Pilot Plant Simulations

Simulations of the pilot plant configuration had previously been performed using both Pro/II and SpeedUp (Chapter 11). However, these models were used for design and did not reflect the actual process exactly. Specifically, the pilot plant is operated by specifying five conditions: three valve positions, the pump speed and the reboiler voltage. This contrasts with the design models where the feed rate, the operating pressure, the reboiler duty, the reflux rate and the bottoms rate (or similar combinations) were specified. The flexibility of the SpeedUp simulation environment was utilised to develop another model of the pilot plant that matched the actual pilot plant inputs. This model was used to provide additional information regarding the operation of the pilot plant and can be used to detected equipment failures (e.g. fouling, blockages, poor heat transfer, etc.) via disparities between the process and the model.

The pilot plant was commissioned with a feed mixture containing predominantly ethanol to simplify handling and storage. The operating conditions are predicted by the simulation and are shown in Table 12.7. More importantly, the simulations can be used to predict the transient response of the process to changes in the key operating variables. Figure 12.6 indicates the process response to closing the reflux control valve by 0.5%, Figure 12.7 shows the response to an increase in the reboiler voltage and Figure 12.8 indicates the response to an increase in the feed pump speed.

Tabic 12.7 - Predicted Operating Conditions for Commissioning Feed

INPUT CONDITIONS

ETHANOL ISOBUTENE ETBE

N-BUTENE

FEED COMP (wt percent)

71 . 1

3

. 5

0

25 . 4

FEED COMP (mole percent)

75 . 0

3

. 0

0

22 . 0

FEED TEMPERATURE

15 . 0

C

FEED RATE

0 . 51

L/min

30

.0

percent

COLUMN PRESSURE

800 .0

kPa-g

REFLUX RATE

1.25

L/min

REBOILER DUTY

7 .24

kW

170 .

. 0

V

REFLUX CONTROL VALVE

95 . 0

percent

closed

DISTILLATE CONTROL VALVE

99 . 5

percent

closed

CONDENSER FLOODING

31. 5

percent

OUTPUT CONDITIONS

TEMPERATURE AND COMPOSITION PROFILES (mole percent)

TEMP

ETHANOL

ISOBUTENE

ETBE

N-BUTENE

CONDENSER

75 .

.1

23 .

.6

8 ,

.6

0 .

. 5

67 .3

TRAY

2

(RECT)

101.

. 9

84 .

. 9

1 ,

, 5

0 .

. 8

12 . 8

TRAY

3

(RXN)

133 .

. 8

96 .

. 9

0 ,

. 3

0 .

.4

2 . 4

TRAY

4

(RXN)

138.

, 2

98 .

.2

0.

. 2

0 .

.2

1.4

TRAY

5

(RXN)

138 .

. 7

9 0 .

. 3

0 ,

. 1

0 ,

.2

1.4

TRAY

6

(STRP)

138 .

. 7

98 ,

.4

0.

. 2

0 ,

. 1

1.3

TRAY

7

(STRP)

144 ,

. 8

99 ,

. 8

0 .

. 0

0 ,

. 0

0 . 1

TRAY

8

(STRP)

145 .

.4

100 .

. 0

0 ,

. 0

0 ,

. 0

0 . 0

TRAY

9

(STRP)

145 .

. 5

100 .

. 0

0 ,

. 0

0 ,

. 0

0 . 0

REBOILER

145 .

. 5

100 ,

.0

0.

. 0

0 .

. 0

0 . 0

PRODUCTS (wt percent)

RATE (L/min) ETHANOL ISOBUTENE ETBE N-BUTENE

DISTILLATE 0.262 20.14 8.97 0.99 69.89

ISOBUTENE CONVERSION

0.12 mole percent

0.40

0.39

Btms Rate

Valve Opening

0.37

0.36

0.35

Figure 12.6 - Transient Response of the Pilot Plant to Closing the Reflux Control Valve

CD O

CD O

1

1 1 1 1 1 1 1 1 1 1 i 1 ! 1 ! 1

i 1 1 1 i 1

1 1 i 1 1 1 ! 1 I 1 Btms Rate |

1 1 1 1 1 1

1 1 1

Vj/^-f — r— I— ! 1 1 1 ill i 1 1 1

1 1

1 1 Voltage 1

1 1 1 1 ! 1 ! !

1 1 1 1 1 [ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0.36

Time (mins)

0.40

0.36

0.35

Time (mins)

I igure 12.7 - Transient Response of the Pilot Plant to Decreasing the Keboiler Voltage

Time (mins)

Figure 12.8 - Transient Response of the Pilot Plant to Increasing the heed Pump Speed

Time (mins)

Figure 12.8 - Transient Response of the Pilot Plant to Increasing the heed Pump Speed

Figures 12.6-12.8 indicate a dominant time constant of approximately 5 minutes. This is convenient since the effects of changes in the feed conditions and in the manipulated variables will be fully evident within a short time thereby allowing several tests to be conducted each time the pilot plant is operated. Figures 12.6-12.8 also indicate significant overshoot in the responses to changes in the reflux rate, reboiler duty and the feed rate. It will be important to consider this effect and allow the column to stabilise sufficiently before collecting data from the pilot plant.

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