Page 159 - Plant design and economics for chemical engineers

COMPUTER-AIDED DESIGN 133

b)

1

t'

FIGURE 4-8
(a) Simultaneous and (b) separate convergence of nested recycle loops for Example 4.

inner loop (unit 10) during each iteration of the outer loop as suggested by Fig.
4-8b. The more appropriate choice, in terms of minimum computation time and
convergence, is not readily apparent. It is often assumed that simultaneous
convergence will be more efficient and that approach is illustrated here. All flow
rates are in lb/h; stream numbers are from Fig. 4-5.

Start with feed stream SO1 = 112.8 lb EB/h
Assume that recycle stream S14 = 0
SO2 and SO3 are then 112.8
The steam feed SO4 = 112.8* 14* 18/106 = 268
The reactor effluent plus recycle SO5 = 380.8 and includes 112.8 * 0.5 = 56.4
lb EB/h. It has changed and not converged yet
SO6 is 380.8. Continuing through the separations, the EB recycle stream
S14 = 56.4

The just-calculated values of the recycles, SO5 and S14, are now used and the