Changing the Order of Reactant Additions

A pharmaceutical plant made process chemistry modifications to minimize the emissions of an undesirable byproduct, isobutylene, from a mature synthesis process. The process consisted of four batch operations (see Figure 3.3.5). Emissions of isobutylene were reduced when the process conditions that led to its formation in the third step of the process were identified.

In the first reaction of the process, tertiary butyl alcohol (TBA) was used to temporarily block a reactive site on the primary molecule. After the second reaction was complete, TBA was removed as tertiary butyl chloride (TBC) by hydrolysis with hydrochloric acid. To improve process economics, the final step involved the recovery of TBA by reacting TBC with sodium hydroxide. However, TBA recovery was incomplete because isobutylene was inadvertently formed during the TBA recovery step.

An investigation indicated that the addition of excess NaOH caused alkaline conditions in the reactor that favored the formation of isobutylene over TBA. When the order of adding the NaOH and TBC was reversed and the NaOH addition rate was controlled to maintain the pH between 1 and 2, the isobutylene formation was almost completely eliminated. Therefore, installing add-on emission controls was unnecessary, and the only capital expense was the installation of a pH control loop.

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