chemical reaction engineering
TRANSCRIPT
Chemical Engineering Plug Chemical Engineering Plug Flow & CSTR ReactorFlow & CSTR Reactor
PRESENTED BYPRESENTED BY
PREM BABOOPREM BABOOM.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 for An Expert for www.ureaknowhow.comwww.ureaknowhow.com
ResourcesResources
• Book– O.Levenspiel: “Chemical Reaction Engineering”– S.Fogler: “Elements of Chemical Reaction Engineering”
– Internet
Reactor PerformanceReactor PerformanceInformation 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.
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?
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
Source of the essential data for reactor Source 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 studiesThe reactors used is larger than bench scale
3. Operating data from commercial scale reactorThe data come from another company and it can be used to design reactor. Unfortunately, data are often incomplete, inaccurate,
Reactor TypeReactor TypeBatch 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
AdvantagesAdvantages1. 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
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.)
Ideal Batch ReactorIdeal Batch Reactor- design equations -- design equations -
reactor the in
reactant of
onaccumulati
of rate
reactor the in
reaction chemical
to due loss
reactant of rate
reactor of
out flow
reactant
of rate
reactor
into flow
reactant
of rate
reactor the in
reactant of
onaccumulati
of rate
reactor the in
reaction chemical
to due loss
reactant of rate
Ideal Batch ReactorIdeal Batch Reactor- design equations -- design equations -
fluid of volumefluid) of ume(time)(vol
reacting A moles
VrA )(
dt
dN A
dt
dNVr A
A )(
reactor the in
reactant of
onaccumulati
of rate
reactor the in
reaction chemical
to due loss
reactant of rate
Ideal Batch ReactorIdeal Batch Reactor- design equations -- design equations -
dt
dNVr A
A )(
dt
dXN
dt
XNd
dt
dN AA
AAA0
0 )]1([
dt
dXNVr A
AA 0)(
AX
A
AA Vr
dXNt
00 )(
design equation
= time required to achieve conversion XA
0AN
tarea
Ideal Batch ReactorIdeal Batch Reactor- design equations / special cases -- design equations / special cases -
AX
A
AA Vr
dXNt
00 )(Const. density
AA X
A
AA
X
A
AA
r
dXC
r
dX
V
Nt
000
0
)()(
A
A
A C
CA
AX
A
AA r
dC
r
dXCt
0 )()(000AC
tarea
tarea
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
Some important aspects of the CSTR Some 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.
Advantages and DisadvantagesAdvantages and Disadvantages
Kinds of Phases Present
Usage Advantages Disadvantages
1. Liquid phase2. Gas-liquid rxns3. 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 control5. Simplicity of
construction6. Low operating
(labor) cost7. Easy to clean
1. Lowest conversion per unit volume
2. By-passing and channeling possible with poor agitation
CSTR ReactorCSTR Reactor- design equations -- design equations -
reactor the in
reactant of
onaccumulati
of rate
reactor the in
reaction chemical
to due loss
reactant of rate
reactor of
out flow
reactant
of rate
reactor
into flow
reactant
of rate
reactor the in
reaction chemical
to due loss
reactant of rate
reactor of
out flow
reactant
of rate
reactor
into flow
reactant
of rate
VrA )(
CSTR ReactorCSTR Reactor- design equations -- design equations -
000 )1( AAA FXF
000 AA CvF
flow volumetricv 0flow molarFA 0
sm /3
smol /
reactor into flow
reactant of rate smol /
reactor of out flow
reactant of rate )1(0 AAA XFF
VrXFF AAAA )()1(00 design equation
FA 0XA ( rA )V
smol /
Ideal Flow ReactorIdeal Flow Reactor- space-time / space-velocity -- space-time / space-velocity -
1
s
time required to process one reactor volume
of feed measured at specified conditions
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
number of reactor volumes of feed at specified
conditions which can be treated in unit time
5 hr-1 – 5 reactor volumes of feed at specified conditions are fed into reactor per hour
Ex.
Ex.
Ideal Flow ReactorIdeal Flow Reactor- space-time / space-velocity -- space-time / space-velocity -
1
s
CA 0V
FA 0
moles A entering
volume of feed
volume of reactor
moles of A entering
time
V
v0
reactor volume
volumetric feed rate
Residence time
CSTR ReactorCSTR Reactor- design equations -- design equations -
V
FA 0
CA 0
XA
rA
FA 0XA ( rA )V
1
s
CA 0V
FA 0
V
v0
Design equation:
Residence time:
area V
FA 0
CA 0
A 0
V
v0
CA 0V
FA 0
CA 0XA
rA
CSTR ReactorCSTR Reactor- design equations / general & special - design equations / general & special
case -case -
V
FA 0
XA
rA
CA CA 0
CA 0( rA )
XA 1CA
CA 0
Special case - constant density:
V
v0
CA 0XA
rA
CA CA 0
rA
Feed entering partially converted:
V
FA 0
XAf XAi
rA f
VCA 0
FA 0
CA 0(XAf XAi)
rA f
A 0
Plug Flow ReactorPlug Flow ReactorDefinition.
“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)
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.
Advantages and disadvantages Advantages 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
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:
onaccumulatireaction by
ncedisappearaoutputinput
Material balance:
=0
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
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
AV
A r
dX
F
dV00
0
design equation
Steady-State Plug Flow ReactorSteady-State Plug Flow Reactor- design equations -- design equations -
AfX
A
AV
A r
dX
F
dV00
0
AfX
A
A
AA r
dX
CF
V0
00
AfX
A
AA
A
A
r
dXC
F
VC
v
V00
0
0
0
000 AA CvF
flow volumetricv 0flow molarFA 0
sm /3
smol /
A 0
If the feed enters partially converted
Af
Ai
X
XA
A
AA r
dX
CF
V
00
Af
Ai
X
XA
AA
A
A
r
dXC
F
VC
v
V0
0
0
0
Af
Ai
Af X
X
X
0
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
Fluidized bed reactor Fluidized 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.
Advantages Advantages • 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 due to the removal of startup conditions in batch process.
Disadvantages Disadvantages • 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 and pipes.
•