Ind. Eng. Chem. Res. 1987,26, 1468-1472
1468
that the power of this flocculant was good for two reasons: (1) a small dosage of only about 22 ppm was enough to flocculate the suspension; (2) the restabilization phenomenon caused by an overdose of the flocculant was not observed. Nomenclature A = absorbance A,, = standard value of A C = concentration of polycation m = mass of suspended solids N = equivalent moles of polyelectrolyte or PEI-PVSK compound T = residual turbidity To = standard value of T X = adsorbed amount of polyelectrolyte V = volume of flocculation system
PVSK = potassium polyvinyl sulfate PEI-PVSK = complex of PEI and PVSK Registry No. PEI.HC1, 26338-45-4;PVSK, 26837-42-3. Literature Cited Healy, T. W.; La Mer, V. K. J. Colloid Sci. 1965, 9, 545. Jirgensons, B.; Straumanis, M. E. In A Short Textbook of Colloid Chemistry; 2nd revised ed.; Pergamon: London, 1962a; Chapter 5. Jirgensons, B.; Straumanis, M. E. In A Short Textbook of Colloid Chemistry; 2nd revised ed.; Pergamon: London, 1962b; Chapter 6. Kashiki, I.; Suzuki, A. Znd. Eng. Chem. Fundam. 1986, 25, 120. Kashiki, I.; Suzuki, A.; Gotoh, K. Kagaku Kogaku Ronbunshu 1982, 8, 73. Linke, F.; Booth, R. B. Trans. Am. SOC.Mech. Eng. 1960,217,364. Sakaguchi, K.; Nagase, K. Bull. Chem. SOC.Jpn. 1966, 39, 88. Senju, R. Koroido Tekitei-hou; Nankodo: Tokyo, 1969.
Subscripts PEI = polyethylenimine hydrochloride
Received for review October 7, 1985 Accepted April 15, 1987
Oxidation Kinetics of FeII-edta and FeII-nta Chelates by Dissolved Oxygen Eizo Sada,* Hidehiro Kumazawa, and Hiroshi Machida Department of Chemical Engineering, Kyoto University, Kyoto 606, Japan
It has been found in our previous work t h a t the degree of removal of NO by aqueous solutions of Na2S03with added Fe'I-edta mainly depends on the concentration of Fe2+,and the concentration of Fe2+is determined from a balance of the rates of oxidation and reduction of iron. Thus, the reaction kinetics for oxidation of Fe"-edta or Fe'I-nta, which is another promising chelating agent t o Fe2+ for removal of NO by dissolved oxygen, were investigated by using a bubble column reactor. T h e oxidation reaction of Fen-edta was found to be first-order with respect to dissolved oxygen and about half-order with respect t o Fe"-edta. T h e oxidation was suppressed by about 30% by adding 20% EDTA in excess. T h e oxidation rate of Fe"-nta was shown t o be 1st-order in dissolved oxygen concentration and approximately 0.7-order in FeLnta concentration. The oxidation, however, could not be suppressed at all by adding NTA in excess. Wet scrubbing processes have a potential of effectively removing both nitrogen and sulfur oxides (NO, and SO,) from stationary combustion facilities simultaneously. Aqueous solutions of Na2S03with added FeEedta chelate seem to be promising absorbents to fulfil such a possibility. To establish the procedure for inevitable regeneration or treatment of used absorbent, it is necessary to clarify the whole scheme of the complex liquid-phase reactions. In our previous work (Sada et al., 1984,1986),the pathways of the liquid-phase reactions were completed and presented in the form of a map. It has been shown that the degree of removal of NO depends on the concentration of Fe2+, which is determined from a balance of oxidation and reduction of iron. Furthermore, it was found that SO2 coexisting with Fez+in the solution reflected the decrease in pH of the absorbent through the reaction of SO2 with S032-and that the removal of NO during the simultaneous absorption of NO and SO2 can be predicted from corresponding experimental results for the absorption of only NO into an absorbent of the same pH value. Regarding reaction kinetics, both the reduction of Fe3+ to Fez+by HS03- with coexisting EDTA and the oxidation of Fez+ to Fe3+by NO in the presence of Na#03 were investigated. It should be emphasized that Fez+ is oxidized by NO in the presence of Na2S03,though it is not oxidized at all in the absence of Na2S03. The rate of the reduction can be 0888-S885/ 87 12626-1468$01.50/ 0
expressed as first-order with respect to both Fe'Qdta and HSO - concentrations and minus first-order with respect to FJI-edta concentration. The rate of the apparent oxidation can be expressed as first-order with respect to both Fe(NO)(edtaY+ and Na2S03 concentrations and minus first-order with respect to Fe"'(edta)- Concentration. Flue gases emitted from stationary combustion sources normally contain several percent oxygen. In view of the above-stated information that the degree of femoval of NO mainly depends on the Fez+concentration, the oxidation kinetics of Fe"-edta by dissolved oxygen should be established more completely. For the sake of comparison, the oxidation kinetics of Fe"-nta were also investigated. nta stands for nitrilotriacetic acid, which is another promising chelating agent to Fe2+for NO removal (Lin et al., 1982). Experimental Section The experiments on oxidation of aqueous Fe"-edta solutions by dissolved oxygen were carried out in a bubble column reactor which is similar to one used in previous work (Sada et al., 1986). The bubble column was operated continuously with respect to the gas phase and batchwise with respect to the liquid phase. The gas sparger was a ball filter (G3,lO mm in diameter). The liquid volume free of gas bubbles was 1000 cm3. The total gas flow rate was 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1469 maintained a t 50.7 cm3 (STP)/s. The concentration of oxygen in an influent stream was varied from 2 to 7 vol 7%. The reaction was followed by sampling a desired amount of reaction liquid for a certain time and determining the final concentration of Fez+ by the ophenanthroline method. For the sake of comparison, absorption of oxygen into aqueous solutions of Fen-nta was carried out by using the same bubble column a t 50 "C. The chelate solutions of Fe"-edta or F e ' h t a were prepared by adding equimolar amounts of FeS04 and EDTA2Na (disodium salt of ethylenediaminetetraacetic acid) or NTA2Na (disodium salt of nitrilotriacetic acid) to distilled water. The initial pH of the solution was adjusted by NaOH to 6-8. To estimate the physical liquid-side volumetric masstransfer coefficients in the same bubble column, some experiments were performed on absorption of pure oxygen into aqueus solutions of NaZSO3without any added catalyst, and the time courses of the concentration of NaZSO3 were measured. The ionic strength of the solution was adjusted to be the same as that used for oxidation of Fen-edta and Fen-nta by dissolved oxygen. The dissolved oxygen reacts with Na2S03to produce NaZSO3in the bulk liquid (Na2S03+ '/202 Na2S04),so that the volumetric mass-transfer coefficient kLoais evaluated from the measured time course of the concentration of NaZSO3,viz.,
02 VOl.% A
3
0
4
0
5
-
A
Time, min
Figure 1. Time dependence of Fez+ concentration in oxidation of Fe"-edta by dissolved oxygen at different oxygen concentrations.
-
-d[Na2S03]/dt kLoa =
(1)
2cAi
Theoretical Background for Evaluation of Reaction Kinetics
Time, min
When the oxidation reaction of Fe"-edta by dissolved oxygen is assumed to be first-order with respect to oxygen and nth-order with respect to Fez+,the rate of absorption of oxygen per unit volume of dispersed phase is expressed as rA = kLoUc$(CAi- CAO)= kCA&Bn = VkCAOCB"
Figure 2. Time dependence of Fez+ concentration in oxidation of Fe"-edta by dissolved oxygen at different initial FeII-edta concentrations.
(2)
where 4 refers to an enhancement factor and v refers to an overall effectiveness factor. The oxidation reaction is stoichiometrically described as 4Fe"-edta2-
+ O2 + 2Hz0
-
4Fe"I-edta-
+ 40H-
(a)
Accordingly, the oxidation rate of Fez+ defined by dCB rB
=
-dt
A?'
(3)
reduces to
Figure 3. Test of eq 5 and determination of reaction rate constant a t 40 "C. Keys as in Figure 1.
k, can be determined through eq 5. where c$ is replaced with unity in the right-hand member. This assumption will be reconfirmed in the next section. By equating eq 3 to the right side of eq 4 and integrating from t = 0 to t, one gets
Here, C, is assumed to be constant in the axial direction of the bubble column because the solute gas (oxygen) is sparingly soluble in aqueous solutions of FeII-edta or Fen-nta. If the time dependence of Fez+is measured, both the order of the reaction, n, and the reaction rate constant,
Experimental Results and Discussion Typical examples of the time dependence of Fez+concentration in the reacting solution are given in Figures 1 and 2. The effects of oxygen concentration and initial Fe"-edta concentration on the oxidation reaction were indicated in Figures 1and 2, respectively. The oxidation reaction was found to be independent of pH of the solution within the pH range covered here. To represent such experimental results on the basis of eq 5, the numerical values of the order of reaction, n, should be preliminarily known. The value of n was determined to be 0.5 so as to apparently give a linear relationship between the left-hand member and the quantity in the parentheses of the right side of eq 5. Figures 3-5 shows the plots of the left side
1470 Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987
-0
0.05 0.10 4 L t -1Cse- Cd/k;a,
0.15 mol.s/dm3
0.20
0.25
Figure 4. Test of eq 5 and determination of reaction rate constant at 30 "C. Keys as in Figure 2.
"h.9
3.0
3.1
3.2 3.3 103/T, K"
3.4
5
Figure 6. Arrhenius plot of rate constant for (1,0.5)-order oxidation reaction of Fe"-edta by dissolved oxygen.
0.20 02 5 V O l %
50 O C 02 5 VOI %
50 'C
[EDTA], mol/dm3 0.01 3
\ i j
ii
0.5
2
7
*\
a
*\ *'
?
0.011
0.012 0.015 0.02
[FeSO,];O.OI
mol/dm3
L A
20
Figure 5. Test of eq 5 and determination of reaction rate constants at different temperatures.
vs. the quantity in the parentheses of the right side of eq 5 for different oxygen concentrations, initial FeII-edta concentrations, and reaction temperatures. To calculate the quantity in that parentheses CAiand kLoa should be estimated. The value of CAiwas calculated from the known solubility of oxygen in water without any correction owing to a salt effect. This assumption is considered to be reasonable because the ionic strength of the solution is less than 0.12 mol/dm3. But even in such a low ionic strength, it has a significant effect on the volumetric liquid-side mass-transfer coefficient. The volumetric liquid-side mass-transfer coefficients, kLoa, were measured in the system using the physical absorption of pure oxygen into aqueous NaZSO3solutions of the same ionic strengths as those for Ferr-edta oxidation runs. The volumetric mass-transfer coefficient depended on the ionic strength of the solution and the temperature. For example, the volumetric mass-transfer coefficient ranged from 0.036 to 0.046 s-l a t 50 "C as the ionic strength varied from 0.036 to 0.12 mol/dm3. A straight line passing through the origin can be drawn in Figures 3 and 4,irrespective of the oxygen concentration or the initial concentration of Fe'I-edta. A straight line passing through the origin can be drawn at every reaction temperature in Figure 5. The slope of a straight line implies a value of the reaction rate constant, k . The values of the rate constant of the (1,0.5)-order reaction a t different temperatures calculated from the slopes of the straight lines are shown in Figure 6. From the slope of the Arrhenius plot, the activation energy was derived to be 18.0 kJ/mol. The solid curves depicted in Figures 1 and 2 were Calculated by eq 5 with the derived reaction rate constants and using the order of reaction with respect to Fez+,n = 0.5. The reaction-diffusion modulus
40 Time, min
60
80
Figure 7. Time dependence of Fez+concentration in oxidation of Fe"-edta by dissolved oxygen in excess of EDTA. 50 "C
'
1
Figure 8. Time dependence of Fe2+concentration in oxidation of Fe"-nta by dissolved oxygen a t different oxygen concentrations.
which is defined by (kCm0.5DA)1/2/kL0 is rouqhly estimated to be less than 0.2, so that 4 should be nearly equal to 1. Moreover, the optimum values of the order of reaction, the frequency factor, and the activation energy appearing in k = A exp(-E/Rn (6) were calculated using all of the experimental data for the time dependence of Fez+ concentration by the Simplex algorithm. The optimum values derived were n = 0.536, A = 1.09 X lo4 dm3n/(moln-s),and E = 23.3 kJ/mol. So far, the Fe"-edta chelate solution was an aqueous equimolar mixture of EDTA2Na and FeS04. It is of interest now to check whether the oxidation by dissolved oxygen is suppressed by adding an excess amount of EDTA2Na to FeS04 or not. Figure 7 shows such experimental results where EDTA2Na is added to create an excess as great as 100%. The oxidation rate was found to
Ind. Eng. Chem. Res., Vol. 26, No. 7, 1987 1471
n = 0.714 and k = 4.94 dm2.242/(mo10.714-s) at 50 "C.)Figure 11shows the time dependence of Fez+concentration in the reacting solution when 100% excess NTA2Na was added. The oxidation could not be suppressed a t all. As the oxidation reaction proceeded, the pH of the Fen-nta solution gradually decreased, whereas the pH of the Fe"-edta solution gradually increased. The scheme of oxidation reaction for Fe"-nta chelate is thought to be different from that for Fe"-edta chelate. In what follows, an oxidation mechanism for both chelates will be suggested. In view of the effects of changing pH observed during the oxidation reaction and the order of reaction with respect to Fe'Ledta or Fe"-nta, which ranges from 0.5 to 1, the following reaction scheme is considered to be valid:
50 C'
\ op
5
VOl.%
__
Figure 9. Time dependence of Fez+concentration in oxidation of Fen-nta by dissolved oxygen at different initial Fe'Lnta concentrations.
LFep+
k
k0
+
0,
(-
LFeZ+O=O)
LFe3+OO- ( X )
(b)
kl
LFe3+OO-
+
3LFeZ+
+
8
k
2H20 2 2(LFe3+'
'O\ (alation)
\Fe3+L)
(c )
H
i H (e)
I
' 0
where L stands for ligand, i.e., either edta or nta. In nta, chelate oxolation takes place to deprotonation through reaction (d); in edta chelate, OH- is formed through reaction e. Moreover, if the steady-state approximation is applied to intermediate X, one gets
i 0.10
0.05
Q15
-4x1 - - k1[Oz]- k,'[X] dt
''O-
I
vO"x
$\OZ
I\
kl[O,l
(NTA], moVdm3 0
0.01
o
aoii
k
