Ind. Eng. Chem. Prod. Res. Dev. 1984, Braca, G.; Sbrana, G.; Valentini, G. (to C.N.R.) [tal. Patent Appl. . . 21 462, A/ 82, May 25 1982a. Braca, G.; Sbrana, G.; Valentlni, 0.; Clnl, M. J . Mol. Catal. 1982b, 77, 323. Braca. 0.; Sbrana, G.; Valentlni, G.; Barberlnl, C. C1 Mol. Chem. 1984, 7, 9. Bradley, J. S. J . Am. Chem SOC. 1979, 70797419. Bryant, F. J.; Johnson, W. R.; Singleton, T. C. Presented at the 165th National Meeting of the American Chemical Society, Dallas, TX, 1973; General Papers, Petrochem; pp 193-197. Drent, E. (to Shell) Eur. Patent Appl. 31 606, Dec 1, 1980. Ehrler. J. L.; Juran, B. Hydrocarbon Process 1982, 67(2), 109. Gauthier-Lafaye, J.; Perron, R. (to RhonaPoulenc Ind.) Eur. Patent Appl. 31 784, Dec 9, 1980. Grey, R. A.; Pez, 0. P.; Wallo, A. J . Am. Chem. SOC. 1981, 703, 7536. Gualnai, 0.; Valentini, 0. Chim. Ind. (M/&n) 1983, 65, 285. lmhausen Chemle Gmbh. Ger. Patent Appl. 2 731 962, Feb 1, 1979. Isogai. N.; Okawa, T.; Hosokawa, M.;Wakui, N.; Watanabe, T. (to Mltsublshl Gas Chem. Co) U.K. Patent Appl., 2078219, June 5, 1981.
417
23,417-421
Jenner, 0.; Kheradmand, H.; Klennemann, A.; Deluzarche, A. J . Mol. Catal. 1983, 18, 61. Keim, W.; Berger, M.; Schlupp, J. J . Catal. 1980, 6 1 , 359. Kelster, J. P.; Gentile, R. J . Organomet. Chem. 1981, 222, 143. Kheradmand, H.; Jenner, G.; Klennemann, A,; Deluzarche, A. Chem. Lett. 1982, 395. Knlfton, J. F. J . Mol. Catal. 1981, 7 1 , 91. Mltsublshi Gas Chem. Co; Jpn. Patent Appl. 80/167 233, Dec 26, 1980. Rathke, J. W.; Feder, H. M. J . Am. Chem. SOC. 1978, 700, 3623. Roper, W. R.; Taylor, G. E.; Waters, J. M.; Wright, L. J. J . Organomet. Chem. 1979, 782, C46. Tonner, S. P.; Trlnn. D. L.; Wainwright, M. S. J . Mol. Catal. 1983, 78, 215.
Received for reuiew November 28, 1983 Accepted March 13, 1984
Cleaning of Flue Gas by Use of Molten Salts. 1. Simultaneous Reduction of SO, and NO, with V205 Catalyst Tsutomu Shlkada; Takamasa Oba, Kaoru Fujlmoto, and Hlro-o Tomlnaga Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Simultaneous reduction of SO, and NO, from flue gas by use of molten salts of the NH4HS04/NaHS0,system containing metal sulfates or oxides as catalysts was studied in a temperature range from 130 to 220 OC. It was found that V205showed a relatively high catalytic activity for the reduction of both SO, and NO, by reaction with ammonia. Addition of transition metal sulfates such as Ti(S04)2,CuSO,, or Zr(S04)2led to a marked increase in the SO, reduction. In the reaction system SO2 was absorbed in an unstable form In the absence of catalyst but was trapped as a more stable form, probably as NH,HSO, in the presence of V205catalyst, while NO was reduced by ammonia to nitrogen in the presence of the catalyst.
Introduction For removing sulfur oxides and nitrogen oxides, flue gases emitted from stationary sources such as petroleumor coal-fired heating furnaces and iron-ore sintering furnaces are being processed by two consecutive steps (Todo and Ueno, 1978; Moser, 1981; Siddiqi and Tenini, 1981; Kurita, 1982). Por example, NO, in flue gas is reduced with solid catalyst to nitrogen by ammonia and then SO, is fixed as sulfates by a wet-type desulfurization process. Although the process has been technically well-established, the plant and its operation are expensive due to the complicated dual reactors and equipment. Furthermore, the wet-type desulfurization step is carried out at low temperatures ranging from 50 to 70 "C and hence the purified stack gas must be reheated by use of heat-exchange or by burning fuel gas. The process has another disadvantage in that a large amount of water is required to make up the loss of water from the wet-type desulfurization equipment. It is desirable, therefore, to develop a flue gas cleaning technique where by SO, and NO, are simultaneously removed in one reactor in the temperature range from 100 to 200 "C. The authors have attempted the simultaneous reduction of SO, and NO, by use of molten hydrogen sulfates as reaction media based on the following three findings: (1) Supported vanadium oxide catalysts show high activities for the reduction of NO with ammonia even when they are covered with several tenths of a percent by weight of NH4HS04(Shikada et al., 1978,1981). (2) An equimolar mixture of NH4HS04and NaHS04 is a liquid with a low viscosity in the temperature range from 100 to 200 O C . (3)
Various metal sulfates and oxides are highly soluble in the molten salt. It was found that V2O5 showed a relatively high catalytic activity for the reduction of both SO, and NO,, and the addition of various transition metal sulfates led to a marked increase in the reduction of SO, (Shikada et ai., 1983b,c).
Experimental Section Materials Used. The gases, NO, SO2,02,and N2,from commercially available cylinders were used without further purification. An ammonia water was supplied by use of a microfeeder. All chemicals used for reaction media and catalysts were reagent grade. Catalyst consisting of V205-NH4Br-Ti02-Si02 (1:0.2:0.2:1, weight ratio) was prepared by a conventional impregnating method described previously (Shikada et al., 1983d). Measurement of Physical Properties of Molten Salts. The melting points of hydrogen sulfates were determined with a differential thermobalance (Sinku Riko TGD-300). The measurement of the viscosity of NH4HS04/NaHS04containing 5% by weight V205was carried out with a rotation viscometer (Rion VT-03). Identification of Product from NO. Figure 1shows the batch reactor (volume, 300 cm3) used for the identification of reaction product derived from NO. About 25 cm3 of a molten salt containing 0.5% by weight of V205 was placed in the reactor and heated in an oil bath with evacuation. The gases with prescribed concentrations of NO, NH3, and O2 were then introduced to it and both liquid and gas phases were stirred. The gases in the reactor
Q196-4321/84/1223-Q417~Q1.~QlQ 0 1984 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 l
200,
227'
Molar fraction of
NOH?&
Figure 3. Melting point diagram for NH4HS04/NaHS04system.
Figure 1. Apparatus for measurement of product from N O (a) magnet; (b) thermocouple; (c) gas-phase stirrer; (d) liquid-phase stirrer; (e) capillary; (f) mass spectrometer. All dimensions are in millimeters.
0' 100
130 160 Temperature I'C)
190
1
Figure 4. Viscosity of NH4HS04/NaHS04with dissolved V20+ 600
Figure 2. Agitated bubble type reactor: (a) gas inlet; (b) gas outlet; (c) ammonia water; (d) stirrer; (e) baffle; (f) thermocouple. All dimensions are in millimeters.
were continuously sampled by a capillary tube at the rate of about 1cm3/minand introduced into a quadrupole mass spectrometer (NEVATE-lW), where the change in the gas composition with reaction time was analyzed. Apparatus, Procedhre, and Analysis for the Continuous Flow Reaction. A continuous flow type reaction apparatus was employed at atmospheric pressure. Figure 2 shows the reactor, which is an agitated, bubble type equipped with two stirrers. After the molten salt reaction medium and catalyst were place in the reactor and heated to a prescribed temperature with stirring,the gases of NO, SOz, 02,and N2 and an ammonia water were introduced to the bottom of the reactor through the pipe used for stirring to start the reaction. The gases a t the outlet of the reactor were dried by passing through a Liebig condenser. Sulfur dioxide and nitrogen oxides were analyzed by a controlled potential electrolysis SO2 meter (Koritsu Rika KS-300)and a chemiluminescence NO/NO, meter (Beckman 9511, respectively. Reaction conditions, unless cited otherwise, were as follows: amount of molten salt,
!
600
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 419
5
I
300
st;;!ing,u-!
c
T
250
O
1
2
3
5
4
6
Time on streom khl
250
' 0
1
2
3
Figure 8. Effects of stirring and heating on outlet SOzconcentration in the presence of Vz06catalyst. Medium: NH4HS04/NaHS04(l:l, molar ratio); catalyst concentration: 5 wt % .
I 5
4
Time on stream ( h l
Figure 6. Dynamic response to ammonia water feed in the absence of catalyst. Medium: NH4HS04/NaHS04(l:l,molar ratio); reaction temperature: 160 "C;inlet gas composition: 400 ppm of SOz, 300 ppm of NO, 0 or 450 ppm of NH8, 5% 02,0 or 10% H 2 0 in N2. IC-
160°C ---+
ic 190°C
I \
I\
'
+
"
2 50
I
Heoting uc
Off
0
1
2
3
4
5
'
C
10
20
Tire
I 300 b r j n g
1
0c'
6
Tlme on stream ( h l
Figure 7. Effects of stirring and heating on outlet SO2concentration in the absence of catalyst. Medium: NH4HS04/NaHS04(l:l,molar ratio).
in a solution, the catalysis by solvent itself or absorption of SO, and NO, to the solvent is not negligible. Blank tests were performed in molten salt alone. The results are shown in Figure 5. The concentration of NO at the outlet of the reactor was equal to that of the inlet gas over the range of the molten salt temperatures from 130 to 220 O C , indicating that either NO was not absorbed at all in the molten salt or quickly reached a saturation level. On the other hand, SO2concentration in the outlet gas was equal to that in the inlet gas at or above 190 "C but was lower a t temperatures below 190 "C, suggesting that SO2 was trapped in the molten salt in some form. Further study was made to clarify this point. Figure 6 shows the dynamic response of SO2 concentration in the effluent gas with and without ammonia water feed at 160 O C . When 400 ppm of ammonia and 10 vol % water vapors were present in the feed, SO2 concentration in the outlet gas was 310 ppm. When the feed of ammonia water was stopped, SO2 concentration in the outlet gas increased gradually to reach a steady-state concentration of 400 ppm, which was equal to the concentration of SO2 in the inlet gas. Although no change in SO2 concentration was observed when water was fed, its concentration decreased to reach a steady state when ammonia water was fed again. It is apparent from the above results that ammonia water is essential for the capture of SOz. h shown in Figure 7, while the outlet SO2 concentration almost instantly increased to 370 ppm and became stable when stirring was stopped, it increased to 500 ppm and then decreased gradually to reach a steadystate concentration of 310 ppm when stirring was resumed. By heating the molten salt up to 190 OC,the SO2 concentration in the outlet gas increased markedly to higher than that of the inlet gas and then decreased. Thus, SO2 is reversibly absorbed in the media; considering the results
40
30
50
60
on streom ( h i
Figure 9. Change in gas composition with reaction time. Medium: NaHS04/LiHS04(l:l, molar ratio); catalyst: V206(0.5 w t %); temperature: 160-170 "C; initial gas composition: 27.2% NO, 8.0% NH,, 8.8% 02, 56.0% He; initial pressure: 97 kPa.
in Figures 6 and 7, SO2is probably trapped as (NH4)2S03 or NH4HS03in the case of catalyst-free molten salt. The assumption is supported by the fact that both (NH4)2S03 and NH4HS03 are unstable and decompose easily on heating. Reaction Products. Figure 8 shows the effect of stirring and heating on the outlet SO2 concentration in the presence of Vz05catalyst. A marked increase in the outlet SO2concentration with stirring or heating up was hardly observed. Therefore, in the presence of catalyst SO2 is trapped as a more stable form, probably as NH4HS04. The formation of (NH4I2SO4is considered to be negligible because it decomposes to NH4HS04and NH3 at temperatures above 120 "C. The removal of SO2 in the present reaction system is shown in the following equations.
--
SO2 + 1/202 SO3 SO3 + NH,
+ H20
NH4HS04
(1) (2)
On the other hand, the identification of the product derived from NO was carried out with a batch reactor by measuring the change in the gas composition with reaction time. A molten salt of NaHS04/LiHS04was used as reaction medium because the use of NH4HS04might cause ambiguity due to the formation of N2 or of NH3. In addition, 0.5% by weight of V205 was used as catalyst to regulate the reaction in order to visualize the change with reaction time. The results are shown in Figure 9. The concentrations of NO, NH,, and O2 in the gas phase decreased rapidly with the time. The sole reaction product was N2,while no formation of NO2 or N20 was observed. Thus, in the reaction system NO is reduced by NH3 to N2 as is the case of the NO reduction with NH3 on supported V205 catalysts (Fujimoto et al., 1977), as shown in the following equation. NO
+ NH, + ' / 4 0 2
-
N2 + 3/2H20
(3)
The formation of N2 increases with time but is far smaller
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984
Table I. Activities of Various Catalystsa conversion, so, NO cat.* VzO5~NH4Br-TiO2-SiO2 32.5 4.7 31.1 6.1 NH4V03 33.0 22.4 vZo5 1.1 3.0 VOS04 32.6 0 Ti(S04)2 13.1 0 cuso4 a Medium: NH4HS04/NaHS04 (l:l, molar ratio). *Concentration: 5 w t %. cReaction temperature: 160 "C; inlet gas composition: 400 ppm of SO2, 300 ppm of NO, 450 ppm of NH,, 5% Oz, 10% HzO, Nz balance.
____
j '
5c
~~
I
0
i z r
V,C,
I,
5
a
' io
13
ioncentrotion (wtll
Figure 11. Effect of Vz05concentration on SO2 and NO conversions. Medium: NH4HS04/NaHS04(l:l,molar ratio); reaction temperature: 163 "C. ,/
overall rate of the SO2 reduction is approximated by eq 4 (the term of oxygen is neglected) ?.so*
-
A-r
-jl
7
I
t';irr
_ _ ~ -L
tr-
Figure 10. SOz and NO conversions aa a function of reaction temperature. Medium: NH4HS04/NaHS04(13,molar ratio); catalyst: vzo5(5 wt %),
than that of the stoichiometry calculated from the observed disappearance of NO or NH3 in the gas phase. This might be due to the remaining of fairly large parts of NO and NH3 dissolved in the medium. In fact, the rate of the reaction between them is slow in the low concentration of V205as will be described later. Catalytic Activities of Metal Salts and Oxides. Table I shows the results by use of a flow reactor on salt of NH4HS04NaHS04 (l:l, molar ratio) for the reduction of SO2and NO. V205-NH4Br-Ti02-Si02 catalyst, which has been shown to be highly active at temperatures as low as 150 "C for the reduction of NO with ammonia by the present authors (Shikada et al., 1983d), was powdered to sizes smaller than 100 mesh and dispersed in the reaction medium. All other compounds added were soluble in the medium. From the results shown in Table I, VZO, showed a high activity for the reduction of NO while all compounds tested, except VOS04 and CuS04, showed nearly the same activities for the reduction of SOz. SO2 is reduced by reaction medium alone (22.5% conversion at 160 "C) and hence the added catalyst does not seem to be so effective. However, SO2 is trapped as a more stable form in the presence of catalyst as described above. Thus, V205 was found to be effective as the catalyst for the simultaneous reduction of SO2and NO. Reaction Temperature. Figure 10 shows the activity of V205 catalyst for the reduction of SO2 and NO as a function of reaction temperature. The conversion of SOz decreased monotonously with a rise in the temperature while that of NO increased with increasing temperature and an NO conversion level of 65.7% was obtained at 220 "C. Apparent activation energy was -5.4 and 30.1 kJ/mol for the reduction of SO2 and NO, respectively. In the present reaction system the reduction of SO2 is considered to proceed in three steps as follows: (1)dissolution of SO2 into the reaction medium, (2) oxidation of SOz to SO3 in the medium, and (3) formation of NH4HS04by reaction of SO3 and ammonia water. the rate of step (3) is sufficiently larger than those of steps (1)and (2) and hence the
kcso, = kKPso,
(4)
where k is the rate constant of oxidation of SO2 to SO3, Csozis the concentration of SO2in the liquid phase, Psoz is the partial pressure of SO2 in the gas phase, and K is the gas-liquid equilibrium constant. In general, high temperature favors the oxidation of SO2( k is large) while low temperature favors the absorption of SO2(Kis large) as can be seen from Figure 5. Therefore, the oxidation of SO2 in the medium and the absorption of SO2 into the medium might be rate-determining steps at low and high temperatures, respectively. Further details of the rate of absorption and reaction in the media have been studied and will be reported. V205Concentration. Figure 11shows the conversions of SO2and NO as a function of V205concentration. For reducing SO2 the addition of 1% by weight of V2O5 was found effective but not significantly so. Conversion of SO2 increased slightly with an increase in the concentration of V205and was independent of it above 6 w t %. Conversion of NO increased markedly with an increase in the Vz05 concentration up to 7 wt % when it reached a constant level. On the other hand, the viscosity of the molten salt containing dissolved V205increased with increasing V205 concentration. The dissolution of V2O5 in the medium was saturated at around 160 "C. Therefore, the concentration of 5% by weight of V205is suitable for the present reaction system. Effect of Additives. The addition of various transition metal sulfates has been tried in order to increase the rate of the SO2 and NO reduction. The experiments were carried out in the media containing 5% by weight of V205 and 1% by weight of metal sulfate. The results are shown in Table 11. The addition of the sulfates promoted the reduction of SO2by about a factor of 2 in SO2conversion. The simultaneous addition of two kinds of sulfates was not very effective. A marked increase in SOz conversion by the addition of the sulfates might be attributed to the modification of V505by these metal ions for the oxidation of SO2but details are not clear. On the other hand, the addition of the sulfates was scarcely effective for the enhancement of NO reduction. Effect of Molten Salt Reaction Media. Table I11 shows the extent of SO2 and NO reduction in several molten salt mixtures containing dissolved V,05. When KHSO, was used instead of NaHSO,, higher SO2 conversion and lower NO conversion were obtained. The decrease in NO conversion is not large in comparison with solid catalyzed system in which the activity of Vz05for NO reduction with ammonia is suppressed remarkably by
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 3, 1984 421
Table 11. Effect of Sulfate Additives" conv. of SOz, % 130 "C 160 "C 190 "C 34.9 33.0 27.8 Ti(S04)z 61.1 62.3 45.3 cuso4 60.6 58.7 59.9 Zr (SO& 62.4 62.4 52.9 PbS04 60.9 60.9 58.1 SnSO, 58.6 58.6 58.9 70.0 Ti(S04)z cuso, 72.2 72.2 Ti(SO& Zr(SO& 58.9 44.4 Ti(S04)~ FeSO, 61.4 63.6 62.5 "Medium: NH4HS04/NaHS04(l:l, molar ratio); catalyst: Vz05(5 wt %). add. 1. (1wt %)
add. 2. (1 wt %)
130 "C 13.7 16.3 13.3 14.1 12.1 12.3 11.6 8.8
conv. of NO, % 160 "C 22.4 22.8 22.7 24.5 21.3 16.7 20.4 14.7 19.1
190 "C 40.0 48.1 40.0 45.3 42.4 40.1 42.0 34.4 35.3
Table 111. Effect of Media on SO2 and NO Conversions" conv. of SO2, % 160 "C 190 "C 33.0 27.8 51.2 50.0 34.1 32.9 46.4 46.4
medium, (1:l) molar ratio) 130 "C NH,HSO,/NaHSO, 34.9 NH;HSO:/KHSO, NH4HS04/LiHS04 34.3 NaHS04/LiHS04 42.9 "Catalyst: V2Ob(5 wt %).
-
looY
80
I
I
O
EJ -60 B c 0
c
-40 ; ; i
> u
0
'0
-
200
400 600
800
130 "C 13.7 43.3 60.3
conv. of NO, % 160 "C 190 "C 22.4 40.0 11.7 28.3 59.3 73.5 71.1 80.1
220 "C 65.7 39.7
reduction of SO2 and NO, respectively.
O
- 20
220 "C 27.8 46.5
u 0
1000 1200
S t l r r l n g w e e d (ml
Figure 12. Effect of stirring speed. Medium: NaHS04/LiHS04 (l:l, molar ratio); catalpt: V20, (5 wt %); reaction temperature: 160 "C.
added potassium compounds (Shikada et al., 1983a). When LiHS04 was used in place of NaHS04, NO conversion was much higher while SO2 conversion remained almost unchanged. Using the molten salt composed of NaHS04 and LiHS04,a higher NO conversion, 80%, was obtained a t 190 "C. In addition, a slightly higher SO2 conversion was observed compared with NH4HS04/LiHSO1. Thus, it can be said that LiHS04 is favorable for higher NO conversion. Similar action of lithium salts has been observed for the solid catalyzed reduction of NO with ammonia (Shikada et al., 1983a). However, the role of lithium salts for promoting the NO-NH, reaction is not clear. Effect of Stirring Speed. Figure 12 shows the conversions of SO2 and NO as a function of stirring speed. Conversion of SO2 was hardly affected by the stirring speed while that of NO increased linearly with an increase in the stirring speed. The results lead to the conclusion that the oxidation of SO2in the reaction media and the dissolution of NO into the media are rate-determining steps in the
Conclusion It was found that V206 and other metal oxides and sulfates were soluble in a molten salt of NH4HS04/NaHSO4 in a low viscosity liquid state at temperatures above 120 "C. By the molten salt containing dissolved V2OS,the simultaneous reduction of SO2 and NO by reaction with ammonia was achieved at relatively high levels in the temperature range from 150 to 200 "C. The addition of transition metal sulfates such as Ti(S04)2,CuS04,or Zr(SO,),enhanced the reduction of SO2 markedly. The molten salt dissolving V206was also found to be highly soluble in water. The fact might have significance in the practical application for stack gas cleaning. In the present reaction system, not only SO, and NO, but also the dust in flue gas is expected to be trapped in the media. Alkali compounds in the dust may be converted into the reaction media themselves, and other insoluble compounds, after being trapped, can be easily separated from the media by washing with water. Literature Cited Fujlmoto, K.; Shlkada, T.; Kunugl, T.; Tomlnaga, H. " y o Kyokalshl 1077, 56, 666.
Kurlta, M. " y o Kyokaishi1982, 67, 896. Moser, R. E. Hydrocarbon Process. 1081, 60, 86. Shlkada, T.; Fujlmoto, K.; Kunugl, T.; Tomlnaga, H. " y o Kyokalshi 1078, 57, 991.
Shlkada, T.; Fujlmoto, K.; Kunugl, T.; Tomlnaga, H.; Kaneko, S.; Kubo, Y. Ind. Eng. Chem. Prod. Res. Dev. 1081. 20. 91.
Shlkada, T.; Fujimoto, K. Chem. Lett. 1083a. 77. Shlkada, T.; Oba, T.; Fujlmoto, K.; Tomlnaga, H. Chem. Lett. 1983b, 511. Shlkada, T.; Oba, T.; Fujlmoto, K.; Tomlnaga, H. Chetn. Lett. 1083c, 721. Shlkada, T.; Ogawa, H.; Fujlmoto, K.; Kunugi, T.; Tomlnaga, H. J . Chetn. Tech. Biotechnol. 1083d,33A, 446. Siddlql, A. A.; Tenlnl, J. W. mrocarbon Process. 1981, 60, 115. Todo, N.; Ueno, A. ShokubelI978, 20, 333.
Received for review December 2, 1983 Revised manuscript received May 14, 1984 Accepted May 30, 1984
