Introduction

Chapters 10 and 11 address flows in which substantial changes in density occur. The changes arise from processes which are dynamical (e.g. density changes from pressure variations associated with fluid accelerations) or thermodynamic (density changes primarily from bulk heat addition due to chemical reaction or phase change) or a combination of the two. This chapter focuses primarily on situations with density variations due to dynamical effects; as we saw in Section 2.2, this means flows with Mach numbers significant compared to unity. Chapter 11 discusses flows with density variations primarily due to heat addition.

Much of the material is based on quasi-one-dimensional gas dynamics. Characterization of quasione- dimensional analysis as “the secret weapon of the internal fluid dynamicist” (Heiser, 1995) is an apt aphorism indeed. This type of treatment enables useful engineering estimates in a wide variety of situations and is a powerful tool for providing insight into the response of compressible flows to alterations in area, addition of mass, momentum, and energy, swirl, and flow non-uniformity. This is true not only for simple duct and channel flows but also for more complex problems, for example those arising in the matching of gas turbine engine components (Kerrebrock, 1992; Cumpsty, 1998).

Many computational techniques now exist to address internal flows in complex geometries. As such, we spend little time in discussion of approximations that were necessary in the past to attack compressible flow problems. One-dimensional analysis, however, is still very much a part of modern approaches to grappling with internal flow problems, even though its use as a detailed design tool has been supplanted by more accurate computations.

Introduction

Unsteady flow phenomena are important in fluid systems for several reasons. First is the capability for changes in the stagnation pressure and temperature of a fluid particle; the primary work interaction in a turbomachine is due to the presence of unsteady pressure fluctuations associated with the moving blades. A second reason for interest is associated with wave-like or oscillatory behavior, which enables a greatly increased influence of upstream interaction and component coupling through propagation of disturbances. The amplitude of these oscillations, which is set by the unsteady response of the fluid system to imposed disturbances, can be a limiting factor in defining operational regimes for many devices. A final reason is the potential for fluid instability, or self-excited oscillatory motion, either on a local (component) or global (fluid system) scale. Investigation of the conditions for which instability can occur is inherently an unsteady flow problem.

Unsteady flows have features quite different than those encountered in steady fluid motions. To address them Chapter 6 develops concepts and tools for unsteady flow problems.

The inherent unsteadiness of fluid machinery

To introduce the role unsteadiness plays in fluid machinery, consider flow through an adiabatic, frictionless turbomachine, as shown in Figure 6.1 (Dean, 1959). At the inlet and outlet of the device, and at the location where the work is transferred (by means of a shaft, say), conditions are such that the flow can be regarded as steady. We also restrict discussion to situations in which the average state of the fluid within the control volume is not changing with time.

Introduction

In this chapter, we discuss the types of thin shear layers that occur in flows in which the Reynolds number is large. The first of these is the boundary layer, or region near a solid boundary where viscous effects have reduced the velocity below the free-stream value. The reduced velocity in the boundary layer implies, as mentioned in Chapter 2, a decrease in the capacity of a channel or duct to carry flow and one effect of the boundary layer is that it acts as a blockage in the channel. Calculation of the magnitude of this blockage and the influence on the flow external to the boundary layer is one issue addressed in this chapter. Boundary layer flows are also associated with a dissipation of mechanical energy which manifests itself as a loss or inefficiency of the fluid process. Estimation of these losses is a focus of Chapter 5. The role of boundary layer blockage and loss in fluid machinery performance is critical; for a compressor or pump, for example, blockage is directly related to pressure rise capability and boundary layer losses are a determinant of peak efficiency that can be obtained.

Another type of shear layer is the free shear layer or mixing layer, which forms the transition region between two streams of differing velocity. Examples are jet or nozzle exhausts, mixing ducts in a jet engine, sudden expansions, and ejectors. In such applications the streams are often parallel so the static pressure can be regarded as uniform, but the velocity varies in the direction normal to the stream.

Objectives

1. Identify the three major driving forces for membrane separations.

2. Define permeability, permeance, selectivity, and rejection.

3. List the transport mechanisms for membrane separations.

4. List some environmental applications of each type of membrane separation.

5. Describe the advantages and disadvantages of membrane technology.

Membrane definition

A membrane can be defined as [1]:

There are three important points to note with respect to this definition. First, a membrane is defined by what it does (function), not by what it is. So, a wide range of materials are potentially useful as membranes. Second, the membrane separation mechanism is not specified. So, again there could be several choices.

Objectives

1. Define the concepts of mass transfer zone, breakthrough, and exhaustion.

2. Use the scale-up approach and the kinetic approach to design fixed-bed adsorption columns based on laboratory or pilot column data.

Background

Adsorption is a process whereby a substance (adsorbate, or sorbate) is accumulated on the surface of a solid (adsorbent, or sorbent). The adsorbate can be in a gas or liquid phase. The driving force for adsorption is unsaturated forces at the solid surface which can form bonds with the adsorbate. These forces are typically electrostatic or van der Waals interactions (reversible). Stronger interactions involve direct electron transfer between the sorbate and the sorbent (irreversible). The strength of this interaction dictates the relative ease or difficulty in removing (desorbing) the adsorbate for adsorbent regeneration and adsorbate recovery. The selective nature of the adsorbent is primarily due to the relative access and strength of the surface interaction for one component in a feed mixture. The solid is the mass-separating agent and the separating mechanism is the partitioning between the fluid and solid phases. An energy-separating agent, typically a pressure or temperature change, is used to reverse the process and regenerate the sorbent.

Adsorption processes are used economically in a wide variety of separations in the chemical process industries. Activated carbon is the most common adsorbent, with annual worldwide sales estimated at $380 million [1].

Separation – the process of separating one or more constituents out from a mixture – is a critical component of almost every facet of chemicals in our environment, whether it is remediation of existing polluted water or soil, treatment of effluents from existing chemical processes to minimize discharges to the environment, or modifications to chemical processes to reduce or eliminate the environmental impact (chemically benign processing). Having said this, there is no text today for this subject which describes conventional processing approaches (extraction, ion exchange, etc.) as well as newer techniques (membranes) to attack the serious environmental problems that cannot be adequately treated with conventional approaches. Existing texts for this subject primarily focus on wastewater treatment using technology that will not be suitable in the larger context of environmental separations. Interestingly, most chemical engineering texts on separations technology are primarily based on whether the separation is equilibrium or rate based. Thus, it is difficult to find one source for separations technology in general.

This text is meant as an introduction to chemical separations in general and various specific separations technologies. In Chapter 1 we give a generalized definition of separation processes and their environmental applications. Following this, the approach to the organization of this text is to first discuss, in Chapter 2, the generic aspects of separations technology as unit operations.

The information in the previous chapter provides an important introduction to the environmental applications of chemical separations technology. This chapter will be devoted to an introductory description of the concept and analysis of a unit operation as applied to separation processes. Subsequent chapters will present some necessary fundamentals of separations analysis and discuss specific separation methods.

Objectives

1. Define the concept of a unit operation and state the design significance.

2. Describe the two basic mechanisms for separations.

3. Discuss factors important in selecting an exploitable property difference.

4. Give examples of equilibrium and rate properties that are used as the basis for separation.

5. Give examples of mass- and energy-separating agents.

6. List the two ways that a separating agent is used to obtain a different compound distribution between two phases.

7. List the four ways that separating agents generate selectivity.

8. Discuss the applications of reversible chemical complexation to separations.

9. Define cocurrent and countercurrent operation.

10. List factors important to the selection of a particular separation process for a given application.

11. List several reasons for implementing a unified view of separations technologies.

Unit operations

Initially, it is useful to introduce the concept of a unit operation and explain how it relates to chemical separations. Figure 2.1 shows a generic unit operation in which a feed stream is separated into two exit streams with different compositions by means of a separating agent. Multiple feed streams into a process and multiple exit streams are also possible.