Flash drums. Vessels into which flow a mixture of liquid and vapor. The goal is to separate the vapor and liquid. For design calculations it is normally assumed that the vapor and liquid are in equilibrium with one another and that the vessel is adiabatic (no heat lost or gained).
Surge tanks. These are storage tanks between units, and can serve a variety of purposes. They can dampen fluctuations in flow rate, composition or temperature. They can allow one unit to be shut down for maintenance without shutting down the entire plant.
Accumulators. These are storage tanks following distillation column condensers. For partial condensers, this flow may be a mixture of vapor and liquid. The outlet flow may be regulated by a level controller in order to avoid the tank either flooding (liquid out the top) or going dry (vapor out the bottom).