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Ind. Eng. Chem. Res. 1996, 35, 1668-1672
Kinetics of NO Absorption in Aqueous Iron(II) Bis(2,3-dimercapto-1-propanesulfonate) Solutions Using a Stirred Reactor Yao Shi,† David Littlejohn, and Shih-Ger Chang* Energy and Environment Division, Ernest Orlando Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720
The iron(II) bis(2,3-dimercapto-1-propanesulfonate) (Fe2+(DMPS)2) complex is a newly developed metal thiochelate for the removal of NOx from flue gas. The mass transfer and kinetics of the absorption of NO in aqueous Fe2+(DMPS)2 solutions have been studied in a stirred reactor. Factors influencing the NO absorption rate, such as pH, Fe2+(DMPS)2 concentration, and the ratio of Fe2+ to DMPS have been investigated. The forward rate constant of the Fe2+(DMPS)2NO complexation reaction has been determined to be 1.1 × 108 L/mols at pH 7.2 and 50 °C. Introduction The use of metal chelate additives in wet flue gas desulfurization (FGD) systems for combined removal of NOx and SO2 has been investigated by several groups (Tsai et al., 1989; Smith et al., 1992; Harriott et al., 1993). The Fe2+(EDTA) additive has been extensively studied for the removal of NO from flue gas. However, this additive suffers several significant drawbacks, such as the formation of undesirable nitrogen-sulfur byproducts and an unstable concentration of Fe2+(EDTA) due to rapid oxidation of the ferrous ion by oxygen. Recently, we have developed a new ferrous thiochelate additive, Fe2+(DMPS)2 (where DMPS is 2,3-dimercapto1-propanesulfonate), which can circumvent the problems associated with the EDTA-based additive (Pham and Chang, 1994). The equilibrium constant for the reaction of Fe2+(DMPS)2 with NO to form a nitrosyl complex has been determined in our laboratory from bubble column experiments (Pham and Chang, 1994). While the equilibrium constant is now known, the reaction rate constant has not been studied. The rate constant is needed to make predictions of NO removal efficiency in full-scale systems. The equilibrium constants for the formation of Fe2+(DMPS)2(NO) and Fe2+(EDTA)(NO) under typical FGD scrubber conditions (i.e. 300-600 ppm NO, 55 °C, pH 6) are 2.1 × 107 and 1.0 × 106, respectively. This indicates that Fe2+(DMPS)2 solutions have a high affinity for NO absorption. In the previous study, a 100 mm diameter turbulent contact absorber (TCA) system was employed to perform integrated tests, including chemical regeneration of the iron-based additive (Shi et al., 1995). The results indicated that a sustainable high level of NO removal efficiency can be easily obtained with the Fe2+(DMPS)2 system. This is attributed to the fact that the thiochelate is substantially more resistant to oxidation than Fe2+(EDTA). It is important to emphasize that Fe2+(DMPS)2(NO) does not react with bisulfite/sulfite ions to yield undesirable nitrogen-sulfur byproducts. The present work involves an investigation of mass transfer and kinetics for the complexation of NO to Fe2+(DMPS)2. The absorption of dilute NO into aqueous solutions of Fe2+(DMPS)2 was studied using a stirred * Corresponding author. † On leave from the Chemical Engineering Department, Zhejiang University, Hangzhou, China.
reactor with a planar gas-liquid interface. Using the theory of mass transfer with chemical reaction, the absorption rates and the forward rate constant for NO absorption by Fe2+(DMPS)2 have been determined from the measurements. The results have been used for planning a pilot plant test of the new additive for nitrogen oxide removal. Experimental Section Materials. DMPS (95%, Janssen), FeSO4‚7H2O (AR grade, Aldrich), sodium citrate dihydrate (AR grade, J.T. Baker), and disodium EDTA (99%, Aldrich) were all obtained commercially and used as received. The gases used included 2.5% NO in nitrogen, 0.3% SO2 in nitrogen, and nitrogen, which were obtained from Matheson. Apparatus and Procedure. A schematic diagram of the experimental apparatus is shown in Figure 1. All of the experiments for NO absorption were carried out in a double-stirred reactor with a gas-liquid interface that was planar to close approximation. The cell has a water jacket through which water from a constant temperature bath is circulated to maintain the desired temperature. The stirred cell is 8.0 cm in diameter and is equipped with four vertical baffles and two stirring blades. The stirring blades are independent, driven by separate motors, and used to stir the gas and liquid phases. The stirring introduced only small deviations to the gas-liquid interface, which was treated as planar for the calculations discussed here. Most of the experiments were performed with a liquid stirring speed (nL) of 150 rpm and a gas stirring speed (nG) of 900 rpm. The gas phase consisted of a mixture of NO and/or SO2 in nitrogen. The 2.5% NO in nitrogen was supplied from a cylinder and was further diluted with N2 to the desired concentration before being fed to the absorber. The SO2 was delivered in a similar manner when it was used. The feed concentration of NO ranged from 100 to 600 ppm, and when used, the feed concentration of SO2 was from 1000 to 2500 ppm. Before the gas mixture was fed into the absorber, it was heated to 50 °C and humidified by bubbling it through hot water. The absorber was operated with a continuous gas flow of about 1000 mL/min. Gaseous NO was absorbed through the free gas-liquid interface. The liquid phase was an aqueous solution of Fe2+(DMPS)2. The Fe2+(DMPS)2 solution was prepared by adding FeSO4‚7H2O and DMPS with 1:2 stoichiometry
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Figure 1. Schematic diagram of the experimental apparatus. 1: NO gas cylinder. 2: N2 gas cyclinder. 3 and 4: Gas flowmeters. 5: Transformer. 6: Heating tape. 7: Water vapor saturator. 8: Water bath. 9 and 10: Driving motors. 11: Stirred cell reactor. 12: Stirring speed controller. 13: Water vapor trap. 14: NO analyzer. Table 1. Physical Properties for the Fe2+(DMPS)2-NO System temp (°C)
[Fe2+(DMPS)2] (mol/L)
109DNO (m2/s)
HNO (atm L mol-1)
25 50
0.01 0.01
2.51 4.1
519 799
k°LCO2 ) 1.635 × 10-6nL0.56
to deionized water, and the solution pH was adjusted by adding small amounts of concentrated NaOH. The volume of the aqueous solution in the cell was always 310 mL. Unlike the gas phase, the solution was not flowed and used batchwise. The inlet and outlet gas concentrations of NO and SO2 were monitored with a Thermoelectron Model 14A chemiluminescent NOx analyzer and a Thermoelectron Model 40 fluorescent SO2 analyzer. A cold trap was used for removal of moisture before the gas entered the analyzers. The concentrations of ferrous and ferric ions were determined by 1,10-phenanthroline colorimetry. The absorption rate was derived from the inlet and outlet gas concentrations, the effective interfacial area, and total gas flow rate. It is reasonable to assume the effective interfacial area is the same as the geometric interfacial area in this type of absorber. During the experimental runs, the temperature was usually maintained at 50 °C, which simulated the temperature in existing FGD scrubbers. The total pressure in the system was 1 atm. Physical Properties of the Fe2+(DMPS)2-NO System. The liquid phase diffusivities of NO can be estimated from the equation developed by Wilke and Chang (1955), and the solubility of NO in the liquid can be evaluated by the method of van Krevelen and Hoftijzer (Danckwerts, 1970). In our study, the concentration of Fe2+(DMPS)2 was relatively low and the contribution of Fe2+(DMPS)2 to the total solubility of NO is negligible. Some of the parameters for the Fe2+(DMPS)2-NO system are listed in Table 1. Results and Discussion Most of the experimental results obtained from this study are shown here as plots of absorption rate as a function of interfacial concentration in the liquid phase or other factors. The observed absorption rates of NO were converted to chemical reaction enhancement factors using the following equation:
NA ) kGA(pA0 - pAi) ) k°LAEACAi
correlations which will be described later. CAi is the liquid phase concentration of the gaseous species A at equilibrium with a partial pressure pAi. The interfacial concentrations of NO and SO2 in liquid phase were assumed to be equal to those in water since the concentrations of Fe2+(DMPS)2 and SO32- were relatively low at the experimental conditions considered here. The interfacial concentration of NO in liquid was estimated using Henry’s Law. The outlet concentration of NO was up to 15% lower than the inlet concentration. The values of the inlet and outlet concentrations were averaged to calculate the interfacial NO concentration. While this introduces some uncertainty into the calculations, the results of the measurements using Fe2+(EDTA) agreed well with values in the literature. Liquid-Side and Gas-Side Mass Transfer Coefficients. The liquid-side mass transfer coefficient (k°L) of the cell was determined by measuring the rate of physical absorption of pure CO2 into water at 25 °C and then correlated to the liquid phase stirring speed, nL. Under these conditions, the gas-side mass transfer coefficient is unimportant.
(1)
Here, the gas-side and liquid-side mass transfer coefficients, kGA and k°LA, are obtained from empirical
(m/s, 25 °C) (2)
The gas-side mass transfer coefficient for the system was determined by absorption of dilute SO2 into an aqueous 0.5 M NaOH solution and correlated to the gas phase stirring speed, nG. Under the conditions for dilute SO2 absorption into aqueous NaOH solutions, the gasside mass transfer coefficient determines the transfer rate and the overall mass transfer coefficient is approximately equal to the gas-side mass transfer coefficient, KG ) kG
kGSO2 ) 5.878 × 10-8nG0.70
(kmol/m2 s kPa, 25 °C) (3)
The values of liquid- and gas-side mass transfer coefficients for a gaseous species I can be calculated by the following correlations involving the diffusivities of the compounds of interest and experimentally determined values of kLCO2 and kGSO2 respectively.
k°LI ) k°LCO2(DI/DCO2)2/3
(4)
kGI ) kGSO2(DI/DSO2)2/3
(5)
In our case, the value of KGNO is 2.36 × 10-5 kmol/m2 s kPa at 150 rpm and 50 °C and KLNO is 4.60 × 10-5 m/s at 900 rpm and 50 °C. Absorption of NO into Aqueous Solutions. The Henry’s Law constant for NO in water is very small, about 1.25 × 10-3 mol/L atm at 50 °C. However, the complexing reaction in the liquid phase will enhance the NO solubility dramatically. Nitric oxide reacts with Fe2+(DMPS)2 to form a nitrosyl complex as follows:
NO(g) a NO(aq)
(6)
Fe2+(DMPS)2(aq) + NO(aq) a Fe2+(DMPS)2(NO)(aq) (7) The complexation reaction may also be illustrated by the equation shown in Figure 2. The structure of the ferrous-ligand complex permits the formation of two types of nitrosyl complex: terminally-bound NO as
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1670 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996
Figure 2. Illustration of the complexation of NO to Fe2+(DMPS)2.
Figure 4. The NO absorption rate as a function of the ratio of Fe2+ to DMPS. Table 2. Effect of pH on the Enhancement Factor for the 10 mM Fe2+(DMPS)2 System EA
pH
EA
pH
257 530
4.5 5.4
1210 1051
6.8 9.8
Figure 3. The effect of pH on the NO absorption rate.
represented in Figure 2 and a combination of both terminal NO and bridging NO (Pham and Chang, 1994). The reactions between a metal chelate and NO should be second order for the forward reaction, i.e. first order in both NO and in the metal chelate (Sada et al., 1980; Yih and Lii, 1988). This must be confirmed experimentally for the ferrous-DMPS system. The value of the equilibrium constant for eq 7 is 2.1 × 107 L/mol at 55 °C (Pham and Chang, 1994). If the concentration of Fe2+(DMPS)2 in the bulk liquid, CB0, is much larger than the concentration of NO at the interface, CAi, as satisfied in the present experiments, the kinetics of the reaction becomes pseudo-first-order when 3 < Ha < Ei/2, where Ha is the Hatta number, defined by
HA ) (DAk2CB0)1/2kL
(8)
and Ei is the enhancement factor for instantaneous reaction, defined by
Ei ) 1 + DBCB0/zDACAi
(9)
For the present reaction, the stoichiometric coefficient of B is z ) 1. If DA ) DB and CB0/CAi . 1, then Ei ) CB0/CAi. Moreover, as Ha > 3, to a close approximation EA ) Ha, and the absorption rate for a pseudo-mth order reaction is given by
NA ) [(2/m + 1)k2CAim+1CB0]1/2
(10)
A plot of log NA versus CAi gives the reaction order for NO ) m, whereas the rate constant k2 can be determined from EA ) Ha. Details of the theory have been summarized by Danckwerts (1970). The effect of pH on NO absorption rates is shown in Figure 3. The Fe2+(DMPS)2-NO system appears to be strongly influenced by the pH of the solution, as are conventional ferrous chelate systems. The experiments were performed at a temperature of 50 °C for both gas and liquid phase, with 500 ppm inlet NO concentration and 10 mM Fe2+(DMPS)2. We observed that Fe2+(DMPS)2 solutions at pH 2-3 did not absorb significant
amounts of NO. This is presumably because both mercapto groups of DMPS are protonated and the chelate does not bind ferrous ions effectively (Ospanov et al., 1989). The NO absorption rate increases substantially as the pH increases from 4 to 7. The maximum NO absorption rate occurs around pH 7. The absorption rate decreases gradually at higher pH conditions, which may be due to competition from a ferrousDMPS-hydroxyl complex. From the experiments, it was observed that the dark green color of the solution increased with pH. From eq 11, the enhancement factor can be evaluated with pH, as shown in Table 2. The values of the enhancement factor were calculated from the measured
EA ) NA/(k°LACAi)
(11)
values of absorption rate NA, liquid phase mass transfer coefficient without chemical reaction, k°LA, and estimated NO concentration of gas-liquid interface. The results indicate that the appropriate pH range is very important in maintaining high NO removal efficiency. Figure 4 depicts the NO absorption rates obtained with different molar ratios of Fe2+ and DMPS. The experiment was run by adding additional Fe2+ to 10 mM Fe2+(DMPS)2 solutions at 50 °C, pH 6.0, and with a 500 ppm inlet concentration of NO. The NO absorption rates slightly increase when the ratio of Fe2+/DMPS increases from 0.5 to 1.0. The figure also shows that additional Fe2+ beyond this ratio does not benefit NO absorption. Moreover, in a real flue gas system with about 5% residual oxygen, the additional uncomplexed Fe2+ would be rapidly oxidized. According to our previous findings (Shi et al., 1995), the Fe3+ produced will be reduced to Fe2+ by DMPS. This may create the problem of excessive consumption of DMPS. It is thus recommended that the range of the ratio of Fe2+/DMPS be maintained at or slightly above 0.5. On the basis of the mass transfer model (eq 10), there should be a linear relationship between NO absorption rate and square root of the concentration of Fe2+(DMPS)2 for a second-order reaction. This prediction
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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1671
Figure 5. The effect of the concentration of Fe2+(DMPS)2 on the rate of NO absorption.
Figure 6. A comparison of the NO absorption rates for Fe2+(DMPS)2 and Fe2+(EDTA) as a function of concentration. Triangle: 10 mM Fe2+(DMPS)2, pH 7.2, 50 °C. Circle: 10 mM Fe2+(EDTA), pH 7.2, 50 °C. Diamond: 10 mM Fe2+(DMPS)2, pH 5.2, 50 °C. Table 3. Forward Rate Constants of Fe2+(DMPS)2 and Fe2+(EDTA) chelates
conditions
(k2)avg (L/mol)
Fe2+(DMPS)2 Fe2+(DMPS)2 Fe2+(EDTA)
50 °C, pH 7.2 50 °C, pH 5.2 50 °C, pH 7.2
1.1 × 108 1.1 × 107 1.2 × 108
was confirmed by the results shown in Figure 5. The experimental conditions for the measurements were 25 °C, pH 5.2, and an inlet NO concentration of 360 ppm. The figure shows a slightly nonzero intercept. This could be due to the presence of some oxidized DMPS in the solution, which would lead to an active Fe2+(DMPS)2 concentration slightly less than the prepared concentration. The results indicate that increasing the concentration of Fe2+(DMPS)2 is an effective way to improve NO removal efficiency in a scrubber system. To evaluate the forward second-order rate constant of the Fe2+(DMPS)2-NO reaction, the absorption rate of NO over a range of NO interfacial concentrations in the liquid phase was determined with the stirred reactor at 50 °C and pH 7.2 and compared with the results for Fe2+(EDTA). The results are plotted in Figure 6. Using the mass transfer model described above, the values for the second-order rate constant can be derived from the results and are shown in Table 3. By extrapolating the values of the equilibrium constant determined by Pham
Figure 7. The effect of citrate concentration on the NO absorption rate by Fe2+(DMPS)2.
and Chang (1994) to 50 °C, we can calculate the reverse rate constant (kr ) k2/Keq) as 4.6 s-1. The rate constant for Fe2+(DMPS)2 is almost the same as that of Fe2+(EDTA) at a pH of 7.2 and 50 °C. The rate constant of the Fe2+(EDTA)-NO system has been reported in a number of previous studies. The absorption of NO in aqueous solutions and slurries containing Fe2+(EDTA) and MgSO3 was studied by Sada et al. (1980). With no sulfite present, pH ) 7.0, and Fe2+(EDTA) ) 0.01-0.05 M, the average reaction rate constant was 1.22 × 108 L/mols at 25 °C. In the study of Teramoto et al. (1978), the rate constant for the Fe2+(EDTA) reaction with NO was 1.4 × 108 L/mols at 25 °C. Yih and Lii (1988) reported the Fe2+(EDTA) rate constant was 1.24 × 108 L/mols at 25°C and increased only slightly with temperature (1.43 × 108 at 60 °C). Their results are in reasonable agreement with those reported here. It is interesting to note the differences in preparation of Fe2+(DMPS)2 and Fe2+(EDTA) solutions for the experiments. Fe2+(DMPS)2 solutions are very easy to handle. The appropriate amounts of FeSO4‚H2O and DMPS are weighed out and then dissolved in the deionized water at 50 °C. The concentration of Fe2+ measured by 1,10-phenanthroline colorimetry is the same as that expected from the amount weighed out. In the case of Fe2+(EDTA) preparation, when Fe2+ and EDTA are added to 50 °C deionized water to prepare an 20 mM solution, the measured Fe2+ concentration is only about 10-11 mM due to oxidation from the presence of oxygen. Therefore, it is necessary to prepare solutions of Fe2+(EDTA) carefully to exclude oxygen and avoid oxidation. These observations indicate that the Fe2+(DMPS)2 complex is much more stable and oxidation resistant than Fe2+(EDTA) in aqueous solutions. The effect of citrate on the NO absorption rate by Fe2+(DMPS)2 solutions was investigated, and the results are shown in Figure 7. Sodium citrate was used as a precipitation inhibitor, since it can form complexes with excess ferrous and ferric ions. The results indicate that 50 mM Fe2+(citrate)2 alone increases NO absorption only slightly above that of water. When there is additional unbound Fe2+ in the Fe2+(DMPS)2 system, citrate will improve the NO absorption slightly by forming a ferrous citrate complex which can form a nitrosyl, as in the case of ferrous citrate alone. Without additional uncomplexed Fe2+, the NO absorption rate decreases slightly in the presence of citrate. This may be due to interference in the ferrous ion-DMPS complex structure by citrate.
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1672 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 H ) Henry’s Law constant, M atm-1 Ha ) Hatta number, (DAk2CB0)1/2/kL k2 ) second-order forward rate constant, L/mol s kG ) gas phase mass transfer coefficient, kmol/m2 s kPa kL ) liquid phase mass transfer coefficient, m/s m ) reaction order NA ) absorption rate, mol/m2 s nG ) gas phase stirrer speed, rpm nL ) liquid phase stirrer speed, rpm p ) partial pressure, kPa z ) stoichiometric coefficient Subscripts A ) NO B ) absorbent in liquid phase i ) gas-liquid interface 0 ) bulk liquid Figure 8. The effect of sulfite concentration on the NO absorption rate by Fe2+(DMPS)2.
Superscripts ° ) without chemical reaction
In systems for combined removal of NOx and SO2, there will be SO32- ions present in solution, which may affect the absorption of NO due to interactions of sulfite with the ferrous complex. The absorption rate of NO at various concentrations of sulfite ion was measured in a 10 mM Fe2+(DMPS)2 solution at 50 °C. The results (Figure 8) indicate that absorption rate decreases as the SO32- concentration increases. In a typical limestone FGD system, there is only a few millimolar of sulfite ion in the scrubbing slurry. Consequently, SO32- will only slightly reduce the NO absorption rate. Conclusions The results of our stirred reactor experiments show that NO absorption rates are strongly influenced by the pH of the Fe2+(DMPS)2 system. The absorption rate increases as the pH increases from 4 to 7 and decreases slightly as the pH increases above 7. The most suitable molar ratio for Fe2+:DMPS seems to be close to 0.5 (i.e. Fe2+(DMPS)2). The NO absorption rate increases linearly with the square root of concentration of Fe2+(DMPS)2, which is consistent with the prediction of the pseudo-first-order reaction model. The forward secondorder rate constant for the complexation of NO to Fe2+(DMPS)2 was determined from experimental results and found to be 1.1 × 108 L/mol s at pH 7.2 and 50 °C, about the same as the value for Fe2+(EDTA). Acknowledgment This work was supported by the Assistant Secretary for Fossil Energy, U.S. Department of Energy, under Contract DE-AC03-76SF00098 through the Pittsburgh Energy Technology Center, Pittsburgh, PA. Nomenclature C ) liquid phase concentration, mol/L D ) molecular diffusivity, m2/s E ) enhancement factor for absorption with chemical reaction, dimensionless
Literature Cited Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970. Harriott, P.; Smith, K.; Benson, L. B. Simultaneous Removal of NO and SO2 in Packed Scrubbers or Spray Towers. Environ. Prog. 1993, 12, 110-113. Ospanov, Kh. K.; Vasil'ev, V. P.; Garavin, V. Yu.; Nukhin, A. N. Acid-Base Equilibrium in Aqueous Solutions of Unithiol. Koord. Khim. 1989, 15, 1619-1621. Pham, E. K.; Chang, S. G. Removal of NO From Flue Gases by Absorption to an Iron(II) Thiochelate Complex and Subsequent Reduction to Ammonia. Nature 1994, 369, 139-141. Sada, E.; Kumazawa, H.; Kudo, I.; Kondo, T. Individual and Simultaneous Absorption of Dilute NO and SO2 in Aqueous Slurries of MgSO3 with FeII-EDTA Ind. Eng. Chem. Process Des. Dev., 1980, 19, 377-382. Shi, Y.; Littlejohn, D.; Chang, S. G. An Additive in Wet Limestone Systems for Combined Removal of SO2 and NOx from Flue Gas. SO2 Control Symposium, Book 3; EPRI/EPA/DOE: Miami, FL, 1995; Session 7B. Smith, K.; et al. Enhanced NOx Removal in Wet Scrubbers using Metal Chelates. Report to Pittsburgh Energy Technology Center, U.S. Department of Energy Contract DE-AC2290PC90362, 1992. Teramoto, M.; Hiramine, S.; Shimada, Y.; Sugimoto, Y.; Teranishi, H. Absorption of Dilute Nitric Monoxide in Aqueous Solutions of Fe(II)-EDTA and Mixed Solutions of Fe(II)-EDTA and Na2SO3. J. Chem. Eng. Jpn. 1978, 11, 450-457. Tsai, S. S.; Bedell, S. A.; Kirby, L. H.; Zabick, D. J. Field Evaluation of Nitric Oxide Abatement with Ferrous Chelates. Environ. Prog. 1989, 8, 126-129. Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Dilute Solutions. Am. Inst. Chem. Eng. J. 1955, 1, 264-270. Yih, S. M.; Lii, C. W. Absorption of Nitrogen Oxide and Sulfur Dioxide in Fe(II)-EDTA Solutions. I. Absorption in a Double Stirred Vessel. Chem. Eng. Commun. 1988, 73, 43-53.
Received for review October 26, 1995 Accepted March 4, 1996X IE950655M
X Abstract published in Advance ACS Abstracts, April 15, 1996.
