Hydrocarbons are the most important energy source on earth. When burned, they release significant amounts of energy. Crude oil is the predominant source of hydrocarbons, which are the essential components of diesel. Esters are compounds usually formed by the reaction between an acid and an alcohol with elimination of water. Transesterification is a chemical reaction in which one ester is converted into another. Transesterification is essential to the production of biodiesel.
After alternatives to traditional, petroleum-derived diesel became more common, industry experts referred to diesel as petrodiesel in order to avoid confusion. Petrodiesel is made by a process known as fractional distillation. Fraction distillation is the separation of a compound liquid (crude oil in this case) at varying degrees of heat so that components separate. Crude oil contains various types of hydrocarbons. Different hydrocarbons have different boiling points. To separate out the diesel, manufacturers heat crude oil at a temperature of 482 degrees to 660 degrees Fahrenheit at normal atmospheric pressure. The resulting fumes are collected and allowed to cool into a liquid state which we call diesel fuel.
There are different grades of diesel fuel, depending on sulfur content. According to DieselNet: “Since the 1990s, fuel quality has been increasingly more regulated by the US EPA under the authority of the Clean Air Act. In the context of the increasingly more stringent diesel emission standards, the most important fuel property regulated by the EPA became the sulfur content.”
There are three ways to make biodiesel: through catalyzed transesterification of the oil; through direct acid catalyzed transesterification of the oil; or through conversion of the oil to its fatty acids and then to biodiesel. The basic process is essentially the same for the three methods: an alcohol and a base are mixed, producing a chemical reaction. Later, the manufacturer separates the resulting glycerin and biodiesel?and then removes the alcohol. Using various chemicals, the manufacturer then neutralizes the glycerin and washes the biodiesel with a methyl ester. Some people mix biodiesel and petrodiesel to compensate for the fact that biodiesel tends to solidify into a gel in cold temperatures.
Synthetic diesel (also known as BML or Biomass to Liquid) is made from dried and gasified garbage, sewage-sludge wood, straw, corn, and hemp. The resulting gas is then purified in the Fischer-Tropsch process, a set of chemical reactions that converts mixtures of carbon monoxide and hydrogen (in this case, the gas produced from the garbage, straw, corn, etc.,) into liquid hydrocarbons. The resulting diesel is low in sulfur, making it an attractive alternative to petrodiesel.
The Fischer-Tropsch process is also the key to producing diesel from hydrocarbon-rich gases such as methane. The methane is partially oxidized, and the resulting excess carbon dioxide and water are removed. The remaining liquid is then reacted over a cobalt or iron catalyst to produce longer chains of hydrocarbons, out of which diesel can be removed by fractional distillation.