Thermodynamic study of binary systems containing sulphur dioxide: Measurements and molecular modelling

Abstract

In the present work, we report isothermal vapour–liquid equilibrium data for two binary systems: nitrogen–sulphur dioxide and oxygen–sulphur dioxide at four temperatures (323.15, 343.15, 373.15, and 413.15 K) and pressures up to 85 MPa. These data were simultaneously obtained from experimental measurements and molecular simulations. The compositions of coexisting phases were experimentally determined using an apparatus based on the “static-analytic” method. At the same time, Gibbs ensemble Monte Carlo (MC) simulations were performed on these binary mixtures, using force fields based on pure component properties. An original force field is proposed for sulphur dioxide molecule, involving three Lennard–Jones centres and three electrostatic charges. The experimental and simulation results appear in good agreement, allowing reliable and accurate predictions at higher pressures with molecular simulations.

Introduction

Within the fight against global warming and the reduction of greenhouse-gas emissions, investigations have been particularly intense over the last years to develop efficient processes of carbon dioxide capture. Carbon dioxide capture and storage (CCS) is a process where carbon dioxide (CO₂) is captured from gases produced by fossil fuel combustion, compressed to approximatively 10 MPa, transported and injected deep underground into geological formations for quasi-permanent storage (thousand years or more).

Depending on the industrial sector (energy production, cement factory, refining, etc.) and on the type of capture process, the composition of gases accompanying CO₂ (also called contaminant gases) can considerably vary, on both qualitative and quantitative levels. Indeed, along with CO₂ and water, many other compounds such as O₂, N₂, Ar, SO_x, NO_x, H₂ and CO can be present at different levels of concentration in flue gases. Furthermore, contaminant gases can be mixed with natural gases in depleted hydrocarbon reservoirs. Some gases present in mixtures are responsible for induced mineral dissolution–precipitation reactions and/or modifications of PVT properties. The impact of such contaminant gases on the phase equilibrium of the reservoir must be understood under geological storage conditions at high pressures and high temperatures.

The innovating aspect of this paper is to initiate a study concerning these contaminant gases. Our study will provide useful information on their possible geological storage with CO₂ in the near or remote future. We focus here on the thermodynamic behaviour of systems containing SO₂ for which available data in the literature are too scarce. Except the H₂O–SO₂ mixture that has been studied by several authors (see for instance Maass and Maass [1], Lindner [2], Morgan and Maass [3], Lavrova and Tudorovskaya [4], Li and Chen [5], Rumpf and Maurer [6], Campbell and Maass [7], Sherwood [8] among others), only very few studies have up till now investigated the thermodynamic properties of other mixtures like CO₂–SO₂, hydrocarbon–SO₂ or other contaminant–SO₂ systems (Caubet [9], Bluemcke [10], Lachet et al. [11], Dean et al. [12], Satterfield et al. [13], Bowden [14]). In the case of N₂–SO₂ system, to the best of our knowledge, the only experimental data available are that reported by Dean and Walls [12], Tsiklis [15] and Dornte and Ferguson [16]. Dornte and Ferguson have also published experimental data for the O₂–SO₂ binary mixture. This lack of experimental data is certainly related to the toxicity of sulphur dioxide, which makes any experimentation delicate and expensive.

In the present work, the thermodynamic behaviour of two SO₂-containing binary mixtures, N₂–SO₂ and O₂–SO₂, was studied through experimental measurements and molecular simulation calculations. Isothermal vapour–liquid equilibrium (VLE) data were generated for these binary systems for temperatures from 323 to 413 K. Monte Carlo (MC) molecular simulations using the Gibbs ensemble were carried out at the same temperatures.

This article is organized as follows. Section 2 presents the experimental setup used to investigate VLE of binary mixtures. In Section 3, the description of the simulation method and expressions used to compute intermolecular potential energy (i.e. force field) are given. The force fields for N₂ and O₂ are also presented in Section 3 together with the development of a new SO₂ force field. All these force fields were tested with respect to their ability to reproduce thermodynamic properties of pure compounds. In Section 4, we present and discuss a comparison between experimental and simulated results obtained for the studied binary mixtures. This paper ends with a fifth section which gives our conclusions.