Decomposition of Acetone by Hydrogen Peroxide/Ozone Process in a Rotating Packed Contactor Young Kul*, Yun-Jen Huang', Hua-Wei Chenr2 , Wei-Ming Houl
ABSTRACT: The direct use of ozone (03) in water and wastewater treatment processes is found to be inefficient, incomplete, and limited by the ozone transfer between the gas-liquid interface because of its low solubility and instability in aqueous solutions. Therefore, rotating packed contactors were introduced to improve the transfer of ozone from the gaseous phase to the solution phase, and the effect of several reaction parameters were investigated on the temporal variations of acetone concentration in aqueous solution. The decomposition rate constant of acetone was enhanced by increasing the rotor speed from 450 to 1800 rpm. .Increasing the hydrogen peroxide (H20 2)/OQ molar ratios accelerated the decomposition rate until a certain optimum H202/03 molar ratio was reached; further addition of H20 2 inhibited the decomposition of acetone, possibly because excessive amounts of H4202 added might serve as a scavenger to deplete hydroxyl free radicals. Water Environ. Res., 83, 588 (2011). KEYWORDS: acetone, H202/03 process, ozone, hydrogen peroxide, rotating packed contactors, ozonation. doi:10.2175/106143010X1285 1009156961
Introduction Acetone is a commonly used solvent, which probably is not considered to be very toxic to the environment. The lethal dose, 50% (LD 50 ) of acetone was reported to be 14.2 mg/L, depending on the pathway of intake (Budavari et al., 1996). However, it is of environmental concern primarily because of its volatility and high water solubility to cause air and water pollution. Decomposition of organic contaminants by advanced oxidation processes (AOPs) has been studied extensively in recent years, which involve the application of various combinations of ozone (03) (Vandersmissen et al., 2008), hydrogen peroxide (H20 2) (Suh and Mohseni, 2004), UV light (Mezzanotte et al., 2007), and catalysts (Qi et al., 2009). Alaton et al. (2002) investigated the decomposition of a reactive dyebath effluent by means of AOPs, including 03, H2 0 2/ UV-C, and TiO 2/UV-A, and reported that ozonation and H/UV-C were beneficial to degrade dyebath effluent in view of the electrical energy efficiency. Ku et al. (2010) presented that the decomposition of aniline by the photocatalytic process was 1 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. 2 Department of Cosmetic Application & Management, St. Mary's Medicine Nursing and Management College, Yi-lan, Taiwan.
* National Taiwan University of Science and Technology, 43, Keelung
Road, Section 4, Taipei 106, Taiwan; e-mail:
[email protected]. 588
enhanced by applied bias potential, because the photogenerated electron-hole pairs were inhibited by the application of bias potential. The generation of hydroxyl radicals (.-O) defines AOPs. These radicals result in the decomposition of organic compounds to innocuous intermediates and end products with high reaction rates (Pera-Titus et al., 2004). Among these AOPs, the applications with ozone are gaining extensive attention, as a result of the powerful oxidizing capability of ozone molecules. Esplugas et al., (2002) investigated the degradation of phenol by different AOPs and revealed that ozonation was the focus of phenol degradation because of the high degradation rate and lower cost. However, the direct use of ozone in water and wastewater treatment processes is found to be inefficient, incomplete, and limited by the ozone transfer between the gas-liquid interface because of its low solubility and instability in aqueous solutions (Hsu et al., 2001; Shin et al., 1999). Therefore, the dissolution of gaseous ozone is of particular importance for cost-effective ozone application. In general, packed-bed columns are more efficient for ozonation with higher reaction rates; however, bubble columns are used for ozonation with slower reaction rates. Rotating packed contactors (RPCs) recently were introduced as a countercurrent contactor to improve gas-liquid transfer by contacting gas and liquid in a high gravity field using centrifugal acceleration (Burns and Ramshaw, 1996; Chen et al., 2005; Ku et al., 2008). The application of RPCs might improve the gas-liquid transfer coefficient by up to I order of magnitude and, thus, dramatically reduced the equipment size. High gas-liquid volumetic mass-transfer coefficients were obtained for the application of RPCs in various gas-liquid transfer operations, such as distillation, absorption, and stripping (Chen et al., 2005; Ku et al., 2008; Lin and Liu, 2003). The combination of ozone with H2 0 2 actually acts like a homogeneous catalyst to enhance the oxidizing ability of ozone, which has attracted much research recently (Ma et al., 2003; Suh and Mohseni, 2004; Vandersmissen et al., 2008) and is considered to be a promising alternative for the removal of organic pollutants in aqueous solutions. In this study, the decomposition of acetone in aqueous solutions by the H2 0 2 /0 3 process in an RPC was examined. The objective of this study was to introduce RPCs as ozonation contactors to improve ozone transfer from the gaseous phase to the solution phase and investigate the effect of several reaction parameters on the temporal variations of acetone concentration in aqueous solutions. The reaction rates of acetone and various reacting species were characterized using a simplified pseudo-first-order kinetic model. Water Environment Research, Volume 83, Number 7
Ku et al. 1,6
E
er
n•tlai
dPump
Controller PH metxr
p
iM-eometer
di
Sensor
Reaction time (min) Thormostht
e.smef
.
ST
stnstl
Stirrar plate
Figure 2-Effect of solution pH on the decomposition rate of acetone by the H2 02/0 3 process in RPC. r
Figure 1-Recirculation RPC reactor system. Experimental Methods Acetone and other chemicals used for analysis were reagentgrade, and all experimental solutions were prepared with deionazed water. The experimental system of RPC was designed for totanreflux operation, and the schematic diagram of tde experimental setup was reported previously (Ku et al., 2008) and is shown in Figure m. The RPC is made of SS314 stainless steel, with an axial height of 3.5 cm. The inner radius And outer radius of The packing material respectively. tam, the RPC were 36 and 77 used in the RPC was stainless-steel pall rings with a specific surTace area of 774 M2/i3 and porosity of 0.69. A motor was used to drive the RPC, with a rotor speed between 300 and 1800 rpm measured with a Saint Wien PM490R4A60 digital panel meter
pl-1 value of the experimental solution was kept constant by the addition of sodium hydroxide or sulfuric acid solutions to the storage tank with a Kyoto Electronic AT-200 autotitrator (Kyoto Electronics Co., Ltd., Kyoto, Japan). At designated time intervals, an appropriate amount of reaction solution was sampled from the storage tank for further analysis. The concentration of acetone present in the reaction solutions was determined by a China Chromatography 8900F gas chromatograph (China Chromatography Co., Ltd., Taipei, Taiwan) equipped with a Stabilwax column and using a flame ionization detector.
Results and Discussion Less than 4% of acetone present in the aqueous solution was lost by volatilization at an inlet airflow rate of 2 L/min for 60 minutes. Decompositions of acetone in aqueous solutions were found to be less than 3% by the additions of H 20 2 . Therefore, both the volatilization and H2 0 2 decomposition rates of acetone are (Saint Wien Enterprise Inc., Ltd., Taipei, Taiwan). An isothermal assumed to be negligible compared with the decomposition rate of storage tank kept at 25°C was. used to hold 10 L of deionized acetone in aqueous solutions by the H2 0 2/0 3 process. aarious Effect of Solution pH. Hydrogen peroxide is slightly acidic in doses of aqueous solution containing 30 mg/L acetone and aqueous solutions (pKa = 11.6) and partially dissociates in alkaline Hss2. The solution was then putped at a fixed flowrate and sprayed radially from the center of the RPC. Ozone containing gas solutions to generate HO2- species. which consequently reacts generated fmom dry oxygen gas with a Fisher 500 ozone generator favorably with ozone molecules to generate hydroxyl free radical. te intactor ed Germany) was flown iientent (Fischer, an) H0 2 from the outer edge of the contactor. The flowrate of the ozoneH 200-,H++ 2 (1) containing gas stream was manipulated by a mass-flow controller. The ozone concentration in the gaseous stream was maintained at 20 mgA. for all experiments by controlling the power input to the ozone generator, The aqueous solution and ozone -contai ni ng gas subsequently was contacted in the RPC. The solution then was transferred from the RPC back to the storage tank. A Seki model SQZ-6000 ozone analyzer (Seki Electronics., Ltd., Tokyo, Japan) was used to determine the ozone content in the inlet and outlet gaseous streams. The dissolved ozone concentration in aqlueous solution was determined by the indigo blue nmethod. The July 2011
H02- +O3+-*-OH+02+02--
(2)
In this study, the temporal decomposition behavior of acetone in aqueous solution by the H202/03 process was influenced markedly by solution pH. As shown in Figure 2, nearly complete decompositions (greater than 99%) of acetone by ozonation in the RPC were observed within 30 minutes of reaction time for experiment conducted at pH 11.0. The temporal behavior of reacting species during the decomposition of acetone by H 20 2/0 3 was described 589
Ku et al. 0.12
Table 1-Pseudo-first-order reaction rate constants for the decomposition rate of acetone at various pH values by H2 0 2/0 3 process in the RPC.
E
pH
0
Acetone removal
3 5 7 9 11
(kAcetone)
0.038 0.048 0.061 0.075 0.090
0.11
"HAOM/O oxidation of acetone system in the rotating packed bed .T=25i+22C [acetone], = 30 mg/L [0to ]
e)0.10
[rsMAN/1e0,]
"Rotor speed =
0.09
o,
= 20 + 0.2 mg/L
"G[L=0.67 (G
2 tLmin) = 0.s (M/M) 1200 + 6 rpm
S0.08 0 .
0.07
0
S0.06 reasonably by the simplified pseudo-first-order kinetic model for all experiments conducted. The pseudo-first-order reaction rate constants for acetone decomposition for experiments conducted at various pH values are summarized in Table 1. The calculated decomposition rate constants of acetone were correlated almost linearly to the solution pH values, as depicted in Figure 3. Various researchers (e.g., Ku et al., 2008; Ma et al., 2003) reported similar experimental results suggesting that the decomposition of several organic compounds in alkaline solution by H 2 0 2 /0 3 process was more favorable. Effect of Rotor Speed. The higher centrifugal force generated for RPCs operated at higher rotor speed could decrease the thickness of the liquid films and the size of droplet and then increase the interfacial area and decrease the resistance of ozone mass transfer. The centrifugal acceleration for the RPC was varied with rotor speed (wv) of the RPC. The mean centrifugal acceleration, a,, is calculated as follows (Burns and Ramshaw, 1996; Ku et al., 2008):
"( 2
2`0 ,5
Where R& and Rj = outer and inner radii of the RPC, respectively (in). As indicated in Figure 4, the decomposition rate constant of acetone at pH 3.0 by the H 20 2 /0 3 process in the RPC was enhanced from 0.014 to 0.055 min- 1 by increasing the rotor speed of the RPC from 450 to 1800 rpm. This observation suggests that ozone transfer is the limiting step for the decomposition rate constant of acetone by the H2 0 2/0 3 process; therefore, centrifugal force should be considered as an effective external force for enhancing the efficiency of ozonation. The pseudo-first-order decomposition rate constants were correlated roughly linearly to the rotor speeds, as depicted in Figure 5. However, the selection of optimum operating rotor speed should include the consideration of electrical power consumption of the rotor, because the centrifugal acceleration and the electrical power consumption is 2-order, correlated to the rotor speed, as indicated in eq 3. The decomposition rate constant of acetone apparently was not enhanced by the rotor speed for experiments conducted with rotor speed less than 750 rpm. Effect of Gas/Liquid Flowrate Ratio. Gas/liquid flowrate ratio is defined as the flowrate of the aqueous solution to the flowrate in gaseous stream. Pseudo-first-order reaction rate constants for acetone decomposition for experiments conducted in the RPC with different gas/liquid flowrate ratios of 0.4, 0.5, 590
S0.05 *00
.6
0,04
"
0.03 ^ ^
2
3
4
5
6
7
8
9
10
11
12
pH value Figure 3-Effect of solution pH and rotor speed on the pseudo-first-order reaction rate constants of acetone by the H2 0 2/0 3 process in the RPC. 0.67, 1.0, and 2.0 at the same rotor speed of 1200 rpm and pH 3.0 are summarized in Table 2. The decomposition rate constant of acetone by the H2 0 2/0 3 process in the RPC increased with decreasing gas/liquid ratio (increasing liquid flowrate for experiments conducted at the same gas flowrate). The possible reason for this phenomenon was that a high liquid flowrate provided more liquid-side mass transfer area that was caused by more liquid droplets spreading over the packing surface. 1.6
S1.01
"0,8 4, , 0.6
0
5
10
15
20
25
30
35
40
Reaction time (min)
Figure 4--Effect of rotor speed on the decomposition rate of acetone by the H2 0 2103 process in the RPC. Water Environment Research, Volume 83, Number 7
Ku et al. D.OBD u.0150 U.O,/O, oxidation of acetone system in the rotating packed bed
"a)
)=251-2
0.055
0
C
"pH-=3 .0±O.t
C
,acetone], = 30 mg/L
0.050 1.0,1(, - 20 + 0.2 mg/L G/L = 0.67 (G - 2 Umnin) 0.045 4I-1,0j, , [O3]1 = 0.5 (M/M) 0
0.040
R
0.035
//U
Table 3-Pseudo-first-order reaction rate constants for acetone decomposition at various gas/liquid flow rate ratios (G/L) and rotor speeds by H2 0 2/0 3 process in the RPC. GIL 0.4
0.677
1.0
2.0
600
0.019
0.015
0.014
0.009
900 1200 1800
0.031 0.055 0.066
0.024 0.038 0.055
0.020 0.028 0.034
0.014 0.021 0,025
0)
0 4. Co 0.030 12
0,025
03 molar flowrate in the gaseous stream, [H202],q/[O31] 0.020 0.015 U
U.0U0U
300
450
600
750
900
1050 1200 1350 1500 1650 1800 1950
Rotor speed (rpm)
Figure 5-Effect of rotor speed on the pseudo-first-order reaction rate constants of acetone by the H2 0 2/0 3 process in the RPC. Another set of experiments was conducted at various rotor speeds (600, 900, 1200, and 1800 rpm) and various gas/liquid flowrate ratios (0.4, 0.67, 1.0, and 2.0). As shown in Table 3, the reaction rate constants at pH 3 were affected greatly by the rotor speed for experiments conducted at a lower gas/liquid flowrate ratio (L=5 L/min), implying that the decomposition rate of acetone by H2 0 2/0 3 process was limited significantly by the ozone transfer from the gaseous stream into the aqueous solution. As shown in Figure 6, The decomposition of acetone was enhanced more significantly by increasing the rotor speed for experiments conducted with lower gas/liquid flowrate ratio (L = 5 L/min). However, for experiments conducted at higher gas/ liquid flowrate ratios, the contact of ozone-containing gas and liquid was sufficient to provide reasonable ozone transfer; therefore, the rotor speed had only a slight influence on the reaction rate constant for experiments conducted at a higher gas/ liquid flow rate ratio (L= I L/min). Consequently, the decomposition rate of acetone by H1202/03 process possibly was limited by the reaction between the acetone molecules, with -OH free radicals generated from the dissolved ozone species in the presence of H20 2 . Effect of Fll202](.q)/[O3](g) Molar Ratio. The effect of the aqueous H2 0 2 molar flowrate in aqueous solution on the feeding
ratio, on the decomposition rate constants of acetone in the RPC for the H20 2/0 3 process at different solution, pH values is shown in Table 4. It was found that acetone was decomposed significantly faster with the addition of H 2 0 2 . However, no linearity was observed between decomposition rate constants and the [H202]aq/ [0 3]g ratio, as depicted in Figure 7. The decomposition rate constant of acetone increased with the [H202]aq/[O3]g ratio, until an optimum ratio was reached; further increased additions of H20 2 might decrease the reaction rate. The optimum [H202]aq/ [0 3], ratio was determined to be 0.5 in the research for experiments conducted at a solution pH greater than 7.0. The existence of an optimum [H202]4q/[O311 molar ratio indicates that
the formation of -OH radicals possibly reaches a maximum for experiments conducted with an optimum [H2202]aq/[)O3]g ratio;
further increased additions of H2 0 2 might decrease the reaction rate, as a result of competition between acetone and H2 0 2 for -OH radicals (Bellamy et al., 1991). For experiments conducted in acidic solutions, the decomposition rate constant of acetone increased with the [H202]aq/[O3]g ratio until the [H202]aq/[O3]g ratio reached 2.0. An explanation for this observation is that H20 2 0.10
.
AHa,O1O oxidation of acetone system In the rotating packed bed
0.09 T:=25_+2°C
S0.08 a-)
o
pH -t3.0+ 0.1 [acetone]o - 30 mg/L
"= 20 + 0.2 mg/L 1H0o'" I [10,1W0.5 (MIM)
0.07
S0.06 0 r
f
-0-rotor speed = 600 + 6 rpm -..rotor speed = 900 ± 6 rpm
0.4
S0.03
rotor speed = 1200 ± 6 rpr
-A
0
Table 2-Pseudo-first-order reaction rate constants for the decomposition rate of acetone at various gas/liquid flowrate ratios by H2 0 2/0 3 process in the RPC. Gas/liquid flowrate ratio
Acetone removal (k Acetone)
S0.02
"o 0.01 a) U.UUJ
0.2
0.40 0.50 0.67 1.00 2.00
July 2011
0.055 0.046 0.038 0.028 0.021
s
0.05
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
20
2.2
G / L flow rate ratio (G= 2 UJmin) Figure 6-Effect of gas/liquid flowrate ratio and rotor speed on the pseudo-first-order reaction rate constants of acetone by the H2 0 2/0 3 process in the RPC. 591
Ku et al.
Table 4-Pseudo-first-order reaction rate constants for acetone decomposition at various [H202](aq) / [O3](g) molar ratios and pH values by the H2 0 2/0 3 process.
ed almost linearly to the solution pH values. The decomposition rate constant of acetone in the RPC was enhanced by increasing the rotor speed. The decomposition rate constant of acetone by H20 2/0
Molar ratios pH
Without H2 0 2
0.5 (M/M)
1.0 (MIM)
2.0 (M/M)
3 7 9 1
0.013 0.021 0.025 0.031
0.038 0.061 0.075 0.090
0.048 0,055 0.059 0.067
0.055 0.041 0.035 0.025
3
process in the RPC increased with increasing liquid
flowrate for experiments conducted at the same gas flowrate. Increased the H2 0 2/0 3 molar ratios accelerated the decomposition rate until a certain optimum H2 0 2/0 3 molar ratio was reached; further addition of H202 inhibited the decomposition of acetone, possibly because excessive amounts of H202 added might serve as a scavenger for .OH free radicals. However, the effect of H1202 addition is highly dependent on the pH level of the solution.
added to acidic solutions is presented predominantly as the H20 2 molecule; the reaction between H 20 2 and 03 molecules is much slower than that between H20- species and 03 (Ma et al., 2003; Suh and Mohseni, 2004; Vandersmissen et al., 2008). The decomposition rates were apparently increased with solution pH for experiments conducted around the optimum [H202]aq/[O3]g molar ratio; however, overdosed amounts of H202 might serve as scavengers for .OH free radicals to inhibit the decomposition of acetone by the H 20 2 /03 process, which is more evident for experiments conducted in alkaline solutions.
Credit This research was supported, in part, by Grant NSC-92-2221-E011-014 from the National Science Council, Republic of China.
Conclusion Application of the H20 2/0 3 process in an RPC has been shown to be feasible for achieving a high degree of acetone decomposition in aqueous solutions. The centrifugal force provided by RPC markedly enhanced the degradation of acetone in aqueous solution. The temporal behavior of various species could be described by a simplified pseudo-first-order kinetic model. In most experiments, the decomposition of acetone was favored to occur in alkaline conditions, indicating that the decomposition of acetone is mainly initiated by -OH free radical attack. The calculated decomposition rate constants of acetone were correlat-
Treatment of VOC-Contaminated Groundwater by Hydrogen Perox-
Submittedfor publication March 28, 2010; revised manuscript submitted October 23, 2010; accepted for publicationJanuary 3. 20111.
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0)
c
0.075
0
0.060
"U3
Ma, H.W.; Hsu, Y.C.; Yang, H. C.; Kuo, M.C. (2003) The Effectiveness of a
a)
a
0.045 0.030 0.015
0.000
S.
.
-0.2
0.0
.
.
0.2
.
.
0.4
.
.
0.6
i
I
0.8
I
.
1.0
.
.
1.2
.
.
1.4
i
---
pH = 3.0 + 0.1
---A-
pH = 7.0 + 0.1 pH = 9.04 0.1 pH = 11.0+± 0. 1
.
1.6
.
.
1.8
.
.
2.0
.
2.2
[H202,1ý5 ) / [O03 molar ratio (M/M)
Figure 7-Effect of [H1O2](aql/[O3](g) molar ratio and solution pH on the pseudo-first-order reaction rate constants of acetone by the H2 0 2/0 3 process in the RPC. 592
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Author: Ku, Young; Huang, Yun-Jen; Chen, Hua-Wei Title: Decomposition of Acetone by Hydrogen Peroxide/Ozone Process in a Rotating Packed Contactor Source: Water Environ Res 83 no7 Jl 2011 p. 588-93 ISSN: 1061-4303 DOI:10.2175/106143010X1285 1009156961 Publisher: Water Environment Federation 601 Wythe Street, Alexandria, Va 22314-1994
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