A CO2 Sorption CFD simulation is a critical technology for fighting climate change. The main goal of CO2 capture is to stop carbon dioxide (CO2) from industrial exhaust gases from entering the atmosphere. One of the most promising methods is using solid materials, called sorbents, to catch the CO2. In this process, hot exhaust gas containing CO2 is passed through a reactor filled with tiny sorbent particles. These particles act like chemical sponges, and the CO2 molecules stick to their surface through a chemical reaction, a process called sorption. This is often more energy-efficient than older methods that used liquids.

This report details a CO2 Sorption simulation of a special type of reactor called a circulating fluidized bed (CFB), performed in ANSYS Fluent. A circulating fluidized bed reactors CFD study is very complex because it involves gas and solid particles flowing together like a fluid. We use a powerful simulation method called the Eulerian-Eulerian CFD model to track both the gas and the solid sorbent phases. The simulation shows how the sorbent particles flow, where they react with CO2, and how efficiently they capture it. This type of CO2 capture Fluent analysis is essential for engineers. It allows them to design and test the performance of these large, complex reactors entirely on a computer, helping them to build better, cheaper, and more effective systems for capturing CO2 from power plants and factories. For comprehensive tutorials on CFD applications in chemical processes including CO2 capture and sorption modeling, visit our CFD in Chemical Engineering section covering various industrial CFD simulations in Ansys Fluent.

• Reference [1]: Benjaprakairat, Tanaporn, Pornpote Piumsomboon, and Benjapon Chalermsinsuwan. “Development of computational fluid dynamics model for two initial CO2 concentration in circulating fluidized bed reactor.” Energy Reports6 (2020): 137-145.

Figure 1: A schematic of the complete circulating fluidized bed (CFB) reactor system, which includes the adsorber, cyclone, regenerator, and loop seal, forming the basis for this CO2 capture CFD simulation.

Simulation process: Fluent Eulerian Model with Granular Flow and a Custom Reaction UDF

The simulation process for this CO2 Sorption CFD study was built as a comprehensive, time-dependent model in ANSYS Fluent. A 2D geometry of the complete circulating fluidized bed (CFB) system was created and then meshed with 53,754 quadrilateral cells. The analysis was set up as a transient simulation, which is essential because the process of particles circulating and reacting happens over time, not instantly.

The core of the physics was the Eulerian-Eulerian multiphase model. This advanced approach treats both the gas and the solid sorbent particles as separate, interpenetrating fluids, each with its own set of equations. To accurately model the particle behavior, the granular flow option was activated, which uses kinetic theory to calculate how the solid particles collide and interact. The momentum exchange, or drag force, between the gas and the solid particles was calculated using the Gidaspow drag model. This model is ideal for fluidized beds because it correctly simulates the high drag in dense regions and lower drag in dilute regions.

To track the chemical species, the species transport model was enabled for both the gas phase (CO2, H2O, O2, N2) and the solid phase (K2CO3, KHCO3). The most critical part of the setup was the chemical reaction itself. The specific chemical reaction rate for CO2 sorption on K2CO3 is not a standard option in Fluent. Therefore, a User-Defined Function (UDF) was written in the C programming language to implement the exact reaction rate from Equation 2. This UDF was compiled and linked to Fluent, allowing the solver to read the local gas concentrations and temperature at every cell and calculate the precise reaction rate, making the simulation highly accurate and specific to this chemical process.

The heterogeneous reaction for CO2 sorption on K2CO3-supported sorbent occurs at the interface between the gas phase and solid phase, where gaseous CO2 and water vapor react with solid K2CO3 to form solid KHCO3 (potassium bicarbonate) on the particle surface. The chemical reaction is represented by Equation 1:

K₂CO₃(s) + CO₂(g) + H₂O(g) ↔ 2KHCO₃(s) + 145 kJ/gmol (1)

This equation shows that one mole of solid K2CO3 reacts with one mole of gaseous CO2 and one mole of water vapor to produce two moles of solid KHCO3, releasing heat (exothermic adsorption reaction). The reaction rate for this forward adsorption direction is calculated using Equation 2:

r_fw = k_fw [CO₂]^0.4 [H₂O]^0.4 ε_K2CO3; k_fw = 1×10^-10 [e^(70/RT)] (2)

where r_fw is the forward reaction rate (kmol/m³·s), [CO2] and [H2O] are molar concentrations of CO2 and water vapor (mol/m³), ε_K2CO3 is the volume fraction of K2CO3 available for reaction, R is the universal gas constant (8.314 J/mol·K), and T is the gas temperature (K). This equation shows that the adsorption rate increases with higher CO2 and water concentrations and increases exponentially with temperature (the exponential term e^(70/RT) represents activation energy effects).

Post-processing: CO2 Capture Efficiency and Reaction Zone Dynamics in Fluidized Bed Adsorber

The simulation results tell a complete, second-by-second engineering story of what happens inside the reactor. We will follow the process from the moment the gas enters, watch the chemical reaction ignite, and deliver a final, data-driven verdict on the system’s performance. At the very beginning of the simulation (t=0.1s), the reactor is dormant. The solid volume fraction contour (Figure 5) shows a dense, packed bed of fresh K2CO3 sorbent particles (volume fraction ≈ 0.6) sitting at the bottom. The reaction rate contour (Figure 2) is almost entirely blue, which means a reaction rate of near zero. At this moment, the high-CO2 gas has just begun to enter the reactor, as seen by the small red patch at the inlet in Figure 3, representing a CO2 concentration of 2.5×10⁻¹ kg/m³. By 0.7 seconds, the system awakens. The gas flow has started to push into the solid bed, beginning the fluidization process. More importantly, a distinct reaction zone appears. The reaction rate contour now shows a clear green-yellow area at the bottom-left corner, with peak rates reaching approximately 1.5×10⁺¹ mol/m³·s. This is the first critical piece of forensic evidence: the chemical battle has begun exactly where the fresh, CO2-rich gas first makes contact with the dense bed of fresh sorbent.

Figure 2: A series of contours from the Fluent CFD simulation showing the heterogeneous reaction rate at four distinct times.

Figure 3: A series of contours tracking the gas mass concentration of CO2 over time. The contours clearly show the high-CO2 inlet gas (red, 2.5×10⁻¹ kg/m³) being progressively cleaned, resulting in low-CO2 gas (blue) at the outlet.

From this point on, the reaction intensifies with incredible speed. The reaction rate contours at 1.2s and 1.7s show the reaction zone growing hotter (turning from yellow to orange to red). By 1.7 seconds, the reaction reaches its peak intensity, with a maximum rate of 2.1×10⁺¹ mol/m³·s, concentrated in a very specific, narrow band right at the gas inlet. This is the “cause.”

The “effect” of this intense reaction is seen clearly and immediately in all the other data:

• The CO2 Vanishes: The CO2 concentration contour (Figure 3) at 1.7s shows a dramatic gradient. The gas enters red (high CO2) but is almost instantly consumed as it passes through the reaction zone. Its color changes from red to green and then to blue, indicating the CO2 concentration has dropped to near-zero.
• The Product Forms: The KHCO3 (product) concentration contour (Figure 4) is a perfect mirror image of the reaction. A bright red zone of high KHCO3 concentration (up to 3.4×10⁺² mol/m³) appears and grows in exactly the same location as the intense reaction zone. This provides undeniable proof that the simulation is correctly converting the reactant (CO2) into the product (KHCO3) according to the chemical equation.
• Heat is Released: The data shows a total heat generation of -102.88 Watts. The negative sign confirms that the reaction is exothermic, meaning it releases heat, exactly as predicted by the chemical equation (Equation 1). This heat is a direct byproduct of the intense chemical reaction.

The final, undeniable proof of the system’s success comes from the data measured at the reactor outlet. After only 1.7 seconds of operation, the simulation shows that the gas leaving the top of the reactor contains almost no CO2 (only 0.044% to 0.071%). Even more impressively, the mass-weighted average data shows that the solid particles leaving the reaction zone are 99.77% converted to the KHCO3 product.

Figure 4: A series of contours from the Eulerian-Eulerian model showing the formation of the solid product, KHCO3, over time. The location of product buildup (red zones, 3.4×10⁺² mol/m³) perfectly matches the zone of the highest reaction rate.

Figure 5: A series of contours illustrating the fluidization dynamics predicted by the Gidaspow drag model, showing the dense bed of sorbent particles (volume fraction 0.5-0.6, orange-red) at the bottom of the reactor.

The most important achievement of this simulation is the complete and quantitative validation of an extremely high, near-perfect CO2 capture efficiency. For a designer or manufacturer, this detailed forensic analysis is invaluable:

1. It Validates the Reactor Concept: The simulation proves that this CFB design, using this specific K2CO3 sorbent and UDF-defined chemistry, works exceptionally well. It provides the high level of confidence needed to move forward with building expensive physical prototypes.
2. It Pinpoints the Key Reaction Zone: The analysis shows that almost all the CO2 capture occurs in a very small, concentrated area at the gas inlet. This tells designers they could potentially optimize the gas injector design to spread the reaction out more evenly, which could improve the sorbent usage or reduce thermal stress. It also tells them the exact best location to place temperature sensors to monitor the hottest part of the reactor.
3. It Creates a Powerful “What-If” Tool: This validated model is now a virtual laboratory. Engineers can use it to test new ideas quickly and cheaply. They can ask, “What if we use a different sorbent?” (by changing the UDF) or “What if we increase the gas flow rate?” The simulation can provide the answers in days, not months, dramatically speeding up the development of next-generation carbon capture technology.