546
The Chemistry and Technology of Petroleum
above the foaming level and to keep the asphalt phase in a fluid state. A flash drum is used to
separate the solvent vapor from asphalt with the design being such to prevent carryover of asphalt
into the solvent outlet line and to avoid fouling the downstream solvent condenser. The solvent
recovery system from asphalt is not usually subject to the same degree of variations as the solvent
recovery system for the DAO and operation at constant temperature and pressure with a separate
solvent condenser and accumulator is possible.
Asphalt from different crude oils varies considerably, but the viscosity is often too high for fuel
oil although, and in some cases, they can be blended with refinery cutter stocks to make No. 6
fuel oil. When the sulfur content of the original residuum is high, even the blend fuel oil will not be
able to meet the sulfur specification of fuel oil unless stack gas cleanup is available.
The DAO and solvent asphalt are not finished products and require further processing or blending,
depending on the final use. Manufacture of lubricating oil is on possibility, and the DAO may also be
used as a catalytic cracking feedstock or it may be desulfurized. It is perhaps these last two options
that are more pertinent to the present text and future refinery operations.
Briefly, catalytic cracking or hydrodesulfurization of atmospheric and vacuum residua from
high-sulfur/high-metal crude oil is, theoretically, the best way to enhance their value. However,
the concentrations of sulfur (in the asphaltene fraction) in the residua can severely limit the performance of cracking catalysts and hydrodesulfurization catalysts (Speight, 2000). Both processes
generally require tolerant catalysts as well as (in the case of hydrodesulfurization) high hydrogen
pressure, low space velocity, and high hydrogen recycle ratio.
For both processes, the advantage of using the deasphalting process to remove the troublesome
compounds becomes obvious. The DAO, with no asphaltene constituents and low metal content,
is easier to process than the residua. Indeed, in the hydrodesulfurization process, the DAO may
consume only 65% of the hydrogen required for direct hydrodesulfurization of topped crude oil.
As always, the use of the material rejected by the deasphalting unit remains an issue. It can be
used (apart from its use for various types of asphalt) as feed to a partial oxidation unit to make
a hydrogen-rich gas for use in hydrodesulfurization processes and hydrocracking processes.
Alternatively, the asphalt may be treated in a visbreaker to reduce its viscosity thereby minimizing
the need for cutter stock to be blended with the solvent asphalt for making fuel oil. Or, hydrovisbreaking offers an option of converting the asphalt to feedstocks for other conversion processes.
SDA has the advantage of being a relatively low-cost process that has the flexibility to meet a
wide range of quality of the DAO. The process has very good selectivity for asphaltene constituents and metals rejection, some selectivity for rejection of coke formers, but less selectivity for
sulfur and nitrogen. The process is best suited for the more paraffinic vacuum residues as opposed
to vacuum residues with high asphaltene content, high metal, and high carbon content of cokeforming constituents. The disadvantages of the separation process are that it performs no conversion and produces the high-viscosity byproduct asphalt (deasphalter bottoms, SDA pitch). When
high-quality DAO is required, the SDA process is limited in the quality of feedstock that can be
economically processed.
The economics of the process are highly dependent on the refinery’s ability to further upgrade
the DAO and the differential between the value of the cutter stocks and the price of high-sulfur
residual fuel oil. In those situations, where there is an economic outlet for the pitch and conversion capacity exists to upgrade the DAO, SDA can be a highly attractive option for the refiner.
One such economic outlet is cogeneration of steam and power using pitch, both to supply the
refiner’s needs and for export to nearby users. Other applications include gasification of the
deasphalter bottoms for hydrogen and/or power generation, and road asphalt production (Elliott
and Stewart, 2004).
Lower boiling solvents and higher solvent ratios precipitate larger quantities of resin constituents
and asphaltene constituents, thereby providing separation of these feed components from saturates
and aromatics components. Low-boiling liquid paraffin solvents show extraction selectivity not only
The Chemistry and Technology of Petroleum
above the foaming level and to keep the asphalt phase in a fluid state. A flash drum is used to
separate the solvent vapor from asphalt with the design being such to prevent carryover of asphalt
into the solvent outlet line and to avoid fouling the downstream solvent condenser. The solvent
recovery system from asphalt is not usually subject to the same degree of variations as the solvent
recovery system for the DAO and operation at constant temperature and pressure with a separate
solvent condenser and accumulator is possible.
Asphalt from different crude oils varies considerably, but the viscosity is often too high for fuel
oil although, and in some cases, they can be blended with refinery cutter stocks to make No. 6
fuel oil. When the sulfur content of the original residuum is high, even the blend fuel oil will not be
able to meet the sulfur specification of fuel oil unless stack gas cleanup is available.
The DAO and solvent asphalt are not finished products and require further processing or blending,
depending on the final use. Manufacture of lubricating oil is on possibility, and the DAO may also be
used as a catalytic cracking feedstock or it may be desulfurized. It is perhaps these last two options
that are more pertinent to the present text and future refinery operations.
Briefly, catalytic cracking or hydrodesulfurization of atmospheric and vacuum residua from
high-sulfur/high-metal crude oil is, theoretically, the best way to enhance their value. However,
the concentrations of sulfur (in the asphaltene fraction) in the residua can severely limit the performance of cracking catalysts and hydrodesulfurization catalysts (Speight, 2000). Both processes
generally require tolerant catalysts as well as (in the case of hydrodesulfurization) high hydrogen
pressure, low space velocity, and high hydrogen recycle ratio.
For both processes, the advantage of using the deasphalting process to remove the troublesome
compounds becomes obvious. The DAO, with no asphaltene constituents and low metal content,
is easier to process than the residua. Indeed, in the hydrodesulfurization process, the DAO may
consume only 65% of the hydrogen required for direct hydrodesulfurization of topped crude oil.
As always, the use of the material rejected by the deasphalting unit remains an issue. It can be
used (apart from its use for various types of asphalt) as feed to a partial oxidation unit to make
a hydrogen-rich gas for use in hydrodesulfurization processes and hydrocracking processes.
Alternatively, the asphalt may be treated in a visbreaker to reduce its viscosity thereby minimizing
the need for cutter stock to be blended with the solvent asphalt for making fuel oil. Or, hydrovisbreaking offers an option of converting the asphalt to feedstocks for other conversion processes.
SDA has the advantage of being a relatively low-cost process that has the flexibility to meet a
wide range of quality of the DAO. The process has very good selectivity for asphaltene constituents and metals rejection, some selectivity for rejection of coke formers, but less selectivity for
sulfur and nitrogen. The process is best suited for the more paraffinic vacuum residues as opposed
to vacuum residues with high asphaltene content, high metal, and high carbon content of cokeforming constituents. The disadvantages of the separation process are that it performs no conversion and produces the high-viscosity byproduct asphalt (deasphalter bottoms, SDA pitch). When
high-quality DAO is required, the SDA process is limited in the quality of feedstock that can be
economically processed.
The economics of the process are highly dependent on the refinery’s ability to further upgrade
the DAO and the differential between the value of the cutter stocks and the price of high-sulfur
residual fuel oil. In those situations, where there is an economic outlet for the pitch and conversion capacity exists to upgrade the DAO, SDA can be a highly attractive option for the refiner.
One such economic outlet is cogeneration of steam and power using pitch, both to supply the
refiner’s needs and for export to nearby users. Other applications include gasification of the
deasphalter bottoms for hydrogen and/or power generation, and road asphalt production (Elliott
and Stewart, 2004).
Lower boiling solvents and higher solvent ratios precipitate larger quantities of resin constituents
and asphaltene constituents, thereby providing separation of these feed components from saturates
and aromatics components. Low-boiling liquid paraffin solvents show extraction selectivity not only