Investigation of the ultrasound assisted CO2 absorption using different absorbents
Introduction
Global warming and climate change have received considerable attention due to the increase in the CO2 concentration in the atmosphere resulting from fossil fuel combustion (Irani et al., 2019). Nowadays, fossil fuel supplies nearly 85 percent of the world energy (Aghel et al., 2018). Furthermore, it takes a long time for green energy sources to replace fossil fuels on the industrial scale (Dharmalingam et al., 2018). It is anticipated that fossil fuels might be used even in the next 20 years, and CO2 emissions into the atmosphere would be continued (Aghel et al., 2018). Therefore, developing some methods is necessary to reduce and eliminate carbon dioxide emissions. In this regard, many projects are performed and designed (Prentza et al., 2018). The principal technologies offered are pre-combustion, oxy-fuel combustion, and post-combustion (Leung et al., 2014). The most important of them is post-combustion without the need for changing the operational units (Garcia et al., 2018).
Numerous post-combustion technologies, namely chemical and physical absorption, adsorption, membrane separation, hydrate separation, and cryogenic separation, have been investigated (Chen et al., 2019). Based on available information, absorption is considered as one of the most efficient and suitable methods for removal of CO2 from the atmosphere (Zheng et al., 2011). Absorption methods, either chemical or physical, are commonly used in industrial applications (Abu-Zahra et al., 2016). The chemical absorption is concentrated on the reaction between carbon dioxide and absorbent, mostly applied to capture CO2 under normal process conditions (T, P). Nevertheless, no reaction is done in the physical absorption and recommended at a high partial pressure of CO2 and low temperature (Abu-Zahra et al., 2016; Kim et al., 2019; Liang et al., 2019). Additionally, physical absorption has less separation efficiency than chemical absorption (Atchariyawut et al., 2007).
In the case of physical absorption, some common solvents are water, propylene carbonate, methanol, N-methyl pyrrolidone, and dimethyl carbonate (Chen et al., 2019; Gui et al., 2010; Liu et al., 2014). The solvents generally used for chemical absorption include alkanol-amine and alkaline solutions. DEA is a secondary amine and have some significant advantages over other amines, including high CO2 loading capacity at low cost (Tirandazi et al., 2017), much lower regeneration energy, and less corrosive than MEA (Laribi et al., 2019). Gomes et al. (2015) studied the amine-based absorbents for the absorption of CO2. They have reported that the best choice for CO2 capture by chemical absorption is diethanol-amine due to the best cost/loading capacity. Another preferred option for CO2 absorption is the use of inorganic chemical reagents like alkali solvents (Toan et al., 2019). Aqueous solutions of K2CO3, NaOH, and Na2CO3 have shown high stability, less environmental impact, lower cost, while they are more abundant compared to the amines (Li et al., 2019; Rodríguez-Mosqueda et al., 2018; Ye et al., 2015). The CO2 absorption capacity of NaOH is higher than amines, such as MEA and DEA solutions, which are considered as the benchmark and economically amine-based solvents, respectively (Li et al., 2019; Tirandazi et al., 2017). Besides, the reaction of CO2 with NaOH is faster than K2CO3 (Ye et al., 2015). K2CO3 offers higher CO2 capture capacity and reaction rate as compared to Na2CO3. However, Na2CO3 appears as a cheaper and more available material than K2CO3 (Amiri and Shahhosseini, 2018; Dong et al., 2015; Kazemi et al., 2020). As regards the above-mentioned, sodium hydroxide and sodium carbonate are selected to compare the performance of di-ethanol-amine with inorganic bases. In addition, water is chosen as a physical solvent to investigate and compare with the selected chemical solvents.
Ultrasonic atomization is an effective technology for producing fine particles of liquids (Gaete-Garretón et al., 2018). The use of high-frequency ultrasonic irradiation in the liquid leads to ultrasonic streaming, and the emergence of a fountain of fine droplets on the liquid surface (Loh et al., 2002; Mai et al., 2019). These droplets are smaller than 100 nm providing a larger surface area for the mass transfer process (Kobara et al., 2010; Kudo et al., 2017). Hence, this practical method has been used in various processes such as combustion of fuel, humidification, aroma diffusion, synthesis of nanoparticles and powder production (Mai et al., 2019; Yasuda et al., 2014). Nevertheless, the actual mechanism of ultrasonic atomization is not fully elucidated (Deepu et al., 2018). In general, three main theories have been proposed in the last few years, namely capillary-wave theory, cavitation theory, and a combination of capillary-wave and cavitation theories (Deepu et al., 2018; Kudo et al., 2017). According to the capillary wave theory, by applying ultrasonic waves, capillary waves are created in the gas-liquid interface, causing instability, and mist droplet generation. The cavitation theory states that the violent collapse of cavitation bubbles produces hydraulic shocks that are associated with atomization. However, the combination of the two mentioned theories is widely accepted. This theory indicates that hydraulic shocks resulting from cavitation, affect the capillary waves and so, the droplets are pinched off the liquid surface (Deepu et al., 2018; Kudo et al., 2017; Mai et al., 2019; Yasuda et al., 2014).
Lately, a few studies have reported that high-frequency ultrasonic waves could be used for the CO2 absorption. Tay et al. (2016) investigated the influence of high-frequency ultrasound on CO2 absorption and desorption processes. It was reported that the high-frequency ultrasonic waves were more favorable for absorption as compared to desorption process. Moreover, the gas-liquid mass transfer was highly enhanced in the presence of ultrasound. They also studied the CO2 chemical absorption into the MEA solution (Tay et al., 2017a). The obtained results showed a remarkable increase in the mass transfer coefficient by employing ultrasonic waves. Another similar study from the same research group for the chemical absorption using potassium carbonate showed that the application of high-frequency ultrasonic waves reduced the time needed to reach the CO2 loading up to 0.9 (Tay et al., 2017b). Their results proved that high-frequency ultrasonic waves could be a promising option to improve CO2 absorption efficiency. Consequently, this subject deserves further investigation in the future.
To date, only a few works of CO2 absorption using high-frequency ultrasonic systems have been published, while no detailed investigation of the effects of the solvents on the process performance has been reported. Therefore, in this study, several experiments were conducted using high-frequency ultrasonic and four different solvents. In addition, the effects of operational variables such as power, initial solvent concentration, temperature, and initial pressure on the CO2 absorption efficiency were studied. The process was also optimized using response surface methodology (RSM).
Section snippets
Materials
All the purchased materials and supplies are listed in Table 1. Distilled water was used to prepare the chemical solutions.
Experimental apparatus and procedure
The schematic diagram of the ultrasonic assistance device for CO2 absorption is shown in Fig. 1. In this work, the dynamic pressure method has been used to determine the absorption rate in a batch process. The experimental set-up consists of a gas storage, and an ultrasonic reaction cell, made of SS-316, with volumes of 680 cm3, and 480 cm3 respectively. The ultrasonic
Results of statistical analysis
The experiments were carried out using the Box–Behnken experimental design. The ANOVA was applied for estimating the coefficients of the regression equation. Furthermore, it was used to check the model significance and adequacy. For assessing the predicted model, the F-value, P-value, the lack-of-fit value, and other adequacy measures were investigated (Kumar et al., 2008). The aim of the optimization was to maximize the absorption rate.
Conclusion
The effects of different solvents such as DEA, NaOH, Na2CO3, and H2O on the ultrasound-assisted CO2 absorption system were investigated. The statistical methodology, Box–Behnken RSM, was applied to determine the optimum conditions of a large experimental domain for a maximum CO2 absorption rate. The effects of operating conditions, including pressure, temperature, initial solvent concentration, and input power, were also studied using a 1.7 MHz high-frequency ultrasound. The results revealed
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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