Intro Notes

Chemical Reactor Design An engineer seeks to design reaction

vessels, to carry out a particular chemical reaction or set of reactions This requires a knowledge of the reaction and

hence reaction kinetics Questions relate to operating conditions, heat

release/absorption/ reaction rate/ reaction order/phase change

A knowledge of the appropriate type of reactor to achieve this goal Scale, duration of reaction, conditions

CHG 3127: Reaction Engineering © E.J. Anthony 2012 1

Contact Details I have two e-mail accounts you can use

[email protected] or

[email protected]

I can be contacted by telephone, but this is not reliable Tel: 613 996 2868

mailto:[email protected]


Course Marking Structure Tutorials and Assignments will be worth

15% of the mark (5% Tutorials and 10% Assignments) 6 Assignments remit in groups of 2

Mid-term 35% of the Mark Final 50% of the Mark

CHG 3127: Reaction Engineering E.J. Anthony 2012 3

TA for Class Xue Wang Email address: [email protected] Office: CBY D218 Office hours to be announced.

CHG 3127: Reaction Engineering © E.J. Anthony, 2012 4


I. Reviewing the Basics Introduction

basic definitions, industrial reactors

Mole Balances and Stoichiometry fractional conversion & extent of reaction mole balance equation stoichiometric tables systems with phase changes

CHG3127: Reaction Engineering © E.J. Anthony 2012 6

Basic Definitions

Chemical Species any chemical compound or element with a given identity (determined by kind, number, and configuration of atoms)

Chemical Reaction said to occur when detectable number of molecules of one or more species have lost their identity and assumed a new one

There may be intermediates, which maintain a steady concentration during the process of conversion

CHG 3127: Reaction Engineering EJ Anthony 2012 7

Basic Definitions Decomposition Reaction

Combination Reaction

Isomerization Reaction

C H C H H2 6 2 4 2

C H H C H2 4 2 2 6

cis trans 2 butene 2 butene

CHG3127: Reaction Engineering E.J. Anthony 2012 8

Basic Definitions

Homogeneous Reaction Reaction involving only one phase

Heterogeneous Reaction Reaction involving more than one phase, with the reaction usually occurring at or near the interface between phases


Basic Definitions

Series Reactions

Parallel Competitive Reactions

Independent Reactions

A B C

A BA C

A BC D


Basic Definitions Chemical Reaction Rate, rj

rate of formation of species j, by chemical reaction, per unit volume (mass of catalyst, surface area of catalyst)

N.B. the reaction rate is not necessarily equal to dCj /dt This is true only for a constant volume batch reactor


Basic Definitions

Elementary Reaction a reaction in which the reaction order of each

species is identical with the stoichiometric coefficient of that species within the reaction equation as written Note here that -rA is the rate of consumption of species A

A B Cr k C CA A B

22


Basic Definitions

Non-elementary Reaction reaction rate is not discernible directly from

overall stoichiometry of reaction

22

21221

22 2

BrHBrkBrHkr

HBrBrH

HBr


Basic Definitions

Molecularity the number of atoms, ions, or molecules involved

(colliding) in the rate-limiting step of the reaction

unimolecular: reaction involving one atom or molecule (e.g., radioactive decay)

bimolecular: reaction involving two atoms or molecules

termolecular: reaction involving three atoms or molecules


Basic Definitions

Reaction Order relates to the exponents of the concentrations in the reaction rate expression of the general form reaction is order with respect to A, order with respect to B, order overall N.B. some complex reaction rate expressions yield meaningful orders under limiting conditions only

r kC CA A B ...


Basic Definitions The Arrhenius law

assume the rate law given by

specific reaction rate constant, k, accounts for temperature dependence of reaction rate (Arrhenius expression):

N.B. in some complex cases k and F functions not separable:

k T Ae E RT /

r k T F C CA A B, ,...

2

2

2 *1 O

ONON Ck

kCr


Classifying Industrial Reactors Classification by Reactor Operation

Batch Reactor all reactants enter at start high conversions, high labour costs limited scale-up potential

Continuous Reactor reactants introduced while products removed

continuously used extensively in large scale operations reactor may be tank, tube or vertical tower


Classifying Industrial Reactors Classification by Reactor Operation

Semi-Continuous Reactor

either reactants or products removed continuously

remaining components entered batch-wise limitations of batch, except better control of

unwanted side reactions through control of specific component concentrations

used in two-phase reactions where gas is bubbled through liquid


Classifying Industrial Reactors

Homogeneous batch reactor

hand holes for recharging reactor

connection for heating or cooling jacket

agitator

From Fogler, Elements of Chemical Reaction Engineering, 3rd Edition



Semi-batch reactor

reactant B

reactant A and product

heater or cooler


Classifying Industrial Reactors Classification by Reactor Design

Features

Tank Reactor most common reactor type; ideal: CSTR operated in batch, continuous, or semi modes frequently includes mixing devices and heat transfer

components

Tubular Reactor common in gas phase reactions can contain packing, catalysts heat transfer jackets or shell and tube systems ideal form: plug flow reactor


Classifying Industrial Reactors Classification by Reactor Design Features

Slurry Reactor vertical tower containing fine catalyst particles

suspended in liquid slurry gas phase reactant bubbled through high heat capacity of slurry maintains temperature control used in hydrocracking of fuel oils and in coal liquifaction


Classifying Industrial Reactors CSTR

. Courtesy of Pfaudler, Inc



Longitudinal tubular reactor

From Fogler, Elements of Chemical Reaction Engineering, 3rd Edition


Classifying Industrial Reactors Circulating fluidized bed reactor

feed

product

settling hopper

riser

standpipe


Mole Balances and Stoichiometry Consider the reaction

generalized stoichiometric equation

extent of reaction fractional conversion

0 v B v C v S v Tb c s t... ...

v Ai ii

0

n nv

i i

i

0 X n nn

i i

i

0

0


Mole Balances and Stoichiometry

relationship between and X

Xnv

lim,

lim

0

Amit Ubhi

solved with substiution from extent of reaction and fractional conversion


Mole Balances and Stoichiometry Mole balance on species j in system of

volume V well-mixed system

not well-mixed

F F GdNdtj j j

j0

G r Vj j

G r V r dVj ji ii

m

jV

1

Amit Ubhi

first question to ask. what are these terms. like what reactor? is it well mixed?

Amit Ubhi

if not well mixed then we have an iterative structure

Amit Ubhi

therefore this is the first thing to do, to see what things can be removed and simplified


Mole Balances and Stoichiometry General integral mole balance on

species j in system of volume V F F r dV

dNdtj j j

V

j0


Stoichiometric Tables Objective: to find expressions for rj and

Nj

Consider the reaction

A as basis:

Rate of formation of C:

In general:

aA bB cC dD

A ba

B ca

C da

D

AAC racr

acr

ra

rb

rc

rd

A B C D

Amit Ubhi

this deals with stoichiometry not rate.these types of reactions are assumed to be controlling


Stoichiometric Tables

Consider the previous reaction with a batch system Moles A reacted at time t: Moles A remaining at time t:

A ba

B ca

C da

D

N XA0

N t N N X N XA A A A 0 0 0 1

Amit Ubhi

Amit Ubhi

Amit Ubhi

Amit Ubhi

rates are -ve when things are disappearing and +ve when appearing



Consider further the batch system with reaction Moles B reacted at time t: Moles B remaining at time t:

A ba

B ca

C da

D

moles reactedmoles reacted

moles reacted =BA

A ba

N XA 0

N t N ba

N XB B A 0 0


Stoichiometric Tables Species Initial Change Remaining

A NA0 N XA0 N N XA A0 0

B NB0 ba

N XA0 N ba

N XB A0 0

C NC0 ca

N XA0 N ca

N XC A0 0

D ND0 da

N XA0 N da

N XD A0 0

I NI 0 — NI 0

Total NT0 N d c b a N XT A0 01

Amit Ubhi

will need to produce these. same thing as an ICE table. Initial Concentration Equilibrium. A summary table of what you start with, changing and remaining.

Amit Ubhi

fractional change, everything is related to one material, in this case 'a'



Total change in number of moles: Define parameter

represents change in total number of moles per mole of A converted

N N da

ca

ba

N XT T A

0 01

da

ca

ba

1

Amit Ubhi

Amit Ubhi

makes things simplier



Concentration of species j given by For species B, for example,

CNVj

j

CN b

aN X

VB

B A

0 0

Amit Ubhi

number of moles divided by volume



Define new variable to represent the inlet molar flow rate of species i relative to the inlet flow rate of the reference (limiting) reactant For species B, for example

ii

A

NN

0

0

CN b

aX

VB

A B

0


Stoichiometric Tables For a constant volume process

Cases of constant volume process: dilute liquid phase reactions organic liquid phase reactions (except

polymerization reactions) gas phase reactions where number of reacting

molecules equals number of product ones, operating at constant T, P

CN

ba

X

V

Nba

X

VC

ba

XB

A B A B

A B

0 0

00

Amit Ubhi

most of the liq-liq or liq-sol are constant problems bc the solids wont compress, or the liq

Amit Ubhi

polymerization would be diff, depending on whats precipitating out

Amit Ubhi

ex: combustion



For flow problems, replace with

N Nj j and 0

F Fj j and 0

Amit Ubhi

Amit Ubhi

flow may or may not change depending on vol.


Stoichiometric Tables Species Initial Change Remaining

A FA0 F XA0 F XA0 1

B FB0 ba

F XA0 F ba

XA B0

C FC0 ca

F XA0 F ca

XA C0

D FD0 da

F XA0 F da

XA D0

I FI 0 — FA I0

Total FT0 F XA0 F F XT A0 0

Amit Ubhi

the change is always going to be in one component, in this case A0



Local concentrations calculated from local molar flow rates and total volumetric flowrate C F

vii

Amit Ubhi

when dealing with volumetric flowrates we deal with local flow rates



In the case of negligible volume changes (most liquid phase reactions)

CFv

Fv

F Xv

C XAA A A

A

0

0

00

11



In the case of significant volume changes (most gas phase reactions) or Define a new symbol

N N N XT T A 0 0

NN

y XT

TA

001

y A0

Amit Ubhi

ex:CH4 + 2H20 = Co2 + 4H2if we wanted the reaction to go to the right



Calculate the volume (volumetric flow rate) using the gas law: or, in terms of fractional conversion

V VPP

TT

zz

NN

T

T

0

0

0 0 0

V VPP

TT

zz

X

0

0

0 0

1

Amit Ubhi

we'll always assume ideality



Concentration given by or, neglecting compressibility effects

0

00 1

/PP

TT

XXav

CC jjAj

XzzPPTTV

XavNV

XavNVN

C jjAjjAjj

1//

0000

00

CHG 3127: Reaction Engineering © E.J. Anthony 44

Stoichiometric Tables For a flow system

Concentration given by or, neglecting compressibility effects

v vPP

TT

zz

X

0

0

0 01

C C

v a XX

TT

PPj A

j j

0

0

01

CFv

F v a Xv

F v a X

v T T P P z z Xjj A j j A j j

0 0

0 0 0 0 1

Amit Ubhi

wont need to remember these equations, he'll give the basic equations but expects us to write stuff.

Stoichiometric Tables In summary

For a batch system:

For a flow system:

For both:

CHG 3127: Reaction Engineering © E.J, Anthony 2012 45

V VPP

TT

zz

X

0

0

0 0

1

v vPP

TT

zz

X

0

0

0 0

1

C Cv a X

XTT

zz

PPj A

j j

0

0 0

01


Alternatives to X Fractional conversion not

appropriate for following situations: membrane reactions gas phase multiple reactions unsteady-state flow reactors


Alternatives to X Express concentrations in terms of

molar flows in these cases

0

00 P

PTT

FF

CCT

jTj

00

0

0

00 RTZ

PvFC T

T

0

0

0

0

0

0

0

0

TT

PP

CF

zz

TT

PPF

Fvv

T

TT

T

(z=z0=1)


Analysis of Reactions with Phase Change

Consider the reaction

Condensation occurs when

beyond this point each mole of D produced in the reaction condenses

A g B g C g D g l 2 ,

y PPD e

v

T,

CHG 3127: Reaction Engineering E.J. Anthony 2012 49


Create additional column in the stoichiometric table to account for balances after condensation begins

Let XC represent the conversion of A at which condensation begins

Use definition of mole fraction of D at condensation along with expressions for total molar flow rate to calculate XC


Consider the example

Assume reactants only are introduced into a flow reactor according to their stoichiometric ratios:

A g B g C g D g l 2 ,

F FF F

B A

C D

0 0

0 0

20




Species Remaining(Before Cond.)

Remaining(After Cond.)

A F XA0 1 F XA0 1

B F XA0 2 2 F XA0 2 2

C F XA0 F XA0

D F XA0 y FD e T,

Total F F XT A 0 3

F y F F F X

F F X yT D e T A A

T A D e

,

,. /

3 2

2 15 10 0

0

CHG 4127: Reaction Engineering © David G. Taylor, 2000 52

Calculate yD,e from operating conditions:

Equate total molar flow rates from two columns at point of condensation and solve for yD,e in terms of XC: or

y XXD eC

C,

3

y PPD e

vD

T,

F X F

XyA C A

C

D e0 03 2

151

.

,



Analysis of Reactions with Phase Change to calculate the concentrations after

condensation we note that, at constant pressure and temperature

C CT T 0

CC

Fv

Fv

vFF

vT

T

T

T

T

T0 0

00

0


Analysis of Reactions with Phase Change Knowing FT0 and FT, we can calculate

concentrations: where

T

TAAA F

FXvF

vFC 0

0

0 1

F FX

yF FT A

D eT A

2

151

30 0 0

.,

,


Reversible Elementary Reactions Consider the reaction

assume rate constants written in terms of

benzene rate of disappearance of benzene by forward

reaction:

rate of appearance due to reverse reaction:

2 6 6 12 10 21 2C H C H Hk k,

r k CB forward B, 12

r k C CB reverse D H, 2


Reversible Elementary Reactions

net rate of formation of benzene given by

net rate of consumption of benzene then equilibrium constant :

r r r r

r k C k C CB B net B forward B reverse

B B D H

, , ,

12

2

HDBHDBB CC

kkCkCCkCkr

1

2212

21

K kkc

1

2


Summarizing So Far... Basic definitions:

chemical species, and reaction; types of reactions (e.g., decomposition, combination, etc.); reaction rate; elementary and non-elementary reactions; molecularity and reaction order; Arrhenius law

Characterizing industrial Reactors: by operation

batch, continuous, and semi-batch

by design tank , tubular, tower, fluidized bed, slurry


Summarizing So Far...

generalized stoichiometric equation, extent of reaction, and fractional conversion

generalized mole balance equation well-mixed not well-mixed


Summarizing So Far...

relationships between reaction rates stoichiometric tables for batch systems tables for continuous (flow) systems definitions for and defining inlet moles (molar flow rates) in

terms of limiting reactant constant and variable volume processes


Summarizing So Far... Use stoichiometric table to analyze phase

change Calculate critical mole fraction from vapour

pressure and operating pressure Add column for after condensation Use two expressions for total moles (or proper

ratio of molar flows) to determine critical X Expressions for reversible, elementary

reactions

Acknowledgement Slides taken and modified from David

Taylor


Intro Notes

Documents