Tc

9.2.2 Irreversible Reaction with Heavy Product

The ethylene glycol reactive distillation system is an example of a reactive distillation system with two reactants that are consumed in a fast and irreversible reaction.

Ethylene oxide + water —> ethylene glycol

Figure 9.3 shows the system and an effective control structure. Ethylene oxide is very volatile, and ethylene glycol is very heavy, so the product is removed from the bottom of the column. The ethylene oxide concentrates in the top of the column. No distillate product is removed. The water feed is introduced to hold the liquid level in the reflux drum. This level loop achieves the necessary balancing of the reaction stoichiometry by adjusting the makeup water flowrate to exactly match the water consumption by reaction with ethylene oxide. Production rate is set by flow-controlling the ethylene oxide.

9.2.3 Neat Operation versus Use of Excess Reactant

If the reaction involves two reactant feedstreams, two basic flowsheets that are used. Consider the reaction A + B C + D. One way to design the process is to feed an excess of one of the reactants into the reactive distillation column along with the other reactant. Figure 9.4 shows a system in which an excess of reactant B is fed. In most cases this excess must be recovered. A second distillation column is used in Figure 9.4 to achieve this recovery. The fresh feed of reactant B is mixed with the recycle of B coming from the recovery column.

The control of this system is fairly easy. The total flow of B to the reactive column is controlled by manipulating the fresh feed of reactant B. The fresh feed of reactant A sets

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