214
The Chemistry and Technology of Petroleum
the temperature changes and these portions of the distillate should be distilled again to amplify the
purification that has already occurred.
Distillation is a common method for the fractionation of petroleum that is used in the laboratory
as well as in refineries. The technique of distillation has been practiced for many centuries, and the
stills that have been employed have taken many forms (Speight and Ozum, 2002; Speight, 2011). It
was recognized in the early days of the refining industry, that desirable product (kerosene) was for
use as lamp oil. Thus, it is not surprising that distillation became the process of choice for petroleum
refining and the process has evolved from the simple distillation units to the complex multi-plate
still used in the refining industry (Chapter 17).
A modern petroleum refinery uses units that provide continuous distillation in which the feedstock is sent to the still all the time and products are drawn out at the same time. The idea in continuous distillation is that the amount going into the still and the amount leaving the still should
always equal each other at any given point in time. Older refineries (and even some modern process
units) employ batch distillation in which the amount of feedstock entering the still and the amount
of products leaving the still is not usually the same all the time and the still is refilled from time
to time. Thus, the distiller fills the still pot at the start, then heats it, as time goes by the vapors are
condensed to yield the products. When the designated quantities of products have been collected,
the distiller stops the still and empties it out ready for a new batch.
Separation by distillation takes place according to volatility, not necessarily according to
molecular weight. Comparison of the boiling points of 2-hydroxypyridine (280°C/760 mmHg,
535°F/760 mmHg) and 4-hydroxypyridine (257°C/10 mm Hg, 495°F/10 mmHg) illustrates that
molecular structure (and the consequences of this structure) influences boiling point. In more
general terms, and without any attendant bonding influences, the boiling point usually increases
with molecular weight in each homologous series. However, the differences between boiling
points in the different homologous series are quite substantial. For example, the two-ring naphthene decahydronaphthalene (decalin) with 10 carbon atoms (molecular weight 138) boils at
195°C (383°F), but the 10-carbon-atom normal paraffin decane (molecular weight 142) boils
at 174°C (345°F).
Thus, if a liquid is contained in a closed space, it emits vapor until a pressure of the vapor is
reached that is related to the temperature of the system; the vapor is then saturated. The vapor pressure of a liquid substance in contact with its own liquid is constant and is independent of the amount
of liquid and of vapor present in the system. The vapor pressure is usually expressed in terms of the
height of a mercury column (in millimeters or inches) that produces an equivalent pressure.
The vapor pressure of a liquid increases with increasing temperature, and when the vapor pressure is equal to the total pressure exerted on the surface of the liquid, the liquid boils. Thus the boiling point of a liquid may be defined as the temperature at which the vapor pressure of the liquid is
equal to the external pressure exerted on the liquid surface. This external pressure may be exerted
by atmospheric air, by other gases, by vapor and air, and so on. The boiling point at a pressure of
760 mm air is usually referred to as the normal boiling point.
The boiling point of a pure liquid has a definite and constant value at a constant pressure, but
the boiling point of an impure liquid depends to some extent on the nature of the impurities. If
the impurities are nonvolatile, the liquid boils at a constant temperature and the impurities remain
behind when the liquid has been distilled. If, however, the impurities are volatile, the boiling point
rises gradually as the liquid distills or may remain constant at a particular stage of the distillation
because of the formation of a constant boiling point mixture of two or more components.
The common feature of all distillation processes is the tendency for the concentration of the
more volatile component in the vapor phase to be greater than the concentration in the liquid phase
when the two phases have been in contact. In simple distillation, the enrichment of the more volatile
component is achieved by partially vaporizing a liquid mixture, either by raising the temperature or
by reducing the pressure, and allowing the two phases to separate.
The Chemistry and Technology of Petroleum
the temperature changes and these portions of the distillate should be distilled again to amplify the
purification that has already occurred.
Distillation is a common method for the fractionation of petroleum that is used in the laboratory
as well as in refineries. The technique of distillation has been practiced for many centuries, and the
stills that have been employed have taken many forms (Speight and Ozum, 2002; Speight, 2011). It
was recognized in the early days of the refining industry, that desirable product (kerosene) was for
use as lamp oil. Thus, it is not surprising that distillation became the process of choice for petroleum
refining and the process has evolved from the simple distillation units to the complex multi-plate
still used in the refining industry (Chapter 17).
A modern petroleum refinery uses units that provide continuous distillation in which the feedstock is sent to the still all the time and products are drawn out at the same time. The idea in continuous distillation is that the amount going into the still and the amount leaving the still should
always equal each other at any given point in time. Older refineries (and even some modern process
units) employ batch distillation in which the amount of feedstock entering the still and the amount
of products leaving the still is not usually the same all the time and the still is refilled from time
to time. Thus, the distiller fills the still pot at the start, then heats it, as time goes by the vapors are
condensed to yield the products. When the designated quantities of products have been collected,
the distiller stops the still and empties it out ready for a new batch.
Separation by distillation takes place according to volatility, not necessarily according to
molecular weight. Comparison of the boiling points of 2-hydroxypyridine (280°C/760 mmHg,
535°F/760 mmHg) and 4-hydroxypyridine (257°C/10 mm Hg, 495°F/10 mmHg) illustrates that
molecular structure (and the consequences of this structure) influences boiling point. In more
general terms, and without any attendant bonding influences, the boiling point usually increases
with molecular weight in each homologous series. However, the differences between boiling
points in the different homologous series are quite substantial. For example, the two-ring naphthene decahydronaphthalene (decalin) with 10 carbon atoms (molecular weight 138) boils at
195°C (383°F), but the 10-carbon-atom normal paraffin decane (molecular weight 142) boils
at 174°C (345°F).
Thus, if a liquid is contained in a closed space, it emits vapor until a pressure of the vapor is
reached that is related to the temperature of the system; the vapor is then saturated. The vapor pressure of a liquid substance in contact with its own liquid is constant and is independent of the amount
of liquid and of vapor present in the system. The vapor pressure is usually expressed in terms of the
height of a mercury column (in millimeters or inches) that produces an equivalent pressure.
The vapor pressure of a liquid increases with increasing temperature, and when the vapor pressure is equal to the total pressure exerted on the surface of the liquid, the liquid boils. Thus the boiling point of a liquid may be defined as the temperature at which the vapor pressure of the liquid is
equal to the external pressure exerted on the liquid surface. This external pressure may be exerted
by atmospheric air, by other gases, by vapor and air, and so on. The boiling point at a pressure of
760 mm air is usually referred to as the normal boiling point.
The boiling point of a pure liquid has a definite and constant value at a constant pressure, but
the boiling point of an impure liquid depends to some extent on the nature of the impurities. If
the impurities are nonvolatile, the liquid boils at a constant temperature and the impurities remain
behind when the liquid has been distilled. If, however, the impurities are volatile, the boiling point
rises gradually as the liquid distills or may remain constant at a particular stage of the distillation
because of the formation of a constant boiling point mixture of two or more components.
The common feature of all distillation processes is the tendency for the concentration of the
more volatile component in the vapor phase to be greater than the concentration in the liquid phase
when the two phases have been in contact. In simple distillation, the enrichment of the more volatile
component is achieved by partially vaporizing a liquid mixture, either by raising the temperature or
by reducing the pressure, and allowing the two phases to separate.