(153a) Thermodynamic Models for Solid – Liquid – Gas Equilibrium of CO2 mixtures 

Fossil fuels are currently the most widely used sources for power and heat generation. Their extensive consumption contributes significantly to the increased levels of greenhouse gases in the atmosphere, which subsequently leads to environmental problems such as global warming. The most important greenhouse gas, in terms of quantity and impact, is carbon dioxide (CO₂). As the global energy demand has increased, CO₂ levels have risen significantly, from the preindustrial levels of 280 ppm to 405 ppm in March 2016. Moreover, fossil fuels will continue to play an important role in power and heat production and also be used in large industrial operations in the foreseeable future. Unless major measures are taken for the reduction of CO₂ emissions, the CO₂ concentration is projected to rise even more over the next 25 years as global demands for energy are anticipated to increase.

Significant amount of research has been conducted for the development of new technologies that aim to reduce the levels of CO₂ in the atmosphere. The most mature technology today is Carbon Capture and Sequestration (CCS), which is the process of capturing CO₂ from the flue gas of a large point source (typically a power plant), transporting it to a sequestration site and then depositing it to a geological formation, which can be a saline aquifer or a depleted oil well. The CCS process can be divided into three main parts: CO₂ capture, transport and storage. There are several different methodologies for the capture process, including pre-combustion, post-combustion and oxy-combustion process. These processes result in streams that contain different gases such as N₂, CH₄, O₂, Ar, SO₂, H₂S and H₂ at various concentrations.

An important part of the CCS process is the transportation of the CO₂-rich stream from the capture plant to the sequestration site. In most cases, transporting CO₂ via pipelines is the most cost effective way of transport. Preliminary conceptual design, detailed design, simulation and optimization of the transport process require, among others, accurate knowledge of the physical properties of the chemical system involved as a function of temperature, pressure and composition. Quite often, the system exists in more than one phase (i.e., liquid, vapor and/or solid) and as a result process design calculations have to take into account the phase equilibrium conditions and also the composition of the relevant phases and the respective physical property values. The two challenges that arise are the accurate prediction or correlation of the physical properties of the system and the conditions of instability, where the system is going to split into two or more coexisting phases.

Vapor â?? liquid equilibrium (VLE) of CO₂ mixtures has attracted lots of attention both in terms of experimental measurements and modeling using equations of state (EoS). On the other hand, relatively little work has been performed to measure and predict solid-fluid equilibrium (SFE) of CO₂ mixtures, which is critical to the design and operation of CO₂ pipelines and storage facilities. Hazard assessment studies associated with CO₂ transport include scenarios of accidental releases with sharp expansion, where solid â?? vapor (SVE) as well as solid â?? liquid (SLE) equilibria may occur. CO₂ exhibits a relatively high Joule â?? Thomson expansion coefficient and a pipeline depressurization will lead in rapid cooling which can reach very low temperatures and as a result, solid formation can be expected. Taking this into account, it is easily understood that the formation of dry-ice resulting from SFE can largely affect the safety of CCS facilities during equipment depressurization, process shutdown or other process upsets.

In this work, solid phase thermodynamic models of different complexity are applied to model the SFE of pure CO₂ and also of CO₂ mixtures with other compounds. These models include an empirical correlation model, a model based on thermodynamic integration and a solid phase EoS for pure CO₂. The different models are coupled with three fluid phase EoS (PR, SRK and PC-SAFT) and the performance of each combined model is evaluated for various binary mixtures. In total, 7 different models are examined i.e. the empirical correlation and the thermodynamic integration models each one coupled with SRK, PR and PC-SAFT EoS and an EoS developed for solids coupled with an EoS developed for fluids. The solid phase EoS used for this purpose was the EoS proposed by Jäger and Span (J. Chem. Eng. Data, 57, 590, 2012). Finally, in order to examine the general validity of the proposed scheme, mixtures where the solid forming component is not CO₂ are examined.

Our calculations reveal that a model that successfully reproduces the pure solid former triple or normal melting point will predict more accurately the solid â?? liquid â?? gas (SLG) locus of the mixture. In this context, the thermodynamic integration model and the Jäger and Span EoS provide in general better predictions of the SLGE, when all the binary interaction parameters (BIPs) are zero for the mixtures of CO₂ with N₂ and H₂. For these two mixtures, the empirical correlation model for the solid phase is comparable to the other two models only when coupled with PC-SAFT which accurately reproduces the pure CO₂ triple point. The use of BIPs, regressed from binary VLE data at low temperature, significantly improves the prediction of the SLG behavior for most models. All models provide very similar results for the mixture of CO₂ with CH₄ and very low deviations have been achieved with the use of BIPs regressed from binary VLE data over a wide temperature range. The use of these BIPs also enabled a successful unified description of both the SLGE and the LG critical locus of the mixture with the PR EoS.

The thermodynamic integration model shows clear superiority in the mixtures where naphthalene and phenanthrene form the solid phase, where the successful representation of the normal melting point of the heavy compound is crucial for the overall fit of the SLG line. The empirical correlation model in these cases is unsuccessful in reproducing the normal melting point of the pure component except when coupled with SRK EoS for the two mixtures of naphthalene. For the mixture of phenanthrene, the overall fit of the SLGE is very poor with this model, but a satisfactory correlation of the behavior was achieved with the thermodynamic integration model coupled with PC-SAFT.