Molecular dynamic simulation of carbon dioxide, methane, and nitrogen adsorption on Faujasite zeolite

Abstract

Removing impurities such as carbon dioxide and nitrogen from natural gas is a technical challenge and one of the major concerns in natural gas treatment process. In this study, adsorption of CH₄, N₂, and CO₂ on the Faujasite (FAU) zeolite has been studied using molecular dynamics simulation at temperatures of 293, 308, and 323 K and pressures up to 1 MPa. COMPASS force field was used to model the interactions between zeolite and guest molecules. Ewald and atom-based summation methods were used for the calculation of electrostatic and van der Waals forces, respectively. Simulated results were modeled using Langmuir, Freundlich, Toth, and Sips adsorption isotherms. Sips isotherm for CO₂, and Toth isotherm for CH₄ and N₂ pure compounds showed the best performance. Heat of adsorption for CH₄, CO₂, and N₂ were calculated to be −15.48, −24.1, and −13.31 kJ⋅mol⁻¹, respectively. A comparative study showed that the simulation model was successful in predicting the overall trend of the adsorption with acceptable accuracy.

Introduction

Natural gas, as a fuel source, provides a quarter of the world’s total energy. It is estimated that the demand for natural gas will rise by 50 percent in the next 20 years [1]. Natural gas constituents depend heavily on the source from which it was extracted, but essentially, it consists of methane (typically 80%−95%), C₂₊ hydrocarbons, and a small percentage of nitrogen and carbon dioxide as impurities [2]. Some sources have higher concentrations of impurities, such as CO₂ [1]. Levels of carbon dioxide in the atmosphere have increased by about 30% after industrialization, and is anticipated to become doubled or tripled by the end of the 21st century [3]. Necessity of removing these impurities has encouraged researchers to study natural gas purification processes [4], [5]. Presence of carbon dioxide and nitrogen in natural gas decreases the energy density of natural gas and causes corrosion in pipelines. As a result, the amount of N₂ and CO₂ should be less than 2% and 4%, respectively [5], [6]. Methane produces less carbon dioxide when burned, compared to coal and oil (45% and 30% respectively), and is more attractive financially [2]. Adsorption using aqueous amine solutions [7], membranes [8], and using nanoporous materials, including zeolites, is recommended for CH₄/CO₂ mixture separation. Adsorption processes are also used in CH₄ separation from coal bed and landfill gas treatment [9], [10], [11]. Due to the financial and environmental concerns, and difficulties in conducting experiments, separation of carbon dioxide and methane mixture is quite challenging. Researchers are trying to find new materials which are not only effective adsorbents but also inexpensive and easy to produce. Among many materials that have been tested, zeolite has proven to be a good candidate. Zeolites can act as heterogeneous catalysts, adsorbents, and molecular sieves in gas separation process [12], [13]. Many studies have focused on the separation of CO₂ and CH₄ using zeolites [2], [14]. Among different zeolites, zeolite 13X is the most efficient and most common adsorbent due to its polar surface, high specific surface area, and big pore volume. Accuracy of adsorbent models can be evaluated by comparing the simulation results and the experimental data [15], [16].

In recent years, adsorption of CH₄, N₂, and CO₂ on zeolites has been studied using atomistic molecular dynamics simulation methods such as Monte Carlo method [17], [18], [19]. Molecular dynamics simulation has an important role in helping us understand how gas is adsorbed on zeolites, at molecular level [20], [21], [22]. Macedonia et al. [23] studied the adsorption of methane, ethane, and argon on mordenite, using Monte Carlo simulation method. Evaluating the results using experimental data, they found the simulation results consistent with empirical data. Ahunbay et al. [24] surveyed methyl tertiary butyl ether (MTBE) adsorption on silicalite-1 using Grand Canonical Monte Carlo (GCMC) simulation and reported good agreement with experimental data except for lower loadings, which was caused by the limitations of the Polymer Consistent Force Field (PCFF). Purdue et al. [25] studied the adsorption of wet flue gas on zeolite 13X in the pressure of 1 atm, and temperature range of 298–348 K using molecular dynamics simulation. Simulated equilibrium isotherm data showed that presence of small amounts of steam in the gas mixture has a profound effect on the loading of adsorbed gas species on zeolite 13X. Recent studies revealed that Monte Carlo is a great method for simulating adsorption and separation of gases using zeolite [15], [26].

Aim of this work is to simulate adsorption of CH₄, N₂, and CO₂ on zeolite FAU at 293, 308, and 323 K and pressures up to 1 MPa with molecular dynamics. Adsorption isotherms were modeled using Langmuir, Freundlich, Langmuir-Freundlich, and Toth methods, and deviations for each model was calculated. In the end, adsorption enthalpy was calculated for CH₄, N₂, and CO₂.