540
The Chemistry and Technology of Petroleum
Thus, although a crude oil might be subjected to atmospheric distillation and vacuum distillation,
there may still be some valuable oils left in the vacuum residuum. These valuable oils are recovered
by solvent extraction, and the first application of solvent extraction in refining was the recovery
of heavy lube oil base stocks by propane (C 3 H 8 ) deasphalting. In order to recover more oil from
vacuum-reduced crude mainly for catalytic cracking feedstocks, higher molecular weight solvents,
such as butane (C 4 H 10 ), and pentane (C 5 H 12 ), have been employed.
20.2.1 deAsPHAltIng ProCess
Following either atmospheric or vacuum distillation, SDA is a separation process that takes an
additional step in the minimization of residual by-product fuel. This process takes advantage of the
fact that the maltene fraction present in residual oils is more soluble in light paraffinic solvents than
asphaltene constituents (Chapters 9 and 12). This solubility increases with solvent molecular weight
and decreases with temperature.
In the process, the feedstock is extracted with a low-boiling liquid hydrocarbon solvent in an
extraction tower. The DAO separator recovers solvent at supercritical conditions and an asphalt flash
drum also recovers solvent. Products are steam-stripped prior to further downstream processing. As
with vacuum distillation, there are constraints with respect to how deep a solvent deasphalting unit
can cut into the residue or how much DAO can be produced. In the case of SDA, the constraints are
typically: (1) the quality specifications of the DAO required by downstream conversion units and (2)
the ultimate disposition of the asphalt (Elliott and Stewart, 2004).
The deasphalting process is a mature process, but, as refinery operations evolve, it is necessary
to include a description of the process here so that the new processes might be compared with new
options that also provide for deasphalting various feedstocks. Indeed, several of these options, such
as the residuum oil supercritical extraction (ROSE) process, have been on-stream for several years
and are included here for this same reason. Thus, this section provides a one-stop discussion of
solvent recovery processes and their integration into refinery operations.
The separation of residua into oil and asphalt fractions was first performed on a production scale
by mixing the vacuum residuum with propane (or mixtures of normally gaseous hydrocarbons)
and continuously decanting the resulting phases in a suitable vessel. Temperature was maintained
within about 55°C (l00°F) of the critical temperature of the solvent, at a level that would regulate
the yield and properties of the DAO in solution and that would reject the heavier undesirable components as asphalt.
Currently, deasphalting and delayed coking are used frequently for residuum conversion. The
high demand for petroleum coke, mainly for use in the aluminum industry, has made delayed
coking a major residuum conversion process. However, many crude oils will not produce coke
meeting the sulfur and metals specifications for aluminum electrodes, and coke gas oils are less
desirable feedstocks for fluid catalytic cracking than virgin gas oils. In comparison, the SDA process can apply to most vacuum residua. The deasphalted oil is an acceptable feedstock for both
fluid catalytic cracking and, in some cases, hydrocracking. Since it is relatively less expensive to
desulfurize the deasphalted oil than the heavy vacuum residuum, the solvent deasphalting process offers a more economical route for disposing of vacuum residuum from high sulfur crude.
However, the question of disposal of the asphalt remains. Use as a road asphalt is common and
as a refinery fuel is less common since expensive stack gas clean-up facilities may be required
when used as fuel.
In this process (Figure 20.1), the feedstock is mixed with dilution solvent from the solvent
accumulator and then cooled to the desired temperature before entering the extraction tower.
Because of its high viscosity, the charge oil can neither be cooled easily to the required temperature nor will it mix readily with solvent in the extraction tower. By adding a relatively small portion of solvent upstream of the charge cooler (insufficient to cause phase separation), the viscosity
problem is avoided.
The Chemistry and Technology of Petroleum
Thus, although a crude oil might be subjected to atmospheric distillation and vacuum distillation,
there may still be some valuable oils left in the vacuum residuum. These valuable oils are recovered
by solvent extraction, and the first application of solvent extraction in refining was the recovery
of heavy lube oil base stocks by propane (C 3 H 8 ) deasphalting. In order to recover more oil from
vacuum-reduced crude mainly for catalytic cracking feedstocks, higher molecular weight solvents,
such as butane (C 4 H 10 ), and pentane (C 5 H 12 ), have been employed.
20.2.1 deAsPHAltIng ProCess
Following either atmospheric or vacuum distillation, SDA is a separation process that takes an
additional step in the minimization of residual by-product fuel. This process takes advantage of the
fact that the maltene fraction present in residual oils is more soluble in light paraffinic solvents than
asphaltene constituents (Chapters 9 and 12). This solubility increases with solvent molecular weight
and decreases with temperature.
In the process, the feedstock is extracted with a low-boiling liquid hydrocarbon solvent in an
extraction tower. The DAO separator recovers solvent at supercritical conditions and an asphalt flash
drum also recovers solvent. Products are steam-stripped prior to further downstream processing. As
with vacuum distillation, there are constraints with respect to how deep a solvent deasphalting unit
can cut into the residue or how much DAO can be produced. In the case of SDA, the constraints are
typically: (1) the quality specifications of the DAO required by downstream conversion units and (2)
the ultimate disposition of the asphalt (Elliott and Stewart, 2004).
The deasphalting process is a mature process, but, as refinery operations evolve, it is necessary
to include a description of the process here so that the new processes might be compared with new
options that also provide for deasphalting various feedstocks. Indeed, several of these options, such
as the residuum oil supercritical extraction (ROSE) process, have been on-stream for several years
and are included here for this same reason. Thus, this section provides a one-stop discussion of
solvent recovery processes and their integration into refinery operations.
The separation of residua into oil and asphalt fractions was first performed on a production scale
by mixing the vacuum residuum with propane (or mixtures of normally gaseous hydrocarbons)
and continuously decanting the resulting phases in a suitable vessel. Temperature was maintained
within about 55°C (l00°F) of the critical temperature of the solvent, at a level that would regulate
the yield and properties of the DAO in solution and that would reject the heavier undesirable components as asphalt.
Currently, deasphalting and delayed coking are used frequently for residuum conversion. The
high demand for petroleum coke, mainly for use in the aluminum industry, has made delayed
coking a major residuum conversion process. However, many crude oils will not produce coke
meeting the sulfur and metals specifications for aluminum electrodes, and coke gas oils are less
desirable feedstocks for fluid catalytic cracking than virgin gas oils. In comparison, the SDA process can apply to most vacuum residua. The deasphalted oil is an acceptable feedstock for both
fluid catalytic cracking and, in some cases, hydrocracking. Since it is relatively less expensive to
desulfurize the deasphalted oil than the heavy vacuum residuum, the solvent deasphalting process offers a more economical route for disposing of vacuum residuum from high sulfur crude.
However, the question of disposal of the asphalt remains. Use as a road asphalt is common and
as a refinery fuel is less common since expensive stack gas clean-up facilities may be required
when used as fuel.
In this process (Figure 20.1), the feedstock is mixed with dilution solvent from the solvent
accumulator and then cooled to the desired temperature before entering the extraction tower.
Because of its high viscosity, the charge oil can neither be cooled easily to the required temperature nor will it mix readily with solvent in the extraction tower. By adding a relatively small portion of solvent upstream of the charge cooler (insufficient to cause phase separation), the viscosity
problem is avoided.