AN OVERVIEW OF NEW APPROACHES FOR REMOVING SULFUR FROM REFINERY SYSTEMS

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AN OVERVIEW OF NEW APPROACHES FOR REMOVING
SULFUR FROM REFINERY SYSTEMS
K.SHANTHI Department of Chemistry, Anna
University, Chennai-600025
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INTRODUCTION

• Due to depleting supplies of quality petroleum
  crudes, refineries world-wide are increasingly
  being forced to use inferior quality heavy oils
  (HO) for producing clean transportation fuels.
• Unfortunately, the low grades HO are considerably
  more difficult to process and can significantly
  reduce the efficiency of clean fuels production.
• From the viewpoint of continual efficient supply
  of clean fuels, it is therefore critical to
  improve key HO processes such as sulphur and
  nitrogen removal.

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Contd..

• Overall, new and more effective approaches and
  continuing catalysis and processing research are
  needed for producing affordable ultra-clean
  (ultra-low-sulfur and low-aromatics)
  transportation fuels and non-road fuels, because
  meeting the new government sulfur regulations in
  20062010 (15 ppm sulfur in highway diesel fuels
  by 2006 and non-road diesel fuels by 2010 30 ppm
  sulfur in gasoline by 2006) is only a milestone.
• The society at large is stepping on the road to
  zero sulfur fuel, so researchers should begin
  with the end in mind and try to develop long term
  solutions.

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REFINING

• Refining is the process, wherein, one complex
  mixture of hydrocarbons is classified into a
  number of other complex mixtures of hydrocarbons.
  
• or
• Petroleum refining is an important activity that
  the product at the end of the process is a source
  of transportation and involves heating of fuels
  and petrochemicals.
• Increasing awareness of the impact of
  environmental pollution by automobiles has
  shifted the responsibility of pollution control
  to the refiners side.

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PROCESSES IN REFINERIES
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Market share of main catalysts technology
divisionsin percentage in terms of sales value
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HYDROTREATING

• Hydrodenitrogenation (HDN) occurs simultaneously
  with hydrodesulfurization HDS),
  hydrodeoxygenation (HDO), hydrogenation (HYD) and
  hydrodemetallization (HDM) during
  hydroprocessing. Effects of these reactions upon
  each other are rather complex.
• The extent of the mutual effects depends on the
  origin of feed, type of catalyst, and operating
  conditions.
• The HDN has been the focus of attention because
  nitrogen removal is required to attain the level
  of sulfur (S) required by fuel specifications. If
  not removed, nitrogen (N)-compounds would inhibit
  HDS and other reactions because of their
  preferential adsorption on catalytic sites.

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• Hydrotreating A process used in the oil industry
  to remove objectionable elements such as nitrogen
  sulfur, oxygen and metals from petroleum
  distillates by reacting them with H2 over a
  catalyst.
• Hydrodenitrogenation (HDN) is the removal of
  nitrogen from nitrogen containing feeds in the
  form of NH3. The resulting products are
  hydrogenated .
• Hydrodesulfurisation (HDS) is the removal of
  sulfur from sulfur containing feeds in the form
  of H2S. The resulting products are hydrogenated .
• Hydrodeoxygenation (HDO) and hydrodemetalization
  are the removal of oxygen and metals from the
  feed.

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Nitrogen And Sulfur Content Present in Different
Crude
Compound Sulfur in wt. Nitrogen in ppm level
Gas oil 1.87 wt. 1000 ppm
Medium cycle oil (MCO) 0.49 wt. 695 ppm
Coal liquid 2.5 wt. 5600 ppm
Vacuum gas oil (VGO) 1.7 wt. 125 ppm
Desulfurized vacuum gas oil (DS-VGO) 0.289 0.028
Light cycle oil (LCO) 2.19 wt. -
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IMPORTANCE

• HDN
• Nitrogen containing compounds severely reduce the
  activity of cracking, hydrogenation,
  isomerisation, reforming and HDS catalysts
• High nitrogen concentrations are detrimental to
  product quality
• To meet the NOx emission restrictions.
• If present, N-compounds affect the stability of
  fuels.

• HDS
• Prevention of poisoning of the metal catalysts by
  sulfur
• Control of pollution by SO2 produced in the
  combustion of gasoline
• Removal of the unpleasant odour of lube oil
  caused by the presence of sulfur

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COMPOUND STRUCUTRE
Six-membered
Pyridine
Piperidine
Quinoline
Tetrahydroquinoline
Acridine
Five-membered
Pyrrole
Indole
Indoline
Carbazole
Nonheterocyclic
Aniline
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Typical Sulfur Compounds and their Hydrotreating
pathway
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Reactivity of various organic sulfur compounds in
HDS
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Classification of Desulphurization Technology

• When organo sulfur compounds are decomposed,
  gaseous or solid sulfur products are formed and
  the hydrocarbon part is recovered and remains in
  the refinery streams. Conventional HDS
• Sulfur compounds are separated from refinery
  stream without decomposition
• Organo sulfur compounds are separated from the
  streams and simultaneously decomposed in a single
  reactor unit rather than in a series of reaction
  and separation vessels

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Contd..
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Desulfurization technologies classified by nature
of a key process to remove sulfur
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CONVENTIONAL HDS

• Sulfided CoMo/Al2O3 and NiMo/Al2O3 catalysts
• Their performance in terms of desulfurization
  level, activity and selectivity depends on
• 1. The properties of the specific catalyst
  used (active species concentration, Support
  properties, synthesis route),
• 2. 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.

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Hydrotreating catalyst

• Hydrotreating model catalyst systems are
  synthesized by impregnating and spin-coating Mo
  and Co precursor compounds onto flat discs with
  an oxidic layer as support, a process much like
  real catalyst preparation.
• Subsequent sulfidation results in the formation
  of CoMoS or MoS2 particles

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Schematic picture of different phases present in
a sulfided alumina-supported CoMo catalyst
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Co is present in threedifferent phases. (i)
The active CoMoS nanoparticles. (ii)
A thermodynamically stable cobalt
sulfide, Co9S8. (iii) Co dissolved in the
Al2O3 support.Only the CoMoS particles are
catalytically active
Schematic representation of the the CoMoS model
under reaction conditions
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Various Co compounds on Co-Mo catalyst

• CoAl2O3. Co atoms are dissolved in the alumina
  support
• Co9S8. The thermodynamically stable cobalt
  sulfide
• CoMoS. A bimetallic sulfide-compound of Co, Mo
  and S. The compound has an MoS2-like texture,
  into which Co atoms are incorporated. The phase
  is non-stoichiometric with respect to the Co/Mo
  ratio, and no unit cell can be defined in the
  crystallographic sense.
• Of the three phases, only the last CoMoS
  structures are associated with an appreciable
  catalytic activity, and is therefore the
  structure of prime interest.

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Contd..

• CoMoS clusters are described as being essentially
  MoS2-like, but with additional Co atoms embedded
  into the MoS2 lattice at the perimeter of the
  cluster.
• It is proposed that Co atoms located at edge
  positions create new and more active sites.
• The promoting role of Co is, however, still
  extensively discussed, and the exact location of
  Co has not been identified.
• A prerequisite for a thorough elucidation of this
  seems to be a better understanding of the
  morphology and atomic-scale structure of CoMoS
  clusters.

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Contd..

• The ternary CoMoS phase is non-stoichiometric and
  thus has no thermodynamically stable counterpart.
• It has, however, been established both
  experimentally and theoretically that the CoMoS
  phase can be formed independently of any support,
  and it should thus be possible to form CoMoS
  clusters and study them independently.
• In the literature it is suggested that the number
  of sulfur vacancies is the main factor
  controlling the catalytic activity. This is
  mainly based on a number of studies dealing with
  trends in the hydrodesulfurization of
  transition-metal sulfides (TMS)

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ROLE OF THE PROMOTER

• The intercalation model
• The pseudo-intercalation or decoration model
• The remote control or contact synergy model
• The so-called CoMoS model, in which Co atoms
  decorate the edges of MoS2-slabs. This model was
  first proposed by Ratnasamy and Sivanskar.

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Different models proposed for active phase Co-Mo-S
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Surface structure models of a conventional HDS
catalyst and the designed catalyst
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Contd..
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HDS PROCESS
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NEED FOR NEW AND EFFICIENT CATALYST

• 1999 500ppm
• 2001 50 ppm
• 2003 10 ppm
• Keeping up with changes in the environmental
  requirements later the final target has been
  changed to develop catalysts which attain 10ppm S
  or less by the end of 2003

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NEW GENERATION HDS CATALYSTS

• Environmental restrictions on petroleum products
  to limit the sulfur level in fuels to 50 ppm or
  lower necessitated new generation
  hydrodesulfurization catalysts.
• In addition, preparing hydrocarbon fuel feeds to
  the fuel cell set up requires sulfur reduction to
  0.1 ppm. Such a demanding task requires catalysts
  that are several times more active than the
  present catalysts used to achieve 500 ppm sulfur.
  
• It is not only the high activity but they should
  also have different activity profiles with
  respect to different functionalities. In order to
  modify the activity to achieve the above said
  objectives several approaches have been pursued
  among which variation of support is an important
  one.

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DEEP DESULFURIZATION

• What is deep desulphurization of the fuels ?
• More and more of the least reactive sulfur
  compounds must be converted to H2S.
• Why is deep desulphurization ?
• DBT and/or DBT derivatives that are known to
  be the most refractory S-containing compounds
  show reactivities 50-fold lower as compared to
  others.
• The concentration of the most refractory
  sulphur compounds in straight-run diesel oil and
  light cycle oil approaches 3000 and 5000 ppm,
  respectively.

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Contd..

• How to approach deep desulfurization?
• The modification of the physicochemical
  properties of the supports is one of the still
  preferred modes of increasing catalytic activity.
  
• The synthesis of mesoporous molecular
  sieves with high surface area and relatively
  ordered pore structure offers new possibilities
  of using these materials as modifiers of the
  porous support structure.
• Deep desulfurization of refinery streams
  becomes possible when the severity of the HDS
  process conditions is increased. Instead of
  applying more severe conditions, perhaps HDS
  catalysts with improved activity and selectivity
  can be synthesized.

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Contd..

• 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 standards.
• The use of novel mesoporous supports for
  catalysts may help larger molecules to have
  access to the pores thereby enhancing the
  activity and minimizing the S N content

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DESULPHURISATION ROUTE OF 4,6-DMDBT
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Typical Reactivity pattern observed in HDS
Catalysis
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General classification of the catalysts
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Methods to improve DDS of 4,6-DMDBT

• One way of reducing the steric hindrance of the
  methyl groups is to shift these groups from 4,6
  to 3,7 or to 2,8 positions through an
  isomerization reaction
• The complete removal of one or both methyl groups
  through a dealkylation reaction offers another
  possibility.
• The scission of the single C-C bond in the
  thiophenic ring (isomerization, dealkyalation,
  and C-C bond scission reactions) as
  non-hydrogenative routes for desulfurization.
• This can take place by the following 2 ways.
• 1.The saturation of one of the phenyl rings
  depends primarily on the hydrogenation
• 2. By incorporating a suitable metal such as Ni,
  W, Pt, Pd, Ru, etc., and/or by providing a
  suitable support.

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APPROACHES FOR DEVELOPING BETTER CATALYSTS
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ADVANCED HDS CATALYSTS

• Different approaches have resulted in new
  catalyst formulations with improved performances
• To improve catalyst performance, all steps in the
  catalyst preparation-choice of a precursor of the
  active species, support selection, synthesis
  procedure and post-treatment of the synthesized
  catalysts-should be taken into account

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CHOICE OF SUPPORTS

• Conventionally used industrial hydrotreating
  catalyst Co (Ni)-Mo /?-Al2O3
• Additives to ?-Al2O3
• Silica, Carbon
• Mixed Oxides
• Clays, Zeolites like Y and USY
• Mesoporous Material MCM-41, HMS, mesoporous
  Alumina and
• SBA-15 large pores and bimodal structure
  consisting of micro and mesopores

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FUNCTIONS OF SUPPORT - GENERAL

• The strength of the interaction with the support
  controls the dispersion, reducibility, acidity
  and catalytic activity.
• The support mesoporosity is important for better
  dispersion of sulfide layer.
• Support design increase significantly the HDS,
  HYD and HDN functionalities of hydrotreating
  catalysts.
• The nature of the support affects sulfidation of
  the active species, leading to better-promoted
  active sites and dispersion of the catalysts.

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Effects of various additives on the properties of
alumina-supported HDS catalysts
APPROACHES FOR DEVELOPING BETTER CATALYSTS
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Comparison of first-order rate constants for
theHDS of 4,6-DMDBT over alumina, HZSM-5 mixed
alumina,and HY mixed alumina-supported CoMo
catalysts
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Enhancement in flexibility of the partially
hydrogenated 4,6-DMBT molecule for approaching
the active sites of the catalyst
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MESOPOROUS MOLECULAR SEIVES

• The special features of SiMCM-41
• (Conventional)
• Mild acidity
• High surface area
• Medium uniform pore size

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Contd..
The special features of AlMCM-41 (spherical)

• Spherical MCM-41 - well structured with Aluminium
  in tetrahedral framework .
• No charge balancing cations other than ammonium
  and proton.
• Possess active Bronsted acid sites.
• Provides an easier access to their adsorption
  sites due to the presence of short channels of
  MCM-41(S)

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AlMCM-41 (Spherical) as a support

• As host material for catalytic active species
  offers approach for the synthesis of innovative
  catalyst due to textural advantages.
• There will be more no of large pore mouths
  present on the MCM-41 (S) external surface.
• The amount of isolated Si-OH groups and the
  topology of the diverse structures determine the
  types of MoO3 species and prevent agglomeration.
• The presence of metals (Ti Zr) on the surface
  of pores can modify the interaction of the active
  phases with the support changing their reduction
  properties and their dispersion leading to
  catalysts more active for the HDS reaction.

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Contd..

• Stabilize the metal oxide particles and does not
  allow metal oxide such as MoO3 to grow to large
  size. Also minimise stacking of active phases.
• The particle growth was limited because species
  movement to the external surface was hindered by
  the non interacting porous texture of MCM-41
  material.
• The wall structure of MCM-41 as an ordered meso
  porous silica resembles amorphous silica

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DBT and 4,6-DMDBT conversion for
NiMo/P-MCM-41(R)catalysts with NiMo/Al2O3
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HDS over sulfided Co-Mo/MCM-41 (50) and
Co-Mo/Al2O3
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AlSBA-15

• SBA-15 possesses abnormal hydrothermal stability
• The large hexagonal pores (40-100A) and bimodal
  structure consisting of micro and mesopores.
• The high surface area can be exploited for
  achieving good dispersion of catalytically active
  transition metal oxides.

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Contd..

• The large pores of these materials may help
  larger molecules to have access to the pores
  thereby enhancing the activity and minimizing the
  S N content.
• Diffusion of large molecules like 4,6-DMDBT will
  be slow in Alumina supported catalyst and the
  reaction is diffusion controlled.
• With these new and novel supports like nano
  spherical MCM-41 and AlSBA-15 the process are
  made non diffusional. HDS takes place through two
  routes. One by HYDS and the other by DDS.
• Sterically hindered compounds can be desulfurised
  by hydrogenation using new and novel supported
  catalysts.

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NiMo/Al-SBA-15 for HDS of 4,6-DMDBT

• The interaction of Ni and Mo species with the
  support becomes stronger with Al loading into the
  SBA-15. Both framework and extraframework Al3
  species participate in the interaction with the
  deposited Mo species acting as anchoring sites
  for Mo. In line with this, the dispersion of
  oxidic and sulfided Mo species increases leading
  to an increase in the catalytic activity of NiMo
  catalysts.
• NiMo catalysts supported on Al-containing SBA-15
  materials with Si/Al molar ratio between 30 and
  10 show high activity in HDS of
  4,6-dimethyldibenzothiophene.
• This can be attributed to both good dispersion of
  Ni and Mo active phases and to the bifunctional
  character of these catalysts, namely, to the
  participation of Bronsted acid sites of the
  support in the catalytic transformations of
  4,6-DMDBT prior to its desulfurization

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SBA-15 - MODIFICATIONS BY POST ALUMINATION

• Chemical grafting of aluminum(III) chloride on
  the surface of SBA-15 is a suitable synthetic
  method for the preparation of mesoporous
  silicoaluminates of SBA-15 type with weak
  Bronsted acidity. This method resulted in the
  preparation of Al-SBA-15(X) materials without
  significant changes in the original pore
  structure and the long-range periodicity order of
  the parent SBA-15 sample.

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COMPARISION OF DDS OF VARIOUS SUPPORTS FOR MO
DDS
HYD
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Comparison of SBA-15 and Al-SBA-15 supported
catalysts with g-Al2O3 for HDS of thiophene
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SBA Alumina supported Co-Mo Catalysts

• SBA-15 and Al-SBA-15 supported gt g-Al2O3
  supported
  co-Mo catalysts
• SBA-15 Al-SBA-15 supported catalysts for HDS
• For Hydrogenation reaction Al-SBA-15 is a better
  support for Mo, CoMo and NiMo than SBA-15.
• It appears high molybdenum dispersion on isolated
  Al sites in Al-SBA-15 and consequent increase of
  anion vacancies at the edge sites of Mo as a
  function ofSi/Al ratio appears to be responsible
  for the outstanding activities of SBA-15 and
  Al-SBA-15 supported catalysts.

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.
Proposed reaction network for the
hydrodesulfurization of 4,6-DMDBT over
NiMo/Al-SBA-15(X) catalysts.
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Multi-walled carbon nanotubes as efficient
support to NiMo hydrotreating catalyst for HDS
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HDS of Thiophene over Pt/AlSBA-15
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Schematic representation of tungstenin the
surface and wall of WO3-SBA-15(A) low tungsten
content and (B) high tungsten content.
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Impact of high-throughput techniques in the
development and launching of a new product.
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CONCLUSION

• 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.

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Thank You