The crude distillation unit is the first processing unit in the oil refinery. The crude oil distillation process aims to separate the crude oil into various petroleum fractions according to their boiling points. As the crude oil is separated into various fractions, therefore, crude oil distillation is called fractional distillation. The crude oil refining process is also called atmospheric distillation as the distillation is carried out slightly above the atmospheric pressure. Separated crude oil products are either sent to storage tanks as products or to the downstream treatment or conversion sections, depending upon the refinery configuration. The process description of a crude oil distillation unit can be seen below.
The basic process of a typical oil refinery can be seen in my previous blog” Petroleum Refinery Process“. The schematic process flow diagram of a crude oil distillation unit or shown below, in Fig. 01
The crude oil distillation process can be divided into the following steps;
Crude Oil de-salting
Crude oil heating and pre-flashing
Crude oil distillation
Products draw from crude column
1. Crude Oil Desalting Process
The desalting process of crude oil removes salts, sand, and water (brine) from crude oil. The importance of the desalting process in an oil refinery is very critical. The high salt content in crude oil can lead to corrosion and fouling in the downstream piping, equipment, and catalytic processing units.
Crude oil from the storage tank is pumped to the crude distillation unit. It is first preheated in the train of heat exchangers by counteracting with the hot process fluids from the crude column, up to 90~150 oC. Water 4~10 % is injected into preheated crude then mixed downstream of the mixing valve. In electrical desalting, the crude oil with diluted salts is then passed to a separating vessel called Desalter, where the proper separation between the aqueous and oil phases is achieved in the presence of an electric current. Water containing salts is removed from the vessel and send to the wastewater treatment section. For two-stage desalting more than ~99 % efficiency is achieved.
2. Crude Oil Preheating and Pre-Flashing
The desalted crude is further preheated and send to the Preflash drum to remove lighter ends and is routed directly to the distillation tower. The desalted crude from the preflash column oil is pumped, further pre-heated and finally passed through a fired heater to a temperature of about 350~400 °C depending upon the crude quality, and product quality requirements. Heavy crude oil needs more temperature for separation as compared to light crude oil.
3. Crude Oil Distillation Column
After heating the crude oil up to the required temperature, it is flashed in an atmospheric distillation tower at 1.2~1.5 atmosphere pressure. In the crude oil distillation column, the main fractions having specific boiling-point ranges and volatility are separated into gases, light distillates, middle distillates, gas oils, and residuum. The fractions removed from the side of the distillation tower at various points between the column top and bottom are called sidecuts. Each of the sidecuts (i.e., the kerosene, light gas oil, and heavy gas oil) is cooled by exchanging heat with the incoming crude oil feed. Most of the fractions (i.e., the overhead naphtha, the sidecuts, and the bottom residue) are sent directly to the downstream processing units called straight run products. While some products may also be stored in intermediate storage tanks before being processed further. Moreover, some part of the straight run products is also blended with the final product.
Hot flashed vapors in crude oil move up in the distillation tower and interact with the condensed liquid coming down. The lightest vapors of gases and naphtha move upwards while heavy vapors are condensed and move downward. Cold streams of reflux in the tower cause condensation of heavy fractions and cause them to move downwards. Further, condensed fractions are taken off at different parts of the tower according to their condensation temperature. The un-condensed gases and naphtha are removed from the top while the un-vaporized part of crude oil leaves from the bottom is called the residue.
Low pressure stripping steam is provided at the bottom of the column to remove any light hydrocarbons from the residue and enhance the flashing of the crude oil. Stripping steam reduces the partial pressure of hydrocarbon vapors and causing them to boil at low temperatures. The stripped residue called reduced crude, leaves from the tower bottom and is either utilized for heat recovery and cooled down as a product or passed to the downstream vacuum distillation unit for further processing.
The top reflux consists of condensed Naptha is used to control the tower top temperature. A high flow of reflux will reduce the top temperature and will reduce the endpoint of the Naphtha product. But top temperature must be controlled sufficiency higher (10~15 degrees C) than the dew point of the overhead vapors.
Re-fluxes play a vital role in the product separation and heat integration of the distillation tower. The pump around the section takes the hot product (Kero, diesel) for heat recovery and returns back as cold reflux above the draw-off tray. In addition, hot reflux also returns back below the daw-off tray. The purpose of hot and cold refluxes is to condense the hot vapors as maximum as possible to reduce the vapor load on the tower and ensure maximum separation of the products.
Furthermore, separated streams are provided side steam strippers for the removal of lighter ends from the products. These lighter vapors and stripping steam are delivered above the side stream draw-off tray.