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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 YourselfC4H8 + 6 O2 --------> 4 CO2 + 4 H2O
1. Is the stochiometric equation balance? Yes
2. What is stochiometric coefficient for CO2
43. 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.
%100n
n - n Excess Percentage
n
n - n Excess Fractional
stoich
stoichfeed
stoich
stoichfeed
ExampleC2H2 + 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)
%100fed mole
reacted moles f ,Conversion Percentage
fed mole
reacted moles f ,Conversion Fractional
Extent of Reaction
Extent of Reaction, ξ
ξ = extent of reactionni = moles of species i present in the
system after the reaction occurrednio = moles of species i in the system
when the reaction startsvi = stoichiometry coefficient for species i
in the particular chemical reaction equation
iioi
iioi
vnn
vnn
or
ExampleN2 + 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.
Unreacted H2 or H2 outlet= (1-0.6) 300 = 120 mol H2/s
Solve for extent of reaction : 60 mol/s
2
1
100
3300
3
2
2
NH
Ar
N
H
n
n
n
n
Test Yourself Page 1192 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
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 thermodynamics2. 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 DiscussionExample 4.6-2
CO + H2O <----> CO2 + H2nco = 1-ξnH2O =2- ξn CO2 = ξnH2 = ξntotal = 3
K=yCO2 yH2 / y CO y H2O=1
yY CO2= ξ/3yH2= ξ/3y CO= (1- ξ)/3y 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 loss2. Less of desired product is obtained for a given quantity of raw
materials3. 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
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
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.
ijjjiioi vnn
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=============================