The most common basic types of chemical reactors are tanks (where the reactants mix in the whole volume) and pipes or tubes (for [[laminar flow reactor]]s and [[plug flow reactor]]s)
Both types can be used as continuous reactors or batch reactors, and either may accommodate one or more solids ([[reagent]]s, [[catalyst]]s, or inert materials), but the reagents and products are typically fluids (liquids or gases). Reactors in [[continuous process]]es are typically run at [[steady-state (chemical engineering)|steady-state]], whereas reactors in [[batch production|batch processes]] are necessarily operated in a [[transient state (chemical engineering)|transient state]]. When a reactor is brought into operation, either for the first time or after a shutdown, it is in a transient
state, and key [[process variable]]s change with time.
There are three idealised models used to estimate the most important process variables of different chemical reactors: