Page 41 - Chemical engineering design

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x
x
5
3
f
f 1 f 2 CHEMICAL ENGINEERING
f
4
3
x 6 Assumed
value
Calculated
x x x x
4
1
2
6
value
Figure 1.14.
x 3
x 4 x 1
x 5 f 4 f 2 f 1
x 4 x 2
x 6 x
6
f 3
x 3
x 5

Figure 1.15.

information to a minimum. Lee and Rudd (1966) and Rudd and Watson (1968) give an
algorithm that can be used to help in the selection of the best design variables in manual
calculations.
The recycle of information, often associated with the actual recycle of process material,
will usually occur in any design problem involving large sets of equations; such as in the
computer simulation of chemical processes. Efficient methods for the solution of sets of
equations are required in computer-aided design procedures to reduce the computer time
needed. Several workers have published algorithms for the efficient ordering of recycle
loops for iterative solution procedures, and some references to this work are given in the
chapter on flow-sheeting, Chapter 4.

1.10. OPTIMISATION

Design is optimisation: the designer seeks the best, the optimum, solution to a problem.
Much of the selection and choice in the design process will depend on the intuitive
judgement of the designer; who must decide when more formal optimisation techniques
can be used to advantage.
The task of formally optimising the design of a complex processing plant involving
several hundred variables, with complex interactions, is formidable, if not impossible.
The task can be reduced by dividing the process into more manageable units, identifying
the key variables and concentrating work where the effort involved will give the greatest