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BTC PTEC Biodiesel
Workshop
August 7 8, 2006
Session 2 Chemical Background
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Agenda for second session
Biodiesel production
Chemistry background
Chemical compounds Chemical reactions in the production of
biodiesel
Material balance
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Biodiesel Production
Transesterification
Esterification
Pretreatment
Biodiesel
Washing
Biodiesel
Stripping
ASTM Quality
Biodiesel
Gums / Waxes / Insolubles
Glycerine
Neutralization
Glycerine
Stripping
Glycerine
Distillation
USP Grade
Glycerine
Acid Catalyst
Base Catalyst
Water
Oil, Fat or Grease Feedstock
Methanol
Methanol
and Water
Distillation
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Chemical background
Biodiesel is made from a reaction of a
vegetable oil or animal fat with an alcohol
This reaction is called transesterification
and produces an ester plus a glycerol
We will first look at some chemical
structure for compounds of interest in the
making of biodiesel
Then we will look at the reaction
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Chemical compounds
Vegetable oils and animal fats (triacylglycerols)
O
CH2OCR1 O CHOCR2 O CH2OCR3
R groups are from fatty acids
of the form
O
HOCR
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Chemical compounds
Oils and fats are composed of Fatty acids
Saturated fats (no double bonds, C C only)
Good cetane numbers and stability
Poor cold weather properties
Unsaturated fats (one or more double bonds,
C = C)
Can be oxidized Better cold weather properties
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Chemical compounds
Fatty acids
CH3(CH2)14COOHpalmitic acid
CH3(CH2)16COOHstearic acid
CH3(CH2)7-CH=CH-(CH2)7COOH oleic acid CH3(CH2)7-CH=CH-CH2-CH=CH-
(CH2)4COOHlinoleic acid
CH3(CH
2)7-CH=CH-CH
2-CH=CH-CH
2-
CH=CH-CH2-COOHlinolenic acid
CH3(CH2)7-CH=CH-(CH2)11- COOH erucicacid
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Chemical compounds
Things that we will use to make biodiesel are: Alcohols
CH3OHmethanol
CH3CH2OHethanol
CH3CH2CH2OHn-propanol
OH
CH3CHCH3iso-propanol
Bases NaOHsodium hydroxide
KOHpotassium hydroxide
NaOCH3sodium methoxide (sodium methylate, 25% active agent inmethanol)
(We can determine the amount of catalyst needed by titrating asample of the vegetable oil with a base)
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Chemical compounds
The reaction will produce:
Glycerols
CH2OH
CHOH
CH2OH O
Soaps (Naor K)OC - R
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Chemical compounds
And the biodiesel products we want are:
Esters (examples)
O
CH3-C-O-CH3methyl acetate (methyl ester)
O
CH2-C-O-CH2CH3ethyl acetate (ethyl ester)
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Other products
Soaps O O
Na
O
C
R
CH2
O
CR
Mono and diglycerides CHOH OCHOH
Free fatty acids HO - C - R
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The transesterification reaction
O
CH2OCR1 O CH2 - OH
CHOCR2 + 3 CH3OH = 3 CH3OOCRi+ CHOH O CH2 - OHCH2OCR3
Triacylglycerol + alcohol = mixture of fatty acid esters
(biodiesel) + glycerol
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Phases
Biodiesel (upper phase)
Contains esters and some methanol (60:40 split with
glycerine phase)
Water not soluble in this phase Glycerine (lower phase)
Also contains contaminants such as soaps
90+% of soap formed
And unreacted chemicals 95+% of catalyst added
Alcohol split with biodiesel phase
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Fatty acid reactions
Side reactions also occur such as:
Reaction with base to form a soap
RCOOH + KOH = RCOOK + H2O
A pretreatment reaction we might use is a FA
with acid catalyst (H2SO4) and methanol to form
an ester
RCOOH + CH3OH = RCOOCH3+ H2O
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Reactions of esters
Other side reactions may be:
Reaction with bases in water or water to form
free fatty acids and acylates
O O
XOH + RO-C-R = XOR + HO-C-R
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Reaction considerations
Need excess of reactant (100% molar excess of alcohol)
a catalyst (acid or base)
moderate temperature (60 - 65 deg C, 140 150 deg
F) mixing
residence time (2 - 4 hours)
Problems may occur from the presence of
Free glycerol (inhibits reaction) moisture (hydrolysis of FA esters at > 0.5%)
excess catalyst (soap formation)
free fatty acids (soap formation)
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Biodiesel from high FFA feedstocks
To remove free fatty acids (FFA) to
prevent soaps, we can use
Acid catalyzed esterification to reduce FFA to
< 0.5 1% and follow this with
Alkali catalyzed transesterification
Or we can just let them form soaps and
hope for the best (no emulsion formationand not too much loss of product)
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Example mass balance
Reactants 100 pounds of vegetable oil (canola)
23 pounds of methanol (100% excess)
0.4 pounds of sodium hydroxide
Products 100 pounds of ester (assuming 100% yield more
commonly it would be 75% for one step and 98% fortwo steps)
11 pounds of glycerine
12 pounds of methanol (unreacted)
0.4 pounds of sodium hydroxide
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Volume balance
Reactants
13 gallons of vegetable oil (canola)
3.5 gallons of methanol (100% excess)
Products
13 gallons of ester
1 gallon of glycerine
1.7 gallons of methanol (unreacted)
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Transesterification Material Balance
Oil Feedstock
100 lb
Catalyst
0.5 to 1.5 lb
Methanol
10 lb + excess
Acid
Water
1 to 100 lbGlycerine Esters
FFA
0 to 1 lb
Reaction and Separation
WashingAcidulationWaste Water
0 to 100 lb
Excess Methanol
50 to >99%Methanol Removal Methanol Removal
Crude Glycerine
10 lb (pure basis)
Biodiesel
95 to 100 lb
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Other steps in production
Water wash (1 100 pounds)
Methanol recovery
Glycerine recovery or disposal Water treatment and disposal
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Alternative reactant comparison
Alcohol Costs (methanol often cheapest)
Ethanol may be more difficult to recover than methanol, alsoneed more but it is renewable
Propanol and higher alcohol derived biodiesels have lower
freezing points Base catalyst
Sodium hydroxide most common in US due to lower cost
Potassium hydroxide more effective and is common in Europe,residue can be used as a fertilizer
Methoxides used for large scale operations (>5 milliongallons/year) do not form water, most active catalyst
Acid catalyst (sulfuric acid), cheap, does not makesoaps, very slow reaction
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Other chemical issues
Extended storage (>1 year) to result in Oxidation (rancidity)
Polymerization
Reactions catalyzed by metals and favored by contact
with air, water or sunlight Inhibited by anti-oxidants
Microbial attack
Polyunsaturated fatty acids most susceptible tooxidation
Safety Chemicals (Methanol, base, acid)
Disposal of wastes
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