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Nitric acid removal via mini-extractors
Chemical Engineering & Processing: Process Intensification 144 (2019) 107637
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Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Nitric acid removal via mini-extractors Tingliang Xie, Laijun Wang, Cong Xu
T
⁎
Institute of Nuclear and New Energy Technology, Collaborative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University, Beijing, 100084, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Acid removal Solvent extraction Mini-extractor Co-current Counter-current Passive mode
Industrial effluents often contain toxic, corrosive, and/or oxidizing acids that have a significant risk of causing major injuries to people or damage to the environment. The risk increases with increases in residence time and volume. Rapid and inherently safe methods, i.e. high-efficiency and small-volume devices, are therefore necessary to remove these acids from the effluents. In the study, two mini-extractors with very small volumes were developed and demonstrated to remove corrosive and oxidizing nitric acid from an aqueous solution using solvent extraction. A co-current mini-extractor having a residence volume of 0.172 mL was initially designed using the Coanda effect and experimentally investigated. The experimental results indicated that a nitric acid extraction extent of 97.5% was achieved even at a high total throughput of 120 mL/min. Furthermore, the cocurrent mini-extractor was adapted to construct a passive and high-throughput counter-current extraction system with a residence volume of 7.51 mL. The nitric acid recovery efficiency was 2.30 times that in the cocurrent extraction equilibrium, and its total throughputs were one to three orders of magnitude higher than those in the reported passive counter-current extraction systems using micro-extractors. Hence, it indicates that mini-extractors are available for the rapid removal of hazardous acids from effluents.
1. Introduction Industrial effluents are often harmful to people’s health and the environment because of containing toxic, corrosive, oxidizing, and explosive materials, e.g. hydrofluoric acid, nitric acid, perchloric acid (or perchlorate), and nitro-compounds. [1] These hazardous materials have to be removed before the effluents are discharged to the environment. Conventionally, rectifying columns, extraction columns (or mixer-settlers), and tank reactors with large dimensions are often used to complete removal of these hazardous materials. [2] The large residence volume leads to an increase in the total hazardous materials and the residence time, and thus significantly increases risks caused by leakage and explosion. Namely, large-scale devices have no inherent safety feature when they are used to treat industrial effluents containing hazardous materials. Recently emerging microfluidics have been proven to have excellent performance such as high-efficiency mass transfer and heat transfer because of micro- or mini-scales. [3,4] The very small volumes also enable the inherent safety feature when using micro- and mini-devices to treat the above hazardous effluents. Nitric acid (HNO3) is a highly corrosive, oxidizing, and toxic strong mineral acid, and has been widely used as a dissolving agent in metallurgical, chemical, and nuclear industries. [5,6] Consequently, a large volume of nitric acid solutions associated with various metals/
⁎
salts have been generated from those industries. Nitrate contamination in drinking water can lead to methemoglobinemia. [7] Sometimes, a small number of organic materials can also dissolve or float in oxidizing HNO3 solutions, resulting in a serious hidden risk of explosion when the solutions are heated. For example, in the reprocessing process of spent nuclear fuels, HNO3 is used to dissolve the spent fuels. Radioactive fissile metal elements (e.g. plutonium and uranium) and other nonradioactive nuclides (e.g. zirconium) dissolve in a nitric acid-water solution and then separated from each other by solvent extraction. As a result, an effluent containing nitric acid and various metal ions is generated, sometimes associating with an explosive organic material called “red oil”. It may lead to an explosion when the temperature is larger than about 130 ℃. To reduce the above risks, it is necessary to remove HNO3 from effluents using micro- or mini-devices with small scales. Lots of researches have been focused on the removal or recovery of HNO3 from industrial effluents and can be classified into three methods: material adsorption [7,8], ion exchange [9–11], and solvent extraction [1,12–17]. Among the three methods, solvent extraction has been widely used to extract and separate HNO3 from the industrial effluents because of high efficiency and low cost. [1,12–17] TRPO (Trialkylphosphine Oxide) [12], TBP (tributyl phosphate) [1,13,15–17], and TOP (tri-octyl phosphate) [14] were used as extractant to selectively
Corresponding author. E-mail address: [email protected] (C. Xu).
https://doi.org/10.1016/j.cep.2019.107637 Received 19 April 2019; Received in revised form 3 August 2019; Accepted 17 August 2019 Available online 17 August 2019 0255-2701/ © 2019 Published by Elsevier B.V.
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extract HNO3 from various aqueous solutions. As for extraction devices, traditional macro-scale extractors such as pulsed column [12,13], mixer settler [14], and centrifugal extractor [15] were adopted in these studies. However, over the past two decades, micro/mini-extractors are widely used in solvent extraction to enhance mass transfer. [18–22] These micro-/mini-extractors exhibit higher surface-to-volume ratios and shorter diffusion lengths for mass transfer by restricting fluids into small channels with dimensions ranging from micro-millimeters to millimeters. [20] Thus, the mass transfer rate is significantly increased. More importantly, the very small inventories of micro/mini-extractors make them attractive for solvent extraction involving hazardous chemicals. In case of an accident, potential harm to the operator and environment is limited. [23] However, few studies have focused on applications of micro-/mini-extractors in removing hazardous materials from industrial effluents. [24] It is owing to two disadvantages of existing micro-/mini-extractors. The first is low throughput that is unable to satisfy industrial production requests. The second is how to break through the mass transfer limit by extraction equilibrium. In this paper, we developed two mini-extractors, a co-current and count-current mini-extractors, demonstrating how to overcome the above disadvantages respectively. Here an HNO3 aqueous solution (a simulated effluent) and a 30% TBP-70% kerosene solution (extraction solvent) were used as the demonstration system. The co-current miniextractor with a characteristic size in the millimeter range was generated by expanding a co-current micro-extractor with a characteristic size ranging from tens to hundreds of micrometers. The method is termed as the Dimension Scale-Out to retain benefits of microfluidics while enabling high-throughput. A challenge faced during the expanding process involves methods to share benefits including the high area-to-volume ratio, short diffusion distance, and narrow residence time distribution of microfluidics. Furthermore, the counter-current mini-extractor (CCME) was designed and demonstrated to remove HNO3 with higher extraction efficiencies than one limited by extraction equilibrium.
Table 1 Densities and viscosities of the experimental systems. Feed materials
Density (kg/m3)
Viscosity (mPa·s)
Red aqueous solution Kerosene with surfactant 3 M HNO3 solution 30% TBP-kerosene
1002.0 745.2 1101.8 818.1
1.39 1.56 1.48 1.85
acidic condition, it does not show interfacial crud formation. [25] The kerosene was purchased from Jinzhou Refinery Factory (Liaoning, China). Analytically pure TBP and nitric acid were purchased from Beijing Chemicals Factory (Beijing, China). The HNO3 concentrations in the raffinates and eluents were determined via an automatic titrator (Metrohm China Ltd., 905) with NaOH as the titrant. All tests were performed within a temperature range of 22–23 °C. The densities and viscosities of the two phases were measured by using a densitometer (Den Di-1, Beijing YILUDA Co. Ltd., Beijing, China) and a rotary viscometer (DV-1, Spain Fungilab Co., Barcelona, Spain), respectively. An interface tensiometer (BZY-1, Shanghai Heng PingInstrument Factory, China) was used to measure the surface tension between the two phases. The density and viscosity of the two phases are listed in Table 1. The interfacial tension of the 30% TBP/ kerosene-HNO3 solution was 8.0 mN/m. 3. Co-current extraction 3.1. Co-current mini-extractor The first stage of the study involves developing a co-current miniextractor with the high throughput and high extraction efficiency (relative to phase equilibrium) based on a co-current micro-extractor. Here a dimension scale-out method was adopted and the flow pattern and mass transfer experiments are used to verify the feasibility of the method.
2. Set-up fabrication and materials 2.1. Fabrication
3.2. Dimension scale-out
Two layers of transparent poly(methylmethacrylate) (PMMA) plates were used to fabricate all the extractors in the study. The mini-channels were machined on a PMMA plate by using a commercial precision end mill (Kejie Machinery Automation Co., Ltd. Of Guangdong, JTGK-600). The slotted plate and other smooth plate were clamped with a stainlesssteel clamp and bonded by using the solvent sticking method via ultrasonic oscillation. A detailed description of the fabrication processes of the extractors is reported in previous studies. [24]
A novel co-current micro-extractor, namely the oscillating feedback micro-extractor (OFM) as shown in Fig. 1-A, is adopted as the basis of the scale-out. The OFM is designed based on an oscillating flow. When a fluid flows into a suddenly enlarged chamber (3 in Fig. 1-A) from a narrow passage (2 in Fig. 1-A) at high speed, it can entrain its surrounding fluid to form a low-pressure region on each side of the chamber. One of the low-pressure regions attracts the fluid and deflects it forming a wall-attachment flow. Due to the pressure difference between the two ends of the feedback channel, much more fluid recirculates into the low-pressure region through the feedback channel. The pressure of the low-pressure region increases producing a larger transverse force on the wall-attachment flow, consequently. The fluid is pushed towards the opposite wall and thus a new wall-attachment flow is formed. Similarly, the recirculation through the feedback channel of this side can also push the wall-attached flow away from this wall. As a result, the fluid swings in the chamber to produce a high-frequency oscillating flow. The OFM was investigated in a previous study and exhibited excellent results with respect to the mass transfer between two phases. [27] As shown in Fig. 1-A, the OFM is scaled out to form an enlarged mini-extractor shown in Fig. 1-B. The mini-extractor consisted of a T-type inlet, a mixing chamber, a splitter, two feedback channels, and an outlet. The widths of the inlet and feedback channel as well as the length of the mixing chamber were increased to 1 mm, 2 mm, and 18.8 mm from original measurements corresponding to 200 μm, 400 μm, and 4420 μm, respectively. Additionally, all the depths of the mini-extractor corresponded to 0.5 mm remaining at a micro-scale. The volume of the co-current mini-extractor was only 0.172 mL much lower
2.2. Materials In the flow pattern experiments, in order to clearly observe the flow patterns and capture the droplets size evolution, a red aqueous solution was used as the dispersed phase and the colorless fully hydrogenated kerosene was used as the continuous phase. Additionally, 0.30 wt% of Span 80 surfactant (sorbitan monooleate, J&K Scientific Ltd., China) was added to the kerosene. The red aqueous solution was produced by dissolving 1 g of red dye (Sanolin Ponceau 4 RC 8, Clariant International Ltd., Switzerland) in 300 g of deionized water. In the extraction experiments, an aqueous nitric acid (3 M HNO3) solution was used as the simulated effluent and dispersed phase, and a 30% Tributyl phosphate (TBP)/kerosene solution was used as the extraction solvent (extractant) and continuous phase. TBP has been widely used as an extractant to extract either anionic or neutral metal species via ion-pairing or direct solvation into an organic phase from an aqueous phase. [25,26] In addition, TBP is low cost and easy to be operated owing to its low viscosity. [25] Furthermore, even under an 2
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Fig. 1. Diagram of the micro/mini extractors available for the co-current flow arrangement: (A) configuration of the oscillating feedback microextractor (depth, 1.0 mm), (B) configuration of the mini extractor, and (C) dimensions of the mini extractor (depth, 0.5 mm). 1: inlet port; 2: inlet channel; 3: mixing chamber; 4: barrier; 5: feedback channel; 6: splitter; 7: converging chamber; 8: outlet channel; 9: outlet port.
When Qaq = Qor =35 mL/min (Fig. 3-d), the number of the red aqueous phase clusters in the left feedback channel significantly reduced due to enhanced feedback flow. However, a few of the red aqueous phases were not broken into small droplets on the left side of the splitter. As Qaq = Qor =40 mL/min (Fig. 3-e), the red aqueous phase was effectively broken into small droplets on the left side of the splitter due to increases in the shear force. However, the number of droplets on the left side still exceeded that on the right side. When Qaq = Qor =50 mL/min (Fig. 3-f), the red aqueous phase was fully broken into small droplets and distributed over the entire mini-extractor. Thus, a high area-tovolume ratio was achieved to enhance mass transfer. It should be noted that the Reynolds numbers (Rei=ρiUiD/μi, i = aq or or, and D is the hydraulic diameter of the inlet channel) Reaq and Reor for the aqueous and organic phases range between 320.4–881.1 and 212.3–583.8, respectively. The Reynolds numbers at the inlet channel always corresponded to a maximum within the entire mini-extractor. Thus, the results confirmed that the two-phase flows in the mini-extractor always remain in the laminar flow regime. A few conclusions were derived from the flow patterns as follows: the flows were still laminar in the mini-extractor, which is formed via the dimension scaleout of the micro-extractor of an OFM, and (2) the dispersed phase was effectively dispersed even in the laminar flows due to the intensive disturbances caused by the vortex, feedback, and oscillating flows as reported in previous studies on OFMs. [27]
than that of traditional extractors such as columns and mixer-settlers (0.1-10 m3).
3.3. Flow patterns The experimental flow chart is shown in Fig. 2. The two immiscible liquids were simultaneously pumped into the mini-extractor by using two precise syringes (Baoding Longer Precision Pump Co., Ltd., LSP011BH, stroke resolution 0.156 μm). A microscope (Shanghai Changfang Optical Instrument Factory, CMM-50E) combined with a digital singlelens reflex camera (Canon, EOS650D; pixels: 5184 × 3456) was used to visualize and record the flow patterns in the mini-extractor. The flow patterns in the co-current mini-extractor with different flow rates (20–55 mL/min) were investigated. A selected set of results are shown in Fig. 3. The aqueous phase flow rate (Qaq) and organic phase flow rate (Qor) were always set as identical. As shown in Fig. 3-a (Qaq = Qor =20 mL/min), a stratified flow is observed, and an evident interface between the two phases is observed. In this case, the mass transfer mainly depended on the thermal motion of molecules. Given Qaq = Qor =25 mL/min (Fig. 3-b), a part of the red aqueous phase was dispersed into droplets and distributed in the mixing chamber and two feedback channels. However, only a small number of droplets were observed in the right feedback channel. Given Qaq = Qor =30 mL/min (Fig. 3-c), significantly more red aqueous droplets were produced when compared with that in Fig. 2-b. However, a few of the red aqueous phase clusters (slug flow) were observed in the left feedback channel. 3
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Fig. 2. Schematic diagram of the co-current extraction experiments using the mini extractor.
3.4. Extraction performance Extraction experiments in the co-current mini-extractor were performed to further verify the feasibility of the dimension scale-out. The extraction extent E is defined as follows.
E=
D De
(1)
D=
Cor,out Caq,out
(2)
Fig. 3. Flow patterns of the feedback mini extractor (a) Qaq = Qor =20 mL/min; (b) Qaq = Qor =25 mL/min; (c) Qaq = Qor =30 mL/min; (d) Qaq = Qor =35 mL/ min; (e) Qaq = Qor =40 mL/min; and (f) Qaq = Qor =50 mL/min. 4
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Where D denotes the ratio of the acid concentration of the organic phase to that of the aqueous phase at the extractor outlet. Given the definition of D, De is defined as the acid concentration ratio of the two immiscible phases in equilibrium. E represents the extent to which the exit streams are close to the phase equilibrium, i.e., the mass transfer limit. Furthermore, the mass transfer rate (R*) and overall volumetric mass transfer coefficient (ka) are calculated by using Eqs. (3) and (4) as follows: [28]
R* = Q aq (Caq,in − Caq,out ) = kaV ΔCm ka =
Table 2 Comparison of extraction efficiencies in the OFM and mini-extractor.
(3)
Q aq (Caq,in − Caq,out )
Where k denotes the overall mass transfer coefficient, a represents the mass transfer area between the two phases per unit volume, and V denotes the volume of the mini extractor. Additionally, △Cm denotes the average concentration difference that is calculated as follows:
ΔCm =
* ) − (Caq,out − Caq,out * ) (Caq,in − Caq,in (Caq,in − C *
)
aq,in ⎤ ln ⎡ (C * )⎦ ⎣ aq,out − Caq,out
(5)
Where Caq, in denotes the acid concentration in the inlet aqueous phase. * denotes the equilibrium acid concentration in the inlet Moreover, Caq,in aqueous phase that corresponds to the acid concentration in the inlet * denotes the organic phase and is zero in the study. Specifically, Caq,out equilibrium acid concentration in the outlet aqueous phase that corresponds to the acid concentration in the outlet organic phase. Additionally, the residence time (tR) is given as follows:
tR =
V Q aq + Qor
Emicro (%)
Qtotal-mini (mL/min)
Emini (%)
52 60 72 80 100 120
88.6 94.9 - (Emul.) - (Emul.) - (Emul.) - (Emul.)
56 64 72 80 100 120
49.8 61.0 69.2 83.8 96.3 97.5
was invalid when the total flow rates Qtotal exceeded 60 mL/min. This is because emulsification occurred and the mixture out of the OFM was difficult to achieve phase separation in a short time. Conversely, a high extraction extent of E = 97.5% was achieved in the mini-extractor even at a total flow rate of 120 mL/min because emulsification was absent. The extraction extent of the mini-extractor was comparable with that of the OFM and was close to the mass transfer limit determined by the phase equilibrium. Additionally, the mini-extractor achieved a total throughput of 120 mL/min that was twice the maximum throughput of the OFM without emulsification. Therefore, the micro-extractor based on the dimension scale-out method was valid to increase the throughput significantly and retain high extraction efficiency. However, it should be noted that when the two phases reach equilibrium, the co-current extraction achieves its maximum extraction efficiency. Here the maximum recovery efficiency for co-current extraction (Xcocurrent=(Caq,in-Caq,out)/Caq,in)) was only 17.7%, a value not satisfying the production requests.
(4)
V ΔCm
Qtotal-micro (mL/min)
4. Counter-current mini-extractor (6) In order to break through the recovery efficiency limited by phase equilibrium, the co-current mini-extractor shown in Fig. 1 was further modified to form a bidirectional mini-extractor based on the requirements of the counter-current extraction system. Furthermore, a countercurrent extraction system was established by using several bidirectional mini-extractors. Finally, extraction experiments were performed to verify the extraction performance of the CCME system.
Fig. 4 illustrates the extraction extent (relative to phase equilibrium) E and overall volumetric mass transfer coefficient ka with respect to different residence times. The results indicated that both E and ka increase with decreases in residence time tR. This was because decreases in the residence time (i.e., increases in the flow rates) effectively break the aqueous phase into small droplets as shown in Fig. 3. Thus, the mass specific surface area increased and the mass transfer was enhanced. Hence, an extraction extent of 97.5% was achieved with a residence time of less than 0.1 s. Additionally, Table 2 compares the extraction extents between the mini-extractor (Emini) and its scale-out base, i.e., the OFM (Emicro) (Fig. 1-A) under different flow rates. The results indicate that the OFM
4.1. Bidirectional mini-extractor and CCME system
• Bidirectional mini-extractor
Fig. 4. Extraction performance of the mini extractor. 5
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Fig. 5. Configurations and dimensions of the passive bidirectional mini extractor (Depth, 2000 μm): (A) schematic of design S, (B) dimensions of design S, (C) design NS, and (D) design AS. 1-bottom inlet/outlet channel; 2-bottom mixing chamber; 3-bottom barrier; 4-bottom feedback channel; 5-bottom splitter; 6-top inlet/outlet channel; 7-top mixing chamber; 8-top barrier; 9-top feedback channel; 10-top splitter; 11-bottom section; 12-top section.
maintained at 60 kPa.
Based on the above co-current mini-extractor, three bidirectional mini-extractors are developed to construct a CCME process as shown in Fig. 5. The bidirectional mini-extractor mainly consists of two parts, namely the upper mixing stage and bottom mixing stage. Each mixing stage exhibits structures similar to those of the co-current mini-extractor in Fig. 1-B. For convenience, the bidirectional mini-extractors with a splitter and two symmetrical feedback channels are denoted as S as shown in Fig. 5-A. Its dimensions are shown in Fig. 5-B. When compared with S, the bidirectional mini-extractors without splitters are denoted as NS as shown in Fig. 5-C. Bidirectional mini-extractors with two asymmetrical feedback channels with two splitters are denoted by AS (shown in Fig. 5-D). The volumes are only 7.51 mL, 7.69 mL, and 7.93 mL for S, NS, and AS, respectively.
4.2. Extraction performance The counter-current recovery efficiency X is defined as follows.
X=
Caq _ in − Caq _ raf Caq _ in
(7)
Where Caq_in denotes the initial nitric acid concentration for the raw liquid in the organic phase settler, and Caq_raf denotes the nitric acid concentration of the raffinate obtained from the aqueous phase layer in the aqueous phase settler. The phase equilibrium recovery efficiency for the co-current extraction Xe is calculated as follows:
• CCME system and working strategy
Xe =
A CCME system is established as described in Fig. 6. The system mainly includes two parts, namely the control model and mini extraction model. The control model contains four solenoid valves (ZCC-1 P, Shanghai Juliang Solenoid Manufacturing Co., China), a timing controller, and a pressure regulating valve (Type 70, Marsh Bellofram Co.). The flow flux is adjusted via controlling the timing controller. The pressure regulating valve is used to adjust the pressure in the compressed gas buffer tank. With respect to the extraction model, it mainly consists of six bidirectional mini-extractors and two settlers. The six bidirectional mini-extractors are connected to each other from end to end to form a mini-extraction section. The feeding port of the aqueous phase settler is connected to the top inlet/outlet of the mini-extraction section (port II) while the feeding port of the organic phase settler is connected to the bottom inlet/outlet of the mini-extraction (port I). The structure and dimensions of the two settlers are shown in Fig. 6-B and Fig. 6-C. As showed in Fig. 7, the working strategy of the CCME system includes four stages: (1) organic phase feeding stage, (2) droplets aggregation stage I, (3) aqueous phase feeding stage, and (4) droplets aggregation stage II. The four stages keep cycling to establish a countercurrent flow arrange and can be realized by controlling the four valves V-LC, V-LV, V-HC, and V-HV. Table 3 shows the opening or closing of the four valves corresponding to the four stages. As a result, the two phases are transported along opposite directions based on the alternative feeding of two phases, and the mixture of the two phases is effectively separated after mixing. During the operating cycle, the dispersed phase is effectively broken into small droplets to increase the area-to-volume ratio and obtain high mass transfer. A detailed description of the operation steps for the control system is given in previous studies. [29] The pressure for the pulsed system is always
Caq _ in − Caq _e Caq _ in
(8)
Where Caq_e denotes the nitric acid concentration in the aqueous phase reaching phase equilibrium. Furthermore, Xe is determined by shake tests and a detailed description to measure the value of Xe is indicated in our previous study. [29] Additionally, the extraction performance index λ is adopted to further evaluate whether the counter-current extraction process is successfully achieved. Additionally, λ is expressed as λ = X/Xe. A detail description [29] for the evaluation is shown in Table 4. It should be noted that back-mixing causes a counter-current extraction to change to a co-current extraction, and thus the values of X and λ decrease. 4.2.1. Effects of structure parameters In the section, the effects of the extractor structure on the hydraulics (shown Table 5) and extraction performance (shown in Fig. 8) for the counter-current extraction is investigated. The duration times for the organic phase feeding stage (toc) and the aqueous phase feeding stage (tac) are both set to 1.0 s. The duration times for the droplets aggregation stage I and II are both set to 5.0 s. The flow rates of the organic and aqueous phases For and Faq (phase transfer rates) are defined as follows:
For =
Faq =
Vor t
(9)
Vaq t
(10)
Where Vor denotes the transferred volume for the organic phase from the aqueous phase settler to the organic phase settler within a complete operating cycle, Vaq denotes the transferred volume for the aqueous phase from the organic phase settler to the aqueous phase settler within a complete operating cycle, and t denotes the lasting time for a 6
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Fig. 6. Schematic of the counter-current mini extraction system: (A) flowchart, (B) organic phase settler, and (C) aqueous phase settler.
significantly enhance the internal recirculation flow through the feedback channels and oscillation flow in the mixing chamber. [24,27] Thus, the highest area-to-volume ratio for mass transfer is achieved, and AS exhibits the highest extraction performance when compared with NS and S. Additionally, NS does not exhibit splitters and asymmetrical feedback channels, and thus it is difficult to break the aqueous phase into small droplets. Hence, NS exhibits the worst extraction performance. However, all the extraction performance indexes (Fig. 8B) for the three different mini-extractors exceed 1.0. Therefore, successful counter-current extraction is achieved in all three structures (i.e., NS, S, and AS).
complete operating cycle that includes four stages. The total throughput (total flow rate) Ftotal is defined as Ftotal = Faq + For, and the flux ratio (R) is defined as R = For / Faq. As listed in Table 5, the total throughput of NS was the highest, and this is followed by S and AS. This is because splitters that reduce the flow resistance are absent. However, AS exhibits the highest recovery efficiency (Fig. 8-A) and extraction performance index (Fig. 8-B), and this is followed by S and NS. The phenomenon is because the dispersed phase is more effectively broken into small droplets in the feeding stage in AS due to the splitter and asymmetrical feedback channels. Extant studies indicated that the splitters and asymmetrical feedback channels 7
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Fig. 7. Working principle of the counter-current extraction system: (A) droplet aggregation stage I, (B) organic phase feeding stage, (C) droplet aggregation stage II, and (D) aqueous phase feeding stage. Table 3 Different operating stages controlled via different states of the solenoid valves. Operating stages
V-LC
V-LV
V-HC
V-HV
Organic phase feeding stage Droplet aggregation stage I Aqueous phase feeding stage Droplet aggregation stage II
Close Close Open Close
Open Open Close Open
Open Close Close Close
Close Open Open Open
Table 4 Evaluation of the counter-current extraction. λ λ < 1 λ=1 λ > 1
Evaluation of the counter-current extraction Poor extraction and phase equilibrium is not achieved Phase equilibrium extraction is achieved Phase equilibrium is broken and is successful. Counter-current extraction is achieved.
Table 5 Results of the hydraulics for the counter-current mini extractors with different structures. Type of structure
Faq (mL/min)
Faq (mL/min)
Ftotal (mL/min)
R
NS S AS
0.9625 0.416 0.3595
0.8055 0.5425 0.4195
1.768 0.9585 0.779
0.84 1.34 1.17
4.2.2. Effects of operating times The NS is selected to investigate the effects of the different phase feeding times and droplets aggregation times. The hydraulic results are shown in Table 6, and the extraction performance is shown in Fig. 9. In general, phase separation is a throughput-limiting step for multistage counter-current extractors. A comparison of T1 with T2 shown in
Fig. 8. Changes in (A) recovery efficiency and (B) extraction performance index in the mini counter-current microextractors with different structures. 8
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Table 6 Results of the hydraulics for the counter-current mini-extractors with different operating times. Time series
Faq (mL/min)
Faq (mL/min)
Ftotal (mL/min)
R
T1: (tac = toc = 1.0 s; tI = tII = 3.0 s) T2: (tac = toc = 1.0 s; tI = tII = 5.0 s) T3: (tac = toc = 2.2 s; tI = tII = 5.0 s)
1.2705 0.9625 2.0596
0.9225 0.8055 1.7104
2.193 1.768 3.770
0.84 1.34 0.83
(total flow rate) of 3.77 mL/min is achieved. As shown in Fig. 9, the recovery efficiency and extraction performance index do not exhibit evident changes with increases in the phase feeding time and droplets aggregation time. This is because a high level of mixing is achieved in the mini- extractor due to the vortex, feedback, and oscillating flows. [24] Therefore, the proposed simple and robust passive CCME system could break through the mass transfer limit caused by phase equilibrium. The experimental results proved mini-extractors available for removing hazardous materials from effluents. 4.2.3. Comparison of the throughput Based on the mass transfer mode and flow arrangement, countercurrent micro/mini-extractor can be classified into two types: continuous counter-current micro/mini-extractor [30,31] and multistage micro/mini-extractor [32–34]. As for the former, solutes are transferred between the two immiscible phases in a differential contact mode without needing a complete phase separation. As for the multistage counter-current micro/mini-extractor, solutes are transferred between two phases in a stage-wise contact mode. Here phase separation is important in order to renew the specific surface area of mass transfer as well as reduce fluid back-mixing. However, check valves [33] or interstage pumps [34] are indispensable for controlling the flow direction for multistage counter-current micro/mini-extractors. These active components at the macro-scale are difficult in integrating with micro/ mini-extractors and make the controlling system complicated. The proposed CCME system is passive and thus, we just make a comparison between the passive counter-current micro/mini- extractors in the literature and the proposed CCME system. Fig. 10 compares the throughput between the proposed CCME system and those reported in extant studies for passive counter-current micro-extractors. The results indicate that both the lower and upper limits of Ftotal that are observed in the proposed CCME system are one to three orders of magnitude higher than those in other passive countercurrent micro-extraction models such as continuous models [30,31] and multistage models [32]. Additionally, a maximum total throughput of 3.77 mL/min is achieved in the proposed CCME system. These experimental results show that the CCME system could break through the mass transfer limit caused by phase equilibrium and significantly improved the throughput by the dimension scale-out method.
Fig. 9. Changes in (A) recovery efficiency and (B) extraction performance index in the mini counter-current extractors with different operating time series.
Table 6 indicates that the flow rates decrease with increases in the droplet aggregation time. This is because increases in the droplet aggregation time are advantageous to phase separation and reduce backmixing during the phase feeding stages. This suggests that the organic (aqueous) phase conveyed during the organic (aqueous) phase feeding stage increases, and thereby increases the throughput. However, the total time also increases with increases in the droplet aggregation time. Finally, the net effect to increase the aggregation time corresponds to decreases in the flow rates. Actually, complete phase separation is not necessary for the presented CCME system. As described in Fig. 6, the counter-current flow arrangement can be realized as long as an aqueous phase-rich layer and an organic phase-rich layer are formed in the bottom and the top of every bidirectional mini-extractor during the aggregation stages. Namely, the presented CCME system can work with a rather short aggregation time (phase separation time) because it does not require complete phase separation. Actually, only a few seconds can ensure phase separation available for the CCME system, as shown in Table 6. With respect to series T2 and T3 in Table 6, the droplet aggregation times (tI = tII = 5.0 s) remain unchanged although the lasting times of the phase feeding stages are different. It is observed the flow rates increase with increases in the phase feeding times. A high throughput
5. Conclusion In this paper, a passive co-current and counter-current mini-extractors were developed based on the oscillating feedback microextractor and demonstrated for removing nitric acid from effluents using solvent extraction. Two problems, i.e. low throughput and mass transfer limit, were overcome. Boh the co-current and counter-current mini-extractors are rapid and inherently safe because of their very small volume. The co-current mini-extractor has a residence volume of 0.172 mL and a nitric acid extraction efficiency of 97.5% was achieved even at a high total throughput of 120 mL/min. The counter-current mini-extractor has a residence volume of 7.51 mL in which the extraction efficiencies were 2.30 times that in the co-current extraction equilibrium and its total throughputs were one to three orders of magnitude higher than those in the reported passive counter-current extraction systems using micro-extractors. The passive, high9
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Fig. 10. Comparison of the throughput.
throughput, and high-efficiency mini-extractors have great potential in removing hazardous materials from effluents. However, it should be noted that the counter-current mini-extractor only achieved the maximum total throughput of 3.77 mL/min that still is relatively low compared with production requests. This problem can be solved using the parallelization of a micro-/mini-extractor (numbing-up) in the future.
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Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgment This work was supported by the Natural Science Foundation of China (No.21576149, No.21776152). References [1] M.A. Muhsan, S. Ilyas, H.A. Cheema, S. Masud, N. Shabbir, Recovery of nitric acid from effluent streams using solvent extraction with TBP: a comparative study in absence and presence of metal nitrates, Sep. Purif. Technol. 186 (2017) 90–95. [2] S. Kumar, B. Kumar, M. Sampath, D. Sivakumar, U.K. Mudali, R. Natarajan, Development of a micro-mixer-settler for nuclear solvent extraction, J. Radioanal. Nucl. Chem. 291 (3) (2012) 797–800. [3] M. Darekar, K.K. Singh, P. Sapkale, A.K. Goswami, S. Mukhopadhyay, K.T. Shenoy, On microfluidic solvent extraction of uranium, Chem. Eng. Process. Process. Intensif. 132 (2018) 65–74. [4] P. Mondal, S. Ghosh, G. Das, S. Ray, Phase inversion and mass transfer during liquid–liquid dispersed flow through mini-channel, Chem. Eng. Process. Process. Intensif. 49 (10) (2010) 1051–1057. [5] N. Sen, M. Darekar, K.K. Singh, S. Mukhopadhyay, K.T. Shenoy, S.K. Ghosh, Solvent extraction and stripping studies in microchannels with TBP nitric acid system, Solvent Extr. Ion Exch. 32 (3) (2014) 281–300. [6] D. Mandal, S.K. Ghosh, Reduction of nitrates present in aqueous waste, Environ. Geochem. Health 8 (2005) 244–249. [7] E. Zakharchenko, O. Mokhodoeva, D. Malikov, N. Molochnikova, Y. Kulyako, G. Myasoedova, Novel sorption materials for radionuclide recovery from nitric acid solutions: solid-phase extractants and polymer composites based on carbon nanotubes, Procedia Chem. 7 (2012) 268–274.
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[30] A. Aota, M. Nonaka, A. Hibara, T. Kitamori, Countercurrent laminar microflow for highly efficient solvent extraction, Angew. Chemie 119 (6) (2007) 896–898. [31] J. Hereijgers, M. Callewaert, X. Lin, H. Verelst, T. Breugelmans, H. Ottevaere, G. Desmet, W. De Malsche, A high aspect ratio membrane reactor for liquid–liquid extraction, J. Memb. Sci. 436 (2013) 154–162. [32] J. Hereijgers, N. van Oeteren, J.F. Denayer, T. Breugelmans, W. De Malsche, Multistage counter-current solvent extraction in a flat membrane microcontactor, Chem. Eng. J. 273 (2015) 138–146. [33] W. Lan, S. Jing, S. Li, G. Luo, Hydrodynamics and mass transfer in a countercurrent multistage microextraction system, Ind. Eng. Chem. Res. 55 (20) (2016) 6006–6017. [34] A. Holbach, N. Kockmann, Counter-current arrangement of microfluidic liquid-liquid droplet flow contactors, Green Process. Synth. 2 (2) (2013) 157–167.
6551–6558. [25] H.A. Cheema, S. Ilyas, S. Masud, M.A. Muhsan, I. Mahmood, J.C. Lee, Selective recovery of rhenium from molybdenite flue-dust leach liquor using solvent extraction with TBP, Sep. Purif. Technol. 191 (2018) 116–121. [26] A. Ishfaq, S. Ilyas, A. Yaseen, M. Farhan, Hydrometallurgical valorization of chromium, iron, and zinc from an electroplating effluent, Sep. Purif. Technol. 209 (2019) 964–971. [27] C. Xu, Y. Chu, An oscillating feedback microextractor with asymmetric feedback channels, Chem. Eng. J. 253 (2014) 438–447. [28] Z. Chen, W.T. Wang, F.N. Sang, J.H. Xu, G.S. Luo, Y.D. Wang, Fast extraction and enrichment of rare earth elements from waste water via microfluidic-based hollow droplet, Sep. Purif. Technol. 174 (2017) 352–361. [29] T. Xie, C. Xu, High-throughput countercurrent microextraction in passive mode, Lab Chip 18 (10) (2018) 1471–1484.
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Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Nitric acid removal via mini-extractors Tingliang Xie, Laijun Wang, Cong Xu
T
⁎
Institute of Nuclear and New Energy Technology, Collaborative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University, Beijing, 100084, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Acid removal Solvent extraction Mini-extractor Co-current Counter-current Passive mode
Industrial effluents often contain toxic, corrosive, and/or oxidizing acids that have a significant risk of causing major injuries to people or damage to the environment. The risk increases with increases in residence time and volume. Rapid and inherently safe methods, i.e. high-efficiency and small-volume devices, are therefore necessary to remove these acids from the effluents. In the study, two mini-extractors with very small volumes were developed and demonstrated to remove corrosive and oxidizing nitric acid from an aqueous solution using solvent extraction. A co-current mini-extractor having a residence volume of 0.172 mL was initially designed using the Coanda effect and experimentally investigated. The experimental results indicated that a nitric acid extraction extent of 97.5% was achieved even at a high total throughput of 120 mL/min. Furthermore, the cocurrent mini-extractor was adapted to construct a passive and high-throughput counter-current extraction system with a residence volume of 7.51 mL. The nitric acid recovery efficiency was 2.30 times that in the cocurrent extraction equilibrium, and its total throughputs were one to three orders of magnitude higher than those in the reported passive counter-current extraction systems using micro-extractors. Hence, it indicates that mini-extractors are available for the rapid removal of hazardous acids from effluents.
1. Introduction Industrial effluents are often harmful to people’s health and the environment because of containing toxic, corrosive, oxidizing, and explosive materials, e.g. hydrofluoric acid, nitric acid, perchloric acid (or perchlorate), and nitro-compounds. [1] These hazardous materials have to be removed before the effluents are discharged to the environment. Conventionally, rectifying columns, extraction columns (or mixer-settlers), and tank reactors with large dimensions are often used to complete removal of these hazardous materials. [2] The large residence volume leads to an increase in the total hazardous materials and the residence time, and thus significantly increases risks caused by leakage and explosion. Namely, large-scale devices have no inherent safety feature when they are used to treat industrial effluents containing hazardous materials. Recently emerging microfluidics have been proven to have excellent performance such as high-efficiency mass transfer and heat transfer because of micro- or mini-scales. [3,4] The very small volumes also enable the inherent safety feature when using micro- and mini-devices to treat the above hazardous effluents. Nitric acid (HNO3) is a highly corrosive, oxidizing, and toxic strong mineral acid, and has been widely used as a dissolving agent in metallurgical, chemical, and nuclear industries. [5,6] Consequently, a large volume of nitric acid solutions associated with various metals/
⁎
salts have been generated from those industries. Nitrate contamination in drinking water can lead to methemoglobinemia. [7] Sometimes, a small number of organic materials can also dissolve or float in oxidizing HNO3 solutions, resulting in a serious hidden risk of explosion when the solutions are heated. For example, in the reprocessing process of spent nuclear fuels, HNO3 is used to dissolve the spent fuels. Radioactive fissile metal elements (e.g. plutonium and uranium) and other nonradioactive nuclides (e.g. zirconium) dissolve in a nitric acid-water solution and then separated from each other by solvent extraction. As a result, an effluent containing nitric acid and various metal ions is generated, sometimes associating with an explosive organic material called “red oil”. It may lead to an explosion when the temperature is larger than about 130 ℃. To reduce the above risks, it is necessary to remove HNO3 from effluents using micro- or mini-devices with small scales. Lots of researches have been focused on the removal or recovery of HNO3 from industrial effluents and can be classified into three methods: material adsorption [7,8], ion exchange [9–11], and solvent extraction [1,12–17]. Among the three methods, solvent extraction has been widely used to extract and separate HNO3 from the industrial effluents because of high efficiency and low cost. [1,12–17] TRPO (Trialkylphosphine Oxide) [12], TBP (tributyl phosphate) [1,13,15–17], and TOP (tri-octyl phosphate) [14] were used as extractant to selectively
Corresponding author. E-mail address: [email protected] (C. Xu).
https://doi.org/10.1016/j.cep.2019.107637 Received 19 April 2019; Received in revised form 3 August 2019; Accepted 17 August 2019 Available online 17 August 2019 0255-2701/ © 2019 Published by Elsevier B.V.
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extract HNO3 from various aqueous solutions. As for extraction devices, traditional macro-scale extractors such as pulsed column [12,13], mixer settler [14], and centrifugal extractor [15] were adopted in these studies. However, over the past two decades, micro/mini-extractors are widely used in solvent extraction to enhance mass transfer. [18–22] These micro-/mini-extractors exhibit higher surface-to-volume ratios and shorter diffusion lengths for mass transfer by restricting fluids into small channels with dimensions ranging from micro-millimeters to millimeters. [20] Thus, the mass transfer rate is significantly increased. More importantly, the very small inventories of micro/mini-extractors make them attractive for solvent extraction involving hazardous chemicals. In case of an accident, potential harm to the operator and environment is limited. [23] However, few studies have focused on applications of micro-/mini-extractors in removing hazardous materials from industrial effluents. [24] It is owing to two disadvantages of existing micro-/mini-extractors. The first is low throughput that is unable to satisfy industrial production requests. The second is how to break through the mass transfer limit by extraction equilibrium. In this paper, we developed two mini-extractors, a co-current and count-current mini-extractors, demonstrating how to overcome the above disadvantages respectively. Here an HNO3 aqueous solution (a simulated effluent) and a 30% TBP-70% kerosene solution (extraction solvent) were used as the demonstration system. The co-current miniextractor with a characteristic size in the millimeter range was generated by expanding a co-current micro-extractor with a characteristic size ranging from tens to hundreds of micrometers. The method is termed as the Dimension Scale-Out to retain benefits of microfluidics while enabling high-throughput. A challenge faced during the expanding process involves methods to share benefits including the high area-to-volume ratio, short diffusion distance, and narrow residence time distribution of microfluidics. Furthermore, the counter-current mini-extractor (CCME) was designed and demonstrated to remove HNO3 with higher extraction efficiencies than one limited by extraction equilibrium.
Table 1 Densities and viscosities of the experimental systems. Feed materials
Density (kg/m3)
Viscosity (mPa·s)
Red aqueous solution Kerosene with surfactant 3 M HNO3 solution 30% TBP-kerosene
1002.0 745.2 1101.8 818.1
1.39 1.56 1.48 1.85
acidic condition, it does not show interfacial crud formation. [25] The kerosene was purchased from Jinzhou Refinery Factory (Liaoning, China). Analytically pure TBP and nitric acid were purchased from Beijing Chemicals Factory (Beijing, China). The HNO3 concentrations in the raffinates and eluents were determined via an automatic titrator (Metrohm China Ltd., 905) with NaOH as the titrant. All tests were performed within a temperature range of 22–23 °C. The densities and viscosities of the two phases were measured by using a densitometer (Den Di-1, Beijing YILUDA Co. Ltd., Beijing, China) and a rotary viscometer (DV-1, Spain Fungilab Co., Barcelona, Spain), respectively. An interface tensiometer (BZY-1, Shanghai Heng PingInstrument Factory, China) was used to measure the surface tension between the two phases. The density and viscosity of the two phases are listed in Table 1. The interfacial tension of the 30% TBP/ kerosene-HNO3 solution was 8.0 mN/m. 3. Co-current extraction 3.1. Co-current mini-extractor The first stage of the study involves developing a co-current miniextractor with the high throughput and high extraction efficiency (relative to phase equilibrium) based on a co-current micro-extractor. Here a dimension scale-out method was adopted and the flow pattern and mass transfer experiments are used to verify the feasibility of the method.
2. Set-up fabrication and materials 2.1. Fabrication
3.2. Dimension scale-out
Two layers of transparent poly(methylmethacrylate) (PMMA) plates were used to fabricate all the extractors in the study. The mini-channels were machined on a PMMA plate by using a commercial precision end mill (Kejie Machinery Automation Co., Ltd. Of Guangdong, JTGK-600). The slotted plate and other smooth plate were clamped with a stainlesssteel clamp and bonded by using the solvent sticking method via ultrasonic oscillation. A detailed description of the fabrication processes of the extractors is reported in previous studies. [24]
A novel co-current micro-extractor, namely the oscillating feedback micro-extractor (OFM) as shown in Fig. 1-A, is adopted as the basis of the scale-out. The OFM is designed based on an oscillating flow. When a fluid flows into a suddenly enlarged chamber (3 in Fig. 1-A) from a narrow passage (2 in Fig. 1-A) at high speed, it can entrain its surrounding fluid to form a low-pressure region on each side of the chamber. One of the low-pressure regions attracts the fluid and deflects it forming a wall-attachment flow. Due to the pressure difference between the two ends of the feedback channel, much more fluid recirculates into the low-pressure region through the feedback channel. The pressure of the low-pressure region increases producing a larger transverse force on the wall-attachment flow, consequently. The fluid is pushed towards the opposite wall and thus a new wall-attachment flow is formed. Similarly, the recirculation through the feedback channel of this side can also push the wall-attached flow away from this wall. As a result, the fluid swings in the chamber to produce a high-frequency oscillating flow. The OFM was investigated in a previous study and exhibited excellent results with respect to the mass transfer between two phases. [27] As shown in Fig. 1-A, the OFM is scaled out to form an enlarged mini-extractor shown in Fig. 1-B. The mini-extractor consisted of a T-type inlet, a mixing chamber, a splitter, two feedback channels, and an outlet. The widths of the inlet and feedback channel as well as the length of the mixing chamber were increased to 1 mm, 2 mm, and 18.8 mm from original measurements corresponding to 200 μm, 400 μm, and 4420 μm, respectively. Additionally, all the depths of the mini-extractor corresponded to 0.5 mm remaining at a micro-scale. The volume of the co-current mini-extractor was only 0.172 mL much lower
2.2. Materials In the flow pattern experiments, in order to clearly observe the flow patterns and capture the droplets size evolution, a red aqueous solution was used as the dispersed phase and the colorless fully hydrogenated kerosene was used as the continuous phase. Additionally, 0.30 wt% of Span 80 surfactant (sorbitan monooleate, J&K Scientific Ltd., China) was added to the kerosene. The red aqueous solution was produced by dissolving 1 g of red dye (Sanolin Ponceau 4 RC 8, Clariant International Ltd., Switzerland) in 300 g of deionized water. In the extraction experiments, an aqueous nitric acid (3 M HNO3) solution was used as the simulated effluent and dispersed phase, and a 30% Tributyl phosphate (TBP)/kerosene solution was used as the extraction solvent (extractant) and continuous phase. TBP has been widely used as an extractant to extract either anionic or neutral metal species via ion-pairing or direct solvation into an organic phase from an aqueous phase. [25,26] In addition, TBP is low cost and easy to be operated owing to its low viscosity. [25] Furthermore, even under an 2
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Fig. 1. Diagram of the micro/mini extractors available for the co-current flow arrangement: (A) configuration of the oscillating feedback microextractor (depth, 1.0 mm), (B) configuration of the mini extractor, and (C) dimensions of the mini extractor (depth, 0.5 mm). 1: inlet port; 2: inlet channel; 3: mixing chamber; 4: barrier; 5: feedback channel; 6: splitter; 7: converging chamber; 8: outlet channel; 9: outlet port.
When Qaq = Qor =35 mL/min (Fig. 3-d), the number of the red aqueous phase clusters in the left feedback channel significantly reduced due to enhanced feedback flow. However, a few of the red aqueous phases were not broken into small droplets on the left side of the splitter. As Qaq = Qor =40 mL/min (Fig. 3-e), the red aqueous phase was effectively broken into small droplets on the left side of the splitter due to increases in the shear force. However, the number of droplets on the left side still exceeded that on the right side. When Qaq = Qor =50 mL/min (Fig. 3-f), the red aqueous phase was fully broken into small droplets and distributed over the entire mini-extractor. Thus, a high area-tovolume ratio was achieved to enhance mass transfer. It should be noted that the Reynolds numbers (Rei=ρiUiD/μi, i = aq or or, and D is the hydraulic diameter of the inlet channel) Reaq and Reor for the aqueous and organic phases range between 320.4–881.1 and 212.3–583.8, respectively. The Reynolds numbers at the inlet channel always corresponded to a maximum within the entire mini-extractor. Thus, the results confirmed that the two-phase flows in the mini-extractor always remain in the laminar flow regime. A few conclusions were derived from the flow patterns as follows: the flows were still laminar in the mini-extractor, which is formed via the dimension scaleout of the micro-extractor of an OFM, and (2) the dispersed phase was effectively dispersed even in the laminar flows due to the intensive disturbances caused by the vortex, feedback, and oscillating flows as reported in previous studies on OFMs. [27]
than that of traditional extractors such as columns and mixer-settlers (0.1-10 m3).
3.3. Flow patterns The experimental flow chart is shown in Fig. 2. The two immiscible liquids were simultaneously pumped into the mini-extractor by using two precise syringes (Baoding Longer Precision Pump Co., Ltd., LSP011BH, stroke resolution 0.156 μm). A microscope (Shanghai Changfang Optical Instrument Factory, CMM-50E) combined with a digital singlelens reflex camera (Canon, EOS650D; pixels: 5184 × 3456) was used to visualize and record the flow patterns in the mini-extractor. The flow patterns in the co-current mini-extractor with different flow rates (20–55 mL/min) were investigated. A selected set of results are shown in Fig. 3. The aqueous phase flow rate (Qaq) and organic phase flow rate (Qor) were always set as identical. As shown in Fig. 3-a (Qaq = Qor =20 mL/min), a stratified flow is observed, and an evident interface between the two phases is observed. In this case, the mass transfer mainly depended on the thermal motion of molecules. Given Qaq = Qor =25 mL/min (Fig. 3-b), a part of the red aqueous phase was dispersed into droplets and distributed in the mixing chamber and two feedback channels. However, only a small number of droplets were observed in the right feedback channel. Given Qaq = Qor =30 mL/min (Fig. 3-c), significantly more red aqueous droplets were produced when compared with that in Fig. 2-b. However, a few of the red aqueous phase clusters (slug flow) were observed in the left feedback channel. 3
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Fig. 2. Schematic diagram of the co-current extraction experiments using the mini extractor.
3.4. Extraction performance Extraction experiments in the co-current mini-extractor were performed to further verify the feasibility of the dimension scale-out. The extraction extent E is defined as follows.
E=
D De
(1)
D=
Cor,out Caq,out
(2)
Fig. 3. Flow patterns of the feedback mini extractor (a) Qaq = Qor =20 mL/min; (b) Qaq = Qor =25 mL/min; (c) Qaq = Qor =30 mL/min; (d) Qaq = Qor =35 mL/ min; (e) Qaq = Qor =40 mL/min; and (f) Qaq = Qor =50 mL/min. 4
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Where D denotes the ratio of the acid concentration of the organic phase to that of the aqueous phase at the extractor outlet. Given the definition of D, De is defined as the acid concentration ratio of the two immiscible phases in equilibrium. E represents the extent to which the exit streams are close to the phase equilibrium, i.e., the mass transfer limit. Furthermore, the mass transfer rate (R*) and overall volumetric mass transfer coefficient (ka) are calculated by using Eqs. (3) and (4) as follows: [28]
R* = Q aq (Caq,in − Caq,out ) = kaV ΔCm ka =
Table 2 Comparison of extraction efficiencies in the OFM and mini-extractor.
(3)
Q aq (Caq,in − Caq,out )
Where k denotes the overall mass transfer coefficient, a represents the mass transfer area between the two phases per unit volume, and V denotes the volume of the mini extractor. Additionally, △Cm denotes the average concentration difference that is calculated as follows:
ΔCm =
* ) − (Caq,out − Caq,out * ) (Caq,in − Caq,in (Caq,in − C *
)
aq,in ⎤ ln ⎡ (C * )⎦ ⎣ aq,out − Caq,out
(5)
Where Caq, in denotes the acid concentration in the inlet aqueous phase. * denotes the equilibrium acid concentration in the inlet Moreover, Caq,in aqueous phase that corresponds to the acid concentration in the inlet * denotes the organic phase and is zero in the study. Specifically, Caq,out equilibrium acid concentration in the outlet aqueous phase that corresponds to the acid concentration in the outlet organic phase. Additionally, the residence time (tR) is given as follows:
tR =
V Q aq + Qor
Emicro (%)
Qtotal-mini (mL/min)
Emini (%)
52 60 72 80 100 120
88.6 94.9 - (Emul.) - (Emul.) - (Emul.) - (Emul.)
56 64 72 80 100 120
49.8 61.0 69.2 83.8 96.3 97.5
was invalid when the total flow rates Qtotal exceeded 60 mL/min. This is because emulsification occurred and the mixture out of the OFM was difficult to achieve phase separation in a short time. Conversely, a high extraction extent of E = 97.5% was achieved in the mini-extractor even at a total flow rate of 120 mL/min because emulsification was absent. The extraction extent of the mini-extractor was comparable with that of the OFM and was close to the mass transfer limit determined by the phase equilibrium. Additionally, the mini-extractor achieved a total throughput of 120 mL/min that was twice the maximum throughput of the OFM without emulsification. Therefore, the micro-extractor based on the dimension scale-out method was valid to increase the throughput significantly and retain high extraction efficiency. However, it should be noted that when the two phases reach equilibrium, the co-current extraction achieves its maximum extraction efficiency. Here the maximum recovery efficiency for co-current extraction (Xcocurrent=(Caq,in-Caq,out)/Caq,in)) was only 17.7%, a value not satisfying the production requests.
(4)
V ΔCm
Qtotal-micro (mL/min)
4. Counter-current mini-extractor (6) In order to break through the recovery efficiency limited by phase equilibrium, the co-current mini-extractor shown in Fig. 1 was further modified to form a bidirectional mini-extractor based on the requirements of the counter-current extraction system. Furthermore, a countercurrent extraction system was established by using several bidirectional mini-extractors. Finally, extraction experiments were performed to verify the extraction performance of the CCME system.
Fig. 4 illustrates the extraction extent (relative to phase equilibrium) E and overall volumetric mass transfer coefficient ka with respect to different residence times. The results indicated that both E and ka increase with decreases in residence time tR. This was because decreases in the residence time (i.e., increases in the flow rates) effectively break the aqueous phase into small droplets as shown in Fig. 3. Thus, the mass specific surface area increased and the mass transfer was enhanced. Hence, an extraction extent of 97.5% was achieved with a residence time of less than 0.1 s. Additionally, Table 2 compares the extraction extents between the mini-extractor (Emini) and its scale-out base, i.e., the OFM (Emicro) (Fig. 1-A) under different flow rates. The results indicate that the OFM
4.1. Bidirectional mini-extractor and CCME system
• Bidirectional mini-extractor
Fig. 4. Extraction performance of the mini extractor. 5
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Fig. 5. Configurations and dimensions of the passive bidirectional mini extractor (Depth, 2000 μm): (A) schematic of design S, (B) dimensions of design S, (C) design NS, and (D) design AS. 1-bottom inlet/outlet channel; 2-bottom mixing chamber; 3-bottom barrier; 4-bottom feedback channel; 5-bottom splitter; 6-top inlet/outlet channel; 7-top mixing chamber; 8-top barrier; 9-top feedback channel; 10-top splitter; 11-bottom section; 12-top section.
maintained at 60 kPa.
Based on the above co-current mini-extractor, three bidirectional mini-extractors are developed to construct a CCME process as shown in Fig. 5. The bidirectional mini-extractor mainly consists of two parts, namely the upper mixing stage and bottom mixing stage. Each mixing stage exhibits structures similar to those of the co-current mini-extractor in Fig. 1-B. For convenience, the bidirectional mini-extractors with a splitter and two symmetrical feedback channels are denoted as S as shown in Fig. 5-A. Its dimensions are shown in Fig. 5-B. When compared with S, the bidirectional mini-extractors without splitters are denoted as NS as shown in Fig. 5-C. Bidirectional mini-extractors with two asymmetrical feedback channels with two splitters are denoted by AS (shown in Fig. 5-D). The volumes are only 7.51 mL, 7.69 mL, and 7.93 mL for S, NS, and AS, respectively.
4.2. Extraction performance The counter-current recovery efficiency X is defined as follows.
X=
Caq _ in − Caq _ raf Caq _ in
(7)
Where Caq_in denotes the initial nitric acid concentration for the raw liquid in the organic phase settler, and Caq_raf denotes the nitric acid concentration of the raffinate obtained from the aqueous phase layer in the aqueous phase settler. The phase equilibrium recovery efficiency for the co-current extraction Xe is calculated as follows:
• CCME system and working strategy
Xe =
A CCME system is established as described in Fig. 6. The system mainly includes two parts, namely the control model and mini extraction model. The control model contains four solenoid valves (ZCC-1 P, Shanghai Juliang Solenoid Manufacturing Co., China), a timing controller, and a pressure regulating valve (Type 70, Marsh Bellofram Co.). The flow flux is adjusted via controlling the timing controller. The pressure regulating valve is used to adjust the pressure in the compressed gas buffer tank. With respect to the extraction model, it mainly consists of six bidirectional mini-extractors and two settlers. The six bidirectional mini-extractors are connected to each other from end to end to form a mini-extraction section. The feeding port of the aqueous phase settler is connected to the top inlet/outlet of the mini-extraction section (port II) while the feeding port of the organic phase settler is connected to the bottom inlet/outlet of the mini-extraction (port I). The structure and dimensions of the two settlers are shown in Fig. 6-B and Fig. 6-C. As showed in Fig. 7, the working strategy of the CCME system includes four stages: (1) organic phase feeding stage, (2) droplets aggregation stage I, (3) aqueous phase feeding stage, and (4) droplets aggregation stage II. The four stages keep cycling to establish a countercurrent flow arrange and can be realized by controlling the four valves V-LC, V-LV, V-HC, and V-HV. Table 3 shows the opening or closing of the four valves corresponding to the four stages. As a result, the two phases are transported along opposite directions based on the alternative feeding of two phases, and the mixture of the two phases is effectively separated after mixing. During the operating cycle, the dispersed phase is effectively broken into small droplets to increase the area-to-volume ratio and obtain high mass transfer. A detailed description of the operation steps for the control system is given in previous studies. [29] The pressure for the pulsed system is always
Caq _ in − Caq _e Caq _ in
(8)
Where Caq_e denotes the nitric acid concentration in the aqueous phase reaching phase equilibrium. Furthermore, Xe is determined by shake tests and a detailed description to measure the value of Xe is indicated in our previous study. [29] Additionally, the extraction performance index λ is adopted to further evaluate whether the counter-current extraction process is successfully achieved. Additionally, λ is expressed as λ = X/Xe. A detail description [29] for the evaluation is shown in Table 4. It should be noted that back-mixing causes a counter-current extraction to change to a co-current extraction, and thus the values of X and λ decrease. 4.2.1. Effects of structure parameters In the section, the effects of the extractor structure on the hydraulics (shown Table 5) and extraction performance (shown in Fig. 8) for the counter-current extraction is investigated. The duration times for the organic phase feeding stage (toc) and the aqueous phase feeding stage (tac) are both set to 1.0 s. The duration times for the droplets aggregation stage I and II are both set to 5.0 s. The flow rates of the organic and aqueous phases For and Faq (phase transfer rates) are defined as follows:
For =
Faq =
Vor t
(9)
Vaq t
(10)
Where Vor denotes the transferred volume for the organic phase from the aqueous phase settler to the organic phase settler within a complete operating cycle, Vaq denotes the transferred volume for the aqueous phase from the organic phase settler to the aqueous phase settler within a complete operating cycle, and t denotes the lasting time for a 6
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Fig. 6. Schematic of the counter-current mini extraction system: (A) flowchart, (B) organic phase settler, and (C) aqueous phase settler.
significantly enhance the internal recirculation flow through the feedback channels and oscillation flow in the mixing chamber. [24,27] Thus, the highest area-to-volume ratio for mass transfer is achieved, and AS exhibits the highest extraction performance when compared with NS and S. Additionally, NS does not exhibit splitters and asymmetrical feedback channels, and thus it is difficult to break the aqueous phase into small droplets. Hence, NS exhibits the worst extraction performance. However, all the extraction performance indexes (Fig. 8B) for the three different mini-extractors exceed 1.0. Therefore, successful counter-current extraction is achieved in all three structures (i.e., NS, S, and AS).
complete operating cycle that includes four stages. The total throughput (total flow rate) Ftotal is defined as Ftotal = Faq + For, and the flux ratio (R) is defined as R = For / Faq. As listed in Table 5, the total throughput of NS was the highest, and this is followed by S and AS. This is because splitters that reduce the flow resistance are absent. However, AS exhibits the highest recovery efficiency (Fig. 8-A) and extraction performance index (Fig. 8-B), and this is followed by S and NS. The phenomenon is because the dispersed phase is more effectively broken into small droplets in the feeding stage in AS due to the splitter and asymmetrical feedback channels. Extant studies indicated that the splitters and asymmetrical feedback channels 7
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Fig. 7. Working principle of the counter-current extraction system: (A) droplet aggregation stage I, (B) organic phase feeding stage, (C) droplet aggregation stage II, and (D) aqueous phase feeding stage. Table 3 Different operating stages controlled via different states of the solenoid valves. Operating stages
V-LC
V-LV
V-HC
V-HV
Organic phase feeding stage Droplet aggregation stage I Aqueous phase feeding stage Droplet aggregation stage II
Close Close Open Close
Open Open Close Open
Open Close Close Close
Close Open Open Open
Table 4 Evaluation of the counter-current extraction. λ λ < 1 λ=1 λ > 1
Evaluation of the counter-current extraction Poor extraction and phase equilibrium is not achieved Phase equilibrium extraction is achieved Phase equilibrium is broken and is successful. Counter-current extraction is achieved.
Table 5 Results of the hydraulics for the counter-current mini extractors with different structures. Type of structure
Faq (mL/min)
Faq (mL/min)
Ftotal (mL/min)
R
NS S AS
0.9625 0.416 0.3595
0.8055 0.5425 0.4195
1.768 0.9585 0.779
0.84 1.34 1.17
4.2.2. Effects of operating times The NS is selected to investigate the effects of the different phase feeding times and droplets aggregation times. The hydraulic results are shown in Table 6, and the extraction performance is shown in Fig. 9. In general, phase separation is a throughput-limiting step for multistage counter-current extractors. A comparison of T1 with T2 shown in
Fig. 8. Changes in (A) recovery efficiency and (B) extraction performance index in the mini counter-current microextractors with different structures. 8
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Table 6 Results of the hydraulics for the counter-current mini-extractors with different operating times. Time series
Faq (mL/min)
Faq (mL/min)
Ftotal (mL/min)
R
T1: (tac = toc = 1.0 s; tI = tII = 3.0 s) T2: (tac = toc = 1.0 s; tI = tII = 5.0 s) T3: (tac = toc = 2.2 s; tI = tII = 5.0 s)
1.2705 0.9625 2.0596
0.9225 0.8055 1.7104
2.193 1.768 3.770
0.84 1.34 0.83
(total flow rate) of 3.77 mL/min is achieved. As shown in Fig. 9, the recovery efficiency and extraction performance index do not exhibit evident changes with increases in the phase feeding time and droplets aggregation time. This is because a high level of mixing is achieved in the mini- extractor due to the vortex, feedback, and oscillating flows. [24] Therefore, the proposed simple and robust passive CCME system could break through the mass transfer limit caused by phase equilibrium. The experimental results proved mini-extractors available for removing hazardous materials from effluents. 4.2.3. Comparison of the throughput Based on the mass transfer mode and flow arrangement, countercurrent micro/mini-extractor can be classified into two types: continuous counter-current micro/mini-extractor [30,31] and multistage micro/mini-extractor [32–34]. As for the former, solutes are transferred between the two immiscible phases in a differential contact mode without needing a complete phase separation. As for the multistage counter-current micro/mini-extractor, solutes are transferred between two phases in a stage-wise contact mode. Here phase separation is important in order to renew the specific surface area of mass transfer as well as reduce fluid back-mixing. However, check valves [33] or interstage pumps [34] are indispensable for controlling the flow direction for multistage counter-current micro/mini-extractors. These active components at the macro-scale are difficult in integrating with micro/ mini-extractors and make the controlling system complicated. The proposed CCME system is passive and thus, we just make a comparison between the passive counter-current micro/mini- extractors in the literature and the proposed CCME system. Fig. 10 compares the throughput between the proposed CCME system and those reported in extant studies for passive counter-current micro-extractors. The results indicate that both the lower and upper limits of Ftotal that are observed in the proposed CCME system are one to three orders of magnitude higher than those in other passive countercurrent micro-extraction models such as continuous models [30,31] and multistage models [32]. Additionally, a maximum total throughput of 3.77 mL/min is achieved in the proposed CCME system. These experimental results show that the CCME system could break through the mass transfer limit caused by phase equilibrium and significantly improved the throughput by the dimension scale-out method.
Fig. 9. Changes in (A) recovery efficiency and (B) extraction performance index in the mini counter-current extractors with different operating time series.
Table 6 indicates that the flow rates decrease with increases in the droplet aggregation time. This is because increases in the droplet aggregation time are advantageous to phase separation and reduce backmixing during the phase feeding stages. This suggests that the organic (aqueous) phase conveyed during the organic (aqueous) phase feeding stage increases, and thereby increases the throughput. However, the total time also increases with increases in the droplet aggregation time. Finally, the net effect to increase the aggregation time corresponds to decreases in the flow rates. Actually, complete phase separation is not necessary for the presented CCME system. As described in Fig. 6, the counter-current flow arrangement can be realized as long as an aqueous phase-rich layer and an organic phase-rich layer are formed in the bottom and the top of every bidirectional mini-extractor during the aggregation stages. Namely, the presented CCME system can work with a rather short aggregation time (phase separation time) because it does not require complete phase separation. Actually, only a few seconds can ensure phase separation available for the CCME system, as shown in Table 6. With respect to series T2 and T3 in Table 6, the droplet aggregation times (tI = tII = 5.0 s) remain unchanged although the lasting times of the phase feeding stages are different. It is observed the flow rates increase with increases in the phase feeding times. A high throughput
5. Conclusion In this paper, a passive co-current and counter-current mini-extractors were developed based on the oscillating feedback microextractor and demonstrated for removing nitric acid from effluents using solvent extraction. Two problems, i.e. low throughput and mass transfer limit, were overcome. Boh the co-current and counter-current mini-extractors are rapid and inherently safe because of their very small volume. The co-current mini-extractor has a residence volume of 0.172 mL and a nitric acid extraction efficiency of 97.5% was achieved even at a high total throughput of 120 mL/min. The counter-current mini-extractor has a residence volume of 7.51 mL in which the extraction efficiencies were 2.30 times that in the co-current extraction equilibrium and its total throughputs were one to three orders of magnitude higher than those in the reported passive counter-current extraction systems using micro-extractors. The passive, high9
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Fig. 10. Comparison of the throughput.
throughput, and high-efficiency mini-extractors have great potential in removing hazardous materials from effluents. However, it should be noted that the counter-current mini-extractor only achieved the maximum total throughput of 3.77 mL/min that still is relatively low compared with production requests. This problem can be solved using the parallelization of a micro-/mini-extractor (numbing-up) in the future.
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