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Effect of temperature on SO2 absorption into sulphuric acid solutions containing hydrogen peroxide
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Chemical Engineering and Processing 47 (2008) 1603–1608
Effect of temperature on SO2 absorption into sulphuric acid solutions containing hydrogen peroxide Sandrine Colle, Jacques Vanderschuren, Diane Thomas ∗ Chemical Engineering Department, Facult´e Polytechnique de Mons, Rue de l’Epargne, 56, B-7000 Mons, Belgium Received 19 October 2006; received in revised form 30 August 2007; accepted 31 August 2007 Available online 12 September 2007
Abstract A mathematical model previously developed for the absorption at 20 ◦ C of sulphur dioxide into sulphuric acid solutions containing hydrogen peroxide was adapted to take the effect of the temperature into account. This model was exploited to determine at 35 and 50 ◦ C the overall kinetic parameter relative to the absorption–oxidation of SO2 , for increasing H2 SO4 molarities (from 1 to 6.4 M) and in the presence of a variable concentration of H2 O2 (1.2–4.4 M) from absorption test runs performed in a cables-bundle laboratory scrubber. It was clearly observed that an increase in temperature enhances the global process of SO2 absorption (solubility + diffusion + reaction), whereas at a given temperature, an increase in the sulphuric acid content slows down the process. © 2007 Elsevier B.V. All rights reserved. Keywords: Sulphur dioxide absorption; Sulphuric acid; Hydrogen peroxide; Liquid-phase reaction kinetics
1. Introduction The framework of this work is the development of a reduction process of sulphur dioxide present in industrial flue gases. Our oxidising process is a wet one achieving the abatement of SO2 using aqueous hydrogen peroxide solutions. This treatment can be interesting if it leads to valuable or reusable sufficiently concentrated sulphuric acid solutions, while limiting the liquid wastes. The fundamental scientific data (thermodynamic and kinetic) relative to this process, however, already patented and applied, in particular in large-scale metallurgical units coupled with the production of sulphuric acid, are very scarce in the literature. Some very general information can only be found [1–4]. The kinetics of SO2 absorption into H2 SO4 solutions (from fairly to strongly concentrated solutions) containing H2 O2 , has been the object of our research for a few years [5–8]: specific absorption tests have been carried out at laboratory and pilot scales in order to determine, by means of the traditional techniques of chemical engineering, the essential data for the design of industrial contactors, valid in a wide range of operating conditions.
∗
Corresponding author. E-mail address: [email protected] (D. Thomas).
0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.08.014
Our laboratory investigation at 20 ◦ C was presented in author’s previous papers, discussing largely the influences of H2 O2 and H2 SO4 concentrations on the absorption rate of SO2 [5]. An overall kinetic parameter (OKP), function of cH2 SO4 , was deduced from numerous experiments conducted in a small cable scrubber at 20 ◦ C and validated satisfactorily in a pilot-scale packed column [6]. The aim of this study was to obtain at higher temperatures, namely 35 and 50 ◦ C, a correlation (Arrhenius-type correlation) for this overall kinetic parameter versus the H2 SO4 concentration of the scrubbing solution, by achieving absorption experiments in the laboratory scrubber. 2. Experimental set-up and procedure 2.1. Semi-continuous absorption tests: SO2 absorbed into sulphuric acid solutions containing hydrogen peroxide The experimental equipment, included essentially an absorption reactor, namely a cables-bundle laboratory scrubber with an inside diameter of 0.045 m and an effective height of 0.54 m. This contactor is well suited to kinetic studies as its specific surface is quite insensitive to the kinetic flow rate and the viscosity. However, a modification of the device was operated while dividing by two the number of twisted polypropylene yarns (from 12 to 6),
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stretched vertically, in parallel and constituting the packing of the gas–liquid microreactor. This has the advantage of preventing the formation of liquid films hung between two cables, the liquid flow observed remaining more stable. Nevertheless, this has also the drawback of a gas–liquid interfacial area reduced by half, involving a similar decrease of the quantity of SO2 absorbed. The gas phase, made of nitrogen humidified in a saturator, and in which SO2 is added to obtain the desired concentration, enters and flows out the column axially, through perforations milled in the bottom and top devices. Absorption performances of the sulphur dioxide were observed at 35 and 50 ◦ C and at atmospheric pressure, deduced from the SO2 gas contents measured simultaneously at both ends of the column by a two-channel UV analyser, allowing to calculate the SO2 fractional absorption A: A=
pSO2 in − pSO2 out pSO2 in
(1)
The scrubbing liquid, containing given sulphuric acid and hydrogen peroxide concentrations, is fed to the top distributing chamber of the contactor; thanks to a gear pump. It is distributed around each cable through individual holes. As the flow becomes steady, the liquid film around the yarn has a cylindrical shape. The liquid feed temperature was controlled by means of a heat exchanger operating counter-currently with water supplied from a thermostatic bath. The absorption column was furthermore insulated with a 10-mm thick sheet of elastomer. No appreciable temperature rise was noted during absorption. The liquid and gas temperatures at both ends of the column were checked by thermometers. As far as the experimental procedure followed in our absorption test runs is concerned, the absorber was operated counter-currently and batchwise with respect to the scrubbing liquid, which was recycled from a tank and brought in contact with the gas loaded with SO2 . Due to a short duration of the test (only a few minutes), quasi-steady-state measurements were made, indicated by no significant changes of column temperature and input and output concentrations. At the end of the test, a liquid sample was withdrawn at the same times than SO2 gas measurements at the inlet and the outlet of the column. For each liquid sample, the sulphuric acid and hydrogen peroxide concentrations were measured, respectively, by an acid–base titration and an iodometric method. A wide range of operating conditions, pertinent to flue gas purification processes, was investigated: liquid flow rate (uL = 0.2–0.33 cm/s) and gas flow rate (uG = 0.20–0.41 m/s), gas-phase composition (ySO2 = 200–5500 ppm), acidity of the solution (H2 SO4 = 1–6.4 M) and H2 O2 concentration (H2 O2 = 1.2–4.4 M). Some hundred tests were carried out at 35 and 50 ± 0.3 ◦ C. Finally, each kinetic test is characterised by a set of seven data (L, G, pSO2 in , pSO2 out , cH2 SO4 , cH2 O2 and temperature) required for the subsequent determination of the overall kinetic parameter.
2.2. Tests for the estimation of the mass transfer characteristics of the packing For the determination at 35 and 50 ◦ C of the volumetric gasphase mass transfer coefficient kG a, as a function of the operating conditions of gas and liquid flow rates, SO2 absorption efficiencies were measured with sodium hydroxide solutions (molarity 1 M), allowing to assume the total cancellation of the liquidphase resistance due to an instantaneous reaction between SO2 and OH− . The results of such absorption experiments were also used for comparison with the SO2 absorption efficiencies measured in the sulphuric acid solutions containing hydrogen peroxide. As far as the volumetric liquid-phase mass transfer coefficients kL a are concerned, they were estimated by extrapolation, at 35 and 50 ◦ C, of the results obtained at 20 ◦ C by absorbing pure carbon dioxide into deionised water, for various liquid flow rates. Lastly, the estimation of the specific interfacial area a, varying with the liquid flow rate and the cinematic viscosity of the solution, was performed at the temperature of 20 ◦ C by absorbing CO2 diluted in air into sodium hydroxide solutions of increasing concentrations (0.5, 2.5, 3.9 and 5.1 M) in the conditions of a fast chemical reaction regime. The reader will find details on the application of this technique and the choice of the kinetic constant for the reaction between CO2 and hydroxide ions OH− in Colle et al. [6]. An extrapolation of these results, using the values of dynamic viscosities of the solutions of H2 SO4 , was carried out to estimate the values of a at 35 and 50 ◦ C. 3. Modelling of SO2 absorption and effect of temperature The mathematical model previously developed by Colle et al. [7] is based on the two-film theory for SO2 absorption accompanied by an irreversible chemical reaction between the solute and the liquid reactant H2 O2 , namely the oxidation reaction SO2 + H2 O2 → H+ + HSO4 − which is first-order with respect to both SO2 and H2 O2 . Depending on the respective concentrations of H2 SO4 and H2 O2 in the scrubbing solution, the chemical reaction regime can be slow, moderately fast or fast, based on a Hatta number criteria. In our case, due to the kinetic characteristics of the reaction, this Hatta number Ha is defined as: k2 DSO2 /H2 SO4 cH2 O2 Ha = (2) kL where DSO2 /H2 SO4 is the diffusion coefficient for SO2 in acid solutions and k2 is the kinetic constant of the SO2 oxidation reaction, depending strongly on the H2 SO4 concentration. Our study, rather general, is related to the moderately fast and fast regimes, but the work presented here reports exclusively tests carried out in the fast regime. The moderately fast chemical reaction regime (0.3 < Ha < 3) was observed for lower H2 O2 or higher H2 SO4 concentrations, studied in a pilot column and specifically reported elsewhere [8].
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For sufficiently high H2 O2 contents, it was previously shown that the fractional absorption is independent on the partial pressure of SO2 in the inlet gas, actually in the presence of an excess of oxidising agent, which confirms the first-order with respect to reactant SO2 . With the conditions of a liquid bulk H2 O2 , concentration higher than the interfacial concentration of SO2 and large Hatta numbers (3 < Ha < Ei /2 with Ei being the value of E for the instantaneous reaction) the reaction becomes pseudo-first-order and fast, and the enhancement factor E is almost equal to Ha. Eq. (3) was then used to compute the liquid side absorption flux RSO2 [9]: k2 DSO2 /H2 SO4 √ cH2 O2 pSO2i RSO2 = EkL cSO2i = H √ = OKP cH2 O2 pSO2i (3) In this relation, pSO2i and cSO2i are the SO2 partial pressure and the interfacial concentration, respectively (assuming a Henry’s equilibrium law at the interface). The SO2 concentration in the liquid film is virtually null, due to the reaction proceeding. In this case of a fast pseudo-first-order reaction, the absorption flux of SO2 is independent of the liquid-phase mass transfer coefficient, but takes into account an overall kinetic parameter (OKP) lumping the kinetics of oxidation reaction, the diffusion as well as the solubility of SO2 in the liquid phase. As already mentioned, at the temperature of 20 ◦ C,√the variation of OKP (which we will note thereafter simply k D/H) as a function of the sulphuric acid content of the solution was determined from test results carried out in a laboratory scrubber and validated with data obtained in a pilot installation, and was the subject of previous publications [5,6]. The determination of the variation of the OKP value at 35 and 50 ◦ C was the aim of this work and required, for the interpretation of the experimental tests newly carried out, the introduction, in the simulation program, of the following modifications: - the estimation of the mass transfer characteristics; - SO2 diffusion and solubility properties in the sulphuric acid solutions, both necessary for the precise estimation of the Hatta numbers. For this last point, the diffusion coefficient of SO2 in water at 20 ◦ C (DSO2 /water = 1.51 × 10−9 m2 /s [10]) was corrected to take the presence of acid and the temperature variation into account, thanks to the classical Stokes–Einstein relation D μ/T = constant. The variation of this coefficient with the H2 SO4 molarity is illustrated in Fig. 1a and b for the temperatures of 35 and 50 ◦ C, respectively. Physicochemical properties, namely viscosities and densities, of the H2 SO4 solutions [10,11] are also presented in the same figures. Lastly, with regard to the Henry’s law constant of SO2 in the sulphuric acid solutions, we achieved a comprehensive thermodynamical study of the system SO2 /H2 O/H2 SO4 including the experimental determination of the solubilities of SO2 in the
Fig. 1. Physicochemical properties (density and viscosity) and SO2 diffusivity as functions of the sulphuric acid concentration of the scrubbing solution at 35 (a) and 50 ◦ C (b).
acidity range 2–7 M (range for which one can reasonably assume that there is no dissociation of the tetravalent sulphur species). This provided us the variation of the Henry’s law constant as illustrated in Fig. 2 for the temperatures of 20, 35 and 50 ◦ C and in the whole acidity range. At a given temperature, the Henry’s law constant increases sharply with the concentration of sulphuric acid, the solubility of the SO2 species being reduced. At a given acidity, the ratio H/H◦ (equal to the activity coefficient of dissolved SO2 ) increases with temperature.
Fig. 2. Increase in the Henry’s law constant of SO2 with the acidity at 20, 35 and 50 ◦ C.
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The Henry’s law constant H◦ (atm m3 /kmol) for the physical dissolution of SO2 into water was found in literature [12]: 30861.36 −0.32612T +209.74 lnT H 0 =10 exp −1203.75+ T (4) 4. Results discussion 4.1. Mass transfer characteristics of the column The following correlation was found for the estimation of the specific interfacial area: 0.46 a ∝ u0.17 L ν
(5)
where uL is the superficial velocity of liquid and ν the kinematic viscosity of the solution (ν = μ/(1000d) given as a function of temperature and acidity, in Fig. 1a and b, for 35 and 50 ◦ C, respectively). Fig. 3 illustrates the effect of uL and ν on the specific interfacial area values. For the gas-phase mass transfer coefficients kG a assessment, the following relation was considered for various gas and liquid superficial velocities, whose importance was estimated by a multi-parametric regression on results obtained at 35 and 50 ◦ C: 0.47 kG a ∝ u0.26 L uG
(6)
where uG represents the gas superficial velocity. Lastly, the estimation of the liquid-phase mass transfer coefficients kL a was based on the values measured by absorbing CO2 into water at 20 ◦ C and leading to: kL a ∝ u0.73 L
(7)
The results were corrected for SO2 and extrapolated with the sulphuric acid solutions at the various temperatures as follows: 0.28 ◦ kL a(t) ∝ u0.73 L (νH2 SO4 (t)/νwater (20 C))
× (DSO2 /H2 SO4 (t)/DSO2 /water (20 ◦ C))0.5
(8)
Fig. 3. Effect of superficial liquid velocities and kinematic viscosities on the interfacial area. Comparison of experimental values and values predicted by correlation (Eq. (5)).
relation based simultaneously on Eq. (5), on experimental results −0.18 D0.5 ) for kL a(kL a ∝ u0.73 L ) and theoretical results (kL ∝ ν L obtained from the application of the penetration theory and the Navier–Stokes equations’ integration, expressed in cylindrical coordinates. 4.2. Kinetic parameters for SO2 absorption into H2 SO4 –H2 O2 solutions In the tests achieved with the scrubbing solutions containing H2 SO4 and H2 O2 , we observed absorption efficiencies always lower than those measured with the NaOH solution (for the determination of the kG a), a mass transfer resistance remaining in the liquid phase. However, it was confirmed in our previous studies that this resistance, for a fixed content of H2 O2 , increases with the acidity of the solution but decreases with an increase in the content of peroxide, for a given concentration of H2 SO4 . The results of our absorption test runs were interpreted; thanks to a simulation model of the column including the expression of the absorption flux presented here above, and steady-state mass balances written in small height incremental volumes. The calculation of OKP is carried out by minimising the difference between the measured and calculated values of SO2 outlet partial pressures. Details are published in ref. [7]. √ Values of OKP (actually the inverse parameter H/ k D √ which represents, together with cH2 O2 , a main component of the liquid resistance) that we determined are presented in Fig. 4 for three temperatures: 20 ◦ C (previous works), then 35 and 50 ◦ C. As previously observed at 20 ◦ C, we can note, at 35 and 50 ◦ C, a regular and steep increase of the value of OKP−1 with the sulphuric acid concentration. The increase of the Henry’s law constant, together with the fall of the diffusivity and of the kinetic constant were found to lead to this increase. For each absorption test, we examined a posteriori that the Hatta number, deduced from the prediction of kL and of H, is higher than 3 and lower than Ei /2, and that the H2 O2 concentration is largely higher than the interfacial SO2 concentration, in order to check the validity of the assumptions made in the model, namely a fast pseudo-first-order but non-instantaneous reaction.
Fig. 4. Overall kinetic parameters versus H2 SO4 concentrations at three temperatures (20, 35 and 50 ◦ C): experimental values (points) and predicted values (continuous lines) by Eq. (7).
S. Colle et al. / Chemical Engineering and Processing 47 (2008) 1603–1608
Fig. 5. Influence of temperature on the OKP values at various acidities.
Though a broad operation range of hydrodynamic conditions (liquid and gas flow rates) and kinetic conditions (gas and liquidphase concentrations) was investigated, only a slight dispersion of the experimental OKP values round the average lines (regression results at the different temperatures) can be observed. This is a confirmation that these parameters are theoretically independent of the conditions prevailing in the contactor. At any acidity, the comparison makes it possible to show that the kinetics of SO2 absorption–oxidation rises (decrease of OKP−1 ) very strongly as the temperature increases. It was thought to be desirable to develop Arrhenius-type correlation for OKP in the range covered in our work. The following relationship plotted in Fig. 5 was found to hold, illustrated for three different acidities: The following relation for OKP was obtained by regression: 1 ln OKP = −(562.77cH2 SO4 + 475.38) +(1.39cH2 SO4 −3.03) T (9) It can be used only in the range of the operating conditions investigated in our experimental tests, that is, to say temperature: (20–50 ◦ C) and H2 SO4 concentration: (1–6.4 M). An “energy of activation” of the overall process (solubility + diffusion + reaction) was obtained, included in the factor between brackets multiplying 1/T, which grows linearly with the acidity of the solution. By testing the predictions of the effect of temperature on the absorption rates, for packed towers whose the effective interfacial area is a function of the viscosity of the scrubbing solution, it is clear that with strong acidities, an increase in temperature is completely favourable to the efficiency of the SO2 abatement, involving the addition of H2 O2 . 5. Conclusions The SO2 absorption into fairly and strongly concentrated sulphuric acid solutions, with addition of hydrogen peroxide, was studied in a cables-bundle lab-scale scrubber at 35 and 50 ◦ C. The mathematical model previously developed, based on the theory of absorption accompanied by a chemical reaction in the fast kinetic regime, was adapted at the two temperatures newly investigated in this work and led to the determination
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of a global kinetic parameter, which strongly decreases with an increase of the sulphuric acid concentration. This result confirms the conclusion already drawn at the temperature of 20 ◦ C. The comparison of the OKP values obtained at 20, 35 and 50 ◦ C allows to conclude to an acceleration of the global process of absorption–oxidation with an increasing temperature. The global kinetic parameter is a fundamental parameter, independent of the hydrodynamic conditions, and combined with the estimations of hydrodynamic and mass transfer characteristics of the used column, is essential for the simulation or design of industrial scrubbers for SO2 abatement with acid solutions containing H2 O2 . With the developed rate-based steady-state model including our original values of the kinetic parameters at three different temperatures, we are now able to simulate the gas treatment process in a more wide range of practical industrial operating conditions. Acknowledgements The FNRS (National Fund for Scientific Research) of Belgium is acknowledged for the grant of “Aspirant” (S. Colle), and for the financial support (D. Thomas) for the purchase of the SO2 Ultra-Violet analyser. Appendix A. Nomenclature
a A cA d D E G H Ha H0 kG kL k2 L pSO2 RA T uG uL
specific interfacial area (m−1 ) fractional absorption = (pSO2 in − pSO2 out )/pSO2 in concentration of component A (kmol/m3 = mol/l) density diffusion coefficient (m2 /s) enhancement factor volumetric gas flow rate (m3 /s) Henry’s coefficient of SO2 for sulphuric acid solutions (atm m3 /kmol) Hatta number Henry’s coefficient of SO2 for water (atm m3 /kmol) gas-phase mass transfer coefficient (kmol/s m2 atm) liquid-phase mass transfer coefficient (m/s) second-order kinetic constant (m3 /s kmol) liquid flow rate (m3 /s) partial pressure of sulphur dioxide (Pa) absorption flux (kmol/s m2 ) temperature (K) superficial velocity of gas (m/s) superficial velocity of liquid (m/s)
Greek letters μ dynamic viscosity of the liquid (Pa s) ν kinematic viscosity (m2 /s) Subscripts i interface in inlet of the column out outlet of the column
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References [1] P.V. Deo, The use of hydrogen peroxide for the control of air pollution, Chem. Prot. Environ., Stud. Environ. Sci. 34 (1988) 275–292. [2] F. Gohara Wadie, W. Johnson Dennis, Hydrogen peroxide for flue gas desulfurization, US Patent 5,674,459 (1997). [3] M. Hammond, High-strength acid containing H2 O2 to scrub SO2 , US Patent 3,760,061 (1973). [4] H. Soldavini, W. Von Wedel, Epuration des gaz de fum´ees par le peroxyde d’hydrog`ene, Info Chimie 334 (1991) 181–183. [5] D. Thomas, S. Colle, J. Vanderschuren, Kinetics of SO2 absorption into fairly concentrated sulphuric acid solutions containing hydrogen peroxide, Chem. Eng. Process. 42 (6) (2003) 487–494. [6] S. Colle, J. Vanderschuren, D. Thomas, Pilot-scale validation of the kinetics of SO2 absorption into sulphuric acid solutions containing hydrogen peroxide, Chem. Eng. Process. 43 (2004) 1397–1402.
[7] S. Colle, D. Thomas, J. Vanderschuren, Process simulation of sulphur dioxide abatement with hydrogen peroxide solutions in a packed column, Trans. IChemE (Part A), Chem. Eng. Res. Des. 83 (A1) (2004) 1–7. [8] S. Colle, J. Vanderschuren, D. Thomas, Simulation of SO2 absorption into sulfuric acid solutions containing hydrogen peroxide in the fast and moderately fast kinetic regimes, Chem. Eng. Sci. 60 (2005) 6472– 6479. [9] C. Roizard, G. Wild, J.C. Charpentier, Absorption avec r´eaction chimique, Techniques de l’Ing´enieur J-1079 (1996). [10] R.H. Perry, D.W. Green, Perry’s Chemical Engineer’s Handbook, seventh ed., Mc Graw Hill, 1997. [11] P. Pascal, Nouveau Trait´e de chimie min´erale, Masson et Cie Editeurs, Paris, 1956. [12] B. Rumpf, G. Maurer, Solubilities of hydrogen cyanide and sulfur dioxide in water at temperatures from 293 to 413 K and pressures up to 2.5 MPa, Fluid Phase Equilibr. 81 (1992) 241–260.
Chemical Engineering and Processing 47 (2008) 1603–1608
Effect of temperature on SO2 absorption into sulphuric acid solutions containing hydrogen peroxide Sandrine Colle, Jacques Vanderschuren, Diane Thomas ∗ Chemical Engineering Department, Facult´e Polytechnique de Mons, Rue de l’Epargne, 56, B-7000 Mons, Belgium Received 19 October 2006; received in revised form 30 August 2007; accepted 31 August 2007 Available online 12 September 2007
Abstract A mathematical model previously developed for the absorption at 20 ◦ C of sulphur dioxide into sulphuric acid solutions containing hydrogen peroxide was adapted to take the effect of the temperature into account. This model was exploited to determine at 35 and 50 ◦ C the overall kinetic parameter relative to the absorption–oxidation of SO2 , for increasing H2 SO4 molarities (from 1 to 6.4 M) and in the presence of a variable concentration of H2 O2 (1.2–4.4 M) from absorption test runs performed in a cables-bundle laboratory scrubber. It was clearly observed that an increase in temperature enhances the global process of SO2 absorption (solubility + diffusion + reaction), whereas at a given temperature, an increase in the sulphuric acid content slows down the process. © 2007 Elsevier B.V. All rights reserved. Keywords: Sulphur dioxide absorption; Sulphuric acid; Hydrogen peroxide; Liquid-phase reaction kinetics
1. Introduction The framework of this work is the development of a reduction process of sulphur dioxide present in industrial flue gases. Our oxidising process is a wet one achieving the abatement of SO2 using aqueous hydrogen peroxide solutions. This treatment can be interesting if it leads to valuable or reusable sufficiently concentrated sulphuric acid solutions, while limiting the liquid wastes. The fundamental scientific data (thermodynamic and kinetic) relative to this process, however, already patented and applied, in particular in large-scale metallurgical units coupled with the production of sulphuric acid, are very scarce in the literature. Some very general information can only be found [1–4]. The kinetics of SO2 absorption into H2 SO4 solutions (from fairly to strongly concentrated solutions) containing H2 O2 , has been the object of our research for a few years [5–8]: specific absorption tests have been carried out at laboratory and pilot scales in order to determine, by means of the traditional techniques of chemical engineering, the essential data for the design of industrial contactors, valid in a wide range of operating conditions.
∗
Corresponding author. E-mail address: [email protected] (D. Thomas).
0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.08.014
Our laboratory investigation at 20 ◦ C was presented in author’s previous papers, discussing largely the influences of H2 O2 and H2 SO4 concentrations on the absorption rate of SO2 [5]. An overall kinetic parameter (OKP), function of cH2 SO4 , was deduced from numerous experiments conducted in a small cable scrubber at 20 ◦ C and validated satisfactorily in a pilot-scale packed column [6]. The aim of this study was to obtain at higher temperatures, namely 35 and 50 ◦ C, a correlation (Arrhenius-type correlation) for this overall kinetic parameter versus the H2 SO4 concentration of the scrubbing solution, by achieving absorption experiments in the laboratory scrubber. 2. Experimental set-up and procedure 2.1. Semi-continuous absorption tests: SO2 absorbed into sulphuric acid solutions containing hydrogen peroxide The experimental equipment, included essentially an absorption reactor, namely a cables-bundle laboratory scrubber with an inside diameter of 0.045 m and an effective height of 0.54 m. This contactor is well suited to kinetic studies as its specific surface is quite insensitive to the kinetic flow rate and the viscosity. However, a modification of the device was operated while dividing by two the number of twisted polypropylene yarns (from 12 to 6),
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stretched vertically, in parallel and constituting the packing of the gas–liquid microreactor. This has the advantage of preventing the formation of liquid films hung between two cables, the liquid flow observed remaining more stable. Nevertheless, this has also the drawback of a gas–liquid interfacial area reduced by half, involving a similar decrease of the quantity of SO2 absorbed. The gas phase, made of nitrogen humidified in a saturator, and in which SO2 is added to obtain the desired concentration, enters and flows out the column axially, through perforations milled in the bottom and top devices. Absorption performances of the sulphur dioxide were observed at 35 and 50 ◦ C and at atmospheric pressure, deduced from the SO2 gas contents measured simultaneously at both ends of the column by a two-channel UV analyser, allowing to calculate the SO2 fractional absorption A: A=
pSO2 in − pSO2 out pSO2 in
(1)
The scrubbing liquid, containing given sulphuric acid and hydrogen peroxide concentrations, is fed to the top distributing chamber of the contactor; thanks to a gear pump. It is distributed around each cable through individual holes. As the flow becomes steady, the liquid film around the yarn has a cylindrical shape. The liquid feed temperature was controlled by means of a heat exchanger operating counter-currently with water supplied from a thermostatic bath. The absorption column was furthermore insulated with a 10-mm thick sheet of elastomer. No appreciable temperature rise was noted during absorption. The liquid and gas temperatures at both ends of the column were checked by thermometers. As far as the experimental procedure followed in our absorption test runs is concerned, the absorber was operated counter-currently and batchwise with respect to the scrubbing liquid, which was recycled from a tank and brought in contact with the gas loaded with SO2 . Due to a short duration of the test (only a few minutes), quasi-steady-state measurements were made, indicated by no significant changes of column temperature and input and output concentrations. At the end of the test, a liquid sample was withdrawn at the same times than SO2 gas measurements at the inlet and the outlet of the column. For each liquid sample, the sulphuric acid and hydrogen peroxide concentrations were measured, respectively, by an acid–base titration and an iodometric method. A wide range of operating conditions, pertinent to flue gas purification processes, was investigated: liquid flow rate (uL = 0.2–0.33 cm/s) and gas flow rate (uG = 0.20–0.41 m/s), gas-phase composition (ySO2 = 200–5500 ppm), acidity of the solution (H2 SO4 = 1–6.4 M) and H2 O2 concentration (H2 O2 = 1.2–4.4 M). Some hundred tests were carried out at 35 and 50 ± 0.3 ◦ C. Finally, each kinetic test is characterised by a set of seven data (L, G, pSO2 in , pSO2 out , cH2 SO4 , cH2 O2 and temperature) required for the subsequent determination of the overall kinetic parameter.
2.2. Tests for the estimation of the mass transfer characteristics of the packing For the determination at 35 and 50 ◦ C of the volumetric gasphase mass transfer coefficient kG a, as a function of the operating conditions of gas and liquid flow rates, SO2 absorption efficiencies were measured with sodium hydroxide solutions (molarity 1 M), allowing to assume the total cancellation of the liquidphase resistance due to an instantaneous reaction between SO2 and OH− . The results of such absorption experiments were also used for comparison with the SO2 absorption efficiencies measured in the sulphuric acid solutions containing hydrogen peroxide. As far as the volumetric liquid-phase mass transfer coefficients kL a are concerned, they were estimated by extrapolation, at 35 and 50 ◦ C, of the results obtained at 20 ◦ C by absorbing pure carbon dioxide into deionised water, for various liquid flow rates. Lastly, the estimation of the specific interfacial area a, varying with the liquid flow rate and the cinematic viscosity of the solution, was performed at the temperature of 20 ◦ C by absorbing CO2 diluted in air into sodium hydroxide solutions of increasing concentrations (0.5, 2.5, 3.9 and 5.1 M) in the conditions of a fast chemical reaction regime. The reader will find details on the application of this technique and the choice of the kinetic constant for the reaction between CO2 and hydroxide ions OH− in Colle et al. [6]. An extrapolation of these results, using the values of dynamic viscosities of the solutions of H2 SO4 , was carried out to estimate the values of a at 35 and 50 ◦ C. 3. Modelling of SO2 absorption and effect of temperature The mathematical model previously developed by Colle et al. [7] is based on the two-film theory for SO2 absorption accompanied by an irreversible chemical reaction between the solute and the liquid reactant H2 O2 , namely the oxidation reaction SO2 + H2 O2 → H+ + HSO4 − which is first-order with respect to both SO2 and H2 O2 . Depending on the respective concentrations of H2 SO4 and H2 O2 in the scrubbing solution, the chemical reaction regime can be slow, moderately fast or fast, based on a Hatta number criteria. In our case, due to the kinetic characteristics of the reaction, this Hatta number Ha is defined as: k2 DSO2 /H2 SO4 cH2 O2 Ha = (2) kL where DSO2 /H2 SO4 is the diffusion coefficient for SO2 in acid solutions and k2 is the kinetic constant of the SO2 oxidation reaction, depending strongly on the H2 SO4 concentration. Our study, rather general, is related to the moderately fast and fast regimes, but the work presented here reports exclusively tests carried out in the fast regime. The moderately fast chemical reaction regime (0.3 < Ha < 3) was observed for lower H2 O2 or higher H2 SO4 concentrations, studied in a pilot column and specifically reported elsewhere [8].
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For sufficiently high H2 O2 contents, it was previously shown that the fractional absorption is independent on the partial pressure of SO2 in the inlet gas, actually in the presence of an excess of oxidising agent, which confirms the first-order with respect to reactant SO2 . With the conditions of a liquid bulk H2 O2 , concentration higher than the interfacial concentration of SO2 and large Hatta numbers (3 < Ha < Ei /2 with Ei being the value of E for the instantaneous reaction) the reaction becomes pseudo-first-order and fast, and the enhancement factor E is almost equal to Ha. Eq. (3) was then used to compute the liquid side absorption flux RSO2 [9]: k2 DSO2 /H2 SO4 √ cH2 O2 pSO2i RSO2 = EkL cSO2i = H √ = OKP cH2 O2 pSO2i (3) In this relation, pSO2i and cSO2i are the SO2 partial pressure and the interfacial concentration, respectively (assuming a Henry’s equilibrium law at the interface). The SO2 concentration in the liquid film is virtually null, due to the reaction proceeding. In this case of a fast pseudo-first-order reaction, the absorption flux of SO2 is independent of the liquid-phase mass transfer coefficient, but takes into account an overall kinetic parameter (OKP) lumping the kinetics of oxidation reaction, the diffusion as well as the solubility of SO2 in the liquid phase. As already mentioned, at the temperature of 20 ◦ C,√the variation of OKP (which we will note thereafter simply k D/H) as a function of the sulphuric acid content of the solution was determined from test results carried out in a laboratory scrubber and validated with data obtained in a pilot installation, and was the subject of previous publications [5,6]. The determination of the variation of the OKP value at 35 and 50 ◦ C was the aim of this work and required, for the interpretation of the experimental tests newly carried out, the introduction, in the simulation program, of the following modifications: - the estimation of the mass transfer characteristics; - SO2 diffusion and solubility properties in the sulphuric acid solutions, both necessary for the precise estimation of the Hatta numbers. For this last point, the diffusion coefficient of SO2 in water at 20 ◦ C (DSO2 /water = 1.51 × 10−9 m2 /s [10]) was corrected to take the presence of acid and the temperature variation into account, thanks to the classical Stokes–Einstein relation D μ/T = constant. The variation of this coefficient with the H2 SO4 molarity is illustrated in Fig. 1a and b for the temperatures of 35 and 50 ◦ C, respectively. Physicochemical properties, namely viscosities and densities, of the H2 SO4 solutions [10,11] are also presented in the same figures. Lastly, with regard to the Henry’s law constant of SO2 in the sulphuric acid solutions, we achieved a comprehensive thermodynamical study of the system SO2 /H2 O/H2 SO4 including the experimental determination of the solubilities of SO2 in the
Fig. 1. Physicochemical properties (density and viscosity) and SO2 diffusivity as functions of the sulphuric acid concentration of the scrubbing solution at 35 (a) and 50 ◦ C (b).
acidity range 2–7 M (range for which one can reasonably assume that there is no dissociation of the tetravalent sulphur species). This provided us the variation of the Henry’s law constant as illustrated in Fig. 2 for the temperatures of 20, 35 and 50 ◦ C and in the whole acidity range. At a given temperature, the Henry’s law constant increases sharply with the concentration of sulphuric acid, the solubility of the SO2 species being reduced. At a given acidity, the ratio H/H◦ (equal to the activity coefficient of dissolved SO2 ) increases with temperature.
Fig. 2. Increase in the Henry’s law constant of SO2 with the acidity at 20, 35 and 50 ◦ C.
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The Henry’s law constant H◦ (atm m3 /kmol) for the physical dissolution of SO2 into water was found in literature [12]: 30861.36 −0.32612T +209.74 lnT H 0 =10 exp −1203.75+ T (4) 4. Results discussion 4.1. Mass transfer characteristics of the column The following correlation was found for the estimation of the specific interfacial area: 0.46 a ∝ u0.17 L ν
(5)
where uL is the superficial velocity of liquid and ν the kinematic viscosity of the solution (ν = μ/(1000d) given as a function of temperature and acidity, in Fig. 1a and b, for 35 and 50 ◦ C, respectively). Fig. 3 illustrates the effect of uL and ν on the specific interfacial area values. For the gas-phase mass transfer coefficients kG a assessment, the following relation was considered for various gas and liquid superficial velocities, whose importance was estimated by a multi-parametric regression on results obtained at 35 and 50 ◦ C: 0.47 kG a ∝ u0.26 L uG
(6)
where uG represents the gas superficial velocity. Lastly, the estimation of the liquid-phase mass transfer coefficients kL a was based on the values measured by absorbing CO2 into water at 20 ◦ C and leading to: kL a ∝ u0.73 L
(7)
The results were corrected for SO2 and extrapolated with the sulphuric acid solutions at the various temperatures as follows: 0.28 ◦ kL a(t) ∝ u0.73 L (νH2 SO4 (t)/νwater (20 C))
× (DSO2 /H2 SO4 (t)/DSO2 /water (20 ◦ C))0.5
(8)
Fig. 3. Effect of superficial liquid velocities and kinematic viscosities on the interfacial area. Comparison of experimental values and values predicted by correlation (Eq. (5)).
relation based simultaneously on Eq. (5), on experimental results −0.18 D0.5 ) for kL a(kL a ∝ u0.73 L ) and theoretical results (kL ∝ ν L obtained from the application of the penetration theory and the Navier–Stokes equations’ integration, expressed in cylindrical coordinates. 4.2. Kinetic parameters for SO2 absorption into H2 SO4 –H2 O2 solutions In the tests achieved with the scrubbing solutions containing H2 SO4 and H2 O2 , we observed absorption efficiencies always lower than those measured with the NaOH solution (for the determination of the kG a), a mass transfer resistance remaining in the liquid phase. However, it was confirmed in our previous studies that this resistance, for a fixed content of H2 O2 , increases with the acidity of the solution but decreases with an increase in the content of peroxide, for a given concentration of H2 SO4 . The results of our absorption test runs were interpreted; thanks to a simulation model of the column including the expression of the absorption flux presented here above, and steady-state mass balances written in small height incremental volumes. The calculation of OKP is carried out by minimising the difference between the measured and calculated values of SO2 outlet partial pressures. Details are published in ref. [7]. √ Values of OKP (actually the inverse parameter H/ k D √ which represents, together with cH2 O2 , a main component of the liquid resistance) that we determined are presented in Fig. 4 for three temperatures: 20 ◦ C (previous works), then 35 and 50 ◦ C. As previously observed at 20 ◦ C, we can note, at 35 and 50 ◦ C, a regular and steep increase of the value of OKP−1 with the sulphuric acid concentration. The increase of the Henry’s law constant, together with the fall of the diffusivity and of the kinetic constant were found to lead to this increase. For each absorption test, we examined a posteriori that the Hatta number, deduced from the prediction of kL and of H, is higher than 3 and lower than Ei /2, and that the H2 O2 concentration is largely higher than the interfacial SO2 concentration, in order to check the validity of the assumptions made in the model, namely a fast pseudo-first-order but non-instantaneous reaction.
Fig. 4. Overall kinetic parameters versus H2 SO4 concentrations at three temperatures (20, 35 and 50 ◦ C): experimental values (points) and predicted values (continuous lines) by Eq. (7).
S. Colle et al. / Chemical Engineering and Processing 47 (2008) 1603–1608
Fig. 5. Influence of temperature on the OKP values at various acidities.
Though a broad operation range of hydrodynamic conditions (liquid and gas flow rates) and kinetic conditions (gas and liquidphase concentrations) was investigated, only a slight dispersion of the experimental OKP values round the average lines (regression results at the different temperatures) can be observed. This is a confirmation that these parameters are theoretically independent of the conditions prevailing in the contactor. At any acidity, the comparison makes it possible to show that the kinetics of SO2 absorption–oxidation rises (decrease of OKP−1 ) very strongly as the temperature increases. It was thought to be desirable to develop Arrhenius-type correlation for OKP in the range covered in our work. The following relationship plotted in Fig. 5 was found to hold, illustrated for three different acidities: The following relation for OKP was obtained by regression: 1 ln OKP = −(562.77cH2 SO4 + 475.38) +(1.39cH2 SO4 −3.03) T (9) It can be used only in the range of the operating conditions investigated in our experimental tests, that is, to say temperature: (20–50 ◦ C) and H2 SO4 concentration: (1–6.4 M). An “energy of activation” of the overall process (solubility + diffusion + reaction) was obtained, included in the factor between brackets multiplying 1/T, which grows linearly with the acidity of the solution. By testing the predictions of the effect of temperature on the absorption rates, for packed towers whose the effective interfacial area is a function of the viscosity of the scrubbing solution, it is clear that with strong acidities, an increase in temperature is completely favourable to the efficiency of the SO2 abatement, involving the addition of H2 O2 . 5. Conclusions The SO2 absorption into fairly and strongly concentrated sulphuric acid solutions, with addition of hydrogen peroxide, was studied in a cables-bundle lab-scale scrubber at 35 and 50 ◦ C. The mathematical model previously developed, based on the theory of absorption accompanied by a chemical reaction in the fast kinetic regime, was adapted at the two temperatures newly investigated in this work and led to the determination
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of a global kinetic parameter, which strongly decreases with an increase of the sulphuric acid concentration. This result confirms the conclusion already drawn at the temperature of 20 ◦ C. The comparison of the OKP values obtained at 20, 35 and 50 ◦ C allows to conclude to an acceleration of the global process of absorption–oxidation with an increasing temperature. The global kinetic parameter is a fundamental parameter, independent of the hydrodynamic conditions, and combined with the estimations of hydrodynamic and mass transfer characteristics of the used column, is essential for the simulation or design of industrial scrubbers for SO2 abatement with acid solutions containing H2 O2 . With the developed rate-based steady-state model including our original values of the kinetic parameters at three different temperatures, we are now able to simulate the gas treatment process in a more wide range of practical industrial operating conditions. Acknowledgements The FNRS (National Fund for Scientific Research) of Belgium is acknowledged for the grant of “Aspirant” (S. Colle), and for the financial support (D. Thomas) for the purchase of the SO2 Ultra-Violet analyser. Appendix A. Nomenclature
a A cA d D E G H Ha H0 kG kL k2 L pSO2 RA T uG uL
specific interfacial area (m−1 ) fractional absorption = (pSO2 in − pSO2 out )/pSO2 in concentration of component A (kmol/m3 = mol/l) density diffusion coefficient (m2 /s) enhancement factor volumetric gas flow rate (m3 /s) Henry’s coefficient of SO2 for sulphuric acid solutions (atm m3 /kmol) Hatta number Henry’s coefficient of SO2 for water (atm m3 /kmol) gas-phase mass transfer coefficient (kmol/s m2 atm) liquid-phase mass transfer coefficient (m/s) second-order kinetic constant (m3 /s kmol) liquid flow rate (m3 /s) partial pressure of sulphur dioxide (Pa) absorption flux (kmol/s m2 ) temperature (K) superficial velocity of gas (m/s) superficial velocity of liquid (m/s)
Greek letters μ dynamic viscosity of the liquid (Pa s) ν kinematic viscosity (m2 /s) Subscripts i interface in inlet of the column out outlet of the column
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References [1] P.V. Deo, The use of hydrogen peroxide for the control of air pollution, Chem. Prot. Environ., Stud. Environ. Sci. 34 (1988) 275–292. [2] F. Gohara Wadie, W. Johnson Dennis, Hydrogen peroxide for flue gas desulfurization, US Patent 5,674,459 (1997). [3] M. Hammond, High-strength acid containing H2 O2 to scrub SO2 , US Patent 3,760,061 (1973). [4] H. Soldavini, W. Von Wedel, Epuration des gaz de fum´ees par le peroxyde d’hydrog`ene, Info Chimie 334 (1991) 181–183. [5] D. Thomas, S. Colle, J. Vanderschuren, Kinetics of SO2 absorption into fairly concentrated sulphuric acid solutions containing hydrogen peroxide, Chem. Eng. Process. 42 (6) (2003) 487–494. [6] S. Colle, J. Vanderschuren, D. Thomas, Pilot-scale validation of the kinetics of SO2 absorption into sulphuric acid solutions containing hydrogen peroxide, Chem. Eng. Process. 43 (2004) 1397–1402.
[7] S. Colle, D. Thomas, J. Vanderschuren, Process simulation of sulphur dioxide abatement with hydrogen peroxide solutions in a packed column, Trans. IChemE (Part A), Chem. Eng. Res. Des. 83 (A1) (2004) 1–7. [8] S. Colle, J. Vanderschuren, D. Thomas, Simulation of SO2 absorption into sulfuric acid solutions containing hydrogen peroxide in the fast and moderately fast kinetic regimes, Chem. Eng. Sci. 60 (2005) 6472– 6479. [9] C. Roizard, G. Wild, J.C. Charpentier, Absorption avec r´eaction chimique, Techniques de l’Ing´enieur J-1079 (1996). [10] R.H. Perry, D.W. Green, Perry’s Chemical Engineer’s Handbook, seventh ed., Mc Graw Hill, 1997. [11] P. Pascal, Nouveau Trait´e de chimie min´erale, Masson et Cie Editeurs, Paris, 1956. [12] B. Rumpf, G. Maurer, Solubilities of hydrogen cyanide and sulfur dioxide in water at temperatures from 293 to 413 K and pressures up to 2.5 MPa, Fluid Phase Equilibr. 81 (1992) 241–260.