Selecting the Correct Phase Envelope

In a previous “Tip of the Month” we discussed several methods of heavy ends characterization and as an example, for a rich natural gas, we tuned the heavy end parameters to match the experimentally measured saturation pressure. After tuning the heavy end parameters, we obtained a phase envelope for each method that passed through the experimental dew point; however, their shapes and specifically, their cricondentherm points were different. At the end, we were faced with the question of “which one is the right phase envelope?”

In this tip, we will explain a procedure for selecting an appropriate C₆₊ characterization method which results in a broader match with the experimental information. For further detail, please refer to Gas Conditioning and Processing, Volume 3, Advanced Techniques and Applications.

Let’s consider a lean natural gas with the composition shown in the first two columns of Table 1. This lean gas contains only 0.067 mol% C₆₊ and even though the amount of C₆₊ is very small, we will see that it has a large impact on the condensate. The detail laboratory analyses [1] of C₆₊ are shown in the 1^st and 2^nd columns of Table 1. We used the SRK EoS in GCAP Software for Vol. 3 of Gas Conditioning and Processing and determined the C₆₊ properties of MW=94.12, SG=0.738, andNBP=195 °F. All other calculations were performed using theSRK EoS.

Table 1. Characterization/Distribution of C₆₊ by different methods

We have plotted the experimentally measured Potential Hydrocarbon Liquid Condensed (PHLC) at 594.7 psia as a function temperature in Figure 1, and by extrapolation, a dew point temperature of 52.1 °F was obtained. We used the methods discussed in the previous tip of the month to characterize the C₆₊ by matching the experimentally determined dew point. For each method, we have presented the tuned MW and distribution of components in Table 1 and the predicted PHLC as a function of temperature and the phase envelope were plotted in Figures 1 and 2, respectively. It should be pointed out that the cricondentherm for the lumped C₆₊ method was below 15 °F; therefore, zero values for PHLC were predicted.

Figure 2 indicates that three of the characterization methods practically generate the same phase envelope. Are all of these phase envelopes presenting the true phase behavior of this lean gas? The answer is “Yes” and “NO”. Figure 1 indicates that, for temperatures close to the dew point, all three methods predict the PHLCs very close to the experimental values; however, at lower temperatures (about 20 °F) the deviation from experimental values increases. Only the normal alkane distribution method gives accurate values even at lower temperatures.

In summary, for sound process design and/or operation we suggest using at least one experimental saturation measurement such dew point or bubble point near the potential operating conditions to characterize the heavy ends. Once the characterization is established, additional experimental measurements should be utilized to verify the accuracy and validity of the tuning technique.

By: Dr. Mahmood Moshfeghian

Reference:

1. Derks, P. A. H., van der Meulen-Kuijk, L., and Smit, A. L. C., “Detailed Analysis of Natural Gas for an Improved Prediction of Condensation Behavior,” Proceedings, 72^nd Annual Convention, Gas Processors Association, 1993.
2. Riazi, M.R. and T.E. Daubert, Hydr. Proc. P. 115, (March) 1980