Petrochemical Processes

Fractionation

• Fractionation, or distillation, is a petrochemical process and one of the first steps in petrochemical refining. It involves separating crude oil into fractions, or cuts, of varying volatilities. The volatility of an oil refers to the weight loss it experiences due to vaporization at a certain temperature. An atmospheric distillation tower may be used to draw off the different products as they rise up within the tower upon being heated and pressurized. Heavy oil residue is taken from the bottom while lighter products and gases are removed from the top through repeated cycles of evaporation and condensation. A vacuum distillation tower is similar in setup except that it operates under lower pressure to prevent thermal cracking.

Conversion

• Other petrochemical processes convert a hydrocarbon into a different molecular state, such as through breaking its structure via "cracking," which can be done either thermally or catalytically. Latter day versions of the thermal type of cracking include visbreaking, coking and steam cracking. Visbreaking is a mild form of cracking that changes the viscosity of a product without modifying the boiling point. Coking is a more severe method that creates lighter products out of heavy residues. Steam cracking is a method used to obtain materials like ethylene from a feedstock. Catalytic cracking also transforms molecular structure, but under much less strenuous conditions than the thermal type. A catalyst such as zeolite, bauxite or silica-alumina accelerates the process of deriving lighter products from heavier ones.

Treatment

• Treating is a petrochemical process that takes out the unwanted compounds in a petroleum product that affect the quality of the final product. Foremost among contaminants is sulfur, as well as dissolved metals and soluble salts. Sweetening removes the sulfur to improve the color and odor of the intermediate or final products, whether it be gasoline, kerosene or jet fuel. Sulfur-containing compounds are made to react with various chemicals such as caustic liquid or amine compounds so they are rendered into less noxious disulphides. Carbon dioxide levels are also reduced. Treatments may involve chemical separation via solvents and oxidizing agents as well as simultaneous drying using molecular sieves.

Considerations

• Petrochemical processing encompasses a broad range of activities that take into account the logistical mechanisms of the refining plant itself. To be able to perform the treating, coking and other operations, the processing system must be able to handle waste and wastewater effectively, as well as provide storage, transportation and cooling of products. It must have a means of generating steam and power, provide steady streams of certain gases such as nitrogen, and have the facilities to test and sample the products obtained.