Oil refineries are complex industrial facilities that convert crude oil into a variety of valuable products, including gasoline, diesel, jet fuel, and numerous petrochemicals. One of the fundamental aspects of refining is the separation of crude oil into its components based on their distinct physical and chemical properties. This separation is done through various processes, each of which is specifically designed for certain fractions of crude oil.
The first phase in petroleum refining operations is the separation of crude oil into its major constituents using 3 petroleum separation processes:
- Atmospheric distillation
- Vacuum distillation
- Light ends recovery (gas processing)
Crude oil consists of a mixture of hydrocarbon compounds including paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of impurities including sulfur, nitrogen, oxygen, and metals. Refinery separation processes separate these crude oil constituents into common boiling-point fractions. Below is an overview of the primary separation processes used in oil refineries:
1. Distillation
A. Atmospheric distillation
This initial step in the refining process involves heating the crude oil and introducing it into the distillation column. Here the oil is separated into different parts according to their boiling point. Lighter fractions, such as gases and naphtha, rise to the top of the column, while heavier fractions, including gas oils and residues, settle to the bottom.
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 C (36 F) between summer and winter.
Typical products from the unit are:
- Gases
- Light straight run naphtha (also called light gasoline or light naphtha)
- Heavy gasoline (also called military jet fuel)
- Kerosene (also called light distillate or jet fuel)
- Middle distillates called diesel or light gas oil (LGO)
- Heavy distillates called atmospheric gas oil (AGO) or heavy gas oil(HGO)
- Crude column bottoms called atmospheric residue or topped crude.
B. Vacuum distillation
For heavier fractions that cannot be effectively separated at atmospheric pressure, vacuum distillation is used. By reducing the pressure in the distillation column, the boiling points of these heavier components are lowered, allowing them to evaporate at lower temperatures and minimizing the risk of thermal cracking.
Topped crude withdrawn from the bottom of the atmospheric distillation column is composed of high boiling-point hydrocarbons. When distilled at atmospheric pressures, the crude oil decomposes and polymerizes and will foul equipment. To separate topped crude into components, it must be distilled in a vacuum column at a very low pressure and in a steam atmosphere.
To extract more distillates from the atmospheric residue, the bottom from the atmospheric CDU is sent to the vacuum distillation unit. The vacuum unit distillates are classified as light vacuum gas oil (LVGO), medium vacuum gas oil (MVGO), and heavy vacuum gas oil (HVGO).
In addition a vacuum residue is produced. If the distillates are feed to down stream conversion process, their the sulphur, metal and asphaltene content should be reduced by hydrotreating or hydroprocessing. In some refineries the whole atmospheric residue is hydroprocessed before vacuum distillation. The vacuum unit can also be used to produce lubrication oil grade feed stocks. This depends on the quality of the crude oil feed to the refinery as only special types of crude can produce lube grade feed stocks.
The atmospheric residue can be sent directly to the vacuum unit after heat extraction in the crude preheat exchangers train. If it is sent to storage, the temperature should not be below 150 C (300 F) to control the viscosity necessary for proper flow. It is then heated in several exchangers by the hot products and pumparounds of the vacuum unit. Final heating to 380–415 C (716–779 F) is done in a fired heater. To minimize thermal cracking and coking, steam is injected in the heater tube passes. The feed enters the vacuum tower at the lower part of the column. As in the case of atmospheric distillation, a 3–5 vol% overflash is maintained (i.e., 3–5 vol% vapours are produced more than the total products withdrawn above the flash zone).
Vacuum distillation units have a system to create the vacuum that uses either ejectors or a combination of ejectors and liquid ring pumps. Ejectors recompress the gases through a nozzle where vapours from the column are sucked into the venturi section of the nozzle by a stream of medium or low pressure steam. The vapour phase at the ejector exit is partially condensed in an exchanger with cooling water. The liquid phase is then sent to the overhead drum. The vapour phase goes from the condenser to another ejector-condenser stage. Liquid ring pumps are similar to rotor gas compressors. One pump can replace two or three stages of ejectors in dry or wet type vacuum distillation. They do not use steam and can significantly reduce hydrocarbon-rich aqueous condensates in a system using ejectors. Systems with ejectors are much more flexible and rapid to put into operation. The higher investments required by liquid ring pumps are offset by reduced steam consumption and lower installation costs.
3. Light ends recovery (gas processing)
Light end recovery is a critical process in oil refining that focuses on the separation and recovery of light hydrocarbon fractions from crude oil or refinery intermediate streams. Commonly referred to as the “light end,” these light hydrocarbons typically include methane, ethane, propane, butane, and light naphtha. Light end recovery is essential to maximize the value of the refining process, optimize product yield and ensure efficient refinery operation.
The liquid sidestream withdrawn from the tower will contain low-boiling components which lower the flashpoint. These ‘‘light ends’’ are stripped from each sidestream in a separate small stripping tower containing four to ten trays with steam introduced under the bottom tray. The steam and stripped light ends are vented back into the vapor zone of the atmospheric fractionator above the corresponding side draw tray.
The light end recovery unit uses absorption and distillation steps to remove propane and heavier components from the refinery gas stream before it is used as fuel gas. The recovered C3+ components are then separated and used in different product streams.
In the absorption stage, naphtha and kerosene are used to absorb heavier hydrocarbons producing fuel gas and heavier liquid stream. The naphtha-rich liquid product that exits the bottom of the absorber enters a series of columns that step by step remove the lighter products.
These sub-columns are named in a self-explanatory way:
- Deethanizer: ethane recovery
- Depropanizer: propane recovery
- Debutanizer: Recovery of butanes
- Deisobutanizer: separates overhead isobutane and regular butane as a side draw
The remaining C5/C6 fraction is typically utilized for gasoline blending, whereas the heavy naphtha fraction is directed to catalytic reforming processes to generate high-octane gasoline. It is important to note that one or more of these light end columns may present challenges when expanding, reconfiguring, or altering refinery feedstocks. However, these challenges can be effectively addressed through the implementation of modern upgraded internals and/or the re-rating of rotating equipment.”