Process Calculations
Supplementary Notes
Material Balance with Chemical Reactions
Dr. Maher Al-Jabari
Think of Environmental applications!
Think of related chemistry topics and Terminology?
Think of Form of material balance equation
Input + Generation = Output + Consumption
Review and Definitions
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Stoichiometry
Stoichiometric Coefficient / Ratio
Stoichiometry proportions
Limiting Reactant
Excess Reactant
Fractional Excess
Fractional Conversion
Reaction Equilibrium
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Reversibility?
Reversible versus irreversible reactions
Equilibrium
Reaction Yield
Multiple Reactions
– Reaction Selectivity
Chemical Equation-1
n1 A + n2 B  n3 C + n4 D
Contains qualitative and quantitative information
Stoichiometry: theory of proportions in reactions
What are the stoichiometric coefficients in the above equation?
Stoichiometric Ratio of A/B is
n1 / n 2
Example
C7H16 + 11 O2  7 CO2 + 8 H2O
Check number of atoms in both sides of the equation – reaction balance?
Stoichiometric Ratio of O2 / C7H16 is
With masses given– change to moles…
11/1 = 11
Chemical Equation-2
Same Example
C7H16 + 11 O2  7 CO2 + 8 H2O
If a feed contains 1 mole C7H16 of and 11 mole of O2
The reactant present: choose?
a) with excess oxygen
b) in Stoichiometry proportions
c) none of the above
 The reaction proceeds to completion = all reactants are consumed
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f a feed contains 1 mole C7H16 of and 16 mole of O2
Which is the limiting reactant (run out)?
Which is the excess reactant?
What is the fractional excess (of the …………. Reactant)?
Fractional Excess
Same Example
C7H16 + 11 O2  7 CO2 + 8 H2O
Fractional Excess of …….. = (16-11)/11
Fractional Excess =
moles of excess reactant in the feed – moles of excess reactant needed to react with all moles of the limiting reactant
moles of excess reactant needed to react with all moles of the limiting reactant
Fractional Conversion
Same Example
C7H16 + 11 O2  7 CO2 + 8 H2O
Fractional Conversion f (for incomplete reaction)
f = moles reacted of A
moles fed of A
If the feed contains 10 moles of C7H16 and
if fractional conversion is 80%. What is
The amount of C7H16 reacted =
The amount of C7H16 leaving (unreacted) =
Reaction Extent (x)
Starting with (moles): 20 + 50
50
Reacted (Known)
Reacted (calculated)
30/2
At the end
30
30/2
5
20
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Reaction Extent (x)
Try this question at home – Reaction Stoichiometry
Reaction Equilibrium
Distinguish between
Chemical Equilibrium (final amounts / composition)
Reaction Kinetics (Time needed)
What is the difference between
reversible and irreversible reactions.
What is the Equilibrium Constant
K = f (Temperature)
Equilibrium Calculations
For a sample reaction – extent analysis – x
• Given Initial Moles of Reactants (and Products)
• Suppose x for reaction
• Determine Final (Equilibrium) moles of all– (reactants and products)
• Estimate total number of moles
• Determine Composition of each reactant and product
• Substituting in expression for K
• Solve for extent x – and choose the reasonable answer.
• Determine number of moles of each reactant and product
• Estimate other required variables e.g. fractional conversion
• See example 4.6.2
• Try this reaction on board
Example- Follow on Board in the class
CO is toxic, it can be converted to CH3OH by reaction with hydrogen
For a reactor containing initially equimolar amounts of CO and H2
but no CH3OH, the following reversible reaction occur:
CO + 2 H2 CH3OH
The reaction proceeds till reaching equilibrium, with K = 1.11
Calculate
a) the molar composition of the components in the final products
b) The fractional conversion of CO
Solve….
Answers:
X = 0.156 (& 0.844?) and YCO= 0.5, YH2= 0.408 …etch
f = 0.156
Try this question at home - Reaction Equilibrium
For the reaction
At T = 1300 K, K = 0.55
Reaction Design aims at
- Maximizing rate of desired reaction (1)
- Minimizing rate of undesired reactions (2 and 3)
 Maximize Profit
Thus, define Yield and Selectivity
Yield Definitions may vary:
Moles of desired product / moles of reacted (Fed or consumed)
Example- Follow on Board in the class
Given
Fractional conversion of CH4 = 95%
Fractional yield of HCHO = 90%
Determine
a) Output Compositions
b) Selectivity of HCHO relative to CO2
HW 3 – Mass balance with Reactions
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Try this question at home -
Balance on Atomic and Molecular Species
• What are the types of Balances you can make?
– Balance on total masses: input = output
– Balances on any of the molecules – Form for each?
Input + Generation? = Output + Consumption?
– What about balances for H-atoms and C-atoms?
input = output
Atoms can neither be created no destroyed? In a …. Reaction
Hydrogen Balance – H versus H2 balance
Follow example-text in the book
How many moles of C in C2H6?
How many moles of H in C2H6?
What about this balance?
Atomic Balances – Simple?
• What is the type of this balance - Molecular or Atomic Balances?
Independent Species
• For non-reactive systems
No. of Equations = No. of independent Species?
• Are Oxygen and Nitrogen in air independent species?
• Read
Analyze Independent Species
The process is
Physical-chemical?
Analysis Techniques
Mass Balance Problems with Reactions
Three methods
1. Molecular species balances (approach with no reactions)
2. Atomic species
3. Extent of Reaction
Do they lead to SAME results?
Choose the most convenient - which for which?
Based on problem type (or your convenience?)
Cases: one reaction, equilibrium, multiple reactions
See example 4.7.1
Read Sections 4.7 c, d and e – and practice methods
Independent Reactions/Equations
Are these 2 equations independent
For Reactive systems:
the degree of freedom analysis must count only independent reactions
Molecular Species Balances
Read p.128
Atomic Species Balances
Read p.129
Extent of Reaction - Balance
Read p.130
Follow the book – in three methods
- Notice the common starting steps (for methane and nitrogen)
- Compare simplicity.
Combustion Reactions-1
Fuel + O  CO2, CO, H2O & SO2 (NOx at High T) + Heat
Comment on Environment – Energy
Types of Fuel
- Coal - C (with H and S content)
- Fuel Oil
- Natural Gas (mainly CH4)
- Liquefied petroleum gas (e.g. C3H12)
Types of Combustion reactions:
complete versus incomplete (partial) combustion
Combustion Reactions-2
Oxygen Supply
From air
79% N2 and 21% O2
N2/O2 = 3.76 (dependent species?)
Also contains humidity (water vapor).
The product gas – Flue gases- Stack gases
Dry basis: without including water vapor in calculations
Wet basis: with including water vapor in calculations
How you convert from given basis to another – Recall conversion between
mass and molar composition! See example 4.8.1 – can be a quiz
See example 4.7.1 on combustion
Combustion Process
Fuel - Expensive
Limiting
Combustion
Chamber
air (free)
excess
Theoretical air?
%excess air?
Example – Methane Combustion
Flue gases
CO2
CO
H2O
N2
Unreacted Fuel
NOx (at High T)
SO2 (from sulfur)
Use wet or dry basis
Combustion Process – Air Supply
Fuel
Combustion
Chamber
Flue gases
air
C  CO2
and
H  H2O
Combustion Calculations
Given
Find
Fuel and entering air
Composition of Flue gases
Composition of Flue gases
Find Fuel and air rate, excess ratio
Similar Mass Balance Calculations
- Use Molecular Species Balance
- Use Atomic Species Balance (more convenient with multiple reactions)
Example – Combustion of Ethane – Follow on the board
Find Composition of Flue gases
In class
using molecular species balance
At home
using atomic species balance
If nitrogen is not withdrawn from the system (purged)
At the same rate it enters the process
Where shall it go?
HW 4 – Mass balance with Reactions
Read section 4.9 P.151
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