material balances with chemical reaction
DESCRIPTION
material balance with chemical rnTRANSCRIPT
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
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