Phase stability conditions of clathrate hydrates for methane + aqueous solution of water soluble organic promoter system: Modeling using a thermodynamic framework

doi:10.1016/j.molliq.2016.09.084

Journal of Molecular Liquids

Volume 224, Part B, December 2016, Pages 1117-1123

https://doi.org/10.1016/j.molliq.2016.09.084 Get rights and content

Highlights

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The considering systems were methane + water + water-soluble organic promoter.
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A correlation was presented to estimate the hydrate phase boundary of this system.
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This correlation calculates the enthalpy of hydrate dissociation.

Abstract

A thermodynamic model is presented for predicting the phase stability conditions of clathrate hydrates for methane + water-soluble organic promoter aqueous solution. A new equation is then proposed to estimate the enthalpy of hydrate dissociation for methane + aqueous solution of water-soluble organic promoter using Genetic Programming (GP) and Teaching–Learning-Based Optimization (TLBO) evolutionary algorithm. The model reliably predicts the hydrate dissociation conditions for methane + aqueous solutions of tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane and acetone. The van Laar model is used to calculate the activity coefficient of water in aqueous solution of water-soluble organic promoter. About 30% of the reported experimental data were used for finding the empirical relationships to estimate the enthalpy of hydrate dissociation and the remaining 70% was used to test the accuracy and the predictive capability of the correlation. The average absolute error for methane hydrate dissociation temperatures was found to be 0.33 K, which indicates the accuracy of the model.

Introduction

Gas hydrates, or clathrate hydrates, are crystalline compounds formed through the combination of water and suitably sized molecules, which are generally gases and some volatile liquids. This compound has a lattice structure formed as the result of the hydrogen bonds of water molecules that create cavities in the form of lattice. This structure, known as empty lattice hydrate, is unstable, however, can become stable at appropriate temperatures and pressures in the presence of various components of suitable size and shape. There is no chemical bond formed between the water and guest molecules trapped in this crystal structure. The stabilization of the crystals is as the result of creation of hydrogen bonds between water molecules and the van der Waals forces [1], [2]. Examples of these types of molecules are natural gases components such as methane, ethane, propane and carbon dioxide.

Humphrey Davy originally discovered gas hydrates in 1810. They can occur in three typical types of structures: I, II and H depending generally on the guest molecules [1]. Gas hydrates rather than caused pipelines occlusion [2], have not always been considered as an undesirable end product. Indeed, this phenomenon may have many industrial applications. Gas hydrates have been utilized to separate mixtures of gases [3] and carbon dioxide from flue/fuel gas [4], desalinate seawater [5], [6], as well as the storage and transportation of natural gases [7]. Given the importance of gas hydrate formation, some methods have been used to expedite its production. Variety of chemicals as promoters has also been used to improve structural stability and gas storage capacity.

Today, two types of promoters are generally used: kinetic promoters and thermodynamic promoters. Kinetic promoters can reduce the hydrates formation time, increase gas storage, and increase the stability of the hydrate produced at ambient condition [8], [9], [10], [11]. Thermodynamic promoters enhance the conditions for gas hydrate formation to low pressures and high temperatures. A group of the latter promoters include water-soluble organic promoters generally forming type II structure. This is partly due to the molecular size of the water-soluble organic promoter. These molecules naturally do not have the capacity to occupy the small cavities of structure II and they normally occupy the larger cavities of structure II and acquire stability. Representatives of water-soluble organic promoters are tetrahydrofuran (THF), 1,3-dioxolane, 1,4-dioxane and acetone [12], [13], [14], [15]. Approximately, six mole percent of these promoters increase the dissociation temperature of methane hydrate and then, at higher concentrations and due to the decreasing of water activity, this effect slowly cools off [12], [13], [14], [15], [16].

Hydrate dissociation/equilibrium conditions for methane + aqueous solution of acetone system have been extensively studied by Mainush et al. [12]; Ng and Robinson [14] and Seo et al. [15]; and for methane + 1.4-dioxane + water system by Jager et al. [13] and Seo et al. [15], for methane + 1.3-dioxolane + water system by De Deuged et al. [16], and for methane + tetrahydrofuran + water system by De deuged et al. [16], Seo et al. [15], Zhang et al. [17] and Mohammadi and Richon [18].

Mainusch et al. [12] measured hydrate dissociation conditions for methane + acetone + water system. The data were described by a model that took into account the variation of the enthalpy of hydrate dissociation as a function of pressure and acetone concentration. The model for this system could well predict hydrate dissociation temperature [12]. Later, Jagger et al. [13] used the van der Waals-Platteeuw solid solution theory and assumed the occupancy of the large cavities of structure II by 1,4-dioxane to predict temperatures of methane hydrate dissociation in the presence of 1,4-dioxane aqueous solution and concluded that the model cannot predict methane hydrate dissociation in concentrations up to 20 mol% of 1,4-dioxane [13]. De Deuged et al. [16] used the van der Waals-Platteeuw solid solution theory to predict hydrate dissociation temperatures for methane + acetone, 1,4-dioxane, 1,3-doxolane or tetrahydrofuran + water system. They also compared the promoting property of these promoters and found that the promoting property was in the order of tetrahydrofuran > 1.3-dioxolane > 1.4-dioxane > acetone [16]. Seo et al. [15] used the van der Waals-Platteeuw solid solution theory, SRK equation of state and UNIFAC activity model to predict hydrate dissociation temperatures in methane/nitrogen + 1.4-dioxane/tetrahydrofuran/acetone/propylene oxide + water system with acceptable results [15]. Using the model established by Jager et al. [13], Javanmardi et al. [19] proposed a model to predict hydrate dissociation temperatures in methane + acetone + water system [19]. Mohammadi and coworkers could well represent/predict hydrate dissociation temperature of methane, carbon dioxide or nitrogen + tetrahydrofuran + water system using an artificial neural network algorithm [20]. Pahlavanzadeh et al. [21] suggested a thermodynamic model to predict the hydrate dissociation temperatures of methane/hydrogen/carbon dioxide + tetrahydrofuran + water system and achieved satisfactory results from the model [21].

To the best of our knowledge, no method has been proposed for using enthalpy of dissociation to predict hydrate dissociation conditions in the presence of various types of water-soluble organic promoters. In this study, Genetic Programming (GP) and Teaching–Learning-Based Optimization (TLBO) evolutionary algorithms were used to develop a simple formula to estimate the enthalpy of hydrate dissociation in methane + water-soluble organic promoter + water system, including acetone, 1,3-dioxolane, 1,4-dioxane, and tetrahydrofuran. Finally, the results are compared with the reported experimental data and satisfactory accuracies are observed.

Section snippets

Thermodynamic model

To calculate the hydrate dissociation temperature in the presence of aqueous solution of an additive, Eq. (1), originally proposed by Pieroen [22], can be used. This approach was originally applied by Mainusch et al. [12] for the systems containing acetone and by Maddox et al. [23], Javanmardi et al., and Ghiasi and Mohammadi [24] for the systems containing alcohols and electrolytes. $ln a_{w} = \frac{- Δ H}{NR} (\frac{1}{T} - \frac{1}{T_{w}})$ where, T_w represents the dissociation temperature of methane hydrate in the presence of pure

Results and discussion

Gas hydrate dissociation conditions in the presence of water soluble organic promoters depend on the size and polarity of these compounds. The number of atoms in a ring, the number of oxygen atoms of promoter and the molecular weight of promoter define the size of promoter. Moreover, the dipole moments of promoter may be used for determining the polarity of promoters.

The molecules of water-soluble organic promoter do not have hydrogen bonds at normal conditions, but when dissolved in water,

Conclusions

A reliable method for prediction of hydrate dissociation conditions for methane + aqueous solution of water-soluble organic promoter is presented. The GP and TLBO algorithms were used to develop a correlation for calculation of enthalpy of hydrate dissociation for methane + aqueous solution of water-soluble organic promoter. The parameters of this correlation were optimized and reported. The studied additives are tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane and acetone. The van Laar and NRTL

Nomenclature

Kihara potential parameter, spherical potential hard core radius $(\overset{o}{A})$

heat capacity temperature dependency (J/mol·K²)

a_w

water activity

A_ij

binary parameters of van Laar

C_mj

Langmuir constant of type j hydrate former in type m cavity (kPa^− 1)

ΔC_pw⁰

reference heat capacity difference (J/mol·K)

ΔC_pw^l

different specific heat capacity

the donor of hydrogen bond of promoter

fugacity (kPa)

G_ij

energy interaction parameters in the Boltzmann from in the NRTL model

Δh_w⁰

reference difference (J/mol)

Δh_w^l

enthalpy

Acknowledgment

The authors acknowledge gratefully the financial support of Shiraz University of Technology.

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Phase stability conditions of clathrate hydrates for methane + aqueous solution of water soluble organic promoter system: Modeling using a thermodynamic framework

Highlights

Abstract

Introduction

Section snippets

Thermodynamic model

Results and discussion

Conclusions

Nomenclature

Acknowledgment

Appl. Therm. Eng.

J. Pet. Sci. Eng.

Fuel

Fuel

Fluid Phase Equilib.

Fluid Phase Equilib.

Chem. Eng. Sci.

Fluid Phase Equilib.

Comput. Aided Des.

Clathrate Hydrates of Natural Gases

Hydrates of hydrocarbons

Denuding hydrocarbon liquids of natural gas constituents by hydrate formation

J. Pet. Technol.

Recovery of CO2 from flue gas using gas hydrate: thermodynamic verification through phase equilibrium measurements

Environ. Sci. Technol.

The hydrate process

Chem. Eng. Prog.

Surfactants as hydrate promoters?

Energy Fuel

Effect of surfactants on hydrate formation kinetics

Experimental determination and modeling of methane hydrates in mixtures of acetone and water

J. Chem. Eng. Data

New developments in the measurement and prediction of hydrate formation for processing needs

Ann. N. Y. Acad. Sci.

Mixed hydrates of methane and water-soluble hydrocarbons modeling of empirical results

AICHE J.

Recovery of CO₂ from flue gas using gas hydrate: thermodynamic verification through phase equilibrium measurements