Chapter 9
Balances on Reactive Processes
„Dr
M. A. A. Shoukat Choudhury
„Department
epa t e t o
of C
Chemical
e ca Engineering
g ee g
„BUET, Dhaka – 1000
Heat of Reaction or Enthalpy of reaction
• The heat of reaction,
(T, P), is the
enthalpy change for a process in which
stoichiometric quantities of reactants at
temperature
p
T and Pressure P react
completely to form products at the same
temperature and pressure.
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Total Enthalpy Change
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Hess’s Law for Calculation of Heats of reaction
• Hess’s law states that if you can obtain a stoichiometric
equation as a linear combination of the stoichiometric equations
y can determine its heat of reaction byy
for other reactions,, you
performing the same operations on the heats of the other
reactions. For example, suppose we experimentally determine
the following two standard heats of reaction:
• We want to determine the heat of the reaction A = B
+ 2D but can’t carry out that reaction experimentally.
We observe
observe, however
however, that we can obtain that
stoichiometric reaction as [1] + 2x[2]:
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„Example
9.4-1 Calculate the standard heat of reaction for the
„The dehydrogenation of ethane
C2H6 = C2H4 + H2
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Energy Balance
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Energy Balance – Open System
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EB – HEAT of REACTION METHOD (Reference
– compounds at 25 oC and 1 atm
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EB – HEAT of FORMATION METHOD
(Reference – Elements at 25 oC and 1 atm
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Energy Balance
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Two Methods for Energy Balance
• Heat of Reaction Method – Good if there is
only one reaction
• Heat of Formation Method – Good for
multiple reaction
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Heat of Reaction Method
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Heat of Reaction Method (cont’d)
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„An
Example
„25
C3H8 (g) + 5 O2 (g)
=
3 CO2 (g)
+ 4 H2O (l)
Example - Heat of reaction method
„25
„0
„0
Heat of Formation Method
Example - Heat of formation method
„0
„0
Problem 9.13
The production of most of the steel manufactured in USA
begins with the reduction of hematite ore (mostly ferric oxide)
with coke (carbon) in a blast furnace to obtain pig iron. The
basic reaction is
Fe2O3(s)+3 C(s)
2 Fe (s) +3CO(g): ΔHr(770F) =
2.111×105
2.111
105 Btu/lb
Btu/lb-mol
mol
Suppose that stoichiometric amounts of ferric oxide and carbon are
p
, the iron emerges
g as a
fed at 77 0F,, the reaction is complete,
liquid at 28000F and CO emerges at 5700F. Perform the
following calculations for a basis of 1 ton of iron produced.
Draw and label a flowchart and perform all the material balance
calculations needed to determine the amounts (1b-mole) of
each feed and product stream component.
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Problem 9.13 (cont’d)
b)
Taking the reactant and product species in their normal states
at 770F as references, prepare an inlet-outlet enthalpy table
and calculate and fill in all unknown component specific
enthalpies (Btu/lb-mole).
Use the following physical property data for iron:
Fe(s): Cp[Btu/(1b-mole.0F)] = 5.90 + 1.50×10-3 T (0F)
Tm = 27940F, ΔHm (Tm) = 6496 Btu/1b-mole
Fe(l): Cp [Btu/(1b-mole.
[Btu/(1b-mole 0F)] = 8.15
8 15
c) Estimate the furnace heat requirement (Btu/ton Fe produced).
d) List the assumptions that make the value calculated in part (b)
only an approximate estimate of the furnace heat
requirement. (One of the assumptions has something to do
with the reactor pressure).
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9.13 (solution) – part d)
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Problem 9.21
Ethanol is produced commercially by the hydration of ethylene:
C2H4(g) + H2O(v)= C2H5OH(v)
Some of the product is converted to diethyl ether in the undesired side
reaction
2 C2H5OH(v) = (C2H5)2O(v)+H2O(v)
The combined feed to the reactor contains 53.7 mole% C2H4, 36.7% H2O
and the balance nitrogen which enters the reactor at 310 0C. The
reactor operates isothermally at 310 oC.
C An ethylene conversion of 5%
is achieved, and the yield of ethanol (moles ethanol produced/mole
ethylene consumed) is 0.90.
Data for Diethyl Ether
ΔH0f = -272.8 kJ/mol for the liquid
ΔH0v = 26.05 kJ/mol (assume independent of T)
Cp[kJ/mol.
[kJ/mol oC]
C]=0
0.08945+40.33
08945+40 33*10
10-5 T (oC) – 2.244
2 244*10
10-7 T2
(a) Calculate the reactor heating or cooling requirement in kJ/mol feed.
(b) Why would the reactor be designed to yield such a low conversion of
ethylene? What processing step (or steps) would probably follow the
reactor in a commercial implementation of this process?
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9.6 Fuels and Combustion
• Heat from combustion reaction produces
steam which drives turbine to produce
steam,
electricity
• Analysis of fuels, combustion reaction and
reactor is an important activity for chemical
engineers
• Three types of fuels
- Solid ((coal,, coke,, wood,, solid waste))
- Liquid (hydrocarbons, coal tars, bio-ethanol)
- Gaseous fuels (NG, acetylene, hydrogen)
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Heating Value of a Fuel
„HHV
„For
= LHV + n * ΔHv (H2O)
a mixture of Fuel: „HV = Σ x (HV)
i
i
„Do
o
Yourself
ou se Example
a pe 9
9.6-1
6
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Heating Values of Common Fuels
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Adiabatic Flame Temperature
„
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Problem 9.66 (F&R, 3rd edition)
Methane is burned with 25% excess air in a
continuous adiabatic reactor
reactor. The methane enters the
reactor at 25 oC and 1.1 atm at a rate of 5.50 L/s ,
and the entering
g air is at 150 oC and 1.1 atm.
Combustion in the reactor is complete, and the reactor
effluent emerges at 1.05 atm. Calculate (a) the
temperature and
d (b) the
h degrees
d
off superheat
h
off the
h
reactor effluent (consider water to be the only
condensable species in the effluent)
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9.66 Solution
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Known Q and unknown outlet temperature
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Waste Heat Boiler
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