Vapour-liquid equilibrium of acetone-CO₂ mixtures of different compositions at the vicinity of the critical point

Highlights

• Vapour-liquid equilibrium of acetone−CO₂ mixtures is studied by computer simulation.

• High temperature near-critical states are considered in the entire composition range.

• The critical parameters are determined in the entire composition range.

• Temperature dependence of the surface tension is determined in the states considered.

Abstract

The vapour-liquid equilibrium of acetone−CO₂ mixtures is studied by computer simulation at 11 different compositions, ranging from neat CO₂ to neat acetone, in a 50–100 K wide range of temperatures at the vicinity of the critical point. The composition dependence of the critical parameters is determined, for the first time, in the entire composition range. It is found that while the critical temperature changes monotonically with the composition, the critical pressure goes through a maximum around the acetone mole fraction value of 0.3, and the critical density might also exhibit a maximum in the acetone mole fraction range of 0-0.2. Temperature dependence of the surface tension is also determined in the entire composition range. The obtained results agree, in general, well with experimental data; their deviation remains below the range within which different experimental data sets deviate from each other. Since experimental data in this respect exist, unfortunately, only in limited ranges of compositions (at low acetone mole fractions) and temperatures (data above about 335 K are scarce), the present study largely extends the range of thermodynamic conditions in which we have reliable information on the liquid-vapour equilibrium and critical conditions of acetone−CO₂ mixtures.

Introduction

Supercritical CO₂ (scCO₂) provides an environmentally friendly alternative to many toxic solvents in a number of industrial processes. Further, mixing scCO₂ with suitably chosen polar co-solvents can substantially increase the solubility of several targeted solutes, especially if the weak acid CO₂ is complemented by a weak base, such as acetone. Further, physico-chemical and solvation properties of such mixtures, often called as CO₂-expanded liquids [1], can be fine tuned through their composition. Due to this increasing interest, the properties of CO₂-acetone mixtures have been investigated both by experimental [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]] and computer simulation methods [17,20,[26], [27], [28], [29], [30]] in the past decades. The majority of these studies focused on the properties of the one-phase mixtures [6,[11], [12], [13],18,20,22,23,27,29,30], including selective solvation of various solutes in such systems [20,30], or on their vapour-liquid equilibrium in a narrow temperature range, usually between 291 and 313 K [25,7,9,10,14,16,17,19,21,26,28]. Experimental data concerning the vapour-liquid equilibrium above this temperature range are scarce. Stievano and Elvassore measured the saturated liquid density at four pressures also at 323 K [14]. Hsieh et al. reported the composition of the liquid phase in liquid-vapour equilibrium at several pressures along the 313, 333, and 353 K isotherms. [25]. Traub et al. measured the composition of the coexisting liquid and vapour phases at a single pressure at 333.K [3], while Bamberger and Maurer measured it along the 323 and 333 K isotherms [10]. Han et al. measured liquid-vapour equilibrium data in the temperature range between 333 and 393 K [15]. Sato et al. reported the bubble point of mixtures of different compositions at temperatures between 313 and 353 K [24], while Wu et al. measured both the bubble and the dew point up to about 480 K [16]. Little is known about the vapour-liquid equilibrium of these mixtures at near-critical conditions, and, in particular, about the composition dependence of the location of their critical point itself. The critical point of the mixture was determined up to the acetone mole fraction of 0.07 by Reaves et al. [8], up to 0.10 by Chen et al. [11], and up to 0.22 by Han et al [15]. We are not aware, however, of any study concerning the location of the critical point in mixtures of higher acetone content, and addressing the vapour-liquid equilibrium at near-critical conditions in such mixtures. The temperature range relevant in this respect extends from the critical temperature of neat CO₂ of 304.2 K [10] to that of neat acetone of 508.1 K [10,31]. Accurate information about the location of the critical point in the entire composition range would, however, be of key importance in the use of these mixtures in supercritical fluid technology [32].

Besides the critical point itself, the surface tension of such systems is also a quantity of key importance. Clearly, the surface tension of a fluid is a relevant factor in the formation of vapour-liquid equilibrium. CO₂-expanded liquids are often used as extracting agents, and the capillary rise of the fluid inside the solid carrier, a quantity that is strongly related to its extracting efficiency, is also governed by its surface tension. Further, such liquids are often used for aerogel drying, where accurate information about the surface tension would be essential to ensure preservation of the aerogel structure. [33,34] However, accurate prediction of the behaviour of the surface tension is rather difficult to be predicted at thermodynamic conditions close to the critical point of the mixture. We are certainly not aware of any surface tension measurement of acetone−CO₂ mixtures in an about 100 K vicinity of the critical point.

Computer simulation methods [35] can offer a convenient tool to complement experimental investigations in this respect, since they provide such a deep, molecular level insight into the system studied that cannot be obtained by any experimental technique. However, computer simulations only access a suitably chosen model rather than the real system of interest itself, therefore, the reliability of the chosen model needs to be validated against existing experimental data whenever possible. In this paper, we present results of extensive computer simulations of the vapour-liquid equilibrium of acetone−CO₂ mixtures in the entire composition range between the two neat liquids, covering an about 80–100 K broad temperature range below the critical point at each composition. The critical temperature and density is determined from the temperature dependence of the coexisting liquid and vapour densities, while the critical pressure is estimated from the temperature dependence of the saturated vapour pressure. Further, the dew and bubble points of the systems are calculated at various temperatures and pressures, and the surface tension and its temperature dependence is also determined in the entire composition range.

The paper is organized as follows. In sec. 2 the details of the simulations performed and potential models used are provided. Then the obtained results are presented and discussed in detail in section 3. Finally, in section 4 the main conclusions of this study are summarised.