J. Environ. Eng. Manage., 20(2), 63-68 (2010)
63
STUDY ON THE DECOMPOSITION OF ISOPROPYL ALCOHOL BY USING
MICROWAVE/Fe3O4 CATALYTIC SYSTEM
Yu-Jie Chang,1 Ching-Hsing Lin,1 Mei-Yin Hwa,1 Yung-Hsu Hsieh,2
Ta-Chih Cheng3 and Chen-Yu Chang4,*
1
Department of Safety Health and Environmental Engineering
Tungnan University
Taipei County 222, Taiwan
2
Department of Environmental Engineering
National Chung-Hsing University
Taichung 402, Taiwan
3
Department of Tropical Agriculture and International Cooperation
National Pingtung University of Science and Technology
Pingtung, Taiwan
4
Department of Biotechnology
Mingdao University
Changhua County 523, Taiwan
Key Words: Microwave, Fe3O4, catalytic packed column, isopropyl alcohol (IPA), volatile organic
compounds (VOCs)
ABSTRACT
Isopropyl alcohol (IPA) is the major organic emission in the semiconductor manufacturing
industry. A microwave/Fe3O4 catalytic system was proposed for treatment of IPA. This system
comprises a household microwave oven modified as the reaction chamber, which was fitted with a
vertical, cylindrical quartz reactor comprising a catalytic packed column filled with granular Fe3O4.
Experimental results showed that the destruction and removal efficiency of IPA by microwave alone
was close to zero, but with the microwave/Fe3O4 system, the temperature of the catalytic packed
column increased rapidly and reached thermal balance within 10-15 min. Analysis of the rear gas
after combustion showed that most of IPA was thermally oxidized into CO2 and H2O. The successful
application of the proposed microwave/Fe3O4 system to thermal destruction of IPA promises a new
technology for treatment of volatile organic compounds.
INTRODUCTION
The semiconductor industry is of the major economic interest in Taiwan, but it also brings significant
amounts of pollutants. In the semiconductor manufacturing process, a large number of high concentrations
of volatile organic solvents are used. The exhaust gas
and wastewater containing more complex components
are emitted by these processes. Isopropyl alcohol
(IPA), acetone, propylene glycol monomethyl ether
acetate, ethyl lactate, phenol, benzene and xylene are
the common organic reagents present at high concentrations in the wastewater [1] and most of them are
volatile organic compounds (VOCs). Most VOCs are
not only volatile but also toxic. In view of their harm*Corresponding author
Email: [email protected]
ful effects on the environment and potential threat to
human health, the U.S. Environmental Protection
Agency enacted the Clean Air Act Amendment in
1990 and classified many VOCs as toxic air pollutants.
Common control techniques for VOCs include combustion, adsorption, absorption, and condensation
processes. While the conventional thermal treatment
and some advanced oxidation processes have shown
greater efficiency in removing VOCs and enhancing
public safety, they require both larger capital investment and processing costs. Also, adsorption and absorption approaches only achieve phase transformation only, rather than reduction in quantity or toxicity
of the VOCs [2-4].
Microwave was first applied to heating food. In
64
J. Environ. Eng. Manage., 20(2), 63-68 (2010)
recent years, its applications have grown rapidly in
diverse fields such as concentration, extraction and
synthesis in chemistry [5-8]. This study focused on
employing microwave as the source of energy
radiating on the catalyst of Fe3O4 to treat VOCs such
as IPA. Microwave processing technique not only
saves time but also requires no additional fuel. Thus, it
is more environmentally friendly and causes no
secondary pollution. In this study, a high-temperature
combustion system is established by applying
microwave of 2450 MHz to a catalytic Fe3O4 packed
column, which is then employed to destroy and
remove VOCs in waste gas. Results of this research
confirm the feasibility of this approach to removal of
air pollutants [9] and the treatment system has already
been granted a patent (No. M309457) in Taiwan [10].
1. Air compressor
2. Mass flow meter
3. Syringe
4. Buffer flask
5. Microwave oven
6. Quartz reactor
7. Thermocouple thermometer
8. Temperature recorder
9. Microwave power modulator
Fig. 1. Schematic diagram of continuous microwave/
Fe3O4 system.
EXPERIMENTAL METHODS AND
MATERIALS
1. Design of Microwave Catalytic Packed Column
Figure 1 is a schematic diagram showing the
proposed continuous microwave radiation (CMWR)
system. A household microwave oven (Panasonic
Taiwan, NE-R30A) with frequency of 2450 MHz and
continuous power output was modified as a reaction
chamber. The rotating plate of a microwave oven was
originally designed to overcome the potential problem
of irregular microwave absorption due to geometric
shapes. In this study, the rotating plate was replaced
by a vertical, cylindrical reactor comprising quartz
tubes of dimensions 50 mm I.D. × 53 mm O.D. × 27.5
cm L (Shan-Long Glass Co., Taiwan). These quartz
tubes of the reactor were first filled with granular
Fe3O4 (5 mm I.D. × 7 mm L, Osaka Co., Japan) to a
height of 25 cm, serving as the catalytic packed column. The images of reactor in CMWR and the packed
catalyst, Fe3O4, are illustrated in Fig. 2. The structure
of granular Fe3O4 was rather uniform and the diameter
was about 100 nm. The K-type thermocouples
(KTS1320, Star Co., Taiwan) were then inserted into
the packed column at a distance of 2.5 cm from the
bottom of the reactor. Inflow gas was pumped from
the outside air using an air compressor of 1.1 kW.
With water, oil and suspended particles filtered and
removed, the purified air then flowed through the
mass flow meter (GFM17, AALBORG Co., Germany)
for controlling the air flow. A simulative IPA (Aldrich)
gas at fixed concentration was injected by the work
bee controller (Bioanalytical Systems Inc., Indiana,
USA) marked as “Syringe” in Fig. 1. Flowing along
the heated pipes, the injected IPA became completely
vaporized. It was then mixed with various proportions
of inflow gas and flew through two buffering flasks
into the reactor. To ensure complete mixing and
reaching the designed concentration, the mixed gas
was sampled by a gas chromatography equipped with
(a) reactor of CMWR system
(b) size of Fe3O4 catalyst
(c) SEM images of Fe3O4 catalyst
(10,000 x)
Fig. 2. Images of reactor and Fe3O4 catalyst in CMWR
system.
flame ionization detector (GC-FID; Agilent Co.,
Model HP6890 Series) and DB – WAX chromatography columns (30 m × 0.53 mm × 1.0 μm; Supelco,
Inc.) before entering the reactor.
2. Operation of Microwave Catalytic Combustion
System
First, the microwave power output was calibrated according to TEPA No. 0910019876 (Environment Protection Agency, Taiwan). Furthermore,
the microwave readout (MD-200, Digital readout Co.,
USA) was employed to monitor leakage of microwave
and ensure safety during the experiment. When the
experiment began, power was turned on and IPA was
fed into the reactor. The temperature of the Fe3O4
catalytic packed column of different heights was
measured and recorded automatically (PCLD-8710,
Bo-Shine Techno. Co., Taiwan) every 5 seconds. The
Chang et al.: IPA Decomposition with Microwave/Fe3O4
1. Effect of Microwave Power
Figure 3a shows the effect of different microwave powers (210-760 W), on temperature of the
Fe3O4 catalytic packed column. As can be seen, the
higher the microwave power is, the higher the rate of
increase in temperature is, which follows the order of
760 W (1.08 °C s-1) > 520 W (0.68 °C s-1) > 310 W
(0.51 °C s-1) > 210 W (0.35 °C s-1). The result was
similar to the previous study [11] and confirmed the
influence of microwave powers. On the other hand,
regardless of the microwave power, the temperature of
the packed column would gradually reach a steady
state of thermal balance. A possible reason is that
Fe3O4 catalyst first absorbed microwave and became
heated, but there was also heat loss due to evaporation
of water in air and inflow gas fed into the reaction system; hence, the final thermal balance is the rise in
temperature as a result of heating by microwave minus heat loss during the process. Moreover, upon
reaching the thermal balance temperature, increase in
duration of microwave radiation cannot increase the
temperature any further. It is because the column is
filled with a single medium, Fe3O4, with a fixed dissipation factor. Therefore, in practical application, the
polluted air should be fed into the reactor and pass
through the packed column upon reaching its thermal
balance temperature for the target air pollutants to be
destroyed and removed.
2. Effect of Space Velocity
In this study, space velocity (SV, h-1) is defined
as the reciprocal of the time inflow gas remains inside
the reactor, which can be expressed as follows:
SV = QV-1
where Q = amount of inflow gas entering the reactor,
m3 h-1 and V = effective volume of reactor, m3.
Accordingly, SV affects the removal efficiency
of VOCs. In actual treatment process of industrial
waste gas, space velocities range between 104 and 105
(a)
900
Temperature (°C)
800
700
600
500
400
300
210 W
310 W
520 W
760 W
200
100
0
1000
(b)
900
800
Temperature (°C)
RESULTS AND DISCUSSION
1000
700
600
500
400
-1
SV = 2040 h
SV = 4080 h -1
SV = 6120 h -1
300
200
100
0
800
(c)
700
Temperature (°C)
concentrations of CO and CO2 in the rear gas after
treatment were also monitored and recorded (EGA200, Shine-Joint Techno. Co., Taiwan). In addition,
the concentration of IPA in the rear gas was analyzed
using GC-FID to estimate the destruction and removal
efficiency (DRE) of IPA. The entire reaction time was
set to be 30 min. The power was turned off
automatically by the timer of the microwave oven
upon termination of the experiment, followed by natural cooling to room temperature at 25 ± 0.5 °C. All
experiments were repeated in triplicate under the same
conditions. The errors of these experiments were less
than ± 4%.
65
600
500
400
Normal air
Dehydrated air
300
200
100
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Time (s)
Fig. 3. Temperature profiles as function of reaction time:
(a) effect of microwave power, (b) effect of space
velocity, (c) effect of relative humidity.
h-1. However, this research was only a simulation
study with the reaction chamber fitted inside a modified household microwave oven; hence, the time for
inflow gas to remain inside the reactor was controlled
to be less than 1.0 s and the space velocities examined
included 2040, 4080 and 6120 h-1.
In general, the higher the SV of inflow gas, the
shorter gas retention time is. The time flowed through
the catalyst is relatively less and reduced the opportunities of contact between IPA and catalyst. In additional, the greater amount of inflow gas carries away
the more surface heat of the Fe3O4 catalytic packed
column that will result in a cooling effect and produce
the lower DRE of VOCs [12]. Figure 3b shows the effects of different SVs on temperature of the Fe3O4
catalytic packed column. The difference in temperature between SVs of 4080 and 6120 h-1 is small, as
evidenced by the overlapping of the two respective
J. Environ. Eng. Manage., 20(2), 63-68 (2010)
temperature curves. This can be attributed to the vertical conductive effect of temperature and the turbulence of the inflow gas, which undermines the cooling
effect achieved by SV of 6120 h-1. In view of this, the
SV of 6120 h-1 should be chosen in the subsequent
experiments. However, in order to avoid the vertical
conductive effect and the interference of turbulence
and consider the stability of the operation, the subsequent experiment still selects the SV of 4080 h-1.
1100
Concentration (ppmv)
66
3. Effect of Relative Humidity (RH)
4. Effect of Microwave/Fe3O4 System
For understanding the influence of microwave
combined with Fe3O4, the background test of microwave alone was first performed. Experimental results
(Fig. 4) showed that IPA was not good absorbent for
microwave and the DRE of IPA by microwave alone
was close to zero. Figure 5 shows the DRE of IPA at
different concentrations (500, 1000, 1500, and 2000
ppmv) achieved by the continuous microwave/Fe3O4
system with SV of 4080 h-1 and microwave power of
760 W. As can be seen, when reaction time reached
10-15 min, the DRE of IPA at different concentrations
all ≥ 99%, indicating that the microwave/Fe3O4 system is efficient in destroying and removing IPA regardless of their concentrations.
1000
950
900
0
10
20
Time (min)
30
40
Fig. 4. The background test of microwave alone
(microwave power = 590 W, IPA = 1000 ppmv,
SV = 4080 h-1).
100
80
DRE (%)
For most catalytic oxidation processes, too high
RH will result in active radicals on the surface of catalyst being covered by water molecules, which will in
turn inhibit catalytic activity and reduce treatment efficiency [13,14]. In this study, with SV of 4080 h-1
and microwave power of 760 W, air dehumidified by
dehydrated reagent (RH < 3%) and normal air (RH at
60-75% in Taiwan) were fed into the reaction system
to examine the effect of RH on temperature of the
Fe3O4 catalytic packed column. According to the results, RH showed no significant influence on increase
in temperature of the packed column (Fig. 3c). For
this phenomenon, the reasonable inference is that
Fe3O4 was an efficient absorbent for microwaves and
it did not keep moisture similar as activated carbon
absorbed. Therefore, Fe3O4 adsorbed microwave rapidly and converted them into thermal energy within a
short time, thus leading to the temperature of the catalytic packed column rapidly rose above 100 °C. Hence,
the water molecules present in the inflow gas were
evaporated long before the packed column reached
thermal balance temperature. Thus, RH exerts no influence on the increase in temperature. In view of the
above, the RH of inflow gas was not strictly controlled in the subsequent experiments of this study. In
additional, it should be emphasized that this conclusion is suitable for most normal RH and low moisture
content of VOCs.
1050
60
40
500 ppmv
1000 ppmv
1500 ppmv
2000 ppmv
20
0
0
5
10
15
20
Time (min)
25
30
Fig. 5. DRE of IPA at different concentrations by
microwave/Fe3O4 system.
5. Concentration of CO and CO2
Figure 6 shows the concentrations of CO and
CO2 in the rear gas with IPA of 1000 ppmv treated by
the microwave/Fe3O4 system under SV of 4080 h-1
and microwave power of 760 W. As can be seen, the
concentrations of CO and CO2 were close to the background value in the initial 2 min when the power was
just turned on. While the concentration of CO2 was
about 340 ppmv, that of CO was less than 5 ppmv and
hence undetected by the instrument (≤ 5 ppmv).
However, the concentrations of both CO and CO2 increased when IPA was thermal oxidized by the microwave/Fe3O4 system. The increase in concentration
of CO2 from the initial background concentration of
348 ppmv to 5940 ppmv was more marked than that
of CO, which rose to 31 ppmv within first 10 min and
then decreased to about 19 ppmv. Calculating the
mass balance of IPA also evidenced the same result,
i.e., IPA was thermal oxidized mostly into CO2 and
small traces of CO. The concentration of oxygen was
maintained within 18-19% v/v throughout the experi
Chang et al.: IPA Decomposition with Microwave/Fe3O4
REFERENCES
6000
5000
Concentration (ppmv)
67
4000
3000
2000
CO
CO2
1000
0
0
5
10
15
20
25
30
Time (min)
Fig. 6. Concentration of CO and CO2 in rear gas after
treatment of IPA by microwave/Fe3O4 system
(IPA = 1000 ppmv, microwave power = 760 W,
SV= 4080 h-1).
ment. The results obtained show that the supply of
oxygen was sufficient during the reaction, which facilitate the thermal oxidation of IPA by the microwave/Fe3O4 system.
CONCLUSIONS
In this study, microwave power and SV of inflow gas were more significant parameters affecting
temperature. The higher the microwave power is, the
greater the rate of increase in temperature and the
higher the thermal balance temperature are. Moreover,
higher SV of inflow gas carries away more heat from
the catalytic packed column, leading to greater decrease in temperature. In sum, the application of the
microwave/Fe3O4 system to catalytic destruction and
removal of IPA promises a new technology for treatment of VOCs. The time required for temperature rise
is short, with the reaction system reaching 800-900 °C
within 10-15 min. Such temperature exceeds the ignition point of most VOCs and can thus rapidly destroy
and remove VOCs in waste gases. The increase in
temperature requires no extra fuel added, and both operation and maintenance are easy. As long as the VOC
concentration is controlled to be less than 25% of the
lowest explosion level, the system does not present
any operation and safety risks. Therefore, the method
proposed in this study has excellent potential for further development and wide application.
ACKNOWLEDGMENT
We thank the National Science Council for the
financial support to this research (NSC 96-2622-E451-002-CC3).
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
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Manuscript Received: April 11, 2009
Revision Received: August 29, 2009
and Accepted: September 4, 2009