Effects Of Supercritical Fluids Research Articles

The effect of supercritical fluids on Nb-Ta fractionation in subduction zones: Geochemical insights from a coesite-bearing eclogite-vein system

Supercritical fluids are ideal media for mass transfer from the subducting slab into the mantle wedge. However, little is known about the role of natural supercritical fluids in subduction zones. A combined study of petrology, geochemistry and zirconology was carried out for a coesite-bearing eclogitic vein and its surrounding eclogites from the Dabie orogen, one of the typical continental subduction zones on Earth. The results are used to reveal the composition and source of supercritical fluids, the P-T conditions and timing of their formation and their geochemical effects in subduction zones. The eclogitic vein is composed of garnet, omphacite, quartz, amphibole, rutile and apatite, with minor amounts of epidote, plagioclase and zircon. Coesite was identified in omphacite in the vein, demonstrating its formation under ultrahigh-pressure (UHP) metamorphic conditions that correspond to subarc depths in oceanic subduction zones. Zircons from the vein give concordant U-Pb ages of 225 ± 5 Ma and flat HREE patterns for newly grown rims, consistent with their crystallization at the UHP eclogite facies. The vein shows similar Hf-O isotope compositions to the host UHP eclogites, indicating that the UHP vein-forming fluid is internally buffered within the UHP eclogites. Minerals in the vein contain not only higher contents of Cr, Ni, Sr, REE and HFSE, but also multiphase crystal inclusions (such as omphacite, quartz, epidote, apatite, amphibole, plagioclase, mica, rutile, calcite, and anhydrite) as well as liquid and gas phases of H2O. The calculated fluid composition of major elements is 31 wt% SiO2, 20 wt% CaO, 10 wt% (SO4)2−, 8 wt% (CO3)2−, 8 wt% FeO, 7 wt% Al2O3, and 9 wt% H2O, with traces of Na2O, K2O and TiO2. This composition is responsible for the UHP veining from supercritical silicate-rich fluids in equilibrium with peak UHP minerals. The P-T-t path of the UHP eclogite-vein system indicates that the second critical endpoint of the basalt-H2O system is located close to 3.4 GPa and 770°C. The high contents of sulfate and carbonate in the vein suggest that supercritical fluids were oxidized and likely a predominant agent to transfer sulfur and carbon at subarc depths. High Nb/Ta ratios of rutile crystals in the vein and quantitative modelling indicate that dehydration of subducting eclogites at subarc depths can produce supercritical fluids with suprachondritic Nb/Ta ratios. The UHP eclogites and mantle wedge peridotites metasomatized by supercritical fluids can acquire suprachondritic Nb/Ta ratios and thus provide a complementary reservoir to balance the subchondritic reservoirs on Earth. Based on a statistics of Nb-Ta data, two indices are proposed to identify the existence of supercritical fluids in fossil subduction zones: (1) rutile crystallized from supercritical fluids shows lower Nb contents than that from aqueous solutions and hydrous melts; (2) UHP eclogites associated with supercritical fluids exhibit a distance >0.1 in their Nb-Ta compositions to the line defined by basalts in the plot of log[Nb] vs log[Ta]. Therefore, the existence of supercritical fluids during UHP metamorphism at subarc depths can be recognized from their effect on the mobility of fluid-immobile trace elements in UHP metamorphic rocks.

Structural and morpholological properties of titanium dioxide-based sol-derived materials synthesized in different supercritical organic solvents

TiO2-based sol-derived materials were synthesized by supercritical drying assisted sol–gel process. The effects of supercritical fluid (acetone, ethanol, and methanol) on structural and morphological properties of TiO2 nanocrystallites were investigated by XRD, infrared spectroscopy, SEM, and specific surface area measurements. The samples synthesized in acetone and ethanol are pure anatase and the brookite phase visibly appears when the methanol is used as supercritical fluid. The structural parameters are found to be dependent on supercritical drying conditions of fluids. The presence of Ti–O and Ti–O–Ti bonds was confirmed by FTIR measurements. EDAX analysis showed that the sample elaborated in ethanol is oxygen rich with higher cell parameters. SEM images show that the samples synthesized in acetone and methanol are formed by spherical and dispersed particles whereas the particles of the sample synthesized in supercritical ethanol are heavily agglomerated to form a denser monolithic block. BET measurements showed that the lowest pore diameter, the biggest pore volume, and the highest specific surface area are found in the TiO2-based sol-derived materials synthesized in supercritical methanol. The adsorption isotherm of the sample synthesized in supercritical methanol is found to be composed of two hysterisis loops probably due to the presence of two types of pores which increase the global pore volume. This result indicates that the adsorption capacity can be generally improved by increasing the specific surface area and pore volume of catalysts.

Effects of supercritical fluids in catalytic upgrading of biomass pyrolysis oil

The catalytic upgrading of biomass pyrolysis oil (bio-oil) was performed under supercritical conditions. The behaviors of solvents with MgNiMo/AC were investigated for use of methanol, ethanol, butanol, hexanol, and formic acid as supercritical fluids. The production of gases including hydrogen, carbon monoxide, carbon dioxide, and methane was analyzed, and changes in the compositions of esters, alcohols, and phenols were examined. When the catalyst dosage and temperature were increased, the production of hydrogen increased. Among the alcohols, methanol produced the highest amount of hydrogen, and butanol produced the lowest amount of hydrogen. Formic acid showed good hydrogen-production ability. After catalytic upgrading of bio-oil, there was a decrease in oxygen and moisture content and an increase in the higher heating value (HHV). With an extended reaction time, the oxygen content was lowered further. During the supercritical ethanol reaction at 340 °C under an initial 30 bar of N2 gas for 2 h the oxygen content decreased from 33.9 to 14.6 wt% and the HHV increased from 25.2 to 35.3 MJ/kg.

Dynamic and Thermal Effects in Supercritical Fluids under Various Gravity Conditions

Some problems of supercritical fluid dynamics and heat transfer under various gravity conditions are solved experimentally and numerically. Ground-based experiments coupling with numerical simulations are performed to investigate dynamic and thermal effects in supercritical fluid subjected to heat supply. Numerical simulations of the piston effect and thermal gravity-driven convection in the fluids with variable physical properties are carried out. The effect of variability of properties caused by density and temperature inhomogeneities on the rate of the piston effect and convective patterns is discussed.

Investigation of the supercritical conditions for Fischer–Tropsch reaction over an industrial Co–Ru/γ-Al2O3 catalyst

Abstract The effects of supercritical fluid (SCF) on Fischer–Tropsch synthesis over an industrial well-characterized Co–Ru/γ-Al2O3 was studied in a laboratory fixed bed reactor. The influence of reaction conditions (such as temperature (240–260 °C), pressure (50–80 bar), syngas feed ratio (H2/CO ratio of 0.5–2.5), and contact time and syngas flow rate ( 3000 – 6000 Ncc g cat - 1 h - 1 ) ) on the FTS activity, selectivity, and hydrocarbon product distributions in the supercritical fluid media was studied. The opportunity to selectively control and maximize the production of the desired fuel fractions from the FTS hydrocarbons spectrum, by tuning either reaction temperature and reaction pressure, was investigated. This study also covers an attempt to understand and model the enhanced chain growth probability in the supercritical phase FTS that resulted in significant deviations from the standard Anderson–Shultz–Flory (ASF) model (specifically within the middle distillate hydrocarbons). The influence of solvent (pure n-hexane and n-pentane) at constant temperature (240 °C) and density (0.3 g/cm3) on the selectivity of FTS in the product hydrocarbon as a function of carbon number was investigated under supercritical conditions. Similar hydrocarbon distribution was obtained, however CO conversion in supercritical hexane was obviously lower than that in supercritical pentane medium. The reaction performances of the Fischer–Tropsch synthesis in the supercritical phase was also compared with those in the conventional gas phase FTS. The results indicate that in most cases, supercritical FTS showed higher activity and better selectivity towards the most desired products compared to conventional gas phase FTS.

Low‐temperature, Selective Catalytic Deoxygenation of Vegetable Oil in Supercritical Fluid Media

The effects of supercritical fluids on the production of renewable diesel-range hydrocarbons from natural triglycerides were investigated. Various supercritical fluids, which included CO2 (scCO2 ), propane (scC3 H8 ) and n-hexane (scC6 H14 ), were introduced with H2 and soybean oil into a fixed-bed reactor that contained pre-activated CoMo/γ-Al2 O3 . Among these supercritical fluids, scC3 H8 and scC6 H14 efficiently allowed the reduction of the reaction temperature by as much as 50 °C as a result of facilitated heat and mass transfer and afforded similar yields to reactions in the absence of supercritical fluids. The compositional analyses of the gas and liquid products indicated that the addition of scC3 H8 during the hydrotreatment of soybean oil promoted specific deoxygenation pathways, decarbonylation and decarboxylation, which consumed less H2 than the hydrodeoxygenation pathway. As a result, the quantity of H2 required to obtain a high yield of diesel-range hydrocarbons could be reduced to 57 % if scC3 H8 was used. As decarboxylation and decarbonylation are mildly endothermic reactions, the reduced heat transfer resistance in scC3 H8 may drive the deoxygenation reaction to thermodynamically favourable pathways.

Supercritically exfoliated ultrathin vanadium pentoxide nanosheets with high rate capability for lithium batteries

Ultrathin V2O5 nanosheets were successfully prepared through supercritical solvothermal reaction followed by annealing treatment. The formation of ultrathin nanosheets is owing to Ostwald ripening and the effect of supercritical fluids. As cathode material for lithium batteries, the ultrathin V2O5 nanosheets exhibit a capacity of 108 mA h g(-1) at a high rate of up to 10 C at 2.4-4 V and excellent cyclability with little capacity loss after 200 cycles. The enhanced rate performance is attributed to the shortened diffusion distance and the increased electrode-electrolyte contact area of the ultrathin nanosheet structure. It is also demonstrated that the supercritical solvothermal method is effective and facile to scalably synthesize ultrathin nanomaterials for lithium batteries.

Preparation, characterisation and catalytic property of ultrafine Ce–La–Mn mixed oxides

Using cerium carbonate, lanthanum oxide and manganese nitrate as raw materials, ultrafine particles of Ce–La–Mn mixed oxides were prepared by the sol–gel method combined with supercritical drying technology. The prepared materials were characterised by thermo-gravimetric and differential thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy and transmission electron microscope. The catalytic properties of ultrafine Ce–La–Mn mixed oxides were tested by the reaction ‘2CO + 2NO = 2CO2 + N2’. The key aim was to examine the effect of supercritical fluid drying technology and Ce-doping on the crystal structure, morphology and catalytic activity of ultrafine Ce–La–Mn mixed oxides. The results show that at 260°C, Ce–La–Mn mixed oxides are brown loose flocculent powders with good dispersibility. The particles are spherical about 10 nm in size. The main crystal components are CeO2, MnO, La5O7NO3 and La(OH)2NO3. After thermal treating at 850°C, Ce–La–Mn mixed oxides were made up of plenty of quasi-global grains smaller than 20 nm; the main crystal components of the Ce–La–Mn mixed oxides are LaMnO3+ λ , La2O3 and CeO2; heat treatment can enhance the crystallinity of the materials. Cerium-doped mixtures just exist as crystal CeO2, which contributes to the crystallisation of Ce–La–Mn mixed oxides in the process of supercritical fluid drying at 260°C, enhancing the catalytic activity of ultrafine Ce–La–Mn mixed oxides which are thermal treated at 850°C.

Luminescent characteristics of Vicia faba L. bean leaves treated with SCF extracts of Reynoutria sachalinensis

The effect of supercritical fluid (SCF) extracts of giant knotweed (Reynoutria sachalinensis) on the photosynthetic activity of Vicia faba L. bean leaves was studied by the slow fluorescence induction (SFI) and thermoluminescence (TL) methods. The treatment of plants enhanced their photosynthetic activity. The maximum stimulating effect was observed for an extract prepared using carbon dioxide containing 2% ethanol as a cosolvent. The observed stimulating effect is believed to be associated with an increase in the number of electron carries in the photosynthetic electron transport chain of plant leaves due to the flux of quinonic compounds into plant cells.

Thermoacoustic effects in supercritical fluids near the critical point: Resonance, piston effect, and acoustic emission and reflection

We present a general theory of thermoacoustic phenomena in one phase states of one-component fluids. Singular behavior is predicted in supercritical fluids near the critical point. In a one-dimensional geometry we start with linearized hydrodynamic equations taking into account the effects of heat conduction in the boundary walls and the bulk viscosity. We introduce a coefficient Z(omega) characterizing reflection of sound with frequency omega at the boundary in a rigid cell. As applications, we examine acoustic eigenmodes, response to time-dependent perturbations, and sound emission and reflection. Resonance and rapid adiabatic changes are noteworthy. In these processes, the role of the thermal diffusion layers is enhanced near the critical point because of the strong critical divergence of the thermal expansion.

CFD studies on particle-to-fluid mass and heat transfer in packed beds: Free convection effects in supercritical fluids

Free convection effects of heat and mass transfer are of prime importance when modeling the behavior of supercritical fluids in packed bed reaction equipment and in solid extractors. Density gradients, caused by temperature or composition variations, tend to control the overall flow pattern of the fluid, making the buoyancy terms of the Navier–Stokes equations an important term to model when doing a computational fluid dynamics (CFD) simulation under the described conditions. The purpose of this work is to visualize by means of CFD techniques the influence of the buoyancy forces over flow patterns and the convective flow in a packed bed filled with a supercritical fluid, because the experimental option is very expensive and time demanding. Computations were made using a three-dimensional CFD modeling strategy. Carbon dioxide in the supercritical conditions was selected as a flowing fluid. Its transport properties at high pressure were incorporated within a CFD commercial code in order to estimate them online within the simulation process. Particle-to-fluid mass transfer in supercritical conditions was analyzed, and transport coefficients were obtained and validated. Recently, Guardo et al. [2006. CFD study on particle-to-fluid heat transfer in fixed bed reactors: convective heat transfer at low and high pressure. Chemical Engineering Science 61, 4341–4353] have presented a correlation for particle-to-fluid heat transfer in supercritical conditions based on an analogy with the correlation proposed by Stüber et al. [1996. Supercritical fluid extraction of packed beds: external mass transfer in upflow and downflow operation. Industrial & Engineering Chemistry Research 35, 3618–3628] for mass transfer. The obtained numerical results presented in this work validate the idea that the modified correlation presented by Guardo et al. [2006. CFD study on particle-to-fluid heat transfer in fixed bed reactors: convective heat transfer at low and high pressure. Chemical Engineering Science 61, 4341–4353] can be used to describe heat transfer phenomena in a packed bed under mixed convection regime at high pressures.

Effect of Supercritical Fluids on C11β-Hydroxylation Activity of Absidia coerulea

The viability and C11beta-hydroxylation activity of Absidia coerulea were measured after treatment with compressed or supercritical CO2 and C2H4 under various initial pH and temperature conditions. The specific activity of A. coerulea on C11beta-hydroxylation can reach 23% and 75%, respectively, after treatment with 7.5 MPa of CO2 and C2H4, leading to the feasibility of enhancing both the solubility of the reactants and the beta-hydrocortisone yield for the hydroxylation of Reichsterin's substance acetate by using supercritical C2H4 as an alternative to the organic solvent.

超臨界流体を用いたポリマーの混練，成形，加工，リサイクル技術 超臨界流体を用いた成形加工のための基礎物性

Solubility, diffusivity and viscosity in supercritical fluid and polymer systems are reviewed. Supercritical fluids such as carbon dioxide dissolve in polymers at high pressures and plasticize them. The increase in diffusivity and the decrease in viscosity of polymers with dissolved supercritical fluids are due to the plasticizing effect of supercritical fluids. In this article general tendency, data source and theoretical calculation methods of solubility, diffusivity and viscosity are presented for supercritical carbon dioxide and nitrogen in polymers.

The Effect of Supercritical Fluids on Solid Acid Catalyst Alkylation

The alkylation of isobutane with trans-2-butene was explored over six solid acid catalysts in the liquid, near-critical liquid, and supercritical regions through the addition of an inert cosolvent to the reaction feed mixture. The addition of supercritical cosolvents did not result in sustained catalytic alkylation activity. A modest improvement in product yield was obtained with the addition of methane in the modified-liquid region; however, catalyst longevity and product selectivity were decreased compared to cosolvent-free liquid conditions. This paper describes the catalyst screening and selection process, an exploration of catalyst performance with varying concentrations of methane, and an examination of the effects of seven supercritical fluids on catalyst performance. The catalysts included two zeolites, two sulfated metal oxides, and two Nafion catalysts. Three hydrocarbons, two fluorocarbons, carbon dioxide, and sulfur hexafluoride were explored as inert cosolvents added to the reaction mixture.

Hydrolysis and Oxidation in Subcritical and Supercritical Water: Connecting Process Engineering Science to Molecular Interactions

Key engineering issues influencing the development of supercritical water oxidation (SCWO) for waste treatment were reviewed. Major chemical pathways and kinetics for hydrolysis and oxidation reactions of model organic wastes were discussed. In selective examples, results from extensive laboratory-scale measurements were compared with molecular simulations of solvation and reaction effects in supercritical water. Connections between reaction chemistry and observed corrosion in SCWO processing equipment were discussed to underscore the importance of understanding electrochemical phenomena over a wide range of temperature and density conditions. Research needs for improved understanding of physical and chemical effects in supercritical fluids were identified.

Supercritical Fluid Extraction of Polycyclic Aromatic Hydrocarbon Mixtures from Contaminated Soils

Highly contaminated (with PAHs) topsoils were extracted with supercritical CO2 to determine the feasibility and mechanism of supercritical fluid extraction (SFE). Effects of supercritical fluid (SCF) density, temperature, and cosolvent type and amount and of slurrying the soil with water were investigated. Experiments were conducted at 308−343 K and 450−850 kg/m3 CO2 density. Water and methanol were used as cosolvents in some experiments, at 0.3−6 wt %. For extractions at an ∼1000 phase ratio (wt of CO2/wt of soil), near-complete contaminant removal was achieved. For an ∼200 phase ratio at 323 K and 13.8 MPa, >95% of two-ring PAHs and >80% of three-ring and heavier PAHs were extracted. Using soil slurries with water had little effect on the total recoveries. Changes in SCF density also had little influence on SFE, while temperature increases improved both initial rates of extraction and total recoveries. Addition of cosolvents generally hindered extraction of the contaminants. These results are used to show that, at typical conditions for CO2-based soil extractions, the solubility of the contaminant in the SCF does not limit SFE. Other internal resistances are important, such as desorption from and diffusion within the polymeric soil organic matter.

Chemical-physical interpretation of cosolvent effects in supercritical fluids

Cosolvent effects in supercritical fluids can be considerable for systems where the cosolvent interacts strongly with the solute. It is shown in a companion paper that strong interactions occur between naproxen ((S)-6-methoxy-α-methyl-2-naphthaleneacetic acid) and various polar cosolvents in supercritical CO 2 . A chemical-physical model proposed by Ekart and Eckert that incorporates chemical equilibria into the Soave-Redlich-Kwong equation of state was used to study these systems. The model was found to describe these systems well, and the associated equilibrium constants obtained could be correlated with the cosolvent α, β, and π * solvatochromic parameters by a linear free energy relationship. This correlation suggests that both physical and chemical forces are important in the solvation process of naproxen in polar cosolvent-supercritical CO 2 mixtures. The model coupled with the correlation represents a step toward predicting solubilities in cosolvent-modified supercritical fluids using nonthermodynamic data. This method of modeling cosolvent effects allows a more intuitive interpretation of the data than either a purely physical equation of state or ideal chemical theory can provide

Py scale in vapor phase and in supercritical carbon dioxide. Evidence in support of a three-density-region model for solvation in supercritical fluids

With a fluorescence study of pyrene, Py ( I 1/ I 3) values are determined in the vapor phase at 70/dgC and in supercritical CO 2 at 45°C over a wide density range ( p r from 0.025 to 1.9.). Noise-elimination for weak pyrene vapor-phase spectra is accomplished by the application of principal component analysis. A characteristic density dependence of Py scale becomes clear with the determination of Py values at low CO 2 densities, which strongly supports a three-density-region solvation model for solvent effects in supercritical fluids.

Quantitative resolution of dual fluorescence spectra in molecules forming twisted intramolecular charge-transfer states. Toward establishment of molecular probes for medium effects in supercritical fluids and mixtures

A thermodynamic equilibrium between the locally excited state and the twisted intramolecular charge-transfer (TICT) state in p-(dimethylamino)benzonitrile and ethyl p-(dimethylamino)benzoate is used to probe unusual solute-solvent interactions in supercritical trifluoromethane and carbon dioxide mixtures. Well-defined locally excited state fluorescence spectra of the two molecules are obtained through application of principal component analysis