Description of Plug Flow Reactor & Tubular Reactors, Chemical Reaction Engineering

1. Chemical Engineering PlugChemical Engineering Plug
Flow & CSTR ReactorFlow & CSTR Reactor
PRESENTED BYPRESENTED BY
PREM BABOOPREM BABOO
M.Sc.B.Tech(Chemical Engineering),M.Phil, M.B.A.M.Sc.B.Tech(Chemical Engineering),M.Phil, M.B.A.
Fellow of Institution of Engineer (India)Fellow of Institution of Engineer (India)
An Expert forAn Expert for www.ureaknowhow.comwww.ureaknowhow.com

5. ResourcesResources
• Book
– O.Levenspiel: “Chemical Reaction Engineering”
– S.Fogler: “Elements of Chemical Reaction Engineering”
– Internet

6. Reactor PerformanceReactor Performance
Information needed to predict the reactor behaviour:
KINETICS
how fast things happen?
input output
CONTACTING
PATTERNS
how materials flow &
contact each other?
Output = f (input, kinetics, contacting)Performance equation
• very fast - equilibrium
• slow - rate, mass, heat• flowing patterns
• contact
• aggregation etc.

7. The Nature of the Reactor Design ProblemThe Nature of the Reactor Design Problem
1. What is the composition of the feedstock, conditions, and
purification Procedures?
2. What is the scale and capacity of the process?
3. Is Catalyst needs?
4. What is operating condition?
5. Continuous or batch process?
6. What type of the reactor best meets the process
requirement?
7. What size and shape reactor should be used?
8. How are the energy transfer?

8. How to choose the reactorHow to choose the reactor
• Yield (should be large)
• Cost (Should be economic)
• Safety Consideration
• Pollution
How to Reactor Design
Firstly; You have to know reaction rate expression
Secondly; fluid velocity, temperature process,
composition and characteristic of species

9. Source of the essential data for reactorSource of the essential data for reactor
designdesign
1. Bench scale experiment (Laboratory Scale)
The reactors is designed to operate at constant temperature,
under condition (minimize heat transfer and mass transfer)
2. Pilot plant studies
The reactors used is larger than bench scale
3. Operating data from commercial scale reactor
The data come from another company and it can be used to
design reactor. Unfortunately, data are often incomplete,
inaccurate,

10. Reactor TypeReactor Type
Batch Reactors (Stirred Tanks)
1. The Batch reactor is the generic term for a type of vessel (Cylinder
Tank) widely used in the process industries.
2. A typical batch reactor consists of a tank with an agitator and
integral heating/cooling system. Heating/cooling uses jacketed
walls, internal coil, and internal tube.
Batch reactor with
single external
cooling jacket
Batch reactor with
half coil jacket
Batch reactor with
constant flux
(Coflux) jacket

11. AdvantagesAdvantages
1. Batch reactor Can be stopped between batches, so the production
rate is flexible
2. Batch reactors are more flexible, in that one can easly use different
compositions in different batches to produces product with different
spesification
3. If the process degrades the reactor in some way, a batch reactor can
be cleaned, relined, etc. between batches. Where continuous
reactors must run a long time before that can be done.
4. If the reactant are stirred, a batche reactor can often achieve better
quality than a plug flow reactor, and better productivity than a CSTR

12. Batch Reactor typesBatch Reactor types
semi-batch reactor
• flexible system but more difficult to analyse
• good control of reaction speed
• applications:
• calorimetric titrations (lab)
• open hearth furnaces for steel production (ind.)

13. Ideal Batch ReactorIdeal Batch Reactor
- design equations -- design equations -














+














+














=














reactorthein
reactantof
onaccumulati
ofrate
reactorthein
reactionchemical
todueloss
reactantofrate
reactorof
outflow
reactant
ofrate
reactor
intoflow
reactant
ofrate














−=














reactorthein
reactantof
onaccumulati
ofrate
reactorthein
reactionchemical
todueloss
reactantofrate

14. Ideal Batch ReactorIdeal Batch Reactor
- design equations -- design equations -
( )fluidofvolume
fluid)ofume(time)(vol
reactingAmoles






VrA )(−
dt
dNA
−
dt
dN
Vr A
A −=− )(














−=














reactorthein
reactantof
onaccumulati
ofrate
reactorthein
reactionchemical
todueloss
reactantofrate

15. Ideal Batch ReactorIdeal Batch Reactor
- design equations -- design equations -
dt
dN
Vr A
A −=− )(
dt
dX
N
dt
XNd
dt
dN A
A
AAA
0
0 )]1([
−=
−
=
dt
dX
NVr A
AA 0)( =−
∫ −
=
AX
A
A
A
Vr
dX
Nt
0
0
)(
design
equation
= time required to
achieve conversion XA
0AN
t
area =

16. Ideal Batch ReactorIdeal Batch Reactor
- design equations / special cases -- design equations / special cases -
∫ −
=
AX
A
A
A
Vr
dX
Nt
0
0
)(
Const. density
∫∫ −
=
−
=
AA X
A
A
A
X
A
AA
r
dX
C
r
dX
V
N
t
0
0
0
0
)()(
∫∫ −
=
−
=
A
A
A C
C
A
A
X
A
A
A
r
dC
r
dX
Ct
0 )()(0
0
0AC
t
area =
tarea =

17. Continuous Stirred Tank ReactorContinuous Stirred Tank Reactor
• In a CSTR, one or more fluid reagents are
introduced into a tank reactor equipped
with an impeller. The impeller stirs the
reagents to ensure proper mixing
Impeller

18. Some important aspects of the CSTRSome important aspects of the CSTR
• At steady-state, the flow rate in must equal the mass flow
rate out, otherwise the tank will overflow or go empty
(transient state).
• All calculations performed with CSTRs assume perect
mixing.
• The reaction proceeds at the reaction rate associated with
the final (output) concentration.
• Often, it is economically beneficial to operate several CSTR
in series. This allows, for example, the first CSTR to
operate at a higher reagent concentration and therefore a
higher reaction rate. In these cases, the sizes of the
reactors may be varied in order to minimize the total
capital investment required to implement the process.
• It can be seen that an infinite number of infinitely small
CSTR operating in series would be equivalent to a PFR.

19. Advantages and DisadvantagesAdvantages and Disadvantages
Kinds of Phases
Present
Usage Advantages Disadvantages
1. Liquid phase
2. Gas-liquid rxns
3. Solid-liquid rxns
1. When
agitation is
required
2. Series
configurations
for different
concentration
streams
1. Continuous
operation
2. Good
temperature
control
3. Easily adapts
to two phase
runs
4. Good control
5. Simplicity of
construction
6. Low operating
(labor) cost
7. Easy to clean
1. Lowest
conversion per
unit volume
2. By-passing
and
channeling
possible with
poor agitation

20. CSTR ReactorCSTR Reactor
- design equations -- design equations -














+














+














=














reactorthein
reactantof
onaccumulati
ofrate
reactorthein
reactionchemical
todueloss
reactantofrate
reactorof
outflow
reactant
ofrate
reactor
intoflow
reactant
ofrate














+














=














reactorthein
reactionchemical
todueloss
reactantofrate
reactorof
outflow
reactant
ofrate
reactor
intoflow
reactant
ofrate
VrA )(−

21. CSTR ReactorCSTR Reactor
- design equations -- design equations -
000 )1( AAA FXF =−
000 AA CvF =
flowvolumetricv =0
flowmolarFA =0
( )sm /3
( )smol /






reactorintoflow
reactantofrate
( )smol /






reactorofoutflow
reactantofrate )1(0 AAA XFF −=
VrXFF AAAA )()1(00 −+−= design
equation
FA0XA=(−rA)V
( )smol /

22. Ideal Flow ReactorIdeal Flow Reactor
- space-time / space-velocity -- space-time / space-velocity -
τ=
1
s
=
timerequiredtoprocessonereactorvolume
offeedmeasuredatspecifiedconditions






 Performance measures of flow reactors:
2 min – every 2 min one reactor volume of feed at specified
conditions is treated by the reactor
s=
1
τ
=
numberofreactorvolumesoffeedatspecified
conditionswhichcanbetreatedinunittime






5 hr-1
– 5 reactor volumes of feed at specified conditions are
fed into reactor per hour
Ex.
Ex.

23. Ideal Flow ReactorIdeal Flow Reactor
- space-time / space-velocity -- space-time / space-velocity -
τ=
1
s
=
CA0V
FA0
=
molesAentering
volumeoffeed





volumeofreactor( )
molesofAentering
time






=
V
v0
=
reactorvolume
volumetricfeedrate
Residence time

24. CSTR ReactorCSTR Reactor
- design equations -- design equations -
V
FA0
=
τ
CA0
=
XA
−rA
FA0XA=(−rA)V
τ=
1
s
=
CA0V
FA0
=
V
v0
Design equation:
Residence time:
area=
V
FA0
=
τ
CA0
εA≠0
τ=
V
v0
=
CA0V
FA0
=
CA0XA
−rA

25. CSTR ReactorCSTR Reactor
- design equations / general & special- design equations / general & special
case -case -
V
FA0
=
XA
−rA
=
CA−CA0
CA0(−rA)
XA =1−
CA
CA0
Special case - constant density:
τ=
V
v0
=
CA0XA
−rA
=
CA−CA0
−rA
Feed entering partially converted:
V
FA0
=
XAf −XAi
−rA( )f
τ=
VCA0
FA0
=
CA0(XAf −XAi)
−rA( )f
εA=0

26. Plug Flow ReactorPlug Flow Reactor
Definition.
“Each and every particle having same residence time, back
mixing not allowed.”
The plug flow reactor (PFR) model is used to describe
Chemical Reaction in continuous, flowing systems. One
application of the PFR model is the estimation of key
reactor variables, such as the dimensions of the reactor.
PFRs are also sometimes called as Continuous Tubular
Reactors (CTRs)

27. Plug Flow ReactorPlug Flow Reactor
• The PFR model works well for many fluids: liquids, gases, and
slurries.
• Fluid Flow is sometimes turbulent flow or axial diffusion, it is
sufficient to promote mixing in the axial direction, which
undermines the required assumption of zero axial mixing.
However if these effects are sufficiently small and can be
subsequently ignored.
• The PFR can be used to multiple reactions as well as reactions
involving changing temperatures, pressures and densities of the
flow.

28. Advantages and disadvantagesAdvantages and disadvantages
• Plug flow reactors have a high volumetric unit conversion,
run for long periods of time without labor, and can have
excellent heat transfer due to the ability to customize the
diameter to the desired value by using parallel reactors.
• Disadvantages of plug flow reactors are that temperatures
are hard to control and can result in undesirable
temperature gradients. PFR maintenance is expensive.
Shutdown and cleaning may be expensive.
Applications
Plug flow reactors are used for some of the following applications:
•Large-scale reactions
•Fast reactions
•Homogeneous or heterogeneous reactions
•Continuous production
•High-temperature reactions

29. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor
- definition -- definition -
 The composition of the fluid varies from point to point
 No mixing or diffusion of the fluid along the flow path
 Material balance – for a differential element of volume dV (not the whole
reactor!)
Characteristics:
( ) ( ) ( )onaccumulati
reactionby
ncedisappeara
outputinput +





+=
Material balance:
=0

30. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor
- material balance -- material balance -
Input of A [moles/time] AF
Output of A [moles/time] AA dFF +
Disappearance of A by rxn. dVrA )(−
dV

31. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor
- material balance -- material balance -
( ) dVrdFFF AAAA )(−++=
dV
( ) ( ) ( )ncedisappearaoutputinput +=
[ ] AAAAA dXFXFddF 00 )1( −=−=)1(0 AAA XFF −=
dVrdF AA )(−=−
dVrdXF AAA )(0 −= ∫∫ −
=
AfX
A
A
V
A r
dX
F
dV
00
0
design
equation

32. Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor
- design equations -- design equations -
∫∫ −
=
AfX
A
A
V
A r
dX
F
dV
00
0
∫ −
==
AfX
A
A
AA r
dX
CF
V
0
00
τ
∫ −
===
AfX
A
A
A
A
A
r
dX
C
F
VC
v
V
0
0
0
0
0
τ
000 AA CvF =
flowvolumetricv =0
flowmolarFA =0
( )sm /3
( )smol /
εA≠0
 If the feed enters partially converted
∫ −
==
Af
Ai
X
X
A
A
AA r
dX
CF
V
00
τ
∫ −
===
Af
Ai
X
X
A
A
A
A
A
r
dX
C
F
VC
v
V
0
0
0
0
τ∫∫ →
Af
Ai
Af X
X
X
0

33. Fixed Bed ReactorFixed Bed Reactor
• Solids take part in reaction  unsteady state or semi-batch
mode
• Over some time, solids either replaced or regenerated
1 2
CA,in
CA,out
Regeneration

34. Fluidized bed reactorFluidized bed reactor
• A fluidized bed reactor (FBR) is a type of reactor that
can be used to carry out a variety of multiphase chemical
reactions. In this type of reactor, a fluid (gas or liquid) is
passed through a granular solid material (usually a
catalyst possibly shaped as tiny spheres) at high enough
velocity to suspend the solid.

35. AdvantagesAdvantages
• Uniform Particle Mixing: Due to the intrinsic fluid-like behavior
of the solid material, fluidized beds do not experience poor mixing
as in packed beds. This complete mixing allows for a uniform
product that can often be hard to achieve in other reactor designs.
The elimination of radial and axial concentration also allows for
better fluid-solid contact, which is essential for reaction efficiency
and quality.
• Uniform Temperature: Many chemical reactions produce or
require the addition of heat. Local hot or cold spots within the
reaction bed, often a problem in packed beds, are avoided in a
fluidized situation such as a FBR. In other reactor types, these
local temperature differences, especially hotspots, can result in
product degradation. Thus FBR are well suited to exothermic
reactions. Researchers have also learned that the bed-to-surface
heat transfer coefficients for FBR are high.
• Ability to Operate Reactor in Continuous State: The fluidized
bed nature of these reactors allows for the ability to continuously
withdraw product and introduce new reactants into the reaction
vessel. Operating at a continuous process state allows
manufacturers to produce their various products more efficiently

36. DisadvantagesDisadvantages
• Increased Reactor Vessel Size: Because of the expansion of the bed materials
in the reactor, a larger vessel is often required than that for a packed bed
reactor. This larger vessel means that more must be spent on initial startup
costs.
• Pumping Requirements and Pressure Drop: The requirement for the fluid
to suspend the solid material necessitates that a higher fluid velocity is
attained in the reactor. In order to achieve this, more pumping power and thus
higher energy costs are needed. In addition, the pressure drop associated with
deep beds also requires additional pumping power.
• Particle Entrainment: The high gas velocities present in this style of reactor
often result in fine particles becoming entrained in the fluid. These captured
particles are then carried out of the reactor with the fluid, where they must be
separated. This can be a very difficult and expensive problem to address
depending on the design and function of the reactor. This may often continue
to be a problem even with other entrainment reducing technologies.
• Lack of Current Understanding: Current understanding of the actual
behavior of the materials in a fluidized bed is rather limited. It is very difficult
to predict and calculate the complex mass and heat flows within the bed. Due
to this lack of understanding, a pilot plant for new processes is required. Even
with pilot plants, the scale-up can be very difficult and may not reflect what
was experienced in the pilot trial.
• Erosion of Internal Components: The fluid-like behavior of the fine solid
particles within the bed eventually results in the wear of the reactor vessel.
This can require expensive maintenance and upkeep for the reaction vessel