How to Generate a Phase Envelope?

In a previous “Tip of the Month” we briefly discussed the need for understanding a phase diagram in a gas processing system. We also defined the areas of a light mixture phase envelope and the terms necessary to “talk intelligently” about the shape of a mixture phase diagram. This allowed us to look at the methods of calculation and their limitations in another tip and eventually defined our areas of risk in the operation or design of a facility based on the phase diagram.

In this Tip we will explain how a phase envelope is generated and what factors affect the shape and accuracy of a phase envelope. There are two methods of generating a phase envelope: a) by conducting a series of bubble point and dew point measurements in a PVT laboratory b) Using a cubic EoS such as SRK or PR and performing a series of bubble point and dew point calculations. The triangle symbols in Figure 1 present a phase envelope measured in a laboratory for a synthetic natural gas.

For the same mixture, we used GCAP for Volume 2 of Gas Conditioning and Processing Software to generate the phase envelope using SRK and PR EoSs. The dashed line represents the SRK EoS and the solid line represents the PR EoS. As can be seen, for this case the SRK EoS gives a perfect match with the experimentally measured dew point curve but PR predicts a lower cricondentherm point. The built in and in–house binary interaction parameters for SRK and PR were used to generate these two diagrams; however, an experienced engineer is able to produce a close match for either of these two EoSs by tuning the binary interaction parameters and/or heavy end properties.

The best practical method for generating an accurate phase envelope by any commercial software is to utilize a limited number of VLE measurements and tune one or more properties of the heavy ends (C₇₊). In other words, we suggest using combination of methods “a” and “b”.

For real natural gas mixtures, the components and composition of the heavy ends are not well defined and laboratory measurements are not accurate enough. Therefore, different techniques as described in the literature and Volume 3 of Gas Conditioning and Processing are used to properly characterize the heavy ends resulting in an overall acceptable match.

Since the laboratory reported values of molecular weight (MW) and specific gravity (SG) of C₇₊ are questionable, in the proceeding section we will demonstrate the impact of these two properties on the shape of overall phase envelope.

Figure 2 presents the impact of molecular weight of C₇₊ on the phase envelope of a rich natural gas mixture. As the C₇₊ molecular weight increases, the cricondentherm temperature and cricondenbar pressure will increase and the two phase region expands.

Figure 3 shows that as the C₇₊ specific gravity increases, the cricondentherm temperature and cricondenbar pressure will also increase and the two phase region expands.

From these two diagrams, one can see that improper characterization of heavy ends may result in a bad design or troublesome operation (e.g. designing a dry gas pipeline instead of two phase gas-liquid flow).

In the next tip of the month we will demonstrate how to tune computer software to generate proper and relatively accurate phase envelopes.

By: Dr. Mahmood Moshfeghian

Reference:

1. C. Jarne, S. Avila, S. T. Blanco, E. Rauzy, S. Otin, I. Velaso, Ind. Eng. Chem. Res. 43 (2004) 209-217.