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Distillation
Lube oil high-vacuum units are specifically designed to produce high-quality distillate fractions
for lube oil manufacturing. Special precautions are therefore taken to prevent thermal degradation
of the distillates produced. The units are of the wet type. Normally, three sharply fractionated distillates are produced (spindle oil, light machine oil, and medium machine oil). Cut points between
those fractions are typically controlled on their viscosity quality. Spindle oil and light machine
oil are subsequently steam stripped in dedicated strippers. The distillates are further processed to
produce lubricating base oil. The short residue is normally used as feedstock for the solvent deasphalting process to produce deasphalted oil, an intermediate for bright stock manufacturing. Highvacuum units for asphalt production are designed to produce straight-run asphalt and/or feedstocks
for residuum blowing to produce blown asphalt that meets specifications. In principle, these units
are designed on the same basis as feed preparation units, which may also be used to provide feedstocks for asphalt manufacturing.
Deep cut vacuum distillation involves a revamp of the vacuum distillation unit to cut deeper into
the residue is one of the first options available to the refiner. In addition to the limits of the major
equipment, other constraints include the following: (1) the VGO quality specification required by
downstream conversion units, (2) the minimum flash zone pressure achievable, and (3) the maximum
heater outlet temperature achievable without excessive cracking. These constraints typically limit
the cut point (true boiling point) to 560°C–590°C (1040°F–1100°F), although units are designed for
cut points (true boiling point) as high as 627°C (1160°F).
17.4 EQUIPMENT
Distillation is the first step in the processing of crude oil, and it takes place in a tall steel tower
(fractionation column), the inside of which is divided at intervals by horizontal trays. The column is
kept very hot at the bottom (the column is insulated) but as different hydrocarbons boil at different
temperatures, the temperature gradually reduces toward the top, so that each tray is a little cooler
than the one below. The crude needs to be heated up before entering the fractionation column and
this is done at first in a series of heat exchangers where heat is taken from other process streams that
require cooling before being sent to rundown. Heat is also exchanged against condensing streams
from the main column.
17.4.1 Columns
Distillation columns (distillation towers) are made up of several components, each of which is used
either to transfer heat energy or to enhance material transfer. A typical distillation column consists
of several major parts:
1. A vertical shell, which houses the column internals and together with the condenser and
reboiler constitutes a distillation column (Figure 17.7), where separation of the components
is carried out
2. Column internals such as trays, or plates, or packings that are used to enhance component
separation
3. A reboilier to provide the necessary vaporization for the distillation process
4. A condenser to cool and condense the vapor leaving the top of the column
5. A reflux drum to hold the condensed vapor from the top of the column so that liquid
(reflux) can be recycled back to the column
In a petroleum distillation unit, the feedstock liquid mixture is typically introduced near to the
mid-point of the column to a tray known as the feed tray. The feed tray divides the column into a top
(enriching, rectification) section and a bottom (stripping) section. The feed flows down the column
where it is collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapor.
Distillation
Lube oil high-vacuum units are specifically designed to produce high-quality distillate fractions
for lube oil manufacturing. Special precautions are therefore taken to prevent thermal degradation
of the distillates produced. The units are of the wet type. Normally, three sharply fractionated distillates are produced (spindle oil, light machine oil, and medium machine oil). Cut points between
those fractions are typically controlled on their viscosity quality. Spindle oil and light machine
oil are subsequently steam stripped in dedicated strippers. The distillates are further processed to
produce lubricating base oil. The short residue is normally used as feedstock for the solvent deasphalting process to produce deasphalted oil, an intermediate for bright stock manufacturing. Highvacuum units for asphalt production are designed to produce straight-run asphalt and/or feedstocks
for residuum blowing to produce blown asphalt that meets specifications. In principle, these units
are designed on the same basis as feed preparation units, which may also be used to provide feedstocks for asphalt manufacturing.
Deep cut vacuum distillation involves a revamp of the vacuum distillation unit to cut deeper into
the residue is one of the first options available to the refiner. In addition to the limits of the major
equipment, other constraints include the following: (1) the VGO quality specification required by
downstream conversion units, (2) the minimum flash zone pressure achievable, and (3) the maximum
heater outlet temperature achievable without excessive cracking. These constraints typically limit
the cut point (true boiling point) to 560°C–590°C (1040°F–1100°F), although units are designed for
cut points (true boiling point) as high as 627°C (1160°F).
17.4 EQUIPMENT
Distillation is the first step in the processing of crude oil, and it takes place in a tall steel tower
(fractionation column), the inside of which is divided at intervals by horizontal trays. The column is
kept very hot at the bottom (the column is insulated) but as different hydrocarbons boil at different
temperatures, the temperature gradually reduces toward the top, so that each tray is a little cooler
than the one below. The crude needs to be heated up before entering the fractionation column and
this is done at first in a series of heat exchangers where heat is taken from other process streams that
require cooling before being sent to rundown. Heat is also exchanged against condensing streams
from the main column.
17.4.1 Columns
Distillation columns (distillation towers) are made up of several components, each of which is used
either to transfer heat energy or to enhance material transfer. A typical distillation column consists
of several major parts:
1. A vertical shell, which houses the column internals and together with the condenser and
reboiler constitutes a distillation column (Figure 17.7), where separation of the components
is carried out
2. Column internals such as trays, or plates, or packings that are used to enhance component
separation
3. A reboilier to provide the necessary vaporization for the distillation process
4. A condenser to cool and condense the vapor leaving the top of the column
5. A reflux drum to hold the condensed vapor from the top of the column so that liquid
(reflux) can be recycled back to the column
In a petroleum distillation unit, the feedstock liquid mixture is typically introduced near to the
mid-point of the column to a tray known as the feed tray. The feed tray divides the column into a top
(enriching, rectification) section and a bottom (stripping) section. The feed flows down the column
where it is collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapor.