Acid Solubility and Dropout Formation in CO2 Systems

Table of Contents

Objective

This article provides a step-by-step guide on how to configure a few cases in OLI Studio to simulate acid dropout in CO₂ transport systems caused by reaction between key CO₂ impurities. It also demonstrates how OLI Studio can be used to evaluate the solubility of acid in CO₂-rich phases (gas or liquid), both before and after enabling redox.

Users will learn how to:

• Define a CO2 transport composition with impurities
• Calculate the (acid) dropout quantities and its chemistry
• Evaluate the solubility of acid in rich-CO2 phase (with or without redox enabled)

Disclaimer

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

Background 

Carbon Capture, Transportation, Utilization, and Storage (CCTUS) systems are essential for reducing CO₂ emissions. While dry, pure CO₂ poses no corrosion risk, captured CO₂ often contains impurities. Reactive impurities, especially H₂O, H₂S, NO₂, SO₂, and O₂, can interact during transport to form aqueous or solid acid phases that coalesce out of the dense CO₂ phase. These coalesced phases, known as “dropout,” can cause severe corrosion in CCTUS infrastructure. Modeling the formation and chemistry of such acid dropouts is therefore critical for safe CO₂ transport. In this article, you will learn how to use OLI Studio to calculate acid dropout quantities and their chemistry, as well as evaluate the solubility of the dropped-out acid from CO₂-rich phase, both with and without redox enabled. 

Calculations in CO₂ Transport

This article provides some of the commonly used calculations in OLI Studio V12.5 for CO₂ transport applications. It helps users understand how to calculate the amount of acid dropout (L1) that may form from a CO₂ stream (L2 or Va) at a given transport pressure and temperature and identify the chemistry of that dropout formed due to reactions between impurities. The article also shows how to determine whether acid dropout occurs, quantify how much sulfuric acid or other acids separate from the rich CO₂ phase, and evaluate the amounts of acid or other impurities that remain in the liquid (L2) or vapor phase (no dropout).

Note: In this article, all simulations assume that reactions proceed to equilibrium under thermodynamic control. An alternative approach for simulating dropout formation exists when the process is not governed by redox equilibrium but is instead controlled by a rate-limiting kinetic reaction step. Users interested in implementing a kinetically controlled reaction as the controlling step can refer to this article, which explains kinetic-controlled acid dropout simulation using OLI Studio V12.5. Depending on the system and modeling objectives, users can choose the appropriate approach (thermodynamic non-redox, redox, or kinetic), to simulate the dropout behavior.

Example 1: Dropout Formation (Redox Equilibria) 

In this example, CO₂ containing the impurity composition shown in the table below is transported under pipeline conditions:

Table 1. CO₂ stream composition transported under pipeline condition

Entering Data for CO₂ Impurities Input 

Procedure

❶ Open OLI Studio and add a Stream and rename it to ‘Dropout Simulation’

❷ Keep the units in Metric and enter the composition of the impurities and Stream Parameters given in Table 1 in Inflow section and keep the Inflow units in (mol) and make sure the MSE Databank is selected

❸ Add CO₂ to the Inflow stream and enter the value of 1E6 mole in the CO₂ cell, this will make the impurity concentrations in approximate ppm-mol

Note: We entered the inflows in moles and specify a total of 1 million moles of CO₂. This allows us to input the composition of the other impurities to represent ppm-mol, where CO₂ is the solvent. 

Oxidation States and Phases to Consider

Because rich-phase CO₂, whether liquid or vapor, can form a non-aqueous organic liquid under certain transport conditions, and because an aqueous phase or even solids may separate from the CO₂, the OLI’s Mixed Solvent Electrolyte (MSE) model is well suited to represent these multiphase behaviors and their associated speciation. Therefore, the calculations should include all phases: solid, vapor, aqueous liquid, and non-aqueous liquid CO₂).

❹ Click on the ‘L2’ phase to include liquid CO₂ phase into the calculations. Make sure that ‘L1’, ‘Va’, ‘So’ are also included in the calculations

Note: ‘L1’ phase represents this aqueous acidic (or sometimes non-acidic) dropout, the ‘L2’ phase represents the non-aqueous liquid CO₂, ‘Va’ represents the gas phase CO₂, and ‘So’ represents the solid phase.

Since the CO₂ dropout is presumably formed through reactions among the impurities, the calculations should account for redox reactions in addition to standard equilibrium reactions.

❺ Enable the Redox (Re), Go to Chemistry > Model Options > Redox > expand Nitrogen and select N(-3), N(+2), N(+4), and N(+5), representing ammonium-bearing species, NO, NO₂, and nitrate-bearing chemistries such as nitric acid, respectively, assuming that, based on experimental evidence, the presence of such species is supported. Then, Expand the Sulfur and select S(-2), S(0), S(+4), and S(+6), representing sulfide-bearing species such as H₂S, elemental sulfur, SO₂, and sulfate/bisulfate-bearing chemistries such as sulfuric acid, respectively.

❻ Go to the Add Calculation button and select Single Point calculation Change the SinglePoint name to ‘Acid dropout’ using the <F2> key and Select Isothermal as Type of Calculation

❼ We are ready to perform the calculation. Click on the Calculate button or press the <F9> key. It is time to save your file (File > Save as…) or using the save icon in the tool bar

Data Analysis 

Note: It should be noted that with Inflows defined as 1×10⁶ mole units of CO₂ and impurities, the following result reflects all values in ppm‑mol units.

❽ After completion of calculations, review the Summary Box. The calculated amount of Acid Dropout is displayed in the Summary Pane, Phase Amounts section as the Aqueous phase quantity, with a value of ~ 156 ppm-mol acid dropout (156 mole per 1×10⁶ mole CO₂). In other words, 156 ppm-mol of L1 (aqueous) is predicted to coalesce from the second liquid phase (L2 or liquid CO₂). For this particular case, no vapor or solid phase is predicted.  

❾ By selecting the Report tab and scrolling down to Species Output (True Species) Table, you can view the full speciation and composition of the dropped-out liquid, which is predominantly composed of bisulfate and hydronium species, making it strongly acidic.

Example 2: Acid Solubility (Without Redox)

Upon the formation of sulfuric acid dropout, it is critical to quantify how much of that acid remains soluble in the rich CO₂ phase, whether in the liquid CO₂ (L2) or the gas CO₂ (Va). There are two approaches for modeling acid solubility: one prior to impurity reactions (no redox) and one following cross-chemical reactions among the impurities forming dropout (with redox). In this example, we present the first scenario for a simple CO₂ and sulfuric acid mixture to demonstrate how to determine the solubility of the acid in a total of 2 moles of a mixture containing 63 wt.% H₂SO₄ and 34 wt.% CO₂ over a pressure range of 1 to 200 atm at two temperatures, -25 °C and 25 °C, without redox.

Procedure

❶ Open OLI Studio and add a Stream and rename it to ‘Acid-Solubility’

❷ Click on Inflow units and change the Inflow variables unit to Mass Fraction. Enter 63 wt.% H₂SO₄ and 34 wt.% CO₂ and make sure the MSE Databank is selected

❸ Click on the ‘L2’ phase to include liquid CO₂ phase into the calculations. Make sure that ‘L1’, ‘Va’, ‘So’ are also included in the calculations and make sure that Redox (Re) is OFF

❹ Go to the Add Calculation button and select Survey calculation Change the Survey name to ‘Acid-Solubility-No-Redox’ using the <F2> key and Select Isothermal as Type of Calculation

❺ Click on the Survey By button and select Pressure and Then by Temperature (as shown below)

❻ Click on the Specs button. This will open the Survey Options window and now click on the Survey Range tab. Enter the Pressure range from 1 to 200 atm, by Number Steps 100 and Click OK

❼ The next step is specifying the Temperature range. Go to the Specs button next to the Temperature option. This will open a new Survey Options window and select the Point List option to create two points with values of -25 °C, and 25 °C. Then Click OK.

❽ Click on the Calculate button or press the <F9> key. It is time to save your file (File >Save as…) or use the save icon in the tool bar

Data Analysis

❾ Click on the Plot tab and Click on the Variables button which will open the Select Data to Plot window. Double click or use the << button to remove the pH variables and look for MBG Totals – Liquid-2 and click on the  box to show all the available variables

❿ Select S(+6) Liq2 and put it in the Y1 Axis using the >> button

⓫ Look for MBG Totals – Vapor and click on the  box to show all the available variables and select S(+6) Vap and put it in the Y1 Axis using the >> button

Note: S(+6) Liq2 and S(+6) Vap represent sulfuric acid soluble in liquid CO₂ and gas CO₂, respectively

⓬ Then click OK.  To see the plot more clearly, right click on any number in the Y axis, and select the Logarithmic Scale option

Note: It should be noted that with Inflows defined as mole units, the following plots reflect all values in mole units

The resulting plot shows the mole solubility of sulfuric acid in rich CO₂ phase either in its gas (olive green color curves) or liquid state (turquoise color curves).

Example 3: Acid Solubility (With Redox)

In this example, we are going to use the inputs and parameters from the table below to create the stream’s composition and calculate the solubility of sulfuric acid formed due to the reaction between impurities (or with Redox enabled) in two different temperatures of -25 °C and 25 °C.

Table 2. Impurity composition for calculating the dropout solubility in rich-CO₂ phase

Follow steps ❶ through ❺, as explained in Example 1, to create the stream composition shown in Table 2.

Then follow steps ❺ through ⓬, as explained in Example 2, to create a Survey, run the Calculation, and plot S(+6) Liq2 and S(+6) Vap, representing sulfuric acid soluble in liquid CO₂ and gas CO₂, respectively, on a Y-Logarithmic Scale, and save the results.

Data Analysis

Note: It should be noted that with Inflows defined as 1×10⁶ mole units of CO₂ and impurities, the following plots reflect all values in ppm‑mol units.

The resulting plot shows the ppm-mol solubility of sulfuric acid dropout in rich CO₂ phase either in its gas (olive green color curves) or liquid state (turquoise color curves). This is the sulfuric acid drop out formed due to the reaction among impurities with redox enabled.

Conclusion

This article guides users through some of the common thermodynamic equilibria calculations for CO₂ transport applications in OLI Studio V12.5. It demonstrates how to simulate the acid dropout formation from a given CO₂ stream, evaluate the acid solubility in dense phase CO₂, quantify sulfuric acid or other phases dropping out of the rich CO₂ phase and evaluate the composition of those dropouts.

Additional Resources

For more information on modeling carbon capture, transportation, and storage processes in OLI, please refer to the following Support Center articles:

• User-Defined Kinetics of Reaction Between Impurities in CCTUS
• Best Simulation Practices for CO2 Transport & Injection Chemistry
• CO2 Impurities Available in MSE and MSE-SRK