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PROS and CONS of REACTORS
Reactor 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 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 reactor• Yield (should be large)• Cost (Should be economic)• Safety Consideration• Pollution
How to Reactor DesignFirstly; You have to know reaction rate expressionSecondly; fluid velocity, temperature process, composition and characteristic of species
Source of the essential data for reactor design
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 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
Advantages1. Batch reactor Can be stopped between batches, so the production
rate is flexible2. 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 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 Reactor- design equations -
reactor the inreactant of
onaccumulatiof rate
reactor the in reaction chemical
to due lossreactant of rate
reactor ofout flow
reactantof rate
reactorinto flow
reactantof rate
reactor the inreactant of
onaccumulatiof rate
reactor the in reaction chemical
to due lossreactant of rate
Ideal Batch Reactor- design equations -
fluid of volumefluid) of ume(time)(vol
reacting A moles
VrA )(
dtdN A
dtdNVr A
A )(
reactor the inreactant of
onaccumulatiof rate
reactor the in reaction chemical
to due lossreactant of rate
Ideal Batch Reactor- design equations -
dtdNVr A
A )(
dtdXN
dtXNd
dtdN A
AAAA
00 )]1([
dt
dXNVr AAA 0)(
AX
A
AA Vr
dXNt00 )(
design equation
= time required to achieve conversion XA
0ANtarea
Ideal Batch Reactor- design equations / special cases -
AX
A
AA Vr
dXNt00 )(
Const. density
AA X
A
AA
X
A
AA
rdXC
rdX
VNt
0000
)()(
A
A
A C
CA
AX
A
AA r
dCr
dXCt0 )()(00
0ACtarea
tarea
Continuous 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
• 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 DisadvantagesKinds 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 Reactor- design equations -
reactor the inreactant of
onaccumulatiof rate
reactor the in reaction chemical
to due lossreactant of rate
reactor ofout flow
reactantof rate
reactorinto flow
reactantof rate
reactor the in reaction chemical
to due lossreactant of rate
reactor ofout flow
reactantof rate
reactorinto flow
reactantof rate
VrA )(
CSTR Reactor- 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 Reactor- space-time / space-velocity -
1s
time required to process one reactor volumeof 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 specifiedconditions 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 Reactor- space-time / space-velocity -
1s
CA 0VFA 0
moles A enteringvolume of feed
volume of reactor
moles of A enteringtime
Vv0
reactor volume
volumetric feed rate
Residence time
CSTR Reactor- design equations -
VFA 0
CA 0
XA
rA
FA 0XA ( rA )V
1s
CA 0VFA 0
Vv0
Design equation:
Residence time:
area V
FA 0
CA 0
A 0
Vv0
CA 0VFA 0
CA 0XA
rA
CSTR Reactor- design equations / general & special
case -
VFA 0
XA
rA
CA CA 0
CA 0( rA )
XA 1 CA
CA 0
Special case - constant density:
Vv0
CA 0XA
rA
CA CA 0
rA
Feed entering partially converted:
VFA 0
XAf XAi
rA f
VCA 0
FA 0
CA 0(XAf XAi)
rA f
A 0
Plug 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 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 • 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.
ApplicationsPlug 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 Reactor- 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 Reactor- 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 Reactor- material balance -
dVrdFFF AAAA )(
dV
ncedisappearaoutputinput
AAAAA dXFXFddF 00 )1( )1(0 AAA XFF
dVrdF AA )(
dVrdXF AAA )(0 AfX
A
AV
A rdX
FdV
000
design equation
Steady-State Plug Flow Reactor- design equations -
AfX
A
AV
A rdX
FdV
000
AfX
A
A
AA rdX
CFV
000
AfX
A
AA
A
A
rdXC
FVC
vV
000
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 rdX
CFV
00
Af
Ai
X
XA
AA
A
A
rdXC
FVC
vV
00
0
0
Af
Ai
Af X
X
X
0
Fixed 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 • 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 • 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 • 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.
•
