Fixed control strategy

Constraint control strategies can be classified as steady-state or dynamic. In the steady-state approach, the process dynamics are assumed to be much faster than the frequency with which the constraint control appHcation makes its control adjustments. The variables characterizing the proximity to the constraints, called the constraint variables, are usually monitored on a more frequent basis than actual control actions are made. A steady-state constraint appHcation increases (or decreases) a manipulated variable by a fixed amount, the value of which is determined to be safe based on an analysis of the proximity to relevant constraints. Once the appHcation has taken the control action toward or away from the constraint, it waits for the effect of the control action to work through the lower control levels and the process before taking another control step. Usually these steady-state constraint controls are implemented to move away from the active constraint at a faster rate than they do toward the constraint. The main advantage of the steady-state approach is that it is predictable and relatively straightforward to implement. Its major drawback is that, because it does not account for the dynamics of the constraint and manipulated variables, a conservative estimate must be taken in how close and how quickly the operation is moved toward the active constraints. 

Although we do not necessarily know the relationships involved in the external world, they do exist and determine the values of the relevant load disturbances. Consequently, we can say that the external world removes as many degrees of freedom as the number of disturbances. In Fig. 7.9 we are introducing a flow control loop to keep F constant, and a temperature control loop with a preheater to maintain qF. The feed composition is fixed by a relationship that we do not know—but all the same must exist. The control objectives are achieved by fitting suitable control systems to the plant and we can say that these control systems remove as many degrees of freedom as the number of control objectives in the overall control strategy. 

An alternative control strategy fixes the reactor-inlet toluene flow rate [16]. Fresh toluene is fed into the condenser drum of the last distillation column, on level control. Production-rate changes can be achieved by changing the setpoint of the toluene reactor-inlet flow, or the setpoint of the reactor-inlet temperature controller. When this control structure is used, the whole range of conversion becomes stable. Drawing of this control structure is left as an exercise to the reader. 

Figure 9.4 Plantwide control strategy fixing reactor-inlet flow rates.

However, design constraints may limit our ability to exercise this strategy concerning fresh reactant makeup, An upstream process may establish the reactant feed flow sent to the plant. A downstream process may require on-demand production, which fixes the product flowrate from the plant. In these cases, the development of the control strategy becomes more complex because we must somehow adjust the setpoint of the dominant variable on the basis of the production rate that has been specified externally. We must balance production rate with what has been specified externally. This cannot be done in an open-loop sense, Feedback of information about actual internal plant conditions is required to determine the accumulation or depletion of the reactant components. This concept was nicely illustrated by the control strategy in Fig. 2.16, In that scheme we fixed externally the flow of fresh reactant A feed. Also, we used reactor residence time (via the effluent flowrate)... 

Fixing a flowrate in a recycle stream does not conflict with our discussion of picking a dominant reactor variable for production rate control in Step 4. Flow controlling a stream somewhere in all recycle loops is an important simple part of any plantwide control strategy. 

Similarly to our approach, Luyben and co-workers [4, 5] proposed to fix the reactor-inlet flow rate of acetic acid and to use the fresh feed to control the inventory in the bottom of the acetic-acid distillation column. The two control strategies are equivalent... 

Control structure 2 overcomes these problems. No reactor composition measurement is used, and throughput is directly fixed by flow-controlling the fresh feed Fqa. This control scheme is intuitively appealing and is often proposed in developing control strategies for this type of process. Unfortunately, as we demonstrated in the previous section, it does not work. 

The process considered here is identical to the one analysed in section 3.1.1, but a different control strategy is used (Fig. 4a). Now the reactor-inlet flow including fresh reactant and recycle is fixed, say at the value F, while the fresh reactant is fed on level control in a surge vessel. An alternative would be to feed the reactant in the reflux drum of the distillation column. This strategy achieves the regulation of reactant inventory, because any imbalance is reflected in a holdup change. 

As for the overall control strategy of the process, it is found that fixing of the two reflux ratio does not work for - -20% changes of the feed IPA composition. The recommended overall control strategy is the one that ratios the reflux flowrates of the two columns to the fresh feed flowrate. The high-purity of the two products can be maintained despite very large feed disturbance variations. 

---