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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?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
Reactor Design
• Basics of Reactor Design
• Mixing in Industrial Reactors
• Heat Transfer in Industrial Reactors
• Vapor-Liquid Reactors
• Reactors for Liquid Catalysis
“ 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
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)
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
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
First Order Approximation
• Very often we can write:
R = k eff CA
• CA is the concentration of one of the reagents (the limiting reagent)
• k eff 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 = k eff (CA – CA*) C
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).ΔHrxn
• Heat of reaction must be calculated at reaction temperature and pressure
• Sensible heat changes: dH = m.C p.dT
• Latent heat due to phase changes: dH = δm.ΔHL
• In industrial practice, all of these are usually estimated using process simulation software:
Reactor Design
• Basics of Reactor Design
• Mixing in Industrial Reactors
• Heat Transfer in Industrial Reactors
• Vapor-Liquid Reactors
• Reactors for Liquid Catalysis
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
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.
Impeller Types
Straight Blade Screw Rushton Turbine Anchor Helical Ribbon Propeller (Turbine) Hydrofoil Pitched BladeBaffles
• 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
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
µ
ρ
N D a 2Re
=Da = agitator blade diameter, m N = agitator speed, revs/s
ρ = density, kg/m3 μ = viscosity Ns/m2
Power Consumption
• Power consumption P (in W or Nm/s) can be made into
dimensionless power number, N p, which can be correlated against impeller Reynolds number
5 3 p N a D N P ρ =
• For Re > 103, power number is roughly constant and mainly a
function of impeller type
• See Perry’s Handbook or vendors for correlations
Re N p
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
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
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
Reactor Design
• Basics of Reactor Design
• Mixing in Industrial Reactors
• Heat Transfer in Industrial Reactors
• Vapor-Liquid Reactors
• Reactors for Liquid Catalysis
Non-Isothermal Liquid Phase Reactors
• Low heat duties can be achieved with a jacketed vessel:
– Q ≈ U A ΔT
• 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
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 = α Reβ Pr 0.33
• Ch 19 (section 19.18) has values for different impellers: – α is in range 0.36 to 1.4,
– β is in range 0.5 to 0.75, typically 0.67
– Re is the impeller Reynolds number
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, ρ ~ 700 kg/m3, μ ~ 0.6 × 10-3 Ns/m2
• 60 rpm = 1 rps, so Re = (0.22)×700×1/0.6 × 10-3 = 46700
• From Ch 19, Nu = 0.36 Re0.67 Pr 0.33 = 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 π.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
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
Reactor Design
• Basics of Reactor Design
• Mixing in Industrial Reactors
• Heat Transfer in Industrial Reactors
• Vapor-Liquid Reactors
• Reactors for Liquid Catalysis
How would you get a vapor to
react with a liquid?
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
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
Sparger as Agitator
• If gas flow rate is large then gas flow can be used as primary means of agitation
• Perry’s 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
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)
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
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
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 A i A L C k C C a k Liquid Vapor B δ A CA,∞ CA,iRate of reaction = k 2 C AC B ≈ k 1C A
Vapor-Liquid Reaction Kinetics
We can define two regimes:
• k 1 << ak L, rate ≈ k 1C A,i
– Known as slow kinetic regime
– Reaction rate occurs with concentration that would be predicted by phase equilibrium
– Design is insensitive to increase in area a
• k 1 >> ak L, rate ≈ a k LC A,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
( ) ( ) ( L) i A L L L i A L L i A A k a k k C k a k a k k a C k k a k k a C C + = + = + = ∞ 1 1 , 1 , 1 1 , , flux) (or reaction of rate so : Solving
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 y diffusivit is where , / and , 0 if ) ( bulk in Reaction film in Reaction 2 1 , , , , 1 << = ≈ − << << ∞ ∞ L L A A i A L i A k k D D k D C C C k a C k a δ δ L k k D Ha = 1