CH 4253 : Process Design

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Chemical Reactor Design S,S,L W
Chapter 7
CH 4253 Process Design November 2009
2
Chemical Reactor Design Problem Change in focus
from profitability
1973 and 1979 Energy Crisis
Clean Air Act (1970)
Dudukovic, et al. ,Frontiers in Reactor
Engineering, Science 325, 698 (2009)
3
Reactor Design Onion Model and Extraneous Issues
The choice of the proper reactor type and
operating conditions for a given process
chemistry is the key factor in determining
volumetric productivity and selectivity. Hence,
although the reactor typically represents between
5 and 15 of capital and operating costs of the
plant, its choice determines the number and load
on prereactor and postreactor separation units
and dictates the cost of the whole process.

• Prof. Rings Lecture - 2 Powerpoint
• Lerou J.J. , Chemical Reaction Engineering A
  Multiscale Approach to a Multiobjective task,
  Chemical Engineering Science,Vol. 51, No. 10,
  pp.1595-1614
• Dudokovic M.P. , Relevance of Multiphase
  Reaction Engineering to Modern Technological
  Challenges, Ind. Eng. Chem. Res. 2007, 46,
• 8674-8686

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• Batch Processes vs. Continuous Processes
• General Indications
• Batch Processes (Industries Fine Chemicals,
  Pharmaceuticals)
• Used for small production rates and long
  reaction times1
• Used where rapid fouling/contamination occurs1
• Low investment costs1
• Extensive Manual Operation and supervision1
• Significant amount of waste generation
• One reactor can be useful for production of
  multiple products
• Involves higher energy use
• Continuous Processes
• Used for high production rates1
• Useful for gas and liquid phase reactions
• Useful when hazardous substances are being dealt

• Westerterp K.R., Van Swajj, W.P.M., Beenackers,
  A.A.C.M. (1984), Chemical Reactor Design and
  Operation, John Wiley and Sons

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Role of Chemical Equilibrium and Chemical Kinetics

• Chemical Equilibrium
• A Chemical Reaction proceeds in the direction
  of minimization of Gibbs Free Energy.
• At equilibrium G has a minimum value.
• Application of Second Law of Thermodynamics
  ( -RT ln K ?G ?HR - T?S ) helps
• to determine the a maximum conversion for a
  particular reaction.
• Knowledge of the above helps us to answer the
  following questions
• In which direction will the reaction proceed and
  the energy evolved for a given extent of reaction
  ?
• What would be the conditions in which the desired
  product yield would be obtainable?
• (Parameters T, P and Composition of
  Reaction Mixture)
• Examples of design decisions
• Use of high pressure in NH3 synthesis
• Use of excess air in catalytic oxidation of SO2
  to SO3
• c) Use of excess steam in water-gas reaction
  (CO H2O ? CO2 H2)

• Westerterp K.R., Van Swajj, W.P.M., Beenackers,
  A.A.C.M. (1984), Chemical Reactor Design and
  Operation, John Wiley and Sons
• Levenspiel , O. (1999), Chemical Reaction
  Engineering, John Wiley and Sons , 3rd ed.

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Analysis - Role of Chemical Equilibrium and
Chemical Kinetics

• Chemical Kinetics
• Degree to which equilibrium is approached depends
  on
• Conversion
• Time during which the volume elements of the
  reacting mixture are exposed to reaction
  conditions

Temperature, Pressure and Concentration have an
effect on kinetics
Examples of design insights
Process Design Problem
Specify the input The
kinetics and flow pattern are known Obtain the
output
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Levenspiels four fundamental questions
Levenspiel, O., Chemical Reaction Engineering
,Ind. Chem. Eng. Res. ,38 (1999) , 4140 4143
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Ideal Reactor Models and influence on design
In science it is always necessary to abstract
from the complexity of the real world, and in its
place to substitute a more or less idealized
situation that is more amenable to analysis.


Kenneth Denbigh
Levenspiel, O., Modeling in chemical
engineering , Chemical Engineering Science ,57
(2002) 4691 4696
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Ideal Reactor Model Continuous Stirred Tank
Reactor (CSTR)

• Key consequences for Design
• Complete Mixing
• Operation at uniform temperature and
  concentration levels
• Steady State Material and Energy balances
  represented by algebraic equations
• Reaction rate is lower throughout the reactor
  than at any point in a PFR being operated at the
  same conversion.
• Comment (pp. 188 S,S,LW) Fluidized Bed
  Reactors

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Ideal Reactor Model Plug Flow Reactor (PFR)

• Key Consequences for Design
• Convective transport dominates over diffusive
  transport
• Conditions are closely satisfied for narrow and
  long tubular reactors at low viscosity
• Best approximation for fully turbulent flow, when
  velocity profiles are flat
• Steady State Material and Energy balances
  represented by ordinary differential equations
• Reaction rate decreases as reactants are consumed
  so the rates are higher at the inlet than at the
  outlet.
• Good for modeling Packed Bed Catalytic reactors
• Comment (pp. 187 S,S,LW) Packed Bed
  Catalytic Reactors

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• Levenspiel , O. (1999), Chemical Reaction
  Engineering, John Wiley and Sons , 3rd ed.

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Choice of type of reactor depends on 1/-rA vs. XA
curve

• Levenspiel , O. (1999), Chemical Reaction
  Engineering, John Wiley and Sons , 3rd ed.

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Cost of Reactors
Length and Diameter of the Reactor
Pressure of the reactor
Kinetics and thermodynamics
Material of Construction gives ?metal
Vc pDL Vhead From tables
based on D
Mass of vessel ?metal (VC2VHead)
Cost of catalyst and packing cost
Cp FMCv(W)
PFR Pressure Vessel CSTR Pressure Vessel
Mixer (Design Pressure given by Hydrostatic Head)
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Cost of Reactors
Hoop-stress formula for wall thickness
t vessel wall thickness, in. P design
pressure difference between inside and outside of
the vessel, psig R inside radius of steel
vessel, in. S maximum allowable stress for the
steel. E joint efficiency (0.9) tccorrosion
allowance 0.125 in.
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Overview of CRE Aspects related to Process
Design

• Levenspiel , O. (1999), Chemical Reaction
  Engineering, John Wiley and Sons , 3rd ed.

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Overview of CRE Aspects related to Process
Design
Vant Hoff Relation

• Levenspiel , O. (1999), Chemical Reaction
  Engineering, John Wiley and Sons , 3rd ed.

17
Overview of CRE Aspects related to Process
Design

• Levenspiel , O. (1999), Chemical Reaction
  Engineering, John Wiley and Sons , 3rd ed.

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Overview of CRE Aspects related to Process
Design
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Review Catalytic Reactors Brief Introduction
Major Steps
Bulk Fluid
CAb
External Surface of Catalyst Pellet
CAs
Internal Surface of Catalyst Pellet
A ? B
4. Surface Reaction
Catalyst Surface
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Catalytic Reactors Implications on design

• What effects do the particle diameter and the
  fluid velocity above the catalyst surface play?
• What is the effect of particle diameter on pore
  diffusion ?
• How the surface adsorption and surface desorption
  influence the rate law?
• Whether the surface reaction occurs by a
  single-site/dual site / reaction between
  adsorbed molecule and molecular gas?