Contemporary Catalysis

Science, Technology, and Applications

Paul C J Kamer, Dieter Vogt, Joris Thybaut

• Publisher: Royal Society of Chemistry
• Year: 2017
• eBook ISBN: 9781788012331
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

Encompassing an integrated approach to the various aspects of catalysis, covering heterogeneous, homogeneous, organo-, bio-, and computational catalysis, as well as reaction and reactor engineering on an advanced level, this textbook is ideal for graduate students with diverse backgrounds, including catalysis, engineering, and organic synthesis. The basic principles of the various fields of catalysis are introduced in a concise way, preparing the reader for the more advanced chapters. Organometallic chemistry, surface science, biochemistry, nanoscience, transport phenomena and kinetics, reactor and reaction engineering are presented, spanning from the underlying science to industrial applications. Several important case studies on industrial applications are given. It includes catalyst preparation and characterisation and explores recent developments in the understanding of catalytic mechanisms, exploring advanced techniques such as operando spectroscopy.

Section VII

Reaction and Reactor Engineering

25 Reaction and Reactor Engineering^†

Jeroen Lauwaert^a, Joris W. Thybaut^*b, Alexandre Goguet^*c, Christopher Hardacre^*d, Oluwafunmilola Ola^e and Mercedes Maroto-Valer^*e

^a Ghent University, Industrial Catalysis and Adsorption Technology, Valentin Vaerwijckweg 1, 9000 Ghent, Belgium

^b Ghent University, Laboratory for Chemical Technology, Technologiepark 914, 9052 Ghent, Belgium

^c Queen’s University Belfast, CenTACatSchool of Chemistry and Chemical Engineering, David Keir Building, Stranmillis Road, Belfast, BT9 5AG, UK

^d The University of Manchester, School of Chemical Engineering & Analytical Science, The Mill (C15), Sackville Street, Manchester, M13 9PL, UK

^e Centre for Innovation in Carbon Capture and Storage (CICCS), School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK

^*E-mail: [email protected], [email protected], [email protected], [email protected]

Introduction

Chemical engineering aims, on the one hand, at simulating and predicting phenomena with respect to chemical reactions, such as intrinsic reaction kinetics, mass transport, sorption effects, thermodynamic and hydrodynamic phenomena and, on the other hand, at the design, construction and optimization of the corresponding reactors in which these reactions are performed. The present chapter starts with explaining how intrinsic reaction rates of chemical transformations occurring on a heterogeneous catalyst surface may be disguised by mass and heat transfer phenomena and how the occurrence of such limitations can be diagnosed. Subsequently, adsorption phenomena are described and it is explained how to account for them in a kinetic model. The third section of this chapter comprises a strategy to extrapolate gas phase kinetics towards liquid or three-phase reactions envisaging the up-scaling from ideal laboratory scale conditions to realistic commercial applications. Next, the focus moves from reaction towards reactor engineering. First the traditional reactor types, i.e., batch, semi-batch, plug flow and continuous stirred tank reactors, are discussed. Subsequently, microreactors, which are characterized by a much larger surface-to-volume ratio and, hence, exhibit an enhanced mass and heat transfer, are discussed. Finally, various methods of energy input are reported. Some specific reactor types such as monolith and membrane reactors, which are able to dramatically decrease the pressure drop, are discussed in more detail in the fifth section. The final section of this chapter aims at reactor and process design. It starts with a discussion on the hierarchical design strategy of chemical processes. Subsequently, reactor selection based on the specific boundaries of the indented application is addressed. The chapter wraps up with a discussion on the phenomena which should be accounted for while designing the selected reactor, i.e., capillary condensation, the catalyst wetting efficiency, the flow regime and axial and radial dispersion.

25.1 Diffusion Limitations in Heterogeneous Catalysis

Getting access to the effective rates of reactant conversion and product formation is crucial for reactor design for heterogeneously catalyzed reactions. The observed rates will be related to chemical transformations but, under certain circumstances, they may be controlled by physical phenomena. At the chemical level, several consecutive steps are involved and the ‘slowest’, i.e., the limiting one, will control the rate at which the overall reaction can proceed. This rate determines the intrinsic reaction rate. The physical phenomena that may disguise this intrinsic reaction rate are, e.g., transport of the reactants and products to and from the active sites.^1–4

The overall reaction rate takes into account all chemical and physical phenomena and constitutes the apparent rate. Consequently, two extreme cases can exist for a given reaction:

1. – The limiting step belongs to the chemical steps; in this case, the reaction is controlled by the chemistry and the intrinsic and apparent rates are the same.
2. – The limiting step belongs to the physical transport processes. The reaction is controlled by transport and the intrinsic and apparent rates are different.

The chemistry is, by definition, the same irrespective of the considered reactor type. As long as the conditions at the level of the catalytic active sites are identical, an identical intrinsic rate will be observed. The same does not hold true considering transport.

Transfer coefficients are affected by the geometry, e.g., the size of the catalyst particle and the velocity of the fluid. By direct consequence, also the transport rates will vary. This means that there can be as many apparent rates as there are different reactor configurations for the same chemistry using the same reaction conditions. The consequences of such behavior are far reaching for the extrapolation from laboratory to the commercial scale.

At the level of the laboratory, experiments typically aim at unravelling reaction mechanisms by interpreting the response of a catalyst to variations in the reaction conditions. Implicitly, such responses should correspond to responses of the chemical part of the process, since it is the chemical reaction mechanism that is investigated. If the reactor is operating at conditions where transport is limiting, it is clear that any conclusion with respect to the chemistry is very likely to be wrong.

During scale up, having access to intrinsic rate data is essential, since it is the only rate that is not scale dependent and the only one that can serve as a common ground. Once the intrinsic kinetics have been acquired, the apparent rate that will result from the change of scale can be determined by applying ...

Table of contents

• Cover
• Title
• Copyright
• Preface
• Contents
• Section I Introduction
• Section II Underlying Principles for Catalysis
• Section III Fundamental Catalysis
• Section IV Catalyst Preparation
• Section V Catalyst Characterization
• Section VI Operando Techniques
• Section VII Reaction and Reactor Engineering
• Section VIII Separation
• Section IX Process Intensification
• Section X Industrial Processes
• Section XI Life Cycle Analysis
• Subject Index