Inconsistencies in dew points from different algorithm types possible causes and solutions

Calculations of natural gas dew points are quite important in the gas processing industry. A number of software packages used by the industry have multiple options for calculations of dew points. One option is to fix the pressure and the software will calculate the dew point temperature. Another option is to have the software generate the phase envelope for the natural gas mixture. A third option is for the user to perform flash calculations at a fixed pressure and varying temperatures until the amount of condensed liquid is arbitrarily small. Sometimes, these three different options yield three different calculated dew points. Because the three different dew point options involve nested iterative loops which solve for liquid and vapor densities and component fugacities as well as the temperature or vapor fraction, the three options should agree within some measure of the different convergence criteria used by the three options, provided the methodology used in a given option does not fail. Examples of dew point calculation differences by the three options are presented and some analysis of which option best represents the equation of state used is given. In addition, possible methods for reducing the inconsistencies are discussed.