Psychrometric charts for water vapour in natural gas

Psychrometric charts present in a graphical form physical property data for gas systems involving one or more non-condensing gas components and a single condensing vapour component. Originally limited to studying the humid air system of water vapour in dry air, the theory behind preparing psychrometric charts for water vapour in natural gas is presented. To illustrate the technique two charts are prepared for a system in which the natural gas is 80.0 mol% methane, 15.0 mol% ethane and 5.0 mol% propane. One chart is prepared for the temperature range of 0 to 60 °C at a system pressure of 101.3 kPa and the other chart for the range of 40 to 110 °C at a system pressure of 400.0 kPa. The charts are based upon semi-theoretical equations and make use of published physical property data and correlations. The behaviour of the gas mixture is characterized by the virial equation of state truncated at the third term. The charts are constructed with the dry bulb temperature and absolute humidity scales as the orthogonal axes. Curves of constant adiabatic saturation temperature, constant relative humidity, constant gas specific volume and constant enthalpy deviation are plotted on the charts. Using the theory presented psychrometric charts for any temperature range, system pressure and natural gas composition may be prepared.