545
Solvent Processes
the oil-rich phase. When the operating temperature is near the critical temperature of the solvent,
control of the extraction tower becomes difficult, since the rate of change of solubility with temperature becomes very large at conditions close to the critical point of the solvent. Such changes in
solubility cause large amounts of oil to transfer between the solvent-rich and the oil-rich phases that,
in turn, causes flooding and/or uncontrollable changes in product quality. To mitigate such effects,
the upper limits of operable temperatures must lie below the critical temperature of the solvent in
order to insure good control of the product quality and to maintain a stable condition in the extraction tower.
The temperature gradient across the extraction tower influences the sharpness of separation of
the DAO and the asphalt because of internal reflux that occurs when the cooler oil/solvent solution
in the lower section of the tower attempts to carry a large portion of oil to the top of the tower. When
the oil/solvent solution reaches the steam-heated, higher temperature area near the top of the tower,
some oil of higher molecular weight in the solvent solution is rejected because the oil is less soluble
in solvent at the higher temperature. The heavier oil (rejected from the solution at the top of the
tower) attempts to flow downward and causes the internal reflux. In fact, generally, the greater the
temperature difference between the top and the bottom of the tower, the greater will be the internal
reflux, and the better will be the quality of the DAO. However, too much internal reflux can cause
tower flooding and jeopardize the process.
The process pressure is usually not considered to be an operating variable, since it must be
higher than the vapor pressure of the solvent mixture at the tower operating temperature to maintain
the solvent in the liquid phase. The tower pressure is usually only subject to change when there is a
need to change the solvent composition or the process temperature.
Proper contact and distribution of the oil and solvent in the tower are essential to the efficient
operation of any deasphalting unit. In early units, days, mixer-settlers were used as contactors but
proved to be less efficient than the countercurrent contacting devices. Packed towers are difficult to
operate in this process because of the large differences in viscosity and density between the asphalt
phase and the solvent-rich phase.
The extraction tower for SDA consists of two contacting zones: (1) a rectifying zone above the
oil feed and (2) a stripping zone below the oil feed. The rectifying zone contains some elements
designed to promote contacting and to avoid channeling. Steam-heated coils are provided to raise
the temperature sufficiently to induce an oil-rich reflux in the top section of the tower. The stripping
zone has disengaging spaces at the top and bottom and consists of contacting elements between the
oil inlet and the solvent inlet.
A countercurrent tower with static baffles is widely used in SDA service. The baffles consist of
fixed elements formed of expanded metal gratings in groups of two or more to provide maximum
change of direction without limiting capacity. The rotating disk contactor has also been employed
and consists of disks connected to a rotating shaft are used in place of the static baffles in the tower.
The rotating element is driven by a variable speed drive at either the top or the bottom of the column
and operating flexibility is provided by controlling the speed of the rotating element and, thus, the
amount of mixing in the contactor.
In the deasphalting process, the solvent is recovered for circulation and the efficient operability
of a deasphalting unit is dependent on the design of the solvent recovery system.
The solvent may be separated from the DAO in several ways such as conventional evaporation or the use of a flash tower. Irrespective of the method of solvent recovery from the DAO, it
is usually the most efficient to recover the solvent at a temperature close to the extraction temperature. If a higher temperature for solvent recovery is used, heat is wasted in the form of high
vapor temperature, and, conversely, if a lower temperature is used, the solvent must be reheated
thereby requiring additional energy input. The solvent recovery pressure should be low enough
to maintain a smooth flow under pressure from the extraction tower.
The asphalt solution from the bottom of the extraction tower usually contains less than an equal
volume of solvent. A fired heater is used to maintain the temperature of the asphalt solution well
Solvent Processes
the oil-rich phase. When the operating temperature is near the critical temperature of the solvent,
control of the extraction tower becomes difficult, since the rate of change of solubility with temperature becomes very large at conditions close to the critical point of the solvent. Such changes in
solubility cause large amounts of oil to transfer between the solvent-rich and the oil-rich phases that,
in turn, causes flooding and/or uncontrollable changes in product quality. To mitigate such effects,
the upper limits of operable temperatures must lie below the critical temperature of the solvent in
order to insure good control of the product quality and to maintain a stable condition in the extraction tower.
The temperature gradient across the extraction tower influences the sharpness of separation of
the DAO and the asphalt because of internal reflux that occurs when the cooler oil/solvent solution
in the lower section of the tower attempts to carry a large portion of oil to the top of the tower. When
the oil/solvent solution reaches the steam-heated, higher temperature area near the top of the tower,
some oil of higher molecular weight in the solvent solution is rejected because the oil is less soluble
in solvent at the higher temperature. The heavier oil (rejected from the solution at the top of the
tower) attempts to flow downward and causes the internal reflux. In fact, generally, the greater the
temperature difference between the top and the bottom of the tower, the greater will be the internal
reflux, and the better will be the quality of the DAO. However, too much internal reflux can cause
tower flooding and jeopardize the process.
The process pressure is usually not considered to be an operating variable, since it must be
higher than the vapor pressure of the solvent mixture at the tower operating temperature to maintain
the solvent in the liquid phase. The tower pressure is usually only subject to change when there is a
need to change the solvent composition or the process temperature.
Proper contact and distribution of the oil and solvent in the tower are essential to the efficient
operation of any deasphalting unit. In early units, days, mixer-settlers were used as contactors but
proved to be less efficient than the countercurrent contacting devices. Packed towers are difficult to
operate in this process because of the large differences in viscosity and density between the asphalt
phase and the solvent-rich phase.
The extraction tower for SDA consists of two contacting zones: (1) a rectifying zone above the
oil feed and (2) a stripping zone below the oil feed. The rectifying zone contains some elements
designed to promote contacting and to avoid channeling. Steam-heated coils are provided to raise
the temperature sufficiently to induce an oil-rich reflux in the top section of the tower. The stripping
zone has disengaging spaces at the top and bottom and consists of contacting elements between the
oil inlet and the solvent inlet.
A countercurrent tower with static baffles is widely used in SDA service. The baffles consist of
fixed elements formed of expanded metal gratings in groups of two or more to provide maximum
change of direction without limiting capacity. The rotating disk contactor has also been employed
and consists of disks connected to a rotating shaft are used in place of the static baffles in the tower.
The rotating element is driven by a variable speed drive at either the top or the bottom of the column
and operating flexibility is provided by controlling the speed of the rotating element and, thus, the
amount of mixing in the contactor.
In the deasphalting process, the solvent is recovered for circulation and the efficient operability
of a deasphalting unit is dependent on the design of the solvent recovery system.
The solvent may be separated from the DAO in several ways such as conventional evaporation or the use of a flash tower. Irrespective of the method of solvent recovery from the DAO, it
is usually the most efficient to recover the solvent at a temperature close to the extraction temperature. If a higher temperature for solvent recovery is used, heat is wasted in the form of high
vapor temperature, and, conversely, if a lower temperature is used, the solvent must be reheated
thereby requiring additional energy input. The solvent recovery pressure should be low enough
to maintain a smooth flow under pressure from the extraction tower.
The asphalt solution from the bottom of the extraction tower usually contains less than an equal
volume of solvent. A fired heater is used to maintain the temperature of the asphalt solution well