Reactants inventory

Control the size of the enumerated library As the name implies, combinatorial libraries can explode in size very quickly. Therefore one must perform reactant-level selections before product enumeration in most design cases. As shown in the example library, molecular weight (MW) is an effective filter to cut down number of reactants, so is reactant availability inside the reactant inventory system. As a matter of principle, more expensive computational approaches (e.g., protein-ligand docking and scoring) should be applied only to smaller subsets of reactants or products. 

In recycle systems, the design of the chemical reactor and the control of the reactants inventory are interrelated [8]. Figure 4.2 shows two different ways of controlling the inventory in a simple system. The first strategy consists of setting the feed on flow control. Consider the simple example presented in Figure 4.2(a). When more fluid is fed to the vessel, the level increases. The outlet flow rate, which is proportional to the square root of the liquid level, will also increase. After some time, the feed and outlet flows are equal, and a state of equilibrium is reached. 

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 first inequality characterizes recycle systems with reactant inventory control based on self-regulation. It occurs because the separation section does not allow the reactant to leave the process. Consequently, for given reactant feed flow rate F0, large reactor volume V or fast kinetics k are necessary to consume the whole amount of reactant fed into the process, thus avoiding reactant accumulation. The above variables are grouped in the Damkohler number, which must exceed a critical value. Note that the factor z3 accounts for the degradation of the reactor s performance due to impure reactant recycle, while the factor (zo — z4) accounts for the reactant leaving the plant with the product stream. 

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. 

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. 

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. 

The flow rates of fresh reactants can be set at arbitrary values, but within stoichiometric constraints. Then, the internal flow rates and concentrations adjust themselves in such a way that, for each reactant, the net consumption rate equals the feed flow rate. Therefore, the reactant inventory becomes self-regulating. When N = R, Eq. (14) has a unique solution, and in consequence the reaction rate constants or the reactor volume do no influence neither the selectivity nor the production rate. 

In this section, we will show that strategies based on controlled-regulation of reactant inventory have two important advantages ... 

Control StniCturB CSS. As mentioned in the previous section, it is desirable to use inferential temperature measurements instead of direct composition measurements whenever possible. However, the results show that control structure CS7 is not an effective control structure, at least with the optimum steady-state design studied here. It appears that some direct composition information about the reactant inventory inside the system is required for a more effective control system because the column is designed for neat... 

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