Molar balance is very important concept for chemical engineering undergraduate students. For reactor systems in chemical engineering, this phenomenon is widely applied. This theory is helped to calculate flow rates, reaction rates in industrial applications.

In the previous chapter we learned about types of reactor systems used in chemical engineering.

- Batch reactor
- Continuous stirred tank reactor ( CSTR )
- Plug flow (tubular) reactor (PFR)

There may be a inflow and a outflow in a reactor and reactions are happening inside the reactor. Therefore molar balance equation includes several terms to describe each of those processes. General molar balance equation is figured below.

In the molar balance equation, there are two terms such as**molar generation** and **molar consumption**. Both of
them exists in opposite directions in the equations. Instead of both of them we only use molar generation which represent both
generation and consumption together.

We represent

Generation as **+ Generation**

Consumption as **- Generation**

Now our equation is reduced as below.

Below figure shows us reactor system with inflow and outflow. We choose one species A for our demonstration.

First, A enters into the system through inflow. Then one or several reactions are happened in the system. Finally remaining A comes out from the system.

For calculate generation we consider small part of volume in the system.

If A is disappearing/ reducing / consumption (due to reacting)

Generation term become negative

Some reactantant or product can be accumulated inside the reactor. Assume that there are N_{A} moles at time t in the reactor. We can write the accumulation rate as below.

Finally, we write the general equation for any reactor system as below.

**Assumptions**

If rate of reaction ( r_{A} ) is same in everywhere in the reactor ( no spatial variation ), r_{A} becomes
constant

If process is in steady state,

According to those assumptions out **molar balance equation for continuous stirred tank reactor**

Conditions of the PPR change along the reactor.

Select a small volume part and we take our selected specie as A. Inflow of reactor is F_{A}|_{V} and
outflow is F_{A}|_{V+ ΔV}

**Assumptions**

He we consider steady state. ( dN_{A}/dt = 0)

According to those assumptions out **molar balance equation for continuous stirred tank reactor**

There are no inflow or outflow in the batch reactor. Initially, we filled the reactor by reactants
and take out products (reactants may be present in products if reaction had not completed. ) after the reaction. Therefore F_{A} = 0 and F_{AO} = 0

But reaction rate (r_{A}) is different in everywhere in the reactor