Ind. Eng. Chem. Res. 2005, 44, 9989-9995
9989
Reaction Behavior of Sulfur Dioxide with Ammonia Yanxia Guo,†,‡ Zhenyu Liu,*,† Zhanggen Huang,† Qingya Liu,† and Shijie Guo† State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
To understand the sulfur recovery behavior during regeneration of SO2-adsorbed sorbents in NH3, the reactions of SO2-NH3-H2O in the absence of O2 are studied in a temperature range of 10-80 °C and feed NH3/SO2 molar ratios of 1-3. The solid products are analyzed by thermal gravimetric analysis, the element analysis, and the wet chemical analysis. The results show that the reactions are very fast and the products are mixtures of various types of ammoniumsulfites with (NH4)2SO3 as the main component. A lower temperature, a greater NH3/SO2 molar ratio, and a higher water vapor content favor the SO2 conversion to the sulfites. The amount of the sulfites collected and their composition are correlated to temperature and the feed NH3/SO2 molar ratios. 1. Introduction
Liu et al. introduced ammonia into CuO/AC catalysts regeneration and suggested the following reactions:7
SO2 and NOx in flue gas are the major air pollutants. Many techniques have been developed or are under investigation to effectively and economically remove them. Simultaneous SO2 and NOx removal by catalystsorbents under dry conditions is considered to be promising; it is capable of capturing SO2 onto the catalyst-sorbents’ surface and selectively reducing NOx into N2 in the presence of NH3.1-4 One of the key steps in such a process is desorption of the adsorbed SO2 upon saturation of the catalyst-sorbents and conversion of the desorbed gaseous SO2 into products of market value and/or suitable for storage and transportation for further processing. Thermal regeneration has been used to desorb the SO2 by heating the catalyst-sorbents to temperatures much higher than that of SO2 and NOx removal, such as that used for activated carbon catalyst-sorbents, where the SO2 and NOx removal is carried out at ∼140 °C while the regeneration of the catalyst-sorbents is at ∼400 °C. This method consumes some of the activated carbon and generates gaseous SO2 that needs to be treated further.4 Another method is to use a reductive gas in the regeneration, such as H2, CO, or CH4, which converts the adsorbed sulfur species also into gaseous SO2, but at temperatures relatively lower than that of the thermal regeneration. Our earlier work found that NH3 is a better reagent for regeneration of many types of catalyst-sorbents, such as CuO/Al2O3, CuO/AC, and V2O5/AC.5,6 The advantages of using NH3 include reduced regeneration temperature, prevention of overreduction of the catalyst-sorbents, and, more importantly, the formation of solid ammonium-sulfur salts upon cooling of the regeneration effluent to temperatures 2.5 and temperatures of >40 °C, the products contain mainly the sulfites with an N/S ratio of 2.
Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 9993
Figure 8. Molar fraction of products with an N/S ratio of 2 at various temperatures and NH3/SO2 ratios.
Figure 9. Relation of the specific temperature and NH3/SO2 molar ratio.
It is important to note that the products identified in Table 3 can be reasonably simplified into two categories, (NH4)2S2O5, with an N/S ratio of 1, and (NH4)2SO3 and (NH4)2SO3‚H2O, with an N/S ratio of 2. On the basis of this categorization, the product composition can be determined directly from the data in Figure 7. Figure 8 shows that the fractions of the products with an N/S ratio of 2 (denoted as n) increase with increasing temperature, linearly with decreasing 1/T, at lower temperatures and then reach 1 at higher temperatures, which can be correlated well by eq 3,
Table 4. Element Analysis and Wet Chemical Analysis of the Products Obtained at Various Water Vapor Contents samples
S (%)
N (%)
H (%)
O (%)
P50-1-8.4 P50-2-0.2 P50-2-0.9 P50-2-3 P50-2-5.2 P50-2-8.4 P50-3-0.8 P50-3-2.4
29.4 27.6 27.7 27.0 27.3 27.8 26.5 27.1
22.9 23.6 23.5 23.6 23.7 23.8 23.2 24.0
6.7 6.7 6.7
40.9 42.0 42.1
6.8
41.5
a
1296.5 + A (for n >1, set n ) 1) n)T
(3)
N/Sa
remarks
1.80 1.95 1.94 2.00 1.98 1.96 2.00 2.00
element analysis element analysis element analysis wet chemical analysis wet chemical analysis element analysis wet chemical analysis wet chemical analysis
Molar ratio.
1 ) 0.002 847 + 0.000 037 exp(RNH3/SO2/1.23) (5) T*
where the majority of the products were (NH4)2SO3‚H2O at water vapor contents of >5 vol % or (NH4)2SO3 at water vapor contents of 1, set n ) 1) (6)
(NH4)2S2O5(s) T 2SO2(g) + 2NH3(g) + H2O(g) (vi)
where the value of A depends on the feed NH3/SO2 ratio. Clearly the lowest temperature (denoted as T*) at which n reaches 1 is an important parameter from both a theoretical and practical point of view, which can be expressed, by setting n ) 1, as
1 A-1 ) T* 1296.5
(4)
Regression of the T* with the feed NH3/SO2 ratio, shown in Figure 9, results in eq 5,
n)
3.2.3. Effect of Water Vapor Content. Table 4 shows the products composition, determined by the wet chemical analysis and the element analysis, at 50 °C and at various water vapor contents. Besides P50-1-8.4, a product formed at an RNH3/SO2 of 1, all the other products show N and S contents similar to that of (NH4)2SO3 (see Table 2), indicating no significant water effect on the product composition in a water vapor content range of 0.2-8.4 vol %. These data agree with the calculation shown in Table 3 based on eq 2, but are somewhat different from that reported by Bai et al.,17
This simplification makes it possible and useful, especially from the practical point of view, to correlate the SO2 conversion with the reaction conditions based on the experimental data. The thermodynamic equilibrium equations for reactions v and vi can be written, respectively, as
K1Θ ) PSPN2PH
(7)
K2Θ ) PS2PN2PH
(8)
where PS, PN, and PH denote the fractional pressures of SO2, NH3, and H2O, and K1Θ and K2Θ are the equilib-
9994
Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005
rium constants of reactions v and vi, respectively. Since K1Θand K2Θ can be correlated with temperature (T) in the form of:
ln K1Θ ) A1 +
B1 T
(9)
ln K2Θ ) A2 +
B2 T
(10)
where A1, A2, B1, and B2 are constants (assuming constant enthalpies), we have
ln K1Θ ) ln(PSPN2PH) ) A1 +
B1 T
(11)
ln K2Θ ) ln(PS2PN2PH) ) A2 +
B2 T
(12)
If the inlet fractional pressures of SO2, NH3, and H2O are PS,0, PN,0, and PH,0, the SO2 conversion is X, and the molar fraction of (NH4)2SO3 in the products is n as defined in Figure 8 or eq 6, then we have Figure 10. Linear regression results of reactions v and vi based on eqs 13 and 14 (lines indicate results from eqs 15 and 16).
PS ) PS,0(1 - X) PN ) PN,0 - [2nPS,0X + PS,0X(1 - n)] ) PN,0 - PS,0X(1 + n)
[
PH ) PH,0 - nPS,0X +
(1 -2 n)P X] ) S,0
(1 +2 n)
PH,0 - PS,0X and
{
ln PS,04(1 - X)[RNH3/SO2 -
(
(1 + n)X]2 RH2O/SO2 -
1+n X 2
)} ) A
1
-
{
B1 (13) T
ln PS,05(1 - X)2[RNH3/SO2 -
(
(1 + n)X]2 RH2O/SO2 -
1+n X 2
)} ) A
2
-
B2 (14) T
where RNH3/SO2 and RH2O/SO2 are feed ratios of NH3/SO2 (or PN,0/PS,0) and H2O/SO2 (or PH,0/PS,0), respectively. On the basis of the experimental data and linear regression, the following two correlations, eqs 15 and 16, are determined and plotted as the lines in Figure 10, and compared with the data points calculated using the right-hand sides of eqs 13 and 14.
ln K1Θ ) 12 -
10 124 T
(15)
lnK2Θ ) 15 -
13 058 T
(16)
It is important to note that eqs 15 and 16 are very different from those reported by Scargill and St. Clair (Table 1). The activation energies determined in this work for (NH4)2SO3 and (NH4)2S2O5 are small, 84.2 and 108.6 kJ/mol, respectively, in comparison to those in Table 1, 254.4 and 325.5 kJ/mol, respectively. The small activation energies obtained in this work indicate that
Figure 11. Comparison of SO2 conversions from the experiments and the calculation.
the data obtained here do not reflect the reaction equilibrium but are influenced by agglomeration of the ammonium-sulfites aerosol formed from the fast reactions of SO2-NH3-H2O. This deduction is not surprising and is consistent with the water effect on the product collection shown in Figures 4 and 5. This finding is important because it indicates that a key factor, in practical operations, to the ammonium-sulfites collection is the rate of agglomeration. Figure 11 compares the calculated SO2 conversions based on eqs 13-16 with the experimental data. The reasonable agreements suggest that the equations are applicable in practice. 4. Conclusions The reactions of SO2-NH3-H2O in the absence of O2 are studied in a temperature range of 10-80 °C at feed NH3/SO2 ratios of 1-3 and water vapor contents of 0.28.4 vol % to meet the needs of sulfur recovery from SO2 adsorbed catalyst-sorbents. The major findings are as follows:
Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 9995
(1) The reactions are very fast to reach thermodynamic limits. The amounts of the solid products collected increase with increasing NH3/SO2 molar ratio and water vapor content in the feed and decreasing temperature. (2) The products contain mainly (NH4)2SO3, especially at temperatures >50 °C and/or NH3/SO2 ratios >2. The minor components are (NH4)2SO3‚H2O and (NH4)2S2O5. The amount of (NH4)2SO3 in the overall products (denoted as n) increases with decreasing temperature and increasing NH3/SO2 molar ratio as shown by
n)
-1296.5 + 4.692 + 0.048 exp(RNH3/SO2/1.23) T (for n > 1, set n ) 1)
which results in a relationship between the lowest temperature (T*) at which the product contains only (NH4)2SO3 and the feed NH3/SO2 ratio:
1 ) 0.002 847 + 0.000 037 exp(RNH3/SO2/1.23) T* (3) Water vapor has no obvious effect on the product composition but promotes the agglomeration of the aerosol ammonium-sulfites up to a water vapor content of 1 vol %. (4) In a temperature range of 10-80 °C, the SO2 conversion can be determined by the correlation for n and the following two equations, which reflect mainly the agglomeration rate of the ammonium-sulfites aerosol formed from the fast reaction of SO2-NH3-H2O in the absence of O2.
ln K1Θ ) 12 -
10 124 T
ln K2Θ ) 15 -
13 058 T
Literature Cited (1) Armor, J. N. Environmental catalysis. Appl. Catal., B. 1992, 1, 221. (2) Bosch, H.; Janssen, F. Formation and control of nitrogen oxides. Catal. Today. 1988, 2, 369. (3) Rosenberg, H. S.; Curran, L. M.; Ando, J.; Oxley, J. H. Post combustion methods for control of NOx emissions. Prog. Energy Combust. Sci. 1980, 6, 287. (4) Olson, D. G.; Tsuji, K.; Shiraishi, I. The reduction of gasphase air toxics from combustion and incineration sources using
the MET-Mitsui-BF activated coke process. Fuel Process. Technol. 2000, 65-66, 393. (5) Xie, G.; Liu, Z.; Zhu, Z.; et al. Reductive regeneration of sulfated CuO/Al2O3 catalyst-sorbent in ammonia. Appl. Catal., B. 2003, 45, 213. (6) Liu, S.; Liu, Z.; et al. Regeneration of CuO/AC Desulphurizer. Chin. Envi. Sci. 2000, 19, 313. (7) Liu, S.; Liu, Z.; et al. Selectively reductive regeneration of the used CuO/AC desulfurizer with NH3. Hua Xue Tong Bao 2001, 9, 573. (8) Ma, J.; Liu, Z.; et al. Regeneration of used V2O5/AC catalystsorbent. Chin. J. Catal. 2005, 26, 463. (9) Vance, J. L.; Peters, L. K. Aerosol Formation Resulting from the Reaction of Ammonia and Sulfur Dioxide. Ind. Eng. Chem. Fundam. 1976, 15, 202. (10) Scott, W. D.; Lamb, D. Two solid compounds which decompose into a common vapor. Anhydrous reactions of ammonia and sulfur dioxide. J. Am. Chem. Res. 1970, 92, 3943. (11) Scargill, D. Dissociation Constants of Anhydrous Ammonium Sulphite and Ammonium Phrosulphite Prepared by GasPhase Reactions. J. Chem. Soc. A 1971, 2461. (12) Landreth, R.; de Pena, R. G.; Helcklen, J. Thermodynamics of the reaction of ammonia and sulfur dioxide in the presence of water vapor. J. Phys. Chem. 1975, 79, 1785. (13) Tock, R. W.; Hoover, K. C.; Faust, G. J. SO2 Removal by transformation to solid crystals of NH3 complexes. AIChE Symp. Ser. 1979, 75, 62. (14) Hartely, E. M.; Matteson. M. J. Sulfur Dioxide Reactions with Ammonia in Humid Air. Ind. Eng. Chem. Fundam. 1975, 14, 62. (15) Hjuler, K.; Dam-Johansen, K. Mechanism and kinetics of the reaction between sulfur dioxide and ammonia in flue gas. Ind. Eng. Chem. Res. 1992, 31, 2110. (16) Bai, H.; Biswas, P.; Keener, T. C. Particle formation by ammonia-sulfur dioxide reactions at trace water conditions. Ind. Eng. Chem. Res. 1992, 31, 88. (17) Bai, H.; Biswas, P.; Keener, T. C. SO2 Removal by NH3 Gas Injection: Effects of Temperature and Moisture Content. Ind. Eng. Chem. Res. 1994, 33, 1231. (18) Hirota, K.; Ma¨kela¨, J.; Tokunaga, O. Reactions of Sulfur Dioxide with Ammonia: Dependence on Oxygen and Nitric Oxide. Ind. Eng. Chem. Res. 1996, 35, 3362. (19) State Environmental Protection Administration. Kongqi he feiqi jiance fenxi fangfa; Chinese Publishing Company of Environmental Sciences: Beijing, China, 1990. (20) Perry, R. H.; Green, D. W. Perry’s Chemical Engineer’s Handbook; Mcgraw-Hill Companies, Inc.: New York, 2001.
Received for review June 20, 2005 Revised manuscript received August 27, 2005 Accepted September 29, 2005 IE050734Q
