An Investigation of Various Methods and Correlations of VLE of Hydrocarbon Mixtures (That Can Be Used As Substitute for Freon-12 in Refrigeration)

Various paths and models have been tested for more accurate VLE predictions for 36 binary systems with 376 data points and 14 ternary mixtures with 368 data points,especially for mixtures of industrial interest in refrigeration units. This lead to select a model for calculation VLE data of hydrocarbon systems with no adjustable parameter and other with one adjustable parameter depending on the accuracy requirement of industrial design .  The first path deals with the calculation of VLE of hydrocarbon systems by applying PR-EOS and PRSV-EOS with different mixing rules in vapor and liquid phases.These mixing rules have different forms with a different number of adjustable parameter.The results of this path shows AS mixing rules gives approximately the best VLE calculated data for both binary and ternary systems when it is compared with other used mixing rules types in these equations of state (PR and PRSV). The overall AAD% by applying AS-mixing rules with PRSV-EOS for 36 binary systems is 1.70525. While for 14 ternary system the overall AAD% is 2.86145. The second path deals with the calculation of VLE data of hydrocarbon systems by applying PRSV-EOS with AS-mixing rules for the vapor phase (since AS-mixing rules with PRSV-EOS gives the better VLE data calculation for both phases) and activity coefficient model for the liquid phase. Where UNIFAC and Wilson models are used to represent the liquid phase activity coefficient. The calculated VLE data of hydrocarbon systems have some limitation during its application, near the critical region with supercritical condition and with systems under high pressure, and then this path will be no more applicable. So, that the number of systems which adapted by this path are reduced to 23 binary systems and only to 5 ternary systems. In this path the overall AAD% for binary systems by applying UNIFAC and Wilson activity coefficient models are 9.31151 and 3.07893 respectively. While for the ternary systems the overall AAD% when UNIFAC 3.07893 respectively. While for the ternary systems the overall AAD% when UNIFAC model applied is 16.36047 and when Wilson model is used the overall AAD% are 3.6366.Also, the UNIFAC activity coefficient model has an additional limitation through its application, when methane component is present in the system mixture where VLE calculated data will be poor. While the overall ADD% for systems which do not contain methane in binary and ternary systems are 2.91742 and 2.78714 respectively Third path: In this path PRSV-EOS with new mixing rules are applied for both system phases (vapor and liquid phase). This mixing rule is derived from Gibbs free energy equation at infinite dilution which in turn is derived from the activity coefficient at infinite dilution condition. The third path links between PRSV-EOS and Gibbs free energy at infinite dilution which adopts UNIFAC and Wilson models. This linking in EOS mixing rules with Gibbs free energy at infinite dilution makes it possible to take advantageous of an EOS route over the activity coefficient route which has the possibility of treating systems containing supercritical compounds. The overall AAD% for 36 binary systems by applying UNIFAC and Wilson models are 2.60706 and 2.01276 respectively. While, for 14 ternary systems by using UNIFAC and Wilson models AAD% are 6.08109 and 5.20716 respectively. These results are obtained without using any adjustable parameter (kij=0). The results of the first path with AS-mixing rules gives approximately the same accuracy when it is compared with the results of the third path without using any adjustable parameter for binary systems. While, for ternary systems AS-mixing rules in PRSV-EOS gives a higher accuracy in VLE data calculation. So, the third path is modified in order to give a higher accuracy for ternary systems. Different modifications in mixing rules are tried. The best results are obtained when the mixing rules is modifying to a quadratic form with one adjustable parameter (kij). This modification allows accurately calculate phase equilibrium of hydrocarbon components even in the presence of polar components which is existed in the system as traces. The over all AAD% for binary systems by applying UNIFAC and Wilson models are 2.22391 and 1.97773 respectively. While, for the ternary systems they are 3.63341 for UNIFAC model and 3.55501 for Wilson model. This path is found to be superior when it compared with the other two paths which are adopted in this work for VLE calculation of hydrocarbon systems especially for ternary systems where high accuracy without or with minimum number of adjustable parameters are rare.