Estimation of solubility of solids in supercritical carbon dioxide

Peng-Robinson equation of state(PR EOS)was chosen for modeling the thermodynamic be-havior of supercritical(SC)-CO_2/Solid systems.The necessary critical constants and acentric factorof the solute were obtained by the Sigmund and Trebble(1992)method based on the molecular weightand boiling temperature,and the vapor pressure of the solute was calculated by its meltingtemperature and heat of fusion.This approach compared very favorably with the conventional corres-ponding state theory,but without using critical constants and vapor pressure of solutes.Four mixingrules were tested for the calculation of solid solubility in SC-CO_2.van der Waals(vdW)mixing rulewith one parameter was considered to be most suitable for the estimation of solubility.A simplecorrelation was developed for the SC-CO_2/solid binary interaction coefficient k_(ij) with the meltingtemperature of pure solutes.The solubilities of solids in SC-CO_2 were estimated for eleven binarysystems at various temperatures,the total absolute average     

Three-phase catalytic hydrogenation of α-methylstyrene in supercritical carbon dioxide

The three-phase catalytic hydrogenation (TPCH) of α-methylstyrene using supercritical carbon dioxide (scCO₂) in a slurry reactor is reported. Kinetic data are presented for the reaction at 323 K over the range of pressure from 7.0 to 13.0 MPa using a carbon-supported palladium catalyst. The experimental data are fitted to a first-order power-law model. A detailed explanation of the methodology used to isolate the effect of CO₂ on the rate of reaction is presented. Particular attention is given to the phase behaviour of the reaction system and the volumetric expansion of the liquid phase with CO₂. It is shown that scCO₂ significantly enhances the rate of reaction. This effect is attributed to the enhancement of the solubility of hydrogen in the liquid phase.     

Surface hierarchical porosity in poly (ɛ-caprolactone) membranes with potential applications in tissue engineering prepared by foaming in supercritical carbon dioxide

This article describes the preparation of porous poly (ɛ-caprolactone), PCL, membranes by supercritical CO₂ (SCCO₂) foaming, displaying surface hierarchical macroporosity which could be tailored by careful control of the pressure, in the range of 150–250 bar, and depressurization processes in several steps, showing also pore interconnectivity between both membrane faces. The membranes exhibited two distinct types of surface macroporosity, the larger with diameter sizes of 300–500 μm were surrounded by and also composed of smaller pores of 15–50 μm (same size as inner pores). Membranes were prepared by solvent casting and submitted to different SCCO₂ foaming. Parameters such as membrane thickness, CO₂ flow, foaming time, pressure, temperature and the depressurization processes (rate and profiles), were varied to determine their influence on final porosity and to decipher which parameters were the most critical ones in terms of surface hierarchical pore organization. No remarkable changes in PCL crystallinity were found when membranes were processed under SCCO₂. Finally, biological evaluation of the porous membranes was achieved by seeding human skin fibroblasts on the prepared membranes. The results, in terms of cell adhesion, spreading, proliferation and metabolic activity indicate that these membranes could hold promise for the fabrication of meshes with controlled porosity for tissue engineering applications.     

Generation of nano-sized Ar–N2 compound particles by homogeneous nucleation and heterogeneous growth in a supersonic expansion

The generation of nano-sized particles was numerically investigated for the heterogeneous growth of nitrogen on homogeneously nucleated argon seed particles in a supersonic expansion. Gas dynamic equations for 1D supersonic flow were solved together with the classical nucleation theory and Hill's growth model.Heterogeneous growth on homogeneously nucleated seeds effectively increased the final particle size to the range of 10–200 nm. The new process could increase the particle mass by a factor of 10 relative to the case of pure gas expansion under comparable pressure and temperature conditions. Increase in particle size resulted predominantly from the enhanced heterogeneous condensation of nitrogen, and the growth by argon condensation could even be reduced relative to the case of pure gas expansion. Final particle size easily reached 200 nm, which has been very difficult to achieve by any of the existing homogeneous growth processes.     

Continuous oxidation of benzyl alcohol in “supercritical” carbon dioxide

Supercritical (dense) carbon dioxide has been applied as solvent for the partial oxidation of benzyl alcohol with molecular oxygen in a fixed-bed reactor. High rate and good selectivity to benzaldehyde (93–97%) has been achieved with 0.5 wt% Pd/alumina or 0.5 wt% Pd/C, at around 100C and 100 bar, using only moderate excess of oxygen. The by-product benzoic acid has an autocatalytic effect on the hydration of benzaldehyde, and the subsequent oxidative dehydrogenation leads to benzoic acid, and benzyl benzoate by esterification. Promotion of Pd by Pb improves the selectivity. No catalyst deactivation or metal leaching has been observed. The method provides reasonable yields at much lower temperature than that applied in conventional gas phase oxidation, showing a potential for the synthesis of thermolabile, water-insoluble aromatic aldehydes.     

Solubility of Δ-tetrahydrocannabinol in supercritical carbon dioxide: Experiments and modeling

The solubility of Δ⁹-tetrahydrocannabinol (Δ⁹-THC) in supercritical carbon dioxide has been determined at 315, 327, 334 and 345 K and in the pressure range from 13.2 to 25.1 MPa using an analytical method with a quasi-flow apparatus. Prior to performing these measurements, the method was validated by measuring anthracene solubilities and comparing these with literature values. The molar solubility for Δ⁹-THC ranged from 0.20 to 2.95 × 10⁻⁴. The data were correlated using the Peng-Robinson equation of state in combination with quadratic mixing rules. Deviations between calculated results and the experimental data ranged from 4.1 to 13.3% absolute average relative deviation (AARD).     

In vitro release profiles of β-carotene encapsulated in PHBV by means of supercritical carbon dioxide micronization technique

The main objective of this work was to investigate the characteristics of the in vitro release of β-carotene encapsulated in poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) using the solution enhanced dispersion by supercritical fluids (SEDS) technique. The release tests were performed using encapsulated complex with solute loading from 2.24 to 27.5% and encapsulation efficiency from 7.75 to 55.54%. The release profile assays were performed in ethyl acetate, n-hexane and anhydrous ethanol, and monitored by UV-vis spectrophotometer concentration analysis. Results indicated higher initial release rates in ethyl acetate and in n-hexane, with cumulative release percentage varying from 31.50 to 69.58% and from 42.08 to 55.96%, respectively. For anhydrous ethanol the maximum concentration was reached at 180 min, 300 min and 10 days, depending on the initial amount of β-carotene, with cumulative release ranging from 45.27 to 88.22%. In general, the β-carotene release can be controlled by the organic solvent used and by the initial amount of solute encapsulated, aspects that help the selection of the conditions to achieve the desired release profiles for a specific application.     

A numerical study of capillary pressure–saturation relationship for supercritical carbon dioxide (CO2) injection in deep saline aquifer

Carbon capture and sequestration (CCS) is expected to play a major role in reducing greenhouse gas in the atmosphere. It is applied using different methods including geological, oceanic and mineral sequestration. Geological sequestration refers to storing of CO₂ in underground geological formations including deep saline aquifers (DSAs). This process induces multiphase fluid flow and solute transport behaviour besides some geochemical reactions between the fluids and minerals in the geological formation. In this work, a series of numerical simulations are carried out to investigate the injection and transport behaviour of supercritical CO₂ in DSAs as a two-phase flow in porous media in addition to studying the influence of different parameters such as time scale, temperature, pressure, permeability and geochemical condition on the supercritical CO₂ injection in underground domains. In contrast to most works which are focussed on determining mass fraction of CO₂, this paper focuses on determining CO₂ gas saturation (i.e., volume fraction) at various time scales, temperatures and pressure conditions taking into consideration the effects of porosity/permeability, heterogeneity and capillarity for CO₂–water system. A series of numerical simulations is carried out to illustrate how the saturation, capillary pressure and the amount of dissolved CO₂ change with the change of injection process, hydrostatic pressure and geothermal gradient. For example, the obtained results are used to correlate how increase in the mean permeability of the geological formation allows greater injectivity and mobility of CO₂ which should lead to increase in CO₂ dissolution into the resident brine in the subsurface.     

Enzymatic synthesis of poly(ɛ-caprolactone) in supercritical carbon dioxide medium by means of a variable-volume view reactor

Poly (ɛ-caprolactone) (PCL) is a biodegradable polyester approved for applications in the human body such as drug delivery devices and sutures. Conventional synthesis of PCL involves metal catalysts and organic solvents that may leave toxic residues in the products and contribute to environmental pollution. Polymerization processes catalyzed by enzymes are becoming more attractive due to the importance of clean processes, which produces substances free of residues, ideal for pharmaceutical and food applications. The aim of this work was to investigate the enzymatic ring-opening polymerization (e-ROP) of PCL in supercritical carbon dioxide (scCO₂) solvent medium through a set of experiments assessing the influence of pressure (120–280 bar), solvent/monomer ratio (2:1–1:2 mass ratio) and enzyme percentage related to monomer (5–15 wt%) on the reaction yield, number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (P.I.). The results of these first experiments were used in the selection of the conditions for the kinetic experiments evaluating the influence of catalyst content and temperature on reaction yield, M_n, M_w, P.I. and on the characteristics of the polymer produced. This study also evaluates the enzyme reuse in order to reduce the impact of the enzyme cost on the process. Results for ANOVA statistical analysis for the first set of experiments show that the pressure or the solvent density has no significant influence over the parameters evaluated, while the solvent/monomer mass ratio presented significant effect on M_n and on M_w, with the best results obtained for the solvent/monomer mass ratio of 1:2. As expected, the enzyme content affects significantly all parameters evaluated. Polymerization results for the kinetic experiments indicate reaction yields up to 90 wt%, M_n up to 13,700 Da and M_w up to 22,200 Da, with P.I. ranging from 1.2 to 1.7. Taking into account the reaction productivity, the conditions for the reuse assays were chosen: 120 bar, 1:2 solvent/monomer ratio, 3 wt% of enzyme, 65 °C and 12 h of reaction. The enzyme recycling experiments suggested viability up to the second cycle as an alternative to improve the enzyme use. The variable-volume view reactor was adequate to provide simultaneous control of the process variables.     

Manufacturing of pulverised nanocomposites—Dosing and dispersion of additives by the use of supercritical carbon dioxide

In this work, the manufacture of particles from polybutylenterephthalate (PBT) and of composites from PBT and zinc oxide and bentonite nanoparticles is described. From the admixing of nanoparticles, improved properties are expected. For the production, the PGSS process with supercritical carbon dioxide as auxiliary medium is used. Spherical particles can be obtained from pure PBT as well as from the composites. The degree of separation of the additives is visualised in TEM pictures. SEM pictures show, that in dependence of the spraying conditions, particles with different morphologies are formed. The shape ranges from the perfect sphere to completely irregular particles or fibres. The apparent density appears to be an indicator for the sphericity of the particles. A suggestion for a correlation between spraying conditions and the apparent density is given.     

Computation of maximum rate of water–sulphuric acid nucleation in diesel exhaust

The origin of nucleation mode observed in the diesel engine exhaust is unclear. In this work, the mechanism of simple classical homogeneous water–sulphuric acid nucleation was studied using the parameterization of Vehkamäki, Kulmala, Lehtinen, and Noppel [2003. Modelling binary homogeneous nucleation of water–sulfuric acid vapours: Parameterisation for high temperature emissions. Environmental Science and Technology, 37, 3392–3398]. In the simple model used, condensation and coagulation were taken into account as sink terms in respective equations. The focus of the study was on the total amount of stable clusters formed, which provides an upper limit for nucleation mode number concentration. It was seen that the nucleation can be achieved even with relatively low sulphuric acid concentrations (of the order of $5\times {10}^{17}{\mathrm{m}}^{-3}$). However, the efficiency depends strongly on the cooling and dilution experienced by the exhaust. According to the results obtained, the assumption of homogeneous sulphuric acid–water nucleation depicted by Vehkamäki parameterization gives physically meaningful results with low-sulphur content fuels if the sulphur-to-sulphuric acid conversion factor is close to 100%. Nonetheless, there are published results of nucleation mode in diesel exhaust which cannot be explained by the sulphuric acid–water mechanism.     

Hydrothermal degradation of model sulfonic acid compounds: Probing the relative sulfur–carbon bond strength in water

Development of heterogeneous catalysts for the biorenewable industry requires hydrothermal degradation resistance. However, the relationship between hydrothermal stability and the immediate electronic hybridization of the carbon atoms adjacent to the sulfonic acid active group is not fully known. We systematically tested model compounds containing sulfonic acid groups linked to aromatic, alkane, or cycloalkane carbon atoms. We subjected them to hydrothermal conditions. The compounds' structural integrity was monitored with solution NMR. The aromatic-sulfonic acid compounds degraded readily, while the hydrolysis of the alkyl sulfonic acid linkages was negligible. Therefore, hydrothermally stable sulfonic-acid catalysts need sulfonic acid attached via alkyl linkers.     

Methanation of carbon dioxide over mesoporous Ni–Fe–Al2O3 catalysts prepared by a coprecipitation method: Effect of precipitation agent

Mesoporous nickel (30 wt%)–iron (5 wt%)–alumina (denoted as NiFeAl–X) catalysts were prepared by a coprecipitation method with a variation of precipitation agent (X = (NH₄)₂CO₃, Na₂CO₃, NH₄OH, and NaOH), and they were applied to the methane production from CO₂ and H₂. Metal particle size of reduced NiFeAl–X catalysts decreased in the order of NiFeAl–NaOH > NiFeAl–NH₄OH > NiFeAl–Na₂CO₃ > NiFeAl–(NH₄)₂CO₃. In the methanation of CO₂, yield for CH₄ increased in the order of NiFeAl–NaOH < NiFeAl–NH₄OH < NiFeAl–Na₂CO₃ < NiFeAl–(NH₄)₂CO₃. This indicates that the catalytic performance in the methanation of CO₂ was strongly influenced by the identity of precipitation agent.     

Selective hydrogenation of styrene oxide to 2-phenyl ethanol over polyurea supported Pd–Cu catalyst in supercritical carbon dioxide

2-Phenyl ethanol (2-PEA) is an important chemical which finds several applications in perfumes, deodorants, soaps and detergents. It is prepared by different polluting and dangerous routes. The current work is concerned with production of 2-PEA by hydrogenation of styrene oxide using polyurea encapsulated catalysts (EnCat) in methanol and supercritical carbon dioxide (scCO₂). Bimetallic Pd–Cu catalyst encapsulated with polyurea, Pd–Cu EnCat, is the best catalyst. The epoxide ring in styrene oxide is selectively hydrogenated to give 2-PEA in scCO₂ without formation of any isomerization or deoxygenated products, which are formed in methanol. A complete conversion of styrene oxide with 100% selectivity to 2-PEA was obtained without addition of any promoter. Effects of various parameters were studied and a bifunctional site Langmuir–Hinshelwood–Hougen–Watson kinetic model was found to be in good agreement with the experimental data. The process is clean and green.     

Unique efficiency of copper–indium catalyst in octanoic acid reduction

Octanoic acid (OA), as model reactant, was hydroconverted in a flow through reactor at 21 bar total pressure and 240–380 °C over various copper catalysts composed with indium or its neighbors in the periodic table (gallium, tin, thallium and cadmium) for comparison. The In-doped sample was proven to be much more advantageous than the other bimetallic analogs tested.     

Methanation of carbon dioxide over mesoporous Ni–Fe–Ru–Al2O3 xerogel catalysts: Effect of ruthenium content

Mesoporous nickel (35 wt%)–iron (5 wt%)–ruthenium (x wt%)–alumina xerogel (denoted as 35Ni5FexRuAX) catalysts with different ruthenium contents (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by a single-step sol–gel method for use in the methane production from CO₂ and H₂. Conversion of CO₂, yield for CH₄, metal surface area, and the amount of desorbed carbon dioxide of the catalysts showed volcano-shaped trends with respect to ruthenium content. Experimental results revealed that metal surface area and the amount of desorbed carbon dioxide of 35Ni5FexRu catalysts were well correlated with conversion of CO₂ and yield for CH₄.     

The relationship between the hydrophobic capacity and extraction efficiency of organic–inorganic hybrid mesoporous silicas for the determination of dibutyl phthalate by SPME–HPLC

Three kinds of organofunctionalized mesoporous silicas, including methyl-MCM-41, propyl-MCM-41 and octyl-MCM-41, were synthesized via one-step co-condensation and for the first time employed as the coatings of solid-phase microextraction fibers coupled with HPLC to extract dibutyl phthalate (DBP). The samples were characterized by fourier transform infrared, small-angle X-ray diffractometer, transmission electron microscope, thermogravimetry analysis and N₂ adsorption–desorption. The results showed that the organofunctionalized mesoporous silicas still preserve a desirable two-dimensional P6mm hexagonal structure with large pore volume, good thermal stability and strong hydrophobicity after introducing alkyl groups of various carbon chains to the framework of MCM-41. When the concentration of DBP in standard solution is 1.0 × 10^?9 mol L^?1, the extraction efficiency of octyl-MCM-41, propyl-MCM-41 and methyl-MCM-41 is 5.7, 4.6 and 4.1 times higher than that of the pure MCM-41, respectively.     

Hydrogenation of cinnamaldehyde over Pt/RHCs in supercritical carbon dioxide – Influence of support pretreatment and phase behavior

The selective hydrogenation of cinnamaldehyde (CAL) was investigated using rice husk-based porous carbon (RHCs) supported platinum catalysts in supercritical carbon dioxide (scCO₂). The effects of surface chemistry treatment of the support and the reaction phase behavior have been examined. The Pt/H–RHCs (HNO₃-pretreated) was more active for CAL hydrogenation compared with Pt/NH₃–RHCs (NH₃ · H₂O-pretreated). The Pt/RHCs catalyst exhibited a higher selectivity to cinnamyl alcohol (COL) compared with commercial catalyst of Pt/C, which is relative to the micro–mesoporosity structure of the RHCs. Under the reaction conditions, the product distribution changes markedly with the variation of the phase behavior. A higher selectivity to hydrocinnamyl alcohol of 90% was obtained in two-phase reaction system of CO₂–riched gas phase (CO₂, H₂, dissolved cinnamaldehyde) and solid phase (catalyst); while, the main product changed to cinnamyl alcohol with a selectivity of 88% in three-phase reaction system of CO₂–riched gas phase (CO₂, H₂, dissolved cinnamaldehyde), cinnamaldehyde–riched liquid phase (cinnamaldehyde, dissolved CO₂ and H₂) and solid phase (catalyst). It is attributed to the influence of the molecule interactions and the substrate concentration effects.