Methanol Recovery Column C3

Figure 14.8 shows the control structure used in the C3 column that is separating methanol and water. A constant reflux ratio is maintained in this column by adjusting the reflux flow-rate based on the distillate flowrate. The temperature on stage 7 is controlled by the reboiler heat input.

There are two key plantwide material balance loops associated with column C3. The level in the reflux drum provides a good indication of the inventory of methanol in the system. If this level is going down, more methanol is being consumed in the reaction than is being fed into the process. Therefore, the control structure maintains the reflux-drum level in C3 by manipulating the methanol fresh feed.

Note that the flowrate of the total methanol (D3 plus fresh methanol feed) is fixed by the two downstream flow controllers setting the flowrates to the reactor and to column C1. This means there is an immediate effect of the fresh feed flowrate on the reflux-drum level. Distillate flow D3 changes inversely with the fresh feedflow because the downstream flowrate is fixed. Thus, the reflux-drum level changes instantaneously for changes in the flowrate of the methanol fresh feed.

At the other end of the column, the base level provides a good indication of the inventory of water in the system. Most of the extraction water just circulates between the extractive column and the recovery column. However, there is a small amount of water lost in the

Figure 14.12 Aspen Dynamics process flow diagram.

Figure 14.12 Aspen Dynamics process flow diagram.

overhead from column C2, which leaves in the aqueous stream from the decanter. A water makeup stream is used to control the liquid level in the base of C3. This makeup flow is small compared to the water circulation, so the base of column C3 must be sized to provide enough surge capacity to ride through disturbances.

Relay-feedback testing and Tyreus-Luyben settings give KC = 11.6 and t/ = 5.3 min with a temperature transmitter span of 100 K and a reboiler heat input range from 0 to twice the steady-state value.

All liquid levels are controlled by proportional controllers with gains of 2. Column pressure controllers use default controller settings and manipulate condenser heat removal (not shown in Figs. 14.7 and 14.8). The faceplates for the 21 controllers in this process are shown in Figure 14.11. Note that flow controllers FCC1meoh, FCRmeoh, and FCw are on cascade because they get their setpoint signals from other controllers or multipliers. The Aspen Dynamics process flow diagram is given in Figure 14.12 with all of the control elements installed and loops closed.

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