Page 256 - Chemical engineering design
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CHEMICAL ENGINEERING
(a)
(b)
Figure 5.19. (a) Control of one fluid stream (b) By-pass control
Figure 5.20. Vaporiser control
3. Cooling water pressure and header temperature.
4. Ambient conditions, which cause changes in internal reflux (see Chapter 11).
The compositions are controlled by regulating reflux flow and boil-up. The column overall
material balance must also be controlled; distillation columns have little surge capacity
(hold-up) and the flow of distillate and bottom product (and side-streams) must match the
feed flows.
Shinskey (1984) has shown that there are 120 ways of connecting the five main parts of
measured and controlled variables, in single loops. A variety of control schemes has been
devised for distillation column control. Some typical schemes are shown in Figures 5.22a,
b, c, d, e (see pp. 234, 235); ancillary control loops and instruments are not shown.
