Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
Field Of The Invention
The invention relates to an improvement in the treat-
ment of hydrocarbon distillates, more particularly to an
improved method of sweetening sour hydrocarbon distillates by
oxidizing mercaptans in the distillate to disulfides in the
presence of a phthalocyanine catalyst on a charcoal carrier
in the presence of a basic medium and oxygen.
Prior Art
Sweetening of sour hydrocarbons is well known in the
petroleum refining arts. Processes abound relating to the
10 treatment of petroleum distillates such as sour gasoline,
cracked gasoline, straight run gasoline, naphtha, jet fuel,
kerosene, fuel oil, etc.
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The prime offender in many hydrocarbon dis-
tillates is mercaptan sulfur, RS~. Mercaptan sulfur can be
successfully removed by hydrotreating, using a catalyst con-
taining Co, Mo, etc., on a carrier such as alumina, at high
temperatures under high hydrogen pressures. This hydrotreating
will convert mercaptan sulfur to H2S which can be removed from
normally liquid hydrocarbon fractions by distillation.
Hydrotreating is relatively expensive, and
many pertoleum products can contain relatively high sulfur levels,
as long as the sulfur is not in the form of a mercaptan. The
mercaptans are objectionable because of their strong odor, and
because they are more corrosive. For many processes, it is ~ -
sufficient if the mercaptans are converted to disulfides, RSSH,
or RSSR.
A process for the fixed bed sweetening or hydro-
carbon distillates is shown in U.S. Patent 2,988,500 (Gleim et al.,
June 13, 1961). In this patent, a novel catalyst was used to
oxidize mercaptans to disulfides. The novel catalyst disclosed
in this patent was cobalt phthalocyanine sulfonate composited `~
with a charcoal carrier. A mixture of sour kerosene, aqueous
NaOH solution, and air were passed over the catalyst to convert
mercaptan sulfur to a level low enough that the kerosene product
recovered would be doctor sweet. The treating reaction was
effected in the presence of an alkaline reagent. The patentee
taught that any suitable alkaline reagent could be used, and
taught that the preferred reagents were sodium hydroxide and
potassium hydroxide. Other reagents considered possible were
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aqueous solutions of lithium hydxoxide, rubidium hydroxide, and
cesium hydro~ide,
Another treating process was disclo~ed in U.S.
Patent 2,744,854 (P. Urban, Jr., May 8, 1956). In this patent,
a similar sweetening process occurred, but the sweetening
reaction was always accomplished in storage tanks, rather than
in a reactor vessel. Thus, reaction times of several days
would be necessary to complete the conversion of mercaptan
sulfur to disulfides. There is extremely detailed and broad
teaching in this patent as to the type of basic reagent which
may be used to facilitate the sweetening reaction. Both organic
and inorganic bases are taught, though from the examples, use of
a phenylene diamine is preferred. Optionally, a metal chelate
may be added to speed up the sweetening which occurs in the
storage tank. In the specific teachings on basic compounds
which may be used in addition to sodium hydroxide or potassium
hydroxide, the patentee teaches over 50 different compounds and
classes of compounds which serve as basic reagents.
Sweetening of hydrocarbon distillates by placing
them in stora~e tanks is, in general, not the preferred way to
convert mercaptans to disulfides. Refineries prefer to use a
more positive treating step to obtain a doctor sweet product.
The fixed bed sweetening process has enjoved worldwide
commercial success. Despite the great acceptance of fixed
bed sweetening by refining industry, there are still a few
areas in which
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attempts have been made to improve the process. Specifically,
the practice of using aqueous sodium hydroxide solutions to
provide the basic medium required for oxidizing mercaptans to
disulfides has resulted in a caustic disposal problem. Even-
tually the caustic solution used in a fixed bed unit becomes
unsuitable for further use. Most common reason for discard-
ing of caustic solutions is that various toxins or catalyst
poisons, generated by the oxidation reaction, accumulate in the
caustic. Thus, for a number of reasons the caustic commonly
~ ~ used in fixed bed sweetening processes must be discarded. Al-
though sodium hydroxide is a very inexpensive chemical to buy,
it is becoming a relatively expensive chemical to throw away,
because of concern about pollution.
~ Also of concern to refiners is the danger that some
of the caustic solution will somehow find its way into the
flnal product. For some uses, e.g., jet fuel, neither sodium
hydroxide nor water may be tolerated in the product. Elabor-
ate measures are taken to make sure that the kerosene product
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destined for use as jet fuel will not contain either water or
NaOH. The solution commonly used is to water-wash the kerosene
effluent from the fixed bed treating process to remove sodium
hydroxide solution. The water-washed kerosene is then passed
through a bed of salt, so that the salt will react with any
water contained in the hydrocarbon, and form a brine which will
remain behind. Finally, the kerosene is passed through a bed
of clay or sand to remove ~he last traces of water or brine
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solution which may be in the product. Although effective, such
elaborate measures add to the cost oE treating and increase
; the ca~ital expenditure required to build a plant for the treat-
ing of fuels where the presence of small amounts of aqueous
sodium hydroxide solutions is objectionable.
O~her probl~ms which have been encolmtered in -the fixed
bed sweetening process are the occasional plugging of the cata-
lyst bed due to formation of soaps. A number of hydrocarbon
distillates contained naphthenic acids, and the naphthenic acids
0 reacted with aqueous sodium hydroxide to form a soap which forms
a gel with the hydrocarbon which in turn plugged the charcoal
bed. It has been necessary to put in caustic prewashes to
remove these naphthenic acids from feeds containing them, so
that the feed to the fixed bed sweetening unit will;be substan-
,
tially free of naphthenic acids. The typical naphLhenic acid
prewash is a large vessel filled with a dilute solution of
sodium hydroxide. While such a vessel is efficient, and rela-
; tively inexpensive, it still adds to the cost of operating a
fixed bed treating process.
Because of these difficulties encountered with some
feedstocks, and some product specifications, I tried to find
some way to eliminate these problems entirely, rather than add
on an extra step upstream or downstream of existing fixed bed
treating units. ~Iy investigation showed that most of the
problems were caused by either something in the feed reacting
with the aqueous sodium hydroxide solution used as a basic
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medium, or caused by remnants of the basic medium appearing
in the product. I discovered a replacement for the sodium
hydroxide solutions currently used in fixed bed treating pro-
cesses. The replacement provided a uniquely satisfactory sub-
stitute for customarily used basic solutions. The material
I discovered was tetra-alkyl guanidines.
E~RIEF SUMMARY OF THE INVENTION
Accordingly the present invention provides in a
process for trea~ing a sour hydrocarbon distillate containing
mercaptans by passing the distillate and an oxidizing agent
over a fixed bed of a phthalocyanine catalyst composited with
a carbon carrier in the presence of an alkaline medium, the
improvement which comprises use of a tetra-alkyl guanidine
as the alkaline medium.
In addition to eliminating some of the problems
caused by the prior art sodium hydroxide solutions, I found
that there was an unexpected benefit obtained by using tetra-
alkyl guanidine as a basic medium. This benefit was an unex-
pected and surprising increase in apparent catalytic activity
of the fixed bed sweetening unit. The use of tetra-alkyl
guanidine permitted significantly improved mercaptan conver-
sion to be effected in a fixed bed sweetening process. Use
of tetra-alkyl guanidines is also beneficial in that the quan-
idines remain in the hydrocarbon phase and pass into storage
tanks used for the hydrocarbons. The guanidines act to suppress
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color degradation in storage, and may also act as a corrosion
inhibitor. Further, the guanidines do not change the color
of the hydrocarbon product, this is contrast to some of the
phenylene diamines which impart a red color to the product.
D AILED DESCRIPTION
An excellent method of carrying out the fixed
bed of hydrocarbons is disclosed in U.S. Patent 2,988,500,
previously mentioned. All thin~s taught in this patent can be
used to good effect in practicing the present invention, with the
substitution of a tetra-alkyl guanidine for the alkaline reagent
of that patent.
The tetra-alkyl guanidine is preferably tetra-
methyl guanidine. Instead of four methyl groups, four ethyl,
propyl, butyl, etc., groups may be used, or guanidines containing
alkyl groups of varying chain lengths can also be used. Tetra-
methyl guanidine is preferred because it is readily available
and inexpensive. Another advantage of the tetramethyl guanidine
is that it can react with light and heavy naphthenic acids,
phenols, etc., without forming soap-like salts. The reaction
product of the guanidine and the naphthenic acids is soluble in
hydrocarbon medium, so it does not plug-up the catalyst bed.
This is in contrast to the reaction product of naphthenic acids
with sodium hydroxide in aqueous solution, which forms soaps
and gels which completely plug-up and render ineffective a
catalyst bed. Also, emulsion problems are eliminated because
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the sodium salts are eliminated. Emulsions cause water to be
carried into storage tanks causing an excess of water in the
tank. These soaps can also carry sodium and water into the
finished product whichare not desirable.
Because of the vast number and variety of crude stocks
which are being treated, it may be desirable to use heavier
alkyl guanidines to treat very heavy charge stocks. In general,
the longer the alkyl groups the more soluble will be the guani-
dine in the hydrocarbon. My experiments have shown, however,
that even the lightest of the tetra-alkyl guanidines can do a
very effective sweetening job at such low concentrations that
it is completely soluble in the hydrocarbon oil treated, such
as a kerosene.
The concentration of the tetra-alkyl guanidine should
be sufficient to provide the basic medium necessary for these
catalytic sweetening reactionsto occur. The tetra-alkyl
guanidine is preferably added continuously to the hydrocarbon
to be treated, or alternatively, it may be placed in an aqueous
or alcoholic solution which is periodically pumped over a fixed
bed of catalyst to wet the surface thereof, with basic solution.
The catalyst used can be any catalyst which will
speed up the rate of mercaptan oxidation in the presence of
an alkaline reagent enough to permit sweetening of a sour hy-
drocarbon distillate over a fixed bed of the catalyst. Some
metal chelates possess sufficient activity to permit their
use as in such a process. Preferred among the metal chelates
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are the phthalocyanines. Especially preferred are the mono-
sulfonated derivatives of cobalt phthalocyanine. The sulfona-
tion of the cobalt phthalocyanine makes the material soluble
- enough in various solvents to permit the impregnation of a
fixed bed of charcoal with the catalyst. The monosulfonate
derivative is preferred because the more highly sulfonated de-
rivatives are more soluble in the hydrocarbon means to be
treated, permitting the leaching away of catalyst from the bed.
Recent work done with polyphthalocyanine catalysts, and mixtures
of different metal phthalocyanines, indicates that these cata-
lysts too may be acceptable for use in the present invention,
although forming no part thereof.
The catalyst material may be composited with any
suitable form of charcoal by conventional means. An excellent
way of preparing the catalyst is to dissolve, e.g., cobalt
phthalocyanine monosulfonate in methanol and pass the mèthanol-
catalyst solution repeatedly over a bed of activated charcoal.
The precise type of catalyst used, its method of preparation and
its incorporation onto a bed of charcoal support form no part
of the present invention.
EXAMPLES
To evaluate the effectiveness of the tetra-alkyl
guanidine of the present invention, a number of experiments
were run. A kerosene which was very difficult to sweeten was
used as the reference feed stock. The kerosene contained 180
wt ppm mercaptan sulfur.
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The test that was used was not meant to be indicative
of commercial operation, rather it was meant to be a simplified
procedure which would quickly separate good alkaline reagents
from bad ones. The test procedure was to put 2 grams of cata-
lyst, wetted with 5 ml of the alkaline reagent being tested,plus 100 ml of feedstock in a 300 ml flask which was then capped.
The flasks were then placed in an automated shaking device.
Temperature was not measured, but all tests were conducted at
ambient temperature in a room maintained at about 25 C, so
changes in temperature are not believed to be significant. The
contents of the flasks were sampled at uniform intervals and
the mercaptan sulfur content of the hydrocarbon was determined.
To insure the validity of the test, a number of
blanks were run, i.e., operation with charcoal whlch contained
no metal phthalocyanine catalyst on it, and operation with
conventional alkaline reagent (sodium hydroxide solution).
The same charcoal material was used throughout the test, a
vegetable derived charcoal sold by the Nuchar Co. The catalyst
was prepared by impregnating the charcoal with a cobalt phthalo-
cyanine monosulfonate. The catalyst was prepared by dissolving0.15 grams of cobalt phthalocyanine sulfonate in 100 cc of
methanol. The cobalt phthalocyanine was difficult to dissolve,
so to insure that all of it went into solution, the dissolu-
tion proceeded stepwise, i.e., one-fourth of the alcohol was
mixed with the phthalocyanine, then decanted, then the next
one-fourth portion was added to the cobalt phthalocyanine re-
maining in the bottom of the flask with grinding of the cobalt
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compound. This was repeated a third and a fourth time to make
sure that all of the active material was dissolved in the alco-
hol. The alcohol was then placed in a container with 15 grams
(100 cc) of charcoal, stirred slightly, and allowed to stand
overnight. The alcohol was then drained from the materialj and
the charcoal dried under a water pump vacuum. The filtrate
had only a faint blue color, but did not contain any significant
amount of cobalt, so the catalyst contained 1 wt. ~ of the co-
balt phthalocyanine sulfonate. This catalyst was divided into
several 2 gram portions for use in carrying out the activity
tests. The basesused, and the results of the test are reported
in the following table.
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A dash indicates that the mercaptan content was not
tested. The results reported under test 3, i.e., use of aqueous
NaOH solution, may be considered the standard activity for a
conventional fixed bed process. Surprisingly, the use of an
alcoholic NaOH solution gives much better results than use of
an aqueous NaOH solution; however, the use of an alcoholic
sodium hydroxide solution forms no part of the present inven-
tion. Not all solutions showed an improvement in going from
an aqueous to an alcoholic phase, as can be observed by compar-
ing the results of aqueous NH40H to alcoholic NH40H. The al-
coholic NH40H appeared to give slightly higher initial activity,
but after a 60 minute period, the mercaptan content was 10 to
20~ higher for the alcoholic solution than for the aqueous
solution.
A number of organic bases were tested. The results
are presented in Table II.
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The process of the present invention is illustrated
in the example wherein the base was alcoholic tetra-methyl
guanidine. The last test, alcoholic trimethyl tallow ammonium
hydroxide, is an illustration of a basic medium which does work
to convert mercaptan sulfur, but which is not acceptable for
use in petroleum refining. The base used in that example im-
parted a deep green color to the kerosene tested, and resulted
in the formation of an emulsion when the kerosene was given the
doctor test. Either property alone, i.e., color formation or
emulsion formation, would disqualify that particular base from
use as a commercial petroleum additive.
Accordingly, it can be seen that the process of the
present invention provides a way to treat even difficult to
sweeten kerosenes without the use of an aqueous sodium hydroxide
solution. Further, the basic reagent of the present invention
provides a more effective sweetening process than aqueous NaOH
solutions or several organic bases suggested by the prior art.
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