Supercritical Phase Research Articles

A neural network correlation for molar density and specific heat of water: Predictions at pressures up to 100 MPa

The IAPWS-95 formulation is a set of equations that calculates water properties with high precision. To obtain a certain thermodynamic property, one or more iterative procedures are needed, which demand high computational effort. The aim of this work was to obtain an Artificial Neural Network based equation to predict the IAPWS-95 formulation values of density (ρ) and specific heat (Cp) of water in liquid, vapor, and supercritical phases, including the saturation line. Data at temperatures up to 1275 K, and pressures up to 100 MPa were considered. Different sets of input variables were tested and best results were obtained using: temperature, pressure, and speed of sound (used to differentiate liquid from vapor at the saturation line). The network 3-20-15-2 was 99.70% faster than the IAPWS formulation, and presented an overall mean percentage errors of 0.23% and 0.51% for ρ and Cp, respectively, which were lower than those obtained using known correlations.

Frenkel line crossover of confined supercritical fluids

We investigate the temperature evolution of dynamics and structure of partially confined Lennard Jones (LJ) fluids in supercritical phase along an isobaric line in the P-T phase diagram using molecular dynamics simulations. We compare the Frenkel line (FL) crossover features of partially confined LJ fluids to that of the bulk LJ fluids in supercritical phase. Five different spacings have been chosen in this study and the FL crossover characteristics have been monitored for each of these spacings for temperatures ranging from 240 K to 1500 K keeping the pressure fixed at 5000 bar. We characterize the FL crossover using density of states (DoS) function and find that partially confined supercritical fluids (SCF) exhibit a progressive shift of FL crossover point to higher temperatures for smaller spacings. While the DoS perpendicular to the walls shows persistent oscillatory modes, the parallel component exhibits a smooth crossover from an oscillatory to non-oscillatory characteristics representative of FL crossover. We find that the vanishing of peaks in DoS parallel to the walls indicates that the SCF no longer supports shear mode excitations and could serve as an identifier of the FL crossover for confined systems just as is done for the bulk. Layer heights of density profiles, self-diffusivity and the peak heights of radial distribution function parallel to the walls also feature the FL crossover consistent with the DoS criteria. Surprisingly, self-diffusivity undergoes an Arrhenius to super-Arrhenius crossover at low temperatures for smaller spacings as a result of enhanced structural order evidenced via pair-excess entropy. This feature, typical of glass-forming liquids and binary supercooled liquids, is found to develop from the glass-like characteristic slowdown and strong caging in confined supercritical fluid, evidenced via mean squared displacement and velocity autocorrelation function respectively, over intermediate timescales.

Preparation and characterization of cellulose acetate-Laponite® composite membranes produced by supercritical phase inversion

Supercritical phase inversion was used to prepare new polymeric membranes, based on cellulose acetate and Laponite®, for tissue engineering applications. These composite membranes were characterized by a porous structure (porosity around 85%) with an average pore size ranging from about 9 to 13 μm depending on the initial weight ratio cellulose acetate/Laponite. This ratio also controlled thermal and mechanical properties of these membranes: Laponite modified membrane decomposition step and increased the membrane elasticity modulus up to 4 MPa while decreasing the percentage elongation at break from 240% to 180%. Laponite also increased material cells adhesion: thanks to its homogeneous inclusion into the membranes, a remarkable cells adhesion of around 80%, six times higher than the original material, was obtained.

Self-diffusion coefficients of methane/n-hexane mixtures at high pressures: An evaluation of the finite-size effect and a comparison of force fields

Mass transport coefficients play an important role in process design and in compositional grading of oil reservoirs. As experimental measurements of these properties can be costly and hazardous, Molecular Dynamics simulations emerge as an alternative approach. In this work, we used Molecular Dynamics to calculate the self-diffusion coefficients of methane/n-hexane mixtures at different conditions, in both liquid and supercritical phases. We evaluated how the finite box size and the choice of the force field affect the calculated properties at high pressures. Results show a strong dependency between self-diffusion and the simulation box size. The Yeh-Hummer analytical correction [J. Phys. Chem. B, 108, 15873 (2004)] can attenuate this effect, but sometimes makes the results depart from experimental data due to issues concerning the force fields. We have also found that different all-atom and united-atom models can produce biased results due to caging effects and to different dihedral configurations of the n-alkane.

Modeling the fluid dynamics of a high-pressure extraction column

The fluid dynamics within a Sulzer CY packing were modeled with CFD. Volume of Fluid (VOF) was used to track the interface of the phases. The single-phase pressure drop and mean residence time of the supercritical phase were calculated as well as the liquid holdup resulting from multiphase flow simulations modeling water as liquid phase. A comparison of simulated and experimental results from this work and from literature show good agreement. The CFD results suggest that aqueous systems in a supercritical CO2 extraction column form droplets, hence these systems can be modeled like a liquid–liquid extraction column. No liquid film wetting the gauze wire packings was observed.

Experimental study of the influence of burst parameters on the initiation of CO2 BLEVE

A small-scale experimental system was constructed to simulate the initiation of CO2 BLEVE process in different phase state. Dynamic pressure parameters after disc burst were analysed to supplement the influence rules of vent size and burst pressure to the intensity of CO2 BLEVE. Comparisons between the supercritical phase state CO2 and liquid CO2 on the pressure parameters were made to reveal the phase state influence to the initiation of CO2 BLEVE. The vent size of 8 mm and 15 mm were two critical parameters under different burst pressures. The changing of pressure became irregular when the vent size surpassed 8 mm, and the pressure rise value will never surpassed the pressure drop value when the vent size reached or surpassed 15 mm. The supercritical state CO2 was unstable compared to the liquid state CO2. A more complex phase transition in the discharge process caused the pressure parameters changing became irregular; especially when the inner pressure reached 10 MPa and CO2 became a supercritical state fluid. After burst, the fluid releasing form the vessel instantly and limited space was reserved for the fluid to swell in the vessel, causing the inner pressure can’t rebound in time, which eventually led the overpressure disappear.

Process simulation of impurity impacts on CO2 fluids flowing in pipelines

Captured carbon dioxide flowing in pipelines is impure. The impurities contained in the carbon dioxide fluid impact on the properties of the fluid. The impact of each impurity has not been adequately studied and fully understood. In this study, binary mixtures containing carbon dioxide and one impurity, at the maximum permitted concentration, flowing in pipelines are studied to understand their impact on pipeline performance. A hypothetical 70 km uninsulated pipeline is assumed and simulated using Aspen HYSYS (v.10) and gPROMS (v.5.1.1). The mass flow rate is 2,200,600 kg/h; the internal and external diameters are 0.711 m and 0.785 m. 15 MPa and 9 MPa were assumed as inlet and minimum pressures and 33 °C as the inlet temperature, to ensure that the fluid remain in the dense (subcritical or supercritical) phase. Each binary fluid is studied at the maximum allowable concentration and deviations from pure carbon dioxide at the same conditions is determined. These deviations were graded to rank the impurities in order of the degree of impact on each parameter. All impurities had at least one negative impact on carbon dioxide fluid flow. Nitrogen with the highest concentration (10-mol %) had the worst impact on pressure loss (in horizontal pipeline), density, and critical pressure. Hydrogen sulphide (with 1.5-mol %) had the least impact, hardly changing the thermodynamic properties of pure carbon dioxide.

Effect of CO2 on the degradation of cement-bonded particleboard based on the treatment to conventional cured board

This study aimed to evaluate and clarify the effect of CO2 at either the gaseous or supercritical phase on the degradation of cement-bonded particleboard (CBP) based on the treatment to conventional cured board, while the estimation of long term degradation of CBP is also discussed. The properties of CBP are improved by CO2 treatment in short period in both the gaseous and supercritical phases even after the conventional curing process. These properties, however, over the longer treatment time degraded in rapid rate against CO2 treatment in both the gaseous and supercritical phases. It was considered that the degradation of CBP increased with increasing the CO2 concentration, CO2 pressure and treatment time. With the longer treatment time against supercritical CO2 in higher concentration and pressure, the residual values of mechanical properties (IB strength, MOR, and MOE) were decreased, indicating CBP performance was degraded rapidly compared to treatment in the lower concentration and pressure of CO2 or under gaseous phase. High coefficient determination values between the residual values of CBP properties, and the treatment time and CO2 concentration were observed and the simple linear model was found to estimate the long term degradation of CBP among the properties and affecting factors.

Improved self-assembled micelles based on supercritical fluid technology as a novel oral delivery system for enhancing germacrone oral bioavailability.

The main objective of this study was to develop a novel self-assembled micelle system to improve the oral bioavailability of the water-insoluble and thermal instable germacrone (GEM). Micelles were prepared with an improved supercritical reverse phase evaporation (ISCRPE) method, and the thin-film hydration (TFH) method was used for comparison. Physicochemical characterization confirmed the nanospherical morphology of the micelles prepared by the ISCRPE method (GEM@SDL-SCF Micelles) had smaller particle size and greater encapsulation efficiency than that of the micelles prepared by the TFH method (GEM@SDL-THF Micelles). Dilution resistance and stability experiments were performed to assess the structural integrity. In vitro GEM release was analyzed by the dialysis diffusion method, and the micelle system showed sustained and cumulative release. Furthermore, the fluorescence results from the cellular uptake study revealed improved drug absorption of GEM since the green fluorescence intensity of C6@SDL-SCF Micelles were stronger than that of C6@SDL-TFH Micelles. The transcellular transport study showed a distinct increase in the apparent permeability coefficient (AP → BL) in a Caco-2 cell transport model, and free GEM solution, GEM@SDL-TFH Micelles and GEM@SDL-SCF Micelles had permeability coefficients of 7.30 × 10-6, 8.01 × 10-6, and 10.57 × 10-6 cm·s-1, respectively. The pharmacokinetics study in Sprague-Dawley rats showed that the oral absorption capacity of the GEM@SDL-SCF Micelles was obviously enhanced, with a relative oral bioavailability of 298% and 147% compared with that of free GEM solution and GEM@SDL-TFH Micelles, respectively. The gastrointestinal safety assessment demonstrated that the micelles used in the experiment did not cause gastrointestinal toxicity. These results indicated that self-assembled micelles are promising nanocarriers aimed at enhancing the oral absorption of drugs with thermal sensitivity and low aqueous solubility, and micelles prepared by supercritical fluid (SCF) technology showed better performance than those prepared by the TFH method.

Heterogeneity and Delayed Activation as Hallmarks of Self-Organization and Criticality in Excitable Tissue.

Self-organized critical dynamics is assumed to be an attractive mode of functioning for several real-life systems and entails an emergent activity in which the extent of observables follows a power-law distribution. The hallmarks of criticality have recently been observed in a plethora of biological systems, including beta cell populations within pancreatic islets of Langerhans. In the present study, we systematically explored the mechanisms that drive the critical and supercritical behavior in networks of coupled beta cells under different circumstances by means of experimental and computational approaches. Experimentally, we employed high-speed functional multicellular calcium imaging of fluorescently labeled acute mouse pancreas tissue slices to record calcium signals in a large number of beta cells simultaneously, and with a high spatiotemporal resolution. Our experimental results revealed that the cellular responses to stimulation with glucose are biphasic and glucose-dependent. Under physiological as well as under supraphysiological levels of stimulation, an initial activation phase was followed by a supercritical plateau phase with a high number of global intercellular calcium waves. However, the activation phase displayed fingerprints of critical behavior under lower stimulation levels, with a progressive recruitment of cells and a power-law distribution of calcium wave sizes. On the other hand, the activation phase provoked by pathophysiologically high glucose concentrations, differed considerably and was more rapid, less continuous, and supercritical. To gain a deeper insight into the experimentally observed complex dynamical patterns, we built up a phenomenological model of coupled excitable cells and explored empirically the model’s necessities that ensured a good overlap between computational and experimental results. It turned out that such a good agreement between experimental and computational findings was attained when both heterogeneous and stimulus-dependent time lags, variability in excitability levels, as well as a heterogeneous cell-cell coupling were included into the model. Most importantly, since our phenomenological approach involved only a few parameters, it naturally lends itself not only for determining key mechanisms of self-organized criticality at the tissue level, but also points out various features for comprehensive and realistic modeling of different excitable systems in nature.

Optimization of process parameters for pipeline CO2 transportation with impurities

According to the actual engineering experience of many years of abroad, and impurities will have an impact on pipeline transportation parameters. This paper takes the 300,000 tons/year CCUS project of an oilfield in China as an example. The Pipe phase simulation software was used to simulate the supercritical dense phase carbon dioxide pipelines with different inlet diameters under the same inlet parameters, and analyze the most suitable pipeline transportation process parameters for impurities containing supercritical dense phase carbon dioxide. The feasibility plan of carbon dioxide pipeline transportation has laid a foundation for the comprehensive construction of carbon dioxide pipeline network in China.

W-shaped permeability evolution of coal with supercritical CO2 phase transition

Gaseous CO2 becomes a supercritical liquid when temperature and pressure transit the critical point (31.1 °C and 7.4 MPa) – a typical state for CO2 storage for carbon sequestration and CO2 injection in enhanced coalbed methane/shale recovery. Therefore, it is essential to define the evolution of coal permeability inclusive of this phase transition. This study presents experimental measurements of coal permeability across the CO2/SCCO2 transition over typical ranges of confining stress (3–15 MPa) and pore pressures (1–13 MPa) at a constant temperature of 40 °C. The results show that contrary to the typical U-shaped permeability -vs- pressure evolution in the subcritical region, a second permeability minimum occurs in the vicinity of the critical pressure – even after the impacts of viscosity transition are accommodated. Permeability to SCCO2 is almost two-orders-of-magnitude smaller than its initial permeability to subcritical-CO2. The phase transition from subcritical to supercritical state controls the W-shaped coal permeability profile. Permeability in the same coal is indexed relative to non-sorbing Helium (He) and slightly-adsorbing Nitrogen (N2) to probe the origins of the W-shaped permeability profile around the SCCO2 phase transition. Observed irreversible changes in mechanical properties driven by the SCCO2 phase transition are linked to the observed permeability reduction. First, observed reductions in coal strength and stiffness after SCCO2 saturation indicate a significant weakening effect - SCCO2 acts as a strong plasticizer enabling rearrangement of the physical structure and adding molecular mobility. The softened coal responds to increased compressional deformation and cleat closure under the same effective stress, resulting in a net permeability reduction in the supercritical region countering any influences of permeability-enhancing microcrack formation. Second, the SCCO2 permeability is further reduced by a large increase in sorption-induced swelling observed after the transition. Measured swelling strains indicate a > 90% increase in SCCO2 sorption-induced swelling relative to Langmuir response at 13 MPa. The enlarged SCCO2 adsorption capacity presumably results from the slightly polar nature of the SCCO2 dimer and the increased SCCO2 density. Permeability reduction due to plasticization and increased swelling is shown to dominate over permeability increases driven by brittle damage.

The Neumann problem of special Lagrangian equations with supercritical phase

In this paper, we establish the global C2 estimates of the Neumann problem of special Lagrangian equations with supercritical phase and the existence theorem by the method of continuity.

A compactness approach to Hessian estimates for special Lagrangian equations with supercritical phase

We derive interior Hessian estimates for special Lagrangian equations with supercritical phase in general dimension through a compactness approach.

Accurate internal energy of argon fluid from a state-of-the-art ab initio potential with uncertainty estimations

The internal energy of argon fluid is computed using an accurate ab initio potential by molecular dynamics simulations for the temperature range from 90 K to 2000 K and at densities up to 30 mol·L−1. The uncertainty of the calculated internal energy values of this work is evaluated by considering the contributions carefully estimating from the ab initio potential and the simulation method, respectively. Excellent agreement with the NIST data shows that the computed internal energy values of this work in gas phase and supercritical phase can be used as reference data. The deviations for liquid phase are well explained by introducing the corrections for three/many-body interactions from the literature. The computed internal energy values of argon with estimated uncertainties are tabulated in the Supplement material and our results at other temperatures and densities are available upon request.

Numerical Simulation of the Impact of a CO2 Flood Pattern Size on Oil Recovery and Carbon Storage

Climate change legislation will not be able to incentivize energy stakeholders nor environmentalist to adopt carbon dioxide sequestration in the short term because of the cost associated with the technology. However, CO2 Enhanced Oil Recovery (CO2-EOR) has the potential to provide a win-win situation. One of the areas that can consider the implementation of this process are heavy oil reservoirs. The global heavy oil estimate is very large. For some countries it can help sustain their daily production simply by understanding reservoir behavior when CO2 is injected into the reservoirs. In recent times, other potential energy issues such as depleting resources and greenhouse gas emissions are playing a bigger role in defining what needs to be contemplated by many nations to obtain energy sustainability. This paper seeks to utilize numerical simulation using a commercial software to evaluate pattern size as a strategy for improving recovery in a heavy oil reservoir on the south east peninsula of Trinidad and Tobago (T&T). Some of these heavy oils can be recovered using conventional oil production followed by currently used enhanced oil recovery techniques. We focus our research on a injecting CO2 into a reservoir labelled field X with analogous data was used from fields in south-west Trinidad. Initially, a homogenous reservoir model was built using the compositional fluid model GMG-GEM and placed on primary production. These models were populated with vertical production and injection wells. Sensitivity analysis was then performed on three development scenarios: 160, 40, and 10 acre five-spots. We then developed a heterogeneous model using geostatistically-generated permeability. Based on the results shown, the 160 acre well spacing would be the most economic option; having a positive NPV and Cumulative Profit after taxes. Although the production was the lowest when compared to the other two patterns, coupled with the low CAPEX, the 160-acre spacing over the 55-year period would be best suited if we consider primary recovery only. While pipelines were found to be the better option for transport for the total 10-year period, the results reveal that trucking was the more attractive initial option; because of its low capital cost and flow-rates of CO2. Trucking initially is a more economic option for shorter injection periods and CO2 flow-rates when compared to pipelines however in the longer term, pipeline would be the better option. The CO2 utilisation rate ranges from 2-16MSCF/bbl over the 10 year injection period. In terms of CO2 storage, although the higher the injection volume, the greater the amount of CO2 stored for the scenarios that we have studied, the most economic case with storage would be the 160 acre spacing injecting a total of 40MSCF/D. However, if at any time there can be economic benefits derived from CO2 storage activities, the economics would drive the higher injection rates. From a storage point of view it is important to choose the right well spacing for not only oil production but for storage projects utilising depleted oil reservoirs. The results showed that on average the heterogeneous cases stored approximately 3% more CO2. For larger projects therefore, will have an impact on storage volumes. Once the reservoir remains unaffected by any breach, the CO2 will remain trapped mainly in the supercritical phase for at least 1000 years.

Generation of field-emitting surface dielectric barrier discharges in Ar and N2

Field-emitting modes of surface dielectric barrier discharges (DBDs) have been generated thus far only in high-pressure CO2, including its liquid and supercritical phases, and in silicone oil. In this study, a generalized discussion with a one-dimensional Townsend-based theory is proposed to predict the accessibility of the field-emitting mode in various media. The field-emitting modes of surface DBDs are demonstrated experimentally in high-density Ar and N2 using Fowler–Nordheim coordinates and image observations.

Impact of Injecting CO2 and Light Hydrocarbon Fractions in a Condensate Reservoir

Greenhouse gas emissions have grown significantly in recent times globally. One of the most prevalent greenhouse gases is carbon dioxide (CO2). Over the years, pilot projects along with reservoir simulations have sought to strengthen our confidence in the use of geologic storage as a mitigation strategy, but a lot more related research still has to be done. One of the key issues when producing condensate reservoirs is that of liquid dropout as the reservoir drops below the dew point pressure. Condensate can therefore become trapped in the reservoir due to the effect of capillary pressure. As a result, flow within porous media can become restricted, thus reducing the efficiency of condensate and gas recovery. CO2 injection can assist with both these challenges, i.e. pressure maintenance and the sequestering of the CO2. The depleted hydrocarbon pore spaces in the reservoir provide suitable storage sites for the injected CO2. Current technologies involve injecting produced natural gas from source wells into other wells to help increase reservoir pressure and improve recovery efficiency. This paper examined the effects of injecting both CO2 and light hydrocarbon fractions (C1-C2) into condensate reservoirs to optimize condensate recovery and CO2 sequestration. The results from the associated simulation studies show that the use of CO2 to increase condensate recovery from the reservoir is feasible with the additional benefit of CO2 sequestration. It also demonstrated that it is highly possible to increase condensate production by injecting CO2 with some percentages of lighter fractions (C1-C2), with these lighter fractions assisting to increase condensate recovery. It is possible to store between 11 and 15% of injected CO2 using the different injection compositions. The injection of CO2 had a positive effect on the re-vaporization of condensate dropout in the reservoir. Increasing the injection pressure yielded higher condensate recoveries. More than 95% of the injected CO2 stored in the reservoir was trapped in the supercritical phase, whilst 3-5% was stored due to hysteresis. As the injection pressure increased, the amount of CO2 stored by hysteresis decreased in all cases. Once the reservoir seals were not breached, the CO2 will remain trapped for over 1000 years.

Characterization of Choked Conditions Under Subsonic to Supersonic Flow in Single‐Phase (Supercritical to Gaseous CO2 or Liquid H2O) and Multiphase (CO2 and H2O) Transport

AbstractIn geologic media, fluids exist in gas, liquid, and supercritical phases, generating multiphase and multicomponent systems. As fluids migrating through geologic fractures reach the speed of sound, choked flow can be developed in microfractures. To elucidate such choked flow, thermodynamic analysis and numerical simulations were conducted with CO2, H2O, and CO2‐H2O mixtures at various phases ranging from supercritical to gaseous CO2 and liquid H2O. Compressible CO2, with a relatively low speed of sound (~225 m/s at 31.1 °C and 7.38 MPa), demonstrated significant changes in thermodynamic properties with small pressure and temperature variations. In contrast, H2O, having a relatively high speed of sound (1,524 m/s), showed little thermodynamic variation. For CO2‐H2O mixtures, a small addition of CO2 (or H2O) dramatically reduced the speed of sound relative to those for pure H2O or CO2. For an idealized converging‐diverging microfracture with CO2 flow, choked flow and a shock wave were generated as outlet pressure was decreased to less than 6.8 MPa. The H2O flow did not generate choked flow at any outlet pressures. For CO2‐H2O mixtures, choked flow was generated when the CO2 void fraction was greater than 0.7 with an outlet pressure of 6.5 MPa, indicating that presence of H2O inhibited occurrence of choked flow. Choked flow and shock waves can occur in various geologic environments including geologic CO2 sequestration, geothermal energy development, geysers, and volcano eruptions.

Critical water content for corrosion of X65 mild steel in gaseous, liquid and supercritical CO2 stream

CO2 could be transported by pipelines in gaseous, liquid or supercritical phase depending on the operating temperature and pressure. Safety concerns might arise from CO2 transportation pipeline, especially crossing the densely populated areas. Water content is critical to understand corrosion mechanism of CO2 pipeline. Corrosion behavior of API 5L X65 mild steel exposed to CO2 environment with different relative humidity (RH), temperature and pressure was investigated by weight-loss and surface analysis techniques to determine critical water content in gaseous, liquid and supercritical CO2 stream. Surface morphology of coupons and composition of corrosion product layers were analyzed by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). Results showed that general corrosion rates remained steady with the increasing of RH before a separate water phase was observed, and it experienced a remarkable increase in free water for all test conditions. Localized corrosion or severe general corrosion was observed when the relative humidity was beyond 60% at 25 °C and 35 °C, 8 MPa and 80% at 35 °C, 4 MPa. The critical water content in CO2 stream was determined to be 60% RH for liquid and supercritical and 80% RH for gaseous CO2 transportation due to the safety concern of X65 pipeline on the basis of corrosion study.