Unit operations controlling

F. Greg Shinskey/ B.S.Ch.E./ Consultant (retiredfrom Eoxboro Co.), North Sandwich, NH. (Fundamentals of Process Dynamics and Control, Unit Operations Control)... 

In unit operations control, the individual column variables are treated only as constraints and so long as the values of these constraints are within acceptable limits, the column is controlled (optimized) to maximize production rate, profitability, etc. Economics of individual fractionators may continually change throughout the life of the plant, because energy savings can be important at one particular time, whereas product recovery can be more important at other times. 

Only after the total process mass balance has been satisfied can we check on the individual component balances in Step 7. That then settles the plantwide issues. We now apply our knowledge of unit operation control in Step 8 to improve performance and remain consistent with the plantwide requirements. Finally, Step 9 addresses higher level concerns above the base regulatory control strategy. 

Whatever the reasons may be for the lack of references on reactor control, we found ourselves in a difficult position in writing this chapter. The initial intent was to give a brief overview of the subject and show-some typical unit operation control strategies to be used in Step 8 of... 

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. 

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. 

Even though unit operation control is not addressed in our plantwide control design procedure until Step 8, it is important to understand up front what all the dominant variables are and their relationship with potential manipulators. This is particularly true if appropriate manipulators are unavailable, in which case design changes must be made. The dominant variables influence several steps in the design procedure, in particular our choice of controlling reactor temperature, production rate, and recycle stream compositions. 

Armed with the thermodynamic fundamentals of heat management, we now take a closer look at the unit operation control loops for heat exchangers. We start with utility exchangers. These are used when heat is supplied to, or removed from, the process. Examples are steam-heated reboilers, electric heaters, fuel-fired furnaces, water-cooled condensers, and refrigerated coolers. 

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