I n d . Eng. C h e m . Res. 1988,27, 2092-2095
2092
Ruthven, D. M.; Goddard, M. "Correlation and Analysis of Equilibrium Isotherms for Hydrocarbons on Zeolites". In Fundamentals of Adsorption; Myers, A. L., Belfort, G., Eds.; American Institute of Chemical Engineers: New York, 1984; pp 536, 537. Tien, C. "Incorporation of the IAS Theory in Multicomponent Adsorption Calculations". Chem. Eng. Commun. 1986, 40, 265.
Wang, S.-C.; Tien, C. "Further Work on Multicomponent Liquid Phase Adsorption in Fixed Beds". AZChE J. 1982, 28, 565.
Received for review February 18, 1988 Revised manuscript received June 10, 1988 Accepted July 6, 1988
Polymeric Iron Chelates for Nitric Oxide Removal from Flue Gas Streams S t e p h e n A. Bedell,* S u s a n S. Tsai, and R o b e r t R. G r i n s t e a d + Dow Chemical U.S.A.,Building B-250, Freeport, Texas 77541
A major problem encountered in the use of ferrous chelates for removal of nitric oxide from flue gases is the loss of the chelate in the purge stream of the built-up salt products. The use of polymeric chelating agents which can hold iron and be separated from waste streams by ultrafiltration is reported. NO absorption is most effective for the Fibrabon 35/ED3A (the reaction product of ethylenediaminetriacetic acid and an epichlorohydrin/bis(6-aminohexyl)aminecopolymer) iron chelate. Removal levels of greater than 90% have been obtained for this system. Comparison of the polymeric chelates with the ferrous chelate of ethylenediaminetetraacetate (EDTA) has been made. Scrubbing of sulfur dioxide from flue gases can be accomplished by contacting the gas with an alkaline absorbent, such as caustic or a limestone slurry. Nitric oxide removal presents some unique problems. Unlike SO2,NO is not very soluble in conventional scrubbing solutions. As developed in the 1970s in Japan, solutions containing ferrous EDTA-type chelates can be used to simultaneously remove SOz and NO from flue gas streams. The formation of a ferrous-nitrosyl complex greatly increases the solution's capacity for NO, as shown for hydrated ferrous ion in eq 1. Chelation of the iron by EDTA not only helps Fe11(H,0)6+ NO + Fe"(H,O),(NO) K,, = lo2.' (1) to keep the iron from precipitating, but also results in greater affinity for NO through ternary complex formation (eq 2). Once fixed in solution, the NO can be readily FeIIEDTA + NO + Fe"(EDTA)(NO) Keq = 106.2(2) reduced by sulfite produced in the SO2 scrubber (eq 3). Though earlier literature reports discrepancies in product Fe"(EDTA)(NO) + S032Fe'I(EDTA) + S,N products (3)
-
identification, Chang and Littlejohn (1985a,b) have determined several species of mixed sulfur-nitrogen salts along with molecular nitrogen and nitrous oxide. Distribution of these products is dependent on reaction conditions. The effectiveness of this chemistry in abating NO to low levels has been well documented (Koizumi et al., 1974; Faucett et al., 1977; Hishinuma et al., 1978). The greatest advantage of the ferrous chelate based NO, processes over many other technologies is the potential for convenient retrofitting of existing SO2 scrubbers. Though plans to install a ferrous-based NO, process in a 750-MW West German coal fired power plant were recently announced (Leimkuehler et al., 1986), a major problem hindering
* Author t o whom correspondence should be addressed. +Current address: Dow Chemical U.S.A., P.O. Box 9002, Walnut Creek, CA 94596.
widespread commercialization of this type of process is the high cost of chelate blowdown losses. As wet FGD waste products (calcium sulfite or sulfate plus salts of the nitrogensulfur products) are removed in a slip stream from the process, some iron chelate will also be purged. This loss of chelate could be minimized if all the iron was in the active ferrous form. Oxygen in the flue gas will oxidize the ferrous chelate (eq 4) to form a ferric complex which has 4Fe"(EDTA)
+ 4H+ + Oz
-
4Fe"'(EDTA)
+ 2H20 (4)
little affinity for nitric oxide. Oxidation of Fe(I1) by NO can also occur (Sada et al., 1986). Though reduction of the ferric chelate by sulfite (eq 5) will compete with the Fe"'(EDTA)
+ SO3,-
-
Fe"(EDTA)
+ '/S206z- (5)
ferrous oxidation, the majority of the iron will remain in the ferric state. Since this will require more total iron to achieve NO abatement, losses during waste product removal will also increase. The key to development of this technology depends on one or a combination of two process improvements (Bedell et al., 1986): (1)efficient, cost-effective means of ferric chelate reduction; (2) full or partial recovery of the iron chelate from the waste purge. The first strategy has been shown by Tsai et al. (1987) to be technically feasible with the use of an electrochemical cell for iron reduction. The current study was undertaken to evaluate the NO removal ability of novel polynuclear iron chelates, large enough to be retained by an ultrafiltration membrane, while FGD product salts are passed through for waste disposal. The recovered iron chelate can then be returned back to the scrubbing device for reaction with additional NO. Figure 1 shows a scheme of this process. Experimental Section Preparation of Chelates. Poly(N-(carboxymethyl) ethyleneimine)ferrate(III)(Fe-CMPEI-150). PEI-150 (45.3 g) (a 33% aqueous solution of 10000 molecular weight polyethyleneimine from Virginia Chemicals) w8s added to 200 mL of water. Fifty-two grams of bromoacetic acid was added to 50.0 g of H 2 0 followed by 42.0 g of 50% KOH solution. The potassium bromoacetate solution was then 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2093
A
GUS O U T L E T
E X I T GRS
1 y ID G L R S S COLUMN
RBSORBER
1 F O O T OF P R C K I N G ( 1 / 0 " GLRSS H E L I C E S )
ULTRRFILTER THERNOMETER
u
NITROGENlSULFUR S R L T S PURGE
MRGNETIC STIRRER
P-1
Figure 1. Process scheme.
added to the PEI-150 solution, and the pH was maintained at 10 or slightly above with KOH. When the pH stabilized, a solution of ferric ammonium sulfate was added and the pH was adjusted to 7.0. The solution was centrifuged to remove some precipitated iron. The solution contained 4900 ppm Fe. Fibrabon 35/ED3A-Fibrabon 35 (a registered trademark of Diamond Shamrock Chemical Company), a water soluble copolymer of epichlorohydrin and bis(6-aminohexyl)amine, was obtained from Diamond Shamrock Chemical Company. Molecular weight ranges, as determined by evaporating and weighing ultrafiltration permeates, are as follows: 33% (0-lOOOO), 10% (1000030000), and 57% (above 30000). Lot A. Ethylenediaminetriacetate (ED3A) was prepared by the reaction of chloroacetate and ethylenediamine. Four hundred milliliters of an ED3A solution (prepared from 0.4 mol of ethylenediamine) was adjusted to a pH of about 6 with HC1. This solution contained approximately 0.24 mol of ED3A, together with ethylenediaminetetraacetate (EDTA) formed in the ED3A synthesis. Ninetyeight grams of Fibrabon 35 was diluted 1:2 with water and added slowly to the ED3A solution, along with some methanol, which helped to maintain a clear solution. A total of about 300 mL of methanol was added, yielding a final solution which was about 30% methanol. The pH was increased to 9 with concentrated NaOH solution and heated. The temperature and pH were maintained at about 55 "C and pH 9.5 for about 2 h. Air was blown over the warm solution for several hours to remove most of the methanol and reduce the volume. This solution was ultrafiltered to remove salts and EDTA. Titrations with FeC& showed a concentration of 0.08 M ED3A (attached to the polymer). The iron complex of this compound was prepared prior to use by adding an equivalent amount of ferrous sulfate. Lot B. Fifty-two grams of Fibrabon 35 was diluted to 350 mL, and to this was added 120 g (2 mol) of ethylenediamine. The system was heated at 55 "C for 2 h, refluxed for 5 h, and finally diafiltered with a 10000 MW cut-off membrane to remove excess ethylenediamine. To the polymer solution was added 23 g (ca. 0.25 mol) of chloroacetic acid, and the system was heated a t 60 "C for 5 h, maintaining a pH of 9-10 by addition of 50% NaOH. The system was didiltered again, and the concentrate was used directly to prepare the iron chelate solution. Lot C. Fifty-four grams of Fibrabon 35 was added to 120 g (2 mol) of ethylenediamine in 150 mL of water and refluxed for 1h. The system was evaporated in vacuo to remove excess ethylenediamine and redissolved in water, and 47 g (0.5 mol) of chloroacetic acid was added. The system was heated at 60 "C for 5 h, keeping the pH at 9-10 as in lot B. This system was diafiltered to remove excess chloroacetic acid and sodium chloride and then used directly to prepare the iron chelate. Lot D. Prepared exactly as lot A.
Figure 2. Laboratory apparatus.
Laboratory Process Rig. The laboratory apparatus used for the NO scrubbing experiments is shown in Figure 2. It consisted of a thermostated water-jacketed 800-mL round-bottomed flask and a l-in.-i.d. glass pipe packed with 12 in. of 1/8-in.glass helices. Flue gas was simulated by mixing gases from three compressed gas cylinders: a CO2/O2/N2mixture, SO2/NZ,and NO/N2. Gas flow rates were controlled by electronic mass flowmeters and were usually controlled a t 3 SCFH. The simulated flue gas typically contained 210 ppm NO, 500 pm SO2, 10% C02, 5% Oz,and the balance nitrogen. NO concentrations were determined by an Interscan Model 154LD NO monitor and SO2 concentrations by standard Draeger tubes. Ferric and ferrous chelate concentrations were determined by colorimetric methods based on thiocyanate and 1 , l O phenanthroline complex formation. The scrubbing liquor consisted of the appropriate amount of iron chelate in a solution containing 0.2-0.5 M initial total sulfur salts. These salts were added as sodium sulfite, and the pH was adjusted to 7.0. As the run proceeded, the sulfur salts were increased slightly by the amount of SOz scrubbed and consisted mostly of sulfite, bisulfite, and oxidized sulfite. The process solution was typically maintained a t 55.0 f 0.5 "C. This solution was pumped a t 50-100 mL/min by P-1 to the top of the absorber where it flowed through the packing countercurrent to the simulated flue gas. P-1 was a peristaltic pump with variable-speed control. Ultrafiltration. The ultrafilter used was an Amicon HlP3-20 or HlP10-20 hollow fiber cartridge with a 3000 or 10OOO molecular weight cut-off membrane, respectively. Solution was pumped by P-2 from the round-bottomed flask through the ultrafilter. The retentate was recycled to the round-bottomed flask, and the permeate was removed from the process. Pressure through the ultrafilter cartridge could be adjusted to regulate the quantity of permeate. Results and Discussion NO was used, though it is realized that a small portion of the NO, will be NO2. NO2, more soluble than NO, should be easily removed by the bisulfite solution. Figure 3 shows the results of NO absorption runs utilizing Fe"EDTA as the iron source. Since the iron(I1) concentration decreased as the run proceeded due to its ready oxidation to Fe(III), NO removal efficiencies for several Fe(I1) levels could be determined in one run. Figure 3 is composed of data from three runs made a t various total iron concentrations. These results confirm earlier reports that NO absorption increases with Fe(1I) concentration and is independent of total iron concentration. At low ferrous levels, small changes in ferrous concentration result in large changes in NO absorption. After about 80% removal is attained, abatement efficiency becomes markedly less sensitive to ferrous concentration. A t these higher
2094 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
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hCOC \
c\
" > , ,-N ,
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Figure 5. Structures of chelating polymers.
CONC , M
Figure 3. NO absorption versus FeIIEDTA concentration (solution temperature = 55 OC, [FeEDTA] = 0.002-0.025 M, [Nafi03]t,o = 0.5 M, liquid circulation rate = 100 mL/min). too
i
,
q.
f
so *
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-
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,
, I
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Figure 6. NO absorption versus Fe(II)/Fibrabon 35 concentration. (0) Lot A, total r e = 0.025 M, liquid circulation rate = 50 mL/min; (V) lot A, total Fe = 0.025 M, liquid circulation rate = 100 mL/min; ( 0 )lot D, total Fe = 0.05 M, liquid circulation rate = 50 mL/min.
7-7 -
0 02
0 06
0.04 Le
(11)
0 08
01
CONC, M
Figure 4. NO abatement versus Fe"CMPE1-150 concentration (solution temperature = 55 "C, [FeCMPEI-150] = 0.0254.1 M, [NazSOs]t,o = 0.2-0.5 M, liquid circulation rate = 100 mL/min).
levels of Fe(II), absorption kinetics may be limited by reactions not involving iron, such as mass transfer. When these reactions were allowed to "line out" or reach a steady state (ferrous oxidation by 0,; ferric reduction by S032-), less than 10% of the total iron was in the active, ferrous state. It is in those regions above 60% removal that the large iron concentrations required will result in costly blowdown losses of iron chelate unless measures (such as electrolytic reduction) are taken to ensure complete reduction to the ferrous state. Figure 4 shows the response of NO absorption to Fe(I1) concentration for Fe-CMPEI. The same general trend of increased abatement efficiency with increased ferrous concentrations was seen a t the lower Fe(I1) levels. As with EDTA, higher levels of ferrous complex do not increase NO removal. Though the CMPEI provided a potentially inexpensive polymeric chelant, the random placing of the aminocarboxylate complexing groups probably did not provide an optimum environment around the ferrous ion for nitrosyl binding. Measurements of equilibrium constants for the reaction of ferrous aminocarboxylates with NO (Chang and Littlejohn, 1985a,b) show that the more thermodynamically stable complexes have greater affinity for NO. Thus, NO binds more effectively to Fe"EDTA than to Fe"NTA (nitrilotriacetate) and Fe"IDA (iminodiacetate). Following this argument, CMPEI, because it contains a variety of IDA- and glycine-type groupings, should have a lower iron stability constant than EDTA and decreased affinity toward NO. For that reason, the Fibrabon 35/ ED3A system was studied. The structures of CMPEI and
100
P Q@
20 10
o+ 0
1
i
0 01
~
- - l
T - - i
0 02
T 0 03
,
-7
-1
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Figure 7. NO absorption versus Fe(II)/Fibrabon 35 concentration. (0) Lot B, total Fe = 0.02 M, liquid circulation rate = 50 mL/min; ( 0 )lot C, total Fe = 0.05 M, liquid circulation rate = 50 mL/min.
Fibrabon 35/ED3A are shown in Figure 5. Each ED3A group is similar to EDTA except that it lacks one carboxymethyl group, which has been replaced by a point of attachment to the polymer backbone. The results of the Fibrabon 35/ED3A runs are presented in Figures 6 and 7. The polymer used for the runs in Figure 6 was different than that used for the Figure 7 runs. Slightly better efficiencies seem to have been obtained when the ethylenediamine was first attached to the backbone and then carboxymethylated. Though the abatement efficiencies at various levels of Fe(I1) in the Fibrabon/ED3A runs are somewhat less than those for the EDTA system, a steady increase in NO absorption does occur as Fe(I1) levels are increased. At high concentrations,
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2095 The loss can be attributed to the overlap in the pore size distribution of the membrane with the molecular weight (and size) distribution of the polymeric chelate. 70
I
G
Conclusions A major problem in the use of ferrous chelates to remove NO, has been the uneconomical losses of the chelate in the blowdown or purge of the sulfur salts. This study has demonstrated that polymeric iron chelates can be made which will participate effectively in the NO, solution chemistry and which are capable of being separated from a purge stream by ultrafiltration. Though the chemistry here was demonstrated on a sodium-based SOz scrubbing solution, the polymeric chelate/ultrafiltration system should also work effectively in limestone-type scrubbers. Undoubtedly, other problems such as prefiltration of fly ash and particulates, membrane fouling, and potential poisons must be addressed when this process is scaled up. Recently we completed a field demonstration using a nonpolymeric chelate (coupled with electrolytic iron reduction) and did not encounter any of these problems. That work w ill be reported in more detail in another paper. Though it should be kept in mind that the abatement efficiencies reported here are also a function of scrubber design and hence can be increased, performance of the synthetically simple CMPEI 150 was not too remarkable. The Fibrabon 35/ED3A system, however, shows much promise as a fully recoverable iron chelate NO, removal catalyst (Grinstead, 1987). Literature Cited Bedell, S. A.; Kirby, L. H.; Buenger, C. F.; McGaugh, M. C. “Chelation Chemistry for Flue Gas Treatment”. Coal Technol. 1986, 2, 265. Chang, S. G.; Littlejohn, D. “The Potential of a Wet Process for Simultaneous Control of SOp and NOx in Flue Gas”. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1985a, 30, 119. Chang, S. G.; Littlejohn, D. “Kinetics of Combined S 0 2 / N 0 Removal in Flue Gas Cleanup”. Prepr. Pap.-Am. Inst. Chem. Eng. 198513, March, 1. Faucett, H. L.; Maxwell, J. D.; Burnett, T. A. “Technical Assessment of NOx Removal Processes for Utility Application”. Electric Power Research Institute, Palo Alto, CA, Nov 1977. Grinstead, R. R. US Patent 4 708 854, Nov 24, 1987. Hishinuma, Y.; Akimoto, H.; Kaji, R.; Nakajima, F.; Arikawa, Y. US Patent 4 081 509, March 28, 1978. Koizumi, M.; Tanaka, T.; Ishihara, Y. “Wet Process for Nitrogen Oxides Removal from Flue Gases, Part I. Additives for Accelerating the Reaction Rate of Nitrogen Oxides Removal with Sodium Sulfite Solution”. Denroyku Chuo Kenkyusho Gijutsu Dai Zchi Kenkyusho Hokoku 1974, 74021. Leimkuehler, J.; Ellison, W.; Makansi, J. “Strict European NOx Code Brings SCR into the Limelight”. Power 1986, 53. Sada, E.; Kumazawa, H.; Hikosaka, H. “A Kinetic Study of Absorption of NO into Aqueous Solutions of NapSO3with Added Fen-edta Chelate”. Ind. Eng. Chem. Fundam. 1986, 25, 386. Tsai, S. S.; Bedell, S. A.; Kirby, L. H.; Zabcik, D. J. “Nitric Oxide Abatement with Ferrous Chelates”. Prepr. Pap.-Am. Inst. Chem. Eng. 1987, Aug, 1.
Received f o r review February 8, 1988 Revised manuscript received June 24, 1988 Accepted July 12, 1988
