Brown coal conversion under the action of supercritical water

A test bench was developed and the conversion of the organic matter of coal (OMC) in supercritical water (SCW) was studied under conditions of a continuous supply of a water-coal suspension to a vertical flow reactor at 390–760°C and a pressure of 30 MPa. From 44 to 63% OMC was released as liquid and gaseous products from coal particles (from the water-coal supension) during the time of fall to the reactor. This stage was referred to as the dynamic conversion of coal. The particles passed through the stage of the dynamic conversion of coal did not agglomerate in the reactor in the subsequent process of batch conversion in a coal layer at T = 550–760°C. The volatile products of the overall process of the dynamic and batch conversion of coal included saturated hydrocarbons (CH₄ and C₂H₆), aromatic hydrocarbons (C₆H₆, C₇H₈, and C₈H₁₀), synthesis gas (H₂ and CO), and CO₂. At T < 600°C, CO₂ and CO were the degradation products of oxygen-containing OMC fragments, whereas they also resulted from the decomposition of water molecules at higher temperatures in accordance with the reaction (C) + H₂O = CO + H₂. The mechanisms were considered, and the parameters responsible for the dynamic conversion of coal were calculated.     

Coupled Supercritical Fluid Extraction/Chromatography of Polycyclic Aromatic Hydrocarbons: Correlation with Solubility Calculations

The solubility of: naphthalene, fluorene, phenanthrene, pyrene, and chrysene in supercritical CO₂ at two temperatures and pressures up to 200 atm were calculated from fugacities derived via the Peng-Robinson equation of state. The calculated solubilities were correlated with the results of experiments in which polycyclic aromatic hydrocarbons (PAH) in fuel mixtures were extracted with supercritical CO₂ and analysed by supercritical fluid chromatography and by gas chromatography.     

Changes in pore structure of anthracite coal associated with CO2 sequestration process

Pore structure changing of coal during the CO₂ geo-sequestration is one of the key issues that affect the sequestration process significantly. To address this problem, the CO₂ sequestration process in an anthracite coal was replicated using a supercritical CO₂ (ScCO₂) reactor. Different coal grain sizes were exposed to ScCO₂ and water at around 40 °C and 9.8 MPa for 72 h. Helium pycnometer and mercury porosimetry provide the density, pore size distribution and porosity of the coal before and after the ScCO₂ treatment. The results show that after exposure to the ScCO₂-H₂O reaction, part of the carbonate minerals were dissolved and flushed away by water which made the true density increased as well as total pore volume and porosity most importantly in the micro-pore range. Hysteresis between mercury intrusion and extrusion was observed. Ink bottle shaped pores can be either damaged or created compared with the ScCO₂ treated coal samples. This suggests that the ScCO₂ treatment most likely increase the volumes of pores in anthracite coal, which also contributed to the increase in porosity of the treated samples. Therefore the CO₂ sequestration into coal appears to have the potential to increase significantly the anthracite microporosity which is very advantageous for CO₂ storage.     

Supercritical carbon dioxide essential oil extraction of Lamiaceae family species: Mathematical modelling on the micro-scale and process optimization

In this paper the essential oil supercritical carbon dioxide extraction from leaves of Lamiaceae family species was studied. Recent investigations of Lamiaceae family essential oil storage have shown that most of the oil is found in peltate glandular trichomes on the leaf surface. The effect of supercritical CO₂ on the peltate glands was investigated by Scanning Electron Microscopy. It was observed that exposure to supercritical CO₂ led to disruption of the peltate glands and essential oil release. This phenomenon was used as a basic hypothesis of the mathematical model of the supercritical fluid extraction with CO₂. The model was applied to simulate basil, rosemary, marjoram and pennyroyal supercritical CO₂ extraction on the existing experimental data. An average deviation from the experimental data was less than 0.83%. The model results indicated a possibility of a decrease in the supercritical CO₂ consumption by modified and optimized processing of Lamiaceae family herbaceous material.     

The synergy of supercritical CO2 and supercritical N2 in foaming of polystyrene for cell nucleation

This study examines the foaming behaviour of polystyrene (PS) blown with supercritical CO₂–N₂ blends. This is achieved by observing their foaming processes in situ using a visualization system within a high-temperature/high-pressure view-cell. Through analyzing the cell nucleation and growth processes, the foaming mechanisms of PS blown with supercritical CO₂–N₂ blends have been studied. It was observed that the 75% CO₂–25% N₂ blend yielded the highest cell densities over a wide processing temperature window, which indicates the high nucleating power of supercritical N₂ and the high foam expanding ability of supercritical CO₂ would produce synergistic effects with that ratio in batch foaming. Also, the presence of supercritical CO₂ increased the solubility of supercritical N₂ in PS, so the concentration of dissolved supercritical N₂ was higher than the prediction by the simple mixing rule. The additional supercritical N₂ further increased the cell nucleation performance. These results provide valuable directions to identify the optimal supercritical CO₂–N₂ composition for the foaming of PS to replace the hazardous blowing agents which are commonly used despite their high flammability or ozone depleting characteristics.     

Economic Assessment of the Hydrogenation of CO2 to Liquid Fuels and Petrochemical Feedstock

To remove high concentrations of CO₂ from the off‐gas of coal‐driven power plants, a new process was proposed. The catalytic hydrogenation of the CO₂ leads to the production of C₂ – C₄ (petrochemical feedstock) and liquid C₅₊ hydrocarbons (fuel). Thus, environmentally harmful CO₂ may be converted sustainably to useful products. On the basis of a process flow sheet, the costs for processing the CO₂ are estimated for different plant sizes. The price of hydrogen contributes significantly to the overall production costs. Further price reductions may be achieved by final engineering optimization of the process as a whole and specific unit operations.     

Equilibrium distributions of key components of spearmint oil in sub/supercritical carbon dioxide

Effects of temperature (at 35, 45 or 55°C) and pressure (10–110 atm) on the relative distribution coefficients of the twelve key components of spearmint oil (essential oil ofMentha cardiaca; Scotch spearmint) at equilibrium in dense CO₂ were investigated under conditions ranging from subcritical to supercritical regions. Effects of vapor pressure, molecular weight and polarity of the key components on their equilibrium distributions in sub/supercritical CO₂ are discussed. At 35°C, all key components of spearmint oil are equally soluble in dense CO₂ within the 12–102 atm pressure region. At 45 and 55°C, the key components are equally soluble for pressures greater than about 60 atm. However, around either 45°C/27 atm or 55°C/35 atm conditions, the relative distribution coefficients of all monoterpene hydrocarbons and of isomenthone (an oxygenated monoterpene) exhibit maxima, which are due to significantly higher vapor pressures of these components and significantly lower solvating power of the dense-gas solvent at these particular temperatures and pressures. Vapor-pressure effects, coupled with the decrease in solvating power, dominate the effects of polarity and molecular mass of the key components. Deterpenation of spearmint oil with dense CO₂ is possible around either 45°C/27 atm or 55°C/35 atm, where the monoterpene hydrocarbons tend to concentrate in the CO₂-rich phase.     

Miscibility in blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and poly(?-caprolactone) induced by melt blending in the presence of supercritical CO2

Blends of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(?-caprolactone) (PCL) have been produced by melt blending in the presence of supercritical CO₂. Infrared spectroscopy has shown that supercritical CO₂ can induce melting in PHBV at temperatures below the melting point. The miscibility of the PCL-PHBV blend system produced by both mechanical and supercritical means has been characterised by a combination of differential scanning calorimetry and dynamic mechanical thermal analysis. It has been shown that PHBV-PCL blends produced using mechanical means were immiscible, whereas the same blends produced using supercritical methods were found to be miscible as evidenced by a decrease in the glass transition temperature of the PHBV component. The development of miscibility is discussed in terms of enhanced interdiffusion resulting from the action of supercritical CO₂. In addition, the infrared spectrum of the blends produced using supercritical CO₂ showed negligible levels of the degradation product crotonic acid. Whereas in the samples produced using mechanical blending without supercritical CO₂, there was a significant increase in the level of crotonic acid, which was interpreted as evidence of degradation.     

Cloud point behavior for poly(isodecyl methacrylate)+supercritical solvents+cosolvent and vapor-liquid behavior for CO<Subscript>2</Subscript>+isodecyl methacrylate systems at high pressure

Experimental cloud-point data of binary and ternary mixtures for poly(isodecyl methacrylate) [P(IDMA)] in supercritical carbon dioxide, dimethyl ether (DME), propane, propylene, butane and 1-butene have been studied experimentally using a high pressure variable volume view cell. These systems show the phase behavior at temperature of 308 K to 473 K and pressure up to 255 MPa. The cloud-point curves for the P(IDMA)+CO₂+isodecyl methacrylate (IDMA) are measured in changes of the pressure-temperature (P-T) slope, and with cosolvent concentrations of 0-60.1 wt%. Also, experimental data of phase behaviors for IDMA in supercritical carbon dioxide is obtained at temperature range of 313.2–393.2 K and pressure range of 5.8–22.03 MPa. The experimental results were modeled with the Peng-Robinson equation of state. The location of the P(IDMA)+CO₂ cloud-point curve shifts to lower temperatures and pressures when DME is added to P(IDMA)+CO₂ solution. The P(IDMA)+C₄ hydrocarbons cloud-point curves are ca. 16.0 MPa lower pressures than the P(IDMA)+C₃ hydrocarbons curves at constant temperature. This article is dedicated to Professor Chul Soo Lee in commemoration of his retirement from Department of Chemical and Biological Engineering of Korea University.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

Influence of CO2 co-feeding on Fischer–Tropsch fuels production over carbon nanofibers supported cobalt catalyst

Nowadays, the syngas which is obtained from the reforming of coal, biomass or natural gas contain significantly amounts of CO₂ that cannot be separated and consequently, it can take part into the Fischer–Tropsch (FTS) catalytic activity. Therefore, the presence of CO₂ in the syngas flow should be taken into account. In the present study, the FTS CO hydrogenation process was compared to that of CO₂ on a carbon nanofibers supported Co catalyst. The influence of CO₂ content in the feed stream (H₂/CO/CO₂ ratio) on the reaction performance in terms of conversion and selectivity to the different products was described. Both the support and the prepared catalyst were characterized by nitrogen adsorption–desorption, temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Results showed that CO hydrogenation was controlled by a Fischer–Tropsch regime, whereas CO₂ hydrogenation was controlled by a methanation process. When feed was composed of CO and CO₂ mixtures, the catalytic activity decreased with respect to that obtained with a CO₂-free feed stream. Moreover, the presence of CO₂ in feed stream favored the formation of lighter hydrocarbons and could block the production of further CO₂ via Water-Gas-Shift (WGS) reaction.     

Effects of supercritical carbon dioxide on waste banana peels for heavy metal removal

The effects of supercritical carbon dioxide (CO₂) on waste banana peels for copper adsorption were evaluated. Supercritical CO₂ was employed both in a solvent extraction for antioxidant compound recovery and in an emerging biomass treatment to increase the subsequent heavy metal-removal step; the latter is termed “explosion with supercritical CO₂”. This lignocellulosic biomass was analyzed before and after being subjected to both processes by scanning electron microscopy and X-ray patterning. Thermal gravimetric and differential scanning calorimetry analyses were performed to understand the different effects of supercritical carbon dioxide employed in these two processes on banana peels. The explosion with supercritical CO₂ process resulted in a more pronounced effect on the vegetable structure. Nevertheless, no increase in the copper-removal capacity was achieved. The adsorption studies showed similar behaviors for fresh and extracted samples, demonstrating that banana peels previously extracted with supercritical CO₂ retained their adsorption capacity for subsequent heavy metal removal.     

Modeling the adsorption of pure gases on coals with the SLD model

The simplified local density/Peng-Robinson model (SLD-PR) was modified to improve its predictive capability when dealing with near-critical and supercritical adsorption systems of the type encountered in coalbed methane recovery and CO₂ sequestration. The goal was to develop efficient equation-of-state (EOS) computational frameworks for representing adsorption behavior, as well as to improve our understanding of the phenomenon. The ability of the modified SLD-PR to correlate accurately data for supercritical adsorption systems is demonstrated using adsorption measurements on activated carbon, Illinois #6 coal, Fruitland coal, and Lower Basin Fruitland coal. The results indicate that the modified SLD-PR model, which incorporates a modified repulsive parameter “b” for the PR EOS, is capable of modeling the adsorption of pure methane, nitrogen, and CO₂ at coalbed conditions. Inclusion of a slit geometry in the adsorbent matrix yields results superior to our previous two-dimensional EOS models for the adsorbates considered. The results also indicate that accounting for the adsorption surface structure within the SLD-EOS framework is effective in improving modeling capability for high-pressure adsorption phenomena. An explanation is offered as to why the adsorbed-phase densities are close to the EOS reciprocal co-volumes. Further, the model (a) generates direct estimates for the adsorbed-phase densities (which facilitate reliable prediction of absolute gas adsorption) and (b) readily describes the observed maximum in Gibbs-adsorption isotherms of CO₂ at the temperatures and pressures encountered in coalbeds.     

High-pressure sorption isotherms and sorption kinetics of CH4 and CO2 on coals

Using a manometric experimental setup, high-pressure sorption measurements with CH₄ and CO₂ were performed on three Chinese coal samples of different rank (VR_r = 0.53%, 1.20%, and 3.86%). The experiments were conducted at 35, 45, and 55 °C with pressures up to 25 MPa on the 0.354-1 mm particle fraction in the dry state. The objective of this study was to explore the accuracy and reproducibility of the manometric method in the pressure and temperature range relevant for potential coalbed methane (CBM) and CO₂-enhanced CBM (CO₂-ECBM) activities (P > 8 MPa, T > 35 °C). Maximum experimental errors were estimated using the Gauss error propagation theorem, and reproducibility tests of the high-pressure sorption measurements for CH₄ and CO₂ were performed. Further, the experimental data presented here was used to explicitly study the CO₂ sorption behaviour of Chinese coal samples in the elevated pressure range (up to 25 MPa) and the effects of temperature on supercritical CO₂ sorption isotherms.The experiments provided characteristic excess sorption isotherms which, in the case of CO₂ exhibit a maximum around the critical pressure and then decline and level out towards a constant value. The results of these manometric tests are consistent with those of previous gravimetric sorption studies and corroborate a crossover of the 35, 45, and 55 °C CO₂ excess sorption isotherms in the high-pressure range. The measurement range could be extended, however, to significantly higher pressures. The excess sorption isotherms tend to converge, indicating that the temperature dependence of CO₂ excess sorption on coals at high-pressures (>20 MPa) becomes marginal. Further, all CO₂ high-pressure isotherms measured in this study were approximated by a three-parameter excess sorption function with special consideration of the density ratio of the “free” phase and the sorbed phase. This function provided a good representation of the experimental data.The maximum excess sorption capacity of the three coal samples for methane ranged from 0.8 to 1.6 mmol/g (dry, ash-free) and increased from medium volatile bituminous to subbituminous to anthracite. The medium volatile bituminous coal also exhibited the lowest overall excess sorption capacity for CO₂. However, the subbituminous coal was found to have the highest CO₂ sorption capacity of the three samples. The mass fraction of adsorbed substance as a function of time recorded during the first pressure step was used to analyze the kinetics of CH₄ and CO₂ sorption on the coal samples. CO₂ sorption proceeds more rapidly than CH₄ sorption on the anthracite and the medium volatile bituminous coal. For the subbituminous coal, methane sorption is initially faster, but during the final stage of the measurement CO₂ sorption approaches the equilibrium value more rapidly than methane.     

Supercritical CO2 extraction of pigment components with pharmaceutical importance from Chlorella vulgaris

BACKGROUND: Chlorella vulgaris is a green microalgae that contains various pigment components of carotenoids and chlorophylls. Supercritical CO₂ is widely used for extraction of pharmaceutical compounds because it is non‐oxic and easily separated from extracted material by simply depressurizing. In this work, pharmaceutical compounds from Chlorella vulgaris have been extracted using supercritical CO₂ with or without entrainer at various extraction conditions. RESULTS: Based on high performance liquid chromatography (HPLC) analysis, the extracts contained pigment components, such as lutein, β‐carotene, chlorophyll a and b. Higher extraction pressure and temperature promoted higher lutein extraction by supercritical CO₂. The optimum pressure and temperature for extraction were obtained as 50 MPa and 80 °C. Ethanol as an entrainer was more effective than acetone for the extraction of pigment components. Pigment components in the extract obtained by supercritical CO₂ with and without entrainer were compared with the extract obtained by a conventional extraction method. CONCLUSION: Supercritical CO₂ has been successfully applied for the extraction of pigment components from Chlorella vulgaris. Supercritical CO₂ enabled high selectivity for lutein extraction; however, the lutein yield was lower than that obtained by extraction using supercritical CO₂ with ethanol and soxhlet. Copyright © 2008 Society of Chemical Industry     

Carbon-13 NMR chemical-shift imaging study of dewatering of green sapwood by cycling carbon dioxide between the supercritical fluid and gas phases

Carbon-13 chemical-shift imaging (CSI) was used to study the distribution of CO₂ in green pine sapwood that was partially dewatered by a process in which CO₂ was cycled between the supercritical fluid and gas phases. Proton magnetic resonance imaging (MRI) was used to characterise the corresponding distribution of water. The CSI experiment showed strongest signals from cells with weakest proton MRI signals. This was consistent with a mechanism in which latewood bands provide pathways for supercritical CO₂ to penetrate into the interior of a specimen. Supercritical CO₂ also penetrated earlywood exposed on surfaces of the specimen.     

Fischer‐Tropsch Synthesis Over Co‐Ru/γ‐Al2O3 Catalyst in Supercritical Media

The Fischer‐Tropsch synthesis (FTS) in gaseous and supercritical phases was examined in a continuous, high‐pressure fixed‐bed reactor by employing a cobalt catalyst (Co‐Ru/γ‐Al₂O₃). The kinetic modeling of the FTS was investigated in the reactor over a 60–80 mesh cobalt catalyst. The Langmuir‐Hinshelwood kinetic equation was used for both the Fisher‐Tropsch (FT) and water gas shift (WGS) reactions. The kinetic model was applied for simulation of the reactor with 16–20 mesh cobalt catalyst. The simulation results showed a good agreement with the experimental data. The experimental data showed that higher CO conversion and lower CH₄ and CO₂ selectivities were achieved in supercritical media compared to the gaseous phase. The BET surface area and pore volume enhancement results provided evidence of the higher in situ extraction and greater solubility of heavy hydrocarbons in supercritical media than in gaseous phases. Furthermore, the effects of supercritical solvent such as n‐pentane, n‐hexane, n‐heptane and their mixtures were studied. Moreover, the influence of reaction temperature, H₂/CO ratio, W/F_(CO+H2) and pressure tuning in the supercritical media FT synthesis were investigated, as well as the effect of the supercritical fluid on the heat transfer within the reactor. The product carbon distribution had a similar shape for all types of solvents and shifted to lighter molar mass compounds with increasing temperature, H₂/CO ratio, and W/F_(CO+H2). Finally, the product distribution shifted to higher molar mass hydrocarbons with increasing pressure. As a result, one may conclude that a mixture of hydrocarbon products of the FTS can be used as a solvent for supercritical media in Fischer‐Tropsch synthesis.     

Extraction of epicuticular wax and nonacosan-10-OL fromEphedra herb utilizing supercritical carbon dioxide

Experimental results concerning the processing feasibility of using supercritical CO₂ were reported for the extraction of epicuticular wax fromEphedra herb. Subsequently the isolability of nonacosan-10-ol from the total extract of epicuticular wax was evaluated by TLC and gas chromatography. Also, Soxhlet extractions ofEphedra herb by n-hexane and chloroform were performed and these results were compared with those obtained by supercritical CO₂ extraction. As a result, we could demonstrate that supercritical CO₂ can be an economical alternative to the organic solvents in processing the medicinal plant.     

Extraction of boric acid from colemanite mineral by supercritical carbon dioxide

The use of H₂SO₄ in boric acid production from colemanite mineral has several problems, related to product impurities, corrosion and environmental discharge limits. To overcome these problems and to increase extraction efficiency of boric acid, heterogeneous reaction between colemanite and CO₂ dissolved in H₂O was studied at and above supercritical CO₂ conditions. Supercritical conditions enhanced the extraction efficiency of boric acid from colemanite mineral, with 96.9% boric acid extraction efficiency being obtained from CO₂–colemanite reaction at 60 °C, for 2 h of reaction time for particles in the range of +20–40 μm. A powder crystallized from filtrate of reaction was determined as H₃BO₃ and the solid formed at the end of reaction was characterized mostly as CaCO₃ according to FTIR, XRD, TGA and SEM analyses. The use of supercritical CO₂ as a leaching agent in colemanite does not only produce boric acid but also helps to reduce the amount of CO₂ in the atmosphere. Based on these facts, supercritical CO₂ as extractant makes this process green and sustainable for recovering boric acid from boron minerals.     

Synthesis of barium titanate using supercritical CO2 drying of gels

BaTiO₃ powders from sol-gel derived gels were prepared using two different drying methods. In addition to the conventional drying of gels in air at 90°C the supercritical CO₂ drying method was also used. Results showed that the properties of BaTiO₃ powder produced by supercritical drying with CO₂ are better. The grain surface is less contaminated as a result of the supercritical drying and the microstructure development during sintering leads to a homogeneous fine-grained microstructure.