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.

Modelling of technical production facilities, plants and processes is an integral part of engineering and process technology development, planning and construction. The successful implementation of modelling tools is strongly related to one's understanding of the physical processes involved. Most important in the context of chemical and process technologies are momentum, heat and mass transfer during production. Projection, or scaling, of the unit operations of a complex production plant or process, from laboratory-scale or pilot-plant-scale to production-scale, based on operational models (in connection with well-known scaling-up problems) as well as abstract planning models, is a traditional but important development tool in process technology and chemical engineering. In a proper modelling approach, important features and the complex coupled behaviour of engineering processes and plants may be simulated from process and safety aspects viewpoints, as well as from economic and ecologic aspects. Model applications, in addition, allow subdivision of complex processes into single steps and enable definition of their interfaces, as well as sequential investigation of the interaction between these processes in a complex plant. From here, realization conditions and optimization potentials of a complex process or facility may be evaluated and tested. These days, in addition to classical modelling methods, increased input from mathematical models and numerical simulations based on computer tools and programs is to be found in engineering practice. The increasing importance of these techniques is reflected by their incorporation into educational programmes at universities within mechanical and chemical engineering courses.

Having divided the atomization and spray process modelling procedure into three main areas:

1. atomization (disintegration),

2. spray, and

3. compaction,

in this chapter we look specifically at the disintegration process as it applies to the case of molten metal atomization for spray forming. We begin by breaking down disintegration into a number of steps:

• the melt flow field inside the tundish and the tundish melt nozzle,

• the melt flow field in the emerging and excited fluid jet,

• the gas flow field in the vicinity of the twin-atomizer,

• interaction of gas and melt flow fields, and

• resulting primary and secondary disintegration processes of the liquid melt.

Several principal atomization mechanisms and devices exist for disintegration of molten metals. An overview of molten metal atomization techniques and devices is given, for example, in Lawley (1992), Bauckhage (1992), Yule and Dunkley (1994) and Nasr et al. (2002). In the area of metal powder production by atomization of molten metals, or in the area of spray forming of metals, especially, twin-fluid atomization by means of inert gases is used. The main reasons for using this specific atomization technique are:

• the possibility of high throughputs and disintegration of high mass flow rates;

• a greater amount of heat transfer between gas and particles allows rapid, partial cooling of particles;

• direct delivery of kinetic energy to accelerate the particles towards the substrate/deposit for compaction;

• minimization of oxidation risks to the atomized materials within the spray process by use of inert gases.

A common characteristic of the various types of twin-fluid atomizers used for molten metal atomization is the gravitational, vertical exit of the melt jet from the tundish via the (often cylindrical) melt nozzle.

Coupling of several mechanisms into an integrated model for the spray forming process is the final aim of simulation. Such an integral spray forming model has been investigated, for example, by Bergmann (2000), Minisandram et al. (2000) (which has already been introduced in Section 6.3) and Pedersen (2003).

Connection between those submodels aforementioned has been performed, for transient temperature material behaviour, from melt superheating in the tundish to room temperature in the preform via cooling and solidification, in a three-stage approach in Bergmann (2000). Tundish melt flow and a thermal model are accounted for in the first stage. The local separation method is employed in the second stage to determine temperature-averaged properties of the spray and solid fractions in the particle mass at the centre-line of the spray. The temporal behaviour of a melt element is derived in terms of the averaged mean residence time of the particle mass. By combination of these data with calculated temporal cooling and solidification distributions of a fixed volume element inside the deposit, one yields the mean thermal history of the material at a specified location in the deposit.

The transient thermal and solidification distributions are shown in Figures 7.1 and 7.2. Results are illustrated separately for the three different modelling areas (with different time scales) for: (a) melt flow in the tundish, (b) particle cooling in the spray, (c) growth and cooling of the deposit. The process conditions used are for a steel spray forming of a Gaussian-shaped deposit (see Table 6.5).