Ind. Eng. Chem. Res. 2000, 39, 5003-5011
5003
Absorption of Nitrogen Oxides in Columns Equipped with
Low-Pressure Drops Structured Packings
Edoardo Decanini, Giuliano Nardini, and Alessandro Paglianti*
Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa,
I-56126 Pisa, Italy
The absorption of nitrogen oxides was investigated experimentally in a column equipped with
HelieR structured packing for different values of operative conditions (NOx inlet concentration,
specific gas, and liquid flow rate). The experimental data were compared with the absorption
efficiency predicted by three models, the first obtained by the open literature and the others
developed in this work. Some conclusions about the relative importance of the different masstransfer mechanisms involving the absorption process were deduced.
Introduction
The study of absorption into water of mixtures of
various nitrogen oxides, which for the sake of the
brevity, are named NOx, has attracted considerable
interest because this process is fundamental in the
production of nitric acid and in other important processes. Furthermore, the removal of NOx from industrial
gas streams has received increasing attention because
of a stringent body of legislation for air pollution control
and reduction: for instance, the abatement of so-called
NOx fuel and NOx thermal generated in a burning plant,
the scrubbing of gas produced in the recovery of precious
metals by treatment with nitric and hydrochloric acid
solutions, and the NOx recovery from gas produced in
organic nitration. The removal of NOx from industrial
gas streams can also be achieved by means of alternative technologies such as selective catalytic reduction
(SCR), which employs ammonia, and nonselective thermic reduction (NSTR), which employs gaseous hydrocarbons; these technologies, however, are not as advantageous as absorption.
Moreover, the study of NOx absorption is really
important also from a theoretical point of view because
it is surely one of the most complex absorption process:
in fact, the NOx absorption involves numerous masstransfer mechanisms with reactions in both liquid and
gas phases.
Several authors, who have studied the mechanisms
of NO and NO2 absorption in water and in nitric acid
solutions at different concentrations, have contributed
to the study of the process to clarify its mechanisms
(Andrew and Hanson,1 Corriveau and Pigford,2 Dekker
et al.,3 Koval and Peters,4 Kramers et al.,5 Lee and
Schwartz,6 Lefers and van den Berg,7 Weisweiler and
Deiss,8); these authors valued the mass-transfer kinetics
between liquid and gas phases of two oxides and of their
mixtures. Bodestein9 studied the oxidation in the gas
phase of NO to NO2 while Crawford and Counce10 and
Komiyama and Inoue11 investigated the decomposition
kinetics of HNO2 produced by hydration of the nitrogen
oxides absorbed in water; the latter authors also studied
* To whom correspondence should be addressed: Department of Chemical Engineering, University of Pisa, Via Diotisalvi n. 2, I-56126 Pisa, Italy. E-mail: [email protected].
Phone: +39-050-511225. Fax: +39-050-511266.
the transport through the liquid phase of NO produced
from HNO2 decomposition. In the literature special
attention has also been given to the development of
absorption models that allow us to simulate the process
in plate columns, in packed columns, and in spray
towers (Jethani et al.,12 Ramanand and Phaneswara,13
Suchak et al.,14 Suchak and Joshi,15) developing detailed
models to take into account all possible absorptions.
Finally, some other authors, such as Counce and Perona16 and Selby and Counce,17 evidenced the importance of the mechanisms of absorption that involves NO2
and N2O4, in comparison to the other mechanisms,
giving a simplified model of the process.
The aim of the present work has been to study
experimentally the process of absorption of NO2 in water
working with columns filled with low-pressure drop
structured packing (Launaro and Paglianti18).
Experimental Apparatus and Procedures
The experimental loop used in this work is schematically described in Figure 1. The air has been supplied
by a compressor and, before entering a column, has been
mixed in a NO2 stream extracted from a cylinder: the
NO2 stream has been heated and diluted with technical
air to avoid its condensation (Teb ) 21 °C). The air flow
rate has been measured with a rotameter and has been
regulated with a manual valve while the dioxide flow
rate has been measured and controlled, employing a
mass flowmeter and controller. In present experiments
only NO2 (in equilibrium with its dimer N2O4), mixed
with air, has been fed to the absorption column without
introducing, since the beginning, others kind of nitrogen
oxides. A manual valve (V1) has been set on the pipe
outlet to regulate the absolute pressure in the equipment. The gas temperature has been measured with two
thermometers on the inlet and the outlet flow lines; also,
the relative humidity was measured by means of two
hygrometers.
The water for absorption has been pumped from the
tank D1 by means of a centrifugal pump and has been
fed to the column C; the water has been discharged from
the bottom after flowing countercurrently through the
column with respect to gas. The water flow rate has
been measured by means of a rotameter and has been
regulated by means of a manual valve, V2. The inlet
and outlet liquid temperatures have been measured
with two thermometers.
10.1021/ie000270q CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/04/2000
5004
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
Figure 3. Experimental absorption data at various NOx inlet
concentrations and for different values of specific liquid flow rates
(kg/m2‚h) at constant specific gas flow rate ug ) 0.415 m3/m2‚s.
Figure 1. Experimental loop. Symbols: C, absorption column;
D1, D2, storage tank; FC, flow controller; FI, flow indicator; P1,
centrifugal pump; P2, fan; PI, pressure indicator; V1, V2, valve.
An analysis of sampling solutions has been done
employing an ionic chromatograph liquid-phase DIONEX 2000: the analysis equipment allows one to
measure the concentrations of sodium nitrites and
nitrates; knowing the solution volume and the sampling
time, it has been possible to evaluate nitrogen oxides
concentrations and their partial pressures in the gas
phase. Therefore, to quantify the absorption for each
experiment, the following indices have been estimated:
η)
(in)
(out)
- pNO
pNO
x
x
(in)
pNO
x
NTU/OG
∫
1
)
Z
(out)
pNO
x
(in)
pNO
x
0
dpNO
x
0
-pNO
x
(in)
pNO
x
1
) log (out) (1)
Z
p
NOx
Figure 2. Single HelieR structured packing element.
The first index expresses the efficiency of absorption,
while the second one, tightly related to the first one, is
a modification of the number of transfer unity definition
NTUOG because, for the absorption of nitrogen oxides,
the current definition is not applicable because the
process takes places with different mass-transfer mechanisms.
Table 1. Geometric Characteristics of HelieR Packing
Experimental Work and Results Obtained
property
value
diameter
thickness
weight
specific area
void fraction
elements per unit volume
1.5 in.
1.2 mm
7.34 g
210 m2/m3
0.936
17511 1/m3
The absorption column has been filled with 25 elements of HelieR structured packing: the overall height
of packing has been 0.985 m, and the elements have
been connected in the 3 × 3 configuration according to
the indications furnished by Launaro and Paglianti.18
A single element of structured packing is represented
in Figure 2 while Table 1 shows some geometric details.
Measurements of absorption efficiency have been
performed using two sampling points on the gas line:
each gas sample has been drawn off the sampling point,
after being measured, and has been sent to a bubbler
containing an absorption solution of 0.1 M NaOH in
water: in this way the NOx in the gas has been fixed in
solution like sodium nitrites and nitrates.
The experiments of absorption have been conducted
at room conditions (P = 1 atm and T ) 293 K) and the
specific flow rates of gas and liquid phases have been
chosen in the range of values commonly employed in
industrial practice, assuming ug ) 0.197-1.26 m3/m2‚
s) and ul ) 4900-39 500 kg/m2‚h. The concentrations
of NOx introduced into column have been assumed
instead in the range cNOx ) 233-60 676 mg/m3. During
the experimental work no significant temperature variations have been measured between inlet and outlet gas
and liquid streams for each run so it can be considered
that the column operated at isothermal conditions: it
can be deduced that heat effects caused by absorption
and evaporation inside the column are not significant
in the range of operative conditions used in this work.
The experimental results are reported in Figures 3
and 4: it is important to point out that the abatement
efficiency of nitrogen oxides depends sensitively on the
working conditions; particularly, it has been found that
the absorption efficiency increases with decreasing
specific gas flow rate and increases with increasing both
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 5005
All the reactions and all the mass-transfer mechanisms that have been taken into account to describe the
whole process of absorption are shown in Figure 5.
Mass Transfer between the Phases and
Reactions in the Liquid and Gas Phase
The molar flow through gas film for a cubic meter of
column volume can be written as
Figure 4. Experimental absorption data at various NOx inlet
concentrations and for different values of specific gas flow rates
(m3/m2‚s) at constant specific liquid flow rate ul ) 19 800 kg/m2‚
h.
Figure 5. NOx absorption mechanisms into water.
the specific liquid flow rate and concentration of nitrogen oxides at the column inlet. The reduction of the
abatement efficiency with increasing specific gas flow
rate can be attributed to the reduction of contact time
between the two phases: in this case this effect prevails
on the increase of gas-side mass-transfer coefficients due
to higher volumetric specific flow rate. The increase of
absorption efficiency with increasing the specific liquid
flow rate depends both on the increase of the liquidside mass-transfer coefficients but, mainly, on the
increase of the effective interfacial area. Finally, the
increase of the abatement efficiency with nitrogen oxides
concentration at the column inlet is a characteristic of
the absorption process of nitrogen oxides: in fact,
increasing the overall nitrogen oxides concentration in
the gas phase, the partial pressures of more soluble
species, and particularly N2O4, increase (Counce and
Perona16).
Process Model
Developing a model of the process is necessary to
introduce some simplifying hypotheses as follows:
i. The gas and liquid phases flow countercurrently
through the apparatus approaching a plug flow.
ii. The liquid holdup is uniform and mass-transfer
coefficients are constant throughout the column.
iii. The gas follows the ideal gas law.
iv. The absorption in the column is isothermal: the
raising of temperature owed to the dissolution and the
reaction with the water of the chemical species absorbed
is neglected; this hypothesis is entirely allowed because
of the low concentrations of NOx in present absorption
experiments.
vi. The column operates at steady state.
JNO,g ) kg,NOa[piNO - poNO]
(2)
o
i
- pNO
]
JNO2,g ) kg,NO2a[pNO
2
2
(3)
o
i
JN2O4,g ) kg,N2O4a[pN
- pN
]
2O4
2O4
(4)
o
i
JN2O3,g ) kg,N2O3a[pN
- pN
]
2O3
2O3
(5)
o
i
JHNO3,g ) kg,HNO3a[pHNO
- pHNO
]
3
3
(6)
o
i
JHNO2,g ) kg,HNO2a[pHNO
- pHNO
]
2
2
(7)
i
o
- pH
]
JH2O,g ) kg,H2Oa[pH
2O
2O
(8)
It is important to point out that NO is desorbed from
the liquid phase (see eq 8) contrarily to all other oxides.
In fact, in the present experimental work, the gas phase
fed to the column is constituted by air mixed only with
NO2 (in equilibrium with N2O4) and NO is produced only
by the reaction nitrous acid decomposition.
In liquid-phase NO2, N2O4 and N2O3 react with water
as follows:
2NO2(l) + H2O(l) f HNO2(l) + HNO3(l)
(9)
N2O4(l) + H2O(l) f HNO2(l) + HNO3(l)
(10)
N2O3(l) + H2O(l) f 2HNO2(l)
(11)
while HNO2 and HNO3 are absorbed only physically.
The flow of these species through the liquid film can be
written as
i
JNO2,l ) ENO2kl,NO2acNO
)
2
a(HNO2)3/2
x23k
i
3/2
idr,NO2DNO2,l(pNO2)
(reaction order n ) 2) (12)
i
JN2O4,l ) EN2O4kl,N2O4acN
)
2O4
i
aHN2O4xkidr,N2O4DN2O4,l(pN
)
2O4
(reaction order n ) 1) (13)
JN2O3,l )
i
EN2O3kl,N2O3acN
2O3
)
i
)
aHN2O3xkidr,N2O3DN2O3,l(pN
2O3
(reaction order n ) 1) (14)
b
i
JHNO2,l ) kl,HNO2a(cHNO
- cHNO
)
2
2
(15)
i
b
- cHNO
)
JHNO3,l ) kl,HNO3a(cHNO
3
3
(16)
JNO,l ) kl,NOa(cbNO - ciNO)
(17)
5006
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
Table 2. Kinetic Constants for Hydrolysis Reactions in
the Liquid Phase
concentration in the gas and liquid phases that can be
expressed by means of Henry’s law as follows:
kidr,NO2 ) 1 × 105 m3/mol‚s (Lee and Schwartz6)
ciNO ) HNOpiNO
HN2O4xDl,N2O4kidr,N2O4 ) 3.9 × 10-5 mol/m2‚s‚Pa (Schifano20)
HN2O3xDl,N2O3kidr,N2O3 ) 1.57 × 10-5 mol/m2‚s‚Pa (Corriveau2)
In the present work the acceleration factors of the
equations (12)-(14) have been computed as suggested
by Doraiswamy and Sharma,19
Ej )
xΦj = Φ
x j
tanhxΦj
(18)
xΦj )
i
i
cN
) HN2O4pN
2O 4
2O4
i
i
cN
) HN2O3pN
2O 3
2O3
i
i
cHNO
) HHNO2pHNO
2
2
i
i
cHNO
) HHNO3pHNO
3
3
where for pseudo n-order reactions is
i
pH
) f(T)
2O
x
(n-1)/2
(Hj pij)
i
i
cNO
) HNO2pNO
2
2
2
D k
n + 1 j,l idr,j
kl,j
The approximation in (18) is valid because for the three
oxides it can be verified that xΦj > 3 so that tanhxΦj
= 1.
The kinetic constants used in the present work are
shown in Table 2.
It is important to point out that it has been supposed
that the mass transfer of NO2, N2O4, and N2O3 through
liquid film depends only on the concentration, and so
on the partial pressure, that it is assumed to be close
to the interface, neglecting possible presence inside the
liquid phase. To verify that this hypothesis is correct,
it is enough to evaluate the equilibrium partial pressures of three oxides when the concentration of nitric
and nitrous acids in the liquid phase is the same as that
in experimental work. The concentration of two acids
in the liquid phase is known by chromatographic
analysis and the equilibrium partial pressures of three
oxides can be evaluated by means of constants of
heterogeneous equilibrium for reactions of the formation
of nitric and nitrous acids in the liquid phase [(9)-(11)]
(see Joshi et al.21): the resulting equilibrium pressures
are <1% of the respective pressures at the interface. It
can be concluded that it is correct to neglect the
concentrations in the bulk of the liquid phase, writing
mass-transfer flow of three oxides through liquid film
as a function of only interface conditions.
To compute mass-transfer flow of each chemical
species through the gas-liquid interface, it is now
enough to write the continuity equations:
JNO,g ) JNO,l
(19)
JNO2,g + 2JN2O4,g ) JNO2,l + 2JN2O4,l
(20)
JN2O3,g ) JN2O3,l
(21)
JHNO2,g ) JHNO2,l
(22)
JHNO3,g ) JHNO3,l
(23)
To solve the above equations system, it is necessary to
know some equilibrium conditions on the interface
(24)
The mass transfer of water through the liquid film has
not been taken into account because it has been supposed that its partial pressure of the interface was equal
to the equilibrium partial pressure in the bulk of liquid
at working temperature. It has also been supposed that,
in the gas film, NO2 and N2O4 are in equilibrium;
therefore,
Keq )
i
pN
2O4
(25)
i
(pNO
)2
2
The equations (2)-(25) allow one to determine the masstransfer flow for each chemical species through the
interface.
In the solution with water the nitrous acid decomposes according to the following reaction (Komiyama
and Inoue11):
3HNO2 f HNO3 + 2NO + H2O
(26)
and the decomposition kinetics is given by
Rdec,HNO2 ) kdec,HNO2
b
(cHNO
)4
2
(cbNO)2
(27)
where the kinetic constant assumes the value kdec,HNO2
) 3.93 × 10-6 L/mol‚s (Abel et al.22).
In the gas phase NO is oxidized by the oxygen
according to the following reaction:
1
NO(g) + O2(g) f NO2(g)
2
(28)
At the working temperature this reaction can be considered irreversible. Reaction kinetics can be described
by the equation
Rox,NO )
kox,NO o 2 o
(p ) p RT NO O2 g
(29)
where the kinetic constant is given by Bodestein:9
log10 kox,NO )
652.1
- 0.7356 (atm-2 s-1)
T
(30)
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 5007
Several equilibria exist between nitrogen oxides in the
gas phase (see Figure 5); the following equilibrium
equations allow one to evaluate partial pressures in the
bulk of gas for each component,
and for the liquid phase,
b
dcHNO
2
)
dz
S1
+ JN2O4,l + 2JN2O3,l + JHNO2,l - Rdec,HNO2
J
L 2 NO2,l
(40)
[
∆Gof,NO(T) + RT log yNO + λN + λO ) 0
o
(T) + RT log yNO2 + λN + 2λO ) 0
∆Gf,NO
2
b
dcHNO
3
o
(T) + RT log yN2O4 + 2λN + 4λO ) 0
∆Gf,N
2O4
o
(T)
∆Gf,N
2O3
]
dz
-
+ RT log yN2O3 + 2λN + 3λO ) 0
)
1
S1
+ JN2O4,l + JHNO3,l - Rdec,HNO2 (41)
J
L 2 NO2,l
3
[
]
dcbNO
S2
- JNO,l
)R
dz
L 3 dec,HNO2
[
o
(T) + RT log yHNO2 + λN + 2λO + λH ) 0
∆Gf,HNO
2
o
(T) + RT log yHNO3 + λN + 3λO + λH ) 0
∆Gf,HNO
3
o
(T) + RT log yH2O + λO + 2λH ) 0
∆Gf,H
2O
(31)
To resolve the differential equations system (33)-(42),
it is necessary to specify the boundary conditions, which
for a column working countercurrently, are
{
yNO + yNO2 + 2yN2O4 + 2yN2O3 + yHNO2 + yHNO3 ) ytot
N*
o
o
o
poNO ) pN
) pHNO
) pHNO
)0
2O3
2
3
o
pH
2O
yH2O ) ytot
O*
tot
yHNO2 + yHNO3 + 2yH2O ) yH*
(32)
Mass Balances in Liquid and Gas Phases
The above equations allow one to estimate the flow
of each species through the interface and the conditions
of chemical equilibrium in the gas; now, it is possible
to write the equations of mass balance over a differential
section dz of column; for the gas phase,
dz
dyN2O4
dz
dyN2O3
dz
dyHNO2
dz
dyHNO3
dz
dyH2O
))-
S
- Ross,NO]
[J
G NO2,g
S
‚J
G N2O4,g
S
) - ‚JH2O,g
dz
G
{
(out)
cacids
)
(45)
for z ) Z
cbNO
+
{
and
b
cHNO
2
(in)
yNO
)
x
(35)
(37)
S
‚J
G HNO3,g
where
(34)
S
‚J
G HNO2,g
)-
(in)
(out)
(in)
(out)
+ Lcacids
) GyNO
+ Lcacids
GyNO
x
x
(33)
(36)
(44)
It is important to point out that in the present experiments the water used for absorption does not contain
dissolved nitrogenous compounds.
The integration of model equations allows one to
estimate the concentrations of nitrogenous compounds
both in a liquid and in a gas at the outlet of the column,
knowing the inlet conditions. Moreover, the results
obtained must satisfy the general mass-balance equation on packing that can be written as
(in)
cacids
)0
S
) - ‚JN2O3,g
G
)-
)
(43)
o,(in)
pH
2O
b
b
) cHNO
)0
for z ) Z w cbNO ) cHNO
2
3
yNO + 2yNO2 + 4yN2O4 + 3yN2O3 + 2yHNO2 + 3yHNO3 +
dyNO2
(42)
for z ) 0 w
o
o
o
o
o
pNO
+ 2pN
) pNO
and pN
) Keq(pNO
)2
2
2O4
x
2O4
2
and
dyNO
S
) - [Ross,NO - JNO,g]
dz
G
]
(out)
)
yNO
x
+
b
cHNO
3
for z ) 0
(in)
pNO
x
P
(out)
pNO
x
(46)
P
while NOx partial pressures both at the inlet and at the
outlet of the column are defined as follows:
{
(in)
o
o
pNO
) pNO
+ 2pN
x
2
2O4
(38)
(39)
(out)
pNO
x
)
poNO
+
o
pNO
2
o
2pN
2O3
+
+
for z ) 0
o
2pN
2O 4
o
pHNO
2
+
+
for z ) Z
(47)
o
pHNO
3
Finally, knowing gas inlet and outlet conditions, it is
5008
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
Table 3. Physical Properties of the Gas and Liquid
Phase (Perry and Green23)
liquid
gas
Fl ) 1000 kg/m3
µl ) 0.001 kg/m‚s
σ ) 72 × 10-3 N/m
Fg ) MairP/RT ) 1.185 kg/m3
µg ) 1.813 × 10-5 kg/m‚s
Table 4. Standard Thermodynamic Properties, Henry’s
Coefficients, and Gas-Phase and Liquid-Phase
Diffusivities for Each Chemical Species at Working
Temperature
∆Hof
∆Gof
H
(kcal/mol) (kcal/mol) (mol/m3‚Pa)
NO
NO2
N 2O 3
N 2O 4
HNO2
HNO3
H 2O
21.60
7.96
20.01
2.23
-11.67
-31.99
-57.80
20.72
12.26
33.32
23.41
-10.93
-17.57
-54.64
Dg
(m2/s)
1.51 × 10-5 2.27 × 10-5
6.91 × 10-5 1.83 × 10-5
1.42 × 10-5
1.38 × 10-2 2.30 × 10-5
0.484
1.75 × 10-5
2090
1.53 × 10-5
2.57 × 10-5
Dl
(m2/s)
3.83 × 10-9
3.47 × 10-9
3.16 × 10-9
1.91 × 10-9
1.9 × 10-9
Figure 6. Comparison between experimental data and absorption
efficiency predicted by Selby and Counce17 model. Specific liquid
flow rate ul ) 19 800 kg/m2‚h.
possible to compute the efficiency of absorption evaluating the indexes η and NTUOG/.
Evaluation of Mass Transfer Coefficients,
Interfacial Area, and Transport Properties
To estimate the effective interfacial area and the
mass-transfer coefficients, the correlations suggested by
Launaro and Paglianti18 for HelieR structured packing
have been used.
The evaluation of physical properties of the system
has been effected using the available data in the
literature, assuming the values shown in Table 3. The
values of Henry’s constants, of the standard free energies and of enthalpies of formation for each component
used in the present model, are drawn by Joshi et al.21
and are shown in Table 4. The liquid-vapor equilibrium
relationship of water used in the present model is from
Reid et al.24 Diffusivities in gas and liquid phase have
been evaluated at the working temperature, applying
the Fuller correlation (Perry and Green23) and Wilke
and Chang correlation shown in Reid et al.,24 respectively; the values employed in this work are shown in
Table 4.
Analysis of the Process
To predict the experimental data of abatement efficiency, it has been necessary to develop the complex
model, described previously, that takes into account
different absorption mechanisms. This choice has been
necessary because the employment of simplified models
does not allow one to obtain good agreement between
measured and calculated absorption data for a large
range of NOx inlet concentrations. For this purpose
observe Figure 6 , which shows the comparison between
experimental data and absorption curves obtained from
the simplified model of Selby and Counce:17 the absorption efficiency is represented in the ordinate by means
of the NTUOG/ index while the concentration of total
oxides at the column inlet is represented in the abscissa
as nitrogen dioxide (generally in the literature NO2/).
This simplified model has been proposed by Selby and
Counce17 to estimate the scrubber performance in
packed columns when NOx feed content is 125-2500
ppm (=230-4700 mg/m3). It can be seen, from Figure
6, that the predicted values for absorption by this
simplified model are lower than the experimental data
at a low concentration of nitrogen oxides while at high
Figure 7. Comparison between experimental data and absorption
efficiency predicted by the Selby and Counce modified model.
Specific liquid flow rate ul ) 19 800 kg/m2‚h.
concentration the calculated values are too high: the
prediction is strictly correct only for a tight range of
concentration (1000-3000 mg/m3). This result partially
agrees with Selby and Counce’s17 conclusions because
their model reproduces properly the experimental data
for a feed NOx concentration range tighter than they
have suggested. It is possible to conclude that the
simplified hypotheses adopted by Selby and Counce17
to describe the process can give large errors when used
to predict abatement efficiency at lower and higher NO2/
concentrations.
In the present work the model of Selby and Counce17
has been modified by introducing also the gas-side masstransfer resistance and supposing that NO2 absorption
was accelerated by chemical reaction with water (Lee
and Schwartz6). The results of the modified model have
been compared to the experimental results in Figure 7.
In this case, for concentrations of nitrogen oxides at the
inlet of a column higher than 5000 mg/m3, the modified
model of Selby and Counce reproduces properly the
experimental data whereas large errors remain at lower
concentrations yet. It is important to observe that the
modified model describes the process taking into account
NO2 and N2O4 absorption only and neglecting the
contribution to the process of the other possible absorption mechanisms. On the other hand, when the concentration of NO2/ is high, it is possible to assume that NO2
(and N2O4) are prevalent on the other nitrogenous
compounds throughout the packing; particularly, it is
possible to neglect the presence of HNO2 and HNO3.
Besides, the comparison between the original model of
Selby and Counce17 and the modified model shows that
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000 5009
Figure 8. Gas-phase equilibrium between NO2 and N2O4. Pressure P ) 1 atm, absolute temperature T ) 293 K.
the resistances in the gas film cannot be neglected when
the NO2/ concentration is higher than 5000 mg/m3.
Important considerations about the mechanism of
absorption that involves NO2 and N2O4 can be drawn
observing Figure 8 , which shows the concentrations of
gas-phase chemical equilibrium between NO2 and N2O4
for different total concentrations of NO2/ when the
temperature is T ) 293 K. This equilibrium is largely
favorable toward the formation of NO2 so that the
transfer of this compound through gaseous film is more
important than the transfer of N2O4. On the other hand,
the experimental data, Figures 3 and 4, show that the
efficiency of absorption increases with increasing concentration of the total oxides. This effect can be attributed to the increment of N2O4 that influences the
process because of its high solubility in water compared
to the solubility of NO2 (Joshi et al.21).
To obtain a model that describes properly the experimental data all over the range of inlet concentrations,
and particularly at low concentration, it is necessary to
take into account all the possible absorption mechanisms neglected in the simplified models by Selby and
Counce.17 In the present work, the presence of nitrogen
compounds (particularly nitric and nitrous acid), in the
gas phase, in addition to other oxides, which are
introduced deliberately into the column, has also been
taken into account. The reactions in the liquid phase
have also been taken into account: in fact, according to
the results of some authors (Andrew and Hanson,1
Koval and Peters,4 and Joshi et al.,21), depletion in liquid
of nitrous acid product in a reaction between the oxides
and water is not instantaneous; so, inside the liquid,
there is nitrous acid with a concentration that could
reduce the driving force in its absorption from the gas.
Furthermore, the decomposition kinetics that has been
studied by some authors (Komiyama and Inoue11 and
Crawford and Counce10) cannot be neglected in mass
balances in the liquid phase.
The results obtained from the simulation of the
process by means of a complete model are shown in
Figures 9 and 10 . There is good agreement between
the calculated and measured absorption efficiency: the
mean absolute error is 21%.
The present results show that, in comparison to
simplified models, improvement in the results obtained
in predicting the absorption efficiency at low concentrations of inlet oxides is due to the absorption of nitric
and nitrous acid being taken into account. In fact, at
low concentration of total nitrogen oxides in the bulk of
gas, the partial pressure of NO2 and mainly of N2O4 at
the gas-liquid interface is not sufficiently high: the
Figure 9. Comparison between experimental data and absorption
efficiency predicted by the complete model. Specific liquid flow rate
ul ) 19 800 kg/m2‚h.
Figure 10. Comparison between all experimental data and
absorption efficiency predicted by complete model.
absorption mechanism that involves only these two
compounds is not predominant in comparison to the
other mechanisms that, therefore, cannot be neglected.
Finally, it is necessary to point out that HNO2 and
HNO3 are very soluble in the liquid phase (Table 2) and
therefore their absorption is controlled by the gas film
as a simple parametric analysis of the model could show.
The model that has been developed can be used to
evaluate the absorption of nitrogen oxides in columns
equipped with packing other than HelieR packing: in
fact, the model requires the knowledge of gas-side and
liquid-side mass-transfer coefficients and the interfacial
area at the operative conditions for the packing employed in the absorption process. Also, the model allows
one to simulate the process when the gas fed to the
column contains not only NO2 and its dimer as in this
experimental work but also other nitrogen oxides (e.g.,
NO); the liquid stream too can be different than pure
water and can contain, for example, nitric acid: for this
purpose it is necessary only to modify adequately the
boundary conditions, taking into account different compositions of inlet streams.
The greatest limitation of this model is that the
process is considered isothermal without introducing an
energy balance and so neglecting the heat effects caused
by absorptions and evaporation: in fact, this hypothesis
is allowed at operative conditions used in the experimental work when no changes of temperature were
measured between the inlet and outlet streams, but can
be uncorrected if the NOx inlet concentrations are higher
than the present work; in this case the heat produced
in the process can be considerable, modifying signifi-
5010
Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
cantly the temperatures and altering equilibria and
transport properties.
Conclusions
In the present work some possible models of the
absorption process of nitrogen oxides were analyzed.
The model of Selby and Counce17 was compared with
the experimental data and was modified to obtain good
agreement between measured and calculated absorption
values at high concentrations of nitrogen oxides; another
model, more complex than the previous, was developed: this model describes properly the process for a
large range of inlet concentrations. As far as the
absorption mechanism is concerned, the following conclusions are obtained:
i. The process of absorption can be properly described
using the simplified model by Selby and Counce17 when
the NOx concentration is in the range of 1000-3000 mg/
m3.
ii. The process of absorption can be simply described
modifying the model by Selby and Counce17 to take into
account the gas-side mass-transfer resistance. This
modified model can be used when the NOx concentration
in the gas phase is higher than 5000 mg/m3, whereas
with lower concentration the absorption efficiency is
underestimated.
iii. If an accurate description of the process is required, it is then necessary to take into account both
HNO2 and HNO3 whose absorptions are controlled by
the gas film.
The present experimental and theoretical results
agree with the other published works (Dekker et al.,3
Andrew and Hanson,1 Hoftizer and Kwanten,25 Kramers
et al.,5 Lefers and Van der Berg,7 Carberry,26 Suchak
et al.,14 Counce and Perona,16 Ramanand and Phaneswara,13 and Komiyama and Inoue,27). Nevertheless, this
work allows one to estimate, by means of comparative
analysis of three models with experimental data, the
relative importance of several absorption mechanisms,
pointing out their contribution in the various ranges of
NOx concentrations considered in the experimental
work.
The complete model could be a useful tool because it
can be easily modified for simulating the absorption
process when the compositions of both the inlet gas and
the liquid streams are different than those in the
present experimental work (only absorption in water of
mixtures of NO2 and N2O4 in air). Nevertheless, the
complex model presents some drawbacks because it does
not allow one to analyze the process with the simplicity
of the model of Selby and Counce.17
Acknowledgment
The authors thank Ing. G. Petrillo from Polcon
Italiana s.r.l., Via F.lli Cervi 77 Cantalupo, Milan for
having provided the HelieR packing.
Nomenclature
a ) effective interfacial area per unit volume of column,
m2/m3
cacids ) total concentration of nitrous and nitrogen acid in
the liquid stream, mol/m3
cj ) molar concentration of j species in the liquid phase,
mol/m3
cNOx ) global nitrogen oxides concentration in the gas
phase, mg/m3
De,j ) diffusivity of j species in the liquid phase, m2/s
dz ) differential height, m
Ej ) acceleration factor for hydrolysis of the j species
G ) molar gas flow rate, mol/s
H/ ) total hydrogen in the column element
Hj ) Henry’s constant for the j species, mol/m3‚Pa
kdec,HNO2 ) kinetic constant of nitrous acid decomposition,
L/mol
Keq ) equilibrium constant in eq 25, atm-1
kg,j ) gas-side mass-transfer coefficient of the j species, mol/
m2‚s‚Pa
kidr,j ) hydrolysis kinetic constant for the j species, m3/mol
kl,j ) liquid-side mass-transfer coefficient of the j species,
m/s
kox,NO ) kinetic constant of NO oxidation, atm-2 s-1
L ) volumetric liquid flow rate, m3/s
Mair ) air mean molecular weight, g/mol
N/ ) total nitrogen in the column element
NO2/ ) superior nitrogen oxides (NO2 + 2N2O4)
NOx ) total nitrogen oxides (NO + NO2 + N2O3 + N2O4 +
HNO2 + HNO3)
NTUOG ) number of overall gas-transfer units
NTU/OG ) modified number of overall gas-transfer units,
m-1
n ) reaction order
O/ ) total oxygen in the column element
P ) total pressure, Pa
pj ) partial pressure of j in the gas-phase interface, Pa
R ) universal gas constant, J/mol‚K
Rdec,HNO2 ) nitrous acid decomposition rate, mol/m3‚s
Rox,NO ) NO oxidation rate, mol/m3‚s
Jj,g ) molar rate of j absorption through the gas film per
unit volume, mol/m3‚s
Jj,l ) molar rate of j absorption through the liquid film per
unit volume, mol/m3‚s
S ) column section, m2
T ) absolute temperature, K
ug ) specific gas flow rate, m3/m2‚s
ul ) specific liquid flow rate, K‚g/m2‚h
yj ) molar fraction of j in the bulk of gas
Z ) column height, m
Greek Letters
o
) standard free energy for the j component, kcal/mol
∆Gf,j
g ) void fraction
η ) absorption efficiency
λj ) constant in eq 31
µ ) viscosity of gas, kg/ms
F ) density of gas, kg/m3
σ ) surface tension, N/m
Φj ) Hatta number for hydrolysis of the j species
Subscripts and Superscripts
b ) bulk of liquid
g ) gas phase
i ) gas-liquid interface
in ) in
l ) liquid phase
o ) bulk of gas
out ) out
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Received for review February 22, 2000
Accepted September 8, 2000
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