Article pubs.acs.org/IECR
Effect of Partitioning on Sonochemical Reactor Performance under 200 kHz Indirect Sonication Kandasamy Thangavadivel,*,† Gary Owens,‡ Kenji Okitsu,*,† and Muthupandian Ashokkumar§ †
Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Environmental Contaminants Group, Mawson Institute, University of South Australia, Mawson Lakes Campus, Mawson Lakes, SA 5095, Australia § School of Chemistry, University of Melbourne, Melbourne, VIC 3010, Australia ‡
S Supporting Information *
ABSTRACT: A rectangular Pyrex glass reactor was indirectly sonicated with a 65-mm-diameter 200 kHz transducer under batch and flow conditions at 20 °C and then partitioned using Perspex plates and sonicated again. Both the degradation of methyl orange (MO) and the calorimetric power decreased with partitioning. Under batch conditions, as the number of compartments increased from 1 to 7, the MO removal efficiency (RE) decreased from 83% to 45%, and the pseudo-first-order rate constant (k) decreased from 0.097 to 0.034 min−1. Under flow conditions the MO RE also decreased from 66% to 27% as the number of compartments increased, and k decreased from 0.096 to 0.016 min−1. Partitioning reduced the reactor volume and contributed to acoustic energy attenuation, resulting in decreased reactor performance, where MO RE was affected by the cross-sectional area, flow velocity, and solution volume.
1. INTRODUCTION It is well-known that medium-frequency ultrasound (200−600 kHz) produces excellent chemical effects in aqueous media.1 At 300 kHz, sonolysis of water produces 22 μM H2O2 min−1.2 To harness the immense benefits of the chemical effects of medium-frequency ultrasound, sonochemical reactor design is of great importance.3 Compared to conventional reactors, sonochemical reactors have unique features that need to be considered to make the best use of the applied acoustic energy and obtain optimal sonochemical efficiency.4 Transducer arrangement, reactor size and shape, and hydrodynamic aspects all play important roles in sonochemical reactor design.5 For batch studies, conical, cylindrical, and rectangular glass reactors with volumes of less than 1000 mL have predominantly been used.1,4−7 Typically, for most organic contaminants with initial concentrations in the millimolar range, sonochemical treatment times vary from 20 to 90 min.1,8,9 The formation of more standing waves and, hence, an increased cavitation bubble population is responsible for the higher chemical yield at medium and higher frequencies.10 A combination of medium-frequency sonochemical reactors (490 kHz) with mechanical mixing increases the sonochemical efficiency by altering the pattern of standing waves.11 Bussemaker and Zhang12 previously recommended that mixing was suitable only for low-frequency reactors and that mixing can attenuate the acoustic energy. They also noted an increased H2O2 yield with 376 and 995 kHz sonochemical reactors when the flow rate was 34 mL min−1.12,13 In addition to being employed for contaminant degradation, single- and multiple-frequency ultrasonic baths are currently being extensively used in laboratories for the extraction of chemicals from soil and other matrixes, for the mixing of chemicals, and for cleaning and degassing.5 Typically, glass sample bottles, retained in a rubber or plastic holder, are simply © 2014 American Chemical Society
placed within a sonicator bath for a defined period of time. These sample holders, apart from attenuating acoustic energy, also act as a barrier, restricting the movement of solutions within the bath. Because the way samples are placed within the sonicator bath can affect the contaminant degradation efficiency, effects of partitioning within a sonochemical reactor is a topic that needs further detailed clarification. A reactor with a higher buffering capacity is normally most appropriate for contaminant remediation, where a continuous stirred tank reactor (CSTR) is one suitable option. By partitioning a large reactor, a series of CSTRs can be obtained under flow conditions that will reduce additional pumping and space requirements, making operation easier. Through partitioning, the flow velocity within the reactor can be increased many fold without affecting the overall residence time and volume of the reactor. However, it is important to understand the effects of reactor partitioning on the reaction kinetics, as well as the residence times of the individual compartments. To our knowledge, no reports are currently available on the effect of reactor partitioning on medium-frequency reactor performance. In this study, we used methyl orange (MO) as a model dye pollutant to evaluate the performance of a Pyrex glass rectangular reactor with internal partitions using 200 kHz indirect sonication under both batch and flow conditions. Received: Revised: Accepted: Published: 9340
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Figure 1. Schematic representations of (A) the sonochemical reactor setup for three-compartment partitioning with two Perspex plates and (B) the flow process. Diagrams are not to scale. In view A, green arrows indicate flow into reactor/compartment; green circles indicate holes. Components in view B: a, storage solution; b, peristaltic pump; c, transducer; d, constant-temperature water bath; e, coupling medium; f, reactor with liquid; g, optical fiber UV−vis sensor; h, optical fiber; i, UV−vis instrument; j, exit to drain.
Table 1. Reactor Compartment Dimensions and Corresponding Residence Times and Flow Velocities no. of plates
compartment distance (mm)
no. of compartmentsa
compartment arrangement (left to right)
RAb (%)
RVc (%)
residence timed (s)
0 2 4 6
65.0 21.6 13.0 9.3
1 3 5 7
R S1, M, S′1 S1, S2, M, S′2, S′1 S1, S2, S3, M, S′3, S′2, S′1
0 3.1 6.2 9.2
0 3.3 6.6 9.8
1230 396 229 158
flow velocityd (m s−1) 5.3 1.5 2.4 3.4
× × × ×
10−5 10−4 10−4 10−4
a Compartment name: R, reactor; S1, left side; S2, adjacent to S1; S3, between S2 and M; M, middle; S′x, right side. bRA is the percentage of Perspex plate area touching the bottom plate of the reactor (n × 59 mm × 1 mm, where n is the number of Perspex plates) relative to the are of the reactor bottom plates (65 mm × 60 mm). cRV is the percentage volume of Perspex plate (n × 59 mm × 1 mm × 105 mm) relative to the total reactor volume (65 mm × 60 mm × 105 mm). Compartment height was 105 mm. Glass reactor volume (without compartment) was 410 mL. dResidence time and flow velocity were computed based on 20 mL min−1.
2. EXPERIMENTAL SECTION 2.1. Chemicals. MO (C14H14N3NaO3S) and analyticalgrade sulfuric acid (H2SO4) were obtained from Wako Pure Chemical Industries Ltd., Osaka, Japan. Milli-Q water, with a resistivity of 18.2 MΩ cm, was used to prepare all aqueous solutions. H2SO4 was used to adjust pH. 2.2. Ultrasonic Reactor. A custom-made rectangular Pyrex glass reactor was used in all experiments. The internal dimensions of the reactor were as follows: length, 65 mm; width, 60 mm; height, 160 mm; wall thickness, 2 mm; internal diameter of the bottom inlet (bottom-plate top level) and discharge (105-mm height from the bottom plate) ports, 5 mm. As indicated in Figure 1A, both ports were located in the horizontal direction of the reactor to provide maximum residence time, and the reactor was partitioned using Perspex plates. The Perspex plates were held vertically, with a guide of fine glass beads (fine glass beads embedded in the wall of the reactor) on the internal wall of the reactor and were placed at the bottom plate of the reactor. Up to six Perspex plates, 59 mm (length) × 120 mm (height) × 1 mm (thickness), were used with holes having a 5-mm internal diameter at the corner of the plates to partition the reactor (Figure 1A and Table 1). The notation in Table 1 was used to represent the compartments. For example, the left-side, middle, and rightside compartments of the the three-compartment reactor are denoted as S1, M, and S′1, respectively. The ultrasound source was a 65-mm-diameter transducer (Kaijo 4611, mfg. no. 00102) with a frequency of 200 kHz and a nominal power of 200 W (Kaijo model 4021, lot. no. 11EA00105). The water bath (26 cm × 36 cm × 15 cm) was maintained at 20 ± 1 °C using a TAITEC cool pump (model
CP-150R). Ultrasound was applied indirectly to the glass reactor (Figure 1B). During sonication, the reactor was maintained 8 mm above the transducer. The transducer and the 2-mm-thick Pyrex glass bottom plate of the reactor were positioned parallel to each other and center-line-aligned along the vertical axis throughout the experiments. In each case, the compartments had equal dimensions; for example, when two Perspex plates were used, the three resulting compartments each had a length of 21.6 mm (Table 1). The holes in the Perspex plates were arranged to ensure the maximum residence time in each of the compartments and also to be consistent with the Pyrex glass reactor construction, especially the inlet and outlet port arrangements. 2.3. Calorimetric Power (Pcal). Calorimetric power under batch and flow conditions was determined using the calorimetric technique reported previously.14 Briefly, the reactor was filled with Milli-Q water to a height of 105 mm and allowed to stabilize at room temperature over 30 min in an air atmosphere without temperature control. Ultrasound was then applied to the reactor, and the temperature was continuously recorded for 90 s using a data logger (every 1 s) through a thermocouple (1-mm diameter, type C, part no. 55052, KATOH) at the side compartment (S1, Table 1, Figure 1A) of the reactor. When multiple Perspex plates were used to divide the reactor into equal-sized compartments (Figure 1 A), the temperature was consistently measured in the last compartment adjacent to the discharge port (S1) under both batch and flow conditions. The calorimetric power (Pcal) was determined using the equation15,16 9341
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Table 2. Summary of Pr, Re, and Convective Heat-Transfer Coefficient Values in the Middle Compartment heat-transfer coefficient (W m−2 K−1) no. of compartments 1 3 5 7 a
Pr
characteristic length (m)
7 7 7 7
0.0624 0.0318 0.0214 0.0161
−1
acoustic stream velocity (m s ) 0.00050 0.0016 0.0028 0.0040
Re 31 51 60 64
convective
overall for acrylic
28 70 130 180
overall for glass
a
NA 51.7 78.4 96.0
26.3 62.2 105 140
Not available.
Table 3. Variations of Pcal,a,b k, and SE with Degree of Compartmentalization for Both Batch and Flow Conditions no. of compartments
Pcal (W)
CEIVc (W L−1)
1 3 5 7
44 41 30 24
± ± ± ±
2 2 1 2
110 100 74 59
1 3 5 7
37 34 28 20
± ± ± ±
1 2 1 3
90 84 68 49
CEIAc (W cm−2)
kd (min−1)
Batch Conditions 1.10 0.097 1.10 0.070 0.77 0.047 0.62 0.034 Flow Conditions (20 mL min−1) 0.95 0.096 0.88 0.052 0.72 0.024 0.51 0.016
R2
SEe (ng J−1)
MO RE (%)
0.9942 0.9910 0.9981 0.9934
67 67 65 72
83 71 51 45
60 42 36 39
66 60 37 27
a Assumed to be the same throughout all compartments in the reactor. bApparent calorimetric power was measured under flow conditions. cCEI = calorimetric energy intensity; V, based on reactor volume; A, based on reactor cross-sectional area. dk = pseudo-first-order rate constant computed for a CSTR under steady-state conditions. eSE for the batch reactor computed based on MO removed within 18 min.
Pcal = MCp
dT dt
2.4. MO Degradation. Batch Conditions. MO solution (10 mg L−1, pH 2) was transferred into the reactor, ensuring that the solution level was just above the top edge of the discharge port (Figure 1A). The solution was sonicated for 21 min at 20 °C under an air atmosphere. During sonication, the MO concentration was periodically monitored (every 3 min) in terms of the absorbance of MO at 510 nm near the discharge point of the reactor (S1, see Table 1) using a UV−visible spectrophotometer (model DH-2000-BAL, sensor model USB2000+UV−vis−ES, Ocean Optics) coupled to a dataprocessing and data-storage device (computer). Under air atmosphere, sonolysis of water produces HNO2 and HNO3, which reduce the solution pH with sonication time. Because MO is stable around pH 2, this pH was chosen to obtain accurate MO measurements. For the three-compartment reactor, readings were also taken in the middle compartment (M), whereas for the fivecompartment reactor, readings were also taken in the side (S1), adjacent (S2), and middle (M) compartments. However, for the seven-compartment reactor, readings were taken only in the side compartment (S1) because there was no additional space to place the optical fiber sensor with a sensor mirror. The MO removal efficiency was determined as
(1)
where M is the mass of water (g), Cp is the specific heat capacity of water (4.18 J g−1 K−1), and dT/dt is the initial rate of solution temperature increase. Results are reported as averages of five independent measurements. Convective heat-transfer coefficients (h) depend on the prevailing hydrodynamic conditions within the reactor, and thus determination of the convective heat-transfer coefficients allows for reactor performance to be better understood in terms of heat transfer. Convective heat-transfer coefficients (h) were determined using the equation Nu =
hL k
(2)
where Nu is the Nusselt number, h is the convective heattransfer coefficient (W m−2 K−1), L is the characteristic length (m), and k is the thermal conductivity of water (W m−1 K−1). Thus, for length/width ratios of 1.0, 1.4, 2.0, 3.0, 4.0, and 8.0, Nu equals 3.0, 3.1, 3.4, 4.0, 4.4, and 5.6, respectively, for laminar flow in noncircular tubes (internal flow) with a uniform surface temperature.17 The Reynolds number (Re) and Prandtl number (Pr) were also determined to better understand the prevailing flow regime within the reactor17 according to the expressions Re = UL/γ and Pr = γ/α, where U is the velocity (m s−1), γ is the kinematic viscosity of water (m2 s−1), and α is the thermal diffusivity of water (m2 s−1). Heat-transfer coefficients were previously determined under sonication using empirical equations and the acoustic stream velocity.18 In our case, the acoustic stream velocity of the middle compartment under different partitioning conditions was estimated from the acoustic stream velocity profile of Chouvellon et al.19 The computed Re and Pr values are summarized in Table 2.
MO removal efficiency (%) =
(C0 − Ct ) × 100 C0
(3)
where C0 and Ct are the MO concentrations in the bulk solution (mg L−1) at times 0 and t min, respectively. Flow Conditions. MO solution (10 mg L−1, pH 2) was used throughout the experiments. As shown in Figure 1B, initially, the flow was established through the reactor system. Briefly, the MO solution in the storage tank (2 L) was pumped at 20 mL min−1 to the bottom port of the reactor with a peristaltic pump. From the overflow port of the reactor, solution was driven by the peristaltic pump and discharged into the drain. This continuous discharge was necessary to avoid liquid accumu9342
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and 28 W m−2 K−1, respectively, corresponding to acoustic stream velocities of 0.0040, 0.0028, 0.0016, and 0.00050 m s−1 and computed Re values of 64, 60, 51, and 31, respectively (Table 2). It was noted that, even with seven compartments, the maximum velocity obtained for 20 mL min−1 of flow through the reactor was 3.4 × 10−4 m s−1, which was lower than the acoustic stream velocity (Tables 1 and 2). Acoustic streaming and the agitation effect of acoustic bubbles become the main source of enhanced heat transfer.22 Because of the very high-velocity jet (0.37 m s−1), low-frequency ultrasound (30 W) also has Re values in the range from 104 to 105, which are very effective in heat-transfer applications18,22 [where the calculations of heat-transfer coefficients used empirical equations derived for a point heat source and a 60.7 kHz ultrasonic horn (6-mm diameter with 1-mm nozzle diameter)]. We used a 65-mm-diameter 200 kHz transducer with an acoustic intensity profile that was much higher at the center axis of the transducer, where the velocity and temperature profiles also followed the acoustic intensity profile. With a reduction in compartment width (i.e., with increased number of compartments), convective heat transfer increased at the central compartment because acoustic intensity was high. The overall heat-transfer coefficients for both glass and Perspex sheet were high when the compartment number was high (Table 2). However, this overall effect could decrease as a result of increased partitioning because of the reduction in acoustic streaming velocity in the adjacent compartments. The relatively lower thermal conductivity of the Perspex sheet (Table 4) might have affected the initial rate of temperature measurement, which was used to compute calorimetric power.
lation within the reactor due to blockage of the discharge tube by air bubbles. Once the flow was stable, sonication commenced at full power at 20 °C. Sonication was performed for 30 min, and the MO absorbance was recorded every 3 min at the discharge compartment (S1). All experiments were duplicated, and the average results are presented. MO Degradation Kinetics. Because MO degradation under sonication is a pseudo-first-order reaction, the rate constant k was estimated from the gradient of a plot of ln(Ct/C0) versus t for all batch studies. Under steady-state conditions, the concentration relationship for i identical CSTRs in series is20 Ci =
C0 (1 + kτ )i
(4)
where τ = V/Q is the residence time (min), V is the reactor volume (L), Q is the flow rate (L min−1), and Ci is the discharge concentration of the ith reactor (mg L−1). Equation 4 can be rearrange to yield ⎡⎛ ⎞1/ i ⎤ C Q k = ⎢⎜ 0 ⎟ − 1⎥ ⎢⎣⎝ Ci ⎠ ⎥⎦ V
(5)
from which the steady-state k value can be determined for each CSTR under different compartmentalization schemes (Table 3). Based on these k values, the concentration and MO degradation per joule (sonochemical efficiency, SE) for each compartment were computed. For the batch reactor, MO degradation after 18 min was used to compute SE.
Table 4. Acoustic Impedance26 and Thermal Conductivity at 20 °C
3. RESULTS AND DISCUSSION 3.1. Calorimetric Power. To understand the performance of an ultrasonic reactor, it is important to know the power delivered to the reaction medium. The calorimetric power measures the amount of electrical energy converted into mechanical energy within the reactor.16 The calorimetric power under both batch and flow conditions decreased with the degree of compartmentalization (Table 3). The calorimetric energy intensity (CEI), which is computed based on calorimetric power per either reactor volume (CEIV) or reactor radiating area (CEIA), is widely used to understand acoustic cavitation conditions within reactors. The CEI ranged from 59 to 110 W L−1 or from 0.62 to 1.1 W cm−2 under the range of operating batch conditions employed. Under similar conditions, the flow process consistently gave lower calorimetric power than the batch process, which can be attributed simply to a dilution effect related to flow.14 With an increase in the number of Perspex plates added, both the percentage reactor area (RA) for irradiation of the liquid and the percentage volume (RV) of liquid in the reactor decreased (Table 1). For seven compartments (addition of six Perplex plates), the decreases in area and volume relative to the original noncompartmentalized reactors were approximately 10%. The addition of Perspex might also affect the acoustic wave patterns. With an increasing number of compartments, under flow conditions, flow velocities within the compartment (Table 1) increased, which affected the performance of the reactor.21 When the compartment widths were 9.3, 13.0, 21.6, and 65.0 mm, the estimated heat-transfer coefficient were 180, 130, 70,
description
acoustic impedance (kg m−2 s−1)
thermal conductivity k (W m−1k−1)
water Perspex Pyrex glass
1.48 × 106 3.21 × 106 12.6 × 106
0.58 0.20 1.14
To further understand reactor performance, we investigated the degradation of MO under batch and flow conditions with varying numbers of compartments. 3.2. MO Degradation. Batch Conditions. Sonochemical degradation of MO under batch conditions mainly occurs through reaction with OH radicals.23,24 To investigate the degradation of MO in the present sonication system, MO concentrations during sonication were measured at S1, where calorimetric temperature was also measured. In all cases under batch conditions, the MO concentration decreased with increasing reaction time (Figure 2A). However, as the number of compartments (number of plates) increased, the rate of MO degradation decreased (Figure 2A). After 21 min of treatment in a compartmented reactor, the final concentrations of MO were 1.3, 2.5, 3.2, and 5.1 mg L−1 for compartments 1, 3, 5, and 7, respectively. In addition, the pseudo-first-order rate constant (k) decreased from 0.1 to 0.03 min−1 as the number of compartments increased from 1 to 7; simultaneously, the MO removal after 18 min decreased from 83%% to 45% (Figure 3, Table 3). For the batch reactor, SE did not change when the number of compartments was increased from 1 to 5. However, for a seven-compartment reactor, SE jumped to 72 ng J−1 (Table 3), which might indicate an issue with calorimetric 9343
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same between the middle (M) and side (S1) compartments (Figure 4A). When the number of compartments increased to
Figure 2. Degradation of MO with sonication time under 200 kHz sonication in (A) batch and (B) flow reactors (flow rate = 20 mL min−1, initial MO concentration = 10 mg L−1 at pH 2, 20 °C). Number of compartments: green squares, 1; blue circles, 3; red triangles, 5; black diamonds, 7. Figure 4. Variation of MO concentration with sonication time under batch conditions (10 mg L−1 at pH 2, 200 kHz, 20 °C). (A) Three compartments: red squares, side (S1); black triangles, center (M). (B) Five compartments: red squares, side (S1), green circles, second compartment (S2); black triangles, center (M).
5, the same tendency as for three compartments was observed (Figure 4B). However, the MO removal efficiency (at S1) obtained with the seven-compartment reactor was 45% (Figure 3), which was lower than that obtained with the fivecompartment reactor (51% RE). This decrease in reactor performance can be directly attributed to how the number of Perspex plates contribute to the overall reduction in both the irradiating area and volume of the reactor. Because the acoustic impedances of Pyrex glass and Perspex sheets are different, 12.6 × 106 and 3.21 × 106 kg m−2 s−1, respectively (Table 4), localized heating might have occurred at the interface of the two materials, consequently reducing the available acoustic energy. Such behavior of compartmented reactors might be relevant for practical applications such as soil or other sample extraction or treatment within a sonicator bath. Attention should be given to the sample bottle holder material (plastic or rubber) and its volume and how samples are physically restrained in the sonicator bath to enable better sample processing efficiency under batch sonication. Flow Conditions. The batch study results for compartmented reactors indicated that the relative performance decreased with increasing number of compartments. To further understand the role of compartmented reactor performance,
Figure 3. Variation of MO degradation rate constant (k) and removal efficiency (RE) with the number of Perspex plates under batch conditions with 200 kHz sonication [10 mg L−1 initial MO concentration at pH 2 and 20 °C, RE computed based on an 18min treatment, where concentration was measured in the side compartment (S1)]. Black triangles, k; red squares, RE.
power measurements for seven compartments. When the number of compartments is high, although the heat transfer is high in the middle compartment, it is lower in adjacent compartments, and this uneven heat transfer might result in the initial measured temperature gradient being lower than actual, leading to a lower observed calorimetric power and a higher SE. However, overall, the observed SE reduction with increasing compartments was relatively small. How the Perspex plates affected the batch reactor performance through increased partitioning was compared for different compartments. When three compartments were used, MO removal (7.7 mg L−1 in 21 min of treatment) was almost the 9344
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Figure 5. Variation of MO removal efficiency (%) with (A) cross-sectional area (perpendicular to flow direction), (B) flow velocity (perpendicular to flow direction) for flow conditions, and (C) reactor volume (effect of Perspex plates) for batch (red circles) and flow (black squares) conditions. MO removal efficiency (%) was calculated after 18 and 30 min of ultrasonic treatment for batch and flow conditions, respectively.
the behavior of compartmented reactors was further studied under flow conditions. With the inclusion of compartments, although the flow velocity increased within the reactor, it was still much lower than the reported acoustic streaming velocity, which was 0.01− 0.1 m s−1.21 Under flow conditions, MO concentrations were primarily measured adjacent to the outlet port (S1), thus ensuring that the overall performance of the reactor as a whole was recorded, with or without compartments, and the MO concentrations in each compartment were also determined (Table S1, Supporting Information). Irrespective of the number of compartments added, the MO concentration decreased with treatment time under all flow conditions (Figure 2B; Figure S1, Supporting Information). As the number of compartments increased (i.e., the number of plates increased), the reduction in MO concentration also decreased (Figure 2B). After ultrasonic treatment for 30 min, the residual MO concentration was 3.4 and 7.3 mg L−1 for the one- and seven-compartment reactors, respectively, corresponding to decreases of 66% and 27%. The observed decreases in MO concentrations under flow conditions were also consistently lower than those previously observed under batch conditions for the same number of compartments (Figure 2A). Therefore, it was clearly confirmed that flow conditions did not increase the overall degradation of MO. The SE trend of the flow reactor also followed that of the batch reactor, but SE was always much lower than that of batch reactor for similar compartments. In compartment 7, SE slightly increased compared to compartment 5. This clearly indicates that calorimetric temperature measurement could be the possible reason for this observation. With no plates added, the reactor simply performed as a continuous stirred tank reactor (CSTR).25 However, when the number of compartments increased, it acted as a series of CSTRs. When the numbers of compartments were 1, 3, 5, and 7, the numbers of CSTRs in series were 1, 3, 5, and 7, respectively. For example, consider the three-compartment reactor: It had three equal-volume CSTRs in series (S′1 → M → S1), with each having a volume of 132 mL, a residence time of 396 s, and a velocity of 1.5 × 10−4 m s−1 for a solution flow of 20 mL min−1. Because of the added Perspex plates, for these three reactors (S′1 + M + S1), the total volume and residence time were slightly lower than those of noncompartmented reactor. Even though each CSTR had an equivalent irradiating area, because of the added Perspex plates, the total irradiation area was lower than that of the noncompartmented reactor. Compared to a single CSTR, a series of CSTRs should normally give better overall contaminant removal efficiency.20,25
When a reactor was partitioned, the bulk movement of liquid within the reactor was restricted. Because of the variation of the acoustic pressure profile, the acoustic streaming velocity varied among the compartments. Because of their much lower acoustic impedance (3.21 × 106 kg m−2s−1) (Table 4), the Perspex plates can also absorb the acoustic energy. For better performance of the batch reactor, the ratio of the reactor diameter to the transducer diameter could be an important factor. However, in our case, the applied ultrasonic power was always the same (one transducer of 65-mm diameter), and only the compartments were changed. Table S1 (Supporting Information) reports the concentration profiles of MO within the compartments. For the CSTRs, the MO reduction decreased toward the feed entry point for all CSTRs with different numbers of compartments. The compartment cross-sectional areas varied under both batch and flow conditions with the number of plates and, consequently, with the number of compartments. For example, when one, three, five, and seven compartments were used, the cross-sectional areas perpendicular to flow were 6.3 × 10−3, 2.3 × 10−3, 1.3 × 10−3, and 9.8 × 10−4 m2, respectively. Therefore, it is important to identify the relationship between this parameter and the overall MO removal efficiency. Figure 5A shows that the MO removal efficiency increased with compartmental cross section (perpendicular to flow) for both batch and flow processes with Perspex plate compartments, suggesting that reducing the compartment cross-sectional area had some effect on MO removal. Figure 5B shows that the MO removal efficiency under flow conditions decreased with increasing flow velocity (Table 1) within the compartmented reactor. A linear correlation (R2 > 0.97) was obtained between the MO removal efficiency and the flow velocity. This clearly indicates that compartmenting the reactor had an adverse effect on MO removal. Also, if external sonicator bath cooling were employed, sample sonication performance might reduce further compared to batch conditions. The MO removal efficiency was also linearly correlated (R2 > 0.95) with reactor volume under both batch and flow conditions (Figure 5C), indicating a decrease in MO removal efficiency with an increase in the number of plates. Generally, under similar batch conditions, contaminant degradation increases when the reactor volume is decreased. However, in the current study, the MO degradation increased linearly with reactor volume (Figure 5C). This was due to the reduction in acoustic energy with the number of Perspex plates, which also reduced the volume. This observation is directly relevant to bath sonicators, which can have varying numbers of sample holders. If enough care is not taken, inconsistencies between 9345
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samples can occur as a result of the variation in partitioning between different runs.
4. CONCLUSIONS The MO removal efficiency of a rectangular Pyrex glass reactor was found to be sensitively affected by the number of Perspex plates in the reactor under both batch and flow conditions. Under batch conditions, when the number of compartments increased from 1 to 7, the MO removal efficiency decreased from 83% to 45%, and the pseudo-first-order rate constant decreased from 0.097 to 0.034 min−1. Under flow conditions, the MO removal efficiency also decreased from 66% to 27% as the number of compartments increased from 1 to 7, and k decreased from 0.096 to 0.016 min−1. SE also followed a similar general trend, with the exception that the SE observed in compartment 7 was slightly higher than that in compartment 5, which might indicate a poor overall heat transfer in multicompartment reactors affecting calorimetric power measurement. It was also observed that the MO removal efficiency increased with increasing cross-sectional area, decreasing flow velocity, and increasing reactor volume. Under the conditions used in this study, very low acoustic streaming (
