2b' Gasoline and Diesel properties

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2b. Gasoline and Diesel properties
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GASOLINE AND DIESEL FROM PETROLEUM REFINING

• Crude oil wide range of hydrocarbons (1-60 C
  atoms), organometallics, S, N.
• Refinery distills oil into various fractions,
  processes individual fractions and blends them to
  obtain products with desired products

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REFINERY PROCESSES

• Catalytic cracking - breaks larger,
  higher-boiling, hydrocarbons into gasoline range
  product that contains 30 aromatics and 20-30
  olefins.
• Hydrocracking - cracks and adds hydrogen to
  molecules, producing a more saturated, stable,
  gasoline fraction.
• Isomerisation - raises gasoline fraction octane
  by converting straight chain hydrocarbons into
  branched isomers.
• Reforming - converts saturated, low octane,
  hydrocarbons into higher octane product
  containing about 60 aromatics.
• Alkylation - reacts gaseous olefin streams with
  isobutane to produce liquid high octane
  iso-alkanes.

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MOTOR VEHICLE FUEL PROPERTIES

• Chemical composition and Heating value
• Physical and performance properties
• Volatility
• Vapour pressure
• Distillation profile
• Vapor/Liquid ratio, V/L (by volume)
• Driveability Index
• Octane and cetane number
• Others

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GASOLINE AND DIESEL PROPERTIESChemical
Composition

• Mixture of 200-300 hydrocarbons
• Alkanes (paraffins, saturated), CnH2n2
• C-C and C-H bonds
• Straight chain n-butane, n-octane etc.
• Branched iso-butane,
• iso-octane (2,2,3 trimethyl pentane)
• Alkenes (olefins, unsaturated),
• C-C, CC and C-H bonds
• CnH2n for mono-olefins (one CC bond)
• ethylene, propylene, isobutylene etc.
• Alkynes, carbon to carbon triple bond, acetylene

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GASOLINE AND DIESEL PROPERTIESChemical
Composition

• Naphtenes (cycloparaffins)
• A ring of C-C bonds, CnH2n , cyclohexane c6H12
• Aromatics benzene ring
• Benzene, ethyl benzene, 1,2,4, trimethyl benzene
• toluene (methyl benzene),
• ortho-, meta- and para- xylene (di-methyl
  benzene)

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MINOR FUEL CONSTITUENTS AND ADDITIVES

• Sulfur present in crude oil, removed by
  hydrodesulfurization in refining, residual
  quantities in gasoline (ppm level)
• Reduces efficiency of catalytic convertor
  emission control for gasoline engines
• contributes to SO2 , SO4 and H2S emissions
• ethylene dichloride and ethylene dibromide added
  concurrently with TAL as lead scavenger, some
  adverse effects on engine and exhaust maintenance
  costs, as well as environmental concerns

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ADDITIVES

• Achieve desired effect at ppm levels
• Antioxidants prevent gum formation through
  oxidation with air
• Corrosion inhibitors prevent corrosion of parts
  that come in contact with fuel
• Metal deactivators inhibit the catalytic
  activity of Cu and Zn for the oxidation of
  gasoline
• Demulsifiers promote water separation, inhibit
  water-fuel emulsion formation
• Deposit control additives prevent deposits on
  fuel injectors and carburetors, intake valves and
  ports, combustion chamber

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OXYGENATES IN GASOLINE AND DIESEL

• Alcohols
• Methanol CH3OH (MeOH)
• Ethanol C2H5OH (EtOH)

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OXYGENATES IN GASOLINE AND DIESEL

• Ethers, from treatment of alcohols with strong
  dehydrating agents
• MTBE, methyl tertiary butyl ether, CH3-O-C4H9
  ETBE, ethyl tertiary butyl ether, C2H5-O-C4H9
• TAME, tertiary amyl methyl ether, C5H9-O-CH3
• DME, dimethyl ether, CH3-O-CH3
• Esters, from reaction of alcohols with acids
• RME, rapeseed oil methyl ester

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VAPOUR PRESSURE

• Pure compounds
• Mixtures of similar compounds

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GASOLINE VOLATILITY

• RVP, Reid Vapour Pressure, Vapour pressure at 100
  F (38 C)
• Adjusted seasonally and geographically at the
  refinery by relative abundance of C4 compounds
  (butane and isobutane)
• Maximum RVP 56 kPa (8.1 psi) for Lower Fraser
  River Valley (Vancouver) in Summer

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EVAPORATIVE EMISSIONS FROM GASOLINE DISTRIBUTION

• Filling (displacement) The gas being displaced
  is typical saturated with the gasoline VOCs it
  has been in contact with
• Breathing (diurnal) Temperature increase causes
• increase in vapour pressure
• expansion of the liquid
• expansion of the vapor
• expansion of the tank
• Quantification of all these factors enables us to
  estimate emissions of gasoline VOCs due to
  daily fluctuations in temperature.

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Figure 10 .2 de Nevers

• Displacement losses

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Figure 10.5 de Nevers

• VOC emissions from refuelling operations, vapour
  return system, Stage 1 control

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Figure 10.6 de Nevers

• Dual vapour return system, Stage 2 control

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DISTILLATION CURVE

• IBP initial boiling point
• Upon heating a mixture of hydrocarbons, lighter
  (more volatile) compounds are driven off first
  remaining mixture has higher boling point.
• Two ways of expressing key points along the
  distillation curve
• T10, T50, T90 The temperature at which the
  indicated (by volume) has evaporated
• E70, E150 (C) The percentage (volume)
  evaporated at the indicated temperature
• End point

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Figure 10.3 de Nevers

• Vapour pressure and molecular weight change
  during distillation

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VAPOUR/LIQUID RATIO (volume)

• If we enclosed the mixture of hydrocarbons in a
  variable volume container such that the pressure
  is always atmospheric, the vapour fraction will
  increase with increasing temperature.
• The temperature at which V/L 20 is used as a
  key indicator of vapour lock tendency the
  malfunctioning of a vehicle because there is too
  much vapour in the fuel delivery system.

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GASOLINE DRIVEABILITY INDEX

• An attempt to quantify cold start and warmup
  performance
• DI 1.5(T10) 3.0(T50) T90
• (for conventional gasolines, i.e. no oxygenates)
• Typical values 850 - 1300
• Varies with gasoline grade (regular, premium) and
  season

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AUTOIGNITION (KNOCK) IN GASOLINE ENGINES

• Combustion started by spark plug
• Flame travels through air-fuel mixture
• Temperature and pressure increase at all points
  in the combustion chamber
• The unburned air -fuel mixture can autoignite
  before the flame front arrives
• The combined effect of the spark-initiated flame
  front and the autoignited gases is an extremely
  rapid pressure rise that can be perceived as a
  knock
• Results in deterioration of engine pistons and
  can result in complete failure

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• CETANE NUMBER AND OCTANE NUMBER Cetane number
  (diesel fuel) and octane number (gasoline) both
  measure the tendency of the fuel to ignite
  spontaneously. In the cetane number scale, high
  values represent fuels that ignite readily and,
  therefore, perform better in a diesel engine. In
  the octane number scale, high values represent
  fuels that resist spontaneous ignition and,
  therefore, have less tendency to knock in a
  gasoline engine. Because both scales were
  developed so that higher numbers represent higher
  quality for the respective use, high cetane
  number fuels have low octane numbers, and vice
  versa.

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GASOLINE OCTANE RATING

• A measure of the fuels resistance to
  autoignition (knock) during combustion
• Measured relative to a mixture of n-heptane
  (rating of 0) and iso-octane (rating of 100),
• or iso-octane and tetra-ethyl lead (TEL) for
  ratings greater than 100
• Ignition characteristics of both reference and
  test fuel are determined in a single cylinder
  test engine with variable compression ratio
  according to ASTM procedures

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RON MON

• Research Octane Number (RON) determined at 600
  rpm engine speed with spark timing set at 13
  degrees BTDC , represents mild driving conditions
  without consistent heavy loads
• Motor Octane Number (MON) determined at 900 rpm
  engine speed with spark timing adjusted inversely
  to compression ratio, represents severe,
  sustained high speed, high load driving
• RON - MON Sensitivity

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OCTANE RATING, INDEX, NUMBER

• Control Octane Number (CON), also known as Anti
  Knock Index, AKI (RONMON)/2
• This is the one quoted in fuel retail in the
  U.S. and Canada
• Minimum AKI In Canada
• Regular (87), Mid-grade (89), Premium (91)

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ROAD OCTANE NUMBER

• Research Octane Number (RON) and Motor Octane
  Number (MON) determined in single cylinder test
  engine in the laboratory
• Road Octane Number (RdON) measures anti-knock
  performance of multi-cylinder engine in an actual
  vehicle under road driving conditions
• Requires multiple road tests with multiple
  vehicles for statistically significant results
  costly procedure
• Estimation RdON a(RON) b(MON) c

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FUEL COMPONENTS AND KNOCK RESISTIVITY

• Straight chain paraffins - WORST
• Highly branched paraffins - BEST
• Olefins better than corresponding paraffins
• Aromatics comparable to branched paraffins

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OCTANE ENHANCEMENT - LEAD

• Addition of Tetra alkyl lead (TEL and TML)
• 0.4 g/L gives RON increase of 10 (not
  linear)
• Adverse effects, directly on health
• reduced mental capacity in children with high
  blood lead levels (BLL 25 microgram/dL )
  resulting from high ambient lead aerosol
  concentrations (5 microgram/m3)
• high blood pressure and strokes in adults
• Adverse effects on vehicle exhaust emission
  control technology for CO, HC, and Nox
• Unleaded gasoline Maximum 0.013 g/L lead

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OCTANE ENHANCEMENT - AROMATICS

• Increase high octane components (e.g. branched
  paraffins and aromatics) of gasoline by refinery
  process modifications
• Adverse effects, directly on health
• benzene is a known carcinogen
• present in gasoline
• partial product of combustion of other aromatics
• Benzene limit (maximum) in gasoline 1 (vol)

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OCTANE ENHANCEMENT - OXYGENATE ADDITIVES

• Ethanol, methanol, tertiary butyl alcohol (TBA),
  methyl tertiary-butyl ether (MTBE), tertiary-amyl
  methyl ether (TAME)
• Increased octane rating
• Decreased emissions of CO in winter,
• evaporative emissions in summer may increase with
  alcohol blends, ethers (MTBE) preferred

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REFORMULATED GASOLINE

• Reduced volatility, sulfur, aromatics and
  olefins, addition of oxygenates

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DIESEL PHYSICAL PROPERTIES

• Heavier HC components than gasoline
• C10-C20 vs C4-C10
• Boiling range 180-400 C vs 30-220 C
• Volatility not an issue
• Cetane rating
• A measure of the fuels ability to ignite
  following injection into compressed air (opposite
  of octane rating, what makes good diesel makes
  poor gasoline and vice versa)
• Measured relative to a mixture of n-cetane
  (n-hexadecane, cetane number 100) and
  2,2,4,4,6,8,8 heptamethylnonane (cetane number 15)

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DIESEL CETANE NUMBER

• Ignition characteristics of both reference and
  test fuel are determined in a single cylinder
  test engine according to ASTM procedures
• Cetane index is an approximation to the cetane
  number that can be obtained from empirical
  relations based on the density and distillation
  characteristics of the fuel, thus avoiding the
  engine test.

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• Cetane Number  varies systematically with
  hydrocarbon structure (see Figure 4-7). Normal
  paraffins have high cetane numbers that increase
  with molecular weight. Isoparaffins have a wide
  range of cetane numbers, from about 10 to 80.
  Molecules with many short side chains have low
  cetane numbers whereas those with one side chain
  of four or more carbons have high cetane numbers.

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• Naphthenes generally have cetane numbers from 40
  to 70. Higher molecular weight molecules with one
  long side chain have high cetane numbers lower
  molecular weight molecules with short side chains
  have low cetane numbers.
• Aromatics have cetane numbers ranging from zero
  to 60. A molecule with a single aromatic ring
  with a long side chain will be in the upper part
  of this range a molecule with a single ring with
  several short side chains will be in the lower
  part. Molecules with two or three aromatic rings
  fused together have cetane numbers below 20.

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CETANE NUMBERS

• EUROPE 43 - 57, average 50
• U.S. lower, minimum 40, average 43
• Higher cetane correlates with
• improved combustion
• improved cold starting
• reduced noise, white smoke, HC, CO and
  particulate emissions, especially during early
  warm-up phase

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FUEL DENSITY

• Density usually less than that of water, i.e.
  fuel floats on water
• specific gravity, s.g. density / water density
• American Petroleum Institute (API) gravity