Variables Affecting the In Situ Transes-terification of Microalgae Lipids

While microalgae show great potential as a future source of biomass for biofuel production, there still exist several obstacles for commercial production associated largely with the high costs of biomass production and fuel conversion routes. The majority of current research into fuel conversion involves the extraction of lipids from biomass and their subsequent conversion to fatty acid alkyl esters (FAAE) and glycerol. This research has traditionally used alkaline catalysts for the transesterification process, but these are limited because they result in partial saponification when used with oil reactants that have free fatty acid (FFA) content above 0.5% w/w. The use of hydrochloric and sulfuric acid as catalysts has been explored as a low-cost alternative that is not affected by FFA content. Ehimen et al. (2010) sought to determine how to maximize the reaction rate by varying conditions, including temperature, reacting alcohol volume, reaction time, and moisture content. They found that fatty acid methyl ester (FAME) conversions rates were positively correlated with volume and temperature, and that equilibrium FAME conversions approached asymptotic reaction time limits of 8 hours for almost all temperatures investigated. —Karen de Wolski
Ehimen E., Sun Z., Carrington C., 2010. Variables affecting the in situ transesterification of microalgae lipids. Fuel 89, 677–684.
Ehimen et al. (2010) researched how certain reaction variables affect biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production from microalgae lipids in an acid-catalyzed in situ transesterification process. While most research into microalgae-based biodiesel production has focused on alkaline catalyzed transesterification, this type of catalyst is not viable for large-scale commercial biodiesel production because it results in saponification when used with oils that have an FFA content above 0.5% w/w. The use of inorganic acids as catalysts for this reaction has been explored, as biodiesel producing transesterification and esterifiction reactions can be catalyzed in this manner. Previous studies have shown that acidic catalysts result in higher fatty acid methyl esters (FAME) yields than alkaline catalysts, and these yields can be optimized under certain reaction conditions.
          In order to reduce biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production costs, several alternatives to conventional transesterification reactions have been explored. This study focused on “in situ” transesterification, a process which reduces cost by eliminating the need for the solvent extraction step, as the biomass oil is converted directly to FAAE. This method could be particularly efficient for microalgae-based biodiesel production because the alcoholysis of the oil in the biomass directly increases biodiesel yields when derived from microalgae biomass oil. Because microalgae lipids tend to have a high FFA content, acid catalysts were used in this study, which sought to find optimal reaction conditions for microalgae-based in situ transesterification. The researchers varied reacting alcohol volume, temperature, reaction time, and process mixing, seeking how to create a reaction which would have the lowest process costs and the best biodiesel yield.
          Microalgae oil was extracted from dried Chlorella biomass via previously established methods. Several analyses were conducted on the oil sample, including specific gravity (SG), gas chromatography, and titration to determine acid value and FFA content. The researchers carried out the in situ transesterification process with a sulphuric acid catalyst at the different alcohol volumes in question. The SGs of the extracted FAME products were measured to determine the extent of conversion during reaction, as a decreasing SG signifies that the reaction has reached equilibrium conversion between microalgae lipids and methyl esters. The extent of conversion was further verified by a GC, and a calibration curve of the relationship between SG and FAME conversion was created.
          After setting up the in situ transesterification reaction, several conditions were varied to maximize output. Five different methanol volumes and four different temperatures were tried, and the FAME product yields and SGs were measured as described previously. Additionally, the researchers tested for the effect of reaction time at each temperature, running a total of eight different reaction times at each experimental temperature. The researchers were also interested in how moisture content and stirring affects the in situ transesterification process. They therefore studied the effects of moisture by air drying and oven drying Chlorella samples to nine different moisture contents and subjecting these samples to the process at a constant temperature and alcohol volume. They then subjected the in situ transesterification process to four stirring variations at a constant temperature and alcohol volume and analyzed the product yields as previously described.
          The Chlorella grown under the culture conditions of this study was found to have a total transesterifiable lipid fraction of 0.276 g oil/g biomass. The SG of the extracted oil was measured at 0.914 at 25 °C. Using the GC analysis of the extracted oil and the chemical equation of the reaction (MMoil=[3MMFA+ MMglycerol] –3MMwater), average molecular mass of the oil was calculated to be 880 g/mol. A calibration curve was obtained by correlating concentration of FAME species and SG of purified product. This curve covers a range of 0% conversion (pure extracted oil) to 92.22% conversion (lowest measured SG), and could therefore be used to predict corresponding percentage FAME conversions for the other reactions run in this study. A strong negative correlation between SG and percent FAME conversion was demonstrated by this curve. The acid value of the oil was found to be 10.21 mg KOH/g, and the FFA content was calculated at 5.11%. Because this is a relatively high FFA content, the use of an acidic catalyst for in situ transesterification reactions was justified for this microalgae oil.
          The results of the experiments varying temperature and alcohol volume show that
increasing both variables generally increases microalgae oil conversion to FAME. However, no significant trends were observed for temperature levels of 60 and 90 °C. For the experiments investigating the interaction of reaction time and temperature, asymptotic FAME conversion values were not reached in the 12 hour time boundary of this study at room temperature. Highest equilibrium conversion levels were reached in the shortest time (70% at 15 min, 90% at 1 hour) for the 90 °C reaction temperature. Similar asymptotic values at reaction times of 2 and 4 hours were found for the 60 and 90°C conditions, and the researchers state that 60 °C may therefore be ideal because process heating and pressure requirements could be inhibited at the higher temperature. The authors were interested in the effect of moisture content on FAME conversion because the drying process represents a significant cost in biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production. However, the results indicate that drying cannot be avoided, as there was a strong negative correlation between moisture content and FAME conversion. The researchers also sought whether stirring could be avoided as a cost-cutting measure. While stirring the reaction for only one hour did produce high equilibrium conversion levels, the yield was still only 91.3% of a continuously stirred system. Intermittent one hour stirring produced results closer to the continuously stirred reaction, but nevertheless fell short, and the investigators therefore concluded that stirring (at least intermittently) is likely a necessary cost of biodiesel production for the in situ transesterification process.
          The authors conclude that increasing temperature, reaction time, and alcohol volume may favor biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production for this particular process. However, energetic cost of the recovery of excess alcohol reactants and increases in FAME purification requirements could potentially limit any cost reductions achievable by these variables. Therefore, further optimization of this process, including studying filtration, evaporation, and extraction, should be studied to make in situ transesterification a viable biodiesel production method. 

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