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Feedstock Quality Issues for Biodiesel Production
Keywords: Biodiesel, feedstock, free fatty acids, acid number, triglycerides, vegetable oil, biofuel, soap, glycerin, saponification, transesterification
Quality of the raw oil (feedstock) has crucial role in determining the quality and quantity of the end product – biodiesel. Impurities present in the oil significantly affect the conversion process. Under similar environmental conditions, you can expect 67% to 84% conversion into biodiesel esters while using crude vegetable oils as compared to 94% to 97% using refined oils. This is because impurities like free fatty acids (FFAs) in the oil interfere with the catalyst favoring soap formation. Concentrations of these FFAs depend on the source and the use of oil. Virgin oil contains approximately 0.3% FFAs. Used vegetable oil has high concentrations of FFAs due to high heating in the frying process causing hydrolysis of oil (triglycerides). Similarly, different long chain fatty acid components in the oil have different physical properties. For instance, rapeseed oil contains higher levels of unsaturated fatty acids, which give liquid esters (biodiesel) at ambient temperatures. Tallow and hard fats on the other hand have higher levels of saturated fats and give an ester product which would solidify below 20oC, affecting the cold flow properties of biodiesel product.
Biodiesel feedstock should, among other qualities, mainly be screened for the following contents:
As mentioned above, free fatty acids are formed when vegetable oil is subjected to high temperature during frying. One molecule of oil can yield three molecules of free fatty acid. FFAs are not favored by transesterification reaction. As the name indicate, FFA is an acid and will react with a base, forming salt (soap is a salt of fatty acids – the saponification process). FFAs are hence critical to biodiesel reaction, which can lead to waste of a part or whole batch. The ranges of FFAs commonly found in biodiesel feedstock are: refined vegetable oils < 0.05%; crude vegetable oil 0.3-0.7%; restaurant waste grease 2-7%; animal fat 5-30%; and trap grease 40-100%. The transesterification reaction can easily assimilate FFAs level less than 1%, while levels less than 0.5% can be ignored. For FFAs levels above 1%, extra alkali catalyst is added to neutralize the FFAs by forming soap, while still leaving enough to act as the reaction catalyst. The additional amount is based on grams of catalyst required to neutralize FFAs in one liter of feedstock. FFA level is determined by acid number/titration test before making biodiesel in order to compensate for the catalyst, which will eventually be used to neutralize the FFAs. For extremely high FFA levels, acid esterification is the most favored reaction to esterify the FFAs into biodiesel.
Water is one of the three important reactants for making soap – water, base and vegetable oil. Water would break apart the base (e.g. KOH) molecule, producing free K+ ions, which could then combine with free fatty acids to produce soap. If the K+ ions are bound up in potassium methoxide, when the vegetable oil is broken apart, the methyl group in alcohol will preferentially bond with the fatty acid, rather than potassium – resulting in biodiesel rather than soap. The presence of water in the feedstock has a greater negative effect than that of the free fatty acids. Water content kept below 0.06% ensures highest yield in the process. Particular attention is hence required for most efficient utilization of waste vegetable oils and crude oils since they generally contain water and free fatty acids.
Moisture in the feedstock is determined by centrifugation or weighing & heat test. Weighing & heat test is simply heating the oil sample to evaporate the moisture. The easy way to detect the presence of water is the bubbling, which is fairly hard in large containers. Using a small sample in a glass beaker/container gives easy determination. An easy way to remove the water from the feedstock is to employ Mother Nature – gravity settling. Letting the oil settled in a tank, preferably with a cone bottom, takes care of at least 85% of the water and solids in two days. Solids and moisture settle to the bottom while leaving clean oil on the top. A drop from any level of moisture down to 0.08% in three days (~94% removal of moisture) has been observed at the W.A. Callegari Center.
To avoid soap formation, it is necessary to dry the oil as much as possible. The process can easily assimilate 1% water. The drying process can be accomplished by heating the oil to 65oC for six hours before the reaction. If using methanol, the temperature of the feedstock has to be less than 64oC to avoid boiling methanol from the reaction mixture.
The other major problem associated with water contamination is that it contributes to microbial growth. Some species of yeast, tingi and bacteria will grow at the at the bottom of a storage tank. The organisms produce sludges and slimes while the water can turn acidic. Some of the organisms can convert the sulfur to sulfuric acid, which can corrode metal, if any, in the dispensing line. Frequently draining the water from storage tanks, ensuring that vents and seals do not allow rainwater to enter and not drawing from the bottom of the tank should prevent large amounts of free water from entering the system.
Solids are considered contaminants in the waste vegetable oil because of their indirect effect on biodiesel process due to its tendency to hold moisture. Filtering large solids is a necessary step to avoid interfering solids coming from the feedstock. There are different approaches used to remove solids and moisture from the feedstock. Filtering is the most common method; however, the inexpensive and clean method is to use gravity settling. Collected oil is transferred to a large tank and let settle for two or three days. Solids and moisture sink to the bottom of the tank while the supernatant oil is ready to use for biodiesel production. The longer the settling time, the cleaner the oil is. However, the oil is monitored for microbial growth. Almost 100% of the solids settle to the bottom in two days, ensuring a good biodiesel product meeting the ASTM specifications. Settling time for solids and water is dependent on the temperature of the oil and quality of the oil collected. For instance, oil with higher mono- and diglycerides would take longer to settle because of their higher affinity toward moisture as compared with pure oil with triglycerides. During the frying process, animal fats are dissolved in the oil. The cloud point of these fats is around 12oC and tends to form wax at lower temperatures during cold winter. Centrifuging a sample collected from the bottom of the settling tank indicated 65% oil and 35% solids. That shows that the oil was still in the process of separation. Heating the solid part to 35oC yielded 60% oil. The results showed that at least 85% of the settled solids could easily be converted into biodiesel at higher ambient temperature if the moisture is boiled off first. Gravity settling also takes care of oil that tends to gel at lower temperature during winter even before processing into biodiesel. However, biodiesel made of such oil also gels at lower temperature.
Summary For an efficient transesterification reaction to produce biodiesel, it is very important to screen the feedstock for quality issues. Good quality of the feedstock means better quality of your biodiesel meeting the ASTM specifications, hence ensuring the best performance of your engine. Among other parameters, free fatty acids in your feedstock, water content and solids are the most important determining factors for your biodiesel production.
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| Last Updated: 8/29/2011 2:46:55 PM |
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