Page 137 - Plant design and economics for chemical engineers

112 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

tion is an enormous task because of the large number of substances and
mixtures of possible interest and the virtually unlimited conditions (of tempera-
ture, pressure, and composition) at which they may be needed. SoftWare to
supply these properties is one of the most useful of all computer aids to
chemical engineers. There are many computer programs available for supplying
or predicting properties, primarily for pure components.
Process synthesis and flow-sheeting programs are those most specifically
intended for design use. Process synthesis (or invention) involves generating a
flow sheet for a process to produce a particular product or slate of products
from specified raw materials. The process flow sheet identifies the chemical
reactors and unit operations required and their sequence, the material and
energy streams in the process, those streams to be recycled, and some character-
istics of the equipment. This flow sheet should be a reasonable approximation of
the best economic flow sheet for the product slate. Process synthesis software, a
type of expert system program, has begun to appear in the late 1980s. Process
synthesis is introduced in this chapter, but rapid changes are to be expected in
this field.
“Flow-sheeting,” as used in computer-aided-design, means performing on
a specified flow sheet the calculations necessary to simulate the behavior of the
process or to design the equipment and to determine values for key operating
conditions. These calculations include mass and energy balances, process-
equipment parameters, and cost estimation for the equipment and plant as well
as an economic analysis of the process. This is chemical process design; it is
what this textbook is all about. The potential of computerized flow-sheeting was
recognized early by chemical engineers. Development of these programs began
in the late 1950s and still continues. Flow-sheeting methods are emphasized in
this chapter.

SPREAD-SHEETING
Spread-sheet software has achieved great popularity because of its availability
for microcomputers at reasonable cost, the ease of learning and using the
software, and its flexible application to many problems. Solutions for many
engineering problems can be obtained more rapidly by spread sheet than by
writing a language code program, such as FORTRAN or BASIC.
The term spread sheet refers to the row and column organizational form of
the input and output of the software. Key features of spread-sheet software that
are necessary or desirable for engineering problem solving include:

Mathematical functions (arithmetic, logical, relational operators, loga-
rithms, exponents, etc.)
User definable functions (to use functions not in the library)
Formula and data replication (for ease of copying entries)