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The iodide iodate reaction method: The choice of the acid
ARTICLE IN PRESS Chemical Engineering Science 65 (2010) 1897–1901
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Shorter communication
The iodide iodate reaction method: The choice of the acid a, ¨ A. Kolbl , S. Schmidt-Lehr b,1 a b
Institute for Micro Process Engineering, Karlsruhe Institute of Technology, P.O. Box 3640, D-76021 Karlsruhe, Germany Institute of Technical and Macromolecular Chemistry, Universit¨ at Hamburg, Bundesstr. 45, D-20146 Hamburg, Germany
a r t i c l e in fo
abstract
Article history: Received 26 September 2009 Received in revised form 14 November 2009 Accepted 19 November 2009 Available online 2 December 2009
Mixing processes are frequently characterized with competing chemical reaction schemes. The most popular reaction scheme for continuous flow mixers is the iodide iodate reaction method, which traditionally uses sulfuric acid as proton source for both reactions involved in the scheme. Experimental evidence is provided which indicates that for a quantitative treatment of the experimental data either the experiments need to be carried out with a strong acid such as perchloric acid or the dissociation constants of sulfuric acid need to be included in the models linking the primary experimental results with quantitative measures such as mixing times. Another chemical test reaction system, the acetal cleavage method, traditionally carried out with hydrochloric acid was applied for comparison. In that case, the use of perchloric acid showed no significant impact on the experimental results. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Hydrodynamics Mass transfer Mixing Kinetics Mixing times Iodide iodate reaction
1. Introduction 1.1. Principles of the test reactions Competing chemical reaction systems are popular means for chemical engineers to characterise mixing processes either in stirred tanks or in continuous flow mixers (Ba"dyga and Bourne, 1999). The most popular reaction scheme for continuous flow mixers is the iodide iodate reaction method (Falk and Commenge, 2009). Two reactions are competing for H + in this scheme: (1) The neutralisation of dihydroboric acid ions to form hydroboric acid and (2) the comproportionation of iodide and iodate ions to form iodine (also referred to as the ‘Dushman reaction’ Dushman, 1904). Iodine formed in the course of the mixing process further reacts with excess iodide to form triiodide according to equilibrium (3). Triiodide absorbs strongly at a wavelength of 353 nm. Using the UV spectra of the resulting mixtures, mixing processes and devices can be characterised (Villermaux et al., 1991). H2BO3 + H + -H3BO3
(1)
5I + IO3 + 6H + -3I2 + 3 H2O
(2)
I2 +I =I3
(3)
The reactions involved in the iodide iodate reaction method differ in their reaction rate. Reactions (1) and (3) are considered to be infinitely fast compared to the physical mixing process, while the rate of the Dushman reaction is kept in the range of the rate of the mixing process in order to yield mixing sensitive results. The rate of the Dushman reaction can be adjusted by choosing appropriate reactant concentrations in order to yield maximum ¨ sensitivity as recently demonstrated experimentally (Kolbl et al., 2008a, b). In case of perfect mixing, the product distribution is solely governed by the kinetics of the reactions involved, and thus, no iodine is found in the resulting mixtures. Any formation of iodine is attributed to imperfect mixing. Hence, the method can be used for parametric studies. The use of the iodide iodate reaction method as a quantitative method is under debate, mainly since the kinetic examinations on the Dushman reaction were undertaken at reactant conditions far from those relevant to mixing studies (Guichardon et al., 2000; Bourne, 2008). Another parallel-competitive reaction scheme is the acetal cleavage method, the competition of the acid-catalysed cleavage of 2,2-dimethoxpropane (DMP) (4) with the neutralisation of HCl with NaOH (5).
Corresponding author. Tel.:+ 49 7247 82 6653; fax: + 49 7247 82 3186.
¨ E-mail address: [email protected] (A. Kolbl). Present address: Tesa SE, Quickbornstraße 24, BF 664, 20253 Hamburg, Germany.
ð4Þ
1
0009-2509/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.11.032
NaOH+ HCl-NaCl+ H2O
(5)
ARTICLE IN PRESS 1898
A. K¨ olbl, S. Schmidt-Lehr / Chemical Engineering Science 65 (2010) 1897–1901
Faster mixing processes prefer the faster neutralization reaction. With slower mixing processes more of the DMP is converted into acetone and methanol. More details on the reaction method can be found in scientific literature for stirred vessels (Walker, 1996; Ba"dyga et al., 1998) and continuous flow mixers (Johnson and Prud’homme, 2003). H + for both reactions is provided by hydrochloric acid.
Table 1 pKa values of acids relevant to this contribution.
1.2. The iodide iodate reaction method in scientific literature
authors, the IEM parameters deliver ‘‘the order of magnitude of mixing time for practical applications (Falk and Commenge, 2009)’’. According to Falk and Commenge, this model parameter offers a reactant concentrations-free measure of the ‘mixing quality’ (as opposed to the segregation index XS) yet. An experimental verification of this assumption has not been provided in scientific literature, except for the variation of the concentration of H + for stirred laboratory vessels reported by Assirelli et al. (2008), who found that varying the concentration of H + did not affect the segregation indices calculated for similar hydrodynamic conditions. In a recent publication (Baccar et al., 2009) numerical simulations and experiments for a ‘hollow fibre membrane contactor’ were compared by means of the iodide iodate reaction method. Two segregation indices obtained at similar conditions (the same device, the same reactant concentrations, comparable throughput) were compared; one derived from numerical simulations (XS = 1.17 10 3) another derived from experimental investigations (XS = 4.42 10 3). The differences between those two values are in the range of the overall effect achieved with microstructured devices (Men et al., 2007; Kockmann et al., 2004a, b, 2005; Engler et al., 2005). This finding indicates that further work needs to be done to achieve better matches between mixing models and experimental results. This manuscript might contribute to that goal.
The iodide iodate reaction method was developed for stirred laboratory tanks (Fournier et al., 1996a, b; Guichardon et al., 2000; Guichardon and Falk, 2000) where a small volume of concentrated sulphuric acid is mixed into a large volume containing the rest of the necessary chemicals, which are placed in the vessel prior to mixing experiments. In the scientific literature on the experimental characterisation of stirred laboratory vessels, a segregation index is typically derived from the UV spectra of the resulting mixtures (Fournier et al., 1996a; Guichardon et al., 2000; Assirelli et al., 2002, 2005; Fang and Lee, 2000; Nouri et al., 2008). The segregation index derived for mixtures obtained in stirred vessels is defined by Eqs. (6)–(8) (Fournier et al., 1996a). XS ¼
Y YST
Y ¼2
ð6Þ
cI3 þ cI2 cH þ
ð7Þ
6cIO3 ;0 6cIO3 ;0 þ cH2 BO3 ;0
ð8Þ
0
YST ¼
The segregation index attempts to indicate the yield of iodine compared to the maximum yield of iodine, which would be obtained in case of an infinitely slow mixing process. The segregation index should yield values between 0 (in case of perfect mixing) and 1 (worst case scenario) and should thus be comparable to a classically defined selectivity. In some of the literature on stirred laboratory vessels, ‘mixing times’ were derived from a simple mixing model: the incorporation model (Fournier et al., 1996b; Guichardon et al., 2000; Fang and Lee, 2000). With the same mixing model, ‘mixing times’ were derived for mixing processes in Kenics static mixers by Fang and Lee (2001). A low flow of concentrated sulphuric acid was continuously dosed into a large main flow containing the other essential chemicals. A first suitable set of concentrations for microstructured mixing devices was proposed by Panic´ et al. (2004). Since then, the iodide iodate reaction method has gained a large popularity ¨ for the characterization of those devices (Kolbl et al., 2008, 2008b; Schneider et al., 2004; Hecht, 2007; Hecht et al., 2007a, b, 2008; ¨ et al., 2005; Werner et al., 2005; Men et al., 2007; Nagawasa Lob et al., 2005; Kockmann et al., 2004a, b, 2005; Engler et al., 2005). In most cases, the absorbance values of the resulting mixtures were compared at characteristic wavelengths (286 or 353 nm). In some cases (Men et al., 2007; Kockmann et al., 2004a, b, 2005; Engler et al., 2005) a segregation index was derived from the spectra of the resulting mixtures according to a definition similar to the Eqs. (6)–(8), modified for the needs of mixtures obtained in static mixers. Falk and Commenge (2009) have recently reviewed previous work on the characterization of microstructured mixing devices and calculated ‘mixing times, tm’ from experimental data published in the reviewed work. The derived ‘mixing times’ are parameters of the IEM model (interaction by exchange with the mean) and should not be confused with the mixing time, required to achieve a specific degree of homogeneity. According to the
Acid
pKa,1
pKa,2
H2SO4 HCl HClO4
2 7 10
+ 1.92 / /
1.3. The choice of the acid A problem, which is hardly addressed in scientific literature, is the choice of the acid, which provides H + for the competing reactions. pKa values of acids relevant to this contribution are listed in Table 1 (Atkins, 1990). Equilibrium calculations using pKH2 SO4;2 show that HSO4 is not fully dissociated, even for diluted sulphuric acid as used for mixing experiments with static mixers. Engler et al. (2005) pointed out that the amounts of available protons change dynamically over the mixing process, which make theoretical considerations difficult. Later, Bourne (2008) wrote that the dissociation constant pKH2 SO4;2 should be included in an appropriate mixing model for achieving reliable mixing times.
2. Experiments and discussion 2.1. Experimental setup and operating conditions The experimental examination of the continuous flow mixers was done in a setup constructed in stainless steel at the Institute for Micro Process Engineering (Karlsruhe Institute of Technology). For the characterization of continuous flow mixers two flows are combined at a volumetric ratio of 1/1. Mixing was achieved in a V type mixer. Those mixers are obtained by stacking and bonding micromachined foils. The examined mixer consists of 60 micromachined channels (cross section 100 mm 70 mm). The reactant concentrations for the iodide iodate reaction method correspond to twice a set of standard concentration defined in a previous ¨ publication (Kolbl et al., 2008a) (solution 1: diluted acid is mixed
ARTICLE IN PRESS A. K¨ olbl, S. Schmidt-Lehr / Chemical Engineering Science 65 (2010) 1897–1901
with solution 2: cNaH2 BO3 ¼ 227 mol=m3 , cKI =79.8 mol/m3, cKIO3 ¼ 16:0 mol=m3 ). For a reference experiment diluted sulphuric acid of a concentration of 30 mol/m3 (potential proton concentration of cH + =60 mol/m3) was employed. This experiment is compared to experiments using perchloric acid in a concentration of 30 mol/m3 (the same concentration of acid molecules) and 60 mol/m3 (the same concentration of potential H + ). More details ¨ on the setup and the mixer can be found in a publication by Kolbl et al. (2008a). With the same experimental setup, the same mixer was examined with the acetal cleavage method. The experimental conditions, the nature of the solvent and reactant concentrations (solution 1: cHCl =200 mol/m3; solution 2: cNaOH = 210 mol/m3, cDMP =200 mol/m3) are the same as those described in a publication by Johnson and Prud’homme (2003). In order to examine the influence of the choice of the acid in the case of a laboratory stirred tank, mixing experiments in a baffled laboratory tank using a Rushton turbine were carried out at the Institute of Technical and Macromolecular Chemistry of Hamburg University. Sulphuric acid ðcH2 SO4 ¼ 30 mol=m3 Þ was compared to perchloric acid ðcHClO4 ¼ 60 mol=m3 Þ. Both solutions have a potential H + concentration of 60 mol/m3. Both solutions were applied to characterize the baffled tank at three different stirring speeds using a Rushton turbine (80, 160, and 240 rpm). The 3 mL of the respective acid were injected over a needle (1 mm inner diameter) at a rate of 1 mL/min at the stirrer plane half distance between the outer end of the stirrer (stirrer diameter 4 cm) and the wall of the tank (tank diameter 10 cm). Prior to the mixing experiment, the tank was filled with a solution (1L) of KI (c= 17.4 mol/m3), KIO3 (c =2.5 mol/m3), H3BO3 (c =34.8 mol/m3), and NaOH (c= 17.4 mol/m3). 2.2. The iodide iodate reaction method In order to test the possible influence of the dissociation equilibrium on the experimentally achieved results, measurements on a V-type mixer with sulphuric acid have been compared to measurements achieved with perchloric acid. The experimentally obtained product selectivities are given in terms of conversion of the key compound iodate (Eq. (9)). XIO3 ¼
cIO3 ;0 cI3 cI2 3 cIO3 ;0
ð9Þ
Lower XIO3 corresponds to lower absorbance values in the UV spectrum of the resulting mixtures and to improved mixing. The experimentally achieved results are given in Fig. 1. The choice of the acid has a clear impact on the experimental results. For a quantitative modification of the iodide iodate reaction method according to Bourne’s (2008) findings, either
Fig. 1. Experimentally obtained conversion degrees of iodate using sulphuric acid and perchloric acid in a V-type mixer.
1899
the dissociation constant of the hydrogen sulphate ion needs to be included in the model assumptions, or the experiments need to be carried out with a very strong acid such as perchloric acid. As discussed by Engler et al. (2005), the concentration of HSO4 changes dynamically over time (with the consumption of H + by the reactions (1) and (2)), i.e. the protons are liberated at a time, when the degree of segregation (according to the definition of Danckwerts (1958), not to be confused with the segregation index as defined by Eqs. (6)–(9)) in the mixture is already reduced, which leads to lower iodine yields than obtained with a very strong acid. Hence, a faster mixing process is assumed when utilizing sulphuric instead of perchloric acid. Smaller numbers of the mixing time would be calculated from simple mixing models, which do not consider the dissociation equilibrium. It is observed that the ratio of the conversions using sulphuric acid and perchloric acid is constant over the examined mass flow range ðXH2 SO4 =XHClO4 ¼ 2:4 70:1Þ. This finding suggests that the experimental results reflect the intrinsic kinetics of the reactions involved in this examination method, free of mass transport influence. Furthermore, the experimental results indicate a strong influence of the H + concentration on the experimentally determined conversion degrees (Fig. 1). This finding clearly indicates the large impact of the H + concentrations on the kinetics of the Dushman reaction. The reaction order of H + is commonly assumed to be 2 (Fournier et al., 1996a, b). 2.3. Experimental results in a stirred laboratory tank The comparison between the use of sulphuric acid and perchloric acid is depicted in Fig. 2. The experimental results show that the choice of the acid also shows an effect upon the experimental results in a stirred laboratory tank. The achieved effect seems to be smaller. The experimental characterisation of mixing processes in stirred tanks is typically carried out at lower reactant concentrations than the characterisation of continuous flow mixers. This also holds for the present study. Although the same acid concentrations were employed for the V type mixer and the stirred laboratory vessel, the results depicted in Fig. 1 can not be directly compared to the results shown in Fig. 2, since in case of the stirred vessel only a small volume of sulphuric acid is used, which gets quickly diluted during the mixing experiment due to the large amount of liquid present in the vessel. 2.4. The acetal cleavage method As for the iodide iodate reaction method, the acetal cleavage method has been tested for a possible influence of the choice of the acid. Perchloric acid and hydrochloric acid differ significantly in the pKa-value (Table 1). Furthermore, the high ethanol content of the solvent might influence the dissociation equilibrium.
Fig. 2. Experimental comparison for utilising sulphuric acid and perchloric acid for mixing experiments in stirred laboratory vessels.
ARTICLE IN PRESS 1900
A. K¨ olbl, S. Schmidt-Lehr / Chemical Engineering Science 65 (2010) 1897–1901
References
Fig. 3. Experimentally obtained conversion of DMP in a V-type mixer using hydrochloric acid and perchloric acid.
The experimental results are depicted as conversion degrees of the key compound DMP over total mass flow rates in Fig. 3. The resulting mixtures were analysed with UV spectroscopy. The experimental results indicate that the choice of an acid stronger than the frequently employed hydrochloric acid does not impact the experimentally determined reactant concentrations in the resulting mixtures for the present experimental conditions.
3. Conclusions The experimental results presented in this contribution clearly indicate that the choice of the acid, which is the proton source for the competing reactions of the iodide iodate reaction method, has a severe effect on the product selectivities of the resulting mixtures. For a quantitative treatment of the experimental data, the dissociation equilibrium of the hydrogen sulphate ion needs to be included in the respective models or the experiments need to be carried out with a strong acid such as perchloric acid. This experimental finding presented here was recently predicted by Bourne (2008). The application of a ‘segregation index, XS’ as defined in scientific literature for mixing experiments achieved with stirred tanks or continuous flow mixers lacks sense when utilising sulphuric acid. The iodine yield (as part of the segregation index) refers to the amount of H + theoretically available (2 mol H + per mol H2SO4). Since the amount of H + in the reacting mixture dynamically changes over the mixing process, less iodine is produced in the course of the mixing process compared to experiments utilising perchloric acid, where the acid molecules are fully ionised from the beginning of the mixing experiment. It has been shown that the presented effect is also found in stirred laboratory vessels. The ratio of the UV signals obtained utilising perchloric acid and sulfuric acid is independent of the flow throughput. This finding suggests that the experimental results are independent of the hydrodynamics, and the observed differences can be attributed to the influence of the H + concentration upon the kinetics of the competing reactions involved. A second competitive reaction scheme, the acetal cleavage method, does not show sensitivity to the choice of the acid (perchloric acid vs. hydrochloric acid). This indicates that HCl is sufficiently ionized due to its higher acid strength. An impact of the nature of the solvent cannot be shown. However, the use of hydrochloric acid is not appropriate for the iodide iodate reaction method due to the formation of mixed halide species (Fournier et al., 1996a, b).
Assirelli, M., Bujalski, W., Nienow, A.W., Eaglesham, A., 2002. Study of micromixing in a stirred tank using a Rushton turbine: comparison of feed positions and other mixing devices. Transactions of the Institute of Chemical Engineers 80A, 855–863. Assirelli, M., Bujalski, W., Eaglesham, A., Nienow, A.W., 2005. Intensifying micromixing in a semi-batch reactor using a Rushton turbine. Chemical Engineering Science 60, 2333–2339. Assirelli, M., Wynn, E.J.W., Bujalski, W., Eaglesham, A., Nienow, A.W., 2008. An extension to the incorporation model of micromixing and its use in estimating local specific energy dissipation rates. Industrial & Engineering Chemistry Research 47, 3460–3469. Atkins, P.W., 1990. Physical Chemistry. Oxford University Press, Oxford a.o.. Baccar, N., Kieffer, R., Charcosset, C., 2009. Characterization of mixing in a hollow fibre membrane contactor by the iodide–iodate method: numerical simulations and experiments. Chemical Engineering Journal 148, 517–524. Ba"dyga, J., Bourne, J.R., Walker, B., 1998. Non-isothermal micromixing in turbulent liquids: theory and experiment. The Canadian Journal of Chemical Engineering 76, 641–649. Ba"dyga, J., Bourne, J.R., 1999. Turbulent Mixing and Chemical Reactions. Wiley, GB, Chichester a.o.. Bourne, J.R., 2008. Comments on the iodide/iodate method for characterising micromixing. Chemical Engineering Journal 140, 638–641. Danckwerts, P.V., 1958. The effect of incomplete mixing on homogeneous reactions. Chemical Engineering Science 8, 93–102. Dushman, S., 1904. The rate of the reaction between iodic and hydriodic acids. Journal of Physical Chemistry 8, 453–482. Engler, M., Kiefer, T., Kockmann, N., Woias, P., 2005. Effective mixing by the use of convective micro mixers. In: Proceedings of the Eighth International Conference on Microreaction Technology (IMRET 8), Atlanta, GA, U.S.A., 10–14 April 2005. Falk, L., Commenge, J.-M., 2009. Characterization of mixing and segregation in homogeneous flow systems. In: Hessel, V., Renken, A., Schouten, J.C., Yoshida, J.-I. (Eds.), Micro Process Engineering, vol. 1: Fundamentals, Operations, and Catalysts. WILEY-VCH, Weinheim, Germany, pp. 147–173. Fang, J.-Z., Lee, D.-J., 2000. Quantitative estimation of segregation indices in CSTR—effects of feed stream and output port. Journal of Chemical Engineering of Japan 33, 923–926. Fang, J.Z., Lee, D.J., 2001. Micromixing efficiency in static mixer. Chemical Engineering Science 56, 3797–3802. Fournier, M.-C., Falk, L., Villermaux, J., 1996a. A new parallel competing reaction system for assessing micromixing efficiency—experimental approach. Chemical Engineering Science 51, 5053–5064. Fournier, M.-C., Falk, L., Villermaux, J., 1996b. A new parallel competing reaction system for assessing micromixing efficiency—determination of micromixing time by a simple mixing model. Chemical Engineering Science 51, 5187–5192. Guichardon, P., Falk, L., 2000. Characterisation of micromixing efficiency by the iodide–iodate reaction system. Part I: experimental procedure. Chemical Engineering Science 55, 4233–4243. Guichardon, P., Falk, L., Villermaux, J., 2000. Characterisation of micromixing efficiency by the iodide–iodate reaction system. Part II: kinetic study. Chemical Engineering Science 55, 4245–4253. ¨ Hecht, K.J., Kolbl, A., Kraut, M., Schubert, K., 2007. Micromixer characterization with the consecutive-competitive bromination of 1,3,5-trimethoxybenzene: experimental procedure and comparison to other mixing-sensitive reactions. A.I.Ch.E. Annual Meeting, Salt Lake City, 4–9 November 2007. Hecht, K.J., 2007. Characterization of mixing processes through the consecutivecompetitive bromination of 1,3,5-trimethoxybenzene, Master thesis, University of Utah, Salt Lake City, UT, USA. ¨ Hecht, K., Kraut, M., Kolbl, A., 2007. Microstructured mixing devices: an efficient tool for the determination of chemical kinetic data? A.I.Ch.E. Spring Meeting, Houston, TX, USA, 22–27 April 2007. ¨ Hecht, K.J., Kolbl, A., Kraut, M., Schubert, K., 2008. Micromixer characterization with the competitive-consecutive bromination of 1,3,5-trimethoxybenzene. Chemical Engineering and Technology 31, 1176–1181. Johnson, B.K., Prud’homme, R.K., 2003. Chemical processing and micromixing in confined impinging jets. A.I.Ch.E. Journal 49, 2264–2282. Kockmann, N., Engler, M., Woias, P., 2004a. Theoretische und experimentelle ¨ ¨ Untersuchungen der Mischvorgange in T-formigen Mikroreaktoren—Teil 3: Konvektives Mischen und chemische Reaktionen. Chemie Ingenieur Technik 76, 1777–1783. Kockmann, N., Engler, M., Woias, P., 2004. Convective mixing and chemical reactions in T-shaped Micro reactors. A.I.Ch.E. Anual Meeting, Austin, TX, U.S.A., 7–12 November 2004. Kockmann, N., Kiefer, T., Engler, M., Woias, P., 2006. Convective mixing and chemical reactions in microchannels with high flow rates. Sensors and Actuators B 117, 495–508. ¨ Kolbl, A., Kraut, M., Schubert, K., 2008a. The iodide iodate method to characterize microstructured mixing devices. A.I.Ch.E. Journal 54, 639–645. ¨ Kolbl, A., Kraut, M., Schubert K., 2008b. Characterisation of mixing devices. The iodide–iodate reaction method: influence of the reactant concentrations. In: 18th International Congress of Chemical and Process Engineering (CHISA 2008), Prague, Czech Republic, 24–28. August 2008.
ARTICLE IN PRESS A. K¨ olbl, S. Schmidt-Lehr / Chemical Engineering Science 65 (2010) 1897–1901
¨ ¨ Lob, P., Hessel, V., Lowe, H., Werner, B., Men, Y., 2005. A novel production microstructured mixer—characterization of mixing efficiency. In: 8th International Conference on Microreaction Technology (IMRET 8), Atlanta, GA, USA; 10–14 April, 2005. ¨ P., Lowe, ¨ Men, Y., Hessel, V., Lob, H., Werner, B., Baier, T., 2007. Determination of the segregation index to sense the mixing quality of pilot- and production-scale microstructured mixers. Chemical Engineering Research and Design 85, 605–611. Nagawasa, H., Aoki, N., Mae, K., 2005. Design of a new micromixer for instant mixing based on the collision of micro segments. Chemical Engineering and Technology 28, 401–407. Nouri, L., Legrand, J., Benmalek, N., Imerzoukene, F., Yeddou, A.-R., Haled, F., 2008. Characterisation and comparison of the micromixing efficiency in torrus and batch stirred reactors. Chemical Engineering Journal 142, 78–86.
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Panic´, S., Loebbecke, S., Tuercke, T., Antes, J., Boˇskovic, D., 2004. Experimental approaches to a better understanding of mixing performance of microfluidic devices. Chemical Engineering Journal 101, 409–419. Schneider, M.-A., Maeder, T., Ryser, P., Stoessel, F., 2004. A microreactor-based system for the study of fast exothermic reactions in liquid phase: characterization of the system. Chemical Engineering Journal 101, 241–250. Villermaux, J., Falk, L., Fournier, M.-C., Deterez, C., 1991. Use of parallel competing reactions to characterize micromixing efficiency. A.I.Ch.E. Symposium Series, vol. 88, pp. 6–11. ¨ rasch Walker, B.M., 1996. Einfluss der Temperatursegregation auf die Selektivitat ¨ ¨ ablaufender Reaktionen, Dissertation, ETH Zurich, Zurich, Switzerland. ¨ P., 2005. Mixers with microstructured foils for chemical Werner, B., Hessel, V., Lob, production purposes. Chemical Engineering and Technology 28, 401–407.
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Shorter communication
The iodide iodate reaction method: The choice of the acid a, ¨ A. Kolbl , S. Schmidt-Lehr b,1 a b
Institute for Micro Process Engineering, Karlsruhe Institute of Technology, P.O. Box 3640, D-76021 Karlsruhe, Germany Institute of Technical and Macromolecular Chemistry, Universit¨ at Hamburg, Bundesstr. 45, D-20146 Hamburg, Germany
a r t i c l e in fo
abstract
Article history: Received 26 September 2009 Received in revised form 14 November 2009 Accepted 19 November 2009 Available online 2 December 2009
Mixing processes are frequently characterized with competing chemical reaction schemes. The most popular reaction scheme for continuous flow mixers is the iodide iodate reaction method, which traditionally uses sulfuric acid as proton source for both reactions involved in the scheme. Experimental evidence is provided which indicates that for a quantitative treatment of the experimental data either the experiments need to be carried out with a strong acid such as perchloric acid or the dissociation constants of sulfuric acid need to be included in the models linking the primary experimental results with quantitative measures such as mixing times. Another chemical test reaction system, the acetal cleavage method, traditionally carried out with hydrochloric acid was applied for comparison. In that case, the use of perchloric acid showed no significant impact on the experimental results. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Hydrodynamics Mass transfer Mixing Kinetics Mixing times Iodide iodate reaction
1. Introduction 1.1. Principles of the test reactions Competing chemical reaction systems are popular means for chemical engineers to characterise mixing processes either in stirred tanks or in continuous flow mixers (Ba"dyga and Bourne, 1999). The most popular reaction scheme for continuous flow mixers is the iodide iodate reaction method (Falk and Commenge, 2009). Two reactions are competing for H + in this scheme: (1) The neutralisation of dihydroboric acid ions to form hydroboric acid and (2) the comproportionation of iodide and iodate ions to form iodine (also referred to as the ‘Dushman reaction’ Dushman, 1904). Iodine formed in the course of the mixing process further reacts with excess iodide to form triiodide according to equilibrium (3). Triiodide absorbs strongly at a wavelength of 353 nm. Using the UV spectra of the resulting mixtures, mixing processes and devices can be characterised (Villermaux et al., 1991). H2BO3 + H + -H3BO3
(1)
5I + IO3 + 6H + -3I2 + 3 H2O
(2)
I2 +I =I3
(3)
The reactions involved in the iodide iodate reaction method differ in their reaction rate. Reactions (1) and (3) are considered to be infinitely fast compared to the physical mixing process, while the rate of the Dushman reaction is kept in the range of the rate of the mixing process in order to yield mixing sensitive results. The rate of the Dushman reaction can be adjusted by choosing appropriate reactant concentrations in order to yield maximum ¨ sensitivity as recently demonstrated experimentally (Kolbl et al., 2008a, b). In case of perfect mixing, the product distribution is solely governed by the kinetics of the reactions involved, and thus, no iodine is found in the resulting mixtures. Any formation of iodine is attributed to imperfect mixing. Hence, the method can be used for parametric studies. The use of the iodide iodate reaction method as a quantitative method is under debate, mainly since the kinetic examinations on the Dushman reaction were undertaken at reactant conditions far from those relevant to mixing studies (Guichardon et al., 2000; Bourne, 2008). Another parallel-competitive reaction scheme is the acetal cleavage method, the competition of the acid-catalysed cleavage of 2,2-dimethoxpropane (DMP) (4) with the neutralisation of HCl with NaOH (5).
Corresponding author. Tel.:+ 49 7247 82 6653; fax: + 49 7247 82 3186.
¨ E-mail address: [email protected] (A. Kolbl). Present address: Tesa SE, Quickbornstraße 24, BF 664, 20253 Hamburg, Germany.
ð4Þ
1
0009-2509/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.11.032
NaOH+ HCl-NaCl+ H2O
(5)
ARTICLE IN PRESS 1898
A. K¨ olbl, S. Schmidt-Lehr / Chemical Engineering Science 65 (2010) 1897–1901
Faster mixing processes prefer the faster neutralization reaction. With slower mixing processes more of the DMP is converted into acetone and methanol. More details on the reaction method can be found in scientific literature for stirred vessels (Walker, 1996; Ba"dyga et al., 1998) and continuous flow mixers (Johnson and Prud’homme, 2003). H + for both reactions is provided by hydrochloric acid.
Table 1 pKa values of acids relevant to this contribution.
1.2. The iodide iodate reaction method in scientific literature
authors, the IEM parameters deliver ‘‘the order of magnitude of mixing time for practical applications (Falk and Commenge, 2009)’’. According to Falk and Commenge, this model parameter offers a reactant concentrations-free measure of the ‘mixing quality’ (as opposed to the segregation index XS) yet. An experimental verification of this assumption has not been provided in scientific literature, except for the variation of the concentration of H + for stirred laboratory vessels reported by Assirelli et al. (2008), who found that varying the concentration of H + did not affect the segregation indices calculated for similar hydrodynamic conditions. In a recent publication (Baccar et al., 2009) numerical simulations and experiments for a ‘hollow fibre membrane contactor’ were compared by means of the iodide iodate reaction method. Two segregation indices obtained at similar conditions (the same device, the same reactant concentrations, comparable throughput) were compared; one derived from numerical simulations (XS = 1.17 10 3) another derived from experimental investigations (XS = 4.42 10 3). The differences between those two values are in the range of the overall effect achieved with microstructured devices (Men et al., 2007; Kockmann et al., 2004a, b, 2005; Engler et al., 2005). This finding indicates that further work needs to be done to achieve better matches between mixing models and experimental results. This manuscript might contribute to that goal.
The iodide iodate reaction method was developed for stirred laboratory tanks (Fournier et al., 1996a, b; Guichardon et al., 2000; Guichardon and Falk, 2000) where a small volume of concentrated sulphuric acid is mixed into a large volume containing the rest of the necessary chemicals, which are placed in the vessel prior to mixing experiments. In the scientific literature on the experimental characterisation of stirred laboratory vessels, a segregation index is typically derived from the UV spectra of the resulting mixtures (Fournier et al., 1996a; Guichardon et al., 2000; Assirelli et al., 2002, 2005; Fang and Lee, 2000; Nouri et al., 2008). The segregation index derived for mixtures obtained in stirred vessels is defined by Eqs. (6)–(8) (Fournier et al., 1996a). XS ¼
Y YST
Y ¼2
ð6Þ
cI3 þ cI2 cH þ
ð7Þ
6cIO3 ;0 6cIO3 ;0 þ cH2 BO3 ;0
ð8Þ
0
YST ¼
The segregation index attempts to indicate the yield of iodine compared to the maximum yield of iodine, which would be obtained in case of an infinitely slow mixing process. The segregation index should yield values between 0 (in case of perfect mixing) and 1 (worst case scenario) and should thus be comparable to a classically defined selectivity. In some of the literature on stirred laboratory vessels, ‘mixing times’ were derived from a simple mixing model: the incorporation model (Fournier et al., 1996b; Guichardon et al., 2000; Fang and Lee, 2000). With the same mixing model, ‘mixing times’ were derived for mixing processes in Kenics static mixers by Fang and Lee (2001). A low flow of concentrated sulphuric acid was continuously dosed into a large main flow containing the other essential chemicals. A first suitable set of concentrations for microstructured mixing devices was proposed by Panic´ et al. (2004). Since then, the iodide iodate reaction method has gained a large popularity ¨ for the characterization of those devices (Kolbl et al., 2008, 2008b; Schneider et al., 2004; Hecht, 2007; Hecht et al., 2007a, b, 2008; ¨ et al., 2005; Werner et al., 2005; Men et al., 2007; Nagawasa Lob et al., 2005; Kockmann et al., 2004a, b, 2005; Engler et al., 2005). In most cases, the absorbance values of the resulting mixtures were compared at characteristic wavelengths (286 or 353 nm). In some cases (Men et al., 2007; Kockmann et al., 2004a, b, 2005; Engler et al., 2005) a segregation index was derived from the spectra of the resulting mixtures according to a definition similar to the Eqs. (6)–(8), modified for the needs of mixtures obtained in static mixers. Falk and Commenge (2009) have recently reviewed previous work on the characterization of microstructured mixing devices and calculated ‘mixing times, tm’ from experimental data published in the reviewed work. The derived ‘mixing times’ are parameters of the IEM model (interaction by exchange with the mean) and should not be confused with the mixing time, required to achieve a specific degree of homogeneity. According to the
Acid
pKa,1
pKa,2
H2SO4 HCl HClO4
2 7 10
+ 1.92 / /
1.3. The choice of the acid A problem, which is hardly addressed in scientific literature, is the choice of the acid, which provides H + for the competing reactions. pKa values of acids relevant to this contribution are listed in Table 1 (Atkins, 1990). Equilibrium calculations using pKH2 SO4;2 show that HSO4 is not fully dissociated, even for diluted sulphuric acid as used for mixing experiments with static mixers. Engler et al. (2005) pointed out that the amounts of available protons change dynamically over the mixing process, which make theoretical considerations difficult. Later, Bourne (2008) wrote that the dissociation constant pKH2 SO4;2 should be included in an appropriate mixing model for achieving reliable mixing times.
2. Experiments and discussion 2.1. Experimental setup and operating conditions The experimental examination of the continuous flow mixers was done in a setup constructed in stainless steel at the Institute for Micro Process Engineering (Karlsruhe Institute of Technology). For the characterization of continuous flow mixers two flows are combined at a volumetric ratio of 1/1. Mixing was achieved in a V type mixer. Those mixers are obtained by stacking and bonding micromachined foils. The examined mixer consists of 60 micromachined channels (cross section 100 mm 70 mm). The reactant concentrations for the iodide iodate reaction method correspond to twice a set of standard concentration defined in a previous ¨ publication (Kolbl et al., 2008a) (solution 1: diluted acid is mixed
ARTICLE IN PRESS A. K¨ olbl, S. Schmidt-Lehr / Chemical Engineering Science 65 (2010) 1897–1901
with solution 2: cNaH2 BO3 ¼ 227 mol=m3 , cKI =79.8 mol/m3, cKIO3 ¼ 16:0 mol=m3 ). For a reference experiment diluted sulphuric acid of a concentration of 30 mol/m3 (potential proton concentration of cH + =60 mol/m3) was employed. This experiment is compared to experiments using perchloric acid in a concentration of 30 mol/m3 (the same concentration of acid molecules) and 60 mol/m3 (the same concentration of potential H + ). More details ¨ on the setup and the mixer can be found in a publication by Kolbl et al. (2008a). With the same experimental setup, the same mixer was examined with the acetal cleavage method. The experimental conditions, the nature of the solvent and reactant concentrations (solution 1: cHCl =200 mol/m3; solution 2: cNaOH = 210 mol/m3, cDMP =200 mol/m3) are the same as those described in a publication by Johnson and Prud’homme (2003). In order to examine the influence of the choice of the acid in the case of a laboratory stirred tank, mixing experiments in a baffled laboratory tank using a Rushton turbine were carried out at the Institute of Technical and Macromolecular Chemistry of Hamburg University. Sulphuric acid ðcH2 SO4 ¼ 30 mol=m3 Þ was compared to perchloric acid ðcHClO4 ¼ 60 mol=m3 Þ. Both solutions have a potential H + concentration of 60 mol/m3. Both solutions were applied to characterize the baffled tank at three different stirring speeds using a Rushton turbine (80, 160, and 240 rpm). The 3 mL of the respective acid were injected over a needle (1 mm inner diameter) at a rate of 1 mL/min at the stirrer plane half distance between the outer end of the stirrer (stirrer diameter 4 cm) and the wall of the tank (tank diameter 10 cm). Prior to the mixing experiment, the tank was filled with a solution (1L) of KI (c= 17.4 mol/m3), KIO3 (c =2.5 mol/m3), H3BO3 (c =34.8 mol/m3), and NaOH (c= 17.4 mol/m3). 2.2. The iodide iodate reaction method In order to test the possible influence of the dissociation equilibrium on the experimentally achieved results, measurements on a V-type mixer with sulphuric acid have been compared to measurements achieved with perchloric acid. The experimentally obtained product selectivities are given in terms of conversion of the key compound iodate (Eq. (9)). XIO3 ¼
cIO3 ;0 cI3 cI2 3 cIO3 ;0
ð9Þ
Lower XIO3 corresponds to lower absorbance values in the UV spectrum of the resulting mixtures and to improved mixing. The experimentally achieved results are given in Fig. 1. The choice of the acid has a clear impact on the experimental results. For a quantitative modification of the iodide iodate reaction method according to Bourne’s (2008) findings, either
Fig. 1. Experimentally obtained conversion degrees of iodate using sulphuric acid and perchloric acid in a V-type mixer.
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the dissociation constant of the hydrogen sulphate ion needs to be included in the model assumptions, or the experiments need to be carried out with a very strong acid such as perchloric acid. As discussed by Engler et al. (2005), the concentration of HSO4 changes dynamically over time (with the consumption of H + by the reactions (1) and (2)), i.e. the protons are liberated at a time, when the degree of segregation (according to the definition of Danckwerts (1958), not to be confused with the segregation index as defined by Eqs. (6)–(9)) in the mixture is already reduced, which leads to lower iodine yields than obtained with a very strong acid. Hence, a faster mixing process is assumed when utilizing sulphuric instead of perchloric acid. Smaller numbers of the mixing time would be calculated from simple mixing models, which do not consider the dissociation equilibrium. It is observed that the ratio of the conversions using sulphuric acid and perchloric acid is constant over the examined mass flow range ðXH2 SO4 =XHClO4 ¼ 2:4 70:1Þ. This finding suggests that the experimental results reflect the intrinsic kinetics of the reactions involved in this examination method, free of mass transport influence. Furthermore, the experimental results indicate a strong influence of the H + concentration on the experimentally determined conversion degrees (Fig. 1). This finding clearly indicates the large impact of the H + concentrations on the kinetics of the Dushman reaction. The reaction order of H + is commonly assumed to be 2 (Fournier et al., 1996a, b). 2.3. Experimental results in a stirred laboratory tank The comparison between the use of sulphuric acid and perchloric acid is depicted in Fig. 2. The experimental results show that the choice of the acid also shows an effect upon the experimental results in a stirred laboratory tank. The achieved effect seems to be smaller. The experimental characterisation of mixing processes in stirred tanks is typically carried out at lower reactant concentrations than the characterisation of continuous flow mixers. This also holds for the present study. Although the same acid concentrations were employed for the V type mixer and the stirred laboratory vessel, the results depicted in Fig. 1 can not be directly compared to the results shown in Fig. 2, since in case of the stirred vessel only a small volume of sulphuric acid is used, which gets quickly diluted during the mixing experiment due to the large amount of liquid present in the vessel. 2.4. The acetal cleavage method As for the iodide iodate reaction method, the acetal cleavage method has been tested for a possible influence of the choice of the acid. Perchloric acid and hydrochloric acid differ significantly in the pKa-value (Table 1). Furthermore, the high ethanol content of the solvent might influence the dissociation equilibrium.
Fig. 2. Experimental comparison for utilising sulphuric acid and perchloric acid for mixing experiments in stirred laboratory vessels.
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A. K¨ olbl, S. Schmidt-Lehr / Chemical Engineering Science 65 (2010) 1897–1901
References
Fig. 3. Experimentally obtained conversion of DMP in a V-type mixer using hydrochloric acid and perchloric acid.
The experimental results are depicted as conversion degrees of the key compound DMP over total mass flow rates in Fig. 3. The resulting mixtures were analysed with UV spectroscopy. The experimental results indicate that the choice of an acid stronger than the frequently employed hydrochloric acid does not impact the experimentally determined reactant concentrations in the resulting mixtures for the present experimental conditions.
3. Conclusions The experimental results presented in this contribution clearly indicate that the choice of the acid, which is the proton source for the competing reactions of the iodide iodate reaction method, has a severe effect on the product selectivities of the resulting mixtures. For a quantitative treatment of the experimental data, the dissociation equilibrium of the hydrogen sulphate ion needs to be included in the respective models or the experiments need to be carried out with a strong acid such as perchloric acid. This experimental finding presented here was recently predicted by Bourne (2008). The application of a ‘segregation index, XS’ as defined in scientific literature for mixing experiments achieved with stirred tanks or continuous flow mixers lacks sense when utilising sulphuric acid. The iodine yield (as part of the segregation index) refers to the amount of H + theoretically available (2 mol H + per mol H2SO4). Since the amount of H + in the reacting mixture dynamically changes over the mixing process, less iodine is produced in the course of the mixing process compared to experiments utilising perchloric acid, where the acid molecules are fully ionised from the beginning of the mixing experiment. It has been shown that the presented effect is also found in stirred laboratory vessels. The ratio of the UV signals obtained utilising perchloric acid and sulfuric acid is independent of the flow throughput. This finding suggests that the experimental results are independent of the hydrodynamics, and the observed differences can be attributed to the influence of the H + concentration upon the kinetics of the competing reactions involved. A second competitive reaction scheme, the acetal cleavage method, does not show sensitivity to the choice of the acid (perchloric acid vs. hydrochloric acid). This indicates that HCl is sufficiently ionized due to its higher acid strength. An impact of the nature of the solvent cannot be shown. However, the use of hydrochloric acid is not appropriate for the iodide iodate reaction method due to the formation of mixed halide species (Fournier et al., 1996a, b).
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