Installing the Control Structure

The controllers are now added in the usual way. Base levels are held by bottoms flow rates. Reflux drum level in column C2 is held by the distillate flow rate (the RECYCLE stream back to column Cl). 

The control of the aqueous level would appear to be straight forward. A level controller would manipulate the valve V5. Conventional logic says that this controller should be Direct acting. If the level goes up, the flow rate of the aqueous stream should be increased and more fed to column C2. Very surprisingly, this set-up was found to not work. The 

Decanter Models in Aspen. Up to this point, we have been using the Decanter model in the simulations. It assumes that there are only two phases, both liquid, and that the decanter pressure is fixed at the specified design value (pressure does not change with temperature or composition). These assumptions do not reflect reality but appear to suffice for steady-state design. In a real decanter, there is a vapor space above the two liquid phases, and pressure will vary with compositions and temperatures unless an inert gas is used to keep a constant pressure via a vent-bleed split-ranged valve setup. 

Aspen has another model that is more realistic, the Flash3 model under Separators. This model has two hquid outlet streams and a vapor outlet stream. 

10% increase in feed flashS modei direct action 

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