Calculation of natural gas isentropic exponent

A detailed numerical procedure for the calculation of the isentropic exponent of a natural gas is derived. The procedure follows the Redlich-Kwong solution using the AGA8/1992 extended virial-type characterization equation (AGA8-92DC equation) for the determination of the compression factor. The AIChE generic ideal heat capacity equations are used to derive the molar heat capacities of the natural gas mixture. The compression factor and its derivatives are calculated on the basis of ISO-12213-2. Two calculation examples are given and the results are graphically illustrated. The calculation procedure is implemented in a digital computer using an object-oriented programming approach. The effect of the isentropic exponent on the accuracy of orifice-based flow-rate measurements is demonstrated and discussed.