**4.4 Free fatty acids**

434 Biodiesel – Feedstocks and Processing Technologies

reduce the complexity of the process by reducing the methanol-to-oil ratio and the amount of recycled methanol. (iii) MG and DG should be the focus for reducing bound glycerol. Steps in the direction of (ii) have been hinted by D'Ippolito et al. (2007) and Manuale et al. (2011) for the supercritical method. The first proposed using 2 reaction steps with a low methanol-to-oil ratio (6-10), retaining glycerol and glycerides in packed bed adsorbers and recycling them to the reactor. The second indicated that the combination of 1-step reaction, a methanol-to-oil ratio of 15-20 and silica refining could produce EN14214 grade biodiesel.

Liquid-liquid equilibrium studies of biodiesel-methanol-glycerol mixtures have been undertaken in the past by Kimmel (2004), Negi et al. (2006) and Zhou & Boocook (2006). They determined that the equilibrium glycerol content in biodiesel depends strongly on the residual content of methanol acting as a cosolvent. When methanol is completely removed the free glycerol content depends only on the temperature, being approximately 0.2% at 25 °C and increasing linearly with temperature (Kimmel, 2004). Even if methanol is not present hydrophilic glycerol can be solubilized in the oil phase by amphiphilic MG and DG. These glycerides can separate from the oil during storage and precipitate as a result of temperature changes or long residence times. Glycerol then precipitates as a consequence of the reduced solubility, leading to the formation of deposits. Soluble glycerol is also a problem because glycerol polymerizes on hot surfaces (cylinders, injectors) with formation of deposits or

Glycerol removal by adsorption was early performed by Griffin and Dranoff (1963) using sulfonic resin beads. Glycerol adsorption over polar surfaces is favored if dissolved in organic media that have little affinity for the adsorbent. Nijhuis et al. (2002) reported that adsorption of organic esters (e.g. biodiesel) over polar surfaces such as those of silica and Nafion resins, is negligible. Yori et al. (2007) studied the reversible adsorption of glycerol from biodiesel and reported that silica has a great capacity for glycerol removal, its saturation capacity being 0.13 g of glycerol per gram of adsorbent. When operated in packed beds, for a glycerol concentration of 0.11−0.25% typical of biodiesel streams issuing from gravity settling tanks, an effluent limit of *C/C0*=0.01 and an entrance velocity of 11 cm min-1, a 2 m high silica bed with 1/8" beads would have a net processing capacity of 0.01−0.02

biodiesel kgsilica-1. Much of the good performance of silica is related to the favorable thermodynamics of adsorption, since glycerol-silica displays an almost irreversible, square

Soaps are produced by the reaction of FFAs during the first steps of caustic refining of the fatty feedstock or by the reaction of the remaining FFAs with alkaline homogeneous catalysts in the transesterification reactor. These reactions lead to the formation of estearates, oleates, palmitates, etc. of sodium and potasium, that are amphiphilic substances that bring phase separation and plugging problems downstream the reactor. Other salts of sodium or potasium come from the neutralization of acid homogeneous catalysts in the acid-catalized process. These inorganic salts lead to corrosion in lines and vessels and they must also be

Metals are minor components in all oils as they are present as oligoelements in highly specialized molecules such as chlorophylls (magnesium) and porphyrins (magnesium, iron, manganese). Other sources of metals are the contamination from iron and copper surfaces

completely eliminated in the final biodiesel product because of quality issues.

"tarnishes". For all these reason glycerol should be thoroughly removed.

**4.2 Glycerol** 

m3

isotherm (Yori, 2008).

**4.3 Soaps, salts and metals** 

FFAs have negligible values in biodiesel produced by the alkaline method. Depending on the efficiency of esterification they can be present in non-negligible amounts in biodiesel produced by the acid-catalyzed method or the supercritical method. Manuale et al. (2011) reacted different feedstocks with acidities ranging from 0.08 to 23.6% and found that the esterification with supercritical methanol (280 °C, 20=methanol-to-oil ratio) reduced the FFA content to 1-2.5% after 1 h and 0.4-0.6% after 1.5 h of reaction time. Reduction of the FFA content to values lower than those of the international norms can be done by washing. Adsorption however can prove simple, robust and efficient. For these application silicas are found to be superior than other adsorbents in both bleaching capacity and bleaching rate.


Table 3. Adsorbents capacity for FFA removal from biodiesel (Manuale et al., 2011).
