Development of an Electrolyte CPA Equation of state for App in the Petroleum and Chemical Industries

Complex mixtures of associating/polar components and electrolytes are often encountered in the oil- and gas and chemical industry. It is well-known in the oil- and gas industry that electrolytes have a substantial effect (typically decreasing) on e.g. solubilities of gases in water-hydrocarbon mixtures (salting-out effect), and furthermore, the presence of electrolytes may enhance the inhibitory effect of glycols on the formation of gas hydrates in natural gas pipelines, thus allowing for problem-free flow. In the pharmaceutical and biochemical industry, electrolytes, associating and polar compounds are important for the purification of complex chemicals. Electrolytes are also very important to the energy industry e.g. with regards to wet flue gas desulphurization or CO2 capture from coal-fired power plants using aqueous solutions of alkanolamines.

Despite this great importance, advanced thermodynamic models have been separately developed over the last years for hydrogen bonding mixtures e.g. water-alcohol-hydrocarbons and salt-solutions e.g. NaCl-water, but relatively few systematic approaches have been proposed for mixtures which contain both polar compounds and salts or other electrolytes. Many of the proposed models have an overwhelming number of parameters and they have not been tested to the mixed solvent systems of interest to oil applications.

The purpose of this project is to develop an electrolyte association equation of state, an extension of CPA (Cubic-Plus-Association) to electrolytes, as CPA has been proven to be a successful model for mixtures containing hydrocarbons, water and alcohols/glycols. As the target is to apply the e-CPA to mixed solvent systems, one electrolyte term (Debye-Huckel or MSA theory) may not be sufficient. The role of the Born term, the concentration dependency of the dielectric constant and of the solvation of ions in general will be thoroughly studied. The “art “ of the developed approach should be to obtain reliable multicomponent predictions using as few model parameters as possible, which are as carefully characterized as possible and as physically meaningful as possible. Such requirements have in reality not been fulfilled by the few existing approaches.
The proposed model will be applied to water-alcohol-hydrocarbon (gas)-salt solutions including hydrate formation curves in presence of both salts and polar inhibitors (methanol, glycols,...). Parameters will be obtained for the relevant components and compared to literature results.

The project includes the implementation of an innovative generic framework for development of new thermodynamic models. An equation of state may be derived from different contributions to the residual Helmholtz energy and mixing rules for calculation of the pure component contributions to the properties of the mixture. All other thermodynamic properties, including fugacities for calculation of phase equilibrium, may be derived from the residual Helmholtz energy and derivatives of this property. The thermodynamic library will consist of implementations of different contributions of the residual Helmholtz energy, and a thermodynamic model will be defined from addition of the different contributions to the total residual Helmholtz energy. This software enables assessment of contributions of the individual terms to the total residual Helmholtz energy, and can be used to compare any thermodynamic property calculated using the equation of state to experimental data, and thereby enable researchers to assess the relative importance of different terms.

The software solution will include handling of experimental data, “smart” parameter estimation for pure component and mixture properties, calculation of thermodynamic properties and phase equilibria and different deployment options allowing the model to be used in other applications. The software will be developed using state-of-the-art software engineering paradigms, and aims to become a generic tool to assist in the development and implementation of new consistent thermodynamic models.

Supervisor: Prof. Georgios Kontogeorgis,

Co-supervisor: Assoc. Prof. Kaj Thomsen: