Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
F-3112
PROCESS FOR DEWAXING
HEAVY DISTILLATES AND RESIDUAL LIQUIDS
The dewaxing of hydrocarbons to liquids of lower pour point
is a process of great commercial significance. The use oF
shape-selective catalysts such as ZSM-5 to selectively convert those
paraffins that contribute the most to high pour points has many
advantages over other methods.
Catalytic dewaxing of hydrocarbon oils to reduce the
temperature at which precipitation of waxy hydrocarbons occurs is a
known process and is described, for example9 in the Oil and Gas
Journal, January 6, 1975, pages 69 73. U. S. Patent No. Re 28~398
describes a process for catalytic dewaxing wi-th a catalyst
comprising ZSM~5 and a hydrogenation/dehydrogenation component. A
process for hydrodewaxing a gas oil with ZSM-5 is described in U. S.
Patent No. 3,956,102. A mordenite catalyst containing a Group VI or
Group VIII me-tal may be used to dewax a distillate from a waxy
crude, as described in U. S. Patent No. 4,100,056. U. S. Patent No.
3,755,138 describes a process for mild solvent dewaxing to remove
high quality wax from a lube stock, which is then catalytically
2~ dewaxed to specification pour point.
Catalytic dewaxing processes may be followed by other
processing steps such as hydrodesulfuriza-tion and denitrogenation in
order to improve the qualities of the product. U. S. Patent No.
3,668,113 describes a catalytic dewaxing process employing a
mordenite dewaxing catalyst which is followed by a catalytic
hydrodesulfuriza-tion step over an alumina-based catalyst. U. S.
Patent No. 4,400,265 describes a catalytic dewaxing/hydrodewaxing
process using ZSM-5 wherein gas oil is catalytically dewaxed
followed by hydrodesulfurization in a cascade systern.
o~
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In catalytic dewaxing processes using shape-selective
catalysts, such as ZSM-5, the waxy components, particularly-the
n-paraffins, are cracked by the zeolite into lighter products, such
as paraffins, olefins and aromatics, some of which remain in the
lube oil boiling range. Some lighter products are produced in the
naphtha boiling range (boiling at less than 20~C (~100F)).
Olefinic fragments are unstable to oxidation so the dewaxed oil may
be subsequen-tly hydrogenated over catalysts to saturate -the olefins
and improve the oxididation stability of the oil. The hydrogenation
catalysts generally used are mild hydrogenation catalysts, such as a
CoMo/A1203 type. The color of the oil may also be improved in
this hydrofinishing.
U. S. Patent No. 4,~28,819 to Shu et al discloses a process
for hydrofinishing a catalytically dewaxed oil in which the residual
wax content of the dewaxed oil is isomerized over a hydroisomeriza-
tion catalyst.
Typically, heavier lube fractions (boiling above 316C
(600F)) contain waxy components comprising normal paraffins,
branched paraffins and cyclo paraffins. When a shape-selective
catalyst, such as HZSM-5, is used to dewax these feeds, the normal
paraffins crack much faster than the branched paraffins and
cycloparaffins.
Recent experience with ZSM-5 based catalytic dewaxing has
shown that the dewaxing of such heavier lube Fractions pose
significantly greater problems than that experienced with lighter
feeds.
Heavier feeds cause catalysts to display a more rapicl loss
of activity. This loss oF activity results in higher catalyst aging
rates, so the reactor temperature must increaSe more rapidly.
Some of the reasons why heavier feeds are harder to process
have now been discovered. Specifically, the same degree oF pour
point reduction requires the conversion of substantially greater
proportions of branched paraffins and o-ther shape selectively
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hindered species for the heavier feeds. Thus, waxes From heavy
neutral or bright stock raffinates, for example, contain smaller
proportions of n-paraffins while light neutral derived waxes are
largely n-paraffins. Secondly, it is now found that the branched
paraffins may be classified into different groups with unique
reactivity characteristics. ~ecause of shape-selective
considerations, n-paraffins or end-methyl branched paraffins are
significantly easier to convert -than other waxes. Thirdly,
mid-methyl branched paraffins and larger sterically hindered high
molecular weight waxes, such as poly-branched paraffin waxes, are
harder to convert than n-parrafin or end-methyl branched paraffins.
In fact, the conversion of the harder to convert waxes is inhibited
by the presence of large quantities of the easier to convert waxes
and the lower molecular weight analogs (primary products) derived
from the molecular cracking of the easier to convert waxes. These
primary products appear to be able to interact with remaining high
molecular weight materials to cause rapid catalyst aging. Thus,
small amounts of easy to convert light hydrocarbons, such as
paraffins and olefins derived from primary conversion of n-paraf~ins
and end-methyl branched paraffins, substantially inhibit the
conversion of the less easily converted poly-branched and mid-methyl
branched paraFfin waxes.
It was discovered that a substantially improved dewaxing
process for heavy feeds was possible by separating the easy and
difficult conversion stages, and by removing the relatively light,
cracked products intermediate easy and difficult conversion stage.
Fig. 1 is a block flow diagram of an embodiment of the
invention showing a first stage dewaxing unit, a separatlon unit,
and a second stage dewaxing unit;
Fig. 2 shows a block flow diagram of a preferred embodiment
comprising at least one reactor in each dewaxing stage;
Fig. 3 is a block flow diagram of a second embodiment of
the invention showing a single stage dewaxing unit, a separation
unit, tankage and an intermittent recycle stream to the single
dewaxing unit;
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F-3112 ~~ 4 ~~
Fig. 4 is a block flow diagram of a third embodiment of the
invention showing a single dewaxing unit, a separation unit and a
continuous recycle stream to the single dewaxing unit;
Fig. 5 is a plot of pour point versus days on stream for 3
feeds with varying amounts of 204C- (400F-) hydrocarbons; and
Fig. 6 is a plot of pour point versus days on s-tream
comparing one-stage dewaxing -to two-stage dewaxing for a single
overall space velocity.
The present process is applicable to feedstocks, including
lube stocks, when a low wax content is desired in the final
product. This process is especially useful for feeds with pour
points higher than 21C (70F). The feeds may be whole crudes or
fractions, and may have been subjected to other refinery processes.
A feedstock 2, as shown in Fig. 1, comprising high pour
point waxy feed, passes through a preheater (not shown) and contacts
a dewaxing catalyst contained in a first stage dewaxing unit 4.
First stage dewaxing unit 4 operates at a temperature of 204 to
427C (400 to 800F), and pressure of 1,5ûO to 7,000 kPa (200 to
1000 psig), and liquid hourly space velocity (LHSV) between 0.5 and
~ hr 1. The feedstock 2 comprises n-paraffin waxes, end methyl
branched waxes, poly-branched waxes and mid-methyl branched waxes.
In the first stage dewaxing unit 4, the easily converted waxes, such
as the n-paraffin waxes and end-methyl branched waxes, are cracked
to lighter products, such as C3 gases and light paraffinic and
olefinic fragments, some of which remain in the lube oil boiling
range, but most of which are in the naphtha range. An effluen-t
stream 6 from the dewaxing unit 4 discharges into separator 8.
Separator 8 separates stream 6 into a vapor stream 10 and a liquid
stream 12. The separation may be accomplished by lowering the
3~ pressure and flashing the effluent stream 6 or by distilling the
effluent stream 6 or by allowing vapor liquid separa-tion to occur at
an elevated pressure and temperature. The separator removes those
materials boiling below 204C (400F), and preferably those boiling
below 371C (700F). The composition of the liquid s-tream 12 and
vapor strearn 10 can be adjusted by adjusting the tempera-ture and
pressure in separator 8.
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F-3112
The vapor stream 10 may be sent to downstream processing,
such as distillation, while the liquid stream 12 passes into the
second stage dewaxing unit 14 which may operate within the same
ranges of temperature and pressure specified for the first dewaxing
unit.
The relative operating conditions in the second stage
dewaxing unit 14 are preferably more severe than those of the first
stage dewaxing unit 4, to obtain a product stream 16 that meets pour
point speciFications by cracking the diFficult to convert waxes,
such as the poly-branched waxes and mid-methyl branched waxes, in
liquid stream 12. Preferably, the ratio of L~SV in the first stage
dewaxing unit 4 relative to dewaxing unit 14 will be 5:1 to about
0.5:1. The second stage dewaxing unit 14 then produces the product
stream 16 which is passed to downstream processing, such as
hydrofinishing into final product.
This promotes removal of the primary reaction products of
the cracking of n-paraffin waxes and end-methyl branched waxes from
the feed to the second stage dewaxer. The primary products inhibit
the cracking of remaining uncracked stock. It is also theorized that
the primary products react ~ith the remaining uncracked stock
because the primary reaction products are often olefins which can
cyclize and/or alkylate to heavier components in the stock. The
primary reaction products, such as light hydrocarbons, especially
naphtha boiling range products, may inhibit the reaction of the
heavier uncracked stock because they are more rapidly absorbed into
catalyst volume, thus in eFfect accelerating the measured rate oF
catalyst aging For dewaxing to the desired product.
The catalysts employed in the first and second stage
dewaxing units ~, 14 may be the same type or di-Fferent. PreFerably,
they possess shape-selective paraFfin cracking ability. Catalysts
that have shape-selective qualities include crystalline zeolite
catalysts and crystalline silica alumina phosphate (SAP0)
catalysts. These rnaterials may be bound in a variety of matrices,
F-3112 -- 6 -
such as silica alumina or silica and alumina alone. The catalysts
should contain a hydrogenation/dehydrogenation component hereafter.
The preferred hydrogenation components are the noble metals of Group
VIII, especially pla-~inum and palladiurn, but other noble metals,
such as irridium, ruthenium or rhodium, may also be used.
Combinations of noble metals with non-noble metals, such as nickel,
rhenium, tungsten, chromium and molybdenum are of interest.
Combinations of Group VIB and Group VIII are also of interest. Base
metal hydrogenation components may also be used, especially nickel,
cobalt, molybdenum, tungsten, copper or zinc. Up to 15% mekal may
be added, though usually much less noble metal promoter is needed.
The metal may be incorporated into the catalyst by any
suitable method such as impregnation or exchange onto the zeolite.
The metal may be incorporated in the form of a cationic, anionic or
a neutral complex, such as Pt(NH3)4~, and cationic complexes
of this type will be found convenient for exchanging metals onto a
zeolite. Anionic complexes are also useful for impregnating metals
into the zeolites.
A portion of zeolites useful herein are termed medium pore
zeolites and are characterized by an effective pore size of
generally less than about 7 angstroms, and/or pore windows in a
crystal formed by lû-membered rings. The medium pore zeolites
include ZSM-5, ZSM-ll, ZSM-23, ZSM-35, ZSM-38, ZSM-48 and TMA
Offretite.
Another class of zeolites important -to the present
invention are large pore zeolites. These have a pore size
suFficiently large to admit the vast majority of components normally
f`ound in a feedstock, generally in excess of 7.5 angstroms and/or
formed by 12-membered rings. The ]arge pore zeoli-tes are
represented by ZSM-4, ZSM-12, ZSM-20, Zeolite Beta, Mordeni-te, TEA
- Mordenite, Dealuminized Y, and Rare Earth Y. Additionally, the large
pore component may include a low sodium Ultrastable Y molecular
sieve (USY).
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F-3112
ZSM-4 is described in U. S. ~,923,639.
ZSM-5 is described in U. S. 3,702,886.
ZSM-ll is described in U. S. 3,709,976.
ZSM 12 is described in U. S. 3,832,449.
ZSM-20 is described in U. S. 3,972,983.
ZSM-23 is described in U. S. 4,076,842.
ZSM-35 is described in U. S. 4,016,245.
ZSM~38 is described in U. S. 4,046,859.
ZSM-48 is described in U. S. 4,397,827.
Zeolite Beta is described in U. S. 3,308,069 and Re. 28,341.
USY is described in U. S. 3,293,192 and 3,449,070.
The first and second stage dewaxing units 4, 14
respectively, may be part of the same reactor, may be in separate
vessels, or may each consist of a plurality of vessels. A preferred
embodiment, as shown in Fig. 2, comprises a series of two or more
reactors 40, 42, 44, 46 and 48 for each stage. As shown in Fig. 2,
the first stage dewaxing unit 4 comprises two reactors 40, 42, and
the second stage dewaxing unit 14 comprises two other reactors 44,
46. However, the units 4, 14 could operate with only one reactor
per unit. The reactors 40, 42, 44 and 46 contain dewaxing catalysts
and are operated in series. While the reactors 40, 42, 44 and 46
are in operation, the catalyst in the remaining reactor 48 could be
reactivated/ regenerated. A solid line, shown in Fig. 2, represents
the path of hydrocarbons through the reactors 40, 42, 44 and 46. A
feedstock passes into the first reactor 40 to produce an outlet
stream 50 which passes into the second reactor 42. The outlet
stream 52 from the second reactor 42 becomes an effluent stream 6
which passes into the separation unit 8 to form a vapor stream 10
and a liquid stream 12. The liquid stream 12 passes into the third
reactor 44 to produce an outlet stream 54 which passes into the
fourth reactor 46 to produce an outlet stream 56 which forms the
product stream 16.
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F-3112 -- 8 --
As shown by Fig. 27 headers 24, 26 and 28 are provided so
that the reactors may be rotated, thus allowing for on-line
reactivation/regeneration of catalysts in any one of the reactors.
The headers 24, 26 and 28 allow flashing or distillation between any
two or the reactors ~0, 42, 44, 46 and 48. The feedstock 2 feeds
the feed header 24 which is attached to each of the reactors 40, 42,
44, 46 and 48. The outlets 50, 521 54, 56 and 58 from each reactor
40, 42, 44, 46 and 48, respectively, can feed either the product
header 28 or the separation unit header 26. The effluent 6 from the
first stage dewaxing unit passes into the separation header 26 and
subsequently into the separation unit 8. The product stream 16
passes into the product header 28 and subsequently to downstream
processing. This arrangement will have an added benefit because
catalysts that age far enough to be unsuitable for use in the more
severe second stage dewaxing unit 14 could be switched to duty in
the first stage dewaxing unit 4, allowing another reactor with its
catalyst charge to be freed for rotational reactivation/
regeneration. Appropriate valving (not shown) would be provided to
direct flow to the correct headers and units.
Fig. 3 shows an alternative embodiment of the invention, in
which a feedstock 2 passes into a dewaxing unit 30 under the first
set of operating conditions outlined above. The effluent 6 then
passes to a separation unit to form a vapor stream 10 and a liquid
stream 12. The liquid stream 12 is then stored in tankage 18, such
as any suitable tankage storage area located on a plant site. Then,
after all of the feedstock 2 has been run through the dewaxing unit
30, alternately to catalytic dewaxing of the feedstock 2, an
effluent 20 comprising the hydrocarbons from the liquid stream 12 is
catalytically dewaxed in the dewaxing unit 30, which operates at the
second set oF operating conditions outlined above. The effluent 6
would then form a product stream 16 which passes to downstream
processing.
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F-3112 9
An alternate embodiment of the invention is shown in Fig.
4, in which a feedstock 2 and a recycle stream 22 are catalytically
dewaxed in a dewaxing unit 30 to produce an effluent 6. The
dewaxing unit 30 temperature is 204 to ~126C (400F to 800F),
pressure is 1,500 to 7,000 kPa (200 to 1000 psig), LHSV is 0.25 to 5
hr~l, based on feedstock 2, and the recycle ratio of recycle
stream 22 to feedstock 2 is 0.5 to 20. The dilution of fresh feed
by once processed and partially dewaxed stock ensures that the level
of light hydrocarbons, developed as primary products, present in the
reactor will be substantially reduced. The effluent 6 passes to a
separation unit 8 to form the liquid stream 12 and the vapor stream
10. The liquid stream 12 is then separated into the product stream
16 and the recycle stream 22 is combined with the feedstock 2 and
recycled to the dewaxing unit 30. It should be understood that in
all of the above embodiments catalytic dewaxing may occur in the
presence or absence of added hydrogen.
By dewaxing hydrocarbons using dewaxing units under two
sets of conditions, dewaxing may be accomplished by an easy
conversion step and a relatively more difficult conversion step. By
separating the easy and difficult conversion steps one can control
the temperature in the two steps, allowing significantly lower
overall aging rates and thus, higher capacity factors. Separating a
vapor stream from a dewaxing unit effluent prior to a second pass
over dewaxing catalyst removes components which inhibit Further
dewaxing and accelerate catalyst aging.
Examples
Laboratory tests, described below, were conducted on a
bright stock comprising a furfural extracted, propane deasphalted
vacuum resid having the -Following properties:
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F-3112 -- 10 --
TABLE 1
~right
Properties Stock
Specific Gravity 0.90
Viscosity
KV at 100C 29.71
KV at 149C (300F) 9.31
Aniline Point F/C 251/122
Elemental Analysis, Wt. %
Carbon 85.53
Hydrogen 13.16
Sulfur 1.31
Nitrogen (ppm) 130
Basic Ni (ppm) 100
ParaFfins 18.8
Naphthenes 42.0
Aromatics 39.2
Distillation
W _ F C_
Initial 800 427
912 489
944 507
998 537
1030 554
__
_ _
__
The tests were conducted at a constant space velocity, pressure and
temperature and record the chanye in pour point versus the number oF
days on stream. Such comparisons allow both an es-timate oF dewaxing
ability and aging rates to be determined simultaneously. Example 1,
shown in Fig. 5, is a plot oF pour point versus days on stream and
F-3112 -- 11 --
compares feed run by itself over catalyst against feed run, with 3
and 6~ added hydrocarbons boiling below 204C (400F). These were
mixtures of paraffins boiling in the naphtha range, i.e., liquids
boiling below 204C. These tests were run at a temperature of 354C
(670F), a pressure of 2,900 kPa (400 psig) and a space velocity of
1 hr 1. As explicitly shown in Fig. 5, the light hydrocarbons
caused an inhihition of catalyst activity and acceleration of the
aging rate. These results demonstrate that small proportions of
easily cracked and diffusionally favored light hydrocarbons can have
significant inhibitive and deactiva-ting effects on the catalyst
relative to the rnain process objective, which is the conversion of
waxes in heavier hydrocarbons to produce a product with a desired
pour point speciFication.
Example 2, shown in Fig. 6, compares the pour point versus
days on stream at constant space velocity and temperature of a feed
dewaxed in a single dewaxing stage as opposed to a feed dewaxed at
the same overall space velocity, temperature and pressure in two
stages with separation and removal of a vapor stream in between the
two stages. The test was run at 299C (570F), pressure of 2,900
kPa (400 psig) and 1.0 LHSV for the single stage unit and 2.0 LHSV
per stage for the two-stage units. Both the one-stage and two-stage
units contain the same amount of catalyst. The catalyst was a
Ni-ZSM-5. The separation was a laboratory fractionation, to remove
204C (400F) and lighter rnaterial from the liquid feed to the
second stage. This shows lower pour points and extended catalyst
life when two stages and a separation s-tage are used.
