Fuel Science 431

GLOBAL CARBON CYCLE
see diagram
PHOTOSYNTHESIS MECHANISM
see diagram
the link joining two cyclic sugars is called ___, ___, and ___
acetal linkage
saccharide linkage
glycoside linkage
catalyst
enhances the rate of the rxn without being consumed or permanently altered
catalysts usually work by…
…reducing activation energy, usually by providing a different rxn mechanism
catalysts operate on ___
kinetics, not thermodynamics, so they can’t change the position of equilibrium, only help to get there faster
homogeneous catalysts
1. in the same phase as reactants and products
2. no problem getting the reactants to the catalyst, but at the end, you have to separate them
3. biochemical rxns (enzymes)
heterogeneous catalysts
1. in a different phase than the reactants and products
2. trouble getting the reactants to the catalyst, but at the end, separation needed
3. industrial rxns
protein structure
1*: sequence of amino acids
2*: alpha coils and beta pleats
3*: 3-D conformation
4*: multiple proteins joined together
rating a catalyst
1. activity- how fast it works
2. turnover frequency- (# of rxns per catalyst site)/time
3. selectivity- (desired product)/(all products)
homogeneous vs. heterogeneous catalysts
homogeneous catalysts (enzymes) have very high turnover frequency (~1000/sec) and good selectivity (~100%)
CONVERSION OF POLYSACCHARIDES TO ETHANOL
see diagram
ETHANOL PRODUCTION
see diagram
fermentation rxn
C6H12O6 -> 2C2H6O + 2CO2
ethanol fermentation details
catalyst is yeast
max ethanol conc. is 8-12%, because above that, the alcohol kills the yeast
after that, distillation is used to increase the concentration
95% azeotrope: options
1. use directly as fuel (slight energy penalty because of the 5% water) 2. make 100% ethanol by A. add a third component like benzene to shift the equilibrium B. react the water away C. cleverly separate the ethanol and water (membranes, etc.)
ethanol: pro #1
theoretically CO2 neutral
ethanol: pro #2
principally renewable
(concerns: soil depletion and monoculture dependence)
ethanol: pro #3
secure, domestic energy resource
ethanol: pro #4
very high octane number–suitable for high compression engines
ethanol: pro #5
partially oxygenated -> shifts combustion chemistry in the engine to reduce emissions of CO and unburned fuel
ethanol: con #1
infinitely miscible with water
ethanol: con #2
lower volumetric energy density than gasoline (lower MPG)
ethanol: con #3
materials compatibility–ethanol is an alcohol (gasoline is a hydrocarbon) -> different interactions with parts of the fuel system
(not a big deal–largely solveable)
ethanol: con #4
food vs. fuel argument–is it ethical?
ethanol: con #5
low energy balance–1.5 (compared to gasoline’s 20)
waste of energy is due to distillation (water has a high specific heat)
ethanol: con #6
can we grow enough?
energy balance of fuel
(energy liberated by burning)/(energy used to make)
ethanol challenges
1. find hardy plants (low-maintenance, grow on crappy soil)
2. find easy hydrolysis of cellulose at mild conditions
3. find yeast that can tolerate >12% ethanol
4. find low energy separation processes (so that we don’t have to use distillation)
plant oils
long-term energy storage for plants
plant oil/biodiesel con #1
high molecular wt
-> low vapor pressure
-> hard to vaporize for combustion
-> hard to use in cold weather
plant oil/biodiesel con #2
high viscosity
(and viscosity increases as temp. decreases)
plant oil/biodiesel con #3
oil molecules tangle with each other and form gums in the engine
petrodiesel

derived from petroleum;

made of alkanes 12-20 carbons long

biodiesel
take apart the plant oil molecule to get the carbon chains
BIODIESEL PRODUCTION
see diagram
ester hydrolysis of plant oil to get the carbon chains can be ___ or ___
base induced (not a catalyst)
acid catalyzed
base-induced hydrolsis is used over acid-catalyzed hydrolysis for plant oils because…
…the last step of the base-induced rxn is a forward-only rxn
the RCOO- formed in base-induced hydrolysis is acidic and bad for the engine, so…
…it is re-sterified, but with the smallest alcohol, methanol
biodiesel vs. petrodiesel
biodiesel has no sulfur and no aromatic compounds -> very clean burning
the couple of oxygens in biodiesel give a small energy penalty
biodiesel vs. ethanol
biodiesel is much easier to make
but we need to figure out what to do with the glycerol that forms
we also need a plant oil source
plant oil/biodiesel pro #1
easy to make compared to ethanol
plant oil/biodiesel pro #2
more clean-burning than petrodiesel
plant oil/biodiesel pro #3
it can be used in a petrodiesel engine
transesterification
plant oil + methanol -> 3 FAs + glycerol
pyrolysis
use of thermal energy to split molecules
pyrolysis is also called ___ and ___
thermal decomposition
carbonization
why does charcoal have a higher calorific value than wood?
wood is 35-45% oxygen (it’s already partially oxidized, so there’s less to oxidize/burn), while charcoal has most of the oxygen already gone
products of wood pyrolysis
gas: CO2, CO, CH4, small hydrocarbons
liquid: pyroligneous acid
hardwood pyroligneous acid
aqueous phase: methanol, acetic acid, acetone
heavy organic phase
softwood pyroligneous acid
light organic phase: turpentine, pine oil
aqueous phase: methanol, acetic acid, acetone
heavy organic phase: wood creosote
methanol can be made from wood by ___ and ___
pyrolysis
gasification
wood -gasification-> methanol
1. endothermic, so the heat from combustion rxn is used to power it
2. product is synthesis gas (CO and H2), which can be made into methanol
3. 90% yield of methanol
GASIFICATION MECHANISM
see diagram
wood -pyrolysis-> methanol
1. endothermic, so the heat from combustion rxn is used to power it
2. methanol is separated out from the organic and aqueous phases
3. 2% yield of methanol
PYROLYSIS MECHANISM
see diagram
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