Note: Descriptions are shown in the official language in which they were submitted.
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HYDRODESULFURIZATION PROCESS
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to the
hydrodesulfurization of petroleum streams in a distillation
column reactor. More particularly the invention relates to
a process wherein a petroleum fraction is fed to a
distillation column reactor containing a
hydrodesulfurization catalyst in the form of a catalytic
distillation structure where the organic sulfur compounds
contained in the petroleum fraction are reacted with
hydrogen to form H2S which can be stripped from the
overhead product.
Related Information
Petroleum distillate streams contain a variety of
organic chemical components. Generally the streams are
defined by their boiling ranges which determine the
compositions. The processing of the streams also affects
the composition. For instance, products from either
catalytic cracking or thermal cracking processes contain
high concentrations of olefinic materials as well as
saturated (alkanes) materials and polyunsaturated materials
(diolefins). Additionally, these components may be any of
the various isomers of the compounds.
Organic sulfur compounds present in these petroleum
fractions are denoted as, "sulfur". The amount of sulfur
is generally dependent upon the crude source. For instance
the Saudi Arabian crudes are generally high in sulfur as
are certain domestic crudes. Kuwaiti, Libyan and Louisiana
crudes are generally low in sulfur. The type of sulfur
compounds will also depend on the boiling range of a given
stream. Generally the lower boiling fractions contain
mercaptans while the heavier boiling fractions contain
thiophenic and heterocyclic sulfur compounds.
The organic sulfur compounds are almost always
considered to be contaminants. They hinder in downstream
processing and at the very least make obnoxious..S02 gas
when burned. For these reasons it is very desirable to
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remove these compounds. The degree of removal is dependent
upon the use of the fraction. For instance, feed streams
to catalytic reforming require extremely low sulfur con-
centrations (less than 1 wppm) Current EPA regulations
call for combustible motor fuels such as gasoline, kerosene
or diesel to have no more than about 500 wppm sulfur. It
is expected that in the future the sulfur specification
will be lowered to about 50 wppm, especially for gasoline.
The most common method of removal of the sulfur
compounds is by hydrodesulfurization (HDS) in which the
petroleum distillate is passed over a solid particulate
catalyst comprising a hydrogenation metal supported on an
alumina base. In the past this has generally been done by
downflow over fixed beds concurrently with copious
quantities of hydrogen in the feed. The following
equations illustrate the reactions in a prior art typical
HDS unit:
(1) RSH + H2 --- RH + H2S
(2) RCl + H2 ---- RH + HC1
(3) 2RN + 4H2 --- RH + NH3
(4) ROOH + 2H2 --- RH + H20
Additional reactions depend upon the sulfur compounds
present and the source of the fraction. The catalyst used
for hydrodesulfurization necessarily is a hydrogenation
catalyst and the support sometimes is acidic in nature.
The latter characteristics provide for some hydrocracking
and hydrogenation of unsaturated compounds. The
hydrocracking results in a higher volume of a less dense
(lower specific gravity) material than the feed.
Typical operating conditions for the prior art HDS
reactions are:
----------------------------------------------------------
Temperature, F 600-780 Pressure, psig 600-3000
H2 recycle rate, SCF/bbl 1500-3000
Fresh H2 makeup, SCF/bbl 700-1000
After the hydrotreating is complete the product is
fractionated or simply flashed to release the hydrogen
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sulfide and collect the now sweetened fraction.
It should be noted that the conditions or severity of
the operation will depend upon the sulfur compounds present
and the degree of desulfurization desired. For instance
mercaptans are much more easily desulfurized than
thiophenes. The desulfurization of thiophenes and other
heterocyclic sulfur compounds riecessarily involves breaking
and saturation of the rings which requires higher severity.
A method of carrying out catalytic reactions has been
developed wherein the components of the reaction system are
concurrently separable by distillation using the catalyst
structures as the distillation structures. Such systems
are described variously in U.S. Pat. Nos. 4,215,011;
4,232,177; 4,242,530; 4,250,052; 4,302,356 and 4,307,254
commonly assigned herewith. Iri addition, commonly assigned
U.S. Patent 4,443,559, 5,057,468, 5262,012 5,266,546 and
5,348,710 disclose a variety of catalyst structures for
this use. A
distillation column reactor has been utilized wherein a
solid particulate catalyst has been placed within a
distillation column so as to act as a distillation
structure. The distillation column reactor has been found
to be particularly useful in equilibrium limited reactions
because the reaction products have been removed from the
reaction zone almost immediately. Additionally the
distillation column reactor has been found to be useful to
prevent unwanted side reaction:s.
In U.S. patent 4,194,964, Chen, et al propose a process
operated at about 300 psig to 3000 psig, high hydrogen
partial pressures and high hydrogen flow rates (around 4000
SCF/B) for concurrent hydroprocessing and distillation of
heavy petroleum stocks. Essentially Chen, et al disclose
the use of concurrent distillation and hydroprocessing of
the heavy stocks for the standard high pressure treating
and hydrocracking. The range of conditions is fairly
consistent with the prior art processes.
SUMMARY OF THE INVENTION
The present invention uses catalytic distillation in
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hydrodesulfurization at low total pressures below about
300 psig, preferably below about 290 and more preferably in
the range of 0 to 200 psig, low hydrogen partial pressure
in the range of 0.1 to 70 psi and temperatures in the
range of 400 to 800 F. Briefly the invention may be said
to comprise:
feeding (1) a petroleum stream containing sulfur
compounds and (2) hydrogen to a distillation column
reactor;
concurrently in said distillation column reactor
(a) distilling said petroleum stream whereby
there are vaporous petroleum products rising upward through
said distillation column reactor, an internal reflux of
liquid flowing downward in said distillation column reactor
and condensing petroleum products within said distillation
column reactor,
(b) contacting said petroleum stream and said
hydrogen in the presence of a hydrodesulfurization catalyst
prepared in the form of a catalytic distillation structure
at total pressure of less than about 300 psig, hydrogen
partial pressure in the range of 0.1 to less than 70 psi
and a temperature in the range of 400 to 800 F whereby a
portion of the sulfur compounds contained within said
petroleum stream react with hydrogen to form H2S;
withdrawing an overheads from said distillation column
reactor containing said H2S;
separating the H2S from said overheads by condensing a
higher boiling fraction in a partial condenser;
returning a portion of said condensed overheads to said
distillation column reactor as external reflux; and
withdrawing a bottoms product having a lower sulfur
content than said petroleum stream.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation of the distillation
column reactor results
in both a liquid and vapor phase within the distillation
reaction zone. A considerable portion of the vapor is
hydrogen while a portion is vaporous hydrocarbon from the
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petroleum fraction. Actual separation may only be a
secondary consideration. Within the distillation reaction
zone there is an internal reflux and liquid from an
external reflux which cool the rising vaporous hydrocarbon
5 condensing a portion within the bed.
Without limiting the scope of the invention it is
proposed that the mechanism that produces the effectiveness
of the present process is the condensation of a portion of
the vapors in the reaction system, which occludes
sufficient hydrogen in the condensed liquid to obtain the
requisite intimate contact between the hydrogen and the
sulfur compounds in the presence of the catalyst to result
in their hydrogenation.
The result of the operation of the process in the
catalytic distillation mode is that lower hydrogen partial
pressures (and thus lower total pressures) may be used.
As in any distillation there is a temperature gradient
within the distillation column reactor. The temperature
at the lower end of the column contains higher boiling
material and thus is at a higher temperature than the upper
end of the column. The lower boiling fraction, which
contains more easily removable sulfur compounds, is
subjected to lower temperatures at the top of the column
which provides for greater selectivity, that is, less
hydrocracking or saturation of desirable olefinic
compounds. The higher boiling portion is subjected to
higher temperatures in the lower end of the distillation
column reactor to crack open the sulfur containing ring
compounds and hydrogenate the sulfur.
It is believed that in the present reaction catalytic
distillation is a benefit first, because the reaction is
occurring concurrently with distillation, the initial
reaction products and other stream components are removed
from the reaction zone as quickly as possible reducing the
likelihood of side reactions. Second, because all the
components are boiling the temperature of reaction is
controlled by the boiling point of the mixture at the
system pressure. The heat of reaction simply creates more
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boil up, but no increase in temperature at a given
pressure. As a result, a great deal of control over the
rate of reaction and distribution of products can be
achieved by regulating the system pressure. Also,
adjusting the throughput (residence time = liquid hourly
space velocity-1-) gives further control of product =
distribution and to a degree control of the side reactions
such as oligomerization. A further benefit that this
reaction may gain from catalytic distillation is the
washing effect that the internal reflux provides to the
catalyst thereby reducing polymer build up and coking.
Finally, the upward flowing hydrogen acts as a stripping
agent to help remove the H2S which is produced in the
distillation reaction zone.
Petroleum fractions which may be treated to remove
sulfur by the instant process include the full range of
petroleum distillates and include natural gas liquids,
naphthas, kerosene, diesel, gas oils (both atmospheric and
vacuum) and residuums. The fractions may be straight run
material direct from a crude fractionation unit or may be
the result of downstream processing, such as fluid
catalytic cracking, pyrolysis or delayed coking.
The hydrogen rate to the reactor must be sufficient to
maintain the reaction, but kept below that which would
cause flooding of the column which is understood to be the
"effectuating amount of hydrogen " as that term is used
herein. The mole ratio of hydrogen to sulfur compound in
the feed varies according to the type of compound and the
amount of hydrogen expected to be consumed by side reac-
tions such as double or triple bond saturation. Hydrogen
flow rates are typically calculated as standard cubic feet
per barrel of feed (SCFB) and are in the range of 300 to
3000 SCFB.
Surprisingly, a low total pressure, below about 300
psig, for example in the range of 0 to 200 psig is required
for the hydrodesulfurization and hydrogen partial pressure
of less than 70 psi down to 0.1 psig can be employed, e.g.
0.1 to 70 psig preferably about 0.5 to 10 psig. The
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preferred hydrogen partial pressure is less than 50 psig.
This prefera-bly-is a hydrogen partial pressure in the range
of a-bout-0:1 to 10 psia-and even more preferably no more
than 7 psia. Optimal results have been obtained in the
range between 0.5 and 50 psig hydrogen partial pressure.
Where the petroleum stream is a naphtha, typical
conditions are overhead temperature in the range of 350 to
550 F, the bottoms temperature in the range of 500 to 800
F, and the pressure in the range of 25 to less than 300
psig. Where the petroleum stream is a kerosene, typical
..,~ conditions are overhead temperature in the range of 350 to
650 F, the bottoms temperature in the range of 500 to 800
F, and the pressure in the range of 0 to 200 psig. Where
the petroleum stream is a diesel, typical conditions are
overhead temperature in the range of 350 ta 650 F, the
bottoms temperature in the range 500 to 850 F, and the
pressure in the range 0 to 150 psig.
Catalyst which are useful for the hydrodesulfurization
reaction include metals Group VIII such as cobalt, nickel,
palladium, alone or in combination with other metals such
as molybdenum or tungsten on a suitable support which may
be alumina, silica-alumina, titania-zirconia or the like.
Normally the metals are provided as the oxides of the
metals supported on extrudates or spheres and as such are
not generally useful as distillation structures.
The catalysts contain components from Group V, VIB,
VIII metals of the Periodic Table or mixtures thereof. The
use of the distillation system reduces the deactivation and
provides for longer runs than the fixed bed hydrogenation
units of the prior art. The Group VIII metal provides
increased overall average activity. Catalysts containing a
Group VIB metal such as molybdenum and a Group VIII such as
cobalt or nickel are preferred. Catalysts suitable for the
hydrodesulfurization reaction include cobalt-molybdenum,
nickel-molybdenum and nickel-tungsten. The metals are
generally present as oxides supported on a neutral base
such as alumina, silica-alumina or the like. The metals
are reduced to the sulfide either in use or prior to use by
AMENOED SHEET
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exposure to sulfur compound containing streams. The
properties of a typical hydrodesulfurization catalyst in
Table I below.
TABLE I
Manufacture Criterion Catalyst Co.
Designation C-448
Form Tri-lobe Extrudate
Nominal size 1.2 mm diameter
Metal, Wt.%
Cobalt 2-5$
Molybdenum 5-20%
Support Alumina
Broadly stated, the catalytic material is a component of
a distillation system functioning as both a catalyst and
distillation packing, i.e., a packing for a distillation
column having both a distillation function and a catalytic
function.
The reaction system can be described as heterogenous
since the catalyst remains a distinct entity.
A preferred catalyst structure for the present
hydrogenation reaction comprises flexible, semi-rigid open
mesh tubular material, such as stainless steel wire mesh,
filled with a particulate catalytic material in one of
several embodiments recently developed in conjunction with
the present process.
One new catalyst structure developed for use in
hydrogenations is described in US Pat. No. 5,266,546,
Briefly the new
catalyst structure is a catalytic distillation structure
comprising flexible, semi-rigid open mesh tubular material,
such as stainless steel wire mesh, filled with a
particulate catalytic material said tubular material having
two ends and having a length in the range of from about
one-half to twice the diameter of said tubular material, a
first end being sealed together along a first axis to form
a first seam and a second end being sealed together along a
second axis to form a second seam wherein the plane of the
first seam along the axis of said tubular material and the
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Ci
plane of the second seam along the axis of said tubular
material bisect each other at an angle of about 15 to 90'.
US Patent No. 4,242,530 and US Pat. No. 4,443,559,
disclose supported catalyst in a
plurality of pockets in a cloth belt or wire mesh tubular
structures, which is supported in the distillation column
reactor by open mesh knitted stainless steel wire by
twisting the two together into a helix. U.S. Pat. No.
5,348,710, which is incorporated herein, describes several
other suitable structures in the prior art and discloses
new structures suitable for this process. Other catalytic
distillation structures useful for this purpose are
disclosed in U.S. patents 4,731,229 and 5,073,236.
The particulate catalyst material may be a powder, small
irregular chunks or fragments, small beads and the like.
The particular form of the catalytic material in the
structure is not critical, so long as sufficient surface
area is provided to allow a reasonable reaction rate. The
sizing of catalyst particles can be best determined for
each catalytic material (since the porosity or available
internal surface area will vary for different material and
of course affect the activity of the catalytic material).
For the present hydrodestilfurizations the preferred
catalyst structures for the packing are those employing
the more open structure of perraeable plates or screen wire.
EXAMPLES
In the examples 1-3 below a catalyst structure was
prepared as disclosed in U.S. Pat No. 5,431,890,
containing 0.947 pounds of the
catalytic material described in Table I and placed in the
middle nineteen feet of a 20 foot tall 1 inch diameter
distillation column reactor. There were 1/2 foot of inert
packing in a rectifying section above the catalyst and 1/3
foot of inert packing in a stripping section below the
catalyst. Liquid feed was fed to the distillation column
reactor at either at about =the mid point or below the
catalyst bed and hydrogen was fed at the bottom of the
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catalyst bed. In each of the examples there is a showing
of a substantial reduction in the amount of organic sulfur
in both'-the overheads -and bottoms, the removed organic
sulfur that has been converted to H2S and separated
5 overhead by partial condensation of the overheads.
Example 1
A full boiling range naphtha was fed to the distillation
column reactor containing a the catalyst prepared as noted
above. Conditions and results are given in TABLE II below.
10 TABLE II
Run No. 4-25HDS
Hours 605.2
Feed
rate, lbs/hr 1.00 - -
total sulfur, wppm (mg) 925 (419)
H2 -rate, SCFH (SCFB) 11.03 (3243)
Temperature, aF
Overhead 364
Top Catalyst Bed 503
Mid Catalyst Bed 514
Bottom Catalyst Bed 580
Bottoms 679
Feed 401
Total Pressure, psig 200.
Hydrogen Partial pressure, psig 23.9
Overhead
rate, lbs/hr 0.74
total sulfur, wppm (mg) 120 (40)
Bottoms
rate, lbs/hr 0.20
total sulfur, wppm (mg) 203 (18)
Reflux Ratio 10:1
Catalyst above feed, feet 9
Catalyst below feed, feet 10
Conversion of organic S, ~ 86
Example 2
A kerosene fraction was fed to the distillation column
reactor described above. Conditions and results are given
in TABLE III below.
AMEMDED SHEET
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TABLE III
Run No. 4-25HDS
Hours. .- - - - 1757.2
Feed
rate, lbs/hr 1.00
total sulfur, wppm (mg) 1528 (694)
H2 rate, SCFH (SCFB) 5.02 (1476)
Temperature, aF
Overheads 449
Top Catalyst Bed 647
Mid Catalyst Bed 659
Bottom Catalyst Bed 697
Bottoms 784
Feed 450
-.~.~
Total Pressure, psig 100
Hydrogen Partial Pressure, psig 11
Overhead
rate, lbs/hr 0.81
total sulfur, wppm (mg) 38 (14)
Bottoms
rate, lbs/hr - 0.17
total sulfur, wppm (mg) 1577 (122)
Reflux Ratio 10:1
Catalyst above feed, feet 19
Catalyst below feed, feet 0
Conversion of organic S, ~ 80
Example 3
A diesel fraction was fed to the distillation column
reactor as described above. Conditions and results are
given in TABLE IV below.
AMENDED SHEET
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'QEAJISS 0 3 JUL '9e
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TABLE IV
Run No. 4-25HDS
'Hours - = - - - - 1421.2
Feed
rate, lbs/hr 1.00
total sulfur, wppm (mg) 1528 (694)
H2 rate, SCFH (SCFB) 10.03 (2949)
Temperature, F
Overheads 438
Top Catalyst Bed 634
Mid Catalyst Bed 648
Bottom Catalyst Bed 689
Bottoms 801
Feed 450
~ ---- -
Total Pressure, psig 100
Hydrogen Partial Pressure, psig 23
Overhead
rate, lbs/hr 0.77
total sulfur, wppm (mg) 84 (29)
Bottoms
rate, lbs/hr 0.21
total sulfur, wppm (mg) 1278 (122)
Reflux Ratio 10:1
Catalyst above feed,.feet 19
Catalyst below feed, feet 0
~./ Conversion of organic S, $ 78
Example 4
In the following example 18.7 pounds of the catalytic
material of Table I were placed in the catalytic distilla-
tion structure prepared as disclosed in U.S. Pat No.
5,431,890, and were placed in the mid 40 feet of a fifty
foot tall three inch distillation column reactor. Liquid
feed was to about two thirds of the way up the column and
hydrogen was fed below the bed. A second full range fluid
cracked naphtha was feed to the column and the conditions
and results are reported in TABLE V below.
AMENDED SHEET
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TABLE V
Run No. 10~7HDS
Hours 308
Feed
rate, lbs/hr 20.0
total sulfur, wppm (mg) 1242 (11,277)
H2 rate, SCFH (SCFB) 41 (601)
Temperature, F
Overheads 476
Top Catalyst Bed 552
Mid Catalyst Bed 651
Bottom Catalyst Bed 696
Bottoms 749
Feed 297
Total Pressure, psig 200
Hydrogen Partial Pressure, psig 21.23
Overhead
rate, lbs/hr 16.0
total sulfur, wppm (mg) 122 (886)
Bottoms
rate, lbs/hr 4.0
total sulfur, wppm (mg) 35 (64)
Reflux Ratio 4:1
Internal Reflux 6.02
Catalyst above feed, feet 15
Catalyst below feed, feet 25
Conversion of organic S, % 92
