Download 3.B.1-6 Petro as Energy Source

Section B:
Petroleum as an Energy Source
• Man has used oil for over 5000 years
• First drilled in 1859 in PA
• Oil is used in almost every aspect of
modern life: products, shipping,
manufacturing, heating, transportation, etc
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B.1 Energy and Fossil Fuels:
Overview
• Chemical energy, a form of Potential
Energy, is stored in the bonds of
compounds
• In an energy releasing chemical reaction
bonds break and reactant atoms reorganize
into new bonds
• Result has less chemical energy, but process
released energy (heat and light)
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B.1 Energy and Fossil Fuels:
Combustion of Methane
• Energy releasing reaction of burning methane
CH4 + 2 O2  CO2 + 2 H2O + energy
• 2 Steps: Bond breaking, then bond making
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B.1 Energy and Fossil Fuels:
Combustion
• Breaking bonds requires energy: endothermic
change (increases chemical energy)
energy + CH4 + 2 O2  C + 4 H + 4 O
• The forming new bonds releases energy:
exothermic change (decreases chemical energy [>])
C + 4 H + 4 O  CO2 + 2 H2O + energy
• Overall this is exothermic as more is released
when reforming than is required to break bonds
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B.1 Energy and Fossil Fuels:
Combustion (continued)
• Overall reaction maybe exothermic or
endothermic:
– if more energy is added to break bonds than
produced when bond making then endothermic
– opposite, like burning methane, is exothermic
• Generally, if a reaction is endothermic, then
the reverse reaction is exothermic
Actual “reaction mechanism” are more complex
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B.2 Energy Conversion
• Conversion makes energy more useful
• Hair dryer example: oil/potential - generate
steam/thermal – spin turbine/kinetic general electricity/electrical – transmit –
thermal and kinetic in the hair dryer.
• Energy is not consumed, but changes form Law of Conservation of Energy
• But some energy is lost to useful work.
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B.2 Energy Conversion (continued)
• Conversion of chemical energy into heat
and then mechanical typically loses > 50%
• Minimizing conversion is essential to using
fuel efficiently
• Solar cells (convert sunlight to electricity)
and fuel cells (convert chemical energy to
electricity) hold promise in adding
efficiency of petroleum use.
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B.2 Energy Conversion –
Automobile example
• Around 25% of the energy in gasoline
contributes to powering the drive wheels.
• 75% is lost into the environment, mostly in
the form of heat: 33% lost through exhaust,
30% through engine cooling, 6% pumping
combustion air, 3% due to piston ring
friction, …
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B.3 Combustion Lab:
Background
• In this lab we measure how much thermal
energy is released when burning a fuel.
• We burn a paraffin wax candle (long chain
alkane) under a can of water and watch the
water temperature increase.
• We will note the increase in temperature,
while the mass of the candle decreases.
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B.3 Combustion Lab:
Background
• Candle wax burning: (exothermic)
C25H52(s) + 38 O2(g)  25 CO2 (g) + 26 H2O (g) + E
Paraffin wax
(Alkane)
Oxygen
gas
Carbon Dioxide
gas
water vapor
gas
ENERGY
• More energy is liberated by forming bonds
of the CO2 and H2O molecules than is
required to break bonds of C25H52 and O2
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B.3 Combustion Lab:
Background
• Specific heat capacity of a substance is the
quantity of heat needed to raise the
temperature of 1 gram of the substance by
1° C.
• The SHC of water is 4.2 J/(g·°C or joules per
gram per degree Celsius).
• To raise the temperature of 10 g water from
25°C to 30°C: 4.2 J * 10 g * 5°C = 210 J
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B.3 Combustion Lab:
Background
• Heat of combustion is the quantity of
thermal energy given off when a certain
amount of substance is burned.
• This is usually measured when :
– one gram or
– one mole (molar heat of combustion)
of substance is burned.
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Hydrocarbon Table:
Including Heat of Combustion
Short
Methane
-162
1
Heat of
Combustion
formula
(kJ/g)
CH4
55.6
Ethane
-88.6
2
C2H6
52.0
1560
Propane
-42.1
3
C3H8
50.0
2200
Butane
-0.5
4
C4H10
49.3
2859
Pentane
36.1
5
C5H12
48.8
3510
Hexane
68.7
6
C6H14
48.2
4141
Heptane
98.4
7
C7H16
*** 48.2
4817
Octane
125.7
8
C8H18
47.8
5450
Boiling
Name
Point (C) #Carbon
Molar Heat of
Combustion
(kJ/mol)
890
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B.3 Combustion Lab:
Calculations
1. Mass of water in grams = volume of water
(100mL) * 1.0 g/mL
2. Total rise in temperature =
final temperature – initial temperature
3. Heat = mass * Specific heat of substance
(water – 4.2J/(g·°C)) * change in temp.
4. Mass of burned wax = Mass of candle &
card before burn – mass after burn
5. Heat of combustion of paraffin = #3 / #4
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B.4 Using Heats of Combustion
• Hydrocarbon + Oxygen gas 
Carbon dioxide+Water+??Thermal energy??
• 2 C2H6 + 7 O2  4 CO2 + 6 H2O + 3120 kJ
(see the table with heat of combustion entry for ethane-C2H6)
• Notice the table entry is for one mole, we
have 2 here “2 C2H6” so we multiply by 2.
• 12 g of octane would be:
12 * 47.8kJ = 574 kJ
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B.5 Altering Fuels:
Overview
• Due to changes in demand various fractions
of hydrocarbons vary in price over time.
E.G: the gasoline fraction demand is growing
more quickly than other types, such as kerosene
• Chemists and chemical engineers work innovate to
maximize $ for crude oil, by maximizing the
amount of the more valuable fractions while
minimizing less valuable fractions of distillation.
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B.5 Altering Fuels:
Cracking
• Due to cars becoming more popular in the early
1900’s chemists invented cracking.
• Cracking - larger hydrocarbons are broken into
smaller ones. E.G: C16 => 2 C8s (kerosene into gasoline)
• Extra C1-4 are burned to feed the cracking process
• > 1/3 of all crude is cracked, and catalysts are used
such as aluminosilicates for efficiency (< tempature)
• Gasoline is mostly straight chain, but branched
chains burn better in engines – less knocking.
E.G: 2,2,4-trimethylpentane isomer of octane is works well
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B.5 Altering Fuels:
Octane Rating
• Isooctane (2,2,4-trimethylpentane) by definition is 100
octane, straight chain heptane is 0. Gas is rated
this way. (> isooctane > octane rating, > heptane < octane rating)
• Prior to the mid-1970, lead, was added to gasoline
to inexpensively increase the octane rating by
more slowly burning straight chain gasoline
molecules, but the lead was found to be harmful to
the environment, so eliminated.
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B.5 Altering Fuels:
Oxygenated Fuels
• A fuel with an oxygen atom – added to gasoline to raise
octane rating, introduced when lead was removed. (burns cleaner due
to the oxygen)
• Methanol (methyl alcohol, CH3OH): > octane rating
and produced from natural gas, coal, corn or wood
• MTBE (116 octane rating), first used in ’70s as an octane booster
then increased importance as an oxygenated additive. BUT is mixes
readily with water, tastes bad, can be bad for you, and seeps into water
from leaking tanks. (net: good for air, bad for water) Some states have
eliminated the use, some others limiting use.
• MTHF (87 octane rating), is promising.
Generated from renewable
resources, also oxygenates fuel
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B.5 Altering Fuels:
Oxygenated Fuels (continued)
• Octane ratings can also be boosted by
isomerization – convert straight chain
hydrocarbon molecules to branched chain
• Hydrocarbon vapor is heated with a catalyst
to get branched chains.
• Branched C6H14 can be added to gasoline
fractions (C5-12) to produce high-quality gas
• Isomerization and cracking add costs!
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