2016
Ind. Eng. Chem. Res. 1987, 26, 2016-2019
Absorption of Dilute NO into Aqueous Solutions of Na2SOswith Added FeIINTA and Reduction Kinetics of FeIIINTA by Na2S03 Eizo Sada,* Hidehiro Kumazawa, and Hiroshi Machida Department of Chemical Engineering, Kyoto University, Kyoto 606, Japan
The absorption of dilute NO into aqueous solutions of Na2S03with added FeI'NTA was carried out by using a semibatch bubble column at 323 K and 0.10 MPa of total pressure. Chemical reactions accompanying this chemical absorption were investigated through quantitative analyses of gas- and liquid-phase species. The pathways of the liquid-phase reactions were found to be similar to those with added Fe"EDTA. It was further found that the degree of removal of NO mainly depends on the concentration of total Fe2+. Thus, the reduction kinetics of Fe3+ by Na2S03with coexisting NTA were investigated compared with the kinetics in the case with coexisting EDTA. The reduction rate was found to be expressed as first-order with respect to the concentration of FeII'NTA, minusfirst-order with respect to the concentration of Fe"NTA, and half-order with respect to the concentration of Na2S03. The wet processes for removal of nitrogen oxides (NO,) have the advantage of effectively removing sulfur dioxide simultaneously. An aqueous solution of Na2S03 with added FeIIEDTA has been recognized as a promising absorbent for simultaneous removal of NO and SO2, where EDTA stands for ethylenediaminetetraacetic acid. The FenEDTA chelate here is added to compensate for the low solubility of NO in water through the formation of a nitrosyl complex. In our previous works (Sada et al., 1984, 1986), the experiments were performed on absorption of NO diluted with Nz or He into aqueous solutions of Na2S03with added Fe"EDTA using a semibatch bubble column. The reactions accompanying such chemical absorption were investigated in detail through quantitative determination of gas- and liquid-phase species. The mechanisms of the main liquid-phase reduction, oxidation of Fez+ to Fe3+ by NO in the presence of Na2S03,and production of NzO have been deduced and drawn in the form of a map. The degree of removal of NO was found to mainly depend on the concentration of total Fez+,which is determined from a balance of the rates of oxidation and reduction of iron. Furthermore, the reaction kinetics of both the reduction of Fe3+ to Fe2+by HS03- with coexisting EDTA and the oxidation of Fez+to Fe3+by NO in the presence of NaZSO3were investigated. The reduction was found to be first-order with respect to FeInEDTA and HS03- concentrations and minus-first-order with respect to Fe"EDTA concentration. The apparent oxidation was found to be first-order with respect to both Fe"(N0)(EDTA)2- and NaZSO3concentrations and minus-firstorder with respect to Fe"'EDTA concentration. Besides Fe'IEDTA chelate, the Fe"NTA chelate has been found to have ability to effectively form a complex with NO, where NTA stands for nitrilotriacetic acid. The equilibrium constants of the coordination of NO to FenNTA have been investigated by Lin et al. (1982). But, both the mechanism and kinetics of the reduction of the nitrosyl complex by HSO, and SO?- with coexisting NTA have not been studied yet. In the present work, the experiments were carried out for absorption of dilute NO in aqueous mixed solutions of Fe"NTA and NaZSO3by use of a semibatch bubble column. The chemical reactions occurring in this chemical absorption system were investigated through quantitative analyses of gas- and liquidphase species in the same manner as in the case of FeIIEDTA (Sada et al., 1984). Further, the reduction kinetics of the reduction of Fe3+by Na2S03with coexisting 0888-5885/87/2626-2016$01.50/0
NTA were investigated and compared with the kinetics in the case with coexisting EDTA (Sada et al., 1986).
Experimental Section First, experiments were carried out for absorption of NO diluted with Nz in aqueous solutions of Fe"NTA by using a buble column at temperatures ranging from 20 to 60 "C to measure equilibrium constants for the complexing reaction of NO with Fe'INTA. The bubble column was a cylindrical cell, equipped with a ball filter (G3, 30 mm in diameter), which was similar to that used in our previous work (Sada et al., 1984), and operated batchwise with respect to the liquid phase and continuously with respect to the gas phase. The volume of liquid absorbent was lo00 cm3. The chelate solutions of FeI'NTA were prepared by adding equimolar amounts of FeS04 and NTA.2Na (disodium salt of nitrilotriacetic acid) to distilled water. The initial pH value of the solution was adjusted by NaOH solutions. The concentration of NO in the influent stream was maintained at about 1000 ppm, and the total volumetric gas flow rate was maintained at about 60 cm3/s. The diluted NO gas was fed continuously into the solution, and the concentration of NO in the effluent stream was monitored until it became equal to that in the influent stream, that is, until chemical equilibrium was reached. Second, experiments were performed for absorption of dilute NO in aqueous mixed solutions of Na2S03and Fe"NTA at 50 "C by using the same bubble column. The volume of the liquid absorbent was 1000 cm3, and the concentration of NO in the influent stream was fixed at ca. 1000 ppm as well. The concentrations of NO and N20 in the effluent stream were measured. If a desired amount of reaction liquid was sampled periodically, the concentrations of Fez+,HS03-, and HON(SO3)?- were measured. The determination of the species in the gas and liquid phases was followed by the procedures described in our previous article (Sada et al., 1984). Third, experiments were performed to establish the reaction kinetics of the reduction of Fe3+by NaZSO3with coexisting NTA. The same bubble column was used for the reactor. The reactor was charged with 500 cm3of an aqueous equimolar solution of FeC13 and NTA.2Na, through which N2 was fed with a flow rate of ca. 60 cm3/s from the ball filter as a gas sparger for 20 min to remove dissolved oxygen. A reaction was caused by adding 500 cm3of aqueous Na2S03solution. During the reaction, N2 was continuously fed at the same flow rate as well. The 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2017
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