Page 50 - Separation process engineering

Chapter 1. Introduction to Separation Process Engineering

1.1 Importance of Separations

Why does chemical engineering require the study of separation techniques? Because separations are
crucial in chemical engineering. A typical chemical plant is a chemical reactor surrounded by separators,
as diagramed in the schematic flow sheet of Figure 1-1. Raw materials are prepurified in separation
devices and fed to the chemical reactor; unreacted feed is separated from the reaction products and

recycled back to the reactor. Products must be further separated and purified before they can be sold. This
type of arrangement is very common. Examples for a variety of traditional processes are illustrated by
Biegler et al. (1997), Chenier (2002), Couper et al. (2005), Matar and Hatch (2001), Shreve and Austin
(1984), Speight (2002), and Turton et al. (2003), whereas recent processes often are shown in Chemical
Engineering magazine. Chemical plants commonly have from 40% to 70% of both capital and operating
costs in separations (Humphrey and Keller, 1997).
Figure 1-1. Typical chemical plant layout

Since separations are ubiquitous in chemical plants and petroleum refineries, chemical engineers must be
familiar with a variety of separation methods. We will first focus on some of the most common chemical
engineering separation methods: flash distillation, continuous column distillation, batch distillation,
absorption, stripping, and extraction. These separations all contact two phases and can be designed and
analyzed as equilibrium stage processes. Several other separation methods that can also be considered

equilibrium stage processes will be briefly discussed. Chapters 17 and 18 explore two important
separations—membrane separators and adsorption processes—that do not operate as equilibrium stage
systems.
The equilibrium stage concept is applicable when the process can be constructed as a series of discrete
stages in which the two phases are contacted and then separated. The two separated phases are assumed

to be in equilibrium with each other. For example, in distillation, a vapor and a liquid are commonly
contacted on a metal plate with holes in it. Because of the intimate contact between the two phases, solute
can transfer from one phase to another. Above the plate the vapor disengages from the liquid. Both liquid
and vapor can be sent to additional stages for further separation. Assuming that the stages are equilibrium
stages, the engineer can calculate concentrations and temperatures without detailed knowledge of flow
patterns and heat and mass transfer rates. Although this example shows the applicability of the
equilibrium stage method for equipment built with a series of discrete stages, we will see that the staged
design method can also be used for packed columns where there are no discrete stages. This method is a
major simplification in the design and analysis of chemical engineering separations that is used in
Chapters 2 to 14.

A second useful concept is that of a unit operation. The idea here is that although the specific design may
vary depending on what chemicals are being separated, the basic design principles for a given separation