Supercritical N2 Research Articles

Molecular dynamics investigation on supercritical water oxidation of a coal particle

The supercritical water oxidation (SCWO) is a technology with very low environmental impact and significant economic benefits. In present paper, the micro-scale dynamics and the kinetic reaction mechanism of the SCWO of a coal particle is investigated by the reactive force-field (ReaxFF) method for the first time. Comparisons between experimental and analytical results on the activation energy show the coal particle model and the ReaxFF MD can successfully reproduce the SCWO process. Results on the atomistic scale conversion ways of O2, CO2, H or H2 and H2O show the SCWO is not simply the gasification plus the combustion. In SCWO, the direct combustion of the coal is weakened apparently due to the existence of the water, the oxidation reaction between the water or H radicals and the oxygen to generate OH plays an important role in coal conversion. The oxidation between carbon structures and OH to generate CO2 and H2O has substituted for the direct combustion reaction with the oxygen to be the dominant way in coal conversion. This process contains the characteristics of both combustion and gasification process. The role of water molecules is something similar to the catalysis. It is converted to OH radicals first in a series of gasification and oxidation processes, then the OH radicals turn into H2O in the following oxidation reactions. After that, the effect of reaction conditions and the transition of N on the SCWO process are studied. Comparing to the single supercritical water gasification, N2 and NO are found to be the extra products in SCWO. CN, CHN and CHON are the dominant N-containing products before they are converted to N2 and NO. The results in present work give insight into the oxidation mechanism of the coal particle, and the SCWO can effectively reduce the pollution emission in a much cleaner way compared with the conventional coal-fired mode.

A Review on Microcellular Injection Moulding.

Microcellular injection moulding (MuCell®) is a polymer processing technology that uses a supercritical fluid inert gas, CO2 or N2, to produce light-weight products. Due to environmental pressures and the requirement of light-weight parts with good mechanical properties, this technology recently gained significant attention. However, poor surface appearance and limited mechanical properties still prevent the wide applications of this technique. This paper reviews the microcellular injection moulding process, main characteristics of the process, bubble nucleation and growth, and major recent developments in the field. Strategies to improve both the surface quality and mechanical properties are discussed in detail as well as the relationships between processing parameters, morphology, and surface and mechanical properties. Modelling approaches to simulate microcellular injection moulding and the mathematical models behind Moldex 3D and Moldflow, the two most commonly used software tools by industry and academia, are reviewed, and the main limitations are highlighted. Finally, future research perspectives to further develop this technology are also discussed.

Unpicking the interplay of turbulence, diffusion, and thermophysics in cryogenic jets at supercritical pressures

Cryogenic supercritical fluids represent an intriguing category of fluids that combine mechanical and thermophysical properties of both ultralow temperature conditions and phenomena taking place well above the critical point. Recent research has demonstrated that it is a common misconception to consider the supercritical state as one homogeneous state. Instead, these fluids consist of two to four liquid and gas like phases, each with their own unique characteristics. In our work, we investigate numerically single-specie cryogenic fluid jets—initially at subcritical temperatures—which are injected into a supercritical environment (both the pressure and temperature exceed the thermodynamic critical state). For the investigation, a new solver, namely, “CoolFoam,” has been developed, which is designed for compressible non-isothermal two-fluid simulations where diffusive transport of heat and/or mass is accounted for. Real fluid thermodynamics are modeled using a polynomial fitting approach developed in our previous work. We introduce also a new phase characterization framework based on the association of phases with specific temperature ranges (rather than using a single line like the Widom line), which allowed us to better identify the similarity effects between the various conditions. We analyze the inter-dependence of the underlying phenomena: density gradient and diffusive mass transport [molecular and thermo(Soret)-diffusion] and turbulence. We also compared supercritical N2 with subcritical liquid and gas jets to highlight potential differences with respect to how these jets behave. We find that the jet dynamics are largely dictated by the thermodynamic transition of the injected fluid and the associated variation in thermophysical properties.

Enhancing deteriorated heat transfer of supercritical nitrogen in a vertical tube with wire matrix insert

In this study, hiTRANTM wire matrix tube insert was used to improve the heat transfer deterioration of supercritical nitrogen (N2) in a heated tube caused by the effects of thermo-physical property variations and buoyancy force. Heat transfer experiments were carried out with N2 flowing upwardly in a vertical circular tube with and without the wire matrix insert under a sequence of experimental conditions including pressures of 35 and 40 bar, mass flow rate of 27.6 and 41.3 g/min and constant heat flux conditions at 6.8, 8.0 and 9.3 kW/m2, respectively. Experimental results show that N2 exhibits similar heat transfer behavior when transiting across the pseudo-critical point as other fluids such as water and CO2. Due to the addition of the wire matrix insert, that intensified the overall fluid mixing in the test tube, the heat transfer performance of N2 was enhanced by more than 42% and up to 2.35 times while the pressure drop increase was negligible compared to the system inlet pressure. Moreover, it has been demonstrated that there might exist an optimum combination of experimental conditions leading to the maximum performance of the wire matrix insert. Furthermore, as the results show, with the Dittus-Boelter correlation and correlations for water and CO2 falling short of fitting the heat transfer data of N2, a heat transfer correlation, exclusively for N2 going through the pseudo-critical point is needed. The findings in this study also reveal that both supercritical N2 internal flow heat transfer and the use of wire matrix insert to enhance the deteriorated heat transfer of supercritical fluids are promising topics and require more significant attentions in future studies.

Surface modification of polymer textile biomaterials by N2 supercritical jet: Preliminary mechanical and biological performance assessment

Foreign Body Reaction (FBR) is a critical issue to be addressed when polyethylene terephthalate (PET) textile implants are considered in the medical field to treat pathologies involving hernia repair, revascularization strategies in arterial disease, and aneurysm or heart valve replacement. The natural porosity of textile materials tends to induce exaggerated tissue ingrowth which may prevent the implants from remaining flexible. One hypothesized way to limit the FBR process is to increase the material surface roughness at the yarn level. Supercritical N2 (ScN2) jet particle projection is a technique that provides enough velocity to particles in order to induce plastic deformation on the impacted surface. This work investigates the influence of ScN2 jet projection parameters like standoff distance or particle size on the roughness that can be obtained on medical polymer yarns of various diameters (100 and 400 μm) and woven textile surfaces obtained from a 100 μm yarn. Moreover, the mechanical and biological performances of the obtained modified textile material are assessed. Results bring out that with appropriate testing conditions (500 bars jet/500 mm distance between nozzle and PET textile) and particle size around 50 μm, it is possible to generate 20 μm large and 4 μm deep craters on a 100 μm monofilament PET yarn and fabric. Regarding the strength of the textile material, it is only slightly modified with the treatment process, as the tenacity of the yarns decreases by only 10%. Moreover, It is shown that the obtained structures tend to limit the adhesion and slow down the proliferation of human fibroblasts.

A Novel Hybrid Foaming Method for Low-Pressure Microcellular Foam Production of Unfilled and Talc-Filled Copolymer Polypropylenes.

Unfilled and talc-filled Copolymer Polypropylene (PP) samples were produced through low-pressure foam-injection molding (FIM). The foaming stage of the process has been facilitated through a chemical blowing agent (C6H7NaO7 and CaCO3 mixture), a physical blowing agent (supercritical N2) and a novel hybrid foaming (combination of said chemical and physical foaming agents). Three weight-saving levels were produced with the varying foaming methods and compared to conventional injection molding. The unfilled PP foams produced through chemical blowing agent exhibited the strongest mechanical characteristics due to larger skin wall thicknesses, while the weakest were that of the talc-filled PP through the hybrid foaming technique. However, the hybrid foaming produced superior microcellular foams for both PPs due to calcium carbonate (CaCO3) enhancing the nucleation phase.

SURFACE MODIFICATION OF BIOMATERIAL FABRIC USING SUPERCRITICAL N2 JET

Textile biomaterials have been largely used over the last decades as vascular grafts, hernia meshes and heart valve leaflet [1-2]. Once implanted in vivo, the natural porosity of textile materials tends to induce exaggerated tissue ingrowth, which may prevent the implants from remaining flexible [3]. One hypothesized way to limit the foreign body reaction process is to increase the material surface roughness [4]. Supercritical N2 jet particle projection is a novel technique to provide enough velocity to micro particles to induce plastic deformation on the textile impacted surface. The aim of this study is to investigate the influence of micro particles laden supercritical N2 jet projection parameters like jet static pressure, standoff distance and particle size on the roughness of PET fabric surfaces. Results bring out that particles projected by the jet N2 SC generate craters on the surface of monofilament as well as multifilament fabric, allowing topographical modifications at the yarn scale. We found that larger particles induce larger crater diameters. Moreover, increasing the static jet pressure from 300 to 1000 bars further allows increase in the crater diameter. For a pressure of 500 bar, the standoff distance must be greater than 300 mm in order to obtain significant roughness values without breaking the PET monofilament fabrics. Thus, this treatment increased the roughness of the monofilament fabric from 0.78 μm to 1.22 μm. The results obtained in this work show that it is possible to create a roughness on a PET fabric using the N2 jet technology.

Interfacial tension and equilibrium contact angle of corn oil on polished stainless steel in supercritical CO2 and N2

Lipids and bioactive components have been isolated using supercritical fluid fractionation (SFF) and packed columns with carbon dioxide from complex lipid mixtures. Lack of information about the wettability of the packing materials by different lipids was the motivation for this study. The interfacial tension (IFT) and equilibrium contact angle (CAeq) of corn oil in air, supercritical carbon dioxide (SC-CO2) and SC-N2 on polished stainless steel (SS) metal sheet were measured at 20, 40, 60, 80 °C and 0.1, 16, 20, 24 MPa in triplicate. IFT and CAeq were determined using Young-Laplace equation and height/width method, respectively. CAeq values of less than 10° demonstrated the complete wetting of corn oil on polished SS surface under unsaturated SC-N2 and SC-CO2, and saturated SC-CO2, but pressure and temperature had a significant effect on IFT and CAeq under these environments. Bouncing of drops on a completely wettable surface was observed under CO2 environment. The findings contribute to better understanding of wetting behaviour of SS surfaces by lipids, with implications for process design.

Transition from Gas-like to Liquid-like Behavior in Supercritical N2

We have studied in detail the transition from gas-like to rigid liquid-like behavior in supercritical N2 at 300 K (2.4 TC). Our study combines neutron diffraction and Raman spectroscopy with ab initio molecular dynamics simulations. We observe a narrow transition from gas-like to rigid liquid-like behavior at ca. 150 MPa, which we associate with the Frenkel line. Our findings allow us to reliably characterize the Frenkel line using both diffraction and spectroscopy methods, backed up by simulation, for the same substance. We clearly lay out what parameters change, and what parameters do not change, when the Frenkel line is crossed.

Cellular morphology evolution in nanocellular poly (lactic acid)/thermoplastic polyurethane blending foams in the presence of supercritical N2

Currently, the formation of nanocells and complex cellular structure (CCS) in semi-crystalline polymer using supercritical fluids as blowing agent had become a big and newly developing challenge worldwide. In this paper, a facile methodology of polymer blending and batch foaming was proposed to fabricate CCS with large cells in micro-size and small cells in nano-size in semi-crystalline poly (lactic acid) (PLA) foams. Thermoplastic polyurethane (TPU) was introduced into PLA through melt mixing method to improve the viscoelasticity and foaming behaviors of PLA. Differential scanning calorimetry results showed that with the addition of TPU, the cold crystallization temperature of various PLA samples increased and their crystallinity kept unchanged almost. Compared with those of pure PLA, the complex viscosity, and storage modulus of PLA/TPU blends increased. Interestingly, with the increasing TPU content, the CCS appeared and became distinct in PLA/TPU blending foams as well as the thermal conductivity of various PLA foams decreased, respectively. The formation mechanism of CCS in nanocellular PLA/TPU blending foams was proposed and explained by schematic diagram. With the foaming temperature rising from 135 to 140 °C, a transition from nano-cells to micro-cells was also observed in PLA/TPU 5 blending foam, due to the decreased melt viscosity.

Improving microcellular foamability of amorphous supramolecular polymers via functionalized nanosilica particles

This work introduces and investigates a synergistic approach to tailor microcellular morphology and physical properties of soft polymer foams by simultaneously utilizing supramolecular associations and nanoparticle inclusions. For this purpose, supramolecular nanocomposite foams consisting of soft amorphous polyacrylates and silica nanoparticles are successfully prepared and characterized. The polymer precursors as well as the nanoparticles are all functionalized with ureido‐pyrimidinone (UPy) supramolecular groups at various contents. Microcellular foams are produced in a batch process at 90°C after a 5‐h saturation step in 9 MPa, using supercritical N2 as the physical foaming agent. The effects of UPy‐grafting on microcellular morphology and consequently on thermal, viscoelastic, and mechanical properties is investigated. Dispersion of nanoparticles as well as the cellular structure of supramolecular nanocomposite foams is analyzed in relation to supramolecular moiety‐grafting density. It is illustrated that strong quadrupole hydrogen bonding between the filler and matrix not only improves the degree of reinforcement in nanocomposites, but also stabilizes bubble growth and retards cell coalescence or rupture. An optimum average cell size and microcellular density, that is, 6 ± 0.2 μm and 6 × 109 cells/cm3, respectively, is achieved with only 4 wt% UPy‐functionalized nanoparticle loading. These results illustrate, therefore, that supramolecular interactions between the chain backbone and nanoscale inclusions can open new horizons to architect and exploit microcellular materials consist up of amorphous, soft, loosely entangled polymers. POLYM. COMPOS., 40:364–378, 2019. © 2017 Society of Plastics Engineers

Using supramolecular associations to create stable cellular structures in amorphous soft polymers

A new strategy to foaming of Poly(methyl acrylate ‐co‐2‐hydroxyethyl methacrylate) as a polymer with low glass transition temperature (Tg) and strength is proposed by introducing supramolecular interactions via quadruple hydrogen bonding ureido‐pyrimidinone (UPy) units. The supramolecular polymers formed a reversible amorphous network and are foamed in a batch process inside an autoclave using supercritical N2. The thermal and dynamical behaviors of unfoamed and foamed supramolecular polymers are studied by differential scanning calorimeter and dynamic mechanical thermal analysis. The presence of UPy units in poly(methacrylic acid‐co‐2‐hydroxyethyl methacrylate) forms a reversible amorphous network and slows down the chains dynamics and leads to increase in the glass transition temperature as well as elastic modulus before and after foaming. The cellular structures, obtained in different foaming temperature, saturation pressure, and UPy content, were investigated using scanning electron microscopy. The higher density of UPy units induced more stability and higher elastic modulus and developed the more stable, closed, and homogenous cell structure with higher expansion ratio and cell. The obtained results show that the supramolecular polymer can be foamed with appropriate cellular structure integrity and uniform cell size distribution at 90°C, 9 MPa, and 4.5 mol% UPy content.

Experimental aspects of buoyancy correction in measuring reliable high-pressure excess adsorption isotherms using the gravimetric method

Addressing reproducibility issues in adsorption measurements is critical to accelerating the path to discovery of new industrial adsorbents and to understanding adsorption processes. A National Institute of Standards and Technology Reference Material, RM 8852 (ammonium ZSM-5 zeolite), and two gravimetric instruments with asymmetric two-beam balances were used to measure high-pressure adsorption isotherms. This work demonstrates how common approaches to buoyancy correction, a key factor in obtaining the mass change due to surface excess gas uptake from the apparent mass change, can impact the adsorption isotherm data. Three different approaches to buoyancy correction were investigated and applied to the subcritical CO2 and supercritical N2 adsorption isotherms at 293 K. It was observed that measuring a collective volume for all balance components for the buoyancy correction (helium method) introduces an inherent bias in temperature partition when there is a temperature gradient (i.e. analysis temperature is not equal to instrument air bath temperature). We demonstrate that a blank subtraction is effective in mitigating the biases associated with temperature partitioning, instrument calibration, and the determined volumes of the balance components. In general, the manual and subtraction methods allow for better treatment of the temperature gradient during buoyancy correction. From the study, best practices specific to asymmetric two-beam balances and more general recommendations for measuring isotherms far from critical temperatures using gravimetric instruments are offered.

Effect of nanosize CaCO3 and nanoclay on morphology and properties of linear PP/branched PP blend foams

Branched polypropylene (PP) polymer (BPP), two nanoparticles with different aspect ratios, nano CaCO3, and nanoclay (Cloisite 20A), were used for improving the melt strength of linear PP (LPP) and producing microcellular foam via a solid‐state foaming process using supercritical N2 as a physical foaming agent. The effects of BPP and nanoparticles were investigated on the rheological and mechanical properties of the nanocomposite and morphological properties of the foamed samples. The effect of temperature, saturation pressure, and foaming time were studied on the morphological properties of the foams, too. The results showed that the BPP increased the elongation at break of nanocomposite samples, and nano CaCO3 increased tensile strength, elongation at break, and tensile modulus, except at its highest content (15 wt%) and improving the dispersion of nanoclay. The nanoclay improved tensile strength and tensile modulus of nanocomposites, except at its highest content (6 wt%). The nanoclay increased the cell density and reduced the cell size and improved melt strength and cell stability during the foaming process. The nano CaCO3 improved foamability, cell size, and cell density. The simultaneous presence of nanoclay and nano CaCO3 synergistically improved the cell density and decreased the cell size of the foams. The best process conditions were found as 140°C temperature, 80 bar pressure, and 2 h time. The results of the DSC analysis showed that the crystallinity of foamed samples was higher than the nanocomposites and DMTA analysis showed that the tan δ of foamed samples was higher than unfoamed samples. POLYM. COMPOS., 40:E227–E241, 2019. © 2017 Society of Plastics Engineers

Physicomechanical, friction, and abrasion properties of EVA/PU blend foams foamed by supercritical nitrogen

Elastomer foams based on EVA, PU, and EVA/PU blends formulated for shoe‐sole applications were prepared by a supercritical N2 batch foaming process and characterized for physicomechanical, friction and abrasion properties. The blending of EVA with PU was aimed for improving the friction and wear characteristics of the EVA based foams. All of the foams prepared showed spherical cells with closed‐cell morphology and the cell sizes varied with varying the EVA/PU blend ratio and CaCO3 content of the foams. The properties such as hardness and resilience, friction coefficients and abrasion resistance improved for the EVA/PU blend foams compared to the EVA foam, but their compression set, tensile strength, and tear strength were inferior to the EVA foam. The incorporation of CaCO3 filler increased density, hardness, tensile strength, and tear strength of the EVA/PU blend foams but decreased resilience, compression set, friction coefficients, and abrasion resistance. The improvement in friction coefficients and wear resistance obtained in the EVA/PU blend foams was significant for shoe‐sole applications. POLYM. ENG. SCI., 2017. © 2017 Society of Plastics Engineers

Numerical Simulation of Jet Injection and Species Mixing under High-Pressure Conditions

Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES) are performed of a round fluid jet entering a high-pressure chamber. The chemical compositions and temperatures of the jet and that of the fluid in the chamber are initially prescribed. The governing equations consist of the conservation equations for mass, momentum, species and energy, and are complemented by a real-gas equation of state. The fluxes of species and heat are written in the framework of fluctuation-dissipation theory and include Soret and Dufour effects. For more than two species, the full mass diffusion and thermal diffusion matrices are computed using high-pressure mixing rules which utilize as building blocks the corresponding binary diffusion coefficients. The mixture viscosity and thermal conductivity are computed using standard mixing rules and corresponding states theory. To evaluate the physical model and numerical method, LES is employed first to simulate a supercritical N2 jet injected into N2. Time averaged results show reasonable agreement with the experimental data. Then, DNS is conducted to study the spatial evolution of a supercritical N2 jet injected into CO2. Time averaged results are used to compute the length of the potential core and the species diffusion characteristics. Spectral analysis is then applied on a time series data obtained at several axial locations and a dominant frequency is observed inside the potential core.

Supercritical Adsorption Based CO<sub>2</sub> Capture from N<sub>2</sub>/CO<sub>2</sub> Mixture by Activated Carbon and Carbon Molecular Sieve

The adsorption equilibria of pure supercritical N2 and CO2 on the activated carbon Yigao-A and carbon molecular sieve CMS-200 were measured by the gravimetric method (IGA-001, Hiden) in the temperature region of 313.15-393.15 K and pressure region of 0-2 MPa. The adsorption kinetics of N2 and of CO2 on the activated carbon Yigao-A and carbon molecular sieve CMS-200 at 313.15 K in 50 kPa were tested. The Langmuir-Freundlich model was applied to analyze the adsorption isotherms. The adsorption kinetics was investigated with the Fickian model. The analysis of adsorption equilibria showed that a high temperature, low pressure is beneficial to the equilibria based selective adsorption of supercritical CO2 on the activated carbon Yigao-A and carbon molecular sieve CMS-200. The investigation of adsorption kinetics exposed that the carbon molecular sieve CMS-200 is able to separate CO2 from the N2/CO2 mixture by its kinetic effect, and lower temperature is preferred.

Coupling discharge and gas dynamics in streamer-less spark formation in supercritical N2

A two-dimensional cylindrically symmetric model is developed to study the streamer-less spark formation in a short gap on the timescale of ion motion. It incorporates the coupling between the electric discharge and the gas through the heat generated by the discharge and the consecutive gas expansion. The model is employed to study electrical breakdown in supercritical N2. We present the simulation results of gas heating by the electrical discharge and the effect of gas expansion on the electrical discharge.

Numerical and experimental investigation of dielectric recovery in supercritical N2

A supercritical (SC) nitrogen (N2) switch is designed and tested. The dielectric strength and recovery rate of the SC switch are investigated by experiments. In order to theoretically study the discharge and recovery process of the SC N2 switch under high repetition rate operation, a numerical model is developed. For SC N2 with initial parameters of p = 80.9 bar and T = 300 K, the simulation results show that within several nanoseconds after the streamer bridges the switch gap, the spark is fully developed and this time depends on the applied electric field between electrodes. During the whole discharge process, the maximum temperature in the channel is about 20 000 K. About 10 µs after the spark excitation of 200 ns duration, the temperature on the axis decays to Taxis ⩽ 1500 K, mainly contributed by the gas expansion and heat transfer mechanisms. After 100 µs, the dielectric strength of the gap recovers to above half of the cold breakdown voltage due to the temperature decay in the channel. Both experimental and numerical investigations indicate that supercritical fluid is a good insulating medium that has a proved high breakdown voltage and fast recovery speed.

Pyrolysis of heavy oil in the presence of supercritical water: The reaction kinetics in different phases

In the presence of supercritical water (SCW) and N2, the pyrolysis of heavy oil was investigated to distinguish the difference in the reaction kinetics between the upgrading in the SCW and oil phases. The pyrolysis in the SCW phase is faster than that in the oil phase, but the reaction in whichever phase is retarded by vigorous stirring. The pyrolysis can be preferably described by a four‐lump kinetic model consisting of the condensation of maltenes and asphaltenes in series. In the SCW phase, highly dispersed asphaltenes are isolated by water clusters from maltenes dissolved in SCW surroundings, by which the condensation of asphaltenes is drastically accelerated. Benefited from excellent mass transfer environments in SCW, the condensation of maltenes is promoted simultaneously. The introduction of SCW into the pyrolysis of heavy oil results in an effectively increased upgrading efficiency, but its influence on the properties of equilibrium liquid products is minor. © 2014 American Institute of Chemical Engineers AIChE J, 61: 857–866, 2015