(376f) A New Way of Teaching Gas Liquid Reaction Kinetics and Gas-Liquid Reactor Design

The topic of gas-liquid reactions and reactor design is recommended to be included in the course contents of a second course on Chemical Reaction Engineering at a senior UG level either as an elective or compulsory course in some schools or in the course content of a post graduate level PG course. The topic of gas-liquid reactions is dealt to a varying degree of extent in several text books that are available in the literature. the prominent books among them being those authored by Astarita(1967),Astarita et al (1983), Danckwerts(1970), Doraiswamy and Sharma(1984), Westerterp et al ( 1991), Levenpsiel (1972,1984,1999), Froment and Bischoff(2011). The books of Astarita (1967), Astarita et al (1983) and Danckwerts (1970) delve into the theory of gas liquid reactions and summarize the relevant literature available till the date of the publication of the respective book but they address the researcher rather than a class room teacher or student. The text book of Doraiswamy and Sharma (1984) explains the theory and applications of gas liquid reactions quite well but does not enable the teacher to train the student in problem solving with respect to identifying rate of a gas-liquid reaction and writing and solving the design of a chosen type of gas-liquid reactor. The Chemical reactor Omnibook (1984) discusses solution of gas-liquid reactor design problems by using the enhancement factor vs Hatta Modulus plot of Van Krevelen and Hoftijzer (1954) for a (1,1) order reaction taking place in a liquid film but which is obtained for the case where the reaction in bulk is assumed to reach completion.. No examples are given to illustrate the use of the equations but a host of problems are assigned at the end of the chapter on gas liquid reactions some of which may require solution of the film model for the case where reaction in bulk may not reach completion or where there is a finite flux of the solute into the liquid bulk. Neither the second or the third edition of the book by Levenspiel contain good number of examples. The importance of the role of gas film resistance in finding the rate of absorption is emphasized quite well in the book by Levenspiel unlike other text books but the procedures for taking gas film resistance into account require trial and error. Same way the equations required for design of ideal reactors are given with no illustrations but plenty of problems are assigned to the student which require lot more analysis and new derivations which are not hinted in the book. The text book of Westerterp et al (1991) covers the theory of absorption with reaction quite well but unfortunately the theories for gas-liquid reactions and solid catalyzed reactions are dealt in the same chapter even alternately which creates lot of confusion to the reader and teacher when he has to present the text book material in the class room. The analogy between the diffusion reaction equations for the two types of reaction systems is restricted and hence it is easier and simpler if they are treated separately. The text book of Froment and Bischoff (2011) treats the kinetics and design of gas-liquid reactions in separate chapters with some illustrations but is not exhaustive enough to enable the theory given by them to solve problems from the Chemical Reactor Omnibook (1984) of Levenspiel. It is rather surprising that in spite of the fact that research on the theory of gas-liquid reactions started from the application of Whittman's film theory as early as 1928 (Hatta, 1928,1935) or so, research papers appeared in chemical engineering science journal up to 2018(Stepanek et al (2018)) or so discussing the equations for predicting enhancement factor for the simplest case of a (1,1) order irreversible gas liquid reaction. Thus, when it comes to teaching of the topic on kinetics and design of gas-liquid reactions and reactors, all of the above mentioned books appear to be lacking in one respect or another when it comes to imparting the student the ability to tackle problems relating to development of rate equations and reactor design procedures. But we find that relatively a lot more coverage is given to the theory of solid catalyzed gas phase or gas-liquid reactions and reactors or the theory of gas-solid(reactant) reactions and reactors.

In the present work, rate equation for the (1,1) order irreversible gas liquid reaction are presented which consider the most general case where the gas film resistance, reaction in film and bulk, depletion of the liquid phase reactant B in the film are derived and from this most general case by extending the Hikita and Asai (1964) approximation as done by Bhattacharya et al (1987).. Rate equations and criteria for the applicability of these special cases, where not all the resistances mentioned above are important, i.e cases corresponding to different regimes of absorption , are derived. The simple notation of these special cases introduced by Doraiswamy and Sharma (1984) is followed in the present work with a slightly modified definition of regimes and naming the gas film resistance controlled mass transfer as an additional regime. Accordingly , the following classification of regimes is followed in the present work. Regime 1 is defined as the regime of absorption with reaction where gas film resistance is negligible, reaction in film is insignificant, liquid phase mass transfer resistance is negligible and reaction takes place only in the reactor bulk and is not complete in the bulk. Regime 2 is defined as the regime where gas film resistance is negligible, reaction in film is insignificant, reaction in film is complete and the controlling resistance is only the liquid phase mass transfer. Regime 3 is defined as the regime in which gas film resistance is insignificant, reaction in film is complete and there is no flux of the solute from the film into the liquid bulk. Regime 4 is defined as the regime in which gas film resistance is not significant, reaction in film is purely diffusion controlled i.e reaction between the solute and the solvent is instantaneous. Regime 5 is defined as the regime in which gas film resistance is controlling and all other resistances are negligible compared to the gas film resistance irrespective of the other features regarding the zone of reaction i.e film or bulk. Mixed regimes where the features of one or more individual pure regimes mentioned above are to be considered as significant such as regime 1 and 2, regime 1,2, and 3, regime 3, 4, and 5, so on. The equations and criteria for the pure or mixed regimes are derived after developing at first , the rate equation for the most general case where features of all the pure regimes mentioned above are important. From the equations for the most general case, rate equations and the criteria for the validity of the various pure and mixed regimes are derived. The need to identify the controlling resistances arises from the fact that the selection of the reactor type depends on which hydrodynamic parameter is involved in the rate equation for the controlling regime that is identified based on input data for a given problem. The above approach has been used by the author while teaching Chemical Reaction Engineering at the senior undergraduate and post graduate level at Indian Institute of Technology (Delhi).over two decades. Also, application of the above approach to solve all the problems assigned in the Chemical Reactor Omnibook was developed and tested in the classroom and a broader set of exercises are also developed to enable the student to develop rate equations and criteria for new reaction systems.

REFERENCES

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