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The Chemistry and Technology of Petroleum
the ketone is removed by heating the wax and passing it into a fractional distillation tower operated at
atmospheric pressure and then into a stripper where steam removes the last traces of ketone.
An almost identical system of distillation is used to separate the filtrate into dewaxed oil and
ketone. The ketone from both the filtrate and wax slurry is reused. Clay treatment or hydrotreating finishes the dewaxed oil as previously described. The wax (slack wax), even though it contains
essentially no oil as compared to 50% in the slack wax obtained by cold pressing, is the raw material
for either sweating or wax recrystallization, which subdivides the wax into a number of wax fractions with different melting points.
Solvent dewaxing can be applied to light, intermediate, and heavy lubricating oil distillates,
but each distillate produces a different kind of wax. Each of these waxes is actually a mixture of a
number of waxes. For example, the wax obtained from light paraffin distillate consists of a series of
paraffin waxes that have melting points in the range of 30°C–70°C (90°F–160°F), which are characterized by a tendency to harden into large crystals. However, heavy paraffin distillate yields a wax
composed of a series of waxes with melting points in the range of 60°C–90°C (140°F–200°F), which
harden into small crystals from which they derive the name of microcrystalline wax or microwax.
On the other hand, intermediate paraffin distillates contain paraffin waxes and waxes intermediate
in properties between paraffin and microwax.
Thus, the solvent dewaxing process produces three different slack waxes (also known as crude
waxes or raw waxes) depending on whether light, intermediate, or heavy paraffin distillate is processed. The slack wax from heavy paraffin distillate may be sold as dark raw wax, the wax from
intermediate paraffin distillate as pale raw wax. The latter is treated with lye and clay to remove
odor and improve color.
There are several processes in use for solvent dewaxing, but all have the same general steps,
which are (1) contacting the feedstock with the solvent, (2) precipitating the wax from the mixture by chilling, and (3) recovering the solvent from the wax and dewaxed oil for recycling. The
processes use benzene-acetone (solvent dewaxing), propane (propane dewaxing), trichloroethylene (separator-Nobel dewaxing), ethylene dichloride-benzene (Barisol dewaxing), and urea (urea
dewaxing), as well as liquid sulfur dioxide-benzene mixtures.
Urea dewaxing (Scholten, 1992) is worthy of further mention insofar as the process is highly
selective and, in contrast to the other dewaxing techniques, can be achieved without the use of
refrigeration. However, the process cannot compete economically with the solvent dewaxing processes for treatment of the heavier lubricating oils. But when it is applied to the lighter materials that
already may have been subjected to a solvent dewaxing operation, products are obtained that may
be particularly useful as refrigerator oils, transformer oils, and the like.
The process description is essentially the same as that used for solvent dewaxing with the omission of the chilling stage and the insertion of a contactor where the feedstock and the urea (with a
solvent) are thoroughly mixed before filtration. The solvents are recovered from the dewaxed oil by
evaporation, and the urea complex is decomposed in a urea recovery system.
Residual lubricating oils, such as cylinder oils and bright stocks, are made from paraffinic or
mixed-base reduced crude oils and contain waxes of the microcrystalline type. Removal of these
waxes from reduced crude produces petrolatum, a grease-like material that is known in a refined
form as petroleum jelly. This material can be separated from reduced crude in several ways. The
original method was cold settling, whereby reduced crude was dissolved in a suitable amount of
naphtha and allowed to stand over winter until the microwax settled out. This method is still used,
but the reduced crude naphtha solution is held in refrigerated tanks until the petrolatum settles out.
The supernatant naphtha-oil layer is pumped to a still where the naphtha is removed, leaving cylinder stock that can be further treated to produce bright stock. The petrolatum layer is also distilled to
remove naphtha and may be clay treated or acid and clay treated to improve the color.
Another method of separating petrolatum from reduced crude is centrifuge dewaxing. In this
process, the reduced crude is dissolved in naphtha and chilled to −18°C (0°F) or lower, which causes
the wax to separate. The mixture is then fed to a battery of centrifuges, where the wax is separated
The Chemistry and Technology of Petroleum
the ketone is removed by heating the wax and passing it into a fractional distillation tower operated at
atmospheric pressure and then into a stripper where steam removes the last traces of ketone.
An almost identical system of distillation is used to separate the filtrate into dewaxed oil and
ketone. The ketone from both the filtrate and wax slurry is reused. Clay treatment or hydrotreating finishes the dewaxed oil as previously described. The wax (slack wax), even though it contains
essentially no oil as compared to 50% in the slack wax obtained by cold pressing, is the raw material
for either sweating or wax recrystallization, which subdivides the wax into a number of wax fractions with different melting points.
Solvent dewaxing can be applied to light, intermediate, and heavy lubricating oil distillates,
but each distillate produces a different kind of wax. Each of these waxes is actually a mixture of a
number of waxes. For example, the wax obtained from light paraffin distillate consists of a series of
paraffin waxes that have melting points in the range of 30°C–70°C (90°F–160°F), which are characterized by a tendency to harden into large crystals. However, heavy paraffin distillate yields a wax
composed of a series of waxes with melting points in the range of 60°C–90°C (140°F–200°F), which
harden into small crystals from which they derive the name of microcrystalline wax or microwax.
On the other hand, intermediate paraffin distillates contain paraffin waxes and waxes intermediate
in properties between paraffin and microwax.
Thus, the solvent dewaxing process produces three different slack waxes (also known as crude
waxes or raw waxes) depending on whether light, intermediate, or heavy paraffin distillate is processed. The slack wax from heavy paraffin distillate may be sold as dark raw wax, the wax from
intermediate paraffin distillate as pale raw wax. The latter is treated with lye and clay to remove
odor and improve color.
There are several processes in use for solvent dewaxing, but all have the same general steps,
which are (1) contacting the feedstock with the solvent, (2) precipitating the wax from the mixture by chilling, and (3) recovering the solvent from the wax and dewaxed oil for recycling. The
processes use benzene-acetone (solvent dewaxing), propane (propane dewaxing), trichloroethylene (separator-Nobel dewaxing), ethylene dichloride-benzene (Barisol dewaxing), and urea (urea
dewaxing), as well as liquid sulfur dioxide-benzene mixtures.
Urea dewaxing (Scholten, 1992) is worthy of further mention insofar as the process is highly
selective and, in contrast to the other dewaxing techniques, can be achieved without the use of
refrigeration. However, the process cannot compete economically with the solvent dewaxing processes for treatment of the heavier lubricating oils. But when it is applied to the lighter materials that
already may have been subjected to a solvent dewaxing operation, products are obtained that may
be particularly useful as refrigerator oils, transformer oils, and the like.
The process description is essentially the same as that used for solvent dewaxing with the omission of the chilling stage and the insertion of a contactor where the feedstock and the urea (with a
solvent) are thoroughly mixed before filtration. The solvents are recovered from the dewaxed oil by
evaporation, and the urea complex is decomposed in a urea recovery system.
Residual lubricating oils, such as cylinder oils and bright stocks, are made from paraffinic or
mixed-base reduced crude oils and contain waxes of the microcrystalline type. Removal of these
waxes from reduced crude produces petrolatum, a grease-like material that is known in a refined
form as petroleum jelly. This material can be separated from reduced crude in several ways. The
original method was cold settling, whereby reduced crude was dissolved in a suitable amount of
naphtha and allowed to stand over winter until the microwax settled out. This method is still used,
but the reduced crude naphtha solution is held in refrigerated tanks until the petrolatum settles out.
The supernatant naphtha-oil layer is pumped to a still where the naphtha is removed, leaving cylinder stock that can be further treated to produce bright stock. The petrolatum layer is also distilled to
remove naphtha and may be clay treated or acid and clay treated to improve the color.
Another method of separating petrolatum from reduced crude is centrifuge dewaxing. In this
process, the reduced crude is dissolved in naphtha and chilled to −18°C (0°F) or lower, which causes
the wax to separate. The mixture is then fed to a battery of centrifuges, where the wax is separated