Tim min

Figure 9.6 - Stop Change m Feed Composilion wuh the LV Configuration

Figure 9.7 - Step Change in Set Point with the LV Configuration

Time (min)

Figure 9.7 - Step Change in Set Point with the LV Configuration

100%

100%

j Bottoms Temperature

Ether Purity

Time (min)

Figure 9.8 - Slop Increase in Feed with the LB Configuration

Figure 9.8 - Slop Increase in Feed with the LB Configuration

100%

98%

?

o £

>

Q.

C

c o o

94%

Conversion

Bottoms Temperature

I Ether Purity

Time (min)

Figure 9.10 - Step Change in Set Point with the LB Configuration

Time (min)

Time (min)

figure 9.11 - Step Increase in Feed with the DV Configuration

Figure 9.12 - Step Change in 1 eed Composition with the DV Configuration

Time (min)

Figure 9.12 - Step Change in 1 eed Composition with the DV Configuration

Time (min)

Figure 9 13 - Slep Change in Set Point with the DV Configuration

Time (min)

Figure 9 13 - Slep Change in Set Point with the DV Configuration

A final comparison can be made between the various control structures: the ability of each configuration to support operation at an equipment constraint. This can be investigated by considering the magnitude of the control moves required to counteract feed rate and composition disturbances and a set-point change. These values were calculated from dynamic simulation results and have been tabulated in Table 9.4. The A denotes the change in either the reboiler duty or the reflux rate due to the change in set-point. A small value indicates that operation can continue close to the equipment constraint while a larger value indicates that the operation become infeasible (i.e. exceeds an equipment constraint) or results in under-utilisation of the available resources. Clearly, the LV and the LB configurations perform best in this regard.

I able 9 4 - Departure from Equipment Constraints

Feed Rate

Feed Composition

Set Point Change

Step Increase (+8%)

Step Increase (+2%)

(+5°C)

Scheme

AQR

AL

AQr

AL

AQr

AL

LV

+2.6%

0

0

0

+0.2%

0

LB

+2.6%

0

0

0

+0.2%

0

DV

0

-4.0%

0

-0.07%

0

-0.4%

(L/D)V

+8.2%

+8.1%

0

-0.08%

+0.6%

+0.4%

(L/D)(V/B)

+8.2%

+8.1%

0

-0.08%

+0.6%

+0.4%

(D/F)V

+8.3%

+8.2%

-2.8%

-4.6%

+13.6%

+20.4%

Overall» there is little distinction between the LV and the LB control structures. Both provide good dynamic responsiveness and the ability to consistently operate close to equipment constraints and the results of the dynamic simulations suggest that either of these could be used effectively for one-point control of the 10 stage reactive ETBE column described in Chapter 3.

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