**2. Materials and Methods**

#### *2.1. Materials*

Granulated sodium hydroxide (purity 98.0%) was provided by Shanghai Macklin Biochemical Co., Ltd., Shanghai, China. A carbon dioxide cylinder (purity 99.0%) was supplied by Beijing Shunanqite Gas Co., Ltd., Beijing, China. Deionized water was used through the experiments.

#### *2.2. Experimental Apparatuses and Procedure*

The RZB used in this work is schematically shown in Figure 1 [12], and the specifications of the RZB are given in Table 1 [12].

**Figure 1.** Structure of the rotating zigzag bed (RZB): (**1**) liquid outlet; (**2**) rotating disk; (**3**) rotating baffles; (**4**) static baffles; (**5**) liquid inlet; (**6**) insulation materials; (**7**) gas outlet; (**8**) static disk; (**9**) perforations; (**10**) shell; (**11**) gas inlet; (**12**) shaft. (Blue and red lines indicate liquid and gas streams, respectively).


**Table 1.** Specifications of the rotating zigzag bed (RZB).

The rotor is the main component of the RZB. Static and rotating baffles are mounted alternately in the radial direction on the static and rotating disks, respectively, in the rotor. The upper section of the rotating baffles is perforated. The annular space between the static and rotating baffles forms the zigzag passage for fluid streams.

The shell of the RZB was covered with insulation materials to keep the reaction temperature in the RZB at a certain level, and an infrared thermometer (F-380, Shenzhen Flank Electronic Co., Ltd.) was employed to measure the temperature of the insulation materials.

The experimental setup is illustrated in Figure 2 [12]. The RZB was preheated by using a NaOH solution until a designated temperature was reached. A mixed gas stream containing air and CO2 was then directed into the RZB via the gas inlet. When CO2 concentration at the gas inlet reached 4%, the NaOH solution with a preset temperature was pumped into the RZB through the liquid inlet. The gas stream flowed inwards from the periphery of the rotor and made contact in a countercurrent with the liquid stream flowing outwards in the rotor to realize the absorption of CO2 by the NaOH solution. Finally, the liquid and gas flows left the RZB via the liquid and gas outlets, respectively.

**Figure 2.** Experimental setup: (**1**) absorbent container; (**2**) thermostatic bath; (**3**) liquid pump; (**4**) liquid rotameter; (**5**) effluent tank; (**6**) RZB; (**7**) motor; (**8**) dryer; (**9**) CO2 analyzer; (**10**) gas rotameter; (**11**) gas mixing tank; (**12**) air compressor; (**13**) CO2 cylinder.

The experiments were performed at ambient pressure, and the experimental data were collected when a stable state of CO2 absorption process was achieved. The concentration of CO2 in gas streams entering and leaving the RZB was measured by an infrared CO2 analyzer (GXH-3010F, Beijing Huayun Analytical Instrument Institution Co., Ltd., Beijing, China). All the experiments were repeated to validate the reproducibility of the results.

#### **3. Theory**
