The biodiesel acceptable standards around the world normally follow either the American Society for Testing and Materials (ASTM) specification D6751 [5] or the European Specification EN14214 [6]. The variable level of substituted or bound glycerol to fatty acid as tri-, di- and mono-glycerides is summed up and calculated as the total bound glycerin value with an allowable value lower than 0.24 % in biodiesel. The biodiesel specification in Europe EN14214 includes a minimum requirement for the ester content (96.5 %) and individual maximum levels for the mono-, di-, and tri­glycerides. Both specifications limit the level of total glycerin remaining in biodiesel fuel to approximately the same value (0.24 % in ASTM D 6751 and 0.25 % in EN 14214). When measuring the level of free fatty acids (FFAs), the total FFAs should be below 0.25 mg/ml.

18.2 Feedstock’s Decomposition

Virgin oil when exposed to moisture, microbial contamination, heating, and light in the presence of air such as in the case of long-term storage or cooking, undergoes a decomposition process such as oxidation and hydrolysis leading to the formation of FFAs and other low — and high-molecular weight hydrocarbons, alcohols, oxidized monomers, dimmers, and trimers. Moisture and lipases excreted by microorganisms promote the hydrolysis of vegetable oil triglycerides to form FFAs, mono-, and di- acylglycerols, which result in increasing of the refining losses directly related to the free fatty acid content of oils and fats (Fig. 18.1). Oxygen and heat cause oxidation

—OH

RCOOH

Fatly acid

-OH — OCOR’ — OH

1-Monoglyccridc 2-Monoglyccridc Fatly acid Fatly acid

of oil triglycerides which results in the formation of hydroperoxides, leading to volatiles responsible later for the unpleasant odor of spent frying oils. The heat can also initiate cyclization reactions; both intra — and inter-molecular, occurring via electrocyclic Diels-Alder and Ene reactions. Furthermore, deterioration of heated oil used for multiple frying operations, is characterized by the formation of a polymeric dark mass and other low-molecular weight compounds. During frying operations, different compounds which include fatty acids, mono-, di-, and tri-glycerides, low — and high-molecular weight polymeric dark brown materials accumulate in oil. It has been reported that oxidation of oil is mostly responsible for much more of the deterioration of fats and oils than hydrolysis [7].

The decomposed oil loses fatty acids which become detached FFAs, or “used” oil and therefore less expensive feedstock to purchase. In order to increase the overall yield of biodiesel, the high-FFAs oil requires pretreatment processing that means subjecting the oil to acidic conditions, where the FFAs are converted to fatty acid esters and thereby lowered pH to acceptable values in the oil, so that the oil will later be converted to biodiesel by conventional alkaline catalysts (reactions 1 and 3, Fig. 18.2) [8]. The FFAs can also be removed under alkaline conditions (reaction 2, Fig. 18.2) when treated with alkaline reagent such as KOH to yield fatty acid potassium salts or soap which can be removed by water-wash process [9, 10].

The conventional chemical synthesis of biodiesel is typically carried out under al­kaline conditions and to a lesser extentunder acidic conditions [11,12]. The alkaline reaction involves transesteriflcation conditions using vegetable oil (triglyceride and to a lesser extent mono — and di-glycerides), methanol, and a chemical base such as sodium hydroxide (NaOH) (reaction 3, Fig. 18.2). The reaction can also be carried out with sodium methylate (NaOCH3) as a catalyst which produces biodiesel and glycerol with improved reaction yields [13-15]. The transesteriflcation of glycerides

Reaction -1

R—COOII + KOH —————————— R—COOK + П2°

batty acid Potassium Hydroxide Soap Water

Reaction -3

— OCOR

Catalyst

—OCOR + CII3OH ————- ► 3R-COOCH3

—— CX OK

….. … Methanol Methyl ester

I nglycmdc

Fig. 18.2 Formation of fatty acid methyl esters (biodiesel) and fatty acid salts from vegetable oil and fatty acids in the presence of methanol and a catalyst and methanol can also be carried out by acid catalysis to produce fatty acid methyl esters (FAMEs) and glycerol; however, this type of reaction is unfavored due to its low rates and its strict reaction conditions with regards to high temperatures and high excess of methanol.