Biodiesel stability

The Biodiesel Stability (BIOSTAB) project, supported by the European Commission, was initiated in 2001 to establish clear criteria and analytical methods for the monitoring biodiesel fuel stability (Various, 2003 Prankl, 2002). The resulting unified method, EN 14112 (Anon., 2003c) established a means for measuring oxidative stability utilizing the Rancimat or oxidation stability instruments. This test method was essentially developed from standards employed in the fats and oils industry to measure isothermally the induction period for oxidation of fatty derivatives. At present, both biodiesel fuel standards ASTM D 6751 (Anon., 2007a) and EN 14214 (Anon., 2003b) include an oxidative stability specification based on measurement by method EN 14112. 

Bondioli, R, Gasparoli, A., Bella, L. D., Taghabue, S., and Toso, G. 2003. Biodiesel Stability Under Commercial Storage Conditions Over One Year. Eur. J. Lipid Sci. Technol, 105,735-741. 

Stavinoha, L. L., and Howell, S. 2000. Biodiesel Stability Test Methods. In Proa, 7th International Conference on Stability and Handling of Liquid Fuels, Atlanta GA IASH. 

Bondioli, E A. Gasparoli L. Della Bella S. Tagliabue G. Toso. Biodiesel stability under commercial stor e conditions over one year. Eur. J. Lipid Sci. Technol. 2003, 105, 735-7M. 

Experiments showed that high methyl ester yields can be achieved with solid bases and super acids under moderate reaction conditions. The solid bases were more effective catalysts than the solid super acids. High stability can be achieved by an ordinary inexpensive preparation process, and the catalyst can be separated easily from the reaction products in the heterogeneous catalysis process. The costly catalyst removal process can be avoided compared with the homogeneous process. Therefore, the heterogeneous process using a solid catalyst should be more economical for biodiesel production. 

This prompted us to investigate the possibility of selectively hydrogenate highly unsaturated oils, unsuitable for the production of Biodiesel, in order to improve their oxidative stability while keeping the cold properties. 

Hydrotreating has been proposed by Arbokem Inc. in Canada as a means of converting Grade Tall Oil into biofuels and fuel additives. However, this process is a hydrogenation process which produces hydrocarbons rather than biodiesel. Recently a process for making biodiesel from crude tall oil has been proposed. It relies on the use of an acid catalysts or of an acyl halide for the esterification reaction, but no information is given on the properties of this fuel, particularly concerning the oxidative stability. 

In fact, after 5 reaction cycles the entrapped lipase shows a residual activity of the 60%, with respect to the total leaching that occurs after 3 or 2 reaction cycles for the adsorbed enzyme (Figure 2). Moreover, with respect to the biodiesel total productivity of the free lipase, the entrapped RML shows, after 5 reaction cycles, a value that is almost 6 times higher (1.62 mg FAME /mg Enz h and 0.28 mg FAME /mg Enz h, respectively for entrapped and free lipase). Concerning the stability of the inorganic matrix that covers the enzyme, after tested reaction cycles (five), mesoporous structure remains unaltered an stable (Figure 3). This indicates that the lipase leaching is due to the enzyme release and not to the collapse of matrix. 

Another route for biodiesel is to hydrotreat unprocessed bio-oils (from castor, cotton, palm, soy etc.) so that no transesterification is needed to stabilize the biodiesel. 

Biodiesel methyl esters blend quite easily into petroleum based conventional diesel fuel. Biodiesel esters typically have better lubricity properties and higher cetane number ratings than conventional diesel fuel, but have poorer water demulsibility and color stability properties. At sub-zero temperatures, the handling characteristics of biodiesel becomes more difficult to control than conventional diesel fuel. 

Very poor biodiesel and biodiesel blends do not shed water as effectively as conventional diesel fuel fuel haze, gelling, and low-temperature handling problems can develop if biodiesel is contaminated with water in storage and transport. Poor double bonds present in the methyl ester compounds are active sites for oxidation and condensation reactions peroxide values can increase fuel darkening and deposit formation in storage systems can occur the addition of oxidation inhibitors to biodiesel helps improve storage stability. 

On the other hand, the storage stability of biodiesel is adversely affected by the presence of unsaturated alkyl components. The olefinic moieties in biodiesel fuel can undergo oxidative degradation via exposure to air with deleterious results, including formation of solids and gums. The degree of oxidative degradation has been shown to increase with fuel unsaturation. 

These fatty acids and oils, as well as their derivatives, are applied in a broad range of products such as surfactants, lubricants and coatings, and, obviously, biodiesel. Upon epoxidation of the double bonds of the unsaturated fatty acids, very important compounds for the polymer industry are produced, which are used as plasticizers and stabilizers for a broad range of polymers such as polyvinyl chloride (PVC), polyesters, and polyurethanes [71]. Another interesting application has been found in the conversion of epoxidized soybean oil to carbonated soybean oil that can be reacted with ethylene diamine to obtain a polyurethane with interesting properties [72], Traditionally, stoichiometric reagents are used for the epoxidation of these oils and fats, albeit in some cases, with limited results. Therefore, the MTO/H2O2 system has been explored to epoxidize unsaturated fatty acids and oils. 

Although there are numerous publications on the effect of natural and synthetic antioxidants on the stability of oils and fats used as food and feed, until recently relatively little publicly available information was available on the effect of antioxidants on the oxidative stability of biodiesel. One of the earliest studies reporting of the effects of antioxidants on biodiesel was that of Du Plessis et aL (1985), which examined storage stability of sunflower oil methyl esters (SFME) at various temperatures for 90 d. Effects of air temperature, presence of light, addition of TBHQ (see Figure 1.1) and contact with steel were evaluated by analysis of free fatty acid content, PV, kinematic viscosity, anisidine value, and induction period. Addition of TBHQ delayed oxidation of samples stored at moderate temperatures (<30°C). In contrast, under unfavorable (50°C) conditions, TBHQ was ineffective. 

Conversely, SFME exhibited relatively poor improvement in oxidative stability with the use of antioxidants, presumably due to the higher concentrations of linoleic acid methyl esters in sunflower oil in comparison to the other biodiesel samples evaluated by the authors. Therefore, a good correlation was found between the improvement in oxidative stability as measured by OSI when antioxidants are used and the fatty acid composition of the biodiesel sample (Mittelbach and Schober, 2003). 

Stavinoha and Kline (2001) adapted ASTM method D 6186 (Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry [P-DSC]) for analyzing the oxidative stability of SME treated with antioxidants. This report concluded that isothermal P-DSC analysis is suitable for screening the effectiveness of antioxidants for treating biodiesel. 

Dunn, R. 0.2005b. Effect of Antioxidants on the Oxidative Stability of Methyl Soyate (Biodiesel). Fuel Proc. Technol., 86,1071-1085. 

Dunn, R. O. 2006a. Oxidative Stability of Biodiesel by Dynamic Mode Pressurized-Differential Scanning Calorimetry (P-DSC). Trans. ASABE, 49,1633-1641. 

Mittelbach, M., and Gangl, S. 2001. Long Storage Stability of Biodiesel Made from Rapeseed and Used Frying Oil. J. Am. Oil Chem. Soc., 78, 573-577. 

Monyem, A., Canakci, M., and Van Gerpen, J. H. 2000. Investigation of Biodiesel Thermal Stability Under Simulated In-Use Conditions. Appl. Eng. Agria, 16, 373-378. 

Stavinoha, L. L. 1998. Summary Overview. In Report, Oxidative and Thermal Stability Testing Methods for Biodiesel (pp. 1 1). San Antonio TX Southwest Research Institute. 

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