Control system objectives and design considerations

Rigorous dynamic simulation is the third important activity in control system desiga A flexible dynamic simulator allows for rapid evaluation of different control stmctures and their responses to various disturbances. In choosing a control scheme, there are several design considerations to take into account. First, it is important to remember that a distillation column performs two basic functions  

The following equations describe how the various manipulated variables are related and show that virtually any variable pairing can be used to achieve the desired control objectives. However, some pairings will provide significantly better sensitivity and responsiveness. Better sensitivity means that the control scheme will react with smaller changes, whereas a more responsive control scheme reacts more quickly. 

Where Hi is the enthalpy of the feed, is the enthalpy of the bottoms, and is the enthalpy of the distillate. 

The control system must satisfy Equations 8.1 and 8.3 at all times. Eor particular values of Xd and Xb (i.e. composition specifications). Equation 8.4 or 8.5 also has to be satisfied. D,B, Qreb. and Qcond can all be fixed or adjusted dynamically by control valves on the flow rate or utility streams. The reflux flow can also be adjusted dynamically and will directly affect the energy balance. 

One of the most difficult aspects of distillation column control is the interaction effects between the material and energy balance and composition controls. Depending on the inventory controls, a heat input or removal can alter both the material draws and the compositions. This interaction can work for us or against us, depending on the control strategy. 

---