Supercritical Systems Research Articles

Potentiometric cell for measuring pH of supercritical aqueous solutions

A flow-through electrochemical cell for subcritical and supercritical water systems was developed based on the cell of Lvov et al. [J. Electroanal. Chem. 463, 146 (1999)]. The cell consisted of two flow-through platinum hydrogen electrodes and was constructed from a high nickel alloy (Hastelloy-B2). The inner surface of electrochemical cell was coated with Al2O3 adhesive and covered with the Al2O3 tube for corrosion resistance. Orifices were placed near the center of the cell to reduce concentration fluctuations around the test electrode. With the cell, a flow apparatus was used to measure the potentials for HCl+NaCl aqueous solutions at supercritical conditions. The cell potentials were measured at temperatures ranging from 23.9 to 400.2 °C and at pressures ranging from 25.0 to 35.1 MPa and the standard deviations of measured potentials were less than 0.4 mV for all tests. Comparison of the experimentally determined and theoretically calculated pH differences are better than ±0.03 logarithmic units over the range of conditions studied.

Equation of state for high-temperature aqueous electrolyte and nonelectrolyte systems

An equation of state has been developed for the representation of the phase behavior of high-temperature and supercritical aqueous systems containing salts and nonelectrolytes. The equation includes a reference part that is based on a model for hard-sphere ion pairs and dipolar solvent molecules. In addition to the reference part, the equation contains a perturbation part, which is expressed by a truncated virial-type expansion. To enhance the predictive capability of the EOS for normal fluids such as hydrocarbons, the equation has been reformulated using the three-parameter corresponding-states principle. For salt–water systems for which little experimental information is available, a predictive procedure has been developed that relies on similarities in the fluid phase behavior of various salt–water systems. This procedure utilizes the equation of state for NaCl+H 2O as a prototype system and introduces a transformation of parameters for the salt of interest. The equation accurately represents vapor–liquid equilibria, solid–liquid equilibria and densities for systems containing water, salts and hydrocarbons.

Inherent possibilities of improving recovery and selectivity using a long solid phase trap in analytical supercritical fluid extraction

Possibilities of improving recovery and selectivity in supercritical fluid extraction systems based on solid phase collection by using a longer trap (5 x 200 mm) instead of a standard trap (5 x 55 mm) were investigated. Such a long trap may give higher sample capacity, and filled with a chromatographic trapping material it gives a column with higher separation efficiency than the standard trap. The system was tested on thermally labile substances such as fat-soluble vitamins and aromatic amines. Quantitative recoveries of both vitamins and amines were obtained when using the long trap combined with modifier in the extracting fluid and trapping temperatures below the boiling point of the modifier conditions that usually cause breakthrough losses of analytes. Furthermore, the vitamins could be eluted from the long trap well separated from the lipid fraction, while this was not possible using the standard trap. The possibility of obtaining selectivity between substances of similar structures, such as aromatic amines, was investigated on different types of trapping material (ODS, NH 2 , and CN). Fairly good fractionation was achieved using CN. This opens up the possibility to decrease the total analysis time by eluting the samples in fractions.

Inherent possibilities of improving recovery and selectivity using a long solid phase trap in analytical supercritical fluid extraction

Possibilities of improving recovery and selectivity in supercritical fluid extraction systems based on solid phase collection by using a longer trap (5×200 mm) instead of a standard trap (5×55 mm) were investigated. Such a long trap may give higher sample capacity, and filled with a chromatographic trapping material it gives a column with higher separation efficiency than the standard trap. The system was tested on thermally labile substances such as fat-soluble vitamins and aromatic amines. Quantitative recoveries of both vitamins and amines were obtained when using the long trap combined with modifier in the extracting fluid and trapping temperatures below the boiling point of the modifier conditions that usually cause breakthrough losses of analytes. Furthermore, the vitamins could be eluted from the long trap well separated from the lipid fraction, while this was not possible using the standard trap. The possibility of obtaining selectivity between substances of similar structures, such as aromatic amines, was investigated on different types of trapping material (ODS, NH2, and CN). Fairly good fractionation was achieved using CN. This opens up the possibility to decrease the total analysis time by eluting the samples in fractions.

Corrosion Behavior of Nickel-Based Alloys in Supercritical Water Oxidation Systems

There is a need to destroy both military and civilian hazardous waste and an urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. One very effective process for the destruction of such waste is supercritical water oxidation (SCWO). By capitalizing on the properties of water above its critical point (374 °C and 22.4 MPa for pure water), this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern. While Ni and Ni-based alloys are generally considered important for severe service applications, results from laboratory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and localized effects, including stress corrosion cracking and dealloying, are seen in some environments. Although exotic liners such as platinum are currently promoted as a solution to aggressive conditions, some evidence suggests the potential for corrosion control by judicious feed modification. Various alloys were exposed in a SCWO system at 600 °C for 66.2 h. After exposure, samples were coated with a thick outer salt layer and an inner oxide layer. It is considered likely that, at the high supercritical temperature employed during this test, the salt was molten and contained a substantial quantity of gas. The inner oxide layer revealed the presence of numerous defects and a thickness that is proportional to the corrosion rate determined by mass loss, suggesting the oxide layer is nonprotective. Of the alloys tested, G-30 exhibited the highest corrosion resistance. Experiments in which a C-276 tube was instrumented with thermocouples and exposed to a HCl feed indicate for this simple non-saltforming influent that there is a strong correlation between temperature and the extent and form of corrosion, with the most pronounced degradation being at high subcritical temperatures. These experiments corroborate previous results from a failure analysis for C-276, suggesting a corrosion maximum in the subcritical region. Background

High‐resolution adsorption isotherms of supercritical carbon dioxide on activated carbon

AbstractSupercritical fluids are attractive solvents for heterogeneous processes, including catalysis and adsorptive separation. However, adsorption processes in the near‐critical region are poorly understood and exhibit unique behavior where the adsorption of the supercritical solvent plays an important role in the solute adsorption. The behavior of supercritical fluids in confined pores has been studied theoretically, but there are few experimental data on their behavior in industrially important microporous materials. The adsorption of carbon dioxide on Calgon F400 activated carbon over a wide range of pressures (0–20 MPa) at temperatures near the critical point of carbon dioxide (30 to 45°C) was studied. Near‐continuous adsorption and desorption isotherms were measured with a new flow gravimetric apparatus with precise control over pressure and temperature. As pressure is increased, the excess adsorption increases sharply at low pressures; then a broad maximum is observed. At temperatures greater than the critical temperature, there is a sharp drop in excess adsorption near the critical region where the density of the bulk fluid increases sharply. A crossover is observed near the critical region where, below a certain pressure, the excess adsorption decreases with temperature, while above the crossover point the trend is reversed. When analyzed as a function of solvent density, the crossover disappears, revealing an anomalous maximum in total adsorption near the critical point similar to the enhanced local density or “charisma” observed in binary solute–supercritical fluid systems. A 2‐D EOS model using the 2‐D Peng‐Robinson EOS was able to qualitatively describe the adsorption behavior over the entire pressure range, but the quantitative agreement was poor in the near‐critical region.

Comparison of two extraction methods independently developed on two conceptually different automated supercritical fluid extraction systems for the determination of polychlorinated biphenyls in sediments

Two extraction methods that independently have been developed on conceptually different automated supercritical fluid extraction systems, ISCO SFX 3560 (syringe pump and liquid trapping) and Hewlett-Packard 7680T SFE (reciprocating pump and solid-phase trapping), were compared for the extraction of polychlorinated biphenyls from two Swedish sediments. The results demonstrated that the high-temperature ISCO method in some cases yields a more exhaustive extraction, but also less clean extracts due to co-extraction of unwanted matrix components which are all present in the trapping solvent. The medium-temperature Hewlett-Packard method may sometimes cause problems with quantitative recoveries, but on the other hand it yields very clean extracts due to the extra selectivity resulting from collection on a solid-phase trap.

ASCAObservations of the Absorption Line Features from the Superluminal Jet Source GRS 1915+105

We have carried out a precise energy spectral analysis of the superluminal jet source GRS 1915+105 observed with ASCA six times from 1994 to 1999. The source was so bright that most SIS data suffered from event pileup. We have developed a new technique to circumvent the pileup effect, which enabled us to study the spectrum in detail and at high resolution (ΔE/E ≈ 2%). In the energy spectra of 1994 and 1995, resonant absorption lines of Ca XX Kα, Fe XXV Kα, Fe XXVI Kα, as well as blends of the absorption lines of Ni XXVII Kα + Fe XXV Kβ and Ni XXVIII Kα + Fe XXVI Kβ, were observed. Such absorption lines have not been found in other objects, except for iron absorption lines from GRO J1655-40, another superluminal jet source. We carried out a curve of growth analysis for the absorption lines and estimated column densities of the absorbing ions. We found that a plasma of moderate temperature (0.1-10 keV) and cosmic abundance cannot account for the observed large equivalent widths. The hydrogen column density of such plasma would be so high that the optical depth of Thomson scattering would be too thick (NH 1024 cm-2). We require either a very high kinetic temperature of the ions (100 keV) or extreme overabundances (100 Z☉). In the former case, the ion column densities have reasonable values of 1017-1018 cm-2. We modeled the absorber as a photoionized disk which envelops the central X-ray source. Using a photoionization calculation code, we constrain physical parameters of the plasma disk, such as the ionization parameter, radius, and density. Estimated parameters were found to be consistent with those of a radiation-driven disk wind. These absorption line features may be peculiar to superluminal jet sources and related to the jet formation mechanism. Alternatively, they may be common characteristics of supercritical edge-on systems.

A polymer NMR cell for the study of high-pressure and supercritical fluid solutions

Nuclear magnetic resonance (NMR) offers researchers unique, highly localized molecular information. The importance of this technique is well established in studies using chemical shift, spin coupling, and relaxation times providing detailed structural information, determining chemical equilibria and kinetics, and understanding molecular dynamic processes. However, the widespread application of NMR spectroscopy to high-pressure liquids and supercritical fluids has been limited due to the complexity of the necessary instrumentation. One approach to these studies is to build a dedicated high-pressure probe. Another involves the utilization of a high-pressure cell designed to fit in commercially available probes. Here we present the design and implementation of a simple, three-piece, high-pressure NMR cell constructed of high-performance polymers. The present cell has pressure capabilities of up to 400 bar; however, the ultimate temperature and pressure limits will be determined by the specific polymer chosen. High-resolution NMR spectra of methanol modified and tributyl phosphate (IBP) modified supercritical CO2 are presented. An example of supercritical fluid phase behavior monitored with NMR is demonstrated for the TBP system in which the chemical shift changes in the 31P nucleus as a function of density are indicative of solution phase separation. The multinuclear NMR data demonstrate the utility of this cell for studying supercritical fluid solution systems relevant to analytical separations and extractions.

Intermolecular Interactions and Local Density Augmentation in Supercritical Solvation: A Survey of Simulation and Experimental Results

Data from prior simulation and experimental studies (a total of 52 solute/solvent pairs) are collected and analyzed in an attempt to relate the extent of local density augmentation in supercritical fluids to the strength of intermolecular interactions. For this purpose, intermolecular potential functions consisting of pairwise additive atom−atom potentials, with parameters either taken from literature sources or derived from quantum chemical calculations, are constructed and tested against experimental second-pressure virial coefficient data. For the solute−solvent combinations of interest in supercritical systems near room temperature, such potentials are found to reproduce experimental second-pressure virial coefficient data with reasonable accuracy. On the basis of these potentials, a variety of characteristics of solute−solvent and solvent−solvent interactions are computed and compared to simulated and experimental measures of density augmentation. It is found that the extent of augmentation is strong...

環境有害物質処理プロセスと装置材料 超臨界環境における金属材料の腐食

環境有害物質処理プロセスと装置材料 超臨界環境における金属材料の腐食

Supercritical carbon dioxide spray systems

The use of CO2 for spray coating is a promising technique for volatile organic compound reduction in the paint industry. However, the polymer precipitation might be induced by the addition of CO2 into solvent, since CO2 is a poor solvent for most polymer solutions. In this work, a solvent selection guide for CO2 spray coating is proposed based on the solubility parameter from the viewpoint of avoiding the polymer precipitation. The precipitation tests were carried out for polymer (acrylic resin) + solvent (eight kinds) + CO2 systems. The solubilities of CO2 in the polymer + solvent (diethyl ketone, butanol, and methanol) varied from 7 to 33 wt.% CO2 at 40 °C for pressures from 0.9 to 6.4 MPa. It was found that the solubility parameter of the mixture at the precipitation point had similar values that are due to dissolution of the CO2 in the solution.

Full Multiple Scattering Approach to X-Ray Absorption near Edge Structure Spectra of Bromonaphthalene in Supercritical Xenon

Detailed X-ray absorption near edge structure (XANES) analyses are carried out in order to study the local structure around a Br atom of bromonaphthalene dissolved in supercritical fluid Xe by usage of full multiple scattering calculations. We find that about six Xe atoms surround the Br atom at the distance of about 5 Å, which is larger than the sum of the van der Waals radii (4.15 Å). Other important information is the relative thermal fluctuation (Debye-Waller factor) between Br and Xe, which is quite large, ∼0.8 Å. This large thermal fluctuation and clustering are essential characteristic features of supercritical systems.

Theory and simulation of cohesive diffusion in nanopores: Transport in subcritical and supercritical regimes

We have studied a lattice model of self-diffusion in nanopores, to explore how loading, temperature, and adsorbate coupling influence benzene self-diffusion in Na–X and Na–Y zeolites. We propose a simple method for determining how adsorbate–adsorbate interactions modify activation energies of site-to-site jumps. We apply a mean-field approximation that describes transport semiquantitatively for a wide variety of system parameters, simplifying kinetic Monte Carlo simulations. We also derive an analytical diffusion theory that provides semiquantitative apparent activation energies, and qualitatively reasonable loading dependencies. We have found that supercritical systems exhibit three characteristic loading dependencies of diffusion, depending upon the degree of degeneracy of lattice sites. Subcritical diffusion systems are dominated by cluster formation, exhibiting intriguing loading dependencies with broad regions of constant diffusivity. Our model for benzene in Na–X is in excellent qualitative agreement with pulsed field gradient nuclear magnetic resonance (NMR) diffusivities, and in qualitative disagreement with tracer zero-length column (TZLC) data. We suggest that high-temperature TZLC experiments should be performed, to test whether the coverage of maximum diffusivity decreases with increasing temperature.

Supercritical carbon dioxide spray systems

Coatings are not just the first line of defense against corrosion. They also fulfill a variety of other important tasks in pipeline operation such as friction reduction, insulation, cathodic current minimization, and so on. Therefore an understanding of coating technology is essential for the modern pipeline engineer.The first half of this chapter contains a summary of current external and internal coating products applied to transmission pipelines and the different roles they fulfill. It explores at a basic level, the modes of functioning for each product class with the purpose to enhance the reader's understanding of the problems and the solutions thereto. The second half discusses pressing challenges in the field and new products that have been developed to address these challenges.

Corrosion of High-Temperature Alloys in Chloride-Containing Supercritical Water Oxidation Systems

Abstract Supercritical water oxidation (SCWO) is a promising and effective method for the oxidation of organic wastes. Its high solubility for organic compounds and its special physical properties (i.e., density and viscosity) result in a key advantage over standard processes such as incineration. During oxidation of chlorine organics, severe corrosion attack of most of the materials has been observed as a result of the formation of hydrochloric acid (HCl). An investigation was conducted of corrosion of high-temperature alloys such as alloy 214 (UNS N07214), alloy 602CA (UNS N06602), alloy G-30 (UNS N06030), alloy 625 (UNS N06625), and alloy 686 (UNS N06686), exposed at 400 bar (40 MPa) and 420°C for 24 hin a dichloromethane (CH2Cl2)/hydrogen peroxide (H2O2) mixture with ∼ 2,000 wppm chloride (Cl2). Weight losses of all the samples varied between 1.3 mm/y for alloy G-30 and ∼ 31.6 mm/y for alloy 686. Alloy 214 showed increased corrosion for the preoxidized material, whereas the corrosion rate of alloy 602...

Supercritical fluids—a novel approach to magnetic media production?: An introduction to supercritical fluid technologies and how they might be applied to deposition/impregnation of metallics, magnetic inks and protective lubrication

The production of magnetic media requires a combination of substrate surface engineering, magnetic ink or metal deposition, and a thin-film high-performance lubricant to protect the surface and read–write head from abrasive wear. The lubricant, such as a perfluoroether oil or derivative, is normally applied via a dilute solution in fluorocarbon 113 solvent which is now known as a banned ozone depleter. The search for alternatives has led to developments in water-borne coatings but water can be an expensive and troublesome solvent. A novel means of delivering magnetic inks to a surface may be via supercritical carbon dioxide or xenon spray systems. Fluorinated and silicone oils are soluble in supercritical CO 2 and can also be applied by this residue-free technique. Supercritical fluids offer the opportunity to deposit/impregnate metals into surfaces via organometallics. Could this replace the future magnetic media production?

Determination of the Local Environment Surrounding Pyrene in Supercritical Alkanes: A First Step toward Solvation in Supercritical Aviation Fuels

We report quantitative data on the local microenvironment surrounding a model organic solute (pyrene) dissolved in several supercritical components of aviation fuels (n-pentane, n-hexane, n-heptane, n-octane). Toward this end, we use static fluorescence spectroscopy to determine whether these supercritical alkanes influence the local microenvironment surrounding pyrene in a way similar to other supercritical fluids (e.g., CO2, CF3H, C2H6, H2O). Steady-state fluorescence measurements indicate that, in all alkanes studied, there is an increase in local fluid density surrounding the pyrene molecule relative to the fluid bulk density (solute−fluid density augmentation). The maximum in this local density augmentation occurs at approximately one-half the fluid critical density and subsequently decreases at higher fluid densities. These results are fully consistent with observations for pyrene in other supercritical fluid systems (i.e., CO2, H2O). The maximum relative local density augmentation increases with in...

Amplitude-shape method: The quasistatic method revisited

The numerical solution of large stiff systems of ordinary differential equations is very expensive and requires large and fast computers. This is painfully clear to all who attempt to solve numerically nuclear reactor kinetics equations. The discrete form may involve as much as 100 000 ordinary differential equations and the stiffness is supplied by prompt neutrons. No wonder that reactor physicists were for years looking for some sort of simplification and this came about in the middle of the 60' as the widely used quasistatic method. Unfortunately, it had been based upon purely heuristic grounds and never left the realm of the reactor physics remaining completely unknown for the specialists in numerical solution of ordinary differential equations. In this article we show that the amplitude-shape method (ASM) which takes from the quasistatic method the representation of the solution as the product of the fast chaiiging amplitude and slow varying shape function, can be used in tlie reactor kinetics. The ASM whose full exposition is given in [5], has been developed to be applicable for numerical solution of a large class of systems of ordinary differential equations and is particularly useful in case of partial differential equations describing thc evolution of physical systems. In this paper we present the application of the ASM to the reactor kinetics equations. We present the numerical results for model equations and show that the ASM works equally well for subcritical or for supercritical systems. From [5] it follows the ASM can be also used for nonlinear problems of nuclear reactor dynamics. Practical implementation of the ASM in reactor codes is being currently investigated.

Equation of State Mixing Rules That Incorporate Only the Residual Part of the UNIQUAC Model

New mixing rules that incorporate the residual part of the UNIQUAC activity coefficient model and the correct quadratic concentration dependence of the second virial coefficient are presented. Following our recent approach, only the attractive part of the excess Helmholtz energy derived from the equation of state (EOS) is related to the excess Gibbs energy. It is recognized that, in the extension of this concept to the UNIQUAC model, the interpretation of the physical contributions of the combinatorial and residual parts must be considered. We conclude that the attractive forces are related only to the residual part of the UNIQUAC model. This allows the formulation of a class of mixing rules that are independent of the repulsive term in the EOS and are applicable to both cubic and noncubic EOS. The proposed mixing rules are applied to different low-pressure systems, exhibiting vapor−liquid and liquid−liquid equilibria, and to two near critical and supercritical systems.