Optimization of Direct Conversion of Wet Algae to Biodiesel under Super-critical Methanol Conditions

Because conventional processes of biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> production from microalgae are both energy and cost intensive, researchers are seeking alternative methods that would minimize these costs, enabling large-scale microalgae-based biodiesel production. The method studied by Patil et al. (2011) in this paper was the direct conversion of wet algae to biodiesel in supercritical methanol conditions (SCM). Traditional processes require the drying of wet algal biomass, the extraction of the oil with solvents, and the catalyzed conversion of the algal oil to biodiesel. SCM conditions allow for a single-step process that could circumvent these expensive steps, thus greatly lessening biodiesel production costs. The researchers sought in this study to characterize the optimal conditions under which biodiesel could be created in this manner, varying reaction time, temperature, and wet algae to methanol (wt./vol.) ratio. They used response surface methodology (RSM) to analyze the results and found that reactions run at a temperature of 255 °C for 25 minutes with a 1:9 wt./vol. ratio produced the best results, representing a potential economical and efficient method of biodiesel production. —Karen de Wolski
Patil P., Gude V., Mannarswamy A., Deng S., Cook P., Munson-McGee S., Rhodes I., Lammers P., Nirmalakhandan N<!–[if supportFields]>XE “nitrogen, N”<![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “nitrogen” <![endif]–><!–[if supportFields]><![endif]–>., 2011. Optimization of direct conversion of wet algae to biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> under supercritical methanol conditions. Bioresource Technology 102, 118–122.

Patil et al. (2011) set out to elucidate the optimal conditions for the one-step reaction of biodiesel<!–[if supportFields]>XE “biodiesel”<![endif]–><!–[if supportFields]><![endif]–> formation from wet algae with SCM conditions. This process avoids the drying, extraction, and catalyzed conversion processes which represent great costs in traditional biodiesel production. Under SCM conditions, water is used as a co-solvent that accelerates conversion of fats and oils to fatty acid methyl esters (FAMEs) and increases solubility and acidity. The process set forth in this study produces FAMEs from polar phospholipids, free fatty acids (FFAs), and triglycerides by reducing polarity of high energy algal molecules while increasing fluidity and volatility. This allows for a single-step process in which extraction and transesterification of wet algal biomass are carried out simultaneously, requiring modest temperatures and relatively low energy input. This process has been conducted successfully for vegetable oils at about half the cost of conventional transesterification methods, and the researchers in this study sought to both demonstrate that it could be carried out for algal oil and to elucidate optimal reaction conditions.
          Patil et al. first characterized the algal samples through various chemical analyses. Lipid extraction of the Nannochloropsis<!–[if supportFields]> XE “Nannochloropsis” <![endif]–><!–[if supportFields]><![endif]–> sp<!–[if supportFields]> XE “Nannochloropsis sp”<![endif]–><!–[if supportFields]><![endif]–>ecies resulted in triglyceride content at 37.72%, other non-polar hydrocarbons/isoprenoids at 8.72%, and polars, glycolipids, and phospholipids at 3.54%. The researchers also used thin layer chromatography, densitometry, and scanning electron microscopy to further characterize the algal sample. Additionally, an FTIR spectra showed this particular algal species to be highly aliphatic and to have hydroxyl, carboxyl, and carbonyl groups, all identified by specific absorption bands. The triglyceride biosynthetic pathway in microalgae is thought to consist of the formation of acetyl coenzyme A in the cytoplasm, the elongation and desaturation of fatty acid carbon chain, and the subsequent biosynthesis of triglycerides. Methanol’s intermolecular hydrogen bonding is significantly decreased in the supercritical state, reducing its polarity and dielectric constant, and allowing the alcohol to solvate non-polar triglycerides. This results in a single phase lipid/methanol mixture and produces FAMEs and diglycerides that can be transesterified into methyl ester and monoglyceride and eventually glycerol.
          The researchers identified the wet algae to methanol (wt./vol.) ratio, reaction temperature, and reaction time as being the most critical variables affecting product FAME content. They utilized RSM to analyze these variables, a statistical analysis involving a three factorial subset that allows for accurate approximation of true error and significance. Wet algae to methanol ratios between 1:4 and 1:12, reaction times between 10 and 30 minutes, and temperatures between 240 and 260 °C were used in a total of 28 experimental runs. Additionally, Patil et al. implemented a general second order linear model with a deconstructionist approach to facilitate parametric evaluation for the predicted response surface and a least square method to predict the values of the involved parameters.
          For the actual experiment, 4 g samples of wet algae paste were run through non-catalytic SCM in a micro-reactor under a matrix of the previously described reaction conditions at a constant pressure of 1200 psi. The organic contents containing the non-polar lipids were isolated and analyzed by gas chromatography-mass spectroscopy (GC-MS) with methyl heptadecanoate as an internal standard. The FAME content of the final product could then be calculated by comparing the integrals of the FAME peaks with integrals of the standard peak. A general linear model, least squares, and ANOVA<!–[if supportFields]> XE “ANOVA”<![endif]–><!–[if supportFields]><![endif]–> were conducted to analyze the effects of the varied reaction parameters.
          The regression analysis showed all three parameters to significantly influence FAME content, confirmed by both P-values and the correlation coefficient (R2=0.921). The researchers created graphs showing response contours of FAME yield against temperature and wet algae to methanol ratio at the three time intervals. The regression coefficients show that reaction time positively affects response up to 255 °C, while higher temperatures are not conducive to transesterification reactions, possibly due to decomposition<!–[if supportFields]> XE “decomposition” <![endif]–><!–[if supportFields]><![endif]–> of oil/lipids and alkyl esters. The ratio of wet algae to methanol had a positive effect on yield up to 1:9, but negatively influenced yield at higher ratios. This may be explained by the reversible reaction being shifted forward as a result of increased contact area between methanol and lipids. This parameter can also interact with reaction temperature to reduce FAME yield due to either FAME decomposition or the reduction of the critical temperature of the reactant/product. High reaction times allowed for completion of the transesterification reaction and thus higher FAME yields. This effect was especially notable at the wet algae to methanol ratio of 1:9 at 255 °C. The experimental analysis and RSM study showed maximum yields under the aforementioned conditions with a reaction time of 25 minutes.
          The researchers were also interested in the elemental composition of the algal samples. They therefore subjected raw and residual samples to scanning electron microscopy to produce the elemental spectra. These results showed that the algal cell wall structure was disturbed and fragmented under the SCM condition and that the algal biomass was thermally degraded due to high unsaturated fatty acid content.
          The algal biodiesel<!–[if supportFields]> XE “biodiesel” <![endif]–><!–[if supportFields]><![endif]–> samples were analyzed by GC-MS to quantify the product. The FAME content was calculated by comparing the FAME peak integrals to the internal standard peak integrals. The algal biodiesel was found to have a large proportion of mono and poly unsaturated FAMEs. The ATR-FTIR spectra of the algal biodiesel were compared to the ATR-FTIR spectra of camelina biodiesel and petro-diesel. The main components of diesel are aliphatic hydrocarbons, which were observed by various peaks on all three spectra.
          The authors conclude that this single-step biodiesel<!–[if supportFields]>XE “biodiesel”<![endif]–><!–[if supportFields]><![endif]–> production process shows great promise in its shorter reaction time, simple product purification, and maximum FAME conversion. It requires lower energy input than conventional methods, and the process can be successfully optimized by RSM, representing the potential for efficient, relatively low-cost biodiesel production. 

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