{"id":2743,"date":"2019-09-01T09:50:10","date_gmt":"2019-09-01T14:50:10","guid":{"rendered":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/?p=2743"},"modified":"2019-10-02T10:43:22","modified_gmt":"2019-10-02T15:43:22","slug":"investigations-into-co2-frost-point-in-the-presence-of-ch4-c2h6-and-n2","status":"publish","type":"post","link":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/2019\/09\/investigations-into-co2-frost-point-in-the-presence-of-ch4-c2h6-and-n2\/","title":{"rendered":"Investigations into CO2 Frost Point in the Presence of CH4, C2H6, and N2"},"content":{"rendered":"

The main issue for cryogenic processing and transporting of a gas stream containing carbon dioxide is its frost formation. Solid carbon dioxide (dry ice) may form at low pressures when the temperature of a gas stream containing CO2<\/sub>\u00a0drops below the triple point temperature of CO2<\/sub>.\u00a0 The triple point temperature and pressure of CO2<\/sub>\u00a0are -56.57 \u00b0C (-69.83 \u00b0F) and 518 kPa (75.12 psia) respectively. Its critical temperature\u00a0and pressure are 31.10 \u00b0C (87.98 \u00b0F) and 7386 kPa (1,071 psia). Frost point occurs along the solid-vapor equilibria (SVE) curve. \u00a0In addition, CO2<\/sub>\u00a0is corrosive in water wet systems and it reduces the gas heating value and Wobbe Index (number). There are limits on CO2<\/sub>\u00a0concentration in the sales gas and liquid products as a result. The sales-gas\/transportation by pipeline specification limits the CO2<\/sub>\u00a0concentration to 1 \u2013 3 mole %. To avoid frost formation in cryogenic gas processing, removal of CO2<\/sub>\u00a0is often required to meet the downstream processing requirements. Types of processing include:<\/p>\n

\u25ba\u00a0Deep NGL extraction plants: typically, less than 0.5 to 1.0 mole % depending on the process used<\/p>\n

\u25ba\u00a0LNG liquefaction plants: less than 50 ppmv<\/p>\n

\u25ba\u00a0N2<\/sub>\u00a0rejection, He recovery: less than LNG feed<\/p>\n

Therefore, it is important to estimate accurately the temperature where carbon dioxide solidifies. The solid \u2013 vapor equilibria (SVE) of natural gas systems containing carbon dioxide may be predicted accurately using simulation programs.<\/p>\n

In continuing the\u00a0April 2012<\/a>\u00a0and\u00a0April 2018<\/a>\u00a0tip of the months (TOTMs) [1, 2], this study was undertaken to prepare simple charts for estimation of CO2<\/sub>\u00a0frost temperature or pressure of binary and ternary systems containing CH4<\/sub>, C2<\/sub>H6<\/sub>, and N2<\/sub>, and CO2<\/sub>. The charts present frost temperature (or pressure) of CO2<\/sub>\u00a0+ light hydrocarbons and nitrogen mixtures as a function of pressure (or temperature) and CO2<\/sub>\u00a0concentration for a wide range of temperature from -80 \u00b0C to -120 \u00b0C (-112 \u00b0F to -184 \u00b0F). The pressure range was from about 100 kPa to 3500 kPa (14.5 psia to 508 psia) and the CO2<\/sub>\u00a0concentration range was from about 0.1 to 55 mole %. For each case, the accuracy of charts will be compared with experimental data.<\/p>\n

Figure 1 [3] presents the phase diagram for pure CO2<\/sub>. Regions S, L, and V denote the Solid, Liquid, and Vapor phase, respectively. Point C is the critical point and point T is the triple point where the three phases of solid-liquid-vapor coexist. Figure 1A (FPS) is presented in Appendix A. The symbols represent experimental [4] data and the solid curves were predicted by the Nasrifar \u2013 Bolland equation of state [5].<\/p>\n

 <\/p>\n

<\/p>\n

Figure 1.<\/strong>\u00a0Experimental [4] and predicted pure carbon dioxide coexistence curves [5]<\/em><\/p>\n

 <\/p>\n

Validation of Thermodynamic Package<\/strong><\/p>\n

To validate the accuracy of the ProMax [6] simulation program, its Peng \u2013 Robinson equation of state and Freeze Analysis Tool were utilized to predict the solid formation temperature of binary mixtures of CO2<\/sub>\u00a0+ CH4<\/sub>\u00a0as a function of pressure and CO2<\/sub>\u00a0composition. The predicted frost temperatures were compared with the experimental data. Figures 1 and 2 present the frost point temperature as a function of pressure and composition for two sets of experimental data [7, 8] and the ProMax predicted values [6].<\/p>\n

 <\/p>\n

<\/p>\n

Figure 2.\u00a0<\/strong>Experimental [7] and predicted carbon dioxide frost points (CO2<\/sub>\u00a0mole % < 11)\u00a0<\/em><\/p>\n

 <\/p>\n

<\/p>\n

Figure 3.<\/strong>\u00a0Experimental [8] and predicted carbon dioxide frost points (10<CO2<\/sub>\u00a0mole %<55)<\/em><\/p>\n

 <\/p>\n

The average absolute relative error (AARE) % for 42 frost temperatures (in K or \u00b0R) of Figure 2 predicted by ProMax compared to the corresponding experimental values [7] is 1.27%. Similarly, the AARE % for 17 frost temperatures (in K or \u00b0R) of Figure 3 predicted by ProMax compared to the corresponding experimental values [8] is 0.27%. Tables 1A and 2A in Appendix A present the point-by-point comparisons. The error analysis of Figures 2 and 3 indicates that the predicted frost temperatures by ProMax is accurate for facilities equipment design and troubleshooting.<\/p>\n

The frost pressure for the binary system of CO2<\/sub>\u00a0+ CH4<\/sub>\u00a0as a function of CO2<\/sub>\u00a0mole fraction at a specified temperature was estimated by adjusting the pressure using the solver tool of ProMax to match the experimental frost temperature. The estimated frost pressures for six isotherms are compared with the corresponding experimental values [9] in Figure 4.<\/p>\n

 <\/p>\n

<\/p>\n

Figure 4.<\/strong>\u00a0Frost point pressures of CO2<\/sub>\u00a0+ CH4<\/sub>\u00a0as a function of CO2<\/sub>\u00a0content at different isotherms. Experimental data are from Ref. [9].<\/em><\/p>\n

 <\/p>\n

Similarly, Figures 5 and 6 present the experimental [9] and ProMax estimated frost pressure of the ternary system of CO2<\/sub>\u00a0+ CH4<\/sub>\u00a0+ N2<\/sub>\u00a0as a function of CO2<\/sub>\u00a0mole fraction and five isotherms for 3 mole % and 5 mole % N2<\/sub>, respectively. These two figures and Figure 6A (in Appendix A) indicate that the frost pressure is the same at a given CO2<\/sub>\u00a0content and an isotherm for nitrogen content of 3 mole % and 5 mole %.<\/p>\n

 <\/p>\n

<\/p>\n

Figure 5.<\/strong>\u00a0Frost point pressures of CO2<\/sub>\u00a0+ CH4\u00a0<\/sub>+ 3 mole % N2<\/sub>\u00a0as a function of CO2<\/sub>\u00a0content at different isotherms. Experimental data are from Ref. [9].<\/em><\/p>\n

 <\/p>\n

<\/p>\n

Figure 6.\u00a0<\/strong>Frost point pressures of CO2<\/sub>\u00a0+ CH4\u00a0<\/sub>+ 5 mole % N2<\/sub>\u00a0as a function of CO2<\/sub>\u00a0content at different isotherms. Experimental data are from Ref. [9].<\/em><\/p>\n

 <\/p>\n

Similarly, Figures 7 and 8 present the experimental [9] and estimated frost pressure of the ternary system of CO2<\/sub>\u00a0+ CH4<\/sub>\u00a0+ C2<\/sub>H6<\/sub>\u00a0as a function of CO2<\/sub>\u00a0mole fraction and five isotherms for 3 mole % and 5 mole % C2<\/sub>H6<\/sub>, respectively. These two figures and Figure 8A (in Appendix A) indicate that the frost pressure is the same at a given CO2<\/sub>\u00a0content and an isotherm for ethane content of 3 mole % and 5 mole %.<\/p>\n

 <\/p>\n

<\/p>\n

Figure 7.\u00a0<\/strong>Frost point pressures of CO2<\/sub>\u00a0+ CH4\u00a0<\/sub>+ 3 mole % C2<\/sub>H6<\/sub>\u00a0as a function of CO2<\/sub>\u00a0content at different isotherms. Experimental data are from Ref. [9].<\/em><\/p>\n

 <\/p>\n

<\/p>\n

Figure 8.\u00a0<\/strong>Frost point pressures of CO2<\/sub>\u00a0+ CH4\u00a0<\/sub>+ 5 mole % C2<\/sub>H6<\/sub>\u00a0as a function of CO2<\/sub>\u00a0content at different isotherms. Experimental data are from Ref. [9].<\/em><\/p>\n

 <\/p>\n

Summary<\/strong><\/p>\n

Based on the work done in this tip, the following can be concluded:<\/p>\n

1.<\/strong>\u00a0For pure CO2<\/sub>, as pressure increases, the frost temperature increases (Figure 1)<\/p>\n

2.<\/strong>\u00a0CO2<\/sub>\u00a0concentration has a great impact on the mixture frost point pressure and temperature.<\/p>\n

a.<\/b><\/span>\u00a0At constant pressure, as CO2<\/sub>\u00a0concentration increases the mixture frost temperature increases (Figures 2 \u2013 3).<\/p>\n

b.<\/strong>\u00a0At constant temperature, as CO2<\/sub>\u00a0concentration increases the mixture frost pressure decreases (Figures 4 \u2013 8).<\/p>\n

3.<\/strong>\u00a0At constant temperature, concentration of 3 and 5 mole % N2<\/sub>\u00a0or C2<\/sub>H6<\/sub>\u00a0has little or no effect on the frost pressure of gas mixtures containing CO2<\/sub>\u00a0+ CH4<\/sub>\u00a0(Figures 5, 6, 6A, 7, 8, and 8A).<\/p>\n

4.<\/strong>\u00a0ProMax is relatively accurate for frost point estimation for gas mixtures of light hydrocarbons and CO2<\/sub>\u00a0(Tables 1A and 2A and Figures 4 \u2013 8).<\/p>\n

5.<\/strong>\u00a0Simple charts are presented for accurate estimation of frost point for gas mixtures of light hydrocarbons, nitrogen and CO2<\/sub>\u00a0as a function of pressure (or temperature) and CO2<\/sub>\u00a0concentration (Figures 2 \u2013 8). These charts are composition specific, similar charts should be developed for different compositions. In a future TOTM, similar charts will be presented for typical natural gas mixtures.<\/p>\n

6.<\/strong>\u00a0Knowledge of phase boundaries and behavior is essential for frost point calculation.<\/p>\n

 <\/p>\n

To learn more about similar cases and how to minimize operational problems, we suggest attending our\u00a0G4 (<\/strong>Gas Conditioning and Processing)<\/strong><\/a>,<\/strong>\u00a0G5<\/strong>\u00a0(Practical Computer Simulation Applications in Gas Processing)<\/a>\u00a0<\/strong>and\u00a0G6<\/strong>\u00a0(Gas Treating and Sulfur Recovery)<\/a>\u00a0courses.<\/p>\n

Written By: Mahmood Moshfeghian, Ph.D.<\/em><\/p>\n

<\/p>\n

\n

References<\/strong><\/p>\n

1. M. Moshfeghian, \u201chttp:\/\/www.jmcampbell.com\/tip-of-the-month\/2012\/04\/natural-gas-with-dry-ice-phase-behavior\/<\/a>,\u201d PetroSkills tip of the month, Apr 2012<\/p>\n

2. M. Moshfeghian, \u201chttp:\/\/www.jmcampbell.com\/tip-of-the-month\/2018\/04\/impact-of-co2-on-natural-gas-density\/<\/a>,\u201d PetroSkills tip of the month, Apr 2018<\/p>\n

3. Kh. Nasrifar and M. Moshfeghian, \u201cPrediction of carbon dioxide frost point for natural gas model systems,\u201d Submitted for publications, May 2019<\/p>\n

4. NIST Chemistry WebBook;\u00a0http:\/\/webbook.nist.gov\/chemistry\/<\/a>\u00a0[Cited 26 April 2019]<\/p>\n

5. K. Nasrifar, O. Bolland, Prediction of thermodynamic properties of natural gas mixtures using ten equations of state including a new cubic two-constant equation of state, J. Pet. Sci. Eng. 51 (2006) 253-266.<\/p>\n

6. ProMax 5.0, Build 5.0.19050.0, Bryan Research and Engineering, Inc., Bryan, Texas, 2019.<\/p>\n

7. G.M. Agrawal, R.J. Laverman, Phase behavior of the methane carbon dioxide system in the solid-vapor region, Adv. Cryog Eng. 19 (1974) 317-338.<\/p>\n

8. L. Zhang, R. Burgass, A. Chapoy, B. Tohidi, E. Solbraa, Measurement and modeling of CO2<\/sub>\u00a0frost points in the CO2\u00a0<\/sub>\u2013 methane systems, J. Chem. Eng. Data 56 (2011) 2971-2975.<\/p>\n

9. X. Xiong, W. Lin, R. Jia, Y. Song, A. Gu, Measurement and calculation of CO2<\/sub>\u00a0frost points in CH4<\/sub>\u00a0+ CO2<\/sub>\/CH4<\/sub>\u00a0+ CO2<\/sub>\u00a0+ N2<\/sub>\/CH4<\/sub>\u00a0+ CO2<\/sub>\u00a0+ C2<\/sub>H6<\/sub>\u00a0mixtures at low temperatures, J. Chem. Eng. Data 60 (2015) 3077-3086.<\/p>\n

\n
\n

Appendix A<\/strong><\/p>\n

<\/p>\n

Figure 1A.<\/strong>\u00a0Experimental [4] and predicted pure carbon dioxide coexistence curves [5]<\/em><\/p>\n

\n

<\/p>\n

Figure 6A.\u00a0<\/strong>Impact of N2<\/sub>\u00a0content on the frost point pressures of CO2<\/sub>\u00a0+ CH4\u00a0<\/sub>+ N2<\/sub>\u00a0as a function of CO2<\/sub>\u00a0content at different isotherms.<\/em><\/p>\n

 <\/p>\n

\n

<\/p>\n

Figure 8A.<\/strong>\u00a0Impact of C2<\/sub>H6<\/sub>\u00a0content on the frost point pressures of CO2<\/sub>\u00a0+ CH4\u00a0<\/sub>+ C2<\/sub>H6<\/sub>\u00a0as a function of CO2<\/sub>\u00a0content at different isotherms.<\/em><\/p>\n

 <\/p>\n

Table 1A.\u00a0<\/strong>Comparison of ProMax predicted frost temperature with the experimental values [7]<\/em><\/p>\n

<\/p>\n

 <\/p>\n

The Average Absolute Relative Error % (AARE%) is calculated by the following equation.<\/p>\n

<\/p>\n

Where:<\/p>\n<\/div>\n

Cal Temp = Calculated frost temperature, K (\u00b0R)<\/p>\n

Exp Temp = Experimental frost temperature, K (\u00b0R)<\/p>\n

n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0= Number of data points, 42<\/p>\n

 <\/p>\n

Table 2A<\/strong>. Comparison of ProMax predicted frost temperature with the experimental values [8]<\/em><\/p>\n

<\/p>\n

<\/div>\n

\n

The Average Absolute Relative Error % (AARE%) is calculated by the following equation.<\/p>\n

<\/p>\n

Where:<\/p>\n

Cal Temp = Calculated frost temperature, K (\u00b0R)<\/p>\n

Exp Temp = Experimental frost temperature, K (\u00b0R)<\/p>\n

n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 = Number of data points, 17<\/p>\n

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