Heat management means keeping the heat a reaction releases (exothermic) or needs (endothermic) in balance with what the vessel can remove or supply. Slaking 1 kg of quicklime releases about 1,160 kJ, enough to raise 3 kg of water by roughly 93 °C, past boiling from a 20 °C start, so the lime must be added in small amounts. Because heat generation grows with volume and removal with surface, and because reaction rate rises steeply with temperature, a hot reaction that is only slightly under-cooled can run away.
Part of Industrial Chemistry
The heat balance of chemical processes: how to compute a temperature rise, why scale makes cooling harder, and how runaway starts.
Generation Against Removal
Every reaction vessel has two heat flows. Heat is generated by the reaction at a rate that scales with the amount of reacting material, which is the volume. Heat is removed through the wall, at a rate that scales with the wall area and the temperature difference to the coolant. The carrier states the consequence as its first scale-up rule: volume grows faster than surface area (see Scaling Up), so a large vessel retains more heat than a small one.
The carrier’s heat examples are the contact process (SO3 and water are violently exothermic), chlorine and sodium hydroxide mixing, concentrated acid and water, the Haber reaction (written with heat as a product), and lime burning, which needs heat supplied. Burning limestone is endothermic and is covered in Limestone Burning and Lime Kiln; slaking releases heat (Slaking).
The Adiabatic Temperature Rise Is The Worst Case
If no heat leaves, the temperature rise is the heat released divided by the heat capacity of the contents: ΔT = Q ÷ (m × c), where m is the mass and c the specific heat, about 4.18 kJ per kg per °C for water (a recalled value). It is the limit a vessel approaches when cooling fails or the reaction is faster than the wall can serve. It does not need a precise model, only the heat of reaction.
Runaway Is A Rate Problem
Reaction rate rises steeply with temperature; a common rule of thumb says it roughly doubles for each 10 °C. Heat removal through the wall rises only in proportion to the temperature difference. If the reaction heats the mass a little, its rate rises, it heats faster, and the heat generation can outrun removal. That is a runaway. It is the reason a “slightly warm” reaction can end with a vessel at boiling or over pressure.
What You Need
- Heat-of-reaction values for every step, from a source you can name. The corpus gives about 65 kJ per mole for slaking quicklime (Slaking).
- A thermometer in the mass.
- A cooling route: jacket, coil (Heat Exchangers), or an ice bath.
- A way to stop feeding and to dump the batch.
Method: Taming An Exothermic Step
Step 1: Compute the adiabatic rise from the heat of reaction and the contents.
Step 2: If it exceeds the safe temperature, dilute, add the limiting reagent slowly, or cool.
Step 3: Add the reactive ingredient in portions, each after the last has finished and the temperature has settled (semi-batch).
Step 4: Agitate, so heat reaches the cooled wall.
Step 5: Stop the feed the moment temperature runs ahead of plan.
Worked Example: Slaking 1 kg Of Quicklime
Quicklime is CaO: 40 + 16 = 56 g per mole. The slaking page gives roughly 65 kJ per mole.
- Moles in 1 kg: 1,000 ÷ 56 = 17.9 mol.
- Heat released: 17.9 × 65 = 1,160 kJ.
- With 3 kg of water: ΔT = 1,160 ÷ (3 × 4.18) = 1,160 ÷ 12.5 = 92.6 °C, about 93 °C. Starting at 20 °C, the water would reach 113 °C if no heat left, which is above boiling. In practice it boils, as the slaking page says.
- With 10 kg of water: ΔT = 1,160 ÷ (10 × 4.18) = 27.8 °C, a warm bath rather than a boil.
The calculation ignores the lime’s own heat capacity and the heat lost to the air and to evaporation, so it overstates the rise; the direction and rough size are the point. The remedy is to slake in small batches and with plenty of water, as the slaking page directs.
Rate effect of a temperature excursion. If a rise of 30 °C occurs and the rate doubles every 10 °C, the rate multiplies by 2 × 2 × 2 = 8. A 50 °C excursion multiplies it by 2⁵ = 32. The rule of thumb is not exact, but the exponential growth is why alarms are set early.
Common Mistakes
- Estimating temperature from wall temperature. The core is hotter.
- Adding all of a reactive ingredient at once because the first small test was mild.
- Cooling only at the end. Cooling must be in place before the reagent goes in.
- Forgetting endothermic steps need a heat supply, such as lime burning.
Safety
Runaway reactions boil, spray and burst vessels
Never carry out an exothermic step in a sealed vessel, never add water to concentrated acid (add acid to water slowly), and keep a method to stop and to cool before starting. Splash protection is mandatory. See Pressure and Temperature and Protective Gear.
What’s Next
- Learn next: Heat Exchangers — hardware for moving heat
- Combine with: Pressure and Temperature — what heat does to a closed vessel