• More catalyst will need to be used, resulting in higher production costs; leading to higher cost;
• Soap (fatty acid salt) is formed, making washing the finished product more difficult, and expensive;
• Water is formed, which will retard the main reaction; and
• The FFA’s are not converted into fuel, reducing the yield.

FFA content in waste oils and fats can range from 3 to 40%.

However, when the amount of FFA in the feedstock exceeds 0.5%, the use of a traditional homogeneous base-catalyzed transesterification process, which employs NaOH as the catalyst, is not recommended, because the catalyst and the raw material will be consumed by the saponification reaction between FFA and base...The soap causes downstream processing problems in product separation because of emulsion formation. Furthermore, it will require complex downstream neutralization, separation, and washing steps, which make the purification of the biodiesel more challenging.

—Baig and Ng

The use of an acid catalyst that is insensitive to FFA is better than the alkaline catalyst for processing vegetable oil with >1% FFA, the authors note, albeit at a much slower reaction rate.

Acidic and alkaline catalysts have their own advantages and disadvantages in the transesterification of high FFA feedstocks. Therefore, both acidic and alkaline catalysts have been used in a two-step process for the synthesis of biodiesel from refurbished waste oils and fats. However, the two-step process (or similar multi-step process) also faces the problem of catalyst removal in both steps.

Aijaz Baig and Flora Ng developed a single-step solid acid-catalyzed process for the production of biodiesel from high FFA feedstocks. The solid acid catalyst based on a supported heteropolyacid catalyst (PSA) was evaluated for the production of biodiesel from soybean oil (SBO) containing up to 25 wt% palmitic acid (PA). This solid acid catalyst catalyzed simultaneously esterification (the reaction of fatty acids with methanol in the presence of an acid catalyst and water to produce biodiesel) and transesterification (the reaction of triglycerides with methanol in the presence of a catalyst to produce biodiesel).

The palmitic acid was converted to biodiesel with 95% conversion using the solid acid catalyst (PSA), and the soybean oil was successfully transesterified with 99% CBG (chemically bound glycerin) conversion.

The solid acid catalyst was used for simultaneous esterification and transesterification of soybean oil containing 10% PA, achieving more than 95% conversion of palmitic acid and 99% conversion of soybean oil. Analysis based on the ASTM D 974, ASTM D 6584, and EN 14103 standards confirmed the production of high-purity biodiesel from feedstock with high FFA content.

The final product, without any post-treatment, has CBG, free glycerin, total glycerin, and acid number values of 0.616 mass%, 0.767 mass%, 1.383 mass%, and 1.505 mg of KOH/g. These values can be reduced further by water washing.

This single-step solid acid catalyzed process has the potential for the production of biodiesel from high FFA feedstocks.

—Baig and Ng

Resources

• Aijaz Baig and Flora T. T. Ng (2010) A Single-Step Solid Acid-Catalyzed Process for the Production of Biodiesel from High Free Fatty Acid Feedstocks. Energy Fuels, Article ASAP doi: 10.1021/ef901258b

• Feasibility Report: Small Scale Biodiesel Production (Waste Management and Research Center)