Elsevier

Fluid Phase Equilibria

Volume 315, 15 February 2012, Pages 40-45
Fluid Phase Equilibria

Vapor–liquid equilibrium for the ternary carbon dioxide + ethanol + n-hexane and quaternary carbon dioxide + ethanol + n-hexane + thiophene systems

https://doi.org/10.1016/j.fluid.2011.11.009Get rights and content

Abstract

In this work, the experimental vapor–liquid equilibria (VLE) for a ternary and a quaternary system was measured in a static–analytic apparatus from subcritical up to near critical pressures of the systems. The VLE for the ternary carbon dioxide + ethanol + n-hexane system was determined at (313.66, 342.82 and 372.76) K. VLE for the same components in the presence of thiophene was obtained at (313.11, 343.39 and 373.06) K. Combined uncertainties for the measured properties were estimated to be ±0.008 MPa, ±0.03 K, ±0.0019 mol/mol for the vapor phase and ±0.0038 mol/mol for the liquid phase with respect to carbon dioxide composition. A high selectivity of n-hexane over ethanol and thiophene was obtained in both systems in the presence of carbon dioxide for the studied range of temperature and pressure. The vapor–liquid equilibrium data were correlated with the Peng–Robinson equation of state using the classical mixing rule.

Highlights

► VLE data of carbon dioxide + ethanol + n-hexane, and +thiophene from (313 to 363) K. ► A static-analytic apparatus with a ROLSI sampler was used. ► Equilibrium ratios Ki and relative volatilities αij were evaluated for these systems. ► α32 {n-hexane (3) over ethanol (2)} were higher in the quaternary system. ► n-Hexane had higher relative volatilities than ethanol and thiophene.

Introduction

Vapor–liquid equilibrium (VLE) data of systems containing hydrocarbons are essential for engineering design of separation processes and unit operations in chemical industries. The knowledge of thermodynamic properties such as VLE and volumetric properties of multicomponent systems over a wide range of temperatures and pressures are important for the development of empirical, semi-empirical or theoretical models able to represent and predict their behavior [1]. Phase equilibria of multicomponent carbon dioxide + hydrocarbons systems are of great importance in petrochemical and chemical industries. Carbon dioxide has been the most used solvent because it is cheap, non-flammable, non-toxic and with low critical properties (Tc = 304.12 K and pc = 7.374 MPa).

Some researches have been accomplished about fluid phase equilibria in ternary systems carbon dioxide + alkanol + n-alkane. Peters et al. [2], [3] have reported the critical endpoints of carbon dioxide – alkanols (pentanol, hexanol, heptanol, octanol, decanol) – tetradecane at temperatures lower than 317 K. Critical lines, miscibility windows and isothermal phase diagrams are presented for carbon dioxide + linear alkanol + linear alkane systems by Kordikowski and Schneider [4], Pöler et al. [5] and Scheidgen and Schneider [6]. The studied alkanols are from 1-hexanol to 1-dodecanol and the number of carbon atoms for the n-alkanes covers the range of C14–C24 in a wide range of temperature and pressure [5]. However, there is a lack of VLE data for this kind of multicomponent systems with short carbon chains.

Our working group had determined the VLE data of ternary systems using carbon dioxide and hydrocarbons [7], [8]. The solubility of thiophene in carbon dioxide is enhanced when an alkanol (ethanol or 1-propanol) is added as cosolvent [9], [10]. Nevertheless, the knowledge of phase equilibrium in multicomponent systems containing carbon dioxide + alkanol + alkane and a sulfur compound is necessary in order to know the selectivity parameter as a base of an upcoming separation process.

In this work, measurements of vapor–liquid equilibria for a ternary and quaternary systems containing carbon dioxide + ethanol + n-hexane, and + thiophene were performed from (313 to 373) K by the static-analytic method. The sulfur compound was added to the ternary system in order to simulate a model fuel and in consequence understand the selectivity of the solvent over the three solutes. The representation of the vapor–liquid equilibria was carried out with the Peng–Robinson Equation of State [11] using the classical mixing rule. The VLE for both multi-component systems was predicted using interaction parameters of binary mixtures which were fitted from literature VLE data [12], [13], [14], [15], [16].

Section snippets

Materials

Carbon dioxide of supercritical grade and helium were purchased from Air Products-Infra. Sigma-Aldrich supplied n-hexane and thiophene and Merck Chemicals provided ethanol. Certified purities, water content and properties [17] of chemicals are reported in Table 1. All chemicals were used without further purifications. A Karl–Fischer coulometer (model: 831, Metrohm) was used for the water content determinations.

Apparatus and procedure

Measurements were performed in an apparatus based on the static-analytic method. The

Results and discussion

The experimental VLE for the carbon dioxide + ethanol + n-hexane was determined at (313.66, 342.82, 372.76) K. These are summarized in Table 2. The VLE for the carbon dioxide + ethanol + n-hexane + thiophene was obtained at (313.11, 343.39, and 373.06) K and are listed in Table 3. The Peng–Robinson equation of state [11] coupled to the classical mixing rule was the base model for the vapor–liquid equilibrium predictions of the ternary and quaternary system. It is expressed asp=RTvba(T)v(v+b)+b(vb)

The

Conclusions

Isothermal vapor–liquid equilibrium data for multicomponent systems was measured in the range of (313 to 373) K. These contained carbon dioxide + ethanol + n-hexane and carbon dioxide + ethanol + n-hexane + thiophene. The relative volatilities between n-hexane and ethanol were enhanced 1.8 times at low temperature of ∼313 K in the quaternary system compared with those obtained in the ternary system. Meanwhile the relative volatilities for n-hexane over thiophene were lower than the alkane–alkanol

List of symbols

    Ai

    response area of the gas chromatograph

    a, b

    parameters to Eqs. (6), (7), (8).

    ci, di

    parameters to Eqs. (4), (5).

    K

    equilibrium ratio

    kij

    binary interaction parameter

    ni

    mole number of component i

    nt

    total mole number

    Nc

    number of components

    Np

    number of data points

    p

    pressure (MPa)

    R

    ideal gas constant (MPa m3 K−1 kmol−1)

    T

    temperature (K)

    xi

    liquid mole fraction of component i

    yi

    vapor mole fraction of component i

    zi

    mole fraction of component i

    u

    uncertainty

    v

    mole volume (m3 kmol−1)

    Greek letters

    αij

    relative volatility

    ω

    acentric factor

    Subscripts

    c

Acknowledgment

Authors thank to Mexican Institutions (CONACyT and Instituto Politécnico Nacional) for the financial support of this research.

References (19)

  • C.J. Peters et al.

    J. Supercrit. Fluids

    (1996)
  • C.J. Peters et al.

    Fluid Phase Equilib.

    (1995)
  • A. Kordikowski et al.

    Fluid Phase Equilib.

    (1993)
  • A.L. Scheidgen et al.

    J. Chem. Thermodyn.

    (2000)
  • O. Elizalde-Solis et al.

    Fluid Phase Equilib.

    (2005)
  • O. Elizalde-Solis et al.

    Fluid Phase Equilib.

    (2005)
  • Z. Knez et al.

    J. Supercrit. Fluids

    (2008)
  • K.S. Pedersen et al.

    Phase Behavior of Petroleum Reservoir Fluids

    (2007)
  • H. Pöler et al.

    Fluid Phase Equilib.

    (1996)
There are more references available in the full text version of this article.

Cited by (3)

  • Isothermal vapour-liquid equilibrium data for the binary systems of CHF<inf>3</inf> with (n-nonane, n-decane, or n-undecane) and C<inf>2</inf>F<inf>6</inf> with (n-nonane or n-decane)

    2018, Fluid Phase Equilibria
    Citation Excerpt :

    An example of a possible polar SCF solvent is R-23. In the characterisation of the performance of supercritical solvents, the most common technique is to investigate the phase equilibrium conditions of a small number of solutes that approximate the desired separation, yet do not require the measurement of an extensive set of binary systems [9–13]. n-Decane has been used as a ‘base-normal-alkane’ by authors such as Huie et al. [14], Nagarajan et al. [15], Chou et al. [16] and Sánchez-Garcia et al. [12].

View full text