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Electrolysis of supercritical aqueous solutions at temperatures up to 800 K and pressures up to 400 MPa
Abstract
A study on the electrolysis of sub- and supercritical aqueous solutions of sodium hydroxide is presented here. Current density–potential curves are recorded at temperatures up to about 800K and pressures up to 400MPa. The molar concentration of the solutions, as referred to T=298K, is varied from c=(0.01to1)mol·dm−3. The autoclave is described in detail. Several electrode materials are examined. The highest reproducibility is obtained with gold electrodes. Some results are also given for platinum, silver and nickel electrodes. Below T=473K the current–potential curves show the familiar transition from a range of very low currents to an ohmic behaviour with steeply increasing currents above the decomposition potential. The decomposition potential decreases with increasing temperature. Above T=473K the current–potential curves no longer reflect distinct transitions from low-current to high-current behaviour, and at T=773K, almost linear current–potential curves are observed over the entire voltage range. Up to current densities of 50mA·cm−2, the pressure dependence of the current density at a given potential is marginal. At higher current densities, up to 35A·cm−2, a substantial pressure effect is observed.
... The electrochemical reactor is merely a part of the larger hydrogen production framework developed for this project. In previous explorations of high temperature and pressure electrolysis researchers were limited by their reactor designs; they were unable to perform thorough electrochemical investigations due to an inability to manage the pressure caused by the generation of hydrogen gas [34,35]. To address this challenge, we have developed a framework that can be used to monitor and release pressure as necessary. ...
Water electrolysis technologies aim to provide a significant increase in green hydrogen production efficiency. In this work, a framework was developed to explore the use of supercritical water for alkaline electrolysis. This framework was used to perform Arrhenius analysis as a function of potential, and to explore activation energies for sub- and supercritical water electrolysis. An analysis of the conductivity of solution unveiled a discontinuity in the trends between sub- and supercritical potassium hydroxide solution conductivity. Unlike prior work on supercritical water electrolysis, this work investigates trends in electrochemical parameters, the sources of these trends, and how they change between the sub- and supercritical regimes.
... The electrochemical reactor is merely a part of the larger hydrogen production framework developed for this project. In previous explorations of high temperature and pressure electrolysis researchers were limited by their reactor designs; they were unable to perform thorough electrochemical investigations due to an inability to manage the pressure caused by the generation of hydrogen gas [34,35]. To address this challenge, we have developed a framework that can be used to monitor and release pressure as necessary. ...
... And the non-polar gas (e.g. oxygen and hydrogen) under the supercritical state can be highly miscible with water [32] which would result in a very high coefficient of diffusion [33]. The high diffusion coefficient would enhance the ionic strength of electrolyte and meanwhile reduce the mass transfer resistance of electrolytic reactants during the ...
To strengthen the formation of Hydroxyl radical (HO) on anode, pressurized water electrolysis under constant temperature were carried out in this work. The HO formation was verified by electrochemistry-electron spin resonance (ESR) and bauxite desulfurization in the same pressurized reactor with different cell designs. With increasing pressure, the current density was increased. However, the equilibrium potential of anode and the initial potential of oxygen evolution reaction (OER) were negative shifted with increasing pressure. Results revealed that pressure promoted the early steps of OER but inhibited subsequent transformation. The apparent transfer coefficient of the anode (ba and bc) further proved that the pressure play a positive role on the OER reserve reaction and promoted the formation of HO. Rct and Rs were all decreased with increasing pressure, demonstrating that pressure improved the mass transfer and reaction of the OER. The electrolysis desulfurization and ESR experiments have clearly supported the positive role of pressure on strengthening the HO formation. From electrolysis with 0.1 MPa, HO signal was detected by ESR when constant potential was 0.8 V on anode, while the signal arose at 0.7 V under 1.0 MPa, indicating that HO was formed in advance under pressure.
... With increasing temperature and pressure (especially, to achieve zero boundary state), the water dissociation constant (K w ) would in- crease obviously and produce more OH ? and H ? . Under this circumstance, water could be highly miscible with nonpolar gases such as hydrogen and oxygen [22], and the reactants exhibited very high diffusion coefficients [23]. These advantages applied in the electrolysis cell would certainly increase the ionic strength of the electrolyte and reduce the mass transfer resistance of surface chemical reactions. ...
... 37,38 Pressurization is a way which can improve the concentration of dissolved oxygen in the solution 42 and enhance cell performance. 43,44 Furthermore, the pressure can strengthen the ion concentration near the electrode, 45 strengthening the water electrolysis and promoting the OER. 46,47 However, it was still unclear whether the pressurization could simultaneously enhance the delivery of dissolved oxygen and the electrode reaction, and also, whether the pressurization could enhance the mass transfer within the boundary layers is still a question. ...
Oxygen reduction reaction (ORR) not only needs the excellent electrode materials, but also requires quick mass transfer of dissolved oxygen. To strengthen the ORR and O2⁻ oxidation desulfurization, pressurized water electrolysis was carried out in this work. The results showed that the desulfurization ratio increased with increasing pressure, indicating that the pressure has strengthened the O2⁻ generation. As expected, the ORR initial potential shifted from −0.034 V to −0.030 V with increasing pressure from 0.1Mpa to 2Mpa. The peak current, the limit current density and the peak integral area all increased with increasing pressure. On the other hand, the reduction in solution resistance (Rs) and reaction resistance (Rct) further verified that the pressure can strengthen the transfer of dissolved oxygen and the ORR dynamics. More importantly, the pressure has improved O2⁻ generated potential in a large parameter window (the width of the metastable zone, the difference between the initial potential and the peak potential of the ORR). The pressure could enhance the O2⁻ formation, and improve the desulfurization process.
... On the other hand, some research works on water vapor electrolysis have been carried out. Ganley [4] and Boll and co-workers [5] reported that increasing the operating steam electrolysis temperature and pressure up to 400 C and z10 MPa or 500 C and 400 MPa respectively, improved the reaction kinetics at the electrode surface and lowered the applied potentials to provide high current densities. However, increasing bath temperature and pressure drastically decreased the terminal potentials and strongly affected the stability of electrode materials in concentrated alkaline corrosive media. ...
The electrodeposition of ternary NiWMo alloys films from citrate ammonia-free electrolyte at room temperature was studied in an effort to evaluate the effect of applied potential on the composition limits, corrosion resistance and the electrocatalytic properties of the deposits towards the hydrogen evolution reaction (HER) in concentrated alkaline solution. The alloys were potentiostatically electrodeposited onto pure copper sheet substrates. The electrodeposits were characterized by means of field-emission scanning microscopy (FESEM) and energy dispersive X-ray analysis (EDXA). In an electrolyte where MoO42-/WO42-=1:1, at a given deposition potential, there is more Mo than W in the deposits, indicating an advantageous induced co-deposition of Mo compared to W. The nucleation mechanism, studied according to Scharifker-Hills theoretical model, revealed an instantaneous nucleation followed by a three-dimensional growth. On the hand, increasing MoO42-/WO42- ratio in the electrolyte under the same deposition potential reduced both Ni and W content in the deposits. A different trend was observed in an equimolar solution, when applying more negative potentials, both Mo and W contents decreased leading to the enhancement of Ni amount. The stability in corrosive media and the catalytic performances of the coatings depended mainly on the applied overpotentials, A mechanism of induced co-deposition of molybdenum and tungsten with nickel is proposed and discussed.
... 14,15 Subsequent to an initial study indicating high energy efficiencies for water electrolysis in molten hydroxides operating up to 350 • C, 16 there have been surprisingly few studies of water splitting in molten hydroxide electrolytes at higher temperatures (up to ∼400 • C), [17][18][19] and there have been only two studies of water sodium hydroxide mixtures at supercritical conditions. 20,21 This study provides an initial exploration of water splitting in molten hydroxide electrolytes at higher temperatures ranging up to 700 • C, but at 1 atmosphere, as a prelude to the further studies of high temperature and high pressure solar water splitting. Water splitting will be demonstrated at high rates at these elevated temperatures in select electrolytes, which such as lithium and barium hydroxide, remain better-hydrated than sodium and potassium hydroxide electrolytes. ...
This study explores a new high temperature molten hydroxide domain to electrochemically split water into hydrogen fuel. Hydrogen fuel, if produced without greenhouse gas emissions, is a promising fuel for transportation. This study opens a pathway to low energy water splitting and the electrolytic production of hydrogen fuel without carbon dioxide emission. A wide range of pure and mixed alkali and alkali earth hydroxide electrolytes are explored at temperatures ranging from 200 to 700°C. Higher temperature leads to improved (lower voltage) water splitting and improved rates of charge transfer, but carries challenges (increased rates of parasitic side reactions as the molten electrolytes dehydrate with increasing temperature). This study extends the range of hydrogen formation in alkaline electrolysis water splitting to over 600°C, and demonstrates that lithium and/or barium hydroxide electrolytes remain hydrated at high temperatures, and in the high temperature domain are advantageous over sodium or potassium hydroxide electrolytes. In pure LiOH, the coulombic efficiency for hydrogen generation decreases with temperature and is measured respectively at ηH2 = 88%, 21%, 4% and 0%, respectively at 500, 600, 700 and 800°C.
We report on a thermodynamic analysis for water electrolysis from normal conditions (P = 0.1 MPa, T = 298 K) up to heretofore unaddressed temperatures of 1000 K and pressures of 100 MPa. Thermoneutral and reversible potentials are determined using equations-of-state published by the International Association for the Properties of Water and Steam and the National Institute of Standards and Technology. The need for using accurate property models at these elevated temperatures and pressures is exemplified by contrasting results with those obtained via ideal assumptions. The utility of our results is demonstrated by their application in an analysis comparing pressurized electrolysis versus mechanical gas compression. Within the limits of our analysis, pressurized electrolysis demonstrates lower energy requirements albeit with electrical work composing a greater proportion of the total energy input.
Wasser tritt nicht nur bei gewöhnlichen Temperaturen und niedrigen Drücken auf, sondern kann bei sehr hohen Temperaturen weit oberhalb seines kritischen Punkts bei 647 K vorliegen. Unter diesen überkritischen Bedingungen lässt sich Wasser kontinuierlich von gasförmigen zu flüssigkeitsähnlichen Dichten komprimieren. Die daraus resultierenden dichten überkritischen Phasen haben außergewöhnliche Eigenschaften, die durch Druck und Temperatur variiert werden können und die Grundlage innovativer Technologien bilden. Dieser Aufsatz beschreibt den derzeitigen Kenntnisstand über die wichtigsten Eigenschaften des überkritischen Wassers und seiner Gemische mit unpolaren, polaren und ionischen Stoffen sowie über die zugrunde liegenden molekularen Prozesse.
This paper describes experimental work involving the direct-current electrolysis of highly concentrated potassium hydroxide solutions at high temperatures (up to 400 °C) and under various pressures. A high-temperature alkaline electrolysis cell resistant to chemical attack from the highly corrosive electrolyte solution and capable of high-pressure operation was designed and tested. The cell was constructed with a Monel® alloy housing and cathode, while various anode materials were compared. The anode materials tested included nickel, Monel alloy, lithiated nickel, and cobalt-plated nickel. The advantages of operating an alkaline electrolysis cell at high temperatures include increasing the ionic conductivity of the electrolyte and enhancing the rates of electrochemical reactions at the electrode surfaces. Cell operation with increasing steam partial pressure over the solution is also shown to enhance cell performance. The prudent selection of anode material also impacts the required terminal potential for a given current density, and consequently the cell's electric power efficiency. The best cell performance was achieved using a cobalt-plated nickel anode at a temperature of 400 °C and a steam partial pressure of 8.7 MPa.
Water is not restricted to moderate temperatures and low pressures, but can exist up to very high temperatures, far above its critical point at 647 K. In this supercritical regime, water can be gradually compressed from gas-like to liquid-like densities. The resulting dense supercritical states have extraordinary properties which can be tuned by temperature and pressure, and form the basis for innovative technologies. This Review covers the current knowledge of the major properties of supercritical water and its solutions with nonpolar, polar, and ionic compounds, and of the underlying molecular processes.
Experimental results are reported for the fluid phase equilibria of the ternary system H//2O-CH//4-NaCl and to a lesser degree of H//2O-CH//4-CaCl//2. Measurements were extended to 800 K and 250 MPa. A 'synthetic' method, described previously, was used, supplemented with an electrolytic indication. The high pressure autoclave with sapphire windows and variable volume together with its auxiliary equipment is described. Pressure-temperature curves on the three-dimensional PTx-surfaces at constant compositions, x, were obtained ('isopleths'). Data for 21 isopleths with NaCl-concentrations of 0. 6, 1. 7 and 8 weight percent relative to water with CH//4-concentrations ranging from 6 to 71 mol percent are presented together with three isopleths for H//2O-CH//4-CaCl//2 mixtures. Molar volumes at the phase boundary surfaces are given.
Experimental data on the vapor-liquid equilibrium relations for the system NaCl-H2O were compiled and compared in order to provide an improved estimate of the P-T-x surface between 300° to 500°C, a range for which the system changes from subcritical to critical behavior. Data for the three-phase curve (halite + liquid + vapor) and the NaCl-H2O critical curve were evaluated, and the best fits for these extrema then were used to guide selection of best fit for isothermal plots for the vapor-liquid region in-between. Smoothing was carried out in an iterative procedure by replotting the best-fit data as isobars and then as isopleths, until an internally consistent set of data was obtained. The results are presented in table form that will have application to theoretical modelling and to the understanding of two-phase behavior in saline geothermal systems. -Authors
Zunächst werden Dielektrizitätskonstanten und Viskosität des Wassers bei Dichten ϱ zwischen 0 und 1 g/cm³ versuchsweise bis 800° C errechnet. Damit und aus der gemessenen Äquivalentleitfähigkeit des Kaüumchlorids wird die Leitfähigkeit für KCl bei unendlicher Verdünnung ermittelt. Aus diesen ergeben sich Dissoziationsgrade α und Dissoziationskonstanten des KCl Für Lösungen mit dem KCl-Molenbruch 1,8 · 10⁻⁵ steigt z. B, bei 600°C α von 0,2 auf 0,97, wenn ϱ von 0,3 auf 0,8 wächst. Die Konstanten nehmen im gleichen Dichteintervall von etwa 10⁻⁶ auf etwa 10⁻² mol/l zu. Bei konstanter Dichte nehmen sie mit steigender Temperatur stets ab. Die Hydratationszahlen für K⁺- und Cl⁻-Ionen im überkritischen hochkomprimierten Dampf liegen zwischen 6 und 7. Die Hydratationsenergie ergibt sich zu etwa 12 kcal/mol je H2O-Molekel.
Experimental results are reported for the fluid phase equilibria of the ternary system H2O-CH4-NaCl and to a lesser degree of H2O-CH4-CaCl2. Measurements were extended to 800 K and 250 MPa. A “synthetic” method, described previously, was used, supplemented with an electrolytic indication. The high pressure autoclave with sapphire windows and variable volume together with its auxiliary equipment is described. Pressure-temperature curves on the three-dimensional PTx-surfaces at constant compositions, x, were obtained (“isopleths”). Data for 21 isopleths with NaCl-concentrations of 0.6, 1.7 and 8 weight percent relative to water with CH4-concentrations ranging from 6 to 71 mol percent are presented together with three isopleths for H2O-CH4-CaCl2 mixtures. Molar volumes at the phase boundary surfaces are given. In comparison with the binary H2O-CH4 system, investigated earlier, the addition of only 0.53 mol% NaCl shifts the range of partial immiscibility to higher temperatures by more than 100 K (for example from 610 to 715 K at 100 MPa for 51 mol% CH4). The shift produced by CaCl2 appears to reflect incomplete ionic dissociation at high temperatures. Encouraging results of computations of critical curves using pseudo-binary approximations and a new equation of state based on defined interaction models are shown.
The combustion of methane with oxygen in supercritical homogeneous aqueous fluids has been investigated and stationary diffusion flames generated to pressures of 2000 bar. A reaction autoclave with sapphire windows contains high pressure homogeneous mixtures of water and methane to 500°C. A typical mixture composition is 70 to 30 mole percent of H2O and CH4. A quasi-circular fluid flow permits the steady injection of oxygen through a 0.5 mm nozzle at rates of 1 — 10 mm3 s−1 at constant pressures. — Above 400°C spontaneous ignition of flames occurred. The flames were observed and recorded with microscope and video camera. Emission spectra in the UV-region were obtained and samples could be taken for analysis. Below about 400°C flame-less oxidation is detected. The stationary diffusion flames are cone-shaped and typically about 3 mm high. Flame examples for pressures between 300 and 2000 bar are shown. Preliminary temperatures derived from OH-spectra are close to 3200 K. — Water can be replaced by argon.
Liquid-vapor critical temperatures (tc) have been determined visually for a variety of single electrolyte aqueous solutions at several molalities up to 1·5–2·5. The dissolved electrolytes studied were LiCl, NaCl, KCl, KBr, KI, CsNO3, KHSO4, NH4Cl, NH4HCO3, (NH4)2CO3, (NH4)2SO4, CaCl2, MgCl2, Mg(NO3)2, HCl, HClO4, HNO3, CH3COOH, H2SO4, H3PO4 and H3BO3. The alkali halide solutions produced the greatest increase in tc from that of pure water (tc = 374.2°C) where at 2 m all values of tc were in the range of 450 to 480°C, providing an increase of +80 to +110°C. Several of the acids (H2SO4, H3PO4, H3BO3) and divalent salts (CaCl2, MgCl2, Mg(NO3)2, (NH4)2SO4) showed intermediate increases, while particular solutes (HCl, HClO4, (NH4)2CO3, NH4HCO3, CH3COOH) gave very little change from tc for water. It is suggested that these differences in behavior are related to differing electrolyte association, hydration, ion size, decomposition for particular electrolytes (HClO4), and other properties.
Binary mass diffusion coefficients and thermal diffusivities for hydrothermal sodium nitrate solutions as a function of pressure (270 < P < 1000 bar), temperature (400 < T < 500{degree}C), and concentration (0.25 < C < 3.0 m) were measured by the laser-induced grating technique. In concentrated hydrothermal NaNO{sub 3} systems, the critical slowing down was significant as far as 300 bar from the phase-separation pressure, resulting in binary diffusion coefficients near the critical point that are comparable to values at ambient conditions. Further from the critical point the mass diffusion coefficients plateaued at their ordinary values. Ordinary binary mass diffusion was about 15 times faster than at 25{degree}C and atmospheric pressure. The Wilke-Chang correlation also yielded good predictions when the solute molar volume was defined as the volume of the hydrated contact ion pair. Predictions can be improved by about 10% if the degree of association can be calculated. Thermal diffusion coefficients (Soret effect) at 450{degree}C were also measured and are about 250 times faster than at ambient conditions. The laser-induced grating technique was found to be highly complementary to the Taylor dispersion technique for diffusion measurements in hydrothermal systems. 88 refs., 10 figs., 3 tabs.
This work investigates the conductivity of tetraalkylammonium tetraarylborates as a function of concentration in supercritical carbon dioxide (sc CO2) at 70°C and 300 bar and compares the ion dissociation constants with those recently measured in cyclohexane. The double layer capacitance is also reported for the first time in a supercritical fluid and found to fit perfectly to a Helmholz model. These data are used to interpret the voltammetric response of a nickel maleonitrile complex at a platinum microelectrode in sc CO2 and cyclohexane.
This paper reviews and evaluates the experimental works of the static dielectric constant (permittivity) of water and steam over the century. The critically evaluated experimental data are represented by a function of temperature and density. This representation was carefully examined in the light of the criteria for smoothness and physical plausibility. As a result of this work, which was largely stimulated by the activities of the International Association for the Properties of Steam, a new International formulation for the static dielectric constant of water and steam was adopted. This formulation covers a temperature range from 0 to 550 °C and a pressure range up to 500 MPa.
The electrolytic conductance of aqueous solutions of KCl, HC1 and H2SO4 has been determined between 45 and 220°C and up to 8000 bars. The conductivity of hydrogen ions has been derived and the abnormal proton mobility is discussed as a function of temperature, pressure and water density. The second dissociation constant of sulphuric acid is given up to 190°C for 4000 and 8000 bars.
Die Begrenzung des Zweiphasengebietes im System Kohlendioxid-Wasser wurde zwischen den kritischen Temperaturen der beiden Komponenten (+ 31 °C und 374 °C) bis zu Drucken von 3500 bar gemessen. Das Gleichgewicht wurde in einem Hochdruckgefäß aus einer korrosionsbeständigen Legierung mit Hilfe eines elektromagnetischen Rührers eingestellt und durch Analyse kleiner, aus verschiedenen Höhen des Meßvolumens entnommener Proben gemessen. Die Gleichgewichtszusammensetzungen von Dampf und Flüssigkeit werden als Funktion von Druck und Temperatur mitgeteilt. Die aus den Koexistenz-kurven ermittelte kritische Kurve verläuft vom kritischen Punkt des reinen Wassers (374,15°C, 221,3 bar) zu niedrigeren Temperaturen und höheren Drucken, und orreicht bei 266 °C ein Minimum in der kritischen Temperatur bei einem kritischen Druck von 2450 bar und einer kritischen Zusammensetzung von 41,5 Mol-% Kohlendioxid. Mit weiter steigendem Druck wendet sich die kritische Kurve wieder zu höheren Temperaturen. Vorn kritischen Punkt des reinen Kohlendioxids ausgehend erreicht die kritische Kurve schon bei 31,5°C einen unteren kritischen Endpunkt. Das System Kohlendioxid-Wasser zeigt unter hohen Drucken die sogenannte Dichteumkehr der Phasen, wobei die Dampfphase dichter wird als die flüssige Phase. Aus dem Verlauf der kritischen Kurve und der Gleichgewichtskurvon werden einige thermodynamische Schlüsse gezogen.
The self‐diffusion coefficients D of compressed supercritical water have been measured as a function of pressure in the temperature range 400 to 700 °C using the NMR spin–echo technique. The experimental diffusion data were compared to theoretical predictions based on a dilute polar gas model using a Stockmayer potential for the evaluation of collision integrals and a temperature dependent hard sphere diameter. The empirical expression ρDαTn, where n = 0.76, fits the experimental data to within±10% over the entire density and temperature region studied. The value of the exponent n = 0.76 agrees favorably with the n values found for diffusion of other gases. The product ρD is density independent under isothermal conditions which indicates that two‐body collisions dominate the diffusion behavior. This finding is in agreement with our earlier results for proton relaxation in compressed supercritical water which were analyzed using a binary collision approximation and a collision modulated spin–rotation interaction described by a single correlation function which is an exponential function of time. The hydrodynanic Stokes–Einstein equation appears to hold at densities above ρc. As expected, the diffusion data cannot be described in terms of a hard sphere model but an empirical fit to lnρD = (A/T)+B, where A and B are constants, represents the experimental data well over the range of temperatures and densities studied.
A new type of apparatus has been constructed for carrying out electrochemistry in near-critical and supercritical aqueous solutions. The following systems have been studied at a platinum electrode: HâO/Oâ, Iâ»/Iâ, Brâ»/Brâ, and hydroquinone/benzoquinone. The compact alumina flow cell can be heated or cooled quickly and can be recharged with fresh electrolyte solution while at high temperature and pressure. A large reduction in the potential required for the electrolysis of water was observed. Diffusivities have been measured for iodide ions and hydroquinone. General agreement with the Stokes-Einstein model was observed in the temperature range 25-375°C.
An electrochemical cell and electrode system for investigation of aqueous systems under near- and supercritical conditions is described. Cyclic voltammetry of 1.5% w/w KCl and 6% w/w KBr solutions was investigated and log current vs. potential (Tafel) plots for the chlorine and hydrogen evolution reactions are given. A study of the Cu/Cu(I)/Cu(II) system in NaâSOâ and KCl solutions is described and the variation of the standard potentials of the half-reactions with temperature (to ca. 300°C) is given. The diffusion coefficient for Cu(II) was determined by chronoamperometry as a function of temperature (22-245°C) and shown to increase from 5.4 x 10â»â¶ cm²/s at 22°C to 3 x 10â»â´ cm²/s at 245°C. The change of diffusion coefficient with temperature was larger than that predicted by the Stokes-Einstein equation at temperatures above 175°C. 36 references, 12 figures, 1 table.
For more than 25 years there has been disagreement about the phase behavior in the system carbon dioxide-water at high temperatures and pressures. New data are presented in this paper for this system. Dew points for three mixtures (53.0, 62.5, and 78.9 mol % CO2) were measured at temperatures from 225 to 275-degrees-C and pressures from 114 to 311 MPa. These data appear to have resolved the discrepancy.
Die mit einem statischen Verfahren gemessenen Gleichgewichtszusammensetzungen der gasförmigen und flüssigen Phasen im System Äthan/Wasser werden zwischen 200 und 400°C bis zu Drücken von 3700 bar und für das System n-Butan/Wasser bei 355 und 364°C bis zu Drücken von 1100 bar angegeben. In beiden Systemen ist die kritische Kurve unterbrochen. Ihr oberer Zweig verläuft vom kritischen Punkt des Wassers ausgehend zu niedrigeren Temperaturen, erreicht ein Temperaturminimum (350°C bei Äthan/Wasser und 351°C bei n-Butan/Wasser) und wendet sich dann wieder zu höheren Temperaturen. Die kritischen Drücke steigen vom kritischen Druck des Wassers ausgehend monoton an.
Hochverdichteter Wasserdampf bei überkritischen Temperaturen kann Feststoffe wie Alkalisalze, Siliciumdioxyd und andere Oxyde und Hydroxyde in beträchtlicher Menge auflösen. Flüchtige Stoffe wie CO2 und Argon sind oberhalb 500°C noch bis zu mehr als 2000 bar vollständig mit Wasser mischbar. Bei Dichten von mehr als 0,2 bis 0,3 g/cm3 besitzt überkritischer Wasserdampf elektrolytische Eigenschaften. Die Dielektrizitätskonstante wird mit Werten zwischen 5 und 20 hoch genug, um die Ionendissoziation gelöster Stoffe zu ermöglichen. Entsprechend der niedrigen Viscosität des überkritischen Mediums erreichen die Beweglichkeiten der lonen darin das Zehnfache der Normalwerte. Das Ionenprodukt reinen Wassers wächst mit steigender Temperatur und Dichte und ist z. B. bei 600°C und 2000 bar etwa 105mal größer als bei 0°C. Hydrolysereaktionen werden dadurch stark begünstigt. Die Bedeutung von Lösungen im überkritischen Wasser für natürliche Lagerstättenbildungen und technische Probleme wird gezeigt. Dichte, Viscosität, Dielektrizitätskonstante und Lösungsfähigkeit überkritischen Wassers werden quantitativ diskutiert. Leitfähigkeitsmessungen werden beschrieben und Dissoziationskonstanten von Salzen sowie von KOH, HCl, HF und NH3 zwischen 400 und 750°C werden angegeben. Methoden zur Berechnung des Ionenprodukts und von Hydrolysegleichgewichten werden gezeigt.
Experimental results are reported for the fluid phase equilibria of the ternary system H2O—C2H6—NaCl, H2O—n-hexane—NaCl and CH3OH—CH4—NaBr. Measurements were extended to 800 K and 250 MPa and to 450 K and 200 MPa with the methanol system. A “synthetic” method, described earlier, was used with a high pressure autoclave with improved sapphire window arrangement. Pressure-temperature curves on the three-dimensional pTx-phase boundary surfaces at constant compositions, x, were obtained (“isopleths”). Temperatures, pressures and molar densities along 14, 5 and 7 isopleths for the systems with ethane, n-hexane and methanol-methane, respectively, are reported. Salt concentrations between 0.1 and 8 weight percent, relative to water or methanol, are applied. The addition of salt shifts the fluid-fluid two-phase region to higher temperatures and pressures in all three systems. The isobaric and isothermal shifts in certain regions can reach 100 K and more and 500 bar and more with 8 weight percent of salt. Even with only 0.1 weight percent 15 K shifts are observed near the respective critical curves. For the H2O—C2H6—NaCl and the H2O-hexane-NaCl systems computational descriptions of the isopleths, based on a “quasi-binary” approach and a recently derived rational equation of state are given.
The system H2 — H2O has been studied isochorically from 0.5 to 90 mol-% H2 and up to 440°C and 2500 bar pressure using an autoclave containing two sapphire windows through which phase transitions could be observed at elevated temperatures and pressures. — The system was found to exhibit so-called “gas-gas” immiscibility with a critical curve proceeding to higher temperatures and pressures from the critical point of pure water. Within the range of these experiments, the critical temperature of H2—H2O mixtures does not change greatly from that of pure water (e.g. Tc = 381.3°C at pc = 2520 bar for 38 mol-% H2). pVT measurements have been made in the homogeneous region and excess volumes have been calculated at 400°C and at different pressures. At 300 bar, the excess volumes are relatively large and positive (e.g. VE = 58.0 cm3 mol−1 at 40 mol-% H2) whereas at 2500 bar, excess volumes of H2—H2O mixtures indicate only small positive or negative departures from ideality.
The water-benzene mixture system at high temperature and high density is considered as a model system to study the dielectric properties of polar-nonpolar binary mixtures. The static dielectric constant or permittivity ε and the density have been measured at 300, 350 and 400°C between 100 and 2800 bar for 7 different water-benzene mixtures covering the whole range between pure water and pure benzene. Apparatus and procedure are described. An autoclave contains a cell closed with a flexible bellows. Inside the cell is a cylindrical condenser, filled with a sample mixture, separated from the pressurizing fluid. Thus dielectric and density measurements can be performed simultaneously with one sample.—Extensive tables with data are given. The uncertainty is believed to be below ±1% for most data. At 400°C and 2000 bar ε(χw = 1.0) is 19.40 and decreases to 9.67 (χw = 0.9) and to 3.68 (χw = 0.5). For pure benzene the value is 2.1. Several continuum theories are examined and discussed to represent the data. The best result was obtained with the Landau-Lifshitz-Looyenga relation. It needs dielectric constants of the pure partners and the mixture densities but requires no adjustable parameters. On the basis of the present investigation this relation is recommended also to estimate dielectric constants for dense, supercritical binary systems of water with nonpolar gases.
A high pressure autoclave with sapphire windows, auxiliary equipment and means and precautions needed for experiments with high pressure, high temperature oxygen are described. With water-oxygen mixtures of different mole fractions, x, determinations were made of liquid-gas phase equilibria conditions along “isopleths” between 500 and 660 K to 250 MPa. Molar volumes of the mixtures were measured at the three-dimensional (PTx) phase equilibria surface and in the supercritical region at 673 K. The critical curve, an envelope for the isopleths, begins at the critical point of water (647 K), has a temperature minimum (640 K) at about 75 MPa and proceeds to 250 MPa at 663 K. Phase equilibria and critical curve data are given. The H2OO2 critical PT curve is very close to the critical curve recently (10) determined for H2ON2. Values for the Henry-constant from 300 K to 647 K for mixtures dilute in oxygen are presented. The Henry constant at room temperature has only about half the value of the Henry constant for nitrogen in water. At the critical temperature of water (647 K), however, both constants do not differ by more than the uncertainty of the determinations. The excess volume was calculated at 673 K from 30 to 250 MPa. All values are positive. The excess Gibbs energy and activity coefficients are presented. One isopleth for H2O-air with x(H2O) = 0.80 was measured and molar volume values for this composition at 673 K between 33 and 280 MPa are given.
Experimental results are reported for fluid-fluid phase equilibria of the ternary system H2OCO2NaCl. Measurements were extended to 773 K and pressures to 300 MPa. A “synthetic” method was used: Known quantities of the three components were filled into an autoclave at room temperature. The autoclave with a sapphire window and variable volume is described. Transition points to homogeneous one-phase conditions could be determined from recorded p—T-curves at constant volumes and from visual observation. From the transition points p—T-curves on the three-dimensional phase equilibrium boundary surface at constant compositions, “isopleths”, were obtained. Data for 20 isopleths with CO2-concentrations between 0.2 and 85 mol% and with 6 weight YO of NaCl are presented together with a few additional results for higher salt concentrations. Molar volumes were measured at the phase boundary surface and within the one-phase region. Excess molar volumes are given. In comparison with the binary H2O-CO2 system, the addition of NaCl shifts the range of partial immiscibility to higher temperatures by up to 100 K; for example from 573 to 673 K at 100 MPa for 48 mol% CO2. Results are in agreement with earlier data of Takenouchi and Kennedy. Considerations of the ternary phase diagram H2OCO2NaCl within a wide range of conditions are given.
Nearcritical and supercritical water (Tc=647 K, Pc=22.1 MPa) is an excellent solvent for many organic compounds. Furthermore it has outstanding properties as a reaction medium because of the complete solubility of reactants, such as oxygen and hydrocarbons. To avoid the high technical demand for the supply of oxygen by compressing air, a new apparatus was developed, which produces oxygen under high pressure by means of electrolysis. For alkaline water electrolysis, an electrolysis cell was constructed working within a pressure range of 22–25 MPa and a temperature up to 623 K. Factors determining the destruction rate are, beside pressure and temperature, the electrical conductivity and the voltage used in the cell. The electrolysis cell was linked to a continuous high pressure extraction apparatus. Different aqueous waste streams (soil extracts, wool scouring wastewater, bilge water) were fed into the system and the destruction yield was studied as a function of these parameters.
- Wulf