introduction to reactor design
DESCRIPTION
A rough guide to the basics of reactor designTRANSCRIPT
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Chemical Engineering Design 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
Reactor Design
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Chemical Engineering Design
Reactor Design
Prediction of reactor performance, product yields etc. See earlier lecture
Detailed discussion of reaction kinetics, catalysis, deactivation, mass transfer, etc. See reactors classes and textbooks
Focus of this lecture is on how real reactors are designed and sized in industry
Special case of biological reactors is treated in next lecture
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reactor Sizing & Costing Estimate required volume
From residence time for non-catalytic reactors From catalyst space velocity for packed bed catalytic reactors
Space velocity = lbs/h per lb catalyst Hence use catalyst average bed density to estimate catalyst bed volume
From hydraulics & residence time for fluidized and slurry reactors Make allowance for head space, internals, etc.
Decide pressure vessel size and shape See pressure vessel design lecture
Cost reactor shell as a pressure vessel
Add extra costs for mixers, internals, controls, etc.
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Complications of Real Reactor Design
How do we handle multiple
phases?
How do we add or remove
heat?
How do we introduce catalyst?
How do we get good mixing
& segregation?
How tight does RTD have
to be? What gives lowest cost?
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Real Reactor Design
Very often, the design of real reactors is a lot more complicated than just estimating the reactor volume
Much of the cost comes from reactor internals Mixers, agitators, baffles Heat transfer (jackets, coils or external loops) Catalyst handling
The mixing and heat transfer performance of real reactors can be very difficult to model and understand, and can have significant effects on process yields and product purity
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reactor Design
Basics of Reactor Design
Mixing in Industrial Reactors
Heat Transfer in Industrial Reactors
Vapor-Liquid Reactors
Reactors for Liquid Catalysis
Reactors for Solid Catalysis
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Ideal Reactors
WMR or CSTR
Perfect mixing
Product and entire vessel contents are at uniform temperature, concentration
Material sees a distribution of residence times
Plug Flow Reactor
No axial mixing
Sharp residence time distribution
Material flowing through the reactor experiences a profile of concentrations and temperatures
Idealized reactor performance is seldom attained in practice, but is useful as a first approximation
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reactor Performance Plug flow reactor:
Well mixed reactor:
G = molar flow rate V = volume X = conversion R = reaction rate per unit volume
G
dV
Balance across element of reactor: -G dX = R dV
G
V Balance across reactor: G (Xin Xout) = R V R is evaluated at outlet conditions
Integrated form depends on rate expression R(X)
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reaction Kinetics Complications
Reactions are seldom simple first or second order
Most catalytic reactions can be fitted with Langmuir-Hinshelwood expressions Inhibition terms are often significant
Mass transfer, mixing & equilibrium often limit the overall rate
Catalyst deactivation is often significant
Simple first order model is usually adequate for predicting conversion, but not for predicting byproduct yields or understanding catalyst behavior
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Mass Transfer
Mass transfer processes often reduce the overall rate of reaction to a slower rate than intrinsic kinetics
Mass transfer limitations can occur: Between phases (V/L, L/L, L/S, V/S, etc.) Inside catalyst pores
Inter-phase transport is strongly influenced by interfacial area, i.e., particle, droplet or bubble size (hence agitation rate)
See reaction engineering textbooks for numerous examples with neat analytical solutions
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
First Order Approximation Very often we can write:
R = keff CA
CA is the concentration of one of the reagents (the limiting reagent)
keff is effective first order rate constant Includes mass transfer resistances Includes concentrations of reagents that are present in excess and
so roughly constant
For an equilibrium reaction, expression is:
R = keff (CA CA*) CA* = equilibrium concentration
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reactor Heat Balance
Reactor design must account for enthalpy difference between feed and products, which can come from:
Heat of reaction: dH = G.(Xout Xin).Hrxn Heat of reaction must be calculated at reaction temperature and pressure
Sensible heat changes: dH = m.Cp.dT
Latent heat due to phase changes: dH = m.HL
In industrial practice, all of these are usually estimated using process simulation software: dHreactor = Hproducts - Hfeeds
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reactor Design
Basics of Reactor Design
Mixing in Industrial Reactors
Heat Transfer in Industrial Reactors
Vapor-Liquid Reactors
Reactors for Liquid Catalysis
Reactors for Solid Catalysis
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Mixing in Industrial Reactors Tubular Reactors
Tubular reactors are almost always designed to be in turbulent flow
A static mixer is usually placed immediately downstream of any feed point to ensure reactor contents are mixed quickly
Static mixer usually consists of baffles to induce turbulence
Source: Komax Inc. www.Komax.com
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
http://www.komax.com/
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Chemical Engineering Design
Mixing in Industrial Reactors Stirred Reactors
Agitator consists of impeller mounted on shaft driven by motor
Motor is usually mounted above the reactor
Reactor usually contains baffles or other internals to induce turbulence and prevent contents from swirling
2007 Chemineer Inc. Used with permission. www.Chemineer.com
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
http://www.chemineer.com/
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Chemical Engineering Design
Impeller Types
Straight Blade
Screw Rushton Turbine Anchor Helical Ribbon
Propeller (Turbine) Hydrofoil Pitched Blade
2007 Chemineer Inc. Used with permission. www.Chemineer.com 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
http://www.chemineer.com/
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Chemical Engineering Design
Baffles
If the tank has no baffles then the liquid will swirl and develop a vortex:
Usually four baffles are placed around the perimeter to break up swirl Typically, baffles are 1/10 of
diameter and located 1/20 of diameter from wall
Side view Top view
Liquid level
Flow pattern
Flow pattern
Baffle
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Impeller Reynolds Number
Can be used to determine extent of mixing and correlate power consumption and heat transfer to shell (jacket)
Defined as
Different definitions are used for agitators without blades
NDa
2
Re =Da = agitator blade diameter, m N = agitator speed, revs/s = density, kg/m3 = viscosity Ns/m2
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Power Consumption Power consumption P (in W or Nm/s) can be made into
dimensionless power number, Np, which can be correlated against impeller Reynolds number
53pN
aDNP
=
For Re > 103, power number is roughly constant and mainly a function of impeller type
See Perrys Handbook or vendors for correlations
Re
Np
10 102 103
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Non-Ideal Flow and Mixing
In some cases, simple correlations may not be adequate: If dead zones cannot be tolerated for reasons of product purity,
safety, etc. If reactor internals are complex If reaction selectivity is very sensitive to mixing
In these cases, it is usually necessary to carry out a more sophisticated analysis of mixing Use computational fluid dynamics to model the reactor Use physical modeling (cold flow) experiments Use tomography methods to look at performance of real reactor
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Computational Fluid Dynamics Calculate mass, energy and momentum balances
discretely across a 2- or 3-dimensional grid of points as a function of time
Can include effects of heat and mass transfer, bubbles, suspended solids
Boundary conditions on grid are set up to reflect reactor geometry
Results are usually plotted as color coded pictures of velocity, mass transfer coefficient, void fraction, shear, etc., that let the designer see where the weak points of the design may be and propose changes to the design geometry
Commercial software such as Fluent, CFX or FloWizard is used (see www.Ansys.com)
Source: Ansys Inc. www.Ansys.com
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
http://www.ansys.com/http://www.ansys.com/
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Chemical Engineering Design
Reactor Tomography Various methods can be used for non-invasive examination of
reactor in-situ Cat Scanning, Ultrasound, Gamma Scanning Usually carried out by specialist contractors, & not cheap
Cat Scanning of FCC regenerator to validate MTO reactor catalyst distribution
Gamma scanning to validate axial catalyst density profile in FCC regenerator
Cat Scanning of FCC regenerator to validate MTO reactor catalyst distribution
Gamma scanning to validate axial catalyst density profile in FCC regenerator
Source: UOP
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reactor Design
Basics of Reactor Design
Mixing in Industrial Reactors
Heat Transfer in Industrial Reactors
Vapor-Liquid Reactors
Reactors for Liquid Catalysis
Reactors for Solid Catalysis
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Non-Isothermal Liquid Phase Reactors
Low heat duties can be achieved with a jacketed vessel: Q U A T
Intermediate duties require an internal coil But note: coil impacts mixing, fouling and cleaning Q = U A Lmtd U can be estimated using correlations for shell side of S&T HX Coil volume must be added to volume calculated from residence time
High duties require an external heat exchange circuit 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Estimating Heat Transfer Coefficients in Stirred Tank Reactors
Reactor side heat transfer coefficient depends strongly on rate of agitation, reactor internals & coil design Very case specific Detailed understanding requires CFD or physical modeling
First approximation for jacket for design purposes:
Nu = Re Pr0.33
Ch 19 (section 19.18) has values for different impellers: is in range 0.36 to 1.4, is in range 0.5 to 0.75, typically 0.67 Re is the impeller Reynolds number Nu = hd/k, where d is reactor internal diameter
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Example A well-mixed reactor for manufacturing a specialty chemical has
diameter 2m and liquid depth 3m. The agitator is a paddle with diameter 0.2m and speed is 60 rpm. The reactor operates at 75 C, and a cooling rate of 200 kW is required. How would you cool the reactor?
Start by assuming typical organic chemical properties Pr ~ 0.9, k ~ 0.14 W/mK, ~ 700 kg/m3, ~ 0.6 10-3 Ns/m2
60 rpm = 1 rps, so Re = (0.22)7001/0.6 10-3 = 46700
From Ch 19, Nu = 0.36 Re0.67 Pr0.33 = 467, and h = k Nu/d = 0.14 467/2 = 33 W/m2K
Heat transfer coefficient on jacket side using cooling water ~ 800 W/m2K, so U ~ (1/800 + 1/33)-1 = 31 W/m2K
Jacket area is .d.L = 3.14 2 3 = 18.85m2, So cooling duty = 31 18.9 dT ~594dT
If cooling water is available at 45 C, then maximum delta T would be 30 C and maximum cooling rate would be 594 45 = 26.7 kW
Jacket is not adequate and we should increase stirrer speed or agitator length or consider a coil or external loop
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Non-Isothermal Vapor Phase Reactors
Heat transfer coefficients are usually too low to use jackets or internal coils
External heating or cooling loops are most common
For very endothermic processes, reaction is carried out in a fired heater tube Reactor design is same as fired heater design Allow extra residence time in radiant zone if necessary See later
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reactor Design
Basics of Reactor Design
Mixing in Industrial Reactors
Heat Transfer in Industrial Reactors
Vapor-Liquid Reactors
Reactors for Liquid Catalysis
Reactors for Solid Catalysis
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
How would you get a vapor to react with a liquid?
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Vapor-Liquid Reactors
Goal Types of V-L Reactor
Examples
Maintain low concentration of gas component in liquid
- Sparged stirred tank reactor
- Sparged tubular reactor
- Liquid phase oxidations using air
- Fermenters Contact gas and liquid over catalyst
- Trickle bed reactor
- Slurry phase reactor
- Catalytic hydrogenation
React a component out of the gas phase to high conversion
- Multi-stage V/L contactor (reactive absorption column)
- Venturi scrubber
- Chemisorption
- Acid gas scrubbing
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Sparged Reactors
Sparger is a pipe with holes for bubbles to flow out
For smaller bubbles, a porous pipe diffuser can be used instead
Balance between bubble break-up and coalescence is quickly established
If small bubble size must be maintained then additional shear is needed and an agitator is used as well
Designer must allow some disengaging space at top of reactor, or entrainment will be excessive
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Sparger as Agitator
If gas flow rate is large then gas flow can be used as primary means of agitation
Perrys Handbook suggests the following air rates (ft3/ft2.min) for agitating an open tank full of water at 1 atm:
Degree of agitation Liquid depth 9ft Liquid depth 3ft
Moderate 0.65 1.3
Complete 1.3 2.6
Violent 3.1 6.2
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Lift Reactors and Loop Reactors
If sparger is used to provide agitation then a baffle is often added to give better liquid circulation and ensure mixing of feeds
These reactors can be used for very large flowrates, where the liquid flow is driven by the vapor flow
Equipment design is governed by two phase flow hydraulics (see earlier lecture)
Baffle
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Chemical Engineering Design
Example: UOP/Paques Thiopaq Reactor
Biological desulfurization of gases with oxidative regeneration of bugs using air
Reactor at AMOC in Al Iskandriyah has six 2m diameter downcomers inside shell
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Reaction in Vapor-Liquid Contacting Columns
Trayed or packed columns can be used to contact vapor and liquid for reaction See separation columns lecture for
design details
Packing may be catalytically active, or could be conventional inert packing
Design is similar to design of absorption columns, but must allow for enhancement of absorption due to reaction
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Vapor-Liquid Reaction Kinetics
If liquid component B is present in excess then we can assume reaction is psuedo-first order in gas component A
Start by assuming reaction in bulk is >> reaction in mass transfer film
( )
==
=
,1
,,
bulk in reaction of Rate
A
AiAL
CkCCak
Liquid Vapor B
A
CA, CA,i
Rate of reaction = k2 CA CB k1CA
Mass transfer flux through film
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Vapor-Liquid Reaction Kinetics
We can define two regimes:
k1 > akL, rate a kLCA,i Known as slow mass transfer regime Reaction rate occurs at the rate that would be set by mass transfer with zero
concentration in the bulk liquid Design is sensitive to increase in area a
( )
( ) ( )LiALLLiA
L
LiAA
kakkCka
kakkaCk
kakkaC
C
+=
+=
+=
1
1,
1
,1
1
,,
flux)(or reaction of rate so
:Solving
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
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Chemical Engineering Design
Vapor-Liquid Reaction Kinetics For either of the slow regimes to occur we need reaction
to mainly occur in the bulk liquid
We define the Hatta number, Ha as:
If the Hatta number is ~1 or greater then we have the fast or instantaneous regimes and the analysis is more complicated: see reaction engineering textbooks
1
ydiffusivit is where,/ and,0 if)(
bulkin Reaction filmin Reaction
21
,
,,,1
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Chemical Engineering Design
Questions ?
2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy
Reactor DesignReactor DesignReactor Sizing & CostingComplications of Real Reactor DesignReal Reactor DesignReactor DesignIdeal ReactorsReactor PerformanceReaction Kinetics ComplicationsMass TransferFirst Order ApproximationReactor Heat BalanceReactor DesignMixing in Industrial ReactorsTubular ReactorsMixing in Industrial ReactorsStirred ReactorsImpeller TypesBafflesImpeller Reynolds NumberPower ConsumptionNon-Ideal Flow and MixingComputational Fluid DynamicsReactor TomographyReactor DesignNon-Isothermal Liquid Phase ReactorsEstimating Heat Transfer Coefficients in Stirred Tank ReactorsExampleNon-Isothermal Vapor Phase ReactorsReactor DesignHow would you get a vapor to react with a liquid?Vapor-Liquid ReactorsSparged ReactorsSparger as AgitatorLift Reactors and Loop ReactorsExample: UOP/Paques Thiopaq ReactorReaction in Vapor-Liquid Contacting ColumnsVapor-Liquid Reaction KineticsVapor-Liquid Reaction KineticsVapor-Liquid Reaction KineticsQuestions ?