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

Fuel 82 (2003) 607631

www.fuelfirst.com

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.

I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607631608


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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.

I.V. Babich, J.A. Moulijn / Fuel 82 (2003) 607631 609


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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.



9/25

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.



10/25

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.



11/25

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.



13/25

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



14/25

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.



15/25

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

Science and Technology of novel process for deep desulfurization of oil refinery streams

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