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Acid‐catalyzed transesterification offers the advantage of esterifying FFAs contained in the fats and oils and is therefore especially suited for the transesterification of highly acidic fatty materials, such as palm oil or waste edible oils. Used cooking oils typically contain 2–7% FFA, and animal fats contain 5–30% FFA. A few very low quality feedstocks, such as trap grease, can approach 100% FFA level. Also, acid‐catalyzed transesterification enables the production of long‐ or branched‐chain esters, which pose considerable difficulty in alkaline transesterification because the FFA react with the catalyst to form soap and water [126] as shown:

Up to 5% FFA, the reaction can still be catalyzed with an alkali catalyst, but additional catalyst must be added to compensate for that lost to soap. The soap produced during the reaction is either removed with the glycerol or washed out during the water wash. When the FFA level is >5%, the soap inhibits separation of the glycerol from the methyl esters and contributes to emulsion formation during the water wash. Intended for these cases, an acid catalyst such as sulfuric acid can be used to esterify the FFA to methyl esters as shown in the following reaction:

This process can be used as a pretreatment to convert the FFA to methyl esters, thereby reducing the FFA level. In that case, the low FFA pretreated oil can be transesterified with an alkali catalyst to convert the triglycerides to methyl esters [127]. As depicted in the reaction, water is produced and, if it accumulates, it can stop the reaction well before completion. It was projected to allow the alcohol to separate from the pretreated oil or fat after the reaction. Exclusion of this alcohol also removes the water formed by the esterification reaction and permits for a second step of esterification; alternatively, one may proceed directly to alkali‐catalyzed transesterification. It is important to note that the methanol–water mixture will also contain some dissolved oil and FFA that should be recovered and reprocessed. The pretreatment by an acidic ion‐exchange resin has also been described [128]. It was revealed [129, 130] that acid‐catalyzed esterification can be used to produce BD from low‐grade by‐products of the oil refining industry such as soap stock.

Acid catalysts provide high yields, but the transesterification reaction is slow than that by alkali catalysis and requires higher temperatures. The most common acids used are phosphoric, hydrochloric, sulfonic, and sulfuric acids. They are also employed in pretreatment steps, to esterify the FFA, prior to basic catalyzed reaction. Nevertheless, acid catalysis is also affected by the presence of water that inhibits the reaction. Transesterification happens at a faster rate in the presence of an alkaline catalyst than in the presence of the same amount of acid catalyst [131].

The typical reaction conditions for homogeneous acid‐catalyzed methanolysis are temperatures of up to 100 °C and pressures of up to five bars in order to keep the alcohol liquid [132]. A further disadvantage of acid catalysis – probably prompted by the higher reaction temperatures – is an increased formation of unwanted secondary products, such as dialkyl ethers or glycerol ethers [76]. Finally, in contrast to alkaline reactions, the presence of water in the reaction mixture proves absolutely detrimental for acid catalysis. Fonseca et al. reported that the addition of 0.5% water to a mixture comprising soybean oil, methanol, and sulfuric acid reduced ester conversion from 95% to below 90% [127]. At a water content of 5%, ester conversion decreased to only 5.6%. It should also be noted that water released during esterification of FFA might inhibit further reaction, so that very acidic raw materials might give moderate conversion even in acid‐catalyzed alcoholysis. Shu et al. investigated the effect of reaction variables such as feed composition, temperature, and rate of mixing on the kinetics of the acid‐catalyzed transesterification of waste frying oils. The optimal yield of 99% was achieved after a 4 h reaction [133].

For acid‐catalyzed transesterification, the concentrated sulfuric acid is the most frequently used catalytic substance. Its advantages are its low price and its hygroscopicity, which is important for the esterification of FFAs, removing released water from the reaction mixture. Drawbacks include its corrosiveness, its tendency to attack double bonds in unsaturated FAs, and the fact that concentrated H₂SO₄ may cause dark coloring in the ester product [65]. Besides, also the use of various sulfonic acids as homogeneous catalysts is reported. These substances have lower catalytic activity than mineral acids. However, they pose fewer problems in handling and do not attack double bonds within the starting material.