scCO2 Conditions Research Articles

Upgrading vacuum residue in a supercritical CO2 environment with polypropylene and steam: Insights into products distribution and characterization of liquid products

The pyrolysis of vacuum residue (VR) using polypropylene (PP) under supercritical carbon dioxide (scCO2) conditions represents a novel approach to converting this abundant and difficult-to-process feedstock into lighter liquid products. In this paper, a set of experiments was performed to analyze the impact of scCO2, PP and steam on the yields of pyrolysis products (i.e., liquid, gas and coke) and characteristics of the liquid fractions. The findings indicated notable improvements when scCO2 was used instead of subcritical CO2 in the pyrolysis of VR with PP. These included decreased viscosity and improved API gravity, alongside increased maltene yield and reduced coke yield. Furthermore, the inclusion of PP markedly elevated the quality of the obtained liquid, whereas steam + scCO2 primarily affected the product distribution by raising the yield of coke and gas fractions. VR pyrolysis under scCO2 conditions at 380 °C, 8 MPa, PP/VR ratio of 0.23 for 60 min in a batch system, without the use of catalyst, resulted in the highest liquid yield of 83.7 ± 1.9 wt%, and minimum coke yield of 9.8 ± 1.3 wt%. Under these conditions, the upgraded liquid consisted of 75.2 wt% light and middle distillates with boiling points below 350 °C. Additionally, the proportion of oxygenates in the pyrolysis oil reduced by 60.5 %, improving its heating value. Moreover, adding PP to the upgrading process resulted in a higher proportion of isoparaffins, cycloparaffins, and aromatics, coupled with a decrease in olefins, aligning the liquid specifications more closely with fuel standards.

Enhancing Carbon Nanotube Yarns via Infiltration Filling with Polyacrylonitrile in Supercritical Carbon Dioxide.

Carbon nanotube (CNT) fibers are renowned for their exceptional axial tensile strength and modulus. However, in yarn form, they frequently encounter transverse loading in practical applications, which exposes their suboptimal mechanical attributes rooted in inadequate inter-tube interactions and yarn surface defects. Efforts to mitigate micro-slippage among CNTs have encompassed gap-filling methodologies with varied materials, yet the outcomes have fallen short of expectations. This work aimed to enhance the mechanical properties of CNT yarns via infiltration with polyacrylonitrile (PAN) under supercritical carbon dioxide (sc-CO2) conditions. PAN was strategically chosen for its capability to undergo pre-oxidation and subsequent carbonization, leading to robust graphitic reinforcement. Leveraging sc-CO2's swelling and high permeability properties, the infiltration process effectively plugged interstitial spaces, elevating the yarn's tensile strength to 277.50 MPa and Young's modulus to 5094.05 MPa. Additional enhancements were realized after pre-oxidation, conferring a dense, reinforced shell structure that augmented tensile strength by 96.93% and Young's modulus by 298.80%. Scanning electron microscopy (SEM) analyses revealed a homogeneous PAN distribution within the yarn matrix, corroborated by X-ray photoelectron spectroscopy (XPS) evidence of C-N bonding, indicative of a successfully interlaced network. Consequently, this investigation introduces a novel strategy to tackle micro-slippage in CNT yarns, thereby achieving substantial improvements in their mechanical resilience.

Impact of supercritical carbon dioxide on the frictional strength of and the transport through thin cracks in shale

Understanding the mechanical and transport behavior of thin (i.e. small aperture) cracks slipping under supercritical carbon dioxide (sc-CO2) conditions is essential to evaluate the integrity of sealing formations with buoyant sc-CO2 below and the success of waterless fracturing. The two major items of interest in this work are frictional strength and permeability change of the crack. We used a triaxial cell that permits in situ visualization to conduct and monitor slippage along the faces of narrow cracks subjected to triaxial stresses. Such cracks are analogs to small geological faults. We tested carbonate-rich, 1-inch diameter Wolfcamp shale samples that are saw cut 30° to vertical to create a thin crack. Friction coefficients ranged from about 0.6 to 0.8 consistent with expectations for brittle rocks. The sc-CO2 generally did not alter friction coefficient over the time scale of experiments. From a transport perspective, saturating cracks with sc-CO2 substantially decreased permeability of the crack by 26%–52%, while slip resulted in a variety of permeability responses. Overall, the combined impact of sc-CO2 saturation and slip reduced fault permeability for all tests. Our observations support the notion that the sealing capacity of some caprocks improves when saturated with sc-CO2 and that some slip of small fractures is not necessarily detrimental to caprock integrity.

Influence of supercritical carbon dioxide (scCO2) curing on carbonation and strength development of brucite recovered from desalination reject brine

Brucite is a potentially sustainable alternative to reactive magnesium oxide (MgO) cement (RMC) with no calcination requirement. This study investigated the effect of elevated CO2 curing concentrations on CO2 sequestration, the formation of carbonate phases, and the compressive strength of compacted pellets of brucite nanopowders synthesized from reject brine. It was revealed that the compressive strength of the compacted brucite pellets with optimum water-to-binder ratio (w/b) enhanced by 45% within 4 h under supercritical CO2 (scCO2) conditions compared to the maximum strength observed under 20% CO2 and 80% relative humidity (RH) at 30 ºC conditions. The scCO2 curing enabled the formation of a high-density nesquehonite phase primarily responsible for the strength development of the brucite pellets. In addition, the CO2 sequestration in the scCO2-cured brucite pellets reached up to 66% of their maximum storage capacity, indicating their excellent CO2 absorption capability.

Influence of Supercritical Carbon Dioxide on the Activity and Conformational Changes of α-Amylase, Lipase, and Peroxidase in the Solid State Using White Wheat Flour as an Example.

Green technologies using renewable and alternative sources, including supercritical carbon dioxide (sc-CO2), are becoming a priority for researchers in a variety of fields, including the control of enzyme activity which, among other applications, is extremely important in the food industry. Namely, extending shelf life of e.g., flour could be reached by tuning the present enzymes activity. In this study, the effect of different sc-CO2 conditions such as temperature (35-50 °C), pressure (200 bar and 300 bar), and exposure time (1-6 h) on the inactivation and structural changes of α-amylase, lipase, and horseradish peroxidase (POD) from white wheat flour and native enzymes was investigated. The total protein (TPC) content and residual activities of the enzymes were determined by standard spectrophotometric methods, while the changes in the secondary structures of the enzymes were determined by circular dichroism spectrometry (CD). The present work is therefore concerned for the first time with the study of the stability and structural changes of the enzyme molecules dominant in white wheat flour under sc-CO2 conditions at different pressures and temperatures. In addition, the changes in aggregation or dissociation of the enzyme molecules were investigated based on the changes in particle size distribution and ζ-potential. The results of the activity assays showed a decrease in the activity of native POD and lipase under optimal exposure conditions (6 h and 50 °C; and 1 h and 50 °C) by 22% and 16%, respectively. In contrast, no significant changes were observed in α-amylase activity. Consequently, analysis of the CD spectra of POD and lipase confirmed a significant effect on secondary structure damage (changes in α-helix, β-sheet, and β-turn content), whereas the secondary structure of α-amylase retained its original configuration. Moreover, the changes in particle size distribution and ζ-potential showed a significant effect of sc-CO2 treatment on the aggregation and dissociation of the selected enzymes. The results of this study confirm that sc-CO2 technology can be effectively used as an environmentally friendly technology to control the activity of major flour enzymes by altering their structures.

Continuous flow extraction of biodiesel produced in a packed-bed reactor using supercritical carbon dioxide and tetrahydrofuran as solvents

The concurrent process of biodiesel (FAME) extraction and triglyceride methanolysis was carried out by using supercritical carbon dioxide (SC–CO2) and tetrahydrofuran (THF). This study explored the integration of SC-CO2 extraction facilitated catalyst-free transesterification efficiently at low temperatures with practical biodiesel productivity. The work started with studying the transesterification kinetics of triglycerides using different molar ratios of methanol under various SC-CO2 conditions in the presence of THF solvent. The elementary second-order kinetics associated with the mass transfer limiting factor satisfied the experimental results from all different mixing ratios. A four-factorial Box-Behnken Design (BBD) coupled with the ANOVA was employed to optimize the FAME recovery by the reaction-extraction processes with continuous two streams of SC-CO2 and 10% v/v THF-methanol mixture. The optimum predicted FAME recovery was 95.47% w/w at 71.93 °C, 186.67 bar using the substrate ratio 12.16. The supercritical fluid-assisted dispersion feature was characterized through the Peclect number (Pe) obtained from the volumetric dispersion model (VDM). The Pe could indicate suitable substrate mixing for the process operation and design to enhance production efficiency. The feasibility of upscaling the process to the commercial level was proposed with preliminary economic consideration, which could suggest process improvements in further research and technological development.

Solid-state 13C NMR spectroscopic study of supercritical CO2 catalyzation treated polyethylene terephthalate textiles for platinum metallization

In this study, polyethylene terephthalate (PET) textiles were metallized with Pt via supercritical carbon dioxide (sc-CO2) catalyzation, using palladium(II) hexafluoroacetylacetonate (Pd(hfac)2) as the Pd catalyst source. The effects of the treatments on the structural changes and molecular mobility of PET were elucidated by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. The carbonyl and nonprotonated aromatic carbons and the methylene and protonated aromatic carbons were analyzed from the NMR spectra using a three-component model (crystalline, rigid amorphous, and mobile amorphous components), which inferred that the PET polymer textile comprised one crystalline and two amorphous regions. Additionally, when Pd catalyzation was performed on the textile under sc-CO2 conditions, the crystalline peak of the carbonyl carbon exhibited a chemical shift into a higher magnetic field, and the spin–lattice relaxation time increased. These results implied that Pd could exist in the vicinity of the CO carbon. The X-ray diffraction studies also showed crystallinity of PET increased by sc-CO2 catalyzation, while sc-CO2 annealing without Pd(hfac)2 did not increase it. Therefore, Pt films were successfully coated on the PET via catalytic plating using sc-CO2. The catalyzation reduced the molecular mobility of the PET polymer, indicating the incorporation of the catalytic metal into the polymer chain. The carbonyl carbon peak simultaneously shifted to a higher field, implying that the catalytic Pd could approach the carbonyl groups. The detailed analysis of the structure and molecular mobility provides valuable information that can contribute to the development of novel conductive materials.

In Situ Attenuated Total Reflection Infrared Spectroscopic Monitoring of Supercritical CO2 Extraction for Green Process Applications

Supercritical CO2 (scCO2) extraction of valuable chemicals from food, biomass, and residues thereof has recently been recognized as a sustainable process. In this study, we present a new design of an attenuated total reflection infrared (ATR-IR) spectroscopic cell for monitoring the extraction of fatty acid from almonds under scCO2 conditions. The newly designed ATR-IR cell allows in situ monitoring of changes of the composition of the almonds during the scCO2 extraction process at a pressure of up to 450 bar. Extracted components can be spectroscopically followed at a time resolution of 30 s. This allows fast and facile optimization of scCO2 conditions such as temperature, pressure, and extraction time.

Mixed-Layer Illite-Smectite Illitization under Supercritical CO2 Conditions

The long-term safe storage of CO2 in geological reservoirs requires the understanding of the impact of CO2 on clay-rich sealing cap rocks. The reactivity of the mixed layer of illite-smectite was investigated to determine the reaction pathways under conditions of supercritical CO2 (scCO2) conditions in the context of geological CO2 storage. A common clay (blue marl from the Guadalquivir Tertiary basin, southern Spain) was tested under brine scCO2 conditions (100 bar and 35 °C) for 120 and 240 h. The clay sample (blue marl) contains calcite, quartz, illite, smectite, and the corresponding mixed-layer and kaolinite. X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses were performed. The illitization of mixed-layer illite-smectite was observed by XRD and confirmed by a variation in the content of different elements (K, Mg, Na, Ca, and Fe) of the transformation, as well as an increase in the specific surface (SSA) of the clay (36.1 to 38.1 m2/g by N2, 14.5 to 15.4 m2/g by CO2 adsorption). Furthermore, these reactions lead to mineral dissolution and secondary mineral formation along the CO2–water–clay intercalations of the source rock were responsible for a change in porosity (7.8 to 7.0 nm pore size). The implications of illitisation, mineral destruction, and precipitation processes on CO2 storage and clay layer integrity should be explored before deciding on a geological storage location.

Effects of Supercritical Carbon Dioxide Extraction (SC-CO2) on the Content of Triterpenoids in the Extracts from Ganoderma Lucidum

This research investigates the effect of SC-CO2 extraction parameters on the triterpenoid recovery from G. lucidum. The SC-CO2 parameters included extraction pressure, extraction temperature, and extraction time. The extraction pressure was varied between 200, 400 and 600 bar, the extraction temperature between 30, 55 and 80 °C, and the extraction time between 30, 75 and 120 min. In this study, the SC-CO2 parameters were first optimized using a response surface methodology (RSM) for maximum triterpenoid recovery. The results showed that the optimal RSM-based SC-CO2 conditions were 430 bar extraction pressure, 54.8 °C extraction temperature and 78.90 min extraction time, achieving the maximum triterpenoid recovery of 1.56 mg/100g. The kinetic behavior of SC-CO2 process was subsequently characterized using a second-order kinetic model under variable extraction pressures and extraction temperatures, given a SC-CO2 time interval. The second-order kinetic models represented well the experimental results of triterpenoid extraction by SC-CO2 method. At these conditions, the triterpenoid extract also exhibited strong scavenging activities with IC50 values of 0.49 mg/mL for the DPPH radical scavenging activity and 0.26 mg/mL for ABTS radical scavenging activity. Thus, triterpenoids extracted from G. lucidum could be regarded as a potential agent for medicinal treatment. The results also suggest that the SC-CO2 extraction can be a useful extraction method for triterpenoid extraction from G. lucidum.

Machinability improvement of compacted graphite irons in milling process with supercritical CO2-based MQL

Compacted graphite iron (CGI) has become one of the most potential and promising materials for diesel engines contributing to automotive industry applications because of its outstanding mechanical properties over flake graphite iron (FGI). However, the poor machinability of CGI has hindered it to be widely employed in potential automotive industry. In this paper, a supercritical CO2-based MQL technique was introduced to improve the machinability of CGI. The machinability of CGI was systematically evaluated considering the cutting force, cutting temperature, surface integrity and tool wear under various cutting parameters (cutting speed, feed rate, depth of cut) and machining atmospheres (i.e., dry, supercritical carbon dioxide (ScCO2), supercritical CO2-based minimum quantity lubrication with oil-on-water droplets cutting fluid (ScCO2-MQL)) compared to FGI and nodular graphite iron (NGI) in the milling process. The results show that among three machining atmospheres, the lowest values regarding cutting force, cutting temperature, and surface roughness values are obtained under the ScCO2-MQL atmosphere. Furthermore, the tool wear mechanisms were revealed to be abrasion wear and adhesion wear. The ScCO2-MQL exhibited excellent superiority in cutting force and temperature, surface integrity, and tool life compared to dry and ScCO2 conditions due to its outstanding cooling and lubricant effects. The results demonstrated that using the advanced ScCO2-MQL technique can substantially improve the machinability of the compact graphite cast irons.

Carbonation Reaction Mechanisms of Portlandite Predicted from Enhanced Ab Initio Molecular Dynamics Simulations

Geological carbon capture and sequestration (CCS) is a promising technology for curbing the global warming crisis by reduction of the overall carbon footprint. Degradation of cement wellbore casings due to carbonation reactions in the underground CO2 storage environment is one of the central issues in assessing the long-term success of the CCS operations. However, the complexity of hydrated cement coupled with extreme subsurface environmental conditions makes it difficult to understand the carbonation reaction mechanisms leading to the loss of well integrity. In this work, we use biased ab initio molecular dynamics (AIMD) simulations to explore the reactivity of supercritical CO2 with the basal and edge surfaces of a model hydrated cement phase—portlandite—in dry scCO2 and water-rich conditions. Our simulations show that in dry scCO2 conditions, the undercoordinated edge surfaces of portlandite experience a fast barrierless reaction with CO2, while the fully hydroxylated basal surfaces suppress the formation of carbonate ions, resulting in a higher reactivity barrier. We deduce that the rate-limiting step in scCO2 conditions is the formation of the surface carbonate barrier which controls the diffusion of CO2 through the layer. The presence of water hinders direct interaction of CO2 with portlandite as H2O molecules form well-structured surface layers. In the water-rich environment, CO2 undergoes a concerted reaction with H2O and surface hydroxyl groups to form bicarbonate complexes. We relate the variation of the free-energy barriers in the formation of the bicarbonate complexes to the structure of the water layer at the interface which is, in turn, dictated by the surface chemistry and the degree of nanoconfinement.

The Influence of Supercritical Carbon Dioxide on Graham Flour Enzyme Polyphenol Oxidase Activity.

Graham flour is a form of whole wheat flour made by grinding the endosperm and is thus also the most nutritious. Generally, the enzyme polyphenol oxidase (PPO) catalyzes two different reactions in the presence of molecular oxygen: the hydroxylation of monophenols to ortho-diphenol and the oxidation of o-diphenol to o-quinone. The purpose of the work was to inactivate PPO activity to extend the shelf life of graham flour and at the same time preserve all the of its high-quality properties. The influence of supercritical CO2 (scCO2) treatment on PPO activity in graham flour was investigated. First, graham flour was exposed to scCO2 conditions, then the proteins were extracted, and in the last step the concentration of total proteins and the specific activity of the PPO enzyme were determined by spectrophotometric assay. PPO activity decreased with an increase in treatment pressure. Furthermore, the flour quality characteristics that meet all needs for wheat end-use products after scCO2 treatment have been preserved. No major changes in the structure of the granulate or shape of the flour particles were observed. A slightly reduced value of the moisture content in scCO2-treated graham flour also implies an extension of the shelf life.

Optimization of supercritical carbon dioxide extraction of fat and cholesterol from beef floss by response surface methodology

Supercritical carbon dioxide (Sc-CO2) was applied to extract fat and cholesterol from beef floss (BF). A response surface methodology (RSM) based on central composite design (CCD) was employed to optimize the extraction conditions of temperature (30 - 62°C), pressure (7 - 35 MPa), and extraction time (0 - 40 mins). The optimum conditions were estimated to be at 51.0°C and 32.8 MPa for a duration of 32.7 mins. Under such conditions, the percentage of fat and cholesterol reduction plus lightness of Sc-CO2 treated BF (STBF) were 81.12%, 86.17%, and 57.60, respectively. There were no significant differences (p>0.05) between experimental and predicted values, indicating the adequacy of the well-fitting models. Furthermore, the protein and ash content of STBF increased significantly (p<0.05) as a result of the extraction. This study indicated that RSM-CCD can be potentially employed in maximizing the extraction of fat and cholesterol from BF under mild Sc-CO2 conditions.

Process Intensification for Enzyme Assisted Turmeric Starch Hydrolysis at Hydrotropic and Supercritical Condition

The turmeric rhizome (Curcuma longa) contains curcuminoids embedded in the starch matrix. It is important to target starch removal by hydrolysis to enhance the rate and extent of extraction of curcuminoids. Enzymes are green catalysts and their activity can be enhanced by working on certain parameters. In the case of starch hydrolysis, α- amylase is more efficient when the starch is in the gelatinized form than when it is in its natural form. α-Amylase activity improves under supercritical carbon dioxide (scCO2) conditions. Certain surfactants improve α- amylase activity. Hydrotropes are a mild form of the surfactants. However, there are no reports on hydrolysis of turmeric starch in hydrotropic or supercritical (SFC) conditions. The work in this report shows beneficial effect of hydrotrope on the enzymatic reactions. There is marked enhancement in the diffusion coefficient (De) for curcuminoid extraction. The increased starch hydrolysis also improves the solid filtration rates after hydrotropic extraction of the curcuminoids. In the case of hydrotropic extraction of curcuminoids alone otherwise the filtration rate decreases due to the sticky nature of the extraction mass, caused by swelling of the starch. The present work includes hydrolysis of turmeric starch in its natural and gelatinized forms using α-amylase in hydrotrope solution (HS) and scCO2.The starch hydrolysis parameters include α-amylase concentration, solid loading concentration, scCO2 flow rate, presence of modifier in scCO2, residence time and scCO2 backpressure. The optimum hydrolysis parameters are 200 IU.cm-3 of α-amylase, at 328 K, 10 % (w/w) of turmeric loading concentration, 6.5 reaction pH, 900 rpm agitation speed for hydrotropic hydrolysis while the optimum conditions are 5 % w/w turmeric loading concentration, 40 min of scCO2 residence time, 15 cm3 of scCO2 phase modifier and 15 MPa as the scCO2 backpressure. The De for hydrolysis in the HS for turmeric starch in gelatinized form (A) was 4.1 x 10-11 m2s-1 and De for starch hydrolysis methods including in HS for turmeric starch in natural state (B), SFC condition for turmeric starch in natural state (C) and SFC for turmeric starch in gelatinized (D) were approximately 4.1 x 10-12 m2s-1. De for native turmeric starch in the absence of hydrotrope and SFC (E), and in case of gelatinized turmeric starch hydrolysis in absence of SFC and hydrotrope were 4.1x 10-14 and 4.1x 10-13 m2s-1, respectively. The filtration rates for (A), (D), (B), (C), (E) were 2 x 10-4, 1.25 x 10-11, 1.4 x 10-11, 7.3 x 10-11, 8.2 x 10-11 kgm2s-1 respectively. The De and filtration rate were noted in order (A) > (D) > (B) > (C) > (E) > (F). The 2.5 h SFC and 2.15 h HS enzyme treated turmeric powder gave 6.56 and 2.44 x 106 times increased filtration rate for curcuminoid extraction compared to untreated turmeric powder.

Desorption of propylene glycol monomethyl ether acetate from activated carbon in supercritical CO2: Measurement and predictive modeling

The desorption behavior of propylene glycol monomethyl ether acetate (PGMEA), which is a volatile organic compound (VOC) used in semiconductor manufacturing, from activated carbon (AC) was experimentally and theoretically studied over a wide range of supercritical carbon dioxide (scCO2) conditions at T=(313 K to 353 K) and P=(10.0 MPa to 20.0 MPa) for the design of AC regeneration processes using scCO2. The experimental results reveal that the desorption ratio of PGMEA depended on the density of CO2, and is affected by its chemical structure and diffusivity of the VOC. A kinetic model based on the material balances of the VOC in the bulk phase and adsorption equilibria in the adsorbed phase described by the Dubinin–Astakhov equation was newly proposed to predict the desorption behavior. The model successfully predicted the desorption behavior using parameters determined from adsorption equilibrium and kinetic measurements of PGMEA on AC in scCO2.

Can supercritical carbon dioxide be suitable for the green pretreatment of plant fibres dedicated to composite applications?

This work explores the use of supercritical carbon dioxide (sc-CO2) conditions as an innovative and environmentally friendly treatment of plant fibres to optimize their performance for integration into composite materials. This study evaluates, in particular, the influence of this treatment on the mechanical, thermal, hygroscopic properties and biochemical features of industrial hemp bast fibres. Two distinct settings were tested by tuning time, temperature and pressure parameters to assess the influence of the severity of the treatment on the fibre quality. Results show that sc-CO2 treatment induces an increase in the fibre fineness and a decrease in their moisture sensitivity while maintaining their initial resistance to temperature. These changes are consistent with the measured decrease in the relative content of hemicelluloses. A significant decrease in the tensile rigidity and strength is also observed as a function of the severity of sc-CO2 treatment, counterbalancing a little bit the benefits retained on the other properties.

Enzyme-assisted supercritical fluid extraction of antioxidant isorhamnetin conjugates from Opuntia ficus-indica (L.) Mill

A one-pot enzymatic assisted supercritical carbon dioxide (SC-CO2) extraction was used to obtain isorhamnetin conjugates from Opuntia ficus-indica (L.) Mill (nopal). Box-Benhken design, discriminate analysis and principal component analysis were used to studt the influence of pressure, temperature, aqueous ethanol solution and time on the total extraction yield and the isorhamnetin conjugates recovery using the enzymes Rapidase Maxi Fruit (Rapidase) and Viscozyme. Isorhamnetin tri- and diglycoside were found in the extracts along with other phenolic compounds such as 3-O-methyl quercetin, 3-O-methyl kaempferol and luteolin. The isorhamnetin and phenolic profile patterns were dependent on the conditions used. Cellular antioxidant activity was positively correlated to isorhamnetin-3-O-glucosyl-rhamnoside and luteolin content. The best results were obtained by using Rapidase, a currently used enzyme for the winemaking industry. The use of enzymes under SC-CO2 conditions affected the release of isorhamnetin conjugate profiles which induced remarkable differences in the cellular antioxidant activity.

Thermal effects on hydrogen storage capacity of Pd/MWCNT nanoparticles deposited at moderate pressure and temperature supercritical carbon dioxide conditions

Pd nanoparticles were deposited on MWCNT support for the first time at 80 ℃ and under 2500 psi scCO2 conditions via a bipyridyl precursor in the absence co-solvent. After deposition, the samples were heated under different atmospheres. Then their morphological and hydrogen uptake capacities were investigated. An average particle size between 7−9 nm was estimated via HR-TEM images while intense Pd(111) and Pd(200) sites were detected for all heated samples except no thermally treated one which exhibits PdHx sites. The highest hydrogen adsorption at room temperature was recorded for the samples heated under oxygen. At room temperature, Pd loading amount was found to be effective in means of captured hydrogen. Hence, at higher temperatures, hydrogen uptake of samples treated under nitrogen atmosphere was higher. Additionally, in case of the sample which was not thermally treated, a signal referring to hydrogen release between 200−300 °C during reduction was detected.

The effect of supercritical CO2 on the permeation of dissolved water through PDMS membranes

Water vapor permeation under supercritical carbon dioxide (scCO2) conditions through dense polydimethylsiloxane (PDMS) was investigated up to pressure of 185 bars to evaluate the regenerability of scCO2 as desiccant to dehydrate fresh products that are prone to product deterioration during conventional drying. This study experimentally examined the impact of concentration polarization on the H2O vapor permeation through dense PDMS membranes in the presence of sub- and supercritical CO2. The results were compared to a system containing N2 instead of CO2.For the CO2 system, the residual mass transfer resistance, which excludes the membrane layer resistance, decreased down to zero with increasing feed pressure, at 90 bar. This is the result of the convergence of the H2O contents of the feed bulk and permeate, which leads to a change of the main H2O transport mechanism within the feed boundary layer from diffusion to convection. Here the H2O and CO2 molecules are transported with comparable speed towards the membrane surface. For the system with N2, the opposite trend was found, due to the maintained significant difference in transport speed between H2O and N2 even at elevated pressures. Consequently, the water vapor transport rate through the PDMS membrane is governed by the type of matrix fluid (CO2 or N2).