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.

The influence and importance of numerical models and simulations in science and engineering as appropriate tools for:

• analysis of engineering processes, as well as for

• conception and design of processes, and the

• development and analysis of control mechanisms,

has rapidly increased in recent years. In some technical research and development areas, simulation has been employed as an important contribution, given identical ranking as experiment and theory. This influence is valid not only in universities and research laboratories, but also in industry.

This technical progress of numerical simulation tools is based on ongoing rapid developments that have been achieved in hardware and numerics, and also on some important developments in modelling of physical and technical processes. These models can be incorporated and implemented into simulation codes that become easy to use. In recent developments in this area, similar success compared to experimental or physical measurement techniques have been achieved.

The possibility of using a simulation model to decouple some of the physical effects and mechanisms involved in a complex technical process, which may only be sequentially analysed by experimental means, highlights the potential of this new analytical approach. Here physical understanding of complex processes may be derived and used to optimize and develop processes. This contributes not only to scientific understanding, but also to economic and ecological technical innovations.

As an example of modelling and numerical process simulation, in this book fluid atomization processes and the spray forming of metals have been investigated, with particular reference to transport and exchange processes within multiphase flow, including momentum, heat and mass transfer.

In this chapter, fundamental features of the metal spray forming process are introduced in terms of their science and applications. Chapter 1 saw the division of the process into three main steps:

1. disintegration (or atomization),

2. spray establishment, and

3. compaction.

Now, a more detailed introduction to those subprocesses that are especially important for application within the spray forming process, will be given.

The spray forming process

Spray forming is a metallurgical process that combines the main advantages of the two classical approaches to base manufacturing of sophisticated materials and preforms, i.e.:

• metal casting: involving high-volume production and near-net shape forming,

• powder metallurgy: involving near-net shape forming (at small volumes) to yield a homogeneous, fine-grained microstructure.

The spray forming process essentially combines atomization and spraying of a metal melt with the consolidation and compaction of the sprayed mass on a substrate. A typical technical plant sketch and systematic scheme of the spray forming process (as realized within several technical facilities and within the pilot-plant-scale facilities at the University of Bremen, which will be mainly referenced here) is illustrated in Figure 2.1. In the context of spray forming, a metallurgically prepared and premixed metal melt is distributed from the melting crucible via a tundish into the atomization area. Here, in most applications, inert gas jets with high kinetic energy impinge onto the metal stream and cause melt disintegration (twin-fluid atomization). In the resulting spray, the droplets are accelerated towards the substrate and thereby cool down and partly solidify due to intensive heat transfer to the cold atomization gas.

Analysis of turbulent multiphase flow in a spray is of major concern during numerical modelling and simulation, as the turbulence is responsible for a number of subprocesses that affect spray forming applications. These result from coupled transport between drop and gaseous phases, and from extensive transfer of momentum, heat and mass between phases due to the huge exchange area of the combined droplet surface. Physical modelling and description of these exchange and transport processes is key to the understanding of spray proces.

In spray forming, especially, the thermal and kinetic states of melt particles at the point of impingement onto the substrate, or the already deposited melt layer, are of importance. This is the main boundary condition for analysis of growth, solidification and cooling processes in spray formed deposits. These process conditions finally determine the product quality of spray deposited preforms. By impinging and partly compacting particles from the spray, a source for heat (enthalpy), momentum and mass for the growing deposit is generated. The main parameters influencing successful spray simulation in this context are:

• the local temperature distribution and local distribution ratio between the particles and the surface of the deposit,

• particle velocities at the point of impingement, and

• the mass and enthalpy fluxes (integrated rates per unit area and time) of the compacting particles.

Distribution of these properties at the point of impingement is determined mainly by the fragmentation process and by the transport and exchange mechanisms in the spray.