Page 51 - Plant design and economics for chemical engineers

3 4 PLANT DESIGN AND ECONOMlCS FOR CHEMICAL ENGINEERS

Summary
The preliminary design presented in this section was developed to show the
logical step-by-step approach which is quite often followed for each new process
design. The exact procedure may vary from company to company and from one
design engineer to another. Likewise, the assumptions and rule-of-thumb fac-
tors used may vary from one company to the next depending to a large extent on
design experience and company policy. Nevertheless, the basic steps for a
process design are those outlined in this preliminary design covering the
manufacture of a common household item.
No attempt has been made to present a complete design. In fact, to
minimize the length, many assumptions were made which would have been
verified or justified in a normal process design. Neither were any alternative
solutions considered even though some were suggested by the literature survey.
The investigation of these various alternatives is left to the reader.

COMPARISON OF DIFFERENT PROCESSES

In a course of a design project it is necessary to determine the most suitable
process for obtaining a desired product. Several different manufacturing meth-
ods may be available for making the same material, and various processes must
be compared in order to select the one best suited to the existing conditions.
The comparison can be accomplished through the development of com-
plete designs. In many cases, however, all but one or two of the possible
processes can be eliminated by a weighted comparison of the essential variable
items, and detailed design calculations for each process may not be required.
The following items should be considered in a comparison of this type:
1. Technical factors
a. Process flexibility
b. Continuous operation
c. Special controls involved
d. Commercial yields
e. Technical difficulties involved
f. Energy requirements
g. Special auxiliaries required
h. Possibility of future developments
i. Health and safety hazards involved
2. Raw materials
a. Present and future availability
b. Processing required
c. Storage requirements
d. Materials handling problems
3. Waste products and by-products
a. Amount produced
b. Value