Modeling of phase equilibria involving clathrate hydrates in the framework of a unified approach for solid solutions

RODRIGUEZ REARTES, SABRINA BELÉN; PORRAS GIRALDO, ANDRÉS FELIPE; ESCOBAR GARCÍA, DAVID J.; MOLINA, MATÍAS JOSÉ; MARCELO SANTIAGO ZABALOY

Campinas

Conferencia; XII Iberoamerican Conference on Phase Equilibria and Fluid Properties for Process Design (Equifase 2022) / X Congresso Brasileiro de Termodinâmica Aplicada (CBTermo, 2022); 2022

UNICAMP

Clathrate hydrates (CHs) are nonstoichiometric ice-like solid materials (solid solutions) made of water (host) and another component (guest), such as methane. Actually, more than one guest may be present in the CH, e.g., the CH may contain ethane and propane. Plugging of pipelines in the oil and gas industry may be caused by precipitation of CHs. Therefore, knowledge of the conditions that promote the CH formation is in such application useful to avoid the CH precipitation. On the other hand, the formation of CHs may be exploited in applications such as the fractionation of mixtures of gases. Conventionally, in the literature, two different models are combined for describing the CH formation: one to represent the fluid phases, and another one for the CHs, being the van der Waals-Platteeuw model the more frequently used for CHs. A third model is used for representing ice. In contrast, Yokozeki [1] used unified functional forms for the PVT behavior of the three states of matter. A particular unified form is a 4-parameter equation of state (EoS) (and its mixing rules). Yokozeki’s approach is in principle applicable to CHs because of their solid solution nature. However, such approach has a number of drawbacks. We have recently proposed a modelling approach (SSA) for the description of the thermodynamic properties of multi-component solid phases, i.e., of solid solutions, which is also applicable, as a limiting case, to pure-component solids [2]. Equilibrium between multicomponent phases and pure-component solids is not allowed in the solid solution approach (SSA), i.e., the frequently used assumption of precipitation in pure state from a multicomponent phase is never adopted. The SSA is a unified approach for describing the thermodynamic properties of molecular solids. This work is a preliminary assessment of the applicability of the SSA to the formation of CHs. It was carried out under the hypothesis that no modifications to the basic SSA architecture would be required in its application to CHs. The SSA isanalogous in many respects to the equation of state approach for fluids. Therefore, the description of the micro-structures of the solid sate is not available to the SSA. Three-phase lines and quadruple points were computed in this work for the systems water-methane and water-ethane. Some of such lines have an ice phase present. Such phase is typically considered in the literature to be pure water ice. In this work the ice phase is regarded as a water + n-alkane solid solution having a low n-alkane concentration. The performance of the SSA, in the reproduction of phase equilibria involving CHs, made possible, in this work, to reach a preliminary answer to the question of whether the SSA would effectively be able to unify the representation of molecular solids of widely varying nature (e.g., CHs versus multicomponent wax-containing solids). [1] Yokozeki. Int. J. of Thermophysics, 26 (2005) 743-765). [2] A.F. Porras Giraldo, S.B. Rodriguez-Reartes, M.S. Zabaloy. Fluid Phase Equilib., 547 (2021) 113174.