Fluid predictive Soave-Redlich-Kwong

The UNIFAC model has also been combined with the predictive Soave-Redlich-Kwong (PSRK) equation of state. The procedure is most completely described (with background literature citations) by Horstmann et al. [Fluid Phase Equilibria 227 157-164 (2005)]. 

All of the cubic equations of state presented so far are generally termed two-parameter equations of state. In this respect, each one of them predicts a constant compressibility factor at the critical point Zc=PcVd T, irrespective of the nature of the compound for van der Waals Zc = 0.375, for Redlich-Kwong and Soave-Redlich-Kwong Zc = 0.333 and for Peng-Robinson Zc = 0.307. The actual values may vary significantly, especially for polar and associating fluids for methane Zc = 0.286, for propane Zc = 0.276, for pentane and benzene Zc = 0.268 and for water Zc = 0.229. To correct for this deficiency, a number of authors have proposed three or four adjustable parameters to the cubic equation of state. The most popular three-parameter cubic equation of state was proposed by Patel and Teja is given by ... 

Predictions of saturated liquid densities of pure fluids from the Soave-Redlich-Kwong equation of state and, to a lesser extent, the Peng-Robsinson equation of state deviate from experimental data. This should be expected given the... 

The modern cubic equations of state provide reliable predictions for pure-component thermodynamic properties at conditions where the substance is a gas, liquid or supercritical. Walas and Valderrama provided a thorough evaluation and recommendations on the use of cubic equation of state for primary and derivative properties. Vapour pressures for non-polar and slightly polar fluids can be calculated precisely from any of the modem cubic equations of state presented above (Soave-Redlich-Kwong, Peng-Robinson or Patel-Teja). The use of a complex funetion for a (such as those proposed by Twu and co-workers ) results in a significant improvement in uncertainty of the predicted values. For associating fluids (such as water and alcohols), a higher-order equation of state with explicit account for association, such as either the Elliott-Suresh-Donohue or CPA equations of state, are preferred. For saturated liquid volumes, a three-parameter cubic equation of state (such as Patel-Teja) should be used, whereas for saturated vapour volumes any modern cubic equation of state can be used. 

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