Continuous Supercritical Water Research Articles

Continuous supercritical water gasification of isooctane: A promising reactor design

A new design of supercritical water gasification system was developed to achieve high hydrogen gas yield and good gas–liquid flow stability. The apparatus consisted of a reaction zone, an insulation zone and a cooling zone that were directly connected to the reaction zone. The reactor was set up at an inclination of 75° from vertical position, and feed and water were introduced at the bottom of the reactor. The performances of this new system were investigated with gasification of isooctane at various experimental conditions – reaction temperatures of 601–676 °C, residence times of 6–33 s, isooctane concentrations of 5–33 wt%, and oxidant (hydrogen peroxide) concentrations up to 4507 mmol/L without using catalysts. A significant increase in hydrogen gas yield, almost four times higher than that from the previous up-down gasifier configuration (B. Veriansyah, J. Kim, J.D. Kim, Y.W. Lee, Hydrogen Production by Gasification of Isooctane using Supercritical Water, Int. J. Green Energy. 5 (2008) 322–333) was observed with the present gasifier configuration. High hydrogen gas yield (6.13 mol/mol isooctane) was obtained at high reaction temperature of 637 °C, a low feed concentration of 9.9 wt% and a long residence time of 18 s in the presence of 2701.1 mmol/L hydrogen peroxide. At this condition, the produced gases mainly consisted of hydrogen (59.5 mol%), methane (14.8 mol%) and carbon dioxide (22.0 mol%), and a small amount of carbon monoxide (1.6 mol%) and C 2–C 3 species (2.1 mol%). Reaction mechanisms of supercritical water gasification of isooctane were also presented.

Supercritical water oxidation of ion exchange resins: Degradation mechanisms

Spent ion exchange resins are radioactive process wastes for which there is no satisfactory industrial treatment. Supercritical water oxidation could offer a viable treatment alternative to destroy the organic structure of resins and contain radioactivity. IER degradation experiments were carried out in a continuous supercritical water reactor. Total organic carbon degradation rates in the range of 95–98% were obtained depending on operating conditions. GC–MS chromatography analyses were carried out to determine intermediate products formed during the reaction. Around 50 species were identified for cationic and anionic resins. Degradation of polystyrenic structure leads to the formation of low molecular weight compounds. Benzoic acid, phenol and acetic acid are the main compounds. However, other products are detected in appreciable yields such as phenolic species or heterocycles, for anionic IERs degradation. Intermediates produced by intramolecular rearrangements are also obtained. A radical degradation mechanism is proposed for each resin. In this overall mechanism, several hypotheses are foreseen, according to HOO radical attack sites.

Industrial emergy evaluation for hydrogen production systems from biomass and natural gas

Fossil fuel resources are the main source for hydrogen production, and hydrogen production by renewable energy, such as biomass, is under development. To compare the performance in natural resource utilization for different hydrogen production systems, in this paper, two laboratorial hydrogen production systems from biomass and one industrial hydrogen production system from natural gas are analyzed by using industrial emergy evaluation indices. One of the laboratorial systems is a continuous supercritical water gasification system from glucose, and the other is a batch supercritical water gasification system from sawdust. The industrial system adopts American Brown technology. The evaluation results show that although the industrial emergy efficiency (IEE) of the industrial system from natural gas is higher than that of the laboratorial systems from biomass, the industrial emergy index of sustainability (IEIS) of the two laboratorial systems are higher than that of the industrial system. To make the laboratorial biomass system become an industrial system, the system should improve its yield, and reduce its capital investment.

The synthesis and fluorescent properties of nanoparticulate ZrO2 doped with Eu using continuous hydrothermal synthesis

Fluorescent nanoparticles of zirconia doped with europium (ZrO2:Eu3+) have been produced via the co-precipitation of simple metal salt precursors using continuous supercritical water hydrothermal synthesis. The effect of reaction temperature and residence time on both particle size and conversion were investigated. Increasing the reaction temperature from 200 to 400 °C increases the size of the particles: smaller diameters ( 20 nm) at higher temperatures. Varying residence time at constant temperatures (between 3.7 and 7 seconds) did not significantly affect the particle size. The structure of the nanoparticles synthesised at 400 °C was revealed to be cubic by X-ray diffraction. Conversion of the metal precursors to ZrO2:Eu3+ was improved from 4 to 99% on increasing the temperature between 250 and 350 °C. The photoluminescent properties of the nanoparticles were also explored with respect to synthesis temperature. Fluorescence upon UV excitation at 255 nm, originating from Eu3+ doped in ZrO2, was not significantly affected by the synthesis temperature. The emission spectra demonstrated characteristic f → f transitions of Eu3+ ions and can be explained through an effective dispersion of these ions within the ZrO2nanoparticles. Initial cell toxicity studies suggest that ZrO2:Eu3+ nanoparticles do not cause any significant increase in cell death in Chinese hamster ovary cells following an 18 hour incubation at 37 °C. This paper describes a ‘green’ and controlled method for the continuous production of research scale quantities of nanomaterials.

Synthesis of Nanosized Ce3+,Eu3+-Codoped YAG Phosphor in a Continuous Supercritical Water System

Luminescent yttrium aluminum garnet (Y3Al5O12) nanoparticles codoped with Ce3+ and Eu3+ (YAG:Ce3+,Eu3+) were continuously synthesized by directly feeding potassium hydroxide solution and a metal salt solution to supercritical water (SCW). The effects of the Ce3+-to-Eu3+ concentration ratio on the photoluminescence of the synthesized nanoparticles were studied using a continuous SCW tubular reactor. At 20-s reaction time and a pH of 9.10 in the SCW reactor, the average size of the prepared phosphor nanoparticles was 60−150 nm and cubic or hexagonal particles coexisted with the spherical particles. The phosphor nanoparticles presented a broad emission band in the green-yellow spectral region due to Ce3+, as well as a sharp emission peak at around 610 nm in the red spectral region due to Eu3+. Without further thermal treatment, the YAG:Ce3+,Eu3+ phosphor synthesized in the continuous reactor under SCW conditions showed strong luminescence properties. Simultaneously, enhancement of the red spectral emission i...

Investigation on degradation of polyethylene to oil in a continuous supercritical water reactor

The effects of temperature, residence time, and pressure on the degradation of polyethylene (PE) to oil were investigated in supercritical water (SCW) using a 180-mL continuous reactor. The yield of oil product increases with the increase of temperature, reaching 79% at 530°C, and then decreases as the temperature further increases. The yield of gas product increases from 3% to 22% as the temperature increases from 500°C to 550°C. The product distribution shifts toward shorter chain hydrocarbons with the prolongation of reaction time. Furthermore, the ratio of alkenes to alkanes of products increases with the increase of temperature and pressure, and decreases with increasing reaction time. The effect of pressure on both products’ yield and composition is less significant.

Synthesis of nano-sized YAG:Eu 3+ phosphor in continuous supercritical water system

Luminescent yttrium aluminum garnet (Y 3Al 5O 12) nanoparticles doped with Eu (YAG:Eu 3+) were continuously synthesized by directly feeding potassium hydroxide solution and metal salt solution to supercritical water (SCW). Effects of Eu concentration, pH, and residence time on photo-luminescence were studied using a continuous tubular reactor. Residence time played a key role in producing single-phase YAG:Eu 3+ nanoparticles. The residence time of 20 s under SCW conditions (400 °C and 280 bar) was enough to form YAG:Eu 3+ phosphor without any intermediate phases. At this residence time, the Eu concentration and pH condition under SCW contributed to improving the size, morphology and luminescent property of YAG: Eu 3+ nanoparticles. The average size of the prepared phosphor nanoparticles at 10 at.% and pH of 9.10 was 74 nm and the morphology was identified as nearly uniform and spherical-like in shape. Without further thermal treatment, the phosphor YAG:Eu 3+ synthesized in the continuous reactor under SCW conditions showed strong luminescence properties and red emission spectra.

Synthesis of nanoparticulate yttrium aluminum garnet in supercritical water–ethanol mixtures

Single-phase YAG nanoparticles have been produced by the rapid hydrolysis and dehydration of Al 3+ and Y 3+ salts in EtOH–H 2O mixtures using continuous supercritical water hydrothermal synthesis. The effects of the nature of the starting solution, the metal concentration, the EtOH content, the flow rate and the temperature on the particle size were studied. EtOH promotes crystallization and highly crystalline materials are obtained at relatively low temperatures. Furthermore the critical temperature of EtOH–H 2O mixtures is lower than that of pure H 2O, reducing the operating temperature. From the variables studied, the most important parameter seems to be the temperature and much smaller particles have been obtained when experiments are conducted under supercritical conditions (average 50 nm), in comparison to near critical or subcritical conditions (average 150 nm). The method is simple and clean and requires neither addition of a base to the system nor the use of very high reaction temperatures and potentially allows the continuous production of large quantities of material.

A versatile flow visualisation technique for quantifying mixing in a binary system: application to continuous supercritical water hydrothermal synthesis (SWHS)

A versatile image analysis system which can be used to quantify the mixing pattern of a two-fluid/phase system has been developed. This visualisation technique, light absorption imaging (LAI), is based on the absorption of light by dyed fluids. In the context of this paper, the technique is applied to supercritical water hydrothermal synthesis (SWHS), which is used in the production of nano-particulate metal oxides. This case study was carried out in an attempt to highlight potential process pitfalls and to optimise overall reactor design. However, the versatility of this LAI technique makes the scope of possible applications much wider. The SWHS process for the production of metal oxide nano-particulates is a relatively new and promising development. A key aspect that requires fundamental research is the phase interaction or ‘mixing mechanism’ within these continuous reactors. The manner in which the supercritical water stream interacts with the cold metal salt stream is of paramount importance since it will inevitably affect the quality and quantity of product as well as highlighting any potential fundamental flaws in a design of the reactor.

Corrosion on continuous supercritical water oxidation for polychlorinated biphenyls

The corrosion resistance of Hastelloy C-276 and titanium alloy G5 against C1 anion which was generated by supercritical water oxidation (scWO) of 3-chlorobiphenyl (3-PCB) in a flow reactor was investigated over a period of 336 hours at a constant pressure of 30 MPa. The condition of the scWO reaction zone was fixed at 750 K. The concentration of chlorine in the effluent collected after cooling to ambient temperature was about 850 wppm. The Hastelloy C-276 has a nominal ratio of approximately 3.7 Ni/l Cr. The Ni/Cr ratio in the effluent was the same with the nominal ratio, and the Hastelloy C-276 was proved to be corroded on the surface located between the bottom of the reactor and the cooling parts, which are under sub- and near- critical conditions of water. The corrosion rate was found to be 5 to 34mm/y in the zone where the temperature ranged from 543 K to 650 K and the corrosion spread over the whole surface of the alloy; however, one can hardly observe such a serious corrosion except the zone located in the between the reactor and the cooling part.

A combined modeling and experimental approach for achieving a simplified closed ecosystem.

In CELSS both biological and physico-chemical processes have to be used to support the main needs of the crews and to minimize the re-supply of food and air from Earth. The basic idea is to create a complete food chain (an artificial ecosystem), beginning from the crew, with its wastes, and returning to the crew to supply it with food and air. Two main other steps of this food chain are a waste treatment process and a biomass production including higher plants. We set up the connection of these key modules, which we called ECLAS (Ecosysteme Clos Artificiel Simplifie). A growth chamber containing higher plants is connected to a continuous supercritical water oxidation reactor, that converts the harvested biomass into carbon dioxide and enables the photosynthesis of the canopy. To achieve a stable coupling through optimized regulations between the modules, we programmed a modular numerical simulation of the system, in order to assess the involved fluxes and to constrain the last degrees of freedom of the experimental system already built. Simulation results and the first experimental results are here compared.