Page 25 - Plant design and economics for chemical engineers

8 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS
I

for installed pipe

Cost far pumping power

E
Pipe diometer
F I G U R E 1.1
Determination of optimum economic pipe diameter for constant mass-throughput rate.

Optimum Operation Design

Many processes require definite conditions of temperature, pressure, contact
time, or other variables if the best results are to be obtained. It is often possible
to make a partial separation of these optimum conditions from direct economic
considerations. In cases of this type, the best design is designated as the
optimum operation design. The chemical engineer should remember, however,
that economic considerations ultimately determine most quantitative decisions.
Thus, the optimum operation design is usually merely a tool or step in the
development of an optimum economic design.
An excellent example of an optimum operation design is the determina-
tion of operating conditions for the catalytic oxidation of sulfur dioxide to sulfur
trioxide. Suppose that all the variables, such as converter size, gas rate, catalyst
activity, and entering-gas concentration, are tied and the only possible variable
is the temperature at which the oxidation occurs. If the temperature is too high,
the yield of SO, will be low because the equilibrium between SO,, SO,, and 0,
is shifted in the direction of SO, and 0,. On the other hand, if the temperature
is too low, the yield will be poor because the reaction rate between SO, and 0,
will be low. Thus, there must be one temperature where She amount of sulfur
trioxide formed will be a maximum. This particular temperature would give the -