module 7 - p l dhar's web page · module 7 reactive mixtures. lecture 7.1 mass and energy...
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Module 7Reactive Mixtures
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Lecture 7.1Mass and Energy
Balance
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THERMODYNAMICS OF REACTIVE MIXTURES
• Special feature : Inter atomic bonds in molecules of reactants are broken followed by rearrangement of atoms to form new compounds
• Thermodynamic analysis:– Mass balance– Energy Balance– Equilibrium Study
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THERMODYNAMICS OF REACTIVE MIXTURES
• MASS BALANCE … Stoichiometry• Cons. of mass for each species.• Examples : Combustion of fuels• Octane is burnt with 95% of theoretical air;
write its combustion equation• Stoichiometric eq:C8H18+12.5O2=8CO2+9H2O• with 95 % air• C8H18+12.5×.95(O2+3.76N2)=?CO2+?H2O+?
CO+?N2
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THERMODYNAMICS OF REACTIVE MIXTURES
• First Law Analysis
1 2
ProductsReactants
Q
Win = -Wsh
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THERMODYNAMICS OF REACTIVE MIXTURE
SHPinPR WHWHHQ +=−=+ 221
∑∑ +−= SHRiRiPiPi WNhNhQ
Need for absolute values or enthalpies
( ) ( )∑∑ −−−= RiRRPiiPP NhhNhhQ 0102
( ) SHRiRPiP WNhNh +−+ ∑ ∑ 00
Enthalpy of reaction : H0
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THERMODYNAMICS OF REACTIVE MIXTURE
ENTHALPY OF REACTION H0
T1 = T2 = T0
1 2Q∑∑ ∆=−= 0
00 HNhNhQ RiRPiP
0 { 250C 1 atm}
H > 0 -----EndothermicH < 0 -----Exothermic
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THERMODYNAMICS OF REACTIVE MIXTURE
COMBUSTION OF FUELS : Heating value of fuel
- H0 = HHV & LHV
LHV : H2O appears in the gas phaseHHV : H20 appears in the liquid phase
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THERMODYNAMICS OF REACTIVE MIXTURE
ENTHALPY OF FORMATION HF
HF = H0 for the reaction producing the compound from its elements.
( )∑−=∆i
elementsiicompF hhh υ
Molal enthalpy of the compound
Stoichiometric coeff. of ith element in forming a
single mole of the compound
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THERMODYNAMICS OF REACTIVE MIXTURE
Convention : assign a value of zero to the enthalpy of all stable elements at 250C and 1 atm.
∴ hF = hcomp.
( ) ( )∑∑ −=∆∴ RiiPii hnhnH 0
Std. enthalpy of reaction products reactants
Stoichiometric coeff.
( )( ) ( )( )∑∑ ∆−∆=R
iFiP
iFi hnhn
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∴ First law statement for S.S.S.F. chem. reaction :
THERMODYNAMICS OF REACTIVE MIXTURE
( ) ( )∑∑ −−−=R
iRRiP
iPPi hhnhhnQ 0102
( ) ( )∑∑ +∆−∆+R
SHiFiP
iFi Whnhn
∑ ∑ +−=P R
SHiiii WhnhnQ
( ) ( )iFiatmKPTi hhhh ∆+−= 1,298,where
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Example • Steady flow combustion of Propane at 25C
with 100% excess air . Products leave at 500K find the heat transfer.
• C3H8 +10(O2+3.76N2)=3CO2+4H2O+5O2+37.6N2
• applying the SSSF equation
∑ ∑ +−=P R
SHiiii WhnhnQ
( ) ( )iFiatmKPTi hhhh ∆+−= 1,298,where
Substituting values from tables get : Q= -1.738 ×106 kJ
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THERMODYNAMICS OF REACTIVE MIXTURE
ADIABATIC FLAME TEMP.SSSF process with Q = 0; WSH = 0First law eq. => ( ) ( )∑∑ = PiiRii hNhN
( )( ) ( )( )∑∑ ∆+−=∆+−P
FiTiiFiTii iiRhhhNhhhN 0000
Known from given reactant states
Determine T such that its value
equals LHS
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End of Lecture