- Email: [email protected]
nano-sized CO2 bubbles
Journal of CO₂ Utilization 34 (2019) 430–436
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Reduction of Na and K contents in bio-heavy oil using micro-/nano-sized CO2 bubbles
T ⁎
Youkwan Kima,1, Jeong-Ik Ohb,1, Ming Zhangc, Jechan Leed, Young-Kwon Parke, Kyun Ho Leef, , ⁎ Eilhann E. Kwona, a
Department of Environment and Energy, Sejong University, Seoul, 05006, Republic of Korea Advanced Technology Department, Land & Housing Institute, Daejeon, 34047, Republic of Korea c Department of Environmental Engineering, China Jiliang University No. 258, Xueyuan Street, Hangzhou, Zhejiang, 310018, PR China d Department of Environmental and Safety Engineering, Ajou University, Suwon, 16499, Republic of Korea e School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea f Department of Aerospace Engineering, Sejong University, Seoul, 05006, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 Micro-/nano-bubbles Bio-heavy oil Metal removal Waste-to-energy
Co-combustion of bunker-C oil and bio-heavy oil for power generation provides a strategic way for mitigating the environmental burdens (climate change) due to the intrinsic carbon neutrality of bio-heavy oil. Prior to the cocombustion process, the high content of Na and K in heavy oil should be removed for the operational safety. Given that the washing process has been currently practiced for metal removal, it is desirable to develop the chemical-free washing process. Note that H2SO4 was commonly used during the washing process. To this end, this study laid great emphasis on the possible use of CO2 for the washing process. To increase the removal efficiency of Na and K, this study particularly employed micro-/nano-sized CO2 bubbles. To access the in-depth insights, a computational fluid dynamics (CFD) simulation was conducted prior to a lab-scale experimental work. The CFD simulation suggests that CO2 bubbling expedited the mixability during the washing process. Also, the CFD simulation suggested that the CO2 bubbling effects could be enhanced because the high viscosity of bioheavy oil cancelled out the buoyancy force. A lab-scale experimental work was also in good agreement with the CFD simulation. In the presence of CO2 bubbles, 80% of Na and K was removed when the volumetric ratio of water and bio-heavy oil was 8. Even under the volumetric ratio condition of 0.125, the metal removal efficiency was not substantially reduced. Thus, all experimental findings signify that metal removal by micro-/nano-sized CO2 bubbling should be done by a cascade system.
1. Introduction
and/or commercialization has been readily achieved as compared with other biofuels (i.e., biohydrogen and bioethanol) [7,8]. Indeed, TGs have been known as one of the most common chemical compounds in all living organisms, and their ubiquitousness serves a crucial role for securing the supply chain of TGs [9,10]. Moreover, the use of biodiesel has been further expanded by the political legislations, such as renewable fuel standard (RFS) [11]. Accordingly, based on the department of energy (DOE), the global production of biodiesel reached up to 2.89 billion gallons in 2017. Despite many socio-economic benefits from the use of biodiesel, the massive generation of wastes from the biodiesel industry has not been considered with the fully transparent manners [12]. Considering that biodiesel production will continue to increase with the development of 2nd (i.e., biodiesel converted from the inedible lipid feedstocks) and 3rd
Among anthropogenic greenhouse gases (GHGs), carbon dioxide (CO2) (from combustion of fossil fuels) has been considered as one of the main contributors for resulting in the global carbon imbalance [1]. Anthropogenic CO2 brings forth the global environmental issues (i.e., global warming and climate change) because the total amount of CO2 from combustion of fossil fuels is indeed over the Earth’s full capacity to sequester CO2 via the complex natural carbon cycles [2,3]. Therefore, the use of biodiesel as an alternative to petro-diesel provides a strategic venue for soothing the adverse environmental impacts arising from global warming due to its carbon neutrality [4–6]. Given that biodiesel is converted from triglycerides (TGs) via the simple process (the acid/ base catalyzed transesterification process), its practical implementation ⁎
Corresponding authors. E-mail addresses: [email protected] (K. Ho Lee), [email protected] (E.E. Kwon). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jcou.2019.07.031 Received 24 January 2019; Received in revised form 7 June 2019; Accepted 22 July 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
(i.e., biodiesel converted from the microalgal lipid) generation of biodiesel [13–16], seeking an effective and reliable way to dispose wastes from the biodiesel industry is of importance. Note that the substantial amount (3–5 wt.% of the lipid feedstock) of biodiesel wastes (i.e., fatty acids (FAs), mono-, di-, and tri-glycerides), namely “bio-heavy oil” or “bio-heating oil”, has been produced, but an effective disposal method has not been established. Currently, most biodiesel wastes (bio-heavy oil) are being co-combusted with petro-derived heavy oil, such as bunker-C oil, for power generation because co-combustion of bunker-C oil and bio-heavy oil provides a strategic means for satisfying the renewable portfolio standard (RPS) [17]. Harnessing the carbon neutral source in power generation (electricity) has been legislatively recommended by RPS [18]. Moreover, the use of bio-heavy oil offers diverse environmental benefits in terms of air pollution controls (APCs) [19]. Considering the structural and compositional matrix of bio-heavy oil (i.e., FAs, mono-, di-, and tri-glycerides), the oxygen content in the carboxyl group of each fatty acid (FA) provides the favorable conditions for the complete oxidation, thereby resulting in the decrease of air pollutants, such as particulate matters (PMs) [19,20]. Nevertheless, one of the main technical challenges to use bio-heavy oil for power generation is the high content of Na and K because such metals cause the nozzle clogging and other operating troubles during combustion of bio-heavy oil. Note that the high content of Na and K in bio-heavy oil is likely attributed to use of base catalysts (i.e., KOH and NaOH) during the biodiesel conversion process [21]. Thus, Na and K contents should be reduced and/or removed from bio-heavy oil. To this end, the acidic solution (H2SO4 solution) is commonly practiced for metal removal due to the high solubility of metals in acidic water [22]. Nonetheless, one of the technical demerits for the washing process is the generation of acidic wastewater (containing metals) [22]. Therefore, it is highly desirable to reduce water usage during the washing process. More preferably, the chemical-free (H2SO4-free) washing process is necessary in that chemical-free wastewater can be a strategic way for reducing the acidic wastewater treatment cost. Among metal removal techniques, the use of CO2 as reactive gas medium to reduce pH during the washing process by bubbling can be a promising alternative. Large bubbles (> 100 μm diameter) rise rapidly (> 6 mm s−1) to the water surface [23]. Microbubbles (1–100 μm diameter) provide a higher surface area per unit volume than the commonly seen larger bubbles [23]. They are not stable for long period (˜ min), rising fast (10-3-10 mm s−1) to the surface, but smaller ones (≤ 20 μm diameter) will shrink to form more effective/stable nanobubbles [23]. Based on these facts, utilizing micro-/nano-sized CO2 bubbles maximize the CO2 effect during the washing process. Therefore, this study laid great emphasis on the removal of metals in bio-heavy oil using micro-/nano-sized CO2 bubbles. To this end, a lab-scale metal removal by introducing micro-/nano-sized CO2 bubbles was carried out to assess the more realistic experimental data. Prior to the lab-scale experimental work, a computational fluid dynamics (CFD) analysis of CO2 bubbles was conducted to understand the fluid behaviors affected by CO2 bubbles more intuitively (or more clearly) depending on their size.
Fig. 1. Schematic layout depicting a numerical grid of a reservoir for the CFD simulation (grid matrix: ≥ 50,000).
at the left wall of the reactor. To investigate the realistic effects of the CO2 bubbles on the internal flow behaviors in the reactor, the EulerianEulerian approach and the shear-stress transport (SST) k-ω turbulence model were used to simply simulate the two-phase flow of the liquid phase and CO2 bubble [24]. In addition, the laminar or shear-stress transport (SST) k-ω turbulence model was chosen depending on the value of the Reynold number. An unsteady simulation was carried out to consider the distribution of the micro-/nano-sized CO2 bubbles over the time scale of 1,200 s (20 min). We assumed that 1,200 s is sufficient to complete the distribution of CO2 bubbles into the reactor. The computation was assumed to be converged at each time step when all residuals were less than 10−3. The flow parameters and boundary conditions used for the present CFD simulation are summarized in Table 1. The velocity inlet condition was assumed for the inflow of the liquid phase and CO2 bubbles with 200 mL min-1 while the pressure outlet condition was set to the outlet. A volume fraction of the CO2 bubbles was initially assumed as 0.1% in the liquid phase. The degassing condition was applied to the liquid surface to model the removal of the floated bubbles. In sum, a major difference is that the real reactor is three-dimensional while the CFD simulation considers two-dimensional plane. But the width of the reactor reservoir in Fig. 1 has a similar order of magnitude with its height and length and boundary effects of the reservoir walls would be minor because the width is much larger than a boundary layer thickness. In such case, the CFD simulation generally can simplify the three-dimensional real problem with the two-dimensional model. Thus, it can be assumed that the present simulation result would have a good agreement with the three-dimensional, and reduce the computational effort. 2.2. Chemical reagents and materials Bio-heavy oil was obtained from Dansuk (www.dansuk.co.kr) Co. Ltd (Ansan-City, Korea). Nano-bubbler was customized, and its working mechanisms were demonstrated in the literatures [25–27]. The CO2 input to the reactor was 200 mL min−1, and its flow rate was controlled using the mass flow controller (Brook E series, USA). Deionized water (DI water) was prepared using the Milli-DI water purification system (MilliporeSigma, USA). Laboratory agitator (IKA Eurostar 20, Republic of Korea) was used to achieve the complete mixture of bio-heavy oil and DI water. pH meter (Mettler Toledo, Switzerland) was used to monitor the pH change during the metal removal. Note that various volumetric mixtures of bio-heavy oil and water (i.e., volumetric ratio of bio-heavy oil to water: 1:8, 1:1, 8:1) were tested in this study.
2. Materials and methods 2.1. Numerical modeling for computational fluid dynamics (CFD) For a CFD simulation, a two-dimensional (2D) configuration of the CO2 bubble reservoir (a metal removal reactor) was chosen. Its dimension was 0.27 m of length and 0.25 m of height. Note that the dimension in Fig. 1 was determined by the actual configuration of the reactor except 0.17 m of width. As shown in Fig. 1, more than 50,000 structured rectangular grids were being considered for the CFD calculation, which included an inlet and outlet of the CO2 bubbles and liquid phase. The CO2 bubbles and the liquid phase were injected from the inlet at the right wall, and the liquid phase was exhausted to the outlet
2.3. Metal measurement in water To evaluate the metal removal efficiency, the mixture (bio-heavy oil and water) was separated by means of the centrifugation (7000 rpm for 431
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
Table 1 Boundary conditions and flow parameters. Flow parameters Diameter of CO2 bubble [μm] Inlet volume fraction of CO2 bubble [%] Flow rate [mL min−1]
Boundary condition 100, 1, 0.01 0.1 200
Inflow Outflow Water surface
Velocity inlet Pressure outlet Degassing
3 min). The concentration of metals in water was measured using an Inductive Coupled Plasma-Optical Emission Spectrometer (ICP-OES) (Perkin Elmer, USA). Prior to the measurement, the sample was digested using a microwave digestor (Milestone, ETHOS touch control, USA) [28]. Prior to the measurement, the multiple calibrations were performed using the standard solution (ICP-element stand solution, Certipur, Germany).
3. Results and discussion 3.1. CFD simulation The CFD calculation was performed to access the in-depth knowledge on the internal flow behaviors when the CO2 bubbles (200 mL min−1) are introduced for the washing process to remove metals in bioheavy oil. The CFD simulation was conducted to predict the internal flow behaviors based on the CO2 bubble size and the fluid properties (i.e., viscosity). To this end, several bubble diameters (100, 1, and 0.01 μm) and the liquid viscosities were considered as the variables for the CFD simulation. First, the distribution of CO2 bubbles and the bubble velocity were predicted with three different bubble diameters (100, 1, and 0.01 μm) in water. In case of 100 μm diameter of CO2 bubble, the velocity vectors by CO2 bubbles are developed vertically at the inlet of the reactor. As depicted in Fig. 2a, a rapid floatation of CO2 bubbles toward the water surface was observed due to a large buoyancy force, which resulted that CO2 bubbles could not spread in the water. Note that the buoyancy force is proportional to the bubble size [29]. However, Fig. 2c demonstrates that the velocity vectors driven by 0.01 μm diameter of CO2 bubbles are fully developed at the entire region of the reactor. Indeed, the velocity vectors derived from 1 μm diameter of CO2 bubbles at the reactor are well established in Fig. 2b, but the velocity vectors at the bottom of the reactor are not fully developed. Such observations signify that the distribution of the CO2 bubbles at the reactor is highly contingent on the bubble size, which is in good agreement with the general concept for the fluid dynamics. Nonetheless, the simulation in Fig. 2 may not reflect the realistic behaviors for CO2 bubbles in that the viscosity of bio-heavy oil is higher than that of water. Note that the viscosity of fluid is one of the main factors governing the fluid dynamics. To assess the more realistic CFD simulation, the viscosity of soybean oil was chosen as the reference value for the CFD simulation. The fluid density and viscosity of water and soybean oil were given in Table 2 [30], and the CFD simulation results were illustrated in Fig. 3. As well demonstrated in Fig. 3, the overall velocity vectors in the reactor for the case of 100, 1, and 0.01 μm are similar with the cases in Fig. 2. For example, CO2 bubbles in Fig. 3a do not float rapidly to the oil surface as compared with the case of Fig. 2a because of a small buoyancy force. Instead, a large rotation flow of the CO2 bubbles is developed with less than 0.1% volume fraction in the oil. Such observations signify that the higher viscosity of soybean oil relative to the case of water (Fig. 2) leads to a larger friction between CO2 bubble and oil. Therefore, the CFD result in Fig. 3 suggests that the high viscosity of soybean oil seems to effectively restrict the buoyancy effect of CO2 bubbles. In these respects, the magnitude of the velocity vectors begins to decrease, and the more CO2 bubbles were uniformly dispersed in oil through the entire region of the reactor with an extended time. Also, Fig. 3b and c suggest that the bubble size variations do not lead to an
Fig. 2. Volume fraction and velocity vector of the CO2 bubble in water, (a) 100 μm, (b) 1 μm, (c) 0.01 μm.
apparent influence on the internal flow behaviors because the viscosity of soybean oil is more dominant than the buoyancy force of CO2 bubbles. Moreover, Fig. 3b and c present that the bubble velocity vectors at the bottom region of the reactor are not fully developed as compared with the case of Fig. 2c. Such observations suggest that the internal flow behaviors of CO2 at the bottom region of the reactor are not developed due to the high viscosity of soybean oil. Thus, the mechanical agitation 432
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
3.2. pH variations induced by micro-/nano-sized CO2 bubbles
Table 2 Density and viscosity of water and soybean oil. Fluid type
Water (Case 1)
Soybean Oil (Case 2)
Viscosity [kg/m ⋅s] Density [kg/m3]
1.0 × 10−3
57.1 × 10−3
998.2
915.7
To confirm the effectiveness of CO2 bubbles, pH variations were monitored. In. detail, Fig. 4 depicts pH variations as a function of bubbling time. Note that the reference value was obtained by 200 mL min−1 of CO2 injection without considering the bubble size. Fig. 4a depicts that pH is decreasing faster when micro-/nano-sized CO2 bubbles are used, and pH becomes steady in 20 min. Also, Fig. 4a experimentally proves that the faster pH changes are achieved when micro-/nano-sized CO2 bubbles are used. This is in good agreement with the CDF simulation in Fig. 2. In reference to micro-/nano-sized CO2 bubbling, pH variations begin to increase at the bubble time ≥ 50 min when we introduced normal CO2 bubbles. One interesting observation is that variation of pH is negligible for 24 h when we introduced micro-/nano-sized CO2 bubbles (Data not shown in this study). To access the more realistic data, the micro-/nano-sized CO2 bubbles were introduced to water in the reactor for 30 min. 30 min of micro-/nano-sized CO2 bubbling was chosen because pH of water become steady in 20 min in Fig. 4a. And then, bio-heavy oil (i.e., volumetric ratio of water to bio-heavy oil: 8) was added to water. Without providing micro-/nano-sized CO2 bubbles (Fig. 4b), pH of the mixture (water and bio-heavy oil) begins to increase in 20 min. Also, Fig. 4b shows pH of the mixture becomes steady in 20 min when micro-/nanosized CO2 bubbles are introduced the reactor. Despite no supply of micro-/nano-sized CO2 bubbles, Fig. 4b shows that pH of the mixture is lower than the case of micro-/nano-sized CO2 bubbling for 30 min. Such observations imply that CO2 bubbling effectively enhance mixing of water and bio-heavy oil. In other words, mixing between bio-heavy oil and water without introducing micro-/nano-sized CO2 bubbling is hard to be achieved even in the presence of the mechanical agitations. Thus, Fig. 4b implied that CO2 bubbling has the technical merits for lowering pH in line with the enhanced mixability. 3.3. Metal removal by micro-/nano-sized CO2 bubbling over the bubbling time To access the more practical data in terms of metal removal by CO2 bubbling, the concentration of Na, K, and Mg as a function of the CO2 bubbling times was monitored. Note that experimental conditions in Fig. 5 is the identical to the case of Fig. 4b. Fig. 5a demonstrates that Na are removed by CO2 bubbling. In detail, the removal efficiency of Na is rapidly achieved in 1 h, of which removal efficiency is equivalent to 48%. However, Fig. 5a the metal removal rate for Na by CO2 bubbling begins to decrease immediately after 1 h. Note that the removal efficiency of Na is 69, 73, and 79% at 240, 360, and 1440 min, respectively. One of the interesting observations in Fig. 5 is that the concentration profiles (i.e., removal efficiency) of Na and K are nearly identical over the entire CO2 bubbling times. As well demonstrated in Fig. 5a and b, the standard deviation for the removal efficiency of Na and K at the given CO2 bubbling times is less than ± 1%. However, the removal efficiency of Mg by micro-/nanosized CO2 bubbling is inferior to the case of Na and K. Such different removal efficiencies between Na and Mg in Fig. 5 are likely attributed to the different ionic energies. To access the more practical data, the same experimental work was conducted, but the volumetric ratio of water to bio-heavy oil was changed. In detail, the volumetric ratio of water to bio-heavy oil was adjusted to 8, 1, 0.125, and the removal efficiency of Na, K, Ca, and Mg after 24 h micro-/nano-sized CO2 bubbling (according to the different volumetric ratio of water and bioheavy oil) was presented in Fig. 6. As declared before in the earlier section of introduction, saving the use of water during the washing process for metal removal is one of the key technical requirements. Fig. 6 demonstrates that the overall metal removal efficiency for 24 h is proportional to the amount of water. In detail, the high volumetric ratio of water to bio-heavy oil shows the better performance for
Fig. 3. Volume fraction and velocity vector of the CO2 bubble in water, (a) 100 μm, (b) 1 μm, (c) 0.01 μm.
is necessary to develop, and it may be maximized for the case of the mixture of water and bio-heavy oil because the higher viscosity of the mixture (water + bio-heavy oil) provides an effective means for restricting the buoyancy effects of CO2 bubbles. However, the unexpected effect arising from the mechanical agitation (bubble size, stability, etc.) need to be considered. 433
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
Fig. 4. (a) pH variations of water as a function of the CO2 bubbling time, (b) pH variations of the mixture of water and bio-heavy oil (volumetric ratio of water to bioheavy oil is 8) (Error bar was not given due to the less than 1% standard deviation).
metal removal from bio-heavy oil. Note that the solubility of cationic metals is generally proportional to the amount of water. However, even under the low volumetric ratio of water to bio-heavy oil, the overall removal efficiency of Na, K, Ca, and Mg is not substantially reduced. Thus, such observations in Fig. 6 signify that employing the low volumetric ratio of water to bio-heavy oil in line with a cascade for metal removal may have the technical merits for saving the use of water. Note that a cascade is a plant consisting of several similar stages with each processing the output from the previous stage. The removal efficiency of Ca under the low volumetric ratio of water to bio-heavy oil (i.e., ratio of 0.125) is discrepant with the overall metal removal efficiency patterns. Given that the solubility of CO2 in water is highly contingent on the temperatures, the further study for the optimization should be followed in the near future.
flow behaviors of CO2. The CFD simulation suggested that micro-/nanosized CO2 bubbles could be distributed the entire region of the reactor. Such distribution of CO2 bubbles could be enhanced in the presence of bio-heavy oil in that the high viscosity of bio-heavy oil effectively canceled out the buoyancy effect of CO2 bubbles. To access the realistic data, a lab-scale experiment for metal removal from bio-heavy oil was also conducted. All experimental findings from the lab-scale experimental works were in good agreement with the CFD simulation. In the presence of micro-/nano-sized CO2 bubble, ˜80% of Na and K was removed when the volumetric ratio of water and bio-heavy oil was adjusted to 8. Even under the volumetric ratio condition of 1 and 0.125, the metal removal efficiency was not substantially reduced. Thus, all experimental findings signify that metal removal by micro-/nano-sized CO2 bubbling should be done by a cascade system.
4. Conclusions
Declaration of Competing Interest
Prior to a lab-scale experimental work for metal removal from bioheavy oil by employing micro-/nano-sized CO2 bubbling, the CFD simulation were carried out to get the in-depth insights on the internal
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.
Fig. 5. Variation of Na, K, and Mg concentrations over micro-/nano-sized CO2 bubbling time (volumetric ratio of water to bio-heavy oil: 8) (Error bar of the removal efficiency was not given due to the less than 1% standard deviation). 434
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
Fig. 6. Removal efficiency of Na, K, Ca, and Mg by micro-/nano-sized CO2 bubbling for 24 (volumetric ratio of water to bio-heavy oil: 8, 1, and 0.125) (Error bar was not given due to the less than 1% standard deviation, Note that the experimental work was conducted for 24 h).
Acknowledgements
[11] W.E. Tyner, F.J. Dooley, D. Viteri, Alternative pathways for fulfilling the RFS mandate, Am. J. Agric. Econ. 93 (2) (2011) 465–472. [12] Y. Zhang, M.A. Dubé, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: 1. Process design and technological assessment, Bioresour. Technol. 89 (1) (2003) 1–16. [13] R.E.H. Sims, W. Mabee, J.N. Saddler, M. Taylor, An overview of second generation biofuel technologies, Bioresour. Technol. 101 (6) (2010) 1570–1580. [14] M.M.K. Bhuiya, M.G. Rasul, M.M.K. Khan, N. Ashwath, A.K. Azad, M.A. Hazrat, Prospects of 2nd generation biodiesel as a sustainable fuel – part 2: properties, performance and emission characteristics, Renew. Sustain. Energy Rev. 55 (2016) 1129–1146. [15] D.P. Ho, H.H. Ngo, W. Guo, A mini review on renewable sources for biofuel, Bioresour. Technol. 169 (2014) 742–749. [16] E. Jacob-Lopes, T.T. Franco, From oil refinery to microalgal biorefinery, J. CO2 Util. 2 (2013) 1–7. [17] P. Ndayishimiye, M. Tazerout, Use of palm oil-based biofuel in the internal combustion engines: performance and emissions characteristics, Energy 36 (3) (2011) 1790–1796. [18] J.-H. Moon, J.-W. Lee, U.-D. Lee, Economic analysis of biomass power generation schemes under renewable energy initiative with renewable portfolio standards (RPS) in Korea, Bioresour. Technol. 102 (20) (2011) 9550–9557. [19] J. Dharmaraja, D.D. Nguyen, S. Shobana, G.D. Saratale, S. Arvindnarayan, A.E. Atabani, S.W. Chang, G. Kumar, Engine performance, emission and bio characteristics of rice bran oil derived biodiesel blends, Fuel 239 (2019) 153–161. [20] J.P. Szybist, J. Song, M. Alam, A.L. Boehman, Biodiesel combustion, emissions and emission control, Fuel Process. Technol. 88 (7) (2007) 679–691. [21] R. Chamola, M.F. Khan, A. Raj, M. Verma, S. Jain, Response surface methodology based optimization of in situ transesterification of dry algae with methanol, H2SO4 and NaOH, Fuel 239 (2019) 511–520. [22] C.E. Demaman Oro, M. Bonato, J.V. Oliveira, M.V. Tres, M.L. Mignoni, R.M. Dallago, A new approach for salts removal from crude glycerin coming from industrial biodiesel production unit, J. Environ. Chem. Eng. 7 (1) (2019) 102883. [23] G.Z. Kyzas, G. Bomis, R.I. Kosheleva, E.K. Efthimiadou, E.P. Favvas, M. Kostoglou, A.C. Mitropoulos, Nanobubbles effect on heavy metal ions adsorption by activated carbon, Chem. Eng. J. 356 (2019) 91–97. [24] M. Colombo, M. Fairweather, Influence of multiphase turbulence modelling on interfacial momentum transfer in two-fluid Eulerian-Eulerian CFD models of bubbly flows, Chem. Eng. Sci. 195 (2019) 968–984. [25] J.-C. Tsai, M. Kumar, S.-Y. Chen, J.-G. Lin, Nano-bubble flotation technology with coagulation process for the cost-effective treatment of chemical mechanical
This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20163010092290). References [1] W. Cramer, A. Bondeau, F.I. Woodward, I.C. Prentice, R.A. Betts, V. Brovkin, P.M. Cox, V. Fisher, J.A. Foley, A.D. Friend, C. Kucharik, M.R. Lomas, N. Ramankutty, S. Sitch, B. Smith, A. White, C. Young-Molling, Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models, Global Change Biol. 7 (4) (2001) 357–373. [2] D.J. Wuebbles, A.K. Jain, Concerns about climate change and the role of fossil fuel use, Fuel Process. Technol. 71 (1) (2001) 99–119. [3] D. Archer, Fate of fossil fuel CO2 in geologic time, J. Geophys. Res.: Oceans 110 (C9) (2005). [4] C. Kiwjaroun, C. Tubtimdee, P. Piumsomboon, LCA studies comparing biodiesel synthesized by conventional and supercritical methanol methods, J. Cleaner Prod. 17 (2) (2009) 143–153. [5] J.A. Mathews, Carbon-negative biofuels, Energy Policy 36 (3) (2008) 940–945. [6] N. Maheshwari, M. Kumar, I.S. Thakur, S. Srivastava, Carbon dioxide biofixation by free air CO2enriched (FACE) bacterium for biodiesel production, J. CO2 Util. 27 (2018) 423–432. [7] J. Lee, Y.F. Tsang, J.-M. Jung, J.-I. Oh, H.-W. Kim, E.E. Kwon, In-situ pyrogenic production of biodiesel from swine fat, Bioresour. Technol. 220 (2016) 442–447. [8] T.M. Mata, A.A. Martins, N.S. Caetano, Microalgae for biodiesel production and other applications: a review, Renew. Sustain. Energy Rev. 14 (1) (2010) 217–232. [9] J.-M. Jung, K.-H. Kim, E.E. Kwon, H.-W. Kim, Analysis of the lipid profiles in a section of bovine brain via non-catalytic rapid methylation, Analyst 140 (18) (2015) 6210–6216. [10] T.M. Mata, A.A. Martins, S.K. Sikdar, C.A.V. Costa, Sustainability considerations of biodiesel based on supply chain analysis, Clean Technol. Environ. Policy 13 (5) (2011) 655–671.
435
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
T.C.O. Fonseca, V.L. Dressler, É.M..M. Flores, Determination of metals and metalloids in light and heavy crude oil by ICP-MS after digestion by microwave-induced combustion, Microchem. J. 96 (1) (2010) 4–11. [29] G. Sridhar, J. Katz, Drag and lift forces on microscopic bubbles entrained by a vortex, Phys. Fluids 7 (2) (1995) 389–399. [30] V. Rodriguez-Martinez, M. O’Meara, B.E. Farkas, Shreya N. Sahasrabudhe, Density, viscosity, and surface tension of five vegetable oils at elevated temperatures: measurement and modeling, Int. J. Food Prop. 20 (sup2) (2017) 1965–1981.
polishing wastewater, Sep. Purif. Technol. 58 (1) (2007) 61–67. [26] W.B. Zimmerman, V. Tesař, H.C.H. Bandulasena, Towards energy efficient nanobubble generation with fluidic oscillation, Curr. Opin. Colloid Interface Sci. 16 (4) (2011) 350–356. [27] M. Fan, D. Tao, R. Honaker, Z. Luo, Nanobubble generation and its applications in froth flotation (part IV): mechanical cells and specially designed column flotation of coal, Min. Sci. Technol. (China) 20 (5) (2010) 641–671. [28] J.S.F. Pereira, D.P. Moraes, F.G. Antes, L.O. Diehl, M.F.P. Santos, R.C.L. Guimarães,
436
Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Reduction of Na and K contents in bio-heavy oil using micro-/nano-sized CO2 bubbles
T ⁎
Youkwan Kima,1, Jeong-Ik Ohb,1, Ming Zhangc, Jechan Leed, Young-Kwon Parke, Kyun Ho Leef, , ⁎ Eilhann E. Kwona, a
Department of Environment and Energy, Sejong University, Seoul, 05006, Republic of Korea Advanced Technology Department, Land & Housing Institute, Daejeon, 34047, Republic of Korea c Department of Environmental Engineering, China Jiliang University No. 258, Xueyuan Street, Hangzhou, Zhejiang, 310018, PR China d Department of Environmental and Safety Engineering, Ajou University, Suwon, 16499, Republic of Korea e School of Environmental Engineering, University of Seoul, Seoul, 02504, Republic of Korea f Department of Aerospace Engineering, Sejong University, Seoul, 05006, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 Micro-/nano-bubbles Bio-heavy oil Metal removal Waste-to-energy
Co-combustion of bunker-C oil and bio-heavy oil for power generation provides a strategic way for mitigating the environmental burdens (climate change) due to the intrinsic carbon neutrality of bio-heavy oil. Prior to the cocombustion process, the high content of Na and K in heavy oil should be removed for the operational safety. Given that the washing process has been currently practiced for metal removal, it is desirable to develop the chemical-free washing process. Note that H2SO4 was commonly used during the washing process. To this end, this study laid great emphasis on the possible use of CO2 for the washing process. To increase the removal efficiency of Na and K, this study particularly employed micro-/nano-sized CO2 bubbles. To access the in-depth insights, a computational fluid dynamics (CFD) simulation was conducted prior to a lab-scale experimental work. The CFD simulation suggests that CO2 bubbling expedited the mixability during the washing process. Also, the CFD simulation suggested that the CO2 bubbling effects could be enhanced because the high viscosity of bioheavy oil cancelled out the buoyancy force. A lab-scale experimental work was also in good agreement with the CFD simulation. In the presence of CO2 bubbles, 80% of Na and K was removed when the volumetric ratio of water and bio-heavy oil was 8. Even under the volumetric ratio condition of 0.125, the metal removal efficiency was not substantially reduced. Thus, all experimental findings signify that metal removal by micro-/nano-sized CO2 bubbling should be done by a cascade system.
1. Introduction
and/or commercialization has been readily achieved as compared with other biofuels (i.e., biohydrogen and bioethanol) [7,8]. Indeed, TGs have been known as one of the most common chemical compounds in all living organisms, and their ubiquitousness serves a crucial role for securing the supply chain of TGs [9,10]. Moreover, the use of biodiesel has been further expanded by the political legislations, such as renewable fuel standard (RFS) [11]. Accordingly, based on the department of energy (DOE), the global production of biodiesel reached up to 2.89 billion gallons in 2017. Despite many socio-economic benefits from the use of biodiesel, the massive generation of wastes from the biodiesel industry has not been considered with the fully transparent manners [12]. Considering that biodiesel production will continue to increase with the development of 2nd (i.e., biodiesel converted from the inedible lipid feedstocks) and 3rd
Among anthropogenic greenhouse gases (GHGs), carbon dioxide (CO2) (from combustion of fossil fuels) has been considered as one of the main contributors for resulting in the global carbon imbalance [1]. Anthropogenic CO2 brings forth the global environmental issues (i.e., global warming and climate change) because the total amount of CO2 from combustion of fossil fuels is indeed over the Earth’s full capacity to sequester CO2 via the complex natural carbon cycles [2,3]. Therefore, the use of biodiesel as an alternative to petro-diesel provides a strategic venue for soothing the adverse environmental impacts arising from global warming due to its carbon neutrality [4–6]. Given that biodiesel is converted from triglycerides (TGs) via the simple process (the acid/ base catalyzed transesterification process), its practical implementation ⁎
Corresponding authors. E-mail addresses: [email protected] (K. Ho Lee), [email protected] (E.E. Kwon). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jcou.2019.07.031 Received 24 January 2019; Received in revised form 7 June 2019; Accepted 22 July 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
(i.e., biodiesel converted from the microalgal lipid) generation of biodiesel [13–16], seeking an effective and reliable way to dispose wastes from the biodiesel industry is of importance. Note that the substantial amount (3–5 wt.% of the lipid feedstock) of biodiesel wastes (i.e., fatty acids (FAs), mono-, di-, and tri-glycerides), namely “bio-heavy oil” or “bio-heating oil”, has been produced, but an effective disposal method has not been established. Currently, most biodiesel wastes (bio-heavy oil) are being co-combusted with petro-derived heavy oil, such as bunker-C oil, for power generation because co-combustion of bunker-C oil and bio-heavy oil provides a strategic means for satisfying the renewable portfolio standard (RPS) [17]. Harnessing the carbon neutral source in power generation (electricity) has been legislatively recommended by RPS [18]. Moreover, the use of bio-heavy oil offers diverse environmental benefits in terms of air pollution controls (APCs) [19]. Considering the structural and compositional matrix of bio-heavy oil (i.e., FAs, mono-, di-, and tri-glycerides), the oxygen content in the carboxyl group of each fatty acid (FA) provides the favorable conditions for the complete oxidation, thereby resulting in the decrease of air pollutants, such as particulate matters (PMs) [19,20]. Nevertheless, one of the main technical challenges to use bio-heavy oil for power generation is the high content of Na and K because such metals cause the nozzle clogging and other operating troubles during combustion of bio-heavy oil. Note that the high content of Na and K in bio-heavy oil is likely attributed to use of base catalysts (i.e., KOH and NaOH) during the biodiesel conversion process [21]. Thus, Na and K contents should be reduced and/or removed from bio-heavy oil. To this end, the acidic solution (H2SO4 solution) is commonly practiced for metal removal due to the high solubility of metals in acidic water [22]. Nonetheless, one of the technical demerits for the washing process is the generation of acidic wastewater (containing metals) [22]. Therefore, it is highly desirable to reduce water usage during the washing process. More preferably, the chemical-free (H2SO4-free) washing process is necessary in that chemical-free wastewater can be a strategic way for reducing the acidic wastewater treatment cost. Among metal removal techniques, the use of CO2 as reactive gas medium to reduce pH during the washing process by bubbling can be a promising alternative. Large bubbles (> 100 μm diameter) rise rapidly (> 6 mm s−1) to the water surface [23]. Microbubbles (1–100 μm diameter) provide a higher surface area per unit volume than the commonly seen larger bubbles [23]. They are not stable for long period (˜ min), rising fast (10-3-10 mm s−1) to the surface, but smaller ones (≤ 20 μm diameter) will shrink to form more effective/stable nanobubbles [23]. Based on these facts, utilizing micro-/nano-sized CO2 bubbles maximize the CO2 effect during the washing process. Therefore, this study laid great emphasis on the removal of metals in bio-heavy oil using micro-/nano-sized CO2 bubbles. To this end, a lab-scale metal removal by introducing micro-/nano-sized CO2 bubbles was carried out to assess the more realistic experimental data. Prior to the lab-scale experimental work, a computational fluid dynamics (CFD) analysis of CO2 bubbles was conducted to understand the fluid behaviors affected by CO2 bubbles more intuitively (or more clearly) depending on their size.
Fig. 1. Schematic layout depicting a numerical grid of a reservoir for the CFD simulation (grid matrix: ≥ 50,000).
at the left wall of the reactor. To investigate the realistic effects of the CO2 bubbles on the internal flow behaviors in the reactor, the EulerianEulerian approach and the shear-stress transport (SST) k-ω turbulence model were used to simply simulate the two-phase flow of the liquid phase and CO2 bubble [24]. In addition, the laminar or shear-stress transport (SST) k-ω turbulence model was chosen depending on the value of the Reynold number. An unsteady simulation was carried out to consider the distribution of the micro-/nano-sized CO2 bubbles over the time scale of 1,200 s (20 min). We assumed that 1,200 s is sufficient to complete the distribution of CO2 bubbles into the reactor. The computation was assumed to be converged at each time step when all residuals were less than 10−3. The flow parameters and boundary conditions used for the present CFD simulation are summarized in Table 1. The velocity inlet condition was assumed for the inflow of the liquid phase and CO2 bubbles with 200 mL min-1 while the pressure outlet condition was set to the outlet. A volume fraction of the CO2 bubbles was initially assumed as 0.1% in the liquid phase. The degassing condition was applied to the liquid surface to model the removal of the floated bubbles. In sum, a major difference is that the real reactor is three-dimensional while the CFD simulation considers two-dimensional plane. But the width of the reactor reservoir in Fig. 1 has a similar order of magnitude with its height and length and boundary effects of the reservoir walls would be minor because the width is much larger than a boundary layer thickness. In such case, the CFD simulation generally can simplify the three-dimensional real problem with the two-dimensional model. Thus, it can be assumed that the present simulation result would have a good agreement with the three-dimensional, and reduce the computational effort. 2.2. Chemical reagents and materials Bio-heavy oil was obtained from Dansuk (www.dansuk.co.kr) Co. Ltd (Ansan-City, Korea). Nano-bubbler was customized, and its working mechanisms were demonstrated in the literatures [25–27]. The CO2 input to the reactor was 200 mL min−1, and its flow rate was controlled using the mass flow controller (Brook E series, USA). Deionized water (DI water) was prepared using the Milli-DI water purification system (MilliporeSigma, USA). Laboratory agitator (IKA Eurostar 20, Republic of Korea) was used to achieve the complete mixture of bio-heavy oil and DI water. pH meter (Mettler Toledo, Switzerland) was used to monitor the pH change during the metal removal. Note that various volumetric mixtures of bio-heavy oil and water (i.e., volumetric ratio of bio-heavy oil to water: 1:8, 1:1, 8:1) were tested in this study.
2. Materials and methods 2.1. Numerical modeling for computational fluid dynamics (CFD) For a CFD simulation, a two-dimensional (2D) configuration of the CO2 bubble reservoir (a metal removal reactor) was chosen. Its dimension was 0.27 m of length and 0.25 m of height. Note that the dimension in Fig. 1 was determined by the actual configuration of the reactor except 0.17 m of width. As shown in Fig. 1, more than 50,000 structured rectangular grids were being considered for the CFD calculation, which included an inlet and outlet of the CO2 bubbles and liquid phase. The CO2 bubbles and the liquid phase were injected from the inlet at the right wall, and the liquid phase was exhausted to the outlet
2.3. Metal measurement in water To evaluate the metal removal efficiency, the mixture (bio-heavy oil and water) was separated by means of the centrifugation (7000 rpm for 431
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
Table 1 Boundary conditions and flow parameters. Flow parameters Diameter of CO2 bubble [μm] Inlet volume fraction of CO2 bubble [%] Flow rate [mL min−1]
Boundary condition 100, 1, 0.01 0.1 200
Inflow Outflow Water surface
Velocity inlet Pressure outlet Degassing
3 min). The concentration of metals in water was measured using an Inductive Coupled Plasma-Optical Emission Spectrometer (ICP-OES) (Perkin Elmer, USA). Prior to the measurement, the sample was digested using a microwave digestor (Milestone, ETHOS touch control, USA) [28]. Prior to the measurement, the multiple calibrations were performed using the standard solution (ICP-element stand solution, Certipur, Germany).
3. Results and discussion 3.1. CFD simulation The CFD calculation was performed to access the in-depth knowledge on the internal flow behaviors when the CO2 bubbles (200 mL min−1) are introduced for the washing process to remove metals in bioheavy oil. The CFD simulation was conducted to predict the internal flow behaviors based on the CO2 bubble size and the fluid properties (i.e., viscosity). To this end, several bubble diameters (100, 1, and 0.01 μm) and the liquid viscosities were considered as the variables for the CFD simulation. First, the distribution of CO2 bubbles and the bubble velocity were predicted with three different bubble diameters (100, 1, and 0.01 μm) in water. In case of 100 μm diameter of CO2 bubble, the velocity vectors by CO2 bubbles are developed vertically at the inlet of the reactor. As depicted in Fig. 2a, a rapid floatation of CO2 bubbles toward the water surface was observed due to a large buoyancy force, which resulted that CO2 bubbles could not spread in the water. Note that the buoyancy force is proportional to the bubble size [29]. However, Fig. 2c demonstrates that the velocity vectors driven by 0.01 μm diameter of CO2 bubbles are fully developed at the entire region of the reactor. Indeed, the velocity vectors derived from 1 μm diameter of CO2 bubbles at the reactor are well established in Fig. 2b, but the velocity vectors at the bottom of the reactor are not fully developed. Such observations signify that the distribution of the CO2 bubbles at the reactor is highly contingent on the bubble size, which is in good agreement with the general concept for the fluid dynamics. Nonetheless, the simulation in Fig. 2 may not reflect the realistic behaviors for CO2 bubbles in that the viscosity of bio-heavy oil is higher than that of water. Note that the viscosity of fluid is one of the main factors governing the fluid dynamics. To assess the more realistic CFD simulation, the viscosity of soybean oil was chosen as the reference value for the CFD simulation. The fluid density and viscosity of water and soybean oil were given in Table 2 [30], and the CFD simulation results were illustrated in Fig. 3. As well demonstrated in Fig. 3, the overall velocity vectors in the reactor for the case of 100, 1, and 0.01 μm are similar with the cases in Fig. 2. For example, CO2 bubbles in Fig. 3a do not float rapidly to the oil surface as compared with the case of Fig. 2a because of a small buoyancy force. Instead, a large rotation flow of the CO2 bubbles is developed with less than 0.1% volume fraction in the oil. Such observations signify that the higher viscosity of soybean oil relative to the case of water (Fig. 2) leads to a larger friction between CO2 bubble and oil. Therefore, the CFD result in Fig. 3 suggests that the high viscosity of soybean oil seems to effectively restrict the buoyancy effect of CO2 bubbles. In these respects, the magnitude of the velocity vectors begins to decrease, and the more CO2 bubbles were uniformly dispersed in oil through the entire region of the reactor with an extended time. Also, Fig. 3b and c suggest that the bubble size variations do not lead to an
Fig. 2. Volume fraction and velocity vector of the CO2 bubble in water, (a) 100 μm, (b) 1 μm, (c) 0.01 μm.
apparent influence on the internal flow behaviors because the viscosity of soybean oil is more dominant than the buoyancy force of CO2 bubbles. Moreover, Fig. 3b and c present that the bubble velocity vectors at the bottom region of the reactor are not fully developed as compared with the case of Fig. 2c. Such observations suggest that the internal flow behaviors of CO2 at the bottom region of the reactor are not developed due to the high viscosity of soybean oil. Thus, the mechanical agitation 432
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
3.2. pH variations induced by micro-/nano-sized CO2 bubbles
Table 2 Density and viscosity of water and soybean oil. Fluid type
Water (Case 1)
Soybean Oil (Case 2)
Viscosity [kg/m ⋅s] Density [kg/m3]
1.0 × 10−3
57.1 × 10−3
998.2
915.7
To confirm the effectiveness of CO2 bubbles, pH variations were monitored. In. detail, Fig. 4 depicts pH variations as a function of bubbling time. Note that the reference value was obtained by 200 mL min−1 of CO2 injection without considering the bubble size. Fig. 4a depicts that pH is decreasing faster when micro-/nano-sized CO2 bubbles are used, and pH becomes steady in 20 min. Also, Fig. 4a experimentally proves that the faster pH changes are achieved when micro-/nano-sized CO2 bubbles are used. This is in good agreement with the CDF simulation in Fig. 2. In reference to micro-/nano-sized CO2 bubbling, pH variations begin to increase at the bubble time ≥ 50 min when we introduced normal CO2 bubbles. One interesting observation is that variation of pH is negligible for 24 h when we introduced micro-/nano-sized CO2 bubbles (Data not shown in this study). To access the more realistic data, the micro-/nano-sized CO2 bubbles were introduced to water in the reactor for 30 min. 30 min of micro-/nano-sized CO2 bubbling was chosen because pH of water become steady in 20 min in Fig. 4a. And then, bio-heavy oil (i.e., volumetric ratio of water to bio-heavy oil: 8) was added to water. Without providing micro-/nano-sized CO2 bubbles (Fig. 4b), pH of the mixture (water and bio-heavy oil) begins to increase in 20 min. Also, Fig. 4b shows pH of the mixture becomes steady in 20 min when micro-/nanosized CO2 bubbles are introduced the reactor. Despite no supply of micro-/nano-sized CO2 bubbles, Fig. 4b shows that pH of the mixture is lower than the case of micro-/nano-sized CO2 bubbling for 30 min. Such observations imply that CO2 bubbling effectively enhance mixing of water and bio-heavy oil. In other words, mixing between bio-heavy oil and water without introducing micro-/nano-sized CO2 bubbling is hard to be achieved even in the presence of the mechanical agitations. Thus, Fig. 4b implied that CO2 bubbling has the technical merits for lowering pH in line with the enhanced mixability. 3.3. Metal removal by micro-/nano-sized CO2 bubbling over the bubbling time To access the more practical data in terms of metal removal by CO2 bubbling, the concentration of Na, K, and Mg as a function of the CO2 bubbling times was monitored. Note that experimental conditions in Fig. 5 is the identical to the case of Fig. 4b. Fig. 5a demonstrates that Na are removed by CO2 bubbling. In detail, the removal efficiency of Na is rapidly achieved in 1 h, of which removal efficiency is equivalent to 48%. However, Fig. 5a the metal removal rate for Na by CO2 bubbling begins to decrease immediately after 1 h. Note that the removal efficiency of Na is 69, 73, and 79% at 240, 360, and 1440 min, respectively. One of the interesting observations in Fig. 5 is that the concentration profiles (i.e., removal efficiency) of Na and K are nearly identical over the entire CO2 bubbling times. As well demonstrated in Fig. 5a and b, the standard deviation for the removal efficiency of Na and K at the given CO2 bubbling times is less than ± 1%. However, the removal efficiency of Mg by micro-/nanosized CO2 bubbling is inferior to the case of Na and K. Such different removal efficiencies between Na and Mg in Fig. 5 are likely attributed to the different ionic energies. To access the more practical data, the same experimental work was conducted, but the volumetric ratio of water to bio-heavy oil was changed. In detail, the volumetric ratio of water to bio-heavy oil was adjusted to 8, 1, 0.125, and the removal efficiency of Na, K, Ca, and Mg after 24 h micro-/nano-sized CO2 bubbling (according to the different volumetric ratio of water and bioheavy oil) was presented in Fig. 6. As declared before in the earlier section of introduction, saving the use of water during the washing process for metal removal is one of the key technical requirements. Fig. 6 demonstrates that the overall metal removal efficiency for 24 h is proportional to the amount of water. In detail, the high volumetric ratio of water to bio-heavy oil shows the better performance for
Fig. 3. Volume fraction and velocity vector of the CO2 bubble in water, (a) 100 μm, (b) 1 μm, (c) 0.01 μm.
is necessary to develop, and it may be maximized for the case of the mixture of water and bio-heavy oil because the higher viscosity of the mixture (water + bio-heavy oil) provides an effective means for restricting the buoyancy effects of CO2 bubbles. However, the unexpected effect arising from the mechanical agitation (bubble size, stability, etc.) need to be considered. 433
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
Fig. 4. (a) pH variations of water as a function of the CO2 bubbling time, (b) pH variations of the mixture of water and bio-heavy oil (volumetric ratio of water to bioheavy oil is 8) (Error bar was not given due to the less than 1% standard deviation).
metal removal from bio-heavy oil. Note that the solubility of cationic metals is generally proportional to the amount of water. However, even under the low volumetric ratio of water to bio-heavy oil, the overall removal efficiency of Na, K, Ca, and Mg is not substantially reduced. Thus, such observations in Fig. 6 signify that employing the low volumetric ratio of water to bio-heavy oil in line with a cascade for metal removal may have the technical merits for saving the use of water. Note that a cascade is a plant consisting of several similar stages with each processing the output from the previous stage. The removal efficiency of Ca under the low volumetric ratio of water to bio-heavy oil (i.e., ratio of 0.125) is discrepant with the overall metal removal efficiency patterns. Given that the solubility of CO2 in water is highly contingent on the temperatures, the further study for the optimization should be followed in the near future.
flow behaviors of CO2. The CFD simulation suggested that micro-/nanosized CO2 bubbles could be distributed the entire region of the reactor. Such distribution of CO2 bubbles could be enhanced in the presence of bio-heavy oil in that the high viscosity of bio-heavy oil effectively canceled out the buoyancy effect of CO2 bubbles. To access the realistic data, a lab-scale experiment for metal removal from bio-heavy oil was also conducted. All experimental findings from the lab-scale experimental works were in good agreement with the CFD simulation. In the presence of micro-/nano-sized CO2 bubble, ˜80% of Na and K was removed when the volumetric ratio of water and bio-heavy oil was adjusted to 8. Even under the volumetric ratio condition of 1 and 0.125, the metal removal efficiency was not substantially reduced. Thus, all experimental findings signify that metal removal by micro-/nano-sized CO2 bubbling should be done by a cascade system.
4. Conclusions
Declaration of Competing Interest
Prior to a lab-scale experimental work for metal removal from bioheavy oil by employing micro-/nano-sized CO2 bubbling, the CFD simulation were carried out to get the in-depth insights on the internal
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.
Fig. 5. Variation of Na, K, and Mg concentrations over micro-/nano-sized CO2 bubbling time (volumetric ratio of water to bio-heavy oil: 8) (Error bar of the removal efficiency was not given due to the less than 1% standard deviation). 434
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
Fig. 6. Removal efficiency of Na, K, Ca, and Mg by micro-/nano-sized CO2 bubbling for 24 (volumetric ratio of water to bio-heavy oil: 8, 1, and 0.125) (Error bar was not given due to the less than 1% standard deviation, Note that the experimental work was conducted for 24 h).
Acknowledgements
[11] W.E. Tyner, F.J. Dooley, D. Viteri, Alternative pathways for fulfilling the RFS mandate, Am. J. Agric. Econ. 93 (2) (2011) 465–472. [12] Y. Zhang, M.A. Dubé, D.D. McLean, M. Kates, Biodiesel production from waste cooking oil: 1. Process design and technological assessment, Bioresour. Technol. 89 (1) (2003) 1–16. [13] R.E.H. Sims, W. Mabee, J.N. Saddler, M. Taylor, An overview of second generation biofuel technologies, Bioresour. Technol. 101 (6) (2010) 1570–1580. [14] M.M.K. Bhuiya, M.G. Rasul, M.M.K. Khan, N. Ashwath, A.K. Azad, M.A. Hazrat, Prospects of 2nd generation biodiesel as a sustainable fuel – part 2: properties, performance and emission characteristics, Renew. Sustain. Energy Rev. 55 (2016) 1129–1146. [15] D.P. Ho, H.H. Ngo, W. Guo, A mini review on renewable sources for biofuel, Bioresour. Technol. 169 (2014) 742–749. [16] E. Jacob-Lopes, T.T. Franco, From oil refinery to microalgal biorefinery, J. CO2 Util. 2 (2013) 1–7. [17] P. Ndayishimiye, M. Tazerout, Use of palm oil-based biofuel in the internal combustion engines: performance and emissions characteristics, Energy 36 (3) (2011) 1790–1796. [18] J.-H. Moon, J.-W. Lee, U.-D. Lee, Economic analysis of biomass power generation schemes under renewable energy initiative with renewable portfolio standards (RPS) in Korea, Bioresour. Technol. 102 (20) (2011) 9550–9557. [19] J. Dharmaraja, D.D. Nguyen, S. Shobana, G.D. Saratale, S. Arvindnarayan, A.E. Atabani, S.W. Chang, G. Kumar, Engine performance, emission and bio characteristics of rice bran oil derived biodiesel blends, Fuel 239 (2019) 153–161. [20] J.P. Szybist, J. Song, M. Alam, A.L. Boehman, Biodiesel combustion, emissions and emission control, Fuel Process. Technol. 88 (7) (2007) 679–691. [21] R. Chamola, M.F. Khan, A. Raj, M. Verma, S. Jain, Response surface methodology based optimization of in situ transesterification of dry algae with methanol, H2SO4 and NaOH, Fuel 239 (2019) 511–520. [22] C.E. Demaman Oro, M. Bonato, J.V. Oliveira, M.V. Tres, M.L. Mignoni, R.M. Dallago, A new approach for salts removal from crude glycerin coming from industrial biodiesel production unit, J. Environ. Chem. Eng. 7 (1) (2019) 102883. [23] G.Z. Kyzas, G. Bomis, R.I. Kosheleva, E.K. Efthimiadou, E.P. Favvas, M. Kostoglou, A.C. Mitropoulos, Nanobubbles effect on heavy metal ions adsorption by activated carbon, Chem. Eng. J. 356 (2019) 91–97. [24] M. Colombo, M. Fairweather, Influence of multiphase turbulence modelling on interfacial momentum transfer in two-fluid Eulerian-Eulerian CFD models of bubbly flows, Chem. Eng. Sci. 195 (2019) 968–984. [25] J.-C. Tsai, M. Kumar, S.-Y. Chen, J.-G. Lin, Nano-bubble flotation technology with coagulation process for the cost-effective treatment of chemical mechanical
This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20163010092290). References [1] W. Cramer, A. Bondeau, F.I. Woodward, I.C. Prentice, R.A. Betts, V. Brovkin, P.M. Cox, V. Fisher, J.A. Foley, A.D. Friend, C. Kucharik, M.R. Lomas, N. Ramankutty, S. Sitch, B. Smith, A. White, C. Young-Molling, Global response of terrestrial ecosystem structure and function to CO2 and climate change: results from six dynamic global vegetation models, Global Change Biol. 7 (4) (2001) 357–373. [2] D.J. Wuebbles, A.K. Jain, Concerns about climate change and the role of fossil fuel use, Fuel Process. Technol. 71 (1) (2001) 99–119. [3] D. Archer, Fate of fossil fuel CO2 in geologic time, J. Geophys. Res.: Oceans 110 (C9) (2005). [4] C. Kiwjaroun, C. Tubtimdee, P. Piumsomboon, LCA studies comparing biodiesel synthesized by conventional and supercritical methanol methods, J. Cleaner Prod. 17 (2) (2009) 143–153. [5] J.A. Mathews, Carbon-negative biofuels, Energy Policy 36 (3) (2008) 940–945. [6] N. Maheshwari, M. Kumar, I.S. Thakur, S. Srivastava, Carbon dioxide biofixation by free air CO2enriched (FACE) bacterium for biodiesel production, J. CO2 Util. 27 (2018) 423–432. [7] J. Lee, Y.F. Tsang, J.-M. Jung, J.-I. Oh, H.-W. Kim, E.E. Kwon, In-situ pyrogenic production of biodiesel from swine fat, Bioresour. Technol. 220 (2016) 442–447. [8] T.M. Mata, A.A. Martins, N.S. Caetano, Microalgae for biodiesel production and other applications: a review, Renew. Sustain. Energy Rev. 14 (1) (2010) 217–232. [9] J.-M. Jung, K.-H. Kim, E.E. Kwon, H.-W. Kim, Analysis of the lipid profiles in a section of bovine brain via non-catalytic rapid methylation, Analyst 140 (18) (2015) 6210–6216. [10] T.M. Mata, A.A. Martins, S.K. Sikdar, C.A.V. Costa, Sustainability considerations of biodiesel based on supply chain analysis, Clean Technol. Environ. Policy 13 (5) (2011) 655–671.
435
Journal of CO₂ Utilization 34 (2019) 430–436
Y. Kim, et al.
T.C.O. Fonseca, V.L. Dressler, É.M..M. Flores, Determination of metals and metalloids in light and heavy crude oil by ICP-MS after digestion by microwave-induced combustion, Microchem. J. 96 (1) (2010) 4–11. [29] G. Sridhar, J. Katz, Drag and lift forces on microscopic bubbles entrained by a vortex, Phys. Fluids 7 (2) (1995) 389–399. [30] V. Rodriguez-Martinez, M. O’Meara, B.E. Farkas, Shreya N. Sahasrabudhe, Density, viscosity, and surface tension of five vegetable oils at elevated temperatures: measurement and modeling, Int. J. Food Prop. 20 (sup2) (2017) 1965–1981.
polishing wastewater, Sep. Purif. Technol. 58 (1) (2007) 61–67. [26] W.B. Zimmerman, V. Tesař, H.C.H. Bandulasena, Towards energy efficient nanobubble generation with fluidic oscillation, Curr. Opin. Colloid Interface Sci. 16 (4) (2011) 350–356. [27] M. Fan, D. Tao, R. Honaker, Z. Luo, Nanobubble generation and its applications in froth flotation (part IV): mechanical cells and specially designed column flotation of coal, Min. Sci. Technol. (China) 20 (5) (2010) 641–671. [28] J.S.F. Pereira, D.P. Moraes, F.G. Antes, L.O. Diehl, M.F.P. Santos, R.C.L. Guimarães,
436