Absorption of Nitrogen Oxides in Aqueous Sodium Sulfite and Bisulfite

Jul 1, 1977 - Aqueous Absorption and Oxidation of Nitric Oxide with Oxone for the Treatment of Tail Gases: Process Feasibility, Stoichiometry, Reactio...
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Absorption of Nitrogen Oxides in Aqueous Sodium Sulfite and Bisulfite Solutions Hiroshi Takeuchi,' Makoto Ando, and Nobuo Kizawa Department of Chemical Engineering, Nagoya University, Nagoya, 464, Japan

Absorption experiments of NO and NO2 diluted with nitrogen (NO, concentrations over a few hundred ppm) in aqueous solutions of sodium sulfite and/or bisulfite were performed at 25 O C using the agitated vessels with gas dispersion and with a plane interface. The experimental results were discussed based on the film theory of gas absorption accompanied by fast, pseudo mth order chemical reaction. The reaction between NO and S032- was found to be second order with respect to NO and zero order with respect to S032-, and the rate of absorption of NO into the sulfite solution increased as the concentration of NO2 in the gas phase increased. As for the absorption of NO2 in the sulfite solution, it was found that the absorption rates went through a maximum as the sulfite concentration varied, and the reaction mechanism to absorption was with competitive reactions involving the hydrolysis of NO2 and the reaction between NO2 and S032- in the liquid film. The reduction of NO2 was found to be first order with respect to S032- and first order with respect to NO2,and so the value of the second-order rate constant was evaluated, while for the hydrolysis of NO2 the reaction order was second order with respect to NO2. Also the absorption of NO2 into the bisulfite solution was accompanied by competitive reactions as well as in the sulfite solution. In addition, aqueous solutions with various reducing or oxidizing agents were examined for the absorption of NO2. Furthermore, the effect of temperature on the absorption of NO2 in the sulfite solution was studied at 10, 15, and 25 O C .

Introduction It is widely recognized that the emission of nitrogen oxides from fuel combustion causes an air pollution problem besides the sulfur dioxide. For SO? pollution, however, many stack desulfurization processes are in commercial operation in Japan, and hence, it is currently possible to control the emission. For nitrogen oxides (NO,) originated from stationary power plants, in general, the concentration in these stack gases is below a few hundred parts per million, and the greater part of NO, is nitric oxide which is poor in reactivity. In the past several years, many efforts have been made to develop the NO,-emission control system from stack gases, and many different processes are in various stages of development. Some processes which are based on absorption with chemical reaction or a catalytic reaction to effect the NO, removal are presently under test on a commercial scale (Tohata, 1975). In a French patent (SINCAT, 1965), it has been reported that both NO2 and NO are decomposed to N2 by the reaction with sulfite. Later, Noguchi (1976) studied the same system and reported the formation of imidodisulfonate as the reaction product. When an alkaline solution is used as an absorbent for removing the SO2 from stack gases, sulfites or bisulfites are formed a t the gas-liquid interface as the result of absorption of SO2 and those may participate in the reaction with NO,. The absorption of NO? in aqueous solutions has been studied extensively, but the mechanism of absorption is still subject to controversy. The process involves gas phase reaction (for example, Caudle and Denbigh, 1953; Peters et al., 1955), liquid phase reaction (for example, Wendel and Pigford, 1958; Kramers et al., 1959;Andrew and Hanson, 1961),and diffusion of products and reactants to and from the gas-liquid interface (for example, Wendel and Pigford, 1958; Chilton and Knell, 1972). The present study was made to investigate the mechanism of absorption of nitrogen oxides in aqueous solutions of Na2S03 and NaHS03. It was also thought that the above absorption system might prove useful in developing processes for removing SO2 and NO, simultaneously from stack gases by alkaline and sulfite solutions.

Experimental Method Two methods of gas-liquid contact were used as the continuous flow system. In the first method, the gas was introduced above the gas-liquid interface of known area equal to free surface (plane interface). In the second method, the gas was introduced in the liquid through a nozzle, and hence mass transfer occurred in a gas-liquid dispersion (bubble dispersion). Plane Interface. A schematic drawing of the experimental setup for the plane interface contact method is shown in Figure 1. The absorption vessel was the same as that used in the previous work (Takeuchi et al., 1975), being made of 10.0-cm i.d. acrylic resin pipe with four baffles. Separate stirrers for the gas and liquid phases were mounted on concentric double shafts rotating in opposite direction by two electric motors. An impeller for stirring up the bulk of the gas phase was with four inclined blades (60" to the horizontal) just above the liquid surface, while for mixing the bulk liquid there were eight flat blades a t half-liquid depth. This arrangement makes it possible to vary the speed of the liquid side impeller between 50 and 150 rev/min and the speed of the gas side impeller between 100 and 500 rev/min independently. Both shafts were sealed with mercury seals. The clearance between the gas side blades and the liquid surface was controlled by a liquid level controller so as to be just 10 mm. Liquids used were water and aqueous solutions of sodium sulfite and/or sodium bisulfite. The liquid was fed to the vessel a t a flow rate of 120 mL/min and flowed out to a drain tank through an overflow pipe. The liquid depth always was maintained 10.0 cm. The temperatures of the entering and leaving liquids were kept constant a t 25 f 0.1 "C. A 5% NO and/or NO? gas mixture balanced with nitrogen was further diluted in the concentration range of 100-800 ppm with nitrogen saturated with water vapor. Then, the mixed gas was fed to the top of the vessel a t a flow rate of 3 L/min. Temperature of the entering gas to the vessel was not adjusted on purpose, since a series of runs were conducted in the air conditioned room. All the experimental runs were carried out a t 25 "C and atmospheric pressure. Concentrations of NO and NO2 in the gas phase were Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 1977

303

I

.,#I

10

I

I

I

I

1,6”

1

I

,

500

750

1000

Agitation

speed

n

Crpml

Figure 2. Effect of agitation of liquid phase on the rates of NO absorption into 0.37 M NaZS03 solution a t 25 “C.

-catalyst is independent of k L and hydrodynamically depends only upon the gas-liquid contacting area (for example, Onda et al., 1972; Reith and Beek, 1973).

:Thermometer R : Thermo-

-r*c

1 : Vessel

10 : (NO+N2 )-Cylinder

2 : Gas phase impeller 3 : Motor

11 : (NOp +Nt )-Cylinder 12 : Measuring needle Valve 13 : Head tank 14 : Over flow pipe

4 5

: :

Tachometer Mercury seal

6

:

Capillary flow meter

7 : Mixing tube

8 : pump 9 : Nt- Cylinder

15 : Reservoir 16 : Const. temp. tank 17 : Drain tank 18 : Saturator

Figure 1. Schematic diagram of the experimental apparatus with plane interface.

measured by a NO, analyzer (YANACO ECL-7s) which automatically and continuously determined the concentrations of NO and total nitrogen oxides (NO,). The analysis is based on a chemiluminescent method, and hence in principle NO2 is measured after conversion to NO by a catalytic reaction. The rate of absorption was determined from a material balance by measuring concentrations of the entering and effluent gases. It was assumed that the gas-liquid interfacial area was the same as the cross-sectional area of the vessel, 78 cm2, and the gas and liquid phases in the vessel were perfectly mixed. Liquid samples were taken from the entering liquid stream and were analyzed for sulfite concentration by iodimetric techniques. A mist appeared in the effluent gas stream during some NO2 absorption runs. However, its presence had no detectable effect on the absorption of NO2. The appearance of the mist was especially observed when the absorption vessel was operated at lower temperatures. Bubble Dispersion. A mechanically agitated vessel was mainly used for the absorption of NO in sodium sulfite solutions. The agitated vessel consisted of a 20 cm long section of 13.9-cm i.d. stainless tank. The vessel was fully baffled and was equipped with a single, 8-bladed turbine impeller of 4.4 cm diameter (standard vessel-impeller geometry). A gas mixture of NO or NO2 and N2 was introduced into the liquid through an 8.0-mm i.d. glass tube located below the turbine impeller. The gas flow rate was maintained 8.8 L/min, corresponding to a superficial velocity of 1.0 cm/s. A liquid flow rate was 200 mL/min in all runs. The temperatures of the entering and leaving liquids were kept constant a t 25 f 0.5 OC. Interfacial areas in the bubble dispersion were determined by a sulfite oxidation method, in which the rate of oxygen absorption in aqueous Na2S03 solution containing Co2+as a 304

Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 1977

Absorption of NO in NazS03 Solutions The rate of NO absorption in aqueous solution of NaZS03 was measured a t a partial pressure of NO, PNO, of 3.5 X lov4 atm in the bubble dispersion. The lower curve in Figure 2 is a plot of the absorption rate per unit volume of the bubble dispersion, $A‘, as a function of the speed of stirring, n. In a mechanically agitated vessel, in general, the interfacial area of the bubble dispersion increases with an increase of n. Then, the absorption rate per unit of gas-liquid contacting area, NA‘, was calculated using the value of interfacial area obtained by the sulfite oxidation. The dotted line in Figure 2 indicates that NA’ is independent of n , that is, liquid phase hydrodynamic conditions, as was the case with sulfite oxidation. Therefore, the model of gas absorption with a pseudo mth order reaction in the liquid phase would be applicable to the absorption of NO in the sulfite solution. For this case, the film and penetration models predict the same absorption rate when there is no gas-phase mass transfer resistance, and give the following equation. absorption rate of gas A

Figure 3 shows plots of NA’ vs. P N O for the results obtained in the bubble dispersion and the plane interface. In the figure, a series of the experimental points fall on the lines with a slope of 1.5,irrespective of the NO2 concentration in the gas phase. This concentration dependency indicates that the reaction between NO and S032- is second order with respect to NO, that is, the value of m in eq 1 is 2. On the other hand, as could be seen from curves 3 and 4 in Figure 3, the dependency of NA’ on the sulfite concentration CBOis less compared with that on the concentration ratio of NO2 to NO, in the gas phase. In addition, from the results obtained in the bubble dispersion at the NO concentration of 350 ppm, it was found that the dependency of NA’ on CBOwas negligibly small in the range of 0.02 to 0.8 M, that is, n = 0 in eq 1.As a result, the overall mechanism of NO absorption in aqueous Na2S03 solution may be represented as

The solubility of NO, H‘, was estimated to be 2 X mol/cm3 atm from the work of Andrew and Hanson (1961), neglecting the effect of salting-out. Diffusivity of NO in water, DA, was estimated to be 2.5 X 10-5 cm2/s from the work of Wilke and Chang (1955) and was used without any further correction. Then, for curve 6 in Figure 3 the second-order rate

I

01-05M

2 COlM 3 05M 4

(IN0,VINOJ: 07-38)

107-08)

1015)

0 0 5 M I3201

0.4 .1.0M-N c 2 SO, C

._ 0 a L

, , , I

Id Portio1 pressure, P", x

02

04

06 08 I O

(NO)/(NOx)

Figure 3. The rates of NO absorption into Na?SO:i solutions in the bubb!e dispersion (n= 500 rpm) and in the plane interface (nc, = 300 rpm and n ~ =, 100 rpm).

constant k' was found to be lo9 l/mol s a t 25 "C. However, even with such value of k ' , it should be noted that the rate of NO absorption is very low a t parts-per-million concentrations. This fact suggests that, on absorption by aqueous solutions as the primary means of removing nitric oxides from stack gases, it may be of a little significance to search a catalyzer for enhancing the reaction rate in the liquid phase. It would be rather interesting to remove S O after oxidizing it into NO? by a gas phase reaction. Effect of the Presence of NOz. For the gas mixture of NO and NO2 diluted with nitrogen, the absorption of KO into the sulfite solutions of 0.4-1.0 M was carried out by varying NOr concentration, while NO concentration was kept constant a t 200 ppm (nc = 300 rev/min). The results obtained are shown in Figure 4 as a log-log plot of NA' vs. (NO)/(NO,).From the figure, it can be seen that the presence of NO?has a noticeable effect on the absorption of NO as evidenced in the results a t higher concentration ratio of (NO)/(NO, ), but the concentration ratio dependency of N.4' is not linear. During the absorption of NO in the presence of NOn, the N201 formed as the result of a homogeneous reaction +

I

01

J

lb 10"( o t r n )

NO + NO? N203

2

(3)

may participate in a reaction with water or sulfites. Then, unlike the case of the absorption by sodium hydroxide solution, which has been reported by Sherwood and Pigford (1952),the absorption rate in the sulfite solution did not show a maximum when (NO)/(NO?) = 1.0. However, no proof of this behavior will be given here.

Absorption of NO2 in NazS03 Solutions In the case of the absorption of NO2 by aqueous solutiqns of Na2SO3 in the bubble dispersion, removal efficiencies as high as 99% were obtained a t CBOhigher than 0.05 M, and hence it led to a uncertainty in calculating the absorption rate N A . At lower sulfite concentration, the dependency of N Aon the partial pressure of NO? was different with respect to partial pressure of the inlet and outlet gas streams. On the other hand, the plane interface contactor makes it possible to keep the interfacial area and bulk concentrations of the gas and liquid phases constant. From this point of view, the device is useful to make clear the mechanism of gas absorption accompanied by a chemical reaction of any order. Therefore, most of the NO2 absorption runs were made using the agitated vessel with a plane interface.

(-)

Figure 4. Effect of K0-KOI concentration ratio on the rate of NO absorption in the sulfite solutions ([Na$30& = 0.4-1.0 hl, P N O = 2 X atm, nc = 300, and n~ = 100 rpm).

Effect of Stirring Speed. In order to evaluate gas side mass transfer coefficient kc, the absorption of dilute SO2 in 1 M NaOH solution was conducted a t stirring speed of the gas phase, TZG,in the range of 100 to 500 rev/min. In all these runs, the SO1 gas was diluted with N2 to the concentration below 0.5%, and the inlet and outlet gas compositions were determined by means of an infrared analyzer (Beckman Model865). The measured hc were related to n~ by an empirical equation h~ = 1.48 X 1 0 % ~ '

(4)

As for the absorption of NO2 in the sulfite solution, the absorption rate was proportional to TZGO when CBO= 0.5 M, but the dependency decreased with decrease in CBOand became less significant for water, while no effect of the liquid phase agitation was found for the variation of n~ between 50 and 150 rev/min. From these results, it would be considered that the mechanism of NO2 absorption into the sulfite solution is significant of the mass transfer of NO2 across the gas film and with a pseudo mth order reaction in the liquid film. Effect of Gas Flow Rate. Gas flow rate was varied from 1.8 to 6.0 L/min, corresponding to a residence time of about 13 to 4 s in the vessel space, respectively. Then, no appreciable change in N A was found over the varied gas flow rate. Effect of Presence of NO. In a series of NO absorption runs in the range of (NO)/(NO,) from 0.2 to 0.95, the rate of NO2 absorption was also measured. Experimental results showed it to be irrelevant to the concentration of NO coexisting with NO2. Effect of Concentration of NOz. The rate of NO2 absorption was directly proportional to the partial pressure a t higher concentrations of sulfite, but the rate for water could be correlated by a straight line with a slope of 1.5. Therefore, to determine the mechanism of NO2 absorption in the sulfite solution, it is necessary to know the NO2 concentration a t the gas-liquid interface, C,. The partial pressure p , on the interface is obtained from the equation NA= ~

- PJ

G(PG

(5)

where p , is in equilibrium with C,. Under the conditions of the low partial pressure of dissolved gas, Henry's law is applicable to the solubility (C, = H p , ) .The value of h c for the NO2 system can be predicted by eq 4, using the relation that h~ varies as the two-third power of diffusivity. In the present study, diffusivities in the gas phase were estimated from the empirical equation (Gilliland, 1934). Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 1977

305

C,,=(Na,S0310

or C~o=lNaHSO,l, ( M I

Figure 6. Plots of ( N A )vs. ~ CBOand ( N A )vs.~ CBo. Broken lines show the values corrected with the diffusivity which is inversely proportional to the liquid viscosity.

to be 2.16 X cm2/s from Wilke and Chang (1955).By an ~ the sulfite solutions below 0.1 application of eq 1 to ( N A )for M, the value of k l a t 25 "C was found to be 6.6 X lo5 L/mol S.

Figure 5. Plots O f N A for aqueous Na2S03 solutions as a function of interfacial partial pressure of N02. Broken lines show the values of N A calculated by eq 8.

Figure 5 shows plots of N A vs. p i , which corresponds to the relationship between N A and Ci. From the figure and eq 1,a reaction between NO2 and water (that is, the hydrolysis of NO21 is found to be second order with respect to NOz, while with increasing CBO,the dependencies of N A on the NO2 concentration decrease from 1.5 to 1.0. As a result, the absorption process may be accompanied by competitive reactions. That is, the nitrogen dioxides competitively react with SO& and water, the orders of reactions with respect to NO2 being first and second, respectively. Effect of Sulfite Concentration. Assuming the gas absorption with the competitive reactions as mentioned above, the rate of NO2 absorption attributed to only a reaction with ~ , be evaluated by the following equation Na2S03, ( N A ) can (NA)l

=

[(NAl2

- (NA)hyd2I1l2

(6)

where N A and ( N A ) h y d are the overall rate of absorption in the sulfite solution and the absorption rate only in water, respectively. The upper solid curve in Figure 6 shows the value of ( N A ) ~ against CBOfor pi = atm in Figure 5. From the figure, it can be seen that the effect of CBO on ( N A )is~probably due to a fast, pseudo-first-order reaction between dissolved NO2 and S03*- in the liquid film, as pointed out for the C02-NaOH system by Sherwood and Pigford (1952) and Onda and Tak~ through a maxieuchi (1966). The variation of ( N A )going mum as CBOvaries is due to the changes in diffusivity of NO2 in the liquid film. The upper broken line in Figure 6 shows the ~ with the diffusivity, which is invalues of ( N A )corrected versely proportional to the liquid viscosity. From the figure, it can be seen that the solubility of NO2 plays an important role in the absorption, and the experimental data show a maximum rate a t about 0.5 M. However, in the range of sulfite concentration below 0.1 M, ( N A )varies ~ as a square root of CBO.Thus, according to eq 1,the reaction between NO2 and S 0 3 2 - is first order with respect to Na2S03. Therefore, considering it to be first order with respect to NOz, the rate of the reaction may be expressed by an empirical equation T o obtain the second-order rate constant k 1 from eq 1,the solubility of NO2 in water was evaluated from the equilibrium concentration given by Andrew and Hanson (1961). Then, the Henry's law constant H of NOz was obtained to be 4.1 X mol/cm3 atm at 25 "C. The diffusivity of NO2 was estimated 306 Ind. Eng. Chern., Process Des. Dev., Vol. 16, No. 3, 1977

For the hydrolysis of NO2, the second-order rate constant may also be calculated by eq 1 with the values of m and n equal to 2 and 0, respectively. The value of k h y d was found to be 7.4 X lo7 L/mol s. Although the hydrolysis of NO2 has been extensively investigated, there is little literature concerned with N02. Previous studies on the absorption of NO2 in aqueous solution have been conducted a t concentrations of NO2 on the order of a few percent (for example, Kramers et al., 1959; Wendel and Pigford, 1958; Chilton and Knell, 1972), so that dinitrogen tetroxide would play an important role in the hydrolysis. In such system, the reaction has been found to be first order with respect to N204, and a rate constant between about 50 and 300 s-l a t 25 "C (Kramers et al., 1959). The difference of these values is probably due to the solubility data used for analyzing the absorption rate. Turning to the absorption of dilute NO2 in the sulfite solution, the effect of hydrolysis of NO:! on the absorption rate would become significant as CBOdecreases. Therefore, from eq 1 and 6, the overall rate of the absorption may be represented by the equation khyd

Experimental results are compared with the calculated values from eq 8, which are shown by the broken lines in Figure 5. From the figure, it can be seen that the experimental data lie in the vicinity of these lines, which would indicate that the absorption process of NO2 into sulfite solution is referred to the model with competitive reactions. Absorption of NO2 in N a H S 0 3 Solutions The absorption of NOz in aqueous solutions of NaHS03 is of interest for the case of simultaneous removal of NO2 and SOz by alkaline or sulfite solution, which will be discussed in the future study. The experimental runs were conducted with the same method as for NO2 absorption. Figure 7 shows the typical results as plots of N A vs. p ~ o ~I tican . be seen from Figure 7 that the absorption rate for a 0.5 M NaHS03 solution is lowered to about one-third of that for Na2S03 solution with the same molarity, and the overall rate for the lower bisulfite solutions is controlled by the hydrolysis of NOz. From the same assumption as for the NOz-NazS03 system, the rate of NO2 absorption attributed to a reaction with HS03-, ( N A ) can ~ , be evaluated. The variation of ( N A )with ~ the bisulfite concentration CB'Ois also shown by the lower curve in Figure 6, from which it can be seen that ( N A )is~ proportional to the square root of CBQ This concentration dependency also indicates that the reaction is first order with respect to NaHS03. Therefore, as with the case of the sulfite solution, the overall rate of NO2 absorption into the bisulfite

(HSO;)/(HS$+SO~)

pN4,,

I-)

Figure 8. Variations of the specific absorption rate and of p H for the mixed solutions of NazS03 and NaHS03. The rates of NO2 and NO are in the plane interface and in the bubble dispersion, respectively.

(atm)

Figure 7. Plots of N A for aqueous NaHS03 solutions as a function of interfacial partial pressure of NOz. Broken lines show the calculated values of N A .

solution may be represented by replacing hl and CBOin eq 8 with h2 and CB'O,respectively, where h2 is the second-order rate constant in an empirical equation r

~ = 0- h2[NO2][NaHS03] ~

(9)

The value of h2 was found to be 1.5 X lo4 L/mol s a t 25 "C. The calculated values of N Aby eq 8 replaced with h2 and CB'O are shown by the broken lines in Figure 7. Both the experimental and calculated values are in good agreement except for the solution of CB'O higher than 0.2 M, where the saltingout effect on the solubility of NO2 would be considered to be concerned. In addition, the absorption runs in the mixed salt solution of Na2S0,3and NaHS03 were made a t a defined total salt concentration. The typical results are shown in Figure 8 as a plot of the specific rate of absorption against the molar ratio of NaHSO3 to the total salt. The rate of NO2 absorption decreased with an increase in the bisulfite ratio, for 0.1 M mixed atm. solution lowering to about one-third a t p~ = 3.5 X For NO absorption it can be seen from Figure 8 that the effect of bisulfite ratio on the absorption is less than that for N02. Since the pH value of the bisulfite solution is lower than 7 , as seen in Figure 8, sulfur dioxide generated from a decomposition of a part of NaHS03 is liberated to the gas phase in the absorption vessel. When the bisulfite concentration is higher, it may be considered that the effect of the presence of SO2 in the gas film becomes important and is more complicated. For the absorption of NO2 in the mixed solution with Na2SO:3 and NaHS03, however, it would regarded as gas absorption accompanied by competitive reactions, in which nitrogen dioxides react independently with SOa2-, HS03-, and water. Then, the absorption rate may be given as NA=

[ (i2

hhydCAi

-k ~ Z ~ C -k BO k2CB'O)

CAL (10)

Absorption in Various Sulfite Solutions Besides sodium sulfite, potassium and ammonium sulfites were used as the absorbent medium for NO2. The results obtained in 0.1 M solutions of these sulfites are shown in Figure 9 as plots of N Avs. P N O ~ .From the figure, it is obvious that the absorption rate is in the order of NH4+ > K+ > Na+, and the sequence is corresponding to the viscosity of the solutions. This fact also indicates that the diffusivity of NO2 in the liquid film is significant for the absorption in these sulfite solutions.

Partial pressure of NO, x IO?atm) Figure 9. The rates of NO2 absorption into aqueous solutions of various salts and urea as a function of the bulk partial pressure of NOz.

In addition, the absorption of NO2 was conducted in an aqueous solution of urea, which has been reported as a decomposing agent for NO2 by Nakagawa (1974). The results obtained were, however, in good agreement with those for water as shown in Figure 9, and it suggests that under neutral conditions a urea has no activity for decomposition of NO2. Furthermore, alkaline potassium iodide and permanganate solutions were examined as the absorbent for oxidizing the nitrous acid formed by the hydrolysis of NO*. The typical results are also shown in Figure 9. For the mixed solution of 1 M KI and NaOH, as can be seen in Figure 9, the absorption was not controlled by the transport of NO2 across the gas film, but corresponded to the results for 0.05 M Na2S03 solution. This behavior is not in conformity with the information of Chilton and Knell (1972) for NO2 concentrations of a few percent in the gas phase, while the absorption rate for alkaline permanganate solution was no more than about twice that for water. From an absorption viewpoint, therefore, it is considered that the sulfite solution is most effective as an absorbent for removing NO*, as reported by Chappell (1972). However, in practice for removing nitrogen oxides from stack gases, besides oxidizing NO into NOz, the dissipation loss of the sulfite by a reaction with the residual oxygen would be of important significance, as will be discussed in the subsequent study.

Effect of Temperature on Absorption Rate T o study the temperature dependency of the rate of NO2 absorption, the experimental runs were made a t 10,15, and Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 3, 1977

307

Na2S03 and NaHS03, eq 10 was derived assuming that sulfites and bisulfite took part in the reactions competitively with water. A temperature rise from 10 to 25 "C lowered the rate of NO2 absorption into water and Naps03 solutions. However, with increasing sulfite concentration, the increase in reaction rate offset the decreased solubility of NO2.

e o , ,I

Nomenclature C = concentration in liquid phase, mol/cm3 or L D = diffusivity, cm2/s H = Henry's law constant defined by C = H p , mol/cm3 atm k G = gas side mass transfer coefficient, mol/cm2 s atm h' = reaction rate constant referring to NO, L/mol s k h y d = rate constant for hydrolysis of NO2, L/mol s k l , k2 = rate constant for reaction between NO2 and S032-, and for reaction between NO2 and HSOs-, respectively, L/mol s m = reaction order with respect to dissolved gas n = reaction order with respect to solute reactant in liquid or agitation speed N A , NA' = absorption rate of NO2 and NO, respectively, mol/cm2 s p = partial pressure of solute gas, atm r = reaction rate, mol/L s 4 ~ =' absorption rate of NO per unit volume of dispersion, mol/cm3 s Parentheses ( ) = concentration in gas phase Brackets [ ] = concentration in liquid phase

(atm)

F i g u r e 10. T h e rates o f NO2 absorption i n t o aqueous Na2S03 solutions a t 10.15, a n d 25 "C as a f u n c t i o n o f t h e bulk p a r t i a l pressure of

NO?.

25 "C. Typical results are shown in Figure 10 as plots of N A vs. pso2i with temperature as a parameter. Temperature rise from 10 to 25 "C lowers the rate of NO2 absorption in water and aqueous solutions of low sulfite concentration. Wendel and Pigford (1958) have also reported such a decrease in the absorption of Np04 into water. Such an effect may be attributed to the decreased solubility of NO2 in the liquid a t higher temperature. In addition, a dimerization of NO2 is favored in the thermodynamic equilibrium a t lower temperature, the resultant Np04 being of high solubility. However, the higher the sulfite concentration, the smaller the effect of temperature on the overall absorption rate. This is due to the increase in reaction rate a t the higher temperature, which more than offsets the decreased solubility. Furthermore, the absorption runs in NaHS03 solutions were made at the same three temperatures. However, it was found that the temperature rise had no appreciable effect on the absorption, as compared with those in the sulfite solution.

Conclusions The absorption of NO in Na2SO3 solution was accompanied by a fast, pseudo-second-order reaction with respect to NO. The absorption rate was represented by eq 2 with the reaction rate constant of lo9 L/mol s a t 25 "C. The absorption of NO2 in Naps03 solution was accompanied by the competitive reactions involving the reactions of NO2 with S03*- and water. The absorption rate was interpreted by eq 8 nith hl of 6.6 X lo5 L/mol s and k h y d of 7.4 X lo7 L/rrp!-. Further, from the absorption in aqueous solutions of K2S03 and (NH&S03, the effect of liquid viscosity on the absorption rate was also confirmed. As for the reaction between NO2 and HS03-, the rate constant hp was 1.5 X lo4 L/mol s. The absorption rate in bisulfite solution was also interpreted by replacing k 1 in eq 8 with k z . For the absorption of NO2 in the mixed solution of

308

Ind. Eng. Chem., Process Des. Dev., Vol.

16,No. 3, 1977

'

Subscripts A = absorbed gas, NO2 or NO B, B' = nonvolatile reactant, Na2SOs and NaHSOs, respectively G = gas phase i = gas-liquid interface L = liquid phase 0 = bulk liquid Literature Cited Andrew, S. P. S., Hanson, D.. Chern. Eng. Sci., 14, 105 (1961). Caudle, P. G., Denbigh, K. G..Trans. Faraday Soc., 49, 39 (1953). Chappell, G. A,, EPA-R2-72-051 (1972). Chilton, T. H., Knell, E. W., PACHEC 111, 75 (1972). Gilliland, E. R., Ind. Eng. Chern., 26, 681 (1934). Kramers, H., Blind, M. P. P., Snoeck, E., Chern. Eng. Sci., 11, 61 (1959). Nakagawa, S., J. Jpn. Pet. Insf., 17, 392 (1974). Noguchi. K.. J. Chern. Eng. Jpn., 21, 122 (1976). Onda. K.. Takeuchi, H., The 6th Symposium of Chemical Reaction Engineering at Nagoya, p 151, 1966. Onda, K., Takeuchi, H., Maeda. Y., Chern. Eng. Sci., 27, 449 (1972). Peters, M. S.,Ross, C. P., Klein. J. E., A.1.Ch.E. J., 1, 105 (1955). Reith. T., Beek, W. J., Chern. Eng. Sci., 28, 1331 (1973). Sherwood, T. K., Pigford, R. L., "Absorption and Extraction", 2nd ed, p 317, McGraw-Hill. New York, N.Y., 1952. SINCAT, French Patent, N1-454-723 (1965). Takeuchi, H., Maeda, Y., Ito, K., Kagaku Kogaku Ronbunshu(Bull. Chern. Eng. SOC.Jpn.). 1, 252 (1975). Tohata, H.. Kagaku Kagaku(Chern. Eng.), 39, 481 (1975). Wendel, M. M., Pigford, R. L.. A./.Ch.E.J., 4, 249 (1958). Wilke. C. R., Chang, P., A.1.Ch.E. J., 1, 264 (1955).

Received f o r revieu: M a y 10, 1976 Accepted F e b r u a r y 16, 1977 Grateful acknowledgment is made t o the Foundation o n Development of Technology for Preventing NO, emission f r o m I r o n Steel Facilities for p r o v i d i n g financial assistance for t h i s investigation.