Experimental determination and thermodynamic modeling of clathrate hydrate stability conditions in methane + hydrogen sulfide + water system
Graphical abstract
Introduction
Clathrate hydrates or gas hydrates are ice-like inclusion compounds which are composed of water as host and gas and/or some volatile liquids as guest intermolecular contiguity under suitable conditions of pressure and temperature (Ke et al., 2018). When hydrates form, water molecules encapsulate guest molecules situated in their vicinity which lead to stabilizing the hydrate structure. Various cavities types come together at different ratios lead to emerge the larger polyhedral crystal structures. The most widespread structures of gas hydrates are entitled as structure I (sI), structure II (sII), and structure H (sH) and each structure has its own associated cage type (Mohammadi et al., 2008; Kvamme et al., 2017). These lattice parameters are influenced primarily by the type and size of the hydrate former (Meragawi et al., 2016).
Clathrate hydrates have been subject of interest in many research areas such as flow assurance (Sloan et al., 2010), drilling and well operations (Merey, 2019), gas storage and separation (Tumba et al., 2016; Eslamimanesh et al., 2012; Filarsky et al., 2018, 2019; Veluswamy et al., 2016; Kumar et al., 2008), sweetening of natural gas (Ani and Ubani, 2016), CO2 capture (Banafi et al., 2019; Bavoh et al., 2019; Chong et al., 2016), water desalination (Khan et al., 2019; Kang et al., 2014; Babu et al., 2018) and other technological applications. Besides the gas hydrate advantages, some negative features also exist (Sun et al., 2019). Association of natural gas (NG) and saline water in the reservoirs results in extracted NG to be saturated with water. Therefore, the hydrates formation can occur in almost all aspects of the production plan and operating facilities of typical NG that can cause operational impediment, safety and financial problem (Shahnazar and Hasan, 2014). Consequently, engineers who work in the field of petroleum industry need to estimate under which circumstances the hydrate phenomenon happens and causes disruption in their application (Altamash et al., 2017; Adamova et al., 2018; Em et al., 2018).
Since with global incrementing demand in natural gas, the sweet oil and gas reserves are running out over time, exploitation from sour reservoir fluids resources will be inevitable. However, producing sour gas is less economically durable due to low NG costs. Hydrogen sulfide hydrate formation, which has a strong tendency to form stable structure I at low-moderate pressures and temperatures in the vicinity of light hydrocarbons such as methane (as a basic part of NG), is one of these extra expenses, which can arise from the extensive flow assurance challenges (Ward et al., 2015; Malagar et al., 2019; Hu et al., 2017). Besides, because of the toxic and corrosive nature of H2S, there is limited experimental information on hydrates of H2S containing gases (Zare Nezhad and Ziaee, 2013; Sun et al., 2003). Phase equilibria of the systems containing hydrogen bonding compounds, like H2S is a controversial subject in the area of thermodynamics (Ng and Robinson, 1985; Gu et al., 1993). Therefore, the precise assessment of sour gas hydrate equilibrium conditions is deemed to be a key issue in this area (Nasir et al., 2020). Consequently, it is crucial to develop a reliable thermodynamic model to predict the hydrate equilibrium/dissociation conditions of H2S containing gases (Carroll and Mather, 1991).
For typical hydrate formers like methane, a significant amount of laboratory data has been reported in the literature (Stoporev et al., 2018; Merkel et al., 2016). However, as mentioned earlier because of the toxic nature of H2S, there is a scarcity of data on hydrate dissociation conditions of H2S containing systems. This study presents new experimental hydrate dissociation conditions data for CH4 + H2S. The obtained results were compared with some literature data on the hydrate phase behavior of the system of methane + hydrogen sulfide reported by different authors: Noaker and Katz in 1954, Ng and Robinson in 1985, Mohammadi and Richon in 2014, ward and Koh in 2015.
Modeling of water containing systems (such as hydrate systems) with a proper EoS is an important issue in both research and industrial applications. Because of H-bonding and the associated tetrahedral structure of water molecules, precise modeling of the thermodynamic equilibrium condition of aqueous systems is possible only through choosing an appropriate EoS (include association term). In the last few decades, the CPA and PC-SAFT theory EOS which account hydrogen-bonding interactions are two most prosperous and extensively proposed models in the fields of chemical and petroleum engineering. From the authors’ knowledge, there is no developed association based model to predict hydrate dissociation conditions of hydrogen sulfide + methane.
In this study, a thermodynamic model was developed by using two types of equations of state to estimate the dissociation conditions of H2S + CH4 clathrate hydrates. As mentioned earlier, through the existence of non-bonded water molecules in the hydrate set and powerful aggregation effects between water molecules and polar species such as hydrogen sulfide, the classical EoS are unable to correctly predict the thermodynamic conditions of the aforesaid system (De Villiers et al., 2013). Therefore, combinations of the PC-SAFT/CPA EoS and the van der Waals – Platteeuw (vdWP) model were used in this work. The vdW–P theory is utilized to compute the hydrate phase fugacity. The water/gas (vapor) phase fugacity is evaluated using the PC-SAFT/CPA EoS to compare which yields an accurate prediction for hydrate dissociation conditions. For this purpose, the binary interaction parameters (kij) were tuned using the literature data on H2S/CH4 solubility in water. As well as, the reliability of each proposed model was validated through comparison with some selected experimental data on CH4-H2S hydrate system collected from the literature.
Section snippets
Chemicals
Specification of the sour gas used in this work is presented in Table 1. It should be noted that the aqueous phase was composed of pure distilled water in all experiments.
Apparatus
Schematic of the laboratory setup used in the current study is illustrated in Fig. 1. The basic segment of the apparatus is a cylindrical reactor with an effective internal capacity of 100 ± 0.5 cm3 (with an internal diameter of 5.5 cm and a height of 4.2 cm) which can sustain maximum pressure of 20 MPa. The reactor is
The van der Waals–Platteeuw hydrate theory
Van der Waals and Platteeuw developed a model that can predict macroscopic thermodynamic properties of clathrate hydrates such as pressure and temperature by applying microscopic features like intermolecular potential (Sloan and Koh, 2008). In the current study, vdWP theory is used to compute the hydrate fugacity, according to equality of water fugacity in the hydrate phase () in equilibrium with water fugacity in the aqueous phase () as given in the equation below:
For
Results and discussion
As mentioned earlier, water has various inherent properties such as polarity that allows it to bond with other polar substances such as hydrogen sulfide and makes association contribution, as well as, methane should be considered as a non-polar associated compound.
In the proposed thermodynamic model (PC-SAFT/CPA EoS), it is assumed that methane behaves as a non-polar and non-associative compound, and pure water and hydrogen sulfide behave as associating polar compounds. The 2B association
Conclusions
The incrementing worldwide natural gas demand and the accessibility of sour NG reserves establish the requirement of hydrate phase behavior study of the CH4 + H2S containing systems aiming to develop thermodynamic models and, above all else, to perform the accurate process design of NG production and transmission technologies from sour natural gas reserves. In this work, a set of hydrate dissociation conditions measurements for methane + hydrogen sulfide + water system was reported in the
Credit author statement
Mahnaz Aghajanloo: Conceptualization, Methodology, Formal analysis, Data Curation; Zahra Taheri: Resources; Taraneh Jafari Behbahani: Conceptualization, Writing - Review & Editing, Supervision, Writing - Original Draft; Amir H. Mohammadi: Conceptualization, Methodology, Writing - Review & Editing, Supervision; Mohammad Reza Ehsani: Supervision; Hamed Heydarian: Software.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The research was made possible via financial support of Research Institute of Petroleum Industry (RIPI).
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