551
Solvent Processes
carbon residue are sufficiently low, is a desirable feedstock for fluid catalytic cracking and hydrocracking units. Overall, the Demex process is an extension of the propane deasphalting process and
employs a less selective solvent to recover not only the high-quality oils but also high-molecularweight aromatics and other constituents present in the feedstock. Furthermore, the Demex process
requires a much less solvent circulation in achieving its objectives, thus, reducing the utility costs
and unit size significantly. The process selectively rejects asphaltenes, metals, and high-molecularweight aromatics from vacuum residua. The resulting demetallized oil can then be combined with
vacuum gas oil to give a greater availability of acceptable feed to subsequent conversion units.
In the process, the vacuum residuum feedstock, mixed with Demex solvent recycling from the
second stage, is fed to the first stage extractor. The pressure is kept high enough to maintain the
solvent in liquid phase. The temperature is controlled by the degree of cooling of the recycle solvent. The solvent rate is set near the minimum required to ensure the desired separation to occur.
Asphaltene constituents are rejected in the first stage. Some resins are also rejected to maintain sufficient fluidity of the asphaltene for efficient solvent recovery. The asphaltene is heated and steam
stripped to remove solvent. The first-stage overhead is heated by an exchange with hot solvent.
The increase in temperature decreases the solubility of resins and high-molecular-weight aromatics
(Mitchell and Speight, 1973). These precipitate in the second-stage extractor. The bottom stream of
this second-stage extractor is recycled to the first stage. A portion of this stream can also be drawn
as a separate product.
The overhead from the second stage is heated by an exchange with hot solvent. The fired heater
further raises the temperature of the solvent/demetallized oil mixture to a point above the critical
temperature of the solvent. This causes the demetallized oil to separate. It is then flashed and steam
stripped to remove all traces of solvent. The vapor streams from the demetallized oil and asphalt
strippers are condensed, dewatered, and pumped up to process pressure for recycle. The bulk of the
solvent goes overhead in the supercritical separator. This hot solvent stream is then effectively used
for process heat exchange. The subcritical solvent recovery techniques, including multiple effect
systems, allow much less heat recovery. Most of the low-grade heat in the solvent vapors from the
subcritical flash vaporization must be released to the atmosphere requiring additional heat input to
the process.
20.2.2.3 MDS Process
The MDS process is a technical improvement of the SDA process, particularly effective for
upgrading heavy crude oils (Table 20.5; RAROP, 1991, p. 95). Combined with hydrodesulfurization, the process is fully applicable to the feed preparation for fluid catalytic cracking and
hydrocracking. The process is capable of using a variety of feedstocks including atmospheric and
vacuum residua derived from various crude oils, tar sand bitumen, and nonvolatile products from
a visbreaker.
In the process, the feed and the solvent are mixed and fed to the deasphalting tower. Deasphalting
extraction proceeds in the upper half of the tower. After the removal of the asphalt, the mixture of
DAO and solvent flows out of the tower through the tower top. Asphalt flows downward to come
in contact with a countercurrent of rising solvent. The contact eliminates oil from the asphalt, the
asphalt then accumulates on the bottom. Deasphalted oil-containing solvent is heated through a
heating furnace and fed to the DAO flash tower where most of the solvent is separated under pressure. Deasphalted oil still containing a small amount of solvent is again heated and fed to the stripper, where the remaining solvent is completely removed.
Asphalt is withdrawn from the bottom of the extractor. Since this asphalt contains a small amount of
solvent, it is heated through a furnace and fed to the flash tower to remove most of the solvent. Asphalt
is then sent to the asphalt stripper, where the remaining portion of solvent is completely removed.
Solvent recovered from the DAO and asphalt flash towers is cooled and condensed into liquid and sent
to a solvent tank. The solvent vapor leaving both strippers is cooled to remove water and compressed for
condensation. The condensed solvent is then sent to the solvent tank for further recycling.
Solvent Processes
carbon residue are sufficiently low, is a desirable feedstock for fluid catalytic cracking and hydrocracking units. Overall, the Demex process is an extension of the propane deasphalting process and
employs a less selective solvent to recover not only the high-quality oils but also high-molecularweight aromatics and other constituents present in the feedstock. Furthermore, the Demex process
requires a much less solvent circulation in achieving its objectives, thus, reducing the utility costs
and unit size significantly. The process selectively rejects asphaltenes, metals, and high-molecularweight aromatics from vacuum residua. The resulting demetallized oil can then be combined with
vacuum gas oil to give a greater availability of acceptable feed to subsequent conversion units.
In the process, the vacuum residuum feedstock, mixed with Demex solvent recycling from the
second stage, is fed to the first stage extractor. The pressure is kept high enough to maintain the
solvent in liquid phase. The temperature is controlled by the degree of cooling of the recycle solvent. The solvent rate is set near the minimum required to ensure the desired separation to occur.
Asphaltene constituents are rejected in the first stage. Some resins are also rejected to maintain sufficient fluidity of the asphaltene for efficient solvent recovery. The asphaltene is heated and steam
stripped to remove solvent. The first-stage overhead is heated by an exchange with hot solvent.
The increase in temperature decreases the solubility of resins and high-molecular-weight aromatics
(Mitchell and Speight, 1973). These precipitate in the second-stage extractor. The bottom stream of
this second-stage extractor is recycled to the first stage. A portion of this stream can also be drawn
as a separate product.
The overhead from the second stage is heated by an exchange with hot solvent. The fired heater
further raises the temperature of the solvent/demetallized oil mixture to a point above the critical
temperature of the solvent. This causes the demetallized oil to separate. It is then flashed and steam
stripped to remove all traces of solvent. The vapor streams from the demetallized oil and asphalt
strippers are condensed, dewatered, and pumped up to process pressure for recycle. The bulk of the
solvent goes overhead in the supercritical separator. This hot solvent stream is then effectively used
for process heat exchange. The subcritical solvent recovery techniques, including multiple effect
systems, allow much less heat recovery. Most of the low-grade heat in the solvent vapors from the
subcritical flash vaporization must be released to the atmosphere requiring additional heat input to
the process.
20.2.2.3 MDS Process
The MDS process is a technical improvement of the SDA process, particularly effective for
upgrading heavy crude oils (Table 20.5; RAROP, 1991, p. 95). Combined with hydrodesulfurization, the process is fully applicable to the feed preparation for fluid catalytic cracking and
hydrocracking. The process is capable of using a variety of feedstocks including atmospheric and
vacuum residua derived from various crude oils, tar sand bitumen, and nonvolatile products from
a visbreaker.
In the process, the feed and the solvent are mixed and fed to the deasphalting tower. Deasphalting
extraction proceeds in the upper half of the tower. After the removal of the asphalt, the mixture of
DAO and solvent flows out of the tower through the tower top. Asphalt flows downward to come
in contact with a countercurrent of rising solvent. The contact eliminates oil from the asphalt, the
asphalt then accumulates on the bottom. Deasphalted oil-containing solvent is heated through a
heating furnace and fed to the DAO flash tower where most of the solvent is separated under pressure. Deasphalted oil still containing a small amount of solvent is again heated and fed to the stripper, where the remaining solvent is completely removed.
Asphalt is withdrawn from the bottom of the extractor. Since this asphalt contains a small amount of
solvent, it is heated through a furnace and fed to the flash tower to remove most of the solvent. Asphalt
is then sent to the asphalt stripper, where the remaining portion of solvent is completely removed.
Solvent recovered from the DAO and asphalt flash towers is cooled and condensed into liquid and sent
to a solvent tank. The solvent vapor leaving both strippers is cooled to remove water and compressed for
condensation. The condensed solvent is then sent to the solvent tank for further recycling.