Processes and Balances
Energy Balances & Heat
14 min · 2 worked examplesFree
Reactions release or absorb heat at industrial scale, and getting it in or out is usually the hard part of the design.
After this lesson you can
- Account for energy across a process
- Say why counter-current heat exchange wins
- Recognise the conditions for thermal runaway
Accounting for energy
Energy is conserved just as mass is, so the same boundary trick works: energy in equals energy out at steady state, counting every route in and out.
Streams carry sensible heat, which changes with temperature as mcΔT. They carry latent heat when they change phase, which happens at constant temperature and is often much larger than the sensible heat either side of it. Reactions release or absorb heat according to their enthalpy change. Pumps and compressors add work, and heat leaks through the walls.
The number that surprises people is latent heat: vaporising water takes about 2260 kJ/kg, against 4.2 kJ/kg per degree to warm it. Boiling water takes roughly five times more energy than heating it from freezing to boiling, which is why evaporation and distillation dominate the energy bill of a chemical plant.
In the full lesson
- Accounting for energy
- Heat exchangers
- Runaway
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.