How can startup/shutdown phases introduce hazards?

• A. Heat removal is reduced and inhibitors may degrade, increasing runaway risk.
• B. Pressure buildup is negligible during startup.
• C. Startup phases always safely manage hazards.
• D. Inhibitors always stabilize during startup.

Answer: (A)

During startup and shutdown, the system is not in its steady state, so heat and chemical controls behave differently. Heat removal can be limited because cooling circuits, heat exchangers, or cooling water flows may not be fully in service yet, or may be operating at reduced effectiveness. That means heat released by exothermic reactions can accumulate instead of being carried away, pushing the temperature up faster than the system can safely handle. If the temperature climbs and the reaction accelerates, you can get a runaway scenario unless cooling catches up.

Inhibitors, which are added to prevent uncontrolled reaction or polymerization, can degrade or be consumed more quickly during these transition periods. Elevated temperatures, contact with surfaces, impurities, or prolonged residence times can reduce their effectiveness. When inhibitors lose their protective function, the same exothermic processes that threaten safety can run unchecked, increasing the risk of a hazardous release, pressure buildup, or thermal explosion.

So startup and shutdown are particularly risky because the normal, steady heat balance and chemical control are momentarily disrupted, making heat removal and inhibitor effectiveness critical factors. The other statements aren’t reliable: pressure can build during these phases, hazards aren’t automatically avoided, and inhibitors don’t always stay stable.