(154a) Modeling and Simulation of Fixed-Bed Reactors Made of Open-Cell Metallic Foam Pellets

Fixed-bed catalytic reactions – exothermic or endothermic – are widely used in the chemical and process industries. They are effectuated by the slender tubes filled with catalyst pellets and enwrapped with the heat transfer media, e.g. circulating gas, to either supply or remove the heat of reaction.¹ To guarantee stable operation and high productivity in a commercial-scale setup, the selection of an adequate catalyst pellet type is of prime important, which is based on the needs such as lower pressure drop, improved heat transfer and high mechanical stability. Indeed, these requirements have been identified as of contrasting nature, therefore, challenging to satisfy the entire demands with the commonly used ceramic type of pellets.²

In recent years, the advancement in the manufacturing technology has triggered the production of alloyed metallic foam pellets (see Fig. 1a), which are stable even in the high temperature environment.³ Due to the superior properties as high porosity, good mechanical stability, global thermal conductivity and the like; the metallic foam pellets have been considered as a beneficial alternative to the conventional ceramic pellets, since they encourage less energy consumption and process efficiency, by virtue of lower pressure drop and improved heat transfer. The methods of process enhancement in the fixed-bed reactor should be encouraged, as it is heavily employed in many relevant large-scale processes, e.g. production of hydrogen by the SMR process. Hydrogen as a green energy is being prefigured as the solution to many issues related to climate change.

However, the tortuous inner structure of metallic foam pellets added with considerable internal flow causes a complex flow field in a packed bed arrangement compared to non-porous pellets. Additionally, for the case of slender packed beds, the reactor wall imparts particle-ordering-effect in the form of variable radial void fraction with the highest bed voidage very close to the wall, followed by a pattern of several maxima and minima from the reactor wall to the center core of the bed. The transport phenomena as well as heat transfer mechanism are dependent on the local flow behavior, which is regulated by the catalyst shape and its arrangement in a packed bed structure. Therefore, it is important to optimize the shape of the metallic foam pellet that is suitable for particular reaction conditions. Despite the fact, a systematic study pertaining to fixed-beds made of metallic foam pellets is still missing, which discourage the manufacturers for its mass production. Recently, Wehinger et al.⁴ proposed a novel particle-resolved CFD approach to analyze the fixed-bed reactors composed of metal foam pellets. The objective of this contribution is to develop a detailed CFD workflow to investigate the flow characteristics and the heat transfer performance of a packed bed made of metal foam pellets. This modeling framework can act as a suitable plug-in of a Digital Twin, which provides a virtual environment to explore the adequacy of any pellet shapes before any decisions concerning manufacturing; reduces the risk in costly investments.

Methods

The particle-resolved CFD approach that accounts for the actual bed geometry was used. The fluid flow throughout the interstitial regions was fully resolved, whereas flow through the porous foam pellets was considered by the porous-media model. The pressure drop at each pellet level was modeled as per the Lacroix equation⁵, and the effective thermal conductivity by the Schuetz-Glicksman model.⁶ A realistic bed structure was generated with the aid of Rigid Body Dynamics algorithm, integrated with the animation software Blender. The CFD simulations were carried out with Simcenter STAR-CCM+ from Siemens Industry Software Inc. Fig. 1c illustrates the proposed CFD workflow.

A suitable experimental setup was also realized to validate the CFD model, as schematically shown in Fig.1b. The steel reactor has an inner diameter of 69 mm and a height of 2.5 m. The pre-heated Nitrogen gas was fed to the top of the reactor, and is cooled while flows through the bed by the cooling oil circulating along the reactor wall. A thermowell equipped with 16 thermocouples was placed at the center core of the bed to measure the axial temperature from the top of the bed to the bottom. The experiment was carried out at different combinations of flow rates and inlet gas temperature, 250 < Re_p < 2500. The temperature data corresponding to the steady-state condition were recorded.

Results and Discussion

Figs. 2a and 2c show the local velocity field and the temperature distribution in a vertical sectional plane, respectively. The local rise in the velocity along the interstitial regions, and the channelling effects near the wall – reactor and thermowell – are clearly visible. As the porous foam pellets allow flow through the pellets, the local velocity fluctuations in the foam packed beds are lower compared to the solid pellets (not shown here). The temperature distribution in a packed bed is dependent on a number of factors such as effective pellet conductivity, lateral convective mixing, magnitude of flow channelling near the wall, etc. Moreover, the significance of each heat transfer mechanisms – conduction, convection and radiation – depends on the volumetric flow rate and the operating temperature. Fig. 2d shows the comparison of the measured and the predicted axial temperature along the center-axis of the bed, at a flow rate of 45 nm³/h and the inlet gas temperature of about 378 K. The simulated temperature profile shows an excellent agreement with the experimental data. As shown in Fig. 2c, the predicted pressure drop is also in good agreement with the experiment. To come up with an absolute CFD model, the validation process will be carried out for a wide range of operating conditions, and appropriate tuning in the modeling approach will be made, if necessary. Subsequently, the validated CFD model will be used to explore the optimal shape of metal foam pellet.

Conclusion

An adapted particle-resolved CFD approach integrated with Rigid Body Dynamics was used to virtually explore the transport phenomena in a packed-bed made of metallic foam pellets. The simulated results are validated against the axial temperature profile and the pressure drop measured in an experimental reactor setup. The proposed CFD model has found as adequate to investigate the flow characteristics and the heat transfer performance of the foam beds. The ability to obtain accurate predictive results in a manageable timeframe is promising, which allows the exploration of the optimal metal foam pellet shape by achieving a reasonable trade-off between pressure drop and heat transfer performance.

References

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