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

• Stoichiometry • Stoichiometric Coefficient / Ratio• Stoichiometry proportions • Limiting Reactant • Excess Reactant • Fractional Excess• Fractional Conversion

Reaction Equilibrium

• Reversibility?• Reversible versus irreversible reactions• Equilibrium • Reaction Yield • Multiple Reactions– Reaction Selectivity

Chemical Equation-1n1 A + n2 B n3 C + n4 D

Contains qualitative and quantitative informationStoichiometry: theory of proportions in reactions

What are the stoichiometric coefficients in the above equation?Stoichiometric Ratio of A/B is n1 / n2

ExampleC7H16 + 11 O2 7 CO2 + 8 H2O

Check number of atoms in both sides of the equation – reaction balance?

Stoichiometric Ratio of O2 / C7H16 is 11/1 = 11

With masses given– change to moles…

Chemical Equation-2Same 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 If 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 ExcessSame Example

C7H16 + 11 O2 7 CO2 + 8 H2O

Fractional Excess of …….. = (16-11)/11Fractional 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 ConversionSame Example

C7H16 + 11 O2 7 CO2 + 8 H2OFractional 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) 30Reacted (calculated) 30/2 30/2

At the end 5 20 65

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 CalculationsFor 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

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 = 1.11Calculate a) the molar composition of the components in the final productsb) The fractional conversion of CO

Solve….Answers: X = 0.156 (& 0.844?) and YCO= 0.5, YH2= 0.408 …etch

f = 0.156

Example- Follow on Board in the class

Try this question at home - Reaction Equilibrium

For the reactionAt 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%

Determinea) Output Compositionsb) Selectivity of HCHO relative to CO2

HW 3 – Mass balance with Reactions

40 43 46 57

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 isPhysical-chemical?

Analysis Techniques Mass Balance Problems with Reactions

Three methods1. 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

For Reactive systems: the degree of freedom analysis must count only independent reactions

Are these 2 equations independent

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-2Oxygen 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 calculationsWet 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

Fuel - Expensive Flue gasesLimiting air (free) CO2

excess CO H2O

Theoretical air? N2

%excess air? Unreacted Fuel NOx (at High T)

SO2 (from sulfur)

Use wet or dry basis Example – Methane Combustion

Combustion Process

Combustion Chamber

Fuel Flue gases air

C CO2 and H H2O

Combustion Process – Air Supply

Combustion Chamber

Given Find Fuel and entering air Composition of Flue gasesComposition of Flue gases Find Fuel and air rate, excess ratioSimilar Mass Balance Calculations - Use Molecular Species Balance- Use Atomic Species Balance (more convenient with multiple reactions)

Combustion Calculations

Example – Combustion of Ethane – Follow on the board

Find Composition of Flue gasesIn class using molecular species balanceAt 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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