Alternative Control Structures

Once the controlled variables have been specified, the control structure depends only on the choice of manipulated variables. For a given process, selecting different manipulated variables will produce different control structure alternatives. These control structures are independent of the controller structure, i.e., pairing of variables in a diagonal multiloop SISO structure or one full-blown multivariable... 

For example, in a distillation column the manipulated variables could be the flow rates of reflux and vapor boilup (R, V) to control distillate and bottoms compositions. This choice gives one possible control structure. Alternatively, we could choose to manipulate the flow rates of distillate and vapor boilup (D, V). This yields another control structure for the same basic distillation process. 

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Figures 6.5-6.8 show the control structures used for the four alternative tubular reactor systems. In Figures 6.6 and 6.7 (the multistage adiabatic systems with interstage...

Plantwide Control Structure There are several alternative control structures for this process, and there is no claim that the one developed is the best from whatever perspective you consider. What is claimed is that it provides effective base-level regulatory control of this process. 

The availability of new membrane processes such as membrane contactors and catalytic membrane reactors, the progresses in membrane-fouling control and the development of new membranes with well-controlled structures and properties, are recognized as key factors for the design of alternative production systems. 

The control structure discussed in this section is presented in Figure 4.4(a). The reactor-inlet flow rate is fixed at the value l. Reactor effluent controls the reactor holdup V, while the coolant flow rate controls the reactor temperature. Dual composition control is used for the distillation column. The reactant is fed on level control. For illustration purposes, a buffer vessel was considered. This increases the equipment cost and might be unacceptable due to safety or environmental concerns. An alternative is to feed the reactant in the condenser dram of the distillation column. This strategy achieves the regulation of reactant inventory, because any imbalance is reflected by a change of the holdup. 

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. 

In this section we will examine several plantwide control structures. The control objective is to change the production rate by 10% while achieving the selectivity and purity targets. As discussed in Chapter 4, plantwide control structures can be classified with respect to the strategy employed for controlling reactants inventory [22]. Four different alternatives are presented in Figure 5.22. 

We now explore two alternative control structures for this process. 

Variable reactor holdup structure. An alternative control structure is shown in Figure 2.7. This strategy differs from the previous one in two simple but important ways. 

So what can we do in this case If column operation requires that we stick to this control structure, feedforward control will help to reduce the swings in distillate flowrate. However, better plantwide performance can be achieved if we can switch the control structure to one in which the vapor distillate is not used to control pressure. One possible alternative is shown in Fig. 6.27. Condenser cooling is used to control pressure, reflux flowrate controls reflux drum level (with P-only control), and the flowrate of the vapor distillate is ratioed to reflux flowrate. With this structure we allow the disturbances that the column energy... 

As alternative matrix additives for FCC catalysts, several silico-alinninates with controlled structural properties were prepared and characterized. These silico-aluminates have the potential to crack high molecular weight hydrocarbons without a loss of selectivity. In this... 

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