Doi:10.1016/s0378-3812(03)00022-0

Fluid Phase Equilibria 207 (2003) 183–192 Solubility of solid solutes in supercritical carbon dioxide Qunsheng Li, Zeting Zhang, Chongli Zhong, Yancheng Liu, Qingrong Zhou Department of Chemical Engineering, Beijing University of Chemical Technology, P.O. Box 100, Beijing 100029, China Received 4 October 2002; accepted 14 January 2003 Abstract
The solubility of 2-naphthol and anthracene in supercritical CO2 was determined at 308.1, 318.1 and 328.1 K, with and without cosolvent. The influence of three polar or nonpolar cosolvents, acetone, ethanol and cyclohexane, wasstudied at concentrations of 3.6 and 4.0 mol%. The solubility enhancement with these cosolvents is considerable,and the cosolvent effect increases in the order ethanol, acetone, and cyclohexane for 2-naphthol and for anthracene,the order is cyclohexane, ethanol, and acetone. The influence of density and cosolvent on the solid solubility wasstudied and discussed.
2003 Elsevier Science B.V. All rights reserved.
Keywords: Solubility; Supercritical fluid; Solid solute; Cosolvent 1. Introduction
Supercritical fluid extraction (SCFE) is a relatively new and promising separation technology that has a great application potential in many separation and purification processes, such as in food, pharmaceutical,polymer processing and biochemical industries, etc. Since the solubility of solids can be easily tuned withsolvent density near the solvent critical point, it makes supercritical fluids (SCFs) attractive solvent candi-dates for separating heavy compounds, which becomes one of the main applications of SCFE technology.
In SCF technology, carbon dioxide is one of the most commonly used gases because it is an easy gas to handle, it is inert, nontoxic and nonflammable, and it has a convenient critical temperature. Onthe other hand, it has some limitations because of its lack of polarity and the capacity to form specificsolvent–solute interactions. Therefore, there is a great incentive to improve its polarity, and it has beenfound that the addition of a small amount of suitable cosolvent can greatly enhance its solvent power.
A number of investigators have published equilibrium solubility data for various solids in SCFs, how- ever, the measurements on the solubility of solids in SCFs with cosolvents are few although the concept ∗ Corresponding author. Tel.: +86-10-64419862; fax: +86-10-64436781.
E-mail address: [email protected] (C. Zhong).
0378-3812/03/$ – see front matter 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0378-3812(03)00022-0 Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 of adding cosolvent to a SCF has received attention many years ago Kurnik and Reid strated that the solubility of a solid in a SC solvent can be enhanced in some instances by the presenceof a cosolvent. Sulfur dioxide has been used as a cosolvent for benzoic acid Van Alsten et al. reported solid–fluid equilibrium data for a number of systems. Dobbs et al. and correlatedsolid–fluid equilibrium data for benzoic acid, 2-aminobenzoic acid, phthalic anhydride, and acridine inCO2 doped with acetone or methanol. Schmitt and Reid ve studied solubility of monofunctionalorganic solids in chemically diverse supercritical fluids and the solubility of naproxen in supercritical car-bon dioxide with and without cosolvents were studied by Ting et al. and Prausnitz the effects of cosolvents on the solubility of benzoic acid by using the Redlich–Kwong equation of state.
In this work, the flow technique coupled with gravimetric analysis was used to measure the solubility of solid solutes, anthracene and 2-naphthol, in CO2 with and without cosolvents. Both polar (ethanol andacetone) and nonpolar (cyclohexane) substances were adopted as cosolvents to investigate the effects ofcosolvent on the solubility enhancement of solid solutes in CO2. Acetone does not self-associate andis solely hydrogen bond acceptor, while ethanol, on the other hand, is able to be both hydrogen bonddonors and acceptors, which also tends to self-associate even in SCFs. Therefore, the present work caninvestigate the effects of both the concentration and the functionality of the cosolvents.
This work is an important step in our long-term objective to predict the solution properties of multi- component supercritical fluid mixtures based on the molecular interactions. It will provide a basis forfuture attempts to demonstrate that a multicomponent supercritical solvent mixture can be highly selectivefor particular solutes due to specific interactions. Rational utilization of these cosolvents could improvethe existing and newly proposed processes, particularly for those compounds with extremely limitedsolubility in pure fluids.
2. Experimental
2.1. Equipment and experimental method The flow diagram of the equipment used is shown in The syringe pump used was a Nova Model 5542121, with constant pressure operating capability for pure CO2. The equilibrium cell consistsof a 40 mm i.d., 300 mm long stainless steel tube. The pump used for cosolvents is high pressure gaugingpump (BECKMAN, 100A). The system temperature was monitored by a platinum resistance thermometeraccurate to ±0.1 K, and the system pressure was measured by a pressure meter (HEISE, Newtown CONN.)with an accuracy of ±0.5 bar. The equilibrium cell and the preheater coil were placed in a water bathwhich was controlled to ±0.01 K.
The equilibrium cell was packed with solid solute, 2-naphthol or anthracene, and each end was plugged with glass wool to prevent the fine solid powder from plugging the smaller 1/6 in. i.d. interconnectingstainless steel tubing.
CO2 and cosolvent from vessels were compressed into the mixer, then through the connecting tube heated by electricity coil, they were put into the equilibrium cell from the bottom. In the equilibrium cell,the solvent and solute reached equilibrium through mass transfer. The fluid phase reached equilibriumflowed from the top of the cell through a decompress valve into two U type tubes in turn. The solid solutewas settled and weighed up by an analysis scale with an accuracy of ±0.05 mg after drying. The volumewas measured by the wet gas meter with an accuracy of ±1%.
Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Fig. 1. Flow apparatus for solubility measurements in SCF with and without cosolvent.
Table 1Source and purity of the materials used We started the measurements at a flow rate of 100 l/h, then reduced the flow rate until the solubility measured did not change with further decreasing the flow rate. By this way, we determined the suitableflow rate is 40 l/h to ensure that the solid and fluid reach equilibrium in the equilibrium cell. To makesure that all the precipitated solute was collected, two U type tubes were used in turn. From experimentalobservation, nearly all the solute was collected in the first U type tube, and very little precipitated in thesecond U type tube. The experimental error for the solute solubility is estimated to be ±2%.
3. Materials
The sources and purities of the various compounds used are given in These materials were 4. Results and discussion
4.1. Solubility of solids in pure CO2 The solubilities of 2-naphthol and anthracene in pure CO2 were measured at 308.1, 318.1, and 328.1 K, which are listed in depicted in respectively.
Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Table 2Solubility of 2-naphthol and anthracene in pure CO2 In order to verify the reliability and efficiency of the solubility apparatus and the technique employed in this study, the solubility data of anthracene in CO2 at 318.1 K and 10.0–25.0 MPa were comparedin the data of Johnston et al. obtained at 323.1 K and 9.0–27.6 MPa. Furthermore, ourexperimental data of 2-naphthol in CO2 at 318.1 K and 10–30 MPa were compared with the data of Tanand Weng As can be seen from the figures, the apparatus and the technique employed inthis work give agreeable results with that of literature.
4.2. Solubility of solids in CO2 with cosolvent In order to investigate the effect of cosolvent on the solubility of solids in SCF, the solubility of 2-naphthol and anthracene in CO2 with cosolvent was further measured. The cosolvents adopted are Fig. 2. Solubility of 2-naphthol in CO2.
Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Fig. 3. Solubility of anthracene in CO2.
acetone, ethanol and cyclohexane. Measurements for three temperatures, 308.1, 318.1 and 328.1 K, werecarried out, and the experimental results are shown in where the corresponding densitieswere calculated by the Patal–Teja equation of state The addition of a cosolvent to a SCF generally can increase the bulk density of the fluid mixture which would contribute to solubility enhancement. A large variation in density would be expected close to thecritical point where the isothermal compressibility is largest. However, at pressures and temperaturesfurther away from this region, where the fluid is less compressible, the increase in bulk density is not Fig. 4. Comparison of the solubility data of this work and that of Johnston et al. anthracene in CO2.
Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Fig. 5. Comparison of the solubility data of this work and that of Tan and Weng 2-naphthol in CO2.
expected to be very significant and should be within a few percent (0–3% for P > 18 MPa) for thecosolvent concentration range between 1 and 5% The effects of density on the solubility of solids in CO2 with cosolvent are shown in From the figures it can be seen that the solubility of solutes increases with increasing density in general.
Table 3Solubility of 2-naphthol in CO2 with cosolvent of 3.6 mol% ρ (mol/l) y2 (×104) T (K) P (MPa) ρ (mol/l) y2 (×104) T (K) P (MPa) ρ (mol/l) y2 (×104) Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Table 4Solubility of anthracene in CO2 with cosolvent of 4.0 mol% ρ (mol/l) y2 (×105) T (K) P (MPa) ρ (mol/l) y2 (×105) T (K) P (MPa) ρ (mol/l) y2 (×105) Both polar and nonpolar solvents were adopted as cosolvents in this work, and both polar and nonpolar solids were selected as solutes, therefore, the experimental data measured in this work are useful to studythe effects of cosolvents.
Fig. 6. Solubility of 2-naphthol in CO2 with cosolvent of 3.6 mol% acetone.
Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Fig. 7. Solubility of anthracene in CO2 with cosolvent of 4.0 mol% acetone.
Fig. 8. Solubility of 2-naphthol in CO2 with cosolvent of 3.6 mol% cyclohexane.
Fig. 9. Solubility of anthracene in CO2 with cosolvent of 4.0 mol% cyclohexane.
Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Table 5The average effect factors of cosolvent The effect factor of cosolvent can be defined as where ypure and ycosolvent denote the solubility of solids in pure SCF and in SCF with cosolvent, respectively.
The average cosolvent effect factors are listed in Since solubility enhancement by the addition of a cosolvent is mainly caused by the formation of special interactions between the solute and cosolvent molecules, it is not surprising to find that the largestcosolvent effect on polar solute, 2-naphthol, is from ethanol, since ethanol has the strongest interactionswith the solute among the three solvents used. While for the nonpolar solute, anthracene, the largestcosolvent effect comes from the nonpolar solvent, cyclohexane. In this case, the interactions between thesolute and cosolvent molecules come mainly from the dispersion force.
5. Conclusion
A continuous flow apparatus was used to determine the solubility of anthracene and 2-naphthol in supercritical CO2 in the temperature range of 308.1–328.1 K and the pressure range of 10–30 MPa, withand without cosolvent. Both polar and nonpolar solvents were adopted as cosolvents, and the cosolventeffects were studied experimentally. The experimental data are useful to the theoretical modeling research,which also provide fundamental data for SCFE process design and development.
molar fraction of solid in supercritical phase (solid solubility) ycosolvent solubility of solids in SCF with cosolvent Q. Li et al. / Fluid Phase Equilibria 207 (2003) 183–192 Acknowledgements
The financial support of the Ministry of Education of China (Contract: 00017) and the Ministry of Science and Technology of China (Contract: G2000048) is greatly appreciated.
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