Understanding CO2 Transport Chemistry using OLI Studio: Stream Analyzer

Table of Contents

Summary

Introduction

Modeling Approach

Calculation Configuration

• 1. Temperature Survey
• 2. Solubility Curves
• 3. Effect of H₂S

Conclusion

Summary

This article shows how OLI Studio: Stream Analyzer can be used to predict the formation of an acid phase caused by impurities such as NOx and SOx present in dense-phase CO₂ during transportation. Survey calculations to determine the solubility of impurities in the CO₂ dense phase as well as the effect of adding additional impurities are presented. 

Disclaimer: The results presented in this article were obtained using OLI Studio: Stream Analyzer, Version 12.5. Results may vary in other software versions due to ongoing updates to the software and chemistry databanks.

Introduction

During CO₂ transportation, process impurities such as NO₂, SO₂, and H₂S can form highly corrosive acids that may lead to catastrophic failure of carbon steel pipelines or tanks. Understanding the electrolyte chemistry of these impurities enables accurate prediction of the conditions under which they react, as well as the conditions under which they remain benign. Effective simulation requires simultaneous prediction of reaction equilibria and phase equilibria in dense phases dominated by CO₂, to define safe operating windows for CO₂ transport systems.

OLI has partnered with the Norwegian Institute for Energy Technology (IFE) to develop accurate thermodynamic models representing these reactions and phase equilibria, accessible within the OLI software platform. These models predict the formation of corrosive components based on experimental work performed by IFE. The resulting parameters can be used to evaluate safe operating windows for CO₂ transportation pipelines and shipping containers.

Modeling Approach

Table 1 shows a theoretical CO₂ stream containing several impurities and the transport temperature and pressure conditions.

Table 1. Composition of CO2 stream containing impurities

*Note: It is important to point out that this stream representation assumes the system contains 1 million moles of CO₂. This construction enables the user to enter the impurities as moles directly in the OLI software to represent contaminant levels reported in units of ppm-mole. For example, in the system from Table 1, NO₂ is reported in the CO₂-dense phase at 2.5 ppm-mole, H₂O is reported at 20 ppm-mole, SO₂ is reported at 10 ppm-mole, and O₂ is reported at 10 ppm-mole, even though the values are entered in units of “mole” in the software.

Calculation Configuration

• For all calculations, the Mixed Solvent Electrolyte (MSE) model was used. 
• The following phases were enabled: 
  • vapor (Va)
  • acid-rich liquid phase (Liq1)
  • dense liquid phase (Liq2)
  • solid phase (So).
• When impurities such as NOx and SOx are present in the CO₂ dense phase, the following oxidation states should be selected:
  • For sulfur:
    • S(−2)
    • S(0)
    • S(+4)
    • S(+6)
  • For nitrogen:
    • N(−3)
    • N(+2)
    • N(+4) 
    • N(+5)

Table 2 includes a list of reactive components based on their oxidation states.

Table 2. Reactive Sulfur- and Nitrogen-Containing Components based on Oxidation State

Three sets of calculations were performed to study the same CO2 stream composition:

1. A temperature survey from 5 °C to 25 °C. This calculation enables the user to identify the composition of the acid phase that drops out of the CO₂ dense phase as temperature varies. (Calculation labeled as “Survey T-1" in the attached file.)
2. A dual temperature-pressure survey was added to calculate the solubility curve for H₂SO₄ for the given stream composition. (Calculation labeled as “Solubility Survey-1" in the attached file.)
3. A composition survey was added to analyze the effect of adding H₂S to the stream. (Calculation labeled as “Composition Survey-1" in the attached file.)

1. Temperature Survey 

Figure 1 shows the composition of the acid phase that drops out of the CO₂ dense phase based on the input data from Table 1. The predicted Liquid-1 phase contains multiple species contributing to acidity. For example, at 25 °C, H₃O⁺ (~35.7 mol%) and HSO₄⁻ (~35.5 mol%) are the dominant acid species, followed by H₂SO₄ (19%), SO₄²⁻ (0.07 mol%), and HNO₃ (1.5e-3 mol%).

Figure 1. Concentration of main species in the Liquid-1 phase that drops out of the CO₂ dense phase as a function of temperature.

2. Solubility Curves

Solubility curves for H₂SO₄, using the stream composition in Table 1 at three temperatures (5 °C, 25 °C, 50 °C), are shown in Figure 2. Each colored line represents the solubility limit of H2SO4 in the defined CO2 stream. In this case you can see that the solubility limit of H2SO4 in this stream is temperature-dependent and the solubility limit increases as the temperature increases.

Note: The OLI MSE model does not directly classify a fluid as "supercritical." Instead, based on the density of the fluid, it will assign vapor-like behavior to the vapor phase and liquid-like behavior to the liquid-2 phase. 

Figure 2. Solubility curve for H₂SO₄ for the stream composition given in Table 1.

3. Effect of H₂S

The effect of H₂S is shown in Figure 4. Note: the x-axis is displayed in moles, but in practice this corresponds to ppm (mole basis). The key takeaway is that the dominant sulfur product depends on the amount of H₂S present in the system. In other words, the total H₂S level controls whether the environment is oxidizing or reducing, which determines whether sulfur ends up primarily as sulfuric acid (S(+6)), SO₂ (S(+4)), or elemental sulfur (S(0)). Thus, the following scenarios can occur:

• Low H₂S: oxidizing environment
  • Most H₂S (S(−2)) is oxidized to H₂SO₄ (S(+6)).
• Intermediate H₂S (≈ 5-9  ppm): transition to less oxidizing conditions
  • The environment becomes progressively less oxidizing.
  • Less H₂SO₄ forms, and SO₂ (S(+4)) begins to form.
• Higher H₂S (> 9  ppm): reducing environment
  • The mixture approaches reducing conditions.
  • Almost no H₂SO₄ forms, SO₂ begins to decrease, and elemental sulfur (S(0)) starts to form.

Kinetic limitations may be possible and therefore should be considered when interpreting these equilibrium-based results. 

Figure 4. Interplay of oxidizing and reducing species and their effect on acid dropout

Conclusion

This study highlights how impurities in dense-phase CO₂ streams can significantly influence system acidity, creating conditions that may lead to corrosion during transport. By using temperature, pressure, and composition surveys in OLI Studio: Stream Analyzer, users can systematically evaluate how these variables affect acid formation and phase behavior within the CO₂ stream. These survey tools provide a structured approach for identifying corrosive regions and quantifying the impact of specific impurities for CO₂-transport system design.