Superambient Conditions Research Articles

Investigation of Metal–H2O Systems at Elevated Temperatures: Part I. Development of a Solubility Apparatus Specialized for Super-Ambient Conditions

Abstract Metals used in aqueous environments where high temperatures and pressures are present are susceptible to corrosion. This is the case for nuclear power plants, especially CANDUTM reactors, where the liquid water systems can reach over 300 °C at pressures well above 101.325 kPa (1 atm). In such situations, failure to control corrosion has economic and safety consequences. To extend corrosion modeling tools, such as Pourbaix diagrams, to harsh aqueous conditions, there is a need for experimental thermochemical data performed at elevated temperatures. This is particularly true for metal and alloy systems where such information is unavailable or unreliable. A relatively simple approach to obtaining temperature dependent thermodynamic properties is the investigation of solid–liquid phase, or solubility, equilibria. However, this requires specially designed instrumentation that can withstand harsh temperatures and extreme pH conditions, while providing accurate data. In this work, an apparatus developed for super-ambient highly acidic and alkaline solubility experiments is presented. Solubility measurements were made in the zinc oxide system, which has been extensively studied. Using these equilibrium data and comparing to the literature, allowed the instrumentation and analysis process to be validated. In situ pH measurements using a constant volume, batch reactor system are described along with a brief presentation of the ZnO dissolution equilibria results at 85 °C (358.15 K).

Effect of super-ambient conditions on the upper explosion limit of ethane/oxygen and ethylene/oxygen mixtures

The upper explosion limit (UEL) of ethane/oxygen and ethylene/oxygen at elevated pressures up to 2.6 MPa and temperatures up to 543.15 K were measured. A coiled nichrome hotwire placed at the center of the vessel was used as the ignition source. Explosion is said to have taken place if there is a pressure rise of at least 5% of the initial pressure in the vessel after ignition. Experimental data obtained in the 1.5 L cylinder and the 20 L sphere had a difference less than 0.5%. Results showed that the test data obtained in the 1.5 L cylinder were reliable (with the relative standard deviation less than 0.15%), and can be used for industrial applications. Leveling off curves of UEL in oxygen depended on temperature and linear (logarithmic) relationships depended on pressure were shown. The transition pressures of 1.6 MPa (ethane/oxygen) and 1.0 MPa (ethylene/oxygen) which made the slope (increasing rate) of UEL curves change from pressure-sensitive to temperature-sensitive were found. Algebraic equations were established for assessing the data of UEL of ethane/ethylene in oxygen at elevated conditions straightforward.

A Comparative Investigation of Mixing Rules for Property Prediction in Multicomponent Electrolyte Solutions

A mathematical technique is developed to investigate physicochemical property prediction of solution mixtures from the corresponding properties of the pure dissolved systems, as is often expressed in empirical ‘mixing rules’ such as those of Young and of Zdanovskii. A systematic method to distinguish between the inherent characteristics of such rules is needed because experimental studies have proved indecisive. Sound mixing rules must be found to support current efforts in thermodynamic modelling where conventional approaches like the Pitzer equations lack robustness. Density differences relative to pure water, osmotic coefficients and heat capacities are investigated with mixtures including {NaCl + MgCl2}(aq) and {NaCl + Na2SO4}(aq) as specific examples representing common-anion and common-cation asymmetric strong electrolyte solutions respectively. Water activity curves for hydrochloric acid and the alkali metal chloride solutions are also considered. The results confirm that, at the present state of the art, differences between mixing rules are for the most part insignificant at 25 °C, being about the same or less than would be expected from experimental uncertainty. As the predicted differences are even smaller at higher temperature, it can be posited that all reasonably well-established mixing rules in the literature will give approximately equivalent and satisfactory predictions of solution properties under superambient conditions. This is particularly important since the effects of temperature on the magnitude of ternary interactions are not well known from experiment.

Flow calorimetry: a technique for caracterisation of liquid-liquid and liquid-gas systems

The laboratory develops projects on acid gas mitigation by capture and storage of carbon dioxide in post combustion industrial effluents. The systems investigated are complex and involve molecular and ionic species in solution and liquid-liquid or vapor-liquid phase equilibria. Then it is of interest to collect consistent experimental data to optimize parameters of theoretical models. In open literature, experimental data mainly concern phase equilibrium and solubility limits as function of temperature, pressure and phase compositions. The determination of energetic properties is more challenging and literature data become scarce. However, they are necessary for understanding chemical mechanisms (ARCIS). Our laboratory develops several calorimetric techniques for the determination of enthalpy of mixing. The description and principles of the techniques are illustrated by examples of applications in the domain of CO2 capture by dissolution in aqueous solutions of amines. The heat of gas dissolution is determined in dynamic mode (Mathonat Arcis MEA) using customized mixing flow unit adapted to Setaram heat conduction differential calorimeters (C80 or BT215). In order to represent the mechanism of chemical dissolution (Simond) the technique was adapted for the measurement of amine protonation constant. Recently it was used to investigate the gas dissolution in demixing amines of interest for breakthrough capture processes (Raynal). The particularity of such amines is a liquid-liquid phase separation in aqueous solution as function of temperature and CO2 gas loading. Such phase separation can be observed when determining excess enthalpies of binary {water + amine} mixtures. The presentation will report different examples in this field and will emphasis the limitations and difficulties associated to measurements at super ambient conditions. REFERENCES [1] Arcis, H.; Rodier, L.; Coxam, J.-Y. J. Chem. Thermodyn. 2007, 39, 878-887.

Standard molar volumes and heat capacities of aqueous solutions of sodium trifluoromethanesulfonate at temperatures up to 573 K and pressures to 28 MPa

Densities and heat capacities of aqueous solutions of sodium trifluoromethanesulfonate (sodium triflate) of concentrations from 0.025 to 0.3mol·kg−1 were measured with high temperature, high pressure custom-made instruments at temperatures up to 573K and at pressures up to 28MPa. Standard molar volumes and standard molar heat capacities were obtained via extrapolation of the apparent molar properties to infinite dilution. The results for volumetric properties are consistent with earlier literature data, but no previous measurements exist for heat capacities of sodium triflate at superambient conditions. The new data were used for calculating the standard molar volumes and heat capacities for the triflate anion and compared with the results for triflic acid that should be essentially identical within the expected error margins. At temperatures above 473K an effort was made to refine the processing of literature data for HCl(aq), taking into account its partial association, and subsequently to modify the value for Na+ ion calculated from the standard thermodynamic values of NaCl(aq) where its ion pairing was already considered. This approach yields reasonable agreement at high temperatures between the values for triflate ion calculated from its salt and those for triflic acid.

High-Pressure Ion Mobility Spectrometry

The effects of above-ambient pressure on ion mobility on resolving power, resolution, and ion current were investigated using a small, stand-alone ion mobility spectrometer (IMS). This work demonstrates the first example of ion mobility spectrometry at pressures above ambient. Ion mobility spectra of chemical warfare agent (CWA) stimulant dimethyl methylphosphonate (DMMP) and several other standard compounds are shown for superambient conditions. The IMS was operated at pressures from 700 to 4560 Torr. An optimal resolving power was obtained at a specific voltage as a function of pressure, with higher optimal resolving powers obtained at higher voltages, as predicted from standard IMS theory. At high pressures, however, resolving power did not increase as much as theory predicted, presumably due to ion clustering. Nevertheless, an increase in pressure was found to improve resolution in IMS. One example where high pressure improved resolution was the separation of cyclohexylamine (K(0) = 1.83) and 2-hexanone (K(0) = 1.86) (where K(0) is the reduced mobility value). The product ions of these two compounds could not be separated at ambient pressure but could be nearly baseline separated when the pressure of the buffer gas was raised to 2280 Torr. Total ion current was also examined at pressures from ambient up to 4560 Torr. Total ion current, when investigated with pressure, was found to reach a maximum, initially rising with increased pressure.

Data and models for calculating the standard thermodynamic properties of aqueous non-electrolyte solutes under hydrothermal conditions

The standard chemical potentials μs of aqueous solutes over a wide range of temperatures and pressures are needed to calculate phase and chemical equilibria in hydrothermal systems. The standard derivative thermodynamic properties of solutes such as enthalpies, heat capacities and volumes can be integrated to calculate μs at super-ambient conditions. These derivative properties can be obtained by extrapolation to infinite dilution of calorimetric and volumetric data for aqueous solutions measured as a function of concentration at various temperatures and pressures. A large amount of new experimental results has been reported over the past two decades, which allowed improvement of predictions based on equations of Helgeson, Kirkham and Flowers, among others. Several new thermodynamic models for the standard thermodynamic properties have been proposed during this period. This paper focuses on non-electrolyte solutes and provides an overview of the recent sources of experimental data. We also compare the ability of several thermodynamic models to correlate and predict μs. This paper was presented at «Fluid/minerals Interactions in the Crust», a symposium in honour of Martine Lagache, Paris, October 1999

00/01294 Process and apparatus for gasifying solid carbonaceous material

The objective of this paper is to review experimental and theoretical studies of gas-solid fluidization at elevated temperatures and pressures. The survey begins with the low velocity end of operations in the region between minimum fluidization velocity and minimum bubbling velocity and shows how correlations established at ambient temperature and pressure for these two quantities may be used to calculate their values at super-ambient conditions. The application of purely hydrodynamic fluid-bed stability criteria to account for the transition from the non-bubbling to the bubbling state is described and compared with the expected effect of interparticle forces on this transition. The effects of temperature and pressure on the dynamics of gas bubbles in powders of Groups A, B and D are considered next and areas of uncertainty in current theories of bubble motion are highlighted. Correlations for jet penetration are then discussed and recommendations made as to the most reliable of these. Circulating fluidized beds (CFBs) operated at high velocity are then considered and it is shown that many of the observed effects in these systems at superambient conditions can be accounted for in terms of changes in the value of the terminal fall velocity, ut, of the bed particles. The effects of changes in ut on entrainment, elutriation and choking are also considered. The effect of increased pressure in enhancing bed-to-surface heat transfer coefficients in beds of Group A powders is shown to be due to the suppression of bubbling while in beds of Group B materials the enhancement is through an increase in the gas convective component of the transfer coefficient. The small amount of work carried out on heat transfer in CFB combustors is reviewed. Pressure effects on the combustion of char in bubbling beds are considered in terms of an established two-phase theory model and it is concluded that the increased rate of solids burn-out at high presures is due to an increase in the value of the local Sherwood number thereby increasing the rate of mass transfer of oxygen to the surface of the burning particle. The important question of sintering leading to defluidization at elevated temperatures is then examined and attention drawn to the current lack of broadly based mechanistic models to account for and predict the phenomenon. The state of the art in the area of scaling relationships is reviewed and it is shown that while the scaling laws for bubbling beds are by now reasonably well established the same is not so for CFBs, indicating a major area for further work.

Thermodynamics of aqueous acetic and propionic acids and their anions over a wide range of temperatures and pressures

The apparent molar volumes of aqueous acetic acid were determined ia density measurements at 298 K<T<573 K and 10 MPa<p<38 MPa. The results were corrected for acid ionization and extrapolated to the standard state of infinite dilution. The new data have been combined with literature values of the standard molar volumes and standard molar heat capacities of acetic and propionic acids, and of acetate and propionate ions. Three correlation models were tested in the description of the standard thermodynamic properties at superambient conditions for the two acids and their anions. The most dependable model, inspired by the fluctuation solution theory, has been selected for generation of the recommended data. Adjustable parameters of the model were obtained by simultaneous correlation of the volumes and heat capacities together with experimental dissociation constants of the acid. It is shown that this model can be used for precise and consistent calculations of standard thermodynamic properties of the two acids and their conjugate ions over a wide range of temperatures and pressures.

Combined flow-mixing power-compensation calorimeter and vibrating tube densimeter for measurements at superambient condition

A new instrument combining a flow-mixing calorimeter and a vibrating tube densimeter inside a thermostatted environment has been constructed. The instrument is designed for simultaneous measurements of calorimetric and volumetric properties at superambient conditions on mixtures for which both pure components are liquid at room temperature or one component is a liquid and the other is gaseous. The system yields simultaneously two properties: (1) the enthalpy of mixing determined in a heat-leak calorimeter with power compensation; (2) the density difference between a mixture (solution) and a reference liquid measured as a change in frequency of a tube vibrating in a field of permanent magnets. The instrument was tested in the full concentration range using aqueous ethanol and aqueous methanol. The results are presented at temperatures between 348 and 573 K and pressures from 5 to 20 MPa for mixing enthalpies ΔHmix ranging from −160 to 5700 J-mol−1. The corresponding heating powers are between −60 and 850 mW, respectively. The results for ΔVmix of mixing volumes were measurable from 348 K to 523 and between 5 and 13 MPa with the maximum volume change being −4.0 cm3-mol−1. The errors in ΔHmix and ΔVmix near mole fraction of 0.5 are believed to be less than 5% over the temperature and pressure ranges given above.

Experimental Technique for a Study of Volumetric Properties of Dilute Aqueous Solutions at Superambient Conditions.

A new vibrating-tube densitometer for measuring densities of fluids in the temperature range from ambient to 580 K and the pressure range up to 35 MPa is described. A novel design of driving and pick-up systems of a metal vibrating tube was employed. The driving system is based on passing the driving electrical signal directly to the electrically insulated vibrating tube, the photo-electric pick-up system employs glass-fibre optics with light source and sensor. New values of partial molar volumes at infinite dilution in water for several derivatives of benzene obtained by means of the densitometer are presented.

Volumetric properties of, and ion-pairing in, aqueous solutions of alkali-metal sulfates under superambient conditions

Apparent molar volumes for Na2SO4(aq) and K2SO4(aq) have been obtained from our recent densitometric measurements and selected experimental data from literature at temperatures up to 573 K and pressures up to 30 MPa. The Pitzer ion-interaction model was used to correlate the data in order to obtain recommended values as a function of temperature, pressure and concentration and to yield partial molar volumes at infinite dilution. By comparison with the volumetric data of other strong electrolytes, and consistent with their reported ion-association constants, the present data indicate that ion-pairing is significant for Na2SO4(aq) and K2SO4(aq) at higher temperatures. A semiquantitative treatment is presented which enables the partial molar volumes of 1 : 2 (and 2 : 1) electrolytes to be corrected for the effects of ion-pairing.

Strategy, methods and equipment for experimental studies of water-salt systems under superambient conditions

Abstract