Separation:
In the first step, molecules are separated through atmospheric distillation (i.e. at normal atmospheric pressure), according to their molecular weight. During the process, which is also known as a topping (refining), the oil is heated at the bottom of a 60-meter distillation column at a temperature of 350 to 400°C, causing it to vaporize. The vapors rise inside the column while the heaviest molecules, or residuals, remain at the bottom, without vaporizing. As the vapors rise, the molecules condense into liquids at different temperatures in the column. Only gases reach the top, where the temperature has dropped to 150°C. The liquids, which are become increasingly light the higher they are found in the column, are collected on trays located at different heights of the column. Each tray collects a different petroleum cut (fraction), also known as a petroleum cut, with highly viscous preservation (hydrocarbons) like asphalt (bitumen) at the bottom and gases at the top.
The heavy residuals left over after atmospheric distillation still contain many products of medium density. The residuals are transferred to another column where they undergo a second distillation to recover middle distillates like heavy fuel oil and diesel.
Conversion:
There are still many too heavy hydrocarbon molecules remaining after the separation process. To meet the demand for lighter products, the heavy molecules are “cracked” into two or more lighter ones.
The conversion process, which is carried out at 500°C, is also known as catalytic cracking because it uses a substance called a catalyst to speed up the chemical reaction. This process converts 75% of the heavy products into gas, gasoline, and diesel. The yield can be increased further by adding hydrogen, a process called hydrocracking, or by using deep conversion to remove carbon.
The more complex the operation, the more it costs and the more energy it uses. The refining industry’s ongoing objective is to find a balance between yield and the cost of conversion.
Treating:
Treating involves removing or significantly reducing molecules that are corrosive or cause air pollution, especially sulfur. European Union sulfur emission standards are very stringent. Since January 1, 2009, gasoline and diesel sold in Europe cannot contain more than 10 parts per million (ppm), or 10 milligrams per kilogram, of sulfur. The purpose of these measures is to improve air quality and optimize the effectiveness of catalytic converters used to treat exhaust gas. For diesel, desulfurization, or sulfur removal, is performed at 370°C, at a pressure of 60 bar. The hydrogen used in the process combines with the sulfur to form hydrogen sulfide (H2S), which is then treated to remove the sulfur, a substance used in the industry.
Kerosene, butane, and propane are washed in a caustic soda (sodium hydroxide) solution to remove thiols, also known as mercaptans. This process called sweetening.
Source: planete-energies.com
