Introduction to reactor design

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

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


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.



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

Reactors for Solid 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 understanding catalyst behavior



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 = 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



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



Reactor 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


http://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. www.Chemineer.com


http://www.chemineer.com/


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/


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



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



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



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



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


http://www.ansys.com/http://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









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


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



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



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









How would you get a vapor to react with a liquid?




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



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

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



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



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



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

AiAL

CkCCak

Liquid Vapor B

A

CA, CA,i

Rate of reaction = k2 CA CB k1CA

Mass transfer flux through film



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



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


Questions ?


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 ?

Introduction to reactor design

Documents

design of real reactors

towler uop

reactor volume

slurry reactors

flow reactor

reactors classes

educational use

liquid catalysis reactors