Location of Sensors

Basic control

P

hB

K

XD

*B

W

++

+

+

unless 1 >0.5

B

unless VB /B» 1

D

unless RID» 1

+

R

+

+ +

S

+

unless X >0.5

+

unless X >0.5

Figure 6 Possibilities for the control scheme.

Figure 6 Possibilities for the control scheme.

where L and V are the liquid and vapour flow respectively (see Figure 5), Kx is the gain for concentration responses and tx is a large time constant for concentration responses, usually in the order of hours. This time constant is approximately proportional to the square of the number of trays.

For control of the bottom quality, the reflux R or distillate flow D are unsuitable candidates, since there is a large dead time in the dynamic response between the flow and the composition. The steam flow S and coolant flow W are acceptable candidates for bottom quality control, unless, for the same reason as holds for bottom-level control, the parameter:

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