Use of the GERG-2008 Equation of State for Hydrocarbon Dew Point Calculation

摘要

This paper describes how to use the GERG-2008 equation of state (ISO 20765 parts 2 & 3) to calculate the hydrocarbon dew point of natural gas mixtures. The hydrocarbon dew point of a natural gas is the temperature at which a hydrocarbon liquid mixture first forms from the gas phase mixture (at the specified pressure) on cooling. It is an important property of the gas to know, and in the UK there is a network entry specification that it should be no more than-2 °C (at any pressure). There are two basic methods to obtain this property. One is to use a dewscope instrument to measure the temperature at which liquid forms. The other is to analyse the composition of the gas with a gas chromatograph and then calculate the dew point using an equation of state. ISO 23874 (Natural gas-gas chromatographic requirements for hydrocarbon dew point calculation, 2006) gives details of how this can be done using any equation of state that can use user-defined 'fraction' input. This is the method routinely used for UK network entry.