Page 22 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

The selection process can be considered to go through the following stages:
within the external constraints.
Possible designs (credible) INTRODUCTION TO DESIGN 5
Plausible designs (feasible) within the internal constraints.
Probable designs likely candidates.
Best design (optimum) judged the best solution to the problem.
The selection process will become more detailed and more refined as the design progresses
from the area of possible to the area of probable solutions. In the early stages a coarse
screening based on common sense, engineering judgement, and rough costings will usually
suffice. For example, it would not take many minutes to narrow down the choice of raw
materials for the manufacture of ammonia from the possible candidates of, say, wood,
peat, coal, natural gas, and oil, to a choice of between gas and oil, but a more detailed
study would be needed to choose between oil and gas. To select the best design from the
probable designs, detailed design work and costing will usually be necessary. However,
where the performance of candidate designs is likely to be close the cost of this further
refinement, in time and money, may not be worthwhile, particularly as there will usually
be some uncertainty in the accuracy of the estimates.
The mathematical techniques that have been developed to assist in the optimisation of
designs, and plant performance, are discussed briefly in Section 1.10.
Rudd and Watson (1968) and Wells (1973) describe formal techniques for the prelim-
inary screening of alternative designs.

1.3. THE ANATOMY OF A CHEMICAL MANUFACTURING
PROCESS
The basic components of a typical chemical process are shown in Figure 1.3, in which
each block represents a stage in the overall process for producing a product from the raw
materials. Figure 1.3 represents a generalised process; not all the stages will be needed for
any particular process, and the complexity of each stage will depend on the nature of the
process. Chemical engineering design is concerned with the selection and arrangement
of the stages, and the selection, specification and design of the equipment required to
perform the stage functions.

Recycle of unreacted By-products
material
Wastes
Raw Feed Product Product
material preparation Reaction separation purification Product Sales
storage storage
Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6

Figure 1.3. Anatomy of a chemical process

Stage 1. Raw material storage
Unless the raw materials (also called essential materials, or feed stocks) are supplied
as intermediate products (intermediates) from a neighbouring plant, some provision will