ReactorChemical reactors are very important unit operations in industrial practice. The reaction occurred in a reactor can either give off heat (exothermic) or absorb heat (endothermic). The reactor is generally assembled with a jacket or coil in order to maintain the reaction temperature properly. If heat is evolved due to exothermic reaction, a coolant stream is required to pass through the jacket (or coil) to remove the extra heat. On the other hand, if endothermic reaction occurs in the system, the flow of a heating medium is considered through the surrounded jacket (or immersed coil) to maintain the reactor temperature at its expected value.The range of forms of chemical reactors is so wide that it is really difficult to establish a complete, systematic classification. However, the primary classification can be based on the number of phases to be involved. When only one phase is required, the system is said to be homogeneous. If more than one phase are present, the system is a heterogeneous one.The chemical reactors also can be classified based on the feeding mechanism as: batch, semi-batch and continuous flow reactors. In the batch reactor, a certain amount of material is introduced first, but there is no further inflow or outflow of materials as the reaction proceeds. For the case of semi-batch reactor, some components are either fed in or withdrawn as the reaction occurs. The name continuous flow reactor indicates that this reactor runs with continuous flow of reactants and products.Chemical reactors are usually vessel type. There are two main basic vessel type reactors, which are designed as tank reactor (a tank) and tubular reactor (a pipe or tube). Batch, semi-batch and many continuous flow reactors are generally tank reactors. Plug flow reactor (PFR) is an example of tubular reactor. The tubular reactors are operated only with a continuous mode. Indeed, there are two distinct types of reactors in continuous processes: continuous stirred tank reactor (CSTR) and PFR.Also, we may consider the following types of reactors. When a reactor operates at a constant temperature, then that is called as the isothermal reactor. If any exothermic or endothermic reactions are involved in the reactor, the temperature of the reaction mixture varies with time and we need to develop the energy balance equation for this non-isothermal reactor. It is well-known that the term adiabatic means no interchange of heat between the system and the surroundings.So, usually no heating and cooling arrangements are provided with the adiabatic reactor. In addition, no heat transfer is involved in between the well-insulated reactor and the external environment. For the case of non-adiabatic reactor, there is heat flow into or out of the reactor. One interesting feature of a non-isothermal CSTR, in which exothermic reaction takes place, is the multiple steady states (for a particular value of the input variable, several values of the output variable may be obtained). Multiple steady states may be the result of energy feedback and the highly nonlinear behaviour of the reaction rate constant (accordingto Arrhenius law). This steady state multiplicity can result in an unstable operating condition leading to a quench (the reaction stops) or a runaway (the reactor overheats).