BTC PTEC Biodiesel
Workshop
August 7 – 8, 2006
Session 2 – Chemical Background
Agenda for second session
•
•
•
•
Biodiesel production
Chemistry background
Chemical compounds
Chemical reactions in the production of
biodiesel
• Material balance
Biodiesel Production
Oil, Fat or Grease Feedstock
Pretreatment
Acid Catalyst
Gums / Waxes / Insolubles
Esterification
Methanol
Base Catalyst
Water
Glycerine
Neutralization
Transesterification
Biodiesel
Washing
Methanol
and Water
Distillation
Glycerine
Stripping
Biodiesel
Stripping
Glycerine
Distillation
ASTM Quality
Biodiesel
USP Grade
Glycerine
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
Chemical compounds
• Vegetable oils and animal fats (triacylglycerols)
O
║
CH2 – O – C – R1
│
O
│
║
CH – O – C – R2
│
O
│
║
CH2 – O – C – R3
R groups are from fatty acids
of the form
O
║
HO – C – R
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
Chemical compounds
• Fatty acids
– CH3(CH2)14COOH palmitic acid
– CH3(CH2)16COOH stearic acid
– CH3(CH2)7-CH=CH-(CH2)7COOH oleic acid
– CH3(CH2)7-CH=CH-CH2-CH=CH(CH2)4COOH linoleic acid
– CH3(CH2)7-CH=CH-CH2-CH=CH-CH2CH=CH-CH2-COOH linolenic acid
– CH3(CH2)7-CH=CH-(CH2)11- COOH erucic
acid
Chemical compounds
• Things that we will use to make biodiesel are:
– Alcohols
• CH3OH methanol
• CH3CH2OH ethanol
• CH3CH2CH2OH n-propanol
OH
│
• CH3CHCH3 iso-propanol
– Bases
• NaOH sodium hydroxide
• KOH potassium hydroxide
• NaOCH3 sodium methoxide (sodium methylate, 25% active agent in
methanol)
• (We can determine the amount of catalyst needed by titrating a
sample of the vegetable oil with a base)
Chemical compounds
• The reaction will produce:
– Glycerols
CH2 – OH
│
CH – OH
│
CH2 – OH
O
║
– Soaps (Na or K) – O – C - R
Chemical compounds
• And the biodiesel products we want are:
• Esters (examples)
O
║
– CH3-C-O-CH3 methyl acetate (methyl ester)
O
║
– CH2-C-O-CH2CH3 ethyl acetate (ethyl ester)
Other products
– Soaps
O
O
║
║
Na – O – C – R
CH2 – O – CR
│
– Mono and diglycerides CH – OH
│
O
CH – OH
║
– Free fatty acids
HO - C - R
The transesterification reaction
O
║
CH2 – O – C – R1
│
O
│
║
CH – O – C – R2 + 3 CH3OH = 3 CH3OOCRi +
│
O
│
║
CH2 – O – C – R3
CH2 - OH
│
CH – OH
│
CH2 - OH
Triacylglycerol + alcohol = mixture of fatty acid esters
(biodiesel) + glycerol
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
Fatty acid reactions
• Side reactions also occur such as:
Reaction with base to form a soap
R – COOH + KOH = R – COOK + H2O
• A pretreatment reaction we might use is a FA
with acid catalyst (H2SO4) and methanol to form
an ester
R – COOH + CH3OH = R – COOCH3 + H2O
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 + R’O-C-R = XOR’ + HO-C-R
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)
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 formation
and not too much loss of product)
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% for
two steps)
– 11 pounds of glycerine
– 12 pounds of methanol (unreacted)
– 0.4 pounds of sodium hydroxide
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)
Transesterification Material Balance
Catalyst
Oil Feedstock
Methanol
0.5 to 1.5 lb
100 lb
10 lb + excess
Acid
Glycerine
Acidulation
Esters
Reaction and Separation
FFA
Waste Water
0 to 1 lb
0 to 100 lb
Methanol Removal
Excess Methanol
Water
1 to 100 lb
Washing
Methanol Removal
50 to >99%
Crude Glycerine
Biodiesel
10 lb (pure basis)
95 to 100 lb
Other steps in production
•
•
•
•
Water wash (1 – 100 pounds)
Methanol recovery
Glycerine recovery or disposal
Water treatment and disposal
Alternative reactant comparison
• Alcohol
– Costs (methanol often cheapest)
– Ethanol may be more difficult to recover than methanol, also
need 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 million
gallons/year) do not form water, most active catalyst
• Acid catalyst (sulfuric acid), cheap, does not make
soaps, very slow reaction
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 to
oxidation
• Safety
– Chemicals (Methanol, base, acid)
• Disposal of wastes