{"id":2583,"date":"2018-04-02T07:52:11","date_gmt":"2018-04-02T12:52:11","guid":{"rendered":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/?p=2583"},"modified":"2018-04-02T07:53:09","modified_gmt":"2018-04-02T12:53:09","slug":"impact-of-co2-on-natural-gas-density","status":"publish","type":"post","link":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/2018\/04\/impact-of-co2-on-natural-gas-density\/","title":{"rendered":"Impact of CO2 on Natural Gas Density"},"content":{"rendered":"

Due to the importance of CO2<\/sub>\u00a0injection for enhanced oil recovery and the increasing interest in CO2<\/sub>capture and sequestration, this study was undertaken to prepare simple charts for accurately estimating the density for hydrocarbon systems containing nil to 100% CO2<\/sub>.<\/p>\n

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The September 2008 Tip of the Month (TOTM) [1] evaluated the accuracy of Katz [2] and Wichert-Aziz [3] shortcut methods for predicting sour and acid gas density. The tip demonstrated that for binary mixtures of CH4<\/sub>\u00a0and CO2<\/sub>, the Wichert-Aziz method gives\u00a0a more\u00a0accurate result for\u00a0CO2<\/sub>\u00a0content of between 10 and90 mole\u00a0percent.<\/p>\n

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The October 2008 TOTM [4] evaluated the accuracy of density calculations using two process simulation software packages, NIST REFPROP program [5], the GERG-2004\u00a0equation of state [6], and AGA 8 method [7] (in addition to the above shortcut methods) against experimental data. An experimental database was used for the basis of comparison. The sources of experimental data were GPA RR-138 [8] and GPA RR 68 [9]. Table 1 of the October TOTM indicated that REFPROP and GERG 2004 give equally the best results.<\/p>\n

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In continuing the September and October 2008 TOTMs, this study was undertaken to prepare simple charts for accurate estimation of the density\u00a0of hydrocarbon systems containing nil to 100% CO2<\/sub>. The charts present density of CO2<\/sub>\u00a0+ light hydrocarbons mixtures as a function of pressure and\u00a0 CO2<\/sub>concentration for four isotherms of -50 \u00b0C, 0 \u00b0C, 50 \u00b0C, and 100 \u00b0C (-58 \u00b0F, 32 \u00b0F, 122 \u00b0F, 212 \u00b0F). The pressure range was from 0.5 MPa to 30 MPa (72.5 psia to 4350 psia) and the CO2<\/sub>\u00a0concentrations range was from 0 to 100 mole % (0, 10, 30, 50, 70, 90, 100 mole%). Table 1 presents the composition of systems studied. The default equations in REFPROP are used to calculate the phase boundaries and densities [5].<\/p>\n

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Table 1.<\/strong>\u00a0Composition\u00a0of CO2<\/sub>\u00a0+ light hydrocarbons systems<\/p>\n

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The performance of REFPROP program extracted from Table 1 of the October 2008 TOTM [4] is presented in Table 2. This table indicates that the average absolute percent error (AAPE) and average percent error (APE) are 0.46 and 0.23, respectively. Due to the high accuracy of REFPROP program, this TOTM will use REFPROP to calculate the density of CO2<\/sub>\u00a0+ light hydrocarbon systems.<\/p>\n

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Table 2.\u00a0<\/strong>Summary of error analysis for\u00a0the binary\u00a0system of CH4<\/sub>\u00a0+ CO2<\/sub>\u00a0density prediction by REFPROP program<\/p>\n

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We replotted the experimental density data reported in the GPA RR-138 [8] and GPA RR 68 [9] to demonstrate the accuracy of REFPROP program. The results of this evaluation are shown in Figures 1A through 5A (Appendix A), for CO2<\/sub>\u00a0content of 9.83 to 100 mole percent and the temperature and pressure ranges of Table 2…<\/p>\n

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Next, we plotted the calculated density by REFPROP as a function of pressure and CO2<\/sub>\u00a0content for four temperatures in Figures 1 through 4.<\/p>\n

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Figure 1.<\/strong>\u00a0Variation of density with pressure and CO2<\/sub>\u00a0concentration at -50 \u00b0C (-58 \u00b0F)<\/p>\n

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Figure 2.<\/strong>\u00a0Variation of density with pressure and CO2<\/sub>\u00a0concentration at 0 \u00b0C (32 \u00b0F)<\/p>\n

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According to REFPROP the dashed lines in Figure 1 present the\u00a0two-phase\u00a0region for all CO2<\/sub>concentrations except for the case of 100 mole % which is in the gas phase. All solid lines present the liquid phase region. REFPROP indicated that in Figure 2:<\/p>\n

\u25baFor CO2<\/sub>\u00a0concentrations of 0, 10, and 30 mole %, the dashed lines present gas phase for pressures up to 2.5 MPa (362.5 psia) and two-phase for pressures more than 2.5 MPa (362.5 psia). The solid lines present the supercritical region.<\/p>\n

\u25baFor CO2<\/sub>\u00a0concentrations of 50, 70, and 90 mole %, the dashed lines present gas phase for pressures up to 2 MPa (290 psia) and\u00a0two-phase\u00a0for pressures more than 2 MPa (290\u00a0psia). The solid lines present the liquid phase.<\/p>\n

\u25baFor CO2<\/sub>\u00a0concentration of 100 mole %, the dashed line presents gas phase. The solid line presents the liquid phase.<\/p>\n

REFPROP also indicated that all dashed lines in Figures 3 and 4 present the gas phase region and all solid lines present the supercritical region.<\/p>\n

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Figure 3.<\/strong>\u00a0Variation of density with pressure and CO\u00a02<\/sub>\u00a0concentration at 50 \u00b0C (122 \u00b0F)<\/p>\n

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Figure 4.<\/strong>\u00a0Variation of density with pressure and CO2<\/sub>\u00a0concentration at 100 \u00b0C (212 \u00b0F)<\/p>\n

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SUMMARY<\/strong><\/p>\n

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

\u25baCO2<\/sub>\u00a0concentration has a great impact on the mixture density. As CO2<\/sub>\u00a0concentration increases the mixture density increases.<\/p>\n

\u25baREFPROP is relatively accurate for density calculations of pure CO2<\/sub>\u00a0and mixtures of light hydrocarbons and CO2<\/sub>\u00a0(Table 2 and Figures 1A-5A).<\/p>\n

\u25baSimple density charts are presented for accurate estimation of a natural gas (relative density 0.65) as a function of pressure and CO2<\/sub>\u00a0concentration for four temperatures (Figures 1-4). These charts are composition specific (Table 2), similar charts should be developed for different compositions.<\/p>\n

\u25baKnowledge of phase boundaries and behavior is essential for density calculation.<\/p>\n

To learn more about similar cases and how to minimize operational problems, we suggest attending ourG4 (<\/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

PetroSkills\u00a0<\/em>offers consulting expertise on this subject and many others. For more information about these services, visit our website at\u00a0http:\/\/petroskills.com\/consulting<\/a>, or email us at\u00a0consulting@PetroSkills.com<\/a>.<\/p>\n

By: Dr. Mahmood Moshfeghian<\/em><\/p>\n

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