Semi-clathrate hydrate phase equilibria of carbon dioxide in presence of tetra-n-butyl-ammonium chloride (TBAC): Experimental measurements and thermodynamic modeling

Highlights

• Enhancement in semi-clathrate hydrate of CO₂ + TBAC solution stability was observed.

• The lowest level of sensitivity to temperature was observed for TBAC -containing system.

• Gibbs energy minimization method was applied to TBAC + CO₂+water system equilibrium.

• A combination of models was used to find the hydrate and aqueous solution properties.

Abstract

The promotion effect of TBAC on the dissociation condition of the studied system is determined under the present experimental data. A combination of the van der Waals–Platteeuw solid solution theory, binding mean spherical approximation electrolyte model and modified Peng–Robinson equation of state is used for calculation of the hydrate, aqueous solution and gaseous phases properties, respectively. The results are also compared with the experimental data for other salts such as tetra-n-butyl-ammonium bromide (TBAB) and tetra-hydro furan (THF). A good agreement between the model predictions and experimental data is found with an absolute average relative deviation of 13.1%.

Introduction

Gas hydrates are a group of ice-like crystalline compounds which are constructed by physically stable interaction of hydrogen bonded lattice structures made by water molecules and small guest gas molecules trapped in the cages of the lattice [1]. Recently, gas hydrates are become important in many areas such as gas separation, air conditioning, water desalination and natural gas storage and transportation [[2], [3], [4], [5], [6]]. The use of hydrate of gases mainly carbon dioxide is particularly important from environmental point of view. Separation of CO₂ using hydrate leads to a major reduction of the CO₂ from gas mixtures which are important in natural gas industry and environmental protection. Several authors have reviewed the positive application of hydrate formation as a novel approach for separation of carbon dioxide [4,7]. Although gas hydrates are suitable properties for several purposes, however it is still remaining concerns about the scale up to large practical applications. In order to overcome of some these deficiencies, some additives called promoters are added to increase its stability.

Quaternary ammonium salts (QAS) are among additives forms semi-clathrate hydrates. Tetra-n-butyl ammonium chloride (TBAC) is one of QAS which interest due to its stability at near atmospheric pressure and ambient temperature. Anionic part (Cl⁻) will be confined in the lattice made by water molecules and its cation occupy some of the available large cavities. In addition, small gas molecules such as CO₂, CH₄, and etc. Can be trapped in the available small cages [[8], [9], [10], [11]]. This alteration usually promotes the hydrate equilibrium conditions and salt acts as a “promoter”. TBAC partially dissociate in water even at low and moderate concentrations and can form three more conventional structures including: TBAC·24H₂O (H₂₄), TBAC·30H₂O (H₃₀), and TBAC·32H₂O (H₃₂) [12]. While there are a few equilibrium data on the dissociation conditions of semi-clathrate hydrates of this salt, rigorous literature survey shows still it is necessary to generate more experimental data to clarify the promotion effect of this salt [[11], [12], [13], [14], [15]].

In semi-clathrate hydrates, it is highly attractive to institute a relation between change of the dissociation pressure and the temperature in a wide range of temperature, pressure and salt composition. To this purpose, it should be found a reliable way to evaluate the thermodynamic stability of semi-clathrate hydrates. There have been some attempts to achieve a reliable relation [[16], [17], [18], [19]]. Most of the thermodynamic models for predicting the semi-clathrate hydrate dissociation conditions are various modifications of the well-known van der Waals-Platteeuw (vdW–P) model. The main assumption in the available models for prediction of semi-clathrate hydrates phase equilibria is that no association of hydrated anions and cations take place in aqueous solution. While quaternary ammonium salts, partially dissociates in aqueous solution. The ideal ionic solution assumption can be removed by develop a reliable thermodynamic model. For instance, Mesbah et al. [20,21] have been modelled the semi-clathrate hydrates of CH₄, CO₂, N₂, and H₂S in TBAB using Langmuir adsorption theory, Peng-Robinson EoS, and genetic algorithm to represent the nonideality of gas hydrate. They have used Nelder-Mead and Genetic algorithm for tuning their model variables, showing their model could cover a wider range of data.

In this work, new experimental data have been measured for the dissociation conditions of carbon dioxide semi-clathrate hydrates in the presence of TBAC at three different salt mass fractions including 0.050, 0.122 and 0.160. Data are experimentally measured by employing a stepwise heating method in the temperature range of 281.6 K–291.1 K and pressures up to 4.29 MPa. A comparison between the promotion effect of TBAC, TBAB and THF at the different concentrations is also performed. Furthermore, hydrate dissociation conditions for TBAC + CO₂+water system is modelled same as presented in Ref. [17] with some modifications.