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Atmospheric Distillation

The petroleum refining industry converts crude oil into more than 2500 refined products, including liquefied petroleum gas, gasoline, kerosene, aviation fuel, diesel fuel, fuel oils, lubricating oils, and feedstocks for the petrochemical industry.

Distillation under atmospheric pressure is a crucial test method to characterize these refined products. It delivers important information about the tested sample in order to e.g. maximize the yield of different petroleum products obtained from crude oil, based on their boiling range characteristics. Moreover, the distillation behavior provides information about composition, properties, and behavior during storage and use, which affect the safety regulations as well as the handling and the performance of fuels and other hydrocarbon-based solvents.

Refining Industry

Crude oil processing is the first step in the petroleum refinery. Since crude oil is a mixture of various components (including paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of impurities like sulfur, nitrogen, oxygen, and metals) with boiling points at different temperatures, the components can be separated according to their boiling points by fractional distillation.

The distillation process for crude oil is typically performed either under atmospheric pressure or under a vacuum. Low-boiling fractions of crude oil usually vaporize below 400 °C under atmospheric pressure and thus these low-boiling fractions are separated by atmospheric distillation in the refinery crude unit. The purposes of this unit include the rejection of major contaminants and an initial separation into streams/cuts for further processing. Fractions with higher boiling points are separated in a vacuum distillation tower.

Atmospheric distillation of crude oil

Figure 1: Atmospheric distillation of crude oil

After leaving the atmospheric distillation column, the fractions do not yet correspond to the requirements of the market. Gasoline, for example, is further processed in a reformer, where its octane number is increased. Only after adding additives to the gasoline is a high-grade fuel produced which has good combustion characteristics and a low emission of pollutants such as sulfur. With diesel fuel many important quality characteristics can be influenced by adding additives to the fuel. Improving the combustion process of diesel fuel leads to a quieter and gentler operation of the engine and fewer deposits, avoiding the sticking of intake valves and filters. Moreover, the cetane number can be increased for a better performance. Cleaning additives lead to fewer deposits in the engine system.

The process of refining is highly sensitive to differences in crude oil composition. The whole process has to be monitored constantly to immediately react to any changes. Furthermore, the final products like diesel and gasoline have to fulfill normative requirements (such as ASTM D975 and EN 590 for diesel fuel and ASTM D4814 and EN228 for gasoline) to guarantee good drivability and a low emission of pollutants. To guarantee comparable results it is essential to constantly run tests according to the test methods ASTM D86 / ISO 3405 to control the refining process and to certify the final products.

Test Method

The test methods ASTM D86 and ISO 3405 qualitatively determine the boiling range characteristics of petroleum products with a boiling range between 20 °C to 400 °C under atmospheric pressure by using a laboratory batch distillation. The product’s boiling range gives information about composition, properties, and behavior during storage and use. The results are important for controlling the refinery process and certifying the final products. This test can be conducted with manual and automated instruments.

Test Principle

Generally, a distillation consists of two processes: evaporation and condensation. To evaporate the sample, it is filled into a distillation flask and heated until it starts to boil and vapor is formed (see Figure 2). The vapor is then conducted into the condenser line in which it is cooled down and condensed back into liquid form. The condensate is collected in a graduated cylinder.

Traditional measurement setup

Figure 2: Traditional measurement setup

During the test, the vapor temperature is precisely recorded as a function of the recovered volume of the condensate. The result is a distillation curve (see Figure 31).

According to the guidelines of the applicable standards, a given volume of sample (100 mL) is placed in the distillation flask and distilled under conditions preset according to the standard.

Results

The atmospheric distillation unit delivers a distillation curve that describes the boiling range of the tested sample and thus its behavior at certain temperatures (see Figure 31). It gives information about composition, properties, and behavior during storage and use. In the case of fuels, these characteristics influence engine starting, warm up, and vapor lock at high operating temperature and high altitude. Moreover, the result is also crucial for the safety and handling regulations of the sample.

Distillation curve of gasoline. Correlation of distillation profile ranges with gasoline performance

Figure 3: Distillation curve of gasoline. Correlation of distillation profile ranges with gasoline performance

References

  1.  Chevron Corporation (2009) Motor Gasolines Technical Review

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