This chapter examines how to calculate the reactor volume and residence time of material balances, which can be used to design reactors when one reaction is taking place. It also compares the behaviour of the different reactors. For a given conversion, the volume of a plug flow reactor (PFR) will always be less than that of a continuous stirred tank reactor (CSTR) for a positive-order reaction. The PFR tends to operate at higher reactant concentrations than the CSTR, since, in the CSTR, instantaneous dilution with the product takes place. This means that for positive-order reactions, PFRs will exhibit higher overall rates of reaction and therefore will have lower volumes than CSTRs for a given conversion. The chapter then looks at the recycle reactor, in which fresh feed is mixed with a recycle stream and then fed to the PFR.
Calculation of reactor volume and residence time
Ian S. Metcalfe
Chemical Reaction Engineering covers the material required for a basic understanding of chemical reaction engineering. The first chapter introduces the topic. The next chapter considers the materials balance for chemical reactions. The chapter after that examines the calculation of reactor volume and residence time. The chapter that follows is about multiple reactions. The text also covers the energy balance and temperature effects. The last chapter is about non-ideal reactors.
Competing reactions in homogeneous systems
This chapter focuses on competing reactions in homogeneous systems. In process development, a good yield is often key to obtaining satisfactory material costs, purity, and robust operation. The essence of obtaining a good chemical yield is the understanding and control of those processes which compete with the desired reaction. This understanding is necessary to obtain a satisfactory laboratory process and, even more so, to scale-up the process for manufacture. The chapter then identifies the types of competing reaction. A pre-requisite for minimisation of side reactions is to know what they are. The commonest parameters used to manipulate absolute and relative reactivities are concentration, order of reactant addition, and temperature. The chapter also considers parallel/consecutive reactions; pH-rate and selectivity profiles; and methods for predicting reactivity.
This chapter considers cycloadditions as the most useful of all pericyclic reactions in organic synthesis. It describes the wide range of known cycloadditions, identifies the conditions under which they take place, draws attention to their regio- and stereochemistry, and gives the simple rules for which of them take place and which do not. It also mentions the most important type of cycloaddition, the Diels–Alder reaction, which is essentially the reaction between butadiene and ethylene which form cyclohexene. The chapter talks about the carbonyl group, which is a substituent attached to the periphery that affects the rate but does not change the fundamental nature of the reaction. It refers to dipoles which react with alkenes or alkynes, or with heteroatom-containing double and triple bonds that form heterocyclic rings.
Dispersion and mass transfer in multi - phase systems aqueous alkali
This chapter explores dispersion and mass transfer in multi-phase systems. Many reactions involve more than one phase, usually two, commonly three and even four or more. In order for the reaction to proceed, mass must transfer between these phases, and it is easy to overlook the fact that considerable effort may be required to distribute them well enough and promote mass transfer to a sufficient extent that the reaction can proceed at a reasonable rate. A gas bubbled through a liquid will not react at any meaningful rate unless well dispersed as small bubbles, and immiscible liquids will not react unless one is dispersed as fine droplets within the other. It is most common for fine chemical reactions to be carried out in a continuous liquid phase. The chapter then looks at phase boundary diagrams and phase inversion.
This chapter deals with electrocyclic reactions, which are characterized by the creation of a ring from an open-chain conjugated system. It illustrates the σ-bond-forming across the ends of the conjugated system and the opening of a σ-bond with the creation of a longer conjugated system. It also reviews the first few members of the series of neutral polyenes: the equilibria between butadiene and cyclobutene, between hexadiene and cyclohexadiene, and between octatetraene and cyclooctatriene. The chapter discusses the strained ring of the cyclobutene that makes a reaction take place in the ring-opening sense. It mentions the butadiene component in o-quinodimethane that undergoes ring-closure to the cyclobutene and the cyclohexadiene component in the cycloheptatriene that undergoes ring-opening to give the hexatriene.
The energy balance and temperature effects
This chapter demonstrates how reactors need not be isothermal. It looks at how reaction rate depends upon temperature for different classes of reaction, including irreversible reactions, reversible endothermic reactions, and reversible exothermic reactions. The chapter then formulates the energy balance for given reactors and uses this to investigate the variation of temperature and therefore reaction rate with time or position in the reactor. This, in turn, can be used to calculate reactor volumes and residence times for a given duty. The chapter also considers steady-state multiplicity in continuous stirred tank reactors (CSTRs) and multistage adiabatic plug flow reactor (PFR). For a PFR, there exists an optimum temperature profile or line of maximum reaction rate and it is important to try to approach this path. The chapter discusses two different methods for achieving this: interstage cooling and cold-shot cooling.
Equilibria in multiphase systems
This chapter addresses equilibria in multiphase systems. Many processes are operated under conditions where more than one phase is present. This is often due to economic considerations: when it is necessary to contact large water-insoluble reactants with inorganic reagents, reactions are frequently run with either a separate solid phase or else a solution phase containing the inorganic reagent. Workup processes frequently involve the washing of a solution of a water-insoluble product which is dissolved in an organic solvent such as toluene with water in order to remove inorganic residues. The chapter then looks at simple distribution equilibria; solubilities of ionisable substrates; liquid–liquid partition of ionisable substrates; and ternary phase diagrams.
Group transfer reactions
This chapter highlights common six-electron group transfer reactions, such as the broken π-bonds that allow the existing and new σ-bonds to complete a ring in the product. It emphasizes that the ene reaction is similar to the diene reaction but with only one double bond, while the metalla-ene reaction occurs when the atom transferred is a metal atom. It also describes the purely thermal aldol reaction of an enol and a ketone, which is identified as a hetero group transfer reaction. The chapter mentions how the ene reaction was proven to be particularly powerful in synthesis when carried out intramolecularly. It discusses the usual increase in rate for an intramolecular reaction that allows even unreactive partners like hydrocarbons to combine.
This introductory chapter outlines the scope and objectives of this book, and provides an overview of the knowledge required. This book is primarily intended for readers who have already taken a basic course in chemical kinetics. The reader should also have some knowledge of thermodynamics for an appreciation of reaction equilibria, heats of reaction, and energy balances. When material and energy balances for batch, CSTR, and PFR reactors are performed, reaction rate expressions will be required in an algebraic form. Reaction rate is usually expressed in terms of the concentrations or partial pressures of the reactants (and sometimes products) and may be determined empirically or may, in part, be based upon an understanding of the reaction mechanism. Ultimately, this book offers knowledge of the principles of chemical reaction engineering which will enable the reader to design basic chemical reactors.
Mass-transfer and reaction in two-phase systems
This chapter studies mass transfer and reaction in two-phase systems. Two-phase reaction systems are sometimes used because they provide an intrinsically better selectivity than is achievable under homogeneous conditions, but their use may also be enforced by the nature of the reactants, if they are mutually insoluble. For large scale use, water immiscible solvents are often preferred because of their ease of recovery. For practical purposes, there are two mechanisms by which reaction may occur. Uncatalysed or extractive reactions occur following simple partition of the reactive component from a source phase to one where it is reactive. Other processes, in which a catalyst is required to facilitate interphase transport of the reactive species, are known as phase-transfer catalysed reactions.
Material balances for chemical reactors
This chapter discusses how material balances should be performed for the three fundamental reactor types used in reaction engineering. These include the plug flow reactor (PFR), the continuous stirred tank reactor (CSTR), and the perfectly mixed batch reactor. In a batch reactor, all of the reactants are supplied to the reactor at the outset; there is no addition of reactants or removal of products during the reaction. Meanwhile, a PFR is a special type of tubular reactor. Feed is continuously supplied to the reactor and products are continually removed. The CSTR, like the PFR, has a continuous supply of feed while products are continually removed. However, in this case, perfect mixing is achieved, i.e. there are no concentration or temperature gradients within the reactor.
Mixing effects in pseudo homogeneous systems
This chapter assesses mixing effects in pseudo-homogeneous systems. The mixing regimes are classified as macromixing, mesomixing, or micromixing. The names reflect the different length and time scales of the dominating mechanisms. The macro scale related to the scale of the equipment, while the micro scale relates to the size of the smallest turbulent eddies. In order to be able to predict the effects of mixing on reaction selectivity during process development, it is necessary to appreciate a number of concepts and definitions used to characterise fluid flows. In tubular flow and in agitated vessels, the concepts of turbulent, laminar, and transitional flows are important. Laminar flow occurs when the fluid flow pattern is well defined, the fluid moves in streamlines with no movement lateral to the flow direction. Meanwhile, turbulent flow occurs when there is random motion imposed upon the main flow direction, with fluctuating motion in all directions.
This chapter evaluates how the design process must be modified when more than one reaction is occurring. It considers parallel reactions. Treatment must be different when there are reactions of mixed reaction rate orders proceeding at the same time, and the local or instantaneous selectivity depends upon the concentration of reactant. In such a case, the choice of reactor will influence the overall selectivity. For a continuous stirred tank reactor (CSTR), the overall selectivity is equal to the local selectivity as the local selectivity is a constant because of the lack of concentration gradients. Hence, calculation of the product distribution is straightforward. For a plug flow reactor (PFR) or batch reactor, it is more complex to evaluate the overall product distribution as the local selectivity or instantaneous selectivity will vary with position or time.
The nature of pericyclic reactions
This chapter describes pericyclic reactions. These reactions are a third distinct class and have cyclic transition structures in which all bond-forming and bond-breaking takes place in concert. The chapter discusses the three classifications of organic reaction—ionic, radical, and pericyclic—and analyses ionic reactions which involve pairs of electrons that move in one direction. It also mentions the unimolecular reaction, wherein the carbon–halogen bond cleaves with both electrons going to the chloride ion and leaves an electron deficiency behind on the carbocation. The chapter describes the nucleophile, which provides both electrons for a new bond, in contrast to the electrophile, which receives them. It covers radical reactions that involve the correlated movement of single electrons..
This chapter focuses on non-ideal reactors. In practice, plug flow and perfect mixing are never achieved. This is because of effects such as stagnant regions and ‘short circuiting’. In a tubular reactor, these stagnant regions will have a longer residence time than the rest of the process stream and mixing will occur within the stagnant regions. In stirred tank reactors, some elements might ‘short circuit’ or bypass the well-mixed bulk of the reactor. This will result in a larger fraction of the process stream having short residence times and there will be incomplete mixing of all the elements in the reactor. The chapter then looks at the concept of residence time distributions (RTDs). We can model a continuous reactor as a collection of small elements. The RTD is a distribution plot of the fraction of these elements exiting the reactor with different residence times.
Pericyclic Reactions starts with a chapter on the nature of pericyclic reactions and considers how important they are. The following chapter looks at cycloaddition reactions. The text thereafter examines the Woodward–Hoffmann rules and molecular orbitals. There follows a chapter on electrocyclic reactions. Towards the end, the book moves on to sigmatropic rearrangements before turning to group transfer reactions in the final chapter.
This chapter evaluates pre-reaction equilibria. Equilibria involving either organic or inorganic species can play an important part in influencing both the reaction and workup stage of organic syntheses. In the reaction stage, these equilibria can determine the proportion of reactive species present, which may not be the same as the total or stoichiometric concentration, and can thus influence both the rate and selectivity of the reaction. During workup, the same principles can determine the actual concentration of species involved in crystallisation or extraction processes. The chapter then looks at examples of the influence of pre-equilibria, as well as multiple ionisation in aqueous systems. It also considers homoconjugation, the situation where a product of an equilibrium reacts with the starting material. Finally, the chapter explains the more complex systems of nitrosation and diazotisation.
John H. Atherton and Keith J. Carpenter
Process Development starts off by looking at the scope of process development and strategies for process development, before moving onto pre-reaction equilibria. There are chapters on competing reactions in homogeneous media and mixing effects in pseudo-homogeneous systems. Next, the text covers equilibria in multiphase systems. It also explains dispersion and mass transfer in multiphase systems and mass-transfer and reaction in two-phase systems. Finally, the text looks at product isolation and workup and scale-up.
Product isolation and work-up
This chapter discusses product isolation and work-up. A laboratory multi-stage synthesis will often start from purified materials and include isolation and purification of intermediates at each stage. In contrast, in order to reduce waste for treatment and to contain equipment costs within reasonable limits, an industrial process will have fewer isolation stages and operate with crude intermediates as far as possible. The most popular isolation technique in a laboratory process is multiple crystallisation. At full scale, crystallisation is still a very common technique, but by no means always the best. The chapter then looks at the most common isolation technologies, distillation, liquid extraction, precipitation, and crystallisation. It does not describe them in detail, but the basic principles are explained in order to highlight the important issues and provide guidance for process development.