Page 350 - Design of Simple and Robust Process Plants

336 Chapter 8 Instrumentation, Automation of Operation and Control
Table 8.4. Optimal operating distillation conditions and profit for different disturbances
(Skogestad, 1999).

Conditions ® x D x B D/F R/F V/F R/D P/F
Disturbances
¯
Nominal 0.995 0.04 0.639 15.065 15.704 23.57 4.528

F = 1.3 0.995 0.04 0.639 15.065 15.704 23.57 4.528
z F = 0.5 0.995 0.032 0.486 15.202 15.525 31.28 2.978
z F = 0.75 0.995 0.05 0.741 14.543 15.284 19.62 5.620
q F = 0.5 0.995 0.04 0.639 15.133 15.272 23.68 4.571
x D = 0.996 0.996 0.042 0.637 15.594 16.232 24.47 4.443
p D = 30 0.995 0.035 0.641 15.714 16.355 24.51
p V = 0.5 0.995 0.138 0.597 11.026 11.623 18.47
D, B, F and V flows in kmol/min; D = distillate; B = bottom stream;
V = vapor; F = feed
prices; p F, D, B, V = $/kmol; P = profit in $/min;
z F = feed concentration of lights; q F = vapor fraction of feed
Nominal conditions: F = 1.0, z F = 0.65, q F = 1.0, p D = 20, p v = 0.1

Based on the selected disturbances, the losses were calculated for the distillate
specification x D = 0.995 and the selected CVs.
The losses are summarized in Table 5. Some of the conclusions were:

± The losses are small when the bottom concentration x B ; is directly controlled,
this represents the dual composition control which is self-optimizing, but
requires a model-based controller for interaction compensation.
± R is a bad choice, particularly when feed flow variations are involved.
± D/F as well as R/D cannot handle feed composition variations efficiently.
± R/F and V/F appears to be the best choices next to x B , but they are sensitive to
implementation errors.

Selection of best pairing
The three selected CVs for further evaluation are:
1. x D and x B
2. x D and R/F
3. x D and V/F
The belonging MVs are respectively:

1. V and D,B,R/F or reflux ratio
2. V and R/F
3. R and V/F