Chemical Engineering: An Introduction

Citation

Denn, M. M. (2011). Chemical Engineering: An Introduction. Cambridge University Press (Cambridge Series in Chemical Engineering).

Chapter Summary

The book is short: fifteen chapters on about 255 pages, framed by a preface and a two-page postface. In the preface, Denn sets out the role of the introductory course: to carry students from basic science and mathematics into the engineering science courses at the core of a chemical engineering degree, and to give them an overview of the profession, built around rate processes in settings that resemble real practice. The book restricts itself mostly to liquid-phase processes and uses mathematics, in the publisher's words, "as a language at the most elementary level".

Chapter 1: Chemical Engineering

The opening chapter defines chemical engineering as the field of applied science that employs physical, chemical and biochemical rate processes for the betterment of humanity, and unpacks both halves of that definition. Its first illustration of a rate process is a drug taken orally, followed from digestion through the bloodstream to metabolism and excretion. It argues that the popular image of chemical engineers making petrochemicals describes only a small part of the modern profession.

Chapter 2: Basic Concepts of Analysis

This chapter introduces modelling as the core activity: describe the system with equations, predict, compare with reality, revise, and then use the model for design. The method is demonstrated on a single simple process unit with an overall mass balance.

Chapters 3–4: The Balance Equation; Component Mass Balances

Chapter 3 generalises the balance equation to quantities that are not conserved, such as a drug that is metabolised in the body. Its worked application is economic: net present worth as a balance on money, used to find the true cost of future expenditure. Chapter 4 extends mass balances to well-stirred, single-phase systems of several non-reacting species, using salt and water as the example.

Chapter 5: Membrane Separation

The first separation process in the book. The chapter places separations in physiology and industry, from the lungs and kidneys to ethanol recovery and oxygen production, and then analyses membrane processes, in which a thin membrane keeps two streams apart while letting selected species pass. The publisher lists haemodialysis among the book's problems.

Chapters 6–7: Chemically Reacting Systems; Designing Reactors

Chapter 6 develops reaction kinetics in single-phase reactors of simple geometry. Chapter 7 uses design in the process sense, choosing what equipment is needed and how large, and sizes reactors and flow rates from the kinetics of Chapter 6. Denn notes that full process design is normally a final-year capstone course; here it appears early in simplified form, with economic trade-offs between capital and operating cost.

Chapters 8–9: Bioreactors and Nonlinear Systems; Overcoming Equilibrium

Chapter 8 uses the activated sludge process for wastewater treatment as the framework for bioreactor design and its nonlinear behaviour; washout of the microorganisms is one of the problems the publisher highlights. Chapter 9, only four pages long, shows how a membrane reactor removes product as it forms and so achieves conversions beyond the equilibrium limit. Reactive distillation is mentioned but left for later courses because it needs vapour–liquid equilibrium.

Chapters 10–11: Two-Phase Systems and Interfacial Mass Transfer; Equilibrium Staged Processes

Chapter 10 classifies two-phase systems, explains when a mixture can be treated as one continuous phase and when it cannot, and develops mass transfer between phases and the equilibrium stage. Chapter 11 applies the equilibrium stage to liquid–liquid extraction with immiscible solvents. It computes the number of stages needed for a given separation and leaves the mechanical design of contactors aside.

Chapters 12–15: Energy Balances; Heat Exchange; Energy Balances for Multicomponent Systems; Energy Balances for Reacting Systems

The last third of the book adds energy as a variable. Chapter 12 introduces energy balances, Chapter 13 treats heat exchange, and Chapter 14 handles mixtures. Denn warns there that published energy balances are often wrong and cites a commercial reactor model whose error guaranteed a wrongly computed hot spot. Chapter 15 brings in the temperature dependence of reaction rates for well-mixed liquid reactors.

Postface

A short closing note on the courses that follow in a chemical engineering degree and on the value of undergraduate research.

Key Concepts

Chapters 1–4: Foundations

• Rate processes: physical, chemical and biochemical processes that proceed at finite rates, which Denn treats as the defining paradigm of the discipline.
• Modelling: turning a physical situation into equations, testing predictions against reality and revising before using the model to design.
• Balance equation: accumulation equals input minus output plus net generation, applicable to conserved and non-conserved quantities alike.
• Net present worth: the balance equation applied to money, to compare costs that fall at different times.

Chapters 5–9: Separations and reactors

• Membrane separation: selective transport across a thin barrier between two fluid streams.
• Reactor sizing: using kinetics to fix reactor volume and flow rate for a required conversion.
• Washout: loss of the microbial population from a continuous bioreactor when it is diluted faster than it can grow.
• Beating equilibrium: removing product during reaction, as in a membrane reactor, so the reverse reaction cannot proceed.

Chapters 10–15: Multiphase systems and energy

• Interfacial mass transfer: movement of a component between phases towards an equilibrium distribution.
• Equilibrium stage: an idealised contactor whose outlet streams are in equilibrium, used to count stages for a separation.
• Energy balance: the first law applied to process units, including mixing and reaction effects.
• Temperature-dependent kinetics: the strong effect of temperature on reaction rates, a key handle in reactor design.

Critical Analysis

Strengths

• Shows what the discipline does. The preface frames the course around rate processes in realistic settings, and the publisher describes the design and analysis problems as realistic in format and scope. John H. Seinfeld, in the AIChE Journal, called the book a compact introduction to material and energy balances that conveys the kinds of problems chemical engineers grapple with.
• Modern examples. Drug metabolism, haemodialysis, wastewater bioreactors and membrane reactors stand alongside reactors and extraction. A Chemistry World review found the use of newer operations such as membrane separations to illustrate traditional concepts appealing and judged the book modern and concise.
• Design early. Reactor sizing with cost trade-offs (Chapter 7) and stage counting (Chapter 11) bring design decisions into a first course, a step the author says is usually postponed to the capstone.
• Careful about energy balances. Chapter 14 explicitly warns against shortcuts that make multicomponent energy balances wrong. That care is useful because the errors it describes appear in published sources.

Limitations

• Demands well-prepared students. David L. Silverstein, reviewing it in Chemical Engineering Education, qualified his praise with "if your students are well-prepared". Students who are shaky in calculus or chemistry may find the pace of the modelling chapters demanding.
• Liquid phase and simplified physics. The book deliberately restricts itself mostly to liquids and well-mixed vessels and leaves out vapour–liquid equilibrium and reactive distillation. Gas-phase stoichiometry, equations of state and phase equilibria of the kind treated in a standard material-and-energy-balance course are not its focus.
• Uneven chapter lengths. Some chapters are very short (Chapter 9 runs four pages, Chapter 3 six), while the analysis chapter runs to nearly forty. Instructors may need to supplement the shorter ones with problems.

Where It Fits in the Course

Denn and Felder, Rousseau and Bullard's Elementary Principles of Chemical Processes are the two introductory books in the chemical process engineering list, and they suit different purposes. Felder is a full textbook for the systematic material and energy balance course, with units, process variables, gas and phase-equilibrium behaviour and many problems. Denn is a compact book about modelling and the scope of the profession. Students on a balance-based first course should treat Felder as the main text and read Denn alongside it for perspective. Courses that open with a broad survey of the discipline can use Denn as the core and add Felder for practice.

Each part of Denn previews a later course on the list. The separation chapters lead to McCabe, Smith and Harriott's Unit Operations of Chemical Engineering and Geankoplis, Hersel and Lepek's Transport Processes and Separation Process Principles. The reactor chapters lead to Fogler's Elements of Chemical Reaction Engineering, and the design-and-cost thread leads to Towler and Sinnott's Chemical Engineering Design, which is written for senior design work.

How to Use This Book

The book is aimed at first-semester chemical and biochemical engineering students. It also suits scientists and engineers who work alongside chemical engineers: the Chemistry World review recommended it to that audience. First-year calculus and general chemistry are the working background.

• Core: Chapters 1–4 establish the modelling method and the balance equation. Every later chapter depends on them, so work through the analysis chapter carefully.
• Mass-balance applications: Chapters 5–9 can be taken as a set of design studies: separations, reactors, bioreactors and the membrane reactor. A short course can choose two or three of them.
• Multiphase and energy: Chapters 10–11 lead towards separations courses, and Chapters 12–15 introduce energy balances. Read Chapters 12 and 14 together if time is short, keeping in mind the warning about incorrect shortcuts.
• Postface: read it at the end; it explains how the rest of the curriculum builds on these chapters.

For each model, write down the assumptions (well mixed, single phase, constant density) before solving. Then ask which one would fail first in a real plant; this is the habit the analysis chapter sets out to build. Students who need more drill in standard balance calculations should pair the book with Felder.