https://doi.org/10.3390/su14074350
“Rotating packed bed (RPB) has gained attention for the potential of process intensification in CO2 separation. RPB was invented by Ramshaw and Mallinson in 1981 to intensify the mass transfer process, compared to the conventional packed bed [84]. As the conventional packed bed in real industry application faced several challenges, such as a large footprint and low mass transfer coefficient, several studies have been conducted on RPB for the potential of replacing the packed bed in the post-combustion application. The RPB was developed to reduce the column size and the complexity of the CO2 absorption plant. Ramshaw reported that the RPB provides a volume of reduction by 2–3 orders of magnitude; additionally, the height equivalent of a theoretical plate (HETP) of RPB can be as low as 1–2 cm, when compared to the conventional packed-bed column [85].
In the typical RPB for the desorption process, the rich solvent flows through a rotating packed bed from the middle part of the RPB and is distributed through the thin liquid films generated by the centrifuge force, which leads to a greater mass transfer [86]. In addition to the absorption and desorption processes, the RPB has been used in distillation and material production. Figure 8 illustrates the cross-section of the RPB for the desorption process. The standard counter-current RPB is equipped with the casing, liquid solvent distributor, dynamic seal, and rotor shaft with the packing. The liquid solvent enters the RPB via the liquid inlet and is distributed in the packed-bed section. Then, the solvent is pushed out from the packed bed by the centrifugal force. The lean solvent is later collected at the bottom section of the RPB. Meanwhile, the gas flows inward to the packed bed and leaves the RPB through the central section, as shown in Figure 9. A reboiler is required to heat the solvent for a better desorption process rate.
Figure 8. Experimental apparatus of the rotating packed bed for solvent regeneration [90].
Atuman et al. performed simulation work to study the size reduction in the RPB, compared to the conventional packed bed [87]. This simulation was validated using an experimental study conducted by Jasim et al. [88]. The simulation study of the CO2 desorption process from MEA was conducted under some fixed conditions, such as rich-MEA solvent flow rate (0.3 kg/s), temperature (104 °C), pressure (202 kPa), and solvent loading (0.482 mol CO2/mol MEA) for both the RPB and conventional packed bed. It was found that the packing volume for the RPB was 0.015 m3, while the conventional packed bed was 0.659. The results show that there is a 44 times reduction in the packing volume for the RPB, compared to the conventional packed bed. Additionally, the calculated height of the transfer unit for the RPB is 1.7 cm, while the conventional packed bed is 20.8 cm. This result contributes to the smaller size of the RPB, compared to the conventional packed bed [87]. Based on the finding by Agarwal et al., the casing volume of the RPB and the packed bed was 4.5 times its packing volume [89]. Hence, the overall volume of the RPB is concluded to be 9.7 times smaller than the conventional packed bed.
The RPB is also known as the energy-intensive system for the solvent regeneration process. The regeneration energy refers to three types of heat required during the desorption or solvent regeneration process, which are the heat of the reaction with CO2, the sensible heat of the solution, and the heat of vaporization. For the energy reduction strategy, these three types of energy should be minimized to as low as possible. The heat of the reaction with CO2 can be adjusted through the heat energy required for the thermal liquid solvent regeneration. Meanwhile, the sensible heat of the solution and the heat of vaporization can be reduced via the optimization of process conditions with a good process design [90]. Apart from these three types of heat required during generation, the reboiler energy and the power of the rotor rotation are crucial to be included for the overall energy consumption.
Chamchan developed a model to study the energy consumption for conventional packed bed and RPB which have been validated from pilot plat at China Steel Corporation (CSC) [91]. This packed-bed pilot plant was equipped with 16mm of packed SUS pall ring, where the diameter of the packed-bed column was 0.1 m with a 2 m height. Meanwhile, the RPB has a 0.06 m height, 0.36 m outer diameter, and 0.12 m inner diameter, which use a 5428 cm3 wire-mesh packing volume. The comparable energy consumption for the conventional packed bed and RPB were obtained using an ASPEN Plus Rate-Based model. The authors found that the energy consumption for the RPB was slightly higher than the packed bed. The RPB consumed about 7.5 GJ/tons of CO2, while the packed bed consumed about 6.5 GJ/tons of CO2 for the regeneration of the MEA solvent [91]. Additionally, Tan and Chen performed a study to identify the power consumption rate for the RPB by using the equation developed by Singh et al. [92]. The results from the study showed that 1.2 kW of power was required for the flow rate of 90 cm3/min. However, this power consumption value is unacceptable for a techno-economy CO2-removal process [93]. Thiels et al. proposed to combine the RPB with a packed-bed column as a method to reduce the energy consumption of the RPB [94].
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