When a feedstock contains less than 3% or 4% FFA, most people just add extra catalyst, let the FFAs convert to soap, and then remove the soap. From 3% or 4%, up to 10% or 15% FFAs, a common approach is to use vacuum distillation to remove the FFAs from the oil. Then the oil can be processed normally, and the FFAs can be sold as animal feed or esterified separately.

However, used cooking oil and trap grease can contain much more than 15% FFAs. These feedstocks need additional processing before they can undergo traditional alkali-catalyzed transesterification.

Acid Pre-Treatment

One way to deal with a high percentage of FFAs is to use an acid catalyst such as sulfuric or hydrochloric acid to convert the FFAs to esters, followed by an alkali catalyst to convert the triglycerides to esters. One problem with this approach is that the conversion of FFAs to esters causes water formation, which can cause soaps to form during the alkali-catalyzed process.

However, this problem can be overcome by using an acid pre-treatment process to reduce the FFAs of the oil or grease. An acid catalyst and alcohol are added and reacted, the mixture is allowed to reach equilibrium, and the methanol, water, and acid portion that separates is removed. Then, if necessary, more acid and alcohol are added, and the process is repeated until the FFA level is less than 1%. After this pre-treatment process, the reaction is continued with alkaline-catalyzed transesterification.

While the acid pre-treatment is effective in reducing FFAs, the acid can cause damage to metal tanks. Use tanks coated with teflon or plastic or invest in super-alloy tanks.

Another disadvantage of the acid pre-treatment is that extra methanol is required because methanol is added with the acid and then removed with the water/acid layer. This methanol becomes contaminated with water. To recover and reuse the methanol, a distillation tower is required to separate the water from the methanol.

This technique was used to set up a pilot plant in Nevada, Iowa, to process high FFA feedstock into biodiesel.

Glycerolysis

An alternative process called “glycerolysis” can be used with feedstocks containing more than 10% FFAs. This involves adding glycerin at 400°F and letting it react with the FFAs to form monoglycerides, a glycerol molecule to which one free fatty acid has been joined. These monoglycerides can then be processed using a standard alkaline catalyst transesterification process.

Waste glycerin from biodiesel processing can be used in this process.

Glycerolysis can be expensive because of the high heat involved, which requires a high-pressure boiler and trained boiler operator. Also, vacuum must be applied while heating to remove water that is formed during the reaction.

Another disadvantage is that the glycerin will also react with the triglycerides in the oil to convert some of them to monoglycerides. While this does not negatively impact the reaction, it means that more glycerin is required for the process, and therefore more glycerin must be removed at the end of the transesterification.

Solid Acid Catalysts

The use of solid acid catalysts is a relatively new technology in biodiesel production. Solid acid catalysts are packed in a canister. As a mixture of oil and alcohol flows through this canister, the FFAs are converted to esters. The advantage of this process is that the acid does not contaminate the oil and therefore does not need to be removed at the end.

However, a disadvantage is that contaminants in the oil such as water and phosphorus can foul the catalyst and prevent it from reacting with the FFAs. One solution is to install a guard column in front of the acid catalyst canister to remove contaminants from the oil.

Even with a solid catalyst, water is formed by the esterification reaction and will contaminate the excess methanol that is used to force the chemical equilibrium to give a complete reaction. To be economically viable and environmentally sustainable, this methanol must be recycled, which requires a distillation process to remove water.

Supercritical Reactors

Another option is currently under development for processing high free fatty acid feedstocks. This option involves performing the reaction under supercritical conditions (275° to 325°C and high pressure). At high temperature and pressure, the reaction does not require a catalyst, so soap formation is not a problem. Water also does not appear to inhibit the reaction. Both free fatty acids and triglycerides react easily, so there is no need to separate these materials before processing. In fact, even very low quality feedstocks can be processed successfully.

However, the high reaction pressure requires heavy-duty reaction vessels. The reaction conditions are so extreme that many side reactions can occur which produce undesired products. The formation of these non-ester compounds means that the final product will probably need to be distilled to meet ASTM quality requirements. In addition, another drawback is the extra energy needed to achieve and maintain the high temperature.