Page 38 - Separation process principles 2

6 Chapter 1 Separation Processes

Water Recycle ethylene Vent

Ethylene t vent

Water Water
Ethylene
hydration
(fixed-bed pressure
catalytic
reactor)

astewater Figure 1.5 Industrial process
for hydration of ethylene to
Recycle water ethanol.

remaining liquid in the low-pressure flash drum. Vapor from to meet required specifications. Sometimes a separation op-
the low-pressure flash is scrubbed with water in an absorber eration, such as absorption of SOz by limestone slurry, may
to remove alcohol and prevent its loss to the vent gas. Crude, be accompanied by a chemical reaction that serves to facili-
concentrated ethanol containing diethyl ether and acetalde- tate the separation. In this book, emphasis is on separation
hyde is distilled overhead in the crude-distillation (stripper) operations that do not rely on concurrent chemical reactions;
column and then catalytically hydrogenated in the vapor however, reactive distillation is discussed in Chapter 11.
phase to convert acetaldehyde to ethanol. Diethyl ether is Chemical engineers also design products. A significant
removed by distillation in the light-ends tower and scrubbed product that involves the separation of chemicals is the
with water in an absorption tower. The final product is espresso machine for making a cup of coffee that is superior
prepared by distillation in the final-purification tower, where to that made in a filter-drip machine. The goal in coffee
93 wt% aqueous ethanol product is withdrawn several making is to leach from the coffee beans the best oils, leav-
trays below the top tray, light ends are concentrated in the ing behind ingredients responsible for acidity and bitterness
so-called pasteurization-tray section above the product- in the cup of coffee. The espresso machine accomplishes this
withdrawal tray and recycled to the catalytic-hydrogenation by conducting the leaching operation rapidly in 20-30 sec-
reactor, and wastewater is removed from the bottom of the onds with water at high temperature and pressure. If the op-
tower. Besides the separation equipment shown, additional eration is carefully controlled, the resulting cup of espresso,
separation steps may be necessary to concentrate the ethyl- if immediately consumed, has: (1) a topping of creamy foam
ene feed to the process and remove impurities that poison the that traps the extracted chemicals, (2) a fullness of body due
catalysts. In the development of a new process from the lab- to emulsification, and (3) a richness of aroma. Typically,
oratory stage through the pilot-plant stage, experience shows 25% of the coffee bean is extracted and the espresso contains
that more separation steps than originally anticipated are less caffeine than filtered coffee. Cussler and Moggridge
usually needed. [13] and Seider, Seader, and Lewin [14] discuss many other
The above examples serve to illustrate the importance of examples of products designed by chemical engineers, some
separation operations in industrial chemical processes. Such of which involve the separation of chemicals.
operations are employed not only to separate a feed mixture
into other mixtures and relatively pure components, to re-
1.2 MECHANISM OF SEPARATION
cover solvents for recycle, and to remove wastes, but also,
when used in conjunction with chemical reactors, to purify Mixing of chemicals is a spontaneous, natural process that
reactor feeds, recover reactants from reactor effluents for re- is accompanied by an increase in entropy or randomness.
cycle, recover by-products, and recover and purify products The inverse process, the separation of that mixture into its