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1.1 Classification of Transport Processes and Separation Processes (Unit Operations)

1.1A Introduction

In the chemical and other physical processing industries, such as the food and biological processing industries, many similarities exist in the manner in which the entering feed materials are modified or processed into final products. We can take these seemingly different chemical, physical, or biological processes and break them down into a series of separate and distinct steps. These steps are commonly called unit operations. However, the term “unit operations” has sometimes been superseded by the more descriptive term “separation processes.” These separation processes are common to all types of diverse process industries.

For example, the separation process distillation is used to purify or separate alcohol in the beverage industry and other types of hydrocarbons in the petroleum industry. The drying of grain and other foods is similar to the drying of lumber, filtered precipitates, and wool. The separation process absorption occurs in the absorption of oxygen from air in a fermentation process or in a sewage treatment plant and in the absorption of hydrogen gas in a process for the liquid hydrogenation of oil. The evaporation of salt solutions in the chemical industry is similar to the evaporation of sugar solutions in the food industry. The settling and sedimentation of suspended solids in the sewage industry and the mining industry are similar. The flow of liquid hydrocarbons in the petroleum refinery and the flow of milk in a dairy plant are carried out in a similar fashion.

Many of these separation processes have certain fundamental and basic principles or mechanisms in common. For example, the mechanism of diffusion or mass transfer occurs in drying, membrane separation, absorption, distillation, and crystallization. Heat transfer occurs in drying, distillation, and evaporation. The following classification of a more fundamental nature is often made, according to transfer or transport processes.

1.1B Fundamental Transport Processes

1. Momentum transfer. This is concerned with the transfer of momentum that occurs in moving media, such as in the separation processes of fluid flow, sedimentation, mixing, and filtration. Momentum transfer is commonly called fluid mechanics in other disciplines.

2. Heat transfer. In this fundamental process, we are concerned with the transfer energy in the form of heat from one place to another. It occurs in the separation processes of drying, evaporation, distillation, and many others.

3. Mass transfer. Here, material (or mass) is transferred from one phase to another distinct phase; the basic mechanism is the same whether the phases are gas, solid, or liquid. Separation processes dependent on mass transfer include distillation, absorption, liquid–liquid extraction, membrane separation, adsorption, crystallization, and leaching.

1.1C Classification of Separation Processes

The separation processes deal mainly with the transfer and change of energy and the transfer and change of materials, primarily by physical means but also by physical–chemical means. The important separation processes, which can be combined in various sequences in a process and which are covered in this text, are described next.

1. Evaporation. This refers to the evaporation of a volatile solvent such as water from a nonvolatile solute such as salt or any other material in solution.

2. Drying. In this operation, volatile liquids, usually water, are removed from solid materials.

3. Distillation. This is an operation whereby components of a liquid mixture are separated by boiling because of their differences in vapor pressure.

4. Absorption. In this process, a component is removed from a gas stream by treatment with a liquid.

5. Membrane separation. This process involves the separation of a solute from a fluid by diffusion of this solute from a liquid or gas through a semipermeable barrier (i.e., the membrane) to another fluid.

6. Liquid–liquid extraction. In this case, a solute in a liquid solution is removed by contacting with another liquid solvent that is relatively immiscible with the solution.

7. Adsorption. In this process, a component of a gas or liquid stream is removed and adsorbed by a solid adsorbent.

8. Ion exchange. This process removes certain ions in solution from a liquid by the use of an ion-exchange solid.

9. Liquid–solid leaching. This process involves treating a finely divided solid with a liquid that dissolves out and removes a solute contained in the solid.

10. Crystallization. In this process, a solute, such as a salt, is removed from a solution by precipitating the solute from the solution.

11. Mechanical–physical separations. These processes involve the separation of solids, liquids, or gases by mechanical means, such as filtration, settling, centrifugation, and size reduction.

1.1D Arrangement in Parts 1 and 2

This text is arranged in two parts:

Part 1: Transport Processes: Momentum, Heat, and Mass. These fundamental principles are covered extensively in Chapters 1 through 21 in order to provide the basis for study of separation processes in Part 2 of this text.

Part 2: Separation Process Principles. The various separation processes and their applications to process areas are studied in Part 2 of this text.

There are a number of elementary engineering principles, mathematical techniques, and laws of physics and chemistry that are basic to a study of the principles of momentum, heat, and mass transfer, and the separation processes. These are reviewed for the reader in this first chapter. Some readers, especially chemical engineers, agricultural engineers, civil engineers, and chemists, may be familiar with many of these principles and techniques, and may wish to omit all or parts of this chapter.

Homework problems at the end of each chapter are arranged in different sections, each corresponding to the number of a given section in the chapter.

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