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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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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.

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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Hydrodesulfurisation(HDS)

Hydrodenitrogenation(HDN)

Hydrodeoxygenation(HDO) and (HDM)

REFINERY PROCESSES

Catalyticcracking Hydrocracking Catalytic

reformingHydrotreating

PROCESSES IN REFINERIES

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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.

HYDROTREATING

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HYDROTREATINGA 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 Hydrogen Sulfide, H2S. The

resulting products are hydrogenated

Hydrodeoxygenation (HDO) and

hydrodemetalization (HDM) are the removal

of oxygen and metals from the feed. respectively.

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Compound Sulfur in wt.% Nitrogen inppm 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.% -

Nitrogen And Sulfur Content Present in Different Crude

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HDSIMPORTANCE

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 odor of lube

oil caused by the presence of sulfur

HDN• Nitrogen containing compounds

severely reduce the activity of cracking, hydrogenation, isomerization, reforming and HDS catalysts

• High nitrogen concentrations are undesirable to product quality

• To meet the NOx ,mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide)emission restrictions.

• If present, N-compounds affect the stability of fuels (fuel storage degradation and contamination). A fuel is considered unstable when it undergoes chemical changes that produce undesirable consequences such as deposits, acidity

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• 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

Classification of Desulphurization Technology

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Desulfurization technologies classified by nature of a key process to remove sulfur

Sulfided CoMo/Al2O3 and NiMo/Al2O3 catalysts

CONVENTIONAL HDS

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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 nature and concentration of the sulfur compounds present in the feed

stream, and reactor and process design.

(sulfiding protocol, temperature, partial pressure of

hydrogen and H2S),

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• 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

Hydrotreating catalyst

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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 three

different 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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Surface structure models of a conventional HDS catalyst and the designed catalyst

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HDS PROCESS

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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.

NEW GENERATION HDS CATALYSTS

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• 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.

DEEP DESULFURIZATION

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• 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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• 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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Typical Reactivity pattern observed in HDS Catalysis

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General classification of the catalysts

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CHOICE OF SUPPORTS

CHOICE OF SUPPORTS

Conventionally used industrial hydrotreating

catalyst Co (Ni)-Mo /γ-Al2O3

Silica, Carbon

Additives to γ-Al2O3

Mixed OxidesClays, Zeolites like Y and USY

Mesoporous Material MCM-41, HMS, mesoporous

Alumina

SBA-15 large pores and bimodal

structure consisting of micro and mesopores

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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

ADVANCED HDS CATALYSTS

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• 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.

FUNCTIONS OF SUPPORT - GENERAL

26APPROACHES FOR DEVELOPING BETTER CATALYSTS

Effects of various additives on the properties of alumina-supported HDS catalysts

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• 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.

CONCLUSION