PI Control Result

5.2.1 PI Control Result

The two-point composition control strategy as described earlier in section

3.1.2 is implemented to control the distillation column. PI controller parameters are taken from IAE tuning approach (Chiu et al., 1973). Figure 5.3 show the control result for PI controller based on IAE tuning rules for the set point tracking whereas Figures 5.5 and 5.6 illustrated the control results for unmeasured disturbances in feed rate and feed composition respectively.

For the setpoint tracking, as can be seen from Figure 5.3, PI controller based IAE setting show a rapid responds but very sluggish control actions. The Reflux rate (L T ) and Boilup rate (V B ) were increased once the setpoint change is introduced with

a smooth but small magnitude profile. As a consequence, the control results for IAE setting were evident lead to a long settling time. This can be seen clearly in particularly for the bottom composition, X B from Figure 5.3 in which at the end of the simulation (t=600 min), the X B from the IAE tuning rules is still unable to settle down to its setpoint at 0.01.

D 1 ,X n

io it 0.995 o s mp

0.99 oc

te la 0.985 til is

time(min)

time(min)

0.1 ,XB 0.09 n 0.08

io 0.07 o s it 0.06 p m 0.05 o

0.04 c

m 0.03 tto 0.02

time(min)

o ilup r B 3.2

time(min)

Figure 5.3

PI controller (IAE setting) for setpoint change (step setpoint change in

XD at t=5min from 0.99 to 0.995) XD at t=5min from 0.99 to 0.995)

0.99 om

ec 0.985

ti llat is

time(min)

time(min)

0.1 B X 0.09 0.08

it ion, 0.07 os 0.06 p

0.05 om 0.04 c 0.03

tom 0.02 ot 0.01

time(min)

time(min)

Figure 5.4

PI controller (IAE) for disturbance rejection (feed rate change

at t=5min from 1.00 to 1.20)

D 1 ,X n

io it 0.995 s o

mp 0.99 oc

te la 0.985 til is

time(min)

time(min)

0.1 ,XB 0.09 n 0.08

it io 0.07 o s 0.06 p

m 0.05 o 0.04 c

m 0.03 tto 0.02

time(min)

3.3 o ilup r B 3.2

time(min)

Figure 5.5

PI controller (IAE) for disturbance rejection (feed composition change

at t=5min from 0.50 to 0.60)

For the regulatory control, it can be seen clearly from Figures 5.4 and 5.5 that the step change in feed composition, ∆ Z F does not produce a significant deviation in

X D and X B compare to step change in feed rate, ∆ . This happens because the F interactions for the LV configuration (two point control) has strongly amplified the effect of feed rate

F, and reduce the effect of the feed composition, Z F. Also, it can be observed that the disturbance in Z F is more easily rejected compared to the disturbance in

F that has more effect on X D and X B . An increased feed rate goes down to the bottom of the column, and this result, through the action of the bottom level controller, in a corresponding increase in the bottom flow. This would significantly affect the material balance which in turn affected the product compositions. The effect of

F is exaggerated particularly when the controller can not make fast control in both composition loops as can be seen in the Figure 5.4 where the PI controller form IAE setting was unable to provide fast control action and this has resulted that the bottom composition still quite far away from its setpoint even at the end of simulation time ( t=600min).

In overall, as can be seen from Figures 5.3 to 5.5, the X B composition loop obviously took a longer settling time compare to X D composition loop in this PI control scheme. The sluggish control movement shown by PI control also resulted a long settling time for both the X D and X B and this has caused them unable to settle down in setpoint change and feed rate change problem. Thus, it can be concluded that the PI control is able to perform better in a less interactive and nonlinear problem. As can be seen in Figures 5.4 and 5.5, PI control is evident able to deliver a better control results in counteracting the feed composition disturbance than in feed rate change.