Effect of Nitrogen Impurities on CO2 Dense Phase Transportation

In the January and February 2012 tips of the month (TOTM) we discussed the isothermal and non-isothermal transportation of pure carbon dioxide (CO₂) in the dense phase region. We illustrated how thermophysical properties changed in the dense phase and studied their impacts on pressure drop calculations. The pressure drop calculation results utilizing the liquid phase and vapor phase equations were exactly the same. We showed that the effect of the overall heat transfer coefficient on the pipeline temperature is significant. In this TOTM, we will study the same case study in the presence of nitrogen impurities under non-isothermal conditions. The Joule-Thompson expansion effect and the heat transfer between pipeline and surroundings have been considered. Specifically, we will report the effect of nitrogen impurities on the pressure and temperature profiles. The Peng-Robinson equation of state (PR EOS) was utilized in this study.

For a pure compound above critical pressure and critical temperature, the system is often referred to as a “dense fluid” or “super critical fluid” to distinguish it from normal vapor and liquid (see Figure 1 for carbon dioxide in December 2009 TOTM [1]).

Calculation Procedure:

The same step-by-step calculation procedure described in the February 2012 TOTM [2] was used to determine the pressure and temperature profiles in a pipeline considering the Joule-Thompson expansion effect and heat transfer between the pipeline and surroundings.

In the following section we will illustrate the pressure drop calculations for transporting CO₂ in dense phase using the gas phase pressure drop equations. For details of pressure drop equations in the gas and liquid phases refer to the January 2012 TOTM [3].

Case Study:

For the purpose of illustration, we considered a case study [also described in reference 2] for transporting 160 MMSCFD (4.519×10⁶ Sm³/d) CO₂ using a 100 miles (160.9 km) long pipeline with an inside diameter of 15.551 in (395 mm). The inlet conditions were 2030 psia (14 MPa) and 104˚F (40˚C). The following assumptions were made:

1. CO₂, with nitrogen impurities of 0, 1, 5, 10, and 15 mole %.
2. Horizontal pipeline, no elevation change.
3. Inside surface relative roughness’s (roughness factor), ε/D, of 0.00013.
4. The ambient/surrounding temperature,Ts, is 55 ˚F and (12.8 ˚C)
5. Overall heat transfer coefficients of 0.5 Btu/hr-ft²-˚F (2839 W/m²-˚C).

Properties: The dense phase behavior and properties were calculated using the Peng-Robinson equation of state (PR EOS) [4] in ProMax [5] software. ProMax was also used to determine pressure and temperature profiles along the pipeline.

Results and Discussions:

Figures 1 through 4 present the phase envelope, dry ice (CO₂ freeze out) curve, and pipeline pressure and temperature profile for 1, 5, 10, and 15 mole % N₂ impurities, respectively, the relative roughness (ε/D) of 0.00013, and the overall heat transfer coefficient (U) of 0.5 Btu/hr-˚F-ft² (2.839 W/m²-˚C).

Figure 1. Phase envelop and dense phase pipeline pressure-temperature profile for 99 mole % CO₂ + 1 mole % N₂, ε/D=0.00013, and U=0.5 Btu/hr-˚F-ft² (2.839 W/m²-˚C).

Figure 2. Phase envelop and dense phase pipeline pressure-temperature profile for 95 mole % CO₂ + 5 mole % N₂, ε/D=0.00013, and U=0.5 Btu/hr-˚F-ft² (2.839 W/m²-˚C).

Figure 3. Phase envelop and dense phase pipeline pressure-temperature profile for 90 mole % CO₂ + 10 mole % N₂, ε/D=0.00013, and U=0.5 Btu/hr-˚F-ft² (2.839 W/m²-˚C).

Figure 4. Phase envelop and dense phase pipeline pressure-temperature profile for 85 mole % CO₂ + 15 mole % N₂, ε/D=0.00013, and U=0.5 Btu/hr-˚F-ft² (2.839 W/m²-˚C).

The effect of N₂ impurities on the line temperature profile is shown in Figure 5. This figure indicates that N₂ impurities have negligible effect on the pipeline temperature profile.

Figure 6 presents the effect of N₂ impurities on the pipeline pressure profile. This figure indicates that as the N₂ impurities increases the pressure drop increases. This can be explained by the fact as the N₂ impurities increase, the mixture density decreases, consequently the velocity increases. Note the pressure drop is proportional to square of velocity and inverse of density. While viscosity decreases with increase in N₂ impurities, its effect is not as large as the density effect. Table 1 presents variation of the mixture density and viscosity as a function of N₂ mole %.

Table 1. Effect of N₂ impurities on density (ρ) and viscosity (µ) of mixture at the inlet condition of 2030 psia (14 MPa) and 104˚F (40˚C)

Conclusions: 

Analyzing Table 1 and Figures 1 through 6, the following conclusions can be made:

1. For the range 0 to 15 mole % N₂, the effect of the N₂ impurities on the pipeline temperature profile is negligible.
2. As the N₂ impurities increase, the pipeline pressure drop increases due to the change in thermophysical properties of mixture.
3. Care should be taken to use accurate thermophysical properties and the phase envelope should be plotted to avoid any operating problem.

Figure 5. Variation of the pipeline temperature profile with the N₂ impurities and U=0.5 Btu/hr-˚F-ft² (2.839 W/m²-˚C)

Figure 6. Variation of the pipeline pressure profile with the N₂ impurities and U=0.5 Btu/hr-˚F-ft² (2.839 W/m²-˚C)

To learn more about similar cases and how to minimize operational problems, we suggest attending our G40 (Process/Facility Fundamentals), G4 (Gas Conditioning and Processing), P81 (CO₂ Surface Facilities), and 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 your consulting needs to consulting@jmcampbell.com.

By: Dr. Mahmood Moshfeghian

Reference:

1. Bothamley, M.E. and Moshfeghian, M., “Variation of properties in the dese phase region; Part 1 – Pure compounds,” TOTM, http://www.jmcampbell.com/tip-of-the-month/2009/12/variation-of-properties-in-the-dense-phase-region-part-1-pure-compounds/, Dec 2009.
2. Moshfeghian, M., ”Transportation of CO₂ in the Dense Phase,” TOTM, http://www.jmcampbell.com/tip-of-the-month/2012/02/ , Feb 2012
3. Moshfeghian, M., ”Transportation of CO₂ in the Dense Phase,” TOTM, http://www.jmcampbell.com/tip-of-the-month/2012/01/, Jan 2012
4. Peng, D. Y., and Robinson, D. B., Ind. Eng. Chem. Fundam., Vol. 15, p. 59, 1976.

ProMax 3.2, Bryan Research and Engineering