Elsevier

Fluid Phase Equilibria

Volume 429, 15 December 2016, Pages 84-92
Fluid Phase Equilibria

Phase equilibria of ternary systems of carbon dioxide + ethanol, dimethyl sulfoxide, or N,N-dimethyl formamide + water at elevated pressures including near critical regions

https://doi.org/10.1016/j.fluid.2016.08.033Get rights and content

Highlights

  • Isothermal phase equilibrium boundaries were measured via synthetic method.

  • The investigated systems are ternary mixtures of CO2 + ethanol, DMSO, or DMF + water.

  • The critical points of each pseudo-binary system were estimated from experimental results.

  • The Peng-Robinson equation with Wong-Sandler mixing rule was used in this study.

Abstract

A visual and volume-variable high-pressure phase equilibrium analyzer was used for measuring the isothermal vapor-liquid equilibrium (or vapor-liquid-liquid) phase boundaries of CO2 with the mixed solvents of ethanol/water = 9/1, dimethyl sulfoxide (DMSO)/water = 9/1, and N,N-dimethyl formamide (DMF)/water = 7.5/2.5 in molar ratio at temperatures from 298.2 K to 348.2 K over wide composition range, including near critical regions. The isothermal phase equilibrium data obtained from this work are compared with those of water-free binary systems to investigate the effect of phase boundary shifts caused by introducing the phase modifier, water, into the organic solvents. The comparison shows that the bubble, dew, and critical pressures substantially increase in the presence of water for each system, especially for DMSO-containing system. The Peng-Robinson equation of state with Mathias-Copemen alpha function and the Wong-Sandler mixing rule is employed to the phase equilibrium calculation. While the UNIFAC model estimates the excess Helmholtz free energy at infinite pressure, the binary interaction parameters for calculating the second virial coefficient are determined by the phase equilibrium data of constituent binaries. The obtained parameters are directly adopted to the phase boundary predictions for the investigated ternary systems.

Introduction

The phase equilibrium properties are essentially needed in the development of supercritical fluid techniques for supercritical fluid (SCF) extraction [1], reaction [2], fractionation [3], nano-particles formation [4], [5], [6], [7], etc. Carbon dioxide has been recognized as one of environmentally benign solvents and commonly used in a variety of SCF technologies, because it has mild critical conditions (Tc = 304.25 K, Pc = 7.38 MPa), inexpensive, nontoxic, nonflammable, and readily available. Ethanol, dimethyl sulfoxide (DMSO), and N,N-dimethyl formamide (DMF) are often used as solvents in supercritical anti-solvent (SAS) processing to generate ultra-fine particles for a variety of materials [4], [5], [6], [7]. Several investigators [6], [7], [8], [9], [10], [11] pointed out that the phase behavior of solvent + anti-solvent mixtures is a key factor to govern the morphology and the size of the resultant particles produced from the SAS process. In general, nanometric particles, micrometric particles, and dense films could be obtained when the SAS precipitation was conducted in supercritical, superheated vapor, and vapor-liquid coexistence phase regions [11], respectively. As a consequence, the vapor-liquid equilibrium (VLE) phase diagram of solvent + anti-solvent systems is fundamentally important for manipulating precipitation conditions to prepare particulate products having preferable size and morphology. The VLE phase boundaries near the critical region are especially of interest in developing the SCF micronization processes.

Regarding the preparation of pharmaceutically active ingredients for aerosol pulmonary delivery, the size distribution of particles is favorable within 1 μm–5 μm. As noted above, the precipitation condition of the SAS process is suggested to be in the superheated vapor region of the solvent + anti-solvent system. However, this operable window is rather narrow for typical carbon dioxide + organic solvent systems. Perez de Diego et al. [12], [13] claimed that the bubble points shift to higher pressures by introducing water into CO2 + DMSO. Therefore, the presence of water may provide a wider operable window, i.e., wider superheated vapor region, for particle formation by using SAS precipitation. Andreatta et al. [14] investigated the VLE behavior of CO2 + DMSO + water system including three different DMSO/water molar ratios. Among several others, the phase equilibrium data of ternary systems of CO2 + ethanol + water and CO2 + DMF + water are available in literature [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Most of above mentioned literature data are in saturated liquid region (bubble points) and only a few data in saturated vapor region (dew points). Moreover, no available VLE data are found around critical regions. In the present study, a visual and volume-variable phase equilibrium analyzer (PEA) was employed to observe the phase transition boundaries of mixtures changing from single phase into vapor-liquid (or from two-liquid phase to vapor-liquid-liquid) coexistence region. Since the operation of the PEA is a synthetic method, it is especially applicable to determine the phase boundaries near critical region, in which analytic method often fails. The isothermal phase boundaries were measured for three ternary systems composed of CO2 + (DMSO/water = 9/1 M ratio), CO2 +(ethanol/water = 9/1 M ratio) and CO2 + (DMF/water = 7.5/2.5 M ratio) at temperatures from 298.2 K to 348.2 K over wide composition range, including near critical regions. Critical pressure and composition are estimated from each isothermal phase envelope.

In these three investigated ternary systems, there is a common binary pair, CO2-water, which may involve both physically and chemically interactions, simultaneously. Valtz et al. [26] found the Peng-Robinson (PR) equation of state [27] with Mathias and Copeman (MC) alpha function [28], [29] and the Wong-Sandler (WS) mixing rules [30] well represented the phase boundaries for this binary system. Similar model is taken in the present study. While the UNIFAC model [31] is used for estimating the excess Helmholtz free energy at infinite pressure, the binary interaction parameter in the calculation of mixtures' second virial coefficient is determined from the phase equilibrium data of the constituent binaries. The optimal values of the binary interaction parameters are adopted to predict the phase boundaries for the investigated ternary systems.

Section snippets

Materials

Carbon dioxide (purity of 0.995+ mass fraction) was supplied by Liu-Hsiang Co. (Taiwan). DMSO (0.999 mass fraction) was purchased from Arcos (USA), ethanol, HPLC grade (0.9999 mass fraction), from Fisher Scientific (USA), and DMF (0.9995 mass fraction) from Fluka (Germany). All the chemicals were used without further purification, except for degassing. Water was prepared by NANO pure-Ultra pure water system that was distilled and deionized with resistivity of 18.3 MΩ cm. The description of the

Experimental results and discussion

The reliability of the VLE data measured using the PEA has been checked elsewhere [33] by comparing with the literature data of CO2 + 1-octanol and CO2 + dimethyl sulfoxide (DMSO). In the present study, this apparatus was employed to measure the isothermal phase equilibrium boundaries for CO2 + ethanol + water, CO2 + DMSO + water, and CO2 + DMF + water in a temperature range of 298.2 K–348.2 K. Table 3, Table 4, Table 5 report the determined phase boundary data for these three ternary systems.

Correlation of VLE data

Firstly, we try to predict the VLE phase boundaries with the Peng-Robinson (PR) equation of state using the van der Waals one-fluid two-parameter mixing rule. As the binary interaction parameters were determined from the VLE data of the constituent binary systems, the deviations between calculated and experimental values are substantially large, especially near the critical regions. Graphical comparisons are shown in Fig. S1–S3 for the systems containing ethanol, DMSO, and DMF, respectively. To

Conclusions

The vapor-liquid (or vapor-liquid-liquid) phase transition boundaries have been determined experimentally for CO2 + ethanol + water, CO2 + DMSO + water, and CO2 + DMF + water in a temperature range of 298.2 K–348.2 K and pressures up to near critical values by using a visual and volume-variable phase equilibrium analyzer. The critical points have been determined by interpolation of the experimental phase boundary data. Introducing a certain amount of water into ethanol/CO2, DMSO/CO2, and DMF/CO2

Acknowledgment

The authors gratefully acknowledged the financial support from the Ministry of Science and Technology, Taiwan, through grant no. NSC95-2214-E-011-154-MY3.

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