What IS accumulation?
Recall that so far in this text it has been assumed that all systems are at steady state, which means that there is no buildup of mass, energy, or other conserved quantities. However, there are many situations, such as whenever operating levels change, that a system will not be at steady state, and mass and energy will be accumulated over time.
The most important thing to remember about accumulation is that it deals with the actual amount of stuff in the system, not any sort of flow rate. If you remember if you’re dealing with actual system properties rather than flow rates, it will help keep the terms straight in unsteady-state balances.
Unsteady-state Mass Balance