1. Material Balances w/ Chemical Reactions

2. Section 4.6—Chemical Reaction Stoichiometry
Consider the following chemical reactions: C2H6  C2H4 + H2 2H2 + O2  2H2O Notice that there is no Law of Conservation of Moles -- Moles are not conserved

3. Basic Terminology Limiting reactant Excess reactant Fractional excess
The ratio of the amount of excess to the stoichiometric amount required
Ex: 2H2 + O2  2H2O
40 mol/hr H2 & 15 mol/hr O2 fed to reactor
Fractional conversion
The ratio of the number of moles reacted to the number of moles fed to the reactor
Ex: From previous example:
25 moles of H2O recovered

4. Example (Pg. 125)
A feed stream of 100 mol/hr enters a reactor. The feed contains 85% ethane and the balance inerts (nonreactive compounds). The fractional conversion of ethane is 0.501, and the fractional yield of ethylene is Calculate the molar composition of the product gas.

5. Molecular vs. Atomic Species Balance
In the previous example we used basic stoichiometry to relate the number of moles of ethane consumed to the number of moles of both C2H4 and hydrogen produced
This is referred to as a molecular species balance
To use a molecular species balance something has to be specified (either the number of moles of a product or a fractional conversion)

6. Atomic Species Balance
Because the law of conservation of matter does apply, we can look at the number of atoms of each element entering and leaving a system
This must be conserved (i.e. the number of atoms of an element entering and leaving must be the same)
“Input = Output”

7. Example to Illustrate Both Types
C2H6  C2H4 + H2
100 mol/min of ethane enters a reactor and the amount of hydrogen produced from the reaction is measured at 40 mol/min.
Find the molar flow rates of unreacted ethane and C2H4 product formed