468
The Chemistry and Technology of Petroleum
Under these conditions, heavy gas oil may be obtained as an overhead product at temperatures of
about 150°C (300°F). Lubricating oil fractions may be obtained as sidestream products at temperatures of 250°C–350°C (480°F–660°F). The feedstock and resid temperatures are kept below the
temperature of 350°C (660°F), above which the rate of thermal decomposition increases whereupon cracking occurs (Chapters 16 and 18). The partial pressure of the hydrocarbons is effectively
reduced yet further by the injection of steam. The steam added to the column, principally for the
stripping of asphaltic constituents in the base of the column, is superheated in the convection section of the heater.
At the point, where the heated feedstock is introduced in the vacuum column (the flash zone),
the temperature should be high and the pressure as low as possible to obtain maximum distillate
yield. The flash temperature is restricted to about 420°C (790°F), however, in view of the cracking
tendency of the feedstock constituents. Vacuum is maintained with vacuum ejectors and lately also
with liquid ring pumps. In the older type high vacuum units, the required low hydrocarbon partial
pressure in the flash zone could not be achieved without the use of lifting steam that acts in a similar
manner as the stripping steam of atmospheric distillation units. This type of units is called wet units.
One of the latest developments in vacuum distillation has been the deep vacuum flashers, in which
no steam is required. These dry units operate at very low flash zone pressures and low pressure
drops over the column internals. For that reason, the conventional reflux sections with fractionation
trays have been replaced by low-pressure drop spray sections. Cooled reflux is sprayed via a number of specially designed spray nozzles in the column countercurrent to the up-flowing vapor. This
spray of small droplets comes into close contact with the hot vapor, resulting in good heat and mass
transfer between the liquid and vapors phase.
When trays similar to those used in the atmospheric column are used in vacuum distillation,
the column diameter may be extremely high, up to 45 ft. To maintain low-pressure drops across
the trays, the liquid seal must be minimal. The low holdup and the relatively high viscosity of the
liquid represent limits to the tray efficiency, which tends to be much lower than in the atmospheric
column. The vacuum is maintained in the column by removing the non-condensable gas that enters
the column by way of the feed to the column or by leakage of air.
The fractions obtained by vacuum distillation of reduced crude depend on whether the run
is designed to produce lubricating or vacuum gas oils. In the former case, the fractions include
(1) heavy gas oil, an overhead product and is used as catalytic cracking stock or, after suitable
treatment, a light lubricating oil, (2) lubricating oil (usually three fractions: light, intermediate, and
heavy), obtained as a sidestream product, and (3) residuum, the nonvolatile product that may be
used directly as asphalt or to asphalt.
The residuum may also be used as a feedstock for a coking operation or blended with gas oils to
produce a heavy fuel oil. However, if the reduced crude is not required as a source of lubricating
oils, the lubricating and heavy gas oil fractions are combined or, more likely, removed from the
residuum as one fraction and used as a catalytic cracking feedstock.
The continued use of atmospheric and vacuum distillation has been a major part of refinery
operations during this century and no doubt will continue to be employed, at least into the beginning decades of the twenty-first century, as the primary refining operation.
Three types of high-vacuum units for long residue upgrading have been developed for commercial application: (1) feedstock preparation units, (2) lube oil high-vacuum units, and (3) high-vacuum
units for asphalt production.
The feedstock preparation units make a major contribution to deep conversion upgrading and
produce distillate feedstocks for further upgrading in catalytic crackers, hydrocracking units, and
coking units. To obtain an optimum waxy distillate quality, a wash oil section is installed between
feed flash zone and waxy distillate draw-off. The wash oil produced is used as fuel component or
recycled to feed. The flashed residue (short residue) is cooled by heat exchange against long residue
feed. A slipstream of this cooled short residue is returned to the bottom of the high-vacuum column
as quench to minimize cracking (maintain low bottom temperature).
The Chemistry and Technology of Petroleum
Under these conditions, heavy gas oil may be obtained as an overhead product at temperatures of
about 150°C (300°F). Lubricating oil fractions may be obtained as sidestream products at temperatures of 250°C–350°C (480°F–660°F). The feedstock and resid temperatures are kept below the
temperature of 350°C (660°F), above which the rate of thermal decomposition increases whereupon cracking occurs (Chapters 16 and 18). The partial pressure of the hydrocarbons is effectively
reduced yet further by the injection of steam. The steam added to the column, principally for the
stripping of asphaltic constituents in the base of the column, is superheated in the convection section of the heater.
At the point, where the heated feedstock is introduced in the vacuum column (the flash zone),
the temperature should be high and the pressure as low as possible to obtain maximum distillate
yield. The flash temperature is restricted to about 420°C (790°F), however, in view of the cracking
tendency of the feedstock constituents. Vacuum is maintained with vacuum ejectors and lately also
with liquid ring pumps. In the older type high vacuum units, the required low hydrocarbon partial
pressure in the flash zone could not be achieved without the use of lifting steam that acts in a similar
manner as the stripping steam of atmospheric distillation units. This type of units is called wet units.
One of the latest developments in vacuum distillation has been the deep vacuum flashers, in which
no steam is required. These dry units operate at very low flash zone pressures and low pressure
drops over the column internals. For that reason, the conventional reflux sections with fractionation
trays have been replaced by low-pressure drop spray sections. Cooled reflux is sprayed via a number of specially designed spray nozzles in the column countercurrent to the up-flowing vapor. This
spray of small droplets comes into close contact with the hot vapor, resulting in good heat and mass
transfer between the liquid and vapors phase.
When trays similar to those used in the atmospheric column are used in vacuum distillation,
the column diameter may be extremely high, up to 45 ft. To maintain low-pressure drops across
the trays, the liquid seal must be minimal. The low holdup and the relatively high viscosity of the
liquid represent limits to the tray efficiency, which tends to be much lower than in the atmospheric
column. The vacuum is maintained in the column by removing the non-condensable gas that enters
the column by way of the feed to the column or by leakage of air.
The fractions obtained by vacuum distillation of reduced crude depend on whether the run
is designed to produce lubricating or vacuum gas oils. In the former case, the fractions include
(1) heavy gas oil, an overhead product and is used as catalytic cracking stock or, after suitable
treatment, a light lubricating oil, (2) lubricating oil (usually three fractions: light, intermediate, and
heavy), obtained as a sidestream product, and (3) residuum, the nonvolatile product that may be
used directly as asphalt or to asphalt.
The residuum may also be used as a feedstock for a coking operation or blended with gas oils to
produce a heavy fuel oil. However, if the reduced crude is not required as a source of lubricating
oils, the lubricating and heavy gas oil fractions are combined or, more likely, removed from the
residuum as one fraction and used as a catalytic cracking feedstock.
The continued use of atmospheric and vacuum distillation has been a major part of refinery
operations during this century and no doubt will continue to be employed, at least into the beginning decades of the twenty-first century, as the primary refining operation.
Three types of high-vacuum units for long residue upgrading have been developed for commercial application: (1) feedstock preparation units, (2) lube oil high-vacuum units, and (3) high-vacuum
units for asphalt production.
The feedstock preparation units make a major contribution to deep conversion upgrading and
produce distillate feedstocks for further upgrading in catalytic crackers, hydrocracking units, and
coking units. To obtain an optimum waxy distillate quality, a wash oil section is installed between
feed flash zone and waxy distillate draw-off. The wash oil produced is used as fuel component or
recycled to feed. The flashed residue (short residue) is cooled by heat exchange against long residue
feed. A slipstream of this cooled short residue is returned to the bottom of the high-vacuum column
as quench to minimize cracking (maintain low bottom temperature).