Ind. Eng. Chem. Res. 1998, 37, 3515-3518
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RESEARCH NOTES Phase Equilibria for Binary Aqueous Systems from a Near-Critical Water Reaction Apparatus Karen Chandler, Brandon Eason, Charles L. Liotta, and Charles A. Eckert* Schools of Chemical Engineering and Chemistry and Specialty Separations Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
For any valid study of reaction kinetics the phase equilibria involved must be known. We present an apparatus designed to measure both phase equilibria and kinetics for reactions in nearcritical water. Liquid-liquid mutual solubilities are reported for the benzene-water and toluene-water systems from 200 to 275 °C at the three-phase pressures and 172 bar. These data were found to be in good agreement with data reported in the literature. Additionally, the near-critical water reaction apparatus was used to verify one-phase regions for the phenolwater and p-cresol-water systems from 250 to 300 °C at 172 bar. Introduction Since water is clearly the ultimate environmentally benign solvent, many investigations are underway using both near-critical or supercritical water as a reaction medium. Inevitably, most of these high-temperature, high-pressure reactions are run in windowless vessels where no observation of the phase behavior is possible. Although some investigators ascertain the phase behavior by a separate experiment or by models, all too often the naı¨ve assumption of homogeneity is made for the contents of the pressure vessel. In such a case, any multiphase behavior invalidates the kinetic results. It is crucial for studies in near-critical or supercritical water, or in any comparable work, to know phase equilibria to interpret kinetic data. Phase equilibrium data for binary mixtures of water and organic compounds at elevated temperatures and pressures are relatively scarce, but these data are required for the design of a variety of separation and reaction processes. For example, extraction processes based on equilibration of water-rich and organic-rich liquid phases are used extensively during petroleum reservoir production, petroleum refining, and coal gasification to remove organic pollutants from wastewater streams (Anderson and Prausnitz, 1986). Further, the growing interest in reducing the use of environmentally harmful organic solvents has resulted in the development of analytical extraction methods and reaction processes that use environmentally acceptable water as the process solvent. Supercritical and subcritical water extraction has been used to remove organic pollutants from environmental solids, such as soils, sediments, and air particulates (Hawthorne et al., 1994; Yang et al., 1995). Also, many organic reactions have been carried out using water as the reaction solvent including toxic waste destruction in supercritical water (Modell et al., 1982; Thomason and Modell, 1984; Helling and Tester, * Author to whom correspondence is addressed. Phone: (404)894-7070. Fax: (404)894-9085. E-mail:
[email protected].
1988; Yang and Eckert, 1988; Shaw et al., 1991; Thornton and Savage, 1992; Jain, 1993) and chemical processing in supercritical and near-critical water (Kuhlmann et al., 1994; Bagnell et al., 1996; Katritzky et al., 1996; Parsons, 1996; Chandler et al., 1997a,b; Holliday et al., 1997; Xu et al., 1997). The specific purpose of this investigation was to obtain phase equilibria and kinetic data from the same experimental apparatus. Liquid-liquid mutual solubilities for the benzene-water and toluene-water systems were measured and compared to literature data to verify the phase equilibrium measurements from the near-critical water reaction apparatus. The quantitative kinetics for alkylation reactions of phenol and p-cresol were reported in near-critical water (275-300 °C) using the experimental reaction apparatus described here (Chandler et al., 1997b), and this investigation verifies that the alkylation reactions were indeed carried out in a one-phase region. Experimental Methods Materials. HPLC-grade water was obtained from Aldrich Chemical Co., and the nitrogen (high-purity grade) used to deoxygenate the water was obtained from Air Products. Benzene (99.9+%, HPLC grade), toluene (99.8%, HPLC grade), phenol (99+%, ACS reagent), and p-cresol (99%) were obtained from Aldrich Chemical Co. and were used without further purification. Methanol (99.9+%, HPLC grade) and the calibration standards, pentane (99+%, HPLC grade) and benzoic acid (99+%, ACS reagent), were also obtained from Aldrich Chemical Co. Equilibrium Cell. Figure 1 shows a schematic of the near-critical water reaction apparatus used to provide liquid-liquid mutual solubilities and to verify one-phase regions. The near-critical water reactor, which was used as the equilibrium cell, was a commercially available sample cylinder (Whitey Co.) constructed of 316 stainless steel with a pressure rating of 340 bar at elevated temperatures. The cell had an
S0888-5885(97)00882-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/26/1998
3516 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998
Figure 1. Schematic of the near-critical water reaction apparatus.
internal volume of 300 mL and was heated in an aluminum heating block. The temperature of the heating block was controlled to within (1 °C of the equilibration temperature (200-300 °C) using three heavy insulated heating tapes (Omega Engineering, Inc.), which were suitable for direct contact with conductive surfaces, and a temperature controller (Omega Engineering, Inc., model CN9000A). Unless otherwise noted, all fluid transfer lines were 316 stainless steel with an outside diameter of 1.59 mm and an internal diameter of 0.762 mm. Initially, the equilibrium cell was loaded with equal amounts of water and organic on a volumetric basis, and the system was deoxygenated by bubbling pure nitrogen gas through the cell for at least 1 h. This was done to reduce the possibility of cell corrosion. The cell was then heated to the desired conditions, and thermal equilibrium was accomplished by allowing the system to remain at elevated temperature overnight. One end of the cell was connected to a digital pressure transducer (Heise, model 901B), which was used to monitor constantly the system pressure to within (0.3 bar. A pressure relief valve was connected to this line. Also, this end of the cell was connected to a high-pressure syringe pump (Isco, Inc., model 260D) that was filled with deoxygenated water, which allowed the addition of water to maintain a constant pressure as samples were removed from the cell. The other end of the cell was used to sample both phases. Small internal diameter tubing (0.508 mm), which extended approximately 2 cm into the cell, was used as the sample-withdrawal port. Sampling Techniques. Only one end of the equilibrium cell was equipped to remove samples from the system. Therefore, the aluminum heating block was attached to a free rotating shaft, which allowed the one sample-withdrawal port to sample both the liquid phase of lower density, which existed at the top of the cell, and the liquid phase of higher density, which existed at the bottom of the cell. This was accomplished by rotating the cell 180° and allowing several hours for the system to reach equilibrium. Attainment of equilibrium was assured by sampling over a time interval. Mutual solubility measurements are subject to large errors, particularly at elevated temperatures and pressures, because transferring a sample from an equilibrium cell to an appropriate analytical instrument with-
out composition change is very difficult (Anderson and Prausnitz, 1986; Leet et al., 1987). The major problems associated with sampling include entrainment of one phase in the other, adsorption of solutes on the walls of the sample line, and phase changes in the sample line due to changes in temperature and pressure (Anderson and Prausnitz, 1986). The sampling techniques employed in this investigation were designed to eliminate these problems. First, entrainment of one phase in the other phase during sampling was considered not to be a problem, because the samples were taken from the end of the equilibrium cell oriented in a vertical position. The sample tubing extended only 2 cm into the cell, which was 37 cm long, and therefore the distance from the sample port to the second phase was large. Second, the sample lines were rinsed with excess methanol to ensure that all of the water and organic were collected and that adsorption on the sample line walls was not an issue. Third, the sample volume was very small compared to the volume of the equilibrium cell, so that the equilibrium was undisturbed by sampling. Each sample was collected in a 250-µL sample loop (Valco Instruments Co., Inc.). The pressure drop due to sampling was relatively small (
