Page 24 - Plant design and economics for chemical engineers

INTRODUmION 7

methanol, by controlled oxidation of natural gas, or by direct reaction between
CO and H, under special conditions of catalyst, temperature, and pressure.
Each of these processes contains many possible alternatives involving variables
such as gas-mixture composition, temperature, pressure, and choice of catalyst.
It is the responsibility of the chemical engineer, in this case, to choose the best
process and to incorporate into the design the equipment and methods which
will give the best results. To meet this need, various aspects of chemical
engineering plant-design optimization are described in Chap. 11 including
presentation of design strategies which can be used to establish the desired
results in the most efficient manner.

Optimum Economic Design
If there are two or more methods for obtaining exactly equivalent final results,
the preferred method would be the one involving the least total cost. This is the
basis of an optimum economic design. One typical example of an optimum
economic design is determining the pipe diameter to use when pumping a given
amount of fluid from one point to another. Here the same final result (i.e., a set
amount of fluid pumped between two given points) can be accomplished by
using an infinite number of different pipe diameters. However, an economic
balance will show that one particular pipe diameter gives the least total cost.
The total cost includes the cost for pumping the liquid and the cost (i.e., fixed
charges) for the installed piping system.
A graphical representation showing the meaning of an optimum economic
pipe diameter is presented in Fig. l-l. As shown in this figure, the pumping cost
increases with decreased size of pipe diameter because of frictional effects,
while the fixed charges for the pipeline become lower when smaller pipe
diameters are used because of the reduced capital investment. The optimum
economic diameter is located where the sum of the pumping costs and fixed
costs for the pipeline becomes a minimum, since this represents the point of
least total cost. In Fig. l-l, this point is represented by E.
The chemical engineer often selects a final design on the basis of condi-
tions giving the least total cost. In many cases, however, alternative designs do
not give final products or results that are exactly equivalent. It then becomes
necessary to consider the quality of the product or the operation as well as the
total cost. When the engineer speaks of an optimum economic design, it
ordinarily means the cheapest one selected from a number of equivalent
designs. Cost data, to assist in making these decisions, are presented in Chaps.
14 through 16.
Various types of optimum economic requirements may be encountered in
design work. For example, it may be desirable to choose a design which gives
the maximum profit per unit of time or the minimum total cost per unit of
production. , 1