Research activities:

Development of a new group - contribution EoS for the modelling of hydrogen bonding mixtures.

Our previous work has shown that the explicit determination of the hydrogen bonding contributions is necessary for the accurate modelling of associating mixtures. Although CPA is an accurate EoS for the modelling of such systems, here a new model which combines the advantages of the application of advanced mixing rules and the associating term is suggested. The attractive characteristic of this model is its group-contribution nature, since it enables the prediction of more complex systems based on parameters fitted to specific binaries. Furthermore, the use of UNIFAC permits a certain flexibility in the selection of the appropriate interaction parameters in contradiction to the one k_ij value used for example in CPA. The performance of the proposed model is evaluated by comparing the obtained results in the phase equilibria of two ternary mixtures versus the results of the models already existing in literature (UMR-PRU, CPA-PR). It is then extended to handling components of interest to natural gas industry and is evaluated in the prediction of phase equilibria of multicomponent mixtures involving associating compounds.

Phase equilibrium in natural gas mixtures with associating components.

Natural gas is getting more and more attention in the market due to its increasing use for heating and transportation purposes. In order to ensure process and product quality, adequate design and control are of utmost importance both in surface and subsea processing and transportation. Despite the fact that natural gas consists mostly from hydrocarbon components which are generally easy to model, an appropriate thermodynamic framework should be developed since several impurities such as water may intensively affect the thermodynamic properties of the pure gas. Furthermore, certain additives, such as alcohols and glycols, are inserted in several parts of processing either as absorbers or to act as hydrate inhibitors. The availability of a thermodynamic model able to accurately predict the multicomponent phase equilibria contributes significantly to produce reliable process simulations as to avoid hydrate formation.

It is well known that the classic cubic equations of state such as SRK or PR combined with the conventional mixing rules commonly used in industrial practice, fail to describe satisfactorily the interactions between hydrocarbons and highly polar compounds such as hydrate inhibitors and water. In this work, an evaluation of the performance of three different versions of the Peng-Robinson EoS in the description of phase equilibrium of hydrocarbon mixtures with water and hydrate inhibitors (methanol and glycols), is presented: the classic Peng - Robinson EoS with the Van der Waals one fluid mixing rules, the combination of the UNIFAC activity coefficient model with the PR EoS that results in advanced cubic EoS mixing rules (UMR-PRU) and the explicit accounting for association based on Wertheim's theory (CPA-PR EoS).

More information and detailed results can be found in:

• Petropoulou E., Pappa G.D., Voutsas E., Modelling of phase equilibrium of natural gas mixtures containing associating compounds, Fluid Phase Equilibria 433, 2017, 135-148.

Simulation of a natural gas dehydration unit with TEG.

Natural gas is saturated with water at reservoir conditions. A key process during offshore natural gas treatment is the removal of water in order to avoid equipment corrosion and hydrate formation as well as to ensure product quality, namely the water dew point specification. The dehydration of natural gas by absorption, usually using glycols such as TriEthylene Glycol (TEG) is a typical industrial procedure so as to avoid flow blockage and equipment breakdown. Although it is well established in industrial practice, the increasing need of lower water content specification in dry gas at the transportation network renders necessary its accurate modelling. Despite the widespread use of dehydration units, few experimental data are available in open literature and most engineering manuals are based on empirical correlations for the design and for determining the operational parameters of the process.

Our group has developed the UMR-PRU model, an EoS/G^E model which combines the PR EoS with the UNIFAC group contribution model. UMR-PRU has been shown to yield quite accurate results for natural gas mixtures. In the current work, it is extended to mixtures of interest for natural gas dehydration purposes. The UMR-PRU model is then evaluated in the simulation of a typical TEG dehydration unit by its incorporation in commercial software through the CAPE-OPEN standard. The aim of this study is to demonstrate that UMR-PRU can be used to simulate complex mixtures containing natural gas, water, and glycols. To that end, the results obtained are compared in terms of process parameters, such as absorbent consumption and losses, energy requirements etc., with those obtained using the proposed by Aspen Hysys, TST/NRTL model.

More information and detailed results can be found in:

• Petropoulou E.G., Voutsas E.C., Thermodynamic Modeling and Simulation of Natural Gas Dehydration Using Triethylene Glycol with the UMR-PRU Model, Ind. & Eng. Chem. Res., 57 (25), 2018, 8584-8604

Measurement of the VLE of CO₂/methane.
(Partly financed by the ECCSL mobility in science project)
In collaboration with SINTEF Energi

Experimental vapor - liquid equilibrium data for the CO₂/CH₄ mixture have been measured with high accuracy at 293.13 K, 298.14 K and 303.15 K, with emphasis on the mixture critical area. The scaling law of statistical thermodynamics has been fitted to the critical region data of each isotherm and very good estimation of the critical point is achieved with a maximum uncertainty of 10 kPa in critical pressure and 0.0009 in critical molar composition. The measurements have been validated against experimental data taken from the literature, where available, and against the prediction of the GERG-2008 model. The Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR) Equations of State using the classic van der Waals one fluid mixing rules, the perturbed chain statistical association fluid theory (PC-SAFT) and the Universal Mixing Rule - Peng Robinson (UMR-PRU) model have been fitted to the data of each isotherm with very satisfactory results.

More information and detailed results can be found in:

• Petropoulou E., Voutsas E., Westman, S.F., Austegard A., Stang H.G.J. L?vseth S.W., Vapor - liquid equilibrium of the carbon dioxide/methane mixture at three isotherms, Fluid Phase Equilibria 462, 2018, 44-58.