Physical Distillation Techniques for Petroleum Analysis

Petroleum is a complex mixture of organic molecules, including crude oil and natural gas, that exists naturally in the earth and formed millions of years ago.

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Crude oil and natural gas are of limited use in their raw form; their value lies in the products they yield: fuels, lubricating oils, waxes, asphalt, petrochemicals, and pipeline-quality natural gas.

An appropriate technique is required to separate these valuable products from petroleum; distillation is the most common method for this.

Key Distillation Techniques in Petroleum Processing

Various distillation techniques are then required to characterize each of these products, not only for their intended use, but also for information obtained during the separation process.

These procedures can be divided into three categories: crude oil distillation, atmospheric distillation, and vacuum distillation. Each of these categories offers alternative techniques that provide similar characterization information while using more cost-effective methods, such as simulated distillation, microdistillation, or spectroscopy.

Crude oil distillation determines the boiling point characteristics of a batch of crude petroleum to be separated in a refinery. This provides information on the volume of specific components, which can then be used to produce end products such as gasoline or diesel.

Crude oil distillation analysis can cover all stages of the process, including the recovery of light gases; atmospheric-stage liquids, such as naphthas; vacuum-stage liquids, which have a higher boiling point and can be cracked into usable products; and residual material that can be used for asphalt or petrochemical production.

Atmospheric distillation provides the boiling point distribution of the lighter liquids throughout the refining process, allowing monitoring of whether they will meet the final requirements for the desired product usage, as well as a final quality check before certification to specification and market release. It is the most common distillation carried out by laboratories and online process monitors.

Vacuum distillation is typically used to characterize the residual material composition that exists after a specific step in the refining process. Since the first stage of atmospheric distillation separates the lighter-boiling components, the remaining material will require a reduced-pressure distillation to avoid thermal breakdown.

This characterization allows the next stage of processing to be properly prepared, or it may be used when transporting the material to another facility for additional applications, such as petrochemical processing.

Simulated distillation (SIMDIS) is a gas chromatographic method used to characterize boiling points across different sample types. It could be used at all stages of the process, as well as in quality control and final certification to ensure product specifications are met.

SIMDIS correlations have been produced for all phases of the petroleum refining process, including light gases, liquefied petroleum gas (LPG), motor fuels, and aviation fuels, as well as correlations for crude oil distillation and high-boiling-temperature vacuum stages.

The method can provide useful information on distillation characterization in less time and with a smaller sample volume than classic distillation test methods.

Microdistillation is a miniature atmospheric distillation technique that is cost-effective for analyzing fuels during the process, quality control, and final certification stages. It uses a micro-sized sample and a test duration that is one-third that of regular distillation, and has been used on motor fuels, aviation fuels, and even 100% biodiesels.

FTIR-ATR spectroscopy is also used to characterize fuels once spectroscopic models for the various fuel types have been constructed.

FTIR analysis is typically used on completed or certified fuels as a convenient mobile method to check the boiling points of gasolines and diesels to ensure they meet specification limitations. FTIR analyzers are also used in refinery process control for real-time analysis of process streams.

Standardization and Industry Applications

Atmospheric distillation was one of the first standardized test methods, with ASTM D86 Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure being accepted as a preliminary method in 1921.

As fuel compositions have changed over time, the scope of D86 has expanded to encompass new fuel types such as ethanol blends or biodiesel. D86 is one of the most widely used test methods in the business.

About 50 years later, in the 1970s, automated versions of D86 were produced, subsequently standardized as part of existing distillation test techniques, and are now the main equipment used, bringing much-needed respite to testing laboratory operations.

Several modifications to D86 have been made, allowing it to be applied to materials other than petroleum products, such as ASTM D850, Standard Test Method for Distillation of Industrial Aromatic Hydrocarbons and Related Materials, and D1078, Standard Test Method for Distillation Range of Volatile Organic Liquids. These modifications have also incorporated automated apparatus into their standardized test methods.

Alternative procedures for physical distillation have generally kept pace with the expansion of scope to incorporate new fuel types as they become available in the marketplace.

Vacuum distillation, also known as reduced-pressure distillation, was introduced not long after it was discovered that the high temperatures required in atmospheric distillation caused petroleum to degrade and become worthless.

Physically lowering the pressure in a system allows the liquid to boil at a lower temperature and separates more petroleum fractions. More methods for converting petroleum into lighter fractions, such as catalytic cracking, were eventually developed to make better use of petroleum output.

The test techniques for reduced-pressure distillation continue to give information for the operation of these processes. Vacuum-distillation test techniques, like atmospheric-distillation test methods, have incorporated automated apparatus and alternative test technologies, such as high-temperature SIMDIS.

Distillation Methods Summarized

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Source: PAC L.P.

PAC leads the standards community in the ongoing refinement of distillation test procedures to keep up with changing fuel types. The company is currently leading a task group to develop alternative temperature verification processes to replace harmful mercury thermometers.

PAC leads several projects in global standards bodies to ensure that distillation test methodologies are aligned. Michael Collier, the chair of the ISO (International Organization for Standardization) Technical Committee on Petroleum and Related Products, also serves as the project leader for the ISO distillation test methods.

This information has been sourced, reviewed and adapted from materials provided by PAC L.P.

For more information on this source, please visit PAC L.P.