Extraction of lignite with water in sub- and supercritical states

On a semi-continuous apparatus, Dayan lignite was extracted with water in sub- and supercritical states. The experiments were carried out to investigate the effect of temperature on extract formation rate, extract yields and product components at different pressures and end temperatures. The results indicated that the extract formation rate has a maximum with the variation of temperature. The temperature corresponding to the maximum extract formation rate, changing with the pressure, is between 400 and 450°C. The extraction yields vary with the conditions. With the increase of pressure, the conversion and extract yield increase. With the increase of end temperature, the conversion also increases, but the increment is mainly gas and light oil. The main fraction in extract is preasphaltene and the main gas component is CO₂.     

Extraction of Avgamasya asphaltite with sub- and supercritical solvents

The yields and the nature of the products from the solvent extraction of Avgamasya asphaltite of SE Turkey with benzene and toluene under Soxhlet, subcritical (up to 292 °C) and supercritical (350–450 °C) conditions are reported. The subcritical yield increases with temperature but also depends on pressure; the extra yield is mainly of asphaltenes. The 350 °C supercritical toluene extract shows little evidence of thermal degradation and is similar in yield and chemical nature to that obtained under subcritical conditions except that it contains more pentane-soluble material. At 450 °C the yield is increased and a number of pyrolytic effects are observed, including reduction in molecular mass, loss of heterocyclic and alkyl groups and the presence of toluene decomposition products.     

采用超临界/亚临界水回收热固性树脂基复合材料

综述了近年来国内外研究者利用超临界/亚临界水分解热固性树脂基(如不饱和聚酯,酚醛树脂,环氧树脂等)复合材料的研究,该方法可将热固性树脂基复合材料分解为单体或低聚物,具有环保、经济、安全的特点,作为一种新兴技术,具有较好的发展前景。     

Upgrading of bitumen with formic acid in supercritical water

Upgrading of bitumen was examined with formic acid in supercritical water (SCW) from 673 to 753 K and at a water/oil ratio from 0 to 3. Decomposition of bitumen in SCW + HCOOH gave higher conversions of asphaltene and lower coke yields than those of pyrolysis or with only SCW. Decomposition of bitumen was also conducted in SCW + H₂, SCW + CO, toluene and tetralin, which revealed that decomposition of asphaltene was promoted and coke formation was suppressed when using SCW + HCOOH. In SCW + HCOOH, an increase in the water/oil ratio promoted both decomposition of asphaltene and suppression of coke formation. Formic acid in SCW seemed to enhance the conversion of bitumen to lower molecular weight compounds because formic acid seems to produce active species in SCW. The low temperature region (ca. 723 K) was suitable for upgrading bitumen with formic acid in SCW since coke formation was strongly promoted at high temperature (>753 K). A reaction model was proposed and the model predicted that hydrogenation of the asphaltene core was important for the suppression of coke formation.     

Electron spin resonance of VO radical-ion in sub- and supercritical water

Electron spin resonance (ESR) spectra of VO²⁺ radical-ions in sub- and supercritical water are observed. Upon increasing the temperature from 20 to 100 °C, the fine structure line widths in the ESR spectra of the vanadyl ion are observed to be reduced that is associated with the effective averaging of the g-factor anisotropy and the hyperfine interaction. With further increasing the temperature, the spectrum components of the hyperfine structure are broadened significantly resulting in the unresolved low-intensity line in supercritical water with ΔH_pp ∼ 300 G. The data obtained allow behavior peculiarities of the paramagnetic VO²⁺ ions in sub- and supercritical water including rotational dynamics and spin exchange between the radicals to be elucidated. The registration of the unresolved low-intensity line in supercritical conditions points to an increase in the local ion concentration in the system that can be an initial stage for the formation of vanadium-based particles in supercritical conditions. The study demonstrates that ESR is the powerful tool to investigate properties of sub- and supercritical water in situ.     

Extraction of victorian brown coals with supercritical water

Extraction of five Victorian brown coals with supercritical water in a semi-continuous reactor was investigated. At 380°C and 22 MPa conversions of 42–54% and extract yields of 20–26% were obtained, significantly higher than with toluene under the same conditions. The extracts were separated into oils, asphaltenes and pre-asphaltenes and these were studied by NMR spectroscopy combined with elemental, OH and molecular weight analyses. The extacts have a high asphaltene and pre-asphaltene content, together with a high oxygen content, especially in the asphaltene and pre-asphaltene fractions. Long alkyl chains were present in the oils and asphaltenes. Temperature had little effect on the extraction, whereas pressure has a marked effect on both the conversion and the composition of the extract. The major difference between the toluene and water extracts is the higher oxygen content of the latter. There was a considerable improvement in the conversion using 0.5 M NaOH rather than water. Analyses of some of the chars were also carried out.     

Catalytic hydrothermal treatment of pharmaceutical wastewater using sub- and supercritical water reactions

Application of subcritical and supercritical water technology for destruction of pharmaceutical compounds (carbamazepine, metoprolol and sulfamethaxazole) was investigated. The experiments were conducted inside batch reactor at a temperature ranging from 473 to 773 K and with different residence times of 5 to 50 min. The results show that carbamazepine, metoprolol and sulfamethaxazole are destructed by 90.27%, 99.99% and 98.84% after a 20 min exposure to 623 K, 673 K and 573 K, respectively. In comparison with the conventional methods of pharmaceutical waste treatment, the current technology provides a higher destruction efficiency (approximately 90–100%) which is achievable in shorter durations. NaOH and CuSO₄·5H₂O were also applied as catalysts in the temperature range of 473 K to 723 K. Comparing these catalysts, CuSO₄·5H₂O demonstrates a higher destruction efficiency, especially at lower temperatures. Based on the proposed pathway, the products of destructioncan be classified as environmentally-friendly compounds. The results show that this technology can be used as a green alternative for efficient removal of pharmaceutical compounds from wastewater streams.     

Effect of supercritical water on upgrading reaction of oil sand bitumen

The advantages of supercritical water (SCW) as a reaction medium for upgrading oil sand bitumen were investigated through a comprehensive analysis of the output product, which includes gaseous products, middle distillate, distillation residue, and coke. Canadian oil sand bitumen mined by the steam assisted gravity drainage method was treated in an autoclave at 420-450 °C and 20-30 MPa for up to 120 min with three kinds of reaction media: SCW, high-pressure nitrogen, and supercritical toluene. The yields of gaseous products indicated that a very small amount of water was involved in the upgrading reaction. The analytical results of the middle distillate fractions were almost the same using water and nitrogen at 450 °C. The distillation residues produced in SCW had lower molecular weight distributions, lower H/C atomic ratios, higher aromaticities, and consequently more condensed structures compared to those produced in nitrogen. The coke produced using SCW also had lower H/C values and higher aromaticities. Judging from all the analytical results, the upgrading of bitumen by SCW reaction was primarily considered to be physical in nature. As a result, it is possible to highly disperse the heavy fractions by SCW. This dispersion effect of SCW led to intramolecular dehydrogenation of the heavier component and prevention of recombination reactions, and consequently gave the highest conversion.     

Effect of temperature on bitumen conversion in a supercritical water flow

This article deals with a study of bitumen conversion (the gross-formula CH_1.47N_0.01S_0.007) in a supercritical water (SCW) flow continuously supplied at the bottom of the vertically located tubular reactor. At the first stage, bitumen was continuously supplied from the top of the reactor into a counter-current SCW flow (400 °C, 30 MPa) for 60 min. At the second stage (after ceasing the supply of bitumen into the reactor), SCW was pumped through the layer of bitumen residue at uniform (2.5 °C/min) temperature increase from 400 to 700 °C at 30 MPa. The amount and composition of the liquid and volatile conversion products were measured. It is revealed that during bitumen supply into the reactor and subsequent pumping of SCW through the layer of bitumen residue in the temperature increasing mode from 400 to 500 °C, the yields of liquid conversion products are equal to 26.9 and 45.4%, respectively, relative to the weight of bitumen supplied into the reactor. Oils are the major components of these liquid products. Participation of H₂O molecules in redox reactions became evident due to the formation of CO and CO₂ even at 400 °C. A significant increase in the yields of H₂, CH₄, and CO₂ are detected at T > 600 °C. Based on the sulfur balance, it can be stated that the degree of bitumen desulfurization at 400–700 °C due to sulphur removal in form of H₂S accounts for 21.6 wt.% A solid carbonaceous bitumen residue, obtained after SCW conversion, is characterized by high specific surface (224 m²/g).     

Extraction of coal using supercritical water

An experimental apparatus was developed to inject coal into an autoclave containing preheated supercritical water. The supercritical water appears to act as both solvent and reactant in the conversion of coal to gases and liquids. Experiments were carried out with German brown coal, lignite and bituminous coal and with glucose at both subcritical and supercritical water densities. A significantly larger quantity of char was obtained when operating at subcritical densities and when the coal was mixed with water before heating to supercritical conditions. Smaller amounts of char were obtained as density increased and as reaction time increased.     

Oxidation reaction of high molecular weight dicarboxylic acids in sub- and supercritical water

Sebacic and azelaic acids, which are representative of high molecular weight dicarboxylic acids, were oxidized in a batch reactor with H₂O₂ oxidant in water from 300 °C to 400 °C at sub- and supercritical conditions. Intermediate products were identified based on GC–MS, HPLC, and ¹H NMR. The main oxidation products were dicarboxylic acids and the minor products were monocarboxylic acids. (ω − 1)-Keto acids, aldehyde-acids, and γ-lactones with carboxylic groups also were identified.On the basis of the products identified, the oxidation pathways of high molecular weight dicarboxylic acids were formulated. The oxidation of high molecular weight dicarboxylic acids proceed with the consecutive oxidation of higher to lower molecular weight dicarboxylic acids mainly through the oxidation at α-, β-, and γ-carbons to a –COOH group. The oxidation of dicarboxylic acids was also accompanied by oxidative decarboxylation, leading to the formation of monocarboxylic acids.     

Liquefaction of rice straw in sub- and supercritical 1,4-dioxane–water mixture

The critical liquefaction of rice straw in sub- and supercritical 1,4-dioxane–water mixture was investigated in a 500 mL autoclave at temperature of 260–340 °C, resistance time of 0–20 min, and volume ratios 0–100 vol.% (1,4-dioxane:mixture). The yields of oil and PA + A (preasphaltene and asphaltene) were in the range of 29.64–57.30 wt.% and 6.42–22.68 wt.%, depending on the temperature, resistance time and volume ratio. The synergistic capability of 1,4-dioxane–water mixture could allow the great decomposition of the tubular structure of lignocelluloses. It was shown by the results that the “oxygen-transfer” reaction, deoxygenation and decarboxylation may occur in the liquefaction of rice straw with 1,4-dioxane–water mixture, while deoxygenation and decarboxylation may be the main reaction. The oil and PA + A fractions obtained at different volume ratios were analyzed by FTIR and GC–MS to investigate the effect of the ratios on the type of the compounds in the liquid products. It is shown that the nucleophilic and hydrolytic functions of water might be weaken at the higher ratio of 1,4-dioxane runs, resulting the lower amount of phenolic, acidic, hydrocarbon and ester derivatives in the oil and PA + A fractions.     

Thermal decomposition of hydrazine in sub- and supercritical water at 25 MPa

Supercritical water flow-through test facility (SCW-TF) for the study of hydrothermal fluids is described. The hydrodynamic behavior of the flow-through reactor is examined from ambient to supercritical water conditions by performing residence time distribution measurements. The results indicate that at 25 MPa, the employed reactor configuration exhibits plug flow behavior with a small extent of dispersion over the temperature range from 298 to 773 K. The experimentally determined effective volume of the reactor was used for the calculation of mean residence times of the fluid in the “hot zone” of the flow-through system. The thermal stability of hydrazine in aqueous solution was examined along the 25 MPa isobar from 473 to 725 K. The obtained first-order rate constant for the thermal decomposition of hydrazine increases from 3.73 × 10⁻⁴ s⁻¹ at 473 K to about 0.31 s⁻¹ at 725 K.     

Extraction with supercritical gases

Extraction with compressed fluids in the critical region is discussed in terms of the marked effect on solvent properties that can be brought about by small changes in pressure or temperature. The theoretical background and experimental data are described, including the classification of the phase behaviour of binary systems. A number of application studies are quoted, and comparison is made with liquid solvent extraction and distillation. Apart from such topics as the breaking of azeotropes, the main area of study is in performing separations on the basis of volatility where the general level of volatility is low. In the field of natural products these include the removal of undesirable substances such as caffeine and nicotine and the isolation of valuable constituents such as food essences and drugs. For fossil fuels, applications are described in enhanced oil recovery, fractionation of heavy petroleum liquids and extraction of liquids from coal.     

Kinetics of solid acid catalyzed 1-octene reactions with TiO2 in sub- and supercritical water

Acid catalyzed reactions of 1-octene on TiO₂ in sub- and supercritical water were investigated (T = 250-450 °C, P = 11-33 MPa). The main products were 2-octene and 2-octanol. Additionally, other liner C₈ alkenes and liner secondary C₈ alcohols were produced as by-products. Through kinetic analysis, acid catalyzed reactions can divide into the reaction catalyzed by Lewis acidic sites on TiO₂ and the reaction catalyzed by protons produced by the dissociation of water molecules. Each type of the reaction is affected by water density or ionic product of water, respectively, therefore, reaction mechanism changes with temperature and pressure. From the contribution of each reaction type, the temperature dependence of cis/trans ratio of produced 2-octene could also be explained.     

Chemical recycling of plastics using sub- and supercritical fluids

The development of chemical recycling of waste plastics by decomposition reactions in sub- and supercritical fluids is reviewed. Decomposition reactions proceed rapidly and selectively using supercritical fluids compared to conventional processes. Condensation polymerization plastics such as polyethylene terephthalate (PET), nylon, and polyurethane, are relatively easily depolymerized to their monomers in supercritical water or alcohols. The monomer components are recovered in high yield. Addition polymerization plastics such as phenol resin, epoxy resin, and polyethylene, are also decomposed to monomer components with or without catalysts. Pilot scale or commercial scale plants have been developed and are operating with sub- and supercritical fluids.     

Reaction kinetics of d-xylose in sub- and supercritical water

Reactions of d-xylose were investigated with a flow apparatus in water at high temperatures (350 and 400 °C) and high pressures (40-100 MPa) to elucidate the reaction pathway and reaction kinetics. The products obtained from the reaction of d-xylose were furfural, d-xylulose, glyceraldehyde, glycolaldehyde, dihydroxyacetone, pyruvaldehyde, lactic acid and formaldehyde. Experimental results showed evidence of a dehydration reaction pathway, a retro-aldol reaction pathway and a Lobry de Bruyn-Alberta van Ekenstein (LBET) pathway from d-xylulose. The proposed reaction pathway and kinetic model were in accord with the experimental results. The kinetic constants showed dependence with water density (pressure). At 400 °C and water density of 0.52 g/cm³ at 40 MPa, the reaction from d-xylose to d-xylulose occurred by the LBET pathway with the reverse reaction being negligible. At 400 °C, increasing the water density from 0.52 to 0.69 g/cm³ decreased the kinetic rate constant of the forward LBET pathway and increased that of the reverse LBET pathway. The kinetic rate constant of the dehydration of d-xylulose to furfural increased with increasing water density at constant temperature. The kinetic rate constant of the retro-aldol reaction of d-xylose increased, and the retro-aldol reaction of d-xylulose decreased with increasing water density at 400 °C.     

Influence of process parameters on the hydrothermal decomposition and oxidation of glucose in sub- and supercritical water

Supercritical water oxidation (SCWO) of wet waste biomass for energy recovery could be an advantageous alternative to conventional combustion with preceding drying. Therefore the reactions of glucose as a model substance for cellulosic biomass were investigated in sub- and supercritical water. The results of hydrothermal and oxidative experiments carried out in a continuous high-pressure plant with a feed solution of 0.2-1.2% (g g⁻¹) glucose at 24-34 MPa, 250-480 °C and residence times of 2-35 s are presented. In the presence of a stoichiometric oxygen concentration (for total oxidation to carbon dioxide and water) glucose decomposes already at subcritical temperatures readily to carbon monoxide and low molecular liquid substances, chiefly organic acids like e.g. acetic acid and glycolic acid. In turn these are in general more stable and react only slowly with oxygen. The effect of temperature, residence time, pressure, reactant concentration and addition of zinc sulfate on the conversion and the yields of reaction products was demonstrated. Already at 350 °C (24 MPa and 30 s) 99% of the glucose are converted. With increasing temperature the production of CO₂ increases. However, even at 480 °C (34 MPa and 4 s) significant amounts of CO are formed and the reaction of glucose to CO₂ and H₂O is not complete. Higher temperatures or greatly longer residence times are needed for a total combustion of the glucose.     

Palm oil transesterification in sub- and supercritical methanol with heterogeneous base catalyst

An environmentally benign process for the production of methyl ester using γ-alumina supported heterogeneous base catalyst in sub- and supercritical methanol has been developed. The production of methyl ester in refluxed methanol conventionally utilized double promoted γ-alumina heterogeneous base catalyst (CaO/KI/γ-alumina); however, this process requires a large amount of catalyst and a long reaction time to produce a high yield of methyl ester. This study carries out methyl ester production in sub- and supercritical methanol with the introduction of an optimized catalyst used in the previous work for the purpose of improving the process and enhancing efficiency. CaO/KI/γ-Al₂O₃ catalyst was prepared by precipitation and impregnation methods. The effects of catalyst amount, reaction temperature, reaction time, and the ratio of oil to methanol on the yield of biodiesel ester were studied. The reaction was carried out in a batch reactor (8.8 ml capacity, stainless steel, AKICO, Japan). Results show that the use of CaO/KI/γ-Al₂O₃ catalyst effectively reduces both reaction time and required catalyst amount. The optimum process conditions were at a temperature of 290 °C, ratio of oil to methanol of 1:24, and a catalyst amount of 3% over 60 min of reaction time. The highest yield of biodiesel obtained under these optimum conditions was almost 95%.