MTBE UNIT
In the MTBE unit, the advanced controls set 12 DCS controller set-points. Four strategies use the SMPC to control the compositions measured using gas chromatograph analyzers:
The methanol concentration in the effluent from the second reactor is controlled by manipulating the methanol-to-isobutene-feed ratio. The isobutene feed is calculated from the flow rate and measured concentration.
Because the reactor positions can be switched, the controller must read the correct effluent analyzer as the controlled variable. The operator enters the reactor positions via a "flag."
Fig. 7 (45229 bytes) compares the methanol concentration before and after advanced control. This strategy stabilizes the feed composition of the separation section.
The MTBE concentration in the extractor raffinate is controlled by manipulating the setpoint of the MTBE-column overhead temperature. The temperature is pressure-compensated to prevent changes in pressure from disturbing the composition control.
Feed-forward moves are made from the column feed rate and reboiler steam flow. The final manipulated variable is the column reflux, compensated for subcooling.
Fig. 8 (45421 bytes) compares the MTBE concentration before and after advanced control. This strategy allows the loss of MTBE in the raffinate to be controlled and balanced against the increased energy required for recovery.
The total C4 concentration in the MTBE product is controlled by manipulating a lower tray-temperature setpoint. A feed-forward move is made from the column feed rate.
The constraint controller acts to prevent violations of process limits. The final manipulated variable is the MTBE-column reboiler steam flow.
The methanol concentration in the recovery-column bottoms is controlled by manipulating a lower tray-temperature setpoint. Feed-forward moves are made from the column feed rate and reflux flow. The final manipulated variable is the reboiler steam flow.
This strategy stabilizes the extractor operation by providing a constant methanol concentration in the extractor water.