What Is Desulfurization

Desulfurization is the process of removing sulfur from something to prevent

contamination. Also known as hydrodesulphurization or HDS, this chemical process
reduces the sulfur dioxide emissions and converts them to sulfuric acid. The sulfuric
acid is then used in car batteries and fertilizer. The most commonly required
desulfurization process is in natural gas. Additional desulfurizing is required for flue
gas, coal, and oil.
Natural gas desulfurization is typically accomplished by adsorption. A bed of
activated carbon is used as the filter for natural gas pipelines. As the natural gas
runs through the pipeline, it runs through the activated carbon at an established
interval. The sulfur is left behind and adsorbed into the activated carbon. Tests are
performed regularly to ensure the levels of sulfur remain in the acceptable level.
Flue gas is the byproduct of power plants and refers to the exhaust from
burning fossil fuels. Flue gas desulfurization is required to reduce the amount of
sulfur dioxide getting into the air. It is a large factor in the formation of acid rain.

Most Common Methods Of Removal Of Sulphur:

I.Hydrodesulfurization.
II.Chemical Desulphurization.
III.Physical Adsorption Of Sulphur Oxide.
IIII.Wet Sulfuric acid process.
IV. Spray dry scrubbing using similar sorbent slurries.

I.Hydrodesulfurization:

In the hydrodesulfurization process, a mixture of the oil-based raw material and

hydrogen gas is heated to 300-400C and pumped under a pressure of up to 130
atmospheres into a hydrodesulfurization reactor.
Here, the mixture passes over a catalyst which breaks the sulfur-carbon bonds,
allowing the sulfur to react with the hydrogen to form hydrogen sulfide. There are a
number of hydrodesulfurization catalysts, but the one most commonly used consists
of molybdenum sulfide, which contains cobalt on an aluminum oxide base.

The H2S flows out of the reactor, along with excess hydrogen, and into a treatment
unit where it is separated out, allowing the hydrogen to be recycled through the
process. Several cycles may be required to reduce the sulfur content to the required
level.
The hydrogen sulfide produced by HDS is converted to elemental sulfur by a
procedure known as the Claus Process refineries generally have a Claus unit for
this purpose. Much of the sulfur recovered in this way is used in the production of
sulfuric acid. Although sulfur deposits are still mined, most sulfur production today
is from petroleum via the HDS and Claus processes.

II.Physical Adsorption Of Sulphur Oxide

Macrotyloma uniflorum Lam. is commonly known as horse gram, which belongs to

the family Fabaceae. Polyphenols present in seed extract of M. uniflorum were water
soluble, heat stable, polar, non-tannin and nonprotein in nature. Taking all these
factors into consideration M. uniflorum seed powder was selected as an adsorbent.
We tried to examine the possibility of using a well-known physicochemical method
as adsorption for the removal of SO2 from aqueous SO2 solution.
The initial screening study has been carried by mixing a known amount of M.
uniflorum adsorbent into the aqueous solution of SO2 (Angold, 1997).
The adsorption experiment is carried out with respect to contact time between
aqueous solution and adsorbent, with respect to effect of aqueous SO2
concentration, and with respect to adsorbent dosage.

III.Chemical Desulphurization:
Chemical desulphurization and microwave-chemical desulphurization was employed
to remove sulfur in crude oil. Several desulfurizing agents have been selected and
investigated.
Among these desulfurizing agents, DCP, BPO, BBPV, and BPMC are organic
peroxides, while the active oxygen content of organic peroxides is increasing, the
oxidation effects become better and the desulfurizing efficiency of crude oil is
increasing.
BBPV and BPMC are compared with other organic desulfurizing agents, which
perform better. Various influencing factors such as dosage of desulfurizer,
investigated temperature, and optimum reacting conditions were obtained.

The optimized dosage of BBPV, BPMC, and formylhydroperoxide is 1%, 2%, and
15%, respectively. The optimized temperature should be 8090C. Microwave
inducement can improve the effect of chemical desulphurization and better
desulfurizing results were gained.
The desulfurizing efficiencies of peroxy acetic acid, BBPV, and BPMC increased from
18.6%, 21.8%, 28.5%, and 24.3% to 34.7%, 33.3%, 34.5%, and 43.3%, respectively.
The microwave inducement can decompose sulfone to water-soluble sulfate and
sulfite. Thus, organic sulfur was transformed into inorganic sulfur and then removed.

Why we make the process of removal of sulfur from crude oil?

Sulfur in crude oil is mainly present in the form of organ sulfur compounds.
Hydrogen sulfur is the only important inorganic sulfur compound find in crude oil.
Its presence however, is harmful because of its corrosive nature organ sulfur
compounds may generally be classified as acidic and non acidic. Acidic sulfur
compounds are the thiols (mercaptans).
Thiophene, sulfides and disulfides are examples of non-acidic sulfur compounds
found in crude oil fractions.
Extensive research has been carried out to identify some sulfur compounds in a
narrow light petroleum fraction.
Most Common Methods Of Removal Of Sulphur:
I.Catalytic Desulfurization.
II.Chemical Desulphurization.
III.Physical Adsorption Of Sulphur Oxide.
IIII.Wet Sulfuric acid process.
IV. Spray dry scrubbing using similar sorbent slurries.

I. By Catalytic Desulfurization:
Hydrodesulfurization (HDS), the industry standard method of removing sulfur in
petroleum refining operations, uses types of catalysts to add hydrogen in order to
reduce unwanted sulfur compounds. Unfortunately, the HDS process typically
requires expensive, high-pressure (up to 1,000 psig), high-temperature (400
550C) equipment to help produce environmentally friendly fuels.

Hydrodesulfurization (HDS) is the standard catalytic process for the removal of

sulfur from petroleum products. In this process, the sulfurous fractions of the crude
oil are mixed with hydrogen and a catalyst to react to hydrogen sulfide. Typically,
the catalyst consists of an alumina base impregnated with cobalt and molybdenum.
As the oilsupplies get more sour, higher pressures and alternative catalysts are
required for the desulfurization. Recalcitrant aromatic sulfur compounds (e.g. 4,6dimethyldibenzothiophene) cannot be removed using hydrodesulfurization, due to
their low reactivity
II.By Chemical Desulphurization:
a) Acid chromous chloride treatment.
b) Peroxyacetic acid treatment.
II.b)Peroxyacetic acid treatment:
Chemical desulphurisation was carried out according to the method described by
Palmer et al. 13 with some modifications. About 2 g of coal (< 250 m) in 70 mL of
glacial acetic acid and 30 mL of 6% hydrogen peroxide concentration were reacted
at room temperature for a specific reaction time.
The reaction mixture was cooled and the residual coal was filtered and washed with
excess of hot distilled water and dried in vacuum oven set at 40C over night. The
experiment was repeated at various reaction temperatures of 50 and 104C
(refluxing temperature).
The reaction was also conducted with various acids to peroxide volume ratio of
50:50, 30:70 and 60:20 and with 30% hyrogen peroxide concentration.
III.By Physical Adsorption Of Sulphur Oxide:
One of the more promising processes for removal of S02 and NO, simultaneously is
the Bergbau-Forschung process that involves S02 adsorption and catalytic NO,
reduction with ammonia by using carbonaceous adsorbents 1281.
The mechanism and kinetics of adsorption of S02 fiom flue gas on carbon is very
complex due to the presence of water vapour and oxygen, which leads to the
formation of sulphuric acid. The adsorption capacity for S02 is much greater than
that of physical adsorption because of the formation of H2S04.
Lu and Do studied the use of coal reject - a coal mine waste - as an adsorbent. They
found that the coal reject, when previously treated by pyrolysis and activation,
possessed considerable adsorption capacities for both SO2 and NO.
This process presents the advantages of simultaneous removal of S02 and NO, and
utilization of solid waste.

Biological Removal of Hydrogen Sulphide

The most valuable component of biogas is methane (CH ) which typically makes
4

up 60%, with the balance being carbon dioxide (CO ) and small percentages of
2

other gases. However, biogas also contain significant amount of hydrogen

sulfide (H S) gas which needs to be stripped off due to its highly corrosive
2

nature. Hydrogen sulfide is oxidized into sulfur dioxide which dissolves as

sulfuric acid. Sulphuric acid, even in trace amounts, can make a solution
extremely acidic. Extremely acidic electrolytes dissolve metals rapidly and
speed up the corrosion process.
The corrosive nature of H S has the potential to destroy expensive biogas
2

processing equipment. Even if there is no oxygen present, biogas can corrode

metal. Hydrogen sulphide can become its own electrolyte and absorb directly
onto the metal to form corrosion. If the hydrogen sulphide concentration is very
low, the corrosion will be slow but will still occur due to the presence of carbon
dioxide.
The obvious solution is the use of a biogas cleanup process whereby
contaminants

in

the

raw

biogas

stream

are

absorbed

or

scrubbed. Desulphurization of biogas can be performed by biological as well as

chemical methods. Biological treatment of hydrogen sulphide typically involves
passing the biogas through biologically active media. These treatments may
include open bed soil filters, biofilters, fixed film bioscrubbers, suspended
growth bioscrubbers and fluidized bed bioreactors.

Biological Desulphurization
The simplest method of desulphurization is the addition of oxygen or air directly
into the digester or in a storage tank serving at the same time as gas holder.
Thiobacilli are ubiquitous and thus systems do not require inoculation. They
grow on the surface of the digestate, which offers the necessary micro-aerophilic
surface and at the same time the necessary nutrients. They form yellow clusters
of sulphur. Depending on the temperature, the reaction time, the amount and
place of the air added the hydrogen sulphide concentration can be reduced by
95 % to less than 50 ppm.
Most of the sulphide oxidising micro-organisms belong to the family of
Thiobacillus. For the microbiological oxidation of sulphide it is essential to add
stoichiometric

amounts

of

oxygen

to

the

biogas.

Depending

on

the

concentration of hydrogen sulphide this corresponds to 2 to 6 % air in biogas.

Measures of safety have to be taken to avoid overdosing of air in case of pump
failures.

Biofiltration
Biofiltration is one of the most promising clean technologies for reducing
emissions of malodorous gases and other pollutants into the atmosphere. In a
biofiltration system, the gas stream is passed through a packed bed on which
pollutant-degrading microbes are immobilized as biofilm. A biological filter
combines water scrubbing and biological desulfurization. Biogas and the
separated digestate meet in a counter-current flow in a filter bed. The biogas is
mixed with 4% to 6% air before entry into the filter bed. The filter media offer
the required surface area for scrubbing, as well as for the attachment of the
desulphurizing microorganisms. Microorganisms in the biofilm convert the
absorbed H S into elemental sulphur by metabolic activity. Oxygen is the key
2

parameter that controls the level of oxidation.

The capital costs for biological treatment of biogas are moderate and
operational costs are low. This technology is widely available worldwide.
However, it may be noted that the biological system is capable to remove even

very high amounts of hydrogen sulphide from the biogas but its adaptability to
fluctuating hydrogen sulphide contents is not yet proven.

Desulfurisation is a chemical process for the removal of sulfur from a material. This can involve
either the removal of sulfur from a molecule (e.g. A=S A:) or the removal of sulfur compounds
from a mixture such as petrochemicals or flue gases.[1]
These processes are of great industrial and environmental importance as they provide the bulk of
sulfur used in industry (Claus process and Contact process), sulfur-free compounds that could
otherwise not be used in a great number of catalytic processes, and also reduce the release of
harmful sulfur compounds into the environment, particularly sulfur dioxide (SO2) which leads to acid
rain.
Processes used for desulfurisation include hydrodesulfurization, flue-gas desulfurization) and
the Wet sulfuric acid process(WSA Process).

BIODESULPHURISATION

The removal of sulfur or sulfur compounds (as from coal or flue gas) using biological agents is
called as biodesulfurization. A single vertical rotating immobilized cell reactor (VRICR) with the
bacterium R. erythropolis, as a biocatalyst, was developed and used for investigation of
biodesulfurization process with its two successive stages of cell growth and desulfurization

activity. With a rotation speed of 15 rpm and oxygen transfer rate of 90 mM O2.l-1.h-1,
immobilized cell concentration of up to 70.0 g.l-1 was achieved during the first stage and further
used, in the second, to carry out a stable continuous desulfurization of model oil
(dibenzothiophene in hexadecane). A steady state with specific desulfurization rate as high as
167 mM 2HBP.Kg-1.h-1 and sulfur removal efficiency of 100% were maintained for more than
120 h. The proposed integrated biodesulfurization process utilizing the VRICR has the potential
to lower operating costs and support possibilities of commercial application at the expense of
Hydrodesulfurization process currently employed. Sulfur emission through fossil fuel
combustion is a major cause of acid rain and air pollution. In order to compete successfully with
Hydrodesulfurization, Biodesulfurization process with a suitable biocatalytic design has been
developed.
Definition - What does Biodesulfurization (BDS) mean?
Biodesulfurization (BDS) is the process of sulfur removal from fuels by means of living organisms. It
is a non-invasive approach that can specifically remove sulfur from refractory hydrocarbons under
mild conditions and it can be potentially used in industrial desulfurization.
In this process, bacteria remove organosulfur from petroleum fractions without degrading the carbon
skeleton of the organosulfur compounds. During a BDS process, alkylated dibenzothiophenes are
converted to non-sulfur compounds.
Like other desulfurization processes, biodesulfurization helps to reduce fuel corrosion of engines and
increase fuel values. The BDS process is applied to desulfurize the more recalcitrant sulfur
compounds.
Corrosionpedia explains Biodesulfurization (BDS)
Biodesulfurization (BDS) offers an attractive alternative to conventional hydrodesulfurization due to
the mild operating conditions and reaction specificity afforded by the biocatalyst. Biological
desulfurization of petroleum may occur either oxidatively or reductively. In the oxidative approach,
organic sulfur is converted to sulfate and may be removed in process water. Oxidative does not
require further processing of the sulfur and may be amenable for use at the well head where process
water may then be re-injected. In the reductive desulfurization scheme, organic sulfur is converted
into hydrogen sulfide, which may then be catalytically converted into elemental sulfur.
The advantage of BDS is that it can be operated in conditions that require less energy and
hydrogen. BDS operates at ambient temperature and pressure with high selectivity, resulting in
decreased energy costs, low emissions and no generation of undesirable byproducts.
Successful biodesulfurization processes are based on naturally occurring aerobic bacteria that can
remove organically bound sulfur in heterocyclic compounds without degrading the fuel value of the
hydrocarbon matrix. In this process air is used to promote sulfur removal from the feedstock.

ABSTRACT

Biodesulfurization offers an attractive alternative to conventional

hydrodesulfurization due to the mild operating conditions and reaction specificity
afforded by the biocatalyst. The enzymatic pathway existing in Rhodococnrs has
been demonstrated to oxidatively desulfurize the organic sulfur occurring in
dibenzothiophene while leaving the hydrocarbon intact. In order for
biodesulfurization to realize commercial success, a variety of process considerations
must be addressed including reaction rate, emulsion formation and breakage,
biocatalyst recovery, and both gas and liquid mass transport. This study compares
batch stirred to electro-spray bioreactors in the biodesulfurization of both model
organics and actual cnrdes using a Rhodococcus IGTSS biocatalyst in terms of their
operating costs, ability to make and break emulsions, ability to effect efficient
reaction rates and enhance mass transport. Additionally, sulfur speciation in crude
oil is assessed with respect to sulfur specificity of currently available biocatalysts.
KEY WORDS:
crude oil desulhrization, Rhodococcus, electrostatic spraying, dibenzothiophene,
biodesulfurization
INTRODUCTION
Biological processing of fossil fuel feedstocks offers an attractive alternative to
conventional thermochemical treatment due to the mild operating conditions and
greater reaction specificity afforded by the nature of biocatalysis. Efforts in
microbial screening and development have identified microorganisms capable of
petroleum desulhrization (see for example, [l-3]), denitrification [4], demetalization
[4], cracking [SI and dewaxing. Biological desulhrization of petroleum may occur
either oxidatively or reductively. In the oxidative approach, organic sulfur is
converted to sulfate and may be removed in process water. This route is attractive
due to the fact that it would not require further processing of the sulfur and may be
amenable for use at the well head where process water may then be reinjected. In
the reductive desulfurization scheme, organic sulfur is converted into hydrogen
sulfide, which may then be catalytically converted into elemental sulfur, an
approach of utility at the refinery. Regardless of the mode of biodesulhrization, key
factors affecting the economic viability of such processes are biocatalyst activity
and cost, differential in product selling price, sale or disposal of co-products or
wastes from the treatment process, and the capital and operating costs of unit
operations in the treatment scheme.
In all fossil fuel bioprocessing schemes, there is a need to contact a biocatalyst
containing aqueous phase with an immiscible or partially miscible organic substrate.
Factors such as liquid / liquid and gas / liquid mass transport, amenability for
continuous operation and high throughput, capital and operating costs, as well as
ability for biocatalyst recovery and emulsion breaking are significant issues in the
selection of a reactor for aqueous / organic contacting. Traditionally, impeller-based

stirred reactors are utilized for such mixing due to their ease of operation and wide
acceptance in the chemical and biological processing industries. Such mechanically
stirred reactors contact the aqueous and organic phases by imparting energy to the
entire bulk solution, i.e. the impeller must move the contents of the reactor.
Recent advances in the area of contactors for solvent extraction have lead to the
development of electrically driven emulsion phase contactors (EPCTM) for efficient
contact of immiscible phases [6]. In this concept, the differing electrical conductivity
between the aqueous and organic phases causes electrical forces to be focused at
the liquid / liquid interface, creating tremendous shear force. This shear causes the
conductive phase to be dispersed (5 Om droplet size) into the nonconductive phase,
but does so with decreased energy requirements relative to mechanical agitators
due to the fact that energy is imparted only at the liquid / liquid interface and not
the entire bulk solution. In a configuration of the EPCm developed at the Oak Ridge
National Laboratory, the contactor serves to disperse aqueous phase containing
biocatalyst into an organic 23 phase. The EPCW creates droplets of water containing
biocatalyst -5 Om in diameter within an organic phase. r Here, we compare the
performance of the EPCW to that of a batch stirred reactor (BSR), investigate the
required level of biocatalyst activity before the surface area afforded by the EPC"
becomes a factor in reactor performance, and characterize the emulsion formed by
both reactors in the presence of bacteria. We have investigated the emulsion quality
formed in the EPC, evaluated the power requirements and analyzed the mass
transfer issues in comparison to stirred reactors. Results on biodesulfurization of
actual crude oil by wild type Rhodococcus IGTS8 are also included. Finally, we
assess the sulfur specificity of available biocatalysts with respect to sulfur
compounds present in crude oils.
MATEFUALS AND METHODS
The experimental procedures used for studying biodesulfurization in model systems
have been discussed in detail in previous publications [7-91. A detailed description
of oil experiments is provided here.
Biodesulfurization of Van Texas Crude oil
Biodesulfurization of Van Texas crude oil was studied in batch stirred reactors to
evaluate the substrate specificity of the biocatalyst. The experiment was conducted
over a treatment period of 6 days. The crude had an API specific gravity of 31", and
a sulfur content of 0.96 wtoh. The crude oil did not contain volatiles due to
production at elevated temperature (-99C). Experiments were performed in batch
stirred reactors utilizing 50 g of frozen Rhodococcus sp. wild type strain IGTS8 (ATCC
53968) cell paste which were brought up to 750 mL. with 0.156M (pH 7.5)
phosphate buffer. The cells were suspended in the phosphate buffer prior to addi!icn
?c the reactor. The reactor VP.QQC! E-:! *:.zz ii 1-L -ATis urn-Culture fermentor
(model 178657, Gardiner, NY), utilizing a 6-bladed Rushton-type impeller with 2

baffles. The reactor was kept at 3OoC, agitated at 800 RPM, and aerated with room
air at a rate of 0.2 standard liters per minute (SLPM). A water condenser was used
on each reactor to capture volatiles which were expected to be minimal or nonexistent considering the fact that the operating temperature was much less than
that of the oil reservoir. The experiment was conducted with 250 mL of crude oil,
treated with 750 mL of the aqueous phase. Samples (30 mL. from the top of the
organic phase) taken during the course of biological treatment were collected after
ceasing the agitation and aeration for 5 min to allow the aqueous and organic
phases to separate. The reactor contents were emptied at the end of the run and
centrifuged at 6000 rpm in a Beckman Model TJ-6 centrifuge to obtain a sample of
treated crude oil. Closed samples were boiled in a closed container for 30 min to
halt biological activity.
Analytical
Model system experiments
In the experiments reported here, DBT and 2-HBP concentrations in the aqueous
phase were below our levels of detection. DBT and 2-hydroxybiphenyl (2-HBP)
concentrations in nhexadecane were measured by gas chromatography using a
Hewlett Packard 5890 gas chromatograph equipped with a flame ionization detector.
Crnde oil
A GC-SCD method was used to determine the sulfur content of the aromatic
fraction of the oil. To allow facilitated observation of sulfur in the treated oil, whole
oil samples were fractionated according to ASTM method D2007. An extended ASTM
D2887 procedure was used for chromatographic separation of the aromatic fraction
of the crude oil. Sulfur analysis was performed by modifylng the ASTM D2887
procedure by adding a Sievers Chemiluminesence sulfur specific detector after the
flame ionization detector.

The new requirements for sulfur content in liquid fuels demand the use of
novel deep desulfurization processes. 4-methyldibenzothiophene, 4,6dimethyldibenzothiophene and their alkyl-substituted derivatives are the
key
substances that need to be separated from diesel fuel and fuel oil. These
compounds require higher hydrogen consumption in the
hydrodesulfurization
process and the use of additional infrastructure in the treatment facility. The
common hydrogenation catalysts are not very effective for the hydrogenization of these compounds, and new innovation for catalysts is
required.
The desulfurization of fuel oil obtained from oil shale is also becoming
important and has different technological needs than other fuels. This paper
critically discusses the non-hydrodesulfurization processes for liquid fuels,
such as extraction, oxidation, and adsorption. These processes, their
development, and recent advances in this research field are briefly
evaluated
as possible deep desulfurization methods.

Introduction
In Europe the sulfur level in max mass% for liquid fuels is presently limited
at 0.015, 0.035, and 0.2 for petrol (gasoline), diesel fuel, and light fuel oil,
respectively [1]. New sulfur limits of 0.0030.005 mass% (3050 ppm) for
petrol and diesel fuel will be introduced in the European community and
USA in coming years [2, 3]. The current technology of hydrodesulfurization
is quite adequate for the present standards [3], however the hydrotreating
process is limited to the production of ultra-low sulfur fuels, and the
consumptions are too high to meet future requirements.

It must be emphasized that desulfurization processes are also essential for

the production of fuel oil from oil shale and for oil obtained by the
utilization of used tires. The authors of this paper are working in the last
field. Although the desulfurization of non-transportation fuels in stationary
applications can be realized in emissions by binding up sulfur oxide with
calcium oxide [4], problems with the utilization of the obtained toxic
compounds remain. Therefore the desulfurization processes of fuel oils are
of current interest. Requirements for sulfur content will approach extremely
low levels, preferably even zero content, in the near future [3], forcing
intensive research into alternative technologies.
One confounding factor in this research is that the reactivity to hydrodesulfurization of organosulfur compounds in liquid fossil fuels varies
widely. In diesel fuel and fuel oil fractions 4-methyldibenzothiophene,
4,6-dimethyldibenzothiophene and other alkyl-substituted derivatives of
dibenzo-thiophene are relatively inert to hydrotreating [5].
The conventional reaction model of hydrodesulfurization of diesel fuel
and fuel oil does not work effectively in the ultra-deep desulfurization range
down to sulfur content 100 ppm or less. The catalyst volumes must be three
times more in the case of 50 ppm, and four times more if the aim is to reach
the final sulfur content of 30 ppm [6]. A thorough survey concerning the
reactivity of sulfur compounds is given by Cremlyn [7]. Hydrotreating
processes are not discussed in this paper as many monographs [710] and
papers [1114] on the subject are available.
This paper discusses the non hydrodesulfurization processes, such as,
extraction, oxidation, adsorption, the combination of these processes, and
the
combination of these processes with hydrodesulfurization to reduce the
consumption of hydrogen. The development of the mentioned processes is
given, along with a discussion of recent studies in this field.

Extraction
The well-known UOP Merox extraction process [15] for removing
mercaptanes is used for liquefied gases and for all liquid fuel raw fractions.
Mercaptanes are extracted with caustic solution. The solution containing
Merox catalyst and dissolved mercaptanes are oxidized with air to disulfides.
The separated disulfide oil can be hydrotreated or sold as a special product.
Mercaptanes are the most slightly removable sulfur compounds of liquid
raw fuels, and the caustic extraction process is widely applied in the
production of liquid fuels.
Many extraction processes for removing sulfur-containing compounds
have been patented. Some of them are discussed below. Extraction with
polyethylene methylether decreases sulfur content by 30% in petrol, diesel
fuel, gas oil, and other raw oil fractions [16]. Extraction process of gas oil

with aqueous acetone, ethanol, mesityl oxide, and formic acid is also
patented. The reduction of sulfur content was estimated to be in the range
from 86 to 96% [17]. Unpublished experiments conducted at our institute,
found that these solvents resulted in much lower extraction percentages
under the same conditions.
Based on our experiments it may be concluded that it is impossible to
obtain low sulfur content in desulfurization process only with extraction
process. However, an extraction process combined with hydrotreating or
oxidation can be very effective. Patented extractive agents used in combination with hydrotreating include: methanol, acetonitrile, monomethylformamide, dimethylformamide, N-methyl-pyrrolidone, furfural, dimethylsulfoxide [18], and polyethylene glycols [19].
Extraction of sulfur compounds from gas oil using polyethyleneglykol
dimethylether reduces sulfur content by 71%. Deep desulfurization was
obtained using the following hydrotreating [20]. The extraction of raw petrol
using the same extractive agent and only a mild hydrotreatment followed by
complete removal of mercaptanes and thiophenes resulted in 94% removal
[19]. Extraction with heterocyclic nitrogen compounds (pyrrolidones, etc.)
and using the following hydrotreating guarantees sulfur content to be
reduced below 0.01mass% [21].
Analogical results were obtained while using dimethylformamide or
furfural as extractive agents combined with hydrotreating [18]. The
desulfurization yield for light oil and straight-run light gas oil was enhanced
by first extracting the sulfur-containing compounds using acetonitrile
followed by photochemical reaction (UV radiation) combined with air
bubbling through the reaction medium. Sulfur content was reduced from 0.2
to 0.05% [22]. In contrast the above study, most extractive processes are
carried out after an oxidation step, as discussed in the next chapter.

Oxidation
Using the oxidation process it is possible to convert thiophene, benzothiophene, dibenzothiophene and their methyl and higher alkyl derivatives
into sulfones and sulfoxides. These compounds can be removed from the
mixture by extraction with relative polar solvents, adsorption, cooling or
pyrolysis into sulfur dioxide. Desulfurization via oxidation has been studied
already since the beginning of the 1960s. Some concrete examples from this
period are presented below.
Oxidation with peroxides in the presence of a Ni-V catalyst followed by
removal of the oxidized sulfur compounds by thermal decomposition at 300
400 C in the presence of ferric oxide liberates sulfur dioxide. The next step in
the desulfurization process by oxidation was hydrogenation in the presence of

Co/Mo catalyst. From petroleum fraction boiling above 250 C, 40% sulfur
was removed by oxidation and 45% by hydrogenation [23, 24]. Arabian crude
oil containing 2.6 mass% of sulfur was dissolved in petroleum-ether and
treated with butyl hydroperoxide in the presence of molybdenum catalyst, the
mixture was then heated at 90 C for a period of 60 hours. Sulfur content after
separation by cooling was found to be 1.6 mass% [25]. However, the removal
of oxidized sulfur compounds was not complete.
In another example, tert-butyl hydroperoxide was used for oxidation of
crude oil in the presence of vanadium acetylacetonate at 90 C. Conversion
rate of up to 74% was obtained for sulfur compounds [26]. The same results
were obtained by oxidation with peroxides at 400415 C and 6065 atm
[27]. The sulfur conversion rate by oxidation of sulfur compounds from
petroleum vacuum residue using peroxides reached 50% [28]. This result is
not bad for such a heavy oil. Obviously, the removal of oxidized sulfur
compounds was not complete. Presented examples may be quite instructive.
It shows that alternative methods to the widely used hydrogenation process
for reducing sulfur content in raw petroleum and its heavy fractions have
been sought after for thirty years. The legislation of new sulfur limits is now
forcing the issue and promoting new research into this essential technology.
Fuel oil obtained from oil shale or wasted tires is of particular concern. The
current standard allows a maximum of 0.8 mass% sulfur content in the fuel
oil of oil shale. In the near future this norm will be likely lowered and
consequently, the desulfurization of shale oil will be required.
It was demonstrated that it is possible to oxidize thiophene derivatives
completely, using peroxides [29]. This is of primary importance for the
desulfurization of diesel fuel and higher oil fractions, as thiophene
derivatives are the primary sources of sulfur in these fuels. Of academic
interest, molybdenum peroxo complexes can also act as oxidation agents
[30]. During the 1990s, a number of industrially significant processes for
desulfurization of petroleum fractions by oxidation with peroxides and
solvent extraction were studied. These resulted in removal rates of 6580%
[31], 89% [32], and 90% [33].
Peroxides are expensive agents compared to the air. However, oxidation
of sulfur compounds of raw fuels using air, oxygen, or ozone provisionally
gave low conversion of up to 45% [34]. Some relatively cheap oxidants,
such as nitrogen oxides and nitric acid, were studied, but the tests gave
inadequate results [35]. Ten years later in a modified oxidation process with
nitrogen oxides, sulfur was removed from diesel oil in the range of 6697%
and from residual oil 70 86%, respectively [36].
An interesting process was worked out by Guth et al. [37]. Desulfurization of petroleum oils was carried out with nitrogen oxides and oxygen to
convert organic sulfur into sulfur trioxide which was absorbed with concetrated sulfuric acid. The removal of sulfur was, however, relatively low.
Probably one of the first attempts to desulfurize shale oil with nitric acid was

patented by Schulz et al. [38]. Tam and Kittrell et al. [3942] published
many works about oxidizing gas oil and other petroleum fractions with
nitrogen oxides or nitric acid. In all cases the oxidized products were
removed by solvent extraction.
In recent years there have been many publications about deep
desulfurization processes of liquid fuels by oxidation. Deep desulfurization
was reached by oxidation with hydrogen peroxide in the presence of
phospho-tungstenic acid catalyst and a phase-transferic agent [43].
Shiraishi
et al. [44] obtained deep desulfurization by photosensitized oxidation using
a
triplet photosensitizer in an oil/water two-phase liquid-liquid extraction
system. However, two follow up studies, utilizing oxidation and extraction,
failed to remove enough sulfur to reach the deep desulfurization level. For
gas oil, they reached 0.22 mass% [45], and for diesel fuel 0.01 mass% [46].
The most desirable result was obtained by peroxyacid oxidation
(HCOOH/H
2
O
2
). Removal of sulfones and sulfoxides, created during oxidation, was carried out by extraction followed by adsorption with silica gel to
reach sulfur content levels of 7.0 ppm and 0.0000%, respectively [47].
Another effective process has been suggested by Mei et al. [48, 49].
Phosphotungstenic acid, tetraoctyl ammonium bromide and ultrasound were
used at 75 C to complete the oxidation. The oxidized product was then
extracted with acetonitrile. Under these conditions dibenzothiophene and its
derivatives were removed from diesel fuel at a rate of more than 99%.
In recent years many desulfurization processes with peroxides have been
patented: removal of sulfur from 250 ppm to 5 ppm in diesel fuel [50], sulfur
removal in diesel fuel from 0.557% to 0.0008% [51], and from gas oil to
82% [52]. In 2004 a patent application concerning desulfurization with
oxidation was presented. Oxidation of crude oil was carried out by hydrogen
peroxide in the presence of a catalyst, surface-active agent and radiation
with
sonic energy at 125 C and ca 3 atm. The resulting sulfur content was 0.7%,
reduced from 2.5% [53].
Li et al. [54] suggested an interesting oxidation catalyst which is also
phase transfer agent. Oxidation was carried out using hydrogen peroxide for
30 minutes at 60 C and 8 minutes at 90 C. The turnover number was
estimated to be higher than 300. After the process the catalyst was
separated
by centrifugation, and oxidized sulfur compounds were removed with
extraction by N-methyl-2-pyrrolidone. Ultra-deep desulfurization with sulfur
content below 0.1 ppm in diesel fuel was achieved. The feed-stock was
prehydrotreated diesel fuel, containing sulfur 0.053 mass%. In a patent

application [55] it was shown that sulfone obtained from dibenzothiophene,

created in oxidation process, was converted into biphenyl and hydrogen
sulfide by hydrogenation. Dibenzothiophene sulfone was hydrogenated
under milder conditions than dibenzothiophene. A product containing
55 ppm sulfur was obtained from gas oil in a two step process.
In recent years several patent applications concerning oxidation with
ozone and oxygen have been presented. Sherman [56] presents oxidation
with sub-micron size bubbles of ozone for desulfurization of diesel fuel, but
the conversion rates of sulfur are not given. In another application, heterogeneous oxidation of transport fuel with oxygen in the presence of a solid
catalyst at temperature of 160 C and pressure of 14 atm, reduced sulfur
content from 233 ppm to 12 ppm (95% removal) [57].
The treatment of raw shale oil with hydrogen peroxide in formic acid in
the presence of limonate ore (containing iron and nickel) at 70 C presented
in [58] resulted in nitrogen removal of up to 99.5%, sulfur up to 13%, dienes
up to 22%, and alkenes up to 22% [58]. However, this process is not
effective for reducing sulfur content, as the catalyst is not very effective at
oxidizing sulfur compounds. Furthermore, it may be supposed that these
results cannot be carried mechanically over to Estonian shale oil.

Adsorption
Like extraction processes, adsorption alone cannot technologically reach the
deep desulfurization levels for liquid fuels.
Since its development began in 1998, much attention has been paid to the
Phillips S Zorb process [59]. In the cited paper it was written: Reducing
hydrogen consumption by avoiding hydrotreating can save a refinery
significant operating costs.... The Zorb process is carried out in the
presence
of hydrogen and modified zinc oxide on a carrier. Sulfur from the sulfur
compounds is carried over to hydrogen sulfide, which was by chemisorption
bound with zinc oxide as zinc sulfide.
There are many patents and patent applications in the field of desulfurization by chemisorption, production of novel sorbents, and regeneration of
sorbents. One example is a process for producing a sorbent composition,
comprised of zinc oxide, silica and alumina, as a carrier, with a metal or
metal oxide, (preferably nickel) as a promotor [6062]. Similar patents
utilizing zinc titanate [63] and zinc ferrite [64] were presented.
The detailed process for these desulfurization methods is described as
follows. A hydrocarbon containing fluid stream was contacted with

hydrogen (mol ratio 0.2:1 to 1:1), solid sorbent particulates (about 20 to

150 microns), which consist from zinc oxide and a promotor, preferably
reduced nickel, on a carrier. Separated adsorbent, where sulfur was bound in
the form of zinc sulfide, was regenerated with an oxygen-containing stream,
and contacted with a hydrogen-containing stream to reduce the unreduced
nickel [65, 66].
The regeneration and activation processes of sorbent are described in
many patents [6769]. In 2004 a complementary patent application
concerning the above mentioned processes was presented [70]. Another process, in
which oxidation is combined with chemisorption on zinc oxide in the
presence of hydrogen was also presented [71]. The oxidation process was
carried out with peroxy acetic acid in which 4,6-dimethyldibenzothiophene
was completely converted to sulfoxide and sulfone. The separated organic
phase was treated with hydrogen stream on zinc oxide. After this process,
sulfur content reaches 2.45 ppm. The novelty of the process is that
sulfoxides and sulfones are more easily converted to hydrogen sulfide than
thiophene derivates.
Many other oxides in addition to zinc oxide are also presented as
chemisorbent agents. For example: manganese [72], copper [73], tin [74],
and even cadmium [75], niobium [76], gallium [77], silver [78], tungsten
and molybdenum oxides [79], have all been presented as chemisorbent
agents.
Also of note is the use of noble metals as promotors of sorbents [80].
Shell Oil Co has presented a process for deep desulfurization of
petroleum fractions, for example gas oil at 350 C and 20 bar. A zinc oxide,
nickel on alumina catalyst was used. The activity permanence of catalyst
was illustrated with the sulfur removing rate as follows: from 750 ppm to
5 ppm after 9 hours and to 10 ppm after using the catalyst during 100
hours.
After 260 hours the conversion of ZnO to ZnS was 94% [81]. This patent
application is very similar to the Phillips S Zorb process mentioned above.
An adsorptive hydrodesulfurization catalyst was developed and evaluated
for the use of kerosene in for fuel cells [82]. A catalyst containing 13% Ni
and ZnO was certified as an adsorptive hydrodesulfurization agent. Keeping
an average of less than 0.03 ppm of sulfur for one year and 1,110 thousand
km is now considered possible for fuel cell powered vehicles. This process
requires hydropretreated fuel. A nickel adsorbent with magnesia promotor
for deep desulfurization of fuel with low sulfur content, preferably for fuel
cells, was also presented [83] and again preceding hydrotreating was used.
Daimler-Chrysler AG has presented a method for deep desulfurization of
engine fuel by adsorption on board a motor vehicle. The adsorption material
is an oxide (Al
2
O

3
, MgO, SiO
2
or TiO
2
) with metal additives [84, 85].
Ma and Song et al. [86,
87] presented a method for deep desulfurization
of hydrocarbon fuels at ambient or elevated temperatures and pressures by
adsorption only without the need for additional hydrogen. The adsorbents
presented are a transition metal chloride, an activated nickel adsorbent, a
metal ion exchanced zeolite, a metal ion impregnated zeolite, nickel carried
on silica-alumina, sulfided Co/Mo/alumina, and regenerated sulfided metal.
Regeneration of adsorbent was carried out by eluation with solvents.
Concentrated sulfur compounds from the adsorbent were hydrogenated. The
above-mentioned method is very interesting, but in the presented examples
the adsorbent became saturated very quickly and is therefore difficult to put
into practice. A method for preparation of Cu(I)-Y-zeolite and desulfur
ization of diesel fuel with that has been published [8890]. Desulfurization
with this adsorbent occurs through formation of a -complex between
Cu
+
ions and the benzo-condensed thiophenic rings. This complex is much
stronger than a complex with aromatics without sulfur allowing for a 99%
desulfurization (from 297.2 to 0.032 ppm). The capacity of these sorbents
was not very high.

Ionic liquids
Ionic liquids as selective extraction agents of sulfur compounds are
discussed separately, because of their novelty and theoretical interest. The
use of ionic liquids for selective extraction of sulfur compounds from diesel
fuel was described for the first time by Bsman et al. [91]. Provisionally the
best results were obtained with AlCl
3
-1-butyl-3-methylimidazolium chloride
(BMIMCl/AlCl
3
) however, the desulfurization rate was not high (85%).
The same authors have also obtained better results with this agent [92].
Deep desulfurization using a chlorine-free ionic liquid (anion is octylsulfate
and cation N-octyl-N-methylimidazolium) is also presented [93]. This agent

is not as sensitive to water as aluminium complexes. A review on the use of

ionic liquids is presented by Sheldon [94]. In Germany, a survey was
published by Wasserscheid and Keim [95], and a detail review about roomtemperature ionic liquids was presented by Welton in 1999 [96].
Presently, ionic liquids are only of academic interest for desulfurization,
much like as also biomethods for removing of sulfur [97].

Conclusions
Presently the most attractive process for deep desulfurization of petrol and
diesel fuel is the Phillips S Zorb process, however, oxidation processes for
desulfurization offer good possibilities for higher fractions such as fuel oil.
The driving force for the implementation of sulfur removal technology
are the new requirements for sulfur content in fuel oil and fuel oil obtained
from oil shale and from wasted tires.
This review of desulfurization processes is necessary to aid in the
planning of future experiments for the purpose of forwarding development
and innovation in the field of deep desulfurization.