PREDICTION AND CORRELATIONS OF RESIDUAL ENTROPY OF SUPERHEATED VAPOR FOR PURE COMPOUNDS

The calculation of the entropy (S) for gases is essential for design process equipments which involve the calculation of the heat and work requirements for industrial processes.Prediction of accurate values of residual entropy (SR) is necessary step for the calculation of the entropy. In order to obtain accurate values of (SR), attention has been oriented to calculate it using an Equation Of State (EOS).In this study important equations of state as Lee-Kesler equation and Virial equation truncated to second and to third terms were used to calculate the residual entropy for pure superheated vapor. In addition to those  equations, cubic equations of state represented by Soave-Redlich-Kwong (SRK), and Peng-Robison equations were also used. These EOS were tested for the available 2791 experimental data points of 20 pure superheated vapor compounds (14 pure nonpolar compounds + 6 pure polar compounds). The Average Absolute Deviation (AAD) for SR of 2791 experimental data points of the all 20 pure compounds (nonpolar and polar) when using equations of Lee-Kesler,Peng-Robinson, Virial truncated to second and to third terms, and Soave-Redlich-Kwong were 4.0591, 4.5849,4.9686, 5.0350, and 4.3084 J/mol.K respectively. It was found from these results that the Lee-Kesler equation was the best (more accurate) one compared with the others,but this equation is sometimes not very preferable, because it needs more time and not easy to apply as cubic equations of state. Noted that SRK equation was the closest one in its accuracy to that of the Lee-Kesler equation in calculating the residual entropy SR of superheated vapor, but it was developed primarily for calculating vapor-liquid equilibrium and to overcome this problem, efforts were directed toward possibility of modifying SRK equation to increase its accuracy in predicting the residual entropy as much as possible.The modification was made by redefining the parameter α in SRK equation to be a function of reduced pressure, acentric factor, and polarity factor for polar compounds in addition to be originally function of reduced temperature and n parameter –which is also function of acentric factor– by using statistical methods. This correlation is as follows: