5.1 Introduction
In this section, we present a brief overview of the crude distillation process.
The first essential task for the crude oil consisting of more than 108 compounds is to separate its major components based on boiling point differences. This principle is exploited in the crude distillation unit which involves energy intensive operation. Since crude distillation involves the processing of the entire feed, it remains as the most significant operation in a refinery.
Figure. 5.1: Process flowsheet-a conceptual diagram of the crude distillation unit (CDU) along with heat exchanger networks (HEN)
The conceptual process flowsheet for the petroleum refinery is shown in the Figure 5.1. It consists of the following important sub-processes:
- 1. Crude desalte
- 2. Furnace
3. Pre-flash column
4. Crude distillation column supplemented with side columns. These columns produce the desired products
5. Pump around heat exchanger units
6. Heat exchanger network that facilitates energy recovery from hot product and reflux streams to heat the crude oil.
7. We next present the functional role of various sub-processes in the crude distillation unit.
5.1.1 Crude desalter
- • Crude oil consists of dissolved salts and they tend to cause fouling and corrosion in various process equipments. Therefore, dissolved salts need to be removed using a separation process.
• The crude desalting unit is a separation process. Here, water along with other trace chemicals such as caustic and acid are allowed to enter a mixing unit along with the crude oil.
• The mixture of crude oil and water is subsequently passed through an electrostatic precipitator cum gravity settler. The electrostatic field enables the agglomeration of water droplets and aids faster gravity settling.
• An essential issue for the good performance of crude desalter is the temperature of the operation. . Usually, high efficiency of salt removal is possible between 100 – 300° F.
• Therefore, the crude oil is heated to about 250° F before it enters the desalter unit.
• The clean desalted crude oil flows from the top of the gravity settler and the water along with other dissolved impurities is removed as a bottom product from the gravity settler unit.
• A high degree of salt removal is desired (95 – 99% removal of the dissolved salt in the crude oil). Usually, a two stage desalting process is deployed. When higher salt removal efficiencies are desired, three stage units are deployed.