Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REMOVAL OF SULFUR-CONTAINING COMPOUNDS FROM
LIQUID HYDROCARBON STREAMS
FIELD OF THE INTENTION
The present invention relates to a process for removing organic sulfur
compounds
(e.g. thiophenes) from liquid hydrocarbon streams. The process comprises
subjecting the
liquid hydrocarbon stream to oxidation conditions, thereby oxidizing at least
a portion of
the organic sulfur compounds to oxidized organic sulfur compounds (e.g.
sulfones),
followed by catalytically decomposing the oxidized organic sulfur compounds to
provide
a treated hydrocarbon product of reduced sulfur content.
B~1CKGROZ7ND OF THE INTENTION
Sulfur is present in a wide range of mostly organic forms in both straight run
and
refined hydrocarbon streams, including, for example, gasoline, diesel fuel,
and kerosene.
Sulfur contaminants, while ubiquitous in hydrocarbon products, are suspected
of causing
adverse environmental effects when converted to sulfur oxides (SOX) upon
combustion.
SOX emissions are believed to contribute to not only acid rain, but also to
reduced
efficiency of catalytic converters designed to improve motor vehicle exhaust
quality.
Furthermore, sulfur compounds are thought to ultimately increase the
particulate content
of combustion products. Because of these issues, the reduction of the sulfur
content in
hydrocarbon streams has become a major objective of recent environmental
legislation
worldwide. For instance, Canada, Japan, and the European Commission have all
recently
adopted a 0.05 wt-% limit on diesel fuel sulfur.
For the oil refiner, complying with such increasingly stringent specifications
has
primarily meant using more severe hydrotreating conditions. Hydrotreating
refers to a
well-known process whereby hydrogen is contacted with a hydrocarbon stream and
catalyst to effect a number of desirable reactions, including the conversion
of sulfur
compounds to hydrogen sulfide. This reaction product is then separated into a
gaseous
hydrotreater effluent stream and thus effectively removed from the hydrocarbon
product.
Hydrotreating can readily reduce the level of several common classes of sulfur
compounds such as sulfides, disulfides, and thiols (mercaptans), present in
refinery
products. Unfortunately, however, hydroixeating (or hydrodesulfurization)
often fails to
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provide a treated product in compliance with the strict sulfur level targets
demanded
currently. This is due to the presence of sterically hindered sulfur compounds
such as un-
substituted and substituted thiophenes that axe essentially refractory in
hydrotreating
environments. Attempts to completely convert these species, which are more
prevalent
in heavier stocks such as diesel fuel and fuel oil, have resulted in increased
equipment
costs, more frequent catalyst replacements, degradation of product quality due
to side
reactions, and continued inability to comply with sulfur specifications.
Several prior art disclosures address sulfur contamination in refinery
products.
US A- 2,769,760, for example, describes a hydrodesulfurization process with an
additional conversion step that does not further reduce the sulfur level but
converts some
sulfur species to less-corrosive forms, allowing the product to meet acidity
requirements.
Other disclosures are more specifically directed toward essentially complete
sulfur
removal from hydrocarbons. Particularly, the ability to oxidize sulfur
compounds that
are resistant to the aforementioned hydrogenation method is recognized in a
number of
cases. Oxidation has been found to be beneficial because oxidized sulfur
compounds
have an increased propensity for removal by a number of separation processes
that rely
on the altered chemical properties such as the solubility, volatility, and
reactivity of such
compounds. Techniques for the removal of oxidized organic sulfur compounds
therefore
include extraction, distillation, and adsorption.
In US-A- 3,163,593, organic sulfur compounds contained in petroleum fractions
are oxidized by contact with a mixture of H202 and a carboxylic acid to
produce
sulfones, which are then degraded by thermal treatment to volatile sulfur
compounds. In
US-A- 3,413,307, thiophene and thiophene derivatives are oxidized to sulfones
in the
presence of a dilute acid. The sulfones are then extracted using a caustic
solution. In
US-A- 3,341,448, the oxidation and thermal treatment steps are combined with
hydrodesulfurization to greatly reduce the hydrocarbon sulfur content. As
noted
previously, the oxidation and hydrogenation techniques are effective for
converting
different types of organic sulfur-containing species, thereby leading to a
synergistic effect
when these methods axe combined.
In US-A- 3,505,210, sulfur contaminants in a hydrocarbon fraction are oxidized
using hydrogen peroxide or other suitable oxidizing agent to convert bivalent
sulfur to
sulfones. The hydrocarbon, after having been subjected to oxidation
conditions, is then
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contacted in this case with molten sodium hydroxide to produce a treated
product of
reduced sulfur content. Another example of a two-step oxidation and extraction
method
is provided in US-A- 3,551,328, where the extractant is a paraffinic
hydrocarbon
comprising a 3-6 carbon number alkane. Also, EP-0565324 A1 teaches the
effectiveness
of oxidizing sulfur-containing compounds followed by removal according to a
number of
possible separations known in the art.
In contrast to the prior art, applicant has determined that organic sulfur
contaminants in petroleum fractions can be first oxidized and then
catalytically
decomposed to hydrocarbons and volatile sulfur compounds. The hydrocarbons
formed
by this conversion remain in the treated liquid petroleum fraction as valuable
components
while the volatile sulfur is easily separable and can therefore be ultimately
sent for
typical caustic scrubbing and/or sulfur recovery procedures currently
practiced
commercially. The conversion of oxidized organic sulfur compounds such as
sulfones
according to the present invention has been determined to occur in the
presence of a
number of solid catalysts under a wide range of reaction conditions.
Compared to other techniques for the removal of oxidized sulfur compounds
from hydrocarbons, heterogeneous catalytic decomposition offers distinct
advantages.
For instance, in prior art methods for extracting sulfones, liquid extractants
are
continually consumed due to solution losses and invariable contamination of
the treated
hydrocarbon product. Also, the high energy costs and incomplete component
separations
associated with distillative separations, as taught in other disclosures, are
avoided using
the process of the present invention. Lastly, the frequent replacement of
adsorbent beds
when hydrocarbons with high sulfur levels are treated is also overcome.
Regarding the oxidative/adsorptive processes of the prior art in particular,
US-A-
3,945,914 teaches, as a first step, the oxidation of sulfur compounds in
hydrocarbons
using any conventional oxidant to form an oxidized sulfur compound. In a
second step,
the oxidized sulfur-containing hydrocarbon is contacted with a metal to form a
metal-
sulfur-containing compound. This process therefore relies on the adsorption of
oxidized
sulfur compounds from the hydrocarbon using a metal capable of forming a metal
sulfide. The metal is selected from the group consisting of Ni, Mo, Co, W, Fe,
Zn, V,
Cu, Mn, Hg, and mixtures thereof. This process is distinguished from
conventional
hydrodesulfurization in that the sulfur is immobilized in the form of a
metallic sulfur
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compound (e.g. a metal sulfide) rather than converted to hydrogen sulfide. For
this
reason, the addition of free molecular hydrogen, as is required in
hydrodesulfurization, is
overcome. A hydrogen atmosphere, however, is apparently needed to effect the
reduction of oxidized sulfur to the metal sulfur compound, based on Examples I-
III of
this reference.
Adsorptive processes for the removal of oxidized sulfur compounds may provide
a higher degree of overall sulfur reduction than traditional
hydrodesulfurization
processes. However, several disadvantages are associated with this type of
treatment
including the need for an adsorptive metal component, a hydrogen atmosphere,
and high
temperatures and pressures to carry out the desired formation of a metal
sulfur
compound. Furthermore, without frequent regeneration of the metal sulfur
compound
back to the original, useful form of the metal component, the metal becomes
quickly
expended by formation of the metal sulfur compound. Otherwise, to avoid
numerous
regenerations, a large amount of the metal component must be utilized.
To overcome these disadvantages, applicants have found that the oxidized
sulfur
compounds can instead be conveniently converted, using a catalyst, to volatile
sulfur
compounds and sulfur-free hydrocarbons. The catalytic conversion takes place
under
relatively mild conditions without the use of hydrogen atmosphere. Because the
sulfur
does not remain on the catalyst, but is instead released in a vapor phase,
active catalytic
sites are not consumed stoichiometrically upon contact with oxidized sulfur
species.
Furthermore, the need for a metal that is known to be reactive with sulfur,
including
those used normally in hydrodesulfurization catalysts (e.g. molybenum) and
also
described in the aforementioned '914 patent, is avoided. In fact,
hydrodesulfurization-
metal containing catalysts of the prior art are not recommended to carry out
the
conversion of oxidized sulfur compounds to volatile sulfur compounds, in
accordance
with the process of the present invention.
SUMMARY OF THE INTENTION
A primary object of the present invention is to provide a process for treating
a
liquid hydrocarbon feed stream containing an organic sulfur compound, the
process
comprising the steps of contacting the liquid feed with an oxidizing agent at
oxidation
conditions, thereby yielding an effluent stream containing an oxidized organic
sulfur
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compound, and; contacting the effluent stream with a solid decomposition
catalyst at
decomposition conditions effective to decompose the oxidized sulfur-containing
compound, thereby yielding a treated liquid stream and a volatile sulfur
compound.
In a preferred embodiment the present invention is a process for treating a
hydrotreated diesel fuel feed stream containing a thiophene compound or a
derivative
thereof, the process comprising the steps of contacting the liquid feed with
an alkyl
hydroperoxide at oxidation conditions, thereby yielding an effluent stream
containing a
sulfone, and; contacting the effluent stream with a solid decomposition
catalyst selected
from the group consisting of layered double hydroxides, molecular sieves,
inorganic
metal oxides, and mixtures thereof at decomposition conditions effective to
decompose
the oxidized sulfur-containing compound, thereby yielding a treated liquid
stream and a
volatile sulfur compound.
In another embodiment the present invention is a process as described above,
further comprising separating the treated liquid stream from the volatile
sulfur
compound.
DET~IILED DESCRIPTION OF THE INTENTION
The feed to the process of the present invention comprises broadly any liquid
hydrocarbon stream contaminated with an organic sulfur-containing compound.
More
particularly applicable, however, are straight run and cracked oil refinery
streams
including naphtha, gasoline, diesel fuel, jet fuel, kerosene, and vacuum gas
oil. These
petroleum distillates invariably contain sulfur compounds, the concentrations
of which
depend on several factors including the crude oil source, specific gravity of
the
hydrocarbon fraction, and the nature of upstream processing operations.
The present invention has been found to be particularly effective for
converting
sterically hindered sulfur compounds such as thiophenes and thiophene
derivatives, that
are known to be essentially non-reactive in hydrotreating (or
hydrodesulfurization)
reaction environments. For this reason, the oxidation/decomposition method of
the
- present invention may be practiced either before or after conventional
hydrotreating is
performed on any of the aforementioned feed stocks to significantly enhance
overall
sulfur removal efficiency. If hydrotreating is performed first, the liquid
hydrocarbon feed
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stream to the present invention is a hydrotreated naphtha, hydrotreated
gasoline,
hydrotreated diesel fuel, hydrotreated jet fuel, hydrotreated kerosene, or
hydrotreated
vacuum gas oil. Alternatively, hydrotreating can also be performed after the
oxidation
and decomposition steps to yield a high quality sulfur-depleted product.
Specific types of sulfur compounds of utmost concern in the refining industry,
due to their refractory nature in otherwise effective hydrotreating
environments, include
thiophene, benzothiophene, dibenzothiophene and alkylated dibenzothiophenes.
Alkylated dibenzothiophenes include the various isomers of methyl-substituted
dibenzothiophenes such as 4-methyldibenzothiophene; 2,8-
dimethyldibenzothiophene;
and 3,7-dimethyldibenzothiophene. Other more complex sulfur-containing
structures
comprising at least three benzene, thiophene, or saturated rings as described
in Ind. Eng.
Chem. Res. 1991, 30, p. 2022 are also readily converted by the 2-step
oxidationldecomposition method of the present invention.
In the first step of the treatment process, the liquid hydrocarbon stream to
be
treated is contacted with an oxidizing agent at oxidation conditions.
Generally, the
oxidation is carried out under mild conditions, at a temperature from
40°C to 120°C and
an absolute pressure from 50 kPa to 1520 kPa. Suitable oxidizing agents have
been
found to be alkyl hydroperoxides (e.g. t-butyl hydroperoxide), peroxides (e.g.
hydrogen
peroxide), percarboxylic acids (e.g. peracetic acid) and oxygen. These
compounds
generally exhibit sufficient oxidation strength to convert thiophenes in the
hydrocarbon
feed to sulfones. Furthermore, hydroperoxides, peroxides, percarboxylic acids,
and
oxygen are desirable as oxidizing agents due to their acceptable solubility in
the
hydrocarbon feed under oxidation conditions.
In general, the oxidizing agent should be introduced in at least the
stoichiometric
equivalent quantity of the feed sulfur, and preferably in an amount from 1 to
100 moles
per mole of sulfur in the liquid feed. Vigorous mixing of the oxidizing agent
and liquid
hydrocarbon is advantageous in the oxidation step and typically performed
using an
appropriate means of agitation such as a mechanical stirrer. Alternatively,
liquid-liquid
contact can also be enhanced with a static mixer. When oxygen gas is used for
the
oxidation step, a sparger or other type of gas distributor is usually
beneficial at the point
of injection to achieve sufficient mixing to overcome mass transfer
limitations. The
oxidation reaction may be carried out batch wise or continuously. For batch
operation, a
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stirred tank reactor is appropriate, while continuous operation typically
requires a
continuously stirred tank reactor (CSTR). In either batch or continuous
operation, a
reactor residence time of 1 to 48 hours is preferred. In CSTR operation the
residence
time is understood to mean the average residence time of the reactants in the
reactor.
When oxygen is selected as an oxidizing agent, either pure oxygen gas or a
mixture of oxygen and a diluent can be employed. Air is often chosen for
convenience.
With either pure or impure oxygen, it is preferred to carry out the oxidation
step of the
present invention in conjunction with a solid oxidation catalyst. Without
limiting the
scope of the present invention, it is believed that a heterogeneous oxidizing
catalyst
promotes the oxidation (by oxygen) of various species contained in the feed to
form
hydroperoxides in situ. For example, oxygen can react catalytically with
cumene that
exists in the feed to form cumene hydroperoxide, which in turn serves as an
oxidizing
agent for organic sulfur contaminants.
In general, an oxidation catalyst can optionally be used in conjunction with
any of
the oxidizing agents (not only oxygen gas) described previously, including
alkyl
hydroperoxides, peroxides, and percarboxylic acids. Suitable solid oxidation
catalysts
and methods for their preparation are known in the art and include various
metals
dispersed on inorganic metal oxide supports such as silica, alumina, titania,
molecular
sieves, and mixtures thereof. Molecular sieves are described in detail in
Szostak,
Molecular Sieves, P~i~ciples of Synthesis a~cd Ide~ctif catioh, Van Nostrand
Reinhold,
(1989) at pages 2-4. Catalytic metals that have been found to be most
effective in
promoting the oxidation step of the present invention include molybdenum,
tungsten,
chromium, vanadium, niobium, tantalum, titanium, cobalt, and mixtures thereof.
Solid
oxidation catalysts can be employed in any number of configurations known in
the art.
Such configurations include fixed-, moving-, fluidized-, and swing-bed
systems, among
others, although a fixed bed is preferred. For oxidation using a solid
catalyst, the
preferred weight hourly space velocity (WHSV) is from 0.1 to 10 hr-1. As
understood in
the art, the WHSV is the hourly rate of liquid feed weight flow divided by the
catalyst
weight and represents the reciprocal of the average time that a weight of
liquid feed
equivalent to the catalyst bed weight is charged to the catalyst.
Regardless of whether the oxidation reaction is performed heterogeneously in
the
presence of a solid catalyst or homogeneously, the oxidation step converts
thiophenes
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originally present in the liquid hydrocarbon to sulfones. For example,
dibenzothiophene
is readily oxidized to dibenzothiophene sulfone. Other types of organic sulfur-
containing
compounds, including branched alkyl sulfides, are oxidized to sulfoxides and
sulfones. It
is the oxidized form of the organic sulfur species that axe amenable to
decomposition
according to the second step of the method of the present invention.
After oxidation of at least a portion of the organic sulfur compounds in the
liquid
hydrocarbon feed, the second step of the present invention involves a
catalytic
decomposition of the oxidized organic sulfur species. As decomposition
catalysts, both
solid acids and bases have been found to be effective. The characterization of
a
particular catalyst formulation in terms of its acidic or basic properties is
described in
detail in Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, pp.
151-153
(1980). Acidic catalysts effective for the decomposition step include
amorphous
aluminosilicates having various proportions of silica and alumina as well as
crystalline
acidic aluminosilicates such as ZSM-5 and mordenite. Both ZSM-5 and mordenite
are
described in terms of structure and properties in Zeolite Molecular Sieves by
Donald W.
Breck (John Wiley and Sons, 1974). Acidic catalysts effective for the
decomposition of
oxidized organic sulfur compounds also include metal oxides, such as alumina,
and
mixed metal oxides such as Si02 ~ ZrO.
Metal oxides that exhibit basic properties, for example MgO, have also shown
suitability in catalyzing the decomposition of oxidized organic sulfur
compounds. Other
examples of effective basic catalysts include layered double hydroxides such
as
hydrotalcite, a magnesium/aluminum layered double hydroxide. The preparation
of
double hydroxides is well known in the art and described in detail in both J.
Catalysis,
94, 547-557 (1985) and US-A-5318936. The preparation of hydrotalcite, for
example,
can be performed by coprecipitation of magnesium and aluminum carbonates at a
high
pH. Thus magnesium nitrate and aluminum nitrate (in the desired ratios) are
added to
sodium carbonate. The resultant slurry is heated at about 65°C to
crystallize the
hydrotalcite and then the powder is isolated and dried.
Conditions appropriate for the catalytic decomposition of sulfones broadly
include a temperature from 200°C to 600°C and an absolute
pressure from 50 to 2,026
kPa. In contrast to typical hydrodesulfurization or hydrotreating processes,
the preferred
decomposition conditions of the present invention are significantly more mild
and
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include a temperature from about 350° to about 400°C and a
pressure from 500 to 1,013
kPa. Furthermore, a hydrogen, carbon monoxide, or other type of reducing
atmosphere is
not required. In other words, the decomposition step can take place in a non-
reducing
environment, meaning that, not considering vapors from the hydrocarbon feed
itself,
reducing gases such as hydrogen, carbon monoxide, etc are substantially
absent.
Preferably, the decomposition reaction pressure is maintained by the
hydrocarbon
pressure alone, without any supply of added overhead or blanketing gas.
Similar to the oxidation step, the decomposition step can be carried out using
a
fixed-, moving-, fluidized-, or swing bed system, but it is preferred to use a
fixed bed of
catalyst. In carrying out the decomposition step using a solid catalyst, the
effluent
hydrocarbon stream from the oxidization step, containing oxidized sulfur
compounds is
passed continuously through a bed of decomposition catalyst at a WHSV from 0.1
to 10
hr-1. Any of the aforementioned solid decomposition catalysts and oxidation
catalysts (if
used) associated with the present invention may be in the form of pellets,
spheres, or any
other desirable shape. Generally, catalyst particle size and shape are chosen,
as is known
in the art, to prevent undue pressure drop across the bed but permit adequate
diffusion of
reactants to active sites on the catalyst surface or within the catalyst
particle.
Under decomposition conditions, the oxidized organic sulfur compounds are
converted to sulfur-free hydrocarbons and volatile sulfur components. Without
wishing
to be bound to any particular theory or reaction mechanism, applicants propose
that the
catalytic decomposition of oxidized sulfur compounds results in the formation
of sulfur
dioxide according to the following general reaction pathway:
R ~~~ x x
R
S 1
Catalyst
~ / \ ~ /~ + SOz
The sulfur-free hydrocarbon, generated from the decomposition, contributes to
the yield of the treated liquid product, while the volatile sulfur component
is primarily
gas phase with a trace amount dissolved in the liquid. For example, consistent
with the
above explanation, dibenzothiophene sulfone has been shown to decompose to
biphenyl
(and, to a much lesser extent, hydroxybiphenyl) and sulfur dioxide gas. The
aromatic
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reaction product biphenyl is, in most hydrocarbon products marketed
commercially as
fuels, considered a valuable clean-burning energy source.
After decomposition of the oxidized sulfur compounds, the treated liquid
hydrocarbon product is typically reduced in sulfur content to less than 60% of
the sulfur
concentration originally contained in the feed. This level of reduction, of
course,
depends greatly on the nature of the sulfur compounds initially present. It
may be fiuther
desirable to separate residual volatile sulfur that is dissolved in the
treated liquid stream.
Because of the large boiling point disparity between the volatile sulfur and
the
hydrocarbon components in the treated liquid, a simple flash vaporization at
atmospheric
or sub-atmospheric pressure or a distillation technique is very effective.
These separation
techniques are well understood in the art and can in this case be performed at
conditions
mild enough so as not to degrade or significantly alter the quality of the
treated
hydrocarbon product.
The following examples are provided to further illustrate and clarify, but not
to
limit, the present invention.
COMPARATIVE EXAMPLE 1
A sample of hydrotreated diesel fuel was found to contain initially 536 ppm by
weight (wt-ppm) of total sulfur, measured based on X-ray fluorescence (XRF),
analysis.
Of the sulfur present, greater than 90% by weight was in the form of
thiophenes such as
thiophenes, benzothiophene, and dibenzothiophene. The sample was treated as
follows:
The hydrotreated diesel fuel was oxidized at 80°C and 1 atmosphere
absolute
pressure using the oxidizing agent t-butylhydroperoxide in the presence of an
oxidation
catalyst comprising molybdenum on an alumina carrier. The molybdenum was
present in
an amount representing 12% of the weight of the carrier. The oxidation
reaction was
carried out in a batch autoclave using mechanical agitation for approximately
24 hours.
Thus, this oxidation was in accordance with the first step of the present
invention. After
the reaction, the hydrocarbon effluent from the oxidation reaction was
analyzed and
found to contain 567 wt-ppm of total sulfur, again measured by XRF. (The
increase in
total sulfur content is most likely attributable to the volatilization of some
hydrocarbons
during oxidation.) A second analysis of this stream, using gas chromatography
(GC)
equipped with a sulfur-sensitive detector, showed that greater than 97% by
weight of this
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sulfur was in the form of sulfones, demonstrating the effectiveness of the
oxidizing agent
and solid catalyst system for converting thiophenes to sulfones. The product
resulting
from this oxidation of hydrotreated diesel fuel was termed the Reference Feed
and was
used in subsequent experimental work targeting the catalytic removal of the
oxidized
sulfur species.
After the oxidation step, the Reference Feed was passed over a solid bed of
commercial hydrotreating catalyst comprising Ni/Mo on a solid support
comprising a
zeolite. Reaction of the oxidized sulfur species was attempted at a
temperature of 350°C,
an absolute pressure of 689 kPa and a WHSV of 5 hr-l. The reaction pressure
was
maintained using the Reference Feed pressure only, without the use of hydrogen
or other
pressurizing gas. After having been subjected to these conditions, the
reaction effluent
was analyzed and the total sulfur level, compared to the original
concentration, did not
decrease to any measurable extent. Also, the sulfur level of the catalyst
itself was high
(about 2700 ppm), indicating that some adsorption of sulfur had occurred,
which would
be expected since the catalyst contained a known sulfur-reactive metal. Aside
from this
adsorption, however, the hydrotreating catalyst did not prove effective for
removing, over
an extended run time of 36 hours, the oxidized sulfur species under conditions
of low
pressure and also in the absence of hydrogen. Furthermore, based on GC-AED
(atomic
emission detection), about 50% of the sulfone species were converted back to
their
homologous starting thiophene.
COMPAR~4TIT~E EXAMPLE 2
The Reference Feed of Comparative Example 1 was passed over a solid bed of
the same catalyst (12% Mo on alumina) used initially to oxidize the
hydrotreated diesel
fuel. The reaction conditions used to attempt the catalytic removal of the
oxidized sulfur
species were similar to those described in Comparative Example 1, but using a
maximum
reaction temperature of 450°C. Again, the reaction effluent showed
negligible removal
of the oxidized sulfur species, in spite of the fact that some of the sulfur
(3000 ppm
relative to the catalyst weight) was adsorbed onto the catalyst by the sulfur-
reactive metal
(i.e. Mo). Furthermore, the sulfur containing compounds in the Reference Feed
and the
reaction effluent were characterized using GC-AED to determine individual
component
contributions. From this analysis, it was determined that a substantial
portion (>90%) of
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the oxidized sulfur species (dibenzothiophene sulfone) in the Reference Feed
was
converted back to the non-oxidized dibenzothiophene, thereby reversing the
reaction
effected in the oxidation step. Again, this catalyst, which contained a
hydrotreating
function (i.e. Mo) was not effective for removing, over an extended run time
of 48 hours,
the oxidized sulfur species under conditions of low pressure and also in the
absence of
hydrogen, characteristic of the present invention.
EXAMPLE 1
The Reference Feed as described in Comparative Example 1 was passed over a
solid bed of catalyst comprising an amorphous acidic aluminosilicate having a
silica to
alumina (Si02/A1203) molar ratio of about 3. Decomposition conditions included
a
temperature of 475°C, an absolute pressure of 689 kPa and a WHSV of 5
hr-1. After
having been subjected to decomposition conditions about 50 hours, the treated
diesel fuel
was analyzed and the total sulfur level, compared to the original
concentration, decreased
about 40%, to 339 wt-ppm based on XRF analysis. This finding indicated that
the acidic
aluminosilicate was an effective catalyst for the reduction of sulfur in the
hydrocarbon
stream, via the decomposition of sulfones contained therein.
In contrast, the total sulfur level decreased only about 4%, in a similar
experiment
where glass beads were used as the decomposition catalyst, rather than the
acidic
aluminosilicate. In this case, the small amount of reduction in sulfur content
observed
may be attributed mostly, if not totally, to thermal decomposition.
EXAMPLE 2
The experiment described in Example 1 was repeated except that the starting
sulfur level in the hydrotreated diesel fuel was 540 wt-ppm. Also, amorphous
magnesium oxide, a basic inorganic metal oxide, was used in place of the
acidic
aluminosilicate as the sulfone decomposition catalyst.
After having been subjected to decomposition conditions to about 50 hours, the
treated diesel fuel was analyzed and the total sulfur level, compared to the
original
concentration, decreased about 74%, to 140 wt-ppm. This finding indicated that
the
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magnesium oxide was an effective catalyst for the reduction of sulfur in the
hydrocarbon
stream, via the decomposition of sulfones contained therein.
EXAMPLE 3
The experiment described in Example 1 was repeated except that the starting
sulfur level in the hydrotreated diesel fuel was 590 wt-ppm. Also, a layered
double
hydroxide called hydrotalcite was used in place of the acidic aluminosilicate
as the
sulfone decomposition catalyst.
After having been subjected to decomposition conditions to about 50 hours, the
treated diesel fuel was analyzed and the total sulfur level, compared to the
original
concentration, decreased about 53%, to 270 wt-ppm. This finding indicated that
hydrotalcite was an effective catalyst for the reduction of sulfur in the
hydrocarbon
stream, via the decomposition of sulfones contained therein.
EXAMPLE 4
A sample of vacuum gas oil (VGO) was found to contain initially 2% by weight
of total sulfur, measured based on XRF analysis. The VGO was oxidized at
80°C and 1
atmosphere absolute pressure using the oxidizing agent t-butylhydroperoxide in
the
presence of an oxidation catalyst 'comprising molybdenum on an alumina
carrier. The
molybdenum was present in an amount representing 12% of the weight of the
carrier.
The oxidation reaction was carried out in a batch autoclave using mechanical
agitation
for approximately 24 hours. Thus, this oxidation was in accordance with the
first step of
the present invention.
After the reaction, it was impossible to determine the total sulfur level or
extent
of oxidation of the sulfur compounds using GC analysis as described in
previous
examples. This was due to the relatively high boiling point temperature range
of the
particular feed stock chosen for this example. However, the oxidized vacuum
gas oil
was diluted with pure toluene to reduce viscosity, to allow the desired
analytical
measurements. The total sulfur level of the toluene-diluted oxidized VGO was
determined to be 6347 ppm based on XRF analysis.
After having been subjected to oxidation conditions and diluted with toluene,
the
VGO was then passed over a solid bed of catalyst comprising an amorphous
magnesium
13
CA 02477565 2004-08-26
WO 03/074633 PCT/US02/05777
oxide (Mg0). Decomposition conditions included a temperature of 425°C,
an absolute
pressure of 689 kPa and a WHSV of 1 hr-1. After having been subjected to
decomposition conditions to about 50 hours, the treated diesel fuel was
analyzed and the
total sulfur level, compared to the original concentration, decreased about
83%, to 1094
wt-ppm based on XRF analysis. This experiment provides a reasonable basis for
concluding that Mg0 was an effective catalyst for the reduction of sulfur in
the VGO
stream, via the decomposition of sulfones contained therein.
14
