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3차 상태방정식과 다양한 혼합법칙을 이용한 2성분계 혼합물의 임계점 예측

Prediction of Critical Loci in Binary Mixtures by Cubic Equations of State with a Class of Mixing Rules

박종선 권영중^†

Jong Seon Park Yong Jung Kwon^†

강원대학교 공과대학 화학공학과

Department of Chemical Engineering, Kangwon National University, Korea

HWAHAK KONGHAK, April 2000, 38(2), 191-198(8), NONE

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Abstract

역행웅축현상이나 초임계유체 추출(SFE), 초임계유체 크로마토그래피(SFC)등 초임계유체를 이용하는 분야에서 필수적인 혼합물의 임계점 예측을 위하여 부피에 대한 3차 상태방정식을 이용하여 계산을 수행하였다. 본 연구에서는 상태방정식으로는 Peng-Robinson(PR) 상태방정식과 Soave-Redlich-Kwong(SRK) 상태방정식을 사용하였고, 각 상태방정식에 내재하는 파라미터 a와 b에 대해서는 classical 혼합법칙 외에 Huron과 Vidal[1,2]이후로 보다 정확한 열역학적 성질의 예측을 위해 제시된 액상 용액 모델식에 근거한 다양한 혼합법칙(EOS/G^E)을 적용하여 이성분계 혼합물의 전체 조성범위에 대한 임계점 예측을 수행하였다. 액상 용액 모델식에 근거한 대표적인 혼합법칙(HV, MHV1, WS, LCVM)에 사용되는 과인 Gibbs 자유에너지 및 과인 Helmholtz 자유에너지의 계산을 위해 액상 용액 모델식으로 분류되는 NRTL, UNIQUAC 그리고 UNIFAC 식을 각각 적용하였다. 초임계유체로 가장 많이 사용되는 이산화탄소를 포함하는 여러 이성분계 화합물의 임계점 궤적에 대한 계산결과는 어느 특정한 상태방정식과 혼합법칙이 적용된 모든 계에서 탁월한 결과를 보이지는 못했으나, 특정한 계에 대해 큰 오차를 유발시키는 다른 혼합법칙에 비해 대체적으로 WS 혼합법칙이 가장 좋은 예측 결과를 보여 주었다.

Critical loci of mixtures, which play an important role in retrograde condensation, supercritical fluid extraction(SFE), and supercritical fluid chromatography(SFC), can be determined by solving the critical loci equation along with an equation of state chosen for the mixture. In most of cubic equations of state, however, an appropriate mixing rule should be utilized for the parameters a and b inherent in them and it usually causes some error in predicting thermodynamics properties of fluid mixtures. Recently, several approximate and reasonably successful mixing rules have been proposed for the description of vapor-liquid equilibria since Huron and Vidal[1,2] developed the basic idea of so called EOS/G^E mixing rule. In this work, four different mixing rules(HV, MHV1, WS, and LCVM) based on the liquid solution models(NRTL, UNIQUAC, and UNIFAC) as well as the classical mixing rule were used to compare the results on critical loci for several binary mixtures when the SRK and PR equations of state were selected for an equation of state. The overall results on critical loci by different mixing rules used were compared and it was also shown that the WS mixing rule produced slightly better results than others even though any equation of state with particular mixing rule did not give the best results of all.

Keywords

Cubic Equation of State G^E Mixing Rules Critical Loci Liquid Solution Models Excess Gibbs Free Energy Excess Helmholtz Free Energy

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