MORPHOLOGY AND TYPOLOGY OF EXPERIMENTAL HIGH-PRESSURE (SUPERCRITICAL) NONASSISTED JETS

The effect of alkyl group substituents on the degradation kinetics of poly(alkyl methacrylates)namely, poly(n-butyl methacrylate) (PBMA), poly(isopropyl methacrylate) (PPMA), poly(ethyl methacrylate) (PEMA), and poly(methyl methacrylate) (PMMA)under supercritical and subcritical conditions was studied at various temperatures (250−375 °C). The molecular-weight distributions were measured via gel permeation chromatography, and continuous distribution kinetic models were used to determine the degradation rate coefficients. The degradation rate coefficients under supercritical conditions were determined to be significantly higher than that those under subcritical conditions. The degradation rate of poly(alkyl methacrylates) increased as the number of carbon atoms of the alkyl substituents increased and, thus, followed the order PBMA > PPMA > PEMA > PMMA. The activation energy decreased with chain length under both subcritical and supercritical conditions. The degradation of PBMA was investigated at various pressures (20−85 bar) at a temperature of 325 °C, and the effect of pressure on the degradation rate coefficient was modeled using transition state theory. The degradation of PBMA was also investigated in different solvents at 325 °C and 60 bar. The supercritical state and the density of the solvent were determined to be important factors that influence the degradation kinetics.

Fuel injection and turbulent mixing at supercritical pressures determines ignition and combustion in numerous engineering applications. Flow evolution under such conditions is characterized by strong non-linear coupling between dynamics, transport coefficients, and thermodynamics. Experimental studies observe that the jets injected at supercritical pressures exhibit significantly different dynamics from the jets at subcritical conditions, owing to the lack of distinct liquid and gas phases in supercritical state. Thus, the averaged flow quantities such as the potential core length, jet spatial growth rate and velocity decay profiles differ in the two conditions, resulting in different mixed-fluid distributions. In this study, turbulent jet direct numerical simulations (DNS) are performed to examine the variations in flow statistics between injection of Nitrogen (N₂) in Nitrogen (N₂) at both subcritical (perfect-gas) and supercritical conditions. In all cases, isothermal round jets at Reynolds number (Re_{D}), based on jet diameter (D) and jet orifice velocity (U₀), of 5000 are considered. For mixing analyses, a passive scalar transported with the flow is examined.

A comparison of cement wasteform carbonation under subcritical and supercritical CO2 conditions has been conducted, as well as leach tests on the carbonated products. Cement samples were prepared with water containing As, Cs, Sr, Ni and Cl, and dried under vacuum prior to exposure to CO2 gas at sub and supercritical conditions. Carbonation under supercritical conditions (50°C, 1200 psi) showed similar CO2 mass uptake as subcritical carbonation (25°C, 800 psi). The apparent depth of carbonation from microscopic examination, however, appeared greater with the supercritical treatment. Microscopic examination also revealed a dark intermediate zone, likely a chemical transition area, between the uncarbonated and carbonated material. Results from experiments where the initial water to cement ratio (w/c) and curing time were varied indicate that high w/c ratios and short curing times promote greater carbonation. In addition, the pore network in specimens with low w/c ratios (0.45) completely filled with water during carbonation. This caused pore water containing soluble contaminants to exude from the specimens. This does not occur at a w/c of 0.60 because the larger pore network after the drying step easily accommodates all the water produced during the reaction. No measurable arsenic or nickel were released in leach tests conducted on both carbonated and uncarbonated specimens. Calcium, cesium and strontium leachability were reduced by carbonation while the teachability of chloride and nitrate were increased with carbonation. Samples carbonated under supercritical pressure conditions showed similar leaching characteristics as samples carbonated under subcritical conditions.

Energetic evaluation of a CO2 refrigeration plant working in supercritical and subcritical conditions

The effect of high-pressure treatment with supercritical CO2 on the inactivation of Listeria innocua in a fish soup was investigated. The soup was inoculated with L. innocua, packaged in modified atmosphere with 50:50 or 95:5 CO2:N2, high-pressure processed (300, 350, 400 and 600 MPa, 2 min) under subcritical (T < 304 K) or supercritical conditions (T > 304 K) and stored at 4 °C for up to 53 days. Treatment at 400 and 600 MPa had a significant (p < 0.05) effect on L. innocua under both supercritical and subcritical conditions. In contrast, pressurization at 350 MPa and supercritical conditions were needed to significantly (p < 0.05) inactive L. innocua. Increased levels of CO2 in the headspace significantly (p < 0.05) reduced the bacterial load during processing, and supercritical conditions had a significant (p < 0.01) interaction with both CO2 levels and pressure. Increased storage time gave significantly increased levels of L. innocua at 400 and 600 MPa. In addition, high levels of CO2 significantly decreased (p < 0.001) growth. However, 350 MPa under supercritical conditions seemed to set the L. innocua in a permanent lag phase, with slow and steadily decreasing numbers of bacteria during storage. All the design variables resulted in significant inactivation of L. innocua, and supercritical conditions combined with high levels of CO2 inhibited the recovery of L. innocua to a large degree.

Heat and mass transfer for isolated and interacting fluid drops under supercritical conditions

A Kinetic Study ofn-Heptane Catalytic Cracking over a Commercial Y-Type Zeolite under Supercritical and Subcritical Conditions

The surface chemical analysis of corroded alloys in the subcritical and supercritical water oxidation (SCWO) of 2-chlorophenol were studied by using a continuous anticorrosive SCWO system. The corrosion phenomenon of metal in the supercritical condition (250 atm and 380 °C) was different from that in the subcritical condition (220 atm and 320 °C). However, the corrosion of metal alloys contributed to the elevation of destruction efficiencies in both subcritical and supercritical conditions. Furthermore, the degree of improvement of destruction efficiency at the subcritical condition was higher than that at the supercritical condition regardless of alloy types. Although all the tested alloys were severely corroded under both conditions, the catalytic effect of Monel K-500 with Cu on the destruction of 2-chlorophenol was higher than the other alloys in both subcritical and supercritical conditions. The results indicated that well-selected metal alloys could be applied to both subcritical and supercritical oxidation as catalysts by corroding themselves.

Chromatographic techniques were used for measuring the adsorption of toluene and naphthalene on silica gel packed beds under the supercritical and subcritical conditions at 301–318 K, 54–150 atm, and 0.3 < NRe < 7. The moment method was used to determine the equilibrium constants and rate parameters. The results showed a significant reduction of the adsorption equilibrium constant as the pressure changed from subcritical to supercritical; the axial dispersion coefficients at supercritical conditions were intermediate to those of what would be obtained in gases and liquids; in the pressure range where density fluctuations with concentration were large, axial dispersion was influenced by both natural convection and molecular diffusion; diffusion within the pores of the adsorbent was found to be the major resistance to adsorption. In addition to these parameters, partial molar volumes were determined from the chromatographic experiment and were found to agree well with the values reported in earlier work.

ABSTRACTNeutron diffraction measurements of H/D isotopic substitution have been performed for seven H/D substituted methanol-water mixtures of 0.3 mol fraction of methanol (xM) under the supercritical (618 K, 100 MPa) and ambient (298 K, 0.1 MPa) conditions. The seven structure factors obtained were subjected to an empirical potential structure refinement (EPSR) modelling to derive all site-site pair correlation functions, coordination number distributions, spatial density functions, and cluster distributions. Water has a four coordinated structure in the first coordination shell under both ambient and supercritical conditions; however, the spatial density distribution of water molecules in the second coordination shell is delocalised under the supercritical condition. The mean coordination number of all atomic pairs with hydrophilic interactions decreases in the supercritical state. On the other hand, the mean coordination number of interactions between the hydrophobic part of methanol and water molecule is less sensitive to temperature. In the supercritical condition, water clusters with a wide size distribution are generated in a methanol-water mixture as well as in pure water. Since the critical temperature of a methanol-water mixture is lower than that of pure water, it can be concluded that the addition of methanol can generate fragment water clusters at a lower temperature.

In the present study, the protein-extracted grass residue (press cake) was processed through hydrothermal liquefaction under sub and supercritical temperatures (300, 350 and 400 °C) with and without using a potassium carbonate catalyst. The results revealed that bio-crude yield was influenced by both temperature and the catalyst. The catalyst was found to be effective at 350 °C (350 Cat) for enhancing the bio-crude yield, whereas supercritical state in both catalytic and non-catalytic conditions improved the quality of bio-crude with reasonable HHVs (33 to 36 MJ/kg). The thermal behaviour of bio-crude was analysed and higher volatile contents (more than 50% under the range of 350 °C) were found at supercritical conditions. The overall TOC values in the residual aqueous phase varied from 22 to 38 g/L. Higher carbon loss was noticed in the aqueous phase in supercritical conditions. Furthermore, GCMS analysis showed ketones, acids and ester, aromatics and hydrocarbon with negligible nitrogen-containing compounds in bio-crude. In conclusion, the catalytic conversion of grass residue under subcritical conditions (350 Cat) is favourable in terms of high bio-crude yield, however, supercritical conditions promote the deoxygenation of oxygen-containing compounds in biomass and thus improve HHVs of bio-crude.

Biofuels production from hydrothermal decarboxylation of oleic acid and soybean oil over Ni-based transition metal carbides supported on Al-SBA-15

Subcritical flux operation is widely practiced in membrane bioreactors (MBRs) to avoid severe membrane fouling and, thus, to maintain sustainable permeability. Filtration at a constant subcritical flux, however, usually leads to a two-stage increase in the transmembrane pressure (TMP): initially slowly, then abruptly. We have investigated the mechanism of this two-stage TMP increase through analyses of the structure and microbial characteristics of the bio-cake formed on the membrane. The MBR was operated under various subcritical and supercritical flux conditions. Under subcritical conditions, we observed the typical two-stage TMP increase. When a constant flux augmented and reached the supercritical conditions, however, the dual TMP change gradually transformed into a steeper, one-stage TMP increase. The second stage TMP increase under the subcritical flux was closely related to the sudden increase in the concentration of extra-cellular polymeric substances (EPSs) at the bottom layer of the bio-cake; we attribute the one-stage TMP increase under the supercritical conditions to the accumulation of microbial flocs and the reduced porosity of the bio-cake under compression. We explain the variation of the EPS concentration in the bio-cake in terms of the spatial and temporal changes of the live-to-dead ratio along the depth of the bio-cake.

Corrosion phenomena of alloys by subcritical and supercritical water oxidation of 2-chlorophenol

In a sustainability context, using renewable energy sources to hedge against increasing consumption of fossil fuels and reduce greenhouse gas emissions becomes increasingly important. The geothermal resource has a great application prospect due to its rich reserves and convenient utilization, and Organic Rankine Cycle (ORC) is a effective method to convert the low-grade geothermal to electricity. To improve the performance of geothermal ORC system, working fluid selection, system parameter optimization and the cycle design are the main approaches. Zeotropic mixtures may show superiority as ORC working fluids due to the temperature glides during the phase transitions, which leads to better temperature matches between the working fluid and the heat source/sink. Moreover, owing to the changing temperature during the transition from liquid to vapor in the vapor generator, supercritical ORC provides a great potential in geothermal utilization and irreversibility reduction. This paper displays an investigation on the performance optimization and economic analysis of various working fluids under subcritical and supercritical conditions. To avoid the silica oversaturation, the geothermal water reinjection temperature should not be less than 70 °C. Turbine inlet temperature, condenser outlet temperature as well as turbine inlet pressure (for supercritical ORC) are optimized to maximize the net power output. Moreover, economic analysis is conducted by taking heat exchanger area per unit power output (APR) and the specific investment cost (SIC) as indicators under the optimal net power output condition. The results shows that working fluid with a medium critical temperature yields greater net power output in supercritical ORC and mixture produces larger net power output compared with its pure components in subcritical ORC. Compared with isobutane (R600a) under subcritical condition, isobutane/isopentane (R600a/R601a) and isobutane/pentane (R600a/R601) under subcritical condition, R134a and R1234ze(E) under supercritical condition yield 3.9%, 3.8%, 8.5% and 8.8% more net power outputs, respectively. In addition, R600a/R601a and R600a/R601 under subcritical condition own higher APR and SIC while R134a and R1234ze(E) under supercritical condition possess lower APR and SIC.