science and technology of novel process for deep desulfurization of oil refinery streams
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Review Article
Science and technology of novel processes for deep desulfurizationof oil refinery streams: a reviewq
I.V. Babich*, J.A. Moulijn
Faculty of Applied Sciences, Delft University of Technology, Delft ChemTech, Julianalaan 136, 2628 BL Delft, The Netherlands
Received 15 March 2002; revised 15 July 2002; accepted 9 October 2002; available online 14 November 2002
Abstract
Oil refinery related catalysis, particularly hydrodesulfurization (HDS) processes, is viewed as a mature technology and it is often stated
that break-throughs are not to be expected. Although this could be a justified compliment to those who developed this area, at the same time it
could also stifle potential new ideas.
The applicability and perspectives of various desulfurization technologies are evaluated taking into account the requirements of the
produced fuels. The progress achieved during recent years in catalysis-based HDS technologies (synthesis of improved catalysts, advanced
reactor design, combination of distillation and HDS) and in non-HDS processes of sulfur removal (alkylation, extraction, precipitation,
oxidation, and adsorption) is illustrated through a number of examples.
The discussed technologies of sulfur removal from the refinery streams lead to a wealth of research topics. Only an integrated approach
(catalyst selection, reactor design, process configuration) will lead to novel, efficient desulfurization processes producing fuels with zero
sulfur emissions.
q 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Oil refinery; Sulfur removal; Hydrodesulfurization
1. Introduction
A modern refinery is a highly integrated industrial plant,
the main task of which is to efficiently produce high yields
of valuable products from a crude oil feed of variable
composition. Employing different physical and chemical
processes such as distillation, extraction, reforming, hydro-
genation, cracking and blending the refinery converts crude
oil to higher value products. The main products are liquid
petroleum gas, gasoline, jet and diesel fuels, wax,
lubricants, bitumen and petrochemicals. Energy and hydro-gen for internal and external use are also produced in a
refinery.
Currently, refineries meet changing societal needs
concerning product specifications and quality by upgrading
existing technologies and continuously developing
advanced technologies [1]. Changes in refining processes
are made in response to external driving forces taking into
account the inherent limitations of the refinery (Fig. 1).
Environmental restrictions regarding the quality of
transportation fuels produced and the emissions from the
refinery itself are currently the most important issues, as
well as the most costly to meet. The primary goal of the
recently proposed regulations (by the Directive of the
European Parliament [2] and the Environmental Protection
Agency (EPA) Clean Air Act (Tier 2) [3]) is to reduce the
sulfur content of transportation fuels. The CO2 emitted by
the refinery into the atmosphere is limited by the Kyoto
protocol [4]. According to various estimation models, $10
15 billions in the European refinery industry and up to $16
billion in US and Canadian refineries will be invested in
direct response to the new environmental clean-fuel
legislation [5,6].
Gasoline, diesel and non-transportation fuels account
for 75 80% of the total refinery products. Most of the
desulfurization processes are therefore dealing with the
streams forming these end products. Sulfur present in
the fuels leads to SOx air pollution generated by vehicle
engines. In order to minimize the negative health and
environmental effects of automotive exhaust emissions,
0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 1 6 - 2 3 6 1 (0 2 )0 0 3 2 4 - 1
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q Published first on the web via fuelfirst.comhttp://www.fuelfirst.com
* Corresponding author. Present address: Faculty of Chemical
Technology, University of Twente, Postbus 217, 7500 AE, Enschede,
The Netherlands. Tel.: 31-53-489-35-36; fax: 31-53-489-46-83.
E-mail address: [email protected] (I.V. Babich).
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the sulfur level in motor fuels is minimized. New sulfur
limits of 30 50 ppm for gasoline and diesel marketed in the
European community and the USA will be introduced
starting from January 1, 2005 [2,3,5,7,8]. Germany has even
passed legislation limiting the sulfur in diesel and gasoline
to 10 ppm as of November, 2001 [9]. In fact, zero-emission
and, as a consequence, zero levels of S are called for
worldwide in coming 510 years. Such ultra low-sulfur
fuels requirements have consequences for the refinery.
Efficiency of the desulfurization technologies becomes a
key point. Conventional hydrodesulfurization (HDS) pro-
cesses cannot currently produce such zero sulfur level fuels,
while maintaining other fuel requirements such as oxygen
content, vapor pressure, benzene content, overall aromatics
content, boiling range and olefin content for gasoline, and
cetane number, density, polynuclear aromatics content, and
distillation 95% point for diesel fuel [2,3,5,7,8].
1.1. Gasoline
Gasoline is formed by blending straight run naphtha
(isomerate, reformate and alkylate products), naphtha from
fluid catalytic cracking (FCC) units and coker naphtha.
Most sulfur in gasoline comes from the FCC naphtha.
Treatment of FCC gasoline is, therefore, essential. The
sulfur content of the other gasoline forming refinery streams
is not a problem for the current environmental regulations,
but to produce gasoline of#30 ppm S the refinery will be
obliged to treat them as well. A relatively high level of
sulfur removal can be reached by using conventional or
advanced CoMo and NiMo catalysts. However, simul-
taneous hydrogenation of olefins should be minimized
because it reduces the octane number. Also aromatics are
not desired in the final gasoline product. Process applica-
bility is determined by its efficiency in terms of end product
yield and specifications. Instead of further improving
traditionally applied catalysis-based HDS technologies in
small steps, now might be the right time for advanced
desulfurization technologies which provide effective sulfur
removal and simultaneously increase the octane number.
1.2. Diesel
Diesel fuel is formed from straight run diesel, light cycle
oil from the FCC unit, hydrocracker diesel, and coker diesel.Nowadays, diesel is desulfurized by hydrotreating all
blended refinery streams. To get diesel with less sulfur
content the hydrotreating operation has to be more severe.
For straight run diesel, sulfur removal is the only point of
concern in hydrotreating since the other diesel specifications
(e.g. cetane number, density, and polyaromatics content) are
satisfactorily met. Hydrocracker diesel is usually relatively
high in quality and does not require additional treatment to
reduce the sulfur content.
As with gasoline, the diesel produced by the FCC and
coker units contains up to 2.5 wt% sulfur. Both the FCC and
coker diesel products have very low cetane numbers
(slightly above 20), high densities, and high aromatics andpolyaromatics content (about 8090%). In addition to being
desulfurized, these streams must be upgraded by high
pressure and temperature processes requiring expensive
catalysts. Another problem is that at high temperature the
hydrogenation dehydrogenation equilibrium shifts toward
aromatics. As with gasoline desulfurization, there are many
options for developing and applying advanced desulfuriza-
tion technologies with simultaneous upgrading to higher
diesel specifications.
1.3. Non-transportation fuels
Non-transportation fuels are formed from vacuum gas
oils and residual fractions from coking and FCC units. The
sulfur content requirement for non-transportation fuels is
less strict than for gasoline and diesel because industrial
fuels are used in stationary applications where sulfur
emissions can be avoided by combustion gas cleaning
processes. In particular, high temperature solid adsorbents
based on zinc titanate [1012] or manganese/alumina
[1315] are currently receiving much attention. In practice,
the major process is the capture of SOx with CaO producing
CaSO4 [1619]. Of course, for non-transportation fuels
Fig. 1. External and internal factors influencing modern refineries.
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HDS technologies can also be applied without considering
other fuel specifications that must be met for gasoline and
diesel fuels.
It is also important to note that in some cases the HDS
process requirements are between those for transportation
and non-transportation fuels. For instance, in large ships andpower plants ample space is available for dedicated
equipment aiming at reduction of emission of SOx and
soot that makes the requirement to sulfur content less strict.
It has to be expected that the sulfur level requirements
will become more and more strict in the near future,
approaching zero sulfur emissions from burned fuels. The
next generation of engines, especially fuel cell based
engines, will also require fuels with extremely low
(preferably zero) sulfur content. Therefore, scientists and
engineers involved in improving current refinery technol-
ogies and developing advanced technologies should shoot
for complete sulfur removal from refinery products.
The applicability of various desulfurization technologies
should be evaluated taking into account all requirements for
the produced fuels. The most effective options for ultra deep
desulfurization should be chosen since removing all sulfur
from the fuels might be too expensive or result in refinery
CO2 emissions which are too high [9,20].
The aim of this paper is to analyze different desulfuriza-
tion technologies for crude oil and refinery streams and to
formulate challenges for innovative research. We purport
that break-through innovations in oil refinery related
desulfurization are still possible. The desulfurization
processes currently employed in some refineries and in
semi-commercialized and laboratory proven approaches arediscussed.
Special attention is paid to development and application
of new desulfurization reactors and some examples of
advanced options for reactor design are mentioned. In a
separate chapter, structured monolithic catalytic reactors are
discussed since they can be readily applied to desulfuriza-
tion processes.
2. Classification of desulfurization technologies
There is a no universal approach to classify desulfuriza-
tion processes. The processes can be categorized by the fate
of the organosulfur compounds during desulfurization, the
role of hydrogen, or the nature of the process used (chemical
and/or physical).B ased on the w ay in which the organosulfur
compounds are transformed, the processes can be divided
into three groups depending on whether the sulfur
compounds are decomposed, separated from refinery
stream without decomposition, or both separated and
than decomposed (Fig. 2). When organosulfur com-
pounds are decomposed, gaseous or solid sulfur products
are formed and the hydrocarbon part is recovered and
remains in the refinery streams. Conventional HDS is the
most typical example of this type of process. In other
processes, the organosulfur compounds are simply
separated from the refinery streams. Some processes of
this type first transform the organosulfur compounds into
other compounds which are easier to separate from the
refinery streams. When streams are desulfurized by
separation, some desired product can be lost and disposal
of the retained organosulfur molecules is still a problem.
In the third type of process, organosulfur compounds
are separated from the streams and simultaneously
decomposed in a single reactor unit rather than in a
series of reaction and separation vessels. These combined
processes, which provide the basis for many technologies
currently proposed for industrial application, may prove
very promising for producing ultra-low sulfur fuels.
Desulfurization by catalytic distillation is the fascinatingexample of this type of process.
Desulfurization processes can be also classified in two
groups, HDS based and non-HDS based, depending
on the role of hydrogen in removing sulfur. In HDS
based processes, hydrogen is used to decompose
organosulfur compounds and eliminate sulfur from
refinery streams while non-HDS based processes do not
Fig. 2. Classification of desulfurization processes based on organosulfur compound transformation.
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require hydrogen. Different combinations of refinery
streams pre- or post-distilling treatments with hydrotreat-
ing to maintain desired fuel specifications can also be
assigned as HDS based processes since HDS treatment is
one of the key steps.
The two above-mentioned classifications overlap to
some extent. Most sulfur elimination processes, with the
exception of selective oxidation, are HDS based. The
organosulfur compound separation processes are usually
non-HDS based since they do not require hydrogen if
concentrated sulfur-rich streams are not subsequently
hydrotreated.
Finally, desulfurization processes can be classified
based on the nature of the key physico-chemical process
used for sulfur removal (Fig. 3). The most developed andcommercialized technologies are those which catalyti-
cally convert organosulfur compounds with sulfur
elimination. Such catalytic conversion technologies
include conventional hydrotreating, hydrotreating with
advanced catalysts and/or reactor design, and a combi-
nation of hydrotreating with some additional chemical
processes to maintain fuel specifications. Technologies of
this type are discussed further in the Section 3.
The main feature of the technologies of the second type
is the application of physico-chemical processes different
in nature from catalytic HDS to separate and/or to
transform organosulfur compounds from refinery streams.
Such technologies include as a key step distillation,
alkylation, oxidation, extraction, adsorption or combination
of these processes. These processes will be discussed in
Section 4.
3. Catalysis based HDS technologies
3.1. Conventional HDS: catalysts and reactivity
Catalytic HDS of crude oil and refinery streams carried
out at elevated temperature and hydrogen partial pressure
converts organosulfur compounds to hydrogen sulfide (H2S)
and hydrocarbons. Detailed analysis of the HDS process is
presented in the literature [21,22] so we discuss only the
general aspects here.
The conventional HDS process is usually conducted over
sulfided CoMo/Al2O3 and NiMo/Al2O3 catalysts [21]. Their
performance in terms of desulfurization level, activity and
selectivity depends on the properties of the specific catalyst
used (active species concentration, support properties,
synthesis route), the reaction conditions (sulfiding protocol,
temperature, partial pressure of hydrogen and H2S), nature
and concentration of the sulfur compounds present in the
feed stream, and reactor and process design.
Organosulfur compounds are usually present in almost
all fractions of crude oil distillation. Higher boiling point
fractions contain relatively more sulfur and the sulfur
compounds are of higher molecular weight. Therefore, a
wide spectrum of sulfur-containing compounds should be
considered from the viewpoint of their reactivity in the
hydrotreating processes. In Table 1 some of the organo-
sulfur compounds of interest, namely, mercaptans, sulfides,
disulfides, thiophenes and benzothiophenes (BT), and their
alkylated derivatives are mentioned with the proposed
mechanism of sulfur removal. Of course, for deep
desulfurization of refinery streams, polynuclear organic
sulfur compounds are also of interest. However, as they are
rather stable under conventional HDS conditions wedecided not to list them in Table 1. Moreover, their
desulfurization reaction pathway is more complex com-
pared with alkylated dibenzothiophene, and is not well
understood.
The reactivity of organosulfur compounds varies widely
depending on their structure and local sulfur atom
environment. The low-boiling crude oil fraction contains
mainly the aliphatic organosulfur compounds: mercaptans,
sulfides, and disulfides. They are very reactive in conven-
tional hydrotreating processes and they can easily be
completely removed from the fuel. Other processes like
Fig. 3. Desulfurization technologies classified by nature of a key process to remove sulfur.
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Merox can be applied to extract mercaptans and disulfides
from gasoline and light refinery streams [23].For higher boiling crude oil fractions such as heavy
straight run naphtha, straight run diesel and light FCC
naphtha, the organosulfur compounds pre-dominantly con-
tain thiophenic rings. These compounds include thiophenes
and benzothiophenes and their alkylated derivatives. These
thiophene containing compounds are more difficult than
mercaptans and sulfides to convert via hydrotreating. The
heaviest fractions blended to the gasoline and diesel pools
bottom FCC naphtha, coker naphtha, FCC and coker
dieselcontain mainly alkylated benzothiophenes, diben-
zothiophenes (DBT) and alkyldibenzothiophenes, as well as
polynuclear organic sulfur compounds, i.e. the least reactive
sulfur compounds in the HDS reaction.HDS of model organosulfur compounds as well as
industrial fuels have been the subject of many investigations
(see, for example, [21,22,2429]). As reaction conditions,
reactor type, catalyst, and feed composition vary from study
to study, the observed data do not always agree. However,
some general conclusions about reaction mechanism and
catalyst efficiency can be made based on the published data.
HDS of thiophenic compounds proceeds via two reaction
pathways (Table 1). Via the first pathway the sulfur atom is
directly removed from the molecule (hydrogenolysis path-
way). In the second pathway the aromatic ring is
Table 1
Typical organosulfur compounds and their hydrotreating pathway
Type of organic sulfur compound Chemical structure Mechanism of hydrotreating reactiona
Mercaptanes R SHRSH H2 !RH H2S
Sulfides R1
SR2
R1
SR2
H2!
R1
H R2
H H2SDisulfides R1SSR2 R1SSR2 H2 !R
1H R2H H2S
Thiophene
Benzothiophene
Dibenzothiophene
a Reaction pathway for alkylated thiophene, benzothiophene and dibenzothiophene is similar to the reaction of nonalkylated counterparts.
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hydrogenated and sulfur is subsequently removed (hydro-
genation pathway). Both pathways occur in parallel
employing different active sites of the catalyst surface.
Which reaction pathway pre-dominates depends on the
nature of the sulfur compounds, the reaction conditions, and
the catalyst used. At the same reaction conditions, DBT
reacts preferably via the hydrogenolysis pathway whereasfor DBT alkylated at the 4 and 6 positions both the
hydrogenation and hydrogenolysis routes are significant
[26,28].
The reactivity of sulfur compounds in HDS follows this
order (from most to least reactive): thiophene . alkylated
thiophene . BT . alkylated BT . DBT and alkylated
DBT without substituents at the 4 and 6 positions .
alkylated DBT with one substituent at either the 4 or 6
position . alkylated DBT with alkyl substituents at the 4
and 6 positions [25,29]. Deep desulfurization of the fuels
implies that more and more of the least reactive sulfur
compounds must be converted.
Since from study to study the parameters of the HDS
process differ, the reported values of catalyst activity and
selectivity vary a lot. For example, in a continuous-flow
reactor, a NiMo catalyst was found to be more active than a
CoMo catalyst for desulfurizing 4,6-dimethyldibenzothio-
phene (DMDBT) [30]. In contrast, desulfurization of the
same sulfur compounds in a batch reactor has been reported
to be more efficient with a CoMo catalyst [31]. However,
despite the differences in the experimental data, some
general conclusions about the performance of NiMo and
CoMo based catalysts can be made [21,22].
Conventional CoMo catalysts are better for desulfuriza-
tion via the hydrogenolysis pathway since the CoMohydrogenation activity is relatively low and, as a result,
relatively little hydrogen is consumed. This makes CoMo
catalysts attractive in HDS of unsaturated hydrocarbon
streams like FCC naphtha. In contrast, NiMo catalysts
possess high hydrogenation activity. Therefore, they are
preferable for HDS of refinery streams that require
extensive hydrogenation.
3.2. Advanced HDS: catalyst, reactor and processing
Deep desulfurization of refinery streams becomes
possible when the severity of the HDS process conditions
is increased. Unfortunately, more severe conditions result
not only in a higher level of desulfurization but also in
undesired side reactions. When FCC gasoline is desulfur-ized at higher pressure, many olefins are saturated and the
octane number decreases. Higher temperature processing
leads to increased coke formation and subsequent catalyst
deactivation. It is also important to note that in practice the
severity of the operating conditions is limited by the HDS
unit design.
Instead of applying more severe conditions, perhaps
HDS catalysts with improved activity and selectivity can be
synthesized. Ideal hydrotreating catalysts should be able to
remove sulfur, nitrogen and, in specific cases, metal atoms
from the refinery streams. At the same time they must also
improve other fuel specifications, such as octane/cetane
number or aromatics content, which are essential for high
fuel quality and meeting environmental legislation stan-
dards. Hydrotreating efficiency can also be increased by
employing advanced reactor design such as multiple bed
systems within one reactor, new internals in the catalytic
reactor or new types of catalysts and catalyst support (e.g.
structured catalysts). The best results are usually achieved
by a combination of the latter two approaches, namely,
using an appropriate catalyst with improved activity in a
reactor of advanced design.
3.2.1. Advanced HDS catalysts
To improve catalyst performance, all steps in the catalystpreparationchoice of a precursor of the active species,
support selection, synthesis procedure and post-treatment of
the synthesized catalystsshould be taken into account.
Different approaches have resulted in new catalyst formu-
lations (Fig. 4) and some examples are considered here.
Applying a new catalyst manufacturing technology,
Akzo Nobel introduced in 1998 new, highly active CoMo
Fig. 4. Different approaches to improve HDS catalyst performance [3236, 4050].
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and NiMo catalysts [5153] referred to as STARS (Super
Type II Active Reaction Sites). Under usual HDS operating
conditions, these catalysts are claimed to desulfurize
refinery streams down to 2 5 ppm of sulfur and to
significantly reduce polyaromatic content and improve the
cetane number and density of diesel fuels. Both CoMo and
NiMo catalysts can be used for deep desulfurization buttheir efficiency is determined by the feedstock properties
[51]. The CoMo STARS catalysts are preferable for streams
with relatively high sulfur levels of 100 500 ppm and
perform better than NiMo catalysts at low pressure. In
contrast, the NiMo STARS catalysts are especially suitable
for fuels with low sulfur levels (below 100 ppm) at high
pressure. Commercial results of STARS catalysts are
reported to be promising. They show a stable high level of
desulfurization during a long-term run of 400 days on
stream. The CoMo STARS catalyst makes it possible to run
a conventional HDS unit with output sulfur levels of 10
20 ppm for feed rates up to 30% higher than those for which
the unit was designed without revamping the equipment
[53,54].
Another Akzo Nobel catalyst preparation technology has
been claimed to result in extremely active hydrotreating
catalyststhe so-called NEBULA catalysts (NEBULA,
NEw BULk Activity) [55]. In these catalysts, which are
also active in sulfided form, the active phase and the carrier
are different in nature from conventional HDS catalysts. The
hydrogen consumption is relatively high and these catalysts
are suitable for diesel hydrotreating both at medium severity
conditions and at high pressure. NEBULA catalysts have
already been applied in two commercial units.
A similar approachto enhance catalyst activity bymodifying the preparation routewas employed by
Criterion Catalysts and Technologies and resulted in
Criterions CENTINEL catalyst family [56]. The CENTI-
NEL catalysts are claimed to combine superior hydrogen-
ation activity and selectivity. At lower H2 pressures and for
high sulfur content streams, CoMo CENTINEL catalysts are
preferable. For high H2 pressures and low sulfur content
(below 50 ppm) NiMo CENTINEL catalysts are more
useful.
Combining new types of active catalytic species with
advanced catalyst supports such as ASA (amorphous silica
alumina) [37,38] can result in extremely high desulfuriza-
tion performance. The application of ASA-supported noble
metal based catalysts for second-stage deep desulfurization
of gas oil is an example [37,38]. The Pt and PtPd catalysts
are very active in the deep HDS of pre-hydrotreated
straight-run gas oil under industrial conditions. These
catalysts are able to reduce sulfur content down to 6 ppm
while simultaneously reducing aromatics to 75% of their
initial amount [57]. The PtPd/ASA catalysts are excellent
for feeds with low or medium sulfur content and low
aromatics levels (Fig. 5). At higher aromatics levels, the Pt/
ASA catalysts perform better than PtPd/ASA. At high sulfur
levels, the ASA supported noble metal catalysts are
poisoned by sulfur and NiW/ASA catalysts become
preferable for deep sulfur removal and dearomatization.
Application of noble metal catalysts for deep HDS is
limited by their sulfur resistance. Therefore, noble metal
catalysts are usually used when most of the organosulfur
compounds and H2S have been removed from the process
stream. A new concept of HDS catalyst design has been
proposed to increase the sulfur resistance of noble metalhydrotreating catalysts [39]. The proposed catalyst is
bifunctional. It combines catalyst supports with bimodal
pore size distribution (e.g. zeolites) and two types of sulfur
resistant active sites. The first type of active sites, placed in
large pores, is accessible for large organosulfur compounds
and is sensitive to sulfur inhibition (sulfur resistant sites of
the type I). The second type of active sites, placed in small
pores, is not accessible for organosulfur compounds and is
stable against poisoning by H2S (sulfur resistant sites of type
II). Since hydrogen can easily access the sites located in the
small pores, it can be adsorbed dissociatively and
transported between pore systems to regenerate thepoisoned metal sites of type I. Auto regeneration is ensured,
so the HDS activity remains high even for feeds with high
sulfur content.
The concept looks very interesting, although successful
application has not yet been demonstrated. Moreover, a
number of questions of scientific interest should be solved.
Appropriate design of active sites of different sulfur
resistance is one of the key feature of this concept. Supports
with appropriate texture and surface chemistry must be
developed. For example, monolith supports with washcoats
of regular structure (discussed in Section 5) might be
attractive.
3.2.2. New reactor systems
3.2.2.1. Counter-current operation. Aside from improving
the catalysts, upgrading hydrotreating equipment is an
option. Conventionally used hydrotreating reactors are
fixed-beds with co-current supply of oil streams and
hydrogen, resulting in unfavorable H2 andH2S concentration
profiles through the reactor. Due to high H2S concentrationat
the reactor outlet, the removal of the last ppm S is inhibited.
Counter-current operation can provide a more preferable
concentration profile. In counter-current reactor operation
Fig. 5. Classification of ASA based catalysts for deep HDS of the feed of
different composition [57].
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mode, the oil feed is introduced into the reactor at the top and
hydrogen is introduced at the bottom of the reactor, in the
place where its presence is most desired. H2S is removed
from the reactor at the top, avoiding possible recombination
of H2S and olefins at the reactor outlet.
A commercial example of this approach is the hydro-
treating process based on SynSat Technology, whichcombines Criterions SynSat catalysts and ABB Lummus
reactor technologies [58,59]. The general process scheme is
shown in Fig. 6.
In the first stage, the feed and hydrogen co-currently
contact the catalyst bed and the bulk of the organosulfur
compounds is converted. This is followed by the removal of
H2S from the reactant flow. The second stage of the reactor
system operates in the counter-current mode providing more
favorable concentration profiles of H2S and H2 over the
length of the reactor.
Such a configuration allows for application of catalysts
that are intrinsically very active but sensitive to sulfurpoisoning, such as the noble metal based catalysts. The
Scanraffs SynSat gas oil hydrotreating unit in Sweden uses
a noble-metal catalyst in the second stage of the process.
Industrial application of SynSat Technologies illustrates the
ability of the counter-current approach not only to remove
sulfur, but also to remove nitrogen and aromatics as well. It
was reported that a sulfur level of 1 ppm and an aromatics
level of 4 vol% could be attained [58].
3.2.2.2. Ebullated bed reactors. The ebullated bed reactor
[60] is an example of other types of reactors aimed at HDS
of heavy refinery streams, processing of which results in
very fast catalyst deactivation due to coke formation. This
type of reactor also has good heat transfer so overheating of
the catalyst bed is minimized and less coke forms. An
ebullated bed is used by a.o. IFP (Institut Francais du
Petrole, France) in the so-called T-Star process to
desulfurize heavy feedstocks such as deep cut heavy
vacuum gas oils, coker gas oils, and some residues [61].
In this unit, the catalyst particles are fluidized by the feed
and hydrogen and are therefore well mixed with the feedstream. Bed plugging and channeling are avoided and
the unit operates nearly isothermally with a constant
low-pressure drop. It is also very convenient that the
catalyst activity can be controlled by adding and with-
drawing catalyst particles. In comparison with fixed-bed
HDS catalysts, the additional requirement for T-Star
catalysts is that they be mechanically stable and resist
attrition. Integration of the T-Star process with inline
hydrotreating produces diesel with less than 50 ppm sulfur
and FCC feed with 1000 1500 ppm sulfur, which will
result in FCC gasoline sulfur of 3050 ppm [61].
As an example of the processes employing a special
reactor design and modified catalyst system for HDS of a
large variety of feedstocks, the so-called Prime processes
(Prime-G, Prime-G , and Prime-D) developed by IFP
must be mentioned [62,63]. They combine mild operating
conditions with relatively high space velocities utilizing a
dual catalyst system. The Prime HDS technology results in
minimal olefin saturation in the case of FCC gasoline
desulfurization, and polyaromatics reduction and cetane
number improvement in the case of gas oils treatment. The
Prime technology enables over 98% desulfurization of the
entire FCC naphtha cut. Prime reactors fit easily into any
refinery configuration and currently five units are in
operation.
3.2.3. Combinations of hydrotreating with other reactions
Sulfur removal by HD S processes is usually
accompanied by other hydrogenation reactions, which are
particularly undesired for FCC gasoline streams where
olefins are present. Olefin saturation during hydrotreating
results in octane loss of the final gasoline pool. Different
options of FCC gasoline treatment before or after desulfur-
ization in the HDS unit can be considered to compensate for
the loss of octane.
3.2.3.1. Aromatizing and hydrotreating. Aromatizing of the
cracked gasoline before HDS treatment was proposed by
Phillips Petroleum Co. [64]. By combining pre-aromatiza-
tion of FCC gasoline streams with conventional HDS, sulfur
content decreases from 300 to 10 ppm and the octane
number increases from 89 to 100. Despite almost complete
olefin saturation, octane is boosted by increasing the
aromatics amount in the end product up to 68 wt%.
However, it is fair to state that a high level of aromatics
in the final product makes application of the proposed
technology less attractive since new environmental rules
require a limited amount of aromatics in the gasoline.Fig. 6. Co-current/counter-current Syn Technology process scheme.
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3.2.3.2. Hydrotreating and octane boosting (ISAL process1).
The ISAL process, which was specially developed for
hydrotreating FCC gasoline, combines conventional HDS
with post-treatment of the products to minimize the
decrease in octane number [6567]. As in conventional
hydrotreating, it saturates olefins present in the feed, but the
resulting octane loss is compensated by octane-enhancingreactions.
The key point of the process is the catalyst formulation.
Due to improved catalyst desulfurization activity and
nitrogen and sulfur tolerance, the ISAL process employs
one fixed-bed reactor unit with the catalyst system divided
in a multiple bed configuration. For example, typically
combination of CoMoP/Al2O3 and GaCr/H-ZSM-5 cata-
lysts is applied [68].
The flow scheme of the ISAL process is similar to that of
a conventional hydrotreating process. As a result, the ISAL
process can be easily implemented as a new process unit or
as a revamp of existing hydroprocessing units. It was very
efficient at reducing sulfur from 1450 ppm in a naphtha feed
to 10 ppm in the final product with almost no decrease in
octane number [67,68].
3.2.4. Catalytic distillation
To avoid octane loss with deeper desulfurization, the
FCC gasoline stream can be fractionated by distillation
before desulfurization and each fraction can be desulfurized
at appropriately severe conditions. This option is efficient
since the olefins are mainly concentrated in the low-boiling
fraction of the FCC naphtha whereas the sulfur compounds
are mainly present in the high-boiling fraction. Moreover,
the nature of sulfur compounds in light and heavy naphthafractions is different and, therefore, they can be hydrotreated
advantageously at different selective conditions, preserving
olefins in the final product. But realizing this approach
requires multiple hydrotreating reactorsone reactor per
fraction. Combining distillation and reaction in a single
vessel is a breakthrough. The elegant technology of sulfur
removal employing distillation and HDS (catalytic distilla-
tion (CD)) has been introduced by CDTech Company
[6971]. The process is based on simultaneously desulfur-
izing and splitting the FCC naphtha stream into fractions
with different boiling points. The simplified CDHDS
process flow is shown in Fig. 7.The main feature of the process is that, depending on the
FCC naphtha properties and desired product specification, a
distillation column is loaded with a hydrotreating catalyst at
different levels of the column or throughout the whole
column. Desulfurization conditions are different for light
and heavy fractions, their severity being nicely controlled
by the boiling temperature of the naphtha fraction. The
lighter fractions, which contain most of the olefins and
easily removable sulfur compounds, are subjected to
desulfurization at lower temperatures at the top of the
column. That leads to higher desulfurization selectivity andless hydrocracking and/or saturation of olefinic compounds.
The higher boiling portions, containing heavily desulfurized
sulfur compounds, are subjected to desulfurization at higher
temperatures at the bottom of the distillation column
reactor. The reaction zone cannot overheat since the
heat released during the HDS reaction is used to boil
the hydrocarbon stream. This leads to nearly perfect heat
integration.
The CDHDS process efficiency has been demonstrated at
Motivas Port Arthur, Texas Refinery with the application of
a commercially available catalyst loaded in a proprietary
distillation structure provided by CDTech [72]. Over thefirst four months of operation, the technology showed a
stable desulfurization level of about 90% with an average
octane number loss of less than 1.
To improve process feasibility and increase product
yield a two stage CDTechw process including CDHydro
(production of sweet light cut naphtha with very low
mercaptan content and increased octane) and CDHDS
processes has been proposed [70,71]. It is claimed that
the technology of CDTechw is about 25% less expensive
than the conventional HDS process, making it very
attractive for refineries.
4. Non-HDS based desulfurization technologies
Technologies that do not use hydrogen for catalytic
decomposition of organosulfur compounds are discussed
here as non-HDS based desulfurization technologies. The
following approaches are considered to be attractive for
attaining high levels of sulfur removal by shifting the
boiling point of sulfur-containing compounds, separating by
extraction or adsorption, and decomposition via selective
oxidation.
Fig. 7. Simplified flow scheme for CDHDS based technology.
1 The name of the technology comes from isomerization and Salazar -
name of the technology inventor.
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4.1. Shifting the boiling point by alkylation
When the boiling temperature of organosulfur com-
pounds is shifted to a higher value, they can be removed
from light fractions by distillation and concentrated in the
heavy boiling part of the refinery streams. British Petroleum
used this approach in a new advanced technology process
for desulfurizing FCC gasoline streamsolefinic alkylation
of thiophenic sulfur (OATS) [7375].
The process employs alkylation of thiophenic com-
pounds via reaction with olefins present in the stream:2
As a result the boiling temperature of the sulfur
containing hydrocarbon compounds increases. In compari-
son with thiophene (boiling point around 85 8C), alkylated
thiophenes such as 3-hexylthiophene or/and 2-octylthio-
phene have a much higher boiling point (221 and 259 8C,
respectively). This enables them to be easily separated from
the main gasoline stream by distillation. The high-boiling
compounds produced can be blended into the diesel pool
and desulfurized by conventional hydrotreating as theoctane number is not important for diesel.
The OATS technology consists of a pre-treatment
section, an OATS reactor, and a product separation unit
(Fig. 8) [73].
Thiophenic sulfur is alkylated in an OATS reactor
employing acidic OATS catalysts such as BF3, AlCl3,ZnCl2, or SbCl5 deposited on silica, alumina or silica
alumina supports [78].
A fter the OATS reactor, the feed is sent to a
conventional distillation column where it is separated
into a light sulfur-free naphtha and a heavy sulfur-rich
stream. The light naphtha is directly sent to the gasoline
pool and the heavy stream is preferably hydrotreated. The
hydrotreater is not an essential part of the OATS
technology, but its application after the fractionator
increases the product yield. Employing the OATS
technology, over 99.5% of the sulfur can be removed
from the gasoline stream [74,79]. Demonstration exper-iments showed sulfur reduction in gasoline from 2330 ppm
to less than 20 ppm with only two octane number loss [74].
Another advantage of the OATS process is that less
hydrogen is consumed since only a relatively low volume
of the FCC gasoline stream is hydrotreated.
The efficiency of the OATS process can be limited by
competing processesalkylation of aromatic hydrocarbons
and olefin polymerization. Fortunately, under the conditions
employed alkylation of the sulfur-containing compounds
occurs more rapidly than that of aromatics. One of the
disadvantages of the OATS process is that the alkylated
sulfur compounds produced require more severe hydro-
treating conditions to eliminate sulfur.To our knowledge, there is no information in the open
literature about catalyst durability and other key process
characteristics. It seems that many issues must be studied
and prov en bef ore OATS t echnol ogy can be
commercialized.
4.2. Desulfurization via extraction
Extractive desulfurization is based on the fact that
organosulfur compounds are more soluble than hydrocar-
bons in an appropriate solvent. The general process flow is
Fig. 8. The OATS process flow scheme.
2 If CH3I or AgBF4 is used as an additional alkylation agent, S-
alkylsulfonium salts are formed and sulfur is removed from fuel oil as
precipitates [76,77]. As a result, fuel oil can be desulfurized to less than
30 ppm S. The desulfurization level can be further increased by increasing
the alkylating agent/sulfur ratio. Taking into account the high cost of
alkylating agents, this approach does not seem to be economically feasible
on an industrial scale. Another disadvantage is the decrease in olefin
concentration due to their reaction with the alkylating agents.
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shown in Fig. 9. In a mixing tank, the sulfur compounds are
transferred from the fuel oil into the solvent due to their
higher solubility in the solvent. Subsequently, the solvent
fuel mixture is fed into a separator in which hydrocarbons
are separated from the solvent. The desulfurized hydro-
carbon stream is used either as a component to be blended
into the final product or as a feed for further transformations.The organosulfur compounds are separated by distillation
and the solvent is recycled.
The most attractive feature of the extractive desulfuriza-
tion is the applicability at low temperature and low pressure.
The mixing tank can even operate at ambient conditions.
The process does not change the chemical structure of the
fuel oil components. As the equipment used is rather
conventional without special requirements, the process can
be easily integrated into the refinery. To make the process
efficient, the solvent must be carefully selected to satisfy a
number of requirements. The organosulfur compounds must
be highly soluble in the solvent. The solvent must have a
boiling temperature different than that of the sulfur
containing compounds, and it must be inexpensive to
ensure economic feasibility of the process.
Solvents of different nature have been tried, among
which acetone, ethanol [80], polyethylene glycols [81], and
nitrogen containing solvents [82] showed a reasonable level
of desulfurization of 50 90% sulfur removal, depending on
the number of extraction cycles.
The GT-DeSulfSM process is an example of desulfuriza-
tion technology based on organosulfur compound extraction
[83]. This process separates the organosulfur compounds
and aromatics from FCC naphtha by extractive distillation
using a blend of solvents. A desulfurized/dearomatisedolefin rich gasoline stream and an aromatic stream contain-
ing the sulfur compounds are formed after treatment in a
GT-Desulf reactor. The first stream is directly used as a
gasoline blend stock. Unfortunately, available literature
does not contain any information on the level of sulfur
removal from the treated stream. The aromatics fraction
with the sulfur compounds is sent to a HDS reactor. After
treatment in the HDS reactor, aromatics recovery is
proposed as an additional option to increase economic
efficiency of the process. The authors pointed out that
the GT-DesulfSM process is economically favorable due to
an integrated approach to the refinery processing (segre-
gated sulfur removal and aromatics recovery) and lower
hydrogen consumption since less FCC naphtha is treated in
the HDS reactor.
The efficiency of extractive desulfurization is mainly
limited by the solubility of the organic sulfur compounds inthe solvent. Solubility can be enhanced by choosing an
appropriate solvent taking into account the nature of the
sulfur compounds to be removed. This is usually achieved
by preparing a solvent cocktail such as acetoneethanol
[80] or a tetraethylene glycolmethoxytri glycol mixture
[81]. Preparation of such a solvent cocktail is rather
difficult and intrinsically non-efficient since its composition
depends strongly on the spectrum of the organosulfur
compounds present in the feed stream.
Solubility can also be enhanced by transforming the
organic sulfur compounds to increase their solubility in
a polar solvent. One way to do this is by selective oxidizing
the organic sulfur compound (thiophene, BTs, DBTs) to
sulfones possessing higher polarity. This type of desulfur-
ization process can be also considered as oxidative
desulfurization technology. They are mentioned here
because they employ liquid/liquid extraction to separate
sulfur-containing compounds from refinery streams.
4.2.1. Desulfurization via conversion and extraction
Conversion/extraction desulfurization (CED) technology
began in 1996 when Petro Star Inc. combined conversion
and extraction to remove sulfur from diesel fuel [84,85].
Before liquid/liquid extraction, the fuel is mixed with an
oxidant (peroxoacetic acid). The oxidation requires astoichiometric amount of the oxidant and proceeds at
temperatures below 100 8C at atmospheric pressure. In
laboratory-scale experiments straight-run diesel fuel with
4200 ppm S was treated to below 10 ppm S [84]. Other fuel
specifications like cetane number, API gravity and aro-
matics content were also improved.
Reducing the oxidant cost is one way to improve the
economic feasibility of this technology. Again, a solvent
cocktail should be more suitable than an individual solvent,
but additional investigations are required to determine
Fig. 9. General process flow of extractive desulfurization.
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the appropriate composition. The processes for deep extract
treatment to recover sulfur from the concentrated sulfur-rich
extract and to return most of the hydrocarbons to the product
stream must be developed to enhance the CED process
performance.
4.2.2. UniPure aromatic sulfur reduction technology [86]The UniPure process is also based on oxidizing aromatic
sulfur compounds before extracting them. The main
difference from the CED technology is that an aqueous
phase is applied along with a dissolved oxidation catalyst.
Organosulfur compounds are claimed to be converted to
sulfones at nearly atmospheric pressure and mild tempera-
ture (up to 120 8C) within short residence times (about
5 min) [86]. After separation of the aqueous and oil streams,
the process follows the same scheme as the CED process.
The sulfur level is reported to be reduced from 270 to 2 ppm
sulfur. Pilot plant tests have been planned for the second
half of 2001.
Oxidation of the organosulfur compounds is the main
limiting step of the conversion/extraction desulfurization
technologies. Kinetics of the oxidation reaction can be
improved by employing photons or ultrasound. In the
following sections, this will be treated more in detail.
4.2.3. SulphCo desulfurization technology3 [87,88]
The SulphCo technology applies ultrasound energy to
oxidize sulfur compounds in a water fuel emulsion contain-
ing a hydrogen peroxide catalyst.4 Similar to the CED and
UniPure technologies, the SulphCo process operates at
7080 8C under atmospheric pressure. The residence time
for the ultra-sound reactor is reported to be only 1 min.No detailed discussion of the mechanism of the
desulfurization reaction is currently available. The authors
claim desulfurization efficiencies for crude oil and diesel up
to 80 and 98% sulfur removal, respectively. For light diesel
fuels, the proposed technology meets the 10 ppm S
requirement. The SulphCo process is reported to be
economically feasible. In accordance with the preliminary
estimation of Betchel Corp. scientists, the SulphCo unit will
cost about 50% of what an equivalent hydrotreater would
cost [88].
The first ultrasonic desulfurization unit has been installed
at the IPLOM petroleum refinery near Genoa in Italy. Itshowed continuous and successful desulfurization of diesel
fuel at a rate of up to 350 bbl per day.3
No detailed discussion of the mechanism of the
desulfurization reaction is currently available. The authors
do not disclose details of the process because of pending
patents and because they are still optimizing some stages of
the process, like elucidation of the mechanism of oxidation
reactions under ultra-sound excitation, optimization of the
solvent and catalyst composition.
4.2.4. Desulfurization by extractive photochemicaloxidation
Another desulfurization method combines photochemi-
cal reactions with extraction of the organosulfur com-
pounds into an aqueous-soluble solvent [9093]. The
sulfur containing hydrocarbons are suspended in an
aqueous-soluble solvent and irradiated by UV or
visible light in a specially designed photoreactor. This
results in the oxidation of the sulfur compounds. The
polar compounds formed are rejected by the non-polar
hydrocarbon phase and are concentrated in the solvent.
Photochemical reaction is assisted by a photosensitizer
9,10-dicyanoanthracene (DCA). Acetonitrile, which
provides relatively high solubility of initial and
oxidized sulfur compounds, was found to be the
most suitable solvent. After photooxidation, the solvent
and the hydrocarbon phases are separated, as in extractive
desulfurization. In addition, the recovery of aromatics
from the solvent and recovery of the photosensitizer from
the solvent and desulfurized hydrocarbon stream must be
done to increase product yield and economic efficiency.
Aromatics are usually recovered by liquidliquid extrac-
tion using light paraffinic solvents and are subsequently
blended into the desulfurized fuel stream [91,93]. DCA is
removed by adsorption, using a silica gel as an adsorbent.
It can be returned to the process after desorption withaqueous solution of acetonitrile. All of these processes are
rather common refinery processes (though not all of the
chemicals are common) that can be easily integrated into
the refinery and do not require special equipment or
conditions.
This photooxidation method showed a high selectivity
to remove sulfur from light oils [90,91], catalytic-cracked
gasoline [92], and vacuum gas oils [93]. The sulfur
content in commercial light oil can be reduced to below
50 ppm [91]. For fuels with higher aromatics content,
efficiency is slightly lower but it was claimed that above
99% of the sulfur was removed from a vacuum gas oil
[93].
At the present stage, the extractive photooxidation
desulfurization process is rather far from being widely
applied in industry. There are a number of problems that
have to be solved to make the process technically and
economically feasible. A solvent has to be carefully selected
from the viewpoint of sulfur compounds solubility and
aromatic rejection. Combination of a solvent and a
photosensitizer has to be optimized to increase the rate of
the organosulfur compounds phototransformation. Some
opportunities for improvement of separation processes still
exist, in particular the DCA recovery. It is promising to
3 http://www.sulphco.com/technology.htm4 Ultrasonic energy can be also used for catalytic HDS of thiophene.
There is only one preliminary report about ultrasound desulfurization of
thiophene water ethanol mixture employing Ni/Al2O3 or Ni/ZnO
catalysts at low temperature and atmospheric pressure [89]. The
combination of ultrasound and catalyst results in water decomposition to
provide hydrogen for thiophene desulfurization. The reported
desulfurization level is about 3040 mol% of thiophene conversion.
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stabilize a photosensitizer on the surface of a solid carrier
without losing its ability to accelerate photooxidation of the
sulfur compounds. This will simplify the general process
flow, eliminating the process of photooxidant recovery from
the fuel oils and the solvent. Also the reactor design is not
standard. It remains to be proven that reasonable photon
efficiency can be obtained.
4.3. Desulfurization by precipitation
Desulfurization by precipitation is based on the for-
mation and subsequent removal of insoluble charge-transfer
complexes. Preliminary experiments were reported for a
model organosulfur compound (4,6-DMDBT, referred to as
DBT) in hexane and gas oil, using 2,4,5,7-tetranitro-9-
fluorene (TNF) as the most efficient p-acceptor [94,95]. A
suspension of the p-acceptor and sulfur containing gas oil
was stirred in a batch reactor where insoluble charge-
transfer complexes between p-acceptor and DBT deriva-
tives formed. The consecutive steps include filtration to
remove the formed complex from gas oil and the recovery
of the p-acceptor excess using a solid adsorbent.
Currently the efficiency is very low. One treatment
results in the removal of only 20% of the present sulfur.
Moreover, there is a competition in complex formation
between DBT compounds and other non-sulfur aromatics
that results in low selectivity for DBT removal. The
experimental results reported are not very informative
because the role of other compounds that might form p-
complexes (aromatics, N-compounds) has not been
studied. Moreover, a large overstoichiometric amount of
TNF is used to provide good complexing and its excessshould be removed from the oil stream afterwards. It
seems interesting to introduce a complexing agent into a
solid organic or inorganic matrix. This would simplify
the process since the filtration and p-acceptor recovery
steps are avoided.
4.4. Selective oxidative desulfurization
Generally, desulfurization by selective oxidation consists
of two main steps: oxidation of sulfur compounds and
subsequent purification [96]. Some of the processes
employing oxidation as one of the key steps (CED,
SulphCo, UniPure, photochemical desulfurization) have
already been discussed as the extraction-based processes. In
the meantime, other methods like distillation, adsorption or
thermal decomposition can be used for separating oxidized
sulfur containing compounds from fuel streams.
To our knowledge, there is no information in the open
literature about combination of selective oxidation and
distillation to remove sulfur. But this approach is feasible, in
principle, because the oxidation of sulfur compounds to
sulfoxides or sulfones increases their boiling temperature.
The oxidative distillation desulfurization process will be
very similar to normal distillation if organosulfur
compounds will only be separated and their treatment will
be done elsewhere). If transformation of the sulfur
compounds is combined with distillation, the process
scheme might be similar to catalytic distillation desulfur-
ization (CDTech).
The possibility of selective oxidation of hydrocarbon
streams with different oxidizing agents (peroxides, peracids,molecular oxygen, and air) followed by thermal decompo-
sition of oxidized sulfur compounds were already described
more than 30 years ago [97]. Organosulfur compound
oxidation to gaseous sulfur compounds in the presence of
methanol has also been proposed. Sulfur is released mainly
as SO2 at low temperatures, and some H2S is formed if the
temperature of decomposition is above 300 8C. However,
efficiency of the process was low as only about 40% sulfur
removal was reported.
Direct selective oxidation of organosulfur compounds to
gaseous sulfur products and hydrocarbons is also an option.
Using oxygen or air rather than hydrogen to remove sulfur
from refinery streams is attractive due to the availability of
the reacting gas and its low price. The main issues of the
direct selective oxidation process are operation safety and
the formation of by-products (CO2, CO, etc.).
We checked the thermodynamic feasibility of selective
oxidation of thiophene and benzothiophene assuming
the formation of SO2 and hydrocarbons using air as an
oxidant. It appears to be thermodynamically feasible within
a temperature interval relevant for a refinery (typically 200
400 8C). It should be noted that, due to the reaction
stoichiometry, either bonded or molecular hydrogen is
needed. Otherwise the reactions resulting in sulfur elimin-
ation will be accompanied by the formation of unsaturatedcompounds that can lead to undesired polymerization or
coke formation. Water can be considered as one possible
hydrogen source, taking into account the availability and
safety. To make this process efficient, appropriate catalysts
with high selectivity for oxidation and decomposition must
be identified.
4.5. Desulfurization by adsorption on a solid sorbent
Desulfurization by adsorption (ADS) is based on the
ability of a solid sorbent to selectively adsorb organosulfur
compounds from refinery streams. Based on the mechanism
of the sulfur compound interaction with the sorbent, ADS
can be divided into two groups: adsorptive desulfurization
and reactive adsorption desulfurization. Adsorptive desul-
furization is based on physical adsorption of organosulfur
compounds on the solid sorbent surface. Regeneration of the
sorbent is usually done by flushing the spent sorbent with a
desorbent, resulting in a high organosulfur compound
concentration flow. Reactive adsorption desulfurization
employs chemical interaction of the organosulfur com-
pounds and the sorbent. Sulfur is fixed in the sorbent,
usually as sulfide, and the S-free hydrocarbon is released
into the purified fuel stream. Regeneration of the spent
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sorbent results in sulfur elimination as H2S, S, or SOx,
depending on the process applied.
Efficiency of the desulfurization is mainly determined by
the sorbent properties: its adsorption capacity, selectivity for
the organosulfur compounds, durability and regenerability.
4.5.1. Adsorptive desulfurizationAdsorptive desulfurization was studied by Salem and
Hamid [98,99] for removing sulfur from naphtha with a
550 ppm initial sulfur level in a batch reactor using
activated carbon, zeolite 5A, and zeolite 13X as solid
adsorbents. Activated carbon showed the highest capacity,
but a low level of sulfur removal. Zeolite 13X was superior
for sulfur removal from low sulfur streams at room
temperature. Therefore, a two-bed combination was pro-
posed for industrial application. The first bed contains
activated carbon and is able to remove up to 65% of the
sulfur at 80 8C. The second bed is loaded with zeolite 13X
and provides almost 100% sulfur recovery at room
temperature if the sorbent/feed ratio is about 800 g/l. No
data about sorbent regeneration were presented.
Activated carbon, zeolites, CoMo catalysts, and silica
alumina sorbents were tested for adsorptive desulfurization
of a mid-distillate stream with 1200 ppm S in a fixed bed
reactor [100]. The process was specially aimed at the
elimination of refractory 4- and 4,6-substituted dibenzothio-
phenes which are pre-dominantly present in the feed after
hydrotreatment. Activated carbon was claimed to possess
good desulfurization performance at 100 8C for 75 min. It is
not possible to estimate the sorbent capacity from the data
given. To regenerate the sorbent, the column was flushed
with toluene. Sorbent capacity was completely restoredwithin 2 h of flushing at 100 8C.
4.5.2. IRVAD process
An adsorption-based desulfurization technology called
IRVAD (combination of the inventor name IRVine and
ADsorption) was developed by Black and Veatch
Pritchard engineering company [101104]. It is targeted
to remove a wide spectrum of organosulfur compounds
from various refinery streams including FCC gasoline. A
simplified process scheme is shown in Fig. 10.
The process is based on moving bed technology and uses
a solid sorbent, which is counter-currently brought into
contact with a sulfur-rich hydrocarbon stream. The
desulfurized hydrocarbon stream is produced at the top of
the adsorber whereas the spent sorbent is withdrawn at the
bottom. The spent sorbent is circulated into the reactivator
where organosulfur compounds and some adsorbed hydro-carbons are desorbed from the sorbent surface. The
regenerated sorbent is recirculated back to the adsorber.
The IRVAD process employs alumina based selective
sorbents produced by Alcoa Industrial Chemicals. To
increase sorbent capacity and selectivity the initial support
was modified with an inorganic promoter [104]. However,
the sorbent formulation is not disclosed. The process
operates up to 240 8C, at low pressure with a hydro-
carbon/sorbent weight ratio of about 1.4. The reactivation
process requires slightly higher temperature. No hydrogen is
required so sulfur removal is not accompanied by undesired
olefin saturation. The efficiency of the IRVAD process was
demonstrated in pilot plant experiments for FCC feedstock
(1276 ppm S) and coker naphtha (2935 ppm), providing at
least 90% reduction in sulfur content [101].
The performance of the IRVAD process is limited by the
sorbent capacity and its affinity for sulfur compounds. As
adsorption of dibenzothiophene molecules occurs parallel to
the surface of the catalyst via the p-electron of the aromatic
ring [105], the sorbent capacity is rather low. As a result, a
high amount of sorbent is required for effective operation of
the desulfurization units.
In the adsorptive desulfurization processes, organosulfur
compounds are only concentrated. Additional downstream
treatment, preferably high-pressure hydrotreating, isrequired to eliminate sulfur. Efficiency of the process can
be increased by optimizing the sorbent properties in order to
improve hydrocarbon recovery during the reactivation
treatment. Some of the operating parameters, such as
adsorbent particle size, number of adsorptionreactivation
steps, weight ratio of the hydrocarbon feed to the adsorbent,
appropriate adsorption, and reactivation temperature, must
also be optimized before commercial application is possible.
Work on the IRVAD process is currently discontinued
because of a time limit to finalize the technology before
Fig. 10. Simplified adsorptive desulfurization process flow.
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the new sulfur levels are introduced in Europe and the
US [106].
4.5.3. Reactive adsorption desulfurization (Phillips S Zorb
sulfur removal technology)
The general desulfurization pathway of sulfur removalby reactive adsorption desulfurization can be described by
the following scheme [107]:
The sulfur atom is removed from the molecule and is
bound by the sorbent. The hydrocarbon part is returned to
the final product without any structural changes.
Employing the principle of reactive adsorption, Phillips
Petroleum Co., USA has proposed the so-called Phillips S
Zorb technology to remove sulfur from gasoline and diesel
fuels5 [108,109]. The S Zorb process is based on fluid bed
technology and the flow scheme is very similar to the
IRVAD technology, but operating conditions are more
severe T 340410 8C; P 220 bar to provide good
kinetics of the process.
It is claimed that the process removes about 98% of the
sulfur from gasoline (feed 1100 ppm, product 25 ppm)
with only 3% decrease in olefin concentration (0.5 1.5
octane number loss) and almost 100% hydrocarbon
recovery [108]. Less hydrogen is consumed than inconventional hydrotreating processes and the requirements
for hydrogen purity are not so strict. S Zorb application for
diesel fuels is now under development and test results
have confirmed the high level of sulfur removal [110]. The
process was proven on laboratory scale in October 1999.
An industrial pilot plant was designed between October
1999 and February 2000, and commercial start-up was
planned in March 2001 at the Phillips Petroleum Co.
refinery in Texas [108,109]. The key point of the S Zorb
processthe composition of S Zorb sorbentis not fully
revealed, but it can be assumed that zinc and other metal
oxides on suitable supports can be used in the process[109]. Zinc oxide is mentioned as the main component of
the desulfurization sorbent described in patents assigned to
Phillips Petroleum Co. [111,112]. The sorbent also
contains alumina, silica and nickel oxide.
4.5.3.1. Thermodynamic analysis of reactive adsorption
desulfurization. The potential applicability of different
metal oxide sorbents in reactive adsorption can be
evaluated from the results of thermodynamic modeling of
the desulfurization process. We calculated the equilibrium
composition of the reacting system from the criterion that
equilibrium at constant temperature and pressure is reached
if the total Gibbs energy is at a minimum with respect to all
possible changes in composition. The gas phase was
simulated by the presence of the model organic sulfur
compound thiophene6 in a H2/N2 mixture. Several types of
metal oxides, namely bulk or supported ZnO, MoO3, NiO,Co3O4, and MnO, were considered as sorbent candidates.
All sorbents show relatively favorable equilibrium data in
the desulfurization of thiophene. The obtained equilibrium
compositions for bulk ZnO sorbent as a function of
temperature at normal total gas pressure are presented in
Fig. 11 as an example. It is observed that with an excess of
hydrogen (H2/thiophene 10) thiophene decomposition is
thermodynamically feasible over a large temperature range
(Fig. 11(a)). Up to 900 K, the equilibrium composition
contains less than 50 ppm S. All sulfur is fixed in the sorbent
as ZnS (not shown in Fig. 11, because of the scale limit).
The role of the hydrogen in reactive adsorption
desulfurization can be clarified from thermodynamic
modeling as well. With a stoichiometric ratio of hydrogen
to thiophene in the reacting mixture, the process does not
result in a high desulfurization level (Fig. 11(b)). Only when
hydrogen is in excess (H2 to thiophene ratio above
stoichiometric), is thiophene desulfurized almost comple-
tely. These data should not be considered as evidence that a
lot of hydrogen is consumed, but they show that hydrogen
still plays a very important role in the process. To clarify the
role of H2 the mechanism of organosulfur compound
decomposition has to be determined experimentally and
kinetics have to be determined. From earlier work, we
expect that the kinetics are fast [113].Conventional Ni Mo/Al2O3 and Ni/Al2O3 HDS cata-
lysts have been experimentally tested in reactive adsorption
desulfurization of kerosene [114]. The oxide form of the
NiMo/Al2O3 catalyst has been shown to exhibit a higher
desulfurization activity in reactive adsorption in comparison
with the sulfided analogue under hydrotreating conditions. It
provides a very high level of kerosene desulfurization in
pure hydrogen. However, the catalyst sulfur capacity is
limited by the amount of active phase that can be sulfided
(NiMo). We have estimated that, for a feed with 100 ppm
S at LSHV equal to 1 h21, the catalyst will be overloaded by
sulfur in less than one month. Of course this is a rather shorttime for conventional industrial application and a tailored
process configuration including regeneration is, therefore,
required.
Reduced Ni/Al2O3 catalysts have shown even higher
activity in the adsorptive desulfurization in comparison with
5 http://www.fuelstechnology.com/sulfur_removal.htm
6 Thiophene was chosen since it was reported to posses the lowest
reactivity in the reactive adsorption process among alkyl thiophenes and
benzothiophenes [108]. This is clear evidence that sulfur removal in the
reactive adsorption occurs via a mechanism different from hydrotreating
mechanism (hydrogenation/hydrogenolysis of organosulfur compounds).
Benzothiophene and dibenzothiophene show similar, even more favorable,
results in thermodynamic modeling.
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the oxide NiMo catalysts, but their life time on stream is
limited by the limited sulfidation capacity of the nickelsurface layer [115]. Different options to regenerate the
sulfur-poisoned Ni surface species have been tested [116,
117]. The best performance has been observed for Ni/ZnO
system. Due to its higher sulfur accepting potential, ZnO in
this system is acting as an acceptor of sulfur that is released
during regeneration of sulfided nickel surface species. The
overall mechanism can be tentatively described by the
scheme in Fig. 12.
Based on the data of Tawara et al. [114117], a
mechanism for the Z Sorb sorbent can be proposed and
the role of the sorbent components can be clarified. ZnO,
which provides the high sulfur capacity, has to be the maincomponent of the sorbent. Alumina and silica can be used to
increase the mechanical strength and attrition resistance of
the sorbent. Also, NiO promotes the decomposition of
organic sulfur compounds [118,119].
Optimal composition of the Ni/ZnO sorbent is deter-
mined by the balance between the poisoning rate and the
regeneration rate and is found to be equal to 13 wt% Ni. It
has been reported that less than 0.03 ppm of sulfur was
present in the effluent during one year for a kerosene feed
containing 51 ppm of sulfur LSHV 0:
27 h21
:The main limitation of the reactive adsorption desulfur-
ization process is connected with quick overloading of the
sorbent in the case of refinery streams with high sulfur
content. High sulfur content requires either a large amount
of sorbent or a suitable process configuration based on fast
kinetics of the deactivation regeneration reactions. Both
sorbent capacity and sorbent performance can be optimized
by appropriate composition of the sorbent applied.
For low sulfur containing streams (usually concerning
removal of the last ppm S) reactive adsorption requires
treatment of large volumes of sulfur diluted reactant. This
results in a high energy penalty due to pumping of thehydrocarbons through the reactor. In this case, it seems very
efficient to apply structured low-pressure drop reactors and
to combine reactive adsorption desulfurization with pro-
cesses such as catalytic distillation or extraction, which pre-
concentrate sulfur.
The list of processes discussed above is not complete.
Many other processes are already applied or almost ready for
industrial application. We limited ourselves to the discussion
Fig. 11. Thermodynamic modeling of the equilibrium composition in the reactive adsorption desulfurization process. Solid sorbent: ZnO; organosulfur
compound: thiophene. (a) H2/thiophene 10:1; (b) H2/thiophene 1:1.
Fig. 12. Mechanism of reactive adsorption desulfurization process.
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of processes that are attractive for ultra-deep desulfurization
of refinery streams but still have some scientific challenges
that need to be addressed.
5. Monolith reactor/catalysts for refinery stream
desulfurization
For successful development of advanced desulfurization
technologiesHDS as well as non-HDS basedboth the
catalyst and the reactor should be close to perfect. A
fascinating option for highly efficient and innovative
technologies arises from a combination of different
functions in single units, performing more functions
simultaneously [120]. Structured monolithic reactors will
play a key role in the design of novel processes based on
multifunctional reactors [121].
Depending on the point of view, a monolith can be
considered to be a reactor or a catalyst: the borders between
catalyst and reactor vanish [122,123]. A large experimental
program dealing with the application of monolith-based
catalysts/reactors in different chemical processes is carried
out at Delft University of Technology. Examples of very
high activity and selectivity have been reported [124127].
Various types of monolithic catalysts can be distin-
guished (Fig. 13). Ceramic monoliths are by far the most
used. Analogous systems are also produced from corrugated
metal sheets. The optimal morphology and structure
depends on the specific application. For desulfurization,
ceramics are probably to be preferred because of their high
resistance to corrosion by H2S.
Monolithic catalysts can be prepared in various ways.They can be produced by direct extrusion of support
material (often cordierite is used, but various types of clays
or typical catalyst carrier materials such as alumina are
also used) or of a paste also containing catalyst particles
(e.g. zeolites, V-based catalysts) or a precursor of catalyst
active species (e.g. polymers for carbon monoliths). An
advantage of this route is that the catalyst loading of the
reactor can be high.
Alternatively, catalysts, supports, or their precursors can
be coated onto a monolithic support structure by washcoat-
ing. The ceramic monolith that is being used as a support
structure for the catalyst is macroporous. This facilitates the
anchoring of washcoat layers. There is a mass of literature
and patents on coating and a variety of preparation
procedures can be applied. It is expected that, in anevolutionary way, better and better catalysts will be
produced. Washcoating was successfully applied with the
strongly acidic polymeric catalyst Nafion and with BEA
zeolite in several acid catalyzed reactions [128].
Monoliths are the dominant catalyst structures for three-
way catalysts in cars [129131], selective catalytic
reduction catalysts in power stations [132,133], and for
ozone destruction in airplanes. Application of structured
catalysts for desulfurization processes can also be highly
advantageous in comparison with common catalysts and
reactors. Monoliths are applicable not only for single-phase
processes, but it has become clear that they are often
preferable for multiphase processes. If Taylor flow through
a single tube is ensured, the diffusion limitations for gas
liquid processes can be reduced due to internal liquid
recirculation during their transport through a channel
(Fig. 14). This results in one order of magnitude faster
mass transfer than in conventional reactors.
What makes monoliths practically attractive for different
chemical processes and, particularly, for desulfurization?
The large open frontal area and straight channels result in
an extremely low pressure drop essential for end-of-pipe
solutions. The straight channels also prevent the accumu-
lation of dust. That makes monolith catalyst/reactors
applicable in desulfurization processes currently employingmoving or ebullated bed reactors (like T-Star or reactive
adsorption). Compared to random packed beds, monolithic
reactors exhibit more ideal reactor behavior resulting in
higher conversion and selectivity. This is particularly
important for deep desulfurization.
Monolith based reactors are very favorable for processes
that benefit from a counter-current operation, especially for
equilibrium limited reactions and when product inhibition is
Fig. 13. Monolithic structures of various shapes. Square channel cordierite structures (1), (3), (5), (6), internally finned channels (2), washcoated steel
monolith (4).
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stripping or distillation are challenging applications which
are not far out of reach for monoliths.
Several options exist for application of monoliths in oil
refineries. They include, but are not limited to, gas phase
processes for removal of the last ppm S from gasoline and
effluent gases; gasliquid phase processes aimed at deep
desulfurization, denitrogenation and dearomatization andhydrocracking (co- and counter-current) employing cataly-
tic distillation, reactive stripping and reactive adsorption;
and gas liquid solid processes like moving bed appli-
cation for hydrodemetalization and sulfur removal by
reactive adsorption.
Catalyst preparation and extrusion should be developed
further for specific applications, optimizing the structure
and active phase distribution. Hydrodynamics and transport
processes have to be described better to design reliable
processes. In many of the processes described above,
monolithic reactors can be fruitfully applied.
6. Desulfurization technologies: evaluation at the
refinery level
To produce fuels satisfying the new environmental
legislation concerning sulfur levels, refineries need to
consider desulfurization of all streams that are used in end
fuel products. Places where desulfurization units have to be
installed and their exact configuration are determined by the
nature of the refinery stream to be desulfurized, its sulfur
content, and the desired product specifications. A simplified
flow scheme of an oil refinery is shown in Fig. 15 with the
possible locations for desulfurization units. Only streamsforming the main end fuel products are shown. Processes to
convert vacuum residue into more valuable products such as
gasoline, naphtha, diesel, etc. are not shown in Fig. 15,
although desulfurization of these streams is also required.
Their desulfurization is similar to that of gasoline, kerosene
and diesel streams produced by distillation (straight run) or
FCC units.
It is fair to state that a refinery does not initially produce
the right products with the right specifications. In particular,the FCC unit produces the wrong products: although the
boiling points are in the desired range, for practical use they
are too high in aromatics and sulfur content. Thus, the FCC
products used for gasoline blending have to be upgraded
extensively without reducing octane number. The alterna-
tive of applying the FCC products for diesel fuel is even
more problematic because aromatics have very low cetane
numbers. In this case, extensive hydrogenation is unavoid-
able. This applies to a much lesser extent to hydrocracking.
In that case, the products contain much lower concentrations
of sulfur compounds and the aromatic content is lower.
In the future refineries will be much more based onhydrocracking than on FCC. This could lead to the
conclusion that research related to the upgrading of current
refinery streams is not advisable. However, this is not the
case for two reasons. Firstly, FCC units are very robust and
profitable. For several decades to come they will play a
major role in the conversion of crude oil into desired
products. Secondly, they are complementary to hydrocrack-
ing and their robustness makes them very flexible. In view
of the fact that better and better catalysts are being
developed, it might well be that FCC will maintain a
prominent position in the future. Moreover, FCC in
principal might be used for modern feedstock, such asbiomass. Also in that case upgrading of products by
hydroprocessing might be required.
Fig. 15. Simplified flow scheme of an oil refinery with possible locations of hydrotreating units.
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6.1. Light stream desulfurization
Desulfurization of straight-run streams like straight run
gasoline, naphtha