Acid Gas-Water Content

In the past Tips of the Month (TOTM), we discussed the phase behavior of sweet natural gas- water, sour natural gas-water, and acid gas–water systems. They were posted in October 2007 TOTM [1], November 2007 TOTM [2], and December 2007 TOTM [3], respectively. In this TOTM, we will revisit the acid gas-water phase behavior system. Specifically, different methods of predicting water content of acid gas systems are evaluated based on experimental data from the literature. Water content diagrams compatible with the experimental data for pure CO₂, Pure H₂S, pure CH₄ and their mixtures are generated and presented. These charts can be used for facility type calculations and trouble shooting.

Table 1 presents the compositions of several acid gas mixtures evaluated in this study, along with their saturated water contents (in mole percent) from experimental data [4] and from predictions by five methods. The Maddox et al. [5] results were generated using GCAP [6] software and the Erbar et al. [7] results were generated by EzThermo [8] software. The Wichert & Wichert [9] and Yarrison et al. [10] results are from GPA RR-210 [11]. The last column presents the results predicted by SRK in ProMax [12].

Table 1 indicates that as long as the total acid gas concentrations is less than 60 mole percent, all five methods produce results within the accuracy of experimental data. However, for higher concentrations of acid gases, the Yarrison et al. [10] and ProMax [12] methods provide more accurate results.

It should be noted that the results of ProMax in Table 1 are based on the water saturator tool available in ProMax. If a conventional 3-phase separator calculation of ProMax is used, the error percent for the third row from the bottom of table reduces from -44.9 % to -16.6 %. The results for the other cases remained practically the same for these two calculations options.

Table 2 and Figure 1 present the error analysis for prediction of 74 additional gas mixtures containing acid gases. The detail of data points and sources of experimental points are in GPA RR-210 [11]. The five methods under study are Maddox et al. [5], Robinson et al. [13], Wichert & Wichert [9], Yarrison et al. [10], and ProMax [12]. With the exception of the ProMax results, the predicted water contents used for error analysis of the other four methods were extracted from   GPA RR-210 [11].

Figure 1 presents the error analysis graphically for the same 74 gas mixtures presented in Table 2. Based on the error analysis of Tables 1, 2, and Figure 1, the ProMax method was chosen to generate water content diagrams for pure CO₂, pure H₂S, and their mixtures. These diagrams are presented in the proceeding sections.

The phase equilibria in the system H₂S + water and CO₂ + water are key to the discussion of the water content of an acid gas system. Figures 2 (SI) and 2 (FPS) present the water content of pure H₂S predicted by ProMax [12] as a function of pressure and temperature, in the international system (SI) and engineering system of units (FPS, foot, pound and second). The behavior shown on this plot is quite complicated and explained thoroughly by Carroll [14]:

“At low pressure the hydrogen sulfide + water mixture is in the gas phase. At low pressure the water content tends to decrease with increasing pressure, which is as expected. Eventually a pressure is reached where the H₂S is liquefied. On this plot this is represented by the discontinuity in the curve and a broken line joins the phase transition. There is a step change in the water content when there is a transition from vapor to liquid. In the case of hydrogen sulfide the water content of the H₂S liquid is greater than the coexisting vapor. This is contrary to the behavior for light hydrocarbons where the water content in the hydrocarbon liquid is less than the coexisting vapor.”

Within the transition region, the acid gas exists as both liquid and vapor.  Water saturation of the vapor phase is represented by the lower value, whereas the water content of the liquid phase is the higher value.

Figures 3 (SI) and 3 (FPS) present the water content of pure CO₂ and pure CH₄ predicted by ProMax [12] as a function of pressure and temperature, in the international system (SI) and engineering system of units (FPS). When Figures 2 and 3 were superimposed on Figures 20-5 and 20-6, respectively, of the GPSA data book [15] a very good match was obtained. The two figures in the GPSA data book are based on experimental data and the Yarrison et al. model.

In general the phase behavior of the system CO₂ + water is as complex as that of the H₂S + water system. The CO₂-rich liquid phase only occurs for temperatures less than about 32.2°C (90°F). As shown in Figure 3 (as well as in Figure 2 reported by Maddox and Lilly [16]), the water content of CO₂ exhibits a minimum.

Figure 4 presents the phase behavior of pure CO₂, Pure H₂S and three mixtures of them containing 2 mole percent CH₄. Their corresponding water content charts are presented in Figure 5.

Summary:

There are several methods available that can be used to predict the water content of acid gases. Most of these methods are based on equations of state and rigorous thermodynamic models. As described above, the phase behavior is complicated and extra care should be taken to assure a correct prediction.  Although not addressed in this study, hydrates can also form and these can significantly complicate phase behavior.

Different methods of predicting water content of acid gas systems are evaluated based on the literature experimental data. In addition, the water content diagrams compatible with the experimental data for pure CO₂, H₂S, CH₄ and their mixture are generated and presented. These charts can be used for facility type calculations and trouble shooting.

To learn more, we suggest attending our G40 (Process/Facility Fundamentals), G4 (Gas Conditioning and Processing), G5 (Gas Conditioning and Processing-Special), and PF81 (CO₂ Surface Facilities), PF4 (Oil Production and Processing Facilities), courses.

John M. Campbell Consulting (JMCC) offers consulting expertise on this subject and many others. For more information about the services JMCC provides, visit our website at www.jmcampbellconsulting.com, or email us at consulting@jmcampbell.com.

References:

1. Moshfeghian, M. “Water-Sweet Natural Gas Phase behavior,” http://www.jmcampbell.com/tip-of-the-month/2007/10/water-sweet-natural-gas-phase-behavior/, October 2007.
2. Moshfeghian, M., ”Water-Sour Natural Gas Phase Behavior,” http://www.jmcampbell.com/tip-of-the-month/2007/11/water-sour-natural-gas-phase-behavior/, November 2007.
3. Wright, W. and M. Moshfeghian, “Acid Gas-Water Phase Behavior,” http://www.jmcampbell.com/tip-of-the-month/2007/12/acid-gas-water-phase-behavior/, December, 2007.
4. Huang, S.S.S., A.D. Leu, H.J. Ng, and D.B. Robinson, “The Phase Behavior of Two Mixtures of Methane, Carbon Dioxide, Hydrogen Sulfide, and Water” Fluid Phase Equil. 19, 21-32, 1985.
5. Maddox, R.N., L.L. Lilly, M. Moshfeghian, and E. Elizondo, “Estimating Water Content of Sour Natural Gas Mixtures”, Laurance Reid Gas Conditioning Conference, Norman, OK, Mar., 1988.
6. GCAP 8.3, John M. Campbell & Co., Norman, Oklahoma, December 2010.
7. Erbar, J.H., A.K. Jagota, S. Muthswamy, and M. Moshfeghian, “Predicting Synthetic Gas and Natural Gas Thermodynamic Properties Using a Modified Soave-Redlich-Kwong Equation of State,” Gas Processor Research Report, GPA RR-42, Tulsa, USA, 1980.
8. EzThermo, Chemical Engineering Consultants, Inc, Stillwater, Oklahoma, 2010.
9. Wichert, G. C. and E. Wichert, “New Charts Provide Accurate Estimation for Water Content of Sour Natural Gas”, O&G J, pp 64-66, Oct. 27, 2003..
10. Yarrison M., Song, K. Y., Cox, K,, Chronister D. and Chapman, W., “Water Content of High Pressure, High Temperature Methane, Ethane and Methane+CO2, Ethane + CO2,” RR-200, GPA, Tulsa, OK, March, 2008.
11. Song, K. Y., Vo, T., Yarrison M. and Chapman, W., “Acid Gas Water Content, An Update Of Engineering Data Book I,” RR-210, GPA, Tulsa, OK, June, 2012.
12. ProMax 3.2, Bryan Research and Engineering, Inc., Bryan, Texas, U.S.A., 2013.
13. Robinson, J. N., et al., Trans. AIME, Vol. 263, p. 281, 1977
14. Carroll, J.J., “The water content of acid gas and sour gas from 100 to 220 °F and pressures to 10,000 Psia,” Presented at the 81st Annual GPA Convention, Dallas, Texas, USA, March 11-13, 2002.
15. GPSA Data Book, Vol. 2, 13^th Ed., Gas Processors and Suppliers Association, Tulsa, Oklahoma, 2013.
16. Maddox, R.N., L.L. Lilly, “Gas conditioning and Processing, Vol 3: Computer Applications and Production/Processing Facilities”, John M. Campbell & Company, Norman, USA, 1982.