Oxidation Stability in Biodiesel: A Brief Review of Current Technology
June 9, 2009
BY Raj Shah, Devinder Mahajan, Saurabh Patel, James Ball, Vincent Colantuoni and Rudy Maraj
With the renewed interest in alternative fuels, there has been a push for cleaner and more environmentally friendly fuel. The increase in greenhouse gases (GHG) has become the driving force for the shift from fossil fuels to alternative fuels. Numerous regulations have been passed in the United States and by the European Commission recently to tackle these environmental problems and to promote the use of biofuels. The most popular biofuel currently in use is biodiesel. Specifically, the fatty acid methyl ester (FAME) is the most commonly used biodiesel. The cost of its production is relatively high compared to that of fossil fuels. In order to cut down on the cost, utilization of animal and vegetable fatty waste can be used for biodiesel production.
The regulations put in place for biofuel advancement resulted in the rapid development of Testing Standardization. ASTM International developed a standard specification D6751 for B100 biodiesel as a result of U.S. regulations. The specification consists of a number of test methods the biodiesel fuel is required to conform to in order for it to be commercially sold and distributed. This was followed by the release of a specification for biodiesel blends, designated D7467. The diesel fuel specification, D975, has also been revised to allow up to 5 percent biodiesel meeting specification D6751. These testing specifications all include oxidation stability as a major test method requirement. Currently, EN 14112 is the referee test method for oxidation stability of biodiesel within both D6751 and D7467 specifications. However, progress is being made towards putting an ASTM-designated test method for oxidation stability into these specifications. ASTM recently released an oxidation stability test method specifically for B100 biodiesel and blends of biodiesel with middle distillate petroleum fuel designated D7462. In time, this new test method will be incorporated into the D6751 and D7467 standard specifications.
Most biodiesel is produced by the transesterification reaction. The process takes place between a triglyceride and an alcohol to form esters and glycerol. A triglyceride is a glycerin molecule with three long fatty acid chains attached to it. The triglyceride reacts with the alcohol, either methanol or ethanol but most often methanol, in the presence of a catalyst such as sodium hydroxide or potassium hydroxide, to form mono-alkyl ester and crude glycerol. The combination of methanol and potassium hydroxide is preferred in FAME production because biodiesel produced this way is less resistant to oxidation than typical fossil fuel untreated with additives.
The general mechanism of oxidation in biodiesel has been well documented. Fatty wastes in general are more susceptible to oxidation because they vary in level of unsaturation, meaning there are more carbon-carbon double bonds and fewer hydrogen molecules on the fatty acid chains. When biodiesel made from unsaturated oil is exposed to oxygen, the oxygen attaches itself to the bis-allylic site directly adjacent to the two double bonds, which initiates an autoxidation chain reaction sequence. Oxidation stability is not related to the number of double bonds available but rather the number of bis-allylic sites2,3. The initiation step is the formation of a free radical that can react directly with oxygen. This leads to the formation of a peroxide or hydroperoxide molecule. The most reactive site for initial formation is the bis-allylic position. The radials formed at the bis-allylic sites immediately isomerize to form a more stable conjugated structure, which reacts directly with oxygen to form peroxide. The existence of these molecules is an early indication of oxidation taking place, and it is measured in terms of peroxide value2. Later, aldehydes and ketones are formed. Finally, during the polymerization process, resins are produced making the fuel unusable1.
Due to its chemical structure, oxidation rates of FAME can depend on many variables such as temperature, light, radiation intensity, presence of naturally occurring antioxidants and more. The oxidation stability of FAME can be increased by adding additional natural and synthetic antioxidants. However, naturally occurring antioxidants in FAME have shown relatively poor efficiency compared with synthetic antioxidants. An effective concentration of the antioxidants is in the range of 200 parts to 1,000 parts per million (PPM), depending on the type of FAME and the test used to evaluate additive performance. The antioxidants work by binding free radicals and stopping chain reactions. In this way, fatty peroxyradicals are stabilized and the oxidation chain reaction is broken1,4. Oxidation stability testing can be conducted under several different methods. The methods used to test for oxidation stability of biodiesel, and all of the supporting technology, will be reviewed below.
Methods
Rancimat EN 14112 – The Rancimat method expresses the oxidation stability of the tested material in terms of an induction period for the production of volatile organic acids, which are byproducts of fatty acid ester oxidative degradation with heat and oxygen11.
Figure 1 shows a plot of a typical Rancimat test. In this particular example, the induction time is about 4.8 hours. One can see a rapid increase in conductivity at this point. The second derivative is automatically taken to measure the absolute maxim, which is defined as the induction time6. This testing time of a few hours, instead of weeks or months, correlates to the shelf life of a product in years.
Fig.1 A Typical Rancimat Conductivity Plot
Rancimat EN 15751 – The EN 15751 method accounts for volatility of biodiesel and the longer stability of blends by increasing the reaction tube length to 250 millimeters (ml) and increasing the minimum analysis time to 20 hours.
High-pressure Differential Scanning Calorimeter (DSC) – High-pressure Differential Scanning Calorimetry can be used to measure oxidation stability of oils, fats and lubricants. The high-pressure DSC test is conducted using a modification of ASTM D5483, which is the Standard Test Method for Oxidation Induction Time of Lubrication Greases. A small quantity of a sample is weighed and placed in a test cell. The cell is heated to a particular temperature and pressurized with oxygen to 500 pounds per square inch. The cell is held at the specified temperature and pressure until an exothermic reaction occurs. The resulting pressure drop is measured during this time and is reported as the oxidation induction time6.
Photochemilluminesence method – The working principle of the photochemilluminesence method is based on the generation of multiple superoxide anion radicals. The superoxide anion radicals are produced in the instrument by optical excitation of a photosensitive substance. The radicals are partially eliminated from the sample by a reaction with the antioxidants present in the sample. The remaining radicals in the measuring cell cause the detector to luminance. The antioxidant capacity of the sample is then exactly determined by a photomultiplier tube. The antioxidative capacity of the sample is quantified by comparison with the standards of Trolox (an antioxidant) for lipid and water soluble antioxidants9. Lab results show that a direct proportionality between oxidation hour and the antioxidant capacity of the biofuel samples can be measured using the
Photochemilluminesence method. The key feature of this method is the speed of oxidative reaction, which, when compared to normal conditions, is 100 times higher. This is evident in the three-minute timeframe required to perform a single measurement. This drastic decrease in measurement time is made possible by photochemical stimulation of the reacting molecules.
ASTM D4625 – This test method has been used to test the storage stability of middle distillate petroleum fuels. Under the D4625 method, the fuel is stored at 43 degrees Celsius for up to 24 weeks. The sample is typically filtered to determine total insolubles on a weekly basis. After filtration, the sample is analyzed for the formation of sediment in stored B5 and B20 blends6,10. Formations of sediments correspond to the oxidation of unsaturated methyl oleate and methyl linoleate in the compounds. Today, ASTM D4625 is used to help determine if long-term storage of biodiesel will lead to the development of precipitate and gums, which are problematic for engines and turbines1,4.
Fig. 2 Measure of Oxidation Stability as a Function of Pressure and Temperature
ASTM D2274 – In the D2774 method, 350 ml of a filtered blend is aged at 95 degrees C and exposed to oxygen for 16 hours. After aging, the sample is cooled to room temperature before filtering to obtain the filterable insolubles quantity2,6. Adherent insolubles are then removed from the oxidation cell and associated glassware with trisolvent, a mixture of equal parts toluene, acetone and methanol. The trisolvent is evaporated to obtain the quantity of adherent insolubles. The sum of the filterable and adherent insolubles, expressed as milligrams per 100 ml, is reported as total insolubles. The 100 ml of the aged sample is mixed with 400 ml of iso-octane to precipitate, and then is filtered. After filtering the aged sample but prior to solvent washing the filters, an aliquot of the filtrate can be obtained for additional testing. The aliquot is analyzed for total acid number, kinematic viscosity at 40 C, density at 15 C, peroxide content and iodine value4,6.
Conclusion
A tremendous amount of resources and time have been dedicated to the methods being utilized to study oxidation stability of biodiesel. These methods have become the stepping stones for long-term biodiesel storage. The advancements in methodology have allowed us to improve biodiesel quality. As the biodiesel industry continues to develop, the need for better testing equipment will steadily increase. The next generation of testing instruments will need to be far more accurate and faster than instruments currently available. For this reason, the photochemilluminesence method appears to be a promising way forward, but further research is still required.
Raj Shah is a director at Koehler Instrument Company. Reach him at rshah@koehlerinstrument.com. Devinder Mahajan is cochair of chemical engineering at State University of New York at Stony Brook. Saurabh Patel is a graduate student working under Devinder Mahajan. The remaining authors work for Koehler instrument Company and can be reached at (631) 589-3800.
References
1. Oxidation Stability of Biodiesel Fuel Produced from Fatty Wastes. Sendzikiene, E., Makareviciene, V. and Janulis, P. 3, Studentu : Polish Journal of Environmental Studies, 2005, Vol. 14.
2. Several factors affecting the stablity of biodiesel in standard accelerated tests. McCormick, R.L., et al. s.l. : Fuel Processing Technology, 2007, Vol. 88.
3. Gerpen, Jon Van. Fuel Stability. Biodiesel Education. [Online] University of Idaho - Department of Biological an Agricultural Engineering, 2004. [Cited: april 27, 2009.] http://www3.me.iastate.edu/biodiesel/index.html.
4. Effect of commercially available antioxidants over biodiesel/diesel blends stability. Dinkov, Roson, et al. Bulgaria : Fuel, 2008, Vol. 88.
5. Oxidative stability of biodiesel from soybean oil fatty acid ethyl esters. Ferrari, Roseli Ap., Oliveira, Vanessa da Silva and Scabio, Ardalla. 3, Ponta Grossa : Sci. Agric., 2005, Vol. 62.
6. Westbrook, S.R. An Evaluation and Comparison of Test Methods to Measure the Oxidation Stability of Neat Biodiesel. San Antonio : National Renewable Energy Laboratory, 2005.
7. Oxidation Stability - Scope & Limitation of Modified Rancimat Method EN 15751 and Future Alternatives. Ullmann, Jorg and Straub, Gunther. s.l. : Raber Bosch GmbH, 2008.
8. The high-pressure DSC for specific reactions. Netzsch. [Online] Netzsch, 2009. [Cited: April 27, 2009.] www.Netzsh.com.
9. Lambda Advanced Technology. Photochem offers method of measuring antioxidants. Laboratorytalk. [Online] oct 23, 2006. [Cited: april 25, 2009.] http://www.laboratorytalk.com/news/lam/lam123.html.
10. Peyton, Kim, McGinnis, Tim and Bureman, Phil Bureman. Preventing Sediment Formation In Stored Biodiesel Fuel Blends. Biodiesel Magazine. [Online] December 2008. [Cited: April 26, 2009.] http://www.biodieselmagazine.com/article-print.jsp?article_id=2979.
11. Tremblay, Alain and Berthiaume, David. Study of the Rancimat Test Method in Measuring the Oxidation Stability of Biodiesel Ester and Blends. Natural Resources Canada. November 2006. http://www.technopolethetford.ca/FichiersUpload/Softsystem/NRCan-OLEOTEK-StudyoftheRancimatTestMethodinMeasuringtheOxidationStabilityofBiodieselEstersandBlends.pdf
The regulations put in place for biofuel advancement resulted in the rapid development of Testing Standardization. ASTM International developed a standard specification D6751 for B100 biodiesel as a result of U.S. regulations. The specification consists of a number of test methods the biodiesel fuel is required to conform to in order for it to be commercially sold and distributed. This was followed by the release of a specification for biodiesel blends, designated D7467. The diesel fuel specification, D975, has also been revised to allow up to 5 percent biodiesel meeting specification D6751. These testing specifications all include oxidation stability as a major test method requirement. Currently, EN 14112 is the referee test method for oxidation stability of biodiesel within both D6751 and D7467 specifications. However, progress is being made towards putting an ASTM-designated test method for oxidation stability into these specifications. ASTM recently released an oxidation stability test method specifically for B100 biodiesel and blends of biodiesel with middle distillate petroleum fuel designated D7462. In time, this new test method will be incorporated into the D6751 and D7467 standard specifications.
Most biodiesel is produced by the transesterification reaction. The process takes place between a triglyceride and an alcohol to form esters and glycerol. A triglyceride is a glycerin molecule with three long fatty acid chains attached to it. The triglyceride reacts with the alcohol, either methanol or ethanol but most often methanol, in the presence of a catalyst such as sodium hydroxide or potassium hydroxide, to form mono-alkyl ester and crude glycerol. The combination of methanol and potassium hydroxide is preferred in FAME production because biodiesel produced this way is less resistant to oxidation than typical fossil fuel untreated with additives.
The general mechanism of oxidation in biodiesel has been well documented. Fatty wastes in general are more susceptible to oxidation because they vary in level of unsaturation, meaning there are more carbon-carbon double bonds and fewer hydrogen molecules on the fatty acid chains. When biodiesel made from unsaturated oil is exposed to oxygen, the oxygen attaches itself to the bis-allylic site directly adjacent to the two double bonds, which initiates an autoxidation chain reaction sequence. Oxidation stability is not related to the number of double bonds available but rather the number of bis-allylic sites2,3. The initiation step is the formation of a free radical that can react directly with oxygen. This leads to the formation of a peroxide or hydroperoxide molecule. The most reactive site for initial formation is the bis-allylic position. The radials formed at the bis-allylic sites immediately isomerize to form a more stable conjugated structure, which reacts directly with oxygen to form peroxide. The existence of these molecules is an early indication of oxidation taking place, and it is measured in terms of peroxide value2. Later, aldehydes and ketones are formed. Finally, during the polymerization process, resins are produced making the fuel unusable1.
Due to its chemical structure, oxidation rates of FAME can depend on many variables such as temperature, light, radiation intensity, presence of naturally occurring antioxidants and more. The oxidation stability of FAME can be increased by adding additional natural and synthetic antioxidants. However, naturally occurring antioxidants in FAME have shown relatively poor efficiency compared with synthetic antioxidants. An effective concentration of the antioxidants is in the range of 200 parts to 1,000 parts per million (PPM), depending on the type of FAME and the test used to evaluate additive performance. The antioxidants work by binding free radicals and stopping chain reactions. In this way, fatty peroxyradicals are stabilized and the oxidation chain reaction is broken1,4. Oxidation stability testing can be conducted under several different methods. The methods used to test for oxidation stability of biodiesel, and all of the supporting technology, will be reviewed below.
Methods
Rancimat EN 14112 – The Rancimat method expresses the oxidation stability of the tested material in terms of an induction period for the production of volatile organic acids, which are byproducts of fatty acid ester oxidative degradation with heat and oxygen11.
Figure 1 shows a plot of a typical Rancimat test. In this particular example, the induction time is about 4.8 hours. One can see a rapid increase in conductivity at this point. The second derivative is automatically taken to measure the absolute maxim, which is defined as the induction time6. This testing time of a few hours, instead of weeks or months, correlates to the shelf life of a product in years.
Fig.1 A Typical Rancimat Conductivity Plot
Rancimat EN 15751 – The EN 15751 method accounts for volatility of biodiesel and the longer stability of blends by increasing the reaction tube length to 250 millimeters (ml) and increasing the minimum analysis time to 20 hours.
High-pressure Differential Scanning Calorimeter (DSC) – High-pressure Differential Scanning Calorimetry can be used to measure oxidation stability of oils, fats and lubricants. The high-pressure DSC test is conducted using a modification of ASTM D5483, which is the Standard Test Method for Oxidation Induction Time of Lubrication Greases. A small quantity of a sample is weighed and placed in a test cell. The cell is heated to a particular temperature and pressurized with oxygen to 500 pounds per square inch. The cell is held at the specified temperature and pressure until an exothermic reaction occurs. The resulting pressure drop is measured during this time and is reported as the oxidation induction time6.
Photochemilluminesence method – The working principle of the photochemilluminesence method is based on the generation of multiple superoxide anion radicals. The superoxide anion radicals are produced in the instrument by optical excitation of a photosensitive substance. The radicals are partially eliminated from the sample by a reaction with the antioxidants present in the sample. The remaining radicals in the measuring cell cause the detector to luminance. The antioxidant capacity of the sample is then exactly determined by a photomultiplier tube. The antioxidative capacity of the sample is quantified by comparison with the standards of Trolox (an antioxidant) for lipid and water soluble antioxidants9. Lab results show that a direct proportionality between oxidation hour and the antioxidant capacity of the biofuel samples can be measured using the
Photochemilluminesence method. The key feature of this method is the speed of oxidative reaction, which, when compared to normal conditions, is 100 times higher. This is evident in the three-minute timeframe required to perform a single measurement. This drastic decrease in measurement time is made possible by photochemical stimulation of the reacting molecules.
ASTM D4625 – This test method has been used to test the storage stability of middle distillate petroleum fuels. Under the D4625 method, the fuel is stored at 43 degrees Celsius for up to 24 weeks. The sample is typically filtered to determine total insolubles on a weekly basis. After filtration, the sample is analyzed for the formation of sediment in stored B5 and B20 blends6,10. Formations of sediments correspond to the oxidation of unsaturated methyl oleate and methyl linoleate in the compounds. Today, ASTM D4625 is used to help determine if long-term storage of biodiesel will lead to the development of precipitate and gums, which are problematic for engines and turbines1,4.
Fig. 2 Measure of Oxidation Stability as a Function of Pressure and Temperature
ASTM D2274 – In the D2774 method, 350 ml of a filtered blend is aged at 95 degrees C and exposed to oxygen for 16 hours. After aging, the sample is cooled to room temperature before filtering to obtain the filterable insolubles quantity2,6. Adherent insolubles are then removed from the oxidation cell and associated glassware with trisolvent, a mixture of equal parts toluene, acetone and methanol. The trisolvent is evaporated to obtain the quantity of adherent insolubles. The sum of the filterable and adherent insolubles, expressed as milligrams per 100 ml, is reported as total insolubles. The 100 ml of the aged sample is mixed with 400 ml of iso-octane to precipitate, and then is filtered. After filtering the aged sample but prior to solvent washing the filters, an aliquot of the filtrate can be obtained for additional testing. The aliquot is analyzed for total acid number, kinematic viscosity at 40 C, density at 15 C, peroxide content and iodine value4,6.
Conclusion
A tremendous amount of resources and time have been dedicated to the methods being utilized to study oxidation stability of biodiesel. These methods have become the stepping stones for long-term biodiesel storage. The advancements in methodology have allowed us to improve biodiesel quality. As the biodiesel industry continues to develop, the need for better testing equipment will steadily increase. The next generation of testing instruments will need to be far more accurate and faster than instruments currently available. For this reason, the photochemilluminesence method appears to be a promising way forward, but further research is still required.
Raj Shah is a director at Koehler Instrument Company. Reach him at rshah@koehlerinstrument.com. Devinder Mahajan is cochair of chemical engineering at State University of New York at Stony Brook. Saurabh Patel is a graduate student working under Devinder Mahajan. The remaining authors work for Koehler instrument Company and can be reached at (631) 589-3800.
References
1. Oxidation Stability of Biodiesel Fuel Produced from Fatty Wastes. Sendzikiene, E., Makareviciene, V. and Janulis, P. 3, Studentu : Polish Journal of Environmental Studies, 2005, Vol. 14.
2. Several factors affecting the stablity of biodiesel in standard accelerated tests. McCormick, R.L., et al. s.l. : Fuel Processing Technology, 2007, Vol. 88.
3. Gerpen, Jon Van. Fuel Stability. Biodiesel Education. [Online] University of Idaho - Department of Biological an Agricultural Engineering, 2004. [Cited: april 27, 2009.] http://www3.me.iastate.edu/biodiesel/index.html.
4. Effect of commercially available antioxidants over biodiesel/diesel blends stability. Dinkov, Roson, et al. Bulgaria : Fuel, 2008, Vol. 88.
5. Oxidative stability of biodiesel from soybean oil fatty acid ethyl esters. Ferrari, Roseli Ap., Oliveira, Vanessa da Silva and Scabio, Ardalla. 3, Ponta Grossa : Sci. Agric., 2005, Vol. 62.
6. Westbrook, S.R. An Evaluation and Comparison of Test Methods to Measure the Oxidation Stability of Neat Biodiesel. San Antonio : National Renewable Energy Laboratory, 2005.
7. Oxidation Stability - Scope & Limitation of Modified Rancimat Method EN 15751 and Future Alternatives. Ullmann, Jorg and Straub, Gunther. s.l. : Raber Bosch GmbH, 2008.
8. The high-pressure DSC for specific reactions. Netzsch. [Online] Netzsch, 2009. [Cited: April 27, 2009.] www.Netzsh.com.
9. Lambda Advanced Technology. Photochem offers method of measuring antioxidants. Laboratorytalk. [Online] oct 23, 2006. [Cited: april 25, 2009.] http://www.laboratorytalk.com/news/lam/lam123.html.
10. Peyton, Kim, McGinnis, Tim and Bureman, Phil Bureman. Preventing Sediment Formation In Stored Biodiesel Fuel Blends. Biodiesel Magazine. [Online] December 2008. [Cited: April 26, 2009.] http://www.biodieselmagazine.com/article-print.jsp?article_id=2979.
11. Tremblay, Alain and Berthiaume, David. Study of the Rancimat Test Method in Measuring the Oxidation Stability of Biodiesel Ester and Blends. Natural Resources Canada. November 2006. http://www.technopolethetford.ca/FichiersUpload/Softsystem/NRCan-OLEOTEK-StudyoftheRancimatTestMethodinMeasuringtheOxidationStabilityofBiodieselEstersandBlends.pdf
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