**1. Introduction**

The rotating zigzag bed (RZB) is an emerging high-gravity device with an innovative zigzag passage structure to enhance fluid turbulence and mass transfer [1,2]. Liquid disperses and coalesces repeatedly by colliding with the rotating and static baffles continuously in the zigzag passage, leading to a high surface renewal frequency of the liquid [3]. Correspondingly, a significant increase in the gas–liquid contact time and liquid holdup is achieved in the RZB, which is in favor of mass transfer [4]. Compared to the conventional rotating packed bed (RPB), the RZB also exhibits the advantages of no demand for the liquid distributor and static seal, easy realization of the intermediate feed, and configuration of the multi-stage rotor in one casing [5,6]. Therefore, RZBs have been applied to stripping [7], distillation [5], extraction [8], and gas–liquid reaction [9,10].

Li et al. [10] studied the mass transfer characteristics of CO2 absorption into NaOH solution in an RZB and found that the efficiency of liquid-side mass transfer in the RZB was significantly superior to that in the conventional RPB. Other research has demonstrated that the effective interfacial area and the efficiency of gas-side mass transfer were on par with those in RPBs with stainless wire mess packing. In consideration of the rotor structure with zigzag channel, pressure drop and power requirement in RZB are higher than those in RPB [1,11]. However, in terms of the study of Liu et al. [12] on the features of CO2 absorption and mass transfer with diethylenetriamine-based absorbents in an RZB, the

**Citation:** Liu, Z.; Esmaeili, A.; Zhang, H.; Wang, D.; Lu, Y.; Shao, L. Modeling and Experimental Studies on Carbon Dioxide Absorption with Sodium Hydroxide Solution in a Rotating Zigzag Bed. *Processes* **2022**, *10*, 614. https://doi.org/10.3390/ pr10030614

Academic Editors: Elio Santacesaria, Riccardo Tesser and Vincenzo Russo

Received: 21 February 2022 Accepted: 17 March 2022 Published: 21 March 2022

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RZB exhibited a better performance on CO2 absorption and mass transfer with decreased device size and absorbent usage in comparison with an RPB, which could make up for the above disadvantages of RZB and reduce the operating cost. Moreover, a smaller volume of RZB with higher gas–liquid mass transfer efficiency makes it more suitable for application in confined spaces, such as vessels and offshore platforms, relative to conventional columns and RPBs [12], suggesting that the RZB is a promising apparatus for intensifying the gas–liquid contact processes.

The overall volumetric mass transfer coefficient is a crucial index to assess the mass transfer performance of a gas–liquid contact device. Extensive studies on the modelling of the overall volumetric mass transfer coefficient between gas and liquid in RPBs or other high-gravity devices have been conducted [13–17]. Additionally, the modelling of local mass transfer in RPBs has also been proposed [18,19]. However, there are only a few reports on the performance of mass transfer in the RZB, such as experimental investigations on the effective interfacial area and local mass transfer coefficient [10] and qualitative analysis of the mass transfer characteristics [2]. To the best of our knowledge, there is no report on the modelling of overall volumetric mass transfer coefficient in the RZB. Therefore, modelling the gas–liquid mass transfer behavior described by the overall volumetric mass transfer coefficient in the RZB can provide a theoretical basis for process intensification in the RZB.

Herein, based on fluid flow in different regions of the rotor in an RZB, a mechanism model has been proposed for the first time to describe the reactive mass transfer process in the CO2–NaOH system and predict the mass transfer performance in the RZB. The effects of various operating conditions on the overall gas-phase volumetric mass-transfer coefficient (*K*G*a*) were studied. The model was validated by the agreement of the predicted values with the experimental results.
