Plantwide Control Structure

We say that the inventory is self-regulating. Similarly, the plantwide control can fix the flow rate of reactant at the plant inlet. When the reactant accumulates, the consumption rate increases until it balances the feed rate. This strategy is based on a self-regulation property. The second strategy is based on feedback control of the inventory. This consists of measuring the component inventory and implementing a feedback control loop, as in Fig. 4.2(b). Thus, the increase or decrease of the reactant inventory is compensated by less or more reactant being added into the process. 

The feedback-control strategy can be implemented through the following steps [8]  

The important advantage of this strategy is that the reactor behaves as decoupled from the rest of the plant The production is manipulated indirectly, by changing the recycle flows, which could be seen as a disadvantage. However, it handles nonlinear phenomena better, such as for example the snowball effect or state multiplicity. Additionally, this strategy guarantees the stability of the whole recycle system if the individual units are stable or stabilized by local control. 

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The dynamics and control of the reactor-column system studied in Chapter 2 (Section 2.9.3) are investigated in this section. Mathematical models of both the reactor and the column are developed, and a plantwide control structure is evaluated. 

Figure 3.38 Reactor-column plantwide control structure.

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 conclusion of this analysis is that plantwide control structures relying on self-regulation can be used, but the snowball effect is avoided only when the reactor volume is large enough. For a first-order reaction involving one reactant, the reactor could be considered sufficiently large if Da > 3. 

The conclusion of this analysis is that plantwide control structures that use feedback to control reactant inventory do not show the snowball effect. These structures can be applied for both large and small reactors, the difference being the variable manipulated for achieving production-rate changes. 

The reactor/separation/recycle level allows plantwide control issues to be included in the hierarchical approach at an early level of design. In most cases, the Separation is considered as a black-box for which targets are set, for example as species recovery or product purities. A stoichiometric or a kinetic reactor can be used. In the first case, plantwide control structures can only be proposed, while in the second case these can be also evaluated. 

Dynamic simulation starts by proper sizing of units and by the evaluation of the key plantwide control structures. The manipulated variables should have sufficient... 

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. 

Table 5.15 Plantwide control structures of the material balance.

The plantwide control structure is presented in Figure 7.11. The most interesting points are discussed below. Firstly, we fix the reactor-inlet flow rate and feed the fresh EDC on level control. Control of reactor and cooling section does not raise any problem. Since the HCl column operates mainly as a stripper, the temperature in the bottom is controlled by manipulating the steam rate, so as to ensure... 

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. 

From the previous analysis, we conclude that a robust plantwide control structure will fix the combined isobutane + recycle (Fj) and fresh butene flow (F0), as illustrated in Figure 9.4. The desired production rate and selectivity could be achieved in a 3-m3 reactor, operated at 268 K. The operating point shows low sensitivity to errors in the manipulated variable Fj (Figure 9.5). This design seems to ensure feasible operation even if the temperature decreases to 260 K (Figure 9.6) or the catalyst activity becomes 40% of the initial value (Figure 9.7), irrespective of the purity of the butene feed stream (Figure 9.8). 

The large swings in recycle flowrates are undesirable in a plant because they can overload the capacity of the separation section or move the separation section into a flow region below its minimum turndown, Therefore it is important to select a plantwide control structure that avoids this effect. As the example below illustrates and as... 

Case 2 includes many of the example systems studied in this book. For example, reactors with temperature as the only controlled variable fall into this category. Also, the isothermal ternary scheme CS4 shown in Fig. 2.13a has a local composition controller on one of the dominant variables, the composition of component A. However, Case 2 is characterized by the fact that other dominant variables are not controlled at the reactor. Instead, the plantwide control structure plays a significant role in its ability to influence these uncontrolled variables. When the uncontrolled compositions become disturbances and the controlled dominant variables are too weak, we have difficulties. On the other hand, the plantwide control structure can be arranged to provide indirect control of the dominant composition variables, thereby augmenting the unit control loops. The HDA process provides a good illustration. The dominant variables are reactor inlet temperature and toluene composi-... 

Case 3, finally, provides the ultimate challenge for the plantwide control structure. Here, all the dominant variables in the reactor are influenced by the actions of controllers elsewhere in the plant. Now it becomes imperative that the plantwide controllers provide indirect control over all or most of the dominant variables. Several examples in Chap. 2 demonstrated this. As we showed in Chap. 2, it is very easy to configure schemes that turn the dominant variables into reactor disturbances. These schemes don t work at all, Consequently, we do not recommend building plants without local unit operation control for the reactor. 

In this chapter we have presented some fundamental concepts of distillation control. Distillation columns are without question the most widely used unit operation for separation in the chemical industry. Most final products are produced from one end or the other of a distillation column, so tight control of product quality requires an effective control system for the column. However, the column is usually an integral part of an entire plant, so its control scheme must also be consistent with the plantwide control structure. 

Figure 10.2 gives the base-case plantwide control structure developed. Total toluene flowrate to the reaction section is flow-controlled. We will make step changes in this flow controller setpoint. Reactor inlet temperature is controlled by the firing rate in the furnace. No heat-exchanger bypass is shown in Fig. 10.2, but we will look at the effect of bypassing the FEHE. Control structure CS2 discussed in Chap. 5 adds a temperature control loop that controls furnace inlet temperature by manipulating the bypass flowrate around the FEHE. See Fig. 5.25. 

The strong competition in the industrial environment nowadays demands for economical operation of chemical plants. This goal can be achieved in two ways, which do not exclude each other. One approach is to continuously respond to the market conditions through dynamic operation. A second approach is to develop control systems that maintain the steady state or implement the optimal dynamic behaviour. For the first approach, the economical optimality is achieved through dynamic optimization. For the second approach, the development of the plantwide control structures to achieve stable operation is of paramount importance. 

The plantwide control structure (Figure 2) is the same as the one determined to have a stable behaviour in [6] the flowrate of the fresh butene is specified, while the /50-butane is introduced by inventory control. 

Examine plantwide control problems, as the input of reactants, the manipulation of the production rate, and the control of waste and impurities. Develop a plantwide control structure (see Chapter 13). 

Chemical conversion is an effective way to counteract the accumulation of impurities due to positive feedback. Also, changing the connectivity of units may be used to modify the effect of interactions, for example by preventing an excessive increase in recycles due to snowball effects. Effective plantwide control structures may imply controlled and manipulated variables belonging to different but dynamically neighbouring units. The methodology to evaluate the dynamic inventory of impurities consists of a combination of steady state and dynamic flowsheeting with controllability analysis. This is used to assess the best flowsheet alternative and propose subsequent design modifications of units. Case Study 3 in Chapter 17 will present this problem in more detail. 

The set of specifications used in the previous section (Fq, V, Z3, Z4) can be viewed as a conventional plantwide control structure, as displayed in Fig. 13.20a. Plant throughput is set by the reactant feed, the reaction volume is kept constant, and the separation section is dual-composition controlled. For this control structure, the feed disturbances affect the flow rate and composition of the reactor outlet/separation inlet. Hence, manipulated variables internal to separation section are used to reject the disturbances. As a result, disturbances are rejected mainly by changing the reaction conditions. 

The implementation of a plantwide control structure on a given design can follow a step-wise procedure (Luyben Tyreus, 1999). The actions with plantwide character regard energy management, production rate, product quality, safety and environmental protection, and control of impurities. 

The plantwide control philosophy has been discussed already in Chapter 13, and illustrated by Fig. 17.8. The following plantwide control structures is proposed ... 

Steady state controllability analysis. A simple and efficient plantwide control structure can be built with multi-SISO PID controllers. This step enables to evaluate the control structures of decentralised (integral) feedback control. The main actions are Determine steady-state gains for plant and disturbances. 

Interactions through recycles can be exploited to create plantwide control structures that are not possible when a stand-alone approach is adopted. In this case, acceptable control of three key impurities can be achieved with only two control loops. 

USE OF STEADY-STATE SENSITIVITY ANALYSIS TO SCREEN PLANTWIDE CONTROL STRUCTURES... 

So far in this chapter we have considered some specific features and problems concerning plantwide control. In this section we present a design procedure that can be used to generate an effective plantwide control structure. 

We want to develop a process flowsheet and plantwide control structure for a plant in which the reaction A + B —> C + D occurs in a CSTR. Fresh feed makeup streams of pure component A Fqa) and pure component B (Fob) are used. The per-pass conversion is not 100 percent, so the reactor effluent contains all four components. 

Use of Steady-State Sensitivity Analysis to Screen Plantwide Control Structures 190... 

Be able to perform a conceptual synthesis of plantwide control structures (pairings) based on degrees-of-freedom analysis and qualitative guidelines. 

The steady-state design of a two-column extractive distillation system was developed in Chapter 5. Now, we want to design an effective control structure for this system. The process has two distillation columns, and a plantwide control structure must be developed that accounts for the interaction between the columns and for the solvent recycle. 

In this section, we demonstrate how this second recycle loop can be successfully converged in Aspen Dynamics. A plantwide control structure is developed and its... 

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