Membrane Absorption Coupling Process for CO2 Capture: Application

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Environmental and Carbon Dioxide Issues

Membrane Absorption Coupling Process for CO2 Capture: Application of water-based ZnO, TiO2, and MWCNT nanofluids Parisa Zare, Peyman Keshavarz, and Dariush Mowla Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03972 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Membrane Absorption Coupling Process for CO2 Capture: Application of waterbased ZnO, TiO2, and MWCNT nanofluids Parisa Zare†, Peyman Keshavarz*,†, and Dariush Mowla†,‡ †

School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran

‡ Environmental

*

Research Center in Petroleum and Petrochemical Industries, Shiraz 71345, Iran

Corresponding author E-mail address: [email protected] (P. Keshavarz)

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ABSTRACT: In this work, stable water-based Zinc oxide (ZnO), Titania (TiO2) and Multiwall Carbon Nanotube (MWCNT) nanofluids are prepared and examined as CO2 absorbents in a polypropylene hollow fiber membrane contactor. Different operating variables, such as liquid flow rate, gas flow rate and concentration of nanoparticles, and their effects on CO2 molar flux are investigated. The long term stability of nanofluids is monitored using UV-Visible spectroscopy. Also, zeta potential measurements and sediment photography are applied to confirm the results of nanofluids stability. Dynamic light scattering is used to determine the size distribution of dispersed nanoparticles. The results show that the increase in nanoparticle concentration to 0.15 wt.% has a favorable effect on CO2 absorption efficiency due to the increase in Brownian motion and other related mechanisms. However, it adversely affects the CO2 absorption by lowering the nanofluid stability at higher concentrations. The obtained results reveal that ZnO nanofluid is the most effective nanofluid in all experimental conditions. At low liquid flow rates of about 10 ml/min, ZnO nanofluid could augment CO2 absorption efficiency by 130%, while both TiO2 and MWCNT nanofluids could enhance it by 60% with respect to distilled water. Possible mechanisms regarding mass transfer augmentation are also discussed.

Keywords: CO2 capture, membrane gas absorption, mass transfer mechanism, nanofluid stability, climate mitigation


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1. INTRODUCTION CO2, the main contributor (about 68%) to greenhouse gases (GHGs), has been recently brought to the center of attention.1,2 CO2 concentration in the atmosphere, which was 384 ppm in 2007, has drastically increased to 400 ppm in 2015 and to 403.3 ppm in 2016.3 Unceasing rise of greenhouse gas emissions will continue in the future, if the demand for fossil fuels, as a major source of energy, does not subside. Despite many policies formulated for mitigating the CO2 emission, it is anticipated that at least until the next couple of decades, fossil fuels will still be the first candidate for energy source.4,5 Therefore, it is imperative to alleviate the problem of global warming and climate change by developing CO2 capture and storage (CCS) technologies.6 Postcombustion CO2 capture (PCC) is considered the most mature technology, since there is no need to change industrial units, and they can be retrofitted easily for PCC. Different separation methods are used in the PCC approach. Absorption, adsorption, cryogenic process, membrane technology, and CO2 fixation by microalgae are the most important technologies.7 Absorption is divided into two categories: Chemical and physical absorption. One of the most common and deep-rooted methods in the industry for CO2 separation is chemical absorption, and it has high CO2 recoveries. However, this method has many drawbacks such as corrosion issues, oxidative degradation, dispersion between gas and liquid (flooding foaming, weeping and entrainment), high operation costs, particularly at elevated CO2 concentrations, and the high regeneration energy (more than 50% of PCC energy consumption).8-12 Also, scientists in the field of CO2 capture regard membrane technology as a potentially viable alternative to conventional separation processes, due to its special advantages such as compaction and


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modularity, ease of expansion, and low energy requirements. Hence, integration of absorption with membrane seems to augment CO2 removal efficiency.12 Economic issues in applying membranes for gas separation, and the need for hybrid systems, led to a new approach: using hollow fiber membrane contactors (HFMCs). The HFMC only acts as a compact gas-liquid contactor with high specific surface area and it usually does not have any function in gas separation and CO2 selectivity.13 In 1999, Gabelman and Hwang noted that HFMC was a suitable choice for adding gas to liquid.14 HFMCs have some advantages to conventional processes. The most important one is preventing phase dispersion between shell and tube sides. Using membrane contactors reduces the capital costs by 35-40% and operating costs by about 38-42%.15,16 The most significant challenge in porous membrane contactors is wetting phenomena where the absorbents penetrate into the pores. Keshavarz et al. studied the simultaneous removal of CO2 and H2S in HFMC, and showed that even low levels of membrane pore wetting would reduce absorption efficiency significantly.17-19 In theory, all absorbents used in conventional processes are applicable in membrane contactors. However, some criteria for absorbent selection should be taken into account, including chemical stability, high CO2 loading, great CO2 selectivity, low regeneration energy, high surface tension, and compatibility with the membrane material. Last but not least, the absorbent should be economical and commercially available.20 Some researchers have used water in membrane contactors for CO2 absorption. Distilled water is a low cost absorbent with interesting characteristics such as high surface tension and a good CO2 absorption capacity.15,21 However, the absorption rate and CO2 recovery are relatively low with water as absorbent. Considering all evidence, adding some promoters to distilled water can improve CO2 absorption rate and capacity, without encountering membrane wetting problem, which is a common problem when chemical absorbents are used.22 4 ACS Paragon Plus Environment

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Nanofluids have shown great influence on heat transfer increase. Due to the heat and mass transfer analogy and the research gap existing in nanofluids application in mass transfer, it is vital to investigate their effects on mass transport.23 The term “nanofluid” was first defined by Choi.24 Nanofluid is a stable dispersion of nanoparticles in a base fluid.23 Low concentration nanofluids have a remarkable influence on CO2 absorption enhancement. This has encouraged researchers to investigate different nanofluids as potential absorbents for CO2 capture. Lu et al. studied the effect of Al2O3 and CNT nanoparticles on CO2 absorption in a stirred reactor. They found that CNT nanoparticles could increase mass transfer significantly.25 Jiang et al. used SiO2, Al2O3, MgO and TiO2 with MEA and MDEA as base fluids for CO2 absorption. The enhancement order was TiO2> MgO > Al2O3> SiO2. They attributed this to the larger CO2 adsorption capacity of TiO2 nanoparticles.26 Wang et al. implemented SiO2, Al2O3 and TiO2 nanofluids with MEA as the base fluid for CO2 absorption. They also found that TiO2 had the highest enhancement.27 Haghtalab et al. investigated the solubility of CO2 in ZnO and SiO2 water-based nanofluids in a batch stirred vessel. They concluded that ZnO water-based nanofluid was more effective than SiO2 nanofluid.28 Golkhar et al. used MWCNT and SiO2 nanofluids in a HFMC for CO2 absorption. Their results showed that MWCNT had higher absorption enhancements than SiO2 nanofluid.8 Nanofluids can also catalyze CO2 regeneration process.15,27,29-32 Utilizing nanofluids as new solvents for CO2 absorption seems to be a potential way to enhance CO2 absorption rate, and consequently, lower energy consumption.28,33-38 The objectives of this work were to experimentally investigate the effect of ZnO, TiO2 and MWCNT water-based nanofluids on CO2 absorption in a polypropylene (PP) HFMC. This is the first time that the applicability of ZnO and TiO2 nanofluids is investigated in an HFMC for CO2 absorption. The effects of gas flow rate, liquid flow rate and concentration of nanofluids on CO2


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absorption are discussed. The results are presented by the molar flux of CO2 and enhancement factor, indicating the effectiveness of nanofluids with respect to distilled water. Moreover, the stability of nanofluids are investigated by three methods of UV-Visible spectroscopy, zeta potential measurement and sedimentation photography.

2. EXPERIENTAL SECTION In this study, the CO2 absorption efficiency of three different nanofluids was determined experimentally in a PP hollow fiber membrane contactor. The applied materials, preparation of nanofluids and experimental procedure are explained in the next sections.

2.1. Materials The materials used in this work are carboxyl functionalized multi-wall carbon nanotubes (MWCNTs), Zinc Oxide and Titania P25 nanoparticles. All nanoparticles are manufactured and supplied by US Research Nanomaterials, Inc. Since MWCNTs are inherently hydrophobic, they tend to aggregate and not disperse well in water. Therefore, the acid functionalized type was purchased. Table 1 shows the specification of nanoparticles applied in this study. Distilled water is used as the base fluid for nanofluid preparation.

2.2. Nanofluid preparation In order to prepare the nanofluids, a two-step method of preparation was used. First, a specific amount of nanoparticles was weighed precisely and added to the base fluid. The mixture was stirred with a magnetic stirrer for 5 min. Then, the suspension was sonicated for 40 min at 50% amplitude using a Q700 Sonicator (QSonica LLC, Newtown CT, USA, with a nominal power of


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700 W, 20 kHz, and a standard 13-mm diameter horn). The sonication was performed intermittently (60 ON/ 20 OFF) using an ice-water bath to prevent heating. It is important to note that no surfactant was used for nanofluids stabilization.

2.3. Experimental apparatus For this work, a hydrophobic polypropylene hollow fiber membrane module, supplied from Parsian Pooya Polymer Co., was applied as a contactor. The HFMC specifications are listed in Table 2. The liquid absorbent passed through the lumen side of the membrane module and the gas passed through the shell side co-currently. A schematic diagram of the apparatus employed is demonstrated in Figure 1. A peristaltic pump (V3, Shenchen Co., China) was used in order to pump the liquid with precise flow rates. A gas mixture of 21 mole% CO2 and 79 mole% N2 was employed as the feed gas. The molar concentrations of CO2 in the gas mixture were measured by a nondispersive infrared (NDIR) CO2 analyzer (G110, Geotech, UK) which could measure CO2% with 0.1% accuracy.

2.4. Nanofluid stability measurement Despite being an important factor, little research has been done on estimating the stability of nanofluids. There are a number of methods for estimating and quantifying nanofluid stability. It is worth mentioning that applying just one method individually is not enough to obtain reliable results.39 In this work, three different methods are applied in order to analyze the stability of nanofluids both quantitatively and qualitatively. Of stability evaluation methods, Ultraviolet– Visible (UV–Vis) spectroscopy, zeta potential measurement and sedimentation photography were employed in this study. UV-Vis absorbance of the nanofluids was obtained by a


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spectrophotometer (Genesys 10UV, Thermo Electron Corporation, USA) and zeta potential tests were performed by a zeta potential meter (zetasizer) (Microtrac Zeta Check, Germany).

2.5. Hydrodynamic diameter of nanofluids measurement Size distributions of the nanofluids were obtained by dynamic light scattering (DLS) on a Nanotrac Wave (Microtrac Inc, USA).

2.6. Experimental procedure of CO2 absorption Before each test, the flow meters, the CO2 analyzer, and also the peristaltic pump were calibrated. All the connections were checked in order to eliminate any leakage. Additionally, air was passed through the shell side of the membrane by a compressor for about 15-20 min to ensure that there was no liquid drop in the membrane pores. A back-pressure valve was placed after the HFMC on the liquid outlet to eliminate gas bubble formation in the liquid side by controlling the liquid pressure relative to gas pressure. To start an absorption test, the pressure and flow rate of the gas stream were adjusted at some specified values. When the gas reached a steady state in about 10 min, the liquid was pumped through the membrane, and the CO2 molecules were transferred from gas to liquid through the membrane pores. All the experiments were performed at least 3 times to minimize experimental errors.


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3. RESULTS AND DISCUSSION 3.1. Nanofluids stability 3.1.1. Stability evaluation by UV–Vis spectroscopy The long-term stability of the three nanofluids is evaluated by measuring the concentration of nanoparticles in the supernatant at different time intervals. Concentrations of the samples were measured using a UV-Vis spectrophotometer. In this work, initially 0.15 wt.% suspensions of the three nanofluids were prepared based on the procedure explained in the previous sections. After diluting each sample, they were scanned in the spectrophotometer in order to find the wavelength corresponding to the maximum absorbance (λmax). λmax for MWCNT, ZnO and TiO2 was 262 nm, 367 nm and 260 nm, respectively. Then, six different known concentrations of each nanofluid were used in order to find the corresponding absorbance at λmax. The aim was to draw the standard calibration curves for each nanofluid (Figures 2-4). R square for all figures was near 1, which shows the accuracy of these standard curves.

By having the absorbance of nanofluids at different time intervals and applying the standard curves, the concentration of each sample can be obtained versus sedimentation time. The concentration of the supernatant reduces gradually because of the nanoparticles’ aggregation and agglomeration. Therefore, measuring the supernatant concentration at different days is a good criterion for analyzing the colloidal stability of nanofluids. Figure 5 displays the concentration ratio of nanofluids versus sedimentation time. According to Figure 5, MWCNT nanofluid apparently has perfect stability over long periods of time (120 hours). TiO2 nanofluid also has remarkable stability, and as it is shown, the relative concentration of the TiO2 nanofluid was still maintained over 0.9 after 120 hours. 9 ACS Paragon Plus Environment

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As demonstrated in Figure 5, the nanofluids prepared in this work obviously gain remarkable stability. Table 3 shows quantitatively the nanofluids’ periods of stability. Zinc oxide nanofluid enjoys sufficiently good stability, and its relative concentration is higher than 0.9 after about 50 hours. However, its stability becomes moderate after 78 hours. It should be noted that all nanofluids were prepared without any surfactant and they were sonicated in 40 minutes only. Also, CO2 absorption tests were done nearly one hour after the nanofluid preparation. Therefore, all the three nanofluids gained suitable stability during CO2 absorption experiments.

3.1.2. Zeta potential measurement The pH and zeta potential of MWCNT, Titania and zinc oxide nanofluids were measured about 1.5 hours after the nanofluid preparation. Table 4 shows the measured values of pH and zeta potential. The absolute values of zeta potential are obviously more than 30 mv, which indicates the good stability of the nanofluids. By comparing the zeta potentials of these three nanofluids, it should be emphasized that MWCNT is more stable than TiO2 and ZnO nanofluids. Moreover, TiO2 is more stable than ZnO. These confirm the results obtained by UV-Vis spectroscopy method.

3.1.3. Sedimentation photography Since MWCNT nanofluid is too dark, the boundary between the sediment and the supernatant is not visible. Therefore, the sedimentation photography method is improper for evaluating the stability of MWCNT nanofluid. For ZnO and TiO2 nanofluids, the test tubes photographs were taken for 17 and 31 days, respectively. Figure 6 depicts the sedimentation photographs of these


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two nanofluids. It is evident that TiO2 nanofluid is more stable than ZnO nanofluid. Although the sedimentation of ZnO nanofluid after day 7 is clear, it keeps remarkable stability for two days.

3.2. Size distribution of nanofluids The nanoparticle cluster size distributions of TiO2, ZnO and MWCNT nanofluids are demonstrated in Figures 7-9, respectively. The average diameters of TiO2 ZnO and MWCNT clusters in the nanofluid are 42.1 nm, 147.1 nm and 34.8 nm, respectively. The higher stability of MWCNT and TiO2 could also be explained by their smaller cluster size compared to zinc oxide nanofluid.

3.3. CO2 absorption in membrane contactor The results are given in the form of CO2 molar flux, enhancement factor and CO2 removal using the following equations:

J CO2 (mol.m 2 .s 1 ) 

QinCin  Qout Cout A

(1)

A  2    rin  n  l

(2)

QinCin  Qout Cout  100 QinCin

(3)



EnhancementFactor 

Nanofluid CO Basefluid CO

(4)

2

2

Where JCO2 is the mass transfer rate of CO2, calculated by Eq. 1, mol.m-2.s-1, Cin and Cout are CO2 concentrations in the inlet and outlet of the gas phase, mol.m-3; Qin and Qout are the inlet and 11 ACS Paragon Plus Environment

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outlet gas flow rates, m3.s-1; A is the interfacial area between gas and liquid, m2, and is calculated by Eq. 2; n stands for the number of fibers; l is the effective length of the fibers and rin is the fiber inner radius. η is CO2 removal efficiency, which is defined by Eq. 3. Enhancement factor is defined as the ratio of mass transfer rate of CO2 in the presence of nanofluid to that of the base fluid and is calculated by Eq. 4.

3.3.1. Effects of nanoparticles concentration Figure 10 represents the variation in ZnO, MWCNT and TiO2 nanoparticles hydrodynamic diameters by the nanofluid concentration. Apparently, the increase in nanofluid concentration enhances the diameters due to the reduction in the nanofluid stability. The effects of ZnO, MWCNT and TiO2 nanofluids are shown in Figures 11 and 12. The results show that all nanofluids have better CO2 absorption than distilled water, and ZnO nanofluid has the highest effect on CO2 absorption at all nanofluid concentrations. There is a maximum enhancement factor at a specific solid loading of the nanoparticle. Here, the maximum CO2 absorption occurs near 0.15 wt.% for all nanofluids. The existence of this critical concentration in different nanofluids has also been reported in other works.26,40 Higher CO2 absorption in nanofluids, compared to the base fluid, is due to both the hydrodynamic effects of nanoparticles, such as Brownian motion and micro-convections, and the surface adsorption effects of nanoparticles. Hydrodynamic effects could be very important especially at low liquid velocities and low Reynold’s number, a condition which is usually valid in membrane contactors. Adsorption effects mainly depend on the surface interactions of CO2 molecules and nanoparticles. Both effects can be enhanced by increasing the concentration of nanoparticles. However, by increasing 12 ACS Paragon Plus Environment

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nanoparticle concentration in the base fluid to a level more than the critical concentration, some larger aggregates and agglomerates are produced. This is confirmed by the results outlined in Figure 10, which demonstrates the hydrodynamic diameters of nanoparticles. There is another important reason behind the reduction of the enhancement factor; at concentrations higher than the critical concentration, the nanofluid becomes so dense that the inter-particle interactions reduce the hydrodynamic effects and Brownian motion.29 The trade-off between these two contrary aspects leads to the trend demonstrated in Figures 11 and 12. The highest CO2 absorption enhancements for ZnO, MWCNT and TiO2 nanofluids are 48%, 22% and 28%, respectively. This considerable improvement shows that these nanoparticles, especially ZnO, may be good candidates as absorbent promoters for CO2 separation.

3.3.2. Effects of liquid flow rate As shown in Figure 13, irrespective of the type of absorbent, the thickness of the boundary layer decreases by increasing nanofluid flow rate. Therefore, CO2 molar flux increases. Moreover, all nanofluids enhance the mass transfer rate compared to distilled water, and the ZnO nanofluid gains higher CO2 molar flux. Figure 14 emphasizes that all nanofluids have greater effectiveness for CO2 absorption at lower liquid flow rates. This is obviously because of the lower turbulence at low liquid flow rates, and consequently, the higher mass transfer resistance in the liquid phase. Therefore, water is somehow a weak absorbent at low flow rates. By using nanofluids at these low flow rates, hydrodynamic effects (Brownian motion and micro convections) in the fluid flow cause great enhancement in CO2 absorption compared to the base fluid. Moreover, at high flow rates, the velocity of the liquid is high enough to reduce the importance of the hydrodynamic 13 ACS Paragon Plus Environment

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effects of nanoparticles. In practice, because of the high pressure drop in the fibers and membrane wetting probability, low liquid flow rates are more applicable in hollow fiber membrane contactors.27 At low liquid flow rates, ZnO nanofluid performed excellently and enhanced CO2 absorption by 130% (1.3 times greater than distilled water). MWCNT and TiO2 nanofluids have 60% enhancement in CO2 absorption compared to the base fluid.

3.3.3. Effects of gas flow rate Figure 15 indicates the variation of mass transfer rate with gas flow rate for ZnO, MWCNT and TiO2 nanofluids and distilled water. It is clear that for all absorbents, CO2 molar flux rises when gas flow rate increases. This is because more CO2 molecules come to the shell side and in contact with absorbents at higher gas flow rates. In doing so, all nanofluids performed better than distilled water. At higher gas flow rates of about 300 ml/min and more, the trend becomes smoother since the mass transfer resistance of gas phase decreases, and the absorbent becomes nearly saturated where some unabsorbed CO2 molecules leave the contactor from the shell side. In other words, at high gas flow rates, liquid phase resistance is the dominant mass transfer resistance. The variation of enhancement factor with gas flow rate is shown in Figure 16. A maximum enhancement factor can be found for each nanofluid around the gas flow rate of 300 ml/min, which can be attributed to the higher importance of liquid phase resistance compared to gas phase resistance at high gas flow rates. The results in Figure 16 show that ZnO, MWCNT and TiO2 nanofluids could enhance CO2 removal by 59%, 50% and 34%, respectively.


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3.4. Nanofluids feasibility for CO2 absorption The maximum enhancement for CO2 absorption by ZnO nanofluid at low liquid flow rates was about 130% with respect to water. This enhancement was about 60% for MWCNT. Therefore, it can be easily claimed that ZnO nanofluid will be a promising candidate for CO2 absorption if nanoparticles synthesis techniques become industrially feasible. In this work, ZnO nanofluid gained the maximum CO2 absorption capacity which is about 0.039 mol CO2 per a liter of nanofluid. By the price data for 0.15 wt.% nanofluid, this value is equal to 0.00277 mol CO2 per EUR. This value is also calculated for MWCNT nanofluid and all represented in Table 5. The value of CO2 mol per USD for ZnO nanofluid is 17 times as many as that of MWCNT nanofluid. In order to determine the advantage of using CNT nanofluids in heat and mass transfer processes, it is crucial to evaluate economic issues. CNTs, whether functionalized or pristine, are much more expensive than other nanoparticles even at a lower mass content. In other words, the cost of 1gr of MWCNT is about 120USD. In contrast, TiO2 and ZnO cost only 20USD and 8USD, respectively.41 Thus, applying ZnO water-based nanofluid is indeed cost-saving. While some research has been carried out on nanofluids implementation for CO2 absorption enhancement, there is limited number of studies on ZnO nanofluid. According to Table 6 and a recent short review by Zhang et al., the average enhancement ratio for various water-based nanofluids used for CO2 absorption in different works was about 1.31.34 This table shows that in this work, much more enhancement ratios were successfully gained by especially applying ZnO in the hollow fiber membrane contactor.


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3.5. Mechanisms of CO2 absorption by nanofluids in HFMC There are different mass transfer mechanisms for nanofluids presented in previous researches. Nevertheless, an exact comprehensive and sophisticated mechanism presenting the mass transfer enhancement by nanofluids is yet to be suggested.23 Below, the most widely proposed gas absorption mechanisms in the literature for mass transfer enhancement by nanofluids are analyzed to examine their importance in membrane contactors. The first mechanism, bubble coalescence inhibition (Bubble-breaking), is demonstrated in Figure 17. Stable nanoparticles suspended in the fluid collide with CO2 micro bubbles and break them to smaller bubbles. As a result, the gas-liquid contact area increases. Moreover, considering the Laplace-Young equation, the inside bubble pressure increases. Also, according to Kelvin equation, the solubility of the gas increases when the diameter of the bubbles decreases.29,31,42 However, Kim et al. questioned this mechanism, arguing for its implausibility. They countered the bubble breaking effect by detecting bubbles initially produced from the nozzle in the nanofluid. The bubbles coming out of the nozzle, they showed, are smaller in size than those in the base fluid, hence not being cracked by nanoparticles.43 The second mechanism, Grazing or shuttle effect, is depicted in Figure 18. In this mechanism, nanoparticles, which are randomly moving in the liquid, adsorb the gas molecules in the gas-liquid diffuse layer, transporting them in the bulk of the liquid to a lower concentration of adsorbate. This phenomenon catalyzes the mass transfer.44 However, this mechanism is also refuted by some studies.45 This mechanism might be important in membrane contactors, especially where a physical absorbent such as water is used as the base fluid. The third mechanism is the hydrodynamic effect, shown in Figure 19. In this mechanism, the Brownian motion of nanoparticles in the gas-liquid interface and the induced micro convections and velocity disturbances cause some turbulency and shearing actions in the diffusion boundary


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layer. Consequently, the thickness of the boundary layer decreases, and facilitates, the CO2 diffusion to the liquid phase. Kim et al. studied CO2 absorption visually by a high-speed camera and reported that the hydrodynamic effect is the dominant mechanism in the CO2 bubble absorption process. They based their conclusion on the mushroom shape of the CO2 bubbles in the presence of nanofluid.43,46,47 Another possible mechanism of mass transfer enhancement, which has rarely been mentioned, may be the Marangoni convection effect. To explain further, this is a mechanism where the accumulation of dispersed nanoparticles on the gas-liquid interface causes a reduction in surface tension. This surface tension gradient can originate the Marangoni convection at the interface, which causes turbulency and interfacial convection at the gas-liquid interface, which in turn augments gas absorption.48 Hydrodynamic effects can be important mechanisms in membrane contactors, because the liquid flow in these modules is usually laminar with low Reynold’s numbers. Therefore, a micro convection can considerably affect the mass transfer in these contactors. Zinc is a mineral which mostly appears in different forms of zinc oxide, zinc sulfide and zinc carbonate. ZnO is almost abundant and inexpensive compared to other metal oxides, and it has many applications in manufacturing various industrial products. Moreover, carbonic anhydrase is an enzyme which catalyzes the hydration reaction of CO2 to the bicarbonate in the human body and has been recently used in CO2 capture processes.49 The active component in the molecular structure of this enzyme is zinc. Therefore, ZnO may be a good candidate in metal oxides for CO2 adsorption. Moradihamedani et al. used ZnO nanoparticles in Polysulfone membranes for CO2/CH4 separation. They emphasized the molecular sieving effect of ZnO and its selectivity towards CO2 molecules.50 Dilshad et al. applied ZnO in PVA/PEG blended membranes for


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CO2/N2 separation. They found that both CO2 permeability and CO2/N2 ideal selectivity are enhanced by increasing ZnO content in the membrane up to 2 wt.%. Their results show that with a ZnO content of above 0.5 wt.%, N2 permeability starts to decrease while CO2 permeability continues to increase when ZnO content is up to 2 wt.%. They attributed this result to the strong interactions between ZnO and polarizability and quadrupole moment of CO2 molecules.51 According to Galhotra, CO2 molecules’ interaction with ZnO nanoparticles under dry and wet conditions is different. In dry conditions, carbonate, bicarbonates and carboxylates species are formed. Whereas in the presence of co-adsorbed water, only carbonate is formed. Adsorption of H2O on the ZnO nanoparticles in nanofluid highly influences the reaction of CO2 upon ZnO.52 In the presence of an aqueous solution of ZnO and the CO32- anions produced from chemical interaction of CO2 with water, zinc carbonate hydroxide species are easily formed and precipitated (Eqs. 4-6).28 On the contrary, ZnO reaction with CO2 in the form of gas is too slow, and proceeds better in a humid gas.39

2H2O ↔ H3O+ + OH-

(4)

CO2 + 2H2O ↔ H3O+ + HCO3-

(5)

HCO3- + H2O ↔ H3O+ + CO32-

(6)

Taira et al. implemented ZnO for CO2 adsorption from humid gases. According to their results, CO2 chemisorption takes place on ZnO particles, and after an adsorption test, the carbonate species formed on the surface of ZnO particles still remained after being purged by N2 gas.53 Kansha et al. introduced a process for CO2 chemical absorption by ZnO solids in a fluidized bed. 18 ACS Paragon Plus Environment

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Energy & Fuels

Their results show that this process is highly economical compared to conventional amine-based chemical absorption processes.54 Another validation for the CO2 chemisorption on ZnO nanoparticles was gained from Haghtalab et al. They explored the existence of carbonate species experimentally by comparing the SEM images of reacted and unreacted ZnO nanoparticles. They concluded that chemical reaction occurs at the surface of ZnO nanoparticles. As a result, CO2 is converted to carbonate species and the surface Gibbs energy of the nanoparticles depletes.28 Noei et al. and Buchholz et al. investigated the interaction of CO2 with ZnO and hydroxylated ZnO nanoparticles through ultra-high infrared spectroscopy. Their results revealed that different carbonate species such as monodentate, bidentate and tridentate are formed on different active sites of ZnO nanoparticles.55,56 CO2 adsorption on metal oxides such as ZnO may occur on three different surfaces such as 1) a polar surface terminated by Zn atoms (0001), 2) a polar surface terminated by O atoms, and 3) a non-polar surface terminated by both Zn and O atoms. The partial density of states (PDOS) experiments taken by Farias et al. shows the chemical adsorption of CO2 on ZnO (0001) surface, and visualizes that the p orbitals of CO2 are mixed with ZnO valance band.57 By contrast, in another work, Tang et al. concluded that depending on the type of the terminated surface configuration, CO2 may physisorb or chemisorb on the ZnO surface. CO2 molecules chemisorb on the terminated surfaces by mixed O and Zn atoms and physisorb on singly-terminated surfaces.58 Many studies26,27 have reported TiO2 nanofluid as the best CO2 nanoabsorbent among CuO, MgO, SiO2 and Al2O3. TiO2 nanoparticles were chosen in this work since their CO2 adsorption capacity is larger than almost all other nanoparticles. It causes higher enhancements in CO2 mass transfer compared to most nanoparticles. However, as it was shown in this study, both ZnO and


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MWCNT nanofluids had much better CO2 absorption performance than titania in membrane contactors. It is interesting to note that in a study by Mirzazadeh Ghanadi et al., similar nanoparticles had been implemented. They used water-based ZnO, MWCNT and TiO2 nanofluids to enhance the mass transfer rate of liquid-liquid extraction for an n-butanol-succinic acid-water system. The concentration range of the nanoparticles was 0.025-0.1 wt.%. Their results reveal that all nanoparticles enhance the mass transfer rate. The interesting part of these results which has considerable accordance with the result gained in the present work is that ZnO performance in mass transfer enhancement was better than MWCNT and TiO2, and the mass transfer rate in the presence of MWCNT was higher than TiO2 nanofluid. Moreover, as shown by the results of the present work, Mirzazadeh Ghanadi et al. concluded that the effectiveness of nanoparticles is much more distinctive at low liquid flow rates.59 Carbon nanotubes have great potential for use in various fields such as electronics, catalyst support, gas storage and composite materials. According to the previous studies, CO2 molecules are adsorbed on the surface of MWCNT nanoparticles.8,38 From an economic point of view, the most important factor that constrains the widespread use of CNT nanoparticles is their cost.

4. CONCLUSIONS In this study, a membrane gas absorption unit was constructed to investigate the effect of ZnO, TiO2 and MWCNT nanofluids on CO2 absorption in a polypropylene HFMC. The effects of gas flow rate, liquid flow rate and concentration of nanofluids on CO2 absorption were evaluated. It 20 ACS Paragon Plus Environment

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was concluded that, among the three nanofluids implemented in this study, ZnO was the best in performance in addition to having the lowest price. MWCNT performed a little better than TiO2 nanofluid for CO2 absorption. Therefore, ZnO nanofluid is a promising candidate and one of the most economical nanoparticles for CO2 absorption. The largest enhancement in mass transfer was seen in every single nanofluid at low liquid flow rates. At low liquid velocities, CO2 molar flux in the presence of ZnO nanofluid was 1.3 times greater than the amount of the base fluid. Also the two other nanofluids could increase CO2 molar flux by 60%. By increasing nanoparticle concentration (in all nanofluids), CO2 removal increases up to a certain value. This, however, decreases in higher concentrations of nanoparticles. At gas flow rates of about 300 ml/min, CO2 absorption efficiency was increased by 59% for 0.15 wt.% ZnO, 50% for 0.15 wt.% MWCNT and 34% for 0.15 wt.% TiO2 nanofluid with respect to distilled water. All of the nanofluids prepared in this work had perfect long term stability, which was monitored by UV-Visible spectroscopy. However, MWCNT had outstanding long term stability. There was no perceptible change in the UV-visible absorbency of MWCNT and TiO2 nanofluids even after 5 days, while for ZnO nanofluid this period lasted for 2 days. Moreover, all three approaches of stability evaluation authenticate the same result. It is concluded that the presence of a few amount of nanoparticles in the base fluid could highly enhance the absorption efficiency and CO2 mass transfer.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5. REFERENCES (1) Ho, S.-H.; Chen, C.-Y.; Lee, D.-J.; Chang, J.-S., Perspectives on microalgal CO2-emission mitigation systems — A review. Biotechnology Advances 2011, 29, (2), 189-198. (2) Hajilary, N.; Shahi, A.; Rezakazemi, M., Evaluation of socio-economic factors on CO2 emissions in Iran: Factorial design and multivariable methods. J. Clean. Prod. 2018, 189, 108115. (3) WMO Greenhouse Gas Bulletin: The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2016, No. 13, 30 October 2017, http://library.wmo.int/opac/doc_num.php?explnum_id=4022 (4) Yarveicy, H.; Ghiasi, M. M.; Mohammadi, A. H., Performance evaluation of the machine learning approaches in modeling of CO2 equilibrium absorption in Piperazine aqueous solution. Journal of Molecular Liquids 2018, 255, 375-383. (5) Mac Dowell, N.; Fennell, P. S.; Shah, N.; Maitland, G. C., The role of CO2 capture and utilization in mitigating climate change. Nature Clim. Change 2017, 7, (4), 243-249. (6) Kenarsari, S. D.; Yang, D.; Jiang, G.; Zhang, S.; Wang, J.; Russell, A. G.; Wei, Q.; Fan, M., Review of recent advances in carbon dioxide separation and capture. RSC Advances 2013, 3, (45), 22739. (7) Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D., Advances in CO2 capture technology-The U.S. Department of Energy's Carbon Sequestration Program. International Journal of Greenhouse Gas Control 2008, 2, (1), 9-20. (8) Golkhar, A.; Keshavarz, P.; Mowla, D., Investigation of CO2 removal by silica and CNT nanofluids in microporous hollow fiber membrane contactors. Journal of Membrane Science 2013, 433, (0), 17-24. (9) Masoumi, S.; Keshavarz, P.; Ayatollahi, S.; Mehdipour, M.; Rastgoo, Z., Enhanced Carbon Dioxide Separation by Amine-Promoted Potassium Carbonate Solution in a Hollow Fiber Membrane Contactor. Energy & Fuels 2013, 27, (9), 5423-5432. (10) Peyravi, A.; Keshavarz, P.; Mowla, D., Experimental Investigation on the Absorption Enhancement of CO2 by Various Nanofluids in Hollow Fiber Membrane Contactors. Energy & Fuels 2015, 29, (12), 8135-8142. (11) Mehdipour, M.; Keshavarz, P.; Seraji, A.; Masoumi, S., Performance analysis of ammonia solution for CO2 capture using microporous membrane contactors. International Journal of Greenhouse Gas Control 2014, 31, 16-24. (12) Mondal, M. K.; Balsora, H. K.; Varshney, P., Progress and trends in CO2 capture/separation technologies: A review. Energy 2012, 46, (1), 431-441. 22 ACS Paragon Plus Environment

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(13) Rostami, S.; Keshavarz, P.; Raeissi, S., Experimental study on the effects of an ionic liquid for CO2 capture using hollow fiber membrane contactors. International Journal of Greenhouse Gas Control 2018, 69, 1-7. (14) Gabelman, A.; Hwang, S.-T., Hollow fiber membrane contactors. Journal of Membrane Science 1999, 159, (1–2), 61-106. (15) Bazhenov, S. D.; Lyubimova, E. S., Gas–liquid membrane contactors for carbon dioxide capture from gaseous streams. Petroleum Chemistry 2016, 56, (10), 889-914. (16) Gómez-Coma, L.; Garea, A.; Irabien, Á., Hybrid Solvent ([emim][Ac]+water) To Improve the CO2 Capture Efficiency in a PVDF Hollow Fiber Contactor. ACS Sustainable Chemistry & Engineering 2017, 5, (1), 734-743. (17) Keshavarz, P.; Ayatollahi, S.; Fathikalajahi, J., Mathematical modeling of gas-liquid membrane contactors using random distribution of fibers. Journal of Membrane Science 2008, 325, (1), 98-108. (18) Keshavarz, P.; Fathikalajahi, J.; Ayatollahi, S., Mathematical modeling of the simultaneous absorption of carbon dioxide and hydrogen sulfide in a hollow fiber membrane contactor. Separation and Purification Technology 2008, 63, (1), 145-155. (19) Keshavarz, P.; Fathikalajahi, J.; Ayatollahi, S., Analysis of CO2 separation and simulation of a partially wetted hollow fiber membrane contactor. Journal of Hazardous Materials 2008, 152, (3), 1237-1247. (20) Zhao, S.; Feron, P. H. M.; Deng, L.; Favre, E.; Chabanon, E.; Yan, S.; Hou, J.; Chen, V.; Qi, H., Status and progress of membrane contactors in post-combustion carbon capture: A state-ofthe-art review of new developments. Journal of Membrane Science 2016, 511, 180-206. (21) Elhajj, J.; Al-Hindi, M.; Azizi, F., A Review of the Absorption and Desorption Processes of Carbon Dioxide in Water Systems. Industrial & Engineering Chemistry Research 2014, 53, (1), 2-22. (22) Lu, J.-G.; Lu, C.-T.; Chen, Y.; Gao, L.; Zhao, X.; Zhang, H.; Xu, Z.-W., CO2 capture by membrane absorption coupling process: Application of ionic liquids. Applied Energy 2014, 115, 573-581. (23) Pang, C.; Lee, J. W.; Kang, Y. T., Review on combined heat and mass transfer characteristics in nanofluids. International Journal of Thermal Sciences 2014, 87, 49-67. (24) Choi, S. U. S., Enhancing thermal conductivity of fluids with nanoparticles. ASMEPublications-Fed 1995, 231, 99-106.


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(25) Lu, S.; Xing, M.; Sun, Y.; Dong, X., Experimental and Theoretical Studies of CO2 Absorption Enhancement by Nano-Al2O3 and Carbon Nanotube Particles. Chinese Journal of Chemical Engineering 2013, 21, (9), 983-990. (26) Jiang, J.; Zhao, B.; Zhuo, Y.; Wang, S., Experimental study of CO2 absorption in aqueous MEA and MDEA solutions enhanced by nanoparticles. International Journal of Greenhouse Gas Control 2014, 29, 135-141. (27) Wang, T.; Yu, W.; Liu, F.; Fang, M.; Farooq, M.; Luo, Z., Enhanced CO2 Absorption and Desorption by Monoethanolamine (MEA)-Based Nanoparticle Suspensions. Industrial & Engineering Chemistry Research 2016, 55, (28), 7830-7838. (28) Haghtalab, A.; Mohammadi, M.; Fakhroueian, Z., Absorption and solubility measurement of CO2 in water-based ZnO and SiO2 nanofluids. Fluid Phase Equilibria 2015, 392, 33-42. (29) Lee, J. S.; Lee, J. W.; Kang, Y. T., CO2 absorption/regeneration enhancement in DI water with suspended nanoparticles for energy conversion application. Applied Energy 2015, 143, 119129. (30) Lee, J. W.; Torres Pineda, I.; Lee, J. H.; Kang, Y. T., Combined CO2 absorption/regeneration performance enhancement by using nanoabsorbents. Applied Energy 2016, 178, 164-176. (31) Lee, J. H.; Lee, J. W.; Kang, Y. T., CO2 regeneration performance enhancement by nanoabsorbents for energy conversion application. Applied Thermal Engineering 2016, 103, 980988. (32) Nguyen, D.; Stolaroff, J.; Esser-Kahn, A., Solvent Effects on the Photothermal Regeneration of CO2 in Monoethanolamine Nanofluids. ACS Applied Materials & Interfaces 2015, 7, (46), 25851-25856. (33) Mohammaddoost, H.; Azari, A.; Ansarpour, M.; Osfouri, S., Experimental investigation of CO2 removal from N2 by metal oxide nanofluids in a hollow fiber membrane contactor. International Journal of Greenhouse Gas Control 2018, 69, 60-71. (34) Zhang, Z.; Cai, J.; Chen, F.; Li, H.; Zhang, W.; Qi, W., Progress in enhancement of CO2 absorption by nanofluids: A mini review of mechanisms and current status. Renewable Energy 2018, 118, 527-535. (35) Ashrafmansouri, S. S.; Nasr Esfahany, M., Mass transfer in nanofluids: A review. International Journal of Thermal Sciences 2014, 82, (1), 84-90. (36) Yang, L.; Du, K.; Niu, X. F.; Cheng, B.; Jiang, Y. F., Experimental study on enhancement of ammonia–water falling film absorption by adding nano-particles. International Journal of Refrigeration 2011, 34, (3), 640-647.


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(37) Nabipour, M.; Keshavarz, P.; Raeissi, S., Experimental investigation on CO2 absorption in Sulfinol-M based Fe3O4 and MWCNT nanofluids. International Journal of Refrigeration 2017, 73, 1-10. (38) Rahmatmand, B.; Keshavarz, P.; Ayatollahi, S., Study of Absorption Enhancement of CO2 by SiO2, Al2O3, CNT, and Fe3O4 Nanoparticles in Water and Amine Solutions. Journal of Chemical & Engineering Data 2016, 61, (4), 1378-1387. (39) Hashim, A. A., Smart nanoparticles technology: InTech., 2012. (40) Keshishian, N.; Nasr Esfahany, M.; Etesami, N., Experimental investigation of mass transfer of active ions in silica nanofluids. International Communications in Heat and Mass Transfer 2013, 46, 148-153. (41) Yazid, M. N. A. W. M.; Sidik, N. A. C.; Yahya, W. J., Heat and mass transfer characteristics of carbon nanotube nanofluids: A review. Renewable and Sustainable Energy Reviews 2017, 80, 914-941. (42) Kim, W.-g.; Kang, H. U.; Jung, K.-m.; Kim, S. H., Synthesis of Silica Nanofluid and Application to CO2 Absorption. Separation Science and Technology 2008, 43, (11-12), 30363055. (43) Kim, J. H.; Jung, C. W.; Kang, Y. T., Mass transfer enhancement during CO2 absorption process in methanol/Al2O3 nanofluids. International Journal of Heat and Mass Transfer 2014, 76, 484-491. (44) Kluytmans, J. H. J.; van Wachem, B. G. M.; Kuster, B. F. M.; Schouten, J. C., Mass transfer in sparged and stirred reactors: influence of carbon particles and electrolyte. Chemical Engineering Science 2003, 58, (20), 4719-4728. (45) Baltrusaitis, J.; Schuttlefield, J.; Zeitler, E.; Grassian, V. H., Carbon dioxide adsorption on oxide nanoparticle surfaces. Chemical Engineering Journal 2011, 170, (2), 471-481. (46) Krishnamurthy, S.; Bhattacharya, P.; Phelan, P. E.; Prasher, R. S., Enhanced Mass Transport in Nanofluids. Nano Letters 2006, 6, (3), 419-423. (47) Yoon, S.; Chung, J. T.; Kang, Y. T., The particle hydrodynamic effect on the mass transfer in a buoyant CO2-bubble through the experimental and computational studies. International Journal of Heat and Mass Transfer 2014, 73, 399-409. (48) Wu, W.-D.; Liu, G.; Chen, S.-X.; Zhang, H., Nanoferrofluid addition enhances ammonia/water bubble absorption in an external magnetic field. Energy and Buildings 2013, 57, 268-277.


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(49) Yong, J. K. J.; Stevens, G. W.; Caruso, F.; Kentish, S. E., The use of carbonic anhydrase to accelerate carbon dioxide capture processes. Journal of Chemical Technology & Biotechnology 2015, 90, (1), 3-10. (50) Moradihamedani, P.; Ibrahim, N. A.; Ramimoghadam, D.; Yunus, W. M. Z. W.; Yusof, N. A., Polysulfone/zinc oxide nanoparticle mixed matrix membranes for CO2/CH4 separation. Journal of Applied Polymer Science 2014, 131, (16), n/a-n/a. (51) Dilshad, M. R.; Islam, A.; Sabir, A.; Shafiq, M.; Butt, M. T. Z.; Ijaz, A.; Jamil, T., Fabrication and performance characterization of novel zinc oxide filled cross-linked PVA/PEG 600 blended membranes for CO2/N2 separation. Journal of Industrial and Engineering Chemistry 2017. (52) Galhotra, P. Carbon dioxide adsorption on nanomaterials. Ph.D. Dissertation, University of Iowa, 2010. (53) Taira, K.; Nakao, K.; Suzuki, K., CO2 capture in humid gas using ZnO/activated carbon and ZnO reactivity with CO2. Reaction Kinetics, Mechanisms and Catalysis 2015, 115, (2), 563-579. (54) Kansha, Y.; Ishizuka, M.; Mizuno, H.; Tsutsumi, A., Design of energy-saving carbon dioxide separation process using fluidized bed. Applied Thermal Engineering 2017, 126, 134138. (55) Noei, H.; Wöll, C.; Muhler, M.; Wang, Y., Activation of Carbon Dioxide on ZnO Nanoparticles Studied by Vibrational Spectroscopy. The Journal of Physical Chemistry C 2011, 115, (4), 908-914. (56) Buchholz, M.; Weidler, P. G.; Bebensee, F.; Nefedov, A.; Woll, C., Carbon dioxide adsorption on a ZnO(10[1 with combining macron]0) substrate studied by infrared reflection absorption spectroscopy. Physical Chemistry Chemical Physics 2014, 16, (4), 1672-1678. (57) Farias, S. A. S.; Longo, E.; Gargano, R.; Martins, J. B. L., CO2 adsorption on polar surfaces of ZnO. Journal of Molecular Modeling 2013, 19, (5), 2069-2078. (58) Tang, Q.-L.; Luo, Q.-H., Adsorption of CO2 at ZnO: A Surface Structure Effect from DFT+U Calculations. The Journal of Physical Chemistry C 2013, 117, (44), 22954-22966. (59) Mirzazadeh Ghanadi, A.; Heydari Nasab, A.; Bastani, D.; Seife Kordi, A. A., The Effect of Nanoparticles on the Mass Transfer in Liquid–Liquid Extraction. Chemical Engineering Communications 2015, 202, (5), 600-605. (60) Jorge, L.; Coulombe, S.; Girard-Lauriault, P.-L., Nanofluids Containing MWCNTs Coated with Nitrogen-Rich Plasma Polymer Films for CO2 Absorption in Aqueous Medium. Plasma Processes and Polymers 2015, 12, (11), 1311-1321.


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(61) Darabi, M.; Rahimi, M.; Molaei Dehkordi, A., Gas absorption enhancement in hollow fiber membrane contactors using nanofluids: Modeling and simulation. Chemical Engineering and Processing: Process Intensification 2017, 119, 7-15.


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Table 1. Physical properties of used nanoparticles Nanoparticle

Multi wall Carbon

Nano Zinc oxide

Nano Titania P25

nanotube Formula

MWCNT

ZnO

TiO2

APS (nm)

10-20

10-30

21

SSA (m2/gr)

+200

20-60

50 ±15

Color

Black

White

White

Crystal morphology

Tubular

Nearly spherical

Spherical

True density (gr/cm3)

2.1

5.606

5.5-6

Surface treatment

Functionalized by carboxyl -

-

group

Table 2. Specifications of used HFMC Inner diameter of fibers (mm)

0.275

Outer diameter of fibers (mm)

0.375

Effective length of fibers (cm)

18

Average pore size (nm)

200

Porosity

0.6

Contact area (cm2)

1484.4

Number of fibers

350

Inner diameter of module (cm)

1.5


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Table 3. Periods of stability of nanofluids 0.15 wt.%

0.1 wt.%

0.15 wt.%

0.15 wt.%

Type of the

MWCNT

MWCNT

TiO2

ZnO nanofluid

nanofluid

nanofluid

nanofluid (Peyravi

nanofluid

(This work)

(This work)

et al., 2015)10

(This work)

More than 120 h

9h

About 120 h

Period of

About 50 h

stability

Table 4. Zeta potential of MWCNT, Titania and Zinc Oxide nanofluids Nanofluid

Zeta potential (mv)

pH

MWCNT (-COOH)

+41.4

7.10

TiO2

+38.1

6.92

ZnO

+32.0

7.90

Table 5. CO2 absorption amount and price data for ZnO and MWCNT (prices retrieved from Sigma-Aldrich.com as of January 2018) ZnO price

CO2 absorption

MWCNT price

CO2 absorption

(USD/g ZnO)

(mol CO2/USD)

(USD/g MWCNT)

(mol CO2/USD)

11.46 USD/g

0.00224

135 USD/g

0.00013


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 6. Comparison of mass transfer enhancement factors References

Nanofluid

Base fluid

Device

Liquid flow rate

Max. E.

(ml/min)

Factor

This work

ZnO

DW

PP HFMC

10

2.3

This work

MWCNT

DW

PP HFMC

10

1.6

This work

TiO2

DW

PP HFMC

10

1.6

Mohammaddoost et al. (2018) 33

Al2O3

DW

PP HFMC

103

2.75


SiO2

DW

PP HFMC

103

2


TiO2

DW

PP HFMC

103

1.8

Peyravi et al. (2015)10

Fe3O4

DW

PP HFMC

7*103

1.45

Peyravi et al. (2015) 10

MWCNT

DW

PP HFMC

7*103

1.4

Jorge et al. (2015) 60

MWCNT

DW

Bubble column

-

1.36

Haghtalab et al. (2015) 28

ZnO

DW

Batch stirred vessel

-

1.14

Haghtalab et al. (2015) 28

SiO2

DW

Batch stirred vessel

-

1.07

Darabi et al. (2017) 61

CNT

DW

HFMC

104

1.32

Darabi et al. (2017) 61

SiO2

DW

HFMC

104

1.16

Lee et al. (2015) 29

SiO2

DW

Bubble column

-

1.235

Kim et al. (2008) 42

SiO2

DW

Bubble column

-

1.24


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Figure 1. Schematic diagram of the experimental apparatus. 1. Liquid tank; 2. Peristaltic pump 3. HFMC (Membrane module); 4. Waste solvent tank; 5. Gas cylinder; 6. CO2 analyzer; 7. Flowmeter; 8. Pressure gauge; 9. Rotameter


Energy & Fuels

2.5

2

Absorbance

0.5

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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R square: 0.994

0.45 0.4

1.5 200

250

300

Wavelength (nm)

1

0.5

0 0

10

20

30

40

Concentration (ppm) Figure 2. MWCNT nanofluid standard curve and UV-Vis absorbance spectra


50

Page 33 of 47

0.9

0.7

Absorbance

0.6

0.5 Absorbance

0.8

0.3 0.2 0.1

0.5

R square: 0.999

0.4

320

340

360

380

400

Wavelength (nm)

0.4 0.3 0.2 0.1 0 0

10

20

30

40

50

Concentration (ppm) Figure 3. ZnO nanofluid standard curve and UV-Vis absorbance spectra

2 1.8

1.4 1.2

1.04 Absorbance

1.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

R square: 0.994

1.035 1.03 1.025 250

1

255

260

265

Wavelength (nm)

0.8 0.6 0.4 0.2 0 0

5

10

15

20

25

30

Concentration (ppm) Figure 4. TiO2 nanofluid standard curve and UV-Vis absorbance spectra


Energy & Fuels

1.1 1.0 0.9 0.8 C/Co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.7 0.6 0.5

0.15%wt. CNT

0.4

0.15%wt. TiO2 0.15%wt. ZnO

0.3

0.1%wt. CNT stability (Peyravi et al., 2015)

0.2 0

20

40

60

80

100

Time (hr)

Figure 5. Concentration ratio of nanofluids vs. time


120

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Figure 6. Photographs of ZnO nanofluid (on the left) and TiO2 nanofluid (on the right); the numbers in the boxes represent the number of days

100

40 35

80

25

60

20 40

15 10

20

5 0 1

10

100

1000

0 10000

Diameter (nm)

Figure 7. Size distribution of TiO2 nanofluid (Concentration=0.15 wt.%, numbered base)


Channel (%)

30

Passing (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 100

80

15

60 10 40

Channel (%)

Passing (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 20

0 1

10

100

1000

0 10000

Diameter (nm)

Figure 8. Size distribution of ZnO nanofluid (Concentration=0.15 wt.%, numbered base)


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100

40 35

80

25

60

20 40

15

Channel (%)

30

Passing (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10

20

5 0 1

10

100

1000

0 10000

Diameter (nm)

Figure 9. Size distribution of MWCNT nanofluid (Concentration=0.15 wt.%, numbered base)


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Figure 10. Hydrodynamic diameter variation with nanoparticle concentration for ZnO, TiO2 and MWCNT nanofluids


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0.0075 0.007 J CO2 ×102 (mol/m2.s)

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0.0065 0.006 0.0055 ZnO

0.005

Mw-CNT TiO2

0.0045 0

0.05

0.1

0.15

0.2

NP Concentration (%wt.)

Figure 11. Effect of nanoparticle concentration on CO2 molar flux (Qg = 150 ml/min, Ql = 40 ml/min, CiCO2 = 21%mol)


0.25

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Figure 12. Effect of nanoparticle concentration on enhancement factor (Qg = 150 ml/min, Ql = 40 ml/min, CiCO2 = 21%mol)


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Figure 13. Effect of liquid flow rate on CO2 molar flux (Qg = 300 ml/min, Np. Concentration=0.15 wt.%, CiCO2 = 21.1%mol)


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Figure 14. Effect of liquid flow rate on enhancement factor (Qg = 300 ml/min, Np. Concentration=0.15 wt.%, CiCO2 = 21.1%mol)


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Figure 15. Effect of gas flow rate on CO2 molar flux (Ql = 50 ml/min, Np. Concentration=0.15 wt.%, CiCO2 = 21%mol)


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Figure 16. Effect of gas flow rate on enhancement factor (Ql = 50 ml/min, Np. Concentration=0.15 wt.%, CiCO2 = 21%mol)


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Figure 17. Schematic of bubble coalescence inhibition (bubble-breaking) mechanism


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(a)

(b)

Figure 18. Schematic of shuttle (Grazing) effect mechanism. a) Before CO2 absorption b) During CO2 absorption


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Figure 19. Schematic of hydrodynamic effect mechanism


Membrane Absorption Coupling Process for CO2 Capture: Application

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