Note: Descriptions are shown in the official language in which they were submitted.
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Recovery of solvent in hydrocarbon extraction system
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The invention relates to an improved process for the solvent
extraction of a petroleum oi~ fraction containing aromatic
and non-aromatic componen~s. In one of its more specific
aspects, the process relates to an improved method of
recovering solvent from the hydrocarbon extract in a solvent
extraction system. A considerable savings in the energy
requirements of a solvent extraction process, as compared
with processes employing conventional solvent recovery
operations, is realized by the process of this invention.
The process of the invention effects the recovery of sol-
vent from the extract phase in a plurality of separation
steps comprising at least three pressure stages.
It is well known that aromatic and unsaturated components
of a hydrocarbon oil charge stock may be separated from
the more saturated hydrocarbon components by various pro-
cesses involving solvent extraction of the aromatic and
unsaturated hydrocarbons. Foremost among the processes
which have received commercial acceptance are extraction
with furfural, N-methyl-2-pyrrolidone and phenol. The
removal of aromatics and other undesirable constituents
from lubricating oil base stocks improves the viscosity
index, color, oxidative stability, thermal stability, and
inhibition response of the base oils and the ultimate lubri-
cating oil products.
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A number of solvents are known which have an affinity for
at least one component of a mixed oil charge stock and
which are partially immiscible with an oil charge stock
under t~e temperature and pressure conditions employed in
S solvent refining the charge stock forming two liquid
phases in the extraction zone, The two liquid phases gener-
ally consist essentially of an extract phase containing the
major amount of the solvent together with dissolved aromatic
components of the charge stock and a raffinate phase con-
lQ taining non~aromatic components o~ the charge stock togetherwith minor amounts of solvent, Among the solvents which are
known to be useful for solvent extraction processing of
petroleum base lubricating oil stocks are furfural, N-methyl-
2-pyrrolidone, phenols and other various well known organic
and inorganic solvents.
Most recently N-methyl-2-pyrrolidone has displaced furfural
and phenol in importance as preferred solvents for extract-
ing aromatic hydrocarbons from mixtures of aromatic and non-
aromatic hydrocarbons. The advantages of N-methyl-2-
pyrrolidone as a lubricating oil extraction solvent for the
removal of undesirabla aromatic and polar constituents from
lubricating oil base stocks is now well recognized by re-
finers, some of whom currently use or previously used other
solvents, such as phenol or furfural for the refining of
lubricating oil base stocks. N-methyl-2-pyrrolidone is
generally the most preferred solvent because of its chemi~
cal stability, low toxicity, and its ability to produce
refined oils of improved quality~
Typical of prior art solvent extraction processes illustrat-
ing conventional solvent recovery operations are those dis-
closed in U,S. 3,329,606; 3,461,066; 3,470,089; and 4,013,549.
The process of this invention is particularly adaptable to
existing phenol, furfural and N-methyl-2~pyrrolidone refining
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installations employing a single or multiple s~age solvent
recovery system and steam or inert gas stripping of the
solvent from the products. The invention is particularly
suited to ~he conversion of furfural and phenol process
installations to N methyl-2-pyrrolidone solvent systems
with substantial savings in the energy requirements of
the solvent refining processO
In recovering a hydrocarbon extraction solvent, e.g.,
N-methyl-2-pyrrolidone, from the oll-solvent mixtures,
i.e., the extract phase and the raffinate phase, wherein
said solvent is separated from said oil-solvent mixtures
by a combination of distillation and stripping, stripping
with an iner~ gas rat}ler than with steam simplifies
solvent purification and reduces the energy requirements
of the process, as compared with conventional steam
stripping. Steam stripping is common in solvent refining
processes. Inert gas stripping has been disclosed, for
example, in U. S. 2,923,680; 4,013,549 and 4,057,491.
In conventional lubricating oil refining processes, the
solvent extraction step is carried out under conditions
effective to recover about 30 to 90 volume percent of the
lubricating oil charge as raffinate or refined oil and to
extract about 10 to 70 volume percent of the charge as an
aromatic extract. The lubricating oil stock is contacted
with a solvent, such as furfural or N-methyl-2-pyrrolidone,
at a temperature at least 5C, preferably at least 50C,
below the temperature of complete miscibility of said
lubricating oil stock in said solvent.
Particularly preferred solvents are furfural and N-methyl-
2-pyrrolidone, both of which are effective for the solvent
extraction of aromatic components from lubricating oil
charge stocks at relatively lower temperatures and lower
solvent to oil dosages than most other known solvents.
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In the ex-trac-tion step, opera-ting conditions are selected to produce a
primary raEfinate having a dewaxed viscosi-ty index of about 75 to 100, and ;l
preferably about 85 to 96. When employing furfural as sol.ven-t, extraction
temperatures within -the range of about 46 to 110 C (llS to 230F), and prefer-
ably about 60 to 95 C (140 to 205 F), wi-th solvent dosages within the range of
abou-t 100 -to 600 percent are employed in order to provide -the desired VI product.
When N-methyl-2-pyrrolidone is employed as solvent, solvent extraction tempera-
-tures wi-thin the range of 43 -to 100 C ~l.10 -to 212 F), preferably wi-thin -the
range of 54 -to 95 C (130 -to 205 F), with solvent dosages wi-thin -the range of 50
to 500 percent, and preferably within the range of 100 to 300 percent, are suit-
able. Water or wet solvent may be injected into -the bottom of the extractor or
admixed wi-th the recycled solvent to control solvent power and selectivity.
To produce a finished lubricatiny oil base stock, -the primary
raffinate is dewaxed -to the desired pour point. If desired, the refined or
dewaxed oil may be subjected to a finishing treatment for colour and s-tability
improvement, for example, mild hydrogenation.
The present i.nvention provides improvements in the methods of s-trip-
ping solven-t from the ex-tract and raffinate products, eliminating oil contamin-
ation in the solven-t, and controlling the water content of the solvent in the
solvent refining system. The process of this invention simplifies solvent
recovery and purification operations as compared with conventional processes and
effects substantial savings in the energy requiremen-ts of a solvent refining
process.
The present invention provides in a process for solvent refining a
lubricating oil feedstock wherein said lubricating oil feedstock is contacted
with a selec-tive solvent for aromatic constituents of said feedstock in an
extraction zone thereby forming a raffinate phase comprising a minor amount of
said solvent and an ex-tract phase comprising a major amount of said solvent,
said rafEinate phase is separa-ted Erom said extract phase, said solvent is
removed from each of said phases by :Elash vaporization, distillation, rectifi-
cation or a combination thereof, and residual solvent is stripped from said
extract and from said raffina-te with an inert s-tripping gas forming a mix-ture
of solvent vapor and s-tripping gas, the improvement which comprises passiny
said stripping gas containing solvent vapor into contact with fresh lubricating
oil feedstock in a countercurren-t contacting zone prior -to the introduc-tion oE
said feeds-tock to said extraction zone whereby solven-t vapors are absorbed from
said stripping gas by said feedstock and extraneous water contained in said
feedstock is at least partially vaporized to form a mixture of iner-t gas and
water vapor, and passing said mixture of inert gas and water vapor to a first
condensing zone wherein a-t least a portion of said water vapor is condensed,
separa-ting inert gas from condensed water, and passing inert ~as from which
water and solvent have been removed into con-tact wi-th said extract and said
raffina-te as said s-tripping gas for the removal of solvent therefrom.
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The accompanying drawing is a schematic flow diagram illus-
trating a solvent refining process employing a modified sol-
vent recovery operation in accordance with the process of
this invention.
Lubricating oil feedstock, which may contain extraneous
water, enters the system through line 5 and is heated in
heater 6 to a temperature within the range of 65 to 120C
(about 150 to 250F). The preheated feedstock is intro-
ducPd through line 7 into the upper part of an absorber-
stripper column 8, suitably maintained at a pressure within
the range of 100 to 415 kPa (0 to 60 psig), wherein the
feedstock is stripped of water by an inert stripping gas
entering the lower part of stripping column 8 through line 9.
Column 8 is provided with suitable means, for example,
perforated, bubble cap or cascade trays, for insuring inti-
mate countercurrent contact between the lubricating oil
feedstock and the stripping gas. Inert gas containing
water vapor is discharged from the top of column 8 through
line 10. The resulting dehydrated feedstock is withdxawn
fromthelower portion of column 8 and passed by pump 11
through heaker 12 and line 13 to the lower portion of
extraction tower 14 where it is intimately countercurrently
contacted with solvent entering the upper portion of ex-
traction tower 14 through line 17. Wet solvent from sol-
vent purification means 30 enters the bottom of extraction
tower 14 via line 99.
The raffinate mixture, comprising typically 85 percent
hydrocarbon oil admixed with solvent, i5 discharged from
the extraction tower 14 through line 19 and processed for
the recovery of raffinate from the solvent. The raffinate,
after the separation of solvent, is the solvent refined
lubricating oil base stock, i.e., the desired product of
the process.
The major portion of the solvent appears in the extract
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mixture withdrawn from the bottom of extraction tower 14.
In this example, an extract mixture comprising about 85
percent solvent is withdraw~ from tower 14 through line 18.
The extract mixture is processed first for the recovery of
solvent from the extract and then from recovery of the ex-
tract as a marketable product of the process. The ma~ox
portion of the extract mixture, typically containing about
85 percent of the solvent, is passed through heat exchangers
20 and 21 which serve to preheat the extract mixture, and
introduced into a low pressure flash tower 22. Flash tower
22 typically operates at a pressure 170 to 205 kPa (10 to
15 psig). Extract mixture from ~ower 14 is introduced into
the upper part of tower 22 as reflux through lines 31 and 32.
Solvent separated from the ex~ract in flash tower 22 is dis-
charged through line 24 to heat exchanger 20 a~d, after con-
densation of solvent vapors and further cooling in cooler 26,
the solvent is passed through line 27 to solvent purifica-
tion and storage 30 for reuse in the process.
The major portion of the extract mixture, from which part of
the solvent has been removed, is withdrawn from the lower
portion of column 22 by pump 36 and passed through heater 37
and line 38 to high pressure flash tower 39. The high pres-
sure flash tower 39 suitably is operated at a pressure within
the range of 375 to 415 kPa (40 to 45 psig). A minor por-
tion of the extract mixture ~rom the bottom of extraction
tower 14 is passed through lines 31 and 33 to the upper por-
tion of high pressure separator 34 through lines 31 and 33
as r~flux for high pressure separator 34. Alternatively,
solvent mixture from low pressure flash tower 22 may be
supplied to tower 33 and tower 22 as reflux through lines
40, 31, 32 and 33.
The solvent vapors leaving the top of high pressure flash
tower 39 through line 41 are passed through heat exchanger
21 in indirect heat exchange with the extract mixture from
the bottom of extraction tower 14, condensing the solvent
vapors and preheatlng the extract mixture prior to its
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introduction to low pressure flash tower 22. Recovered
solvent is passed through line 42 to a solvent accumulator
purification and storage 30 for reuse in the process.
The hydrocarbon oil extract withdrawn from the bottorn of
high pressure separator 39 through line 44 still contains
some solvent, or example, 5 to 15 volume percent solvent
and 9S to 85 volume percent hydrocarbons. This extrac~
mixture is passed through line 44 to vacuum flash tower 46
for the further recovery of solvent from the e~txact.
Vacuum flash tower 46 typically comprises coun~ercurrent
vapor-liquid contact trays, suitably of the cascade or
bubble-~ray type construction. A portion of the extract
mixture from extraction tower 14 or low pressure fLash
tower 2~ is supplied to the top of vacuum flash tower 46 as
reflux through lines 36 and 47~ The vacuum flash tower may
operate at a pressure within the range of 10 to 100 kPa.
In the ~acuum flash tower 46, additional separation of
extract from solvent takæs place. Solvent vapors are with-
drawn from the top of flash tower 46 through line 48 to a
condenser 49 and solvent accumulator 50. Uncondensed gases
withdrawn from accumulator 50 throu~h line 51 to a suitable
vacuum source, not illustrated, may be discarded or recir-
culated ~hrough line 86.
An extract rich fraction is withdrawn from the bottom offlash tower 46 through line 54 and introduced into the
upper portion of stripper 55. Stripper 55 is typically a
countercurrent vapor-liquid contact column provided with
bubble trays in which the liquid extract flowing downwardly
through the colu~l is contacted with inert stripping gas
introduced into the lower portion of stripper 55 through
line 56. A part of the extract mixture from the bottom of
extraction tower 14 is supplled as reflux to the upper por-
tion of stripper 55 through lines 36 and 57. Alternatively,
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a part of the extract mixture from the bottom of low pres-
sure flash tower Z2 may be supplied as reflux to towers 22,
39, 46 and 55 via line 40.
Extract oil con~aining less than about 50 parts per million
solvent, and typically comprising 80 percent unsaturated
hydrocarbons and abou~ 20 percent saturated hydrocarbons,
is withdrawn from the lower end of stripper 55 by pump 58
and passed through heat exchanger 59 where it is cooled
by indirect heat exchange with the raffinate mixture taken
ovarhead from extractor 14 and discharged from the system
through line 60 as a product o the process~
Inert stripping gas and stripped solvent vapors are dis-
charged from the upper part of stripper 55 through line 62
to condens~er 63 where solvent vapors are condensed, Sol-
vent condensate is collected in condensate accumulator 64.
Inert gas separated from tha condensate is discharged into
line 65 for recirculation to the process as described here-
inafter.
Raffinate mixture ~aken overhead from extracti.on tower 14via line 19 is heated in heat exchanger S9 by indirect heat
exchange with s~ripped extract from extract stripper 55 and
then passed through heat exchanger 67 and heater 68 prior
to introduction into vacuum flash tower 70 wherein solvent
is separated from the raffinate mixture. A minor portion
of the raffinate mixture from line 19 ~y-passes heat ex-
changers 59 and 67 and heater 68 and is introduced into
3Q the upper portion of vacuum flash tower 70 through line 71
as reflux, A further portion of the raffinate mixture from
line 19 by~passes heat exchangers 59 and 62 and heater 63
and is introduced into the upper portion of stripper 75
through line 72 as reflux.
Solvent vapors separated from the raffinate mixture in
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1ash tower 70 are withdrawn from the top of the tower to
line 48 and passed, together with solvent vapors from flash
tower 46, to condenser 49 wherein the solvent vapors are
condensed. The condensate solvent is collected in conden-
sate accumulator 50 and uncondensed gases are withdrawnthrough line 51, as explained hereinabove,
Raffinate, still containing some solvent, is withdrawn from
the lower part o~ vacuum flash tower 70 through line 74 to
the upper part of stripping column 75 wherein the residual
solvent is removed from the raffinate by stripping with
inert gas enterin~ the lower part of stripper 75 through
line 76. Raffinate, substantially free from solvent, is
withdrawn as a product of the process from the lower por-
tion of stripper 75 by pump 77, passed in indirect heatexchan~e with raffinate mixture from line 19 in heak ex-
changer 67, and discharged through line 78 as the refined
lubricating stock, the principal product of the process.
Condensates from accumulator drums 50 and 64 are passed by
pumps 79 and 80, respectively, to solvent purification and
storage system 30. Various process steps may be utiliæed
in the purification of solvent for reuse in the process,
including, for example, distillation, and azeotropic sepa-
ration, absorption, gas stripping, and the like, primarilyfor removal of excess water, if present, and for removal
of polymers, oils, and the like. Excess water from any
extraneous source may be removed from the solvent purifi-
cation and storage system 30 through line 81. Solvent is
recycled to the process by pump 82 through line 83 to line
17, as required~
Inert gas from strippers 55 and 75, after separation of
condensate solvent in condensate separator 64, may still
contain sol~ent vapors The inert gas from separator 64
is repressured by compressor 85 and passed through line 86
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to absorber-stripper 8 which serves as an absorber for sol-
vent vapors remaining in the inert gas stream, The inert
gas stream leaving compressor 85, suitably at a pressure
in the range of 170 to 310 kPa (10 to 45 psig) or higher,
depending upon the pressure in tower 8, is at an elevated
temperature due to the heat of compression in the compressor.
This gas stream may be heated or cooled as required to main-
tain the desired temperature in absorber-stripper tower 8
which is suitably at a temperature within the range of 65
to 150C ~about 150 to about 300FJo
In the absorber-stripper 8, solvent vapors are absorbed
from the inert gas, and water entering the system with the
lube oil feedstream is vaporized into the inert gas~ Inert
gas containing water vapor leaves the a~sorber-stripper
column 8 through line 10 and is cooled in condenser 88 to a
temperature sufficient to condense water vapor from the inert
gas stream. Condensate separated from the inert gas is col-
lected in condensate separator 8g from which condensate com~
prising water is discharged through line 90. Inert gas from
which water and solvent ~apors have been removed is passed
through line 91 and through heater 92 to lines 56 and 76 for
introduction into strippers 55 and 75, respectively~
The stripping gas may comprise a substantially inert gas
including, but not limited to, nitrogen, methane, carbon
dioxide, and the like. Nitrogen is a preferred inert gas
for use in the process.
30 By contacting fresh lubricating oil feedstock with inert
stripping gas previously used in the process for stripping
solvent from the extract and raffinate products, two impor-
tant advantages are obtained. Solvent is recovered from
the stripping gas and water is simultaneously removed from
the feedstock,
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In solvent refining of lubricatin~ oil stocks, there is a
tendency for light oils to accumulate in the solvent. The
accumulation of ligh~ oils in the solvent in the conven-
tional solvent refining process requires additional dis~
tillation in the solvent purification process steps to
remove the accumulated oils. These ligh~ oils usually are
carried over from the extract and raffinate flash towers
and strippers wi~h the solvent and stripping medium. In
the process of this invention, a portion of the extract
mixture from the extrac~ion tower 14 or from flash tower
2~ is employed as reflux to the flash towers 22 and 39 in
the solvent recovery section of the process In the pre-
ferred embodiment illustrated, extract mixture from the
extraction tower 14 or from the low pressure flash tower
column 22 is also utilized as reflux for the extract recov-
ery columns, i.e., flash tower 46 and stripper 55. In this
embodiment, the reflux to vacuum flash tower 65 and stripper
70 in the raffina~e recovery section o~ the process consists
of a part of the raffinate mixture taken overhead from sol-
vent extraction tower 14.
... The use of primary extract mixture from tower 14, or extract
mixture from tower 22 from which a substantial portion of
the solvent has been separated, as reflux to flash towers
22 and 39 substantially eliminates the carryover of lightoil from the towers with the separated solvent. Similarly,
in the other towers, the relatively low volatilities of the
extract fraction and of the raffinate fraction substantially
reduces or eliminates the carryover of light hydrocarbons
in the solvent vapors. At the same time, the raffinate and
extract reflux streams are good absorbents for any light
hydrocarbon oil vapors which might otherwise tend to be
carried over from the various solvent separation towers.
The combination of pretreatment of fresh lubricating oil
feedstock with the inert gas recycle and the reflux of the
various towers with extract and raffinate effectively
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eliminates or substa~ially reduces the amount of solvent
purification required for the process.
In a specific example of the process of the present
invention, 20,400 kg/hr (45,000 pounds per hour) of Wax
distillate (WD-20) containing 45 ppm ~ater is fed to an
absorber-stripper in accordance with the process o~ this
invention where it is stxipped at llS~C and 138 kPa (20
psia) with 250 kg/hr (551 pounds per hour) o~ nitrogen
recycle gas containing 12~7 kg/hr (28 pounds per hour) of
N-methyl-2-pyrrolidone vapors carried over from the
raffinate and extract strippers of a solvent refining unit
as described hereinabove. In the absorber-stripper, the
solvent contained in the recycled inert gas stream is
recovered from the inert gas. At the same time, water
contained in the lubricating oil charge stock is substan-
tially completely vaporized into the nitrogen stripping
gas.
The mixture of nitrogen and water vapor leaviny the absorber-
stripper is cooled at 50C condensing the water vapor. Con-
densate water is separated from the recycle gas stream and
the resulting dry nitrogen heated to 290C and recirculated
to the strippersi52~2 kg/hr (115 pounds per hour) of the
heated dry nitrogen is supplied to the extract stripper and
197.7 kg/hr (436 pounds per hour) to the raffinate stripper.
The raffinate and extract strippers are operated at a bottom
pressure of 35 kPa (5 psia) and an overhead pressure of
20.7 kPa (3 psia). Both strippers are refluxed with N-
methyl-2-pyrrolidone at a temperature of 20C. The extract
feed stream enters the extract stripper at 290C and the
product extract leaves the stripper at 260C. The feed to
the extract stripper contains 2,255 kg/hr (4,971 pounds per
hour) of extract and 435 kg~hr (959 pounds per hour) N-
methyl-2-pyrrolidone. 154.2 kg/hr (340 pounds pr hour)
N-methyl-2-pyrrolidone is supplied to the extract stripper
as reflux. Nitrogen stripping gas supplied to the stripper
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at 290C and 35 kPa (S psia) at the rate of 52.2 kg/hr
(115 pounds per hour) i~ disch~rged from the top of the
stripper at 160C and 20.7 kPa (3 psia) together with
589.2 kg/hr (1,299 pounds per hour) of N~methyl-2-pyrroli-
done. The overhead ~rom the extract stripper is combinedwith overhead from the raffinte stripper which comprises
lg7.8 kg/hr (436 pounds per hour) nitrogen and 836 kg/hr
(1,843 pounds per ho~r) N-methyl-2-pyrrolidone at 20.7
kPa (3 psia) and 155C. Nitrogen and N-methyl-2-pyrrolidone
vapors from the strippers are passed to a condenser where
they are cooled to 75C at 20.7 kPa (3 psia) condensing
1,425~2 kg/hr (3,142 pounds per hour) of solvent. After
the removal of the condensate solvent, the nitrogen, amount-
ing to 250 kg~hr (551 pounds per hour) and containing
12.7 kg/hr (28 pounds per hour) of solvent vapors at 75C,
is passed to a compressor where the pressure of the mix-
ture is rai~ed to 175 kPa (about 25 psia) with an increase
in temperature to 120C. The compressed mixture i~
recycled to the absorber-stripper for recovery of the
solvent vapors prior to reuse in the process.
