Energy is needed to separate a mixture into pure compounds. Some separation operations are highly energy consuming; others are less so. But all separations require a minimum amount of energy. Section 10.1 introduces the concept of minimum energy required for separation, independent of the process employed. This section then illustrates the minimum energy required in various separation processes such as membrane gas permeation, distillation, extraction and adsorption. How to reduce the amount of energy needed for a given separation by particular separation processes is the general topic of Section 10.2. The specific separation processes and topics covered in this section include: evaporation of water for desalination by multiple-effect evaporation, multistage flash evaporation, vapor-compression evaporation; distillation based separation of mixtures by multieffect and heat pump configuration. The role of free energy of mixing in influencing the energy requirement for separation is briefly considered here. Finally, the special considerations needed for processing dilute solutions of solutes of interest are discussed in the context of reducing the total energy required.

Separations in which the feed phase or feed-containing phase has a bulk motion parallel to the direction of the force causing separation have been studied in Chapter 6. In many separation techniques/processes/operations, the feed phase or feed-containing phase has a bulk motion perpendicular to the direction of the driving force. In a number of situations, the feed is introduced in small amounts in a carrier fluid whose bulk motion is perpendicular to the force direction. Such separations will be studied in this chapter.

For separations based on distribution of species between two phases in equilibrium, we study in Section 7.1 primarily those two-phase systems where the second phase (e.g. adsorbent particles in a packed adsorbent bed) is stationary; the first phase, which is more often the feed solution/mixture, moves perpendicular to the direction of chemical potential driving force between the two immiscible phases (Figure 7.0.1(a)). This first phase (the mobile phase) bulk motion is generally in one direction. The benefits of such bulk motion in terms of extreme purification achievable in two-component systems and multicomponent separation capability will be illustrated. In the cyclic processes studied next, the direction of motion of the mobile phase is periodically reversed; the direction of force is also reversed, except it remains perpendicular to the bulk-phase flow direction (Figures 7.0.1(b) and (c)). The mobile phase is sometimes generated by the separation operation, as in the case of the blowdown phase of pressure swing adsorption (PSA). The elution chromatographic process considered next involves injection of a sample to be separated into a carrier fluid flowing perpendicular to the direction of the force between the fluid and the stationary adsorbent phase (Figure 7.0.1(d)); this and other related chromatographic processes are also studied in Section 7.1.5. The imposition of an electrical force parallel to the direction of flow in a packed chromatographic column is studied in Section 7.1.6 in what is called counteracting chromatographic processes.

We have studied in Section 8.1 how a number of double-entry separation stages can be connected together in a countercurrent fashion. We have also learnt that such an arrangement achieves a far greater extent of separation in separation processes such as distillation, extraction, absorption and adsorption compared to what can be achieved in a single stage. Such multistage configurations are generally identified as “cascades,” and in the case of configurations of an overall countercurrent flow as “countercurrent cascades.” Such countercurrent cascades can have significant variations, for example ideal cascades, squared off cascades, tapered cascades, etc. Further, there can be novel countercurrent cascades with single-entry separation stages. In addition, there can be other types of connections between a number of single stages, resulting in, for example, crosscurrent cascades, two-dimensional cascades, etc.

We will briefly focus in Section 9.1 of this chapter on many such configurations of cascades, beginning with countercurrent cascades. Special attention will be paid to single-entry separation stages in, for example, the configuration of an ideal cascade. These cascades are generally employed to enhance substantially the separation achieved in a single stage for a binary mixture. Such cascades can also reduce the amount of energy or mass-separating agent needed for the separation of a binary mixture. In the separation of a multicomponent mixture by distillation, a number of such cascades may be joined together in particular ways – Section 9.2 will provide a brief introduction to this topic. Section 9.3 briefly considers cascades for multicomponent mixture separation involving other separation processes.

The area of separations is extremely broad. Most separation processes/techniques have an extensive literature. Each process has its own universe. Yet there are a few common principles and characteristics that are shared by many separation techniques/processes. This book has provided an elementary introduction to such principles and characteristics, even as it provides some details of many of the most commonly used separation processes. It is hoped that this book will help the reader to be prepared to undertake more extensive studies of individual processes/techniques or of a certain class of techniques or certain sequences of separation processes/techniques.

An important goal of this book is to develop an appreciation that separation processes/techniques may be understood on the basis of a few key features/concepts: the nature of the force causing the selective movement of molecules, macromolecules and particles; the nature of the various regions/phases in the separation system; the four types of bulk flow pattern of the regions/phases vis-à-vis the direction(s) of the forces causing selective movement of species/particles to be separated; the method of feed introduction. Further, for relatively simple systems, one could develop the governing equations that describe the separation achievable from the set of basic governing equations provided in Section 6.2 for most systems described in the book. In addition, a useful introduction to the role played by chemical reactions of certain types for a variety of separations has been provided.

Chemical reactions occur in many commonly practiced separation processes. By chemical reactions, we mean those molecular interactions in which a new species results (Prausnitz et al., 1986). In a few processes, there will be hardly any separation without a chemical reaction (e.g. isotope exchange processes). In some other processes, chemical reactions enhance the extent of separation considerably (e.g. scrubbing of acid gases with alkaline absorbent solutions, solvent extraction with complexing agents). In still others, chemical reactions happen whether intended or unintended; estimation of the extent of separation requires consideration of the reaction. For example, in solvent extraction of organic acids, the extent of acid dissociation in the aqueous phase at a given pH should be taken into account (Treybal, 1963, pp. 38–41). Chemical equilibrium has a secondary role here, yet sometimes it is crucial to separation.

Familiarity with the role of chemical reactions in separation processes will be helpful in many ways. This is especially relevant since a few particular types of chemical reactions occur repeatedly in different separation processes/techniques. These include ionization reactions, acid–base reactions and various types of complexation reactions. The complexation reactions also include the weaker noncovalent low binding energy based bonding/interactions identified in Section 4.1.9. A better understanding and quantitative prediction of separation in a given process is possible, leading to better process and equipment design. In processes where a chemical agent is used, different agents can be evaluated systematically. On occasions, it may facilitate the introduction of reactions to processes for enhancing separation.

In this chapter, we cover those separation processes/techniques where two bulk phases or regions flow through the separation device such that the force(s) driving the separation act perpendicular to the directions of flow of both phases/regions. Section 8.1 describes those separation devices where the two phases or regions have bulk motions countercurrent to each other in the separation device. This section also covers those larger multistage separation devices where the overall pattern of flow vs. force corresponds to countercurrent flow, even through the local flow configurations in a stage may utilize other flow vs. force arrangements for the two flowing phases. Invariably, these separations will be studied under continuous flow of two phases/regions and steady state conditions. The focus will be primarily on phase equilibrium based processes driven by chemical potential gradient such as gas absorption, distillation, solvent extraction, melt crystallization and adsorption. Limited attention has been paid to membrane based processes such as dialysis, electrodialysis, liquid membrane processes, gas permeation and external force driven processes of gas centrifuge (thermal diffusion and mass diffusion are also considered here). Section 8.2 will be concerned with separations where the two phases or regions flow in a cocurrent fashion. Continuous flow of two phases/regions as well as discontinuous flow of one of the phases will be covered. Section 8.3 will focus on those configurations where the two bulk phases are in crossflow in the device. The force(s) will continue to act perpendicular to the flow directions of the two bulk phases in both Sections 8.2 and 8.3.

The basic objective of this chapter is to describe the organization of this book vis-à-vis separations from a chemical engineering perspective. Separation, sometimes identified as concentration, enrichment or purification, is employed widely in large industrial-scale as well as small laboratory-scale processes. Here we refer primarily to physical separation methods. However, chemical reactions, especially reversible ones, can enhance separation and have therefore received significant attention in this book. Further, we have considered not only separation of mixtures of molecules, but also mixtures of particles and macromolecules.

The number of different separation processes, methods and techniques is very large. Further new techniques or variations of older techniques keep appearing in industries, old and new. The potential for the emergence of new techniques is very high. Therefore, the approach taken in this book is focused on understanding the basic concepts of separation. Such an approach is expected not only to help develop a better understanding of common separation processes, but also to lay the foundation for deciphering emerging separation processes/techniques. The level of treatment of an individual separation process is generally elementary. Traditional equilibrium based separation processes have received considerable but not overwhelming attention. Many other emerging processes, as well as established processes dealing with particles and external forces, are not usually taught to chemical engineering students; these are integral parts of this book. To facilitate the analysis of processes over such a broad canvas, a somewhat generalized structure has been provided. This includes a core set of equations of change for species concentration, particle population and particle trajectory. These equations are expected to be quite useful in general; however, separation systems are quite often very complicated, thereby limiting the direct utilization of such equations.

In Section 3.3, we illustrated the thermodynamic relations that govern the conditions of equilibrium distribution of a species between two or more immiscible phases under thermodynamic equilibrium. In Section 4.1, we focus on the value of the separation factor or other separation indices for two or more species present in a variety of two-phase separation systems under thermodynamic equilibrium in a closed vessel. The closed vessels of Figure 1.1.2 are appropriate for such equilibrium separation calculations. There is no bulk or diffusive flow into or out of the system in the closed vessel. The processes achieving such separations are called equilibrium separation processes. Separations based on such phenomena in an open vessel with bulk flow in and out are studied in Chapters 6, 7 and 8. No chemical reactions are considered here; however, partitioning between a bulk fluid phase and an individual molecule/macromolecule or collection of molecules for noncovalent solute binding has been touched upon here. The effects of chemical reactions are treated in Chapter 5. Partitioning of one species between two phases is an important aspect ever present in this section.

The criteria for thermodynamic equilibrium in a single-phase system in a closed vessel subjected to an external force field were also developed in Section 3.3. Based on these criteria, we develop in Section 4.2 estimates of the separation achieved in a single phase in the closed vessel. These estimates are also developed in a closed vessel when an additional property gradient, e.g. density gradient, pH gradient, etc., exists across the vessel length. Focusing is the term often used to characterize the latter separation techniques. In this section, we cover in addition the extent of separation achieved when a temperature gradient is imposed on a single-phase system in a closed vessel not subjected to any external force field.

Separation is a major activity of chemical engineers and chemists. To separate a mixture of two or more substances, various operations called separation processes are utilized. Before we understand how a mixture can be separated using a given separation process, we should be able to describe the amount of separation obtained in any given operation. This chapter and Chapter 2 therefore deal with qualitative and quantitative descriptions of separation. Chapter 2 covers open systems; this chapter describes separations in a closed system.

In Section 1.1, we briefly illustrate the meaning of separation between two regions for a system of two components in a closed vessel. Section 1.2 extends this to a multicomponent system. In Section 1.3, various definitions of compositions and concentrations are given for a two-component system. In Section 1.4, we are concerned with describing the various indices of separation and their interrelationships for a two-region, two-component separation system. A number of such indices are compared with regard to their capacity to describe separation in Section 1.5 for a binary system. Next, Section 1.6 briefly considers the definitions of compositions and indices of separation for the description of separation in a multicomponent system between two regions in a closed vessel. Finally, Section 1.7 briefly describes some terms that are frequently encountered.

We have studied a variety of separation processes and techniques. Our focus was on developing an elementary understanding of an individual separation process/technique. In practice, more often than not, a combination of more than one separation process is employed, regardless of the scale of operation involved. Here we introduce very briefly the separation sequences employed in a few specific industries. The separation sequences of interest are considered under the following headings: bioseparations (Section 11.1); water treatment (Section 11.2); chemical and petrochemical industries (Section 11.3); hydrometallurgical processes (Section 11.4). It is to be noted here that often the separation sequences are reinforced by chemical reactions within such a sequence or before/after the separation steps. The intent here is to provide an elementary view of the complexity and demands of practical systems where certain types of separation sequences are crucial/primary/dominant components.

More often than not, we will find that certain types of separation techniques and processes are much more prevalent in certain industries. For example, solvent extraction and back extraction processes are dominant in the recovery and purification of metals and metallic compounds via hydrometallurgical processes. On the other hand, distillation and, to a much lesser extent, absorption/stripping followed by solvent extraction are the primary separation processes in the chemical/petrochemical industries. Water treatment industries/plants are however, focused much more on deactivation/removal of biological contaminants, suspended materials and dissolved impurities from water via oxidation processes, filtration, membrane techniques and ion exchange processes. Biological separations share some of these characteristics of water treatment processes in terms of the separation techniques; however, since the focus is on recovering/purifying the biologically relevant compound, processes such as chromatography are in great demand.

No explicit information survives about the date when Samia was produced, and we are forced to rely on internal evidence. This has led different analysts to somewhat different conclusions, often based on rather equivocal evidence, but nearly always placing the play somewhere in the first half of Menander's career.

The most cogent evidence for an early date is the presence of satirical references to three contemporary Athenians, Diomnestus, Chaerephon and Androcles (504, 603, 606–8nn.). There are no such references in any of the other plays of Menander preserved on papyri (not even in the virtually complete, and relatively early, Dyskolos), and only ten in the ‘book’ fragments. Of these ten, four (frr. 264–6, 268) come from Orge, Menander's first play; two (frr. 224, 225) from Methe, which must date from 318 or earlier; one from Androgynos (fr. 55), also likely to be early, since it refers (fr. 51) to the Lamian war of 322; and two from Kekryphalos (frr. 215, 216), which probably dates from the first year or two of Demetrius of Phalerum's rule. Thus, apart from Samia, we have only one possible reference to a contemporary Athenian in a play that may be later than c.315 – fr. 385, from Hypobolimaios, which mentions one Amphietides, whose name had become proverbial for imbecility (and who may therefore not have been a contemporary at all). This is very strong evidence that Samia was produced no later than c.314 – that is, in the first seven years or so of Menander's career – and, in the absence of any objective evidence requiring a later date, this evidence should be accepted.

Social Movements and Protest introduces social science students to the shifting terrain of ‘social movement studies’, providing them with a chronology of the field and bringing them up to date with conceptual developments. The book is first and foremost for students studying at the undergraduate level. It aims to encourage them to critically engage with the different ways in which social movements have been conceptualized and researched, and to evaluate the strengths and weaknesses of different approaches. The book argues that critical developments in the field have moved in the direction of a ‘relational approach’ to social movements, which it draws out and supports across the various chapters. It also argues that by engaging with ‘new cases and new contexts’ – ranging from lifestyle movements and terrorism, to globalization, new media, and protest in authoritarian regimes – we can stretch and problematize existing conceptualizations of social movements in ways that force us to rethink ‘what social movements are’. In this respect, the book raises four main issues: the necessity of strains, resources, and organization; the centrality of the state; the desirability of collective identity; and the distinction between ‘unorganized, individual action’ and ‘organized, collective action’.