Chemical Reactions

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Michigan State UniversityMechanical
Engineering FALL 2007 - ME 444 Chemical
ReactionsHarold Schock
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Fuels for the Internal Combustion Engine

• Coal
• Liquid hydrocarbons
• Gaseous hydrocarbons
• Alcohols
• Liquid and gaseous HC fuels are composed of many
  different hydrocarbons

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A Few Definitions for Fuels

• H H H H H H
• I I I I I
  I
• H C C C C H H C C C C H
• I I I I
  I I I I
• H H H H H H
  H H
• (a) (b)
• Molecular structure
  a saturated chain
• b unsaturated chain

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Fuel Definitions (cont.)

• H H Hydrocarbons can also have a
  
• \ / ring structure. Cyclo-alkanes
• C are saturated and have the
• / \ formula CnH2n. They are also
• H C C H called Cyclopropane naphthenes
• / \ Two molecules with the same
• H H number of carbon and hydrogen
  atoms, but with a different
• (Saturated) structure are called isomers.

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Characteristics of SomeHydrocarbon Families
Saturated
Structure
Formula
Family

Yes
Chain
CnH2n2
Alkanes
No
Chain
CnH2n
Olefin
No
Chain
CnH2n-2
Diolefin
Yes
Ring
CnH2n
Naphthene
No
Ring
CnH2n-6
Aromatics Benzene
Adapted from VanWylen Sonntag
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Fuel Composition and Combustion

• Fuels are a mixture of hydrocarbons
• During combustion H-C bonds are broken
  and new bonds are formed with oxygen
• Chemical energy is converted to thermal energy
  during this process and also a formation of (CO2
  H2O)
• Size and geometry of the molecule control the
  chemical properties

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Hydrocarbon Fuels from Crude Oil are Derived
from Distillation and Cracking Processes
Temperature
C
First Drop
End Point

• Per Cent Recovered

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Alcohol Fuels

• H H H
• l I I
• H C O H H C C O H
• l I I
• H H H
• CH3OH C2H3OH
• (methanol) (ethanol)

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Other Oxygenates as Fuels or Additives

• Ethers ignition improvers for diesels
• Dimethylether (DME) proposed as a diesel
  fuel (CH3)2O
• Methyl tertiary butyl ether (MTBE)
• (CH3)3COCH3 Octane improvement in
  gasoline engines
• Problems Production of formaldehyde (CH2OH),
  ground water, CO reduction, particulate reduction
  issues

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45
40
38
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Caloric Value (MJ/kg)
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MJ liter
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34
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32
41
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Density (kg/m3 at 15C)
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A Simple Combustion Process

• Consider the Reaction
• (Reactants) (Products)
• C O2 CO2
• 1 kmole Carbon reacts with 1 kmole O2 to produce
  1 kmole CO2
• or
• 12 kg C react with 32 kg O to produce 44 kg of
  carbon dioxide

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A Simple Combustion Model

• CH4 (methane) 2O2 CO2 2H2O
• Assumed combustion is complete
• C CO2, H H2O
• In an actual combustion process there are
  intermediate products
• Usually, O2 is supplied as air
• Assume N2, Ar and other do not undergo a
  chemical reaction

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A Simple Combustion Model (cont.)

• Consider air to be composed of
• 21 O2 and 79 N2
• If the oxygen for combustion is
• supplied by air we have
• CH4 3O2 2(3.76)N2 CO2 2H2O 7.52N2
• Minimum amount of air required
• to achieve complete combustion
• is called the theoretical air

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A Simple Combustion Model (cont.)

• Complete combustion with theoretical air
• leaves no oxygen in the products.
• Actually complete combination requires excess
  oxygen, thus we supply gt 100 theoretical air.
• Example 150 theoretical air (or 50 excess air)
  means 1.5 x minimum amount needed for complete
  combustion
• or
• CH4 2(1.5)O2 2(3.76)(1.5)N2 CO2 2H2O
  O2 11.28N2

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AF and FA Ratio

• Theoretical A/F Ratio Mass Theoretical Air
• Mass Fuel
• (could be on a mole basis, but usually
  expressed on a mass basis)
• Incomplete Combustion Less than the
• theoretical amount of air is supplied (globally
  or locally.) CO is formed and HCs may remain.

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Enthalpy of Formation

• Previously, in thermodynamic calculations using
  water as an example, we chose the internal
  energy of saturated liquid at 0.01ºC to be zero.
• We can choose any reference state
  as long as there are no inconsistencies in future
  calculations.

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Q -393522 kJ
1 kmol C
1 kmol C
25oC. 0.1 MPa
25oC. 0.1 MPa
1 kmol O2
25oC. 0.1 MPa
Chemical reaction is written
C O2 ? CO2  First Law  Qc.v. HR
Hp   or   Qc.v.   A measurement of heat
transfer would give ?hpr
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• Assume that the enthalpy of all elements
  is chosen to be zero at the reference state.
• then QC.V HP 393522 KJ
• Kmole
• The enthalpy of ideal gas CO2 with
• reference to the zero base element reference
  state is called the enthalpy of formation
• 393522 KJ
• Kmole

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• The enthalpy of CO2 in any other
• state (relative to the defined base)
• can be found as follows
• See V S
• tables A.11, A.13

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Table A.11SI
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Table A.13SIEnthalpy of Formation
Richard Stone, Intro Internal Comb. Engines, 3rd
Edition
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Observations Regarding theEnthalpy of Formation

• Actual enthalpies of formation are calculated
  from statistical thermodynamics using observed
  spectroscopic data.
• In the absence of a nuclear reaction each
  element is conserved in a chemical reaction.
• Certain elements or compounds can exist in
  more than one state at 25ºC, 0.1 MPa.
  Value chosen is for the chemically stable form.
  
• For example, O, O2, O3 all can exist at
  250C.0.1MPa, but O2 is chemically stable

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Miscellaneous Notes

• Table A.13 gives two values for the enthalpy
  of formation of H20 (liquid water and
  ideal gas water)
• IG value used with the ideal gas tables (A.11)
• Liquid water value used in connection with
  real changes as found in the steam tables

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Summary

• A liquid or gaseous hydrocarbon fuel contains
  many HCs
• Combustion involves breaking and reforming of
  chemical bonds as well as energy transformed from
  chemical to heat energy
• Alcohols and other oxygenates can be used as a
  fuel or additive but other issues are present
  (MTBE)
• Simple Combustion Model yield a value for
  theoretical air needed for combustion

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Summary (cont.)

• AF and FA ratio defined
• Enthalpy of Formation
• reference of 25ºC, 0.1 chosen all stable
• elements assigned 0 _at_ 25oC, 0.1MPa
• Tables identified of enthalpy for formation
• for typical molecules
• Preparation for First Law Analysis of
• Reacting Systems Complete

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Example Problem 4.1__ 1
1
slope -n
log P
2
log P
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• When a gas undergoes a reversible process in
  which there is heat transfer such as the
  expansion shown in 1 2 a plot of log P vs
  log V yields a straight line.
  
• This is called POLYTROPIC PROCESS

For an ideal gas with work done at a moving
boundary, during a polytropic process
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