Atmospheric distillation constitutes the most widely used separation unit operation in the chemical process industry, being the dominant technology in applications requiring the fractionation of large volumes of fluids.
Its implementation is particularly relevant in petroleum processing, petrochemical production, natural gas treatment, liquefied gas separation, and the production of hydrocarbons and solvents. However, its greatest technical and operational impact is observed in crude oil refining.
It is estimated that atmospheric distillation is present in approximately 95% of separation processes worldwide. In the United States, more than 40,000 distillation columns are in operation, with investments exceeding USD 8 billion and an energy consumption equivalent to 15% of total annual industrial energy use¹. These figures position distillation as one of the most extensively studied unit operations in chemical engineering.
This article analyzes the refining processes that make up a petroleum refinery, with emphasis on within the global crude oil processing scheme.
Global crude oil refining process
The objective of crude oil refining is to transform petroleum into commercial products through an ordered sequence of refining processes. In general terms, these processes are classified as follows²:
- Physical separation processes (decantation, distillation)
- Transformation or conversion processes (alkylation, cracking)
- Purification or treatment processes (desulfurization, dehydrogenation)
- Blending processes
- Storage and distribution processes
Initially, crude oil, once water and salts have been removed, is preheated in a furnace and fed into a distillation column operating at atmospheric pressure. In this unit, the crude oil is separated into fractions such as light gases, gasolines, heavy naphtha, kerosene, heavy gas oil, and atmospheric residue.
The residue is subsequently routed to a vacuum distillation unit, where vacuum gas oil and heavy crude are obtained, which feed conversion processes such as thermal cracking or delayed coking.
How is distillation integrated into crude oil refining?
These processes are integrated as follows:
The operating scheme of a petroleum refinery is based on the sequential and functional interconnection of physical separation, conversion, and purification processes, whose objective is to maximize crude oil utilization and adjust product quality to market specifications.
Following atmospheric distillation, the resulting streams do not fully constitute final products but instead feed various downstream units. Atmospheric distillates, such as naphthas, kerosenes, and gas oils, may be sent directly to purification processes (hydrotreating, desulfurization) or serve as feedstock for conversion processes, depending on their composition and the adopted refining scheme.
Atmospheric residue, being a high-boiling fraction, is processed in vacuum distillation, where it is separated into vacuum gas oils and heavy residues. The latter represent a key feedstock for deep conversion processes such as thermal cracking, hydrocracking, or delayed coking, aimed at transforming low-value fractions into lighter, more commercially attractive products.
In parallel, gaseous streams generated in both separation and conversion units are routed to gas recovery and treatment systems, enabling the production of LPG, fuel gas, and other intermediate streams that are reintegrated into the refinery’s energy and product balance.
This continuous interaction between separation, conversion, and purification configures a highly integrated system, where each process operationally depends on the previous one and conditions the performance of the next. The following video shows an animation of the integrated crude oil refinery process, courtesy of Chemical Engineering School.
Los procesos de refinación del petróleo explicados de forma sencilla.
Relationship between distillation and conversion processes
During the initial stages of distillation, product separation does not alter the molecular structure of the crude oil, since hydrocarbons are segregated according to their boiling points. However, in subsequent refining processes, such as catalytic cracking or hydrogen-based cracking, heavy molecules are fragmented to obtain products of higher commercial value.
Products from hydrogen cracking are saturated, while those obtained from catalytic cracking require additional treatments such as catalytic reforming or hydrogenation to improve their quality.
Basic principles of the atmospheric distillation process
This is a key process in crude oil refining, through which hydrocarbons are separated into fractions according to their boiling point ranges³. Prior to this, the desalted crude oil is preheated in heat exchanger trains and then its temperature is increased in a direct-fired furnace before entering the distillation column, operated at near-atmospheric pressure and typical temperatures between 343 and 370 °C.
Inside the tower, lighter components vaporize and rise through the column, while heavier fractions condense and descend toward the bottom, where they are recovered as atmospheric residue. This residue constitutes the feed to the vacuum distillation unit, while upper fractions are directed to purification or conversion processes.
Tis is equipped with trays or contact devices that facilitate mass transfer between vapor and liquid phases. This global flow pattern provides countercurrent vapor–liquid contact across all stages of the column⁴. Each stage promotes partial evaporation and condensation of components, enabling progressive and controlled separation. Streams are withdrawn at different heights of the column, defined during process design according to the specifications of each fraction.
The liquid reaching the bottom of the column passes through a reboiler, where it is heated to generate vapor known as boilup, which is sent back to the column, while the remaining liquid is withdrawn as bottom product⁵.
To further explore the process, you are invited to view the following video, which clearly and didactically illustrates crude oil distillation, the flow of streams within the column, and the role this unit plays as the starting point of the global refining process. Source: OMV.
Distillation of crude oil in a refinery.
Material selection and standards in design
Is a fundamental process in the petroleum industry, demands special attention to the selection of materials used in the design of atmospheric distillation towers, in strict compliance with specific regulations. These aspects are necessary to prevent internal corrosion problems and ensure the long-term integrity of the installation, while meeting safety and environmental standards.
The importance of these elements is detailed below, considering the previously mentioned regulations.
Resistance to aggressive environments: The tower must operate under challenging environmental conditions, requiring robust design and corrosion-resistant materials, in compliance with standards such as API RP 939-C for protective coatings and ASME BPVC for pressure vessels.
Prevention of losses and contamination: Standards such as API RP 571 guide the evaluation of corrosion damage, contributing to a design that prevents product losses and avoids contamination. Proper material selection under these guidelines is necessary to ensure corrosion resistance.
Extended service life: Compliance with standards such as NACE SP0169 and SP0296 ensures recommended practices for cathodic protection systems, contributing to material selection and design that extend the service life of the tower.
Operational efficiency: Design and corrosion-resistant materials, in accordance with ASME BPVC, ensure operational efficiency and minimize maintenance costs. Compliance with occupational safety regulations, such as OSHA, is essential to maintain a safe and operational work environment.
Compliance with regulations and standards: The incorporation of corrosion-resistant materials is important to comply with industrial safety regulations and standards, such as NFPA 30 for the safe storage of flammable liquids. Adhering to these regulations avoids regulatory penalties and ensures safe and environmentally responsible operation.
Problems and solutions in atmospheric distillation
The inherent challenges in an plant include variability in feedstock quality, which is a common and critical aspect. The presence of impurities such as water and metals can trigger problems that affect the distillation process and reduce plant efficiency.
- Water contamination
- Problem: The presence of water can cause corrosion, emulsions, and complications in the distillation process.
- Corrective action: Implement effective dehydration systems to remove water from the feed.
- High sulfur conten
- Problem: High sulfur content can affect the quality of distilled products and require additional treatments.
- Corrective action: Perform a desulfurization process prior to distillation to reduce sulfur content.
- Presence of metals
- Problem: Metals in the feedstock act as unwanted catalysts, affecting efficiency and reducing equipment service life.
- Corrective action: Use chemical treatments or demetallization processes to remove or reduce metal content.
- Variability in composition
- Problem: Fluctuations in crude oil composition can affect the distribution of distilled products.
- Corrective action: Perform constant composition analysis and adjust operating conditions as necessary.
- Organic impurities
- Problem: The presence of resins and asphaltenes obstructs equipment, reduces plant performance, and generates corrosion problems.
- Corrective action: Implement efficient filtration and separation systems to remove impurities prior to distillation.
Conclusions
Preventing problems during atmospheric distillation is important to maintain process integrity and ensure the quality of the resulting products. Variability in feedstock composition, the presence of impurities such as water, sulfur, and metals, as well as possible organic contamination, are common issues that can compromise efficiency and profitability.
Implementing preventive measures such as dehydration systems, desulfurization, and continuous monitoring is necessary to prevent corrosion, ensure homogeneous product distribution, and extend equipment service life. Diligent attention to these aspects not only preserves operational integrity but also contributes to sustainable and profitable long-term production in the crude oil refining industry.
Investment in robust design and corrosion-resistant materials, guided by specific standards, is fundamental to preventing internal corrosion problems in towers. This approach not only protects infrastructure but also contributes to efficient performance, sustainability, and long-term operational safety of the installation.
References
- Luque S Rodríguez, Aurelio B. Vega Granda. Advanced simulation and optimization in the chemical and process industry: HYSYS.
- Crude Oil Refining; accessed 31/12/2023: https://es.slideshare.net/WilderLoza/procesos-de-refinacion#2
- King, C. J. (1980). Separation processes. New York: McGraw-Hill.
- Perry, R. H., & Green, D. W. (2008). Perry’s chemical engineers’ handbook. New York: McGraw-Hill, 7th edition.
- Seader, J. D., Henley, E. J., & Roper, D. K. (2011). Separation Process Principles: Chemical and Biochemical Operations. Hoboken, NJ: Wiley.