Continuing the September 2018 tip of the month (TOTM) [1], the phase behavior of ethane and water binary system was studied. Like propane–water system [2], the ethane–water system is complicated for the following two reasons:
1. Very low mutual solubility in liquid phases.
2. At lower temperatures, ice or hydrates is formed.
In this tip, we will evaluate the accuracy of water content predicted by a process simulation software against limited measured experimental data. Second, the tip studies the effect of pressure and temperature on the ethane water content in equilibrium with liquid water, ice, or hydrate phase. In addition, water content charts are presented for isobars of 14.7, 25, 50, 100, 150, and 200 psia (101.3, 172, 345, 699, 1034, 1379 kPa). For each isobar a temperature range of -60 °F to 200 °F (-51 °C to 104 °C) is covered.
Evaluation of the Water Content Prediction Methods
The performance of the ProMax simulation software [3], for estimating the water content of ethane in equilibrium with hydrate or liquid water was evaluated against limited GPA RR 132 experimental data [4]. A summary of water content comparisons for ethane vapor (G) or liquid (LHC) in equilibrium with hydrate (H) and liquid water (LW) is presented in Table 1. Three methods within ProMax were utilized. These three methods are labeled and described as follows:
►ProMax 1: Water content was estimated using a water saturator tool.
►ProMax 2: One mole of pure ethane stream was mixed with a pure water stream at the desired pressure. To determine the water content of the mixed stream, the solver tool of ProMax was used to adjust the pure water stream flow rate to match the system temperature.
►ProMax 3: Water content was estimated by performing flash calculations for a binary system of 50–50 mole % ethane–water at system pressure and temperature.
Table 1. Comparison of vapor or liquid ethane water content (PPM by mole) in equilibrium with liquid water or hydrate by ProMax against the GPA-RR 132 [4] experimental data
* The pressure in parenthesis are the experimental values which were adjusted to form liquid ethane. See below for detail of pressure adjustment.
The SRK EOS (Soave-Redlich-Kwong equation of state) [5] with its ProMax default binary interaction parameters were used. For these set of pressures and temperatures, all three methods give relatively good results. Table 1 indicates that even the ethane water contents are very low (from 2 to 733 ppm by mole), the average absolute percent deviations are relatively low and are from 11 to 18.5 %.
For pressures higher than 496 psia (3421 kPa), to form liquid ethane and liquid water (LHC-LW) phases at equilibrium, the experimental pressures reported in Table 1 were adjusted as follows:
1. For a binary system of 50–50 mole % ethane–water, at the specified temperature and 0 % vapor the pressure was calculated.
2. The calculated pressure was increased slightly, i.e. less than 2 psi (14 kPa).
3. The summary of results for the adjusted pressures is presented in Table 2.
Table 2. Pressure adjustment summary for pressure higher than 496 psia (3420.7 kPa)
Ethane Water Content Charts
The coexistence of equilibrium phases depends on the system pressure and temperature as follows:
a. Ethane vapor phase in equilibrium with liquid water
b. Ethane vapor phase in equilibrium with ice or hydrate
c. Ethane liquid phase in equilibrium with liquid water
d. Ethane liquid phase in equilibrium with hydrate
Figure 1 illustrates the presence of these equilibrium phases as a function of temperature for the isobar of 200 psia (1379 kPa). The propane water content was estimated by the following procedures.
a. For temperatures of 200 °F to about 47.5 °F (93 to ~ 8.6 °C), the ethane vapor is in equilibrium with liquid water phase so the water saturator tool of ProMax was used.
b. For temperatures of about 47.5 °F to -6 °F (~ 8.6 to -21.1 °C), the ethane vapor was in equilibrium with the hydrate phase, so one mole of pure ethane stream was mixed with a pure water stream atpressure of 200 psia (1379 kPa). To determine the water content of the mixed stream, the solver tool in ProMax was used to adjust the pure water stream flow rate to form hydrate at the specified hydrate formation temperature.
c. The ethane vapor phase transition to the liquid phase takes place at -6 °F (-21.1 °C).
d. For temperatures of -6 °F to -60 °F (-21.1 to -51 °C), the ethane liquid is in equilibrium with the hydrate phase, so one mole of pure ethane stream was mixed with a pure water stream at pressureof 200 psia (1379 kPa). To determine the water content of the mixed stream, the solver tool in ProMax was used to adjust the pure water stream flow rate to form hydrate at the specified hydrate formation temperature.
Figure 1. Water content of vapor and liquid Ethane as a function of temperature at 200 psia (1379 kPa)
Table 3 presents the three-phase temperatures of a ethane–water system and the saturatiom temperatures of pure ethane estimated by ProMax.
Table 3. Three phase temperature and saturation temperature for six isobars
Phases: V = Vapor, I = Ice, H = Hydrate, and LW = Liquid Water
Similarly, the water content charts of ethane vapor and liquid phases were prepared for the other isobars and are presented in Figures 2–4. Note in Figures 2 and 4, due to the very small values, the liquid ethane water contents for different isobars fall on the same curve.
Figure 2. Water content of Ethane as a function of temperature for six isobars
Figure 3. Water content of vapor ethane as a function of temperature for several isobars
Figure 4. Water content of liquid ethane as a function of temperature for several isobars
Conclusion
Similar to propane, estimating ethane water content requires a good understanding of the phase behavior. The process simulation programs have several tools or procedures for estimating the water content. Which one should be used to give a correct answer? It depends. In addition to the selection of a suitable equation of state, the selection of the right tool or procedure at a given set of conditions is essential. The choice of a suitable tool changes as the conditions or the equilibrium phases change.
The presented ethane water content charts can be used for facility type calculations and trouble shooting. It is a good practice to test the performance/accuracy of the selected tool against experimental data first. Obviously, for better understanding of ethane–water phase behavior and improving the thermodynamic modeling, more experimental data is needed.
To learn more about similar cases and how to minimize operational problems, we suggest attending ourG4 (Gas Conditioning and Processing) and G5 (Practical Computer Simulation Applications in Gas Processing) courses.
By: Dr. Mahmood Moshfeghian
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References
1. Moshfeghian, M., “Propane – Water Phase Behavior at Low to Moderate Pressures,” PetroSkills TOTM, Sep 2018.
2. Harmens, A. and E.D. Sloan, “The phase Behavior of Propane – Water System: A Review,” The Canadian J of Chem Engr, Vol 68, Feb 1998.
3. ProMax 4.0, Build 4.0.17179.0, Bryan Research and Engineering, Inc., Bryan, Texas, 2017.
4. Song, K and R. Kobayashi, “Water content of ethane, propane, and their mixtures in equilibrium with water and hydrates,” Gas Processor Association Research Report (GPA RR 132), Tulsa, Oklahoma, 1991.
5. Soave, G., Chem. Eng. Sci. Vol. 27, No. 6, p. 1197, 1972.
