CONTROL OF EXCESS REACTANT SYSTEMS

In Chapter 4 we compared the economics of two alternative reactive distillation processes for the quaternary system with the reaction A + B C + D. The first was a single reactive column operating in neat mode. The second process was a two-column flowsheet in which the reactive column was fed with an excess of one of the reactants and the excess was recovered in a second distillation column. The one-column system was shown to be significantly less expensive. 

However, we mentioned that the one-column process may be more difficult to control. There are several reasons for this concern. The one-column system requires a control system that can achieve the necessary precise balancing of the fresh feedstreams so that there is no gradual buildup of one of the reactants, which could cause a loss of conversion and product purities. Some method for measuring or inferring the amount of at least one of the reactants inside the process must be available. A direct composition measurement may be necessary, which has major disadvantages compared to the use of simple and reliable temperature measurements. The one-column system also has the inherent disadvantage of having fewer manipulated variables to achieve control and attenuate disturbances. 

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However, the flowsheets for the first and second types of chemistry come in two flavors. The flowsheet can consist of either a single reactive column or a two-column system with a reactive distillation column followed by a recovery column and recycle of excess reactant back to the reactive column. The type of flowsheet depends on whether we want to operate the reactive distillation column neat (i.e., no excess of either reactant). The excess reactant flowsheet has higher capital and operating costs, but its control is easier. The control of this system is considered in Chapter 11. 

We present quantitative comparisons of the dynamic controllability of these two flowsheets in this chapter. Two different control structures are considered for the one-column neat system and one for the two-column excess reactant system. 

Another very common example of this type of system is in controlling two feed streams to a reactor where an excess of one of the reactants could move the composition in the reactor into a region where an explosion could occur. Therefore, it is vital that the flow rate of this reactant be less than some critical amount, relative to the other flow. Multiple, redundant flow measurements would be used, and the highest flow signal would be used for control. In addition, if the differences between the flow measurements exceeded some reasonable quantity, the whole system would be interlocked down until the cause of the discrepancy was found. 

However, the MEP may be a convenient measure of the progress of a molecule in a reaction, because in general a molecule will move, on average, along the MEP in a well-defined valley, and it is a good approximation of the motion of vibrationally cold systems (e.g., for photochemical reactions in which the excited state reactant has a small/controlled amount of vibrational excess energy). 

We might be tempted to control reflux drum level with one of the fresh reactant feeds, as done above. The problem with this is that the material in the drum can contain a little of component C mixed with either A or B, Simply looking at the level doesn t tell us anything about component inventories within the process and which might be in excess. The sj stem can fill up with either. Some measure of the composition of at least one of the reactants is required to make this system work. Compositions in the reactor or the recycle stream indicate an imbalance in the amounts of reactants being fed and being consumed. If direct composition measurement is not possible, inferential methods using multiple trays temperatures in the column are sometimes feasible (Yu and Luyben, 1984). 

A fresh reactant feed stream cannot be flow-controlled unless there is essentially complete one-pass conversion of one of the reactants. This law applies to systems with reaction types such as A + B -> products and was discussed in Chap. 2, In systems with consecutive reactions such as A - B -> M - C and M + B -> D - C, the fresh feeds can be flow-controlled into the system because any imbalance in the ratios of reactants is accommodated by7 a shift in the amounts of the two products (M and D) that are generated. An excess of A will result in the production of more M and less D. An excess of B results in the production of more D and less M. 

We conclude that most reaction systems in the chemical industries are exothermic. This has some immediate consequences in terms of unit operation control. For instance, the control system must ensure that the reaction heat is removed from the reactor to maintain a steady state. Failure to remove the heat of reaction would lead to an.accumulation of heat within the system and raise the temperature. Forreversible reactions this would cause a lack of conversion of the reactants into products and would be uneconomical. For irreversible reactions the consequences are more drastic. Due to the rapid escalation in reaction rate with temperature we will have reaction runaway leading to excessive by-product formation, catalyst deactivation, or in the worst case a complete failure of the reactor possibly leading to an environmental release, fire, or explosion. 

While the individual reaction rates are the variables that, can be affected in a reacting system, we often express the performance of the reactor in terms of measures derived from the rates. Conversion and yield are such quantities. Conversion refers to the fractional consumption of a reactant in the reactor feed, whereas yield refers to the amount of product made relative to the amount of a key reactant fed to the reactor. In recycle systems the per-pass conversion of the various reactants is a relevant measure. It depends upon the rate of reaction for the specific component but also on the reactor feed. The per-pass conversion of an excess reactant is less than that of a limiting reactant. For example, the per-pass conversion of ethylene in a typical vinyl acetate reactor is only 7 percent whereas the per-pass conversion of oxygen is 36 percent. In Chap. 2 we discussed the plantwide control implications of incomplete conversion. 

If the use of an organic solvent is preferable, an FC-72/organic solvent binary system can be used. The reaction was conducted in a 1 1 mixture of FC-72 and toluene. However, a mixture of equimolar reactants failed to give complete conversion in this protocol. The use of a slight excess of alcohol (1.2-1.3 equiv.) was required for satisfactory yields (>99%). The catalyst was recovered without loss (>99%) from the FC-72 layer. Notably, however, a control experiment without the catalyst afforded only a 23% yield. 

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