Batch Nitration Runaway Risk and Scale-Up
A 5 m3 batch reactor is used for nitration of an aromatic compound with mixed acid. The reaction mass is 4,200 kg with a heat capacity of 2.1 kJ/kg·K. Cooling is by a jacket and an internal coil with 25 m2 area; cooling water supply is 28 C. Adiabatic calorimetry gives a reaction onset at 120 C, a maximum temperature of synthesis reaction (MTSR) of 185 C, and a decomposition onset of 165 C. The reactor relief valve is set at 2.5 barg and vents to a scrubber through a 4-inch line. The operating temperature is 95 C. A recent batch had a cooling-water failure; the temperature rose from 95 C to 142 C in 12 minutes before the operator restored cooling. The batch size is to be increased by 20% using the same reactor and cooling system. The plant proposes to rely on the existing relief valve and add a high-temperature interlock at 150 C.
1. Assess the thermal hazard using the calorimetry data, and explain what the 142 C event indicates.
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Operating at 95 C is 25 K below the reaction onset (120 C), which is a reasonable margin, but the MTSR of 185 C is above the decomposition onset of 165 C. That means a cooling failure at the wrong moment could take the mass into a decomposition regime, not just a synthesis runaway. The observed rise from 95 C to 142 C in 12 minutes is already above the reaction onset and shows the system can generate heat faster than the jacket and coil remove it after failure. It did not reach decomposition onset, but the margin was only 23 K. The relief valve set at 2.5 barg may not protect against a two-phase runaway; the 4-inch vent and scrubber need a relief-sizing basis. The high-temperature interlock at 150 C is too close to the decomposition onset and acts only after significant runaway. A proper MTSR, adiabatic temperature rise, and relief scenario are needed.
2. Evaluate the proposed 20% scale-up with the same cooling system and existing relief valve. What calculations or tests are needed?
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A 20% increase in batch mass raises heat generation by about 20%, while the jacket and coil area stay the same. The cooling capacity per unit mass falls, so the adiabatic time to maximum rate shortens. The observed 47 K rise in 12 minutes would become faster, and the MTSR may not change per unit mass but the path to it is less controllable. The existing relief valve cannot be assumed adequate: venting a reacting nitration mass may be two-phase or gassy, and the scrubber may be undersized. Required: heat-transfer calculations for the scaled batch (UA, jacket delta-T, coil area), adiabatic calorimetry at the new concentration and volume, relief sizing using a recognized method for two-phase flow, and a scrubber capacity check. Also test the interlock at 150 C; it is above the observed 142 C and below decomposition onset, so it may not prevent decomposition. Reduce batch size or improve cooling and interlocks.
3. Recommend a layered control strategy for the scaled-up batch.
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Prevent: keep operating temperature well below onset; limit accumulation of unreacted mixed acid by controlled addition and adequate agitation; verify cooling-water reliability and backup. Detect: add reactor temperature, jacket temperature, and agitation interlocks; set a high-temperature trip at a lower value, for example 130-135 C, with feed cut-off and emergency cooling. Mitigate: provide emergency cooling or quench, a catch tank, and a properly sized relief system with scrubber; consider a dump tank. Procedural: batch record with hold points, cooling-failure drill, and independent alarm. Management: re-run calorimetry and relief design before scale-up; do not rely on a single interlock. If relief cannot be demonstrated, do not increase batch size.