The Demand Of Ultra Low Sulphur Diesel Biology Essay

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The demand of ultra low sulphur diesel oil is exerting pressure on oil refinery industries. The strict regulation against environmental pollution defined by Environmental protection agency (EPA) led to produce ultra low sulphur diesel oil. USA, Japan, and Western Europe adopted diesel oil having a maximum sulphur limit of 500 ppm. Within the European community further reduction to 350 ppm was proposed by the year 2000. Japan again proposed further reduction of sulphur level from 500 ppm to 50 ppm by the end of the year 2004. It was also proposed that practically all transportation fuels should have a sulphur limit of < 10 ppm in the year of 2010 [3]. Global trends in diesel sulphur regulation are show in figure 1.1. Conventional HDS process requires alumina supported Co-Mo or Ni-Mo based catalysts at high temperature and pressure. Other catalysts supported on different supports were also used by many authors [4-6]. The sulphur content of the diesel oil can be typically reduced to a level of 300 - 500 ppmw by using conventional HDS process. Sulphur compounds such as thiophenes and benzothiophenes are easily reducible using conventional approach. The low sulphur diesel (< 50 ppmw) mainly contains refractory sulphur compounds e.g. alkyl substituted thiophenes. The alkyl substituted thiophenes are considered to be more refractory sulphur compounds and they cannot be easily hydrodesulfurized by conventional method. Methyl substituted thiophenes are found in higher concentration than others in low sulphur diesel, and it is believed that due to steric hindrance of substituted methyl groups the sulphur atoms are inaccessible to the active sites of the catalyst [7-8].

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Figure .1 Trends in diesel sulphur regulation [9]

1.1 Different Sulphur Compounds Present in Diesel Oil and Their Possible Reaction Pathway

There are a number of sulphur compounds present in the diesel oil. The sulphur containing compounds present in petroleum oils are generally classified into two categories: non- heterocyclic and heterocyclic. Thiols, sulphides, and disulfides are come in the first category whereas thiophenes with one to several aromatic rings and their alkyl or aryl substituent are come in the latter category. Mohammad Farhat Ali et al. [10] reported that the hydro treated diesel oil they used had 58 organosulfur compounds. The hydro treated diesel oil mainly contained alkyl substituted benzothiophenes and dibenzothiophenes. Some important S-containing compounds present in diesel oil with their possible desulfurization reaction pathway are shown in table 1.1.

A better HDS catalyst can be developed by understanding the mentioned mechanism/pathway in table 1.1. The ring saturation mainly depends on the hydrogenation capabilities of the catalyst which can be enhanced either by incorporating a suitable metal such as Ni, W, Pt, Pd, Ru etc., and/or by providing a suitable support. HDS of refinery oil can also be enhanced by selecting a suitable catalyst having high acidic properties. Acidic properties of catalyst control other possible pathways (isomerisation, dealkylation, and C-C bond scission etc.) in HDS of refinery oils. Support plays an important role in controlling the acidic properties of the catalyst.

1.2 Type of Supported Catalysts and Effect of Acidity of the Supports on HDS of Diesel Oil

Hydrodesulfurization catalysts are prepared by impregnation of active metals (Mo, W) and promoters (Co, Ni) onto the support surface using pore filling or incipient wetness impregnation techniques. Co-impregnation or sequential impregnation procedure is applied during the impregnation of metals. The impregnated sample is then dried and calcined for the dispersion of active metals on the support. The quality of dispersion depends on various parameters such as solute concentration, pH, calcination temperature, impregnation procedure etc. A number of supported catalysts have been developed for getting higher conversion during the HDS of diesel oil.

Table .1 Typical organosulfur compounds and their hydrotreating pathway [1, 11]

Type of Organic Sulphur Compound

Chemical Structure

Mechanism of Hydro Treating Reaction

Mercaptanes

R-S-H

R-S-H + H2 → R-H + H2S

Sulfides

R1-S-R2

R1-S-R2 + H2 → R1-H + R2-H + H2S

Disulfides

R1-S-S-R2

R1 -S-S-R2 + H2 → R1-H + R2-H + H2S

Thiophene

+H2

+ H2

S

-H2S

-H2S

S

S

Benzothiophenes

S

-H2S

+H2

+H2

S

+ H2

S

Dibenzothiophene

S

}

{

-H2S

+H2

+H2

+H2

+H2

+H2

-H2S

+H2

S

S

S

-H2S

4,6-Dimethyl dibenzothiophene

(4,6-DMDBT)

CH3

CH3

S

Direct Desulfurization

C-C bond scission

Isomerisation

Hydrogenation

CH3

CH3

CH3

CH3

S

CH3

CH3

S

CH3

CH3

CH3

CH3

Demethylation

S

CH3

CH3

CH3

CH3

S

CH3

CH3

1.2.1 Oxide Supported Catalyst

Various oxide materials such as Al2O3, TiO2, ZrO2, B2O3 etc. are used as a support material in the preparation of HDS catalyst. These support materials are used either alone or as a mixture with others. Alumina is widely used as a support material in the preparation of HDS catalyst. A combination of metals such as CoMo, NiMO, or NiW supported on alumina has been used by many authors. It has been found from literatures that CoMo based alumina catalysts have low activities for prehydrogenation of the aromatic ring [12-14]. Many authors have tried to increase the activity of CoMo based alumina catalysts by incorporating more hydrogenation capabilities in these materials. Hydrogenation capabilities of the material can be enhanced by increasing active metal loading, better dispersion of active metals, and manipulating acidity level of alumina support. Literatures show that NiMo based alumina catalysts have better aromatic hydrogenation capability than CoMo based alumina catalysts [15, 16]. Lecrenay et al. [17] found that the hydrogenation activity of NiMo based alumina catalyst was almost 2.5 times higher than the CoMo based alumina catalyst. As a result, NiMo based alumina catalyst showed a higher HDS rate. NiW based materials are known to have higher hydrogenating properties. A comparative study among alumina based CoMo, NiMo, and NiW was conducted by Reinhoudt et al. [18]. NiW based catalyst was found to be more active than others due to having higher hydrogenation capability.

1.2.2 Noble metal based catalysts

Owing to having high hydrogenation capabilities, noble metals are one of the better choices for making HDS catalysts. The major drawback for using these metals is very low sulphur tolerance, and hence their use as a HDS catalyst is limited. The effect of various noble metals has been studied in the preparation of HDS catalyst. Noble metals have been used either alone or in a combination with others for the preparation of HDS catalyst. A combination of various noble metals such as Re-Pt, Ir-Pt, Sn-Pt, and Ge-Pt were used by Takashi et al. [19] for the hydrogenation of LCO/SRGO feedstock. Pt or Pt-Pd supported catalyst showed very high activities.

1.2.3 Catalysts Supported on Non-Oxide-Based Materials

Owing to high surface area, acidic properties, and well defined pore structure, zeolites and mesoporous materials have been used as support materials. Recently CoMo/NiMo supported on these materials has attracted much attention in HDS of refinery oils. Higher amount of active metals can be loaded onto these support materials due to having high surface area without affecting dispersion of active metals.

1.2.4 Carbon-Supported Catalysts

Recently carbon has attracted much attention as a support material for HDS catalysts. Carbon materials are mainly composed of elemental carbon. Due to its atomic structure (1S2, 2S2, 2P2) carbon element can bond with other elements as well as with itself. Depending on the hybridization type of carbon atoms, three major allotropic form of carbon (diamond, graphite, and fullerenes) are formed. Some allotropes of the carbon are shown in figure 1.2. Carbon possesses highly desirable properties such as high surface area, controllable pore volume and pore size, and can be modified as per the process requirement. Different forms of carbon are available commercially. These include powdered activated carbon (PAC), Granular activated carbon (GAC), carbon fibres, and carbon fibre cloths etc. Yosuke Sano et al. [20] studied deep desulfurization of diesel oil. They used pitch based activated carbon fibre (ACF). Activated carbon fibres are basically carbonized carbon fibres which are subsequently heat treated in an oxidizing atmosphere, i.e. activation step. ACF offers high BET surface area, low external mass transfer resistance, and allow to surface modification. ACF has certain advantages such as high hydrophobicity, resistance towards aliphatic and aromatic organic solvents, ease of handling, and lower hydrodynamic resistances over PAC, GAC, and activated carbon (AC).

Figure 1.2 Some allotropes of carbon: a) diamond; b) graphite; c) lonsdaleite; d) Fullerenes; e) amorphous carbon; f) carbon nanotube [21]

1.3 Type of Desulfurization processes

1.3.1 Hydrodesulfurization

Hydrodesulfurization process is the back bone of refinery industries for removing sulphur from crude oil and processed oils. Hydrodesulphurization is the catalytic process for removing/reducing sulphur content from refinery oils. In this process H2 reacts with sulphur containing oil in the presence of a suitable catalyst. Usually high temperature and high pressure are required for this process. H2S gas is produced during the reaction. The H2S gas is then converted into byproduct elemental sulphur or sulphuric acid (H2SO4).

1.3.2 Oxygenative Desulfurization

In the oxygenative desulfurization (ODS) process, sulphur containing compounds are oxidized to form sulfoxides and sulfones. These sulfoxides and sulfones are then removed from the oil using extraction or solid adsorption [22-25]. H2O2 is mainly used as an oxidant in this process. There are several advantages of ODS over HDS process: 1) reaction conditions are very mild such as ambient temperature and pressure, 2) expensive H2 gas is not required in ODS process, so capital investment in this process is lower than HDS, and 3) there are high potentials for desulfurization of sterically hindered thiophene derevatives. In spite of having advantages of this process over HDS, there are some disadvantages of this process. These include: 1) the difficulties in separation and recovery of the catalysts after the reactions, 2) the low utilization efficiency of H2O2, 3) their low-oxidation activity and their low selectivity for the sulphides present in fuel oils, and 4) the introduction of other components to the oxidation systems [26]. This process is suitable for small and medium size refineries, especially those that are isolated and not located close to a hydrogen pipeline.

1.3.3 Desulfurization by Ionic Liquids

Liquid-liquid extraction is applied in industries for the separation of mixtures. Currently organic solvents are used for removing sulphur compounds from refinery oils using extraction as a separation technique. The extraction efficiency depends on selected solvent for specific extraction process. The conventional solvents used for extraction are highly volatile, flammable, and toxic. Ionic liquids have replaced organic solvents to improve environmental friendliness of conventional extraction techniques due to having high chemical and thermal stability, non-flammability, non-toxicity, negligible vapour pressure and recyclability. The liquid-liquid extraction using ionic liquids (ILs) is an attractive approach for removing sulphur compounds. This approach is very effective for removing thiophene, benzothiophene, dibenzothiophene etc. from diesel oil. The advantages of this operation are mild process conditions, simplistic operation option, and low energy consumption. ILs can be designed with respect to new physiochemical properties by changing their cation and anion combination [21].

1.3.4 Bio - desulfurization

Bio-desulfurization is the process for removing /reducing sulphur content from petroleum oils using biocatalyst. The conversion rates using bio-desulfurization are lower than HDS technique. Therefore this technique is considered as a complementary process, after removing bulk sulphur using HDS process. The cleavage of carbon-sulphur bonds can be performed either with an aerobic or an anaerobic mechanism. Biocatalysts are specific in nature i.e. most microorganisms can remove sulphur from only one type of compounds. For example bio-desulfurization using Rhodococcus erythropolis IGTS8 can remove sulphur from DBT and its derivatives, and Gordonia sp. 213E can remove sulphur from BT and its derivatives [22]. Some microorganisms have the ability to remove sulphur from both BT and DBT [23].

1.3.5 Desulfurization by Adsorption

Catalytic HDS is not appropriate for reducing sulphur content to a level of ultra deep hydrodesulfurized oil (0.1 - 10 ppm-S) because of more high cost H2 and larger reactor volume are required. The fuel properties may also be changed at this stage such as colour and octane number. Desulfurization by adsorption requires a solid sorbent which can selectively adsorb organosulfur compounds efficiently from refinery oils. Desulfurization by adsorption can be divided into two groups; 'adsorptive desulfurization' and 'reactive adsorption desulfurization'.

Liquid hydrocarbon fuels can be desulfurized using porous materials by adsorptive desulfurization technique. Many authors have studied adsorptive desulfurization using Activated carbon (AC), activated carbon fibre (ACF), and metals loaded AC and ACF [24-26]. Adsorptive desulfurization is carried out at ambient temperature and pressure, which eliminates the need of large volume reactor, provides low operational cost, and fuel properties do not alter. Organosulfur compounds are physically adsorbed onto solid sorbent during the adsorptive desulfurization. Regeneration of sorbent is possible in adsorptive desulfurization and it is done by flushing the spent sorbent with a desorbent. Organosulfur compounds are chemically interacted with solid sorbent during the reactive adsorption desulfurization. Sulphur is attached to sorbent and the S- free hydrocarbon is released into the purified fuel stream. Depending on the process applied; H2S, S, or SOx, may be produced during the regeneration of spent sorbent. Sorbent properties such as adsorption capacity, selectivity for the organosulfur compounds, durability, and regenerability etc. play an important role for determining adsorption efficiency.

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