Measurement of the PVTx properties of n-heptane in supercritical water

By means of a constant-volume piezometer, measurements have been made of the PVTx properties of water-n-heptane mixtures at supercritical conditions. The measurements cover the temperature range from 573 to 673 K and pressures from 2 to 30 MPa. Values of excess, partial, and apparent molar volumes were obtained from these measurements. Tests on the piezometer and consistency tests on the measurements suggest that the results are free from significant ‘dead volume’ error. The PVT data for the pure components (water and n-heptane) obtained using the piezometer are in excellent agreement with results obtained by other investigators. The overall accuracy of the pressure, density, temperature, and mole fraction are ± 0.15%, ± 0.5%, ± 10 mK and ± 0.002, respectively.Analysis of the results for dilute water-n-heptane mixtures show that the partial molar volume of n-heptane (solute) and the excess molar volume of the mixture near the critical point of pure water (solvent) exhibit remarkable anomalies. Our results contribute to the formulation of supercritical solubility in near-critical fluids.     

Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water

In this work, the denitrogenation of hydrocarbons under supercritical water oxidation environment was investigated in a rotated bomb reactor at 623-723 K and 25-35 MPa over sulfided NiMo catalyst. Quinoline was used as a model nitrogen-containing compound. A high reduction of total nitrogen up to about 85% was obtained. The denitrogenation pathway is composed of two consecutive steps: in situ H₂ generation and the hydrogenation of quinoline. The hydrogenation mechanism of quinoline varies with reaction temperature because of the participation of supercritical water in HDN step. The strong adsorption of quinoline and its hydrogenation intermediates on catalyst surface has an adverse influence on total nitrogen reduction rate.     

Corrosion control methods in supercritical water oxidation and gasification processes

Use of supercritical water (SCW) as a medium for oxidation reactions, conversion of organic materials to gaseous or liquid products, and for organic and inorganic synthesis processes, has been the subject of extensive research, development, and some commercial activity for over 25 years. A key aspect of the technology concerns the identification of materials, component designs, and operating techniques suitable for handling the moderately high temperatures and pressures and aggressive environments present in many SCW processes. Depending upon the particular application, or upon the particular location within a single process, the SCW process environment may be oxidizing, reducing, acidic, basic, nonionic, or highly ionic. Thus, it is difficult to find any one material or design that can withstand the effects of all feed types under all conditions. Nevertheless, several approaches have been developed to allow successful continuous processing with sufficient corrosion resistance for an acceptable period of time. The present paper reviews the experience to date for methods of corrosion control in the two most prevalent SCW processing applications: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).     

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.     

Non-catalytic Oppenauer oxidation of alcohols in supercritical water

Non-catalytic Oppenauer oxidation was applied for alcohols, such as benzyl alcohol (4) and benzhydrol (1), in the presence of an excess amount of carbonyl compound, formaldehyde (5a), as an oxidant with and without water. Oppenauer oxidation took place in both reactions of 4 and 1 to afford the oxidation products, benzaldehyde (6) (95%) and benzophenone (2) (64%), concomitant with relatively small amounts of reduction products, toluene (7) (1%) and diphenylmethane (3) (13%), respectively, at 400 °C for 10 min without water in an SUS 316 batch-type tubular reactor. Lower yields of oxidation products 6 (68%) and 2 (30%) were obtained in supercritical water under the conditions of 400 °C, 10 min, and 0.35 g/mL water density, while the formation of the reduction products 7 and 3 was completely suppressed. Thus, water was indispensable for the clean and highly selective Oppenauer oxidation of 4 and 1 to yield 6 and 2.     

Upgrading of asphalt with and without partial oxidation in supercritical water

Supercritical water and supercritical water partial oxidation treatments were applied to the upgrading of asphalt. Asphalt was converted at 613-673 K, 0-0.5 g/cm³ water density under argon or air atmosphere. Under an argon atmosphere and 0.5 g/cm³ water density, both the asphaltene conversion and desulfurization increased with increasing temperature. At 673 K, the asphaltene conversion and the yield of CO₂ increased with an increasing water density. Water apparently participated in the reaction and its hydrogen was used for capping the free radicals generated during the upgrading of asphalt resulting in an increased yield of maltene. Under an air atmosphere at 673 K, asphaltene conversion was lower but desulfurization was higher than those obtained in an argon atmosphere.     

Steam reforming of n-hexadecane over noble metal-modified Ni-based catalysts

Steam reforming of n-hexadecane, a main constituent of diesel, over noble metal-modified Ni-based hydrotalcite catalyst was carried out in a temperature range of 700–950 °C, at an atmospheric pressure with space velocity of 10,000–100,000 h⁻¹ and feed molar ratio of H₂O/C = 3.0. The catalysts were prepared by a co-precipitation and dipping methods. The noble metal-modified Ni-based hydrotalcite catalyst displayed higher resistance for the sintering of active metal than the Ni-based hydrotalcite catalyst prepared by the conventional method. It was found that the Rh-modified Ni-based catalysts showed high resistance to the formation of carbon compared to Ni-based catalysts. The results suggest that Rh-modified Ni-based catalyst can be applied for the steam reforming (SR) reaction of diesel.     

Partial oxidation kinetics of m-hydroxybenzyl alcohol with noble metal catalysts in supercritical carbon dioxide

Partial oxidation of m-hydroxybenzyl alcohol was studied over several supported noble metal catalysts in a temperature range from 373 to 413 K, up to 2 MPa of oxygen pressure and 20 MPa of carbon dioxide pressure. The major product detected was m-hydroxybenzaldehyde. A charcoal supported palladium catalyst gave the highest yield of the aldehyde. For high temperature above 393 K and high oxygen pressure above 0.5 MPa, total oxidation was observed, which caused the selectivity of m-hydroxybenzaldehyde to decrease. Supercritical carbon dioxide medium seemed to improve heat dissipation of the reaction to allow the partial oxidation of m-hydroxybenzyl alcohol to occur under mild conditions. The partial oxidation of benzyl alcohol over a charcoal supported palladium catalyst was also examined for comparison and the major product formed was benzaldehyde. The conversion of benzyl alcohol and the selectivity to benzaldehyde was higher than those for the case of partial oxidation of m-hydroxybenzyl alcohol.     

Partial oxidation of alkanes to oxygenates in supercritical carbon dioxide

The catalytic partial oxidation of propane in supercritical carbon dioxide has been investigated in a stirred batch reactor. Various metals (oxides) have been used as supported catalysts with respect to their activity and selectivity for the formation of oxygenates. The reactions run with a 1:2.3–2.9:68–108 molar ratio of propane:synthetic air:CO₂ at 453–573 K and 80–100 bar. Using a precipitated 2.4 wt.% Co₃O₄–SiO₂ catalyst at 573 K, a total oxygenate (i.e. acetic acid, acetone, acetaldehyde, methanol) selectivity of 59% and a propene selectivity of 21% were obtained at a propane conversion of 12 mol%. The same catalyst has been used to investigate the influence of the supercritical conditions and initial feed composition on the reaction, varying the density of CO₂ and the concentration of synthetic air, respectively.     

Phenol oxidation over CuO/Al2O3 in supercritical water

Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al₂O₃ as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO₂ formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO₂ formation than did bulk MnO₂ or bulk TiO₂. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.     

城市污泥在超临界水中的部分氧化实验研究

为实现城市污泥的资源化利用和无害化处理,利用超临界水间歇式实验系统研究其催化部分氧化特性。结果表明:氧化剂系数增大0.2,H2摩尔分数降低3.1%—6.1%;温度每升高20 K,H2摩尔分数提高2.0%左右;反应时间延长10 min,H2摩尔分数提高1.5%—1.7%,在30 min达到最大;加入活性炭催化剂,H2摩尔分数提高14.5%—16.1%。25 MPa时,拟合反应的动力学方程(基于COD),得到质量分数1%活性炭催化(无催化)时的速率常数、活化能、反应级数分别为6 347.7(4 403.3),105.698 kJ/mol(113.054 kJ/mol),2.023(2.023)。活性炭可促进水气转化反应,提高H2摩尔分数,但对COD去除率无明显影响。     

Transpiring wall reactor in supercritical water oxidation

Reactor corrosion and plugging problems have hindered the commercialization of supercritical water oxidation (SCWO) for wastewater purification. The use of transpiring wall reactor (TWR) is an effective means to overcome the above two problems by forming a protective water film on the internal surface of the reactor to aviod contacting corrosive species and precipitated organic salts. This work mainly aims to objectively review experimental investigations and numerical simulation results concerning TWR. Subsequent investigations for parameters optimizations of TWR are also proposed in order to ultimately build effective regulation methods of obtaining excellent water film properties. All this information is very important in guiding the structure design and operation parameters optimization of TWR.     

A process for generating power from the oxidation of coal in supercritical water

A theoretical study of power generation from oxidation of coal by supercritical water oxidation (SCWO) is presented. Two versions of SCWO power plant are compared to two of the most efficient conventional power plant processes: pulverised coal power plants and pressurised fluidised bed power plant. The effects of steam pressure and temperature on produced (W_p), consumed (W_c) and net work (W_N) are calculated in order to compare the efficiency of these power plants for the same steam conditions. Enthalpies have been calculated using residual enthalpies by Peng-Robinson equation of state. Calculated results show that net work in SCWO power plant is 5% higher than in other power plants, due to the fact that no air surplus is necessary for complete combustion and because steam is produced by direct heating. Energetic efficiency of SCWO increases more quickly with temperature than for the other power plants. The effect of steam pressure is different: until 30 MPa power plant efficiencies increase more quickly in SCWO power plants than in conventional plants, but when steam pressures increases beyond 30 MPa, efficiencies decrease in SCWO power plants.     

Simple, non-catalytic permethylation of catechol derivatives in subcritical and supercritical water

Environmentally benign, non-catalytic, simple, and complete aromatic ring methylation of catechol derivatives by using 1,3,5-trioxane as the source of methyl groups was investigated in subcritical and supercritical water and under solvent-free conditions. Irrespective of the presence of subcritical and supercritical water as a reaction medium, catechol and 4-methylcatechol afforded the permethylation product 3,4,5,6-tetramethylcatechol. Only a small amount of 3,4,5,6-tetramethylcatechol (2% yield) was obtained under solvent-free conditions at 400 °C for 10 min. However, supercritical water considerably accelerated the formation of 3,4,5,6-tetramethylcatechol (13% yield) under the conditions of 400 °C, 10 min, and 0.35 g/mL water density.     

Fast catalytic oxidation of phenol in supercritical water

The catalytic oxidation of phenol in water over a commercial oxidation catalyst, CARULITE 150, was investigated in a fixed bed flow reactor at 250 atm and temperatures from 380°C to 430°C. The phenol and oxygen concentrations at the reactor entrance varied between 0.070 and 1.24 mmol/l, and 9.60 and 39.6 mmol/l, respectively. The reaction conditions produced phenol conversions and selectivities to CO₂ much higher than those produced by non-catalytic oxidation. The kinetics of phenol disappearance and of CO₂ formation were both roughly first-order, and the activation energies were 31 and 47 kcal/mol, respectively. The catalyst did not undergo continuous deactivation during the catalytic oxidation, but rather maintained a high activity even after several days of continuous operation.     

Hydrothermal flames in supercritical water oxidation: investigation in a pilot scale continuous reactor

Hydrothermal flames at 25 MPa supercritical water environment were investigated using a 4800 ml reaction tower, in which the sapphire windows were fitted for optical access. Down flowing hydrothermal flames were observed for oxidation of 2-propanol when the reactor was fed with inlet organic concentration higher than 2 vol% and air ratio higher than 1.8. Flame temperature, as high as 1100 °C, was measured by means of a thermocouple and the temperature was found to be strongly influenced by air ratio. Effective and stable oxidation of organics with TOC removal rate of 99.9% was achieved. Dioxins were also decomposed with a ratio higher than 99.9%, within 1 min reaction time in this reactor configuration.     

Modelling of heat transfer and hydrodynamic with two kinetics approaches during supercritical water oxidation process

Supercritical water oxidation is an innovative and very efficient process to treat hazardous organic waste. In order to better understand the complex physic phenomena involved in this process, and to design more efficient reactors or to insure future efficient scale-up, a simulation with the Computational Fluid Dynamics software FLUENT was carried out for a simple tubular reactor.The turbulent non-reactive flow is well-represented using the k– model. Nevertheless, the k–ω model gives better results when a source term is added to take into account the chemical reaction.Two approaches are used to model the reaction rate: an Arrhenius law and the Eddy Dissipation Concept (EDC) generally used to describe combustion reactions.The results of this simulation using Arrhenius law, are in good agreement with experimental data although a simple thermohydraulic model was used. Moreover, the sensitiveness to the inlet temperature has been demonstrated. It influences the reaction start-up and the shape of the measured wall temperature peak. Equally, the simulated temperature profiles using Eddy Dissipation Concept model are in good agreement with experimental ones. Hence, the two approaches give similar results. Nevertheless, the EDC model predicts more precisely the thermal peak location at the reactor wall.     

Water density effects on methanol oxidation in supercritical water at high pressure up to 100 MPa

Reaction kinetics of methanol oxidation in supercritical water at high pressure condition (420 °C; 34-100 MPa; ρ = 300-660 kg/m³) was investigated. Pseudo-first order rate constant for methanol decomposition increased with increasing water density. Effects of supercritical water on the reaction kinetics were investigated using a detailed chemical kinetics model. Incorporating the effect of diffusion in a reduced model revealed that overall kinetics for SCWO of methanol is not diffusion-limited. Roles of water as a reactant were also investigated. The dependence of sensitivity coefficient for methanol concentration and rate of production of OH radical on water density indicated that a reaction, HO₂ + H₂O = OH + H₂O₂, enhanced the OH radical production and thereby facilitated the decomposition of methanol. It is presumed that concentration of key radicals could be controlled by varying pressure intensively.     

Solvothermal synthesis of vanadium phosphate catalysts for n-butane oxidation

In this paper, we have developed a simple, low-cost, template-free and surfactant-free solvothermal process for synthesis of vanadyl hydrogen phosphate hemihydrate (VOHPO₄·0.5H₂O) with well defined crystal size. The synthesis was performed by reaction of VPO₄·2H₂O with an aliphatic alcohol (isobutyl alcohol, 1-pentanol, 1-hexanol, 1-heptanol or 1-decanol). This afforded well crystallized VOHPO₄·0.5H₂O by solvothermal methods at 120 °C temperature. This new method significantly reduced the preparation time and lowered production temperature (50%) of catalyst precursor (VOHPO₄·0.5H₂O) when compared to conventional hydrothermal synthesis methods. By varying the reducing agent, the solvothermal evolution process from layered tetragonal phase VOPO₄·2H₂O to orthorhombic phase VOHPO₄·0.5H₂O was observed. It was found that the length of carbon chain in an alcohol in the solvothermal condition had a great impact on chemical and physical properties of resulting catalysts. Interestingly, there was no trace of VO(H₂PO₄)₂ an impurity noted to be readily formed under solvothermal preparation condition. Therefore, this study introduces a more facile synthetic pathway to V(III) compounds. In addition, the microwave-synthesized catalysts exhibited some properties superior to those of conventionally synthesized catalyst such as better stability, crystallinity, and catalytic activity in the production of maleic anhydride. The characterization of both precursors and calcined catalysts was carried out using X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), N₂ physisorption, temperature programmed reduction (H₂-TPR) and scanning electron microscopy (SEM). The XRD pattern of the active catalyst prepared by this solvothermal method confirmed the presence of smaller crystal size (between 6 and 13 nm along 0 2 0 planes) of vanadium phosphate catalyst with higher specific surface area. Finally, the yield of maleic anhydride was significantly increased from 29% for conventional catalyst to 44% for the new solvothermal catalyst.