Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 115

The atmospheric equivalent boiling point is the boiling point at which a sample or fraction of a fuel would boil under atmospheric pressure if it was stable and would not decompose. This provides a common basis for the categorization and direct comparison of petroleum components across the entire volatility range accessible by atmospheric and vacuum distillation.

Distillation at lower pressure allows high-boiling fractions to distill at lower temperatures, thereby foregoing the potential of thermal decomposition if attempts were made to distill the fraction at the higher temperatures required at atmospheric pressure. This allows the collection of distillates with an atmospheric equivalent temperature cut point of as high as 560°C (1050°F). The actual observed boiling points during this distillation are, of course, much lower. Despite the low pressure, the reboiler may have to be heated as high as 370°C (700°F) for such high-boiling distillates.

Molecular distillation is a non-equilibrium process and the atmospheric equivalent boiling range of a fuel or a fraction of a fuel that cannot be measured directly. The atmospheric equivalent boiling point (AEBP) concept was developed to compensate for this and can be estimated from one of the following equations:

The mid-AEBP is the temperature for the 50% mass point on the distillation curve of the fraction, and M_n is the molecular weight. Either the specific gravity (sp. gr.) or atomic hydrogen/carbon (H/C) ratio, which can be readily measured in the fractions, can also be used. Using the mid-AEBP concept, even fractions of the non-distillable residue can be included in the boiling range curves and relationships such as the variation of sulfur and nitrogen contents can be derived as a function of the AEBP.

Typically, as the mid-AEBP increases, the sulfur and nitrogen concentrations of the fraction generally also increase. Also, the highest concentrations of both sulfur and nitrogen appear in the non-distillable fractions. This behavior follows the heteroatom behavior observed for refinery distillation cuts. Thus, the higher the boiling range of the fraction, the higher the heteroatom concentration. This also establishes that the heteroatom concentration continues to increase as the volatility of the compounds decreases, which was not known previously for the 540°C⁺ (1000°F⁺) residuum.