Znd. Eng. Chem. Res. 1990,29, 2267-2272 Skelland, A. H. P.; Hemler, C. L. Unpublished work, 1969, reported in Skelland (1985) above, pp 402, 408, 409. Skelland, A. H. P.; Lee, J. M. Agitator Speeds in Baffled Vessels for Uniform Liquid-Liquid Dispersions. Znd. Eng. Chem. Process Des. Deo. 1978, 17, 473-478. Skelland, A. H. P.; Lee, J. M. Drop Size and Continuous-Phase Mass Transfer in Agitated Vessels. AZChE J. 1981, 27, 99-111; 1983, 29, 174. Streitwieser, A.; Heathcock, C. H. Introduction to Organic Chemistry; MacMillan: New York, 1973. Sykes, P.; Gomezplata, A. Particle Liquid Mass Transfer in Stirred Tanks. Can. J. Chem. Eng. 1967,45, 189-196. Toor, H. L.; Marchello, J. M. Film-Penetration Model for Mass and Heat Transfer. AZChE J. 1958, 4, 97-101.
2267
Treybal, R. E. Stirred Tank and Mixers for Liquid Extraction. Znd. Eng. Chem. 1961,53, 597-606. Treybal, R. E. Liquid Extraction, 2nd ed.; McGraw-Hik New York, 1963.
Treybal, R. E. Mass Transfer Operations, 3rd ed.; McGraw-Hill: New York, 1980. Wilhelm, R. H. Rate Process-Application to Reactor Design. Chem. Eng. Prog. 1949,45,208-224. Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Dilute Solutions. AZChE J. 1955, 1, 264-270.
Received for review November 6 , 1989 Revised manuscript received June 22, 1990 Accepted July 4, 1990
GENERAL RESEARCH Adsorption of Nitrogen Monoxide by the Chelate Resin-Immobilized Iron(I1) Complex and Its Application for Simultaneous Removal of Nitrogen Monoxide and Sulfur Dioxide Hiroyuki Asanuma, Akihiko Takemura, Naoki Toshima,* and Hidefumi Hirait Department of Industrial Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
An aqueous dispersion of the resin-iron(I1) complex was prepared from FeS04 and chelate resin containing iminodiacetic acid moieties. The dispersion can simultaneously adsorb nitrogen monoxide and sulfur dioxide from a dilute mixed gas used as a model of a flue gas. The adsorbent dispersion was slightly deactivated by oxygen. The rate of deactivation of the dispersion, however, was much less than that of an aqueous solution of the corresponding monomeric model due to the slow diffusion of oxygen into the resin in the former case. Even after deactivation, the chelate resin can be easily recovered from the deactivated aqueous dispersions by separation by filtration, washing with a hydrochloric acid solution, and neutralization with a sodium hydroxide solution. The recovered resin can be reused as a starting material for the preparation of the fresh adsorbent dispersion by mixing with an aqueous FeS04 solution,
Introduction The concentration of nitrogen oxide in the atmosphere has tended to increase gradually during recent years not only in Japan but also all over the world (OSullivan, 1988), although much effort has been concentrated on removing nitrogen oxide from a flue gas. Moreover, together with sulfur dioxide it causes acid rain. Thus, advanced technology in this field is strongly desired to break through the present situation and improve the global atmospheric circumstance. The technologies that have been investigated for the removal of nitrogen monoxide (NO) from a flue gas can be classified into two categories. One is the development of new catalysts, by which poisonous NO is reduced to innoxious nitrogen by using a gaseous reductant such as ammonia and hydrogen (Tuenter et al., 1986; Juentgen et al., 1988a,b; Davini, 1988). The other is an absorption technique, in which nitrogen oxide is removed
* Author t o whom correspondence should be addressed. Present address: Department of Industrial Chemistry, Faculty of Engineering, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan. 0888-5885/90/2629-2267$02.50/0
with an aqueous absorbent (Teramoto et al., 1978; Hishinuma et al., 1979; Ogura and Ozeki, 1983; Chang et al., 1983; Liu et al., 1988; Sada et al., 1988). The latter method has merit in the disuse of a reductant gas like ammonia, which is also possibly in danger of causing more air pollution. Moreover, the absorption method is more durable for coexisting gases like carbon monoxide and sulfur dioxide (SO,) compared with the former catalytic method. Thus, because of these merits, this new absorption method is expected to be developed further. An absorption method usually involves an aqueous solution of complexes of iron(I1) (Fe(I1)) with polyamine carboxylic acids such as ethylenediaminetetraacetate (EDTA), nitrilotriacetate (NTA), and so on. This method is noteworthy because it is highly capable not only of absorption of NO but also of simultaneous absorption of NO and SO,, since an aqueous (wet) system can usually be applied for the removal of SO2 in contrast with a dry system for the removal of NO(Hirai et al., 1985a,b). However, there are some problems in the wet system for the removal of NO. One problem is its durability with oxygen since the oxidation of Fe(I1) ions to Fe(II1) ions 0 1990 American Chemical Society
2268 Ind. Eng. Chem. Res., Vol. 29, No. 11, 1990 - C H - C H d H -CH5 HOOCH2C\ ,CHzCOOH YCH CH N, HOOCH2C' CHzCOOH
*
a>
@
6
-CH-CHT
:CH?COOH
CH2'CH,C00H (Gj
Figure 1. Chemical structures of (a) ethylenediaminetetraacetic acid and (b) the chelate resin involving iminodiacetic acid moieties.
by oxygen in the flue gas causes the lose of the NO absorbing capability of the Fe(I1) complex (Liu et al., 1988). The other problem is in the recovery of the complex from the aqueous solution for recycling use, since EDTA is expensive and the separation of EDTA from the used absorbent mixtures is not so easy. We have successfully prepared a new aqueous dispersion of the NO adsorbents by immobilizing the Fe(I1) ions on the polystyrene-based chelate resin (CR) involving iminodiacetic acid (IDA) moieties in place of EDTA (Figure 1) in order to obtain easy handling of the complex (Hirai et al., 1985a,b; Toshima et al., 1989). The resulting aqueous dispersion has been found to have enough high adsorbing capability for NO even a t low concentrations. Moreover, the concentrated recovery of NO is also possible by use of the present aqueous dispersion (Hirai et al., 1985b; Toshima et al., 1989). In the practical utilization of the present adsorbent dispersion, the high durability with oxygen and SO, is also strongly desired because these gases are known to affect the removal of NO. Moreover, the resin part is expected to be easily separated from the used adsorbents and be utilized repeatedly. These advantages are characteristic of the resin-immobilized metal complex. In the present paper, we report the results of basic research on the present adsorbent to emphasize the potential utility of the adsorbent in the simultaneous removal of NO and SO,. Experimental Section Materials. A reagent grade of FeS04-7H,0 was purchased from Koso Chemical Co., Ltd., and was used without further purification. The polystyrene-based chelate resin (Mitsubishi Chemical Industry Co. Ltd., chelate resin CR-10, involving sodium iminodiacetate moieties, with a round shape of average diameter of 0.5 mm) was ground in advance with a mortar and pestle to fine particles of 10-pm diameter and dried under vacuum at room temperature. One gram of the dried resin involved 2.78 mmol of iminodiacetic acid moieties, which was determined by pH titration with a NaOH solution. Poly(acrylic acid) resin (Mitsubishi, WK-20) was washed with 1 mol dm-3 of a sodium hydroxide solution and was ground before use with a mortar and pestle to fine particles. Poly(styrenesu1fonic acid) resin (Mitsubishi, PK-208, in sodium form; the degree of cross-linking is 4%) was dried after washing with a large amount of water. Iminodiacetic acid (Tokyo Kasei) was purified by recrystallization from water before use. The nitrogen gas containing the desired amount of SO2 and/or NO was separately prepared by mixing SO2and/or NO at the settled concentration purchased from Takachiho Co. Ltd. and was used without further purification. Measurement of t h e Electronic Spectrum. Electronic spectra of the solid part of the complexes were measured with a Hitachi Model 340 spectrophotometer equipped with an integrated reflection apparatus. Preparation of Adsorbent. The original adsorbents were prepared as follows: 7.53 g of the dried commercial chelate resin (ground into 10-pm particles) was added to 20 cm3 of an aqueous solution of 8.72 g (31.4 mmol) of FeS0,.7H20. The mixtures were stirred for a day under
Nadelength / n r r
Figure 2. Electronic spectra of gaseous NO (-) room temperature.
and SOp (--) a t
nitrogen to immobilize Fe(I1) ions on the chelate resin completely. A small amount of water was added to the mixtures to adjust the total volume of the adsorbent mixture to 50 cm3. In order to examine the effect of the solid part and the supernatant upon the adsorbing capability for NO and SOz, the original adsorbent dispersion was separated into the solid part and the supernatant by centrifugation, and the resulting supernatant was examined as an absorbent. The remaining solid part was also used as an adsorbent dispersion by adding water to adjust the total volume of the dispersion to 50 cm3. To examine the effect of oxygen, the adsorbent mixtures were prepared by using twice as much dried fine chelate resin (15.06 g, involving 42.0 mmol of sodium iminodiacetate moieties) to the same amount (8.72 g) of FeSO,. 7H20in 15 cm3of water. For comparison of the durability of the resin-immobilized metal complexes with oxygen with the usual homogeneous aqueous solution of the monomeric complex, a solution of (iminodiacetato)iron(II)was also prepared by neutralizing 5.59 g (42.0 mmol) of iminodiacetic acid with twice the amount (3.36 g, 84.0 mmol) of sodium hydroxide in water, followed by the addition of 8.72 g (31.4 mmol) of FeSO4-7H20. The poly(acry1ic acid) resin-immobilized Fe(I1) complex was prepared from 8.72 g of FeS04.7H20and 4.27 g of the acrylic acid resin, which was ground and neutralized before use as described above. The poly(styrenesu1fonicacid) resin-immobilized Fe(I1) complex was prepared from 8.72 g of FeS04-7H20and 8.04 g of the poly(styrenesulfonic acid) resin. The total volume of the adsorbent dispersions was adjusted to 50 cm3 by adding a small amount of water. The pH range of adsorbent dispersions described above was between 4.5 and 5.5 because ion exchangeable moieties in the resin work as buffers. The adsorption capability of NO did not change in this pH range for the supplemental experiments. In addition, it was reported by Hishinuma et al. (1979) that the equilibrium constant for the absorbing reaction of NO with Fe(I1)-EDTA solution does not change at the pH range from 3 to 6. The alkaline adsorbent was prepared by removing the supernatant of the original adsorbent mixture, followed by addition of 5 cm3 of a 1mol dm-3 NaOH solution and about 15 cm3 of water in order to adjust the total volume of the adsorbent mixtures to 50 cm3. All these operations described above were carried out under nitrogen to avoid the oxidation of the Fe(I1) ions. Determination of the Concentration of Nitrogen Monoxide and Sulfur Dioxide. The concentrations of NO and SO2 were analyzed simultaneously by sampling 23.8 cm3 of the gas in a 100 mm-quartz cell, followed by measuring the absorbance at 287 and 226.5 nm. The electronic spectra of SO2 and NO are shown in Figure 2. The concentration of SO2 in the mixed gas can be calcu-
Ind. Eng. Chem. Res., Vol. 29, No. 11, 1990 2269
5
1
1
-
1
Figure 3. Schematic illustration of a closed circulation system for the simultaneous adsorption of NO and SOz. (1) Gas buret. (2) Gas holder. (3) Sampling outlet. (4) Adsorbing vessel. (5) Constanttemperature water bath.
lated from the absorbance at 287 nm where NO does not have any absorbance in the electronic spectrum. The concentration of NO was calculated from the absorbance of the mixed gas at 226.5 nm by deducing the absorbance derived from SOz. Adsorption Experiment. The adsorption of NO was carried out as follows: After the closed circulation system as is shown in Figure 3 was evaluated, 6 dm3of the nitrogen gas containing 1000 ppm NO was introduced into the system. The adsorbent was contacted with the sample gas at the rate of 1.6 dm3 min-' by circulating the mixed gas with a gas pump through the bubbler containing the adsorbent mixtures. The simultaneous adsorption of SOzand NO was carried out in the same apparatus by using the mixed sample gas containing 860 ppm NO and 1180 ppm SOz. Seven cubic decimeter of the mixed sample gas was prepared by mixing 1dm3 of nitrogen containing 7130 ppm SOz with 6 dm3 of nitrogen containing 1000 ppm NO in the circulating system. The mixed gas was circulated for 10 min in advance without contact with the adsorbent dispersion to achieve complete mixing before bubbling through the adsorbent mixtures. Complete mixing was confirmed by sampling the mixed gas and measuring the concentration of NO and SOz in it during the circulation of the gas. The adsorption experiments were carried out at 30 "C when nothing was especially commented. Effect of Oxygen. If oxygen is introduced into dilute NO balanced with NP, the NO molecules will be oxidized to nitrogen dioxide, which makes it difficult to analyze the concentration of NO by the electronic spectra. So the effect of oxygen on the adsorbent dispersions was examined for NO adsorption not during the adsorption of NO but before the introduction of NO to the adsorbent dispersions. The experiments for the oxygen effect were carried out by bubbling air through the adsorbent mixture at the rate of 1.6 dm3 min-' for 90 min. This operation made the adsorbent dispersion involving 31.4 mmol of Fe(I1) ions in contact with 1.17 mol of oxygen. Recovery of the Chelate Resin. After contact with oxygen, the solid parts of the adsorbent dispersions were separated from the supernatant by filtration, eluted with 100 cm3 of a 6 mol dm-3 hydrogen chloride solution to remove Fe(II1) ions, and washed with a large amount of water and 200 cm3 of a 1 mol dm-3 sodium hydroxide solution. The resulting solid part gave the resin in the same condition as the original one. Addition of 20 cm3 of the aqueous solution involving 8.72 g of FeS04.7Hz0to the preceding resin under nitrogen and stirring the mixtures for a day resulted in reimmobilizationof Fe(I1) ions on the resin.
Figure 4. Adsorption curves of NO a t 30 "C ( O ) ,40 OC (A), and 60 O C (0) by the adsorbent dispersions prepared from the chelate resin (7.53 g) and an aqueous solution of FeSO, (8.72 g, 31.4 mmol) from 6 dm3 of the nitrogen gas containing 1000 ppm NO.
0.5
0
60
30
t /min
90
120
Figure 5. Adsorption of NO by the aqueous dispersions of the Fe(I1) complex immobilized by the chelate resin (e),poly(acry1ic from 6 dm3 acid) resin ( O ) ,and poly(styrenesu1fonic acid) resin (0) of the nitrogen gas containing 1000 ppm NO.
Results and Discussion Adsorption of Nitrogen Monoxide. Figure 4 depicts the NO adsorption curves of the aqueous dispersions of the chelate resin-immobilized Fe(I1) complexes from the nitrogen gas containing 1000 ppm NO at various temperatures. As is shown in Figure 4, 68% of the charged amount of NO can be adsorbed by the fresh adsorbent dispersions at 30 "C. The equilibrated amount of adsorbed NO decreases with increasing the temperature at which the adsorption experiment is carried out. Therefore, NO is possibly desorbed by treating the NO-adsorbed dispersions with heat. The adsorption-desorption cycles can result in the separation and the concentrated recovery of NO. Other resin-immobilized Fe(I1) complexes were also examined for the adsorption of NO as is shown in Figure 5. In the case of the poly(styrenesu1fonicacid) resin, on which the Fe(I1) ions are immobilized by electrostatic forces, NO was scarcely adsorbed as is shown with open circles in Figure 5. On the other hand, the polyacrylic acid-Fe(I1) complexes adsorb 51% NO within 60 min. These facts suggest that the complex formation of Fe(I1) ions with carboxyl groups is necessary to achieve the property of NO adsorption. In fact, Fe"S04, which can correspond to the sulfonic acid resin-immobilized Fe(I1) complex, scarcely absorbs NO, while iron(I1) acetate or iron(I1) iminodiacetate does absorb NO (vide infra). In the case of the chelate resin, the solid part of the dispersion adsorbs NO and the liquid part does not. Thus, the active part for the adsorption of NO is the solid part. This is also supported by the observation in the electronic spectra. Table I shows the changes in the electronic spectra of the resin-immobilized Fe(I1) complexes corresponding to the solid part of the aqueous dispersions. The chelate resin before the immobilization is white, and its
2270 Ind. Eng. Chem. Res., Vol. 29, No. 11, 1990 Table I. Electronic Spectra of the Solid Parts of the Adsorbent Dispersion before and after Contact with NO material L../nm chelate resin (CR) 280 Fe(I1)-CR 340 NO-Fe(I1)-CR 450, 600 NO-Fe(I1)-EDTA 430, 630
~
reflection spectrum has the maximum peak at 280 nm. After the immobilization of Fe(I1) ions, the color of the resin changes from white to light green and the reflection spectrum shows the new maximum peak to be around 340 nm, which does not exist both in the chelate resin before the immobilization of Fe(I1) and in an aqueous solution of Fe”SO4. These facts clearly demonstrate the complexation of the Fe(I1) ions with the iminodiacetic acid moieties in the resin. The color of this complex changes from light green to dark green on contact with NO, and two shoulder peaks appear at around 450 and 600 nm. Similar changes can also be observed in the case of an aqueous solution of the Fe(I1)-EDTA complex coordinated by NO, which has maximum peaks at around 430 and 630 nm (Hishinuma et al., 1979; Ogura and Ozeki, 1983). These two peaks disappear after the desorption of NO in both cases of the chelate resin-immobilized Fe(I1) complex and the aqueous soultuion of Fe(I1)-EDTA. These facts indicate that the chelate resin-immobilized Fe(I1) complex, the solid part of the adsorbent dispersions, can adsorb NO by coordinating NO on the Fe(I1) ion and that this complexation reaction is reversible. Equilibrium Constants for the Reversible Adsorption of NO. Further analysis for the adsorbing reaction of NO by the present dispersion was carried out by calculating the equilibrium constant of the adsorption of NO, since the adsorption has been demonstrated to proceed reversibly. The adsorption is considered to proceed according to the following scheme by assuming 1:l complex formation of NO to Fe(I1) in the chelate resin (Hishinuma et al., 1979): NO(,) e NO,,,) (1) NO,,,)
+ CR-Fe(I1)
CR-Fe(I1)-NO
Table 11. Equilibrium Constant for the Reversible Adsorption of NO by the Chelate Resin-Immobilized Fe(I1) Complexn a m eauilibrium constant temp/OC Kfl-llatm-’ Kc/dm3mol-’ -AH/kJ mo1-l 25.0 60.1 31.1 x 103 39.5 23.2 14.8 X lo3 45.6 59.8 7.00 5.35 x 103 80.0 2.02 1.67 x 103
(2)
K = [CR-Fe(II)-NO]/([CR-Fell][NO]~,,,) =
([CR-Fe(II)-NOlH)/([CR-Fe(II)lp,,)(3) K = ~ N ~ H / ( [ C R - F ~ ( I I ) I ~ P ~ O ) (4) The complex formation constant K for eq 2 can be obtained from eq 3 where [CR-Fe(I1)-NO] and [CR-Fe(II)] are the amount of NO coordinated on CR-Fe(I1) in mmol (equal to the amount of adsorbed NO, nNO,in eq 4) and the amount of CR-Fe(I1) without NO, respectively. The amount of uncoordinated CR-Fe(II), [CR-Fe(II)], can be calculated by substracting the amount of adsorbed NO, nNO, from that of initially immobilized Fe(II), [CR-Fe(II)],,. Since the immobilized Fe(I1) ions ([CR-Fe(II)],,) are in large excess to the adsorbed NO ([CR-Fe(II)],/nN0 = 221, the amount of uncoordinated Fe(II), [CR-Fe(II)], can be regarded as that of the initially immobilized Fe(II), [CRFe(II)l0. Thus, eq 3 can be expressed as eq 4,where pNO and H are the partial pressure of NO and Henry’s constant, respectively. The validity of eq 4 (1:l complex formation) has been confirmed by the proportionality between pNO and n N O (Toshima et al., 1989). The obtained equilibrium constants at various temperatures are listed in Table 11. The equlibrium constant decreases with an increase in the temperature. The enthalpy change (AH) can be calculated from van’t Hoff
~~
Adsorption of NO was carried out by contact with 6 dm3 of nitrogen containing 1000 ppm N O a t the rate of 1.6 dm3 min-I.
8
“OI
t I min
Figure 6. Adsorption of NO from 7 dm3of a nitrogen gas containing of SOz by the 860 ppm NO in the presence ( 0 )and absence (0) adsorbent dispersions prepared from 8.72 g of FeSO, and 7.53 g of the dried fine chelate resin in water.
plots of the equilibrium constants and was obtained as -45.6 kJ mol-’. The enthalpy change in the absorbing reaction of NO with an aqueous solution of Fe(I1)-EDTA was reported to be -66.1 kJ mol-’ (Hishinuma et al., 1979), which is lower by 20.5 kJ mol-’ than that of CR-Fe(I1). The strong coordination of NO to Fe(I1)-EDTA is considered to be due to the strong back-donation from the Fe(I1) atom to the NO molecule because of the increase in the electron density of the Fe(I1) atom by the presence of EDTA. In the present CR-Fe(I1) complex, the strong electron-releasing effect could not be achieved since the basicity of the CR ligand (pK& = 8.90 (Ando, 1962)) is smaller than that of EDTA (pK, = 10.26 (Schwarzenbach et al., 1954)) due to the benzyl group. Moreover, the decrease in the entropy change is considered to be larger than that of the homogeneous solution of Fe(I1)-EDTA or Fe(11)-IDA solution because the degree of freedom of NO molecules decreases with the coordination to Fe(II), which is bound to the solid polymer through IDA moieties. This decrease in the entropy change will diminish the equilibrium constant compared with that of Fe(I1)-EDTA or Fe(I1)-IDA (AG = AH - TAS). Simultaneous Adsorption of Nitrogen Monoxide and Sulfur Dioxide. The simultaneous removal of NO and SOz is important because both gases are usually involved in flue gases together. At first, the adsorption of NO in the presence of SOz was examined. Figure 6 depicts the adsorption curves of NO in the presence (closed circles) and absence (open circles) of SOz. As Figure 6 shows, the presence of SOz does not suppress the adsorption of NO, but on the contrary, a little promotes the smooth adsorption of NO. Sulfur dioxide was also removed by aqueous adsorbent dispersions at the same time with NO. Up to 93% of the charged SOz can be absorbed at equilibrium within 35 min as shown by open circles in Figure 7. Thus, the simultaneous removal of SOz and NO is successful using the present adsorbent dispersions. The observed increase in the adsorbed NO in the presence of SOz (Figure 6) can be explained by the consideration that dissolved SO2 in an
Ind. Eng. Chem. Res., Vol. 29, No. 11, 1990 2271
L
q
Ps
0
,
30
,
,
60
90
t I min
,I
Table 111. Adsorption of SO2 by the Aqueous Dispersion of the Resin-Immobilized Fe(I1) Complex and Its Components molar ratio of adsorbed SO2 to charged SOz adsorbent a t 15 min at eouilibrium 0.881 0.931 adsorbent dispersion" supernatantb 0.610 0.630 solid part' 0.809 0.892 0.547 0.647 distilled water (50 cm3) "Prepared from 8.72 g of FeS0,.7Hz0 and 7.53 g of the dried fine chelate resin. bSeparated from the adsorbent mixture by removing the solid part with centrifuge. 'Prepared by adding water to the solid part of the original adsorbent dispersion to adjust the total volume of the dispersion to 50 cm3.
aqueous phase brings about HS03- or ,903- ions that further react with NO in an aqueous phase (Chang et al., 1983),
- N20 + Sod2-
+ S03(NO)22-
Since the absorption of SO2by water is known, the effect of the resin-immobilized Fe(I1) complexes upon the adsorption of SO2 was examined. The adsorption experiments were carried out by using each component that was separated from the aqueous dispersions of the adsorbent as described in the Experimental Section. As is shown in Table 111, the amount of SO2 adsorbed by the solid part is larger than that by the supernatant or distilled water. This fact indicates that the resin part also plays an important role in the removal of SO2. Since SO2 is dissolved in water according to eq 5 , the rise in the pH shifts the SO2 + H 2 0 + H+ + HS03-
I/J
120
Figure 7. Adsorption of SOz in the presence of NO (860 ppm) from 7 dm3 of a nitrogen gas containing 1180 ppm SOz by the original and the aqueous dispersion ( 0 )prepared adsorbent dispersions (0) by exchanging the supernatant of the original one with an aqueous solution of sodium hydroxide (1 mol dm-3).
S032- + 2N0
o o2 0
(5) equilibrium to the right side, which increases the amount of adsorbed SO2. In the present adsorbent case, the pH in the resin part is thought to be higher than that in the aqueous phase because the adsorbent dispersion is prepared from the sodium type of chelate resin. Moreover, Fe(I1)-EDTA can form a complex with S032-to produce Fe(II)-S032--EDTA (Teramoto et al., 1978). Thus, the resin part can also adsorb SO2. In this way, the total efficiency for removal of SO2 was raised by the presence of the solid part. In fact, the adsorbent prepared by exchanging the supernatant with an alkaline solution (pH = 8-10) adsorbs more SO2 than the original adsorbent dispersion as is shown in Figure 7. With regard to adsorption of NO instead of SO2, the above supernatant-exchanged dispersion adsorbs more NO at equilibrium than the original one, though the adsorption rate becomes slightly slower by exchanging the supernatant with an alkaline solution as is shown in Figure 8.
Figure 8. Adsorption of NO in the presence of SOp (1180 ppm) from 7 dm3 of a nitrogen gas containing 860 ppm NO by the original and the aqueous dispersion (0)prepared adsorbent dispersion (0) by exchanging the supernatant of the original one with an aqueous solution of sodium hydroxide (1 mol d m 9 .
c
Figure 9. Adsorption of NO by the Fe(I1) complex (21.0 mmol) from 6 dm3 of a nitrogen gas containing lo00 pm NO with the monomeric ligands (IDA, square) and the polymeric ligands (CR, round) before (open) and after (close) bubbling air for 90 min at the rate of 1.6 dm3 m i d .
Thus, the presence of SO2 increases the adsorption activity for NO, and the high pH of the supernatant (pH = 8-10) is favored for the simultaneous removal of SO2 and
NO. Durability with Oxygen. The flue gases usually contain oxygen, which easily oxidizes the Fe(I1) ion to an inert Fe(II1) ion. Thus, the examination of the durability with oxygen is necessary for practical use. When an aqueous solution of the Fe(11)-iminodiacetic acid complex is contacted with air, the color of the solution changes from green to brown due to the oxidation of Fe(I1) to Fe(II1). The Fe(I1)-IDA complex is capable of such a high adsorption of NO in the absence of oxygen that it adsorbs 96% NO within 45 min. However, it deactivates by contact with air for 1.5 h, resulting in the adsorption of only 18% NO as is shown with closed squares in Figure 9. On the contrary, the Fe(I1)-CR complex adsorbs 87% NO a t equilibrium, which is lower than that of the original Fe(11)-IDA complex (96%). After contact with air for 1.5 h, however, it still adsorbs 64% NO, which means that 70% of the original activity remains. Thus, it has been made clear that the resin-immobilized Fe(I1) complex has a higher durability with oxygen than the corresponding monomeric model, an aqueous solution of the Fe(1I)-IDA complex. The durability of the resin-immobilized Fe(I1) with oxygen is probably attributed to the slow diffusion of oxygen into the resin inside. The oxidation of Fe(I1) in the resin undergoes three steps: the dissolution of oxygen into the aqueous phase, the successive diffusion of the dissolved oxygen into the resin, and the reaction of oxygen with Fe(I1). Though the rapid dissolution of oxygen in the solution can be obtained by rigorous bubbling, the ac-
2272 Ind. Eng. Chem. Res., Vol. 29,No. 11, 1990
phase. Using an alkaline solution increases the capability for the simultaneousremoval of NO and SOz. The aqueous dispersions have high durability with oxygen compared with the homogeneous solution of the Fe(I1)-IDA complex, a model compound of the Fe(I1)-CR complex. This durability with oxygen is derived from a slow diffusion of oxygen into the resin. The oxidized complex can be easily recollected from the dispersions and regenerated as fresh adsorbent dispersions. Acknowledgment Cycle
Figure 10. Recycling use of the adsorbent dispersion. Adsorption of NO was carried out by contact with 6 dm3 of nitrogen containing 1000 ppm NO. The chelate resin (15.06g at a dry state) was recovered from the deactivated adsorbent dispersion by filtration and regenerated by the elution with a hydrogen chloride solution, followed by the neutralization with a sodium hydroxide solution. See text for details.
companying diffusion of oxygen into the resin phase is necessary to oxidize the resin-immobilized Fe(I1) ions. The driving force for the diffusion inside the resin is considered to be only the discrepancy in the oxygen concentration, since the motion of the solution inside the solid part of the resin is suppressed. Thus, the oxidation of Fe(I1) in the resin phase goes slowly because of the slow diffusion of oxygen. In the case of the homogeneous solution, on the contrary, the dissolved oxygen can rapidly react with active Fe(I1) ions since both oxygen and the Fe(I1) ion exist in the same solution phase. The present considerations will be supported by the fact that the adsorption of NO by the resin-immobilized Fe(I1) complex proceeds at a slightly slower rate than that by the aqueous solution of the Fe(I1)-IDA complex. Namely, NO as well as oxygen diffuses slowly into the resin-immobilized Fe(I1) complex. The durability with oxygen will be obtained by this sacrificial slow diffusion of NO. Recovery of the Resin. The solid part of the deactivated complex can be easily recollected by centrifugation or filtration without significant loss of the resin compared with the recollection of IDA from an aqueous solution of the Fe(I1)-IDA complex. The recollected complex is regenerated by leaching the Fe ions with hydrochloric acid, followed by washing with an alkaline solution and adding a fresh solution of FeS04. The details are described in the Experimental Section. Figure 10 shows the repeated adsorption of NO by the resin-immobilized Fe(I1) complex that is regenerated by the recyclic use of the resin from the deactivated one according to the method described above. The adsorbent is completely regenerated without a significant decrease in the amount of the adsorbed NO at equilibrium. This fact is derived from easy recollection of the resin and the stability of the iminodiacetic acid moieties of the chelate resin during the adsorption of NO and the regeneration of the adsorbent dispersion. Conclusion The presence of SO, never suppresses the adsorption of NO by the present aqueous dispersions of the chelate resin-immobilized Fe(I1) complex, and moreover, the adsorption of NO is observed to be accelerated by the presence of SO2 probably due to the reaction of NO and SOP. Sulfur dioxide is not only absorbed by the aqueous phase of the dispersions but also is adsorbed by the resin
The present work was partially suppported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture, Japan. Registry No. NO,10102-43-9;SOz, 7446-09-5. Literature Cited Ando, T. Syntheses of Chelating Agents. I. Stability of the Metal Chelate of Benzylamine-N,N-diacetic Acid and its Nitro DerivaJpn. 1962,35,1395-1399. tives. Bull Chem. SOC. Chang, S.-G.; Littlejohn, D.; Lynn, S. Effects of Metal Chelates on Wet Flue Gas Scrubbing Chemistry. Environ. Sci. Technol. 1983, 17, 649-653. Davini, P. Reduction of Nitrogen Oxides with Ammonia: The Activity of Certain Soots. Fuel 1988,67, 24-26. Hirai, H.; Asanuma, H.; Toshima, N. Chelate Resin-Immobilized Iron(I1) Complexes as New Nitrogen Oxide Adsorbents. Chem. Lett. 19858,655-658. Hirai, H.; Asanuma, H.; Toshima, N. Adsorption and Desorption of Nitrogen Oxide by Iron(I1)-Chelate Resin ComDlexes. Chem. Lett. i g m , 192111924. Hishinuma, Y.; Kaji, R.; Akimoto, H.; Nakajima, F.; Mori, T.; Kamo, T.; Arikawa, Y.; Nozawa. S. Reversible Binding of NO to Fe(11)-EDTA. Bull. Chem.’ SOC.Jpn. 1979,52,2863-2865. Juentgen, H.; Richter, E.; Kuehl, H. Catalytic Activity of Carbon Catalysts for the Reaction of Nitrogen Oxides (NOx) with Ammonia. Fuel 1988a, 67,775-780. Juentgen, H.; Richter, E.; Knoblauch, K.; Hoang-Phu, T. Catalytic Nitrogen Oxide (NOx) Reduction by Ammonia on Carbon Catalysts. Chem. Eng. Sci. 198813,43,419-428. Liu, D. K.; Frick, L. P.; Chang, S.-G. A Ferrous Cysteine Based Recyclable Process for the Combined Removal of NO, and SO2 from Flue Gas. Enuiron. Sci. Technol. 1988,22,219-223. Ogura, K.; Ozeki, T. Anodic and Chemical Oxidations of Nitric Oxide Using Metal-Chelate Catalysts. Denki Kagaku 1983,51, 762-766. O’Sullivan, D. Third World Has Most Chemical Contamination. Chem. Eng. News 1988,66 (40),8-9. Sada, E.; Kumazawa, H.; Yoshikawa, Y. Simultaneous Removal of Nitric Oxide and Sulfur Dioxide by Absorption into Aqueous Mixed Solutions. AIChE J . 1988,34,1215-1220. Schwarzenbach, G.; Gut, R.; Anderegg, G. Complexons. XXV. Polarographic Study of Exchange Equilibria. New Data on the Formation Constants of Metal Complexes of Ethylenediaminetetraacetic Acid and 1,2-Diaminocyclohexanetetraacetic Acid. Helu. Chim. Acta 1954,37,937-957. Teramoto, M.; Hiramine, S.; Shimada, Y.; Sugimoto, Y.; Teranishi, H. Absorption of Dilute Nitric Monoxide in Aqueous Solutions of Fe(I1)-EDTA and Mixed Solutions of Fe(I1)-EDTA and Na,SOg. J. Chem. Eng. Jpn. 1978,11,450-457. Toshima, N.; Asanuma, H.; Yamaguchi, K.; Hirai, H. The Adsorption and Desorption of Nitrogen Oxide by the Aqueous Dispersion of the Chelate Resin-Immobilized Iron(I1) Complex. Bull. Chem. SOC.Jpn. 1989,62,563-570. Tuenter, G.; van Leeuwen, W. F.; Snepvangers, Leo. J. M. Kinetics and Mechanism of the Nitrogen Oxide (NOx) Reduction with Ammonia on Vanadium Oxide-Tungsten Oxide-Titanium Oxide Catalysts. Ind. Eng. Chem. Prod. Res. Deu. 1986,25,633-636.
Receiued f o r review October 16, 1989 Revised manuscript received May 21, 1990 Accepted July 10,1990
