Separations as unit operations - Principles of Chemical Separations with Environmental Applications - page 14

2.2 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 separating agent can be either a mass or an energy modification.
Separating agents exploit a physical property difference to facilitate the separation. The
mechanism for separation uses this physical property difference to provide the separation.
Later in this chapter, the concept of a mass-separating agent will be discussed in detail.
The separation that occurs will depend upon process conditions such as feed composition,
phase, temperature, pressure, flowrates, the separating agent, and the separation method
used.
It is important to note that the fate of each stream is important with respect to environ-
mental impact. This includes the separating agent. The use of energy as a separating agent
has a direct impact through energy consumption but also generates pollution due to energy
production. The environmental impact of a mass-separating agent involves the ultimate
fate of the mass. How is it ultimately disposed? Does it enter the product streams?
A unit operation is any single step in an overall process that can be isolated and that
also tends to appear frequently in other processes. For example, a car's carburetor is a
single unit operation of the engine, just as the heart is a unit operation of the human body.
The concept of a unit operation is based on the idea that general analysis will be the same
for all systems because individual operations have common techniques and are based on
the same scientific principles. In separations, a unit operation is any process that uses
the same separation mechanism. For example, adsorption is a technique in which a solid
sorbent material removes specific components, called solutes, from either gas- or liquid-
feed streams because the solute has a higher affinity for the solid sorbent than it does for the
fluid. The mathematical characterization of any adsorption column is the same regardless

Product (1)

Feed(s)

Separation
process

Product (2)

Separating agent
• energy
• mass

Mechanism
• equilibrium
• rate

Property
difference

Figure 2.1 Generic unit operation for a separation process.