다환 방향족 탄화수소 혼합물 포함 계의 고압 상거동

김철중, 원동복*, 한규진*, 박소진*

R&D center, SK corporation

*충남대학교 공과대학 화학공학과

High-Pressure Phase Behavior for System Containing Polycyclic Aromatic Hydrocarbon Mixture

Cheol-Joong Kim, Dong-Bok Won*, Kyu-Jin Han1* and So-Jin Park*

R&D center, SK corporation

*Dept. of Chem. Eng., Chungnam National University

INTRODUCTION

Polycyclic aromatic hydrocarbon mixtures (PAH), such as FCC decant oil (FCC-DO), is the typical raw material of carbon precursor which can be applied as a precursor for high performance and high functional carbon materials[1]. A few studies have been focused on the effects of impurities and molecular distribution pattern of PAH on the properties and performance of high performance carbon materials, and it is concluded that PAH should be highly purified to give further advanced performance to final carbon materials[2].

Usually, solvent extraction and vacuum distillation techniques are used to separate useful or useless fraction from the substance, however low repeatability and selectivity of these techniques nevertheless induce the application of new separation technique which is superior than previous techniques to improve the performance of carbon materials.[3]

Supercritical fluid extraction (SFE) has found application in solvent extraction, in separation processes, and in the upgrading of petroleum feed stocks. Also, it is already well known that supercritical propane can fractionate heavy oil into wax, resin, and asphaltene[4]. The process can be used for removing solid particulate impurities from PAH by dissolving all PAH into the extract phase and removing solid impurities as the raffinate. The success of these techniques shows the applicability of SFE for advanced refining of PAH, however it has not been sufficiently investigated.

In this study, the authors evaluated the applicability of SFE for the purification of PAH based on the investigation for high-pressure phase equilibrium behavior for the toluene/PAH system. Also, the prediction model for phase equilibrium behavior and for extraction selectivity was discussed.

PHASE EQUILIBRIUM MODELING

A successful representation of SFE of heavy oils requires specification of an appropriate equation of state, the development of a pseudo-component lumping scheme to represent the heavy oil, and the calculation of pseudo-critical properties to represent the extraction process.

The critical properties, TC, VC and PC, were calculated by Twu correlations which were composed as a function of mean average boiling point and specific gravity[5]. The specific gravity of each pseudo-components were estimated on the basis of the Kay's mixing rule and Newton numerical method. The accentric factor, v, was calculated with the Lee-Kesler correlation[6]. The Tsang-ASPEN correlation was used to calculate the molecular weight of the pseudo-components. The Peng-Robinson equation of state (PR EOS) with the van der Waals mixing rules were used for the phase behavior calculations.

EXPERIMENTAL SECTION

The PAH used in the study is produced from a FCC (fluid catalytic cracking) process (FCC-DO). Chemical grade toluene (purity: >99.5%) has been used without further purification from purchasing purity.

A simplified scheme of the continuous-flow type experimental system is presented in Fig. 1. The system has been designed by modification of those used by Thies et al.[3]. The feed mixture enters the cell in the middle of the inner chamber and separates into two phases from which are drawn off independently through top and bottom sample ports. The interface level in the cell is determined visually and is adjusted by regulating the flow rate of the bottom phase with the micrometering valve. Temperature and pressure of the extraction cell has been controlled within ± 0.5 K and ± 0.05MPa, respectively.

RESULTS AND DISCUSSION

Vapor-liquid equilibrium composition for ethanol-water mixtures at 423.2 K was measured for comparison with published data to verify operation of the apparatus. As illustrated in Fig. 2, the measured values are in good agreement with those previously reported by Nisen et al. within the error range of ±0.29%[7].

Binary interaction parameters of the Peng-Robinson equation of state are successfully correlated in terms of the ratio of critical molar volumes of toluene and each of pseudo-components. The constants of correlated functions what were used for assigning the interaction parameters between toluene and each of pseudo-components were determined by a trial and error procedure. The constants were adjusted to obtain the best possible agreement between the experimental and calculated vapor-liquid equilibrium compositions.

To give a fairly clear picture of the rather complex behavior of toluene/FCC-DO mixtures over a range of temperatures and pressures, a discussion of certain generalized diagrams seems desirable. Fig. 3 shows a pseudo-binary pressure vs. composition diagram for these isotherms at the several measured pressures[8]. Also, predicted diagrams with PR EOS have been shown together with experiment data for comparison. It is clear from the data that the phase behavior of toluene/FCC-DO system can be predicted very well with the use of PR EOS with considering the molecular interaction parameters between toluene and pseudo-components of FCC-DO.

The phase diagrams shown in Fig. 3 belong to type I category of phase boundary diagram models for binary system. The phase behavior for a system is controlled by the degree of molecular size asymmetry. Some system, which has a low degree of molecular size asymmetry, shows type I phase behavior, and another system with a high degree of molecular size asymmetry, type V behavior. So, it is believed that the molecular size asymmetry of the toluene/FCC-DO system is not too high.

CONCLUSIONS

Phase behavior of the system was well predicted with Peng-Robinson equation of state. The phase diagrams of the system belong to type I category of phase boundary diagram models for binary system because of the low degree of molecular size asymmetry between toluene and FCC-DO.

REFERENCES

1. I. Mochida, Y. Korai, C. Ku, F. Watanabe and Y. Sakai, Carbon, 38 (2000) 305.

2. A. Figuerias, M. Granda, E. Casal, J. Bermejo, J. Bonhomme and R. Menendez, Carbon, 36 (1998) 883.

3. K.W. Hutchenson, J.R. Roebers and M.C. Thies, The Journal of Supercritical Fluids, 4 (1991) 7.

4. M.R. Gray, Upgrading Petroleum Residues and Heavy Oils, Marcel Decker, New York, 1994.

5. C.H. Twu, Fluid Phase Equilibria, 16 (1984) 137.

6. M.G. Kesler and B.I. Lee, Hydrocarbon Processing, 55 (1976) 153.

7. V. Niesen, A. Palavra, A.J. Kidnay and V.F. Yesavage, Fluid Phase Equilibria, 31 (1986) 283.

8. J.S. Rowlinson and F.L. Swinton, Liquids and Liquid Mixtures, Butterworths Scientific, 1982.

Fig. 1. Schematic diagram of the high pressure phase equilibrium measurement apparatus

Fig. 2. Comparison of experimental and reference data for ethanol(1)-water(2) system at 423.2K.