256
The Chemistry and Technology of Petroleum
can have higher boiling points than polycondensed aromatic systems. And this fact is very meaningful
when attempts are made to develop hypothetical structures for asphaltene constituents (Chapter 12).
The boiling points of petroleum fractions are rarely, if ever, distinct temperatures; it is, in fact,
more correct to refer to the boiling ranges of the various fractions. To determine these ranges, the
petroleum is tested in various methods of distillation, either at atmospheric pressure or at reduced
pressure. In general, the limiting molecular weight range for distillation at atmospheric pressure
without thermal degradation is 200–250, whereas the limiting molecular weight range for conventional vacuum distillation is 500–600.
As an early part of characterization studies, a correlation was observed between the quality of
petroleum products and their hydrogen content since gasoline, kerosene, diesel fuel, and lubricating
oil are made up of hydrocarbon constituents containing high proportions of hydrogen. Thus, it is not
surprising that tests to determine the volatility of petroleum and petroleum products were among
the first to be defined. Indeed, volatility is one of the major tests for petroleum products and it is
inevitable that all products will, at some stage of their history, be tested for volatility characteristics.
Distillation involves the general procedure of vaporizing the petroleum liquid in a suitable flask
either at atmospheric pressure (ASTM D86, D216, D285, D447, and D2892) or at reduced pressure
(ASTM D1160), and the data are reported in terms of one or more of the following seven items:
1. Initial boiling point is the thermometer reading in the neck of the distillation flask when the first
drop of distillate leaves the tip of the condenser tube. This reading is materially affected by a
number of test conditions, namely, room temperature, rate of heating, and condenser temperature.
2. Distillation temperatures are usually observed when the level of the distillate reaches each
10% mark on the graduated receiver, with the temperatures for the 5% and 95% marks
often included. Conversely, the volume of the distillate in the receiver, that is, the percentage recovered, is often observed at specified thermometer readings.
3. End-point or maximum temperature is the highest thermometer reading observed during
distillation. In most cases it is reached when the entire sample has been vaporized. If a
liquid residue remains in the flask after the maximum permissible adjustments are made in
heating rate, this is recorded as indicative of the presence of very high boiling compounds.
4. Dry point is the thermometer reading at the instant the flask becomes dry and is for special purposes, such as for solvents and for relatively pure hydrocarbons. For these purposes dry point is
considered more indicative of the final boiling point than end point or maximum temperature.
5. Recovery is the total volume of distillate recovered in the graduated receiver and residue is
the liquid material, mostly recondensed vapors, left in the flask after it has been allowed to
cool at the end of distillation. The residue is measured by transferring it to an appropriate
small graduated cylinder. Low or abnormally high residues indicate the absence or presence, respectively, of high-boiling components.
6. Total recovery is the sum of the liquid recovery and residue; distillation loss is determined
by subtracting the total recovery from 100%. It is, of course, the measure of the portion of
the vaporized sample that does not condense under the conditions of the test. Like the initial
boiling point, distillation loss is affected materially by a number of test conditions; namely,
condenser temperature, sampling and receiving temperatures, barometric pressure, heating
rate in the early part of the distillation, and others. Provisions are made for correcting high
distillation losses for the effect of low barometric pressure because of the practice of including distillation loss as one of the items in some specifications for motor gasoline.
7. Percentage evaporated is the percentage recovered at a specific thermometer reading or
other distillation temperatures, or the converse. The amounts that have been evaporated
are usually obtained by plotting observed thermometer readings against the corresponding
observed recoveries plus, in each case, the distillation loss. The initial boiling point is plotted with the distillation loss as the percentage evaporated. Distillation data are considerably reproducible, particularly for the more volatile products.
The Chemistry and Technology of Petroleum
can have higher boiling points than polycondensed aromatic systems. And this fact is very meaningful
when attempts are made to develop hypothetical structures for asphaltene constituents (Chapter 12).
The boiling points of petroleum fractions are rarely, if ever, distinct temperatures; it is, in fact,
more correct to refer to the boiling ranges of the various fractions. To determine these ranges, the
petroleum is tested in various methods of distillation, either at atmospheric pressure or at reduced
pressure. In general, the limiting molecular weight range for distillation at atmospheric pressure
without thermal degradation is 200–250, whereas the limiting molecular weight range for conventional vacuum distillation is 500–600.
As an early part of characterization studies, a correlation was observed between the quality of
petroleum products and their hydrogen content since gasoline, kerosene, diesel fuel, and lubricating
oil are made up of hydrocarbon constituents containing high proportions of hydrogen. Thus, it is not
surprising that tests to determine the volatility of petroleum and petroleum products were among
the first to be defined. Indeed, volatility is one of the major tests for petroleum products and it is
inevitable that all products will, at some stage of their history, be tested for volatility characteristics.
Distillation involves the general procedure of vaporizing the petroleum liquid in a suitable flask
either at atmospheric pressure (ASTM D86, D216, D285, D447, and D2892) or at reduced pressure
(ASTM D1160), and the data are reported in terms of one or more of the following seven items:
1. Initial boiling point is the thermometer reading in the neck of the distillation flask when the first
drop of distillate leaves the tip of the condenser tube. This reading is materially affected by a
number of test conditions, namely, room temperature, rate of heating, and condenser temperature.
2. Distillation temperatures are usually observed when the level of the distillate reaches each
10% mark on the graduated receiver, with the temperatures for the 5% and 95% marks
often included. Conversely, the volume of the distillate in the receiver, that is, the percentage recovered, is often observed at specified thermometer readings.
3. End-point or maximum temperature is the highest thermometer reading observed during
distillation. In most cases it is reached when the entire sample has been vaporized. If a
liquid residue remains in the flask after the maximum permissible adjustments are made in
heating rate, this is recorded as indicative of the presence of very high boiling compounds.
4. Dry point is the thermometer reading at the instant the flask becomes dry and is for special purposes, such as for solvents and for relatively pure hydrocarbons. For these purposes dry point is
considered more indicative of the final boiling point than end point or maximum temperature.
5. Recovery is the total volume of distillate recovered in the graduated receiver and residue is
the liquid material, mostly recondensed vapors, left in the flask after it has been allowed to
cool at the end of distillation. The residue is measured by transferring it to an appropriate
small graduated cylinder. Low or abnormally high residues indicate the absence or presence, respectively, of high-boiling components.
6. Total recovery is the sum of the liquid recovery and residue; distillation loss is determined
by subtracting the total recovery from 100%. It is, of course, the measure of the portion of
the vaporized sample that does not condense under the conditions of the test. Like the initial
boiling point, distillation loss is affected materially by a number of test conditions; namely,
condenser temperature, sampling and receiving temperatures, barometric pressure, heating
rate in the early part of the distillation, and others. Provisions are made for correcting high
distillation losses for the effect of low barometric pressure because of the practice of including distillation loss as one of the items in some specifications for motor gasoline.
7. Percentage evaporated is the percentage recovered at a specific thermometer reading or
other distillation temperatures, or the converse. The amounts that have been evaporated
are usually obtained by plotting observed thermometer readings against the corresponding
observed recoveries plus, in each case, the distillation loss. The initial boiling point is plotted with the distillation loss as the percentage evaporated. Distillation data are considerably reproducible, particularly for the more volatile products.