Isobaric specific heat capacity of natural gas as a function of specific gravity, pressure and temperature

Abstract

Natural gas engineering entails production, processing, storage and transportation of natural gas. A good handling of the gas requires a sophisticated understanding of how its density, compressibility, pseudo-pressure and specific heat capacity vary with the gas condition. A variety of methods have been presented in petroleum and gas journals and a host of scholastic materials to evaluate other properties over a wide range of temperatures, however, the available correlation for isobaric specific heat capacity is for only 150 °F. We generated 200 samples of natural gas mixture with methane component ranging from 0.74 to 0.9985 using normally distributed experimental design. The variations of the respective specific heat capacity of the components and the effect of composition on the specific gravity and overall specific heat capacity of the gas were taken into consideration. The developed correlation reads in the specific gravity and temperature to generate the ideal gas specific heat capacity of the sample. The result yielded 99.75% accuracy at 150 °F when compared to experimental data, as against the result from isentropic coefficient method which overestimated the ideal gas specific heat capacity by 25% at the same temperature. The ideal gas specific heat capacity developed compared to 6000 data points generated from mixing rule at different temperatures resulted in correlation regression coefficient of 0.9999. To account for the deviation from ideal gas behaviour, this work presents 99.7% R squared value for dimensionless residual specific heat capacity as a function of reduced temperature and pressure compared to that calculated from Starling Carnahan equation of state. This model is the first explicit correlation for the residual specific heat capacity of natural gas to be derived.