ppm. Additionally, the type of rock mineral plays a crucial role in these findings. Therefore, this paper emphasises carbonate rocks, with a particular focus on limestone composed primarily of calcium carbonate (CaCO3). The experiment is conducted under elevated supercritical conditions, encompassing pressures of 10 MPa and 15 MPa, along with temperatures of 305 K and 323 K. Equilibrium contact angles are determined by measuring receding and advancing angles using the Vinci IFT 700 instrument under supercritical conditions. The results reveal a consistent trend wherein contact angles decrease as pH, temperature, and pressure increase, indicating a shift towards a water-wet system at higher pH, temperature, and pressure levels.

The studies on the modeling of processes and physicochemical properties of fluid systems in the near- and supercritical regions of state parameters are reviewed. The specifics of supercritical fluids (SCFs) and their role in various technological processes are discussed. The nonequilibrium and equilibrium processes for which mathematical modeling methods were used are listed. The nonequilibrium processes are grouped as equations of chemical kinetics, hydrodynamic models, and diffusion equations, and also as descriptions of nanoparticle distributions and phenomenological models. The equilibrium models cover the existing spectrum of experimental data interpretations from thermodynamic to microscopic models, which allow for nonideal reaction systems, including those using the molecular dynamics method and quantum-chemical calculations of energy. The reviewed studies mainly concentrated on the equations of state of SCFs and their modeling at the molecular level, reflecting the multicomponent character of mixtures and specifics of interactions of components including their asymmetry. In general, two fields of application of the equations of state can be revealed: the one related to the wide range of thermodynamic parameters within a wide range of pressures and temperatures outside the rarefied gas region and the one related to the near-critical region itself. Real processes occur outside the near-critical region and are retarded within this region in systems with increased density of components. For modeling the processes in SCFs, a single approach is suggested, which provides self-consistent treatment of process equilibrium and kinetics based on a molecular description using the lattice gas model. [ABSTRACT FROM AUTHOR]

The results of a study of tandem reactions - aldol-crotonic condensation and hydrogenation by the mechanism of hydride transfer in acetone-isopropanol mixtures on a МgSnО3 catalyst at 300–400°C and 12.0 MРa are presented. In these conditions the mixture of reactants (acetone and isopropanol) and reaction products is in supercritical state. The temperature dependences of the reactants conversion and the selectivity for condensation and hydrogenation products are considered. It has been established that the hydrogenation of acetone aldol-crotonic condensation products proceeds by the addition of hydrogen from isopropanol molecule to C=O-bonds of isomeric mesityl and phorone oxides and is accompanied by dehydration of alcohols and isomerization of double C=C-bonds. The optimal conditions for the formation of the carbonyl group hydrogenation products are 350°C and the ratio acetone : isopropanol = 7 : 3. At the acetone to isopropanol ratio 1 : 1, methylpentadienes dominate in the products of mesityl oxide conversion. The hydrogenation reaction also proceeds with the participation of isophorones, the condensation products of three acetone molecules. [ABSTRACT FROM AUTHOR]

Literature analysis shows that the main method to model the equilibrium characteristics of reaction systems at elevated pressures, including processes under supercritical conditions, are equations of state describing the non-ideality of the vapor and liquid phases, while the law of mass action is applied to describe the kinetics of the elementary and chemical stages. The mentioned difference in the types of models used to describe the equilibrium and kinetic characteristics of the same experimental system under study violates the second law of thermodynamics formulated by Clausius. The only theoretical method consistent with the second law of thermodynamics is the molecular theory based on the lattice gas model. In order to satisfy the second law of thermodynamics, molecular models must provide the self-consistent description of the rates of the chemical process at the equilibrium and elementary stages. This means that the molecular models must provide a single mathematical apparatus to calculate the states of the system both outside and inside the equilibrium point. The molecular models can differ in both the effective parameters of the interparticle interaction and the methods of refining these models due to taking into account distinctions in sizes, contributions of the vibrational motions of the components, as well as the accuracy of description of the correlation effects. To ensure the self-consistent description of the equilibrium and kinetics, the models must at least reflect the effects of direct correlations. One-particle approximations (mean field, chaotic, density functional) do not correspond to the self-consistency condition and violate the second law of thermodynamics. [ABSTRACT FROM AUTHOR]

[Display omitted] Ethanol over calcium stannate (CaSnO 3) catalyst under supercritical conditions (400 °C, 120 atm) is transformed mainly into ethanal, ethyl acetate and 2-buten-1-ol. The process is accompanied by crystallization of the amorphous catalyst. [ABSTRACT FROM AUTHOR]

This study investigates the thermal stressing characteristics of hydrocarbon fuels to develop thermally stable fuels for supersonic vehicle applications. The thermal stability characteristics of two multicomponent hydrocarbon fuels, namely F-1 and F-2 fuels, under subcritical and supercritical conditions, are examined for a wide range of temperatures (30–500 °C) and pressures (15–50 bar) using a semi-batch reactor. Various analysis techniques, such as UV-Vis spectrophotometry, ASTM D86 distillation, gas chromatography, scanning electron microscopy (SEM), etc., are used to characterize the feed and product properties. The study showed that the fuels are reasonably stable up to around 400 °C and above 40 bar pressure. The stability of the fuels is relatively less under subcritical pressure at elevated temperatures. The amount of gum formation increased with the increase of both temperature and pressure parameters. Further, the SEM analysis showed oval-shaped and ribbon-like structures in the solid deposits. The present investigation may be useful in developing an appropriate fuel for regenerative cooling in supersonic applications. [ABSTRACT FROM AUTHOR]

The existing possibilities for modeling the kinetics of supercritical processes at the molecular level are considered from the point of view that the Second Law of thermodynamics must be fulfilled. The only approach that ensures the fulfillment of the Second Law of thermodynamics is the molecular theory based on the discrete–continuous lattice gas model. Expressions for the rates of the elementary stage on its basis give a self-consistent description of the equilibrium states of the mixtures under consideration. The common usage today of ideal kinetic models in SC processes in modeling industrial chemistry contradicts the non-ideal equation of states. The used molecular theory is the theory of absolute reaction rates for non-ideal reaction systems, which takes into account intermolecular interactions that change the effective activation energies of elementary stages. This allows the theory to describe the rates of elementary stages of chemical transformations and molecular transport at arbitrary temperatures and reagent densities in different phases. The application of this theory in a wide range of state parameters (pressure and temperature) is considered when calculating the rates of elementary bimolecular reactions and dissipative coefficients under supercritical conditions. Generalized dependencies are calculated within the framework of the law of the corresponding states for the coefficients of compressibility, shear viscosity, and thermal conductivity of pure substances, and for the coefficients of compressibility, self- and mutual diffusion, and shear viscosity of binary mixtures. The effect of density and temperature on the rates of elementary stages under supercritical conditions has been demonstrated for a reaction's effective energies of activation, diffusion and share viscosity coefficients, and equilibrium constants of adsorption. Differences between models with effective parameters and the prospects for developing them by allowing for differences in size and contributions from the vibrational motions of components are described. [ABSTRACT FROM AUTHOR]

The catalytic oxidation of benzene with nitrous oxide (N 2 O) over ZSM-5 zeolite has been carried out in a continuous-flow reactor under supercritical conditions and compared with the results of the gas-phase reaction. Aromatic substrates and nitrous oxide under the conditions of supercritical experiments (300–435 °C, 6.0–18.0 MPa) are both reagents and the supercritical medium. It has been established that the productivity of the supercritical oxidation of benzene into phenol significantly exceeds the productivity of the gas-phase process owing to the limited reversible deactivation of the catalyst under supercritical conditions and the in situ removal of the coke precursors by the dense reaction medium. In addition, it has been demonstrated that a successful in situ regeneration of the deactivated oxidation catalyst can be carried out during the transition from gas-phase reaction conditions to supercritical conditions in one experiment. [Display omitted] • ZSM-5 zeolite is a highly active and selective catalyst of benzene oxidation. • Benzene and nitrous oxide are both reagents and the supercritical medium. • Productivity of supercritical benzene oxidation greatly exceeds that of gas-phase oxidation. • Enhanced performance is due to dissolution and removal of coke precursors by the reaction supercritical medium. • Regeneration of deactivated catalyst has been carried out using supercritical conditions. [ABSTRACT FROM AUTHOR]

• Heat transfer of supercritical R1234yf was experimentally investigated in a horizontal micro-fin tube. • Operating parameter effects on heat transfer were analyzed. • Heat transfer comparisons were conducted between R1234yf and R134a. • Prediction accuracy of existing correlations for R1234yf was analyzed. R1234yf is regarded as an ideal substitute for R134a in supercritical organic Rankine cycles. This study experimentally analyzed the convective heat transfer coefficients of supercritical R1234yf in a micro-fin tube. The experiments show the influence of the system operating parameters including pressure, heat flux and mass flux on the heat transfer. Then, this paper compares the heat transfer rates for R1234yf and R134a at supercritical pressures and the ability of existing correlations to predict the heat transfer coefficients with R1234yf. The results show that as q / G increases, the buoyancy increases Nu bottom , while the variations in Nu top are divided into two regions based on the bulk fluid enthalpy. The influence of pressure on the heat transfer is also related to the bulk fluid enthalpy. When the bulk fluid enthalpy is less than a critical value, Nu bottom and Nu top both increase with decreasing pressure and then decrease above this critical enthalpy. The heat transfer coefficient is no longer enhanced at the top with large buoyancy forces. For low mass fluxes, the heat transfer coefficients of R134a and R1234yf are similar. For high mass fluxes, the heat transfer coefficients of R134a are higher than those of R1234yf. The Wang correlation, that was based on R134a data, more accurately predicts the heat transfer coefficients than other correlations for supercritical R1234yf in the horizontal micro-fin tube. Among all the 4050 experimental points, 90.17% of Nu top and 84.49% of Nu bottom were predicted with errors of less than 30% by the Wang correlation. [ABSTRACT FROM AUTHOR]

The aldol condensation of acetone under supercritical conditions at 300–400°C and 11.0 MPa was carried out on BaSnO3-450 and BaSnO3-750 catalysts prepared by calcination of BaSn(OH)6 at 450 and 750°C, respectively. Conducting the reaction under these conditions makes it possible to overcome the problem of catalyst deactivation by coking and to obtain valuable chemicals with high selectivity. At 300°C, both catalysts provide selectivity of 85–87% to isomeric mesityl oxides (C6 products). At 400°C with the BaSnO3-450 catalyst, the selectivity significantly changes from C6 to C9 (phorones). Powder X-ray diffraction and TEM data attest to the multiphase nature of the samples, which contain barium stannates, barium carbonate, and tin oxide. It was found that the catalyst structure rearranges in situ, which greatly affects the catalytic properties and is most pronounced for BaSnO3-450. [ABSTRACT FROM AUTHOR]

CO 2 conversion via hydrogenation over iron-based catalysts on non-carbon supports produces mainly CO or methane by the Sabatier reaction, while the formation of C 2+ hydrocarbons is of greatest interest. C x H y production from CO 2 may be considered as a two-step process with the initial formation of carbon monoxide by the reverse water gas shift reaction followed by the Fischer-Tropsch synthesis (FTS). In the present work CO 2 hydrogenation over iron-based catalysts (Fe, FeCr, FeK) deposited on a carbon carrier has been studied. The catalyst structure has been investigated by XRD, TEM, XPS, Mössbauer spectroscopy and in situ magnetometry. Spinel-type oxide phases (magnetite Fe 3 O 4 ; maggemite γ-Fe 2 O 3, and, in the case of FeCr/C catalyst, iron chromite Fe 1+x Cr 2-x O 4 ) are formed on the catalysts, and they contribute exclusively to the CO production. Iron carbides, active in FTS, are formed on Fe- and FeK-catalysts during pre-activation in reducing environment and then during the reaction. The reaction over the 20Fe1K/C catalyst in supercritical high-density CO 2 /H 2 substrate (400 °C, 8.5 MPa) leads to 72 % selectivity for C 1 -C 12+ hydrocarbons (alkanes and alkenes). Under the same conditions, iron carbides do not form on the FeCr/C catalysts, and CO 2 hydrogenation results in the CO formation with the selectivity of 90-100 %., (© 2024 Wiley-VCH GmbH.)

Global warming and fossil fuel depletion have boosted the search for alternative and renewable fuels with a low environmental impact. Biodiesel exhibits many advantages over conventional diesel including the possibility of being produced from renewable sources such as waste oils and fats. Specifically, waste animal fats are receiving increased attention as an alternative to vegetable oils for biodiesel production. This low-cost feedstock allows the mitigation of environmental pollution and can also improve biodiesel features by increasing cetane number and enhancing oxidative stability. Among the different technologies available for biodiesel production, supercritical processes offer important advantages over conventional catalytic transesterification in terms of process efficiency and reaction time. According to the increasing interest and number of research articles published in this field in the last years, this work focuses on the systematic review of the technology by using the Systematic Reviews and Meta-Analysis (PRISMA) methodology. This work describes the state of the art of non-catalytic supercritical production of biodiesel using animal fats as a feedstock and discusses the key aspects of the process such as the type of fat used, operation variables including reaction time, temperature, solvent excess, pressure, and solvent excess, and the final properties of the synthesized biodiesel. [Display omitted] • Animal fats for biodiesel production in supercritical alcohol • PRISMA method followed to systematically review fat-derived biodiesel production • Analysis of the influence of key operating variables on biodiesel yield • Waste animal fats as a low cost and promising feedstock for biodiesel production [ABSTRACT FROM AUTHOR]

In this paper, we introduce a novel, adapted approach for computing gas adsorption properties in porous materials. Our methodology is based on the Dubinin-Polanyi's adsorption model, and we investigate various frameworks to estimate its required essential components and prediction capabilities. The required components are linked to the physicochemical properties of the adsorbates, such as the vapor saturation pressure and density in the adsorbed phase. To conduct this analysis, we obtain adsorption isotherms for several metal–organic frameworks encompassing a range of pore sizes, shapes, and chemical compositions. We then apply and evaluate multiple combinations of models for saturation pressure and density. After the evaluation of the methods, we propose a working thermodynamic model for computing adsorption isotherms, which entails using the critical isochore as an approximation of the saturation pressure above the critical point and applying Hauer's method with a universal thermal expansion coefficient for density in the adsorbed state. This framework is applicable not only to simulated isotherms but also to experimental data from the literature for various molecules and structures of different natures, demonstrating robust predictive capabilities and high transferability. Our method showcases superior performance in terms of accuracy, generalizability, and simplicity compared to existing methods currently in use. In light of our results, this method, starting from a single adsorption curve and based on physically interpretable parameters, can predict adsorption properties across a wide range of operating conditions. [Display omitted] • The adsorption potential theory can be used for predicting adsorption isotherms. • A combination of adsorbed density and vapor pressure is key to the predictive theory. • The predictive capabilities of the proposed framework are successfully tested. • Equations of state are alternative options to obtain vapor saturation pressure. [ABSTRACT FROM AUTHOR]

Aldol condensation of acetone on strontium stannate is studied at a temperature of 250–400°C and a pressure of 2.5 and 12.5 MPa. The main reaction products are mesityl oxide and isophorone. It is shown that the SrSnO3 catalyst is stable under supercritical conditions and its productivity with respect to mesityl oxide isomers at 350°C and 12.5 MPa equals 9 g h–1 , which is higher than in the gas phase. [ABSTRACT FROM AUTHOR]

This paper examines the treatment efficiency of liquid and solid organic agroindustrial waste in supercritical (SC) conditions. The waste conversion rate is not less than 99.7%. The correlation between the organic matter content and the continuous reactor volume is identified. High waste oxidation rates in SC conditions make a reactor volume of 100–140 L sufficient for a waste capacity of up to 100 kg per hour. The reactor heat density may reach 730 kW/m3. [ABSTRACT FROM AUTHOR]

The product formation sequence and kinetic regularities of vanillin transformations in the propanol-2 medium are investigated at different temperatures and durations of the process in a static mode. The vanillin transformation products are determined by the gas chromatography–mass spectrometry method. At a high degree of vanillin conversion (up to 100%), a relatively small number of compounds are formed, the main ones of which are substituted phenols and their derivatives, such as vanillyl alcohol and its isopropyl ether, guaiacol, creosol (2-methoxy-4-methylphenol), and homocatechol (3,4-dihydroxytoluene). A scheme of reciprocal transformations of intermediate compounds is proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] Increasing atmospheric pollution with greenhouse gases, a large proportion of which are transport pollutants, is forcing the search for new fuels from renewable sources. Biodiesel is currently produced by transesterification of plant oils over heterogeneous catalysts under gentle conditions. The other recent technology dealing with the transesterification with alcohols under supercritical conditions, i. e., at high temperature and pressure, can be more efficient, the cost of the resulting biodiesel having been lower, and lower quality feedstocks having been used. Supercritical transesterification can be performed catalytically or catalyst-free. This paper provides an overview of the catalytic lipid transesterification under supercritical conditions. The influence of raw material, alcohol and catalyst, as well as process parameters on biodiesel yield is analyzed. [ABSTRACT FROM AUTHOR]

Current possibilities of modeling the kinetics of supercritical processes are considered, based on the theory of an absolute rate of the reaction for non-ideal reaction systems, which considers intermolecular interactions that change the effective energy of activation of elementary stages. This allows the theory to describe the rates of elementary stages for arbitrary temperatures and densities of the reagent in different phases. Application of this theory in a wide range of state parameters (pressure and temperature) is examined while calculating elementary bimolecular reactions and dissipative coefficients under supercritical conditions. Generalized dependences within the law of corresponding states are calculated for the compressibility, viscosity, and thermal conductivity coefficients of pure substances and those of the compressibility, self- and mutual diffusion, and viscosity of binary mixtures. The effect density and temperature have on the rates of elementary stages under supercritical conditions is demonstrated for a reaction's effective energies of activation, diffusion and share coefficients, and equilibrium constants of adsorption. Differences between models with effective parameters and the prospects for developing them by allowing for differences in size and contributions from the vibrational motions of components are described, along with ways of improving the accuracy of describing correlation effects. [ABSTRACT FROM AUTHOR]

A comparative study of n-butane conversion into aromatic hydrocarbons on a ZSM-5 zeolite in the H-form and modified with gallium or platinum at 530°C in the gas phase (at 1 atm) or under supercritical conditions (at 190 atm) is carried out. Under supercritical conditions, the catalysts demonstrate high levels of activity, stability, and productivity. Unlike the gas-phase experiments, the modification of a ZSM-5 zeolite with gallium or platinum in the processes under supercritical conditions produces no effect on the conversion of n-butane or the distribution of n-butane aromatization products. [ABSTRACT FROM AUTHOR]

The structure of 2 wt % aqueous solutions of ethanol, phenol, and o-methoxyphenol (guaiacol) was modeled in NVT ensemble using the classical molecular dynamics method at densities of 0.997 and 0.133 g/cm3 corresponding to the normal (298 K, 0.1 MPa) and supercritical (673 K, 23.0 MPa) conditions. The self-diffusion coefficients were calculated for individual components in solutions; the radial distribution functions were calculated for the oxygen atoms of water molecules, oxygen atoms of hydroxyl groups, and centers of mass in phenol molecules. The possibility of clusterization of solute molecules was analyzed. The data obtained suggest heterogeneity of solutions, in which clusters of different compositions and structures can exist. Clusterization of up to seven ethanol and phenol molecules can occur under normal conditions, and dimerization was detected under SC conditions. The structural features of solutions under normal and SC conditions were compared. The difference in the formation of hydration shells of ethanol and phenols molecules was demonstrated. Stable shells of water molecules form around ethanol molecules under normal conditions. For phenols, the solvation shells are unstable, with a pronounced tendency toward clusterization of organic molecules. [ABSTRACT FROM AUTHOR]

The decomposition of methylcyclohexane (MCH) containing suspended functionalized graphene sheets (FGS) was studied using a high-pressure flow reactor, under supercritical conditions over the temperature range of 780 – 825 K at a constant pressure of 4.72 MPa. Experiments showed both fuel conversion rates and C 1 -C 2 product yields were increased by 43.3% and 62.1%, respectively, with the addition of 50 ppmw of FGS at 820 K. The reaction mechanisms between hydrocarbon and FGS were computationally investigated using reactive molecular dynamics (MD) with ReaxFF force fields at the temperatures of 1700, 1800, and 1900 K. The MD simulations revealed that oxygen-containing functional groups attached to the FGS plays an important role in catalyzing the decomposition of the fuel. Heterogeneous dehydrogenation of MCH into a C 7 H 13 radical intermediate led to formation of secondary and tertiary radicals, such as H, CH 3 , and C 2 H 5 , during early extents of reaction promoting additional fuel-consuming steps, including H-abstraction and hydrogenation. [ABSTRACT FROM AUTHOR]

Abstract—The solubility of iodine in sub- and supercritical carbon dioxide was studied using a flow-type extraction setup. It is shown that at 20–80°C and 60–350 atm, the solubility of iodine increases at rising temperature and CO2 pressure and the mole fraction of iodine in its saturated solution ranges from 0.12 × 10–3 to 1.42 × 10–3. [ABSTRACT FROM AUTHOR]

Increasing energy demands lead to the search for sustainable biofuels such as biodiesel from renewable sources such as algal biomass. Specially, microalgae have become increasingly popular as biodiesel feedstock due to their fast-growing rates and relatively high level of lipids. The non-catalytic transesterification reaction of microalgae oil under supercritical conditions in the presence of alcohols is an efficient process for biodiesel production and can overcome some of the limitations posed by subcritical catalytic transesterification. Two-step and one-step processes have been reported for this purpose, in the last case with extraction and transesterification stages being performed simultaneously. This works offer a systematic review using the PRISMA methodology analyzing the status of this research topic and identifying future research strategies. With this work, PRISMA methodology is applied for the first time within technological engineering fields. Moreover, this is the first specific overview on the production of biodiesel from microalgae under supercritical conditions. Several key factors such as microalgae type and operation variables including extraction oil, temperature and reaction time, pressure, water content in biomass, alcohol excess and final biodiesel properties are discussed. Further efforts need to be done in order to study the continuous biodiesel production by the development of integrated processes that involve both microalgae cultivation and biodiesel production. Unlabelled Image • Production of biodiesel from microalgae feedstock in supercritical conditions • Systematic review on biodiesel production from microalgae by the PRISMA method • Study of the effect of the most relevant operational variables on FAME yield • Wet/dry microalgae biomass as promising feedstock for biodiesel production [ABSTRACT FROM AUTHOR]

• A new approach to determine the time of supercritical transition for MD droplets simulation. • Supercritical evaporation characteristics of ammonia droplets in multi-component gaseous environments were revealed. • Polarity has a significant impact on the profile of environment gas diffusing into the ammonia droplet. The evaporation characteristics of ammonia fuel droplets in single and multi-component supercritical environments were studied using molecular dynamics simulations. A new approach for determining the supercritical transition of droplet evaporation modes was proposed from the perspective of atomic pair entropy fingerprints. Some improvements of the previous pair entropy fingerprint formulas were conducted to better analyze the variations in the disorder of fuel molecules during evaporation. The transition moment of droplet evaporation under supercritical conditions was based on the degree of fuel molecular disorder within the vapor–liquid mixing layer. Then, the effects of ambient conditions on the mode transition of droplet evaporation were studied. The findings indicated that the supercritical transition would not be obvious if ambient pressure was high enough. The existence of polar molecules in the environment can accelerate the mode transition. In addition, the diffusion tendency of the ambient gas components into the droplets was investigated. It was found that polar molecules diffused more strongly into the ammonia droplet than non-polar molecules. [ABSTRACT FROM AUTHOR]

[Display omitted] The successful use of a palladium membrane to enhance the yield of benzene–toluene–xylene (BTX) fraction in n -butane aromatization at 530 °C and 95 atm on a Ga/H-ZSM-5 catalyst has been shown. Removal of hydrogen from the reaction zone through the palladium membrane led to an increase in both the n -butane conversion and the selectivity of the BTX fraction formation. [ABSTRACT FROM AUTHOR]

Abstract In this paper, Cu/Al 2 O 3 catalysts with different metal loadings was applied for the direct ethanol conversion into linear α-alcohols in the supercritical regime. 5% Cu/Al 2 O 3 catalyst was found to provide ethanol conversion into 1-butanol, 1-hexanol and 1-octanol with 80.2% selectivity of 1-butanol or total selectivity of linear α-alcohols of 95.5 at 33.5% conversion of ethanol. The supercritical conditions allow to increase catalysts productivity in comparison with subcritical regime. Kinetic features of the reaction and the effect of water content in the feed mixture were investigated. Highlights • The selectivity of the formation of linear α-alcohols from ethanol was 98.4%. • Ethanol conversion and 1-butanol selectivity increased in supercritical regime. • Ethanol conversion decreases proportionately to water content in feeding mixture. • Kinetic parameters of the ethanol self condensation were determined. [ABSTRACT FROM AUTHOR]

Abstract: Results are presented from a comparative study of CO2 hydrogenation under gas-phase and supercritical conditions for CO2 in the presence of 15% Fe/SiO2 catalyst. The reaction is studied in the temperature range of 300-500°C at atmospheric pressure under gas-phase conditions and at a pressure of 95 atm under supercritical conditions at an Н2 : СО2 molar ratio of 2 : 1. It is found that the process proceeding under supercritical conditions lowers CO selectivity from 90-95 to 30-50% over the range of temperatures and raises (up to 60%) the hydrocarbon selectivity. In contrast to gas-phase hydrogenation, the formation of alcohols is observed in the reaction under supercritical conditions. Using a combination of thermogravimetry, differential thermogravimetry, and differential thermal analysis (TG-DTG-DTA), it is shown that the process proceeding under supercritical conditions results in a 2.2-fold drop in the amount of carbon-like deposits on the catalyst surface. X-ray diffraction analysis shows that under gas-phase process conditions, graphite-like structures form on the catalyst surface; this effect is not observed under supercritical conditions. The developed catalyst and the process for CO2 hydrogenation can be recommended for the further modification and improvement of the properties of a catalyst based on iron nanoparticles that is much (10-100 times) cheaper than the previously reported CO2 hydrogenation catalysts. [ABSTRACT FROM AUTHOR]

Two alternative processes for biodiesel production from used/waste cooking oil, the single-step supercritical transesterification and the two-step process consisting of oil hydrolysis and subsequent supercritical methyl esterification, were designed and simulated using the Aspen Plus 8.8 software and assessed from a techno-economic point of view. Detailed operating conditions, material and energy flows, and equipment designs were provided for biodiesel plants with 15,000 t and 30,000 t annual inlet capacities. Results revealed that both technologies offer a techno-economically viable solution for biodiesel production from low quality feedstocks such as used cooking oil. The two-step process requires milder operating conditions, lower investment and overall production costs per unit of biodiesel produced, however, it has higher process energy requirements. Total capital investments for a 15,000 t capacity plant which applies the single- or the two-step process were estimated as 6.1 million EUR or 5.8 million EUR, respectively, whereas the production cost of biodiesel are estimated as 0.824 EUR/kg and 0.786 EUR/kg for the single- and two-step processes, respectively (at used cooking oil price of 0.500 EUR/kg). Higher plant capacities have lower production and capital costs per unit of biodiesel output. Supercritical transesterification should lead to a clean, flexible and modular method to process used cooking oil for the production of a more valuable product. [ABSTRACT FROM AUTHOR]

All atom molecular dynamics simulations of uranyl ions in supercritical water are used to dissect the effects of concentration of uranyl ions and density of water on various structural and dynamic properties of the solutions. The analyses of radial distribution functions as a function of concentration of the uranyl ion and water density reveal that the effect of the former on the local structure is negligible as compared to the same of the later. The number of hydration water of the uranyl ion has been observed to increase with increasing density of the water, but it decreases with the increasing concentration of the uranyl ions. The orientational distributions are observed to be independent of variation in concentration of the uranyl ion, same as the case was with water density. The translational and rotational dynamics of the water molecules have been investigated from the respective mean squared displacements and time correlation functions. Although increase of both the concentration of the uranyl ions and the density of water reduces translational diffusivity of water as well as uranyl ions, the effect of changing water density is more than that of uranyl concentrations. However, orientational relaxation of various molecular vectors of the water molecule is practically unchanged with any variation in concentration of the uranyl ions and it changes only slightly with the change in water density. Unlike at ambient condition, orientational dynamics at supercritical conditions remains virtually unchanged with the change in uranyl ion concentration. [ABSTRACT FROM AUTHOR]

Catalytic reactions of supercritical n -hexane and n -dodecane were studied over H-ZSM-5, H-Y and Al-MCM-41 at pressures between 4 and 6 MPa and temperatures between 558 and 748 K, using a combination of experimental and theoretical approaches. The primary reaction for n -hexane conversion over H-ZSM-5 was cracking, whereas the primary reaction over H-Y and Al-MCM-41 was isomerization. In contrast, cracking was the primary reaction of n -dodecane over all of the aluminosilicates. Turnover frequencies (TOFs) for n -hexane and n -dodecane conversion over H-ZSM-5 were much greater than for H-Y and Al-MCM-41 under all tested reaction conditions. The invariance of TOFs for n -hexane conversion over H-ZSM-5 and H-Y over the pressure range studied indicated that the reaction was nearly zero-order in hydrocarbon at the supercritical conditions used here. Configurational-bias Monte Carlo simulations indicate that the loadings of reactant molecules reached only about half of their saturation values at the experimental conditions, but plateaus in the adsorption isotherms of n -hexane in H-ZSM-5 and of n -dodecane in both H-ZSM-5 and H-Y zeolites account for the nearly zero-order kinetics. At supercritical conditions, the apparent activation energy of n -dodecane cracking over H-ZSM-5, H-Y and Al-MCM-41 as well as that for n -hexane cracking over H-ZSM-5 was associated with the rate constant for C C bond cleavage (156–200 kJ mol −1 ). Density-functional theory calculations show that for n -hexane conversion over H-ZSM-5, β-scission from the bound alkoxide possesses the highest barriers, ranging from 167–177 kJ mol −1 depending on the mode of cracking. The intrinsic activation barriers for isomerization and hydride transfer reactions were calculated to be significantly lower, at 94 and 96–124 kJ mol −1 , respectively, although still higher than the experimentally observed value of 74 ± 6 kJ mol −1 for n -hexane conversion over H-Y. Rate expressions derived for monomolecular cracking, bimolecular cracking, and isomerization reactions suggest that the differences between the apparent barriers determined experimentally and the intrinsic barriers calculated theoretically are likely due to the competing hydrocarbon adsorption or quasi-equilibrated exothermic reaction steps. The increased selectivity to cracking over isomerization for n -hexane in H-ZSM-5 was attributed to the difficulty of product removal from the zeolite and, for n -dodecane, to the greatly reduced β-scission barriers and the much stronger product adsorption. [ABSTRACT FROM AUTHOR]

Lignite-water suspension supercritical gasification was carried out in an electric field with an intensity of 0.5 V/cm in the presence of additives under the pressure of 24 MPa in two temperature ranges: 330–360 and 390–450 °C. From 50 to 78% of hydrogen and oxygen of gas products were of water origin. The application of the electric field switched carbon oxidation over to hydrocracking in the range of the subcritical temperature. At the subcritical temperature, the electric field had moderated the lignite hydrolysis and intensified its hydrocracking. The gasification of lignite-water suspension under the supercritical pressure in the electric field had at least five types of interactions: electrolysis of water, thermal decomposition of coal, carbon oxidation by oxygen of water as well as hydrolysis and hydrocracking of coal. Electrolysis and hydrocracking were dominant for the process at the subcritical temperature on water. Hydrolysis and hydrocracking of C C bonds of coal were the main at the supercritical parameters of water. [ABSTRACT FROM AUTHOR]

Thermophysical properties, such as density, specific heat, viscosity and thermal conductivity, vary sharply near critical point. To evaluate these properties of hydrocarbons accurately is crucial to the further research of fuel system. Comparison was made by the calculating program based on four widely used equations of state (EoS), and results indicated that calculations based on the Peng-Robinson (PR) equation of state achieve better prediction accuracy among the four equations of state. Due to the small computational amount and high accuracy, the evaluation method proposed in this paper can be implemented into practical application for the design of fuel system. [ABSTRACT FROM AUTHOR]

Waste heat recovery (WHR) for internal combustion engines in vehicles using Organic Rankine cycle (ORC) has been a promising technology. The operation of the ORC WHR system in supercritical conditions has a potential to generate more power output and thermal efficiency compared with the conventional subcritical conditions. However, in supercritical conditions, the heat transfer process in the evaporator, the key component of the ORC WHR system, becomes unpredictable as the thermo-physical properties of the working fluid change with the temperature. Furthermore, the transient heat source from the vehicle's exhaust makes the operation of the WHR system difficult. We investigated the performance of the ORC WHR system at supercritical conditions with engine's exhaust data from real city and highway drive cycles. The effects of operating variables, such as refrigerant flow rates, evaporator and condenser pressure, and evaporator outlet temperature, on the performance indicators of the WHR system in supercritical conditions were examined. Simulation of operating parameters and the boundary of the WHR system are also included in this paper. [ABSTRACT FROM AUTHOR]

Abstract—The condensation of acetone under supercritical conditions was studied at different temperatures, pressures, and densities in a static mode. The high yield of mesitylene under the given conditions and its easy isolation from the reaction mixture made it possible to propose a new method for its preparation. [ABSTRACT FROM AUTHOR]

The non-catalytic synthesis of biodiesel (fatty acids methyl esters) from triglycerides and methanol proceeds at elevated pressures above 100 bar and temperatures above 523 K. Kinetic investigations of the system revealed an unusual behavior of the reaction rate constant with increasing temperature and pressure. In order to explain this phenomenon, the phase behavior of the triglycerides-methanol mixture was investigated. The phase equilibria of the binary system sunflower oil-methanol were measured at different temperatures between 473 and 503 K, and a range of pressures between 10 and 56 bar. The experimental data were correlated using the Peng-Robinson, Soave-Redlich-Kwong and Redlich-Kwong-Aspen equations of state and different mixing rules. The best results were obtained with the RK-ASPEN equation of state and the Van der Waals mixing rule (VdW), which were then used to calculate the distribution of the phases at pressures and temperatures usual for the non-catalytic synthesis of biodiesel under high pressures. The obtained data indicated a strong influence of the phase equilibria on the reaction kinetics. .

The non-catalytic synthesis of biodiesel (fatty acids methyl esters) from triglycerides and methanol proceeds at elevated pressures above 100 bar and temperatures above 523 K. Kinetic investigations of the system revealed an unusual behavior of the reaction rate constant with increasing temperature and pressure. In order to explain this phenomenon, the phase behavior of the triglycerides–methanol mixture was investigated. The phase equilibria of the binary system sunflower oil–methanol were measured at different temperatures between 473 and 503 K, and a range of pressures between 10 and 56 bar. The experimental data were correlated using the Peng–Robinson, Soave–Redlich–Kwong and Redlich–Kwong–Aspen equations of state and different mixing rules. The best results were obtained with the RK–ASPEN equation of state and the Van derWaals mixing rule (VdW), which were then used to calculate the distribution of the phases at pressures and temperatures usual for the non-catalytic synthesis of biodiesel under high pressures. The obtained data indicated a strong influence of the phase equilibria on the reaction kinetics.

A small-scale demonstration plant was designed, set up, and operated for the heterogeneously catalyzed synthesis of biodiesel under supercritical conditions. An Aspen Plus® model was applied to minimize the energy demand for the product purification stages. Vacuum distillation followed by a small rectification column was identified as the optimum solution. The determination of the energy demand of the plant revealed the economic viability of the process as far as the energy consumption is concerned. The catalyst incorporated into the plant was selected through a screening procedure applying tricaprin as model substance. γ-Al2O3 modified by acid treatment and La2O3 supported by γ-Al2O3 proved to be the most promising candidates. The stability of these formulations was proven by long-term stability tests. [ABSTRACT FROM AUTHOR]

The experimental research of lignite-water suspension gasification under the supercritical conditions of water was carried out. The conversion of lignite was studied under the pressure 24 MPa in two temperature range: 290–360 and 390–500 °C without and with additives. Calculation of the activation energies of the reactions of gases formations had shown that NiO-MoO 3 -Al 2 O 3 had acted as a catalyst, sodium and calcium hydroxides had behaved as reactants. The general scheme of the mechanism on formation of the gas products containing H 2 , CH 4 and CO 2 was examined. It was shown that at the subcritical temperature the gas products were formed mainly from hydrogen of coal and oxygen of mineralized water. Carbon of lignite behaves like an acid, and water is as a base at the subcritical temperature; they switch their roles at the supercritical temperature. 80% of hydrogen and oxygen of gas products were of water origin at the supercritical temperature. C, O and H conversions at initial rates of reactions with gases distribution have been synchronously analyzed for primary reactions clearing. It had found that the gasification of lignite-water suspension under the supercritical pressure had at least four types of interactions: thermal decomposition of coal, carbon oxidation by oxygen of water as well as hydrolysis and hydrocracking of coal. Oxidation of carbon was dominant for the process at the subcritical temperature on water. Hydrolysis of C C bonds of coal was the main at the supercritical parameters of water. [ABSTRACT FROM AUTHOR]