Here, the two units of N - (M + 1) and (M - 1) stages are treated just like elements. Nine interstreams are created by the combination of elements; so
The number of design variables is
NU = C + 2N + 9 Nvu - NU = (10C + 2N + 27) - (9C + 18)
One set of specifications that is particularly convenient for computer solutions is:
(including reboiler) Pressure of stream leaving condenser Pressure of either stream leaving reflux divider Heat leak for each stage (excluding reboiler) Heat leak for reflux divider Feed stream Reflux temperature Total number of stages N Number of stages below feed M Distillate rate D/F Reflux rate (LN + i/D)
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