Supercritical Alcohol Reaction for Biodiesel Synthesis without Catalysts
This new technology utilizes supercritical alcohol to allow the normally immiscible oil and alcohol to form a single-phase solution. At supercritical alcohol conditions, the solubility parameter of alcohol is reduced and leads to the formation of a homogeneous phase between alcohol and oil.
Supercritical alcohol reaction allows for the use of inexpensive feedstock such as waste oils and fats which contain high proportions of these impurities.
Triglycerides + 3 Methanol =3 FAME+ Glycerol
In addition to the supercritical alcohol reaction, there are many other supercritical methods for carrying out the transesterification reaction.
Two-step SCFs Process for Non-Catalytic Biodiesel Synthesis using Subcritical H2O
An alternative non-catalytic process in supercritical fluids for the synthesis of biodiesel fuel consists of a two steps process. In this method, triglycerides are firstly hydrolyzed to free fatty acids, and in a second step are esterified to the corresponding esters. The reduction of the dielectric constant of water with increasing temperature promotes the miscibility between water and the oil, favoring a hydrolysis process. This allows acid and base-catalyzed reactions to be performed in high-temperature pressurized water with no catalyst. Hence, subcritical H2O is used as both the solvent and the catalyst for the hydrolysis step at 270 °C, 7 MPa at a volumetric water/triglyceride ratio of 1:1. After hydrolysis, two layers are formed, the upper portion containing fatty acids and the lower portion water with glycerol. In a second step, FAMEs are produced in supercritical methanol at 270 °C and 7 MPa.
Step 1 Triglycerides + H2O = 3 Fatty Acids + Glycerol
Step 2 Fatty acids +Methanol = FAMEs +Water
Glycerol Free Supercritical Reactions
Supercritical Methyl Acetate
In order to prevent the production of glycerol as a by-product, methyl acetate instead of methanol is used under supercritical conditions. The supercritical methyl acetate method converts triglycerides into fatty acid methyl esters (FAMEs) and triacetin, instead of glycerol without a catalyst. Triacetin, a valuable fuel additive is formed simultaneously which leads to simplified downstream processes compared to conventional catalytic reactions while the mixture of FAME and triacetin can be utilized as biodiesel, rather than FAMEs only.
Triglycerides + 3 Methyl Acetate = 3 FAMEs +Triacetin
Supercritical Dimethyl Carbonate
Biodiesel can be produced from triglycerides and dimethyl carbonate in a non-catalytic process by using supercritical dimethyl carbonate. Triglycerides, as well as fatty acids, are converted to fatty acid methyl esters (FAMEs). With this methodology, another valuable compound, glycerol carbonate, is obtained as a secondary product, instead of the undesirable glycerol, and a weak acid, such as citramalic acid, as the main by-product.
Triglycerides +3 DMC = 3 FAMEs + Glycerol carbonate + Citramalic acid
Biodiesel is a fuel that is typically made by chemically reacting lipids (triglycerides) with an alcohol producing fatty acid esters. The most common alcohol used in the reaction is methanol which produces fatty acid methyl esters. Conventionally, the transesterification reaction of triglycerides is conducted using homogeneous basic catalysts such as sodium hydroxide and potassium hydroxide or acid catalysts including sulfuric acid and phosphoric acid.
These catalytic reactions have many problems that make them unattractive. Generally, alcohol and vegetable oil do not form a single-phase solution. The poor contact between these two reactants causes reactions to proceed slowly and therefore requires intense mixing. Also, the recovery of glycerol is difficult due to contamination with methanol and catalyst. Moreover, the alkaline or acidic wastewater generated from the separation requires additional disposal costs. Altogether, these problems are the result of using a catalyst in the reaction, but without catalyst, the reaction rate is too slow to effectively produce biodiesel. Therefore, alternative technologies that do not require a catalytic process are the subject of intensive research.