1. Introduction
It is well known that the main energy generation sources are still gas, oil, and fossil fuels all over the world. Pollution has led to global warming, melting ice in the South and North poles, the rising of sea and ocean levels, and climate change. Therefore, researchers are increasingly focusing on finding appropriate and optimal methods for gas treatment, especially carbon dioxide emissions [
1].
There are four approaches to reducing gaseous emissions, namely, electrochemical, oxy-fuel combustion, pre-combustion, and post-combustion methods [
2]. Carbon dioxide is mostly a greenhouse gas that plays a major role in climate change and is often absorbed in post-combustion processes in industries. These processes and other relevant technologies are developed to increase efficiency and decrease costs. Absorption and chemisorption are especially important in industries [
2,
3,
4]. A good patent review on CO
2 removal technologies can be found in [
5]. All technologies have their own advantages and disadvantages when used alone; therefore, to reduce the drawbacks, several intensification approaches are simultaneously applied. Song et al. called these “hybrid processes” [
6]. Other techniques include the formulation of solvents [
7,
8] and process intensification tools such as centrifugal, vibration, mixing, nanoparticle, and magnetic field applications [
9,
10]. Process intensification (PI) helps to reduce equipment size, energy consumption, and effluents. and allows the systems to be compacted in smaller areas with innovations [
11,
12,
13]. Rotating packed beds (RPBs) are a type of PI equipment that uses centrifugal acceleration [
14]. Another alternative PI approach, which was mentioned above, is the use of nanofluids (NFs) that include the suspension of nanoparticles (almost less than 100 nm) in a base fluid (BF) that can transfer heat and improve MT operations such as liquid-liquid extraction [
15]. A great number of investigators have worked on CO
2 capture by using NFs, and a short review of these studies was published by Zhang et al. [
16]. Previous investigation reports illustrated that Al
2O
3, TiO
2, and SiO
2 NFs are most effective MeOH-based NFs in CO
2 absorption. For instance, Pineda et al. [
17] examined the enhancement of Al
2O
3 and SiO
2 and demonstrated that CO
2 absorption increased by more than 9% with these NFs. Lee and Kang [
18] evaluated CO
2 solubility with Al
2O
3/NaCl aqueous NFs and reported 12.5% intensifiction. Jiang et al. [
19] studied amine-based NFs and showed that TiO
2 and Al
2O
3 were more efficient than SiO
2. Haghtalab et al. [
20] studied water-based ZnO and SiO
2 NFs in concentrations of 0.05, 0.1, 0.5, and 1 wt.% and compared the amounts of their enhancement at high pressures. Similarly, Al
2O
3 was examined in a concentration range of 0–0.2 wt.% in pure water and the optimum concentration value of 0.05% wt was reported In the results by Ref. [
21]. Another example is the examination in mass fractions of 0.02, 0.1, 0.5, and 1% of Al
2O
3, SiO
2, Fe
3O
4, and CNT NPs in water and amine solution by Rahmatmand et al. [
22], who reported that Al
2O
3 and SiO
2 were more efficent NFs. Darvanjooghi et al. [
23] evaluated an SiO
2 NF with different sizes at concentrations of 0.005, 0.05, 0.01, and 0.1 and the results indicated that maximum CO
2 removal occured at the concentration of 0.01 wt.% and that the rate of absorption increased with the increase in NP size.
Salami and Salimi [
24] used water-based NFs to enhance the MT rate in packed bed columns and reported that Al
2O
3 and SiO
2 NFs with a nanoparticle (NP) concentration of 0.05 vol.% had MT rates of 14% and 10%, respectively. In addition, some researchers, such as Samadi et al. [
25] and Salimi et al. [
26], applied magnetic NFs and magnetic field induction to achieve higher MT rates. These types of NFs enjoy two benefits: They are controllable in magnetic fields and help with CO
2 capture [
27]. The simple or single-block RPB includes a liquid distributor and a rotor made up of a packing and a shaft. It should be noted that the liquid acceleration is set based on the rotor speed, which can intensify micro-mixing and MT according to its pattern. The change in the flow area is more significant in RPBs than in PBCs; therefore, the MT coefficient varies at different points of the rotor because of gas velocity and surface area changes along the radius [
28]. It is clear that RPBs have a larger surface area than conventional PBCs in the same operation conditions [
10]. Wang et al. [
29] reported that advantages of RPBs were high volumetric MT coefficients (leading to a decreased physical size, as well as lower capital and operation costs), decreased flooding tendency, micro-mixing improvement, suitability for toxic and expensive materials, and skied-mounted applicability. Recently, Luo et al. [
30] studied micro-mixing by applying two intensification approaches (ultrasonic-assisted RPB), and Dashti and Abolhasani [
31] examined physicochemical absorption using MEA and NPs with TiO
2/MEA (0.01–0.1 wt.%) in an RPB with blade packing. They used TiO
2 based on the results of [
19] and reported precipitation of particles not possible in RPBs because of high interactions and very low resistant time of the NFs.
Because of the advantages, RPBs are still designed and developed on laboratory and industrial scales. Generally, this topic is treated and developed in four directions:
- (a)
Examination of chemical solvents like propylene carbonate to evaluate the absorption ability [
32]; mixed-chemical solvents, as in Ref. [
33]; or new advanced-green solvent performance in acid gas treatment, such as ionic liquids [
34] and nanofluids [
31].
- (b)
Optimization of RPB operations by improving contact surface area or mass transfer by changing the hydrodynamic features through:
Modifications to RPB configurations such as “liquid distributor(s).” For instance, Hacking et al. [
35,
36] examined novel redistribution rings in a bed. Wu et al. used a multi-inlet to distribute liquid in a bed [
37] whose efficiency was examined by Zhang et al. [
38], and Wang et al. [
39] evaluated the contacting angle of fluids,
Modifications to the bulk zone by adding baffles/blades, as in Yang et al. [
40], or mesh pin, as in Liu et al. [
41,
42]; or changing the number of mesh screen layers, as in Su et al. [
43]; changing the packing wettability, as Lu et al. [
44]; or changing the kind of packing structure, as reported in [
45].
- (c)
Examination of the intensification by replacing the conventional equipment with RPBs, such as deoxygenation of water and fuel [
46,
47], wastewater treatment [
48], CO
2 absorption [
49], and selective absorption [
50].
- (d)
Focusing on the micro-scale or micro-mixing in a heat-sensible process, controlling polymerization processes, and packing optimization [
51].
All of these development were carried out by experimental methods (tests or high-speed photography), CFD (reviewed by Chen et al. [
52]), and numerical methods, especially the artificial neural network (ANN) method [
53,
54].
Regarding the first category, numerous CO
2 absorption systems in RPBs have been investigated by researchers at different operating parameters. Jiao et al. [
55] examined the performance of an RPB in a Q
L range of 0.5–1 L/min and a Q
G range of 667–1667 L/min at 1 bar and 298 K. Generally, Xing et al. [
56] showed RPB performance at atmospheric pressure in different rotational speeds and a vast gas–liquid ratio (GLR) range of more than 20 and up to 20,000, as summarized in
Table 1. In addition, Xiang et al. [
32] evaluated CO
2 absorption with propylene carbonate in a pilot-scale RPB at a gas flow rate of 8–25 m
3/h and liquid flow rate of 100–300 L/h at high pressure and a rotating speed of 290–1450 rpm.
The higher possible GLR for CO
2 absorption in an RPB is an important parameter to provide economical performance, as reported in [
63], which allows operation with the lowest absorption solvent solution.
Considering the above issues, the present study focused on physical absorption and was carried out with two intensification approaches by an RPB as a contactor that provided centrifugal force and NPs that were applied to increase the effectiveness of water-based NFs as advanced environmentally friendly water-based solvents. Therefore, this study examined the effect of NFs on the MT coefficients at low pressure and CO2 concentrations in a centrifugal field in an RPB. In this device, the cavity zone was removed with a modification of the contactor and was filled by packing to extend the gas–liquid contact area. Injections were mounted on the shaft and the liquid was distributed through the holes on the rotor, which could cool down the bearings in operation. On the other hand, we applied TiO2, Al2O3, and SiO2 nanoparticles in CO2 capture. In this article, first, the setup characteristics, operation, and data processing of CO2 capture by NFs are described, and then the results are evaluated and discussed.
3. Results and Discussion
In the setup of this work, the gravity ratio increased from 30.06 to 219.04 from the inner radius of the packing to the outer radius, and from 53.45 to 389.40 at the low and high rotating speeds of the rotor, respectively. Regarding this issue, it was necessary to survey the effect of the rotating speed on the MT volumetric coefficient.
Figure 3 shows the values of
kLa of pure water and various NFs versus the CO
2 content of the gas feed, which reveals how the concentration of NPs and kind of NPs affected the MT performance. It was also found that the MT coefficient of Al
2O
3 NF- 0.01 wt at high/low rotating speeds (
Figure 3a) and TiO
2 NF-0.05 wt.% were the highest values (
Figure 3b), and the Al
2O
3 and TiO
2 NFs’ MT volumetric coefficient v alues were higher than those of the SiO
2 NF at a concentration of 0.1 wt.% (
Figure 3c).
Figure 3d demonstrates that an increase in the rotating speed of the RPB led to an increase in the
kLa values. The figure indicates that there was a relationship between
kLa and the rotor speed. It also reveals that higher rotor speeds improved
aw. Therefore, the a
e values change along the radius for solvents, as reported by Agarwal et al. [
82], and are influenced by changes in the NF concentration, type of NF, and CO
2 content. This may be due to the microconvection ability of NPs and its dependency on concentration values. A higher amount of CO
2 content also makes for a higher driving force due to further concentration differences between the phases.
To evaluate the effect of Al
2O
3, TiO
2, and SiO
2 concentration on MT performance, concentrations of 0.05, 0.01, and 0.1 were selected and examined at a constant gas flow rate of 10 L/min with different CO
2 content values (10–50 vol.%) and various solvent flow rates, as shown in
Figure 4a–d. The figure demonstrates that
kLa was influenced by the kind of nanoparticle as well as nanofluid concentration and flow rate, but it was not changed by varying the CO
2 content. This figure also indicates that NFs had substantially higher MT performance than water, which may be due to the shuttle effect, as well as hydrodynamic and bubble breaking effects in NFs, which can increase the contact area and reduce the boundary layer’s thickness, which, as discussed by Kim et al. [
83], can intensify the diffusion of CO
2 molecules [
84]. Furthermore, it was found that the concentration of 0.01 wt.% of the NF was more efficient than other concentrations of the suspension. This result confirms Krishnamurthy et al.’s and Kim et al.’s reports [
71,
85], which demonstrated that an increase in the concentration values of NPs intensifies the diffusion up to a certain value and then decreases. In addition, viscosity does not have a significant effect because of low concentrations, as reported by Samadi [
25].
As
Figure 3a and
Figure 4, reveal, the NF concentration of 0.01 wt.% was tested at different flow rates and a constant gas flow rate (10 L/min) with two CO
2 content values (50 and 10 vol.%) which also included Al
2O
3 NFs (II). The results of these tests are shown in
Figure 5.
Figure 5a,b reveal that the TiO
2 and Al
2O
3 NFs (II) were more effective than other NFs and pure water. In addition, as can be seen, the Al
2O
3 (II) NF was more effective than the Al
2O
3 (I) NF, which may have been due to high turbulence that occurs at higher gas and liquid flow rates, and more importantly due to effective surface area values and the microconvection ability of NPs.
Considering the data shown in
Figure 5 and using Equation (11), the MT intensification parameter at high rotating speeds was calculated for Al
2O
3 (II), Al
2O
3 (I), TiO
2, and SiO
2. The average amount of
keff at the high speeds of the RPB were 1.99, 1.47, 1.82, and 1.77 for Al
2O
3 (II), Al
2O
3 (I), TiO
2, and SiO
2, respectively, and the maximum value reached 2.59 for Al
2O
3 (II), which may have been due to its nanoscale mixing abilities in liquid.
In addition, the results shown in
Figure 3,
Figure 4 and
Figure 5 indicate that the performance of metal- oxide NPs was better than that of SiO
2 NF, which could be related to the microconvection, diffusion, and surface-active site ability of NFs, which can adsorb ions. In addition, surface renewal rate or acidic, basic, or amphoteric properties of NF can change the concentrations of H
3O
+ and OH
− [
20].
On the other hand, it can be claimed that the Al
2O
3(II) NF performance was better than that of Al
2O
3(I), which may be related to surface area, as indicated in Refs. [
23,
86]. Based on the study carried out by Ali et al. [
87], who showed that a nanofluid’s thermal conductivity coefficient increases with the size of the NPs, and considering the similarity between heat and mass transfer, it may be argued that the better performance of the Al
2O
3(II) NF was due to the increase in the NPs.
The effect of gas feed and solvent flow rates on the performance of Al
2O
3 (I) and (II) NFs-0.01 wt.% was examined (
Figure 6). It is clear that the amount of
kLa increased with an increase in the gas feed/solvent flow rate, and its values were higher at high rotating speeds than lower ones. Therefore, there was a relationship between the above-mentioned operating parameters and MT performance.
Figure 6a shows the performance of Al
2O
3 (I) and (II) NFs (0.01 wt.%) at high rotating speeds, a constant CO
2 content of 50 vol.%, and two constant solvent flow rates of 24 and 42 L/h based on changes to the gas flow rate through the two upper solid curves along with the upper horizontal and right vertical axes, as well as the two lower dotted lines along with the lower horizontal and left vertical axes.
Figure 6b similarly shows the performance of these two NFs at the same concentration, with a CO
2 content of 50 vol.% and at the constant gas flow rates of 6, 16, and 24 L/h based on changes to the solvent flow rate. It is also shown that sovereignty was related to Al
2O
3 (II). It is possible that the microconvection strength of Al
2O
3 (II) was higher than that of Al
2O
3 (I) in the solvent.
According to the flow regime, to predict the absorption performance of a contactor, it is necessary to determine the liquid-phase MT coefficient and Peclet Number (
Pe) [
72].
Based on the MT correlations reported in [
67], the above reported results, and evaluations of the correlations, it can be concluded that the volumetric MT coefficient was mostly proposed by investigators using Reynolds (
Re), Schmidt (
Sc), Weber (
We), Froud (
Fr), Grashof (
Gr), and
φ. For dimensional analysis, it is necessary to take into account parameters such as the geometry of the system, the process nature, kinematic viscosity, and external forces. Therefore, according to the above experimental results, the fact that external forces affect the turbulent situation of the fluid flow; the packing geometry; the liquid surface tension, which can be affected by the NP concentration and size; and the effect of rotational speeds considered with the
Re,
Sc,
We, and
Gr numbers should be all taken in to consideration. On the other hand, the relationship between
kLa and the above-mentioned parameters can be expressed as follows:
Taking into account the analogy of heat and mass transfer and considering the results of Kristiawan et al. [
88] and Mohammadoost et al. [
64], investigations in mass transfer have shown that when the absorbing solvent is an
NF, the
NF’s Reynolds number and concentration of
NFs can affect the MT performance. Therefore, the above-mentioned issues, critical surface tension, and Brownian Reynolds number (
ReNF), which show the NP’s role in micro-convention, lead to the following equation:
In the present study, the constants of the empirical correlation of MT performance were found with regression analysis of the experimental data of water and
NFs. This equation, known as the predicted model, showed a 29.7% discrepancy with the experimental data that were shown and compared with Rajan et al.’s [
70] correlation (in
Figure 7):
The limitations of dimensionless numbers in the correlation were 5.29 × 10−1 ≤ ReL < 9.26 × 10−1, 1.55 ≤ ReG < 4.13, 1.89 × 10−6 ≤ We ≤ 7.68 × 10−6, 6.66 × 101 ≤ Sc ≤ 3.18 × 10−2, 4.12 × 105+ ≤ Gr ≤ 7.33 × 105, 0.00 ≤ ReNP ≤ 1.31.
kLa had higher ReL and ReNP power compared to other dimensionless groups, which shows that kLa was strongly connected to the liquid flow rate and NP concentration.
Considering the above-mentioned points about dimensionless groups, the above-mentioned relationship form can be used to scale up the RPB.
4. Conclusions
In this investigation, MT performance in a CO2-N2-NF system with centrifugal acceleration and several NPs was examined. The values of kLa for water, TiO2, Al2O3, and SiO2 NFs in various concentrations were obtained and the effect of changing operation parameters was analyzed. The metal oxides of TiO2 and Al2O3 (I) and (II) were more efficient than the mineral oxide of SiO2. Moreover, the concentrations of 0.01 wt.% were more efficient in comparison with the base fluid (water) and other concentrations of the suspension, and the NFs of TiO2, Al2O3, and SiO2 to some extent affected the liquid volumetric interfacial MT coefficient at the high rotating speeds of the RPB. The results clearly show that at high rpms, NPs in suitable concentrations can enhance the MT performance considerably, even when using low-solubility solvents like water at low pressure. In this study, Al2O3 (II) showed better performance and increased the amount of kLa approximately 2.6 times more than water.
Additionally, it can be pointed out that MT coefficients are a function of fluid properties and flow rates and change by variation of these quantities in the flow direction. A new correlation was also proposed (Equation (19)) in this study for kLa prediction at low pressure, which includes both the microconvection of NPs and the surface tension of packings. For future research on physical absorption, examining the process at higher system pressure and CO2 concentration, adding extra intensification tools such as external magnetic forces, the use of the hybrid water-based NFs with Al2O3-TiO2, a higher concentration of NPs and rpm, examination of MT interfacial effective surface area for NP participation, and more tests on other mineral NPs and advanced solvents such as biphasic, phase change, ionic liquids, and deep eutectic solvents are recommended.