(60f) Densities and Phase Loops for Four Natural Gas-like Mixtures

We have measured the densities and phase lops for four mixtures that contain methane through n-pentane, carbon dioxide and nitrogen using a magnetic suspension densimeter and an isochoric apparatus. The temperatures range from 250-450 K (-10 to 350 °F), and the pressures range up to 150 MPa (22,500 psia). We have compared these data to predictions from the equations of state normally used in the natural gas industry.

The results for the phase loops are disturbing to say the least. The equations of state miss the maximum pressure by as much as 2.5-3.0 MPa (~400 psia). This would have a significant impact upon compressor specifications. The densities deviate from AGA 8 by 1.0 % in Region 1, 0.75 % in Region 2, and 0.5 % in Region 3. Most of the data lie outside the stated accuracy of AGA 8 in regions 1 and 2, while all the data fall within the stated accuracy in Region 3.

Based upon these results, it appears that equation of state development is necessary. To accomplish this work with a reasonable hope of obtaining a truly predictive equation would require an extensive data collection project.