1. Chapter 3 Mass Balance Balance on Reactive Processes System

2. Content Stoichiometry
Limiting and Excess Reactant, Fractional Conversion and Extent of Reaction
Chemical Equilibrium
Multiple Reaction, Yield and Selectivity
Balance on Reactive System

3. 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

4. C4H8 + 6 O2 --------> 4 CO2 + 4 H2O
Test Yourself
C4H8 + 6 O > 4 CO2 + 4 H2O
Is the stochiometric equation balance?
Yes
What is stochiometric coefficient for CO2 4
What is stochiometric ratio of H2O to O2 including it unit
4 mol H2O generated/ 6 mol O2 consumed
How many lb-moles of O2 reacted to form 400lb-moles CO2
600 lb-moles O2 reacted
100 mol/min C4H8 fed into reactor and 50% is reacted. At what rate water is formed?
200 mol/min water generated

5. 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.

6. 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

7. Fractional Conversion
Fractional Conversion (f)

8. Extent of Reaction Extent of Reaction, ξ ξ = extent of reaction
ni = moles of species i present in the system after the reaction occurred
nio = moles of species i in the system when the reaction starts
vi = stoichiometry coefficient for species i in the particular chemical reaction equation

9. N2 + 3H2 ------------> 2NH3
Example
N2 + 3H > 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.
Unreacted H2 or H2 outlet= (1-0.6) 300 = 120 mol H2/s
Solve for extent of reaction : 60 mol/s

10. 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?
C2H4
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

11. Great work is done by people who are not AFRAID to be great

12. Class Discussion
Example 4.6-1

13. Chemical Equilibrium
For a given set reactive species and reaction condition, two fundamental question might be ask:
What will be the final (equilibrium) composition of the reaction mixture? – chemical engineering thermodynamics
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

14. CO + H2O <----> CO2 + H2
Class Discussion
Example 4.6-2
CO + H2O <----> CO2 + H2
nco = 1-ξ
nH2O =2- ξ
n CO2 = ξ
nH2 = ξ
ntotal = 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

15. 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:
Economic loss
Less of desired product is obtained for a given quantity of raw materials
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

16. Yield 3 definition of yield with different working definition Yield =
Moles of desired product formed
Moles that would have been formed if there were no side reaction and the limiting reactant had reacted completely
Yield =
Moles of desired product formed
Moles of reactant fed
Yield =
Moles of desired product formed
Moles of reactant consumed

17. 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.

18. Class Discussion
Example 4.6-3

19. Balance of Reactive Processes
Balance on reactive process can be solved based on three method:
Atomic Species Balance
Extent of Reaction
Molecular Species Balance

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21. 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

22. =============================
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

23. 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

24. ANY QUESTION?