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Segmented-Flow Direct Sampling Mass Spectrometry of Biodiesel Headspace

Solving the growing global energy crisis will require a collaboration of many renewable energy sources (wind, solar, biofuels, etc). Biodiesel is a relatively clean burning and renewable fuel source produced from virgin plant oils, animal fats, and used cooking oils. Renewability, reduced emissions, and an increased lubricity offset the lower energy density and the less favorable cold flow. Additionally, in light of sulfur restrictions on petroleum diesel fuel, biodiesel can be blended for increased lubricity. Biodiesel is produced by processing plant oils to transesterify (exchange) the methanol or ethanol for glycerol (triglyceride) under base catalyst. A typical biodiesel production uses excess alcohol to maximize the transesterification. The products of the reaction are the methyl or ethyl monoester (biodiesel) and glycerol. Incomplete reaction results in bound glycerol, which is the mono, di, or triglyceride. Subsequently, product clean-up steps typically use water to separate the organics (biodiesel and bound glycerol) from the aqueous layer (alcohol, glycerol, and the base catalyst). Water used in the final product clean-up and present in the starting plant oil is a biodiesel contaminant. Methanol is a reaction starting material. Both of these adversely effect critical product quality and play significant roles in changing the process efficiency. Water contamination inhibits combustion, supports microbial activity, and raises the gel point. If enough water is present, over time the water will react with the monoesters to form free fatty acids (FFAs). This will cause sludge that can clog the fuel system and diminish engine performance. The solubility of water in biodiesel is approximately 1500 ppm and the current industry acceptable standard (ASTM) is 500 ppm. Excess alcohol, typically methanol, can also negatively affect the product. For example, the normal flash point of biodiesel is 170 8C, but in a solution with just 1% methanol the flash point decreases to less than 40 8C. Lower flash point can impair the performance of the engine, cause early failure of fuel system components, and pose a safety risk when transporting or storing the fuel. Methanol concentration is not directly monitored by ASTM, but the European biodiesel standard has a limit of 0.2% methanol. B100 and blends must adhere to the set industry standards of quality and purity. There are 14 tests in the ASTM method that assure the quality of final product. Quality is verified in the final product but, as with most chemical production, the key to efficient operation is to monitor the process at every step. This allows precise process control as system parameters change over time. The focus of this research is to develop an in-line analytical measurement method for the determination of methanol and water in biodiesel. Consideration is first given to analytical external methods of analysis. Gas chromatography (GC) is the ASTM method for determination of methanol and glycerol (free and bound). High-performance liquid chromatography (HPLC) is also very effective in determining final product impurities as well. These chromatographic techniques require sample preparation, time for analysis, limited detection abilities, and limited robustness. Water measurement in organics is done by Karl Fischer titration, which is highly selective and sensitive to water content. However, it is not practical for a continuous process monitoring technique. Process analysis requires the analysis to occur relatively quickly with minimal sample preparation. Spectroscopic analysis, such as near infrared, has been used because it gives reasonable access to the sample. Fourier transform infrared (FT-IR) spectroscopy has been used to verify the methyl ester (FAME) content of the final biodiesel product. These techniques both potentially respond to the methanol and water, but will require extensive data analysis for adequate selectivity. The vibrational response of the OH and CH are correlated to the by-products and contaminants present (water, alcohol, glycerol, and free fatty acid). The selectivity between these is difficult and further complicated by the biodiesel response, which is present in much greater concentration. Both absorbance techniques have fundamental limits on the sensitivity, complicated by the aforementioned overlap in vibrational energies, and are temperature sensitive, particularly the NIR technique. Related mass spectroscopy (MS) techniques for analysis are direct membrane and headspace. Direct membrane MS analysis has been demonstrated for analysis of organics in water. The reverse (water in organics) is more difficult because in order for a membrane to let water permeate, the pores become large enough that unselective permeation occurs. For example, the Nafiont membrane will swell and become less selective when exposed to water and proceed to let biodiesel past the membrane. Headspace analysis has been used to monitor volatile components in soya oil and to monitor the oxidation alteration of olive oil. The selectivity of mass spectrometry (MS) provides the ability to identify and differentiate molecules in a sample. This Received 5 July 2007; accepted 27 August 2007. * Author to whom correspondence should be sent. E-mail: lecap1dj@ cmich.edu.