Chapter 3
Mass Balance
Balance on Reactive
Processes System
Content
 Stoichiometry
 Limiting and Excess Reactant, Fractional
Conversion and Extent of Reaction
 Chemical Equilibrium
 Multiple Reaction, Yield and Selectivity
 Balance on Reactive System
Stoichiometry


Stoichiometry – theory of proportions in which chemical species
combine with one another.
Stoichiometric equation of chemical reaction – statement of the
relative number of molecules or moles of reactants and products
that participate in the reaction.
2 SO2 + O2 ---> 2 SO3

Stoichiometric ratio
 ratio of species stoichiometry coefficients in the balanced
reaction equation
 can be used as a conversion factor to calculate the amount of
particular reactant (or product) that was consumed
(produced).
2 mol SO3 generated
2 mol SO2 consumed
2 mol SO2 consumed
1 mol O2 consumed
Test Yourself
C4H8 + 6 O2 --------> 4 CO2 + 4 H2O
1. Is the stochiometric equation balance?
 Yes
2. What is stochiometric coefficient for CO2
 4
3. What is stochiometric ratio of H2O to O2 including it unit
 4 mol H2O generated/ 6 mol O2 consumed
4. How many lb-moles of O2 reacted to form 400lb-moles CO2
 600 lb-moles O2 reacted
5. 100 mol/min C4H8 fed into reactor and 50% is reacted. At
what rate water is formed?
 200 mol/min water generated
Limiting Reactant & Excess Reactant



The reactant that would run out if a reaction proceeded to
completion is called the limiting reactant, and the other reactants
are termed excess reactants.
A reactant is limiting if it is present in less than its stoichiometric
proportion relative to every other reactant.
If all reactants are present in stoichiometric proportion, then no
reactant is limiting.
Fractional Excess 
n
Percentage Excess 
n
feed
-n
stoich
n
stoich
feed
-n
n
stoich
stoich
 100%
Example
C2H2 + 2H2 ------> C2H6
Inlet condition: 20 kmol/h C2H2 and 50 kmol/h H2
What is limiting reactant and fractional excess?
(H2:C2H2) o
= 2.5 : 1
(H2:C2H2) stoich = 2 : 1
H2 is excess reactant and C2H2 is limiting reactant
Fractional excess of H2 = (50-40)/40 = 0.25
Fractional Conversion
 Fractional Conversion (f)
moles reacted
Fractional Conversion , f 
mole fed
Percentage Conversion , f 
moles reacted
 100%
mole fed
Extent of Reaction
 Extent of Reaction, ξ
ni  nio  vi 
or
n i  n io  vi 
ξ
ni
nio
vi
= extent of reaction
= moles of species i present in the system after
the reaction occurred
= moles of species i in the system when the
reaction starts
= stoichiometry coefficient for species i in the
particular chemical reaction equation
Example
N2 + 3H2 ------------> 2NH3
Reactor inlet: 100 mol N2/s; 300 mol H2/s; 1 mol Ar/s
If fractional conversion of H2 0.6, calculate extent of reaction
and the outlet composition.
n H 2  300  3
n N 2  100  
n Ar  1
n NH 3  2
Unreacted H2 or H2 outlet= (1-0.6) 300 = 120 mol H2/s
Solve for extent of reaction : 60 mol/s
Test Yourself Page 119
2 C2H4 + O2 ------->2 C2H4O
The feed to a reactors contains 100kmol C2H4 and 100kmol O2.
a) which is limiting reactant?
C 2 H4
b) Percentage of excess?
{(100-50)/50 }x100%=100%
c)
O2 out?
C2H4 formed?
Extent of reaction?
50kmol
100kmol C2H4
50kmol
d) if fractional conversion for limiting reactant is 50%, what is outlet
composition and extent of reaction?
50kmol C2H4;
extent of reaction = 25 kmol;
75 kmol O2
50 kmol C2H4O
e) if reaction proceed to a point where 60kmol O2 left, what is fractional
conversion for C2H4? Fractional conversion of O2 and extent of reaction?
fC2H4=0.8
fO2=0.4
extent of rxn=40 kmol
Great work is done by people who
are not AFRAID to be great
Class Discussion
Example 4.6-1
Chemical Equilibrium

For a given set reactive species and reaction condition, two
fundamental question might be ask:
1. What will be the final (equilibrium) composition of the reaction
mixture? – chemical engineering thermodynamics
2. How long will the system take to reach a specified state short
of equilibrium? – chemical kinetics

Irreversible reaction
 reaction proceeds only in a single direction (from reactants to
products)
 the concentration of the limiting reactant eventually
approaches zero.
Reversible reaction
 reactants form products for forward reaction and products
undergo the reverse reactions to reform the reactants.
 Equilibrium point is a rate of forward reaction and reverse
reaction are equal
However the discussion to get the chemical equilibrium point is
not covered in this text- learn in chemical engineering
thermodynamic


Class Discussion
Example 4.6-2
CO + H2O <----> CO2 + H2
nco
nH2O
n CO2
nH2
ntotal
= 1-ξ
=2- ξ
=ξ
=ξ
=3
K=yCO2 yH2 / y CO y H2O=1
yY CO2= ξ/3
yH2= ξ/3
y CO= (1- ξ)/3
y H2O= (2- ξ)/3
Multiples Reaction, Yield & Selectivity


Some of the chemical reaction has a side reaction which is
formed undesired product- multiple reaction occurred.
Effects of this side reaction might be:
1. Economic loss
2. Less of desired product is obtained for a given quantity of raw
materials
3. Greater quantity of raw materials must be fed to the reactor
to obtain a specified product yield.
selectivity
=
moles of desired product
moles of undesired product
Yield
 3 definition of yield with different working definition
Moles of desired product formed
Yield
Yield
=
=
Moles that would have been formed if there
were no side reaction and the limiting
reactant had reacted completely
Moles of desired product formed
Moles of reactant fed
Yield
=
Moles of desired product formed
Moles of reactant consumed
Extent of Reaction for Multiple Reaction
 Concept of extent of reaction can also be applied
for multiple reaction
 only now each independent reaction has its own
extent.
ni  nio   vi j 
j
ij
Class Discussion
Example 4.6-3
Balance of Reactive Processes
 Balance on reactive process can be solved based
on three method:
1. Atomic Species Balance
2. Extent of Reaction
3. Molecular Species Balance
MUFLIS DALAM HARTA HANYA
KEMISKINAN YANG SEMENTARA
DI DUNIA, MUFLIS DALAM
WATAK ADALAH KEMISKINAN
TERBURUK DI DUNIA. IA AKAN
DI BAWA BERSAMA KE AKHIRAT
Atomic Species Balance
-
No. of unknowns variables
No. of independent atomic species balance
No. of molecular balance on indep. nonreactive species
No. of other equation relating the variable
=============================
No. of degree of freedom
=============================
Extent of Reaction
No. of unknowns variables
+ No. of independent chemical reaction
- No. of independent reactive species
- No. of independent nonreactive species
- No. of other equation relating the variable
=============================
No. of degree of freedom
=============================
Molecular Species Balance
No. of unknowns variables
+ No. of independent chemical reaction
- No. of independent molecular species balance
- No. of other equation relating the variable
=============================
No. of degree of freedom
=============================
ANY
QUESTION?