Supercritical Carbon Dioxide Research Articles

Hydrogels and aerogels of cellulose nanofiber derived from barley straw with addition of chitosan using high-intensity ultrasound and supercritical CO2 drying

In this study, cellulose-rich residue (purity of 90.2%) removed from barley straw was used to obtain cellulose nanofiber hydrogels (1–2 wt%) by high-intensity ultrasound treatments at 1200 W for 10–30 min. The highest water retention value (82.9±0.8 g/g) and fibrillation yield (62.12±0.13 wt%) of hydrogel were obtained by 1 and 2 wt% cellulose nanofibers, respectively, after ultrasound treatment at 1200 W for 20 min. Also, chitosan (1–3 wt%) was added to the cellulose nanofiber hydrogels by ultrasound treatment at 1200 W/20 min. The addition of chitosan reduced the water retention value and fibrillation yield. All hydrogels exhibited a typical elastic gel-like behavior. Then, the hydrogels were dried using supercritical CO2 to form aerogels, and their properties (bulk density, porosity, stiffness, functional groups, morphology, and thermal behavior) were determined. The lowest bulk density (0.056±0.011 g/cm3) and the highest porosity (96.50±0.17%) was achieved using 1 wt% cellulose nanofiber. The addition of chitosan at 3 wt% increased aerogel stiffness to 3200.10±8.80 g/mm.

Influence of Supercritical CO2 Fracturing Fluid on the Permeability of Shale Reservoir and Mechanism Analysis

Influence of Supercritical CO₂ Fracturing Fluid on the Permeability of Shale Reservoir and Mechanism Analysis

Supercritical CO2-Based Extraction and Detection of Phenolic Compounds and Saponins from the Leaves of Three Medicago varia Mart. Varieties by Tandem Mass Spectrometry

A comparative metabolomic study of three varieties of alfalfa (Medicago varia Mart.) was performed via extraction with supercritical carbon dioxide modified with ethanol (EtOH) and the detection of bioactive compounds via tandem mass spectrometry. Several experimental conditions were investigated in the pressure range of 50–250 bar, with ethanol used as a co-solvent in an amount of 1% of the total volume in the liquid phase at a temperature in the range of 31–70 °C. The most effective extraction conditions were as follows: a pressure of 250 Bar and a temperature of 60 °C for M. varia. M. varia contains various phenolic compounds and sulfated polyphenols with valuable biological activity. Tandem mass spectrometry (HPLC-ESI–ion trap) was applied to detect the target analytes. A total of 103 bioactive compounds (59 polyphenols and 44 compounds belonging to other chemical groups) were tentatively identified in extracts from aerial parts of alfalfa. For the first time, twenty-one chemical constituents from the polyphenol group (flavones: Formononetin, Chrysoeriol, Cirsimaritin, Cirsiliol, Cirsilineol, tricin-O-hexoside, Apigenin C-glucose C-deoxyhexoside, Apigenin 7-O-diglucuronide, 2′-Hydroxygenistein 4′,7-O-diglucoside, etc.) and six from other chemical groups (saponins: Soyasaponin II, Soyasaponin gamma g, Soyasaponin I, Soyasaponin Bd, Soyaysaponin beta g, etc.) were identified in the aerial parts of M. varia.

Microcellular long-chain branched polyphenylene oxide (PPO) with excellent comprehensive properties and comprehensive analysis of its dielectric properties

High-frequency and high-speed communications impose stringent requirements on low-dielectric materials with exceptional comprehensive properties. The development of dielectric materials with low dielectric constants and low dielectric dissipation factors holds significant value in reducing interconnect delay, power consumption, and crosstalk in high-frequency communication. In this study, polyphenylene oxide (PPO), known for its excellent properties, was chosen as the base material. Modified PPO with a long-chain branched structure was prepared using the multi-epoxy modifier ADR-4468. The increase in molecular weight and the formation of a long-chain branched structure improved the thermal stability, flexural properties, and foamability of PPO. Introduction of low-dielectric air into the material was achieved through supercritical CO2 foaming, resulting in a series of microcellular PPO foams exhibiting low dielectric properties (with a dielectric constant as low as 1.12 and a dielectric dissipation factor as low as 0.000636). Concurrently, the dielectric properties of microcellular PPO foams were examined at 10 GHz, revealing that the foam’s dielectric properties were minimally impacted by the properties of the raw materials. Based on water absorption analysis, an improved equation for calculating the dielectric dissipation factor was proposed, comprehensively analyzing the influencing factors on the foam’s dielectric dissipation factor. The study unveiled that water absorption significantly affects the foam’s dielectric dissipation factor, and under high expansion ratios, water absorption’s impact on the dielectric dissipation factor exceeds that of air introduction. These microcellular PPO foams, possessing excellent comprehensive properties and low dielectric characteristics, hold promise for applications in the realm of high-frequency and high-speed communications.

Optimization and 4E analysis of integration of waste heat recovery and exhaust gas recirculation for a marine diesel engine to reduce NOx emissions and improve efficiency

This paper proposes an integration of waste heat recovery system and exhaust gas recirculation system to break the inherent trade-offs between NOx and carbon emissions of marine low-speed diesel engine. It is significant important to break these trade-offs, because Energy Efficiency Existing Ship Index may not be met, though Energy Efficiency Design Index and Tier III can be already achieved for old ships. The waste heat recovery system consists of supercritical carbon dioxide Brayton cycle and Kalina cycle, where the integration of supercritical carbon dioxide Brayton cycle and Kalina cycle is achieved in the parallel style. This paper uses the classical thermodynamics in terms of engineering approach to carry out the parameter analysis of combined system first. The optimization is conducted in terms of efficiency, energy density and techno-economy with the introduction of global optimal policy. The results showed that the highest efficiency of 52.32% at Tier III mode can be achieved, at which the net power, exergy efficiency, area per unit power output and levelized energy cost can be achieved by 1568.57 kW, 45.75%, 0.589 m2/kW and 0.114$/kWh at main engine load of 85%. Further, the net power output of waste heat recovery system can reach 1085.76 kW and 2043 kW at Tier II mode and Tier III mode, while the recovering rate of waste heat recovery system can reach over 5% at Tier III mode. Finally, the Energy Efficiency Existing Ship Index of a container ship equipping with such combined system can be reduced from 9.50 to 9.05.

β-phase formation of poly(vinylidene fluoride) foam based on the porous morphology control via supercritical carbon dioxide

The increase of electroactive β phase of poly(vinylidene fluoride) (PVDF) is important in energy harvesting applications. In this work, a green method of supercritical CO2 physical foaming was applied to obtain the PVDF foams with high content of β-crystal by utilizing the biaxial stretching when the cells grew, which were applied in the field of piezoelectric sensors. By changing the foaming condition, the PVDF foams with various structures were obtained, and a comprehensive study on the crystal transformation of them was carried out. The PVDF foam with increased β crystals content was more accessible to obtain at a high saturation pressure, corresponding to the large expansion ratio and small cell size. The acquisition of β crystals mainly came from the α-β solid-solid phase transformation during decompression, rather than the annealing process. Due to the introduction of cell structure, the piezoelectric output performance of the sample was effectively amplified. The produced PVDF foam with the β-crystal content of 30%, exhibited the low density of 0.11 g/cm3 and the highest piezoelectric output of 4.1 V (corresponding to a specific voltage of 35 V‧cm3/g). This foaming strategy provides guidance for lightweight design and high-performance fabrication of piezoelectric materials.

Highly efficient hydrolysis of cellulose to sugars using supercritical CO2 as a green acid catalyst and solvent

Rapid depolymerization of cellulose into processable monomers (e.g., sugars) using solid acid catalysts is an important step for cost-effective biofuel and biochemical production, but has not yet been achieved due to the limited contact between solid cellulose and solid catalysts. Herein, the unique roles of supercritical CO2 (i.e., scCO2) as an in-situ acid catalyst and reaction solvent in achieving the ultra-fast full solid catalytic hydrolysis of cellulose are disclosed for the first time. When the ball-milling pretreated cellulose was hydrolyzed using oxidized carbon catalysts at 150 °C and 100–300 bar-CO2, the hydrolysis kinetics remarkably increased by 3× for conversion and 5× for glucose, resulting in ∼90% conversion and ∼85% total sugar selectivity at 20 min. The hydrolysis rate obtained with scCO2 here was higher than conventional ones with toxic and unrecyclable homogeneous catalysts (e.g., HCl) under harsh reaction conditions (i.e., 180–220 °C and pH of 1–2). A comprehensive reaction engineering study (e.g., temperature, CO2 pressure, stirring speed, catalyst acid properties) combined with the estimation of the solution pH by the CO2 phase equilibrium model and the in-situ and ex-situ monitoring of the phase behavior of the H2O/scCO2 solution were conducted to quantify the activity promotion by scCO2 and understand the acid-solvent roles of scCO2 toward the enhanced hydrolysis of cellulose. Specifically, the formation of the Pickering emulsions at the interface between scCO2 and water and their impact on the enhancement of the cellulose-carbon contact were proposed and verified in detail.

A novel design method for NiCr coating against supercritical CO2 via the ReaxFF molecular dynamics theoretical analysis and experimental verification

In order to design a NiCr coating with the strongest corrosion resistance, the corrosion behavior of Ni, Ni10Cr and Ni20Cr coatings was studied using molecular dynamics, in which Ni20Cr coating exhibited the best corrosion resistance. The corrosion behavior of Fe9Cr heat-resistant steel with or without Ni20Cr coating in a supercritical CO2 at 650 ℃ and 15 MPa for 1000 h was carried out by corrosion test, which significantly reduced the experimental period and cost. The Ni20Cr coating was intact and uniform after exposure for 1000 h. A dense and continuous Cr-rich oxide film with low growth stress could rapidly formed on the Ni20Cr coating. The Ni20Cr coating could effectively enhance the corrosion and carburization resistance of Fe9Cr heat-resistant steel.

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2

The effect of surface grinding on the stress corrosion cracking initiation of 316LN stainless steel in 600 °C supercritical CO2 was studied through constant extension rate tensile test. The ground surface exhibited much higher oxidation resistance than the polished surface as the deformation layer can greatly enhance the diffusion of active elements and promote the formation of more protective oxide scale. Moreover, the intergranular cracking was effectively suppressed on the ground surface as the ingress of oxidant along the original grain boundary was inhibited.

Multistage microcellular waterborne polyurethane composite with optionally low-reflection behavior for ultra-efficient electromagnetic interference shielding

Electromagnetic interference (EMI) shielding materials with superior shielding efficiency and low-reflection properties hold promising potential for utilization across electronic components, precision instruments, and fifth-generation communication equipment. In this study, multistage microcellular waterborne polyurethane (WPU) composites were constructed via gradient induction, layer-by-layer casting, and supercritical carbon dioxide foaming. The gradient-structured WPU/iron-cobalt loaded reduced graphene oxide (FeCo@rGO) foam serves as an impedance-matched absorption layer, while the highly conductive WPU/silver loaded glass microspheres (Ag@GM) layer is employed as a reflection layer. Thanks to the incorporation of an asymmetric structure, as well as the introduction of gradient and porous configurations, the composite foam demonstrates excellent conductivity, outstanding EMI SE (74.9 dB), and minimal reflection characteristics (35.28 %) in 8.2–12.4 GHz, implying that more than 99.99999 % of electromagnetic (EM) waves were blocked and only 35.28 % were reflected to the external environment. Interestingly, the reflectivity of the composite foam is reduced to 0.41 % at 10.88 GHz due to the resonance for incident and reflected EM waves. Beyond that, the composite foam is characterized by low density (0.47 g/cm3) and great stability of EMI shielding properties. This work offers a viable approach for crafting lightweight, highly shielding, and minimally reflective EMI shielding composites.

Development of polyamide foam by supercritical CO2: Low density, anti-shrinkage and thermal insulation

Saving energy and reducing environmental harmful emissions have become a global concern, so the foam thermal insulation materials are of great significance for saving energy, reducing greenhouse effect and ensuring the comfort and safety requirements of buildings. Polyamide12 (PA12) has outstanding weather resistance, mechanical stability and termite resistance, which is a promising building material. Herein, we prepared a series of PA12 foams by using supercritical carbon dioxide (Sc-CO2) as foaming agent, polymethyl methacrylate (PMMA) as a blend modification material, tridehydroglycerol isocyanurate (TGIC) as chain extender improved the cells wall strength and stability. When the PA12 and PMMA blending ratio was 70:30 and TGIC content was 0.2 phr, the foam density was 0.02 g/cm3, and the thermal conductivity of the foam was as low as 0.0302 W/m·K. Moreover, the influence of viscosity gradually increased with the increase of PMMA as shown by tanδ; at the same time, as an amorphous material, PMMA enlarged the melting range of the blends and reduced the enthalpy of melting. Therefore, the introduction of PMMA increased the temperature range of the foaming window to170–190 °C. This study provided an effective method for the design of foams with low density, low shrinkage, excellent mechanical properties and high thermal insulation properties, which has broad PA12 foam application prospects in the fields of buildings, such as wall or roof insulation.

Preparation of tomato peel pomace powder/polylactic acid foams under supercritical CO2 conditions: Improvements in cell structure and foaming behavior

Polylactic acid (PLA) is an eco-friendly material that can help address the problems of petroleum depletion and pollution. Blending renewable biomass materials with PLA to create composite foams with a tunable pore structure, superior performance, and low cost is a green technique for improving the pore structure and mechanical characteristics of single PLA foams. PLA/TP composites were created using melted tomato peel pomace powder (TP), which has a lamellar structure, as a reinforcing agent. Then, the relationship between the vesicle structure, morphology, and properties of the PLA/TP composite foams produced through supercritical CO2 intermittent foaming were investigated. The findings revealed that TP considerably enhanced the rheological characteristics and crystalline behavior of PLA. The PLA/TP composite foam had a better cell structure, compression characteristics, and wettability than pure PLA. The expansion ratio of the PLA/TP composite could reach 18.8, and its thermal conductivity decreased from 174.2 mW/m·K at 100 °C to 57.8 mW/m·K at 120 °C. Furthermore, annealing before foaming decreased the average composite foam blister size from 110.09 to 66.53 μm, and the annealing process also improved compression performance. This study contributes to solving environmental difficulties and creating PLA foams with controlled bubble structures, uniform bubble sizes, and outstanding overall performance.

Comparative study of different layouts in the closed-Brayton-cycle-based segmented cooling thermal management system for scramjets

The segmented cooling thermal management system based on the closed-Brayton-cycle (CBC) is an excellent technology for thermal protection and power generation of scramjets. In this paper, a comparative study is conducted to investigate the system performance under different supercritical carbon dioxide CBC layouts. A zero-dimensional thermodynamic model was established to analyze the effects of compressor inlet temperature (T1), turbine inlet temperature (T3), and compressor outlet pressure (p2) on fuel mass flow rate for cooling (mf) and net power (Pnet). A quasi-one-dimensional flow and heat transfer model was developed to study the cooling effectiveness. Results indicate that higher T3 and p2 reduce mf while increasing Pnet in the simple layout (SL) and simple recuperated layout (SRL). However, in the recompressing layout (RCL), the effects of T1, T3, and p2 on mf are more complex due to the split ratio. Moreover, RCL has the lowest maximum wall temperature among the three layouts. Finally, after optimization, RCL has the lowest mf about 0.332 kg/s, which is a 19.8 % reduction compared to the regenerative cooling system. Correspondingly, RCL achieves the highest Pnet, reaching 109.89 kW. In general, the recompressing layout is most suitable for the CBC-based segmented cooling thermal management system for scramjets.

Simulation and comprehensive study of a new trigeneration process combined with a gas turbine cycle, involving transcritical and supercritical CO2 power cycles and Goswami cycle

Simulation and comprehensive study of a new trigeneration process combined with a gas turbine cycle, involving transcritical and supercritical CO2 power cycles and Goswami cycle

Numerical simulation study on enhanced efficiency of carbon dioxide geological storage with nanoparticles in deep saline aquifer

Geological storage of carbon dioxide (CO2) is a promising technique to reduce large-scale greenhouse gas emissions. There are some candidates for geological storage formation while deep saline aquifers have attracted tremendous attention because they have the largest potential of all possible subsurface storage sites. CO2 injected into deep saline aquifers migrates upward until it reaches a sealing formation due to its buoyancy effect, which causes poor storage efficiency. Over the past few years, some novel research has been published to utilize nanoparticles for CO2 geological storage. The addition of nanoparticles to injecting super-critical CO2 (scCO2) changes its physical properties, such as the density and viscosity, which could mitigate gravity segregation and viscous fingering. However, the studies related to these techniques are still extremely limited, and the potential advantages or their impacts are poorly understood. In this paper, we investigated the effects of the physical properties of scCO2 containing nanoparticles on storage efficiency by using a versatile commercial simulator (CMG-GEM). In particular, the effects of nanoparticle types (aluminum oxide, silicon dioxide, titanium oxide, and zinc oxide) and concentration on CO2 storage efficiency were presented quantitatively by considering possible operation scenarios. The results showed the migration of CO2 to the upper part of the aquifer was suppressed, and the storage of CO2 in the lower part of the aquifer was promoted when scCO2 was injected with nanoparticles. It also showed some effects in delaying breakthrough time and mitigating breakthrough amount when we assumed the pressure control well. It was indicated that the cumulative breakthrough amount after 30 years was reduced by 8.2% in the minimum case and by 79.3% in the maximum case. Although the proposed method still has economic challenges, the innovation in the production processes of nanoparticles or combining with added value will lead to improving feasibility.

The study on mechanism of water–rock reaction in shale oil formation rich in alkaline minerals in Mahu Sag by using supercritical CO2

The Mahu Sag of Junggar Basin, China is rich in alkaline minerals. Generally, the water–rock reactions between hydraulic fluid and alkaline minerals during the hydraulic fracturing process will lead to the precipitation of minerals and block the pore or wellbore, thereby affecting the recovery efficiency. Supercritical carbon dioxide (ScCO2) regarded as a fracturing fluid can significantly improve the recovery of shale oil resources, but there is limited research on the reaction of ScCO2 to water and rock in shale samples rich in alkaline minerals. Therefore, ScCO2 treatment experiments were conducted on shale samples rich in alkaline minerals under 80°C and 10 MPa. Before the experiments, X-ray powder diffraction (XRD) was applied to characterise the mineral properties of the three shale samples. In addition, the duration time of ScCO2 treatment was set at 7 and 20 days to observe the treatment time impacts on the pore structure. Scanning electron microscopy (SEM) analysis and the Mann–Whitney U test were conducted before and after ScCO2 treatment. SEM images show the changes in pore structure. Opening pores are observed due to the interaction between the alkaline mineral and ScCO2. The Mann–Whitney U test shows the changes in fracture toughness. The changes in pore structure also play an important role in fracture toughness. This study investigated the ScCO2 impacts on shale samples rich in alkaline minerals and the analysis of changes in pore structure and fracture toughness can provide theoretical and data evidence to enable on-site optimisation of the ScCO2 fracturing scheme.

Supercritical CO2 extraction increases the recovery levels of omega-3 fatty acids in Tetraselmis chuii extracts

Tetraselmis chuii is a microalgae marketed as ingredient meeting the acceptance criteria for novel foods established by the European Union and can be an important source of healthy fatty acids (FA). The aim of this research was to characterize the FA profile of T. chuii fractions obtained by supercritical carbon dioxide (SCCO2) extraction operating with two sequential co-solvents and to evaluate the effect of biomass pretreatment (freeze/thaw cycles followed by ultrasounds). T. chuii biomass was confirmed to be an important source of omega-3 FA, mainly due to the abundance of α-linolenic acid, and pre-treatment significantly improved the lipid yield. Other omega-3 FA, such as 16:3, 16:4, 18:4, 18:5, 20:3 and 20:5, were also detected in different proportions. When SCCO2 extraction of pretreated and un-pretreated T. chuii was compared with conventional solvent extraction, the nutritional quality indices of the extracts were improved by the use of SCCO2.

Supercritical CO2 extraction of Walnut (Juglans regia L.) oil: Extraction kinetics and solubility determination

Experimental and modelling investigations of supercritical CO2 extraction of oil from Juglans regia L. kernels were conducted at 200 and 400 bar, 313 and 333 K at a CO2 flow rate of 0.1 kg/h. Regardless of the pressure and the temperature, the highest achievable yield was estimated at about 0.7 kgoil / kgbiomass. The extraction kinetics were modeled with Sovová’s broken and intact cells model. Walnut oil apparent solubility in supercritical CO2 was determined and modelled with the Chrastil equation. A retrograde solubility behaviour was observed at 200 bar, the faster extraction kinetics were found at 400 bar and 333 K. The fatty acid composition of the supercritical CO2 extracted oil was compared to the quality of the oil extracted by press and n-hexane Soxhlet extraction.

A recompression supercritical CO2 cycle using heat source storage to achieve low-frequency reactor regulation: Analysis and optimization of load capacity under safe compressor operation

This study focuses on proposing a heat source storage mode for the indirect recompression supercritical CO2 cycle (RSCC) coupled with a sodium-cooled fast reactor, aiming to achieve low-frequency regulation of the reactor and enhance the load capacity of system. A deep neural network-based turbomachinery performance prediction approach is employed to assess the system's part-load and overload performance under several typical control strategies. The turbine inlet temperature of the system under all control strategies except the intermediate heat exchanger (IHX) bypass rises with decreasing load. The system's part-load performance is positively correlated with its corresponding overload power. The findings reveal that the adjustable load ranges of the heat source storage tanks under inventory, turbine bypass, turbine throttle, and IHX bypass controls are 10%–156.10%, 10%–156.10%, 30%–147.30%, and 10%–146.88%, respectively, along with the safe compressor operation. As the near-choke operation of the compressor limits the rise in overload power, a turbine fitted with variable inlet guide vanes is tried. The optimization results demonstrate that the largest overload power of the system can reach 161.18 % without reducing the thermal efficiency. This study provides a heuristic insight into the variable load operation for indirect nuclear-powered Brayton cycles under low-frequency reactor regulation.

Obtaining Bixin- and Tocotrienol-Rich Extracts from Peruvian Annatto Seeds Using Supercritical CO2 Extraction: Experimental and Economic Evaluation.

Currently, Bixa orellana L. extracts are used as a color source in the food, pharmaceutical, and cosmetic industries because they are important as a potential source of antioxidant activity. The extraction is carried out by conventional methods, using alkaline solutions or organic solvents. These extraction methods do not take advantage of the lipid fraction of annatto (Bixa orellana L.) seeds, and the process is not friendly to the environment. In this work, the objective was to obtain an extract rich in nutraceuticals (bixin and tocols) of high antioxidant power from Peruvian annatto seeds as a potential source for a functional food or additive in the industry using supercritical fluid extraction (SFE). Experiments related to extraction yield, bixin, tocotrienols, tocopherols, and antioxidant activity were carried out. The SFE was performed at 40 °C, 50 °C, and 60 °C, and 100, 150, and 250 bar with 0.256 kg/h carbon dioxide as the supercritical solvent (solvent-to-feed ratio of 10.2). Supercritical extraction at 60 °C and 250 bar presented the best results in terms of global extraction yield of 1.40 ± 0.01 g/100 g d.b., extract concentration of 0.564 ± 0.005 g bixin/g extract, 307.8 mg α-tocotrienol/g extract, 39.2 mg β-tocotrienol/g extract, 2 mg γ-tocopherol/g extract, and IC50 of 989.96 μg extract/mL. Economical evaluation showed that 60 °C, 250 bar, and 45 min presented the lowest cost of manufacturing (2 × 2000 L, COM of USD 212.39/kg extract). This extract is a potential source for functional food production.