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

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)

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

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

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

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.

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.

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, …

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.

B.3 Combustion Lab: Background

• Candle wax burning: (exothermic)C25H52(s) + 38 O2(g) 25 CO2 (g) + 26 H2O (g) + EParaffin wax Oxygen Carbon Dioxide water vapor ENERGY

(Alkane) gas gas gas

• More energy is liberated by forming bonds of the CO2 and H2O molecules than is required to break bonds of C25H52 and O2

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

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.

Hydrocarbon Table:Including Heat of Combustion

Boiling Short Heat of Molar Heat of

Name Point (C) #Carbon formulaCombustion

(kJ/g)

Combustion

(kJ/mol)

Methane -162 1 CH4 55.6 890

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

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

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

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.

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

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.

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

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!