When dealing with high acidity feedstocks, in particular waste frying oils or animal fats from the meat or fish processing industry, one needs to perform a pre-treatment to guarantee that the transesterification reaction is performed in an efficient way and that the quality of the biodiesel obtained follows all the applicable norms such as the EN 14214. Also, it is important to know their characteristics and the presence of contaminants that reduce the efficiency and effectiveness of the alkali-catalyzed transesterification.

For the removal of contaminants, of special concern is the presence of moisture that has a strong negative influence in the transesterification reaction. Water content of waste oils and animal fats may vary considerably depending on the origin. Rice et al. [81] reported a range of 1-5% (w/w) of water contents in waste frying oils. The presence of water inhibits the esterification and transesterification reactions, favours the hydrolysis of triglycerides and FFA, lowers the esters yield, and renders the ester and glycerol separation difficult [7, 18]. If the water concentration is greater than 0.5%, the ester conversion rate may drop below 90% [19]. Water also promotes soap formation in the presence of the alkali catalysts, increasing catalyst consump­tion and diminishing its efficiency. The water content in the feedstock should be lower than 0.06% (w/w) [64, 81]. Heating the waste frying oil or tallow over 100°C, to about 120°C, can boil off any excess water present in the feedstock. For other contaminants, other strategies should be employed in a case-by-case scenario.

The waste frying oils may have other impurities such as solid particles resulting from the food frying and sodium chloride that is added to the fried food. Depending on the feedstock characteristics, the separation of these solid particles may be accomplished by filtration, pressing, or centrifugation. The presence of chlorides may cause corrosion problems in the process equipment and piping system.

The acid value of oil is another important parameter to be determined, since it allows one to evaluate which is the most adequate method to produce biodiesel. For example, depending on the oil acidity, one — or two-step process can be used, where in a first step, the level of FFA is reduced to below 3% by acid-catalyzed esterification with methanol as reagent and sulphuric acid as catalyst and, in a second step, trig­lycerides in product from the first step are transesterified with methanol by using an alkaline catalyst to produce methyl esters and glycerol [94] .

The FFA content of waste frying oil and animal fats vary widely. Waste oils typi­cally contain 2-7% (w/w) of FFA [95], while animal fats may contain 15% FFA but can be as high as 40% [18, 93, 94]. In order to maximize the methyl esters yield, Freedman et al. [39] proposed to use vegetable oils with a FFA content lower than

0. 5% (w/w) in order to not affect the yield of transesterification reaction. Rice et al. [81] reported that a reduction of FFA from 3.6to0.5% increased yields from 73to 87%. Canakci and Van Gerpen [19] referred that a FFA level above 5% can lower the ester conversion rate below 90%. A study from the Sustainable Community Enterprises [85] concluded that due to its high acidity, salmon oil requires an esterification pre-treatment to be possible to perform the transesterification

In the presence of FFA and moisture, saponification reactions occur because the fatty acids react with the catalyst to produce soaps, decreasing the methyl esters yield, or even inhibiting the transesterification reaction. Even in small amounts, these contaminants can reduce the reaction rate by orders of magnitude [18] . Moreover, the formation of soap consumes catalyst and causes emulsions to be formed, which limits the mass transfer between phases, significantly reducing the chemical reaction rate and the selectivity to biodiesel. This further complicates the separation of phases after the reaction completion and makes it difficult to recover and purify biodiesel [7] .

The equations (1) and (2) represent, respectively, the saponification of FFA and esters.

R — COOH + NaOH heat > R — COONa + H2O

FFA Metalic alkoxide Salt Water (1)

R — COOR’ + NaOH water > R — COONa + OH — R’

Ester Metalic alkoxide Salt Alcohol (2)

Aryee et al. [9] used FTIR and titrimetric analytical methods for FFA determina­tion in fish oils extracted from salmon skin, concluding that the FFA content of Atlantic salmon skin lipids increased linearly from 0.6 to 4.5% within the 120 days it was stored at 20°C, as a result of auto-oxidation. Wu and Bechtel [96] also found that the FFA level in salmon heads and viscera increases with the storage time and temperature. From a practical point of view, this results show that at least the fish oils should be used immediately after their extraction, limiting somehow the utiliza­tion at a local scale or when the logistical networks are efficient.

Refined vegetable oils normally do not need a pre-treatment in order to produce biodiesel. However, the waste frying oils and the animal fats with high acidity (more than 2.5% w/w of FFA) need a pre-treatment to reduce their FFA content. This is nor­mally done by acid-catalyzed esterification, using H2SO4 as catalyst and methanol as reagent in the proportions of 2.25 g of methanol and 0.05 g of sulphuric acid per each gram of FFA in oil. From the several approaches proposed in literature such as esterification and distillation refining method [99], Bianchi et al. [14] concluded that esterification is the most attractive to lower the FFA content of waste animal fat to 0.5% (from a typical range of 10 to 25%) using a solid acid ion-exchange resin as catalyst.

During esterification, the FFA are converted to methyl esters, but the triglycer­ides remain essentially unconverted to esters for low methanol to oil molar ratios and short reactor residence times [7, 29, 51, 60]. The esterification reaction can be represented as follows

R — COOH + CH3OH acid catalyst > r — COOCH3 + H2O

FFA methanol esters water (3)

Since water is formed as a by-product during esterification, it needs to be removed or the reaction will be quenched prematurely. One possible approach is to remove water while the reaction occurs, for example, using a membrane reactor. Another approach is to perform the reaction in two rounds with the removal of methanol, sulphuric acid, and water phase in between, followed by the addition of more fresh reactant to perform a second-round reaction driving it closer to completion [19-21, 94]. Zhang et al. [100] suggested the addition of glycerine after the second-round reac­tion to remove all the water from the oil stream, having the advantage of removing the acid catalyst which may cause neutralization of the alkali-catalyst during the transesteri fi cation reaction.