Elsevier

Fluid Phase Equilibria

Volume 315, 15 February 2012, Pages 9-15
Fluid Phase Equilibria

Solubility of o-tolidine in pure and modified supercritical carbon dioxide

https://doi.org/10.1016/j.fluid.2011.11.010Get rights and content

Abstract

The equilibrium solubility of o-tolidine in supercritical carbon dioxide with or without cosolvents was measured using a dynamic flow method. The experimental data were taken at 308, 318 and 328 K, and over the pressure range from (11.0 to 21.0) MPa. Ethanol and ethylene glycol were chosen as the cosolvent, and the mole fraction of cosolvent was 0.01, 0.02, and 0.04, respectively. The experimental data indicated that the solubility of o-tolidine can be enhanced by the presence of both the cosolvents, and increases with an increase in the mole fraction of cosolvents. The thermodynamic properties (total enthalpy ΔtotH, sublimation enthalpy ΔsubH and solvation enthalpy ΔsolH) of solid solute were obtained. The experimental data were correlated by Chrastil, Bartle, Méndez-Santiago and Teja, K–J, modified K–J, modified Chrastil and modified Sovova models. All the correlations showed satisfactory agreement with the experimental data.

Highlights

► The solubility of o-tolidine was determined in SCCO2 with and without cosolvents. ► The cosolvent effect was investigated at different temperature and concentration. ► The thermodynamic properties (ΔtotH, ΔsubH and ΔsolH) of o-tolidine were obtained. ► Good agreement was obtained using seven density-based models.

Introduction

Supercritical fluid (SCF) technology has gained a rapid development in the last 30 years and received much attention for applications in different fields, such as pharmaceutical, food, separation, and polymer processes [1], [2], [3], [4]. SCFs have properties intermediate between those of gases and liquids, that is, high diffusivity and low viscosity. Among the SCFs, carbon dioxide has been the best choice for most supercritical fluid technology, primarily because that carbon dioxide is environmentally friendly, inexpensive, nontoxic, nonflammable, and it has a near-ambient critical temperature (Tc = 304 K). Supercritical carbon dioxide (SCCO2) has been proposed as an environmentally benign replacement for the organic solvents because of these advantages. Unfortunately, it does have limitations for dissolving polar substances as a result of its lack of polarity and associated deficiency for specific solvent-solute interactions. It has been found that adding a small amount of polar cosolvent to SCCO2 can have dramatic effects on the solvation ability of carbon dioxide [5], [6]. The specific interactions between the solute and the cosolvent would lead to an enhancement in the solubility of a solute.

High-pressure equilibrium solubility data in a SCF are among the important and fundamental data for SCF process design. Though a number of solid solubility data in SCCO2 have been reported by some workers [7], [8], the available solubility data for solid solute in SCCO2 with or without cosolvents are limited, and more experimental solubility data of various compounds in SCCO2 are still needed.

Compounds with various functional groups usually have great variations in the magnitude of solubility in SCCO2. Our lab has measured the solid solubility of various aromatic compounds with different groups [9], [10]. Diamines, including two amino-functional groups, are an important class of compounds applied in different fields of industry [11], [12]. o-Tolidine (4,4′-diamino-3,3′-dimethylbiphenyl) is an important organic intermediate which is mainly used in the manufacture of dyes such as naphthol AS-G, and it can act as sensitive colorimetric reagent for gold and for free chlorine in water. To the best of our knowledge, no solid solubility data regarding this compound have yet been reported before. The solubility determination of o-tolidine is essential to know the effect of the functional group –NH2 on the solubility.

In this work, we therefore measured the equilibrium solubility of o-tolidine with or without cosolvents in SCCO2 at three temperatures between 308 and 328 K and at five pressures between 11.0 and 21.0 MPa. Five density-based models, Chrastil [13], Bartle [14], Méndez-Santiago and Teja [15], K–J [16] and modified K–J [17] models were used to correlate the experimental data for the binary system (SCCO2 + o-tolidine). Modified Chrastil [18] and modified Sovova [19] models were employed to correlate the solubility for the ternary system (SCCO2 + o-tolidine + cosolvent). The present work is a part of our long-term objective to predict the solubility behavior of solutes in SCFs by analyzing the contribution of different classes of functional groups.

Section snippets

Materials

o-Tolidine (CAS Registry No. 119-93-7, a mass purity of more than 99.0%, analytical grade) was purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd. The physical property [20] and molecular structure about o-tolidine are given in Table 1. Ethanol (CAS Registry No. 64-17-5, with a minimum mass purity of more than 99.7%) and ethylene glycol (CAS Registry No. 107-21-1, with a minimum mass purity of more than 99.5%) were obtained from Beijing Chemical Reagent Factory. High-purity

Solubility of o-tolidine in pure SCCO2

The experimental equilibrium solubility values of o-tolidine in pure SCCO2, together with the density of CO2, are summarized in Table 2 and plotted in Fig. 2. The pure CO2 density values were obtained from the National Institute of Standards and Technology Web site. The solubility is expressed in terms of o-tolidine mole fraction (y2). It can be obviously observed from Table 2 and Fig. 2, the influence of pressure on the equilibrium solubility follows the expected trends that the solubility

Conclusion

The solubility of o-tolidine in SCCO2 in the absence and presence of cosolvents at temperatures from (308 to 328) K and pressures from (11.0 to 21.0) MPa was first measured. The mole fraction of o-tolidine was in the range of (10−6 to 10−5). The crossover pressure region for the binary system (SCCO2 + o-tolidine) was at pressures around (11.5–13.5) MPa and for the ternary system (SCCO2 + o-tolidine + ethanol) was from (12.5 to 15.0) MPa. Chrastil, Bartle, M–T, K–J and modified K–J models were applied

List of symbols

    a0, a1, k

    parameters in the Chrastil and modified Chrastil models

    b0, b1, b2

    parameters in the Bartle model

    c0, c1, c2

    parameters in the Méndez-Santiago and Teja model

    d0, d1, d2

    parameters in the K–J model

    d1, d2, d3, d4

    parameters in the modified K–J model

    k′, m, n

    parameters in the Sovova model

    k′, m, n, f

    parameters in the modified Sovova model

    e

    enhancement factor

    ΔH

    enthalpy

    MW

    molecular weight

    M1

    molecular weight of CO2

    M2

    molecular weight of the solute

    M3

    molecular weight of the cosolvent

    N

    number of data points

    P

Acknowledgements

This research was financially supported by the funds awarded by National Natural Science Foundation of China (No. 21176012), the supports from Petrochina Company Limited through the Applied Research ProProject (No. 2009A-3801-02) and the National High-Tech Program (“863” Plan, # 2006AA030203). The authors are grateful to the support of this research from the Mass Transfer and Separation Laboratory in Beijing University of Chemical Technology.

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