Absorption of NO2 in a Packed Tower
with Na~S03Aqueous Solution
Luke Chen? Jin-Wei Lin? and Chen-luYawb
a Tamkang University, Department of Water Resources and Environmental Engineering, Tamsui, Taipei Hsien, Taiwan
University of Massachusetts Dartmouth, Advanced Technology and Manufacturing Center, 151 Martine Street, Fall River, MA 02723
This paper looks at the development of a two-stage chemical scrubberfor NO, control.In thefirst stage, the mostpractical oxidizing agentsfor NO oxidation are sodium chlorite
and sodium hypochlorite.Although a considerable amount
of work has been done on the reaction kinetics of NO2 with
N a S O 3 there are majorgaps in developing an N a S O
aqueous solutionfor NO2 absorption in the second stage. f
particular importance is the rate of chemical absorption. A
pilot-scale researchprogram was initiated to evaluate the
absorption rate of NO2 with Na#03 aqueous solution in a
packed tower. The research is directed at obtaining height of
a transfer unit (KTJfor NO2 absorption, and to determine
reasonable operation conditionsfor the packed bed scrubber
A sulfite concentration of 0.25 M is essential to have a reasonable HTU in 2 to 5feet for a gas rate between 1,050 to
2,350lb@ hr. The results indicate that the scrubbing effectiveness of NO2 increased with the L/G ratio, and an WG of
more than 3 is requiredfor high NO2 absorption.
d
INTRODUCTION
The Clean Air Act Amendment of 1930 provided a regulatory drive for reducing nitrogen oxides (NO,) from stationary sources. The NO, in flue gases essentially consists
of nitric oxide (NO) and nitrogen dioxide (NO$. NO2 can
be effectively absorbed in some aqueous solutions, but
not NO [l,21. Unfomnately, most (more than 95%) of the
NO, emitted in flue gases are NO. Therefore, in a twostage chemical scrubbing system for NO, emission control, NO oxidation is a crucial first step. The slow oxidation rate of NO in air can be increased by injecting a
strong oxidizing agent, such as ozone(O3) [21, chlorine
dioxide (C102) [3,41, or chlorine (C12) [51 into the flue gas,
or adding an oxidant, such as sodium chlorite (NaC102)
16-81, hydrogen peroxide (H202) DI,sodium hypochlorite
(NaClO) I8, 101, or potassium permanganate (KMn0,Q [2,
101, to the scrubbing solution.
In recent years, some two-stage chemical scrubbing
systems have reached either pilot-scale demonstration,
or full-scale installation. Among them are a hypochlorite/sulfide system from Trih4er 1111 and from Environair 1121, ozone/sodium hydroxide (NaOH) system
from BOC-Cannon [131, chlorite/sulfite I141 and pulsed
Environmental Progress (V01.21, No.4)
corona/sulfite systems from Beltran 115, 161, and
pulsed corona/thiosulfate system from ADA I171.
CHEMISTRY OF NO2 ABSORPnON
The effectiveness of sodium sulfite (Na2S03) aqueous solution for NO2 absorption was documented in
the early 1970s [l,21. Due to its limited applications,
little effort was made to develop sulfite scrubbers for
pollution control. However, research on the reaction
rate and mechanisms of NO2 with sulfur(IV) was pursued to better understand the role of NO, in atmospheric droplets [18-201 and NO2 influence in a limestone flue gas desulfurization system I21-251. The
products and stoichiometry upon bubbling NO2
through a HSO3- solution suggest that the overall
reaction may be described by [181:
2 NO2 + S03-2 + H20 + 2 NO2- + 2 H+ + S04-2 (1)
Essentially no N O or NO3- appeared to be produced. Clifton, et a l . [231 suggest that the reaction
appears to involve the formation of an intermediate
complex, which can undergo subsequent reaction
with NO2 or others. In an atmospheric droplet, the
fate of the intermediate might not simply react with
other N 0 2 , since NO2 will be at such a low concentration. in a flue gas scrubber, particularly when the
gas phase NO is converted to N 0 2 , the intermediate is
much more likely to react with additional NO2 due to
its much higher concentrations. Littlejohn, et a l . [241
believes that the reaction initially produces a nitrite
ion and a sulfite radical:
NO2 + S03-2
+ N 0 2 - + SO3'-
The sulfite radical (SOg'-) can undergo either
recombination or reaction with oxygen. Dithionate ion
(S204-2) was observed as a product in all of the reaction mixtures from studies done without o gen. The
ratio of the two main products (S04-2/S206 1is 1.8. A
feasible mechanism for the production of dithionate is
the recombination reaction of sulfite radicals:
7
December 2002 225
(3)
In the presence of oxygen, sulfite radicals were
consumed by oxygen:
s0g'-
+ 0 2 + so5'-
(4)
The reaction is very fast, approaching the diffusion
control limit. At low sulfite radical concentrations and
large dissolved oxygen concentrations, Reaction 3 is
insignificant compared to Reaction 4. After a complicated mechanism, sulfate ion is the major end product
in the solution.
Absorption of NO2 occurs with simultaneous mass
transfer and fast chemical reaction. Although previous
investigators have studied the reaction of NO2 with
SO -2, those studies were performed under conditions
dif erent from that of a chemical scrubber. Shen and
Rochelle 1251 measured NO2 absorption rate in sodium
sulfite aqueous solution under conditions of a limestone
slurry scrubber. All experiments were performed in a
stirred cell contactor with separately agitated gas and
liquid. Under typical conditions of a limestone slurry
scrubbing, 10 mM total dissolved S(IV) and pH 4 to 5,
the NO2 removal was less than 50 percent.
Yang, et al. [151 studied sulfite scrubbing for NO2
removal in their attempt to develop corona-induced
chemical scrubbers for NOx emission control. At a gas
mass flowrate of 90 lb/ft2 hr in their bench-scale
packed bed scrubber, the NO2 removal was 98.8%
and the outlet concentration was 1.1 ppm.
Although a considerable amount of work has been
done on the reaction of NO2 with Na2S03, there are
major gaps in developing Na2S03 aqueous solution
for NO2 absorption. Of particular importance is the
absorption rate of NO2 in a commercial scrubber. A
pilot-scale program was initiated to evaluate the
absorption rate of NO2 with Na2S03 aqueous solution in a packed tower. The research is directed at
obtaining height of a transfer unit (HTU) for NO2
absorption, and the effects of major operation parameters, such as ORP, gas rate, liquid rate and Na2S03
concentration in the liquid on NO2 absorption.
I
CHEMICAL ABSORPTION IN A PACKED TOWER
Consider a packed tower with the following characteristics. The cross sectional area is S and the deferential volume, with respect to the height, dZ, is SdZ. If
the change in gas molar flow rate Vis neglected, the
amount of gas absorbed in section dZ is -V@, which
is equal to the absorption rate times the differential
volume:
-Vdy = K y
0- y*) SdZ
(5)
This equation is rearranged for integration by grouping together the constant factors V; SdZ, and K y , which
have a constant value with dZ.
226
December 2002
The equation for the column height, Z F can be
written by integrating dZ from 0 to Z;r;as follows:
fV\
(7)
z,=[The integral in Equation 8 represents the change in
vapor concentration divided by the average driving
force and is called the number of transfer units, NTU or
No The other part of Equation 7 has the unit of length
a n l i s called the height of transfer unit, HTU or Ho
The chemical reaction in the liquid phase reguces
the equilibrium partial pressure of the solute over the
solution, which greatly increases the driving force for
mass transfer. If the reaction is essentially irreversible
at absorption conditions, the equilibrium partial pressure is zero, and the N can be calculated just from
OY
the change in gas composition [261. For y* = 0.
The rate of absorption of NO2 can be evaluated by
the overall mass transfer coefficient, K a. The twofilm theory of mass transfer leads to e!It following
equation for K p , where the liquid-film mass transfer
coefficient kx* is multiplied by an enhancement factor
@ when there is a chemical reaction in the liquid film.
(9)
where m is the solubility of the gas phase reactant in
water and k p is the gas film mass transfer coefficient.
The enhancement factor depends on diffusivities and
concentrations of both reactants in liquid and gas phases, and on the reaction rate constant. For a fast irreversible reaction and excess reactant in the solution, the
enhancement factor, @ can be expressed as [271:
where k2 is the reaction rate constant, Bg is the initial
concentration of reactant in the liquid and D A is the
diffusivity of reactant in the gas phase.
EXPERIMENTAL SECTION
The absorption tests were carried out at a pilot
plant built by Kunstoff Manufacturer, Co. Figure 1
shows the schematic of the gas scrubbing pilot plant.
The plant consists of a gas blending system, a gas
scrubber, a chemical injection and control system, and
a NOx monitoring unit. The gas blending system is
Environmental Progress (V01.21, No.4)
Table 1 . Experimental parameters and operating conditions.
Scrubber parameters
Column diameter (ID)
Tower height
Packing height
Packing size (nominal)
Gas flow rate
Gas mass flow rate
Gas temperature (room)
Gas composition (N02/air)
Liquid mass flow rate
Alkalinity (by NaOH)
ORP (by Na2S03)
m
m
m
in
Gas parameters
ft3/min, acfm
lb/ft2 hr
O C
PPm
Liquid parameters
lb/ft2 hr
pH
mV
capable of producing a wide variety of gas compositions by mixing air with high concentration NO2 from
cylinders. The N02-containing air stream is then
passed through the scrubbing tower where the NO2 is
absorbed and oxidized. Samples are taken to determine the inlet and outlet concentrations of NO2 and,
through calculation, removal efficiencies and HTUs.
The gas blending system is capable of a total flow
rate of 45 m3/min (1,600 cubic feet per minute, cfm).
Concentrations are varied by injecting NO2 from a 5%
gas cylinder through a mass flow meter. The system is
made of glass fiber reinforced plastic (FRP), including the
blower, except for the NO2 lines which are polypropylene tubing. After the NO2 is injected into the air stream,
the whole stream is passed into a section of Tellerete
Packing for better mixing. The well mixed N02-containing air stream is then carried into the gas scrubber, where
absorption and chemical reaction occur.
The packed tower is constructed of a 5 meter long
(16.5 ft) and 0.45 meter diameter (1.35 ft) polypropylene column with a section of 1.8 meter (5.9 ft)
packed bed made by randomly packed 3.25 inch, No.
2 K-type Tellerete Packing. The top of the column
holds a demister head packed with No. 1 R-type
Tellerete Packing for removing entrained droplets
from the gas stream. The entire column sits on a vessel which serves as the scrubbing solution reservoir.
The concentrations of Na2S03 in the scrubbing
solutions are monitored and controlled by the oxidation reduction potential (ORP) metedmetering pump
system. A circulating pump withdraws scrubbing solution from the reservoir and pumps it up to the top to
be sprayed down on the packed bed, countercurrent
to the gas flow. The rough pumping rate is controlled
by regulating the recirculation rate, with the final
adjustment being made at the Signet 5500 flow meter
downstream from the pump.
A chemiluminescent NO, analyzer is used to measure NO2 concentrations. Basically, the signal from the
NO, analyzer comes from the light emitted from the
chemiluminescentgas phase reaction of nitric oxide and
ozone. To measure NO concentration, the gas sample is
blended with ozone in a reaction chamber. The ozone
is generated in situ by a high voltage arc ozone generaEnvironmental Progress (V01.21, No.@
0.45 (16.2 in)
5 (16.5 ft)
1.8 (5.9 ft)
3.25
400 - 900
1,000 2,500
25
200
-
-50 - -250
3,000 4,000
11
tor. The resulting chemiluminescence is monitored
through an optical filter by a high sensitivity photomultiplier positioned at one end of the reaction chamber.
The analysis is sensitive only to NO. To measure NO,
concentrations, the sample gas is diverted through a
high-temperature converter, where the NO2 is converted to N O , and the total of NO,, NO, plus N 0 2 , is
detected as NO. The NO2 concentration is the difference between the two readings for NO, and NO. Signals from the NO, analyzer are continuously recorded.
RESULTS AND DISCUSSION
The results from the scrubbing pilot plant research are
given in this section. Experiments were conducted at the
conditions indicated in Table 1. Parameters, such as oxidation reduction potential and Na2SO3 concentrationsin the
scrubbing solutions, gas velocity, and liquid mass flow
rate, as well as gas and liquid flow rates, were studied for
their effect on NO2 absorption. A set of operating conditions was established after these tests, and based on these
operating conditions, the HTU for NO2 absorption in the
packed bed is correlated to gas flow rate for further study
and full-scale scrubber design.
In the first series of tests, the tower was operated at
various gas velocities in order to find out a reasonable
contact time between NO2 and S03-2 in the scrubbing solution. At a gas velocity of 2.7 m/s (8.9 ft/s)
and a liquid rate of 40 L/min (3,000 lb/ft2 hr), the
NO2 removal was 60 percent. The removal rate
increased with the decrease of gas velocity, as shown
in Figure 2. At a higher sulfite concentration, 0.25
molar (M),the NO2 absorption was more than 99%
when the gas velocity was reduced to 1.2 m / s (4 ft/s).
A gas velocity of 1.2 m / s represents a 1.5 second contact time between NO2 and S03-2 in the scrubber. It
is clear that high NO2 absorption would require operation at gas velocities less than 1.2 m / s .
Other runs were made with two volumetric gas rates,
11.3 and 22.7 m3/min (400 and 800 ft3/min, cfm) to test
the effect of S03-2 concentration on NO2 absorption.
The concentration of SOg2 was controlled and monitored by an ORP meter. At a gas rate of 22.7 m3/min,
the NO2 absorption was about 45% at an ORP of -50
mV. The negative ORP indicates that sulfite is a reducing
December 2002 227
T
*
Analyzer
Demiater
nozzl
Packec
bed
NO2
W
Blower
Mixer
Pump
r5!!mp
Overflowl
;
I
45% NaOH
i
12% Na2S03
Figure 1. Schematic of the pilot plant gas scrubbing system.
agent and its concentration is only proportional to the
value. The NO2 absorption increased with the S03-2
concentration, expressed as oxidationheduction potential in millivolts. At a lower gas rate, 11.3 m3/min, NO2
absorption increased from 75% to 96% when OW was
increased from -50 to -240 mV, as shown in Figure 3.
The differences of NO2 absorption between the two gas
flow rates were 13% to 27% over the same range of
OW. The gas rate of 11.3 m3/min represents a 1.5 second contact time between NO2 and S03-2 in the
packed tower while the -240 mV represents a sulfite
concentration of 0.25 M in the scrubbing solution.
The next set of experiments was designed to quantlfy
the effects of liquid flow rate on NO2 absorption. The
gas rate was varied from 1,000 to 2,400 lb/ft2 hr. The
OW in the liquid was maintained constant at -240 mV.
At a liquid rate of 3,000 lb/ft2 hr, NO2 absorption
ranged from 65% to 94%, as shown in Figure 4. The
experiment was repeated with a liquid rate of 4,000
lb/ft2 hr. In the same range of gas rate, when liquid rate
was increased from 3,000 to 4,000 lb/ft2 hr, NO2
absorption increased 3% to 6%. Liquid rate has a smaller
effect on NO2 absorption in the packed tower. This
confirms that absorption rate, in terms of number of
transfer units, varied inversely with gas velocity, and
increased with the 0.4 to 0.6 power of the liquid rate.
A number of experiments were performed at the
conditions indicated in Table 1 with 0.15 and 0.25 M
sodium sulfite scrubbing solution. The gas rate was
varied from 11.3 to 25.4 m3/min (400 to 900 cfm). The
liquid rate was maintained at 4,000 lb/ft2 hr. Thus, the
practical range of 1.7 to 3.7 liquid to gas mass ratio
228
December 2002
(UG) was studied. The actual measurements from
these experiments are plotted in Figure 5. The data
indicate that the scrubbing effectiveness of NO2
increased with the V G ratio, and an V G of more than
3 is required for high NO2 absorption. Operating at a
higher liquid rate can be justified by keeping L/G
greater than 3, even though the effect of liquid rate on
NO2 absorption is smaller than that of gas rate.
For absorption of NO2 in water, the solubility is
only 0.04 M/atm at 25" C . However, the low solubility
can be improved by a big enhancement factor rovided by the rapid reaction of NO2 with SO3- . The
enhancement factor depends on diffusivities and concentrations of both NO2 and S03-2, and on the reaction rate constant. For a fast irreversible reaction, and
with excess S03-2 in the solution, the enhancement
factor, can be estimated from Equation 11 with DA =
2 x 10- cm2/s [281, BO = 0.25 M, kL* 0.01 cm/s, and
k 2 = 11 x lo5 1251.
5
$
=
(11)
@NO,
(1 1 x lo5x 0.25 x 2 x 10-~)0.~
= 235
0.01
The value is large, but it is not large enough to compensate for the much lower solubility. The liquid-film
resistance is still much greater than gas-film resistance.
This hypothesis is supported by the ex erimental data
in Figure 6. The result shows that SO3- in the solution
is still strongly affecting the overall mass transfer rate in
5)
Environmental Progress (V01.21, No.4)
100
NO2 n m d (%)
80
60
40
20
1
1.2
1.4
1.6
1.8
2
2.2
G u ssloelty (mla)
2.4
2.6
2.8
3
0
-300
-250
-200
-150
-100
-50
0
(hidatloo redoction potential (mV)
Figure 2. The effect of gas velocity on NO2 absorption in a packed bed scrubber.
Figure 3. The effect of oxidation reduction potential
on NO2 absorption with gas rates at 400 acfm and
800 acfm, and liquid mass flow rate at 4,000 Ib/ft2 hr.
NO2 removal (%)
100
NO2 r o m d (%)
80
60
40
20
1000
1200
1400
1600
1800
2000
2200
2400
Ou r8te (IblftA2-hr)
0
1
1.5
2
2.5
LIG n t l o
3
3.5
4
Figure 4. The effect of gas mass rate on NO2 absorption at liquid rate of 3,000 and 4.000 lb/ft2 hr.
Figure 5. The effect of liquid-gas mass ratio (VOon NO2
absorption at sulfite concentrations of 0.15 and 0.25 M.
terms of HTLJ, through enhancement factor, and through
liquid film mass transfer coefficient.
ACKNOWLEDGMENT
CONCLUSIONS
The Na2S03 aqueous solution is effective for NO2
removal in a packed bed scrubber. NO2 absorption
occurs with simultaneous mass transfer and fast chemical
reaction. At a sulfite concentration of 0.25 M, and a gasliquid contact time of 1.5 seconds, NO2 absorption was
more than 33%.The Na2SO3 creates an irreversible reaction to drive NO2 to the scrubbing solution. Even with
0.25 M sodium sulfite in the scrubbing solution, the liquid-film resistance still controls the absorption of N02.
The enhancement effect is due to reaction of NO2 with
S03-2. A sulfite concentration of 0.25 M is essential for a
reasonable HTU in 2 to 5 feet for a gas rate between
1,050 to 2,350 lb/ft2 hr. The results also indicate that the
scrubbing effectiveness of NO2 increased with the V G
ratio and an V G of more than 3 is required for high NO2
absorption. Since Na2S02 in the solution increases both
the enhancement factor and solution capacity for NO2
absorption, operating at a higher concentration can be
justified. The ORP can be used to monitor and control
the concentration of Na2S03 in the scrubbing solution.
An ORP of -240 mV is adequate for a 99% NO2 removal.
Environmental Progress (V01.21, No.4)
The authors are grateful to Arthur Lee, President of
Kunstoff Manufacturer Co., Taipei, Taiwan, for support
of this research.
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~~
HTU (ft)
8,
I
I
I
I
I
I
2200
2400
I
1-
0
1000
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1400
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GM rate (IblltAZ-hr)
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