SELECTION AND SIZING OF HOMOGENEOUS AND CATALYTIC REACTORS

1. SELECTION AND SIZING OF
HOMOGENEOUS AND CATALYTIC
REACTORS
Introduction
Chemical reactors are vessels designed to contain chemical
reactions.
An industrial chemical reactor is a complex device in which
heat transfer, mass transfer and diffusion may occur along
with chemical reaction with provisions of safety and
control.
Chemical reactors can be classified as follows:-
1. Simple Batch Homogeneous Reactors: Like Single
Batch Stirred Tank.
2. Semi – Batch Reactors.
3. Continuous Homogeneous Reactors: Like Continuous
Stirred Tank Reactor (CSTR), Plug Flow Reactor (PFR).
4. Continuous Heterogeneous Reactors: Like Fixed Bed
Catalytic Reactor, Fluidized Bed Catalytic Reactor.
Selection and Sizing of Reactors
All chemical processes are centred in a chemical reactor,
thus selection and sizing of appropriate reactor is the most
important factor in determining overall process economics.
The important considerations during selecting and sizing of
reactor are as follows:-

2. 1. The number of phases present at the reaction
conditions, i.e., at that particular thermodynamic
conditions (i.e., whether the system is homogeneous or
heterogeneous).
2. The stoichiometry of reactions; the number of reaction
steps involved and the important reaction products,
intermediates and by-products.
3. Measure the kinetics of each step (Rate constant and
order of reaction).
4. Identification of the heat requirements of the process,
i.e., reaction is adiabatic or non – adiabatic or
isothermal.
5. The purpose of reactor selection, i.e., it is for
evaluation purpose or for commercial purpose.
HOMOGENEOUS REACTORS
Homogeneous Reactors are single phase reactors. The
common examples are:-
A. SINGLE BATCH STIRRED TANKS
These types of reactors are preferred if the process
involves small quantities, intermittent products,
complicated sequence of operation, or extremely long
holding time.
The design equation used for reactor sizing is:-
𝑁 =
𝑑𝑋
𝑑𝑡
= −𝑟 . 𝑉
where, 𝑁 is number of moles of A initially fed to the
reactor (when we are studying the reaction with respect
to A),
X is the number of moles of product formed at time‘t’,
𝑟 is the rate of reaction with respect to A,

3. V is the volume of the reactor required.
Using this equation, we can estimate the size (volume) of
the reactor vessel for the given process. The size of these
reactors varies from a few litres to thousand litres.
B. CONTINUOUS FLOW STIRRED TANK REACTORS
(CSTR)
These are suitable for large scale processes. They are
used where high degree of agitation is necessary; the
reaction requires relatively low holding time and where
backmixing is not detrimental to yield.
The design equation for reactor sizing is:-
𝑉 =
𝐹 . 𝑋
−𝑟
where, 𝑉 is the volume of the reactor required,
𝐹 is the flow rate of the reactant A,
X is the number of moles of product formed,
𝑟 is the rate of reaction with respect to A.
C. PLUG FLOW REACTOR (PFR)
These are longitudinal tubular reactors which are
mainly used homogeneous gas phase reactions.
Usually, they have large length to diameter ratio.
The design equation for reactor sizing is:-
𝑉 = 𝐹
𝑑𝑋
−𝑟
where, 𝑉 is the volume of the reactor required,
𝐹 is the flow rate of the reactant A,
X is the number of moles of product formed,
𝑟 is the rate of reaction with respect to A.

4. As we can see from
and PFR that −𝑟 is a function of conversion of reactant to
product, i.e., −𝑟 =
of reactor by constructing Levensp
as a function of X
Figure
In the plots, shaded region represent the volume of the
reactor. it also shows that
conversion and reaction condition.
Another most widely used parameter for reactant sizing is
‘Residence time (tR
As we can see from the reactor sizing equations of CSTR
is a function of conversion of reactant to
= f (X). Using this, we can size any type
by constructing Levenspiel Plot; where we plot
as a function of X.
Figure 1 Levenspiel Plot for CSTR
Figure 2 Levenspiel Plot for PFR
, shaded region represent the volume of the
also shows that 𝑉 > 𝑉 for same
conversion and reaction condition.
Another most widely used parameter for reactant sizing is
R)’.
𝑡 =
𝑉
𝑄
the reactor sizing equations of CSTR
is a function of conversion of reactant to
Using this, we can size any type
iel Plot; where we plot
, shaded region represent the volume of the
for same
Another most widely used parameter for reactant sizing is

5. where, V is the volume of reactor tank,
Q is the volumetric flow rate.
CATALYTIC REACTORS
These are usually multiphase heterogeneous reactors. The
common examples are:-
A. FIXED BED REACTORS
These are used for study of solid catalyst. These are
cylindrical vessels packed with catalyst pellets of 1 to 10
mm. Their size varies from a few cm in diameter to
several meters. Reactors more than 14 feet diameter are
avoided.
B. FLUIDIZED BED REACTORS
These are extensively used in catalytic cracking process.
Fluidization can only be used with relatively small sized
particles. The size of catalyst pellets is less than 0.1 mm.
The design equation for catalytic reactor sizing, in terms of
mass of catalyst necessary for conversion at constant
pressure is:-
𝑊 = 𝐹
𝑑𝑋
− 𝑟′
where, W is the mass of the catalyst required,
𝐹 is the flow rate of the reactant A,
X is the number of moles of product formed,
𝑟′
is the moles of A that have reacted per g of catalyst.
Another parameter for sizing of catalytic reactor is:-

6. Weight or Mass Space Velocity =
Importance of Selecting and Sizing a
Reactor
 To obtain the maximum possible yield.
 For achieving the economies of scale, i.e., maximum
yield using the smallest reactor.
 To obtain a better yield quality.
References:-
1. Unit Processes in Organic Synthesis; P. H. Groggins.
2. Chemical Engineering Design: Principles, Practice, and
Economics of Plant; Gavin P. Towler, R. K. Sinnott.