Mixing Rules for Accurate Prediction of Vapor-liquid Equilibria of Gas/Large Alkane Systems Using SRK Equation of State Combined with UNIFAC

Abstract

Two new mixing rules have been proposed and tested in this work. Both are more accurate than the modified Huron-Vidal mixing rule (MHV1) for gas/large alkane systems. The new mixing rule 1 (MR1), in which a correction factor to G^E from UNIFAC is introduced in MHV1 and correlated with carbon number, gives very good predictive accuracy for gas/large alkane systems, where MHV1 performs poorly. The present work shows that although MHV1 itself can not predict the VLE of a highly asymmetric system with good accuracy, a simple modification can greatly improve its accuracy, and make it useful for highly asymmetric systems. In addition, MR1 is compared with a recently proposed model named LCVM, which is a linear combination of Vidal (1978) and Michelsen (1990b) mixing rules (Boukouvalas et al., 1994). MR1 gives better results than LCVM, and the advantage of MR1 over LCVM becomes evident with increasing carbon number, which shows that MR1 is more reliable for extrapolating to larger alkanes than LCVM. On the other hand, a new linear mixing rule for a_m/b_m is introduced into MHV1 to replace the linear mixing rule for b_m, which constitutes the new mixing rule 2 (MR2). The results show that MR2 is superior to MHV1, especially for systems with high asymmetry, which leads to the conclusion that a linear mixing rule for a_m/b_m is more suitable for highly asymmetric systems than for b_m.