Supercritical Water Research Articles

Phytochemical Screening, Antimicrobial and In-vitro Cytotoxic Activities of Several Extracts of Six Medicinal Plants (Crossopteryx febrifuga Benth, Khaya anthotheca, Gardenia ternifolia, and Terminalia glaucescens) from Central Africa Region

The qualitative study of secondary metabolites of the root and stem bark of Crossopteryx febrifuga Benth, Khaya anthotheca, Gardenia ternifolia, and Terminalia glaucescens obtained by maceration were estimated by tub reaction. Antimicrobial and cytotoxic activities of the root and stem bark of the same plants obtained by maceration and subcritical water extraction were respectively tested whit antibiogram disc test on Pseudomonas Aeruginosa (ATCC 27853), Enterobacter Hormaechei (ATCC700323), Bacillus Spizizenii (ATCC6633), Enterococcus Casseliflavus (ATCC700327), Staphylococcus aureus subsp. Rosenbach (ATCC 6538), and Salmonella Typhus (ATCC 6539). The Tetrazolium test (MTT test) was realized by using the mouse fibroblast cells (NIH3T3). Extracts based on the bark of Terminalia glaucescens and Crossopteryx febrifuga Benth induce sensitivity to 4/6 bacterial strains, in particular Enterobacter Hormaechei (ATCC700323): 3±2cm; Bacillus Spizizenii (ATCC6633): 5±2 cm; Staphylococcus aureus subsp. Rosenbach (ATCC 6538): 5±2; and Salmonella Typhus (ATCC 6539): 3±2cm. Extracts based on the bark of Khaya anthotheca and those of the bark of Gardenia ternifolia induce only the sensitivity on 2/6 bacterial strains specially Enterobacter Hormaechei (ATCC700323) and Bacillus Spizizenii (ATCC6633) respectively 4±2cm and 5±2cm. Extracts based on the barks and roots of Khaya anthotheca and those of the roots and barks of Gardenia ternifolia only induce sensitivity on 2/6 bacterial strains tested, in particular the strain of Enterobacter Hormaechei (ATCC700323) and Bacillus Spizizenii (ATCC6633). Only extracts based on the bark of Crossopteryx febrifuga Benth and Terminalia glaucescens show sensitivity against Salmonella Typhus (ATCC 6539). All the plant extracts tested show very low cytotoxic power. Plant extracts obtained by the maceration method (Hydro Methanol 90%) induce 19% inhibition of cell growth compared to extracts obtained by supercritical water 125°C 60ATM and 125°C80 ATM. The extracts obtained using the supercritical water 125°C 60ATM method are the least offensive on cell proliferation. The most concentrated extracts show the greatest inhibitions. The extracts of plant E show the greatest inhibition at a concentration of 1 g/ml.

Partial upgrading of bitumen with supercritical water— liquid products characteristics, gas composition and coke morphology

The study aimed to investigate the partial upgrading of bitumen in the absence of water (non-water) and the presence of supercritical water (SCW). Partial upgrading experiments were conducted at the target temperature of 420 °C, with the reaction time varying from zero to 60 min in a batch reactor. The liquid products were analyzed for density, viscosity, composition, total acid number (TAN), olefin content, and elemental analysis. The gas composition and morphology of solid products were also studied. The experimental results revealed that while partial upgrading of bitumen with SCW could accelerate improvement in oil quality, particularly in terms of API and viscosity, it concurrently led to elevated gas and coke formation levels. Moreover, the difference in the distribution of oil fractions between the two environments (absence and presence of SCW) tends to diminish with an extended residence time. Additionally, as the residence time increases, the efficiency of TAN reduction under SCW conditions is less pronounced compared to the non-water conditions. Liquid products obtained in an SCW environment exhibited higher olefin content (between 3 and 4 wt %) compared to non-water conditions (between 1.8 and 2.7 wt %). In both environments, the olefin content was initially increased up to a residence time of 20 min and then decreased. Regarding heteroatoms, the sulphur (S) and nitrogen (N) contents decreased up to 36 % and 72 %, respectively, while a greater reduction was observed in the presence of SCW. More non-hydrocarbon gases (H2S, CO2 and CO) were produced in the presence of SCW. SEM images showed that an extended residence time led to a shift in coke morphology towards a more uniform and compact structure, regardless of the upgrading environment. However, the porous structures of coke samples obtained under SCW are distinguished from those formed without water, which is attributed to the phase inversion of precursors. The findings of this study confirm that SCW plays a beneficial role as a solvent in the partial upgrading of bitumen, expediting the process. However, the increased level of coke formation and olefin contents suggest that it may not function effectively as a hydrogen donor in the process.

Back Cover: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. Int. Ed. 28/2024)

Back Cover: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. Int. Ed. 28/2024)

Rücktitelbild: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. 28/2024)

Rücktitelbild: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. 28/2024)

Widom Line in Supercritical Water in Terms of Changes in Local Structure: Theoretical Perspective.

Performing molecular dynamics simulations with the TIP4P/2005 water model along 9 isobars (from 175 to 375 bar) in the temperature range between 300 and 1100 K, we have found that the loci of the extrema in the rate of change of specific structural properties can be used to define purely structure-based Widom lines. We have examined several parameters that describe the local structure of water, such as the tetrahedral arrangement, nearest neighbor distance, local density around the molecules, and the size of the largest dense domain. The last two parameters were determined using the Voronoi polyhedral and density-based spatial clustering of applications with noise methods, respectively. By analyzing the moments of the associated distributions, we show that along a given isobar, the temperature at which we observe a maximum in the fluctuation, the rate of change of the average values, or in the skewness values unambiguously determines the Widom line that is in agreement with the experimentally detected, thermodynamic response function-based ones.

Treatment of pistachio processing industry wastewaters by supercritical water oxidation in a continuous-flow reactor

ABSTRACT The objective of the present study is the treatment of pistachio processing industry wastewaters (PPIW) using the supercritical water oxidation method. The experiments were conducted within a 400–600°C temperature range and a 30–150 s reaction time range, while maintaining a constant pressure of 25 MPa and using an O2/COD ratio of 1:1. To observe the effects of the initial PPIW and O2 concentrations on the treatment efficiency, experiments were also conducted with O2/COD ratios ranging from 0.5 to 3, while maintaining a constant reaction temperature and time of 500°C and 60 s, respectively. The influence of reaction temperature, reaction time and O2/COD ratio on the total organic carbon (TOC) and total nitrogen (TN) contents of the liquid PPIW effluents were investigated. Treatment efficiencies up to 99.75% regarding TOC conversion and 78.72% regarding TN conversion were obtained in very short reaction times. Additionally, the kinetics of oxidation of PPIW was studied, and reaction rate expressions based on TOC and TN were proposed.

Local number fluctuations in ordered and disordered phases of water across temperatures: Higher-order moments and degrees of tetrahedrality.

The isothermal compressibility (i.e., related to the asymptotic number variance) of equilibrium liquid water as a function of temperature is minimal under near-ambient conditions. This anomalous non-monotonic temperature dependence is due to a balance between thermal fluctuations and the formation of tetrahedral hydrogen-bond networks. Since tetrahedrality is a many-body property, it will also influence the higher-order moments of density fluctuations, including the skewness and kurtosis. To gain a more complete picture, we examine these higher-order moments that encapsulate many-body correlations using a recently developed, advanced platform for local density fluctuations. We study an extensive set of simulated phases of water across a range of temperatures (80-1600K) with various degrees of tetrahedrality, including ice phases, equilibrium liquid water, supercritical water, and disordered nonequilibrium quenches. We find clear signatures of tetrahedrality in the higher-order moments, including the skewness and excess kurtosis, which scale for all cases with the degree of tetrahedrality. More importantly, this scaling behavior leads to non-monotonic temperature dependencies in the higher-order moments for both equilibrium and non-equilibrium phases. Specifically, under near-ambient conditions, the higher-order moments vanish most rapidly for large length scales, and the distribution quickly converges to a Gaussian in our metric. However, under non-ambient conditions, higher-order moments vanish more slowly and hence become more relevant, especially for improving information-theoretic approximations of hydrophobic solubility. The temperature non-monotonicity that we observe in the full distribution across length scales could shed light on water's nested anomalies, i.e., reveal new links between structural, dynamic, and thermodynamic anomalies.

Hydrothermal hydrolysis of triglycerides: Tunable and intensified production of diglycerides, monoglycerides, and fatty acids

This study focuses on utilizing subcritical and supercritical water technology to produce intensive and selective diglycerides, monoglycerides, and fatty acids. The water properties can be easily modified by adjusting pressure and temperature, which promotes an improving solubility of the triglycerides in water and a selective production of glycerides. Triacetin and sunflower oil were used as models for short and long-chain components, exploring a range of temperatures (250 to 400 °C), pressures (22.0 to 25.0 MPa), and reaction times (0.7 to 29 s). A conversion of 98.5 % of sunflower oil is reached at 375 °C and 25.0 MPa in 29 s. The resulting products are 16.3 % diglycerides, 27.9 % monoglycerides, 48.9 % fatty acids, and 5.3 % glycerol. On the other hand, only 13 % of the oil is hydrolyzed at 300 °C, 22.0 MPa, with a reaction time of 23.6 s. The main product at this condition was fatty acids with a yield of 3.7 %. Furthermore, a kinetic model was developed to obtain comprehensive kinetic parameters for both raw materials to evaluate their reactivity during hydrolysis. This study provides valuable insights into the hydrolysis kinetics and the most promising conditions to make valuable products like fatty acids, mono and diglycerides, and glycerol.

Numerical simulation of drag on local particle cluster within a high void fraction environment in supercritical water

The drag of particle cluster within a high void fraction environment in supercritical water (SCW) is investigated by numerical simulation. The results show that the description pattern of particle drag in SCW is the same as that in constant property flow (CPF). The diffusion of pressure in the phenomenon of fluid flow around particle cluster caused that the variation pattern of drag coefficient is contrary to the classical conclusion. The normalized drag coefficient is mainly depending on the thermal properties. The internal flow characteristics of the particle cluster further confirm the effects of fluid flow around it. After separating the effects of thermal properties, the exponent -β representing the effects of void fraction in SCW is similar to that in CPF. A correlation for calculating the particle drag in SCW is developed based on this result.

Prediction of Individual Gas Yields of Supercritical Water Gasification of Lignocellulosic Biomass by Machine Learning Models.

Supercritical water gasification (SCWG) of lignocellulosic biomass is a promising pathway for the production of hydrogen. However, SCWG is a complex thermochemical process, the modeling of which is challenging via conventional methodologies. Therefore, eight machine learning models (linear regression (LR), Gaussian process regression (GPR), artificial neural network (ANN), support vector machine (SVM), decision tree (DT), random forest (RF), extreme gradient boosting (XGB), and categorical boosting regressor (CatBoost)) with particle swarm optimization (PSO) and a genetic algorithm (GA) optimizer were developed and evaluated for prediction of H2, CO, CO2, and CH4 gas yields from SCWG of lignocellulosic biomass. A total of 12 input features of SCWG process conditions (temperature, time, concentration, pressure) and biomass properties (C, H, N, S, VM, moisture, ash, real feed) were utilized for the prediction of gas yields using 166 data points. Among machine learning models, boosting ensemble tree models such as XGB and CatBoost demonstrated the highest power for the prediction of gas yields. PSO-optimized XGB was the best performing model for H2 yield with a test R2 of 0.84 and PSO-optimized CatBoost was best for prediction of yields of CH4, CO, and CO2, with test R2 values of 0.83, 0.94, and 0.92, respectively. The effectiveness of the PSO optimizer in improving the prediction ability of the unoptimized machine learning model was higher compared to the GA optimizer for all gas yields. Feature analysis using Shapley additive explanation (SHAP) based on best performing models showed that (21.93%) temperature, (24.85%) C, (16.93%) ash, and (29.73%) C were the most dominant features for the prediction of H2, CH4, CO, and CO2 gas yields, respectively. Even though temperature was the most dominant feature, the cumulative feature importance of biomass characteristics variables (C, H, N, S, VM, moisture, ash, real feed) as a group was higher than that of the SCWG process condition variables (temperature, time, concentration, pressure) for the prediction of all gas yields. SHAP two-way analysis confirmed the strong interactive behavior of input features on the prediction of gas yields.

Hydrogen-rich syngas upgrading via CO2 adsorption by amine-functionalized Cu-BTC: the effect of different amines.

Syngas produced from supercritical water gasification typically contain a high amount of CO2 along with H2. In order to improve the quality of syngas, amine-functionalized copper benzene-1,3,5-tricarboxylate (Cu-BTC) was synthesized as an effective adsorbent for selective removal of CO2 from syngas to increase the concentration of H2. The amines used in this study included monoethanolamine (MEA), ethylenediamine (EDA), and polyethyleneimine (PEI). The fundamental physicochemical character of adsorbents, CO2 adsorption capacity, and CO2/H2 selectivity were analyzed. The physicochemical characterization indicated that the structure of amine-functionalized Cu-BTC was partially damaged, which resulted in a decrease in specific surface area and pore volume. On the other hand, the enlarged pore size was beneficial for the mass transfer of gas in the adsorbent. Among these adsorbents, Cu-BTC/PEI exhibited the maximum CO2 adsorption capacity of 3.83mmol/g and the highest CO2/H2 selectivity of 19.74. It was found that the adsorption pressure is the most significant factor for the CO2 adsorption capacity. Lower temperature and higher pressure were favored for CO2 adsorption capacity and CO2/H2 selectivity, so physical adsorption by Cu-BTC played a dominant role. Moreover, Cu-BTC/PEI can be well-regenerated with stable adsorption efficiency after five consecutive cycles. These findings suggested that Cu-BTC/PEI could be a promising alternative adsorbent for CO2 capture from syngas.

Hydrogen production from supercritical water gasification of lignosulfonate over CuO-ZnO: Sulfur transport and catalyst regeneration

In this study, sodium lignosulfonate was gasified in supercritical water over CuO-ZnO, and the gasification performance and sulfur transport and fate were investigated experimentally. The products and catalysts were characterized with GC-MS, IC, FTIR, XRD and XPS. It was found that the presence of CuO-ZnO improved the gasification efficiency, and the H2 yield peaked at 9.35 mol/kg with 1.5 g CuO-ZnO. The sulfur element was mainly distributed as H2S and CH3SH in the gas product owing to the reducing reaction atmosphere and SO42- in the liquid product. The presence of CuO-ZnO facilitated the conversion of sulfur from H2S and CH3SH in the gas product to SO42- in the liquid product. And the CuO component was converted into Cu2S, which fixed some sulfur in the solid residues. Three regenerating methods were tested for the sulfur-poisoned CuO-ZnO, and calcination showed superiorities over SCW and SubCW treatment. The calcination eliminated the sulfur and carbon deposited on the catalyst, and also converted Cu2S into CuO by oxidation. The calcined catalyst showed relatively high catalytic activity as the fresh catalyst. With further optimization, CuO-ZnO can be a promising catalyst for SCWG of sulfur-containing organic wastes.

Experiment evaluation and thermodynamic analysis of pulping modification on ordered conversion of bark in supercritical water

Major obstacles of supercritical water gasification (SCWG) of lignocellulosic biomass, including ineffective pulping for energy self-sufficiency and a high conversion barrier significantly inhibited its large-scale development. In this study, the pulping effect of alkaline and salt pulping for high concentration bark slurry were compared systematically. The SCWG performances of bark (non-catalytic/catalytic) and bark slurry were evaluated. Based on energy self-sufficiency, a novel SCWG process of 50 wt% bark slurry was designated with a multistage reactor configuration and pulping process. The findings suggest that alkaline pulping had a wider applicability (reaching 50 wt%) and better rheological properties than salt pulping. Only 30 wt% bark slurry was capable for applying salt pulping because of solid residue with the excessive salt addition. Alkaline pulping resulted in an ordered conversion of 5 wt% and 10 wt% bark slurries during SCWG, with complete gasification of 5 wt% bark slurries prepared by KOH and NaOH at 650 and 680 °C, respectively. Besides, alkaline and salt pulping reduced the 26 % and 33 % carbon emissions, respectively. In the novel process, the optimal ratio of preheated water to bark slurry was 4 (water diversion to the primary and secondary gasification reactors of 1:3), which exhibits in energy and exergy efficiencies of 47.47 % and 48.63 %, respectively. This configuration reduces exergy losses in the gasification reactor and heat exchanger, aiding in the large-scale industrial output of SCWG.

Thermodynamic and environmental analysis of integrated supercritical water gasification of sewage sludge for power and hydrogen production

Supercritical water gasification is a promising process for treating sewage sludge (SS) to obtain hydrogen and electrical energy. Here, a novel thermodynamic equilibrium model for supercritical water gasification of sewage sludge was developed and the thermodynamic and environmental effects of operating parameters on the system were investigated. The sensitivity analysis revealed that the optimum reaction condition for the system was: reaction temperature of 750 °C, preheated water to SS ratio of 1, and SS concentration of 25 %. Under this condition, the system energy efficiency and exergy efficiency were 44.74 % and 46.51 %, respectively, with H2 production of 103.19 kg/h and power generation of 2526.2 kW. The energy and exergy efficiencies could be improved by reducing the preheated water to SS ratio and increasing the reaction temperature and SS concentration. The heat exchangers and the heater were the key units causing exergy losses, accounting for 40.3 % and 19.5 % of the total exergy losses, respectively. When utilizing carbon capture and storage (CCS), the global warming potential (GWP) value was only 0.31 kg CO2-eq under the optimal operating parameters, which was 61.44 % less than without CCS.

Supercritical water depolymerization of black liquor, refining and comprehensive analysis of products including biopolyols

Black liquor (BL) has valuable lignin and its derived compounds, which can be used for many purposes, such as polyol synthesis. Herein, we reveal polyols presence in depolymerized black liquor (DBL) and propose a fast and scalable approach to increase the yield of this fraction. Hydrothermal treatment (HTL) of black liquor (385°C, 26 MPa) was performed in a custom designed supercritical water (SCW) pilot-plant with rapid reaction times of around 0.4 s. Ash-free, total ethyl acetate extracted bio-oil yield was found 77% w/w. In addition to the contribution of the detailed fractionation methods, this study highlights continuous operation (>1 hour), and short reaction times (∼0.4 s) of raw black liquor in SCW to produce biopolyols and aromatic bio-oil.

Investigating Salt Precipitation in Continuous Supercritical Water Gasification of Biomass

The formation of solid deposits in the process of supercritical water gasification (SCWG) is one of the main problems hindering the commercial application of the process. Seven experiments were conducted with the grass Reed Canary Grass with different preheating temperatures, but all ended early due to the formation of solid deposits (maximum operation of 3.8 h). The position of solid deposits in the lab plant changed with the variation in the temperature profile. Since the formation of solid deposits consisting of salts, coke, and corrosion products is a severe issue that needs to be resolved in order to enable long-time operation, inner temperature measurements were conducted to determine the temperature range that corresponds with the zone of solid formation. The temperature range was found to be 400 to 440 °C. Wherever this temperature was first reached solid deposits occurred in the system that led to blockage of the flow. Additional to the influence of the temperature, the influence of the flow direction (up-flow or down-flow) on the operation of the continuous SCWG plant was examined. If salts are not separated from the system sufficiently, up-flow reactors should be avoided because they amplify the accumulation of solid deposits leading to a shortened operation time. The heating concept coupled with the salt separation needs to be redesigned in order to separate the salts before entering the gasification reactors. Outside of the determined temperature zone no deposition was visible. Thus, even though the gasification efficiency was low it could be shown that the operation was limited to the deposits forming in the heating section and not by incomplete gasification in the reactor where T > 600 °C.

Effect analysis on recycling of cathode material from spent ternary lithium-ion batteries via supercritical water oxidation and acid-leaching

This study proposed a recycling method for spent batteries by combining supercritical water oxidation and acid-leaching. In supercritical water oxidation process, the cathode material was separated from the electrode plate, and 98% of lithium in the cathode was leached to achieve the separation of lithium from Ni, Co and Mn. Ni, Co and Mn were then leached during the acid leaching process. The effect of solid-liquid ratio, oxidant mass concentration, reaction time and temperature in supercritical process on metal leaching was analyzed. The leaching rate of Ni, Co and Mn was improved by increasing the mass concentration of oxidant and the reaction temperature. The effect of reaction time on metal leaching is related to the mass concentration of oxidant in the solution. Through the control of paraments, the leaching efficiencies of Ni, Co and Mn were significantly improved from 90.4%, 84.7%, 86.4–99.7%, 96.8% and 98.5%.

Mild-Temperature Supercritical Water Confined in Hydrophobic Metal-Organic Frameworks.

Fluids under extreme confinement show characteristics significantly different from those of their bulk counterpart. This work focuses on water confined within the complex cavities of highly hydrophobic metal-organic frameworks (MOFs) at high pressures. A combination of high-pressure intrusion-extrusion experiments with molecular dynamic simulations and synchrotron data reveals that supercritical transition for MOF-confined water takes place at a much lower temperature than in bulk water, ∼250 K below the reference values. This large shifting of the critical temperature (Tc) is attributed to the very large density of confined water vapor in the peculiar geometry and chemistry of the cavities of Cu2tebpz (tebpz = 3,3',5,5'-tetraethyl-4,4'-bipyrazolate) hydrophobic MOF. This is the first time the shift of Tc is investigated for water confined within highly hydrophobic nanoporous materials, which explains why such a large reduction of the critical temperature was never reported before, neither experimentally nor computationally.

Characterization of the oxide film formed on alumina-forming austenitic stainless steel in deaerated, DH, DO, and OT supercritical water

The effects of water chemistry (deaerated (DO < 10 ppb), dissolved hydrogen (DH 1.6 ppm), dissolved oxygen (DO 2 ppm), and oxygen treatment (OT 300 ppb O2 and 100 ppb NH3) on the corrosion behavior of alumina-forming austenitic stainless steel (AFAs) were systematically investigated. A dual-layer oxide scale composed of an inner Fe/Cr/Al oxide layer and an outer Fe-rich oxide layer was observed on the AFAs. Water chemistry was found to have a negligible effect on the inner Fe/Cr/Al oxide layer; however, the morphology of the outer Fe-rich oxide layer was found to depend significantly on the chemistry. No facetted and dense hematite, which effectively protected the alloy and led to lower weight gain (less oxidation) over prolonged exposure, was formed in the outer oxide layer in oxidizing environments (DO and OT). Conversely, columnar magnetite with a porous structure was formed in outer oxide layer in reducing environments (deaerated and DH) in the absence of oxygen, with a higher weight gain observed. The addition of NH3 increased the pH of the supercritical water (SCW), thereby promoting alumina dissolution and rendering the OT conditions undesirable for use in a supercritical water-cooled reactor (SCWR). This study provides valuable insight into the behavior of AFAs in SCW and highlights the importance of carefully controlling SCW chemistry to optimize material performance.

In situ visualization of salt crystallization in sub-/supercritical water environments

Elevated temperature and pressure conditions in sub-/supercritical water environments hinder the direct observation of salt crystallization phenomena, resulting in indeterminate salt crystal morphologies. In this study, an innovative experimental apparatus has been developed to conduct in situ investigations into the crystal morphology of various types of inorganic salts. The results indicate that the critical temperature of the water-salt system is higher than that of pure water systems, with phase transition thresholds for saturated NaCl-H2O and KCl-H2O systems 671.4 K and 728.1 K, respectively. A distinct “flash crystallization” phenomenon is observed during the crystallization of inorganic salts, accompanied by different crystal morphologies. KCl crystals exhibit a dispersed particle form, NaCl crystals manifest in a finely fragmented accumulation form, and Na2SO4 crystals present transparent feather-like structures. The size of NaCl crystals formed on the surface of a Ni wire at 622.8 K through heterogeneous nucleation ranges from 200.4 μm to 450.9 μm. The crystallization characteristics of salt in sub-/supercritical water are influenced by various factors, including electric dipole moment interactions, common ion effect, the effect of type I salts on type II salts, and homo/heterogeneous nucleation.