Unit Operations

UNIT OPERATIONS

An economical method of organizing much of the subject matter of chemical engineering is based on

two facts: (1) Although the number of individual processes is great, each one can be broken down into

a series of steps, called operations, each of which in turn appears in process after process; (2) the

Individual operations have common techniques and are based on the same scientific principles. For

example, in most processes solids and fluids must be moved; heat or other forms of energy must be

transferred from one substance to another; and tasks such as drying, size reduction, distillation, and

evaporation must be performed. The unit operation concept is this: By studying systematically these

operations themselves--operations that clearly cross industry and process lines—the treatment of all

processes is unified and simplified.

The strictly chemical aspects of processing are studied in a companion area of chemical

engineering called reaction kinetics. The unit operations are largely used to conduct the primarily

physical steps of preparing the reactants, separating and purifying the products, recycling unconverted

reactants, and controlling the energy transfer into or out of the chemical reactor.

The unit operations are as applicable to many physical processes as to chemical ones. For example,

the process used to manufacture common salt consists of the following sequence of unit operations:

transportation of solids and liquids, transfer of heat, evaporation, crystallization, drying, and screening.

No chemical reaction appears in these steps. On the other hand, the cracking of petroleum, with or

without the aid of a catalyst, is a typical chemical reaction conducted on an enormous scale. Here the

unit operations—transportation of fluids and solids, distillation, and various mechanical separations—

are vital, and the cracking reaction could not be utilized without them. The chemical steps themselves

are conducted by controlling the...