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Chemical Reaction Engineering

An Introduction to Industrial Catalytic Reactors

Tarek Moustafa, Ph.D.

November 2011

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Module objectives (TPO)

• To differentiate between various types of catalytic reactors

• To apply the design equations: material, energy and momentum balance equations on ideal and industrial catalytic reactors

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Introduction

• In most of chemical engineering job venues, a good understanding of industrial reactors is essential and important

• The reactors are the heart of most chemical processes and all technologies starts from the reaction part and accordingly the reactor

• Many types of industrial reactors are available depending on the reaction and the process involved

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General Classifications• Catalytic vs. non-catalytic Reactions

- Catalytic reactions are more dominant in chemical industry (especially organic)

- Catalytic reactions are more difficult to handle

• Homogeneous vs. Heterogeneous Catalysts

- Homogeneous catalysts are generally more active but a separation & recycle steps for the catalyst are essential

- Heterogeneous catalysts are most widely used

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Introduction

• Ultimate Objective:

Commercial Reactor– Design and Operate:

Successfully

• Typical Unfortunate News– Catalyst does not perform

well when scaled-up to commercial reactor

– Hot spot, temperature

runaway, explosion

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Phenomena in Commercial Reactors

• Transport Phenomena– Momentum Transfer– Heat Transfer– Mass Transfer

• Chemical Reactions– On Heterogeneous Catalyst

Surface

All Happens Simultaneously !

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Types/Configurations of catalytic reactors

• Fixed Bed Catalytic Reactors- Adiabatic single packed bed- Adiabatic beds in series with intermediate cooling or

heating- Multi-tubular fixed bed- Radial flow bed- Reverse flow bed- Auto-thermal reactors

• Fluidized Bed Reactors• Moving Bed Reactors• CSTR with jacket or coil (usually for liquid phase)

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Reactors’ Schematic

Single Adiabatic

bed

Adiabatic beds in series or staged

beds with intermediate

heating or cooling

Multitubular fixed bed

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Reactors’ Schematic

Radial flow bed

Reverse flow reactors

Auto-thermal reactors

TT0

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Important Phenomena & Considerations

• Adiabatic Packed Bed Catalytic Reactors- Simplest design- Used when reaction is associated with moderate heat

generation / consumption• Multi-tubular fixed bed

- Reaction is associated with high heat generation / consumption

• Radial flow bed- Pressure drop is critical

• Reverse flow bed- Used for endothermic reactions, to produce product and exothermic catalyst regeneration

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Ideal reactors

• CSTR (continuous stirred tank reactor)- Composition and temperature everywhere is the

same and equals that of the outlet- Infinite diffusion and sometimes called one point

reactor

• PFR (Plug flow reactor) - Composition and temperature changing from one point to another along the length of the reactor- No diffusion and flow is only due to bulk flow inside the reactor

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Non-isothermal continuous-flow stirred catalytic reactor

Process Feed

Cooling/Heating fluid inlet

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Non-isothermal continuous-flow stirred catalytic reactor – Design Equations

Q = Fout Cp (T – Tr) - FAo Cpo (To – Tr ) + FAo x HR

• Material Balance

W rA = FAo x

• Rate Law (in case of first order reaction)

rA = ko e-E/RT CA

• Energy Balance

Q = U A (T – Tc)

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Example 101 An isomerization reaction is taking place in a continuous stirred catalytic reactor: A BThe reaction is first order with respect to A and the rate can be expressed as: k = 16.96*1014 e-19400/T m3/kg cat h. It is desired to feed 800 kgmole per hour of pure liquid A to the reactor. If the reactor is operated adiabatically and the inlet temperature and concentration are 140°C and 10 gmol/l respectively. What is the volume required of the catalyst to achieve 20% conversion if the catalyst bulk density is 2 g/cm3. (Hr = 21 kcal/gmole,

Cp A = 32 cal/gmole K and Cp B = 36 cal/gmole K)

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Solution

Q = Fout Cp (T – Tr) - FAo Cpo (To – Tr ) + FAo x HR

• Material Balance

W rA = FAo x W rA = 800 * 0.2

• Energy Balance

• Rate Law

0 = 800*32.8*(T – 298) – 800*32*(413 – 298 ) - 800*0.2*21000

rA = ko e-E/RT CA = 16.96 1014 e-19400/538.2 *10(1-0.2)

= 0.377 kgmol/kgcat h

T = 538.2 K

W = 424.6 kg and V = 0.2123 m3

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Isothermal plug-flow catalytic reactor

• Compositions and possibly pressure are changing along the length of the reactor

• Rate is not constant inside the reactor, and is varying form one location to another

Fs 2

T, P2

Fs 1

T, P1

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Isothermal plug-flow catalytic reactor – Design Equations

• Material Balance

rA dW = FAo dx

• Rate Law

Could be power form or Langmuir-Hinshelwood kinetics

rA = ko e-E/RT CA /(1+KACA+KBCB)

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Non-isothermal plug-flow catalytic reactor

• Compositions, temperature and possibly pressure are changing along the length of the reactor

• Rate is not constant inside the reactor, and is varying form one location to another

Fs 2

T2, P2

Fs 1

T1, P1

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Non-isothermal plug-flow catalytic reactor – Design equations

F Cp dT + rA dW HRo - U A (T – Tc) = 0

• Material Balance

rA dW = FAo dx

• Rate Law (Langmuir-Hinshelwood kinetics)

rA = ko e-E/RT CA /(1+KACA+KBCB)

• Energy Balance

• Momentum Balance

dP/dL = - G (1-) [150(1- ) + 1.75 G]Dp 3 Dp

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References• Missen, R., Mims, C. and Saville, B., Introduction

to chemical reaction engineering and kinetics, Wiley (1999).

• Fogler, S., Elements of chemical reaction engineering, 4th ed., Prentice-Hall (2004).

• Froment, G.F. and K.B. Bishoff, “Chemical reactor analysis and design”, 2nd ed., Wiley (1990).