Page 22 - Chemical process engineering design and economics

Processes and Process Engineering 9

Chemical engineers express their ideas by first constructing a process flow
diagram to describe the logic of the process. At an early stage of the process de-
sign, several flow diagrams are drawn to illustrate process alternatives. Following
this initial stage, a preliminary screening will reduce the many alternatives to a
few of the most promising, which are studied in detail. Process-circuit analysis,
which establishes specifications for the process, will be the subject of a later chap-
ter. These specifications are quantities, such as flow rates, compositions, tempera-
tures, pressures, and energy requirements. Once the process specifications are
established, each process unit is sized. At the beginning of a process design, sim-
ple sizing procedures are sufficient to determine a preliminary production cost. In
fact, it may be poor strategy to use more exact, and therefore more costly design
procedures until the economics of the process demands it. The process design
engineer will have a number of design procedures available, each one differing in
accuracy. He will have to decide which procedure is the more appropriate one for
the moment. To determine the economic viability of a process, the product manu-
facturing and capital costs are estimated first. Using simplified cost estimating
techniques, the most costly process steps are located for a more detailed analysis.
The steps in a process design, listed above, do not have well defined
boundaries, but overlap. New information is fed back continuously, requiring
revision of previous calculations. Process design is a large-scale iterative calcula-
tion which terminates on a specified completion date.

PROCESS STRUCTURE
Because of the numerous process types, it is essential to be able to divide a process
into a minimum number of basic logical operations to aid in the understanding of
existing processes and in the development and design of new processes. The elec-
trical engineer designs electrical circuits consisting of transistors, resistors, capaci-
tors and other basic elements. Similarly, the chemical engineer designs process
circuits consisting of reactors, separators, and other process units. Early in the
development of chemical engineering the concept of unit operations and processes
evolved to isolate the basic elements of a process. Unit operations consist of
physical changes, such as distillation and heat transfer, and unit processes consist
of chemical changes, such as nitration and oxidation. Thus, any process consists
of a combination of unit operations and processes. Trescott [18] discusses the his-
tory of this concept.
A modification of the unit-operations, unit-process division is shown in Ta-
ble 1.3, where a process is divided into nine basic process operations. According
to this division, the unit operations are subdivided into several basic operations
and conversion is substituted for all unit processes for a total of nine process

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