Chemical Reaction Chapter 1

7/28/2019 Chemical Reaction Chapter 1

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Chemical Reaction Engineering (CRE)is the field that studies the rates and

mechanisms of chemical reactions andthe design of the reactors in which they

take place.

Lecture 1

1



Today’s lecture Introduction

Definitions

General Mole Balance Equation Batch (BR)

Continuously Stirred Tank Reactor (CSTR)

Plug Flow Reactor (PFR)

Packed Bed Reactor (PBR)

2



Chemical Reaction Engineering

Chemical reaction engineering is at the heart of virtually every chemical process. It separates thechemical engineer from other engineers.

Industries that Draw Heavily on ChemicalReaction Engineering (CRE) are:

CPI (Chemical Process Industries)

Examples like Dow, DuPont, Amoco,

Chevron

3



4



Chemical Plant for Ethylene Glycol (Ch. 5)

Smog (Ch. 1)

Plant Safety(Ch.

LubricantDesign (Ch.

Cobra Bites

(Ch. 6 DVD-

ROM)

Oil Recovery

(Ch. 7)

Wetlands (Ch. 7 DVD-

ROM)

Hippo Digestion (Ch. 2)

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http://www.umich.edu/~essen/

Materials on the Web and CD-ROM

6



Let’s Begin CRE

Chemical Reaction Engineering (CRE) is the

field that studies the rates and mechanisms of

chemical reactions and the design of the reactors

in which they take place.

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Chemical Identity

A chemical species is said to have reacted whenit has lost its chemical identity.

The identity of a chemical species is determined

by the kind , number , and configuration of that

species’ atoms.

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Chemical Identity

A chemical species is said to have reacted whenit has lost its chemical identity. There are three

ways for a species to loose its identity:

1. Decomposition CH 3CH 3 H 2 +

H 2 C=CH 2

2. Combination N 2 + O2 2 NO

3. Isomerization C 2 H 5 CH=CH 2 CH 2 =C(CH 3 )2

9



Reaction Rate

The reaction rate is the rate at which a specieslooses its chemical identity per unit volume.

The rate of a reaction (mol/dm

3

/s) can beexpressed as either:

The rate of Disappearance of reactant: -r A

or as The rate of Formation (Generation) of product: r P

10



Reaction Rate

Consider the isomerization

A B

r A = the rate of formation of species A per unit

volume

-r A = the rate of a disappearance of species A

per unit volume

r B = the rate of formation of species B per unit

volume

11



Reaction Rate

EXAMPLE: ABIf Species B is being formed at a rate of

0.2 moles per decimeter cubed per second, ie,

r B = 0.2 mole/dm3 /s

Then A is disappearing at the same rate:

-r A= 0.2 mole/dm3 /s

The rate of formation (generation of A) isr A= -0.2 mole/dm3 /s

12



Reaction Rate

For a catalytic reaction, we refer to -r A', which isthe rate of disappearance of species A on a per

mass of catalyst basis. (mol/gcat/s)

NOTE: dC A/dt is not the rate of reaction

13



Reaction Rate

Consider species j:

1. r j is the rate of formation of species j per unit

volume [e.g. mol/dm3s]

2. r j is a function of concentration, temperature,pressure, and the type of catalyst (if any)

3. r j is independent of the type of reaction

system (batch, plug flow, etc.)

4. r j is an algebraic equation, not a differential

equation

(e.g. = -r A = kC A or -r A = kC A2)

14



General Mole Balance

time

mole

time

mole

time

mole

time

mole

dt

dN G F F

jSpeciesof

on Accumulati

Rate Molar

jSpeciesof

Generation

Rate Molar

out jSpecies

of Rate

Flow Molar

in jSpecies

of Rate

Flow Molar

j

j j j

0

F j0 F j G j

System

Volume,

V

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If spatially uniform

G j r jV

If NOT spatially uniform

2V

r j 2G j1 r j1V 1

G j2 r j2V 2

1V

r j1

16




G j r jiV ii1

W

G j lim V 0 n

r ji

V ii1

n

r jdV

Take limit

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General Mole Balance on System Volume V

In Out Generation Accumulation

F A0 F A r A dV

dN A

dt

F A0

F AG A

SystemVolume,

V

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Batch Reactor Mole Balance

F A0 F A r A dV

dN A

dt

F A0 F A 0

dN A

dt

r AV

Batch

V r dV r A A

Well Mixed

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dt dN A

r AV Integrating

Time necessary to reduce number of moles of A

from N A0 to N A.

when t = 0 N A=N A0 t = t N A=N A

A

A

N

N A

A

V r

dN

t 0

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A

A

N

N A

A

V r

dN t

0

NA

t21



CSTR Mole Balance

F A0 F A r A dV

dN Adt

dN A

dt 0

Steady State

CSTR

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F A0 F A r AV 0

V F A

0 F

Ar A

V r dV r A A

Well Mixed

CSTR volume necessary to reduce the molar flow

rate from F A0 to F A.

CSTR Mole Balance

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Plug Flow Reactor

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Plug Flow Reactor Mole Balance

V

V V V

F A

F A

0

0

V r F F

V in

Generation

V V at

Out

V at

In

AV V AV A

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limV 0

F A V V

F A V

V r A

Rearrange and take limit as ΔV0

dF AdV r A


26

This is the volume necessary to reduce the entering

molar flow rate (mol/s) from F A0 to the exit molar

Alt ti D i ti



Alternative Derivation –


00

dV r F F A A A

0dt

dN ASteady State

dt

dN dV r F F

A

A A A

0

PFR

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dF A

dV r A

0 dF AdV

r A

Differientiate with respectto V

A

A

F

F A

A

r

dF

V 0

The integral form is:

This is the volume necessary to reduce the entering

molar flow rate (mol/s) from F A0 to the exit molar

flow rate of F A.

Alternative Derivation –


28

P k d B d R t M l



dt

dN W r W W F W F

A

A A A

A

W AW W A

W

r W

F F

0

lim

0dt

dN ASteady State

PBR

Packed Bed Reactor Mole

Balance

29

P k d B d R t M l



Packed Bed Reactor Mole

Balance

dF A

dW r A

Rearrange:

PBR catalyst weight necessary to reduce the

entering molar flow rate F A0 to molar flow rate

F A.

A

A

F

F A

A

r

dF W

0

The integral form to find the catalyst weight is:

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Reactor Mole Balance Summary

Reactor Differential Algebraic Integral

V F A0

F A

r A

CSTR

V r dt

dN A

A

0

A

A

N

N A

A

V r

dN t Batch

N

A

t

dF A

dV r A

A

A

F

F A

A

dr

dF V

0

PFR

F A

V31



Reactors with Heat Effects EXAMPLE: Production of Propylene Glycol in an Adiabatic CSTR

Propylene glycol is produced by the hydrolysis of

propylene oxide:

CH 2

CH CH 3

H 2O

H 2SO4 CH 2

CH CH 3

O

OH

OH

Fast Forward 10 weeks

from now:

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What are the exit conversion X and exit

temperature T?

Solut ion

Let the reaction be represented by

A+BC

v0

Propylene Glycol

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39

Evaluate energy balance

terms



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Analysis

We have applied our CRE algorithm to calculate theConversion (X=0.84) and Temperature (T=614 °R)

in a 300 gallon CSTR operated adiabatically.

X=0.84

T=614 °R

T=535 °R

A+BC

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Separations

These topics do not build upon one another

Filtration Distillation Adsorption

44



Reaction Engineering

Mole Balance Rate Laws Stoichiometry

These topics build upon one another

45



Mole Balance

Rate Laws

Stoichiometry

Isothermal Design

Heat Effects

46



Mole Balance Rate Laws

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Mole Balance

Rate LawsStoichiometry

Isothermal Design

Heat Effects

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End of Lecture 1

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Supplemental Slides



Additional Applications of CRE

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Supplemental Slides



Additional Applications of CRE

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53

Compartments for perfusion



Compartments for perfusion

Perfusion interactions between

compartments are shown by arrows.

V G, V L, V C , and V M are -tissue water volumes for the gastrointestinal,

liver, central and muscle

compartments, respectively.

V S is the stomach contents volume.

StomachVG = 2.4 l

Gastrointestinal

VG = 2.4 ltG = 2.67 min

Liver

Alcohol

VL = 2.4 l

tL = 2.4 min

Central

VC = 15.3 l

tC = 0.9 min

Muscle & Fat

VM = 22.0 l

tM = 27 min

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Chemical Reaction Chapter 1

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