Part A: Multiphase reactors: chemical reaction engineering

1 Introduction

The purpose of this chapter is to introduce any reader to this book and into the subject of chemical reaction engineering (CRE), regardless of whether the reader is new to the subject, or whether the reader is experienced in reaction engineering.

1.1 Book introduction

There are many good and several truly excellent textbooks on multiphase reactors and CRE in general. This book, however, is different. It has more facets in its focus than the existing textbooks.

The first facet of this focus is on teaching students at universities (applied sciences and academic) on all levels: BSc, MSc, and PhD. Examples and exercises are provided at the end of each chapter, and Chapter 14 provides exercises for the reactor design. The second facet of this focus is on industrial practitioners in research development in process engineering and in operation. Industrial applications with their development trajectory are provided at that end. The third facet of the focus is on academic teachers and researchers. To that end, Part D with education guidelines and complete industrial cases for educational purposes are provided.

The approach is also different from other CRE textbooks. Although not an aim by itself, we wanted a book basically without equations beyond high school level. There is not a single differential equation to be found. The approach is on conceptual understanding, explaining the principles, the complex phenomena occurring in a reactor, and design and scale-up methodologies without resorting to mathematics. It goes without saying that we recognize and embrace the power of mathematics in translating (reaction engineering) concepts into clear and unambiguously defined models. We even have one chapter devoted to modeling.

However, in our experience, for quite a number of students and practitioners alike, the math actually forms a serious barrier. Even an ordinary differential equation can be such a barrier, let alone more lengthy derivations given in detail before ending at a final solution. The final solution may be simple enough and in fact what the practitioner will use for, for example, design or interpretation. What matters most from a CRE learning point of view is not the correctness of the math between model definition and that final equation, but a proper understanding of the concepts, their usefulness, as well as their limitations.

Furthermore, each theoretical subject is treated in such a way that industrial applications are at hand. A major portion of the book therefore comprises "real life" examples from the authors' own direct experience to illustrate how the theory is applied in practice.

The scope is on all important reaction engineering phenomena, residence time distribution (RTD), mass transfer, heat transfer, which affect reactor performance parameters, conversion, selectivity, and product quality. Specific attention is paid to energy management for safe reactor design and operation.

This understanding of phenomena and their relation to reactor performance is then used for elaborate descriptions of reactor type selection, design, modeling, and experimental validation methods. Moreover, we treat in quite some detail the process of "stage-gated innovation." Different strategies and ways of working are required for the different stages between ideation, development, de-risking, detailed engineering, procuring and contracting, and ultimately start-up and deployment. We provide elaborate explanations for most subjects because we think those subjects are important. Some subjects we treat concisely. This is the case if the subject is less important, or when the subject is explained well in another textbook, to which we refer to.

All of the above led to the organization of this book, with four Parts A, B, C, and D, as shown in Figure 1.1.

Figure 1.1: Book content structure.

Part A Introduction: This provides a general introduction on chemical reactor and reaction engineering and shows all major reactor types with their major features as pictures. So readers get a feeling of how multiphase reactors look like. So here already some vocabulary is built up.

Part B Fundamentals: This describes the theory around major phenomena affecting reactor performances and how design and modeling play a role.

Part C Stage-gate innovation methods: This describes methods and guidelines for reactor type selection, reactor modeling, reactor design, and experimental validation of models and designs, for each innovation stage, from ideation up to deployment, including development. The other innovation stages such as engineering, procurement, and construction, start-up, normal operation, and demolition are briefly described.

Part D Education: This provides guidelines, methods, and "real-life" cases for education in academia and industry.

The remainder of this introduction section leads the reader to reaction engineering by conversations. We finish this first chapter with our potted history of the development of CRE as a separate discipline.

1.2 A reaction engineer meets an electronic engineer

Reaction engineering is a spectacular discipline. Jan Harmsen recalls:

I still remember that I told my friend, an electronic engineer, about the effect of residence time distribution on reactor conversion. I told him: "Two reactors with the same feed composition, the same feed flow, the same temperature, the same pressure, and the same residence time of the fluid flowing through the reactor can have wildly different degrees of conversion, just due to different mixing pattern of the fluid."

He said: "you must be kidding."

I then explained him the effect of residence time distribution by just taking first a pipe reactor in which every fluid particle stays the same long time in the reactor reaching 99.999% conversion and then the other reactor, a mixed reactor, in which the average residence time of the fluid particles is the same as in the pipe reactor. In the mixed reactor however, some fluid particles directly swing to the outlet by the mixer blade, so without any conversion. The very deep conversion is not reached at all. "I see" he said.

1.3 Levenspiel's genius Problem 1.1

We treat here the wastewater treatment exercise case Problem 1.1 by the late Professor Octave Levenspiel to reveal a lot of important aspects of CRE and, in particular, of multiphase reaction engineering [1].

While still both working for Shell, we had designed from scratch a company internal course and delivered it many times in Amsterdam, Bangalore, Pernis, Bintulu, Doha, and Houston. It is called Industrial Reaction Engineering and Conceptual Process Design. The target audience is freshly recruited graduates with a master's degree or PhD in chemical engineering. The vast majority of this course comprises real-life case studies from the authors' own experiences. Deliberately, these case studies are quite different compared to "constructed" problems or examples typically practiced at universities.

Nevertheless, the course starts with Problem 1.1: the very first "constructed problem" from the classic Chemical Reaction Engineering textbook by Octave Levenspiel [1]. This is an easily underrated example of the educational genius that Levenspiel was. There are many lessons to be learned from this seemingly innocent student problem.

So we go step by step through the exercise. Prior to coming to our course, the participants are asked to read the short introduction of Levenspiel's book and then solve Problem 1.1 "Municipal wastewater treatment plant." This is their "tiny bit of homework." We start the course with compiling and discussing all the answers.

The problem stated seems simple enough:

Most people in our course, chemists and chemical engineers alike, calculate the rate of reaction by simply taking the difference in biological oxygen demand (BOD) between inlet and outlet concentration and divide that by the 8 h mean residence time. So they calculate the reaction rate to be, for example, 2.17 × 10-4. Some treat the given wastewater flow rate as superfluous information; others use it to determine the tank volume. Most folks - many of them are experienced chemists/engineers - mention not only a number but also a unit (dimension), for example, mol/s m3 belonging to the number above, or they convert it into another unit for time or volume.

The first impression might be that this example is all about the importance of explicitly mentioning the units when communicating, for example, a rate of reaction to colleagues. Levenspiel's introduction before Problem 1.1...