(393g) A Mechanistic Approach for Predicting Mass Transfer in a Rotating Packed Bed Contactor for Carbon Capture

A rotating packed bed (RPB) contactor is a promising technology which can potentially replace a conventional packed bed used in absorption and stripping processes due to its enhanced mass transfer between liquid and vapour phases and reduced equipment size and cost [1]. In particular, RPB has been considered for amine-based carbon capture processes as process intensification, referred to as HiGee. RPB has been reported to be associated with the reduced equipment size at the pilot scale by one-third compared to the conventional packed bed while achieving a similar absorption performance and energy consumption [2]. Furthermore, it has been demonstrated that RPB can have approximately 2.7 times larger mass transfer coefficient than a packed bed [3].

When a liquid stream is distributed into the RPB packing, it is split into thin films and small droplets along the liquid trajectory from the distributor to the liquid exit. This increases the contact area between the liquid and vapour phases, thereby improving absorption rates [4]. Furthermore, the interfacial area is highly dependent on the properties of packing materials and liquid properties, such as surface tension and solvent viscosity that vary with temperature and amine concentration [5]. These properties, along with operating conditions such as liquid and gas flow rates and rotating speed, govern the liquid holdup and flooding in RPB, thereby determining the effectiveness of the interphase mass transfer in RPB. Despite the direct relevance of such variables with mass transfer, little has been revealed, especially the in-depth understanding of multiphase flow characteristics and their associations with mass transfer depending on various packing properties and operating conditions due to the technological novelty and complexity. Most empirical correlations are based on the conventional packed bed (e.g., the correlation of Billet and Schultes for an effective interfacial area [6]), and only a few are derived for RPB, e.g., the Burns correlation for liquid holdup [7].

In this study, we propose a mechanistic modelling approach that can predict liquid holdup and the occurrence of flooding in RPB under various operating and design conditions by accounting for liquid trajectory and force balances for droplets, thereby predicting interphase mass transfer coefficients. The schematic of the model structure can be seen in Fig A. The main elements of the developed model are the models for predicting liquid trajectory and velocity and the dynamic model for the entry zone where initial droplets are born from the liquid jets (Fig B). Within the entry region, several collisions between packing and liquid occur (Fig B), which result in the disintegration of liquid jets into small droplets. Since these variables are highly influenced by the packing configurations, experimental data with different packings available in the literature are used to gain insights into the flow characteristics (i.e., the formation of liquid films and droplets and their movement) [8], as well as to validate the developed model. Based on the proposed modelling approach, the velocity of liquid velocity and resultant residence time can be calculated as shown in Figs C and D and compared with experimental results presented in the literature [9]. Comparison results show that the calculated residence times for different flowrates and rotating speeds are in good agreement with the experimental data. Moreover, due to the limited availability of various packing data, the complex flow characteristics in various packing candidates are elucidated with the aid of computational fluid dynamics (CFD) for multiphase flows. Preliminary results of multiphase flow simulation are shown in Fig E, where the breakup of liquid jets into small droplets can be observed in a net-type packing.

Finally, the developed model for the liquid holdup, flooding and interphase mass transfer can further be combined with thermodynamics, chemical reaction kinetics and mass/energy/momentum transfer models in the bulk phases for simulating absorption and stripping at a process level. The developed mechanistic model is believed to be a tractable and versatile tool that can be used to evaluate the performance of RPBs under various operating and design conditions and to optimise operating protocols and RPB designs for enhanced CO₂ absorption and minimised energy.

References

[1] Wang, Z., Yang, T., Liu, Z., Wang, S., Gao, Y. and Wu, M., 2019. Mass transfer in a rotating packed bed: a critical review. Chemical Engineering and Processing-Process Intensification, 139, pp.78-94.

[2] Adamu, A., Russo-Abegão, F. and Boodhoo, K., 2020. Process intensification technologies for CO2 capture and conversion–a review. BMC Chemical Engineering, 2(1), pp.1-18.

[3] Xie, C., Dong, Y., Zhang, L., Chu, G., Luo, Y., Sun, B., Zeng, X. and Chen, J., 2018. Low-concentration CO2 capture from natural gas power plants using a rotating packed bed reactor. Energy & Fuels, 33(3), pp.1713-1721.

[4] Xie, P., Lu, X., Ding, H., Yang, X., Ingham, D., Ma, L. and Pourkashanian, M., 2019. A mesoscale 3D CFD analysis of the liquid flow in a rotating packed bed. Chemical Engineering Science, 199, pp.528-545.

[5] Zhang, L.L., Wang, J.X., Xiang, Y., Zeng, X.F. and Chen, J.F., 2011. Absorption of carbon dioxide with ionic liquid in a rotating packed bed contactor: mass transfer study. Industrial & engineering chemistry research, 50(11), pp.6957-6964.

[6] Billet, R. and Schultes, M., 1999. Prediction of mass transfer columns with dumped and arranged packings: updated summary of the calculation method of Billet and Schultes. Chemical Engineering Research and Design, 77(6), pp.498-504.

[7] Burns, J.R., Jamil, J.N. and Ramshaw, C., 2000. Process intensification: operating characteristics of rotating packed beds—determination of liquid holdup for a high-voidage structured packing. Chemical Engineering Science, 55(13), pp.2401-2415.

[8] Yang, Y., Xiang, Y., Chu, G., Zou, H., Luo, Y., Arowo, M. and Chen, J.F., 2015. A noninvasive X-ray technique for determination of liquid holdup in a rotating packed bed. Chemical Engineering Science, 138, pp.244-255.

[9] Keyvani M. Operating characteristics of rotating beds. Case Western Reserve University; 1989.