Biodiesel can be produced in a few different ways. The process can be operated either as a batch process or as a continuous process. It is usu­ally performed catalytically, using a strong base or acid as the catalyst. Alternatively, it can be operated noncatalytically, using supercritical meth­anol. The most popular process in the industry currently uses methanol as the alcohol, sodium hydroxide (NaOH) as the base catalyst, and is a con­tinuous process.

Biodiesel is most popularly produced by the transesterification reaction of triglycerides. Triglycerides are found in plant oils and animal fats and the molecular structure of a triglyceride is shown in Figure 2.5. Transesterification occurs when the triglycerides are mixed with an alcohol, typically either methanol or ethanol. As the hybridized terminology of “trans — + esterifica­tion" implies, transesterification is a chemical reaction in which the aliphatic organic group (R-) of an ester is exchanged with another aliphatic organic group (R-) of an alcohol, thereby producing a different ester and a differ­ent alcohol. In other words, the starting ester (triglyceride) and monohydric alcohol (methanol) are converted by the transesterification reaction into a simpler form of esters (biodiesel) and a more complex form of alcohol, that is, trihydric alcohol (glycerin). In this reaction of transesterification of triglyc­erides, three alcohol molecules liberate the long-chain fatty acids from the glycerin backbone by bonding (i. e., esterification) with the carboxyl group carbons in the triglyceride molecule, as shown in Figure 2.6. The products of the transesterification reaction are a glycerin molecule and three long — chain mono-alkyl ester molecules, otherwise commonly known as biodiesel. More specifically, if a biodiesel is produced by transesterification reaction between soy triglycerides and methanol, the resultant biodiesel ester is often referred to as methyl soyate. The transesterification reaction is usually cata­lyzed using a strong base such as NaOH or KOH. The base helps to catalyze this reaction by removing the

Before oils and fats can react to form biodiesel they must go through a pretreatment process. The first stage of the pretreatment process involves fil­tering to remove dirt and other particulate matters from the oil. Next, water must be removed from the oil because it will hydrolyze the triglycerides to form fatty acid and glycerin instead of biodiesel and glycerin. Free fatty acids can directly react with base catalyst to form soap, which is certainly not desir­able for biodiesel manufacture. If soap formation is active, the process would require an additional amount of base catalyst to compensate for the reactive depletion. Finally, the oil must be tested for free fatty acid (FFA) content. Typically, less than 1% FFA in oil is acceptable for processing without further provisional treatments. Free fatty acids are long-chain carboxylic acids that have broken free from the triglycerides, typically from thermal degradation of triglycerides as a result of prolonged exposure to heat. These acids can increase soap formation in the reactor, as mentioned earlier. Too much soap in the reactor causes substantial difficulties: (a) soap formation becomes a reason for an additional amount of base catalyst usage to overcome its reac­tive depletion in the soap formation; (b) additional problems arise in product separation; and (c) as an extreme case, the formed soaps mix with water from the fuel wash stage to create an emulsion that can seriously slow down or even prevent settling of the wash water layer from the product biodiesel layer. There are two ways to deal with the free fatty acids in the oil. An acid can be added to the oil to convert the free fatty acids into biodiesel; this is the case with an acid-catalyzed esterification reaction. Alternatively, they can be neu­tralized, turned into soap, and removed from the oil. After being pretreated the oil is then sent to the reactor. The methanol that reacts with the oil also has to go through some pretreatment. Before the methanol is sent to the reac­tor it goes through a mixer where it is combined with the sodium hydroxide catalyst. The oil and methanol/catalyst mixture are then fed into the reac­tor to undergo the transesterification reaction. Methanol is fed in excess of around 1.6 times the stoichiometric amount and the reactor is kept at around 60°C. With the aid of the base catalyst the reaction is able to proceed at up to 98% conversion. The exit stream from the reactor is fed into a separator. The glycerin by-product has a much greater density (glycerin specific gravity at 25°C = 1.263) than the biodiesel (specific gravity at 60°C = 0.880) and is there­fore easily removed via gravity separation. After the biodiesel is separated from the glycerin by-product it goes through a purification process. The first step is to neutralize the remaining catalyst by adding an acid to the biodie­sel. Then the biodiesel is sent through a stripper to remove any methanol left from the reactor. This methanol is then recycled back to the methanol/ catalyst mixer.

After the methanol removal, the biodiesel goes through a water wash to remove all the soaps and salts (e. g., neutralized salt of NaOH catalyst) gen­erated during transesterification and neutralization. The biodiesel is then dried and stored as the final product. The glycerin by-product, or crude glycerin, also goes through some purification. The crude glycerin contains a considerable amount of methanol, which comes out unreacted due to its excess amount of feed to the reactor. The glycerin goes through a distilla­tion separation that recovers a great deal of the methanol for recycling. The recycled methanol collects most of the water that entered the process and therefore it must go through a separate distillation column to be purified. The glycerin that comes out of the distillation process is pure glycerin that can be marketed for other industries including the pharmaceutical and cosmetics industries. A schematic of the biodiesel manufacturing process via transesterification is shown in Figure 2.7.

As mentioned above, crude glycerin is a mixture of glycerin, methanol, and salts. Crude glycerin can be sold as is or further purified into phar­maceutical-grade glycerin. A marketable grade of crude glycerin is gener­ally at least 80% glycerin with less than 1% methanol. Crude glycerin that has lower levels of glycerin or higher levels of methanol often has little or no value; this is especially true in the current era of an oversupply of glycerin on the market. Although glycerin is overly abundant in the world marketplace due to rapidly increased biodiesel production, the purification of crude glycerin into pure glycerin is quite energy-intensive and costly. Efficient chemical conversion of glycerin or crude glycerin into other value — added chemicals and petrochemicals, in addition to the conventional end — uses established in the food, pharmaceutical, and cosmetics sectors, would help stabilize the market price of glycerin and provide additional income to the biodiesel industry, thus improving the industry’s gross margin and profitability.