Subcritical Conditions Research Articles

Continuous hydrothermal processing of poly(ethylene terephthalate) (PET) under subcritical water conditions: A proof-of-principle closed-loop study

Plastics have undeniably played a crucial role in the technological and societal advancements over the past decades. However, the current linear consumption model and the reliance of the plastic industry on fossil carbon pose pressing environmental and economic challenges that cannot be ignored. In this context, poly(ethylene terephthalate) (PET) has emerged as one of the most studied plastics in the field of chemical recycling, given its extensive use in packaging and textiles, and its susceptibility to undergo solvolysis into its constituent monomers, which facilitates closed-loop recycling of PET waste. This study presents the validation of a first-of-its-kind continuous flow system that utilizes subcritical water for neutral hydrothermal processing of PET at three different reaction temperatures 250, 280 and 310 °C. Regardless of the processing conditions, PET underwent full conversion into monomers and low molecular weight esters like mono(2-hydroxyethyl) terephthalate (MHET) and bis(2-hydroxyethyl) terephthalate (BHET), with an approximate carbon distribution of 20 % and 80 % between aqueous and solid products, respectively. The product composition is strongly related to the processing conditions, with 77.5 % ethylene glycol and 94.2 % terephthalic acid recovered as monomers at 310 °C. Furthermore, solid products were repolymerized to assess the potential of reusing the heterogeneous precursor mixture as starting material for PET production. Results reveal that the solid products obtained at 250 °C yielded polymer chains with decomposition temperature, melting temperature, crystallinity, and average molecular weight (50.4 kDa) closest to commercial PET. These results underscore the potential of neutral hydrothermal processing in a future circular PET value chain.

Enhancing sustainable waste management: Hydrothermal carbonization of polyethylene terephthalate and polystyrene plastics for energy recovery

Hydrothermal carbonization (HTC) of single plastic polymers such as polyethylene terephthalate (PET) and polystyrene (PS) has not yet been explored on a large scale, particularly their thermal behavior, chemical transformations under subcritical conditions, and the energy properties of the resultant hydrochar. This study investigated these aspects by employing techniques, such as thermogravimetric analysis (TGA), Fourier transformed infrared spectroscopy (FTIR), elemental and calorific analysis. The results show that PET hydrochar has a superior energy densification (1.37) and energy yield (89 %) compared to PS hydrochar (1.13, 54 %). Hydrothermal carbonization modifies the chemical structure of the polymers by increasing the number of carbonyl groups (CO) in PET and forming new ones in PS, and by enhancing hydroxyl groups (OH) in PET while retaining them in PS. Both materials preserve their aromatic and aliphatic structures, with the introduction of alkenes groups (CC) in the PET hydrochar. PET hydrochar begins to decompose at lower temperatures (150–270 °C) than PS hydrochar (242–283 °C) but reaches higher peak temperatures (420–585 °C vs. 390–470 °C), with both types achieving similar burnout temperatures (650–800 °C). PET hydrochar recorded a higher activation energy (121–126 kJ/mol) than PS hydrochar (67–74 kJ/mol) with the Mampel first-order reaction model as the best fit.

Isomerization of aldo-disaccharides to keto-disaccharides in arginine solution and phosphate buffer under subcritical conditions

Maltose, cellobiose, and lactose dissolved in 0.01 mol/L arginine solution or 0.01 mol/L phosphate buffer (pH 7.0) at 0.2 mol/L were reacted batchwise at 110 °C. The yields of the corresponding keto-disaccharides were 23.4% and 17.6% for maltose, 19.3% and 13.0% for cellobiose, and 20.0% and 13.7% for lactose in arginine solution at 30 min and phosphate buffer at 150 min, respectively. All the disaccharides were hydrolyzed in the range of 0.5 to 3.5%, the degree of hydrolysis being greater in arginine solution than in phosphate buffer. The monosaccharides produced by hydrolysis were slightly isomerized to other monosaccharides. A decrease in pH and an increase in absorbance at 280 and 420 nm continued even at the almost constant yields of the main products. The browning was particularly pronounced in the latter half of the reaction, suggesting a change of the by-products to substances with high absorption coefficient.

Underwater half-space analysis to oblique 3D seismic waves based on exact free-field response

The seismic behavior of underwater half-space under arbitrary 3D incidence of P- or SV-waves is analyzed by the 2.5D finite/infinite element method. Firstly, the 3D responses of solid and liquid in free field are newly derived exactly, which cover both the sub- and super-critical conditions. Then, the equivalent seismic forces acting on the solid boundary of the 2.5D finite/infinite element method are merely computed from the free-field displacement for given earthquakes. Subsequently, the equation of motions for the underwater half-space under arbitrary 3D incident seismic waves are established in the 2.5D finite/infinite element system, with the key parameters specified. The proposed method was verified against the previous 2D solutions and inversely computed free-field solutions in frequency domain. Major conclusions drawn from the present analysis include: (1) The effects of liquid, concerning enlargement, reduction or disturbance, on the seabed responses become stronger for increasing liquid depth under sub-critical condition; (2) The seabed interior responses are less sensitive to the variation in liquid depth under super-critical condition; and (3) Under the super- and sub-critical conditions, respectively, more attention should be given to the interior response for P-waves and to the seabed surface response for SV-waves.

CO2-enhanced PET depolymerization by catalyst free methanolysis

Depolymerization shows promise as a strategy for converting waste plastic into monomers that can be used for repolymerization. However, catalytic approaches would be more complex and expensive due to catalyst separation, recovery, and recycling. Here, we present a CO2-enhanced subcritical methanolysis method without catalyst that can generate monomer from poly(ethylene terephthalate)(PET). The synergistic effect of CO2 on the subcritical methanolysis process for PET depolymerization was investigated including the CO2 initial pressure, reaction temperature and reaction time. Under subcritical conditions, the optimal reaction parameters with PET conversion ratio of 100% and DMT yield of 79.1% are as follows: temperature of 220℃, methanol to PET mass ratio of 6:1 and reaction time of 50 min. Moreover, through the analysis of experimental data, it was concluded that the depolymerization reaction follows a first-order kinetics, with the reaction rate being 2–3 times faster than the traditional process. However, the apparent activation energy for both reactions is found to be similar, with values of 135.1 kJ·mol−1 and 146.2 kJ·mol−1, respectively.

Catalytic Conversion of Oil Shale over Fe or Ni Catalysts under Sub-Critical Water

Sub-critical water is an environment-friendly solvent. It is widely used for the extraction of various organic compounds. It can be used to dissolve and transport organic matter in oil shale. In this study, the conversion of oil shale was synergistically catalyzed by the addition of Fe or Ni to the Fe inherent in samples under sub-critical water conditions. Oil shale can be converted to gas, oil and residues of oil. Thermogravimetric (TG) analysis results presented that the weight loss of raw oil shale was up to 15.85%. After sub-critical water extraction, the weight loss rate of the residues was reduced to 8.41%. With the application of a metal catalyst, Fe or Ni, the weight loss of residues was further reduced to 7.43% and 6.57%, respectively. According to DTG curves, it was found that there were two weight-loss rate peaks. The decomposition process of kerogen in oil shale could be divided into two cracking processes. One is decomposed at a high velocity at around 420 °C, and another is decomposed at a low velocity at around 515 °C. Gas chromatography (GC) results of gas products indicated that Fe or Ni could contribute to producing normal alkanes, such as methane, ethane, propane, etc., which are produced by the hydrogenation of alkenes via hydrogen transfer during the conversion process of kerogen. Gas chromatography-mass spectrometry (GC–MS) was conducted to analyze the components of the liquid products. The results showed that n-alkanes, iso-alkane, oxygenated hydrocarbons and aromatic compounds were the major components of the kerogen cracking products. When Ni was introduced as a catalyst, the contents of aromatic compounds and oxygenated hydrocarbons in the liquid products were increased from 19.55% and 6.87% to 22.38% and 13.77%, respectively. This is due to the synergistic effect of the addition of Ni with the inherent Fe in oil shale under sub-critical water which ensures kerogen is more easily cracked to produce aromatic compounds and oxygenated hydrocarbons.

Tsunami Debris Damming Drag Forces And Associated Coefficients On Elevated Coastal Structure Columns

Tsunami overland flow induces hydrodynamic loads on coastal structures and may transport various objects located within the inundation zone, which could become debris and exacerbate hydrodynamic loading. In the process of adopting “the first national, consensus-based standard for tsunami resilience” (Chock, 2016) in the form of ASCE 7-16 Chapter 6: Tsunami Loads and Effects, emphasis was placed on evaluating debris transport and impact forces. This is evidenced by the robust body of literature regarding physical model experiments of these processes and thorough design procedures for both load considerations in current structural engineering standards (ASCE, 2022). Debris damming forces, resultant of debris being transported and accumulating against structures, are less thoroughly studied, having only recently begun to transition from steady flow experiments to transient flow conditions representative of coastal inundation events. A recent pair of experiments bridges this gap, comparing debris damming via steady-state, subcritical flow conditions to that caused by a dam-break style hydraulic bore (Stolle et al., 2018). That paper aimed to study debris dam formation, stability, and loading as well as runup of the flow onto idealized structural columns. Another study varied debris quantity, orientation, and arrangement to determine the effect had on damming and impact loads (Shekhar et al., 2020), however neither compared findings to current standards. The experimental work presented herein represents initial findings of a multi-year experimental campaign to better understand the mechanisms that lead to debris damming and increased structural loading. This work builds upon previous studies by using larger scale debris elements, more numerous debris fields, and more trials to better model such a stochastic process as debris damming. Three different incident wave conditions also led to varied hydrodynamics at the column specimen. In later phases, this campaign will also investigate the debris damming consequences of heterogeneous debris, which more accurately represent highly variable debris fields observed in post-event site surveys (Nistor et al., 2017). This paper aims to compare experimental debris dam loading parameters to those in the current ASCE 7-22 standard (ASCE, 2022). Namely, evaluating conservatism of ASCE 7-22 design values for: overall drag force on buildings, minimum closure ratios used in load determination, and empirical rectilinear structure drag coefficients during both debris accumulation and quasi-steady debris damming phases.

Experimental investigation of the supercritical heat transfer of the low GWP refrigerant R1234ze(E): calibration and validation of test rig

Heat transfer of supercritical refrigerants is applied in practical systems such as the transcritical organic Rankine cycle or supercritical heat pump. The concept of supercritical heat transfer is not new and has already been studied and applied for decades in several applications. However, results on one fluid cannot be directly translated to another one. This is because the thermohydraulic behavior of fluids is heavily influenced in the near-supercritical region due to sudden variations in the thermophysical properties when the fluid is heated or cooled close to its critical point. For refrigerants under horizontal flow, the research is limited. Most studies in literature are on small diameter tubes and on R134a, which has a high global warming potential. Therefore, an experimental test rig has been built to measure the supercritical heat transfer of low global warming potential refrigerants over a large temperature and pressure range in the supercritical region. In this work, the calibration and validation of this test rig is presented using the refrigerant R1234ze(E). The test section consists of a 4 m long horizontal tube with an inner diameter of 22.9 mm, equipped with thermocouples at both the tube wall and in the bulk of the flow, and is heated by resistance wires wrapped around the tube. For the calibration, the temperature profiles under both non-heating and heating conditions are evaluated. For the validation, the average Nusselt numbers obtained through experiments under subcritical conditions are compared to the average Nusselt numbers calculated based on the Dittus-Boelter and Gnielinski correlation.

Optimal charge amount for semiconductor chiller applying eco-friendly refrigerant R-744

The refrigerants used to cool the heat generated in semiconductor production are Freons, which are now subject to regulation because of their high ozone depletion potential (ODP) and global warming potential (GWP). Therefore, R-744 is considered an eco-friendly refrigerant. This study experimentally analyzed the effect of the refrigerant charge amount based on the temperature discharged after cooling objects when operating a brine chiller using R-744 under subcritical conditions. The results showed that the optimal charge amount in terms of the coefficients of performance (COP) differed from that in terms of the maximum cooling capacity. In case of the apparatus used in this study, the optimal refrigerant charge amount with maximum cooling capacity was estimated at 8.2 kg when the temperature of the cooling object was in the range of −20 °C and −10 °C and at 9.2 kg when the temperature of the cooling object was 0 °C. However, in terms of COP, the optimal refrigerant charge amount was estimated at 8.2 kg when the temperature of the cooling object was in the range of −20 °C and 0 °C. This difference was attributed to the variation in the required evaporation heat capacity with the target temperature of the cooling object.

Experimental investigation of the total flow resistance in emergent and submerged rigid canopy flows

In canopy flows, flow resistance mainly originates from vegetation drag and depends on vegetation characteristics and flow conditions. In the present study, a series of experiments were performed in various hydraulic scenarios with high stem Reynolds numbers (2641 ≤ Red≤ 17333) using relatively sparse rigid canopies, represented with four different dimensionless vegetation densities (0.0044, 0.0098, 0.0174 and 0.0392), on a smooth bed. A novel drag plate mechanism was developed to measure the total flow resistance due to the emergent and submerged vegetation arrays in a staggered pattern under subcritical flow conditions. Manning’s roughness coefficient and Darcy–Weisbach friction factor were adopted to represent the total flow resistance in the analyses. Simple empirical relationships based on roughness concentration and submergence ratio were derived to determine the total flow resistance parameters within a broad range of stem Reynolds numbers. Although relationships were proposed in a simple form to be used for direct practical applications, they show similar or better performance in the prediction of total flow resistance parameters than the existing equations in the literature, which require considerable computational effort. Additionally, analyses demonstrated that the results of the present study and those of similar studies regarding canopy flow resistance are in good agreement. Accordingly, the novel drag plate looks promising for measuring flow resistance due to vegetation and bed conditions similar to those in nature.

Experimental study of Cryogenic jet injection using centrifugal nozzles at supercritical pressure

The new generation of large-thrust rockets commonly employ cryogenic liquids such as liquid hydrogen (LH2), liquid oxygen (LOX), and methane as propellants. Conventional analytical methods established under subcritical (specifically, low-pressure) conditions are inadequate for addressing the atomization issues under trans-critical and supercritical conditions. Therefore, conducting experimental research on atomization of cryogenic fluids under supercritical conditions holds significant practical value. This paper employs liquid nitrogen (LN2) as the working fluid and focuses on conducting injection atomization experiments on LN2 under supercritical pressure conditions. The research involves the design and construction of an experimental system for supercritical cryogenic fluid injection, with the aim of measuring and studying the spray characteristics of centrifugal nozzles under various conditions. Three major categories of experiments were designed for low-temperature LN2 injection: subcritical nitrogen into subcritical environment (BB), subcritical nitrogen into supercritical environment (BP), and supercritical nitrogen into supercritical environment (PP). The experimental results ultimately reveal that for jets in BB, both the spray cone angle and the breakup length increase with higher injection pressure drop. In the case of BP, the spray cone angle decreases while the penetration distance increases with higher pressure drop. As for the jets in PP, the spray cone angle increases and the penetration distance decreases with higher injection pressure drop. Across different geometric parameters of the centrifugal nozzle, it was observed that with an increase in the geometric parameter, the rate of decrease in the spray cone angle with respect to pressure drop amplifies, and the penetration distance exhibits an increasing trend as the geometric parameter grows.

Study of the chemical components of CO2 extracts from the fruits of Sorbus aucuparia L.

The article presents the results of the study of the chemical composition of Sorbus aucuparia L. СО2 subcritical extract. For the first time in Kazakhstan, 20 grams of brown Sorbus aucuparia L. extract were collected using subcritical carbon dioxide extraction. The current study was directed mainly to the chemical compositions of subcritical Sorbus aucuparia L. СО2 subcritical extract. The Sorbus aucuparia L. extract's chemical compositions were determined using gas chromatography/mass spectrophotometry (GC-MS). The extract included the following main compounds: 5-Methyl-2(3H)-furanone (30.18 %), 5-(3-Ethoxy-4,5-dihydro-isoxazol-5-yl)-5-methyl-imidazolidine-2,4-dione (3.20 %), 4H-Pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6-methyl (2.53 %). Determined fatty acid profile and the moisture content of raw vegetable ingredients of Sorbus substance. Quantitative determination of fatty acids of ethanol extract was carried out. The results of the analysis for fatty acids in the study showed that linoleic (37.7 %) and oleic (50.5 %) were the most prominent fatty acids. The aim of this study is to determine the component composition by using the GC-MS method and fatty acid to study the Sorbus aucuparia L. extract obtained by CO2 extraction, which grows in Kazakhstan. Materials and methods. To determine the possibility of using Sorbus aucuparia L., we carried out the composition and fatty acid of the extract obtained by CO2 extraction in subcritical conditions of Sorbus aucuparia L. by a certain GC-MS method. Results. The raw materials of the plant were collected in accordance with GACP requirements. Conducted subcritical CO2 extraction of plant raw materials showed a 20 g extraction yield. Chemical compounds were discovered, bioactive components were identified, such as 5-Methyl-2(3H)-furanone (30.18 %), 5-(3-Ethoxy-4,5-dihydro-isoxazol-5-yl)-5-methyl-imidazolidine-2,4-dione (3.20 %), 4H-Pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6-methyl (2.53 %). Conclusions. The possibility of using the obtained CO2 extract of Sorbus aucuparia L. in the field of pharmaceutical products as a substance and a drug

Experimental Study of the Effect of the Initial Droplet Diameter on the Evaporation Characteristics of Unsymmetrical Dimethylhydrazine Droplets in a Subcritical Environment

The evaporation characteristics of unsymmetrical dimethylhydrazine droplets with different initial diameters in a subcritical environment were experimentally investigated with the temperature–pressure separation technique. The evaporation processes of unsymmetrical dimethylhydrazine droplets with different initial diameters in this environment have the same general pattern. All the studied droplets exhibit a short transient heating phase and a steady-state evaporation phase obeying d2. Notably, the expansion of the transient heating phase gradually increases with increasing ambient pressure. The change in diameter squared ∆d2max increases from 1.03% at 1 MPa to 12.48% at 5 MPa. Under subcritical conditions, the evaporation rate decreases linearly with decreasing droplet diameter, and the droplet evaporation lifetime increases linearly. Changes in the initial droplet diameter may still have a large effect on droplets smaller than those studied here. When the ambient pressure is not greater than 3 MPa, the change in the steady-state evaporation time for both medium- and large-diameter droplets accounts for more than 70% of the variation in the droplet evaporation lifetime. As the ambient pressure increases to 4 MPa and 5 MPa, the percentage of the change in the transient heating time contributing to the variation in the droplet evaporation lifetime gradually increases to more than 45%.

Equations and methodologies of inlet drainage system discharge coefficients: A review

Abstract Accurate determination of grate inlet discharge coefficients is crucial in reducing modeling uncertainties and mitigating urban flooding hazards. This review critically examines the methods, equations, and recommendations for determining the weir/orifice discharge coefficients, based on the inlet parameters and flow conditions. Reviewing previous studies for inlets showed that the discharge coefficient of rectangular inlets under subcritical flow ranges from 0.53 to 0.6 for weirs and from 0.4 to 0.46 for orifices, while in grated circular inlets, it falls between 0.115 and 0.372 for weirs and between 0.349 and 2.038 for orifices. For circular non-grated inlets under subcritical flow, the weir and orifice coefficients are in the range of 0.493–0.587 and 0.159–0.174, respectively. However, the orifice discharge coefficients of grated and non-grated inlets with unknown Froude number range between 0.14–0.39 and 0.677–0.82, respectively. For supercritical flow, the weir and orifice discharge coefficients of grated and non-grated rectangular inlets are 0.03–0.47 and 1.67–2.68, respectively. Previous studies showed that it is recommended to correlate the discharge coefficients with the approaching flow and Froude number under subcritical and supercritical flows, respectively. Yet, additional studies are recommended for a better understanding of the limits and parameters governing the flow transitional stage between weir and orifice and between subcritical and supercritical conditions. Moreover, further research is required to determine the weir and orifice discharge coefficients of circular inlets under supercritical flow as well as the orifice discharge coefficient range of rectangular non-grated inlets under subcritical flow. Finally, it is recommended to increase the road surface roughness to reduce Froude number, and thereby, increase discharge coefficients of street inlets. The aim of this review is to help inlet designers and authorities promote sustainable cities with resilient urban drainage systems and reduce the environmental, economic, health, and social impacts of urban drainage failure.

Numerical study on the critical characteristics of internal short-circuit hot spot in lithium-ion batteries

High current density resulting from internal short circuit (ISC) in lithium-ion batteries leads to rapid local temperature rise, forming a hot spot. This study quantitatively characterized the critical properties of ISC hot spot in batteries. ISC hot spot in batteries exhibit two different evolution patterns. When the short circuit resistance approaches the internal resistance of the battery, the hot spot temperature reaches its peak at 1165 K. The thermal runaway characteristics of batteries vary depending on the short circuit resistance, with the hot spot growing and spreading rapidly when the resistance value is 30 mΩ. In addition, a modeling study was conducted to examine the critical characteristics of short circuit resistance. At a hot spot radius of 2 mm, the critical resistance was determined to be 281 mΩ, accompanied by a critical hot spot temperature of 562 K. Under subcritical conditions, the SEI film decomposes completely, while the electrolyte undergoes minimal decomposition. Interestingly, it was observed that the critical resistance and critical hot spot temperature decrease as the hot spot radius increases. Finally, the study quantified the effects of hot spot thermal conductivity, convective heat transfer coefficient, and hot spot position on the critical threshold of hot spots. These findings provide valuable references for the design of batteries with high critical thresholds.

Experimental investigation of three-dimensional flow dynamics in a laboratory-scale meandering channel under subcritical flow condition

This study investigates the three-dimensional turbulence flow properties in a meandering channel under subcritical flow conditions, specifically focusing on a few critical parameters such as three-dimensional velocity behaviour, Turbulent kinetic energy, skewness, and kurtosis. A model study has been attempted by considering the same sinuosity, Froude number, and bed material from an Indian peninsular river to understand the basic hydrodynamics of a meandering channel. Understanding these critical parameters is crucial for comprehending the complex flow behaviour and its implications in meandering channels. Experimental measurements were conducted to collect data on Velocity, Turbulent Kinetic energy, skewness, and kurtosis in all three directions. The objective was to analyse their variations and behaviour along the meandering path, providing insights into the flow dynamics and turbulence characteristics. Advanced measurement techniques, such as Acoustic Doppler Velocimeters (ADV), were utilized to obtain accurate and detailed data. The study revealed distinct patterns and trends in the velocity, Turbulent Kinetic energy, skewness, and kurtosis along the flow path of a sand bed meandering channel. These parameters were found to vary significantly in different sections, highlighting the influence of channel geometry and flow conditions. Graphical representations of the flow profiles were employed to visualize the spatial distribution and understand the variations in these turbulence properties. The findings of this study enhance our understanding of the three-dimensional turbulence flow properties in sand bed meandering channels under subcritical flow conditions. The results offer valuable information for designing hydraulic structures, flood control measures, and river restoration projects.

Experimental investigation of a new CO2 refrigeration system arrangement for supermarket applications

A new position of an internal heat exchanger within the thermodynamic cycle is proposed to improve the performance of CO2 refrigeration systems in warm climates, specifically in supermarket applications. The proposed configuration uses the saturated vapor from the liquid receiver to recover the heat that is rejected by a subcooler positioned after the gas cooler. This system results in a reduced evaporator superheat. This configuration is compared with a reference system.The experimental study is conducted on a CO2 laboratory cooling system reproducing a commercial plant. The cooling capacity of the system is 30 kW. The water inlet temperature in the gas cooler (hot source) varies from 15 °C to 35 °C, while the mono-ethylene glycol outlet temperature of the evaporator (cold source) is fixed at -8 °C. The setup is validated through an energy balance.Using a subcooler in this new position improves the coefficient of performance by 10.2 % under transcritical conditions and 6.3 % under subcritical conditions when compared with the reference system.

Critical mass phenomenon for a parabolic–elliptic multispecies chemotaxis system in a two‐dimensional disk

A parabolic–elliptic Keller–Segel model for multipopulations in a two‐dimensional unit ball will be considered in this paper. For the initial boundary value problem with mutually attractive populations, the global existence of bounded solutions has been proved through the logarithmic Hardy–Littlewood–Sobolev inequality for system when the initial masses satisfy the subcritical condition. Moreover, based on the moments technique, there exists a blow‐up solution when the initial masses satisfy the supercritical case. In a word, a critical mass phenomenon has been established in this multispecies chemotaxis system.

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.

Highly efficient and reusable nanostructured porous Ni/La2O2CO3 tandem catalyst for hydrogen and methane production from subcritical hydrothermal treatment of nitrocellulose

Chemical conversion of biomass into fuel gas under subcritical water conditions is a crucial step towards a sustainable and carbon–neutral route for energy storage. However, generation of highly selective gas, such as hydrogen and methane, remains a challenge under such conditions. We present here a reusable tandem catalyst, porous nanostructured nickel-exsolved lanthanum oxycarbonate (Ni/La2O2CO3) for nitrocellulose gasification. Hydrogen (H2) and methane (CH4) production achieves 45.5 % (13.6 mmol) and 9.4 % (2.8 mmol) selectivity, respectively, at 350 ℃, which overwhelms the activity of Raney nickel (41 %,10 mmol and 2.8 %, 0.7 mmol for H2 and CH4, respectively). The study of temperature-dependent structure and the electron paramagnetic resonance spectroscopy further elucidated the origin of its high catalytic efficacy together with the phase change of the catalyst. The reused catalyst after regeneration retains 55 % of its initial activity after five consecutive tests. This study highlights the Ni/La2O2CO3 catalyst that holds significant potential for selective fuel gas generation from various biomass sources.