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BTC PTEC Biodiesel Workshop

This workshop explores the chemical background of biodiesel production, including the chemistry of compounds and reactions involved. Topics covered include esterification, transesterification, glycerine stripping, and ASTM quality standards.

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BTC PTEC Biodiesel Workshop

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  1. BTC PTEC Biodiesel Workshop August 7 – 8, 2006 Session 2 – Chemical Background

  2. Agenda for second session • Biodiesel production • Chemistry background • Chemical compounds • Chemical reactions in the production of biodiesel • Material balance

  3. Biodiesel Production Oil, Fat or Grease Feedstock Gums / Waxes / Insolubles Pretreatment Esterification Acid Catalyst Methanol Transesterification Glycerine Neutralization Base Catalyst Methanol and Water Distillation Biodiesel Washing Glycerine Stripping Water Biodiesel Stripping Glycerine Distillation ASTM Quality Biodiesel USP Grade Glycerine

  4. 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

  5. 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

  6. 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

  7. Chemical compounds • Fatty acids • CH3(CH2)14COOH palmitic acid • CH3(CH2)16COOHstearic 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-CH2-CH=CH-CH2-COOHlinolenic acid • CH3(CH2)7-CH=CH-(CH2)11- COOH erucic acid

  8. 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)

  9. Chemical compounds • The reaction will produce: • Glycerols CH2 – OH │ CH – OH │ CH2 – OH O ║ • Soaps (Na or K) – O – C - R

  10. 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)

  11. Other products • Soaps O O ║ ║ Na – O – C – RCH2 – O – CR│ • Mono and diglycerides CH – OH │ O CH – OH ║ • Free fatty acids HO - C - R

  12. The transesterification reaction O ║ CH2 – O – C – R1 │ O CH2 - OH │ ║ │ CH – O – C – R2 + 3 CH3OH = 3 CH3OOCRi + CH – OH │ O │ │ ║ CH2 - OH CH2 – O – C – R3 Triacylglycerol + alcohol = mixture of fatty acid esters (biodiesel) + glycerol

  13. 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

  14. 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

  15. 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

  16. 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)

  17. 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)

  18. 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

  19. 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)

  20. Transesterification Material Balance Oil Feedstock 100 lb Catalyst 0.5 to 1.5 lb Methanol 10 lb + excess Water 1 to 100 lb Glycerine Esters Acid Reaction and Separation FFA 0 to 1 lb Waste Water 0 to 100 lb Acidulation Washing Excess Methanol 50 to >99% Methanol Removal Methanol Removal Crude Glycerine 10 lb (pure basis) Biodiesel 95 to 100 lb

  21. Other steps in production • Water wash (1 – 100 pounds) • Methanol recovery • Glycerine recovery or disposal • Water treatment and disposal

  22. 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

  23. 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

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