Page 28 - Plant design and economics for chemical engineers

INTRODUCTION 11
arise when the process finally comes to life in the form of a complete plant or a
complete unit.

THE DESIGN APPROACH

The chemical engineer has many tools to choose from in the development of a
profitable plant design. None, when properly utilized, will probably contribute
as much to the optimization of the design as the use of high-speed computers.
Many problems encountered in the process development and design can be
solved rapidly with a higher degree of completeness with high-speed computers
and at less cost than with ordinary hand or desk calculators. Generally overde-
sign and safety factors can be reduced with a substantial savings in capital
investment.
At no time, however, should the engineer be led to believe that plants are
designed around computers. They are used to determine design data and are
used as models for optimization once a design is established. They are also used
to maintain operating plants on the desired operating conditions. The latter
function is a part of design and supplements and follows process design.
The general approach in any plant design involves a carefully balanced
combination of theory, practice, originality, and plain common sense. In original
design work, the engineer must deal with many different types of experimental
and empirical data. The engineer may be able to obtain accurate values of heat
capacity, density, vapor-liquid equilibrium data, or other information on physi-
cal properties from the literature. In many cases, however, exact values for
necessary physical properties are not available, and the engineer is forced to
make approximate estimates of these values. Many approximations also must be
made in carrying out theoretical design calculations. For example, even though
the engineer knows that the ideal-gas law applies exactly only to simple gases at
very low pressures, this law is used in many of the calculations when the gas
pressure is as high as 5 or more atmospheres (507 kPa). With common gases,
such as air or simple hydrocarbons, the error introduced by using the ideal gas
law at ordinary pressures and temperatures is usually negligible in comparison
with other uncertainties involved in design calculations. The engineer prefers to
accept this error rather than to spend time determining virial coefficients or
other factors to correct for ideal gas deviations.
In the engineer’s approach to any design problem, it is necessary to be
prepared to make many assumptions. Sometimes these assumptions are made
because no absolutely accurate values or methods of calculation are available.
At other times, methods involving close approximations are used because exact
treatments would require long and laborious calculations giving little gain in
accuracy. The good chemical engineer recognizes the need for making certain
assumptions but also knows that this type of approach introduces some uncer-
tainties into the final results. Therefore, assumptions are made only when they
are necessary and essentially correct. I ‘