The presence of electrolytes in aqueous solutions has long been recognized as contributing to significant departures from thermodynamic ideality. The presence of ions in process streams can greatly add to the difficulty of predicting process behavior. The difficulties are increased as temperatures and pressures within a process are elevated. Because many chemical companies now model their processes with chemical process simulators it is important that such codes be able to accurately model electrolyte behavior under a variety of conditions. Here we examine the electrolyte modeling capability of ASPEN PLUS™, a widely used simulator. Specifically, we will present our efforts to model alkali metal halide and sulfate systems. We will show conditions for which the models within the code work adequately and how they might be improved for conditions where the simulator models fail.

Molar enthalpies of dilution Delta H-dil(m) of Na2CO3(aq) were measured from molality m = 1.45 mol . kg(-1) to m = 0.008 mol . kg(-1) at seven temperatures from T = 298 K to T = 523 K at the pressure p = 7 MPa, and at four temperatures from T = 371 K to T = 523 K at the pressure p = 40 MPa. Molar enthalpies of dilution Delta H-dil(m) Of NaHCO3(aq) were measured from m = 0.98 mol . kg(-1) to m = 0.007 mol . kg(-1) at the same temperatures and pressures. Hydrolysis and ionization equilibria contribute substantially to the measured enthalpies under many of the conditions of this study. Explicit consideration of these reactions, using thermodynamic quantities from previous studies, facilitates a quantitative representation of apparent molar enthalpies, activity coefficients, and osmotic coefficients with the Pitzer ion-interaction treatment over the ranges of temperature, pressure, and molality of the experiments. (C) 2001 Academic Press.

For the first time, water sorption on representative geothermal reservoir rocks from The Geysers steam field has been determined in the laboratory at actual reservoir temperature. The Oak Ridge National Laboratory (ORNL) isopiestic apparatus has been used to measure quantities of water retained at various temperatures and relative pressures by plug samples of three representative reservoir metagraywacke cores. The measurements were made at 150, 200 and 250°C as a function of relative pressure in the range 0.00 ⩽ p/p0 ⩽ 0.98, where p0 is the saturated water vapor pressure. Both adsorption (increasing pressure) and desorption (decreasing pressure) runs were made in order to investigate the phenomenon of hysteresis. Low-temperature gas adsorption analyses were completed on the same rock samples. Nitrogen or krypton adsorption and desorption isotherms at 77 K were used to obtain BET (Brunauer, Emmet, Teller) specific surface areas and pore volumes and their distributions with respect to pore sizes. Mercury-intrusion porosimetry was also used to obtain similar information extending to very large pores (macropores). A qualitative correlation was found between the surface properties obtained from nitrogen adsorption and the mineralogical and petrological characteristics of the solids. In general, however, there is no direct proportionality between BET specific surface areas obtained from nitrogen adsorption and the capacity of the rocks for water adsorption at high temperatures. An analysis of the temperature dependence of adsorption/desorption indicates that multilayer adsorption rather than capillary condensation is the dominant water storage mechanism in The Geysers reservoir rocks at high temperatures.

We review the recently proposed molecular-based formalism for infinitely dilute high-temperature electrolyte solutions which establishes connections between the microscopic solvent environment around individual ionic species and their macroscopic solvation behavior. Then, we illustrate this formalism by interpreting some recent experimental results for infinitely dilute NaCl aqueous solutions. Finally, we discuss some theoretical implications, guided by the formalism, concerning the modeling of high-temperature aqueous-electrolyte solutions.

Isopiestic molalities of aqueous solutions of Na2HPO4,K2HPO4 , NaH2PO4, and KH2PO4have been measured at intervals from T= 383.15 K to T= 498.15 K, with NaCl (aq) as the isopiestic standard for the calculation of osmotic coefficients. Osmotic coefficients for these electrolytes, already low at T= 298.15 K, decreased even more at elevated temperatures, becoming quite low with equilibrium isopiestic molalities as large as 15 mol · kg−1. Minor modifications of the standard ion-interaction model were necessary in order to obtain an acceptable fit of the experimental osmotic coefficients. Stoichiometric activity coefficients were computed from parameters resulting from the least-squares fit, which was quite good, of the osmotic coefficients. In the case of the di-hydrogenphosphates the relative order of the osmotic and activity coefficients reversed at the higher temperatures, which was not observed for the mono-hydrogenphosphates.

The solvation of infinitely dilute CsBr in high-temperature aqueous solutions is analysed by integral equation calculations, according to the recently proposed molecular-based formalism which connects the solvent environment around individual ionic species and their macroscopic solvation behaviour. Recent experimental data for infinitely dilute CsBr aqueous solutions are interpreted via the same formalism and compared with their analogous integral equation calculations. Finally, some relevant theoretical implications regarding the modelling of high-temperature aqueous electrolyte solutions are discussed and illustrated by integral equation results.

Raman spectra have been used to identify and characterize aqueous hydroxouranyl(VI) complexes from 0.0038 to 0.647M at pH from 0.24 to 14.96 adjusted witheither HCF3SO3 and/or (CH3)4NOH under ambient conditions. In acidic media(0.24 ≤ pH ≤ 5.63), the existence of four species UO2+2,(UO2)2(OH)3+,(UO2)2(OH)2+2, and (UO2)3(OH)+5 was confirmed. At high uranium concentrations(ΣU ≥ 0.1M) and in strongly acidic solutions (pH ≤ 1.94), one additional weakband was observed at 883±1 cm−1. This band was assumed torepresent thespecies UO2+2 with a reduced hydration number.In neutral and basic solutions(5.63 ≤ pH ≤ 14.96), five complexes were postulated: (UO2)3(OH)−7,(UO2)3(OH)2−8,(UO2)3(OH)4−10,(UO2)3(OH)5−11, andUO2(OH)2−4, based on theassigned symmetrical stretching frequencies of the UO2 group in each complex.(UO2)3(OH)−7 is the dominant species over mostof the pH range (4.53–12.78).The stability ranges of the other trinuclear species are:(UO2)3(OH)2−8 (10.97 ≤pH ≤ 13.83), (UO2)3(OH)4−10 (10.97 ≤ pH ≤ 13.85) and (UO2)3(OH)5−11(12.53 ≤pH ≤ 14.10), which were identified for the first time. Finally, the monomericuranate anion OU2(OH)2−4 dominates in highly basic solution (12.48 ≤ pH ≤14.96). The linear correlation between the symmetrical vibrational frequency v1of the linear O = U = O entity and the average number \(\overline n\) of hydroxide ligandscoordinated to each uranium atom in a given species has been reaffirmed andexpanded:\(v_1 ({\text{cm}}^{{\text{ - 1}}} ) = - 22X\overline n + 870\)The v1 correlation was also used to predict the vibration frequencies of theundetected monomers UO2(OH)+, UO2(OH)o2,UO2(OH)−3 at 848±2, 826±2, and804±2 cm±1, respectively. Characteristic band areas for eachuranyl hydrolyzedspecies were determined by Raman spectra decomposition and their hydrolysisquotients log Q, were calculated. Structures of the four triuranylspecies are proposed.

Isopiestic molalities of aqueous solutions of phosphoric acid have been measured at 383.15 to 523.15 K. NaCl(aq) served as the isopiestic standard for the calculation of osmotic coefficients. The ion-interaction model gave an excellent fit to the experimental osmotic coefficients by treating H3PO4(aq) as a 1–1 weak electrolyte. Activity coefficients, both real and stoichiometric, were also obtained from the analysis of the isopiestic results. As expected, the ionization of H3PO4(aq) decreased with increasing temperature. However, at higher molalities, the ionization increased with increasing molality (reionization).

The behavior of the first hydration shell of species in solution and its relevant thermophysical properties are studied by molecular dynamics of infinitely dilute NaCl aqueous solutions at high temperature. The ion-induced effects on the water local properties are assessed in terms of the corresponding radial profiles for the local density, the local pressure, the local electric field, the local dielectric constant, and two alternative types of coordination numbers, along the near-critical reduced isotherm Tr=1.05 and the supercritical reduced isochore ρr=1.5. Simulation results are discussed in the context of their usefulness in enhancing the understanding and the modeling of supercritical aqueous electrolytes.

Our molecular-based formalism for infinitely dilute supercritical nonelectrolyte solutions is extended to electrolyte solutions by establishing rigorous connections between the microscopic behavior of the solvent around individual ionic species and their macroscopic solvation behavior. The formalism relies on the unambiguous splitting of the mixture’s properties into short-ranged (finite) and long-ranged (diverging) contributions, associated with the corresponding solvation and compressibility-driven phenomena, respectively. The salt (solute) and solvent’s residual chemical potentials are linked to the change of the local solvent’s environment around the infinitely dilute anion and cation, and the salt partial molar properties are interpreted in terms of the individual ion partial molar counterparts without introducing any extra-thermodynamic assumption. This is achieved with the use of Kusalik and Patey’s version of the Kirkwood–Buff fluctuation theory of mixtures. Moreover, the salt-and the individual ion-induced effects are connected to the solvent’s electrostriction around the ions, and to the coefficients of the Helmholtz free energy expansion for dilute mixtures. The ion-induced effects are also linked to well-defined excess solvation numbers which do not rely on any choice of solvation shell radius. Finally, some theoretical implications concerning the modeling of high-temperature aqueous-electrolytes solutions are discussed.

The profiles of the potential of mean force for the Cl - - H 3 O + pair, as predicted by two ab initio models, are determined by constraint molecular dynamics simulation at a near-critical condition. The corresponding association constants are then determined and compared with that from conductance measurements to test the reliability of the current simulation models for HCl .

Factor analysis and self modeling were used as new methodologies to investigate uv-vis absorption spectra of uranyl (0.08 M) aqueous solutions (pH = 3.16, at the ambient temperature) as a function of temperature (8 degrees C-75 degrees C). The study indicated that the above system can be described as a two-component system involving equilibrium between UO22+ and (UO2)(2)(OH)(2)(2+). The pure spectrum for the dimeric species is extracted from the broad overlapping features via a self-modeling method. In addition, this method allows spectrophotometric determination of the reaction enthalpy for the dimerization reaction.

Isopiestic molalities of aqueous solutions of the alkali metal bromides have been measured at T = 383.15 K to T = 498.15 K. Aqueous NaCl served as the isopiestic standard for the calculation of osmotic coefficients. The ion-interaction model gave an excellent fit to the experimental osmotic coefficients and provided parameters for the calculation of activity coefficients. Osmotic and activity coefficients both decrease with increasing temperature in this temperature range. A simple model is presented from which activity and osmotic coefficients (at p(sat)) can be calculated as a function of molality for T = 273.15 K to T = 523.15 K. Comparison of the stoichiometric activity coefficients with those of the corresponding alkali metal chlorides reveals quite complicated relative behavior.

Molecular simulation of infinitely dilute NaCl aqueous solutions are performed to study the Na+/Cl− ion pairing in a polarizable and a non-polarizable solvent at supercritical conditions. The simple point charge (SPC), the Pettitt–Rossky and the Fumi–Tosi models for the water–water, the ion–water, and the ion–ion interactions are used in the determination of the degree of dissociation, its temperature and density dependence, and the kinetics of the interconversion between ion-pair configurations in a non-polarizable medium. To assess the effect of the solvent polarizability on the stability of the ion-pair configurations we replace the SPC by the polarizable point charge (PPC) water model and determine the anion–cation potential of mean force at Tr=1.20 at ρr=1.5.

Isopiestic molalities of aqueous solutions of the alkali metal hydroxides have been measured atT=(383.25, 413.12, and 443.09) K. The isopiestic standard for calculation of osmotic coefficients was NaCl(aq). The ion-interaction model gave an excellent fit with the experimental osmotic coefficients and provided parameters for the calculation of activity coefficients. Osmotic and activity coefficients both decrease with increasing temperature in this temperature range. As atT=298.15 K, the order of the results is the reverse of that for the alkali metal chlorides. The model of Simonsonet al. (J. Chem. Thermodynamics1989, 21, 561–584) is an excellent predictor of the experimental results for NaOH(aq). A simple model is presented from which activity and osmotic coefficients (atpsat) can be calculated as a function of molality forT=273.15 K toT=523.15 K.

For three decades, molecular models for water, nature's most important liquid, have been developed and refined by fitting to structure measured by neutron scattering. The decade-old widely accepted structure of water at room temperature and pressure was recently revised as a byproduct of attempts to understand the structure of high-temperature/high-pressure water for which, in a remarkable reversal of roles, molecular models successfully pinpointed inaccuracies in scattering data. Subsequent improvements in analyzing scattering data have led to reevaluation of water structure at normal conditions. This remarkable interplay between molecular modeling and experiment suggests molecular methods can effectively complement scattering experiments.

We report molecular dynamics calculations of the ionic mobility and limiting conductance of NaCl in supercritical water as a function of density along an isotherm 5% above the critical temperature. The number of hydration water molecules around ions is found to dominate the behavior of the limiting conductance in the higher-density region while the interaction between the ions and hydration water molecules is found to dominate in the lower-density region. The different effects in the lower- and higher-density regimes lead to different slopes for the limiting conductance as a function of density in the two regimes.