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CFD Modeling on Hydrodynamic Characteristics of Multiphase Counter-Current Flow in a Structured Packed Bed for Post-Combustion CO_{2} Capture

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## Abstract

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_{2}capture is a promising technology for industrial application. Gas-liquid interfaces and interactions in the packed bed are considered one of the key factors affecting the overall CO

_{2}absorption rate. Understanding the hydrodynamic characterizations within packed beds is essential to identify the appropriate enhanced mass transfer technique. However, multiphase counter-current flows in the structured packing typically used in these processes are complicated to visualize and optimize experimentally. In this paper, we aim to develop a comprehensive 3D multiphase, counter-current flow model to study the liquid/gas behavior on the surface of structured packing. The output from computational fluid dynamics (CFD) clearly visualized the hydrodynamic characterizations, such as the liquid distributions, wettability, and film thicknesses, in the confined packed bed. When the liquid We (Weber number) was greater than 2.21, the channel flow became insignificant and flow streams became more disorganized with more droplets at larger sizes. The portion of dead zones is decreased at higher liquid We, but it cannot be completely eliminated. Average film thickness was about 0.6–0.7 mm, however, its height varied significantly.

## 1. Introduction

_{2}) concentrations, which is attributed to the anthropogenic emissions from the use of fossil fuels, and the use of coal alone accounted for 43% of global CO

_{2}emissions [1]. Post-combustion CO

_{2}capture using solvent-based CO

_{2}absorption technologies is one of the most promising approaches among the current CO

_{2}capture technologies and may be implemented at the large-scale in power generation [2]. A schematic diagram of a solvent-based CO

_{2}absorption process is presented in Figure 1. It includes an absorber, a stripper, and their necessary auxiliary equipment for an aqueous solvent circulating between the two reactors [3]. Both reactors typically use a packed bed to improve the mass transfer by providing an effective contact area between gas and liquid phase reactants [4] due to relatively slow reaction kinetics. The packing materials are in many different forms, but they can be generally divided into random and structured packing. The structured packing can provide a similar effective surface area but much less pressure drop compared to random packing, making it a good candidate for application in post-combustion CO

_{2}capture processing, and is investigated herein [5].

_{2}capture. For example, a CO

_{2}absorber capable of handling 90% capture of CO

_{2}from a 0.7 MWe power system would be approximately 19.5 m in height and have at least 13.7 m of structure packing [6,7]. Therefore, understanding the hydrodynamic characterizations within packed beds is of the essence, which will provide insights for gas/liquid mixing enhancement, channel flow prevention, and surface wettability estimation, and then develop techniques to enhance the mass transfer by intensifying the gas/liquid contact and mixing.

_{2}absorption processes are typically two-phase counter-current unstable flow, and current simulations are insufficient to provide a clear understanding of the dependencies for their 3D hydrodynamics in structured packing. In order to reduce computing complexity, researchers [6,16] used a porous media zone to simulate structured packing. This method could provide certain information in the simulation and design of new geometries. However, it cannot precisely present the interactions between the two phases under counter-current flow. Thus, no information can be acquired to enhance the gas/liquid mixing. Table 1 lists representative CFD models in structured-packing bed hydrodynamic study.

_{2}capture reactions is to build a true packing geometry instead of a porous media configuration, and consider the solvent properties along the packing length. In this study, a 3D CFD model will be built to understand the hydraulic characterizations in the structured bed, and to provide insights to estimate the surface wettability, prevent the channel flow, and identify the way to enhance the gas/liquid mixing. These results expand the CFD model for efficient industrial application.

## 2. Computational Model

#### 2.1. Governing Equations

#### 2.2. Geometry

_{2}absorber processes [28]; the number 250 in its designation indicates a specific surface area of 250 m

^{2}m

^{−3}and the symbol Y means a surface inclined angle of 45° relative to the flow direction. Mellapak 250 Y is made of corrugated metal sheets arranged side-by-side with opposing channel orientations, as illustrated in Figure 2a,b. To save computational time, one slice was chosen as the modeling geometry, thus, a “sandwich-like” packing was built, as shown in Figure 2c.

#### 2.3. Meshing

#### 2.4. Turbulence Model

^{3}), $v$ is its velocity (m/s), and $\mu $ is the viscosity (kg/m/s).

#### 2.5. Boundary Conditions

## 3. Results and Discussion

#### 3.1. Validation of Simulation Model

_{G}is gas velocity.

_{L}is the liquid velocity in m/s, $a$ is the specific surface area in m

^{2}m

^{−3}, g is the gravity in m/s

^{−2}, and $\epsilon $ is the void fraction in %. Liquid holdups using the developed CFD model at low liquid flow rates and three different gas flow rates were compared to this empirical equation as presented in Figure 6 and show that different gas flow rates had only a very weak effect on liquid holdup, a result consistent with data from the empirical model. Such a weak dependency on gas flow rates may be a result of the liquid film becoming thinner as the gas flow rates were increased. As a consequence of the agreement between the 3D simulations and empirical model results, the CFD model for structured packing was considered validated.

#### 3.2. Liquid Distribution and Surface Wettability

^{3}), $v$ is the fluid velocity (m/s), $l$ is its characteristic length—typically the droplet diameter (m), and $\sigma $ is the surface tension (N/m) of the liquid.

_{2}capture mass transfer. Further increasing We to 5.13, the flow pattern did not change too much besides increasing the liquid fraction in the packing space compared to We = 2.21, resulting in higher pressure drop which kept on increasing with the increase of gas velocity at this condition, as shown in Figure 8, and implied the potential flooding as demonstrated by the packing operating curve obtained experimentally.

#### 3.3. Formation of Droplets

_{G}= 1431 and We = 5.13. At t = 0.011 s, the liquid reached the packing surface and flowed along its 45° trajectory. At t = 0.058 s, the liquid wetted the first element and began to flow onto the second element. Several droplets are seen to have been formed at t = 0.058 s at the frontier of the flow stream and turning point of the two packing elements; the area of the neck is smaller than above or below it and, as a consequence, the gas flow velocity is higher in the neck. By t = 0.134 s, the second element had been wetted, and by t = 0.232 s the liquid had flowed throughout the whole domain; droplets can be found in every element region by t = 0.232 s.

#### 3.4. Film Thickness

_{G}= 1431. When We was below 2.21, the packing surface was not fully wetted and the film thickness was not uniform; this result indicated that surface tension dominated the liquid flow. When We was increased to 2.21 and beyond, liquid film increased in thickness, and the film thicknesses became more uniform at We = 2.21. However, as also shown in Figure 13, the liquid film became less uniform with significant protrusions at We = 5.13, which implied strong interactions between the gas and liquid phases. This also implied that the film thickness will be influenced by the time, gas flow rates, and liquid flow rates. However, the average film thickness can be estimated using this model. Figure 14 shows the influence of gas flow rates and liquid flow rates on the average film thickness where film thicknesses were between 0.6–0.7 mm. Larger liquid flow rates created thicker liquid films; for example, the film thickness was near 0.62 mm at We = 0.57 and increased to near 0.7 mm when We = 5.13. However, changing the gas flow rates had no significant effects or result in any consistent trends to arise on film thickness. However, with increasing gas flow rates, the liquid flow will become unstable.

## 4. Conclusions

_{2}capture process. The model was validated by experimental data in the public domain. The hydrodynamics in a structured packing under various flow conditions were examined and discussed. Inertial forces and surface tension affecting the flow patterns, and a higher We number leads to a more uniform liquid distribution, high wettability, and a minimum probability for channel flow. However, a large We number (e.g., high liquid flow rate) may lead to flooding. The wetted surface area and average film thickness were greatly affected by the liquid flow rate while slight effects by the gas flow rate were observed. However, increasing the gas flow rate will enhance the gas–liquid interactions, leading to increased liquid film instability and large amount of liquid droplet generation. Average film thickness was about 0.6–0.7mm. The unparalleled counter flow direction of the gas and liquid is the main reason for droplet formation, where it is more obvious at the turning point of two packing elements.

## Author Contributions

## Funding

_{2}Capture and Geological Storage, Jiangsu Province (China University of Mining and Technology) (NO: 2017A02).), and by the Natural Science Foundation of Jiangsu Province (BK20180645), the author also thanks the support from the Key Project of Scientific Research Frontiers of CUMT (2017XKZD02).

## Conflicts of Interest

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**Figure 1.**Schematic diagram of a post combustion solvent-based CO

_{2}capture process. MEA means monoethanolamine; HEX means heat exchanger.

**Figure 2.**Images of the Mellapak 250 Y structured packing within a bed: (

**a**) overview; (

**b**) top view; and (

**c**) the geometric model.

**Figure 5.**A comparison of pressure drops from Stichlmair’s empirical model and the CFD simulation results for different gas flow rates: (

**a**) $R{e}_{G}=911$, (

**b**) $R{e}_{G}=1106$.

**Figure 7.**Liquid distribution for structured packing at different liquid We numbers from 0.57 to 5.13 and fixed $R{e}_{G}=1431$.

**Figure 9.**Wettability for structured packing at different liquid We numbers from 0.57 to 5.13 and fixed $R{e}_{G}=1431$.

**Figure 11.**Flow development with time at $R{e}_{G}=1431$ and We = 5.13 (red color represents 100 vol. % of liquid and blue color represents 100 vol. % of gas).

**Figure 12.**Gas and liquid flow vectors at $R{e}_{G}=1431$ and We = 5.13. (

**a**) The velocity vectors in the whole domain. (

**b**) Velocity vectors in a neck area or element channel within the packing.

**Figure 13.**Film thicknesses as a function of liquid flow rates at constant $R{e}_{G}=1431$ (a red color represents 100 vol. % of liquid and blue color is 100 vol. % of gas).

Reference | Geometry Models | Contribution |
---|---|---|

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Haroun et al. (2014) [19] | 3-D Vertical Plate Model | Prediction of effective area and liquid hold-up in structured packings. |

Ding et al. (2015) [20] | 3D Structured Model | Characterized the pressure drop trend. |

Fourati et al. (2013) [21] | 1D Model | The Eulerian two-fluid framework with user-defined functions and associated models are taken into account in the liquid dispersion model. |

Haroun et al. (2012) [22] | 2-D Structured Packing Model | Studied mass transfer and liquid hold-up in a 2D cross section structured packing using the VOF method. |

Boundary | Materials | Type | Value | Velocity | Reynolds Number | Weber Number |
---|---|---|---|---|---|---|

Liquid inlet | Water | Mass Flow Rate | 12.2–48.8 m^{3}/(m^{2}·h) | 0.015–0.6 m/s | 15–600 | 0.02–5.13 |

Gas inlet | Air | Velocity m/s | 0.5–1.1 m/s | 0.5–1.1 m/s | 325–1200 | - |

Liquid outlet | - | Pressure-outlet | 0 Pa | - | - | - |

Gas outlet | - | Pressure-outlet | 0 Pa | - | - | - |

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**MDPI and ACS Style**

Yang, L.; Liu, F.; Saito, K.; Liu, K.
CFD Modeling on Hydrodynamic Characteristics of Multiphase Counter-Current Flow in a Structured Packed Bed for Post-Combustion CO_{2} Capture. *Energies* **2018**, *11*, 3103.
https://doi.org/10.3390/en11113103

**AMA Style**

Yang L, Liu F, Saito K, Liu K.
CFD Modeling on Hydrodynamic Characteristics of Multiphase Counter-Current Flow in a Structured Packed Bed for Post-Combustion CO_{2} Capture. *Energies*. 2018; 11(11):3103.
https://doi.org/10.3390/en11113103

**Chicago/Turabian Style**

Yang, Li, Fang Liu, Kozo Saito, and Kunlei Liu.
2018. "CFD Modeling on Hydrodynamic Characteristics of Multiphase Counter-Current Flow in a Structured Packed Bed for Post-Combustion CO_{2} Capture" *Energies* 11, no. 11: 3103.
https://doi.org/10.3390/en11113103