Plant-wide control: recycle streams

To demonstrate the principles of plant-wide control for a recycle system, consider the ethylene glycol plant shown in Figure 10.4. Equivalent amounts of water and ethylene oxide are fed to a reactor, as dictated by the reaction stoichiometry, to produce ethylene glycol. The liquid product stream is sent to a distillation column to separate unreacted water and ethylene oxide from the ethylene glycol. The unreacted feed is sent back through a recycle loop to the reactor. 

Most chemical processes involve two important operations (reaction and separalion) that are typically carried out in different sections of the plant and use different equipment. The reaction section of the process can use several types of reactors [continuous stirred-tank reactor (CSTR), tubular, or batch] and operate under a wide variety of conditions (catalyzed, adiabatic, cooled or heated, single phase, multiple phases, etc.). The separation section can have several types of operations (distillation, extraction, crystallization, adsorption, etc.), with distillation being by far the most commonly used method. Recycle streams between the two sections of these conventional multiunit flowsheets are often incorporated in the process for a variety of reasons to improve conversion and yield, to minimize the production of undesirable byproducts, to improve energy efficiency, and to improve dynamic controllability. 

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