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Managing the CO Oxidation Exotherm: Adiabatic Rise and Heat Recovery

Part of the CO Oxidation: The Complete Guide — this article is one of the detailed pages in the guide.

Direct answer: Every mole of CO oxidized releases roughly 283 kJ of heat — enough to raise a 1% CO stream by 80–90°C adiabatically. The exotherm is both the main operating risk (overheating, sintering) and the main opportunity (free heat for preheating or export). Design starts by computing the adiabatic temperature rise at the peak CO concentration.

Computing the rise

For air-like streams, the adiabatic temperature rise is approximately:

ΔT ≈ 9 × C_CO (°C, with C_CO in % by volume)

So 1% CO ≈ +90°C, 2% ≈ +180°C, and a 5% stream carries +450°C — beyond what any commercial catalyst bed survives. This single number decides staging, dilution and materials.

When the rise is a risk

  • Concentrated streams — above roughly 2–3% CO, the bed can exceed its rated temperature and sinter.
  • Flow interruptions — if the fan trips while CO-rich gas remains, the static bed can overheat locally.
  • Cold-start cycles — preheating a bed that then sees high CO can overshoot on ignition.

Mitigations: multi-stage beds with intercooling or dilution air, temperature interlocks on the fan and fuel supply, and catalyst formulations rated for higher temperature excursions.

When the rise is an asset

  • Preheating the inlet gas — a feed/effluent heat exchanger lifts cold inlet gas to light-off using the outlet stream.
  • Steam or hot water export — a waste heat boiler or economizer downstream converts the rise into process heat.
  • Self-sustaining operation — above roughly 1 g/Nm³ equivalent concentration, the reaction pays for its own preheat at steady state.

Bed thermal design rules

  • Calculate the rise at peak CO, not average.
  • Verify the maximum bed exit temperature against the catalyst's thermal rating.
  • Add high-temperature alarms and, on large systems, a quench or bypass path.
  • In staged designs, size intercooling so no stage exceeds its rating even if one stage underperforms.

Manufacturer perspective

The exotherm calculation is the first step in any CO project — it sets the number of stages, the materials and the heat recovery value. We always ask for the CO range over normal operation, including upset peaks, because those peaks size the safety system.

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