User-Defined Kinetics of Reaction Between Impurities in CCTUS

Table of Contents

Objective

Background

Acid Dropout Under the Control of Kinetics

• Entering Data for CO2 Impurities Input
• Phases to Consider
• Defining the Kinetic Correlations and Variables
• Data Analysis

Conclusion

Additional Resources

Objective

This article provides a step-by-step guide on how to configure a user-defined kinetic case in OLI Studio to simulate acid dropout formation due to reactions between key CO2 impurities in CO2 transport systems.  

Users will learn how to:

• Define a CO2 transport composition with impurities
• Set up the kinetic reactions in the OLI Studio
• Define the kinetic correlations and variables
• Calculate the dropout quantity and its chemistry under the control of the kinetic reactions

Disclaimer

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

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. 

Acid Dropout Under the Control of Kinetics

If the rate-controlling reactions governing the dropout in the CCTUS system are known, users can enter these reactions and their kinetic parameters to predict dropout. This article assumes that experimental evidence identifies the key reactions controlling acid or sulfur dropout, allowing users to apply their own kinetics rather than relying on thermodynamic redox equilibrium calculations.

In this example, we assume that a user with field experience has clear experimental evidence that the following reactions are the main steps controlling acid or elemental sulfur dropout. Therefore, instead of relying on thermodynamic redox, the user wants to implement the following reaction kinetics into the software:

In this example, CO2 with the impurity composition shown in the table below is transported under pipeline conditions. The user wants to use OLI Studio to evaluate, after 50 hours, whether any dropout occurs due to reactions between the impurities in the CO2 stream.

Table 1. CO2 stream composition transported under pipeline conditions

Entering Data for CO2 Impurities Input

Procedure

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

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.

Note: For processes that include reaction kinetics, the software needs to know how the concentrations of all reactants and products change over time. This is determined by the rate law. Because of this, every species that appears in kinetic reactions must be listed in the Inflow. For example, in this case NO, SO3, and elemental sulfur (S8) are products of the reactions. Even if they are not part of the initial Inflow, they still need to be added to the Inflow list with an initial amount of zero so the software can track their formation during the simulation.

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, 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.

❺ Make sure that the Redox button (Re) is OFF (do NOT enable it), because the user will be defining their own redox reactions. 

❻ To enable reaction kinetics,  go to Chemistry > Model Options. This will open the Chemistry Model window. Then, select the Phases tab and check the Kinetics box. Then click OK.

❼ Go to the Add Calculation button and select Single Point calculation and change the SinglePoint name to ‘Kinetic’ using the <F2> key. Select Isothermal as Type of Calculation and change the Kinetics Holdup Time to 50 hours and the Number of Kinetic Steps to 10

Figure 1. Enabling kinetics in OLI Studio

Setting Up the Reactions in OLI Studio

❽ Now, our next step is to define the reaction kinetics. Click on the Specs button. This will open the Calculation Options window. Then, select Kinetics under the Category panel. Click the Add button. This will open a Select a Reaction window. Double click on <Create New Reaction> to add the abovementioned three reactions one by one in the ‘Reactions:’ section as shown below: 

❾ Type in the reactions one by one and select SPEC as the Rate Specification for each, as in this example, we want to define our own rate expression. 

Note: The other option for Rate Specification is STD (Standard Rate Reaction Kinetics), for Arrhenius type of kinetics (not used in this example). For more information, please refer to our Support Center article.

Figure 2. Reactions entered in the Calculation Options - Kinetics window

Defining the Kinetic Correlations and Variables 

Now, we are ready to define the rate expressions in the ‘DEFINES:’ section for each individual reaction. For this example, the following rate law is used to describe the reaction rate:

 where kf and kr represent forward and reverse rate constants, and ar and ap denote activities of reactants and products raised to their corresponding reaction coefficient (ri and pi, respectively).

Note: We can define any variable, though some naming conventions are applied. Any concentration variable such as [SO2aq] is defined as the natural log and is designated with the letter “L”. So [SO2aq] is designated as LSO2AQ. Similarly, activity coefficients γ are also defined on  the natural log basis. So, γSO2 is written as LogeγSO2 = ASO2AQ. Therefore, we now need to add these variables to the kinetics window in the form of:

FRXNn = Forward reaction rate =

RRXNn = Reverse reaction rate =

For example, for reaction (3), the forward and reverse reaction rate should be defined as:

FRXN3 = LH2SAQ+AH2SAQ+0.5*(LO2AQ+AO2AQ)

RRXN3 = 0.125*(LSULFURELAQ+ASULFURELAQ)+LHO2AQ+AHO2

Note: For a reaction rate to be considered in the program, your set of variables should include a RATEn statement where the “n” is the reaction rate equation number

Note: The variable VOLLIQ is the volume of the liquid phase in Liters. Since OLI requires the rate to be in mol/m3 units, we need to divide the volume by 1000. Therefore, multiply your final rate expression by the (VOLLIQ/1000) factor. For each reaction, the Expression inserted in the ‘DEFINES:’ section should be in the form of:

RATEn = (KFn*EXP(FRXNn)-KRn*EXP(RRXNn)))*VOLLIQ/1000

For example, for reaction (3), the net reaction rate should be defined as:

RATE3 = (KF3*EXP(FRXN3)-KR3*EXP(RRXN3))*VOLLIQ/1000

Therefore, the kinetic expressions for FRXNn and RRXNn (forward and reverse reaction rates) need to be defined by users. Similarly, KFn and KRn (forward and reverse reaction rate constants), need to be specified by the user. A summary of the Reaction Rate Parameters  for this example  is shown in Figure 3. Please note, these parameters are case-specific for this example and should not be used for other design work. 

Figure 3. Calculation Options – Kinetics windows for each of the three reactions governed by kinetics in this example

After inserting all reactions and the corresponding rate expressions and variables, click the OK button.

Click on the Calculate button.

Data Analysis

Once the calculation is complete, you can click the Output tab and add the Liquid-1 and Solid sections (not molecular aqueous). This is the composition of the dropout. In this case, both liquid sulfuric acid and solid elemental sulfur drop out of liquid CO2. 

Also, based on the information in the Summary pane,  about 0.0467 ppm-mol of aqueous phase (L1) and 3.74 ppm-mol solid (elemental sulfur) dropped out. That 0.0467 ppm-mol L1 is mainly composed of elemental sulfur, hydronium, sulfate and bisulfate ions, as shown in the Liquid-1 Output section. 

Note:  With Inflows defined with respect to 1×106 mole units of CO2, the results reflect values in ppm‑mol units.

Figure 4. Isothermal calculation results with kinetics applied

Calculation without Kinetics

Add another single point isothermal calculation to the existing stream, rename it to ‘Redox-No-Kinetic’ and uncheck the Kinetics setting from the Chemistry model.   In place of kinetics, click the Redox button (Re) in the OLI Studio Tool Bar. Next, go to Chemistry > Model Options > Redox > expand the Nitrogen and Sulfur options. Enable N(3-), N(+2), N(+4), and N(+5) for Nitrogen and S(-2) , S(0), S(+4), and S(+6) for Sulfur. 

After running the calculation, please review the results in the Output tab (see Figure 5). Here, no kinetic reactions were considered; instead, only redox equilibria were considered. 

Figure 5. Calculation results with thermodynamic redox (no kinetic reactions)

In the previous calculation, the reaction kinetics constrained the thermodynamic equilibria, which provide worst-case scenarios. This resulted in much lower predicted acid dropout compared to the thermodynamic redox calculation. The thermodynamic redox predicts a dropout of 127 ppm-mol acid dropout with no solid formation.

Conclusion

This article describes how to configure kinetic reactions in OLI Studio and how to define the kinetic correlations and variables that govern impurity-induced dropout formation in CCTUS systems.

Additional Resources

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

• Best Simulation Practices for CO2 Transport & Injection Chemistry
• CO₂ Impurities Available in MSE and MSE-SRK
• Understanding CO2 Transport Chemistry using OLI Studio: Stream Analyzer
• Material selection for CO2 injection scenarios in saline aquifers using OLI Studio: Corrosion Analyzer