Outline
Material and Energy Balances
CHEN 2120
Class Meeting #33
April 18th, 2007
• Review: why are we interested in heats of
reaction?
• Heat of combustion
• Intro to Energy Balances on Reactive
Processes
• Example
“Heats of combustion”
Heats of reaction
25°C
A
Adiabatic
Mixer
B
Heats of reaction
2H2 + O2 Æ 2H2O
A,B
T = 25°C
H2
∆H = 0 (if ideal)
2H2 + O2 Æ 2H2O
H2
What will happen over time
if the reactor is adiabatic?
(Exothermic)
Adiabatic
Reactor
O2
H2 O
Reactor
O2
H2 O
Energy
25°C
(Exothermic)
Reactants
Products
T = ??
-The temperature of the
reactor will increase, causing
the reaction to ‘runaway’.
∆H = ??
Heats of reaction
2H2 + O2 Æ 2H2O
H2
Heats of reaction
(Exothermic)
2H2 + O2 Æ 2H2O
H2
H2 O
Reactor
O2
Reactor
(Exothermic)
H2 O
O2
Temp
Runaway reaction
- (Boom!!)
Energy
k = Ae − Ea / RT
To maintain a constant
reactor temperature, what
must be done?
Reactants
Products
Time
1
Heats of reaction
2H2 + O2 Æ 2H2O
H2
Reminders
(Exothermic)
H2 O
Reactor
O2
Energy
Q
Heat must be removed
from the reaction!
Reactants
Products
How are heats of reaction
determined?
1) Calorimetry
2) Using heats of formation:
-
-
Standard heat of formation is the enthalpy
change associated with the formation of 1
mole of the compound at a reference
temperature and pressure (usually 25C and
1 atm)
The standard heat of formation of an
elemental species is zero.
• ∆Hr is negative for exothermic reactions and positive for
endothermic reactions
• At low and moderate temperatures, ∆Hr is nearly independent of
pressure
• The value of the heat of a reaction depends on how the
stoichiometric equation is written:
CH4 + 2O2 Æ CO2 + 2H2O
vs. 2CH4 + 4O2 Æ 2CO2 + 4H2O
• Heat of reaction depends upon state of aggregation:
CH4(g) + 2O2(g) Æ CO2(g) + 2H2O(l)
vs. CH4(g) + 2O2(g) Æ CO2(g) + 2H2O(g)
• Standard heat of reaction: heat of reaction when both the reactants
and products are at a specified reference temperature and pressure,
usually (and always in the text) 25°C and 1 atm.
How are heats of reaction
determined?
ˆ
∆H = ∑ ν ∆Hˆ − ∑ ν ∆Hˆ
r
fi
i
fi
reac tan ts
Example:
CaC2(s) + 5H2O(l) Æ CaO(s) + 2CO2(g) + 5H2(g)
∆H’r = [(1)(-635.6 kJ/mol)+(2)(-393.5 kJ/mol)+(5)(0)]
- [(1)(-62.76 kJ/mol)+(5)(-285.84 kJ/mol)]
= 69.36 kJ/mol (as written, and at 25°C and 1 atm!)
Heats of combustion
• The heat of combustion, ∆H’c, is the heat of the
combustion of that substance with oxygen to
yield specified products [e.g., CO2(g) and
H2O(l)], with both reactants and products at
25°C and 1 atm (Table B.1)
• Assumptions: all carbon in the fuel forms
CO2(g), all hydrogen forms H2O(l), all sulfur
forms SO2(g), and all nitrogen forms N2(g).
• Example: ethanol, ∆H’c = -1366.9 kJ/mol
C2H5OH(l) + 3O2(g) Æ 2CO2(g) + 3H2O(l)
i
products
Non-standard states
• How would you calculate ∆Hr of the
following reaction at 56°C?
56°C
∆Hr
C2H5OH(l) + 3O2(g)
Æ
∆H1
56°C
2CO2(g) + 3H2O(l)
∆Hr = ∆H1 + ∆H°r + ∆H2
Reactants at 25°C
∆H°r
∆H2
Products at 25°C
2
Intro to Energy Balances on
Reactive Processes
H2
H2 O
Reactor
Heat of Reaction Method
1)
2)
3)
O2
• Two methods:
1) Heat of reaction method
- Best for single reactions for which ∆Hr is
known.
2) Heat of formation method
- Best for multiple reactions and single
reactions for which ∆Hr is not known.
4)
5)
6)
7)
Complete the material balance calculations on the reactor to the
greatest extent possible.
Choose reference states for specific enthalpy calculations.
For a single reaction in a continuous process, calculate the extent
of reaction, ξ, from equation 9.1-3:
ξ = nA,r/|υA|
Prepare the inlet-outlet enthalpy table, inserting known molar
amounts (ni) or flow rates for all inlet and outlet stream
components.
Calculate each unknown stream enthalpy.
Calculate ∆H for the reactor. Use the following formula:
∆H = ξ∆Hr’ + ΣnoutH’out – ΣninH’in (single reaction)
Solve the energy balance.
Example: Problem 9.12
Q
3