The objective of this work is the assessment of the feasibility studies of phase equilibria mutual solubility process utilizing subcritical propane. A thermodynamic model based on regular solution theory studies to evaluate activity coefficients expression to each the heavy compound such as (propane and hexane) and the solvent such as propane in order to predict mutual solubility data. The use of equations derived from thermodynamic of the regular solution theory for collecting and predicting mutual solubility discussed with reference to binary pairs (propane / hexane and propane / decane systems). It is concluded that the calculation of some of the parameters required for these calculation would be difficult if the solute or heavy component hexane or decane in LPG or LNG were sensitive to temperature or complex substance about which little was known apart for its structural formula. An alternative procedure is to apply activity coefficients expression of the regular solution theory from which is called Universal Functional Activity Coefficient theory (UNIFAC) to each phase. Calculation along these lines described and the physical basis for applying this method under the relevant condition discussed. The UNIFAC theory approach in particular has been found to be in good estimation for the present studies of these systems LNG and LPG composition.
