Supercritical Environment Research Articles

Prediction and modeling of tie-dye polyester fabric's texture and color feature in supercritical carbon dioxide

To mitigate the water-intensive challenges of tie-dye clothing production and address the absence of objective evaluation for finished products, we have explored the correlation between tie-dye techniques in supercritical carbon dioxide (SC-CO2) and image patterns. Employing digital image processing, we extracted the average HSV three-component value of the valid tie-dye area, the proportion of incompletely dyed area in the HSV color space, and the initial three texture features of the digital tie-dye image using the Tamura method. We scrutinized five factors in the supercritical CO2 dyeing process: rotational speed, dyeing temperature, dyeing pressure, dye concentration, and dyeing time. The single-factor climbing test revealed that temperature and pressure are the predominant factors influencing tie-dyeing performance. We propose a correlation between the texture parameters of tie-dyeing and the swelling effect of polyester fabric in a supercritical CO2 environment. Subsequently, we conducted two-factor tests on temperature and pressure to investigate their specific effects on the parameters of supercritical polyester tie-dyeing. The results indicated significant changes in regularity, saturation, and the proportion of undyed area with variations in dyeing temperature and pressure. Building upon these findings, we developed an artificial neural network for predicting tie-dye dyeing temperature and pressure in supercritical CO2, utilizing features regularity, saturation, and proportion of undyed area as inputs. In the 105st round, experimental results demonstrated an accuracy of 0.99445, substantiating the efficacy of tie-dye pattern features in supercritical CO2 for technique forecasting.

Microscopic investigation and local standard deviation analysis of liquid propane jets in supercritical environment

To improve the thermal efficiency of engines, the temperature and pressure in the combustion chamber are continually increased, potentially reaching the critical points of the fuels. Additionally, light fuels such as liquid ammonia and propane are utilized to reduce carbon emissions. The relatively low critical temperature of light fuels facilitates their transition into a supercritical state, which greatly change the jet characteristics. To gain a deeper understanding of the supercritical transition of fuels, jets of liquid propane are investigated through high-speed microscopic imaging technology, covering subcritical to supercritical environmental conditions. Liquid propane is injected through a transparent flat nozzle to observe the internal flow states within the nozzle. For image and data processing, local standard deviation (LSD) analysis and a Residual Network (ResNet-50) model are employed to identify the structures within the mixing region. Droplets counting and frequency domain analysis are also performed to reveal the features of supercritical transition. The findings highlight the importance of mask size in the jet feature extraction. Large mask highlights large scale structures including jet surface and mixing region structures, while small mask brings about full-scale feature including droplets and all other features. A criterion for mixing regime classification based on reduced temperature (Tr) and reduced pressure (Pr) is established directly from image data using the ResNet-50 model, where a condition of Tr1.6Pr0.4 ≥ 1.067 indicates a transcritical mixing regime. Both droplet counting and frequency domain analysis corroborate this mixing regime criterion. These insights enhance our understanding of supercritical transitions and are useful for simulation verifications.

Atomic insights into the corrosion behavior of Fe-Cr alloys in supercritical CO2 environment

In this work, the corrosion process of Fe-Cr alloys in flowing CO2 was simulated using molecular dynamics within an extended ReaxFF force field. The effects of Cr content, temperature variations, impurity gases and vacancy defects were discussed. The corrosion process was dominated by oxidation and carburization reactions, with the oxidation rate higher than carburization rate, resulting in the formation of a carbide layer within the central region between two oxide layers. The increase in Cr content tended to decelerate corrosion, while raising temperature aggravated the corrosion of Fe-Cr alloy. The presence of O2 and SO2 impurities was found to thicken the corrosion layer, in contrast to the effects of CO and H2O impurities. A notable finding highlighted the positive influence of 2% vacancy defects in enhancing the corrosion resistance of the Fe-Cr alloy. The Al2O3 coating achieved a good corrosion protection by inhibiting the dissociation of CO2.

Investigation of thermodynamic state evolution, phase transition and mixing of the hydrocarbon droplet in convective supercritical environments

Liquid alkane droplets transit to supercritical fluids under supercritical conditions. However, the subsequent thermodynamic state evolution, phase transition, and mixing in this case, which directly influences combustion in actual diesel engines, need further investigation. We studied the thermodynamic state evolution, phase transition, and mixing of n-heptane droplets in supercritical laminar nitrogen streams through a set of conservation equations. Non-ideal thermodynamic and transport properties were calculated to incorporate the dramatic variation of physical properties in the supercritical regime. We focus on the effects of Reynolds number, ambient pressure, and droplet size on the thermodynamic state evolution, phase transition and mixing of droplets. Results showed that increasing the Reynolds number can accelerate the evolution of thermodynamic states, which leads to the faster transition of droplets from the liquid phase and supercritical fluids to gas-phase mixtures. With the increased Reynolds number, the phase transition rate is increased and the mixing homogeneity is improved due to the enhanced mass transfer. Caused by the negative correlation between mass transfer and ambient pressure, in higher pressure, droplets after the supercritical transition require more time to transition from supercritical fluids to vapor-phase mixtures. Increasing ambient pressure inhibited the droplet phase transition process and deteriorated the mixing homogeneity, but no significant influence was shown on the droplet dynamic evolution process. Smaller droplets transition faster from liquid phase and supercritical fluids to vapor-phase mixtures. The faster thermodynamic state evolution and faster dynamic evolution process of the smaller droplet leads to its more rapid homogeneous mixing with ambient gasses, but the mode of dynamics is independent of the droplet size.

Efficient synthesis of γ-valerolactone from ethyl levulinate via catalytic transfer hydrogenation in supercritical isopropanol over nickel aluminum oxide nanosheets

This study reports a novel method for γ-valerolactone (GVL) production from ethyl levulinate (EL) utilizing non-noble metal catalysts within a supercritical isopropanol (IPA) system. Nickel-loaded nickel–aluminum oxide nanosheets (NiXAl-R) were fabricated via hydrothermal treatment of Ni–Al layered double hydroxide precursors, followed by hydrogen reduction at 550 °C for 1.5 h. Using NiXAl-R as the catalyst and IPA as the hydrogen donor, we achieved notable results: 82.7% EL conversion, 73.6% GVL yield, and 1.178 molGVL g−1 h−1 reaction efficiency after 1.5 h in supercritical IPA. The catalytic activity of NiXAl-R remained stable over five catalytic cycles. The surface of NiXAl-R exhibits abundant oxygen vacancies and unsaturated Al species (Alx+), which promote the initial transfer hydrogenation of EL to form the ethyl 4-hydroxypentanoate (4-HPE) intermediate. Simultaneously, the supercritical IPA environment overcomes mass transfer limitations at the solid–liquid interface between 4-HPE and the catalyst, thereby facilitating the Alx+-catalyzed intramolecular esterification of 4-HPE into GVL.

Efficacy of initiators on fuel conversion, heat sink, and coke propensity of hydrocarbon fuel under supercritical environment

The investigation includes the cracking characteristics of a multi-component hydrocarbon fuel with a boiling range of 149–224 °C under supercritical environments. The impact of reactor pressure and the influence of initiators on cracking characteristics, such as fuel conversion, coke deposition, gas and liquid product distribution pattern, and heat sink capacities, of the fuel are examined explicitly. The fuel conversion improved by 70.8% as the reactor pressure increased from 2.5 MPa to 5.5 MPa at 650 °C. However, the coke deposition also increased by 55% for the corresponding pressure range. A decrease in the olefin-to-alkane ratio with pressure signifies the possibility of a reduction in unimolecular decomposition reactions and an increase in bimolecular reactions. The efficacy of two initiators, namely triethylamine (TEA) and di-tert-butyl peroxide (DTBP), on heat sinks and coke deposition was also examined. Between the two initiators, the nitrogen-based TEA initiator showed better performance in terms of fuel conversion and coke deposition rate, and the oxygen-based DTBP offered better heat sink capacity. About 22% increment in endothermicity was obtained with 1 wt% DTBP loading at 650 °C under 5.5 MPa pressure.

Numerical simulation of fluid-particle flow of jet in supercritical water environment

Supercritical water gasification provides a new approach to the green transformation of coal. Nevertheless, the flow characteristics of fluids and particles injected into the reactor through the nozzle still need to be further investigated. In this paper, a three-dimensional numerical simulation of the particle jet in supercritical water environment is carried out coupling Large Eddy Simulation (LES) with Discrete Phase Model (DPM). The simulation focuses on the effects of particle size, Stokes number, mass flux ratio and initial fluid velocity on the flow characteristic of the fluid-particle flow, especially on the vortex structure evolution, particle distribution and velocity profile. The findings are that as the particle size decreases and the initial fluid velocity increases, the evolution of vortex structures become increasingly fierce, such as the destruction of large-scale coherent structures, the disappearance of vortex ribs, and the lateral expansion of vortices in the development section. Particle distribution exhibits characteristics similar to the vortex structure, as particles adhere closely to the carrier fluid at low Stokes number (St < 1). On the contrary, with increasing of Stokes number (St > 1), the particles are freer from fluid interference and keep their original motion state moving forward. The fluid velocity at the center of the transverse interface jet decrease with increased mass flow ratio. For higher initial fluid velocities, particle distribution is more uniform, and more particles are carried to areas with larger radial distances.

Theoretical analysis of the three-stage phase transition process of alkane droplet under supercritical conditions

With the development of heavy-duty diesel engines, the in-cylinder ambient conditions of the diesel engine are well above critical points of the main components of the diesel fuel. Therefore, significant changes arise in the phase transition process of the liquid fuels. However, the entire phase transition process of the liquid fuels in such supercritical environments is not well understood compared with that under subcritical conditions. In this work, A high-pressure transient droplet evaporation model was presented for the theoretical analysis of phase transition processes of the heptane/dodecane/hexadecane-nitrogen binary system under supercritical conditions. Analysis of the thermodynamic pseudo-trajectories reveals that the alkane droplets will undergo a three-stage phase transition process when the ambient condition is much higher than the critical point of alkanes, including classical two-phase mode, transitional two-phase mode, and supercritical single-phase mode. The evaporation dominates the phase transition process in the classical two-phase mode, and the transition from classical two-phase modes to transitional two-phase modes occurs when gas-liquid interfaces deviate from the global thermal equilibrium. In transitional two-phase modes, a gas-liquid interface still exists and separates the liquid phase and vapor phase, but with a questionable adoption of vapor-liquid equilibrium. The transition from the transitional two-phase mode to the supercritical single-phase mode occurs when the thermodynamic state of the binary mixture in the interface reaches the critical mixing state. We distinguished the phase states for the whole phase transition process according to the thermodynamic analysis of binary systems. Analogous to the immiscible interface in classical two-phase mode, a miscible boundary layer containing supercritical fluids exits between the liquid core and gas in the initial period of the supercritical single-phase mode. The pseudo-boiling phenomenon occurs in this boundary layer.

Corrosion Monitoring Techniques in Subcritical and Supercritical Water Environments

A series of advanced equipment exposed to sub-/supercritical water environments at high temperatures, high pressures, and extreme water chemistry with high salt and dissolved oxygen content faces serious corrosion problems. Obtaining on-site corrosion data for typical materials in harsh environments is crucial for operating and maintaining related equipment and optimizing various corrosion prediction models. First, this article introduces the advantages and disadvantages, usage scenarios, and future development potential of several in situ monitoring technologies, including ultrasonic thickness measurement, the infrared thermography method, microwave imaging, eddy current detection, and acoustic emission. Considering the importance of electrochemical corrosion data in revealing microscale and nanoscale corrosion mechanisms, in situ testing techniques such as electrical resistance probes, electrochemical corrosion potential, electrochemical impedance spectroscopy, and electrochemical noise that can be applied to sub-/supercritical water systems were systematically discussed. The testing platform and typical data obtained were discussed with thick and heavy colors to establish a mechanical prediction model for corrosion behavior. It is of great significance to promote the development of corrosion monitoring techniques, such as breaking through testing temperature limitations and broadening the industrial application scenarios and maturity.

Corrosion behavior of eutectic high-entropy alloys in supercritical CO2 environments

This study explores the corrosion resistance of AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) in supercritical CO2 (S–CO2). The corrosion behavior was meticulously examined under conditions of 500 °C and 25 MPa CO2 over a duration of 800 h. The formation of distinct passivation layers on the face-centered cubic (FCC) and body-centered cubic (BCC) B2 phases was observed, leading to a cooperative passivation effect of Al and Cr. The protective Al2O3 oxide layers, which were dense and continuous, played a crucial role in inhibiting carburization and further oxidation, thereby significantly enhancing the corrosion resistance. Interestingly, no carburization was observed in this study.

Supercritical ethanol-assisted catalytic upgrading of bio-tar using mesoporous SBA-15 supported Ni-based catalysts

The bio-tar transformation was conducted utilizing Ni-based catalysts supported on mesoporous SBA-15 within a supercritical ethanol environment. This approach aimed to facilitate the synthesis of biofuels and/or value-added chemical compounds, including alkyl phenols and aromatics. The characterization of the catalysts was accomplished by X-ray diffraction (XRD), N2-sorption, temperature programmed reduction (TPR), Transmission electron microscope (TEM), and Fourier Transform Infrared (FT-IR) techniques. Among the examined catalysts, namely SBA-15, Ni/SBA-15, Ni–Mo/SBA-15, and Mg–Ni–Mo/SBA-15, Mg–Ni–Mo/SBA-15 emerged as the catalyst demonstrating the most pronounced activity for the hydrodeoxygenation (HDO) of bio-tar to produce high-quality liquid products (Total Acid Number (TAN): 3.4 mgKOHg−1, oxygen content: 11.0 wt%, Higher heating value (HHV): 36.7 MJkg−1). The effect of reaction temperature (275–350 °C) and time (0–120 min) on the upgrading of bio-tar over Mg–Ni–Mo/SBA-15 was investigated in a one-pot process. The acids, aldehydes, and oxygenated phenols underwent complete conversion into chemically more stable compounds such as aromatics and alkyl phenols at 325 °C for a 120 min holding time. In particular, the oxygenated phenols found in the bio-tar underwent a complete transformation into alkyl phenols within the liquid products.

Numerical investigation on flow and heat transfer of the rotating spherical particle in a supercritical water crossflow environment

The supercritical water gasification reactor provides a green and efficient method for the utilization of organic resources such as coal and biomass. It is important to note that collision-induced particle rotation in the reactor will affect the flow and heat transfer characteristics between the particles and the fluid. At present, there is a lacking consideration of particle rotation in the supercritical water environment. The purpose of this paper is to investigate the effects of dimensionless rotation rate (Ω∗) and particle Reynolds number (Rep) on drag coefficient (Cd), lift coefficient (Cl) and average Nusselt number (Nu) for the single particle in a supercritical water crossflow under two rotation modes, with rotation axes parallel and perpendicular to the jet direction. The flow field, temperature field, boundary layer, and surface pressure coefficient (Cp) were analyzed to interpret the variation of the particle dimensionless coefficients (Cd, Cl and Nu). It was found that the value of ∣Cp∣ on the particle surface rose at different locations for the two rotation modes, leading to an increase in the pressure drag and dominating the total drag force. However, the two rotation modes produced different deviating effects on the average Nusselt number. Ultimately, new correlation formulas of Cd, Cl and Nu for a rotating spherical particle in a supercritical water environment were proposed based on the results of numerical simulations, with a maximum deviation of 8.52%.

Study of stress corrosion cracking susceptibility of irradiated ferrite-martensitic stainless steel 07Kh12NMFB in supercritical water. Part 2. Development of corrosion crack identification technique and analysis of autoclave test results

The studies of stress corrosion cracking have been carried out for the stainless ferritic-martensitic steel with chromium content of 12% irradiated to a damage neutron dose of ~12 dpa. This steel was chosen as a candidate material for the internals of supercritical water-cooled reactors (SWCR).The first part of the paper was devoted to autoclave testing of specially designed disk specimens under constant load in a supercritical water environment (at 450°C and 250 atm pressure). In this part of the article the developed technique for corrosion cracks identification is presented and analysis of autoclave tests results is performed.As a result of the experiments performed, the threshold stress values below which stress corrosion cracking initiation doesn’t occur. The values of threshold stresses are determined for the studied steel irradiated at temperatures of 390 and 550°C. Possible mechanisms of stress corrosion cracking of the studied steel are analyzed and directions for further research are proposed.

Investigation of Jatropha Oil Transesterification into Methyl Esters under Supercritical Methanol Environment using Advanced Heterogeneous Catalysts

AbstractThe rationale for this study lies in the investigation of the transesterification of crude Jatropha curcas oil into biodiesel using advanced heterogeneous catalysts under supercritical methanol conditions. The oxides of strontium, lanthanum, and zinc (SrO, La2O3, and ZnO) were synthesized with varying loadings (2 wt %, 6 wt %, and 10 wt %) on γ‐Al2O3 using the wet impregnation method followed by calcination at 900 °C. To gain a comprehensive understanding of the prepared catalysts, multiple analytical techniques were employed, including scanning electron microscopy (SEM) with energy‐dispersive X‐ray spectroscopy (EDS), Brunauer‐Emmett‐Teller (BET) surface area analysis, X‐ray diffraction (XRD), and Fourier transform infrared (FT‐IR) spectroscopy. The performance of the catalysts was assessed through the measurement of biodiesel yield, where the SrO/γ‐Al2O3, ZnO/γ‐Al2O3, and La2O3/γ‐Al2O3 systems produced yields of 97.54 %, 97.49 %, and 97.36 %, respectively, under identical reaction conditions of a 1 : 40 oil to methanol molar ratio, 10 wt % catalyst loading amount, 300 °C reaction temperature, 90 bar reaction pressure, and a reaction time of 3 min. Despite a minimal decrease of 0.05 %, zinc oxide catalysts remain highly attractive due to their favorable price position compared to strontium. Additionally, the catalyst can be reused up to 12 times with minimal loss in biodiesel yield (~2 %).

Experimental study of the evaporation characteristics of unsymmetrical dimethylhydrazine in a sub-supercritical environment

In this study, the temperature–pressure separation technique was used to successfully obtain the process of the unsymmetrical dimethylhydrazine droplet diameter variation with time during evaporation in subcritical, supercritical, and far-supercritical environments at temperatures and pressures of 473–773 K and 1–9 MPa, respectively. The results show that unsymmetrical dimethylhydrazine droplet evaporation has two characteristic stages, transient heating and steady-state evaporation, under low-pressure and low-temperature environments; the steady-state evaporation stage influences droplet evaporation life more than the transient heating stage. Under high-pressure and high-temperature environments, droplet evaporation shows only a single feature of steady-state evaporation, and the droplet expansion process in the transient heating stage disappears. The normalized droplet lifetime decreased exponentially with temperature and decreased linearly and monotonically with pressure. The evaporation rate constant increases linearly and monotonically with temperature and exponentially with pressure.

Optimization and kinetic study of biodiesel production from Jatropha curcas oil in supercritical methanol environment using ZnO/γ-Al2O3 catalyst

AbstractIn this study, the conversion of crude Jatropha curcas oil into biodiesel through transesterification was investigated in the presence of heterogeneous solid catalysts under supercritical methanol environment. The principal impetuses catalyzing the expansion in optimal biodiesel production are primarily attributed to the increasing demand for sustainable energy sources, the availability of raw materials, and innovations in production methodologies. To maintain the optimization, 6 wt% and 10 wt% of zinc oxide (ZnO) were incorporated into gamma-alumina (γ-Al2O3) through a wet impregnation method followed by calcination at 900 °C. Furthermore, the study examined the effect of alcohol/oil molar ratio, reaction temperature, and reaction time on the process to achieve maximum biodiesel production. The study revealed that a catalyst consisting of 10 wt% ZnO on γ-Al2O3 exhibited exceptional performance with a biodiesel yield of 95.64% under the reaction conditions of a molar ratio of 1:40 oil to methanol, a temperature of 300 °C, a pressure of 9 MPa, and a residence time of 3 min compared to the yield of 100% under same condition at residence time of 9 min. After thorough investigation, the kinetics of the catalytic transesterification reaction were elucidated, and suitable kinetic parameters were proposed.

Corrosion mitigation behavior of mild steel in supercritical CO2 environments with varying the solution volume

The corrosion behavior of X70 steel in CO2 saturated water at 60 °C and 10 MPa was investigated by limiting the exposed solution volumes. When the specific solution volume to the exposed area was smaller than 10 mL/cm2, it revealed the formation of protective FeCO3 layer, leading to a significant corrosion reduction. The time-sensitive nucleation and the spatial growth of FeCO3 grains governed the overall performance of corrosion product layers. The solution volume dependency was further explained by a diffusion-controlled deposition mechanism. It suggested a possible way to prevent CO2 corrosion in harsh environments by generating stable FeCO3 layers.

Temperature and Reaction Time’s Effects on N80 Steel Corrosion Behavior in Supercritical CO2 and Formation Water Environments

In the present study, an immersion experiment was carried out to examine how N80 steel corrodes when exposed to formation water containing dissolved CO2 and supercritical CO2 (Sc-CO2) along with water vapor. We employed electrochemical and surface analysis methods to examine the influence of various factors, including the temperature and duration of immersion, on the extent of corrosion. The results show that the corrosion patterns of N80 steel in a supercritical CO2 environment and CO2-saturated formation water differed significantly. The presence of similar corrosion features was suggested by the constant structure of the corrosion products identified in the formation water. However, the morphology of the corrosion product was complex in the supercritical CO2 environment, exhibiting features of pitting and localized corrosion. Furthermore, a non-linear trend in the corrosion rate was observed between 40 °C and 120 °C. Specifically, the rate of corrosion declined from 40 °C to 80 °C, but it then resumed its growth from 80 °C to 120 °C. These findings suggest that very high temperatures could lead to the destruction of corrosion products and subsequently enhance the corrosion process.

Molecular dynamics simulation and experimental analysis of nucleation and growth mechanism of mixed inorganic salts in supercritical water

Supercritical water gasification (SCWG) technology is one of the more promising approaches for processing oily wastewater. However, there needs to be more research on the sedimentation of mixed inorganic salts in the supercritical water reactor while treating oily wastewater. In this study, the clustering process of mixed inorganic chlorides containing NaCl, KCl, and CaCl2 in supercritical water was simulated through molecular dynamics simulation. Molecular dynamics simulation was mainly conducted for mixed inorganic salts within the parameter range of 673 K-1073 K and 22 MPa-28 MPa. Combined with Yasuoka and Matsumoto's cluster theory, it can be concluded that the nucleation process of mixed inorganic salts in the supercritical water environment is mainly affected by density. Although the ion diffusion coefficient increases with temperature, the nucleation rate of inorganic salts gradually decreases due to the decrease in density. In the temperature range of 673 K-1073 K, the nucleation rate of mixed inorganic salts decreased from 34.96 to 1.65 1036 m−3·s−1. At the same pressure, the crystal growth rate parameter decreased gradually with increasing temperature, reaching about 168.25–60.09 m·s−1. Finally, the simulation results were verified experimentally, confirming that the crystal growth rate decreases with increasing temperature.

DNS of a non-premixed CH4/O2 flame in a supercritical CO2 environment

3D Direct Numerical Simulation (DNS) is performed to investigate turbulent non-premixed methane oxy-combustion in a CO2-rich environment at supercritical conditions (300 bar). The study focuses on a shear-layer configuration typical of slot burners, where the central slot carries the fuel and the oxidant mixture O2/CO2 flows on both sides of the central jet. The simulation solves the compressible Navier–Stokes equations coupled with the translated volume formulation of the Peng–Robinson cubic equation of state (VTPR-EoS), utilizing a reduced 23-species kinetic scheme derived from the ARAMCO 2.0 complete mechanism. The extremely high pressure results in turbulent scales smaller than those observed at lower pressures. Both premixed and diffusion flame zones are identified, with heat primarily released in non-premixed rich regions, leading to a CO mass fraction of up to 0.55. The flow exhibits uphill (counter-gradient) diffusion for both CO2 and H2O, primarily influenced by the interaction of binary diffusion coefficients and individual species chemical potential gradients. Values in the range of 2.52−2.62 are obtained for the fractal dimension of temperature iso-surfaces. Mixture fraction probability density functions (PDFs) are compared with the standard presumed β−PDF, revealing an underestimation of mixing. Differently from flames at atmospheric pressures, the Dufour effect in the energy diffusion flux, is not negligible. Furthermore, fugacity coefficients must be considered in the calculation of chemical species kinetic rates.