Chemical scrubbing of odorous fumes emitted from hot

115
Sustain. Environ. Res., 25(2), 115-124 (2015)
Technical Note
Chemical scrubbing of odorous fumes emitted from hot-melted
asphalt plants
Ya-Chen Wu, Po-Cheng Chen, Hsiao-Yu Chang* and Ming-Shean Chou
Institute of Environmental Engineering
National Sun Yet-Sen University
Kaohsiung 80424, Taiwan
Key Words: Hot-melted asphalt odors, chemical scrubbing, odor removal, VOCs, sodium hypochlorite
ABSTRACT
Hot-melted asphalt (HMA) plants use sized gravels, asphalt and/or recycled asphalt as raw
materials. In the plants, the materials are heated to certain preset temperatures and blended at fixed
ratios at around 170 °C to prepare the required HMA for road paving. In the asphalt-melting, hotblending and dumping operations, fumes and particulates are emitted from the process equipments
and chimneys. The emitted gases contain various volatile organic compounds and poly aromatic
hydrocarbons which are harmful to the health of the plant workers and nearby residents. Complaints
from the residents also come with the fume and odorous emissions. In this study, an oxidationreduction-in-series scrubbing process was tested to remove odorous compounds in waste gases emitted
from HMA plants. Waste gas samples for test were collected from the vent hole of an oven which
contains heated samples of asphalt or recycled asphalt concrete. NaOCl solution was used to scrub and
oxidize the compounds and H2O2 to reduce the chlorine emitted from the oxidative scrubber. A gas
chromatography with a mass spectrophotometric detector (GC-MSD) was used for the identification of
the odorous species and their concentrations in the waste gases.
GC-MSD indicate that alkanes, arenes, alkenes, halides, esters, and carbonyl compounds are
detected in the test gas. Scrubbing test results indicate that with oxidative solution of 60-120 mg L-1
residual chlorine at pH 7.0-7.5 and reductive solution of 35 mg L-1 H2O2 peroxide at pH > 12, over 90%
of the non-methane hydrocarbon in the tested gas could be removed. Odor intensities could be reduced
from 3,090 (expressed as dilutions to threshold) to 73. Pungent asphalt odor in the test gas was turned
into slight sulfur smell after the scrubbing.
For removing the odors from 500 Nm3 min-1 of the flue gas vented from a HMA plant, a cost
analysis indicates the required total cost for chemicals (NaOCl, H2O 2 and NaOH) added to the
scrubbers is around USD 94 d-1 for a daily operation time of 7.5 h. The cost is far lower than that by the
traditional thermal incineration one (USD 836 d-1) or by the regenerative thermal oxidation one (USD
478 d-1). This study has successfully developed a cost effective chemical scrubbing technology for the
removal of odorous compounds in gases emitted from HMA plants.
INTRODUCTION
Malodor might be thought of as the single or composite of chemical compounds which causes ill feelings
by smelling through the sensory organ. It is often
classified as sensory pollution resulting in damaging
more mentally or psychologically than physically [1].
*Corresponding author
Email: d9033803@student.nsysu.edu.tw
Many of sources causing malodors are found to include
chemical plants, oil refineries, sewage treatment
plants, landfills, livestock facilities, etc. [2-5]. It is
in fact known that some of these compounds, when
accumulated beyond certain concentration ranges, can
exert toxic effects on human beings [6].
Hot-melted asphalt (HMA) plants use sized
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Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
gravels, asphalt and/or recycled asphalt as raw
materials. In the plant, the sized gravels and/or recycled
asphalt cement (RAC) are blended at fixed ratios
and heated to around 170 °C, and then sprayed and
blended with HMA to prepare the required HMA for
road paving. In the asphalt-melting, hot-blending and
dumping operations, fumes and particulates are emitted
from process equipments and chimneys. As cited in AP42 [7,8], emitted gases contain various volatile organic
compounds (VOCs), such as long-chain aliphatic or
oxygenated hydrocarbons with 12-15 carbon number,
benzene, toluene, xylenes, ethyl benzene, cumene,
phenols, formaldehyde, poly aromatic hydrocarbons
(PAHs), chlorinated hydrocarbons, and carbon
disulfide. In addition, when the hot vented gas contacts
with steam or air, the content of PAHs and thiophenic
compounds would increase. The US EPA reported that
the emission factor of VOCs is around 0.003 kg Mt-1
HMA. Nearly all the emitted chemical compounds are
harmful to the health of the plant workers and nearby
residents. In Taiwan, the Environmental Protection
Administration has been receiving numerous complaints
relating to the odors emitted from asphalt plants for a
long time.
At present, waste gases from the hot-blending
and dumping operations are collected, combined and
transported to a control system. In general, a bag
filter is used to remove the mists in the collected gas.
Vent gas from the bag filter still contains various
odorous VOCs. The odorous VOCs should further be
diminished to eliminate possible public complaints.
As reported by the Asphalt Odor Control News™
[9], a plant uses a scrubber and a fabric-over-strainer
sock filter to remove the mists in the gas vented from
asphalt cement storage tanks, and successively directs
the gas to an AC (activated carbon) unit, where all
odorous compounds are adsorbed. The method is
technically and economically feasible for the control of
small flow rates of the odorous gas vented from asphalt
cement storage tanks as cited. However, frequent
replacement of the AC will be required for high rates
of odorous gas vented from HMA hot-blending and
dumping operations. This leads to high AC replacement
costs.
Cook et al. [10] investigated the performance of a
full-scale biofilter for treating off-gases from polymermodified asphalt production. The biofilter was effective
in controlling odor from the production process and
removed 98% of the H2S with concentrations less than
400 ppm. Chemical scrubbing is generally used for the
control of gases with inorganic acid or base pollutants
such as SO2, HCl, HNO3, H2S and NH3 [11,12]. For
the scrubbing control of waste gases with VOCs, an
oxidant is generally added to the scrubbing water to
oxidize the VOCs. Aqueous NaOCl was found to be
among the most effective oxidants.
A few studies focused on the NaOCl oxidation of
aqueous organics such as benzene, toluene, xylenes,
phenolates, aldehydes, and ketones. Chungsiriporn
et al. [13] proposed the removal of toluene from
waste air using a spray wet scrubber combining the
absorption and oxidation reaction of NaOCl solution.
The oxidation reaction yields the oxidation products
including salt and chloride ions as given by the Eq. 1
C
6H5CH3 + 3NaOCl → C6 H5COOH +
3NaCl + H2O
(1)
With conditions of air flow rate of 100 m 3 h -1,
influent toluene concentration of 1,500 ppm (6,160
mg Nm-3), NaOCl concentration of 0.02 M (1,490 mg
L-1), NaOCl solution feed rate of 0.8 m3 h-1 (1.19 kg
NaOCl h-1), the highest toluene removal efficiency was
around 92%. Mirafzal and Lozeva [14] presented phase
transfer catalyzed oxidation of alcohols with NaOCl in
ethyl acetate media with excellent yield of aldehydes or
ketones as oxidized products.
Cheng and Hsieh [15] integrated chemical scrubber
with NaOCl and surfactant to remove hydrocarbons in
cooking oil fume. Results proposed suitable operating
parameters of NaOCl scrubber system at pH 6.5, 200
ppm of NaOCl and 11 L m-3 of liquid/gas ratio. Under
the conditions, NMHC (non-methane hydrocarbon) in
the cooking fume could be removed from 19 ± 13 to
4 ± 2 mg as CH4 m-3. Addition of 0.08 mL surfactant
(22 mg L-1 sodium dodecyl benzene sulfonate) to the
NaOCl solution further increased the NMHC removal
to 86% (NMHC decreased from 11 ± 11 to 2 ± 2 mg as
CH4 m-3). The study did not indicate, however, which
components in the fume gas were removed or oxidized.
Literature data [16,17] indicate that there may be
aromatics, phenols, furfural, PAHs (from naphthalene,
C 10 H 8 , to dibenzo (a,h) anthracenes, C 22 H 14 ), and
carbonyls (C1-C10 aldehydes, acrolein, butanone, and
benzaldehyde) in cooking fumes.
To the authors’ knowledge, there is no report on the
removal of VOCs or odorous compounds emitted from
HMA processes by chemical scrubbing approaches. In
the present study, chemical scrubbing technology using
NaOCl as an oxidant for VOCs and odors and alkaline
H2O2 as a reducing agent for the emitted chlorine was
tested for its applicability to the odor control of the
HMA processes. Emission characteristics, effects of
pH and concentration of the NaOCl on the VOC and
odor removal efficiencies, and effects of pH and H2O2
concentration on the chlorine removal efficiencies will
be presented. Cost analysis and a comparison will also
be made for the developed chemical control process
Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
and other competitive processes.
MATERIALS AND METHODS
1.Experimental Systems
Schematics of the experimental systems are shown
in Figs. 1 and 2. Figure 1 shows a gas generation oven
(Hipoint Precision Oven, Type: OV-40, Taiwan) in
which asphalt or RAC was placed in an aluminum
foil plate and the oven heated to around 250 °C to
generate the fume gas for tests. Fume gas samples were
collected in 25-L bags from the vent hole of the oven
by a gas sampling pump drawing at a fixed flow rate
of 2-3 L min-1. The scrubbing system shown in Fig. 2
consists of a gas pump and 2-1 L scrubbing bottles.
The first bottle (oxidation scrubber) was filled with
600 mL of aqueous NaOCl solution for absorbing and
oxidizing the chemicals from the induced fume gas at a
fixed flow rate of 1 L min-1. The second one (reduction
3 TI
2
1
4
1 Oven
5
4 Gas pump (operated at
2-3 L min-1)
2 Asphalt or recycled
5 Waste gas storage bag
asphalt cement
(25 L)
3 Temperature indicator
Fig. 1. Schematic diagram of the test waste gas generation
and collection system.
3
3
6
1
2
4
5
1 Waste gas storage bag (25 L)
4 Oxidation scrubber
2 Gas pump (operated at
1.0 L min-1)
5 Reduction scrubber
3 Gas sampling port
6 Scurrbed Gas vent
Fig. 2. Schematic diagram of the 2-step chemical scrubbing system.
117
scrubber) contained 600 mL of aqueous H 2O 2 and
NaOH solution for absorbing the chlorine gas stripped
from the aqueous NaOCl solution and reducing it to
NaCl following the chemical reaction:
Cl2 + H2O2 + 2 NaOH → 2 NaCl +
O 2 + 2 H 2O
(2)
Asphalt and RAC used in this study were obtained
from a local HMA plant. The collected RAC was cut
into pieces of approximately 2.54 x 2.54 cm in size.
All chemicals (aqueous NaOCl solution with 12 wt%
available chlorine, 35 wt% aqueous H2O2 solution, 98
wt% H2SO4, and NaOH) are all reagent grades.
2.Scrubbing Solution
Tests were performed with an initial available
chlorine concentration of 60, 120, 240, and 480 mg
L-1. Effects of initial pH on the odor and/or NMHC
removal were then tested with the different initial
available chlorine levels. H2O2 solution in the second
bottle was kept at an initial concentration of 35 mg
L-1 and adjusted to pH 12.5 by adding 25 wt% NaOH
solution. Around 34 mL of the NaOH solution or 10
g pure NaOH was required for 1 L of H2O2 solution.
The scrubbing liquors were eventually drained to the
wastewater treatment system after experiment.
3.Sample Analysis
In the course of reaction, gas samples collected
was analyzed for THC (total hydrocarbon), NMHC,
Cl2, and odor intensity. Scrubbing liquors were also
determined for pH and residual chlorine, if necessary.
THC concentrations in gas samples were analyzed with
a portable flame ionization detector (FID, Thermo,
TVA-1000B, USA). The FID was calibrated monthly
by standard methane gases in the concentration range
from 0.5 to 50,000 ppm.
Compositions of VOCs in gas samples were
analyzed by a gas chromatography (GC, 6890N Series,
Agilent, USA) coupled with a mass spectrometry (MS,
5973 Network Mass Selective Detector, Agilent, USA).
The GC/MS was calibrated by a standard gas with
compounds shown in Table 1 before each sampling
day. The standard gas was stored in a cylinder [18].
NMHC concentrations in gas samples were
analyzed using a GC (GC-14B, Shimadzu, Japan) with
a capillary column (0.53 mm id and 30 m long, coated
with 5 mm-thick polydimethylisioloxane, Alltech. No.
16843) and a FID.
A chlorine analyzer (ToxiRAE II, 045-0516-000,
USA) was used for the analysis of gaseous chlorine
concentrations with the minimum detection limit of
0.1 ppm. A pocket colorimeter (No. 58700-00, HACH,
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Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
Table 1. GC/MS gas calibration compounds
Compound
MDL (ppb)
Acetone
Acetonitrile
0.32
Benzene
Bromodichloromethane
0.25
Bromomethane
0.31
1,3-Butadiene
2-Butanone (MEK)
Carbon Tetrachloride
0.27
Chlorobenzene
0.26
Chloroethane
Chloroethene
0.27
Chlorodifluoromethane
0.26
Chloroform
0.26
Chloromethane
3-Chloro-1-propene
0.17
1,2-Dibromoethane
0.29
Dibromochloromethane
0.22
1,3-Dichlorobenzene
0.25
1,4-Dichlorobenzene
0.24
1,2-Dichlorobenzene
0.23
cis-1,2-Dichloroethene
0.25
trans-1,2-Dichloroethene
0.19
1,2-Dichloroethane
0.24
1,1-Dichloroethene
0.21
1,2-Dichloroethene
0.29
Dichlorodifluoromethane
0.27
Dichloromethane
1,2-Dichloropropane
0.27
trans-1,3-Dichloropropene
0.26
Compound
cis-1,3-Dichloropropene
1,2-Dichloro- 1,1,2,2-tetrafluoroethane
Ethylbenzene
Heptane
Hexane
Hexachlorobutadiene
Methyl methacrylate
4-Methylpentan-2-one
alpha-Methylstyrene
n-Octane
Pentane
Propane
Prop-2-enal
2-Propenenitrile
Styrene
1,1,2,2-Tetrachloroethane
Tetrachloroethene
Trichloroethene
1,1,2-Trichloro- 1,2,2-Trifluoroethane
1,1,1-Trichloroethane
Trichlorofluoromethane
1,1,2-Trichloroethane
1,2,4-Trimethylbenzene
1,3,5-Trimethylbenzene
Toluene
m/p-Xylene
o-Xylene
Vinyl Acetate
MDL (ppb)
0.23
0.29
0.22
0.15
0.17
0.13
0.23
0.17
0.41
0.31
0.28
0.32
0.32
0.31
0.20
0.15
0.27
0.18
MDL: Method Detection Limit
Japan) was used for the detection of residual chlorine
in the scrubbing liquor [19]. Deionized water was used
as a blank to calibrate the colorimeter each time before
the measurement.
4.Olfactory Test
In Taiwan, the “Triangular Odor Bag Method”
used in Japan [20,21] has officially been adopted for
measuring the odor index. The sensory test is conducted
by at least 6 members of the panel. Each panel is
given 3 bags: 1 with a certain dilution of the sample
gas and 2 with odor-free air, and the panel asked to
choose the odorous bag after smelling all the gases in
the bags. If the panel can tell the bag with the odorous
gas, the odorous gas is then further diluted and the test
continued until all panels are unable to identify the bag
with odor. Test data are then used to obtain the odor
index as shown by an example shown in Table 2 [22].
In the Table, by deleting the highest and the lowest of
the individual panel values (Xi), the average threshold
X of the panel and the odor index Y are calculated as
follows:
The odor index Y of 550 represents a dilution of
550 of the odorous gas by fresh air, around 50% of
Table 2. Example of sensory test for sample collected at exhaust port
Dilution ratio 30 100 300 1000 3000 10000 Threshold of each panel (Xi) Excluding maximum/minimum values
Logarithm 1.48 2 2.48 3 3.48
4
A
/  C
2.24
Excluded
B
/   C
2.74
C
/    
3.74
Excluded
C
Panel
D
/   C
2.74
E
/  C
2.24
F
/    C
3.24
Average
2.74
: positive answer, C: negative answer
Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
general audience can detect the odor and the other 50%
cannot.
According to the stationary air pollution source
emission standards [23], gas odor intensities Y should
be not greater than 1,000, 2,000 and 4,000, respectively,
for emission stack heights of lower than 18, 18-50,
and higher than 50 m. Stack heights for most of HMA
plants in Taiwan are in the range of 18-50 m.
RESULTS AND DISCUSSION
1.THC Concentrations in the Generated Gas
Figure 3 shows variations of THC concentrations
in the generated gas with heating time and temperature.
THC concentrations were higher when the asphalt
was heated up to 250 °C then at 200 °C. At the
same heating temperature, RAC yielded lower THC
concentrations than the original one because of the
lower asphaltine contents in the cement. Another
possible reason for the lower THC emissions from the
RAC may be that the VOCs therein have almost all
volatized with a paving time of years. However, in the
plant, RAC is usually crushed, screened, and preheated
to 200-250 °C and blended with sized gravels with
lower preheat and drying temperatures of 110-150 °C.
The relatively higher preheat temperatures of RAC
make the hot emission gas more odorous than that for
gravels. While virgin asphalt is heated only to 140160 °C before blending with the preheated gravels, the
odorous intensities of the gas emitted from the blending
operation are thus less than those from the preheating
of the crushed RAC.
Figure 3 also shows that THC concentrations in the
gases emitted from the heated asphalt and RAC were
roughly stable for heating times of over 25-30 min and
the emitted gases were subsequently discharged into
Fig. 3. Variations of THC concentrations in the generated
gas with heating time and temperature.
119
the scrubbing solutions.
2.Water Scrubbing Tests
Tests were initially performed by scrubbing
the gases only with deionized water filled in both
scrubbers. Results of the water-washing tests shown
in Table 3 indicate that VOC removal efficiency was
approximately 71% for RAC and 68% for regular
asphalt. Although some water-soluble VOCs in both
gas emissions were removed, a significant odor
remained as noted by nose sniffing. Thus, odor removal
efficiency by transferring VOCs from gas to aqueous
phase has not been substantiated. This implies that
water scrubbing without chemical addition is not a
good practice for the odor control of emissions from
HMA plants. Data shown in Table 3 also indicate that
methane accounts around 10% of the THC in both
emitted gases. Water-insoluble methane is of course not
easily removed by water.
3.Dependency of NMHC Removal on the Initial
Available Chlorine Concentration without pH
Adjustment
Results shown in Table 4 and Fig. 4 show an
optimal initial available chlorine concentration ([Cl2]o)
of 60 mg L-1 for NMHC removal. The NMHC removal
efficiencies of gas emissions from RAC and regular
asphalts were 91 and 93%, respectively. The higher
NMHC removal at a [Cl2]o of 60 ppm might result from
the lower pH (initial 9.4 and final 7.3) which gave a
higher ratio of [HOCl]/([HOCl] + [OCl-]). HOCl has a
stronger oxidation power than OCl-.
Chlorine concentrations in the exhaust gas from
the oxidation liquid increased with increasing operation
time and increasing [Cl2]o as shown in Fig. 5. Data
indicate that a lower [Cl 2] o gave a higher chlorine
loss of around 6-7 ppm especially at the end of the
operation because of the decreasing solution pH with
the operation time. Chlorine in the exhaust gas from
the oxidation liquid could be absorbed by the reducing
liquid with initial concentration of 35 mg L -1 H 2O 2
adjusted to pH 12.5. Exhaust gas from the reducing
liquid had a chlorine of as low as < 0.2 ppm which
is slightly lower than the odor threshold of 0.21-0.34
ppm.
Table 3. NMHC removal by 2-satge water scrubbing
Gas concentration NMHC
Heated sample Scrubbing (ppm as methane)
THC CH4 NMHC removal %
Before 20 1.9 18.1
RAC
After
6 1.9 5.3
71
Before 28 3.3 24.7
Asphalt
After
11 2.9 7.8
68
120
Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
Table 4. NMHC removal data by the 2-satge chemical scrubbing
Heated
[Cl2]o (mg L-1) Scrubbing pH of the oxidation liquid**
Sample*
RAC
60
9.40
Before
After
7.34
120
9.66
Before
After
7.78
240
9.88
Before
After
8.32
480
10.1
Before
After
9.36
Asphalt
60
9.42
Before
After
7.52
120
9.87
Before
After
7.83
240
10.0
Before
After
8.50
480
10.1
Before
After
9.26
Gas concentration (ppm as methane) NMHC
THC
CH4
NMHC
removal %
34.2
4.2
30
5.4
2.7
2.7
91
17.4
5.7
11.7
9.7
7.0
2.7
77
21
5.9
15.1
9.1
6.3
2.8
82
30.7
7.9
22.8
14.2
9.1
5.1
78
33.7
8.3
25.4
9.1
7.2
1.9
93
10.2
3.0
7.2
3.9
2.5
1.4
80
28.3
6.2
22.1
10.9
5.8
5.1
77
15.6
4.0
11.6
5.1
3.5
1.6
87
*Heating temperature: 250 °C
**Without pH adjustment
4.Dependency of NMHC Removal on the Initial pH
With Fixed Initial Available Chlorine of 60 mg L-1
of the oxidation treatment is the best with near-neutral
pH, reasonable with acidic, and poor with alkaline
conditions.
According to results without pH control (Fig. 4),
oxidation removal of the NMHC in the emitted gas had
a higher efficiency of over 90% at [Cl2]o of 60 mg L-1.
The efficiency dropped to around 83% at pH 8 due to
an increase in [Cl2]o to 120 mg L-1 with a higher caustic
soda in the added NaOCl solution. Effect of pH on the
oxidation removal of the NMHC might be important.
Figure 6 shows variations of NMHC removal
efficiency from the generated gas with initial pH of the
oxidation liquid with fixed initial available chlorine of
60 mg L-1. Both of the asphalt and RAC gas emission
washing were optimized at pH 6.0-7.5, with NMHC
removal efficiencies of over 90%. Removal efficiency
dropped to less than 90% at pH 8. The effectiveness
Fig. 4. Dependency of NMHC removal on the initial
available chlorine of the oxidation liquid without
pH control.
Fig. 5. Variations of Cl2 in the exhaust gas from the oxidation liquid with scrubbing operation time for the
four runs with different initial available chlorine
concentrations in the oxidation liquid. pH of the
oxidation liquids were not adjusted.
Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
Fig. 6. Variations of NMHC removal efficiency from the
generated gas with initial pH of the oxidation liquid
with fixed initial available chlorine of 60 mg L-1.
5.Dependency of NMHC Removal on the Initial
Available Chlorine with Neutral pH
Figure 7 shows variations of NMHC removal
efficiency from the generated gas from RAC and
asphalt with initial available chlorine concentration in
the oxidation liquid with initial pH adjusted to 7.0 and
7.5. The NMHC removal increased with an increase
in [Cl2]o, however, it leveled off with [Cl2]o above 120
mg L-1 ; for cost effective, the operating concentration
of [Cl2]o is recommended to be 60-120 mg L-1 with pH
controlled to in the range of 7.0-7.5.
6.GC-MSD Examination
It is well perceived carbonyl compounds as major
121
odorous pollutants. Carbonyls are directly discharged
from such primary sources as exhaust gases of motor
vehicles and incomplete combustion of hydrocarbon
fuels in industrial machinery and industrial processes
[24].
GC-MSD results (Table 5) indicate that alkanes,
arenes, alkenes, halides, esters, and carbonyl
compounds are detected in the test gas. Most VOCs are
oxygenated ones among which acrolein and acetone are
major components. For RAC generated gas, around 9899% and 90% of the influent acrolein (1.39 ppm) and
acetone (3.64 ppm), respectively, could be removed by
the 2-stage scrubbing system operated at the optimal
conditions as stated in the previous section. However,
alkanes, alkenes (mainly 1,3-butadiene), aromatics, and
chlorinated hydrocarbons were not effectively removed.
The reasons may be the lower water solubility and less
reactivity of NMHCs with the oxidant.
7.Olfactory Test
Kim and Park [25] show the relationship between
olfactory and indirect instrumental methods for odor
detection. It confirms that the odorant concentration
data measured by instrumental method can be used
effectively to account for the odor intensity estimated
by the sensory method for samples collected from
sources with high activities. In the present study,
sensory tests indicate that the scrubbed gas has no
asphalt odor. One test indicates the odor intensities
could be reduced from 3,090 (expressed as dilutions to
threshold) to 73. Pungent asphalt odor in the test gas
turned into slight sulfur smell after the scrubbing. Since
the stack heights for most of HMA plants in Taiwan
are in 18-50 mm the odor intensity of 73 meets the
requirement [23].
8.Economic and Performance Assessments
Fig. 7. Variations of NMHC removal efficiency from the
generated gas with initial available chlorine concentration in the oxidation liquid with initial pH
adjusted to 7.0 and 7.5, respectively, for RAC and
asphalt.
Solutions of NaOCl, H2SO4, H2O2 and NaOH are
needed to add to the two-stage chemical scrubbers
used in the present study. From the experimental data,
an average of 1 kg of 12% available Cl 2 bleaching
solution is required for oxidizing 1000 Nm 3 of the
emitted gas. According to the equations Cl2 + 2NaOH =
NaOCl + NaCl + H2O and 2NaOCl + H2SO4 = 2HOCl
+ Na2SO4, for neutralization of 1 mol NaOCl (71 g of
available chlorine) to HOCl, it requires 0.5 mol H2SO4
(50.5 g of 97% H2SO4). Accordingly, 0.0854 kg of 97%
H2SO4 [(1 x 0.12)/(71 x 50.5)] ＝ 0.0854) is required
for neutralization of 1 kg of the bleaching solution for
1,000 Nm3 waste gas.
According to data shown in Fig. 5b, effluent gas
from the oxidation liquid with an initial available
chlorine of 60 mg L-1 has Cl2 of 3.66 ppm at 20 min,
Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
122
Table 5. GC-MSD data for the gases before and after the chemical scrubbing treatment
NMHC (ppm)
RAC
Asphalt
Before
After
Before
After
Alkanes
Propane
0.56
0.38
0.34
0.34
n-Pentane
0.27
0.31
0.17
0.19
n-Hexane
0.21
0.23
0.14
0.12
Heptanes
0.09
0.11
0.08
0.06
Octanes
0.04
0.05
0.04
0.03
Sub. sum
1.18
1.08
0.77
0.73
Olefins
1,3-Butadiene
0.066
0.093
0.063
0.08
Sub sum
0.066
0.093
0.063
0.08
Aromatics
Benzene
0.27
0.28
0.11
0.10
Toluene
0.079
0.05
0.05
0.04
Ethyl benzene
0.009
0.007
0.007
0.004
m/p-Xylenes
0.023
0.019
0.020
0.01
o-Xylene
0.009
0.008
0.011
0.004
1,3,5-Trimethyl benzene
< 0.001
< 0.001
< 0.001
ND
1,2,4-Trimethyl benzene
0.003
0.001
0.003
< 0.001
Styrene
0.003
< 0.001
0.002
< 0.001
Sub. sum
< 0.39
< 0.37
< 0.21
< 0.16
ChloroChloromethane
0.030
0.036
0.018
0.021
Chloroethane
0.005
0.006
0.003
0.003
hydrocarbons
Bromomethane
< 0.0011
< 0.001
< 0.001
< 0.001
Vinyl chloride
< 0.001
< 0.001
< 0.001
< 0.001
Dichloromethane
0.003
0.003
0.002
0.002
Chloroform
ND
0.037
ND
0.024
1,1,2,2-tetraethane
ND
ND
0.002
ND
Sub. sum
< 0.045
< 0.087
< 0.029
< 0.054
Oxygenated
Acetonitrile
0.045
ND
0.057
0.003
Acrolein
1.39
0.013
1.17
0.018
hydrocarbons
Acetone
3.64
0.037
0.86
0.083
Methyl ethyl ketone
0.84
0.018
0.78
0.046
4-Methyl-2-pentanone
0.044
< 0.001
0.025
0.002
Sub sum
5.96
< 0.069
2.89
0.152
Esters
Ethyl acetate
0.29
0.022
0.36
ND
Methyl methacrylate
0.007
ND
ND
ND
Sub. sum
0.30
0.022
0.36
ND
Total
7.94
1.72
4.32
1.17
ND: not detected
and the chemicals required for reduction of the Cl2
introduced to the reducing liquid and neutralization
of the reduced product (HCl) can be calculated by
following the stoichiometry of Eq. 2. Taking 1000 Nm3
of the influent gas with 3.66 ppm Cl2 to the reduction
liquid as a base:
Mass of Cl2
emitted
=
=
1000 Nm3/(22.4 Nm3 kmol-1) x
[3.66 ppm/(106 ppm)]
1.63 x 10-4 kmol
Mass of H2O2
required
=
1.63 x 10-4 kmol x 34 kg kmol-1
=
=
0.556 x 10-2 kg pure H2O2
1.59 x 10-2 kg 35% H2O2
solution
Mass of NaOH
required
=
1.63 x 10-4 kmol x 2 x 40 kg
kmol-1
1.30 x 10-2 kg pure NaOH
2.90 x 10-2 kg 45% NaOH
solution
=
=
According to the above calculations, it requires
0.0159 and 0.0290 kg, respectively, of 35% H2O2 and
45% NaOH solutions to treat 1,000 Nm3 of the waste
gas.
In addition to the required chemicals, it is necessary to treat wasted scrubbing liquids from both scrubbers before dumping them into receiving water bodies
or sewage systems. According to Eq. 2, 1 mg L-1 of
available chlorine needs 0.479 mg L-1 of H2O2 (34/71 =
0.479) and 2.50 mg L-1 of 45% NaOH (80/(71 x 0.45) =
2.50) to reduce to NaCl. For the reduction of 60 mg L-1
available chlorine in the waste liquor, it requires 28.7
mg L-1 H2O2. A combination of equal amounts of the
waste oxidation liquid (60 mg L-1 available chlorine)
and reducing liquid (35 mg L -1 available chlorine)
results in total removal of free chlorine in the combined
liquid. It needs only stoichiometric amount of H2SO4
to neutralize the caustic soda in the drained liquid. The
maximum amount of 97% H2SO4 is 0.0165 kg [(0.029
x 0.45)/(40 x 49 x 0.97) = 0.0165) for 1,000 Nm3 waste
gas.
Wu et al., Sustain. Environ. Res., 25(2), 115-124 (2015)
Table 6 lists the requirement of chemicals and the
costs for treating 1000 Nm3 of the waste gas. Current
methods used by HMA plants to remove odor include
traditional thermal oxidation (TO) and regenerative
thermal oxidation (RTO). This study comparatively
evaluates economics of TO, RTO and the developed
chemical scrubbing technology. For removing the
odors from 500 Nm3 min-1 of the flue gas vented from
a HMA plant, cost analysis shown in Table 7 indicates
the required total cost for chemicals (NaOCl, H2SO4,
H2O2 and NaOH) added to the scrubbers and waste
Table 6. Requirement of chemicals and the costs for treating 1000 Nm3 of the waste gas
Amount Unit cost
Cost
Chemical
(kg)
(USD kg-1) (USD)
Bleaching solution
1.00
0.167
0.167
(12% Cl2)
Sulfuric acid (97%)
0.102
0.067
0.00068
Hydrogen peroxide
0.016
0.5
0.008
(35%)
Sodium hydroxide
0.029
0.333
0.0097
(45%)
Total
0.185
Table 7. Economic and performance assessment
Thermal
Chemical
RTO
oxidation
scrubbing
300,000
400,000
100,000
Investment
(USD)
Power
20
60
45
requirement (kW)
Electricity
15
45
34
(7.5 h d-1) (kWh)
Fuel
660
237
0
(7.5 h d-1) (L)
(heavy oil) (diesel)
12% NaOCl
0
0
225
solution
(7.5 h d-1) (kg)
97% H2SO4
0
0
22.9
(7.5 h d-1) (kg)
35% H2O2
0
0
3.6
solution
(7.5 h d-1y) (kg)
45% NaOH
0
0
6.5
solution
(7.5 h d-1) (kg)
Daily electricity
11.2
33.6
25.2
cost (USD kWh-1)
Daily fuel cost
825
444
0
(USD)
Daily chemical
0
0
42
cost (USD)
Sum of daily cost
836
478
93.4
(USD)
Operation and
Simple
Simple
Simple
maintenance
Facility life (yr)
10
10
10
Odor intensity
> 95
> 95
> 95
reduction (%)
123
liquids was around USD 42 d-1 for daily operating time
of 7.5 h. In the estimation, the costs for final disposal
of the spent oxidation and reducing liquors need to be
added. The cost of USD 42 d-1 is far lower than that by
TO (USD 836 d-1) or by RTO (USD 478 d-1). This study
has successfully developed a cost effective chemical
scrubbing technology for the removal of odorous
compounds in gases emitted from HMA plants.
CONCLUSIONS
In this study, an oxidation-reduction-in-series
scrubbing process was tested to remove odorous compounds from waste gases emitted from HMA plants.
GC-MSD examination results indicate that
alkanes, arenes, alkenes, halides, esters, and carbonyl
compounds are detected in the test gas. Scrubbing test
results indicate that with oxidative solution of 60-120
mg L-1 residual chlorine at pH 7.0-7.5 and reductive
solution of 35 mg L-1 hydrogen peroxide at pH > 12,
over 90% of the NMHCs in the tested gas could be
removed. Odor intensities could be reduced from 3,090
to 73. Pungent asphalt odor in the test gas was turned
into slight sulfur smell after the scrubbing.
For removing the odors from 500 Nm 3 min-1 of
the flue gas vented from a HMA plant, cost analysis
indicates the required total cost for chemicals added
to the scrubbers was around USD 42 d -1 for daily
operating time of 7.5 h. The cost is far lower than
that by the traditional TO (USD 836 d-1) or by RTO
(USD 478 d-1). This study has successfully developed
a cost effective chemical scrubbing technology for the
removal of odorous compounds in gases emitted from
HMA plants.
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: August 27, 2014
Revision Received: October 28, 2014
and Accepted: November 19, 2014