U.S. patent application number 10/790909 was filed with the patent office on 2004-09-16 for process for the hydrodesulfurization of naphtha.
This patent application is currently assigned to CATALYTIC DISTILLATION TECHNOLOGIES. Invention is credited to Podrebarac, Gary G..
Application Number | 20040178123 10/790909 |
Document ID | / |
Family ID | 32965714 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040178123 |
Kind Code |
A1 |
Podrebarac, Gary G. |
September 16, 2004 |
Process for the hydrodesulfurization of naphtha
Abstract
A process for the desulfurization of a full boiling range
naphtha stream is disclosed in which a full boiling range naphtha
is concurrently split in catalytic distillation column reactor into
a light and a heavy naphtha and the light nahtha thioetherified.
Only the heavy naphtha is treated to remove organic nitrogen
catalyst poisons then hydrodesulfurized. Both the light and the
light naphtha streams may be subject to a final
hydrodesulfurization in polishing reactor.
Inventors: |
Podrebarac, Gary G.;
(Houston, TX) |
Correspondence
Address: |
KENNETH H. JOHNSON
P.O. BOX 630708
HOUSTON
TX
77263
US
|
Assignee: |
CATALYTIC DISTILLATION
TECHNOLOGIES
|
Family ID: |
32965714 |
Appl. No.: |
10/790909 |
Filed: |
March 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60454259 |
Mar 13, 2003 |
|
|
|
Current U.S.
Class: |
208/211 ;
208/208R; 208/254R |
Current CPC
Class: |
C10G 2400/02 20130101;
C10G 67/06 20130101 |
Class at
Publication: |
208/211 ;
208/254.00R; 208/208.00R |
International
Class: |
C10G 067/06 |
Claims
The invention claimed is:
1. A process for treating a naphtha stream comprising the steps of:
(a) feeding hydrogen and a naphtha stream containing olefins,
diolefins, mercaptans, thiophene, thiophenic compounds and organic
nitrogen compounds to a first distillation column reactor; (b)
concurrently in said first distillation column reactor: (i)
reacting the diolefins with the mercaptans in the presence of a
thioetherification catalyst in a distillation reactor zone to
produce sulfides, and (ii) separating the naphtha into at least two
fractions comprising a first lower boiling fraction having a
reduced sulfur content and a first higher boiling fraction
containing the sulfides, thiophene, thiophenic compounds and
organic nitrogen compounds, (c) removing the first lower boiling
fraction from the first distillation column reactor as overheads;
(d) removing the first higher boiling fraction from the first
distillation column reactor as bottoms; (e) treating the first
bottoms to remove organic nitrogen compounds and to produce an
effluent having a reduced nitrogen compound content; and (f)
feeding hydrogen and the effluent to a hydrodesulfurization reactor
containing a hydrodesulfurization catalyst wherein a portion of the
sulfides, thiophene and thiophenic compounds are reacted with
hydrogen to form hydrogen sulfide.
2. The process according to claim 1 wherein said treating of step
(e) comprises feeding the first bottoms to a nitrogen adsorption
unit.
3. The process according to claim 1 wherein said
hydrodesulfurization reactor comprises a second distillation column
reactor and the effluent is concurrently separated into a second
lower boiling fraction containing the hydrogen sulfide and a second
higher boiling fraction and further comprising the steps of: (g)
removing the second lower boiling fraction from the second
distillation column reactor as a second overheads; (h) removing the
second higher boiling fraction from the second distillation column
reactor as a second bottoms; (i) combining and feeding said second
overheads and said second bottoms to a hydrogen sulfide separation
vessel wherein the hydrogen sulfide is separated as a gas from a
liquid effluent; (j) feeding hydrogen and the liquid effluent from
the hydrogen sulfide separation vessel to a polishing reactor
containing a hydrodesulfurization catalyst wherein additional
sulfides, thiophene and thiophenic compounds are reacted with
hydrogen to form hydrogen sulfide and a reduced sulfur effluent;
and (k) separating the hydrogen sulfide from the reduced sulfur
effluent.
4. The process according to claim 3 wherein said first distillation
column reactor contains two beds of thioetherification catalyst in
said distillation reaction zone and a mid range boiling fraction
containing thiophene is removed from between said beds and combined
with said liquid effluent from said hydrogen sulfide separation
vessel and fed to said polishing reactor.
5. The process according to claim 2 wherein said adsorption unit
comprises solid particulate materials capable of selectively
adsorbing organic nitrogen compounds.
6. The process according to claim 5 wherein said solid particulate
materials capable of selectively adsorbing organic nitrogen
compounds comprise alumina, acid white clay, Fuller's earth, active
carbon, zeolites, hydrated alumina, silica gel, ion exchange resins
and mixtures thereof.
7. A process for treating a naphtha stream comprising the steps of:
(a) feeding hydrogen and a naphtha stream containing olefins,
diolefins, mercaptans, thiophene, thiophenic compounds and organic
nitrogen compounds to a first distillation column reactor
containing two beds of thioetherification catalyst; (b)
concurrently in said first distillation column reactor: (i)
reacting the diolefins with the mercaptans in the presence of the
thioetherification catalyst in a distillation reactor zone to
produce sulfides, and (ii) separating the naphtha into at three
fractions comprising a first lower boiling fraction having a
reduced sulfur content and a first higher boiling fraction
containing the sulfides, thiophenic compounds and organic nitrogen
compounds and a mid boiling range fraction containing thiophene;
(c) removing the first lower boiling fraction from the first
distillation column reactor as overheads; (d) removing the first
higher boiling fraction from the first distillation column reactor
as bottoms; (e) removing the mid range boiling fraction
intermediate said two beds and containing thiophene and a side
draw; (f) feeding the first bottoms to a nitrogen adsorption unit
wherein organic nitrogen compounds are removed from said bottoms to
produce a first effluent having a reduced nitrogen compound
content; and (g) feeding hydrogen and the first effluent to a
second distillation column reactor containing a
hydrodesulfurization catalyst; (h) concurrently in said second
distillation column reactor; (i) reacting the sulfides and
thiophenic compounds with hydrogen to form hydrogen sulfide and
(ii) separating the first effluent into a second lower boiling
fraction containing the hydrogen sulfide and a second higher
boiling fraction; (i) removing the second lower boiling fraction
from the second distillation column reactor as a second overheads;
(j) removing the second higher boiling fraction from the second
distillation column reactor as a second bottoms; (k) combining and
feeding said second overheads and said second bottoms to a hydrogen
sulfide separation vessel wherein the hydrogen sulfide is separated
as a gas from a liquid effluent; (l) feeding hydrogen, the liquid
effluent from the hydrogen sulfide separation vessel and said side
draw to a polishing reactor containing a hydrodesulfurization
catalyst wherein additional sulfides, thiophene and thiophenic
compounds are reacted with hydrogen to form hydrogen sulfide and a
reduced sulfur effluent; (m) separating the hydrogen sulfide from
the reduced sulfur effluent; and (n) combining said reduced sulfur
effluent with said first overheads.
8. In a process for treating a naphtha stream comprising the steps
of: (a) feeding hydrogen and a naphtha stream containing olefins,
diolefins, mercaptans, thiophene, thiophenic compounds and organic
nitrogen compounds to a first distillation column reactor; (b)
concurrently in said first distillation column reactor: (i)
reacting the diolefins with the mercaptans in the presence of a
thioetherification catalyst in a distillation reactor zone to
produce sulfides, and (ii) separating the naphtha into at least two
fractions comprising a first lower boiling fraction having a
reduced sulfur content and a first higher boiling fraction
containing the sulfides, thiophene, thiophenic compounds and
organic nitrogen compounds, (c) removing the first lower boiling
fraction from the first distillation column reactor as overheads;
(d) removing the first higher boiling fraction from the first
distillation column reactor as bottoms; and (e) feeding hydrogen
and the effluent to a hydrodesulfurization reactor containing a
hydrodesulfurization catalyst wherein a portion of the sulfides,
thiophene and thiophenic compounds are reacted with hydrogen to
form hydrogen sulfide; wherein the improvement comprises feeding
the first bottoms to an organic nitrogen compound treatment unit
wherein organic nitrogen compounds are removed from said bottoms to
produce an effluent having a reduced nitrogen compound content.
9. The process according to claim 8 wherein said treating of step
(e) comprises feeding the first bottoms to a nitrogen adsorption
unit.
10. The process according to claim 9 wherein said adsorption unit
comprises solid particulate materials capable of selectively
adsorbing organic nitrogen compounds.
11. The process according to claim 10 wherein said solid
particulate materials capable of selectively adsorbing organic
nitrogen compounds comprise alumina, acid white clay, Fuller's
earth, active carbon, zeolites, hydrated alumina, silica gel, ion
exchange resins and mixtures thereof.
Description
[0001] This application claims priority from the prior U.S.
provisional application, serial No. 60/454,259 filed
03/13/2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the hydrodesulfurization of
a full boiling range naphtha which contains organic nitrogen
compounds. More particularly the invention relates to an
improvement in the process for treating a fluid cracked naphtha to
remove organic sulfur compounds by hydrodesulfurization. Most
particularly the invention relates to catalytic distillation column
reactions and the restriction of organic nitrogen compounds from
contact with the hydrotreating catalyst.
[0004] 2. Related Information
[0005] 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.
[0006] The composition of untreated naphtha as it comes from the
crude still, or straight run naphtha, is primarily influenced by
the crude source. Naphthas from paraffinic crude sources have more
saturated straight chain or cyclic compounds. As a general rule
most of the "sweet" (low sulfur) crudes and naphthas are
paraffinic. The naphthenic crudes contain more unsaturates and
cyclic and polycylic compounds. The higher sulfur content crudes
tend to be naphthenic. Treatment of the different straight run
naphthas may be slightly different depending upon their composition
due to crude source.
[0007] Reformed naphtha or reformate generally requires no further
treatment except perhaps distillation or solvent extraction for
valuable aromatic product removal. Reformed naphthas have
essentially no sulfur contaminants due to the severity of their
pretreatment for the process and the process itself.
[0008] Cracked naphtha as it comes from the catalytic cracker has a
relatively high octane number as a result of the olefinic and
aromatic compounds contained therein. In some cases this fraction
may contribute as much as half of the gasoline in the refinery pool
together with a significant portion of the octane.
[0009] Catalytically cracked naphtha gasoline boiling range
material currently forms a significant part (.apprxeq.1/3) of the
gasoline product pool in the United States and it provides the
largest portion of the sulfur. The sulfur impurities may require
removal, usually by hydrotreating, in order to comply with product
specifications or to ensure compliance with environmental
regulations.
[0010] 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. Additionally copious quantities
of hydrogen are included in the feed. The following equations
illustrate the reactions in a typical HDS unit:
(1) RSH+H.sub.2.fwdarw.RH+H.sub.2S
(2) RCI+H.sub.2.fwdarw.RH+HCI
(3) 2RN+4H.sub.2.fwdarw.2RH+2NH.sub.3
(4) ROOH+2H.sub.2.fwdarw.RH+2H.sub.2O
[0011] Typical operating conditions for the HDS reactions are:
1 Temperature, .degree. F. 600-780 Pressure, psig 600-3000 H.sub.2
recycle rate, SCF/bbl 1500-3000 Fresh H.sub.2 makeup, SCF/bbl
700-1000
[0012] The reaction of organic sulfur compounds in a refinery
stream with hydrogen over a catalyst to form H.sub.2S is typically
called hydrodesulfurization. Hydrotreating is a broader term which
includes saturation of olefins and aromatics and the reaction of
organic nitrogen compounds to form ammonia. However
hydrodesulfurization is included and is sometimes simply referred
to as hydrotreating. After the hydrotreating is complete, the
product may be fractionated or simply flashed to release the
hydrogen sulfide and collect the now desulfurized naphtha.
[0013] The predominant light or lower boiling sulfur compounds are
mercaptans while the heavier or higher boiling compounds are
thiophenes and other heterocyclic compounds. The separation by
fractionation alone will not remove the mercaptans. However, in the
past the mercaptans were frequently removed by oxidative processes
involving caustic washing. A combination oxidative removal of the
mercaptans followed by fractionation and hydrotreating of the
heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the
oxidative removal of the mercaptans the mercaptans are converted to
the corresponding disulfides.
[0014] In addition to treating the lighter portion of the naphtha
to remove the mercaptans the lighter fraction traditionally has
been used as feed to a catalytic reforming unit to increase the
octane number if necessary. Also the lighter fraction may be
subjected to further separation to remove the valuable C.sub.5
olefins (amylenes) which are useful in preparing ethers.
[0015] More recently a new technology has allowed for the
simultaneous treatment and fractionation of petroleum products,
including naphtha, especially fluid catalytically cracked naphtha
(FCC naphtha). See, for example, commonly owned U.S. Pat. Nos.
5,510,568; 5,597,476; 5,779,883; 5,807,477 and 6,083,378.
[0016] It has long been known that the organic nitrogen compounds
inhibit the hydrodesulfurization reaction and to some extent the
hydrogenation reaction. See for example "Poisoning of Thiophene
Hydrodesulfurization by Nitrogen Compounds" by Vito LaVopa and
Charles N. Satterfield published in the Journal of Catalysis,
volume 110 (1988) at pages 375-387. In addition the effect of water
and ammonia on hydrotreating catalysts has been studied. See
"Influence of Water and Ammonia on Hydrotreating Catalysts and
Activity for Tetralin Hydrogenation" published in Catalysis Today
vol. 29 (1996) at pages 229-233.
[0017] It is an advantage of the present invention that the contact
of organic nitrogen compounds of the entire naphtha feed are
restricted from contact with the catalysts used in the
hydrotreating. It is particular advantage that the catalytic
distillation reaction allows the separation of the feed, into a
lighter portion for hydrotreating and concentration of the organic
nitrogen compounds into a heavier portion of the feed comprising
less than the entire feed, thereby allowing a smaller volume of the
feed which constitutes the gasoline product to have to contact the
organic nitrogen compound adsorbent. It is a feature of the present
invention that both the hydrodesulfurization catalyst and the
thioetherification catalyst are protected from contact with the
organic nitrogen compounds. These and other advantages and features
will become obvious from the following descriptions.
SUMMARY OF THE INVENTION
[0018] Briefly the invention comprises the treatment of a full
boiling range naphtha (boiling in the range of C.sub.5 to about
420.degree. F.) under catalytic distillation conditions to remove
organic sulfur compounds in which mercaptans are removed by
thioetherification while at the naphtha is concurrently split into
a light fraction boiling in the range of C.sub.5 to 160.degree. F.
which is taken as overheads and a heavy fraction boiling in the
range of 160-420.degree. F. which is taken as bottoms. The
thioetherification catalyst is located in the upper portion of the
column where the light naphtha fraction migrates and where
mercaptans and olefins, preferably diolefins, are reacted to form
heavier sulfide products which drop into the bottoms comprising the
heavy naphtha fraction. The bottoms are treated to remove organic
nitrogen compounds, preferably by passing it through an adsorber
unit wherein most of the organic nitrogen compounds are adsorbed
from the stream. The effluent from the organic nitrogen removal
unit is then fed along with hydrogen to a second distillation
column reactor containing a hydrodesulfurization catalyst wherein
most of the organic sulfur compounds are converted to hydrogen
sulfide. If desired the bottoms effluent from the second
distillation column reactor and/or amid range naphtha sidedraw from
the first distillation column reactor, after hydrogen sulfide
removal, may be fed to a polishing reactor to obtain the desired
level of desulfurization.
[0019] The removal of the organic nitrogen compounds from the feed
to the hydrodesulfurization reactor benefits the process in three
ways:
[0020] 1. Higher catalyst activity on the order of 20-50% is
possible;
[0021] 2. The selectivity of the system will improve permitting
lower olefin conversion for any given level of desulfurization;
and
[0022] 3. Catalyst life will be improved.
[0023] As used herein the term "distillation column reactor" means
a distillation column which also contains catalyst such that
reaction and distillation are going on concurrently in the column.
In a preferred embodiment the catalyst is prepared as a
distillation structure and serves as both the catalyst and
distillation structure. The term "reactive distillation" is used to
describe the concurrent reaction and fractionation in a column. For
the purposes of the present invention, the term "catalytic
distillation" includes reactive distillation and any other process
of concurrent reaction and fractional distillation in a column
regardless of the designation applied thereto.
BRIEF DESCRIPTION OF THE DRAWING
[0024] The FIGURE is a flow diagram in schematic form of the
preferred embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] The benefit of the of employing a catalytic distillation
mode in conjunction with the nitrogen adsorption, is that the
stream is split and the organic nitrogen compounds which are
characteristic of the naphtha tend to largely fall in the boiling
range of the heavy naphtha. The light naphtha fraction which is
substantially free of the organic nitrogen compounds is not sent
through the adsorption, thus reducing the load on the adsorption
facility which reduces cost of installation and operation. The
nitrogen containing compounds found in full boiling range cracked
naphthas may include nitrites, carbazoles, benzocarbazoles,
indoles, pyridines, quinolines, acridines and tetrahydroquinolines
and are characterized as naturally occurring polar compounds.
[0026] Cracked naphtha is the preferred feed to the process of the
instant invention because it has the desirable light olefins and
the undesirable nitrogen and sulfur compounds. A typical fluid
cracked naphtha boils in the range of C.sub.5 to about 420.degree.
F. and contains typically mercaptans and the lighter thiophenic
compounds. The lower boiling (C.sub.5-about 160.degree. F.) portion
of the naphtha which contains most of the valuable olefins is
therefore not subjected to hydrodesulfurization catalyst but to a
less severe treatment wherein the mercaptans contained therein are
reacted with diolefins contained therein to form sulfides
(thioetherification) which are higher boiling and can be removed
with the heavier naphtha. Typically the reaction is carried out
over a nickel or palladium catalyst in the presence of hydrogen in
a naphtha splitter where a light fraction is taken overhead and a
heavy fraction is taken as bottoms. Advantageously the sulfides are
removed with the bottoms which are eventually subjected to
hydrodesulfurization.
[0027] THIOETHERIFICATION
[0028] A suitable catalyst for the reaction of the diolefins with
the mercaptans is 0.4 wt. % Pd on 7 to 14 mesh Al.sub.2O.sub.3
(alumina) spheres, supplied by Sud-Chemie, designated as G-68C.
Typical physical and chemical properties of the catalyst as
provided by the manufacturer are as follows:
2 TABLE I Designation G-68C Form Sphere Nominal size 7 .times. 14
mesh Pd. wt. % 0.4 (0.37-0.43) Support High purity alumina
[0029] Another catalyst useful for the mercaptan-diolefin reaction
is 58 wt. % Ni on 8 to 14 mesh alumina spheres, supplied by
Calcicat, designated as E-475-SR. Typical physical and chemical
properties of the catalyst as provided by the manufacturer are as
follows:
3 TABLE II Designation E-475-SR Form Spheres Nominal size 8 .times.
14 Mesh Ni wt. % 54 Support Alumina
[0030] 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 diolefins and acetylenes in the feed is at least 1.0 to
1.0 and preferably 2.0 to 1.0.
[0031] HYDRODESULFURIZATION
[0032] The reaction of organic sulfur compounds in a refinery
stream with hydrogen over a catalyst to form H.sub.2S is typically
called hydrodesulfurization. Hydrotreating is a broader term which
includes saturation of olefins and aromatics and the reaction of
organic nitrogen compounds to form ammonia. However,
hydrodesulfurization is included and is sometimes simply referred
to as hydrotreating.
[0033] Catalyst which are useful for the hydrodesulfurization
reaction include Group VIII metals 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.
[0034] 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 exposure to sulfur
compound containing streams. The catalyst may also catalyze the
hydrogenation of the olefins and polyolefins contained within the
light cracked naphtha and to a lesser degree the isomerization of
some of the mono-olefins. The hydrogenation, especially of the
mono-olefins in the lighter fraction may not be desirable.
[0035] The properties of a typical hydrodesulfurization catalyst
are shown in Table I below.
4 TABLE III 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
[0036] The catalysts typically are in the form of extrudates having
a diameter of 1/8, {fraction (1/16)} or {fraction (1/32)} inches
and an LID of 1.5 to 10. The catalyst also may be in the form of
spheres having the same diameters. They may be directly loaded into
standard single pass fixed bed reactors which include supports and
reactant distribution structures. However, in their regular form
they form too compact a mass and must then be prepared in the form
of a catalytic distillation structure. The catalytic distillation
structure must be able to function as catalyst and as mass transfer
medium. The catalyst must be suitably supported and spaced within
the column to act as a catalytic distillation structure. In a
preferred embodiment the catalyst is contained in a woven wire mesh
structure as disclosed in U.S. Pat. No. 5,266,546, which is hereby
incorporated by reference. More preferably the catalyst is
contained in a plurality of wire mesh tubes closed at either end
and laid across a sheet of wire mesh fabric such as demister wire.
The sheet and tubes are then rolled into a bale for loading into
the distillation column reactor. This embodiment is described in
U.S. Pat. No. 5,431,890 which is hereby incorporated by reference.
Other catalytic distillation structures useful for this purpose are
disclosed in U.S. Pat. Nos. 4,731,229, 5,073,236, 5,431,890 and
5,730,843 which are also incorporated by reference.
[0037] The conditions suitable for the desulfurization of naphtha
in a distillation column reactor are very different from those in a
standard trickle bed reactor, especially with regard to total
pressure and hydrogen partial pressure. Typical conditions in a
reaction distillation zone of a naphtha hydrodesulfurization
distillation column reactor are:
5 Temperature 450-700.degree. F. Total Pressure 75-300 psig H.sub.2
partial pressure 6-75 psia LHSV of naphtha about 1-5 H.sub.2 rate
10-1000 SCFB
[0038] 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 petroleum fraction. Actual
separation may only be a secondary consideration.
[0039] 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. In particular, sulfur
species concentrate in the liquid while the olefins and H.sub.2S
concentrate in the vapor allowing for high conversion of the sulfur
compounds with low conversion of the olefin species.
[0040] The result of the operation of the process in the
distillation column reactor is that lower hydrogen partial
pressures (and thus lower total pressures) may be used.
[0041] 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.
[0042] It is believed that in the present distillation column
reaction 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 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. A
further benefit that this reaction may gain from distillation
column reactions 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 H.sub.2S which is produced in the
distillation reaction zone.
[0043] The adsorption unit may be a dual fixed bed with one bed
adsorbing while the other is desorbing. The adsorption unit may
also be a fluidized bed with continuous withdrawal and regeneration
of the adsorbent. The adsorbents are characterized as solid
particulate materials capable of selectively adsorbing organic
nitrogen compounds. Examples of available adsorbents include active
alumina, acid white clay, Fuller's earth, active carbon, zeolite,
hydrated alumina, silica gel and ion exchange resins. The adsorbent
may be regenerated with a hot gas such as hydrogen or nitrogen.
[0044] Referring now to the FIGURE there is shown a flow diagram in
schematic form of one embodiment of the invention. The naphtha is
fed via flow line 101 to a first distillation column reactor 10
containing two beds 12 1nd 14 of thioetherification catalyst in the
upper end or rectification section below the beds. Hydrogen is fed
via flow line 101A as necessary to keep the catalyst in the hydride
state. The mercaptans and diolefins in the naphtha react in the
catalyst beds to form sulfides. At the same time the naphtha is
split into a lower boiling fraction boiling in the range of C.sub.5
to about 160.degree. F. which is taken as overheads via flow line
103. The overheads having most of the mercaptans removed have a
reduced sulfur content. A portion of the overheads is condensed and
returned to the distillation column reactor as reflux (not shown).
A mid boiling range cut containing predominantly thiophene as the
sulfur constituent is removed from between the beds 12 and 14 via
flow line 104.
[0045] The lighter components are stripped from the naphtha in the
stripping section of the distillation column reactor 10 which
contains standard distillation structure 16 such as sieve trays,
bubble caps or structured packing. The lighter components thus
stripped out are removed with the overheads. The bottoms stream
containing organic nitrogen compounds, the higher boiling sulfides
and thiophenic compounds are removed via flow line 102 and fed to
an adsorption unit 20 where organic nitrogen compounds are removed.
The effluent from the adsorption unit 20 in flow line 105 is fed to
a second distillation column reactor 30 containing beds 32 and 34
of hydrodesulfurization catalyst. Hydrogen is fed below the beds
via flow line 105A. The organic sulfur compounds in the effluent
are reacted with the hydrogen to produce hydrogen sulfide. At the
same time the effluent is split into a lower boiling fraction
boiling in the range of about 160-350.degree. F. which includes the
gaseous hydrogen sulfide and any lighter hydrocarbons which result
in the slight hydrocracking of the feed by the catalyst. The lower
boiling fraction is removed as overheads via flow line 106 and the
bottoms are removed via flow line 107. A portion of the overheads
is condensed and returned to the distillation column reactor as
reflux as needed (not shown). The bottoms and overheads are
combined in flow line 108 and passed to separation vessel 40, such
as a standard distillation column, where the hydrogen sulfide is
removed via flow line 109. The liquid form the separation vessel 40
is removed via flow line 110 and combined with the mid boiling
range cut in flow line 104 and fed to polishing reactor 50
containing a bed 52 of hydrodesulfurization catalyst where the
sulfur content is reduced to the desired level, e.g., <50 wppm.
Hydrogen is fed to the polishing reactor as required via flow line
111A. Finally the effluent from the polishing reactor 50 is taken
via flow line 112 to a second separation vessel where the hydrogen
sulfide produced in the polishing reactor is removed via flow line
113 and the desulfurized naphtha is removed via flow line 114 and
combined with the overheads in flow line 103 from the
thioetherification reactor 10 in flow line 115.
* * * * *