Types of Chemical Reactors and their Applications

Department of Chemical Engineering
University of Gujrat, Pakistan

The rate equation
Chemical Reactors and their Applications
The most useful measure of reaction rate for
reactant A for a reaction
aA +bB  rR + sS
Rate of reaction is some times influenced by
composition and energy of material

Energy of material includes temperature, light intensity,
and magnetic field intensity.
So we can write

Reactor System
Homogenous Reaction
Heterogeneous Reaction
Check the reaction possibility of each state
of matter with another???

Introduction
Almost all chemical engineering process contains three
operations.
What does chemical reactor design means ?
Unit
operation
(cleaning,
crushing )
Chemical
reactor
Unit
operation
(separation)
Raw
material
Product

Types of reactors
1.Batch- uniform composition
everywhere in reactor but
changes with time
2. Semi batch- In semi batch one
reactant will be added when
reaction will proceed
3. Continuous reactor
a. Mixed flow- this is uniformly
mixed , same composition
everywhere, within the reactor
and at exit
b. Plug flow- flow of fluid through
reactor with order so that only
lateral mixing is possible.
Differentiateb/wsemi-batch
reactorandsystem???

Ideal Reactor Types
Batch Reactor
Uniform composition every
where in reactor but changes with time
Plug Flow Reactor
Fluid passes through reactor with no
mixing of earlier & later entering fluid,
with no over taking like the fluid moved
in single file through reactor
Mixed Flow reactor
Uniformly mixed same composition
every wherewith in reactor and at
exit Chemical Reactors and their Applications

Reactor design parameter
Reactor design basically means which type, size of
reactor and method of operation we should employ
for a given conversation
Parameters
Volume of reactor
Flow rate
Concentration of feed
Reaction kinetic
Temperature
pressure

Reactor Concepts
– Fixed bed reactors
– Fluidized bed reactors
– Stirred tank reactors
– Slurry loop reactors
– Bubble columns

Fixed Bed Reactors
Concept
– Collection of fixed solid
particles.
– The particles may serve as a
catalyst or an adsorbent.
– Continuous gas flow
– (Trickling liquid)
Applications
– Synthesis gas production
– Methanol synthesis
– Ammonia synthesis
– Fischer-Tropsch synthesis
– Gas cleaning (adsorption)

Fixed Bed Reactors
Challenges/Limitations
– Temperature control
– Pressure drop
– Catalyst deactivation

Fixed Bed Reactors
Temperature control
– Endothermic reactions may die out
– Exothermic reactions may damage the reactor
– Selectivity control
How can we control exothermic & endothermic
reaction in FBR??

Fixed Bed Reactors
Single-Bed Reactor
– All the particles are located in
a single vessel
Advantages/Disadvantages
– Easy to construct
– Inexpensive
– Applicable when the reactions are not very
exo-/endothermic
How can we control hot spots in FB??

Fixed Bed Reactors
Multi-Bed Reactor
– Several serial beds with
intermediate cooling/heating
stages
– Applicable for exo-/endothermic
reactions

Fixed Bed Reactors
SO3 reactorNH3 reactor

Fixed Bed Reactors
Multi-Tube Reactor
– Several tubes of small
diameter filled with particles.
– Expensive
– High surface area for heat
exchange ⇒ Very good very
temperature control
– Applicable for very
exo-/endothermic reactions
Recall Shell & tube heat
exchanger

Fixed Bed Reactors
Reactor height: 30 m
Number of tubes: 40-10000
Tube length: 6-12 m
Tube diameter: 70-160 mm
Steam reformer

Fixed Bed Reactors
Pressure drop
– Friction between the gas and particle phase results in a pressure
drop.
– High pressure drop ⇒ high gas compression costs
– Some systems have low tolerance for pressure drop.
– The pressure drop is mainly dependent on reactor length, particle
diameter, void fraction and gas velocity.

Fixed Bed Reactors
Large particles has to be used (dp>1mm).

Fixed Bed Reactors
Porous catalyst particle
– The particles are porous to
increase the surface area of the
catalyst.
– Reactants are transported
inside the pores by means of
molecular diffusion
– Adsorbption of reactant to the
active sites where the reaction
occurs.
– Desorption of Products
– Diffusion of product back to
the bulk.
– Heat is transported by
conduction.

Fixed Bed Reactors
Catalyst deactivation
– The catalyst gets deactivated if the active sites get contaminated.
– Sulfur compounds deactivate Ni-catalysts
Desulfurization is often necessary prior to reforming.
– Formation of carbon deposits deactivate the catalysts.
Large carbon deposits may clog the tubes, causing hot-spots
that damage the reactor.
– Catalyst regeneration is necessary.

Fixed Bed Reactors
Summary Advantages/Disadvantages
– High conversion is possible
– Large temperature gradients may occur
– Inefficient heat-exchange
– Suitable for slow- or non-deactivating processes

Fluidized Bed Reactors
Concept
– Collection of solid particles dispersed
in a continuous phase.
– The particles may serve as a catalyst,
adsorbent or a heat carrier.
– Continuous flow of gas or liquid
Applications
– Catalytic cracking processes
– Fischer-Tropsch synthesis
– Polymerization
– Waste combustion
– Drying

A fluidized bed exhibits liquidlike behavior

Continuous regeneration

– Conversion may be poor if gas is bypassing.
– Erosion of vessel and pipe lines.
– Uniform temperature
– Efficient heat-exchange
– Can handle rapid deactivating processes.

Stirred tank Reactors
Concept
– Forced mixing by use of impeller.
– Applied in reactive systems when
mixing is the rate determining step.
– Single phase: liquid mixing.
– Two phases: liquid/gas, liquid/particle
– Three phases: liquid/particle/gas
Typical applications
– Chemical component and phase
mixing
– Fermentation reactor
– Food and paper industry
– Natural gas
conversion/polymerization

The mixing is influenced by:
– stirring rate and pumping capacity
– liquid height
– baffle design
 (baffles reduces solid body rotation)
– size and geometry of the tank
– size and geometry of heat equipment
– size and type of impeller
How can we add heat into the FBR, fluidized BR, STR???
solid body rotation??

Impellers
– Radial flow impellers are suitable
for dispersion of gas in liquid.
– Axial flow impellers are suitable
to blend liquids and suspend solids
in liquids.

Exception: slurries with high concentrations of large
particles (difficult mixing).

Slurry loop Reactors
Concept
– Collection of solid catalyst particles
dispersed in a liquid phase (slurry).
– The slurry is circulating at a high
velocity impelled by an axial pump.
– The mixing pattern is very intensive
and well defined.
Typical application
– Polymerization

Slurry loop Reactors
– Very efficient heat-exchange
– Can operate at high polymer concentrations

Bubble Columns
Concept
– Gas dispersed in a continuous
liquid phase.
– Two phases: liquid/gas.
– Three phases: slurry/gas
Typical applications
– Natural gas conversion
– Waste water treatment
– Bio-processes
What is diffuser???

Bubble Columns

Bubble Columns
– Non-uniform product if bubble size distribution is heterogeneous

Chapter 5
Ideal Reactors for a single reaction
(Chemical Reaction Engineering 3rd
Edition by
Octave Levenspiel)

Mole balances for multiple reactionsMole balances for multiple reactions
It is better to solve problems using moles (Nj) or molar flow rates (Fj) rather
than conversion
V
CC
r
dt
dC
FVr
dt
dN
V
C
r
dt
dC
Vr
dt
dN
r
dV
dC
r
dV
dF
r
CC
V
r
FF
V
r
dt
dC
Vr
dt
dN
BB
B
B
BB
B
A
A
A
A
A
A
A
A
A
A
AA
A
AA
A
A
A
A
][ 00
0
0
0
0
0
0
−
+=+=
−==
==
−
−
=
−
−
=
==
ν
ν
ν
ν

Ideal Batch Reactor
Things to supposed
The composition is uniform throughout the reactor at
any instant of time
We may accounting about whole reactor
Noting enters nor leave the reactor
So for component A (Single reaction)
Input = output + disappearance + accumulation
=0
0=

Disappearance = -Accumulation
Disappearance of A by Reaction (Moles/time)
Accumulation of A (Moles/time)

Results
1. Time Required for conversion
This is the general equation showing time required to
achieve conversion XA for isothermal and non isothermal
operation
2. Volume of reacting fluid and reaction rate
Both remain under the integral sign and both change as
reaction proceed

3. For constant density of fluid
Equation 3 can be simplified as

4. Volume of reacting mixtures changes with
conversion
Such as in single gas-phase reaction with significant
density changes
Recall the expression
Density = mass/volume

Space Time
The natural performance measure of a batch reactor
is time “t”
It is denoted by τ
τ = 1/s
Unit = time
“It is time required to process one reactor volume of feed
measured at specified conditions”
Thus a space time of of 3 min means that every 3 minute one
reactor volume of feed at specified conditions is being
treated by the reactor.

Space Velocity
It is reciprocal of space time
It is denoted by s
s = 1/τ
Unit = 1/time
“It is no of reactor volumes of feed at specified conditions
which can be treated in unit time”
Thus a space velocity of 5 hr-1 means that 5 reactor volumes
of feed at specified conditions is being fed into the reactor
per hour

We may arbitrarily select the temperature , pressure,
and state (s, l, g) at which we choose to measure the
volume of material being fed to the reactor
Then space time and space velocity depends upon the
condition selected

Types of Chemical Reactors and their Applications

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