Direct Visualization of Scale-Up Effects on the Mass Transfer

Demonstration pubs.acs.org/jchemeduc

Direct Visualization of Scale-Up Effects on the Mass Transfer Coefficient through the “Blue Bottle” Reaction Patrick M. Piccione,*,†,§ Adamu Abubakar Rasheed,‡ Andrew Quarmby,† and Davide Dionisi‡ †

Process Studies Group, Technology and Engineering, Jealott’s Hill International Research Centre, Syngenta, Bracknell, Berkshire RG42 6EY, United Kingdom ‡ School of Engineering, Materials and Chemical Engineering Group, University of Aberdeen, Aberdeen AB24 3UE, United Kingdom S Supporting Information *

ABSTRACT: This paper summarizes the experience gained at Syngenta and at University of Aberdeen on visualization of gas−liquid mass transfer in agitated vessels. The aim is to show how simple experiments can be used to directly visualize the effect of vessel size, air flow rate, and agitation speed on the rate of mass transfer for gas−liquid reactions without the need for sophisticated probes or indirect calculations. The reaction used for visualization purposes is the “blue bottle” reaction, that is, the oxidation by oxygen of leucomethylene blue to methylene blue. This reaction produces a distinctive blue color, and its rate depends on the rate of oxygen mass transfer from the gas to the liquid phase. Therefore, this reaction can be used to visualize the rate of mass transfer and how this rate is affected by geometrical and operating parameters of agitated vessels. The described demonstration is furthermore highly convenient for educational purposes. This is the first demonstration visually illustrating scale-up effects in chemical processes. KEYWORDS: Upper-Division Undergraduate, Demonstrations, Chemical Engineering, Hands-On Learning/Manipulatives, Kinetics, Industrial Chemistry, Mechanisms of Reactions, Transport Properties, Organic Chemistry

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INTRODUCTION For maximum relevance and applicability of research laboratory work to large-scale process development, equipment at different scales must represent the most important physicochemical processes faithfully. While homogeneous chemical reactions are inherently scale-independent, multiphase reactions and processes often depend on mixing and associated “transport” phenomena, which are sensitive to the physical scale at which they are practiced. Predictive process development thus relies crucially on scale-up/scale-down rules.1 Despite their importance, the practical elements of scale-up are not directly discussed in standard chemistry and chemical engineering curricula. The treatment of mixing phenomena is often heavily mathematical, leading to strong quantitative predictions but more limited insight and intuitive understanding. To effect the latter, visual demonstrations are far and away the most memorable due to the prominence of vision as a primary sense of perception for human beings: in layperson terms, “to see is to believe”.2 Such demonstrations further benefit from the fact that the concepts can be transferred memorably to nonspecialists, even in the absence of theoretical familiarity. No simple experiment was found in the literature to visualize the effect of scale-up effects on chemical processes. In the few transport phenomena demonstrations reported,3−5 the main demonstrated aspects are dependence on parameters at a given scale rather than changes at different scales. Several demonstrations rely on analytical equipment such as dissolved oxygen sensors or spectrophotometers, which visualize phenomena via a computer rather than directly through human perception.3,5 There is a need for a new teaching activity to allow students to develop an intuitive appreciation for the importance of such scale effects. This study reports a new demonstration for the © XXXX American Chemical Society and Division of Chemical Education, Inc.

visualization of the effect of agitation conditions and scale on mass transfer limited gas−liquid reactions. The basis is the “blue bottle” reaction, in which the solution changes from colorless to blue when oxygen transfers from the gas to the liquid phase.

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BACKGROUND

Blue Bottle Reaction

The blue bottle experiment’s history was summarized by Rajchakit and Limpanuparb.6 The demonstration was popularized by Campbell at Harvey Mudd College,7 while the redox reaction itself was known decades earlier.6 The aim of the demonstration is to illustrate the applicability of mechanistic concepts early in the chemistry curriculum. The experiment consists of an approximately half-full bottle of colorless liquid, which upon shaking turns rapidly blue, then more slowly returns to the initial colorless state. Repeated shakings result in apparently identical color changes. The blue species is the cationic form of methylene blue (MB+) in aqueous solution. The color changes are due to its reduction by glucose enediolate ion (G−) to the hyaline leucomethylene blue (MBH), which is colorless, and to the reoxidation of MBH to MB+ by dissolved oxygen. This demonstration is attractive on multiple pragmatic counts: first and foremost, it offers a bright, sharp color contrast. Furthermore, it is robust with respect to changes in concentrations and material purity. Practically, the reagents are cheap, easy to source, and easy to handle safely. It has therefore been Received: August 21, 2016 Revised: January 18, 2017

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DOI: 10.1021/acs.jchemed.6b00633 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

used extensively at several universities.7−9 The pedagogic steps to unravel various aspects of the mechanism were indeed provided by Campbell’s article7 together with notes from students studying the reaction at University College London. More recently, the system has been adapted in various ways leading to variations in reducing agents, dyes, and presentation.6,8,10,11 To summarize, the blue bottle reaction has been used extensively for demonstrations of kinetics, and in particular to explore the dependence of the homogeneous phase reactions on various parameters. The original, best-documented, and mostexhaustively studied blue bottle system is extended in this work to develop a visual demonstration of heterogeneous gas-to-liquid mass transfer and its dependencies. Mass Transfer

The rate of mass transfer is typically described in terms of a mass transfer coefficient (kLa), as the product thereof and of a concentration driving force. Numerous semi-empirical correlations for the mass transfer coefficient are available.12 Assuming geometric similarity and constant volumetric gas flow per liquid volume, the classical forms for sparged vessels lead to a powerlaw dependence on both agitation rate and impeller diameter kLa ∝ N3βD2β + γ

(1)

where N, D, β, and γ are the impeller rotation rate, impeller diameter, and two empirical constants (usually ranging from 0.2 to 0.7), respectively.

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Figure 1. Oxidation of leucomethylene blue at two different agitation speeds (50 and 150 rpm). 50 rpm corresponds to a kLa value of 7.6 h−1; 150 rpm corresponds to 20.2 h−1. This figure was captured from Video 1.

DEMONSTRATION The experimental procedure and reactor setups are described in detail in the Supporting Information. The reactors are set up with an overhead stirrer and submerged gas addition. Different volume reactors are proportionately scaled to be “geometrically similar”. The blue bottle solution of potassium hydroxide, glucose, and methylene blue is sparged with nitrogen gas until the blue color disappears. Agitation is then set at the desired value, and air is introduced at the desired flow rate until the blue color appears, which can be timed with a stopwatch. Sequential runs at different conditions demonstrate the effect of agitation on the mass transfer, while simultaneous runs in two different reactor volumes demonstrate the effect of scale.

Figure 1 compares the color in the vessel at the two agitation rates, at several times from the start of the experiment. Initially, no color is observed, because the oxygen concentration in the vessel is low. At time 0, air begins to be pumped into the vessel. After 13 s, at the slow agitation rate (50 rpm), still no color is observed, while at the fast agitation rate the liquid in the vessel has already turned blue, since the mass transfer of oxygen from the gas to the liquid phase is faster due to the higher agitation rate. After 30 s, both vessels have turned blue, even though the blue color is darker in the vessel agitated at 150 rpm, because of the greater extent of the leucomethylene blue oxidation. The same effect is more clearly evident in Video 1 (Supporting Information). This example, demonstrated at the University of Aberdeen, shows how the blue bottle reaction can provide an immediate visualization of the effect of agitation speed on the mass transfer coefficient.

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HAZARDS Sodium hydroxide and its solutions are corrosive. Protective gloves and a laboratory coat should be worn together with safety eyewear. Loose hair and clothing can be entrained by rotating shafts, and should be tied securely. Standard precautions for the use of electrical equipment should be observed with the motors.

Same Rotation Rate, Different Scales

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Figure 2 shows the experiments carried out at Syngenta, as does Video 2. These experiments show the simultaneous oxidation, started at the same time, of the 10 L vessel (to the left) and 1 L vessel (to the right) over time at the same volumetric flow rate per liquid volume. Experiment 1 shows the effect of maintaining the same agitation rate and same gas flow rate per unit of reactor volume for two vessels with a factor of 10 difference in size. The initial colors are similar (see Video 2 at time 38 s, which corresponds to 14 s after starting to add air), but upon air addition, a dramatic difference is observed: the large vessel changes color much faster, and also achieves a much darker blue hue (see Video 2 at time 68 s, which corresponds to 44 s after starting to add air). According to the mass transfer correlations developed for this system (Supporting Information), for the vessels studied here,

RESULTS AND DISCUSSION Videos of the blue bottle experiments are available as Supporting Information, though the “supporting” nature is due to the limitation of the printed medium: in reality, they are an integral part of this contribution as its very subject is a new visual demonstration. Video 1 shows the experiments carried out at University of Aberdeen, and Video 2 shows the experiments carried out at Syngenta. These experiments are described in the sections below. Same Vessel Size, Different Agitation Rate

The first set of visualization experiments compared mass transfer rates for two agitation speeds, 50 and 150 rpm, using the same vessel and the same agitator. B



Demonstration

Figure 2. Effect of scale and agitation rate on mass transfer. Top row: experiment 1 (same agitation rate). Bottom row: experiment 2 (same kLa values). This figure was captured from Video 2.

Matching kLa Values

under the conditions of this experiment (geometric similarity, same rotation rate, and same gas flow rate per unit volume of liquid), the mass transfer coefficient kLa depends on the agitator diameter according to D1.4. The larger vessel is thus expected to transfer oxygen into the liquid approximately 2.9 times faster, in accordance with the observations. In addition, another important point that can be illustrated is that the conditions of operation determine the final color attained. As shown in the Supporting Information (eq 2 below, see the Supporting Information for the definition of the symbols), the concentration of the methylene blue cation is higher for higher values of kLa; therefore, the final intensity of the blue color will be higher for higher oxygen mass transfer rates (this is true considering either of the two solutions of eq 2):

After having demonstrated poorer mass transfer in the small vessel at similar rotation rates, it is important to prove that similar mass transfer rates in the two vessels can be achieved under different conditions. The exact same solutions were thus reused, at the same volumetric gas flow rates, but two different impeller speeds were used. As described in the Supporting Information, for the system under study, the kLa values are proportional to the agitation rate and the agitator diameter according to N1.2D1.4 for the same fluids and at similar volumetric gas flow rates per volume of liquid. With the small vessel operating at 500 rpm and the large one at 200 rpm, the kLa values are predicted to differ by less than 4%. Indeed, experiment 2 in Video 2 shows fairly similar times for initial color change (see Video 2 at time 85 s, which corresponds to 8 s after starting to add oxygen), as well as a similar color intensity at the end (allowance being made for the shorter visual path in the smaller vessel).

⎛ k [G−] k [G−] ⎞ ⎜⎛ [MB ] = k [G−] ⎜⎜k1 + k1 2 [MB]TOT + 2 ⎟ kLa[O*2 ] [O*2 ] ⎠ 2 k 2a[O*] k1 ⎜⎝⎝ L 2 ⎡⎛ ⎞2 k [G−] k [G−] kLa⎟ [MB]TOT + 2 ± ⎢⎜k 1 + k 1 2 ⎢⎣⎝ kLa[O*2 ] kLa[O*2 ] ⎠ +

Commentary

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⎤1/2 ⎞ k 2[G−] 2 k1 [MB]TOT ⎥ ⎟⎟ −4 ⎥⎦ ⎠ kLa[O*2 ]

The experiments described here, considered together, visually prove the following in short succession: (i) Mass transfer is a strong function of agitation. (ii) Different mass transfer rates can lead to different final states (and hence mass transfer is an important phenomenon to understand for all process technologists, whether chemists or chemical engineers), and (iii) importantly, the scale of operations can alter process behavior significantly and must therefore be carefully considered. (iv) Well-defined scale-up rules can be applied to practice

(2) C



Demonstration

Hendrie, University of Aberdeen, for skillful assistance in the experimental work.

scale-up and scale-down predictively during process development as a science rather than as an art.

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Further Opportunities

A further opportunity, not developed in this work, is that the theoretical expression developed herein for the quasi-steadystate MB+ concentration could be combined with knowledge of the intrinsic homogeneous rate laws and constants to calculate the mass transfer coefficient from spectrophotometric measurements using eq 2. Another opportunity is to perform multiple scale comparisons on the recent adaptations10,11 of the blue bottle reaction system.

(1) Atherton, J. H.; Carpenter, K. J. Process Development: Physicochemical Concepts; Oxford University Press: Oxford, 1999; pp 1−88. (2) Mizuno, T. Personal communication, Maidenhead, UK, 2015. (3) Dickey, A. D.; Dickey, M. D.; Stewart, M. D.; Willson, C. G. An Automated Statistical Process Control Study of Inline Mixing Using Spectrophotometric Detection. J. Chem. Educ. 2006, 83 (1), 110−113. (4) Trujillo-de Santiago, G.; Rojas-de Gante, C.; García-Lara, S.; Ballescá-Estrada, A.; Alvarez, M. M. A Colorful Mixing Experiment in a Stirred Tank Using Non-Newtonian Blue Maize Flour Suspensions. J. Chem. Educ. 2014, 91 (10), 1729−1735. (5) Dietrich, N.; Loubière, K.; Jimenez, M.; Hébrard, G.; Gourdon, C. A New Direct Technique for Visualizing and Measuring Gas−Liquid Mass Transfer around Bubbles Moving in a Straight Millimetric Square Channel. Chem. Eng. Sci. 2013, 100, 172−182. (6) Rajchakit, U.; Limpanuparb, T. Rapid Blue Bottle Experiment: Autoxidation of Benzoin Catalyzed by Redox Indicators. J. Chem. Educ. 2016, 93, 1490−1494. (7) Campbell, J. A. KineticsEarly and Often. J. Chem. Educ. 1963, 40 (11), 578−583. (8) Cook, A. G.; Tolliver, R. M.; Williams, J. E. The Blue Bottle Experiment Revisited. J. Chem. Educ. 1994, 71 (2), 160−161. (9) Anderson, L.; Wittkopp, S. M.; Painter, C. J.; Liegel, J. J.; Schreiner, R.; Bell, J. A.; Shakhashiri, B. Z. What Is Happening When the Blue Bottle Bleaches: An Investigation of the Methylene Blue-Catalyzed Air Oxidation of Glucose. J. Chem. Educ. 2012, 89 (11), 1425−1431. (10) Wellman, W. E.; Noble, M. E. Greening the Blue Bottle. J. Chem. Educ. 2003, 80 (5), 537−540. (11) Rajchakit, U.; Limpanuparb, T. Greening the Traffic Light: Air Oxidation of Vitamin C Catalyzed by Indicators. J. Chem. Educ. 2016, 93, 1486−1489. (12) Lee, J. H.; Foster, N. R. Measurement of Gas-Liquid Mass Transfer in Multi-Phase Ractors. Appl. Catal. 1990, 63 (1), 1−36.

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CONCLUSIONS An experimental setup and procedure have been devised to adapt the well-known blue bottle reaction so as to demonstrate chemical engineering principles related to transport phenomena. The previous advantages remain: robustness, cost of materials, and ease of setting up safely. The new demonstration with simultaneous reactions at two scales offers a striking, colorful visualization of scale effects on mass transfer to the unaided human eye; various conditions can easily be compared to exemplify the importance of the mass transfer coefficient kLa. The demonstration can further be combined with various analytical techniques to extract quantitative kinetics for the various steps. The authors hope that others will use this new demonstration to raise awareness of the importance of scale-up effects.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00633. Extensive discussion, theory details, mechanism, and detailed experimental methods (PDF, DOCX) Video of experiments at the University of Aberdeen (MPG) Video of experiments at multiple scales at Syngenta (MPG)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Patrick M. Piccione: 0000-0002-9164-5474 Adamu Abubakar Rasheed: 0000-0001-9860-9479 Present Address §

Process Studies Group, Technology and Engineering, Syngenta, Breitenloh 5, CH-4333 Münchwilen, Switzerland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Guy Ramsay and Rob Courtney for their assistance in assembling various generations of the Syngenta experimental setup. Patrick M. Piccione and Andrew Quarmby acknowledge Syngenta for support for publishing this work, as well as George Hodges for helpful discussions on the chemical reaction mechanism, and Rob Lind for help with video processing. Davide Dionisi and Adamu Rasheed thank Elizabeth D


Direct Visualization of Scale-Up Effects on the Mass Transfer

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