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254                              Advances in Eco-Fuels for a Sustainable Environment

         moisture content up to 90% and also high FFA content can be used as the feedstock.
         (2) A short reaction time is required to reach high conversion. (3) No catalyst is
         required, thus product separation and purification are simpler and easier [43,
         90–92]. Both two-step transesterification and in situ transesterification can be carried
         out under the critical condition. In a two-step transesterification, subcritical water
         extraction or solvent extraction was carried out in the first step, followed by trans-
         esterification under the supercritical condition in the next step. Because energy con-
         sumption is high due to sequential processes, in situ transesterification under critical
         condition can be the alternative to reduce energy consumption.
            Temperature plays an important role in the sub/supercritical process. Dielectric
         constant and polarity of water/methanol decrease when temperature is near or above
         its critical temperature, so that water/methanol has the ability not only to extract lipids
         but also to convert lipids to biodiesel in the homogeneous mixture [93]. The ability
         of water/methanol to have similar properties as an organic solvent under super/sub-
         critical condition make them feasible to eliminate the extraction step conducted sep-
         arately without the catalyst addition. In situ transesterification under the supercritical
         condition needs a higher operating temperature and pressure than under the subcritical
         condition. To lower the mixture’s critical temperature, adding a cosolvent is neces-
         sary. Cosolvent addition has a positive effect on increasing the extraction efficiency
         of the lipid [94]. Besides temperature, pressure, alcohol amount, and reaction time
         also affect biodiesel conversion in the sub/supercritical process. The higher the tem-
         perature, the higher the biodiesel yield that can be obtained. However, alkyl esters may
         decompose at high temperature so it is necessary to know the decomposition temper-
         ature limit of each microalgae strain [94]. Pressure has a positive effect on biodiesel
         yield, although pressure is temperature dependent. Jazzar et al. [91] reported that pres-
         sure above 15MPa just gave a small increase in biodiesel yield. Excessive alcohol
         amounts are necessary in the supercritical condition. Alcohol acted as a solvent to
         extract the lipid, as a reactant to form biodiesel, and as a catalyst precursor [89]. How-
         ever, extravagant alcohol amounts should be avoided because that can decrease bio-
         diesel yield. In addition, separation of glycerin will be difficult because alcohol is
         soluble in glycerin [95]. Although, the supercritical process needs a short reaction
         to reach completion, it is still important to know the optimum reaction time because
         a reaction shorter than the optimum time will result in an uncompleted reaction [86,
         90] while a reaction time longer than the optimum will decrease biodiesel due to the
         presence of water, which favors the hydrolysis of FAME to FFA [92, 95].
            Temperature and pressure required in the subcritical process are lower than that for
         the supercritical process, but still can obtain a high FAME yield. Thus, the subcritical
         process is an alternative technology in biodiesel production with less energy require-
         ment. There are several studies reported about direct transesterification under the sub-
         critical condition. Tsigie et al. [90] and Sitthithanaboon et al. [43] converted wet
         microalgae (80% moisture content) to biodiesel using methanol as the reactant under
         the subcritical condition. The two works used different algae strains and operating
         conditions, and obtained a maximum FAME yield of 88.65% and 59.28%, respec-
         tively. Factors that affect conversion under the subcritical condition are quite similar
         to those under the supercritical conditions. Table 9.7 shows several reports about bio-
         diesel production from microalgae under sub/supercritical condition.
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