MATERIAL BALANCES WITH

CHEMICAL REACTION

General Material Balance Equation

A balance on a material in a process system

may be written as:

Input + generation - output - consumption =

accumulation

The equation may be written for any material

that enters or leaves any process system

It can be applied to the total mass or total

moles of this material or to any atomic species

involved in the process

• if there is no reaction,

Input =output

• The generation and consumption terms

are applied only when chemical reaction is involved

• It should be noted that in chemical reactions, the total mass of the input remains constant, but the total moles may or may not remain constant.

N2+3H2 2NH3

1 mole of N2 ≡ 3 mole of H2 ≡ 2 mole of

NH3

It is very convenient to use basis of

calculations in molar units

For any reactant material the balance of material can be written as

Material entering= material reacted +material un reacted

For products we can write

Material leaving = material produced by the reaction.

Fundamental concepts

Stoichiometry It is a theory of the proportions in which chemical

species combine with one another.

Stoichiometric equations:

Stoichiometric equation of a chemical reaction is a

statement indicating relative moles of reactant and

products that take part in the reaction

Stoichiometric co-efficient: It is the number that precedes the formula of each

component involved in a chemical reaction

Refers to quantities of reactants and products in a balanced chemical reaction.

aA + bB cC + dD

i.e. a moles of A react with b moles of B to give c moles of C and d moles of D.

a,b,c,d are stoichiometric quantities

Limiting reactant / component:

◦ In practice a reactant may be used in excess of

the stoichiometric quantity for various reasons.

◦ In this case the other reactant is limiting i.e. it will

limit the yield of product.

Limiting Reactant:

◦ It is the reactant that would disappear first if a

reaction goes to completion

◦ Limiting reactant is the reactant which is present

in such proportions that its consumption will

limit the extent to which the reaction can

proceed

Excess Reactant:

Excess reactant is the reactant which is

taken more than the requirement to

combine with other reactant as per the

Stoichiometry.

It is the reactant which will be present in

the product even if the reaction goes to

completion

Definitions to solve material

balances. Percentage excess:

Consider a reaction A + B C

Where B is the excess reactant, then

CONVERSION:

Conversion or fractional conversion or

degree of completion is defined with

respect to limiting reactant

It gives idea regarding degree of

completion of reaction

Consider a chemical reaction

A + B C

Where A = a limiting reactant

B = the excess reactant

YIELD:

Extent of reaction() Also called reaction coordinates

Measures progress of reaction.

If ni = moles of i species taking part in reaction

vi = stoichiometric number of species I

Zi = fractional conversion of species I

Usually limiting reactant.

vi -ve for reactant , +ve for

product

Combustion of solids liquids and

gaseous fuels. Orsat analysis

Flue gas analysis on dry basis.

Measures CO2 CO and O2 in a given flue

gas.

gas

C

O

S

H

N

liquid

C

O

H

solid

C

O

S

H

N

H2O

Ash

Fuel

O2

N2

H2O

Air

CO CO2

O2

SO2 SO3

N2

H2O

Ash C,N,H,S,O

Flue gas

Refuse

How to solve material balance? The total mass of the fuel burned and flue

gas produced : carbon balance The refuse formed : ash balance(inert) The amount of air supplied : nitrogen

balance. The amount of hydrogen in fuel: oxygen

balance

Proximate analysis of coal

Empirical procedure

List the composition in terms of ◦ Moisture: loss in weight when coal is heated at

1050C

◦ Volatile combustible matter: loss in weight when coal is heated at 9500C in crucible for 7 minutes minus weight of moisture

◦ Ash: The weight of residue after complete combustion (700-7500C)

◦ Fixed carbon: calculated as 100 minus sum of all the above.

◦ If sulphur is present which is also reported as percentage

Ultimate analysis of coal

Gives the percentage by weight of the

various elements and ash in the coal

sample.

Provides data for material balance

combustion of fuel.