Experimental determination and thermodynamic modeling of clathrate hydrate stability conditions in methane + hydrogen sulfide + water system

doi:10.1016/j.jngse.2020.103549

Journal of Natural Gas Science and Engineering

Volume 83, November 2020, 103549

https://doi.org/10.1016/j.jngse.2020.103549 Get rights and content

Highlights

•
CH₄ + H₂S hydrate dissociation pressures in presence of pure water were measured from 282.1 to 288.3 K.
•
PC-SAFT/CPA EoS based thermodynamic models were developed to predict the hydrate dissociation conditions.
•
Both models are capable of predicting the hydrate dissociation conditions for mixtures of CH₄ + H₂S.
•
The PC-SAFT EoS based thermodynamic model yields more accurate results.

Abstract

In the petroleum industry, it is essential to comprehend the hydrate equilibrium qualifications in order to prevent or defer this phenomenon. However, the thermodynamic equilibrium information on hydrogen sulfide containing systems reported in the literature is limited and discrepant in comparison with other gases. In this communication, new experimental investigations of hydrate dissociation conditions of methane (0.995 mole fraction) + hydrogen sulfide (0.005 mole fraction) + water system which forms stable structure I were carried out in temperature and pressure ranges from 282.1 to 288.3 K and 5.63 to 12.19 MPa, respectively. The experiments were carried out by using the stepwise heating method with an isochoric pressure search procedure in a 100 cm³ autoclave. The apparatus and method validity were confirmed by comparison of the methane hydrate equilibrium data from the current study with some selected data from the literature. In this study, a thermodynamic model was developed to estimate the dissociation conditions of the binary CH₄+H₂S hydrate system. For this purpose, a systematic comparison is made between results from PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory) and CPA (Cubic Plus Association) equations of state (EoS). In the present work, PC-SAFT and CPA equations of state are utilized to model the gas/vapor and water phases and van der Waals Platteeuw (vdW-P) is used for hydrate phase modeling. In this regard, the binary interaction parameters in PC-SAFT/CPA EoS for the aforementioned system were tuned using literature data on H₂S/CH₄ solubility in water. Model outcomes show that both models are entirely trustworthy over an extensive range of temperatures (273 to 350 K), pressure (2 to 22 MPa) and all H₂S concentrations. In addition, a comparison between experimental data, the models results and HWHydrate (version 1.1) predictions ascertain that the PC-SAFT is relatively precise than CPA in the binary hydrate system of CH₄-H₂S.

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 ( $f_{w}^{H}$ ) in equilibrium with water fugacity in the aqueous phase ( $f_{w}^{L}$ ) as given in the equation below: $f_{w}^{H} (T, P) = f_{w}^{L} (T, P)$

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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Experimental determination and thermodynamic modeling of clathrate hydrate stability conditions in methane + hydrogen sulfide + water system

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Chemicals

Apparatus

The van der Waals–Platteeuw hydrate theory

Results and discussion

Conclusions

Credit author statement

Declaration of competing interest

Acknowledgement

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Experimental measurement and phase behavior modeling of hydrogen Sulfide−Water binary system

Ind. Eng. Chem. Res.

Thermodynamic Modeling study on phase equilibrium of gas hydrate systems for CO₂ capture