Crude distillation unit (CDU) is at the front-end of the refinery, also known as topping unit, or atmospheric distillation unit. It receives high flow rates hence its size and operating cost are the largest in the refinery. Many crude distillation units are designed to handle a variety of crude oil types. The design of the unit is based on a light crude scenario and a heavy crude scenario. The unit should run satisfactorily at about 60% of the design feed rate. Seasonal temperature variation should be incorporated in the design because changes in the cut point of gasoline can vary by 20^oC (36^oF) between summer and winter. The capacity of the CDU ranges from 10,000 barrels per stream day (BPSD) or 1400 metric tons per day (tpd) to 400,000 BPSD (56,000 metric tpd). The economics of refining favours larger units. A good size CDU can process about 200,000 BPSD. The unit produces raw products, which have to be processed in downstream unit to produce products of certain specifications. This involves the removal of undesirable components like sulfur, nitrogen, and metal compounds, and limiting the aromatic contents. This chapter explores the process of crude distillation. When the crude oil enters the unit, it carries with it some brine in the form of very fine water droplets emulsified in the crude oil. The salt content of the crude measured in pounds per thousand barrels can be as high as 2000. Desalting of crude oil is an essential part of the refinery operation. It also describes the product slate from the crude distillation unit (atmospheric and vacuum distillation).

This chapter explores catalytic reforming and isomerization. Catalytic reforming of heavy naphtha and isomerization of light naphtha constitute a very important source of products having high octane numbers, which are key components in the production of gasoline. Environmental regulations limit on the benzene content in gasoline. If benzene is present in the final gasoline it produces carcinogenic material on combustion. Elimination of benzene forming hydrocarbons, such as hexane will prevent the formation of benzene, and this can be achieved by increasing the initial point of heavy naphtha. These light paraffinic hydrocarbons can be used in an isomerization unit to produce high octane number isomers. Catalytic reforming is the process of transforming C₇–C₁₀hydrocarbons with low octane numbers to aromatics and iso-paraffins which have high octane numbers. It is a highly endothermic process requiring large amounts of energy. The process can be operated in two modes: a high severity mode to produce mainly aromatics (80–90 vol%) and a middle severity mode to produce high octane gasoline (70% aromatics content). Isomerization is a mildly exothermic reaction and leads to the increase of an octane number. It is a process in which light straight chain paraffins of low RON (C₆, C₅, and C₄) are transformed with proper catalyst into branched chains with the same carbon number and high octane numbers.

This chapter explores thermal cracking and coking in the process of petroleum refining. Thermal cracking is the cracking of heavy residues under severe thermal conditions. The liquid products of this process are highly olefinic, aromatic, and have high sulfur content. They require hydrogen treatment to improve their properties. Coking is the process of carbon rejection from the heavy residues producing lighter components lower in sulfur, since most of the sulfur is retained in the coke. The thermal treatment of hydrocarbons follows a free radical mechanism where cracking reactions take place in the initiation step. The reactions in the final step result in the formation of heavy fractions and products like coke. There are three classes of industrial thermal cracking processes. (1) Mild cracking (as in visbreaking) in which mild heating is applied to crack the residue just enough to lower its viscosity and also to produce some light products; (2) Delayed coking in which moderate thermal cracking converts the residue into lighter products, leaving coke behind; (3) The final process involves severe thermal cracking: part of the coke is burned and used to heat the feed in the cracking reactor, as in fluid coking. In other version of the process, steam is used to gasify most of the coke (flexicoking).