Darren Rowland

JESS, a Joint Expert Speciation System - IV: A large database of aqueous solution physicochemical properties with an automatic means of achieving thermodynamic consistency

The JESS software package, which is a widely-used tool for modelling chemical speciation in complex aqueous environments, has been extended to allow comprehensive predictions of physicochemical properties for strong electrolytes in aqueous solution. Another large database, this time of physicochemical property data, has been added to the JESS suite, along with the computational methods which automatically turn these diverse literature data into a thermodynamically-consistent calculation for water activities, densities, heat capacities, etc. Given the recent emphasis on the role of water activity in predicting electrolyte mixing behaviour, we expect that this capability will lead to major changes in the way aquatic chemistry is modelled in future.

JESS, a Joint Expert Speciation System—V: Approaching thermodynamic property prediction for multicomponent concentrated aqueous electrolyte solutions

Considerable difficulties persist in modelling the thermodynamics of multicomponent aqueous electrolyte solutions, especially at high concentrations. The widely adopted Pitzer formalism suffers from severe disadvantages, particularly with the combinatorial increase in mixing parameters required in multicomponent systems. As an alternative, the simple mixing rules of Young, of Harned and of Zdanovskii have been employed to predict the properties of mixtures using only the properties of the binary constituents with few or no additional parameters. Among these, Zdanovskiis rule is particularly promising because it constitutes a fundamental criterion for ideal mixing, i.e. when solutions having the same solvent activity are mixed in any proportion, the solvent activity remains unchanged. Many mixtures of strong electrolyte solutions are known from experiment to obey Zdanovskiis rule. This is important because application to aqueous electrolyte systems of practical interest has been hindered due to the process-intensive determination of water activities using the Gibbs-Duhem relation. This paper describes an alternative method which efficiently calculates the water activity of a multicomponent solution obeying Zdanovskiis rule. Some specific examples of the method are presented and various applications considered. In some systems, where deviations from Zdanovskiis rule occur, a single empirical parameter can be obtained and can be easily incorporated into the calculations.

Thermodynamically-robust Pitzer equations for volumetric properties of electrolyte solutions

Pitzer equations are widely employed to correlate and predict the volumetric properties of aqueous electrolyte solutions over broad ranges of pressure and temperature. However, the currently-used pressure and temperature terms are empirical and tend to violate known thermodynamic behaviour. Three functional constraints have been identified that overcome this problem.

Thermodynamic Properties of the Glycine + H2O System

New equations describing the thermodynamic properties of the glycine + H2O system are obtained from previously published measurements. The measured values span a range of temperatures of approximately 273 to 473 K for glycine(aq) and (5 to 310) K for α-glycine(cr). This work provides critically assessed values for the following properties: (1) thermal properties of α-glycine(cr) from 0 to 310 K, (2) the change in excess Gibbs energy for glycine(aq) solutions as a function of temperature, pressure, and molality, valid from 273 to 473 K, pressures up to 40 MPa, and the molality range of 0 to 3.6 mol  kg−1 (or the saturation limit), and (3) standard-state properties for the aqueous solution process.New equations describing the thermodynamic properties of the glycine + H2O system are obtained from previously published measurements. The measured values span a range of temperatures of approximately 273 to 473 K for glycine(aq) and (5 to 310) K for α-glycine(cr). This work provides critically assessed values for the following properties: (1) thermal properties of α-glycine(cr) from 0 to 310 K, (2) the change in excess Gibbs energy for glycine(aq) solutions as a function of temperature, pressure, and molality, valid from 273 to 473 K, pressures up to 40 MPa, and the molality range of 0 to 3.6 mol  kg−1 (or the saturation limit), and (3) standard-state properties for the aqueous solution process.

JESS, a Joint Expert Speciation System – VI: Thermodynamically-consistent Standard Gibbs Energies of Reaction for Aqueous Solutions

Spanning all available combinations of more than 50 000 chemical species pertinent to aqueous solutions at 25 °C, a comprehensive set of reliable, thermodynamically-consistent standard Gibbs energies of reaction and their corresponding equilibrium constants is derived. The number and type of species that can be assessed for consistency is greatly expanded by abandoning conventional use of the chemical elements as the basis set. Using reaction data previously accumulated from the literature for almost 80 000 chemical reactions, equilibrium constants are automatically obtained (by extrapolation to standard conditions as necessary) and brought into thermodynamic consistency by a large-scale ordered Gaussian elimination process, yielding over 2500 basis species and the relative standard Gibbs energies for all other chemical species in known equilibrium with them. This allows the Gibbs energy difference for any relevant reaction to be calculated by summation. Strategies have been developed to ensure the process is dependable, scalable and sustainable. A new hazard with the averaging of thermodynamic parameters is identified. The results, which are openly available on-line, are in good accord with authoritative sources such as the U.S. National Institute of Standards and Technology (NIST) Thermodynamic Web-book and the Chemical Thermodynamics Series from the OECD Nuclear Energy Agency (NEA) but are far more extensive, including for the first time many analytical chelating agents and their metal complexes.

Gas Hydrate Formation Probability Distributions: The Effect of Shear and Comparisons with Nucleation Theory

Gas hydrate formation is a stochastic phenomenon of considerable significance for any risk-based approach to flow assurance in the oil and gas industry. In principle, well-established results from nucleation theory offer the prospect of predictive models for hydrate formation probability in industrial production systems. In practice, however, heuristics are relied on when estimating formation risk for a given flowline subcooling or when quantifying kinetic hydrate inhibitor (KHI) performance. Here, we present statistically significant measurements of formation probability distributions for natural gas hydrate systems under shear, which are quantitatively compared with theoretical predictions. Distributions with over 100 points were generated using low-mass, Peltier-cooled pressure cells, cycled in temperature between 40 and -5 °C at up to 2 K·min-1 and analyzed with robust algorithms that automatically identify hydrate formation and initial growth rates from dynamic pressure data. The application of shear had a significant influence on the measured distributions: at 700 rpm mass-transfer limitations were minimal, as demonstrated by the kinetic growth rates observed. The formation probability distributions measured at this shear rate had mean subcoolings consistent with theoretical predictions and steel-hydrate-water contact angles of 14-26°. However, the experimental distributions were substantially wider than predicted, suggesting that phenomena acting on macroscopic length scales are responsible for much of the observed stochastic formation. Performance tests of a KHI provided new insights into how such chemicals can reduce the risk of hydrate blockage in flowlines. Our data demonstrate that the KHI not only reduces the probability of formation (by both shifting and sharpening the distribution) but also reduces hydrate growth rates by a factor of 2.

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.