Introduction into Reaction Engineering

Usually, a Chemical Engineer is hired to:

• maintain and operate a process

• fix some perceived problem

• increase capacity or selectivity at minimum cost

• Searching for alternate processes to replace old ones

• Finding ways to make a product from different feedstocks

• Reducing or eliminating a troublesome by product

Some of the major parameters in the design of chemical reactors include:

Reaction Rate

This is the rate at which a species loses its chemical identity per unit volume. The rate of a reaction can be expressed as the rate of disappearance of a reactant or as the rate of appearance of a product. Consider species A being converted to product B:

If B is being created at 0.2 moles per decimeter cubed per second, ie,

r_B = 0.2 mole/dm³/s

Then A is disappearing at the same rate:

-r_A = 0.2 mole/dm³/s

For a catalytic reaction, we refer to -r_A, which is the rate of disappearance of species A on a per mass of catalyst basis.

Let’s assume that r_j is the rate of formation of a species j per unit volume e.g., mol/dm³*s. The r_j is:

• a function of concentration, temperature, pressure and the type of catalyst (if any)
• independent of the type of reaction system (batch, plug flow, etc.)
• is an algebraic equation, not a differential equation. 

Catalysts are an important aspect of reaction engineering. They alter the speed of a reaction by lowering the activation energy of a reaction. The activation energy of a reaction is the amount of energy needed to produce a forward or reverse reaction. In the following figure we can see the activation energy of a non catalytic reaction (red line), while the green line represents the same reactions activation energy when a catalyst is used.

Examples 

What are reversible reactions?