Journal of Environmental Protection, 2011, 2, 175-185
doi:10.4236/jep.2011.22020 Published Online April 2011 (http://www.SciRP.org/journal/jep)
175
Removal of Nitrogen Dioxide and Sulfur Dioxide
from Air Streams by Absorption in Urea Solution
Mahmood M. Barbooti1*, Neran K. Ibraheem2, Awni H. Ankosh2
1School of Applied Sciences, Baghdad, Iraq; 2Department of Chemical Engineering, University of Technology, Baghdad, Iraq.
Received November 25th, 2010; revised January 13th, 2011; accepted March 16th, 2011.
ABSTRACT
The study focuses on the absorption rates of NO2, SO2 and a mixture of these two acid gases into urea solution in
packed bed column. The absorption rate was studied as a function of absorbent temperature, urea concentration and
acid gas concentration. The influence of liquid temperature between 10 - 40˚C, urea concentration between 0.1 - 0.5 M
and acid gas concentration NO2 between 100 - 1000 ppm (191 - 1910 mg/m3), SO2 between 500 - 2500 ppm (1310 -
6530 mg/m3) were investigated. The mass gas flow rate of 20.646 (kg/m2.min) at 25˚C and the absorption rate were
determined by measuring the NO2 and SO2 concentrations in the inlet and outlet streams of the absorptioncolumn. The
absorption rate of SO2 increases with the decrease of temperature of absorbent (urea solution) and with the increase of
the urea concentration. The presence of NO2 in the effluent gas stream lowers the absorption rate of SO2 in urea solu-
tion due to the fast reaction of NO2 with urea as compared with SO2. The absorption rate of NO2 decreases as the urea
concentration exceeds 0.4 mol/l and for NO2 gas concentration of 100 ppm due to the decrease the diffusivity of the gas.
The experimental data were analyzed using dimensionless analysis to find the correlation of mass transfer coefficient in
the packed column Sh (H/dp)1.2 = 4.19*10–2 *(G' dp/μg)0.87 (μg/ρg DAB)0.60 The results confirmed the hypothesis that the
absorption is accompanied with chemical reaction. Also it is found the increasing the temperature of absorbent solution
the absorption rate of two gases is decreases. The mass transfer coefficient models are in good agreements with the
Kramer’s equation.
Keywords: Sulfur Dioxide Removal, Nitrogen Dioxide Removal, Column Absorption, Removal of Acid Gases, Air
Pollution Prevention
1. Introduction
The harmful effects of the sulfur dioxide, SOx and nitrto-
gen dioxide, NOx, gases emissions in dense industrial and
urban areas receive increasing attention especially where
their production exceeds neutralization and dispersion
forces [1]. Of the major contributors nitric acid plants
contribute the most NO2 to the environment [2]. The re-
lease of SOx can be controlled by ammonia injection
which relies solely on gas phase reaction in the presence
of moisture to produce ammonium sulfate solid particles
that can be captured by any other particulate collection
device [3]. Heterogeneous reduction process of SOx by
hydrogen sulfide into sulfur can be done in the presence
of suitable solid phase catalysts. Methane can be used for
the reduction of SOx into hydrogen sulfide on alumina as
the catalyst. The hydrogen sulfide produced by this
method is captured by amine scrubbing of the reduced
gas stream. The higher concentration of sulfide obtained
by heating the amine salt may then be easily and eco-
nomically converted to elemental sulfur via the Claus
process [4].
Catalytic oxidation of SOx with air, via the heteroge-
neous contact process or the homogeneous chamber
process, also serves to improve the collection efficiency
of the SO2. Also the collection of SO3 by direct absorp-
tion into water is extremely efficient and the produced
sulfuric acid is a salable commodity [5]. The scrubbing
of SO2 in dilute ammonium hydroxide gives ammonium
sulfate that can be a valuable constituent of fertilizer
formulations [6]. The Wellman-lord process uses the
effective sodium sulfite equilibrium to capture sulfur
dioxide from flue gases [7]. Most of the sodium bisulfite
produced is converted back to sodium sulfite which can
be crystallized out, dried and sold as wood pulping
chemical. The citrate process in which much develop-
ment work has been invested by the U.S Bureau of mines
and by Pfizer use an aqueous solution of citric acid to

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution
176
capture SO2. Uncomplexed citric acid can be regenerated
thermally to obtain stripped citrate solution and stream of
up to 90% SO2 at this stage [8].
The SO2 gas may be entrapped in molten alkali car-
bonate at about 425˚C [9]. Limestone or lime slurry in
water is used in a suitably designed scrubber in an effec-
tive and relatively low cost SO2 removal method. The
affinity of the activated carbon for the acid gases is, in
increasing order, CO2 < SO2 < NO2, [10]. The use of
powdered limestone injection is applied to remove SO2
from flue gases from coal burning. The SO2 reacts with
solid lime to form solid particles of calcium sulfite and
calcium sulfate which are captured in electrostatic pre-
cipitators [11].
Three methods are used to control and reduce NOx
emission, namely: absorption, selective catalytic reduc-
tion and non-sélective catalytic reduction [12]. For
methane reduction, the polluted gas stream is preheated
to about 400˚C and then blended with the appropriate
proportion of methane before passage over platinum or
palladium catalytic surface for reduction [13]. Selective
catalytic abatement uses a catalyst and ammonia fuel to
reduce NOx in preference to combustion with the much
higher levels of oxygen in the gas at temperatures in the
range 210 - 410˚C. Slight excess of ammonia may be
used to leave 5 - 20 (ppmv) in the treated gas stream.
Zhang et al., 2008 [14], studied the NOx removal from
simulated flue gas by chemical absorption-biological
reduction integrated approach in a biofilter. They con-
cluded that maximum elimination capacity (18.78 g·NO
m−3 h
−1) was achieved at a loading rate of 28.68 g·NO
m−3 h−1 and maintained 5 h operation at the steady state.
The aim of the present work is at studying the effects
of the operation parameters on the absorption of SO2 and
NO2 from air streams using dilute urea solution. The op-
erating variables studied are: concentration of the two
gases in air, concentration of absorbent (urea) solution
and temperature of absorbent solution
2. Materials and Methods
2.1. Experimental Set up
Figure 1 shows a photograph of the bench scale system
designed for the absorption of gases. The rig consisted of
three sections: generation, absorption and analysis sec-
tions.
For the generation of the gases a three neck QVF 5
L-flask is connected at its upper part to a glass burette
(100 ml) capacity which contains sulfuric acid or nitric
acid. The inlet of the generation section was connected to
a compressed air source.
The absorption section was a packed column of 7.5 cm
inside diameter and 50 cm height, which is packed with
glass Rashig rings (6 mm inside diameter). The height of
packing is (45 cm). Counter current flow of the gas mix-
ture was maintained at a mass flux of gases (G' = 20.64
kg/m2·min), and the mass flux of absorbent liquid down-
ward = 188 kg/m2·min. The used values for the gas flow
rate and liquid flow rate was checked with loading,
flooding and pressure drop calculation in the column.
Calculations are given in appendix C. The pump is used
to rises the absorbent solution from the 15 liter capacity
(QVF) vessel supported at the middle of column. The rig
was insulated with glass wool to maintain the absorbent
temperature nearly constant.
Calibrated rotameters were used to measure the air and
solvent flow rates.
2.2. Chemicals
For the generation and detection of SO2, Sodium sulfite,
Na2SO3 was supplied from Merck, Germany. Sulfuric
acid, H2SO4, iodine, I2, Iodine ampoule, 0.1 N, potassium
iodide, KI, starch and Sodium thiosulfate ampoule, 0.1 N
were analytical grade reagents from BDH, England. Urea
was an industrial grade product from a local fertilizer
factory.
For the generation and detection of NO2, nitric acid,
sodium hydroxide, phenolphthalein and copper were
supplied from BDH, England; Hydrogen peroxide, H2O2
was a local product.
Figure 1. A photograph of the experimental Rig.

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution177
2.3. Procedures
SO2 Generation was carried out by the reaction of so-
dium sulfite solution with sulfuric acid using various acid
concentrations (5 - 23 wt%). An air stream at constant
mass flux of (20.64 kg/m2·min) was then passed through
the reaction vessel to transport the generated SO2 gas
towards the packed bed absorber. Urea solution was al-
lowed to move downward from the top of the column at
constant mass flux (188 kg/m2·min).
NO2 was generated by the reaction of copper with ni-
tric acid of various concentrations (5 - 35 wt%). An air
stream at constant mass flux of (20.64 kg/m2·min) was
then passed through the reaction vessel to transport the
generated NO2 gas towards the packed bed absorber.
Urea solution was allowed to move downward from the
top of the column at constant mass flux (188 kg/m2·min).
Generation of SO2 and NO2 mixture:
Two reaction vessels were used simultaneously to
generate the two gases as in the procedure above and the
gases were directed at the same time by the same air
stream towards the absorption column. The outlet of the
column was connected to two different traps containing
the detection reagents of residual gases sequentially,
where the first absorber trap contains iodine solution for
SO2 and the second contains hydrogen peroxide for NO2.
2.4. Measurements of Absorption Rate
The weight of SO2 Absorbed was determined by measur-
ing the residual amount by its quantitative reaction with
iodine and titrating excess iodine with sodium thiosulfate.
The number of iodine equivalents is equal to the residual
SO2
in the trap.
The weight of NO2 absorbed by the quantitative
reaction with hydrogen peroxide to form nitric acid
which can be determined by titration with 0.01 N NaOH
solution using phenolphthalein as an indicator [15].
2.5. Experimental Design
Variables acting together may have greater or smaller
effect than individual variables acting alone. A response
surface can be most efficient fitted if proper attention is
given to the choice of experimental design [16]. Box-
Wilson, composite rotatable design is common type of
statistical experiments, especially applicable to optimiza-
tion analysis.
The effect of three variables for each acid gas such as,
liquid temperature, liquid concentration and concentra-
tion of SO2 and NO2 on the absorption rates were inves-
tigated and analyzed. The number of experiments needed
according to design are 15 plus 5 experiments at the cen-
ter point to estimate the experimental error.
For SO2 gas, the ranges of the operating variables are:
1) SO2 concentration, 500 - 2500 ppm (1310 - 6550
mg/ m3) = X1
2) Urea concentration, 0.1 - 0.5 mol/L = X2
3) Liquid temperature, 10 to 40˚C = X3
According to Equation (1) the relationships between
the coded levels and the corresponding real variables as
follows
2
1
1500
577 35
coded
con c.SO
X.

 (10)

2
03
01154
coded
Urea.concmol l.
X.

 (11)

3
C25
866
coded
Tempofsolution
X.



(12)
For NO2 gas, the ranges of the operating variables are:
1) NO2 concentration, 100 - 1000 ppm (191 - 1910
mg/ m3) = X1
2) Urea concentration, 0.1 - 0.5 mol/L = X2
3) Liquid temperature, 10 to 40˚C = X3
According to Equation (1) the relationships between
the coded levels and the corresponding real variables are
as follows
2
1
550
260
Coded
Conc.NO
X  (13)
2
03
0 1154
coded
Conc.urea.
X.

 (14)

3
C25
866
Coded
Temp
X.



(15)
2.6. For SO2 and NO2 Gases
1) SO2 concentration, 500 - 2500 ppm (1310 - 6550
mg/ m3) = X1
2) NO2 concentration, 100 - 1000 ppm (191 - 1910
mg/ m3) = X2
3) Urea concentration, 0.1 - 0.5 mol/L = X3
4) Liquid temperature, 10 to 40˚C = X4
2
1
1500
500
coded
Conc.SO
X  (16)
2
2
550
225
coded
Conc.NO
X  (17)
3
03
01
coded
Conc.urea .
X.

 (18)

4
C25
75
coded
Temp
X.



(19)
The values of the coded and real variables for the ab-
sorption experiments of SO2/NO2 mixtures are given in
Table 1.

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution
178
Table 1. Coded and real variables for SO2 and NO2 absorption experiments.
Coded Variables Real Variables
Run no. X1/X2/X3X4 ConSO2 (ppm) ConcNO2 (ppm) Urea. Conc. (gmol/l) Temp of Absorbent 0C
1 +1/+1/+1/[phone] 0.4 32.5
2 –1/+1/+1/[phone] 0.4 32.5
3 +1/–1/+1/[phone] 0.4 32.5
4 +1/+1/–1/[phone] 0.2 32.5
5 +1/+1/+1/–1 2000 775 0.4 17.5
6 –1/–1/–1/–1 1000 225 0.2 17.5
7 +1/–1/–1/–1 2000 225 0.2 17.5
8 –1/+1/–1/–1 1000 775 0.2 17.5
9 –1/–1/+1/–1 1000 225 0.4 17.5
10 –1/–1/–1/[phone] 0.2 32.5
11 +1/+1/–1/–1 2000 775 0.2 17.5
12 +1/–1/+1/–1 2000 225 0.4 17.5
13 +1/–1/–1/[phone] 0.2 32.5
14 –1/–1/+1/[phone] 0.4 32.5
15 –1/1/–1/1 1000 775 0.2 32.5
16 –1/1/1/–1 1000 775 0.4 17.5
17 2/0/0/0 2500 550 0.3 25
18 0/2/0/0 1500 1000 0.3 25
19 0/0/2/0 1500 550 0.5 25
20 0/0/0/2 1500 550 0.3 40
21 –2/0/0/0 500 550 0.3 25
22 0/–2/0/0 1500 100 0.3 25
23 0/0/–2/0 1500 550 0.1 25
24 0/0/0/–2 1500 550 0.3 10
25 0/0/0/0 1500 550 0.3 25
26 0/0/0/0 1500 550 0.3 25
27 0/0/0/0 1500 550 0.3 25
28 0/0/0/0 1500 550 0.3 25
29 0/0/0/0 1500 550 0.3 25
3. Results and Discussion
The measurement concerned the study of the effect of the
operation variables such as SO2 and NO2 level, urea
concentration and operating temperature on the absorp-
tion rate of acid gas and consequently their removal.
The absorption rate of an acid gas (NO2 or SO2) is ex-
pressed as the equivalent moles of NO2 or SO2 that react
with urea solution per unit interface area per unit time.
This absorption rates were calculated in accordance with
the expressions of Weisweiler and Dieb [17]. The ab-
sorption rates values for SO2 and NO2 are listed in the
Tables 2 and 3, respectively. The mass transfer coeffi-
cient for the packed bed column was estimated by using
the design equation for dilute gas mixture [18].
3.1. Effect of Operating Variables
3.1.1. Effect of Acid Ga s Co nce ntr ation
Figure 2 shows the effect of SO2 concentrations on the
absorption rate for various urea concentrations (0.1 - 0.5
mol/L at fixed absorbent solution temperature of 25˚C.
For NO2 the results are shown in Figure 3. The results
indicate that the absorption rate increases as the SO2
concentration increases. This can be attributed to the
greater driving force available as the SO2 concentration
increased and consequently higher absorption rate are
more easily attained. These results agree with published
report of Basu et al. [19], where the rate of absorption
increased directly with the driving force during the SO2
absorption in caustic soda and dimethylaniline solution.
The results of NO2 indicated that the absorption rate
increases sharply as the concentration of the gas increases.
This behavior may be expected because the reaction be-
tween NO2 and urea solutions in the liquid phase is very
fast. These results confirm the findings of Lefer et al.
[20].
Figure 4 shows the effect of NO2 concentrations on
the absorption rate for various working temperatures (10
- 40˚C) at fixed urea concentration of 0.185 M. For SO2
the results indicated a similar trend. The absorption rate
increases as the SO2 and NO2 concentrations in- creased.
Also, these figures indicate that the absorption rate is
more favored at lower temperatures. Thus the re- action
rate is higher at lower temperatures because the diffusiv-
ity increasing with decreasing temperature.

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution179
Table 2. Experimental results of SO2 absorption and values of interfacial area of packing.
Run No. SO2 input Conc.,
ppm SO2 Output Conc., ppm ap (m2/m3) a(m2) Na *10-3 (mol/m2.s) KG (kg/m3 s (kN/m 2)
1 922.6 156.842 175.3678 0.073 214 0.476 972 0.000 168 352
2 2077.6 276.3208 175.3678 0.073 214 1.121 972 0.000 191 671
3 922.6 52.957 24 175.4981 0.073 268 0.541 277 0.000 271 306
4 2077.6 49.654 64 175.4981 0.073 268 1.262 219 0.000 354 488
5 922.6 204.7618 183.8415 0.076 752 0.426 515 0.000 136 429
6 2077.6 386.2051 183.8415 0.076 752 1.004 968 0.000 152 494
7 922.6 107.0216 183.9731 0.076 807 0.484 242 0.000 195 092
8 2077.6 120.5008 183.9731 0.076 807 1.162 009 0.000 257 866
9 500 130 180.1686 0.075 218 0.224 323 0.000 124 574
10 2500 162.5 180.1686 0.075 218 1.417 175 0.000 252 774
11 1500 170.835 180.0638 0.075 174 0.806 313 0.000 201 026
12 1500 58.695 180.3642 0.075 3 0.872 884 0.000 299 381
13 1500 17.952 171.2891 0.071 511 0.945 113 0.000 430 476
14 1500 280.245 184.3118 0.076 948 0.722 887 0.000 151 648
15 1500 111 180.1686 0.075 218 0.842 12 0.000 240 782
16 1500 111 180.1686 0.075 218 0.842 12 0.000 240 782
17 1500 111 180.1686 0.075 218 0.842 12 0.000 240 782
18 1500 111 180.1686 0.075 218 0.842 12 0.000 240 782
19 1500 111 180.1686 0.075 218 0.842 12 0.000 240 782
20 1500 111 180.1686 0.075 218 0.842 12 0.000 240 782
Table 3. Experimental results of NO2 absorption and values of interfacial area of packing.
Run No. NO2 input Conc.,
ppm NO2 Output Conc., ppm ap (m2/m3) a(m2) Na*10-3 (mol/m2.s) KG(kg/m3.s( kN/m 2)
1 290 55.564 175.3678 0.073 214 0.148 201 0.000 113
2 810 125.631 175.3678 0.073 214 0.432 63 0.000 127
3 290 14.21 175.4981 0.073 268 0.174 214 0.000 206
4 810 21.06 175.4981 0.073 268 0.498 366 0.000 249
5 290 73.5266 183.8415 0.076 752 0.130 538 8.93E-05
6 810 176.5719 183.8415 0.076 752 0.381 971 9.92E-05
7 290 31.2098 183.9731 0.076 807 0.155 944 0.000 145
8 810 40.581 183.9731 0.076 807 0.463 645 0.000 195
9 100 20 180.1686 0.075 218 0.049 225 0.000 107
10 1000 39 180.1686 0.075 218 0.591 318 0.000 216
11 550 53.185 180.0638 0.075 174 0.305 876 0.000 155
12 550 28.7705 180.3642 0.0753 0.320 372 0.000 196
13 550 26.774 171.2891 0.071 511 0.338 638 0.000 211
14 550 102.7565 184.3118 0.076 948 0.269 009 0.000 109
15 550 23.65 180.1686 0.075 218 0.323 871 0.000 209
16 550 23.65 180.1686 0.075 218 0.323 871 0.000 209
17 550 23.65 180.1686 0.075 218 0.323 871 0.000 209
18 550 23.65 180.1686 0.075 218 0.323 871 0.000 209
19 550 23.65 180.1686 0.075 218 0.323 871 0.000 209
20 550 23.65 180.1686 0.075 218 0.323 871 0.000 209

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution
180
3.1.2. Effect of Urea Concentration
Figure 5 shows the effect of urea concentration on the
absorption rate of SO2 at various concentrations in the
range (500 - 2500 ppm) and fixed absorbent temperature
(25˚C). It is clear that as the urea concentration increased,
the absorption rate increases regularly. Also, a similar
effect is obtained at various working temperature (10 -
40˚C) and fixed SO2 concentration (1500 ppm). When
the urea concentration increased, the absorption rate in-
creases, this agrees with the findings in the cases of using
dimethylaniline as an absorbent as reported by Basu et al.
[27]. The activity of urea solution towards the removal of
sulfur dioxide may be due to the possible bonding of two
SO2 molecules to each urea molecule [21].
The results of NO2 absorption at various concentra-
tions (100 - 1000 ppm) and fixed absorbent temperature
(25˚C) showed a clear increase in the absorption rate as
the urea solution increases. However, beyond 0.4 mol/l
of urea, the absorption rate decreases especially at lower
acid gas concentration. This trend may be attributed to
the decrease the diffusivity of gas.
The effect of urea concentration on the absorption rate
of NO2 at different absorption temperature in the range
(10 - 40˚C) and constant nitrogen dioxide concentration
(500 ppm) is shown in Figure 6. It is clear that as the
urea concentration increased, the absorption rate in-
creases. The activity of urea solution toward the removal
of NO2 may be due to fast bonding of NO2 with urea
molecules.
3.1.3. Effect of Temperature of Absorbent
The effect of the temperature of the absorbent on the
absorption rate of SO2 is shown in Figure 7 at constant
acid gas concentration and various urea concentrations in
the range of (0.1 - 0.5) mol/l. The absorption rate is found
to decrease considerably with increasing the temperature.
At constant urea concentration and various NO2 concen-
tration in the range of SO2 (500 - 2500) ppm and NO2
(100 - 1000) ppm, the increase of absorbent temperature
lowers the absorption rate of the two acid gases. Such a
slightly decreasing rate at higher temperatures may be
attributed to the decrease of the solubility and diffusivity
of aid gases in urea solution as the temperature increased.
Figure 2. Absorption rate as a function SO2 concentration
for various urea concentration.
Figure 3. Absorption rate as a function NO2 concentration
for different urea concentration.
Figure 4. Absorption rate as a function NO2 concentration
for different Temperatures of absorbent.
Figure 5. Absorption rate as a function urea concentration
for different SO2 concentration.

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution181
Figure 6. Absorption rate of NO2 as a function urea con-
centration for various temperatures.
Figure 7. Absorption rate of SO2 as a function of tempera-
ture of absorbent for various urea concentrations.
3.1.4. Effect of NO2 on SO2 Absorption Rate
Figure 8 shows the absorption rate of SO2 in the pres-
ence of various NO2 concentrations at fixed temperature
and urea concentration. There existed a minor decrease in
the absorption rate decrease with the increase of NO2
concentration. Table 4 shows the experimental data and
values of interfacial area of packing for NO2 and SO2.
This slight decrease may be due to the fact that the rate
of reaction of urea is faster with NO2 than that with SO2.
3.2. Mass Transfer Coefficient in Packed Tower
The majority of published results for mass transfer coef-
ficients in packed towers are for rather small laboratory
units of 50 - 250 mm diameter, and there is still some un-
certainty in extending these data for use in industrial
units. One of the great difficulties in correlating the per-
formance of packed towers is the problem of assessing
the effective wetted area for interface transfer. It is con-
venient to consider separately the conditions where the
gas film controls the process, and then where the liquid
film controls [18].
3.2.1. Postulating the Model
The mass transfer correlation is usually determined by
using Buckingham method for dimensional analysis. The
mass transfer coefficient is a function of more variable
effect on this important parameter in packed bed column.
The basic equation relating the variables is

GABg

g
K
fdpDG H

  (20)
The variable and the dimensional constant believed to
be involved and their dimensions in the engineering sys-
tem are given below
Mass transfer coefficient G
K
LT (21)
Gas density

g
Gas viscosity

g

M
L.T  (23)
Diffusivity of gas

2
AB
DLT

(24)
Packing size (25)

dp L
Height of packing

H
L (26)
Mass velocity of gas

2
GMT.L


(27)
According to Buckingham theory
73 4Pnm

 (28)
P is equal to number of dimensionless group
Choosing the three virtual variable
dpLLdp

 (29)
22
AB AB
DLTTdpD (30)
3
g
3
g
M
LM dp  (31)


1GA
KdpDB

this dimension is Sherwood number
(32)
Figure 8. Absorption rate as a function SO2 concentration
for various NO2 concentrations.

M
L  (22)

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution
182
Table 4. Experimental results of SO2 and NO2 absorption and values of interfacial area of packing.
Run Inlet SO2
ppm
Outlet SO2
ppm
Inlet NO2
ppm
Outlet NO2
ppm
ap
m2/m3
am
m2
Na SO2
mmol/m2·s
Na NO2
mmol/m2·s
Kg (SO2)
m/s
Kg (NO2)
m/s
1 2000 398.6 775 158.8 184.0660.076 840.9503 0.3710 0.000 146 0.0001
2 1000 297.5 775 141.1 184.0660.076 840.4168 0.3823 0.000 11 0.0001
3 2000 294 225 56.09 184.0660.076 841.0124 0.1061 0.000 174 9.03E-5
4 2000 650.5 775 247.2 183.7940.076 730.8020 0.3317 0.000 102 7.44E-05
5 2000 121.2 775 106.0 176.4700.073 671.1629 0.4205 0.000 265 0.000 135
6 1000 262.5 225 57.84 176.3520.073 620.4568 0.1050 0.000 126 9.22E-05
7 2000 321.2 225 80.28 176.3520.073 621.0398 0.0910 0.000 173 6.99E-05
8 1000 327.5 775 148.8 176.4700.073 670.4162 0.3774 0.000 105 0.000 112
9 1000 169 225 206.1 176.2310.073 570.5150 0.0118 0.000 168 5.94E-06
10 1000 338.7 225 68.68 183.9160.076 780.3927 0.0983 9.81E-05 7.72E-05
11 2000 461.2 775 222.2 176.4700.073 670.9524 0.3328 0.000 139 8.47E-05
12 2000 60 225 39.78 176.2310.073 571.2024 0.1117 0.000 332 0.000 118
13 2000 511.2 225 87.39 184.0660.076 840.8835 0.0828 0.000 123 6.15E-05
14 1000 247.5 775 59.17 183.7940.076 730.4472 0.1041 0.000 127 8.7E-05
15 1000 390 775 197.3 184.0340.076 830.3620 0.3554 8.52E-05 8.9E-05
16 1000 200 775 41.23 176.4270.073 650.4953 0.4510 0.000 152 0.000 199
17 2500 345 550 134.7 180.1910.075 221.3063 0.2557 0.000 183 9.34E-05
18 1500 367.5 1000 194.2 180.3640.075 30.6858 0.4761 0.000 13 0.000 109
19 1500 82.5 550 20.68 179.9710.075 130.8603 0.3256 0.000 269 0.000 218
20 1500 407.5 550 145.9 187.6020.078 320.6360 0.2487 0.000 116 8.46E-05
21 500 113.5 100 57.97 180.1910.075 220.2342 0.3026 0.000 137 0.000 149
22 1500 156.6 550 17.79 180.0630.075 170.8148 0.0530 0.000 209 0.000 115
23 1500 442.5 550 162.7 180.2010.075 230.6410 0.2382 0.000 113 8.09E-05
24 1500 97.5 550 23.87 171.9100.071 770.8911 0.3236 0.000 265 0.000 218
25 1500 265.5 550 85.47 180.1910.075 220.7483 0.2857 0.000 16 0.000 124
26 1500 265.5 550 85.47 180.1910.075 220.7483 0.2857 0.000 16 0.000 124
27 1500 265.5 550 85.47 180.1910.075 220.7483 0.2857 0.000 16 0.000 124
28 1500 265.5 550 85.47 180.1910.075 220.7483 0.2857 0.000 16 0.000 124
29 1500 265.5 550 85.47 180.1910.075 220.7483 0.2857 0.000 16 0.000 124

2

g
gAB
D

 this dimension is Schmidt number
(33)

3

g
Gdp




this dimension is Reynolds number
(34)

4dp H


(35)
These four dimensionless groups are frequently used
in mass transfer coefficient correlation. Functionally, their
relation may be expressed as
 
,,, 0Sh ReScdp H

 (36)
 
1
Or as,,ShReScdpH
It has been found that these dimensionless groups may
be correlated well by equation of the type


 
ab
c
GAB gggAB
K
dp DKGdpDdpH

 
(38)
In which K, a, b, c are experimentally determined by
using Statistical- software Windows version 5.5.

 
08660599085
0
2
419 10..
.
gggAB
Sh.G dpDdpH

 
(39)
Finally the model can be expressed as in the equation:

12 208706
41910
...
Sh H dp.ReSc

 0 (40)
 
(37)

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution183
.
Correlation coefficient, R2 = 0.9838
Variance explained, S = 96.789%
3.2.2. Comparison of the Model with Other
Correlation
The proposed model expressed by Equation (40) is com-
pared with Sherwood equation [22];
083 044..
ShRe Sc

  (41)
where the β' = 0.021 to 0.027 a mean value of 0.023 is
used; The Kramer’s equation [23]:
059 033
0 069. Sh.Re Sc (42)
and Weisweiler, et al. [32] equation:

08 13
03 . Sh.ReScdpH

(43)
These equations were applied for the operating vari-
ables of the present work for acid gas concentration be-
tween (100 - 2500) ppm and temperature range (10 -
40)˚C. The comparison indicated a reasonable agreement
with Kramer’s equation while deviated strongly with
those obtained by Weisweiler using the Sherwood Equa-
tion (41). The absorption rate values obtained in the pre-
sent work for various acid gases concentrations and tem-
pera- tures were in the range of 0.320 × 10-3 - 1.16 × 10-3
mol/m2·s. These values compares well with the published
data from several investigators using various absorption
designs [17,20,24-27]. The details of the comparison can
be seen in Table 5.
4. Conclusions
The following conclusions can be drawn from the present
work:
1) The absorption rate of SO2 increases when the tem-
perature of absorbent (urea solution) decreases and in-
creases as the concentration of absorbent solution.
2) The presence of NO2 in the effluent gas stream in
addition to SO2 decreases the absorption rate of SO2 in
urea solution this may be due to the fast reaction of NO2
with urea as compared with SO2.
Table 5. Comparison of absorption rate values with some published results.
Reference Acid gas con ppm TEMP. ˚C Absorption rate*10−3 mol/m2·s Contact Pattern
250 25 1.114
Dekker et al. (1959) 250 35 1.003 Wetted wall column
250 20 0.77
Kramers et al. (1961) 250 30 0.891 Packed bed absorber
Kameoka and Pigford (1977) 250 25 0.685 Wetted sphere absorber
Weisweiler and Deib (1981) 250 25 0.486 Falling film absorber
250 20 0.496
Lefers and Berg (1982) 400 20 0.182 Wetted wall column
670 30.3 1.562
285 38.5 1.903
85 37.5 2.411
Bubble cap plate column
580 30.8 0.826
390 33.2 0.871
181 32.9 1.464
Miller (1987)
110 30.6 2.411
Sieve plate column
150 25 0.476
150 35 1.394
450 25 0.105
450 35 0.333
Weisweiler
et al. (1990)
600 25 0.018
Mixed column
1500 10 0.94
922.6, 2077 16.5 0.47, 1.12
1500 25 0.8412
922.6,2077.6 33.6 0.42, 1.16
1500 40 0.722
550 10 0.338
290, 810 16.5 0.148, 0.43
550 25 0.3205
This
Work
290,810 33.6 0.1305, 0.381
Packed bed
550 40 0.269

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution
184
3) The absorption rate of NO2 decreases at higher urea
concentration 0.4 mol/l for low gas concentration ppm
due to the decrease the diffusivity of gas.
4) The variables that affect the absorption rates of SO2
and NO2 in urea solution can be formulated using the
dimensionless group analysis and may be expressed by
the following equation
Sh (H/dp)1.2 = 4.19*10−2 Re0.87 Sc0.56
This model agrees well with Kramer’s Equation (42)
with a correlation coefficient, R, of 0.9838.
5. Acknowledgements
One of us (M.M.B.) is grateful for the Institute of Inter-
national Education, IIE, for granting him a scholarship as
a visiting Professor at Montclair State University in New
Jersey.
REFERENCES
[1] T. Denmead, B. Macdonald, I. White, G. Bryant, D. Grif-
fith, A. Kinsela and M. Melville, “Links between Emis-
sions of Nitrogen and Sulfur Gases from Acid Sulfate
Soils: Field Evidence,” 19th World Congress of Soil Sci-
ence, Soil Solutions for a Changing World, Brisbane,
2010.
[2] P. G. Cheremisionff and R. A. Young, “Air Pollution
Control and Design Hand Book,” Marcel Dekker, Inc.
New York, 1977.
[3] C. M. Hansen, “Control of Sox Emission,” Industrial and
Engineering Chemistry Product Research and Develop-
ment, Vol. 16, 1977, p. 266. doi:10.1021/i360063a016
[4] V. S. Soldatov, A. V. Kashinskii, T. A. Korshunova and
V. I. Martinovich, “Removal of Nitrogen Dioxide from
Air Using Aqueous Carbamide on a Fibrous Ion Ex-
changer,” Chemistry and Material Science, Vol. 41, No.
(Suppl. 2), 2005, pp. 760-763.
[5] U.S.DOE, “Advance Technology for the Control of Sul-
fur Dioxide Emission from Coal-Fired Boilers,” Clean
Coal Technology Conference, Knoxville, 1999.
[6] B. H. Potter and T. L. Craig, “Control Sulfur Oxide,”
Chemical Engineering Programme, Vol. 68, No. 8, 1972,
p. 53.
[7] F. S. Chalmer, “Evaluation of Regenerable FGD Proc-
ess,” AIChE Journal, Vol. 23, No. 4, 1977, pp. 538-544.
[8] E. Bekassy-Molnar, E. Marki and J. G. Majeed, “Sulphur
Dioxide Absorption in Air-Lift-Tube Absorbers by So-
dium Citrate Buffer Solution,” Chemical Engineering and
Processing, Vol. 44, No. 9, 2005, pp. 1039-1046.
doi:10.1016/j.cep.2005.02.001
[9] J. H. Russel, J. I. Paige and D. L. Paulson, “Evaluation of
Some Solid Oxides as Sorbent of SOx,” Rep Invest-U.S
Bur. Mines R1 7582, 1971.
[10] P. G. Gray, “A Fundamental Study on the Removal of Air
Pollutants (Sulfur Dioxide, Nitrogen Dioxide and Carbon
Dioxide) by Adsorption on Activated Carbon,” Gas
Separation & Purification, Vol. 7, No. 4, 1993, pp. 213-
224. doi:10.1016/0950-4214(93)80020-W
[11] P. Daniell, A. Soltani-Ahmadi and H. O. Kono, “Reaction
Kinetics of the SO2-CaO System-Pore Closure Model,”
Powder Technology, Vol. 55, 1988, pp. 75-78.
doi:10.1016/0032-5910(88)80090-1
[12] EPAU.S Environmental Protection Agency, “Alternative
Control Techniques Document-Nitric and Adipic Acid,”
EPA Publication, No. 450/3-91-026, 1991.
[13] European Commission, “Technical Note on Best Avail-
able Technologies Not Entailing Excessive Costs for
Heavy Metal Emissions from Non-Ferrous Industrial
Plants,” Final Report - May 1991, ECSC-EC-EAEC,
Brussels Luxembourg, 1994.
[14] S. H. Zhang, L. L. Cai, X. H. Mi, J. L. Jiang and W. Li,
“NOx Removal from Simulated Flue Gas by Chemical
Absorption - Biological Reduction Integrated Approach
in a Biofilter,” Environment Science and Technology, Vol.
42, No. 10, 2008, pp. 3814-3820.
doi:10.1021/es800200g
[15] G. A. Streuli and P. R. Averell, “The Analytical Chemis-
try of Nitrogen and It Is Compound, Part I,” John Wiley
and Sons. Inc, New York, 1970.
[16] D. G. Montgomery, “Design and Analysis of Industrial
Experiments,” John Wiley and Sons, New York, 1976.
[17] W. Weisweiler and K. H. Deib, “Measurement of Ab-
sorption Rate of NO2 and SO2 in Water in a Falling Film
Absorber,” Chemistry Engineering, Vol. 4, 1981, p. 79.
[18] J. M. Coulson and J. F. Richardson, “Chemical Engi-
neering,” Vol. 2, 4th Edition, Pergamon Press, Oxford,
1991.
[19] R. K. Basu and B. K. Dutta, “Kinetic of Absorption of
Sulfur Dioxide in Dimethylaniline Solution,” Canadian
Journal of Chemistry Engineering, Vol. 65, No. 1, 1987,
pp. 27-35. doi:10.1002/cjce.5450650106
[20] J. B. Lefers and P. I. Berg, “Absorption of NO2-N2O4 –
SO2 in to Dilute and Concentrated Aqueous Solution,”
Chemistry Engineering Journal, Vol. 23, 1981, p. 211.
[21] R. E. Khoma, M. I. Gavrilenko and V. I. Nikitin,
“Interaction of Sulfur Dioxide with Aqueous Solutions of
Amides,” Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005,
pp. 771-777.
[22] T. K. Sherwood, R. L. Pigford and R. L. Wilke, “Mass
Transfer,” McGraw-Hill, New York, 1975.
[23] H. Kramers, M. P. Blind and E. Sneck, “Absorption of
NO2, SO2, N2O4 by Water Jet,” Chemistry Engineering
Science, Vol. 14, No. 1, 1961, pp. 115-123.
doi:10.1016/0009-2509(61)85062-8
[24] W. Weisweiler, K. Eidam, M. Thiemann and K. W. Wie-
gand, “Absorption of NO2/N2O4 in Nitric Acid, Gas,”
Chemistry Engineering and Technology, Vol. 13, No. 3,
1990, pp. 97-101. doi:10.1002/ceat.270130113
[25] W. A. A. Dekker, E. Snoeck and Kramer’s. “The Rate of
Absorption of Acid Gas in Water and Aqueous Solution,”
Chemistry Engineering Science, Vol. 11, 1951, p. 61.
doi:10.1016/0009-2509(59)80073-7

Removal of Nitrogen Dioxide and Sulfur Dioxide from Air Streams by Absorption in Urea Solution185
[26] Y. Kameoka and R. L. Pigford, “Absorption of NO2 in
Water Sulfuric Acid, Sodium Hydroxide and Alkaline So-
dium Sulfite Aqueous Solution,” Industrial & Engineering
Chemistry Fundamentals, Vol. 16, No. 4, 1977, p. 153.
[27] D. N. Miller, “Mass Transfer in Nitric Acid,” AIChE
Journal, Vol. 33, No. 8, 1987, p. 1351.
doi:10.1002/aic.690330812