Technical background of petroleum refining

The refining process

Crude oil is a mixture of chemical compounds and needs to be separated into useful products, such as gasoline, kerosene, diesel, jet fuel, fuel oil, lubricants, and so on. Essentially, refining breaks crude oil down into its various components, which then are selectively reconfigured into new products.

Conceptually, all refineries perform three basic functions: separation, conversion and treatment. The refining process involves both physical and chemical processes. The dominant physical process is distillation, which enables the separation of the lighter components from the heavier components. The most widely used conversion method is called cracking, because it uses heat – pressure or catalytic – to “crack” heavy hydrocarbon molecules into lighter ones. Other refinery processes, instead of splitting molecules, rearrange them to add value. Alkylation, for example, makes gasoline components by combining some of the gaseous by-products of cracking.

The outputs from the primary distillation process can rarely meet market requirements, either in quality or in quantity. Modern refineries usually have a series of secondary processing units to increase the output of lighter, cleaner, high-value products. Each of the secondary processing units after the distillation unit has a specific purpose, whether to increase separation; to upgrade low-value products, such as residual fuel oil, to high-value products, such as distillate; to increase octane; or to enhance environmental compliance by removing sulphur and other impurities. A modern refinery typically has the following main processing units.

Atmospheric distillation unit (ADU): The refining process always begins with atmospheric crude distillation. The simplest refinery may have nothing but the distillation process. Figure 5.1 shows a simple version of a refinery with only the distillation process, whereas Figure 5.2 shows the flow diagram of a relatively complex refinery. In this very first step of the refining process, crude oil is piped through a hot column, which is fuelled typically by natural gas or residual fuel oil. As oil is heated, various fractions of oil boil off. The resulting liquids and vapour are discharged to distillation and cooling towers.