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CORRESPONDENCE Comments on “High-Pressure Phase Equilibrium Data for Aromatic Components of Wine: Carbon Dioxide/n-Butanal System” Baoquan Zhang,* Xiufeng Liu, and Hongyu Guo The State Key Laboratory of C1 Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, P. R. China
Sir: The paper published in Ind. Eng. Chem. Res. by Vazquez da Silva and Barbosa1 is devoted to the experimental acquisition of high-pressure phase equilibrium data of aromatic compounds of wine in carbon dioxide. Those data are essential for the design of dealcoholization processes by supercritical fluid extraction (SFE), one of the most promising techniques for dealcoholization. The research on the experimental acquisition of high-pressure phase equilibrium data is of great significance from both theoretical and practical points of view. The experimental data of equilibrium acquired would be widely used in the design of SFE processes such as purification of foods and drinks, dealcoholization, modernization of Chinese traditional medicine, biosynthesis, etc.2 The relevant research work will get more and more attention in the coming years. Although the measuring method and the result procured in the paper are both reliable, there exist some problems to be clarified. Those problems will be stated in the following two sections. Experimental Apparatus Several parts in the schematic diagram shown in Figure 1 (it is also marked as Figure 1 in the original papers1,3) need to be clarified. First, the four circles on the very top of the diagram were not marked. We think they should be PI2, PI3, T1, and PI1 respectively from left to right based on the description of the experimental apparatus in the paper. Second, the two six-way sampling valves were not correctly arranged. Apparently, the marked port in the center of the valve does not exist at all. The solid circle in the center is to picture the switch for changing the connection of ports. According to the structure of six-way valves, the ports connection of the two six-way sampling valves is shown in Figure 2. The solid line is used to illustrate the connection when sampling, whereas the dashed line is used to show the ports connection when the system is working for equilibration. * To whom correspondence should be addressed. Fax: +8622-27403144. E-mail:
[email protected].
There is another arrangement of the two sampling units where two high-pressure six-way valves have to be employed in order to make the samples directly introduced into a gas chromatograph and a highpressure liquid chromatograph. This arrangement would simplify the sampling procedure and the following analysis of samples, except being handled much easier. Application of the Crossover Theory in Vapor-Liquid Equilibrium Prediction The experimental data were fitted by both SoaveRedlich-Kwong (SRK) and Peng-Robinson (PR) equations of state in the paper. Generally speaking, SRK and PR can yield reasonable representations of thermodynamic properties for pure fluids and fluid mixtures. However, those equations would fail to yield an accurate description of the thermodynamic behavior for the system with long-range correlations, such as near a critical point.4,5 That is to say, when the vapor-liquid equilibria in a wide range of temperatures and pressures including the near-critical region need to be predicted, those cubic equations of state would fail in the near-critical region. Part of the experimental data in this paper lies in the near-critical region indeed. The poor fitness of vapor-liquid equilibria in the nearcritical region (as shown in Figure 2 in the paper) presents a visual verification of the above argument. For supercritical fluids, their properties can be adjusted by changing pressure or temperature. This adjustment is especially remarkable in the near-critical region, the utilization of which is strongly recommended. So, the precise prediction of thermodynamic properties for fluids in the near-critical region is the prerequisite to utilizing supercritical fluids in industrial processes. However, the simple cubic equations of state are not good enough to fit or predict the vapor-liquid equilibrium as done in the paper. Sengers and coworkers and Keselev et al. developed crossover equations of state incorporating the scaling law near the critical point into the regular expansion away from the critical point. Recent development of research in phase equilibrium has demonstrated that the crossover theory
10.1021/ie010600m CCC: $22.00 © 2002 American Chemical Society Published on Web 06/04/2002
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Figure 1. Schematic representation of the apparatus.
in this respect could be made by applying novel technologies of experimentation and relevant theoretical outcomes. Literature Cited
Figure 2. Ports connection of six-way valves when sampling.
can correlate the whole range of experimental data including the near-critical region.4,5 It would have given a much better prediction if the experimental data had been interpreted by using the crossover theory. The research on high-pressure phase equilibria, theoretical and experimental, is in great need in consideration of potential industrial applications. Great progress
(1) Vazquez da Silva, M.; Barbosa, D. High-Pressure Phase Equilibrium Data for Aromatic Components of Wine: Carbon Dioxide/n-Butanal System. Ind. Eng. Chem. Res. 2000, 39, 4427. (2) Zhang, B. Q.; Liu, H. J.; Liu, X. F. Application of supercritical fluids in chemical and biochemical processing: past and prospects. Prog. Nat. Sci. 2001, submitted for publication. (3) Pereira, P. J.; Gomes de Azevedo, E. J. S.; Nunes da Ponte, M. Phase Equilibria for {2,3-Epoxypropanol (Glycidol) + Carbon Dioxide} from T ) 292 K to T ) 343 K at pressures up to 27 MPa. J. Chem. Thermodyn. 1997, 29, 197. (4) Jin, G. X.; Tang, S.; Sengers, J. V. Global thermodynamic behavior of fluid mixtures in the critical region. Phys. Rev. E 1993, 47, 388. (5) Kiselev, S. B.; Ely, J. F. Crossover SAFT Equation of State: Application for Normal Alkanes. Ind. Eng. Chem. Res. 1999, 38, 4993.
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