Section 1.2.3: Basic Reactors
The Well-Mixed Reactor
The most common type of chemical reactor consists basically
of a tank fitted with a stirrer. The stirrer is meant to
ensure that the contents of the vessel are well mixed, so that
conditioons everywhere are the same. In mathematical terms
this is thus a lumped system.
This type of reactor may be operated in three different ways.
The other major type of chemical reactor consists of a tube or pipe
down which reactants flow continuously and turn into products, leaving
at the far end. This is called a tubular or plug-flow reactor
or PFR. It is obviously a distributed system since conditions vary
spatially along the tube, and possibly also across it. Except
during start up and shut down it will normally be operated in a steady state
- The vessel is filled with reatants and the reaction
allowed to proceed until a sufficient proportion of these are
converted to product. This is called batch operation.
It is clearly an unsteady-state situation, and so is described
by o.d.e.s with time as the independent variable.
- Reactants are continuously fed to the reactor at a constant rate and
product continuously withdrawn. After the initial startup, this is a steady
state situation described by algebraic equations. In this mode
of operation the reactor is referred to as a continuous stirred
tank reactor or CSTR.
- In a fed batch reactor typically one reactant is initially
charged into the reactor and the second fed to it continuously
over a period of time. No product is withdrawn until the reaction
period has elapsed, when the feed is stopped and the entire contents
removed. The rate at which reactant is supplied may be varied as the
reaction proceeds. Mathematically this is almost the same as batch operation.
The rate of a chemical reaction depends primarily on the amount
of reactants present.
For a unimolecular reaction, e.g.:
A -> products
We can write a rate equation:
R = - V k Can
Rdefines the rate at which A disappears and thus at which products
- R is the extensive rate (extent) of the reaction in kmol/hr
- V is reactor volume in m³
- k is the rate constant on a per hour basis; its units depend
on reaction order
- n is reaction order and may be nonintegral
- Ca is the concentration of species A in kmol/m³
If n=1 the reaction is said to be First Order and
all equations have a particularly simple solution.
For a bimolecular reaction, e.g.:
A + B -> products
Bimolecular reactions are often Second Order and
we can write a rate equation:
R = - V k Ca
Where Cb is the concentration of B.
- A CSTR model for a unimolecular reaction appears in
- The development of a model for a second order fed batch
reactor is described in
and used in handin 5.
Move back to Revision Index