Processes and Balances
Mass Balances
14 min · 2 worked examplesFree
Everything that goes in comes out or accumulates. The simplest idea in the discipline, and the one every calculation rests on.
After this lesson you can
- Write in equals out plus accumulation for any process
- Do a component balance to find an unknown stream
- Say why a recycle loop needs a purge
In equals out plus accumulation
Draw a boundary around any piece of a process and mass is conserved across it:
in − out + generated − consumed = accumulated
For a process at steady state nothing accumulates, so the equation collapses to in = out. For a purely physical operation with no reaction, generation and consumption are zero as well, leaving simply what goes in comes out.
That is the whole tool. Its power is in the choice of boundary: draw it around one unit, around a group of units, or around the entire plant, and you get a different equation each time. Choosing the boundary that makes the unknowns fall out is the actual skill, and it is the same instinct as choosing a pivot when taking moments.
In the full lesson
- In equals out plus accumulation
- Component balances
- Recycle and purge
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.