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 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

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

Class DiscussionExample 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 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

Class DiscussionExample 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?